A PERMANENT MAGNETIC MATERIAL

There is presented a method for providing a permanent magnetic material comprising hexagonal ferrites, which method does not necessitate neither large magnetic fields nor organic solvents. The produced permanent magnetic materials have excellent properties, in particular in terms of energy product, such as in terms of energy product and density. In further aspects, the invention relates to particles for providing the permanent magnetic material, and a corresponding method of manufacture. In particular embodiments of the invention the hexagonal ferrite is given by CaFe12O19, SrFe12O19 or BaFe12O19, such as given SrFe12O19 or BaFe12O19.

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Description
FIELD OF THE INVENTION

The present invention relates to permanent magnetic materials, and in particular relates to a permanent magnetic material and a corresponding method of manufacture.

BACKGROUND OF THE INVENTION

Typically, high performance permanent magnets comprise rare-earth elements, such as neodymium, which may be expensive and/or difficult to access. It would be advantageous to provide new permanent magnetic materials without rare-earth elements, such as magnetic materials without rare-earth elements which have improved properties.

Hence, an improved permanent magnetic material that does not include rare-earth elements would be advantageous, and in particular a permanent magnetic material that does not include rare-earth elements which has improved energy product with respect to prior art permanent magnetic materials that do not include rare-earth elements would be advantageous.

SUMMARY OF THE INVENTION

In particular, it may be seen as an object of the present invention to provide a magnetic material that solves the above mentioned problems of the prior art with being dependent on rare-earth elements. Furthermore, it may be seen as an object of the present invention to provide a permanent magnetic material that does not include rare-earth elements and which has improved energy product with respect to prior art permanent magnetic materials that do not include rare-earth elements. It is a further object of the present invention to provide an alternative to the prior art.

Thus, the above described object and several other objects are intended to be obtained in aspects of the invention described below, such as within a first aspect of the invention by providing a permanent magnetic material and a corresponding method of manufacture.

The invention may be particularly advantageous, such as is particularly, but not exclusively, advantageous for obtaining a method for preparing particles comprising hexagonal ferrite. According to a first aspect of the invention, there is provided a method for preparing particles comprising hexagonal ferrite for a magnetic material, the method comprising

    • Forming a precursor solution comprising elements of the hexagonal ferrite,
    • Feeding the precursor solution, such as a precursor solution containing a precipitate, into a supercritical reactor, so as to carry out a supercritical synthesis of the particles, wherein the particles have an anisotropic shape and wherein the size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, such as the size being smaller than 100 nm, such as the size being smaller than 50 nm.

A possible advantage of this method may be, that it enables providing particles which may be compacted into a permanent magnetic material, such as compacted into a permanent magnetic material with relatively high energy product, such as without the use of high magnetic field and/or organic solvents. Thus, the method enables producing particles which facilitate a relatively cheap and safe way of producing permanent magnetic materials with a high energy product. Another advantage may be, that the method enables dispensing with rare earth elements.

Hexagonal ferrites, also known as hexaferrites, are known in the art, and includes M-type ferrites, such as BaFe12O19 (BaM or barium ferrite), SrFe12O19 (SrM or strontium ferrite), and cobalt-titanium substituted M ferrite, Sr— or BaFe12_2xCoxTixO19 (CoTiM), Z-type ferrites (Ba3Me2Fe24O41) such as Ba3Co2Fe24O41, or Co2Z, Y-type ferrites (Ba2Me2Fe12O22), such as Ba2Co2Fe12O22, or Co2Y, W-type ferrites (BaMe2Fe16O27), such as BaCo2Fe16O27, or Co2W, X-type ferrites (Ba2Me2Fe28O46), such as Ba2Co2Fe28O46, or Co2X, U-type ferrites (Ba4Me2Fe36O60), such as Ba4Co2Fe36O60, or Co2U. Hexagonal ferrites are described in detail in the article “Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics”, Robert C. Pullar, Progress in Materials Science 57 (2012) 1191-1334, which reference is hereby incorporated in entirety be reference.

It may furthermore be noted that hexagonal ferrites, also known as hexaferrites, are known in the art, and includes M-type ferrites, such as BaFe12O19 (BaM or barium ferrite or BaO.6Fe2O3), SrFe12O19 (SrM or strontium ferrite or BaO.6Fe2O3), CaFe12O19 (CaM or calcium ferrite or CaO.6Fe2O3) and furthermore including particle substitution of the type BaxSr1-xFe12O19 or BaxCa1-xFe12O19 or CaxSr1-xFe12O19 or CayBaxSr1-x-yFe12O19, and furthermore including, Z-type ferrites (Ba3Me2Fe24O41) such as Ba3Fe2Fe24O41, or Y-type ferrites (Ba2Me2Fe12O22), such as Ba2Fe2Fe12O22, or Fe2Y, W-type ferrites (BaFe2Fe16O27), such as BaFe2Fe16O27, or Fe2W, X-type ferrites (Ba2Me2Fe28O46), such as Ba2Fe2Fe28O46, or Fe2X, U-type ferrites (Ba4Me2Fe36O60), such as Ba4Fe2Fe36O60, or Fe2U. Hexagonal ferrites are described in detail in the article “Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics”, Robert C. Pullar, Progress in Materials Science 57 (2012) 1191-1334, which reference is hereby incorporated in entirety be reference.

An advantage of employing hexagonal ferrites is that they all have a magnetocrystalline anisotropy (MCA), that is the induced magnetisation has a preferred orientation within the crystal structure by having an easy axis of magnetisation (known as uniaxial hexaferrites). In embodiments of the invention, the hexagonal ferrite has a magnetocrystalline anisotropy, such as an easy axis of magnetization. In embodiments of the invention, the hexagonal ferrite is an uniaxial ferrite.

By ‘anisotropic shape’ may be understood that the particles are not spherical, such as the particles are platelet or plate-like shaped (such as a length along each of two crystal axes being at least a factor of 2, such as 4, such as 6, such as 8, such as 10 times larger than a length along the third crystal axis), such as shaped as hexagonal-plates. The particles may, such as in some embodiments, be at least so anisotropic that the particles may take on, such as takes on, a preferred orientation during uniaxial pressing of the particles. The particles may in some embodiments be at least so anisotropic that most particles, such substantially all particles, such as all particles, take a similar orientation, such as substantially the same orientation, such as the same orientation, during uniaxial pressing of a plurality, such a large number, of particles. An advantage of this may be, that alignment may be realized during pressing without necessitating application of an external magnetic field.

‘Single domain’ is well known in the art. A single-domain particle may be defined as one in which the single-domain state has the lowest energy of all possible states.

Throughout this application reference is made to ‘magnetic material’ and ‘magnet’. It is understood, that a magnet may be composed of magnetic material. In terms of properties, such as energy product, density or crystallite sizes and orientations, it is further understood, that ‘magnet’ and ‘magnetic material’ may be used interchangeably.

In an embodiment of the invention there is provided a method, wherein the supercritical synthesis comprises heating of the precursor solution, and wherein said heating is achieved by raising the temperature at a rate of at least 10° C./second, such as at a rate of at least 20° C./second, such as at a rate of at least 20° C./second. A possible advantage of heating of the precursor material at such relatively high rate, e.g., (>10° C./s) may be that it causes a burst of nucleation.

In an embodiment of the invention there is provided a method, wherein a reaction time period during the supercritical synthesis is 10 minutes or less, such as 8 minutes or less, such as 6 minutes or less, such as 4 minutes or less, such as 2 minutes or less. An possible advantage of said reaction time period being relatively short (e.g., <10 minutes) may be that it reduces or prevents grain growth.

In an embodiment of the invention there is provided a method, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba), such as from the group comprising Strontium (Sr) and Barium (Ba), such as selected from the group consisting of Strontium (Sr) and Barium (Ba). A possible advantage of employing calcium and/or strontium, such as CaFe12O19 or SrFe12O19, may be that it comprises only elements which are relatively harmless in regards to health and environment.

In an embodiment of the invention there is provided a method, wherein a step of forming the precursor solution comprises

    • dissolving
      • a compound comprising iron (Fe), such as a compound selected from the group comprising, such as consisting of:
        • iron nitrate, such as (Fe(NO3)3.9H2O,
        • iron chloride, such as FeCl3, and
        • iron sulphate, Fe(SO4)3,
    • and/or dissolving one or more compounds selected from the group comprising, such as consisting of:
      • a compound comprising strontium (Sr), such as a compound selected from the group comprising, such as consisting of:
        • strontium nitrate, such as Sr(NO3)2,
        • strontium hydroxide, such as Sr(OH)2,
        • strontium chloride, such as SrCl2,
      • a compound comprising Barium (Ba), such as a compound selected from the group comprising, such as consisting of:
        • barium nitrate, such as Ba(NO3)2,
        • barium hydroxide, such as Ba(OH)2,
        • barium chloride, such as BaCl2,
      • a compound comprising Calcium (Ca), such as a compound selected from the group comprising, such as consisting of:
        • calcium nitrate, such as Ca(NO3)2,
        • calcium hydroxide, such as Ca(OH)2,
        • calcium chloride, such as CaCl2.

In exemplary embodiments, a compound comprising iron (Fe) is dissolved together with any one of compound chosen from the group comprising, such as consisting of: Sr(OH)2, Sr(NO3)2, and SrCl2, Ba(OH)2, Ba(NO3)2, BaCl2.

In an embodiment of the invention there is provided a method, wherein a step of forming the precursor solution comprises

    • dissolving
      • a compound comprising iron (Fe), such as a compound selected from the group comprising, such as consisting of:
        • iron nitrate, such as (Fe(NO3)3.9H2O,
        • iron chloride, such as FeCl3, and
        • iron sulphate, Fe(SO4)3,
    • and/or dissolving
      • a compound comprising strontium (Sr), such as a compound selected from the group comprising, such as consisting of:
        • strontium nitrate, such as Sr(NO3)2,
        • strontium hydroxide, such as Sr(OH)2,
        • strontium chloride, such as SrCl2.

In an embodiment of the invention there is provided a method, wherein a step of forming the precursor solution comprises

    • dissolving
      • a compound comprising iron (Fe), such as a compound selected from the group comprising, such as consisting of:
        • iron nitrate, such as (Fe(NO3)3.9H2O,
        • iron chloride, such as FeCl3, and
        • iron sulphate, Fe(SO4)3,
    • and/or dissolving
      • a compound comprising Barium (Ba), such as a compound selected from the group comprising, such as consisting of:
        • barium nitrate, such as Ba(NO3)2,
        • barium hydroxide, such as Ba(OH)2,
        • barium chloride, such as BaCl2.

In an embodiment of the invention there is provided a method, wherein a step of forming the precursor solution comprises

    • dissolving
      • a compound comprising iron (Fe), such as a compound selected from the group comprising, such as consisting of:
        • iron nitrate, such as (Fe(NO3)3.9H2O,
        • iron chloride, such as FeCl3, and
        • iron sulphate, Fe(SO4)3,
    • and/or dissolving
      • a compound comprising Calcium (Ca), such as a compound selected from the group comprising, such as consisting of:
        • calcium nitrate, such as Ca(NO3)2,
        • calcium hydroxide, such as Ca(OH)2,
        • calcium chloride, such as CaCl2.

In an embodiment of the invention there is provided a method, wherein a step of forming the precursor solution comprises

    • dissolving
      • a compound comprising iron (Fe), such as a compound selected from the group comprising, such as consisting of:
        • iron nitrate, such as (Fe(NO3)3.9H2O,
        • iron chloride, such as FeCl3, and
        • iron sulphate, Fe(SO4)3,
    • and/or dissolving at least two, such as two, compounds selected from the group comprising, such as consisting of:
      • a compound comprising strontium (Sr), such as a compound selected from the group comprising, such as consisting of:
        • strontium nitrate, such as Sr(NO3)2,
        • strontium hydroxide, such as Sr(OH)2,
        • strontium chloride, such as SrCl2,
      • a compound comprising Barium (Ba), such as a compound selected from the group comprising, such as consisting of:
        • barium nitrate, such as Ba(NO3)2,
        • barium hydroxide, such as Ba(OH)2,
        • barium chloride, such as BaCl2,
      • a compound comprising Calcium (Ca), such as a compound selected from the group comprising, such as consisting of:
        • calcium nitrate, such as Ca(NO3)2,
        • calcium hydroxide, such as Ca(OH)2,
        • calcium chloride, such as CaCl2.

