SmCo-BASED ALLOY NANOPARTICLES AND PROCESS FOR THEIR PRODUCTION
SmCo-based alloy nanoparticles composed mainly of a SmCo-based alloy containing Sm and Co as constituent elements, wherein the content of metal elements other than Sm and Co is 0.05-20 wt % with respect to the SmCo-based alloy.
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1. Field of the Invention
The present invention relates to SmCo-based alloy nanoparticles and to a process for their production.
2. Related Background Art
A large variety of magnetic materials are used in the fields of magnetic recording media and magnets. SmCo-based alloy magnetic materials have very high coercive force and uniaxial magnetocrystalline anisotropy, and are known to exhibit high magnetic properties even as nanoparticles. SmCo-based alloys, therefore, are promising materials for use as various types of magnetic materials, including magnetic recording media for high-density recording.
Methods for synthesis of SmCo-based alloy particles are known which allow synthesis of SmCo5 alloy particles by the physical method of sputtering (for example, see Reference 1 listed below). Another method that has been proposed is vapor phase deposition with a cluster gun to produce SmCo-based alloy particles (for example, see Reference 2 listed below).
(Reference 1) D. Weller, et al, IEEE Trans. Magn, 36, p. 10-15 (2000)
(Reference 2) S. Stoyanov et al., “High anisotropy Sm—Co nanoparticles: Preparation by cluster gun technique and their magnetic properties”, JOURNAL OF APPLIED PHYSICS, Vol 93, Number 10 (2003), p. 7592
On the other hand, methods for synthesizing SmCo-based alloy particles by chemical means have also been proposed, such as synthesis by polyol reduction (see Japanese Unexamined Patent Publication No. 2006-245313, for example) and synthesis by microwave polyol reduction (see Japanese Unexamined Patent Publication No. 2007-128991, for example).
SUMMARY OF THE INVENTIONHowever, research by the present inventors has revealed that SmCo-based particles obtained by physical synthesis as in References 1 and 2 do not have very high magnetic properties. Then, such physical synthesis require heat treatment and have low SmCo-based particle yields, they are unsuitable for industrial mass production.
On the other hand, the SmCo-based alloy particles disclosed in Japanese Patent Laid-Open No. 2006-245313 have a coercive force (Hc) of as low as 500 Oe at ordinary temperature. Also, the SmCo-based alloy nanoparticles disclosed in Japanese Patent Laid-Open No. 2007-128991 have only been observed to exhibit magnetic properties at cryogenic temperature. The low magnetic properties of the SmCo-based alloy particles disclosed in the aforementioned patent documents are attributed to the fact that the SmCo-based alloy particles that are produced contain large amounts of unreacted samarium salts, since samarium salts are not easily reduced substances.
SmCo-based particles have therefore been desired which exhibit satisfactorily excellent magnetic properties and have sufficiently small particle sizes. A process for production allowing such SmCo-based alloy nanoparticles to be easily produced in mass has also been desired.
It is an object of the present invention to provide SmCo-based alloy nanoparticles with sufficiently small particle sizes and satisfactorily excellent magnetic properties, as well as a production process that allows the SmCo-based alloy nanoparticles to be mass-produced at high yield.
In order to achieve this object, the invention provides SmCo-based alloy nanoparticles composed mainly of a SmCo-based alloy containing Sm and Co as constituent elements, wherein the content of metal elements other than Sm and Co is 0.05-20 wt % with respect to the SmCo-based alloy.
Such SmCo-based alloy nanoparticles can be suitably used as magnetic material because of their sufficiently small particle sizes and satisfactorily excellent magnetic properties. The reason for the satisfactorily excellent magnetic properties of the SmCo-based alloy nanoparticles of the invention is that they have sufficiently low unreacted components and comprise a SmCo-based alloy as the major component. By varying the content of the metal elements other than Sm and Co, it is possible to control the magnetic properties and particle sizes of the SmCo-based alloy nanoparticles of the invention, and to improve the degree of design freedom for magnets or magnetic recording media.
The SmCo-based alloy nanoparticles of the invention preferably contain the aforementioned metal elements at 0.05-10 wt % with respect to the SmCo-based alloy. SmCo-based alloy nanoparticles with this range exhibit even more excellent magnetic properties.
