MATERIAL COMPRISING A SEMI-HEUSLER ALLOY AND PROCESS FOR PRODUCING SUCH A MATERIAL

Info

Publication number: 20140127070
Type: Application
Filed: Nov 4, 2013
Publication Date: May 8, 2014
Applicant: Robert Bosch GmbH (Stuttgart)
Inventors: Michael Schwall (Mainz), Benjamin Balke (Mainz), Martin Koehne (Asperg)
Application Number: 14/071,325

Abstract

A material includes at least two different alloy phases. At least two alloy phases are each formed by at least one thermodynamically stable semi-Heusler alloy. The semi-Heusler alloys of the at least two alloy phases are different from one another. At least two of the semi-Heusler alloys have at least partly sintered particles that have an average particle size D50 in the range of less than or equal to 100 nm. Such a material has particularly good thermoelectric properties. A process is implemented to produce the material.

Description

Description

This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2012 220 306.0 filed on Nov. 8, 2012 in Germany, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to a material comprising a semi-Heusler alloy. The present disclosure further relates to a process for producing such a material.

Thermoelectric materials are known for various applications. An example of volume materials which are possible for thermoelectric elements are semi-Heusler alloys. These are derived from the Heusler alloys which were discovered by Fritz Heusler in 1903. This class of materials of Heusler alloys is characterized, inter alia, by particular Heusler alloys composed of nonmagnetic metals being ferromagnetic. Heusler alloys have the empirical formula X₂YZ.

A separate alloy class having the empirical formula XYZ, known as semi-Heusler alloys, which have a C1b structure consisting of three nested face-centered cubic sublattices is derived from the Heusler alloys.

However, in the field of thermoelectric compounds, few examples of semi-Heusler alloys have become known.

SUMMARY

The present disclosure provides a material comprising at least two different phases, wherein at least two phases are each formed by at least one thermodynamically stable semi-Heusler alloy, the semi-Heusler alloys of the at least two phases are different from one another and at least two semi-Heusler alloys have at least partly sintered particles having an average particle size D₅₀in the range of less than or equal to 100 nm.

For the purposes of the present disclosure, a phase is, in particular, a homogeneous region or a substance within the material having identical physical and chemical properties. For example, such a phase can consist of a semi-Heusler alloy.

For the purposes of the present disclosure, thermodynamically stable means, in particular, that the respective phase or the material of this phase, in particular the semi-Heusler alloy which forms the respective phase or of which the respective phase consists does not transform but remains stable in terms of its atomic composition and atomic arrangement up to a defined temperature. Here, thermodynamic stability can be ensured up to a temperature which can be essentially dependent on the desired field of use or on the production process. For example, thermodynamic stability can be ensured up to a temperature which is above room temperature and can be, for example, in the range of greater than or equal to 500° C.

Here, a semi-Heusler alloy can, as is known per se, be an intermetallic compound or an intermetallic phase which is based on the empirical formula XYZ and which can have, in particular, a C1b structure. Being based on an empirical formula or on a structure can mean, in particular, that additional elements can be provided in addition to the elements in the formula or individual elements can be at least partly replaced. Such a C1b structure can, in particular, consist in a manner known per se of three nested face-centered cubic sublattices. For the purposes of the present disclosure, a semi-Heusler alloy can also be, in detail, a substance which is based on the empirical formula Ti_xZr_yHf_zNiSn in which: x+y+z=1 and thus in particular but not exclusively a semi-Heusler alloy of the n type without this being mentioned separately each time in the following. Thus, semi-Heusler alloys are for the purposes of the disclosure in particular ternary intermetallic compounds. A person skilled in the art will know that the term semi-Heusler alloy is a customary and widespread term for such an intermetallic phase but strictly speaking does not have to refer to an alloy without this being separately mentioned each time in the following.

Furthermore, for the purposes of the present disclosure, an average particle size D₅₀means that at least 50% of the particles present have such a particle size or are in the present particle size range. However, the particle size is, for the purposes of the present disclosure, the size or the particle size which the individual particles have even when they may be adhesively bound to other particles. Thus, for the purposes of the present disclosure, particles are not necessarily completely separate solids but can likewise be solids which were formerly separate in the form of a powder but have become adhesively bound to one another by means of a sintering process.

A material as described here has, in particular, the advantage that it has particularly good thermoelectric properties, in particular a high efficiency index (ZT), and has a very good electrical conductivity combined with a low thermal conductivity.

