KR101452795B1 - Methods for high figure-of-merit in nanostructured thermoelectric materials - Google Patents

Abstract

A thermoelectric material having a high performance index ZT is described. In many cases, this thermoelectric material is believed to help increase the ZT value of the thermoelectric material (e.g., by increasing phonon scattering due to interface at grain boundaries or at grain / inclusions boundaries) ) Nano-sized domains (e. G., Nanocrystals). The ZT value of such a thermoelectric material may be greater than about 1, greater than about 1.2, greater than about 1.4, greater than about 1.5, greater than about 1.8, greater than about 2, and even greater. Such a thermoelectric material can be prepared from a thermoelectric starting material by producing nanoparticles from a thermoelectric starting material, or can be prepared from mechanically alloyed nanoparticles from elements, which nanoparticles are subsequently added to a new bulk material (E. G., Through a direct current hot press). Non-limiting examples of starting materials include bismuth, lead and / or silicon-based materials, which may be alloyed, elemental and / or doped. Various compositions and methods relating to thermoelectric aspects of nanostructures (e. G., Modulation doping) are also disclosed.

 Thermoelectric material, performance index, nanostructure, ZT value,

Description

[0001] The present invention relates to nanostructured thermoelectric materials,

Cross-reference to related application

This application is a continuation-in-part of U.S. Patent Application No. 10 / 977,363, filed October 29, 2004, entitled " Nanocomposites with High Thermoelectric Figures of Merit & &Quot; Methods for High Performance Indices in Thermoelectric Materials of Nanostructures ", entitled " Methods for High-Performance-In-Merit in Nanostructured Thermoelectric Materials, " filed on December 1, 2006 and U.S. Provisional Application No. 60 / 872,242 Claim the benefit of the call. The contents of both of these applications are incorporated herein by reference.

TECHNICAL FIELD This application relates generally to thermoelectric materials and methods for their manufacture, and more particularly to thermoelectric materials exhibiting improved thermoelectric properties.

The thermoelectric properties of a material are given by the following formula: Z = S ² σ / k, where S is the Seebeck coefficient, σ is the electrical conductivity and k is the total thermal conductivity Quot; index ZT "). It is desirable to produce materials with high ZT values (with low thermal conductivity k and / or high power factor S ² sigma). By way of example, such materials may be used to fabricate potentially high quality power generation and cooling systems.

Gist of invention

In one aspect, the present invention is directed to a method of forming nanoparticles from a starting material, such as a thermoelectric bulk material, and consolidating the nanoparticles at elevated temperature under reduced pressure, By forming a thermoelectric material exhibiting a ZT value higher than that of the thermoelectric starting material at a temperature of less than about 600 캜, less than about 200 캜, or less than about 20 캜. In some cases, the peak ZT value of the formed material may be about 25 to about 1000% greater than the peak ZT value of the starting material. In other cases, the peak ZT of the formed material may be substantially greater than 1000% of the peak ZT of the starting material.

The term "nanoparticle" is generally known in the art and is used herein to refer to a nanoparticle having a size (e. G., Average or maximum size) in the range of less than about 1 micron, such as from about 1 to about 1000 nm It is used to mean material particles. Preferably, the size may be in the range of less than about 500 nm, preferably in the range of about 1 to about 200 nm, more preferably in the range of about 1 to about 100 nm. Nanoparticles can be produced, for example, by grinding the starting material into nanoscale pieces (e.g., grinding using dry milling, wet milling or other suitable techniques). As an example, ball milling can be used to achieve the desired nanoparticles. Optionally, in order to reduce the particle size again, cooling may also be used (e.g., cooling the starting material while grinding it), while producing nanoparticles. Other methods of producing nanoparticles can include condensation from a gas phase, wet chemical methods, and other methods of forming nanoparticles. In some cases, nanoparticles of different elemental materials (e. G., Bismuth or tellurium) can be separately generated and then consolidated with the resulting thermoelectric material, as discussed below.