The metal elements in the SmCo-based alloy nanoparticles of the invention preferably include at least one element selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Ir, Os, Cu, Ni, Cr, Al and Mn. SmCo-based alloy nanoparticles comprising these metal elements as the metal elements exhibit even more superior magnetic properties.
The SmCo-based alloy nanoparticles of the invention preferably have particle sizes of 1-30 nm. SmCo-based alloy nanoparticles in this range can be suitably used as magnetic recording media for high-density recording, for example.
The SmCo-based alloy nanoparticles of the invention are preferably obtained by liquid synthesis wherein a strong reducing agent is added to an organic solvent containing a samarium salt, cobalt salt and metal element salts, to reduce the samarium salt, cobalt salt and metal element salts. Such SmCo-based alloy nanoparticles are particularly useful in industry as starting materials for magnetic materials, because of their excellent mass productivity and relatively high particle size distribution.
The SmCo-based alloy nanoparticles of the invention have a core-shell structure comprising a core section and a shell section covering the core section, and preferably the proportion of the metal elements with respect to the SmCo-based alloy in the core section is greater than the proportion in the shell section.
According to the invention there is further provided a process for production of SmCo-based alloy nanoparticles composed mainly of a SmCo-based alloy containing Sm and Co as constituent elements, the process comprising a mixing step in which a starting material containing a samarium salt, cobalt salt and salts of metal elements other than Sm and Co, as well as a protective agent, is mixed with a reducing organic solvent, and a reduction step in which a strong reducing agent is added to the mixture and heated to reduce the samarium salt, cobalt salt and metal element salts.
By the process for production of SmCo-based alloy nanoparticles described above it is possible to accomplish high-yield production of SmCo-based alloy nanoparticles with sufficiently small particle sizes and satisfactorily excellent magnetic properties. The reason for this effect is conjectured by the present inventors to be as follows. Specifically, it is believed that reduction of samarium salts or cobalt salts results in rapid reduction of the salts of the metal elements other than Sm and Co in the starting material, while the metal elements exhibit a catalytic effect whereby they serve as nuclei for crystal deposition which promotes deposition of the SmCo-based alloy crystals. This allows smooth synthesis of SmCo-based alloy nanoparticles with a sufficiently reduced content of unreacted substances. The catalytic effect of the metal elements adequately shortens the reaction time and inhibits grain growth of the produced SmCo-based alloy nanoparticles. It is therefore possible to synthesize SmCo-based alloy nanoparticles that have satisfactorily excellent magnetic properties and sufficiently small particle sizes. Furthermore, since the production process of the invention employs the chemical process of reduction of the starting material to synthesize the SmCo-based alloy nanoparticles, it allows mass production of SmCo-based alloy nanoparticles at a higher yield than physical methods such as sputtering.
The production process of the invention preferably includes a dehydration step in which the mixture obtained from the mixing step is stirred and heated for dehydration and then cooled, prior to the reduction step. This sufficiently removes moisture before the reduction step, thus inhibiting oxidation of Sm and Co and allowing the reduction reaction of the samarium salt and cobalt salt to proceed even more smoothly. In addition, if the mixture is cooled to near room temperature after dehydration and then a strong reducing agent is added prior to further strength and heating, it is possible to prevent the bumping that occurs when reduction reaction proceeds at once, and to thus reduce impurities. That is, by adding a strong reducing agent after cooling, SmCo-based alloy nanoparticles with excellent magnetic properties can be produced at high yield, having a further reduced unreacted substance content and even lower content of impurities other than the SmCo-based alloy.
According to the production process of the invention, the metal elements preferably include at least one element selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Ir, Os, Cu, Ni, Cr, Al and Mn. Since these metal elements are more easily reduced than samarium salts or cobalt salts, they become rapidly reduced as crystal-generating nuclei, thus exhibiting a more superior catalytic effect. Reduction of the samarium and cobalt salts can therefore be further promoted.
The strong reducing agent used in the production process of the invention preferably contains at least one compound selected from the group consisting of LiAlH4, NaBH4, N2H, B2H6 and LiBH(C2H5)3. This will allow SmCo-based alloy nanoparticles to be obtained with a further reduced amount of residual unreacted samarium salt or cobalt salt.