For example, a step of forming the precursor solution comprises dissolving any one of Fe/Sr/Ba, Fe/Sr/Ca, Fe/Ba/Ca, Fe/Ca/Sr/Ba, Sr/Ba, Sr/Ca, Ba/Ca, Ca/Sr/Ba. It may be understood that a hexagonal ferrite XFe12O19 may thereby be provided, wherein X is a mixture comprising the dissolved species, e.g., Sr/Ba (i.e., Sr and Ba), Sr/Ca (i.e., Sr and Ca) or Ba/Ca (Ba and Ca).

In an embodiment of the invention there is provided a method, wherein a step of forming the precursor solution comprises

    • dissolving iron nitrate, such as (Fe(NO3)3.9H2O, and strontium nitrate, such as Sr(NO3)2.

In an embodiment of the invention there is provided a method, wherein the precursor solution has a Sr:Fe ratio of 1:1.

In an embodiment of the invention there is provided a method, wherein the precursor solution has a X:Fe ratio of 1:1 or Rx:1 where Rx is a number within 0.083-5, such as within ( 1/12)-5, such as within 0.083-2, such as within ( 1/12)-2, such as within 0.1-5, such as within 0.1-2 (i.e., X being within 1/10 the amount of Fe and 2 times the amount of Fe), such as within 0.125-5, such as within 0.125-2 (i.e., X being within ⅛ the amount of Fe and 2 times the amount of Fe), such as within 0.25-1.75, such as within 0.3-1.6, such as within 0.5-1.5, such as within 0.7-1.3, such as within 0.8-1.2, such as within 0.9-1.1, such as Rx being substantially 1, where X is chosen from the group comprising, such as consisting of: Strontium (Sr), Barium (Ba) and Calcium (Ca).

In an embodiment of the invention there is provided a method, wherein the precursor solution has a Sr:Fe ratio of 1:1 or RSr:1 where RSr is a number within 0.125-2 (corresponding to the concentration of Sr being within ⅛ the amount of Fe to 2 times the amount of Fe), such as within 0.25-1.75, such as within 0.5-1.5 such as within 0.75-1.25, such as within 0.9-1.1, such as RSr being substantially 1, such as RSr being 1.

In an embodiment of the invention there is provided a method, wherein the precursor solution has a Ba:Fe ratio of 1:1 or RBa:1 where RBa is a number within 0.1-2 (corresponding to the concentration of Ba being within 1/10 the amount of Fe to 2 times the amount of Fe), such as within 0.25-1.75, such as within 0.5-1.5, such as within 0.75-1.25, such as within 0.9-1.1, such as RBa being substantially 1, such as RBa being 1.

In an embodiment of the invention there is provided a method, wherein the precursor solution has a Ca:Fe ratio of 1:1 or RCa:1 where RCa is a number within 0.1-2, such as RCa being 1.

In an embodiment of the invention there is provided a method, wherein the method further comprises adding a base, such as an alkaline solution, to the precursor solution, and wherein a concentration of Fe3+ iron(III) within the precursor solution when adding the base is within 0.05-0.750 M. An advantage of this may be, that it enables controlling the particle size.

In an embodiment of the invention there is provided a method, wherein the method further comprises adding a base, such as an alkaline solution, to the precursor solution, and wherein a concentration of Fe3+ iron(III) within the precursor solution when adding the base is within 0.05-0.750 M, such as within 0.05-0.5, and wherein a final concentration of the precursor is within 0.05-0.50 M, such as 0.05, and is achieved through dilution with base and/or water.

In an embodiment of the invention there is provided a method, wherein an alkaline solution is added in a concentration being at least 1.25 times, such as at least 1.50 times, such as at least 2 times, such as 2 times, the concentration of nitrates from both the iron nitrate and the strontium nitrate. A possible advantage of having the ratio being relatively high may be that impurities, such as α-Fe2O3 may form at lower values of the ratio.

In an embodiment of the invention there is provided a method, wherein the alkaline solution comprises a substance selected from the group comprising: NaOH, KOH and LiOH, such as from the group comprising NaOH and KOH. In an embodiment of the invention there is provided a method, wherein the alkaline solution comprises a substance selected from the group consisting of: NaOH, KOH and LiOH, such as from the group consisting of: NaOH and KOH. A possible advantage of employing these substances may be that they ensure that particles of correct composition are formed. An advantage of employing NaOH or KOH may be that they yield a high purity.

In an embodiment of the invention there is provided a method, wherein the alkaline solution is added drop wise under constant stirring until a dark red precipitate is formed.

In an embodiment of the invention there is provided a method comprising feeding the precursor solution, such as the precursor solution containing the dark red precipitate, into a supercritical reactor.

In an embodiment of the invention there is provided a method, wherein the precursor solution is fed into the supercritical reactor at the flow rate of within 0.5-50 mL/min, such as within 1-10 mL/min, such as 5 mL/min.

In an embodiment of the invention there is provided a method comprising feeding deionized water into the supercritical reactor at a flow rate of within 0.15-150 mL/min, such as within 3-30 mL/min, such as 15 mL/min. In and embodiment, the method comprises feeding the deionized water into the supercritical reactor from a second line.

In an embodiment of the invention there is provided a method comprising

    • feeding the precursor solution, such as the precursor solution containing the dark red precipitate, into the supercritical reactor, at a first flow rate,
    • feeding deionized water into the supercritical reactor at a second flow rate,
    • wherein the ratio of the first flow rate and the second flow rate is between 1:0.3 and 1:30, such as between 1:1 and 1:10, such as 1:3.

In an embodiment of the invention there is provided a method, wherein the precursor solution and the deionized water meet at a mixing point.

According to a second aspect of the invention, there is provided particles comprising hexagonal ferrite for a magnetic material, such as particles prepared according to the first aspect, wherein the particles have an anisotropic shape and wherein the size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, such as the particles being smaller than 100 nm, such as the particles being smaller than 50 nm.

In an embodiment of the invention there is provided particles, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group comprising Strontium (Sr) and Barium (Ba). In an embodiment of the invention there is provided particles, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group consisting of: Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group consisting of: Strontium (Sr) and Barium (Ba).

When referring to “size”, as such, it may be understood that the size ‘as such’ refers to the greatest distance between any two points of a particle.

In an embodiment of the invention there is provided particles, wherein the hexagonal ferrite comprises XFe12O19, where X is combination comprising, such as consisting of, two or three elements selected from the group comprising, such as consisting of, Calcium (Ca), Strontium (Sr) and Barium (Ba), such as any one of Ca/Sr, Ca/Ba, Sr/Ba, or Ca/Sr/Ba.

In an embodiment of the invention there is provided particles, wherein the dimensions of the particles may be described by dimensions along a first crystal axis (a-axis), a second crystal axis (b-axis) and a third crystal axis (c-axis), and wherein the dimensions of the particles are substantially larger along the first crystal axis (a-axis) and/or the second crystal axis (b-axis) relative to the dimension along the third crystal axis (c-axis).

In an embodiment of the invention there is provided particles, wherein the dimensions of the particles are at least 2 times larger, such as at least 3 times larger, such as at least 4 times larger, such as at least 5 times larger, such as at least 10 times larger, such as substantially 10 times larger, along the first crystal axis (a-axis) and/or the second crystal axis (b-axis) relative to the dimension along the third crystal axis (c-axis). The ratio between the largest lateral dimension (along the first and second crystal axis) and the dimension orthogonal thereto (along the third crystal axis) may be referred to as aspect ratio. A possible advantage of having a larger aspect ratio may be that it enables the properties of a permanent magnetic material made from pressing or compacting the particles, to be better. In an embodiment, the particles are plate like, such as both of the dimensions along the first and second crystal axis are substantially larger, such as at least 2 times larger, such as at least 5 times larger, such as at least 10 times larger, such as substantially 10 times larger, than the dimension along the third crystal axis.

In an embodiment of the invention there is provided particles, wherein a dimension of the particles along the first crystal axis (a-axis) is within 20-40 nm, such as substantially 30 nm, such as 30 nm, and wherein a dimension along the second crystal axis (b-axis) is within 20-40 nm, such as substantially 30 nm, such as 30 nm, and wherein a dimension along a third crystal axis (c-axis) is within 2-4 nm, such as substantially 3 nm, such as 3 nm. In an alternative formulation, the size of the particles may be described as being given by [20-40]×[20-40]×[2-4] nm3, such as 30×30×3 nm3. Particles of this size may during a compaction, such as a pressing, grow into sizes, such as [a×b×c]=60×60×12 nm3.

In an embodiment of the invention there is provided particles, wherein a dimension of the particles along the first crystal axis (a-axis) is substantially 30 nm and wherein a dimension along the second crystal axis (b-axis) is substantially 30 nm and wherein a dimension along a third crystal axis (c-axis) is substantially 3 nm. In an alternative formulation, the size of the particles may be described as being given by 30×30×3 nm3.

In an embodiment of the invention there is provided particles, wherein the energy product (BHmax) of the particles, such as the as prepared particles, is at least 0.1 kJ/m3, such as at least 1.0 kJ/m3, such as substantially 1 kJ/m3. By ‘as prepared particles’ is understood particles which have not been compacted, such as particles which have not been subjected to pressure substantially above atmospheric pressure, such as particles which have not been subjected to pressure substantially above atmospheric pressure and/or a temperature substantially above room temperature.

According to a third aspect of the invention, there is provided a method for preparing a permanent magnetic material comprising hexagonal ferrite, the method comprising, such as comprising the steps of, such as comprising the successive steps of:

    • obtaining particles comprising hexagonal ferrite, such as particles prepared according to the first aspect and/or provided according to the second aspect, which particles have an anisotropic shape, such as an anisotropic shape and a size being smaller than or equal to a size enabling individual particles to become single domain magnets, such as the size enabling individual particles to become single domain magnets, such as smaller than 100 nm, such as the particles being smaller than 50 nm,
    • compacting the particles into a permanent magnetic material, such as the step of compacting the particles into a permanent magnetic material comprises applying a uniaxial pressing step, wherein the step of compacting the particles comprises applying a pressure above atmospheric pressure and a temperature above room temperature, such as said temperature being above a blocking temperature of said particles, such as said pressure and temperature being applied in a temporally overlapping manner (such as at least a portion of the pressing above atmospheric pressure is temporally overlapping with at least a portion of the temperature being above room temperature, such as at least a blocking temperature of said particles), such as substantially simultaneously, such as simultaneously, and wherein a size of the particles after the step of compacting are smaller than or equal to a size enabling individual particles to become single domain magnets, such as the size enabling individual particles to become single domain magnets.

An advantage of the present method may be, that it enables preparing a permanent magnetic material, such as a permanent magnetic material comprising multiple particles comprising hexagonal ferrite, such as a high quality permanent magnetic material, which method may be seen as simple, cost efficient, cheap and/or environmentally friendly. For example, the method may facilitate dispensing with a need for applying an external magnetic field during compacting. For example, the method may facilitate dispensing with a need for employing glue and/or solvents during compacting. By ‘compacting’ may be understood a process of transforming a plurality of particles into a single, coherent material comprising said plurality of particles. It may in general be understood, that the process of compacting may last less than 20 minutes, such as less than 15, such as less than 10 minutes, such as 9 minutes or less. An advantage of a relatively short period of compaction, may be that grain growth is accordingly limited. It may in general be understood, that the temperature above room temperature may be at least a blocking temperature of said particles. It may in general be understood, that the pressure above atmospheric pressure suffice in order to make the particles adhere to each other and obtain a preferential orientation at the temperature above room temperature.

In an embodiment of the invention there is provided a method, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba), such as from the group consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba).