In detail, the material has in principle any plurality of different phases but has at least two different phases, where at least two, preferably all, phases are each formed by a thermodynamically stable semi-Heusler alloy, with the semi-Heusler alloys being different from one another. The use of semi-Heusler alloys makes it possible, firstly, to utilize the advantage that such alloys are generally semiconductors having relatively small band gaps, which makes use in which good thermodynamic properties are required very advantageous. In addition, very high efficiency indices (ZT) in respect of thermoelectric properties can, in particular, be achieved by use of such alloys.

As a result of the semi-Heusler alloys having particles which have an average particle size D₅₀in the range of less than or equal to 100 nm, the thermal conductivity can be lowered while maintaining a good electrical conductivity, which can further improve the efficiency index in respect of the thermoelectric properties. This is because the thermal conductivity is an important influencing parameter to which the efficiency index is inversely proportional, so that a reduction in the thermal conductivity can have a positive influence on the thermoelectric properties.

As a result of the semi-Heusler alloys having particles which have an average particle size D₅₀in the range of less than or equal to 100 nm, it is possible to create a structure in which phonons, i.e. pseudo particles by means of which heat is transported and which are thus responsible for thermal conduction, are scattered and uniform thermal conduction is reduced thereby. However, the particles are provided, in particular, with a size which does not hinder the flow of electrons. In this way, the thermal conductivity is reduced without significantly decreasing the electrical conductivity.

Due to the production process, in particular, the respective particles are at least partly sintered to one another. It can thus be seen that the particles are essentially not present as uniform solids which are completely separate from other particles but the particles are instead joined to one another and are partly surrounded by other particles. For example, the prescribed particle size or the prescribed grain size of the semi-Heusler phases used can ensure that a nanostructured microstructure is maintained even during sintering since no significant grain growth occurs during the sintering process. A significant reduction in or complete prevention of grain growth during a sintering process, for example, can in particular be achieved in combination with the above-described average grain size by the provision of two semi-Heusler phases which are not miscible with one another and are both thermodynamically stable. The two different phases, in particular, mutually prevent grain growth. Prevention of such grain growth can not only be made possible in a sintering process during production but also after complete production of the material, for instance in applications in a high temperature range. The material can not only be produced so as to be stable but is also stable and experiences no adverse effect on the thermoelectric properties even during long-term use under harsh conditions, in particular high temperatures.

The particles thus have a prescribed particle size not only before sintering but also after sintering in the finished material. Furthermore, a microstructure which has grains or particles having an average grain size D₅₀of less than or equal to 100 nm is still present during the entire life even at high temperatures even when, as indicated above, the particles can be, for example, adhesively bonded to one another by means of sintering.

Furthermore, the properties can be tailored by possible variation of the phases, e.g. in particular, the semi-Heusler alloys or semi-Heusler composites, present in the material in respect of their precise composition, particle size or particle size distribution, mixing ratio of various phases and also the number thereof, so that a particularly broad and additionally specific field of application is possible.

The above-described material makes it possible, for example, to develop inexpensive, environmentally friendly, effective and resource-saving thermoelectric generators for the recovering of energy from waste heat, for example in mass production. These thermoelectric generators can be used in numerous applications in order to utilize the waste heat, which has hitherto not been utilized in terms of energy content, for power generation. Examples of applications are the generation of power from waste heat in the exhaust system of an automobile or from the waste heat in hot gas from industrial thermal processes.

In an embodiment, at least one semi-Heusler alloy, preferably all semi-Heusler alloys, can have particles which have an average particle size D₅₀in the range of less than or equal to 30 nm, in particular in the range from ≧10 nm to ≦30 nm, for example in the range from ≧15 nm to ≦20 nm. In this embodiment in particular, i.e. when the for example sintered particles have a size in the above-described ranges, it is possible to create a structure in which the thermal conductivity can be particularly effectively reduced while the electrical conductivity is not decreased or not significantly decreased, in particular by scattering of the phonons and transmissivity in respect of electrons. Thus, the thermoelectric properties of the material are, particularly in this embodiment, particularly advantageously configured or a particularly good ZT value can be achieved.

In a further embodiment, the particles can have an at least bimodal particle size distribution. Thus, the particle size can be such that, for example, two particle size maxima are provided in the case of a bimodal particle size distribution. However, a plurality of maxima in the particle sizes or maxima in the proportion at particular particle sizes can equally well be present. In this embodiment, the particularly pronounced thermoelectric properties at various temperatures, for instance at two temperatures in a bimodal distribution, may be able to be modified by adapting the particle size maxima. In this way, particularly good matching to the desired field of use can be achieved since phonons of various wavelengths can be scattered in this way. In detail, the efficiency index (ZT) is defined as follows: ZT=S²*δ/λ, where S corresponds to the Seebeck coefficient, δ corresponds to the electrical conductivity and λ corresponds to the thermal conductivity. The bimodal particle size distribution with formation of finer and coarser particles also enables the phonons having a greater wavelength to be scattered at the larger particles at low temperatures. As an illustrative and nonrestrictive example, ⅓ of the particles in a trimodal particle size distribution can be set to about 10 nm, ⅓ can be set to about 50 nm and ⅓ can be set to about 100 nm.