The nanoparticles may be consolidated at a predetermined temperature and pressure to induce electrical coupling between the nanoparticles sufficient to form the resulting thermoelectric material. (Also known as plasma pressure compression, "plasma pressure compaction" (P ² C), or spark plasma sintering, spark plasma sintering (SPS)), including current induced hot presses, Press and isobaric hot press processes can be used to achieve consolidation of the nanoparticles. The pressure selected may be in the range of, for example, from about 10 to about 900 MPa, or in the range of from about 40 to about 300 MPa, preferably from about 60 to about 200 MPa. The selected temperature can range, for example, from about 200 to about the melting point of a thermoelectric material (e.g., from 200 to about 2000 degrees Celsius) or from about 400 to about 1200 degrees Celsius for a Bi ₂ Te ₃ based material, To about 600 < 0 > C, or from about 400 to about 550 < 0 > C.

In a related aspect, in the method, the consolidation of the nanoparticles compresses the nanoparticles to provide a material exhibiting a density in the range of about 90 to about 100% of the theoretical density (e.g., a porosity of about 10 % Or less than about 1%).

In a related aspect, the thermoelectric material produced by the method of the present invention, as discussed above, has a ZT value (e.g., a peak ZT value) of at least about 1, at least about 1.2, at least about 1.4, About 1.5 or more, and most preferably about 2 or more. Furthermore, in many embodiments, the thermoelectric material is, for example, in certain operating temperature, which may be dependent on the melting point of the material, for example, Bi ₂ Te ₃ system at about 300 ℃ less than the temperature high in the case of a material ZT value. The increased ZT value may also depend on the doping level and / or the microstructure of the material.

In many cases, the starting material of the thermoelectric material (e.g., the starting bulk material or the fluid phase material for particle synthesis) exhibits a ZT value of less than about 1 and optionally greater than or equal to about 0.1 and produces nanoparticles from the starting material, The final thermoelectric material obtained by consolidating these nanoparticles (e.g., grinding the starting material by grinding or other suitable technique) has a thermal conductivity of at least about 1, at least about 1.1, at least about 1.2, at least about 1.3, at least about 1.4, A ZT value of about 1.5 or more, or about 2 or more.

A variety of thermoelectric materials can be used as starting materials in the practice of the present invention. The starting material of the thermoelectric material can be p-doped or n-doped. Examples of starting materials for thermoelectric materials include, but are not limited to, bismuth, lead or silicon based materials. For example, the starting materials for thermoelectric materials include bismuth-antimony-tellurium alloys, bismuth-selenium-tellurium alloys, lead-tellurium alloys, lead-selenium alloys or silicon-germanium (e.g. SiGe) can do. For example, in some embodiments, the starting material of the thermoelectric material can be a Bi ₂ Te _3-x Se _x alloy, where x ranges from about 0 to about 0.8. Alternatively, in some other embodiments, the starting material of the thermoelectric material can be a Bi _x Sb _2-x Te ₃ alloy, wherein x ranges from about 0 to about 0.8. In some embodiments, a starting material of a thermoelectric material having a polycrystalline structure may be used, which may optionally include an average grain size (e.g., greater than about 1 micron).

In another aspect, the nanoparticles have a size (e.g., average or maximum size) of less than about 1000 nm, or less than about 500 nm, or less than about 200 nm, preferably less than about 100 nm, For example from about 1 to about 200 nm, or from about 1 to about 100 nm, preferably from about 1 to about 50 nm. This particle size can be generated by any of the techniques discussed herein, such as grinding the starting material by ball milling or other suitable techniques.

In a related aspect, in the method, the nanoparticles are maintained at elevated temperature under pressure for a time in the range of, for example, from about 1 second to about 10 hours, to produce thermoelectric materials that are the result of the improved thermoelectric properties. . In another aspect, the nanoparticles are applied at a predetermined temperature while maintaining them at low or ambient pressure for a time sufficient to allow the resulting thermoelectric material to be produced. In another aspect, the nanoparticles can be consolidated at room temperature under high pressure to form a sample having a high theoretical density (e.g., about 100%), and then annealed at a high temperature to form a final thermoelectric material.