According to the invention it is possible to provide SmCo-based alloy nanoparticles with sufficiently small particle sizes and satisfactorily excellent magnetic properties, as well as a production process that allows the SmCo-based alloy nanoparticles to be mass-produced at high yield.
A preferred embodiment of the invention will now be explained with reference to the accompanying drawings where necessary.
The SmCo-based alloy nanoparticles of this embodiment have a mean particle size of 1-30 nm. Thus, since the SmCo-based alloy nanoparticles with nanosize particle sizes have excellent magnetic properties and sufficiently small particle sizes, they can be used as magnetic powder for magnetic recording media, to increase the recording density of the magnetic recording media. The particle sizes of the SmCo-based alloy nanoparticles can be measured by observation with a transmission electron microscope (TEM).
The preferred composition for the SmCo-based alloy as the major component of the SmCo-based alloy nanoparticles is SmCo5.
The SmCo-based alloy nanoparticles of this embodiment are composed mainly of a SmCo-based alloy, and contain a metal element other than Sm and Co (hereinafter referred to as “third metal element”) at 0.05-20 wt % with respect to the total SmCo-based alloy. The third metal element may be present in the SmCo-based alloy nanoparticles as a simple metal or a metal compound.
The third metal element content is preferably 0.05-20 wt % and more preferably 0.05-10 wt % with respect to the total SmCo-based alloy. The third metal element content can be confirmed by ICP optical emission spectroscopic analysis or the like.
If the third metal element content is less than 0.05 wt % with respect to the obtained SmCo-based alloy nanoparticles during production of the SmCo-based alloy nanoparticles, the improving effect of the third metal element will not be adequately obtained and the SmCo-based alloy nanoparticle yield will be reduced, while the unreacted substance content will be increased, thus resulting in inferior magnetic properties. If the third metal element content is greater than 20 wt %, on the other hand, the lower proportion of SmCo-based alloy contributing to the magnetic properties will result in reduced magnetic properties.
The third metal element may be a precious metal element such as Au, Ag, Pt, Pd, Rh, Ru, Ir or Os, or a transition metal element such as Cu, Ni, Cr or Mn. Al may be mentioned as an example of a third metal element other than these.
The SmCo-based alloy nanoparticles of this embodiment may have a core-shell structure comprising a core section with the third metal element as the major component, and a shell section containing a SmCo-based alloy as the major component, covering the periphery of the core section.
A process for production of the SmCo-based alloy nanoparticles of this embodiment will now be explained. The process for production of SmCo-based alloy nanoparticles of this embodiment comprises a preparation step in which the samarium salt, cobalt salt and third metal element salt are each dissolved in a reducing organic solvent to prepare solutions, a mixing step in which the samarium salt solution, the cobalt salt solution and the third metal element salt solution are mixed to obtain a mixture, an addition step in which a protective agent is added to the mixture obtained in the mixing step, a dehydration step in which the mixture containing the protective agent is heated for dehydration, and a reduction step in which a strong reducing agent is added to the dehydrated mixture to reduce the samarium salt, cobalt salt and third metal element salt, in order to obtain SmCo-based alloy nanoparticles containing the third metal element. Each of these steps will now be explained in detail.
(Preparation Step)
The Sm (samarium) salt used for this embodiment may be samarium acetylacetonate hydrate ([CH3COCH═C(O—)CH3]3Sm.xH2O), and the Co (cobalt) salt may be cobalt acetylacetonate ([CH3COCH═C(O—)CH3]3Co). These salts are readily soluble in organic solvents and relatively easily reduced, and are therefore preferably used from the viewpoint of obtaining SmCo-based alloy nanoparticles with small particle sizes and high purity.
The third metal element salt may be an acetylacetonate salt. Acetylacetonate salts are preferred for use because they are readily soluble in organic solvents.
The organic solvent used to dissolve the samarium salt, cobalt salt and third metal element salt is preferably one with a high boiling point. A specific example is 1,2-hexadecanediol, which has reducing action.