In an embodiment of the invention there is provided a method, wherein

    • obtaining particles comprising hexagonal ferrite, which particles have an anisotropic shape,
      comprises obtaining particles comprising hexagonal ferrite, where the anisotropic shape is a plate like shape, and wherein the size of the particles (such as the size of the particles before initiation of compacting the particles) is at most 100 nm, such as at most 75 nm, such as at most 50 nm, such as at most 40 nm, such as the sizes along the crystal axes [a; b; c] being within [20-40 nm; 20-40 nm; 2-4 nm].

In an embodiment of the invention there is provided a method, wherein the method comprises reducing or breaking a magnetic interaction between the particles when compacting the particles and/or during compacting the particles, so as to allow alignment of the particles when compacting the particles and/or during compacting the particles. This may be seen as advantageous, since it facilitates alignment of the particles, since a magnetic interaction between the particles may impede the alignment. The magnetic interaction may be reduced or broken by supplying sonic and/or thermal energy, such as by supplying sufficient thermal energy for exceeding the blocking temperature of the particles. An advantage of this may be that it enables a method for preparing a permanent magnetic material without using glue and/or solvents. In an embodiment of the invention there is provided a method, wherein the method does not comprise having glue and/or solvent between the particles during compacting the particles.

In an embodiment of the invention there is provided a method, wherein the method comprises:

    • pre-heating of the particles, wherein said pre-heating comprises applying a pre-heating temperature above room temperature to said particles before compacting the particles, so that a temperature of said particles is the pre-heating temperature when the compacting is initiated.

In an embodiment, the step of compacting the particles is carried out without an external magnetic field applied, such within a magnetic field being 0 T. In an embodiment, the step of compacting the particles is carried out in an external magnetic field applied which is within the range 0-1 T, such as within 0-0.5 T, such as within 0-0.25 T, such as within 0-0.1 T, such as within 0-0.05 T.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material comprising hexagonal ferrite, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group comprising Strontium (Sr) and Barium (Ba). In an embodiment of the invention there is provided a method for preparing a permanent magnetic material comprising hexagonal ferrite, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group consisting of: Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group consisting of: Strontium (Sr) and Barium (Ba).

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material comprising hexagonal ferrite wherein the obtained particles are smaller compared to the particles in the permanent magnetic material, such as wherein the particles are enlarged during the step of compacting the particles.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the step of compacting the particles comprises uniaxial pressing. It may be understood that in uniaxial pressing, the pressure is applied along an axis, such as a 1-dimensional axis. The pressing may be carried out by placing the particles between two punches and moving one or both punches along said axis, such as towards each other in opposite directions, so as to apply a pressure on the particles, such as placing the particles, such as in the form of a powder, between two punches and applying a pressure via the punches along a single direction. A possible advantage of uniaxial pressing may be, that it enables alignment of the anisotropic particles.

In a particular embodiment, the step of compacting the particles comprises uniaxial hot pressing. Uniaxial hot pressing may alternatively be phrased hot uniaxial pressing (HUP). It is understood that in uniaxial hot pressing, heat is supplied to the particles during pressing. In a particular embodiment, the step of supplying heat comprises a step chosen from the group comprising, such as consisting of:

    • resistive heating, such as applying a current through the particles, such as applying a pulsed DC current through the particles,
    • induction heating,
    • heating via a flux of incoming electromagnetic radiation, such as heating via light, such as via LASER,
    • microwave heating.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the step of compacting the particles comprises Spark Plasma Sintering (SPS), such as the method comprising loading the particles into a pressing tool (such as a graphite pressing tool) wherein uniaxial pressure is applied to punches and a pulsed DC current is directed through the pressing tool and the particles. A possible advantage of employing SPS, may be that it enables compacting particles into a permanent magnetic material without having to use a large external magnetic field and/or organic solvents. Another possible advantage of SPS is that it enables fast heating of the particles, which in turn enable that particles are heated above a blocking temperature so fast that the particles do not have enough time to grow too large.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material wherein the pressure is at least 20 MPa, such as at least 40 MPa, such as at least 60 MPa, such as at least 80 MPa, such as 80 MPa, such a below 100 MPa, such as between 20 MPa and 100 MPa. A possible advantage of employing relatively large pressure may be that it may enable yielding a high density of the final permanent magnetic material.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the change from room temperature to said temperature above room temperature, such as a blocking temperature, may comprise temperature changes at a rate of at least 10° C./minute, such as at least 25° C./minute, such as at least 50° C./minute, such as at least 75° C./minute, such as at least 100° C./minute, such as substantially 100° C./minute.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the direct pulsed DC current is applied so as to heat the particles at a rate of at least 10° C./minute, such as at least 25° C./minute, such as at least 50° C./minute, such as at least 75° C./minute, such as at least 100° C./minute, such as substantially 100° C./minute.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the direct pulsed DC current is applied so as to heat the particles to a temperature of at least 800° C., such as at least 875° C., such as 950° C., such as below 1450° C., such as between 800° C. and 1450° C. A possible advantage of employing relatively high temperature may be that it enables sintering the particles. Another possible advantage of heating to a relatively high temperature, such as heating to or above a magnetic phase transition, may be that it reduces or breaks magnetic adhesion of the particles. Breakage of the magnetic adhesion may in turn aid alignment of the particles under the applied pressure.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein at least partially during compacting the particles into a permanent magnetic material

    • the particles are heated to a temperature of at least 800° C., such as at least 875° C., such as 950° C., such as between 800° C. and 1450° C., and
    • the pressure is at least 20 MPa, such as at least 40 MPa, such as at least 60 MPa, such as at least 80 MPa, such as 80 MPa, such a below 100 MPa, such as between 20 MPa and 100 MPa.

An advantage of having both relatively high temperature and relatively high pressure during compacting of the particles, may be that it enables improving the alignment of the particles, since the relatively high temperature may reduce or break a magnetic interaction between the particles, and the applied pressure may then ensure that the anisotropic particles align, thus the temperature and pressure may work synergistically together to improve alignment. It may in general be understood, that the temperature above room temperature may be at least a blocking temperature of said particles.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the temperature above room temperature is held for at least 1 minute, such as at least 2 minutes, such as substantially 2 minutes, such as at least 5 minutes, such as at least 10 minutes, before cooling to room temperature. A possible advantage of employing relatively long time may be that it enables improved properties of the final permanent magnetic material, such as through increasing a size of the particles, such as the grain size of the particles.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the step of obtaining particles, comprises

    • preparing particles according to the first aspect, or
    • obtaining particles according to the second aspect.

In an embodiment of the invention there is provided a method for preparing a permanent magnetic material, wherein the method further comprises annealing the permanent magnetic material, such as annealing for at least 1 hour, such as at least 2 hours, such as at least 4 hours, such as 4 hours, such as between 1-10 hours, such as between 2-6 hours, such as between 2.5-5 hours, such as 4 hours, such as annealing at a temperature of between 800-1200° C., such as annealing at a temperature of between 800-1000° C., such as at a temperature of 850° C., such as annealing for 4 hours at 850° C. A possible advantage of annealing may be that it improves the properties, such as the energy product of the permanent magnetic material. In a specific embodiment, the annealing may be preceded by a heating step, such as heating within 1 hour from 750° C. to 850° C. Another advantage of annealing may be that it renders it possible to obtain particles which are relatively small, such as smaller than size enabling individual particles to become single domain magnets (which entails relatively low blocking temperature), exceeding the relatively low blocking temperature (which may be realized fast and/or in simple equipment since the blocking temperature is relatively low), compacting the particles (which will then be relatively small, such as too small to be single domain magnets), annealing so as to increase the particle size into a size enabling individual particles to become single domain magnets. Thus, besides enabling optimization, annealing also enables utilizing of otherwise too small particles and optionally gain benefits from using such small particles.

According to a fourth aspect of the invention, there is provided a permanent magnetic material comprising particles comprising hexagonal ferrite, such as a permanent magnetic material prepared according to the third aspect, wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, such as the size enabling individual particles to become single domain magnets.

When referring to a permanent magnetic material, such as within the fourth aspect, it may be understood, that the permanent magnetic material is a multi-grain material, such as a macroscopically sized material, such as the material being at least 1 mm3, such as at least 1 cm3.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the particles have an anisotropic shape, such as the particles having a plate like shape, such as hexagonal plate like shape.

In an embodiment of the invention there is provided a permanent magnetic material, wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, such as a size enabling individual particles to become single domain magnets, such as 100 nm or less, such as 50 nm or less,

and wherein the particles have an anisotropic shape, such as the particles having a plate like shape, such as the particles having an aspect ratio of 2 or more, such as having an aspect ratio of 5 or more, such as having an aspect ratio of 10 or more,
and wherein crystallites in the permanent magnetic material have a preferential orientation, such as a texture index of at least 2.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the hexagonal ferrite in the permanent magnetic material occupies at least 90 vol % of the volume, such as at least 93 vol %, such as at least 95 vol %, such as at least 96 vol %, such as at least 97 vol %. An advantage of having the hexagonal ferrite occupying a large volume percentage may be, that it enables better magnetic properties, such as a higher energy product and/or energy density, since less space is wasted around the hexagonal ferrite material.

In an embodiment of the invention there is provided a permanent magnetic material, wherein impurities, such as impurities being material not being said hexagonal ferrite, in the permanent magnetic material contribute to less than 3 wt %, such as less than 2.5 wt %, such as less than 2 wt %, such as less than 1.5 wt %, such as less than 1 wt %, such as less than 0.5 wt %, such as less than 0.25 wt %, such as less than 0.1 wt %. ‘Impurities’ may be understood to relate to any material other than the hexagonal ferrite. For example, glue and/or solvent residues may be considered impurities. An advantage of having no or only a relatively small amount of impurities may be that it enables better magnetic properties, such as a higher energy product and/or energy density, since less space is wasted on impurities.

In an embodiment of the invention there is provided a permanent magnetic material, wherein dimensions of the particles are at least 2 times larger, along a first crystal axis (a-axis) and/or a second crystal axis (b-axis) relative to a dimension along a third crystal axis (c-axis).

In general, a plate like shape may be understood to be a shape wherein both of the dimensions along the first and second crystal axis are substantially larger, such as at least 2 times larger, such as at least 5 times larger, such as at least 10 times larger, such as substantially 10 times larger, than the dimension along the third crystal axis.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group comprising Strontium (Sr) and Barium (Ba). In an embodiment of the invention there is provided a permanent magnetic material, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group consisting of: Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group consisting of: Strontium (Sr) and Barium (Ba).

In an embodiment of the invention there is provided a permanent magnetic material, wherein crystallites in the permanent magnetic material have a preferential orientation, such as the crystallites in the permanent magnetic material being substantially aligned, such as the crystallites in the permanent magnetic material being aligned. An advantage of this may be that it enables providing a collection of oriented single domain nanoparticles, which may be beneficial for producing a larger net magnetisation than non-oriented particles or a bulk material containing multiple magnetic domains.

In an embodiment of the invention there is provided a permanent magnetic material, wherein a texture index of the permanent magnetic material is at least 2, such as at least 2.5, such as at least 3.0, such as at least 3.25, such as at least 3.5, such as at least 4.0, such as at least 4.5, such as at least 5, such as at least 6, such as at least 7, such as at least 8, such as at least 9, such as at least 10, such as at least 11, such as at least 12, such as at least 15, such as at least 17, such as at least 17.2, such as 17.2.

Texture index is well known in the art and is to be understood as is known in the art. A description of the texture index can for example be found in “Texture Analysis in Materials Science—Mathematical Methods”, Bunge H-J, (1982), (London: Butterworths), which is hereby incorporated by reference in entirety, and section “4.8 Texture index” is in particular incorporated by reference.

In an embodiment of the invention there is provided a permanent magnetic material, wherein a ratio Jr(0°)/Jr(90°) is at least is at least 2, such as at least 2.5, such as at least 3.0, such as at least 3.5, such as at least 4.0, such as at least 4.35, such as 4.35, wherein said ratio Jr(0°)/Jr(90°) is a ratio between

    • a first remanence value Jr(0°) obtained at a first orientation of the permanent magnetic material with respect to an external magnetic applied field,
    • a second remanence value Jr(90°) obtained at a second orientation of the permanent magnetic material with respect to the applied external magnetic field, wherein the second orientation is orthogonal to the first orientation.