In a further embodiment, two thermodynamically stable phases whose semi-Heusler alloys or which phases, in the case of the latter consisting of the semi-Heusler alloys, are present in a ratio of from 1:5 to 5:1, for example from 1:4 to 4:1, can be provided. In particular at such ratios of the semi-Heusler alloys used, particularly stable particle sizes can be obtained even in the case of production and use of the material under harsh conditions, for instance elevated temperatures, which allows a material having particularly defined properties and a significant reduction in the thermal conductivity and thus an increase in the thermoelectric efficiency.

In a further embodiment, the phases or the semi-Heusler alloys can be thermodynamically stable up to a temperature of greater than or equal to 950° C., both as such and also in direct contact with one another. In this embodiment, a particularly broad field of use is possible since the material comprising two or more semi-Heusler alloys, for example consisting of these, is not only intrinsically thermodynamically stable up to very high temperatures but, in addition, the phases thereof retain their properties, in particular the thermoelectric properties, for example their efficiency index, when they are in direct contact with one another. In addition, the thermodynamic stability allows production processes for the material which require a high temperature, for example spark plasma sintering or pressure sintering.

In a further embodiment, at least two semi-Heusler alloys can each be based on an alloy selected from the group consisting of Ti_0.68Zr_0.18Hf_0.14NiSn; Ti_0.43Zr_0.28Hf_0.29NiSn; Ti_0.21Zr_0.4Hf_0.39NiSn; Ti_0.24Hf_0.76NiSn; Ti_0.61Hf_0.39NiSn; Ti_0.83Hf_0.17NiSn; Ti_0.83Zr_0.17NiSn; Ti_0.65Zr_0.35NiSn; Ti_0.31Zr_0.69NiSn; Zr_0.78Hf_0.22NiSn; Zr_0.55Hf_0.45NiSn; Zr_0.37Hf_0.63NiSn; TiNiSn; ZrNiSn and HfNiSn. The abovementioned semi-Heusler alloys in particular are thermodynamically stable and do not change or do not significantly change their properties, in particular the thermoelectric properties, up to high temperatures, in particular up to 950° C. or even above. Materials having at least two different alloys of this type, in particular, have a particularly high efficiency index. Thus, the abovementioned examples as specific semi-Heusler alloys all have the advantage of reducing the thermal conductivity in a material according to the disclosure while not having an adverse effect on the electrical conductivity in order to achieve a particularly high ZT value.

In a further embodiment, at least one metal from the group consisting of titanium (Ti), zirconium (Zr) and hafnium (Hf) can be at least partly replaced by a metal from the group consisting of scandium (Sc), yttrium (Y), niobium (Nb), vanadium (V), manganese (Mn), aluminum (Al), silver (Ag), in particular so as to give at least one semi-Heusler alloy based on a composition from the group consisting of Ti_0.64Zr_0.18Hf_0.14Nb_0.04NiSn, Ti_0.39Zr_0.28Hf_0.29Nb_0.04NiSn and Ti_0.17Zr_0.4Hf_0.39Nb_0.04NiSn. Such replacement of the respective metals in the semi-Heusler alloys enables the thermoelectric properties of the material to be tailored in an advantageous way. In detail, the electrical conductivity can be increased in a targeted manner by means of targeted replacement without the thermal conductivity being significantly increased.

In a further embodiment, nickel (Ni) can be at least partly replaced by a metal from the group consisting of manganese (Mn), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn) and silver (Ag). Such replacement of the respective metals in the alloys enables the thermoelectric properties of the material to be tailored in an advantageous way. In detail, the electrical conductivity can be increased in a targeted manner by means of a targeted replacement without the thermal conductivity being significantly increased.

In a further embodiment, tin (Sn) can be at least partly replaced by a metal from the group consisting of antimony (Sb), tellurium (Te), bismuth (Bi), indium (In), gallium (Ga), aluminum (Al), in particular so as to form at least one semi-Heusler alloy based on a composition from the group consisting of Ti_0.68Zr_0.18Hf_0.14NiSn_0.998Sb_0.002, Ti_0.43Zr_0.28Hf_0.29NiSn_0.998Sb_0.002and Ti_0.21Zr_0.4Hf_0.39NiSn_0.998Sb_0.002. The thermoelectric properties of the material can be tailored in an advantageous way by means of such replacement of the respective metals in the alloys. In detail, the electrical conductivity can be increased in a targeted way by means of a targeted replacement without the thermal conductivity being significantly increased.