Dissolution of the samarium salt, cobalt salt and third metal element salt in 1,2-hexadecanediol in the preparation step produces a solution containing the samarium salt, a solution containing the cobalt salt and a solution containing the third metal element salt. In order to prevent oxidation of the Sm, Co and third metal element, the preparation step is preferably carried out in an inert gas atmosphere such as nitrogen or argon.
(Mixing Step)
In the mixing step, the solutions prepared in the manner described above are mixed. There are no particular restrictions on the order of mixing, and the three solutions may be combined simultaneously or two of the solutions may be combined and the remaining one mixed with the obtained mixture.
The amount of third metal element salt solution used is preferably 0.01-0.5 mol in terms of the third metal element with respect to 1 mol of the total of the samarium and cobalt elements in the samarium and cobalt salts.
The mixing step is preferably carried out in an inert gas atmosphere such as nitrogen or argon in order to prevent oxidation of the Sm, Co and third metal element.
(Addition Step)
In this step, a protective agent is added to the mixture obtained in the mixing step. The protective agent has the function of protecting the SmCo-based alloy nanoparticles that are produced, and oleic acid, oleylamine, polyvinylpyrrolidone or polyvinyl alcohol is preferably used. The protective agent may also be used in the form of a solution in an organic solvent such as ether.
The amount of protective agent added is preferably 0.1-20 mol with respect to 1 mol of the total of the samarium and cobalt elements in the samarium and cobalt salts.
(Dehydration Step)
The dehydration step is a step in which the mixture is stirred and heated under an inert gas flow or under reduced pressure after inert gas substitution, for dehydration. Sufficiently reducing the moisture in the mixture can satisfactorily inhibit oxidation of the reduced Sm or Co. It will thus be possible to obtain SmCo-based alloy nanoparticles at an even higher yield.
Heating of the mixture is preferably carried out at 110-220° C. for 1-24 hours. The mixture is then preferably cooled to room temperature, and more preferably cooled to 10-30° C. This will allow reduction with the strong reducing agent in the reduction step described hereunder to be accomplished in a more reliable manner, so that high-purity SmCo-based alloy nanoparticles are obtained. If the addition of a strong reducing agent is under high-temperature conditions (for example, 50° C. and higher) without cooling, the reduction with the strong reducing agent will occur all at once, thus tending to result in production of impurities, such as simple cobalt (Co metal), in addition to the SmCo-based alloy, and lowering the SmCo-based alloy content.
(Reduction Step)
The reduction step is a step in which, after the strong reducing agent has been added to the mixture following the dehydration step, the mixture is stirred and heated to thoroughly reduce the samarium salt, cobalt salt and third metal element salt and produce SmCo-based alloy nanoparticles containing the third metal element.
The strong reducing agent used may be at least one compound selected from the group consisting of LiAlH4, NaBH1, N2H4, B2H6 and LiBH(C2H5)3. These strong reducing agents are preferably added to the mixture after dissolution in an organic solvent such as an alcohol.
After addition of the strong reducing agent, an oil bath or mantle heater is used to keep the mixture at 150-320° C. and preferably 250-280° C. for 1-3 hours for heated reflux, to accomplish reduction with the strong reducing agent and the reducing organic solvent and obtain a reaction mixture. The reduction reaction reduces the samarium salt, cobalt salt and third metal element salt. Upon removal of the solvent from the obtained reaction mixture, SmCo-based alloy nanoparticles containing the third metal element are obtained.
Since the third metal element salt is more easily reduced than the samarium salt and cobalt salt in the reduction reaction described above, it tends to be reduced earlier than the samarium salt and cobalt salt. Therefore, the third metal element presumably acts as nucleus origins for deposition of the SmCo-based alloy. The presence of the third metal element ensures even smoother reduction of the samarium salt and cobalt salt, so that the obtained SmCo-based alloy nanoparticles containing the third metal element have a sufficiently reduced impurity content.
The SmCo-based alloy nanoparticles containing the third metal element can be suitably used as magnetic material because of both their adequately small particle sizes and excellent magnetic properties. The particle sizes of the SmCo-based alloy nanoparticles may be adjusted by varying the amount of protective agent added and the third metal element content within the ranges according to the invention, or by varying the temperature and time for heated reflux in the reduction step.