Any one of the first remanence value and the second remanence value may be obtained, such as measured, using a vibrating sample magnetometer (VSM), such as a vibrating sample magnetometer as is known in the art. It may be understood that the first orientation may be an orientation wherein an easy axis of the permanent magnetic material is aligned, such as is parallel, with the applied external magnetic field and/or that the second orientation may be an orientation wherein a hard magnetic axis of the permanent magnetic material is aligned, such as is parallel, with the applied external magnetic field.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the relative orientation of crystallites within the permanent magnetic material is at least 10%, such as at least 20%, such as at least 30%, such as at least 40%, such as at least 45%, such as 55%, such as the relative orientation being determined by the method of:

    • a. obtaining a powder X-ray diffraction pattern of the permanent magnetic material,
    • b. sum the intensities for the reflections along the 00l-direction starting with 004 and including 006, 008, 0010, 0012 and 0014 for the first 82 directions excluding 002 diffraction planes, i.e., obtain the value ΣI(00l) for the first 82 directions excluding 002 diffraction planes,
    • c. sum the intensities for all which are within the first 82 directions excluding 002, i.e., obtain the value ΣI(hkl) for the first 82 directions excluding 002, starting at the diffraction plane 004 and adding all reflection planes until hkl=315 giving a total of 81 reflections
    • d. calculate the relative orientation as the ratio ΣI(00l)/ΣI(hkl).

This method for quantifying the relative orientation will be referred to as the relative orientation quantification method.

It is understood that the relative orientation may be given as a ratio or a percentage, for example the ratio of 1:2=0.5 may be given in percentage as 50%.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the preferential orientation is with c-axis lattice planes substantially parallel, such as parallel to each other, such as substantially orthogonal, such as orthogonal to a pressing direction employed during a preparation of the permanent magnet. It may be understood that in some embodiments, crystallites with c-axis lattice planes which are aligned less than 25% from a direction being orthogonal to a pressing direction are construed to be substantially orthogonal to the pressing direction. It is understood, that these crystallites may also to some extent contribute to a magnetization along the c-axis.

In an embodiment of the invention there is provided a permanent magnetic material, wherein crystallites in the permanent magnetic material have a length along a third crystal axis (c-axis) of less than 250 nm, such as less than 200 nm, such as less than 150 nm, such as less than 100 nm, such as less than 75 nm, such as less than 50 nm, such as within 2 nm and 150 nm, such as within 2 nm and 100 nm, such as within 2 nm and 75 nm, such as within 3 nm and 150 nm, such as within 3 nm and 100 nm, such as within 3 nm and 75 nm, such as within 25 nm and 75 nm, such as within 50 nm and 70 nm, such as approximately 60 nm. A possible advantage of such dimension may be that it enables that individual particles correspond to single domain magnets. Another possible advantage of such dimension may be that it enables that individual particles correspond to single domain magnets and have a relatively low blocking temperature, which may allow the magnetic adhesion to be reduced or broken at relatively low temperatures, such as during a compaction of a powder of particles into the permanent magnetic material, such as when performing an SPS pressing. The nanoparticles have been made with sizes ranging from 20 nm to 70 nm along the a/b axis and from 2.4 nm to 11 nm along the c axis by varying the concentration of starting precursor material in the solution.

In an embodiment of the invention there is provided a permanent magnetic material, wherein crystallites in the permanent magnetic material have a length along a first crystal axis (a-axis) and/or second crystal axis (b-axis) of less than 250 nm, such as within 25-250 nm, such as less than 200 nm, such as less than 175 nm, such as less than 150 nm, such as less than 100 nm, such as less than 80 nm, such as less than 50 nm, such as within 30 nm and 175 nm, such as within 50 nm and 90 nm, such as within 60 nm and 80 nm, such as within 65 nm and 75 nm, such as approximately 70 nm. A possible advantage of such dimension may be that it enables that individual particles correspond to single domain magnets. Another possible advantage of such dimension may be that it enables that individual particles correspond to single domain magnets and have a relatively low blocking temperature, which may allow the magnetic adhesion to be reduced or broken at relatively low temperatures, such as during a compaction of a powder of particles into the permanent magnetic material, such as when performing an SPS pressing. In general, the crystallites in the permanent magnetic material may have a length along a first crystal axis (a-axis) and/or second crystal axis (b-axis) of 25 nm or more, which lower limit may be combined with any upper limit referred to above, such as within 25-250 nm. An advantage of this lower limit may be that it enables individual particles to form single-domain particles.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material has an energy product (BHmax) of more than 11 kJ/m3, such as more than 15 kJ/m3, such as more than kJ/m3, such as more than 25 kJ/m3, such as at least 26 kJ/m3, such as 26 kJ/m3, such as at least 28.5 kJ/m3, such as 28.5 kJ/m3.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material is shaped into a permanent magnet which has a diameter of 8 mm.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material is shaped into a permanent magnet which has a thickness of 1 mm.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material is shaped into a permanent magnet which has a diameter of at least 5 mm. In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material is shaped into a permanent magnet which has a thickness of at least 1 mm. In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material is shaped into a permanent magnet which has a diameter of at least 1 mm and wherein the permanent magnetic material is shaped into a permanent magnet which has a thickness of at least 0.1 mm. In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material is shaped into a permanent magnet with a volume of at least 1 mm3, such as at least 2, 5, 10, 20, 50, 100, 200, 500, 1000, 10000 or 100000 mm3.

In an embodiment of the invention there is provided a permanent magnetic material, wherein the permanent magnetic material has a density of at least 2.0 g/cm3, such as at least at least 3.0 g/cm3, such as at least at least 4.0 g/cm3, such as at least 4.5 g/cm3, such as at least 4.7 g/cm3, such as 4.7 g/cm3, such as at least at least 5.0 g/cm3, such as at least at least 5.2 g/cm3, such as substantially 5.3 g/cm3. An advantage of having a high density, such as at least 5.2 g/cm3, may be that the energy product may be higher. An advantage of having a high density may be that it enables a higher energy density.

In an embodiment of the invention there is provided a permanent magnetic material, which has little or no glue and/or little or no solvent between the particles. This may be realized by manufacturing according to the third aspect. An advantage of this may be that it enables a relatively high density of the magnetic material, such as a material where little space is wasted on, e.g., glue and/or solvent.

In an embodiment of the invention there is provided a device for inter-converting between electrical energy and kinetic energy, wherein the device comprises the permanent magnetic material, such as the permanent magnetic material prepared according to the fourth aspect. In another embodiment of the invention there is provided a device for inter-converting between electrical energy and kinetic energy, wherein the device comprises permanent magnetic material provided according to the third aspect.

The applications for such a device range is from kitchen appliances to gearless wind turbines and electrical cars. Permanent magnets are the key component in electro motors and dynamos, where the permanent magnets, e.g., together with a current conducting Cu coil, is responsible for the inter-conversion between electricity and motion.

The first, second and third and fourth aspect of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The permanent magnetic material and corresponding method of manufacture and the particles and corresponding method of manufacture according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

FIG. 1 illustrates the design of a permanent magnet on a logarithmic scale.

FIG. 2 shows illustrations of the structure of a single domain magnetic nanocrystal.

FIGS. 3(a)-(b) show photographs of a nanopowder.

FIG. 4 shows a schematic diagram of the hydrothermal flow synthesis setup.

FIGS. 5-6 show TEM and AFM images of nanoparticles of SrFe12O19.

FIG. 7 shows the XRPD pattern of the supercritical synthesized sample.

FIGS. 8-9 shows a hysteresis loop of as prepared SrFe12O19, respectively, hysteresis loops for different samples.

FIG. 10 is a schematic illustration of the working principles of a SPS press.

FIG. 11 illustrates platelets exposed to elevated pressure and temperature.

FIG. 12 shows normalized powder diffraction patterns.

FIGS. 13-14 show X-ray date corresponding to the magnetic material.

FIG. 15 shows the effect on the energy product of annealing.

FIG. 16 shows an image of an SrFe12O19 powder.

FIG. 17 shows a graph of particle size as a function of concentration of Fe(NO3)3.9H2O and Sr(NO3)2 when addition of the base is taking place.

FIG. 18 shows a graph of particle size as a function of the ratio of Ba:Fe.

FIG. 19 is a schematic of the pole figure measurement.

FIG. 20 shows the volume fraction of samples with a given alignment.

FIG. 21 shows reduced pole figures.

FIG. 22 shows the sample setup in the VSM.

FIG. 23 shows hysteresis curves for various samples.

 DETAILED DESCRIPTION OF AN EMBODIMENT Structural Control from Sub-Nanometer to Bulk

The design of permanent magnets is challenging as it involves structural control at all levels from atomic positions to the macroscopic arrangement of nanoparticles with a specific size and shape. On the sub-nanometer scale ferromagnetism predominantly originates from the self-rotation of unpaired electrons in atoms and quantum mechanical interactions may cause the magnetic spin of many atoms to align with respect to each other. For small particles all spins can be easily rotated by thermal energy and such compounds are known as superparamagnetic. At a specific size, which depends on the material, the spins become increasingly difficult to rotate and the particle becomes a single domain magnet Nanoparticles smaller than 25 nm are often superparamagnetic, while particles in the range of 25-250 nm are single domain magnets. Upon further size increase multiple domains with different magnetic orientations are introduced and this reduces the magnetic energy of the system. Thus, a collection of oriented single domain nanoparticles can produce a larger net magnetisation than a bulk material containing multiple magnetic domains. The challenge of the research project can be divided into different size domains: atomic, nanometre and micrometre length scales. At the atomic scale the goal is to synthesize structures giving rise to strong quantum mechanical interactions aligning the magnetic spins. The next step is to ensure that these atomic structures are reproduced in nanoparticles of appropriate size and shape. Finally, a large number of nanoparticles with perfected atomic structure, size, and shape must be compacted into a bulk material, where the individual nanoparticles are ordered with respect to each other. Control at all length scales is beneficial for making such high performance permanent magnets.

Thus, according to an embodiment of the invention there is provided a method for providing a permanent magnetic material, such as a magnet comprising XFe12O19 where X is an element selected from the group comprising, such as consisting of: Calcium (Ca), Strontium (Sr) and Barium (Ba), such as selected from the group comprising, such as consisting of: Strontium (Sr) and Barium (Ba), the method comprising the steps of

    • synthesizing structures giving rise to interactions, such as strong quantum mechanical interactions, aligning the magnetic spins, such as via providing structures exhibiting magnetocrystalline anisotropy,
    • ensuring that these atomic structures are reproduced in nanoparticles of appropriate size, such as enabling the nanoparticles to correspond to a size being smaller than or equal to a size enabling the individual particles to become a single magnetic domain, and appropriate shape, such as the shape being anisotropic, such as platelet shaped, such as the size and shape facilitating a breakage of magnetic interactions between said particles upon a heating of said particles so as to facilitate alignment of the particles,
    • compacting the particles into a bulk material, where the individual nanoparticles are ordered with respect to each other, optionally aided by uniaxial pressure and/or elevated temperature.

FIG. 1 illustrates the design of a permanent magnet on a logarithmic scale from femtometres to centimetres. Starting from the left is shown an unpaired electron 102 spinning, which gives rise to the magnetic moment. Next an iron atom 104 is coordinated to oxygen and this structure is placed in a nanometre sized unit cell 106. The size and shape of the nanoparticles are controlled to create a single domain magnetic particle 108 measuring 25-250 nm. These single domain particles are compacted into micrometre sized particles 110 with their magnetic axis pointing along the same direction. The final magnetic sample 112 is depicted in the illustration as being in the mm-cm range, but may in other embodiments be smaller or larger.

FIG. 2 shows illustrations of the structure of a single domain magnetic nanocrystal.