As regards further technical features and advantages of the material of the disclosure, explicit reference is at this point made to the explanations in relation to the process of the disclosure, the figures and the description of the figures.

The present disclosure further provides a process for producing a material, in particular a material as described above, which comprises the process steps:

a) provision of at least two different thermodynamically stable semi-Heusler alloys in at least one alloy body;
b) optionally comminution of the at least one alloy body to form particles having an average particle size D₅₀in the range of less than or equal to 100 nm;
c) mixing of the particles obtained in process step a) or b) in a predetermined ratio; and
d) sintering of the mixture obtained in process step c).

The above-described process is, in a particularly simple way, suitable for producing a material as described above and thus creating a material which has, in particular, a lower thermal conductivity combined with a good electrical conductivity and as a result has a significantly better efficiency index ZT compared to the materials known from the prior art.

For this purpose, at least two different thermodynamically stable semi-Heusler alloys are provided in at least one alloy body in a process step a). Here, the different semi-Heusler alloys can be provided in a single alloy body or, preferably, be provided in two separate alloy bodies. In the latter case, the alloy bodies are each formed by different semi-Heusler alloys. The at least one alloy body is, in particular, a metallic compound which comprises or consists of the desired thermodynamically stable semi-Heusler alloy. This or these alloy body/bodies can in principle have any suitable shape in this state.

In a further process step b), the alloy body or alloy bodies is/are optionally comminuted to form particles having an average particle size D₅₀in the range of less than or equal to 100 nm. For example, the at least one alloy body can be comminuted to form particles which have an average particle size D₅₀in the range of less than or equal to 30 nm, in particular in the range from ≧10 nm to ≦30 nm, for example in the range from ≧15 nm to ≦20 nm. Such comminution can, for example, be effected by milling. Here, the particles can all have the same particle size or a suitable particle size distribution can be provided. In the latter case, it is possible, for example, to produce various mixtures each having a uniform particle size and these mixtures are in turn blended. It can also be possible for the different semi-Heusler alloys to have particles of different particle sizes or different particle size distributions or for a uniform particle size or particle size distribution to be set for all the semi-Heusler-alloys. Here, a person skilled in the art will be able to see that in production of the particles directly in the desired sizes, the process steps b) and a) can coincide or process step b) can be omitted.

After production of semi-Heusler alloy particles of a predetermined composition, these are mixed in a predetermined ratio in process step c). The ratio of the various semi-Heusler alloy phases in the material to be produced can be set by means of this process step. For example, a ratio of from 5:1 to 1:5 can be set.

When the appropriate mixture has been produced, sintering of the mixture obtained in process step c) is carried out in process step d). In this process step, the particles obtained can be sintered to one another so as to form a stable material which can, in particular, be used as thermoelectric substance, as has been described in respect of the material. Here, nanostructuring of the microstructure can be maintained in the long term at high temperatures by means of the above-described process since the alloy phases which are thermodynamically stable in the presence of one another mutually prevent grain growth. This applies both during production, in particular during sintering, and also during later use at high temperatures.

In an embodiment, at least one thermodynamically stable semi-Heusler alloy can be provided by means of the process steps:

- a1) production of a metal mixture comprising nickel, tin and at least one of titanium, zirconium and hafnium in a predetermined weight ratio; and
- a2) treatment at elevated temperature of the mixture obtained in process step a1).

In this embodiment, the alloy can be made particularly homogeneous. This is advantageous since, in particular, the alloys can be comminuted to a nanostructure in a subsequent process step and, in particular in this embodiment, it can also be ensured that essentially all particles have a uniform composition and the various phases formed are also particularly homogeneous and particularly defined properties can be brought about thereby even in the case of such small nanostructures.

Here, in a first step a1), a metal mixture comprising nickel, tin and at least one of titanium, zirconium and hafnium in a predetermined weight ratio is produced. In particular, the weight ratios can be matched to the desired thermodynamically stable structure to be achieved in the semi-Heusler alloy. Here, the abovementioned metals can in principle be used in any form, with the purity preferably being particularly high in order to aid very precise setting of the thermodynamically stable phases.