The production process of this embodiment allows high-yield synthesis of SmCo-based alloy nanoparticles containing the third metal element. The nanoparticles obtained by the production process of this embodiment may also include nanoparticles composed mainly of a SmCo-based alloy without the third metal element, or nanoparticles of the third metal element.
The embodiment described above is only a preferred embodiment of the invention, and the invention is in no way limited thereto.
EXAMPLESThe present invention will now be explained in greater detail based on examples and comparative examples, with the understanding that these examples are in no way limitative on the invention.
Examples 1-8 Synthesis of SmCo-Based Alloy NanoparticlesA first solution was prepared by dissolving 0.33 mmol of samarium acetylacetonate hydrate ([CH3COCH═C(O—)CH3]3Sm.xH2O) and 0.02-0.5 mmol of the third metal salts listed in Table 1 in 2 ml of 1,2-hexadecanediol (CH3(CH2)13CH(OH)CH2OH) under a nitrogen atmosphere.
Separately from this solution, there was prepared a second solution by dissolving 1.67 mmol of cobalt acetylacetonate ([CH3COCH═C(O—)CH3]3Co) in 2 ml of 1,2-hexadecanediol under a nitrogen atmosphere.
Also, a third solution was prepared by dissolving 3.0 mmol of oleic acid (CH3 (CH2)7CH═CH(CH2)7COOH) and 3.0 mmol of oleylamine (CH3(CH2)7CH═CH(CH2)8NH2) in 40 ml of octyl ether ([CH3(CH2)7]2O) under an inert gas atmosphere (for example, nitrogen or argon).
The first solution, the second solution and the third solution were then mixed for about 12 hours using a mechanical stirrer under a nitrogen atmosphere to obtain a mixture.
In order to remove the water in the mixture, a three-necked flask containing the mixture was heated in an oil bath at 200° C. under a nitrogen stream and these conditions were maintained for about 1 hour, after which it was cooled to room temperature (about 20° C.). To the cooled mixture there was added absolute ethanol dissolving 6 mmol of sodium borohydride (NaBH4), as a strong reducing agent.
The temperature of the oil bath was then raised and the reaction mixture was kept at 250-280° C. for 1-3 hours of heated reflux. After cooling to room temperature, an ultrafilter was used for filtration and dehydrated ethanol or the like was used for solution exchange and washing of the particles. Next, an evaporator was used to remove the solvent attached to the particles, and vacuum drying was carried out for 10 hours at 40° C. to obtain particles for Examples 1-8.
[Evaluation of Particles]
The particles obtained in Examples 1-8 were observed (3,000,000 magnification) with a high-resolution TEM (JEM-3010, trade name of JEOL Corp.). Also, 100 particles were randomly selected from the electron microscope image, and the mean particle size was calculated. The mean particle sizes of the synthesized particles are listed in Table 1.
The obtained particles were also examined by X-ray diffraction (XRD) and ICP optical emission spectroscopic analysis.
The particles of Example 1 and Examples 3-8 were also confirmed by XRD and high-resolution TEM analysis to be SmCo5 particles with adequately reduced contents of impurities such as oxides. The contents of the third metal elements in the particles of Examples 1-8 as determined by ICP optical emission spectroscopic analysis are also listed in Table 1.
The yields of SmCo5 particles in Examples 1-8 are also shown in Table 1. The yields are the total masses of Sm and Co elements as measured by ICP optical emission spectroscopy, with respect to the theoretical produced mass of SmCo5 as calculated from the amounts of Sm and Co used.
The evaluation results described above confirmed that the SmCo5 nanoparticles containing third metal elements were obtained at high yield.
[Evaluation of Magnetic Properties]
The coercive forces (Hc) of the SmCo5 nanoparticles of Examples 1-8 were measured using a VSM (Vibrating Sample Magnetometer) (VSM-5, trade name of Toei Industry Co., Ltd.), under conditions with an applied magnetic field of 20 kOe at 25° C. The results of coercive force measurement are shown in Table 1.