FIG. 2(a) shows a structure of a single domain magnetic nanocrystal 208. The structure may be probed with an X-ray powder diffraction instrument, such as a Rigaku X-ray diffractometer, wherein the X-ray source is mounted on one side of the sample (which is found in the middle), while the detector is on the other side. The emitted X-rays are reflected by the atomic planes in the nanocrystals and the angular positions and intensities of these reflections gives information about their structures at the atomic level.

FIG. 2(b) shows the extracted size and shape of the nanoparticles from the Rietveld refinements, i.e., the approximate dimensions extracted from the powder diffraction model, cf., FIG. 7, which represents an average in contrast to the nanoparticles shown in FIGS. 5-6, which depict individual nanoparticles.

FIG. 3(a) shows a photograph of a nanopowder.

FIG. 3(b) show final mm-cm sized pellets. Regarding the production of the magnetic pellets, relatively inexpensive metallic salts may be dissolved in water and crystallized. The resulting magnetic nanoparticles may be imaged by AFM microscopy and TEM microscopy (see FIGS. 5-6).

FIG. 16 shows an image of an SrFe12O19 powder obtained with an optical microscope. Nano powder of SrFe12O19 is characterised by having a red/brown luster—in contrast to larger sized particles and pressed pellets, which are black.

The following Examples document how to successfully align very small nanoparticles of SrFe12O19, such as significantly smaller than 0.1 μm, by using the anisotropic shape of the nanoparticles. The nanopowders have an aspect ratio of 1/10 between the dimensions along the a/b-axes and the c-axis. Applying an elevated pressure and temperature to these nanoparticles causes alignment of the nanopowders without applying magnetic field or using organic solvents. The elevated temperature facilitates breaking the magnetic interaction and the uniaxial force field facilitates in aligning the platelet shaped nanoparticles. The method proves to produce bulk samples with high energy product, almost 2.5 times larger than that of a conventional pressed powder, treated by the same pressing method. By ‘conventional pressed powder’ is understood conventional powder treated, such as pressed, by the same method, such as conventional powder pressed according to Example II. The preparation of platelet-shaped nanoparticles and pressing them into bulk samples with high energy product using neither magnetic field nor organic solvents presents a method for producing improved magnets. The process consist of two steps, synthesis of nanoparticles from using supercritical synthesis and spark plasma sintering pressing of the obtained powder samples to produce dense pellets with high energy product.

Production of Nanoparticles and Compaction into Magnets

The atomic structure including size and shape of the nanoparticles may be controlled in a single synthesis step. In one exemplary production method, cheap metallic salts are dissolved in water and crystallized by fast heating, such as very fast heating, and subsequent cooling. By very fast heating may be understood more than 20 K/second.

To obtain the right crystalline product a number of parameters may be accurately controlled. For example, such parameters may include the reactor pressure, temperature, heating rates, and reaction time. Furthermore, other relevant parameters may be relevant, such as pH, concentration, stoichiometry, and nature of the ingredients of the metal ion solution.

In an exemplary embodiment, the synthesis may be monitored continuously using X-ray diffraction as a function of time, which gives insight into at least some of the growth parameters.

Example I Supercritical Synthesis

Thin platelets of SrFe12O19 (size ˜30×30×3 nm3) were prepared through supercritical synthesis, where iron and strontium nitrates (Fe(NO3)3.9H2O and Sr(NO3)2) are dissolved in deionized water to obtain precursor solutions with a Sr:Fe ratio of 1:1. The concentration was 0.05 M. NaOH was added in double concentration with respect to nitrates from both Fe(NO3)3.9H2O and Sr(NO3)2. The alkaline solution was added drop wise under constant stirring and resulting in a dark red precipitate. The precursor solution containing the precipitate was fed into the supercritical reactor from a 200 mL injector and pumped into the system at the flow rate of 5 mL/min. Deionized water was feed from a second line at a flow rate of 15 mL/min. Precursor and supercritical water meet at the mixing point, see FIG. 4.

FIG. 4 shows a schematic diagram of the hydrothermal flow synthesis setup. The illustration is extracted from the reference “Glucose-assisted continuous flow synthesis of Bi2Te3 nanoparticles in supercritical/nearcritical water”, by Mi, J. L., et al., Journal of Supercritical Fluids, 2012. 67: p. 84-88, which reference is hereby incorporated by reference in entirety.

At the mixing point the nucleation is initiated and crystallization takes place as the precipitates flow through the reactor. The product is quenched by a cold water jacket and collected at the outlet through a pressure relief valve. The reaction temperature was 390° C. and the system was pressurized to 250 bars. The obtained product was washed, centrifuged and dried. The resulting nanoparticles are very small with an average size of ˜30×30×3 nm3 as extracted from powder X-ray diffraction, TEM and AFM pictures. The magnetic properties of the as synthesized compound were investigated using a vibration sample magnetometer (VSM). FIG. 4 shows the supercritical synthesis apparatus. Images of TEM and AFM are shown in FIGS. 5-6. The images reveal the dimension of the samples along with a regular hexagonal shape of the nanoparticles, with relatively short c-axis compared to the a- and b-axes.

FIGS. 5-6 show nanoparticles of SrFe12O19 prepared by the supercritical flow synthesis method. Both images show agglomerates SrFe12O19 nanoparticles.

FIG. 5 shows a Transmission Electron Microscopy (TEM) image.

FIG. 6 shows an Atomic Force Microscopy (AFM) image.

Finally the powder diffraction pattern is shown in FIG. 7, Rietveld refinements have been used to model the crystal structure along with the size and shape of the nanoparticles.

FIG. 7 shows the powder diffraction pattern of the supercritical synthesized sample, the grey data points (716) represent the observed data (YOBS), the black line (722) represent the Rietveld model (YCALC) and the lower grey line (726) represent the difference (YOBS-YCALC) between the observed data and the model. The vertical light grey lines (724) are signifying the Bragg positions.

FIG. 8 shows a hysteresis loop (816) of as prepared SrFe12O19. The magnetic properties of the as prepared nanoparticles are shown in FIG. 8. Before measuring the “as prepared” sample it was cold compacted and glued with cement to a VSM sample stick—the cement is also beneficial in order to bind the nanoparticles and prevent reorientation of the magnetic grains. The “as prepared sample” has a saturation magnetisation of 30 emu/g, a remenance of 11 emu/g, and a coercivity of 1032 Oe.

Example IV 1st Alternative Supercritical Synthesis

A study of the effect of temperature, pH and concentration demonstrates that the correct phase (SrFe12O19) can be made at reaction temperature of 350° C. The pH have a minor effect and the compound can be made with KOH (instead of NaOH). Other parameters are the same as in EXAMPLE I. The effect of concentration, when adding the base is remarkable and from the table below (TABLE I) it can be seen how the concentration of Fe(NO3)3.9H2O, when adding the base determines the sizes of the prepared particles, such as the prepared nanocrystallites.

TABLE I Fe(NO3)3•9H2O (M)* ab (nm) c (nm) ab/c ratio 0.05 19.1 2.5 7.64 0.075 29.4 4.0 7.35 0.150 41.0 5.5 7.45 0.300 62.1 7.7 8.06 0.500 69.0 9.4 7.34 0.750 62.9 11.2 5.61 *Concentration of Fe(NO3)3•9H2O in the precursor solution at the moment of addition of the base. All solutions were, after the addition of the base, taken to a final Fe(NO3)3•9H2O concentration of 0.05M by adding water.

FIG. 17 shows a graph of particle size as a function of concentration of Fe(NO3)3.9H2O and Sr(NO3)2 when addition of the base is taking place. As can be observed, with increasing concentration of Fe(NO3)3.9H2O and Sr(NO3)2 when addition of the base is taking place, larger particles are formed. The upper curve with square markers represents the size along the ab-axis. The lower curve with circle markers represents the size along the c-axis.

Example V 2nd Alternative Supercritical Synthesis

The possibility of making BaFe12O19 nanoparticles using supercritical flow has also been demonstrated, and here size can be adjusted by varying the Ba to Fe ratio of the precursor solution. Other parameters are the same as in EXAMPLE I. The results show, as equivalently observed with SrFe12O19, an increase of particle size with increasing amount of Ba with respect to Fe. TABLE II shows particle sizes for various sample (where each row corresponds to a sample), where the ratio in the first column corresponds to the ratio of Ba with respect to Fe.

TABLE II SAMPLE ab (nm) c (nm) ab/c 1-1 25.8 3.6 7.1 1-2 32.8 5.2 6.4 1-4 46.8 7.5 6.3 1-6 51.2 8.1 6.4 1-10 83.5 12.5 6.7

FIG. 18 shows a graph of particle size as a function of the ratio of Ba with respect to Fe. The upper curve with square markers represents the size along the ab-axis. The lower curve with circle markers represents the size along the c-axis.

In all cases within Example V, addition of NaOH 16 M was done on a precursor solution with Fe(NO3)3.9H2O concentration of 0.05 M. No more water was added after the base, therefore being 0.05 M the final concentration of Fe(NO3)3.9H2O in the precursor solution. The concentration of Ba(NO3)2 was changed accordingly to result in the presented ratios.

Example II Spark Plasma Sintering

The prepared platelet nanoparticles are pressed into compact magnets by means of spark plasma sintering (SPS) press. The nanoparticles are prepared according to Example I. The nanopowder is loaded into a graphite pressing tool and uniaxial pressure is applied to the punches, meanwhile a pulsed DC current is directed through the graphite pressing tool and the sample.

FIG. 10 is a schematic illustration of the working principles of a spark plasma sintering press.

FIG. 11 illustrates the proposed process, when the thin platelets are exposed to elevated pressure and temperature.

The SrFe12O19 nanopowder was loaded into the graphite pressing tool a with an inner diameter of 8 mm and 0.3 g were loaded into the cylindrical cavity and inserted into the SPS vacuum chamber. A minimal pressure was applied to hold the pressing tool in place. The chamber was sealed and the atmosphere was removed from the chamber. A pressure of 80 MPa was applied to densify the sample. A direct pulsed DC current was applied and the sample was heated by an approximate rate of 100° C./min to a temperature of 950° C., the temperature was held for 2 minutes before cooling to room temperature. FIG. 11 shows an illustration of what is expected to happen as high pressure and temperature is applied.

Powder diffraction pattern of the pellets reveals a significant alignment of the crystallites in the sample and growth of the nanoparticles.

FIG. 12 shows normalized powder diffraction pattern for as prepared nanopowder (1216), SPS pressed commercial powder (1220) and SPS pressed nanopowder (1218). Index is given for some of the main reflections. SPS pressed is understood to refer to pressing according to Example II. For the SPS nanopowder (1218), such as the SPS pressed nanopowder (1218), it is clear that the most pronounced peaks are associated with (00l) given direct information on the preferred orientation of the crystal grains inside the SPS pellet. The index on the powder diffraction pattern for as prepared nanopowder (1216) shows that the reflections originating from a/b axes is much narrower than the other peaks point to the larger dimensions along the a/b axis. The powder diffraction pattern for commercial sample (1220) does not show extraordinary alignment. FIG. 12 is a comparison between the as prepared nano particles and the nano particles after SPS pressing. From the peak width the growth along the c-axis can be estimated to increase from about 3 nm to about 60 nm. The particle size in the a/b direction is currently being investigated. The most pronounced feature for the SPS nanopowder is the high degree of preferred orientation. The peaks observed are all characterized by having an l-index, (e.g. 00l, where 00l=004, 006, 008 etc), therefore the crystallite grains have preferential orientation with the c-axis lattice planes parallel to the surface. In other words the platelets are preferentially ordering with the c-axis planes perpendicular to the pressing direction.

The obtained pellet from SPS pressing was 8 mm diameter and 1 mm thick. A diamond blade saw was used to cut a piece with mass of 12.34 mg from the original disc. The sample was heated to 200° C. in vacuum for 1 hour to remove moisture before being weighted and mounted for VSM measurements.