In a further step a2), the mixture obtained in process step a1) is treated at elevated temperature. In particular, the temperature can be selected so that the various components are melted and a particularly homogeneous and uniform and in particular pore-free microstructure is thus formed. Treatment at elevated temperature can be effected briefly or for relatively long periods of time, for instance for a period of time of up to seven days or else significantly below, with temperatures of from ≧800° C. to ≦1100° C. being able to be employed. After cooling, for example using ice water over a period of, for example, 15 minutes, the corresponding semi-Heusler alloy or phase can be provided in finished form and be used further.

In a further embodiment, process step d) can be carried out by spark plasma sintering (SPS) or pressure sintering. It is also possible to use an SPS process known as Field-Activated Sintering (FAST) and DC sintering, in particular one in which sintering is effected essentially with application of an electric current, in particular direct current. Here, the current is passed directly through the powder to be sintered, with the electric power of the current being converted into heat power by the ohmic resistance of the powder, resulting in the powder heating up. For the purposes of the present disclosure, pressure sintering can be, in particular, a sintering process carried out under a suitable pressure. In this embodiment in particular, a particularly stable microstructure can be advantageously formed and a reduction in grain growth can be achieved since the SPS process takes, for example, very little time.

In a further embodiment, process step a2) can be carried out using an electric arc. In this embodiment, it can be ensured in a particularly simple way or by means of a particularly simple arrangement that the metal or the metal mixture is completely molten and a homogeneous mixture is thus formed. In other words, it is possible to prevent relatively large regions of individual metals from remaining unmelted in the mixture and thus forming a region of a single metal in a subsequent product. Thus, a particularly homogeneous and defined product can also be obtained in this embodiment.

In a further embodiment, process step b) can be carried out using a ball mill, in particular a planetary ball mill. Such setting of the particle sizes or grain sizes makes it possible, in particular, to ensure that uniform and defined particles are formed. As a result, the material to be produced can also have particularly defined properties in respect of electrical and thermal conductivity.

In a further embodiment, the process can be carried out at least partly in an inert gas atmosphere. The use of an inert gas atmosphere makes it possible to prevent the structure of the alloy from being adversely affected or even destroyed by, for example, oxidation processes. As a result, a particularly defined product can be obtained and rejects can be minimized or prevented by use of an inert gas atmosphere. For the present purposes, an inert gas is, in particular, a gas or a gas mixture which reacts with none of the alloy constituents under the respective conditions. Examples of inert gases comprise but are not restricted to nitrogen or argon.

Regards further technical features and advantages of the process of the disclosure, explicit reference is made here to the explanations in respect of the material of the disclosure, the figures and the description of the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and advantageous embodiments of the subject matter of the disclosure are illustrated by the drawings and explained in the following description. It should be noted that the drawings have only a descriptive character and are not intended to restrict the disclosure in any way.

FIG. 1 shows a schematic depiction of a typical atomic structure of a semi-Heusler alloy;

FIG. 2 shows a schematic depiction of a composition triangle showing thermodynamically stable semi-Heusler alloys which can be used according to the disclosure;

FIG. 3 shows a depiction of an XRD pattern of the illustrative semi-Heusler phase Ti_0.21Zr_0.4Hf_0.39NiSn, which can be used according to the disclosure; and

FIG. 4 shows a depiction of the efficiency index of illustrative materials compositions.

DETAILED DESCRIPTION

FIG. 1 shows the crystal structure of a semi-Heusler alloy. Such a structure is formed by a substance based on the empirical formula XYZ, more precisely Ti_xZr_yHf_zNiSn where x+y+z=1. The structure is a C1b structure which consists of three nested face-centered cubic sublattices. Such a structure is a constituent of a material.

In detail, a material comprises at least two different semi-Heusler alloys, where at least two semi-Heusler alloys are formed by a semi-Heusler alloy which is, for example, thermodynamically stable up to a temperature of greater than or equal to 950° C., as indicated above, and at least two semi-Heusler alloys have at least partly sintered particles which have an average particle size D₅₀in the range of less than or equal to 100 nm. For example, at least one semi-Heusler alloy can have particles which have an average particle size D₅₀in the range of less than or equal to 30 nm, in particular in the range from ≧10 nm to ≦30 nm, for example in the range from ≧15 nm to ≦20 nm. The particles here can, in particular, have an at least bimodal particle size distribution and/or the different semi-Heusler alloys of the different phases can be present in a ratio of from 1:5 to 5:1.