Particles were obtained by synthesis in the same manner as Example 2, except that lithium aluminum hydride (LiAlH4) was used as the strong reducing agent instead of sodium borohydride. Evaluation of the particles and their magnetic properties in the same manner as Example 2 revealed a high yield of SmCo5 nanoparticles with excellent magnetic properties, containing Cu as the third metal element. The particle size, third metal element content, yield and coercive force (Hc) of the SmCo5 nanoparticles were measured and the results are shown in Table 1.
Comparative Example 1SmCo5 nanoparticles were synthesized in the same manner as Example 1, except that the third metal salt (palladium acetylacetonate) and sodium borohydride were not used. The particles and their magnetic properties were evaluated in the same manner as Example 1. The lower chart of
Particles were obtained by synthesis in the same manner as Example 1, except that no sodium borohydride was used. The particles and their magnetic properties were evaluated in the same manner as Example 1. The middle chart of
Particles were obtained by synthesis in the same manner as Example 1, except that the third metal element salt (palladium acetylacetonate) was not used. The particle size, yield and coercive force (Hc) of the particles are shown in Table 1. The coercive force (Hc) of the particles of Comparative Example 3 was lower than that of the SmCo5 nanoparticles containing third metal elements of Examples 1-9.
Claims
1. SmCo-based alloy nanoparticles composed mainly of a SmCo-based alloy containing Sm and Co as constituent elements, wherein the content of metal elements other than Sm and Co is 0.05-20 wt % with respect to the SmCo-based alloy.
2. SmCo-based alloy nanoparticles according to claim 1, wherein the content of the metal elements is 0.05-10 wt % with respect to the SmCo-based alloy.
3. SmCo-based alloy nanoparticles according to claim 1, wherein the metal elements include at least one element selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Ir, Os, Cu, Ni, Cr, Al and Mn.
4. SmCo-based alloy nanoparticles according to claim 1, wherein the particle sizes are 1-30 nm.
5. SmCo-based alloy nanoparticles according to claim 1, which are obtained by liquid synthesis wherein a strong reducing agent is added to an organic solvent containing a samarium salt, a cobalt salt and metal element salts, to reduce the samarium salt, the cobalt salt and the metal element salts.
6. SmCo-based alloy nanoparticles according to claim 1, having a core-shell structure comprising a core section and a shell section covering the core section, wherein the proportion of the metal elements with respect to the SmCo-based alloy in the core section is greater than the proportion in the shell section.
7. A process for production of SmCo-based alloy nanoparticles composed mainly of a SmCo-based alloy containing Sm and Co as constituent elements, the process comprising:
- a mixing step in which a starting material containing a samarium salt, a cobalt salt and salts of metal elements other than Sm and Co, as well as a protective agent, is mixed with a reducing organic solvent; and
- a reduction step in which a strong reducing agent is added to the mixture and heated to reduce the samarium salt, the cobalt salt and the salts of the metal elements.
8. A process for production of SmCo-based alloy nanoparticles according to claim 7, further comprising a dehydration step in which the mixture obtained from the mixing step is stirred and heated for uniform dissolution and dehydration and then cooled, prior to the reduction step.
9. A process for production of SmCo-based alloy nanoparticles according to claim 7, wherein the metal elements include at least one element selected from the group consisting of Au, Ag, Pt. Pd, Rh, Ru, Ir, Os, Cu, Ni, Cr, Al and Mn.
10. A process for production of SmCo-based alloy nanoparticles according to claim 7, wherein the strong reducing agent contains at least one compound selected from the group consisting of LiAlH4, NaBH1, N2H4, B2H6 and LiBH(C2H5)3.
Type: Application
Filed: Mar 4, 2009
Publication Date: Oct 15, 2009
Applicants: TDK Corporation (Chuo-ku), Tokyo University of Science Educational Foundation Administrative Organization (Shinjuku-ku)
Inventors: Mamoru Satoh (Tokyo), Naoki Toshima (Tokyo), Mutsuko Kinjo (SanyoOnoda-shi), Haruki Kinjo (Ginowann-shi), Shiho Tokonami (SanyoOnoda-shi)
Application Number: 12/397,786
International Classification: C22C 28/00 (20060101); C22C 19/07 (20060101); C22C 30/00 (20060101); B22F 9/18 (20060101);