FIG. 9 shows a comparison of different samples, in particular the as prepared (which in the figure is labelled ‘As synthesized’) samples (916), the as prepared sample after SPS Pressing (which in the figure is labelled ‘SPS nanopowder’) (918) and commercial sample (which in the figure is labelled ‘SPS commercial’) (920) which is a commercially available SrFe12O19 powder pressed in the same way as the SPS pressed “as prepared” nanopowder, such as pressed according to Example II. The hysteresis curve in FIG. 9 shows the intensity of magnetization as function of the applied magnetic field. The external field was scanned from −1.5 to 1.5 T. Based on the hysteresis curves the energy product BHmax can be calculated. The as prepared powder has a BHmax of 1 kJ/m3, while the commercial sample (which commercial sample has been obtained from TRIDELTA Hartferrite GmbH, type 14T) has a value of 11 kJ/m3 and the aligned SPS pressed nanopowder reveals an energy product of 26 kJ/m3. The SPS pressed “as prepared” sample has a saturation magnetisation of 69 emu/g, a remanence 57 emu/g, and a coercivity of 3788 Oe. The SPS pressed commercial sample has a saturation magnetisation of 62 emu/g, a remenance of 38 emu/g, and a coercivity of 3784 Oe.

The final size of the particles within the permanent magnetic material after SPS pressing have been extracted from data measured at beamline BL44XU, SPring8, Japan. The data extends to much higher scattering values than our data obtained from a standard X-ray source, such as a standard Rigaku X-ray diffractometer. The size (determined from X-ray diffraction measurements conducted at SPring8) along the a and b-axes is approximately 70 nm, and the size along the c-axis is approximately 60 nm. From a Rigaku X-ray diffractometer we estimated the size along the c-axis size to be 58 nm, i.e., in very good agreement with the synchrotron data from Spring8.

FIGS. 13-14 show the refinements of the full data (FIG. 13) and the data corresponding approximately to the range covered with the Rigaku X-ray diffractometer (FIG. 14). It can be observed that the peaks are significantly sharper—than those shown for “as prepared sample” (see ref. sign 716 in FIGS. 7 and 1216 in FIG. 12) confirming that growth has taken place. In an embodiment still larger particles may be obtained via annealing, for e.g., 1 hour, such as 2 hours, such as 3 hours.

The relative orientation of particles within the permanent magnetic material may be determined by taking intensities (in a powder X-ray diffraction spectrum) for reflections along the 00l direction and divide by the total intensity of all reflections as described elsewhere in this document, such as the relative orientation being given by the ratio:


ΣI(00l)/ΣI(hkl),

cf., the relative orientation quantification method described elsewhere in this application. This equation gives a comparable number for the relative orientation in different samples, such as permanent magnetic materials. For a random orientation sample probed at the Spring8, the relative orientation is given as ΣI(00l)/ΣI(hkl)=4%. For the commercially available powder compacted with SPS the relative orientation is 9%. The permanent magnetic material according to an embodiment of the present invention obtains 55%. Comparing these numbers it is clear that the inventive permanent magnetic material is significantly more aligned than the normal commercial sample and that the commercial sample is more aligned than the completely random sample.

To sum up on examples I-II: The Examples demonstrate an exemplary method for producing high energy product (BHmax) SrFe12O19 pellets. The high energy product comes about from the alignment of platelet shaped SrFe12O19 nanoparticles by applying a uniaxial pressure and elevated temperature. The applied pressure and temperature causes the nanoparticles to align and grow into more optimal sizes domains compared to the as prepared nanoparticles. Our nano based magnets show an energy product almost 2.5 times larger than conventional prepared and pressed hexaferrite. Thus, it is shown how to prepare highly aligned bulk magnets from nanoparticles with small grains without the use of magnetic guide field or organic solvents, using a process of aligning nanoparticles using uniaxial pressure combined with heating for the preparation of strong permanent magnetic materials based on hexaferrite material.

Example VI Spark Plasma Sintering with Pre-Heating

In this example, pre-heating has been applied, i.e., the temperature of the particles has been raised before pressing has been initiated.

A permanent magnetic material has been prepared according to Example VI by successively:

    • heating (i.e., pre-heating) the as prepared nanopowder (corresponding to the particles Example IV prepared with a Fe(NO3)3.9H2O concentration of 0.15 M) to 600° C., wherein said heating (i.e., raising the temperature from room temperature to 600° C.) takes 2 minutes,
    • holding the said nanopowder for 1 minute at this temperature (600° C.),
    • then applying a pressure of 100 MPa and subsequently,
    • heating to 1000° C. while still maintaining the pressure of 100 MPa, wherein said heating (i.e., raising the temperature from 600° C. to 1000° C.) takes 4 minutes,
    • holding the said nanopowder for 2 minute at this temperature (1000° C.),
      wherein the total pressing procedure takes 9 minutes, including heating from 600° C. to 1000° C. which takes 4 minutes, and holding the temperature at 1000° C. for 2 minutes.

This procedure results in excellent alignment of the nanoparticles.

Applying heating before applying pressure is an advantage in relation to the alignment as the resulting relatively high temperature may facilitate reducing or eliminating magnetic interaction between the particles and thus make the nanoparticles more susceptible to applied pressure, which may in turn improve the alignment of the samples. It is understood when referring to ‘applying heating before applying pressure’, that ‘before’ may be understood as ‘immediately before’ and/or as ‘applying heating before applying pressure so that the sample has an elevated temperature, such a temperature above room temperature, such as a temperate of at least the blocking temperature, when the application of pressure is initiated’.

The magnetic material prepared by SPS pressing at 100 MPa and 1000° C. of particles prepared (corresponding to the particles prepared according to Example IV) from of Fe(NO3)3.9H2O concentration of 0.15 M and where heating (to 600° C.) prior to applying pressure took place have a saturation magnetisation of 70 emu/g, a remanence magnetization of 61 emu/g, and a coercivity of 2829 Oe. This gives rise to an energy product BHmax of 25 kJ/m3. This is comparable to the results obtainable according to particles obtained as in Example I and compacted according to Example II, such as with a precursor concentration of 0.05 M Fe(NO3)3.9H2O and pressing at 80 MPa and heating to 950° C.

To sum up on examples I-VI: It is demonstrated that alignment of larger particles (this is with concentration 0.15 M for their starting precursor) is possible. Furthermore, observations indicate that a permanent magnetic material according to Examples IV and IV may result in a very well aligned alignment structure. Furthermore, the Examples goes to show that within some range of particle sizes and within some range of pressing parameters, strong magnets can be produced.

Example III Effect of Annealing

A series of annealing tests on a sample that was SPS pressed (according to Example I and II) is carried out. This is done by cutting different samples from the same pellet—i.e. all conditions are kept the same except the time in which the sample has been sitting in the furnace. The Furnace was heated to 750° C. and samples was inserted and further heating to 850° C. this was done within 1 hour. The samples were then held at different times as shown in FIG. 15.

FIG. 15 shows the effect on the energy product of annealing at 850° C. at different time periods.

The annealing process allows the nanoparticles to grow. From FIG. 15 it can be seen that 4 hours give an optimum with an energy product of about 28.5 kJ/m3. Longer times cause the energy product to steadily decrease, such as longer times until 4 hours cause the energy product to steadily decrease (after which it starts dropping).

Samples prepared with different annealing time gives approximate a/b dimensions of 105 nm and c-axis of 70 nm from data collected at Spring8. Data collected with a Rigaku Smartlab corresponds within a small margin with this and gives a/b dimensions of 120 nm and c-axis of 60 nm.

Example VII Providing Pole Figures and Texture Index

Pole figures reveal very high alignment of the nanoparticles after pressing according to Examples II and VI.

A method for estimating grain alignment is provided by measurement of pole figures. From pole figures an alignment distribution can be determined. In other words, what fraction of a sample is aligned with the c-axis parallel to the flat pellet surface.

FIG. 19 is a schematic of the pole figure measurement.

The pole figure is measured by varying the incident beam with respect to the surface of the sample. Practically this is done by rotating the sample around the chi axis (chi is the axis defined as the line between the source and the detector, chi axis is shown as a dashed line in FIG. 19). The chi angle is step at 5° and at every chi—the sample is rotated around the phi axis (the phi axis corresponds to the vector normal to the sample surface, the phi axis in FIG. 19 coincides with the (008) reflections). The pole figure information is collected at the reflections (110), (008), (107), (114), and (203).

FIG. 21 shows reduced pole figures resulting from obtaining pellets according to Example I (FIGS. 21A-B) or Example IV prepared with a Fe(NO3)3.9H2O concentration of 0.15 M (FIGS. 21C-E) and 0.750 M (FIG. 21 F) compacting according to various methods, more particularly (TI refers to ‘texture index’):

FIG. 21(A): cold pressing (resulting in TI=1.60),

FIG. 21(B): direct pressing according to Example II (resulting in TI=3.25),

FIG. 21(C): pressing according to Example VI (resulting in TI=11.7),

FIG. 21(D): pressing according to Example VI (resulting in TI=11.6),

FIG. 21(E): pressing according to Example VI (resulting in TI=8.65),

FIG. 21(F): pressing according to Example VI (resulting in TI=17.2).

FIG. 21 shows that the SPS compacted samples in FIG. 21B-F comprises particles, which are aligned to a larger degree compared with the cold compacted sample in FIG. 21(A), which hardly reveals any preferred alignment. The maximum (“max”) values indicated in the subfigures relates to the texture, since is shows the height of the peak. The minimum values (“min”) gives information about the randomly oriented sample or the background level.

By cold pressing is understood a sample obtained as a cold compacted sample, more particularly a sample (i.e., particles) which was (cold) compacted (into a permanent magnetic material) by pressing at room temperature.

The obtained data is feed into the MTEX software (version 3.4.1) and the orientation distribution function (ODF) is calculated, from the ODF it is possible to extract the texture index (TI) and the reduced pole figure, which takes all collected data into account. Examples of reduced pole figures are shown in FIG. 21—along with the texture index for the different samples. The program MTEX is obtained from the web-address: https://code.google.com/p/mtex/. The following references describe the usage and the algorithms behind MTEX: A novel pole figure inversion method: specification of the MTEX algorithm, Hielscher, Schaeben: J. of Appl. Cryst. (2008), 41(6) (which is hereby included by reference in entirety) and Orientation Distribution Within a Single Hematite Crystal, R. Hielscher, H. Schaeben, H. Siemes: Math. Geosci. (2010), 42, 395-375 (which is hereby included by reference in entirety).

The MTEX software also allows extraction of the volume fraction of the sample with a specific alignment. This is shown in FIG. 20.

FIG. 20 shows the volume fraction of samples with a given alignment, more particularly the fraction of the sample found within a 5° angle. The cold pressed sample (corresponding to the sample in FIG. 21(A)) is relatively close to having random orientation. This is also seen from the texture index of 1.60 being close to the value for a random powder of 1. The direct SPS pressed sample (corresponding to the sample in FIG. 21(B)) shows somewhat better sample alignment, texture index 3.25, however with a broad maximum. The Preheated SPS samples (corresponding to the samples in FIGS. 21(C-F)) have a significantly narrow alignment of the grains and the texture indexes in all cases are above 8. The curves representing samples manufactured using pre-heating are provided with filled markers. The curve representing the direct SPS pressed sample is provided with a non-filled, square marker. The curve representing the cold pressed sample is provided with a non-filled, round marker.

Example VIII Sample Alignment from Magnetic Measurements

An estimate of the samples alignment can also be extracted from oriented magnetization measurements collected in a vibrating sample magnetometer (VSM).