For example, the at least two semi-Heusler alloys can each be selected from the group consisting of Ti_0.68Zr_0.18Hf_0.14NiSn, Ti_0.43Zr_0.28Hf_0.29NiSn, Ti_0.21Zr_0.4Hf_0.39NiSn, Ti_0.24Hf_0.76NiSn, Ti_0.61Hf_0.39NiSn, Ti_0.83Hf_0.17NiSn, Ti_0.83Zr_0.17NiSn, Ti_0.65Zr_0.35NiSn, Ti_0.31Zr_0.69NiSn, Zr_0.78Hf_0.22NiSn, Zr_0.55Hf_0.45NiSn, Zr_0.37Hf_0.63NiSn, TiNiSn, ZrNiSn, HfNiSn. These semi-Heusler alloys which are thermodynamically stable up to 950° C. are shown in the composition triangle of FIG. 2, with the content of the respective composition being able to have an error in the range of up to 3%. The composition triangle shows the composition ZrNiSn at point A, the composition TiNiSn at point B and the composition HfNiSn at point C, with, furthermore, the content of hafnium being indicated in mol % along the edge 1, the content of zirconium being indicated in mol % along the edge 2 and the content of titanium being indicated in mol % along the edge 3.

The production of such an alloy is described by way of example below. The various elements of the thermodynamically stable semi-Heusler alloy to be produced are weighed out in accordance with the stoichiometry which is indicated for the intermetallic compounds No. 1-15 in table 1 below (errors in the respective composition: 3%). Here, the examples having the numbers 1-15 correspond to the examples from the composition triangle.

TABLE 1 Ti Zr Hf Ni Sn % by % by % by % by % by No. Ti_xZr_yHf_zNiSn mass mass mass mass mass 1 Ti_0.68Zr_0.18Hf_0.14NiSn 13.2 6.6 9.3 23.5 47.5 2 Ti_0.43Zr_0.28Hf_0.29NiSn 7.5 9.3 18.8 21.3 43.1 3 Ti_0.21Zr_0.4Hf_0.39NiSn 3.4 12.4 23.7 20.0 40.4 4 Ti_0.24Hf_0.76NiSn 3.5 0.0 41.8 18.1 36.6 5 Ti_0.61Hf_0.39NiSn 10.6 0.0 25.2 21.2 43.0 6 Ti_0.83Hf_0.17NiSn 16.1 0.0 12.3 23.7 48.0 7 Ti_0.83Zr_0.17NiSn 17.1 6.7 0.0 25.2 51.0 8 Ti_0.65Zr_0.35NiSn 12.9 13.3 0.0 24.4 49.4 9 Ti_0.31Zr_0.69NiSn 5.8 24.7 0.0 23.0 46.5 10 Zr_0.78Hf_0.22NiSn 0.0 24.7 13.6 20.4 41.2 11 Zr_0.55Hf_0.45NiSn 0.0 16.3 26.1 19.1 38.6 12 Zr_0.37Hf_0.63NiSn 0.0 10.4 34.7 18.1 36.7 13 TiNiSn 21.3 0.0 0.0 26.1 52.7 14 ZrNiSn 0.0 34.0 0.0 21.8 44.2 15 HfNiSn 0.0 0.0 50.2 16.5 33.4

The weighed out materials are subsequently melted in an electric arc furnace at a current of 70 A. For purification, the sample chamber is evacuated three times to a vacuum of 5*10⁻⁴mbar and flooded with argon (99.999%) after each evacuation before melting. After the triple evacuation and flooding, the materials are melted under an argon atmosphere at about 20 mbar. The electric current for generating the argon plasma is supplied by a DC transformer (U=20V, I=20-100 A).

For melting, the electric arc is slowly moved from sample to sample, with each sample being kept in the melt for about 20 seconds. To achieve better homogenization, the samples are turned after solidification and melted again. This remelting can be carried out up to 10×. The samples are subsequently melted under an argon atmosphere in fused silica and aged at 950° C. for 7 days. The samples are taken from the furnace and immediately quenched in ice water.

As starting materials, the following materials obtainable from Chempur were procured: nickel wire having a diameter of 1.0 mm and a purity of 99.98%; titanium wire having a diameter of 1.0 mm and a purity of 99.8%; hafnium pieces having a size of <55 mm and a purity of 99.8%; zirconium sponge having a size of 3-6 mm and a purity of 99.8%; tin granules having a size of 2-4 mm and a purity of 99.99%. To produce phase mixtures having a nanostructured microstructure, the samples are produced as follows: the individual samples are produced as described above in an electric arc furnace and subsequently milled for 5 hours under an argon atmosphere in a planetary ball mill PM-100 from Retsch. The powders of the various samples are subsequently combined in the desired stoichiometry and sintered at 1050° C. for 10 minutes under a mechanical pressure of 50 MPa and a vacuum atmosphere of 4 Pa in an SPS machine (Spark Plasma Sintering, model HP D 5/1 FCT Systeme GmbH, Germany). The heating/cooling rates are 50° C./min. Samples having a density of >95% of the theoretical density were able to be obtained.