FIG. 22 shows the sample setup in the VSM. The sample is held between two quartz rods placed inside a non-magnetic brass holder. The sample alignment with respect to the applied field (H) (i.e., the external magnetic field) gives the easy axis. For the SPS pressed samples the easy axis is coinciding with the surface normal of the pressed pellets. In the left sub-figure (marked 0°) the easy axis (indicated with arrow 2228) is aligned with respect to external magnetic field. In this case the most square-shaped hysteresis curve is obtained. In the right sub-figure (marked 90°) the sample has been rotated 90°, so that the easy axis (indicated with arrow 2230) is aligned perpendicularly (with an angle of 90°) with respect to the applied field. This 90° rotation produces the hard magnetic axis. If the sample is perfectly aligned, the 0° measurement should produce a square hysteresis curve, while the 90° should produce a curve with polarization (J) as a function of magnetic field (H), where the curve goes through the origo (0, 0). By taking the ratio of the remanence values Jr(0°)/Jr(90°) obtained for the two situations, an estimate of the sample alignment can be extracted. For a completely isotropic magnet Jr(0°)/Jr(90°)=1, while for an aligned anisotropic magnet Jr(0°)/Jr(90°)>1, and for a perfectly aligned anisotropic magnet)) Jr(0°)/Jr(90°) goes towards infinity.

FIG. 23 shows hysteresis curves for the sample labelled ‘SPS48’ (preheated) (a sample where particles are prepared according to Example IV (with a Fe(NO3)3.9H2O concentration of 0.75 M), and where pressing is carried out according to Example VI) rotated in steps of 15 degrees from 0° to 90°. In this case the ratio of Jr(0°)/Jr(90°)=4.35, and it is observed how the curvature of the hysteresis curve in the second quadrant increases with increasing angle. For a sample with direct pressing (where particles are obtained according to Example I and pressing is done according to Example II) the ratio Jr(0°)/Jr(90°)=3.03. The aligned magnetization measurements allow an independent estimate of the sample alignment.

Example IX Magnetic Measurements with Differently Aligned Samples

FIG. 24 shows magnetization measurements of differently prepared SrFe12O19 nanoparticles with respect to sample preparation and pressing conditions, more particularly hysteresis curves for pellets pressed under different conditions and with different alignment. The cold pressed sample, as well as samples labelled ‘SPS27’, ‘SPS45_2h’ and ‘SPS47’ have been prepared from nanoparticles with sizes of about (a×b×c)=(30×30×3 nm3), while SPS48 has been prepared from larger particles with sizes of (a×b×c)=(63×63×11 nm3). The particles for the cold pressed sample are prepared according to Example I. SPS27 correspond to a sample prepared according to Examples I-II. Each of SPS45_2h and SPS47 comprise particles prepared according to Example IV (with a Fe(NO3)3.9H2O concentration of 0.15 M) where pressing is carried out according to Example VI. SPS48 comprise particles prepared according to Example IV (with a Fe(NO3)3.9H2O concentration of 0.75 M) where pressing is carried out according to Example VI. The cold pressed samples shows a very smooth reduction of J in the second quadrant of the coordinate system point to an unaligned samples, where as all SPS samples shows a relative abrupt change in J. The sample SPS47 has a better alignment compared to SPS27 (the squareness of the curve), however the coercivity (Hc) is lower. SPS48 is showing the best alignment of all the samples, but the coercivity is significantly reduced, which may indicate that smaller particles are preferable in the final pellet. The figures derivable from FIG. 24 are inserted in TABLE III

TABLE III Jr (T) Hc (T) BHmax (kJ/m3) Cold press 0.1463 0.0991 2.67 SP S27 0.379 0.379 25.592 SPS 45_2h 0.341 0.348 20.816 SP S 47 0.389 0.279 25.114 SP S 48 0.393 0.182 10.508

To sum up, there is presented a method for providing a permanent magnetic material comprising hexagonal ferrites, which method does not necessitate neither large magnetic fields nor organic solvents. The produced permanent magnetic materials have excellent properties, in particular in terms of energy product, such as in terms of energy product and density. In further aspects, the invention relates to particles for providing the permanent magnetic material, and a corresponding method of manufacture. In particular embodiments of the invention the hexagonal ferrite is given by CaFe12O19, SrFe12O19 or BaFe12O19, such as given by SrFe12O19 or BaFe12O19.

In embodiments E1-E42 of the invention, there is presented:

    • E1.A method for preparing particles comprising hexagonal ferrite, for a magnetic material, the method comprising
      • Forming a precursor solution comprising elements of the hexagonal ferrite,
      • Feeding the precursor solution, such as a precursor solution containing a precipitate, into a supercritical reactor, so as to carry out a supercritical synthesis of the particles, wherein the particles have an anisotropic shape and wherein the size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets.
    • E2.A method according to embodiment E1, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Strontium (Sr) and Barium (Ba).
    • E3.A method according to embodiment E2, wherein a step of forming the precursor solution comprises
      • dissolving
        • a compound comprising iron (Fe), such as a compound selected from the group comprising
          • i. iron nitrate, such as (Fe(NO3)3.9H2O,
          • ii. iron chloride, such as FeCl3, and
          • iii. iron sulphate, Fe(SO4)3,
      • and/or dissolving
        • a compound comprising strontium (Sr), such as a compound selected from the group comprising:
          • i. strontium nitrate, such as Sr(NO3)2,
          • ii. strontium hydroxide, such as Sr(OH)2, and
          • iii. strontium chloride, such as SrCl2.
    • E4.A method according to embodiment E2, wherein a step of forming the precursor solution comprises
      • dissolving iron nitrate, such as (Fe(NO3)3.9H2O, and strontium nitrate, such as Sr(NO3)2.
    • E5.A method according to any one of embodiments E3 or E4, wherein the precursor solution has a Sr: Fe ratio of 1:1.
    • E6.A method according to any one of embodiments E3 or E4, wherein an alkaline solution is added in a concentration being at least 1.25 times, such as at least 1.50 times, such as at least 2 times, such as 2 times, the concentration of nitrates from both the iron nitrate and the strontium nitrate.
    • E7.A method according to embodiment E6, wherein the alkaline solution comprises a substance selected from the group comprising: NaOH, KOH and LiOH.
    • E8.A method according to embodiment E6, wherein the alkaline solution is added drop wise under constant stirring until a dark red precipitate is formed.
    • E9.A method according to embodiment E1, comprising feeding the precursor solution, such as the precursor solution containing the dark red precipitate, into a supercritical reactor.
    • E10. A method according to embodiment E9, wherein the precursor solution is fed into the supercritical reactor at the flow rate of within 0.5-50 mL/min, such as within 1-10 mL/min, such as 5 mL/min.
    • E11. A method according to embodiment E1, comprising feeding deionized water into the supercritical reactor at a flow rate of within 0.15-150 mL/min, such as within 3-30 mL/min, such as 15 mL/min.
    • E12. A method according to embodiment E1, comprising
      • feeding the precursor solution, such as the precursor solution containing the dark red precipitate, into the supercritical reactor, at a first flow rate,
      • feeding deionized water into the supercritical reactor at a second flow rate,
      • wherein the ratio of the first flow rate and the second flow rate is between 1:0.3 and 1:30, such as between 1:1 and 1:10, such as 1:3.
    • E13. A method according to embodiment E1, wherein the precursor solution and the deionized water meet at a mixing point.
    • E14. Particles comprising hexagonal ferrite for a magnetic material, wherein the particles have an anisotropic shape and wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets.
    • E15. Particles according to embodiment E14, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Strontium (Sr) and Barium (Ba).
    • E16. Particles according to embodiment E14, wherein dimensions of the particles may be described by dimensions along a first crystal axis (a-axis), a second crystal axis (b-axis) and a third crystal axis (c-axis), and wherein dimensions of the particles are substantially larger along the first crystal axis (a-axis) and/or the second crystal axis (b-axis) relative to a dimension along the third crystal axis (c-axis).
    • E17. Particles according to embodiment E14, wherein dimensions of the particles are at least 2 times larger, such as at least 5 times larger, such as at least 10 times larger, such as substantially 10 times larger, along a first crystal axis (a-axis) and/or a second crystal axis (b-axis) relative to a dimension along a third crystal axis (c-axis).
    • E18. Particles according to embodiment E14, wherein a dimension of the particles along a first crystal axis (a-axis) is substantially 30 nm and wherein a dimension along a second crystal axis (b-axis) is substantially 30 nm and wherein a dimension along a third crystal axis (c-axis) is substantially 3 nm.
    • E19. Particles according to embodiment E14, wherein an energy product (BHmax) of the particles is at least 0.1 kJ/m3, such as at least 1.0 kJ/m3, such as substantially 1 kJ/m3.
    • E20. A method for preparing a permanent magnetic material comprising hexagonal ferrite, the method comprising:
      • obtaining particles comprising hexagonal ferrite, which particles have an anisotropic shape,
      • compacting the particles into a permanent magnetic material, wherein the step of compacting the particles comprises applying a pressure above atmospheric pressure and a temperature above room temperature, and wherein a size of the particles after the step of compacting are smaller than or equal to a size enabling individual particles to become single domain magnets, such as the size enabling individual particles to become single domain magnets.
    • E21. A method according to embodiment E20, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Strontium (Sr) and Barium (Ba).
    • E22. A method according to embodiment E20, wherein the particles are enlarged during the step of compacting the particles.
    • E23. A method for preparing a permanent magnetic material according to embodiment E20, wherein the step of compacting the particles comprises uniaxial hot pressing.
    • E24. A method for preparing a permanent magnetic material according to embodiment E20, wherein the step of compacting the particles comprises spark plasma sintering (SPS), such as the method comprising loading the particles into a graphite pressing tool wherein uniaxial pressure is applied to punches and a pulsed DC current is directed through the graphite pressing tool and the particles.
    • E25. A method for preparing a permanent magnetic material according to embodiment E20, wherein the pressure is at least 20 MPa, such as least 40 MPa, such as least 60 MPa, such as least 80 MPa, such as 80 MPa, such a below 100 MPa, such as between 20 MPa and 100 MPa.
    • E26. A method for preparing a permanent magnetic material according to embodiment E20, wherein a pulsed DC current is applied so as to heat the particles at a rate of at least 10° C./min, such as substantially 100° C./min.
    • E27. A method for preparing a permanent magnetic material according to embodiment E20, wherein a pulsed DC current is applied so as to heat the particles to a temperature of at least 800° C., such as at least 875° C., such as 950° C., such as below 1450° C., such as between 800° C. and 1450° C.
    • E28. A method for preparing a permanent magnetic material according to embodiment E20, wherein the temperature above room temperature is held for at least 1 minute, such as at least 2 minutes, such as substantially 2 minutes, such as at least 5 minutes, such as at least 10 minutes, before cooling to room temperature.
    • E29. A method for preparing a permanent magnetic material according to embodiment E20, wherein the step of obtaining particles, comprises
      • preparing particles according to the independent method of embodiment E1, or
      • obtaining particles according to the independent product embodiment E14.
    • E30. A method for preparing a permanent magnetic material according to embodiment E20, wherein the method further comprises annealing the permanent magnetic material, such as annealing for at least 1 hour, such as at least 2 hours, such as at least 4 hours, such as 4 hours, such as between 1-10 hours, such as between 2-6 hours, such as between 2.5-5 hours, such as 4 hours, such as annealing at a temperature of between 800-1000° C., such as at a temperature of 850° C., such as annealing for 4 hours at 850° C.
    • E31. A permanent magnetic materialic material comprising particles comprising hexagonal ferrite, wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, such as a size enabling individual particles to become single domain magnets.
    • E32. A permanent magnetic materialic material according to embodiment E31, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group comprising Strontium (Sr) and Barium (Ba).
    • E33. The permanent magnetic materialic material according to embodiment E31, wherein crystallites in the permanent magnetic material have a preferential orientation, such as the crystallites in the permanent magnetic material being substantially aligned.
    • E34. The permanent magnetic materialic material according to embodiment E31, wherein a relative orientation of crystallites within the permanent magnetic material is at least 10%, such as at least 20%, such as at least 30%, such as at least 40%, such as at least 45%, such as 55%.
    • E35. The permanent magnetic material according to embodiment E33, wherein the preferential orientation is with c-axis lattice planes parallel to each other, such as orthogonal to a pressing direction employed during a preparation of the permanent magnet.
    • E36. The permanent magnetic material according to embodiment E31, wherein crystallites in the permanent magnetic material have a length along a third crystal axis (c-axis) of less than 250 nm, such as less than 200 nm, such as less than 150 nm, such as less than 100 nm, such as less than 75 nm, such as less than 50 nm, such as within 3 nm and 150 nm, such as within 3 nm and 100 nm, such as within 3 nm and 75 nm, such as within 25 nm and 75 nm, such as within 50 nm and 70 nm, such as approximately 60 nm.
    • E37. The permanent magnetic material according to embodiment E31, wherein crystallites in the permanent magnetic material have a length along a first crystal axis (a-axis) and/or second crystal axis (b-axis) of less than 250 nm, such as less than 200 nm, such as less than 175 nm, such as less than 150 nm, such as less than 100 nm, such as less than 80 nm, such as less than 50 nm, such as within 30 nm and 175 nm, such as within 50 nm and 90 nm, such as within 60 nm and 80 nm, such as within 65 nm and 75 nm, such as approximately 70 nm.
    • E38. The permanent magnetic material according to embodiment E31, wherein the permanent magnetic material has an energy product (BHmax) of more than 11 kJ/m3, such as more than 15 kJ/m3, such as more than 20 kJ/m3, such as more than 25 kJ/m3, such as at least 26 kJ/m3, such as 26 kJ/m3, such as at least 28.5 kJ/m3, such as 28.5 kJ/m3.
    • E39. The permanent magnetic material according to embodiment E31, wherein the permanent magnetic material is shaped into a permanent magnet which has a diameter of 8 mm.
    • E40. The permanent magnetic material according to embodiment E31, wherein the permanent magnetic material is shaped into a permanent magnet which has a thickness of 1 mm.
    • E41. The permanent magnetic material according to embodiment E31, wherein the permanent magnetic material has a density of at least 2.0 g/cm3, such as at least at least 3.0 g/cm3, such as at least at least 4.0 g/cm3, such as at least at least 4.5 g/cm3, such as 4.70 g/cm3.
    • E42. A device for inter-converting between electrical energy and kinetic energy, wherein the device comprises the permanent magnetic material according to embodiment E31.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A method for preparing a permanent magnetic material comprising hexagonal ferrite, the method comprising: wherein the compacting of the particles comprises applying a pressure above atmospheric pressure and a temperature above room temperature, and wherein a size of the particles after compacting are smaller than or equal to a size enabling individual particles to become single domain magnets.

obtaining particles comprising hexagonal ferrite, which particles have an anisotropic shape and;
compacting the particles into a permanent magnetic material;

2-53. (canceled)

54. The method according to claim 1, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba).

55. The method according to claim 1, wherein

obtaining particles comprising hexagonal ferrite, which particles have an anisotropic shape, comprises obtaining particles comprising hexagonal ferrite, wherein the anisotropic shape is a plate like shape, and wherein the size of the particles is at most 100 nm.

56. The method according to claim 1, wherein the method comprises reducing or breaking a magnetic interaction between the particles when compacting the particles and/or during compacting the particles, so as to allow alignment of the particles when compacting the particles and/or during compacting the particles.

57. The method according to claim 1, wherein the method comprises:

pre-heating of the particles, wherein said pre-heating comprises applying a pre-heating temperature above room temperature to said particles before compacting the particles, so that a temperature of said particles is the pre-heating temperature when the compacting is initiated.

58. The method according to claim 1, wherein the particles are enlarged during the step of compacting the particles.

59. The method according to claim 1, wherein the compacting of the particles comprises uniaxial hot pressing.

60. The method according to claim 1, wherein the compacting of the particles comprises spark plasma sintering (SPS).

61. The method according to claim 1, wherein the pressure is at least 20 MPa.

62. The method according to claim 1, wherein a pulsed DC current is applied so as to heat the particles at a rate of at least 10° C./min.

63. The method according to claim 1, wherein the particles are heated to a temperature of at least 800° C.

64. The method according to claim 1, wherein at least partially during compacting the particles into a permanent magnetic material:

the particles are heated to a temperature of at least 800° C., and
the pressure is at least 20 MPa.

65. The method according to claim 1, wherein a pulsed DC current is applied to heat the particles to a temperature of at least 800° C.

66. The method according to claim 1, wherein the temperature above room temperature is held for at least 1 minute, before cooling to room temperature.

67. The method according to claim 1, wherein the obtaining of particles, comprises:

preparing particles by:
forming a precursor solution comprising elements of the hexagonal ferrite, and feeding the precursor solution, into a supercritical reactor, so as to carry out a supercritical synthesis of the particles, wherein the particles have an anisotropic shape and wherein the size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, or
obtaining particles comprising hexagonal ferrite for a magnetic material, wherein the particles have an anisotropic shape and wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets.

68. The method according to claim 1, wherein the method further comprises annealing the permanent magnetic material.

69. A permanent magnetic material comprising particles comprising hexagonal ferrite, wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, and wherein the particles have an anisotropic shape, and wherein crystallites in the permanent magnetic material have a preferential orientation.

70. The permanent magnetic material according to claim 69, wherein impurities, in the permanent magnetic material contribute to less than 3 wt %, or less than 1.5 wt %.

71. The permanent magnetic material according to claim 69, wherein dimensions of the particles are at least 2 times larger, along a first crystal axis (a-axis) and/or a second crystal axis (b-axis) relative to a dimension along a third crystal axis (c-axis).

72. The permanent magnetic material according to claim 69, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba).

73. The permanent magnetic material according to claim 69, wherein a texture index of the permanent magnetic material is at least 2.

74. The permanent magnetic material according to claim 69, wherein a ratio Jr(0°)/Jr(90°) is at least is at least 2 wherein said ratio Jr(0°)/Jr(90°) is a ratio between

a first remanence value Jr(0°) obtained at a first orientation of the permanent magnetic material with respect to an external magnetic applied field, and
a second remanence value Jr(90°) obtained at a second orientation of the permanent magnetic material with respect to the applied external magnetic field, wherein the second orientation is orthogonal to the first orientation.

75. The permanent magnetic material according to claim 69, wherein a relative orientation of crystallites within the permanent magnetic material is at least 10%.

76. The permanent magnetic material according to claim 69, wherein a preferential orientation is with c-axis lattice planes parallel to each other.

77. The permanent magnetic material according to claim 69, wherein crystallites in the permanent magnetic material have a length along a third crystal axis (c-axis) of less than 250 nm.

78. The permanent magnetic material according to claim 69, wherein crystallites in the permanent magnetic material have a length along a first crystal axis (a-axis) and/or second crystal axis (b-axis) of less than 250 nm.

79. The permanent magnetic material according to claim 69, wherein the permanent magnetic material has an energy product (BHmax) of more than 11 kJ/m3.

80. The permanent magnetic material according to claim 69, wherein the permanent magnetic material has a density of at least 2.0 g/cm3.

81. A device for inter-converting between electrical energy and kinetic energy, wherein the device comprises:

a permanent magnetic material prepared by:
obtaining particles comprising hexagonal ferrite, which particles have an anisotropic shape; and
compacting the particles into a permanent magnetic material;
wherein the step of compacting of the particles comprises applying a pressure above atmospheric pressure and a temperature above room temperature, and wherein a size of the particles after the step of compacting are smaller than or equal to a size enabling individual particles to become single domain magnets; or
a permanent magnetic material comprising particles comprising hexagonal ferrite, wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets, and wherein the particles have an anisotropic shape, and wherein crystallites in the permanent magnetic material have a preferential orientation.

82. A method for preparing particles comprising hexagonal ferrite, for a magnetic material, the method comprising:

forming a precursor solution comprising elements of the hexagonal ferrite, and
feeding the precursor solution, into a supercritical reactor, so as to carry out a supercritical synthesis of the particles, wherein the particles have an anisotropic shape and wherein the size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets.

83. The method according to claim 82, wherein the supercritical synthesis comprises heating of the precursor solution, and wherein said heating is achieved by raising the temperature at a rate of at least 10° C./second.

84. The method according to claim 82, wherein a reaction time period during the supercritical synthesis is 10 minutes or less.

85. The method according to claim 82, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba).

86. The method according claim 82, wherein the forming of the precursor solution comprises dissolving a compound comprising iron (Fe), and/or dissolving a compound comprising strontium (Sr).

87. The method according to claim 82, wherein the forming of the precursor solution comprises dissolving iron nitrate, and strontium nitrate.

88. The method according to claim 82, wherein the precursor solution has a X:Fe ratio of 1:1 or Rx:1, where Rx is a number 0.1-2 and, wherein the precursor solution has a Sr:Fe ratio of 1:1.

89. The method according to claim 82, wherein the method further comprises adding a base to the precursor solution, wherein a concentration of Fe3+ iron(III) within the precursor solution when adding the base is within 0.05-0.750 M.

90. The method according to claim 82, wherein the method further comprises adding a base to the precursor solution, wherein a concentration of Fe3+ iron(III) within the precursor solution when adding the base is within 0.05-0.750 M, and wherein a final concentration of the precursor is 0.05-0.50 M and is achieved through dilution with base and/or water.

91. The method according to claim 82, wherein the forming of the precursor solution comprises: dissolving a nitrate selected from the group consisting of strontium nitrate, barium nitrate, and calcium nitrate, wherein an alkaline solution is added in a concentration being at least 1.00 times, 1.50 times, or 2 times the concentration of nitrates.

dissolving iron nitrate, and/or

92. The method according to claim 82, wherein an alkaline solution is added in a concentration being at least 1.25 times, the concentration of nitrates from both the iron nitrate and the strontium nitrate.

93. The method according to claim 91, wherein the alkaline solution comprises a substance selected from the group consisting of NaOH, KOH and LiOH.

94. The method according to claim 91, wherein the alkaline solution is added drop wise under constant stirring until a dark red precipitate is formed.

95. The method according to claim 82, comprising feeding the precursor solution, into a supercritical reactor.

96. The method according to claim 82, comprising feeding the precursor solution into a supercritical reactor, wherein the precursor solution is fed into the supercritical reactor at the flow rate of within 0.5-50 mL/min.

97. The method according to claim 82, comprising feeding deionized water into the supercritical reactor at a flow rate of within 0.15-150 mL/min.

98. The method according to claim 82, comprising:

feeding the precursor solution into the supercritical reactor, at a first flow rate, and
feeding deionized water into the supercritical reactor at a second flow rate, wherein the ratio of the first flow rate and the second flow rate is between 1:0.3 and 1:30.

99. The method according to claim 97, wherein the precursor solution and the deionized water meet at a mixing point.

100. Particles comprising hexagonal ferrite for a magnetic material, wherein the particles have an anisotropic shape and wherein a size of the particles are smaller than or equal to a size enabling individual particles to become single domain magnets.

101. Particles according to claim 100, wherein the hexagonal ferrite comprises XFe12O19, where X is an element selected from the group consisting of Calcium (Ca), Strontium (Sr) and Barium (Ba).

102. Particles according to claim 100, wherein dimensions of the particles may be described by dimensions along a first crystal axis (a-axis), a second crystal axis (b-axis) and a third crystal axis (c-axis), and wherein dimensions of the particles are substantially larger along the first crystal axis (a-axis) and/or the second crystal axis (b-axis) relative to a dimension along the third crystal axis (c-axis).

103. Particles according to claim 100, wherein dimensions of the particles are at least 2 times larger, along a first crystal axis (a-axis) and/or a second crystal axis (b-axis) relative to a dimension along a third crystal axis (c-axis).

104. Particles according to claim 100, wherein a dimension of the particles along a first crystal axis (a-axis) is within 20-40 nm, and wherein a dimension along a second crystal axis (b-axis) is within 20-40 nm, and wherein a dimension along a third crystal axis (c-axis) is within 2-4 nm.

105. Particles according to claim 100, wherein an energy product (BHmax) of the particles is at least 0.1 kJ/m3.

Patent History
Publication number: 20160167978
Type: Application
Filed: Jul 4, 2014
Publication Date: Jun 16, 2016
Inventors: Mogens Christensen (Aarhus), Matilde Saura Muzquiz (Aarhus), Marian Stingaciu (Risskov)
Application Number: 14/901,949
Classifications
International Classification: C01G 49/00 (20060101); H01F 1/01 (20060101);