The samples produced in this way were analyzed by means of XRD measurements and SEM with EDX (errors of the composition about ±3%) and the thermodynamically stable semi-Heusler phase was identified. FIG. 3 shows the XRD measurements on the thermodynamically stable semi-Heusler phase Ti_0.21Zr_0.4Hf_0.39NiSn, with the scattering angle 20 being plotted on the X axis and the Y axis indicating the intensity.

Apart from the abovementioned compounds, individual metal constituents can also be at least partly replaced. For example, at least one metal from the group consisting of titanium, zirconium and hafnium can be at least partly replaced by a metal from the group consisting of scandium, yttrium, niobium, vanadium, manganese, aluminum, silver, in particular so as to give at least one semi-Heusler alloy based on a composition from the group consisting of Ti_0.64Zr_0.18Hf_0.14Nb_0.04NiSn, Ti_0.39Zr_0.28Hf_0.29Nb_0.04NiSn and Ti_0.17Zr_0.4Hf_0.39Nb_0.04NiSn. Furthermore, nickel can be at least partly replaced by a metal from the group consisting of manganese, iron, cobalt, copper, zinc and silver. In addition, tin can be at least partly replaced by a metal from the group consisting of antimony, tellurium, bismuth, indium, gallium, aluminum, in particular so as to give at least one semi-Heusler alloy based on a composition from the group consisting of Ti_0.68Zr_0.18Hf_0.14NiSn_0.998Sb_0.002, Ti_0.43Zr_0.28Hf_0.29NiSn_0.998Sb_0.002and Ti_0.21Zr_0.4Hf_0.39NiSn_0.998Sb_0.002.

The substituted semi-Heusler alloys can essentially be based on the following empirical formulae:

(Ti_aZr_bHf_c)_1-xM_xNiSn where M=scandium, yttrium, niobium, vanadium, manganese, aluminum, silver where x=0-0.2 and a+b+c=1; (Ti_aZr_bHf_c)Ni_1-xM_xSn where M=manganese, iron, cobalt, copper, tin, silver where x=0-0.2 and a+b+c=1; or (Ti_aZr_bHf_c)NiSn_1-xM_xwhere M=antimony, tellurium, bismuth, indium, gallium, aluminum where x=0-0.2 and a+b+c=1.

Illustrative properties of a material are shown in Table 2. In particular, the Seebeck coefficient, the electrical resistance, the thermal conductivity and the efficiency index ZT are shown. For this material, a material comprising, as alloy phases, 60% of Ti_0.21Zr_0.4Hf_0.39NiSn and 40% of Ti_0.68Zr_0.18Hf_0.14NiSn was produced:

TABLE 2 Seebeck Thermal Temperature coefficient Resistance conductivity [K] [μV/K] [Ohm m] [W/K/m] ZT 339.6 −203.9 4.51E−05 387.4 −217.1 3.67E−05 1.36 0.37 436.1 −225.0 3.06E−05 484.8 −231.7 2.62E−05 1.31 0.76 534.0 −234.8 2.30E−05 583.1 −234.8 2.06E−05 1.30 1.20 632.4 −233.9 1.87E−05 682.4 −231.9 1.71E−05 1.35 1.59 731.2 −230.5 1.62E−05 781.3 −224.9 1.50E−05 1.45 1.81 830.1 −216.6 1.39E−05 879.2 −206.3 1.25E−05 1.65 1.82

It can be seen that such a material has good thermoelectric properties.

The properties can, in particular, be tailored by means of the parameters during production. Examples of variables for setting the desired properties comprise but are not restricted to the number and the respective choice of the semi-Heusler alloy phases, the average particle size or particle size distribution, in particular with a bimodal or multimodal particle size distribution, doping and also the aging conditions during production of the semi-Heusler alloy phases.

Furthermore, in FIG. 4, the temperature in K is plotted on the X axis against the efficiency index ZT of various materials on the Y axis, i.e. the ZT value which can be achieved at particular temperatures. Here, the curve a) describes a material comprising 60% of Ti_0.21Zr_0.4Hf_0.39NiSn and 40% of Ti_0.68Zr_0.18Hf_0.14NiSn, the curve b) describes a material comprising 40% of Ti_0.21Zr_0.4Hf_0.39NiSn and 60% of Ti_0.68Zr_0.18Hf_0.14NiSn, the curve c) describes a material comprising 60% of Ti_0.21Zr_0.4Hf_0.39NiSn and 40% of Ti_0.43Zr_0.28Hf_0.29NiSn and the curve d) describes a material comprising 40% of Ti_0.21Zr_0.4Hf_0.39NiSn and 60% of Ti_0.43Zr_0.28Hf_0.29NiSn.

Claims

1. A material, comprising:

at least two different phases, at least two phases each being formed by at least one thermodynamically stable semi-Heusler alloy, the semi-Heusler alloys of the at least two phases being different from one another and at least two semi-Heusler alloys having at least partly sintered particles that have an average particle size D50 in the range of less than or equal to 100 nm.

2. The material according to claim 1, wherein at least one semi-Heusler alloy has particles having an average particle size D50 in the range from less than or equal to 30 nm.

3. The material according to claim 1, wherein the particles have an at least bimodal particle size distribution.

4. The material according to claim 1, further comprising two thermodynamically stable phases whose semi-Heusler alloys are present in a ratio of from 1:5 to 5:1.

5. The material according to claim 1, wherein the phases are thermodynamically stable up to a temperature of greater than or equal to 950° C.

6. The material according to claim 1, wherein at least two semi-Heusler alloys are each based on an alloy selected from the group consisting of Ti0.68Zr0.18Hf0.14NiSn, Ti0.43Zr0.28Hf0.29NiSn, Ti0.21Zr0.4Hf0.39NiSn, Ti0.24Hf0.76NiSn, Ti0.61Hf0.39NiSn, Ti0.83Hf0.17NiSn, Ti0.83Zr0.17NiSn, Ti0.65Zr0.35NiSn, Ti0.31Zr0.69NiSn, Zr0.78Hf0.22NiSn, Zr0.55Hf0.45NiSn, Zr0.37Hf0.63NiSn, TiNiSn, ZrNiSn, HfNiSn.

7. The material according to claim 1, wherein the at least two semi-Heusler alloys are based on the empirical formula TixZryHfzNiSn, where at least one metal from the group consisting of titanium, zirconium and hafnium is at least partly replaced by a metal from the group consisting of scandium, yttrium, niobium, vanadium, manganese, aluminum, silver.

8. The material according to claim 1, wherein the at least two semi-Heusler alloys are based on the empirical formula TixZryHfzNiSn, where nickel is at least partly replaced by a metal from the group consisting of manganese, iron, cobalt, copper, zinc and silver.

9. The material according to claim 1, wherein the at least two semi-Heusler alloys are based on the empirical formula TixZryHfzNiSn, where tin is at least partly replaced by a metal from the group consisting of antimony, tellurium, bismuth, indium, gallium, aluminum.

10. A process for producing a material having two different phases, comprising:

forming particles of at least one alloy body having at least two different thermodynamically stable semi-Heusler alloys, the particles having an average particle size D50 in the range of less than or equal to 100 nm;

mixing the particles in a predetermined ratio; and

sintering the mixture.

11. The process according to claim 10, wherein at least one thermodynamically stable semi-Heusler alloy is provided by:

producing a metal mixture comprising nickel, tin and at least one of titanium, zirconium and hafnium in a predetermined weight ratio; and

treating the metal mixture at an elevated temperature.

12. The process according to claim 10, wherein the sintering of the mixture includes spark plasma sintering or pressure sintering.

13. The process according to claim 11, wherein the treatment of the metal mixture at the elevated temperature includes using an electric arc.

14. The process according to claim 10, wherein the forming of the particles includes using a ball mill.

15. The process according to claim 10, wherein the process is at least partly carried out in an inert gas atmosphere.

16. The material according to claim 2, wherein the at least one semi-Heusler allow has particles having an average particle size D50 in the range from ≧10 nm to ≦30 nm.

17. The material according to claim 2, wherein the at least one semi-Heusler allow has particles having an average particle size D50 in the range from ≧15 nm to ≦20 nm.

18. The material according to claim 7, wherein at least one semi-Heusler alloy is formed based on a composition from the group consisting of Ti0.64Zr0.18Hf0.14Nb0.04NiSn, Ti0.39Zr0.28Hf0.29Nb0.04NiSn and Ti0.17Zr0.4Hf0.39Nb0.04NiSn.

19. The material according to claim 9, wherein at least one semi-Heusler alloy is formed based on a composition from the group consisting of Ti0.68Zr0.18Hf0.14NiSn0.998Sb0.002, Ti0.43Zr0.28Hf0.29NiSn0.998Sb0.002 and Ti0.21Zr0.4Hf0.39NiSn0.998Sb0.002.