Another aspect relates to a method of forming a thermoelectric material comprising generating a plurality of nanoparticles. As an example, I can also generate particles by grinding one or more bulk element materials. For example, nanoparticles can be produced by grinding at least two different bulk element materials, such as bismuth and tellurium, bismuth and selenium, antimony and tellurium, antimony and selenium, and silicon and germanium at a processable rate . In this case, more than two types of nanoparticles can be formed. If different types of particles are generated separately, these particles may be mixed and re-ground (e.g., by a ball mill) to form mechanically alloyed particles. Alternatively, all of the various bulk materials may be simultaneously grinded to form mechanically alloyed particles. A mixture of nanoparticles formed using mechanically alloyed nanoparticles or separately produced nanoparticles from an element, compound or alloy can be compressed at elevated temperature under pressure to produce a thermoelectric material that is the result of a ZT value of greater than or equal to about 1 have. The dopant may optionally be added to the mixture of nanoparticles. In other embodiments, the nanoparticles may be coated with particles from raw materials having good ZT values (e.g., greater than about 0.5) and / or micron sized particles (e.g., having an average size of from about 1 to about 10, 50 , 100 or 500 microns). ≪ / RTI >

In another aspect, there is provided a thermoelectric material comprising a material structure comprising a plurality of inclusions having an average size ranging from about 1 to about 500 nm, wherein the structure has a ZT value of at least about 1 , Peak ZT value), preferably about 1.2 or more, or about 1.5 or more, and even about 2 or more.

In a related aspect, the thermoelectric material may exhibit the ZT value at a temperature less than about 2000 占 폚, or less than about 1000 占 폚, or less than about 600 占 폚, or less than about 200 占 폚, or less than about 20 占 폚. In addition, the average grain size may range from about 1 to about 500 nm. The structure may be substantially free of grains of greater than about 500 nm (e.g., substantially no grains having an average and / or maximum dimension greater than about 500 nm), larger grain sizes (e.g., about 1 mu m or more).

In another aspect, at least one of the grains comprises at least one precipitation zone or other inclusions in the grains, wherein the precipitation zone or other inclusions are in the range of, for example, from about 1 to about 50 nm, 20 nm. ≪ / RTI > The precipitation zone may feature different phases for the same composition with different composition and / or crystal orientation, and / or for the remainder of the grain.

In another aspect, the thermoelectric material may have a density ranging from about 90% to about 100% of the respective density theoretical value. By way of example, thermoelectric materials can exhibit a porosity of less than about 10%, preferably less than about 1%.

In a related aspect, the thermoelectric materials are selected from small crystalline grains oriented randomly with respect to each other (e.g., having an average size in the range of less than about 500 nm, or less than about 200 nm, preferably in the range of about 1 to about 100 nm ). ≪ / RTI >

One aspect of the present invention relates to a thermoelectric material that may comprise a material structure having a plurality of grains. The grains may have an average size in the range of about 1 to about 10 microns, or in the range of about 1 to about 5 microns, or in the range of about 1 to about 2 microns. At least some of the grains may comprise one or more precipitation areas or other types of inclusions. Such regions may have an average size of from about 1 to about 100 nm, or from about 1 to about 50 nm. The thermoelectric material may have a ZT value of about 1 or more, about 1.2 or more, about 1.5 or more, or about 2 or more. For example, the ZT value may also range from about 1 to about 5. The thermoelectric material may exhibit the ZT value at an operating temperature of less than about 2000 占 폚, or less than about 1000 占 폚, or less than about 600 占 폚, or less than about 200 占 폚, or less than about 20 占 폚. The grains may be formed from a variety of materials, such as any combination of a bismuth-based alloy, a lead-based alloy, and a silicon-based alloy.

Another aspect of the invention is to derive a thermoelectric material comprising a host material having a plurality of inclusions or particles dispersed throughout the host. The particles or inclusions may have a size below the threshold, for example, less than about 20 microns. The host material may comprise one or more grains, wherein at least a portion of the grains have a size of greater than about 1 micron or less than about 1 micron (e.g., a maximum size and / or average size at a particular dimension). In some embodiments, the host material is not unduly doped, as in conventional thermoelectric materials, and is not excessively doped, which is responsible for a large portion of the charge carrier, such as at least 50%, at least 80%, at least 90% 99% or more is due to the presence of these inclusions. In some embodiments, the particles can be doped more than the host material. The thermoelectric material may exhibit a greater carrier concentration and / or charge carrier mobility than the respective carrier concentration and / or charge carrier mobility of the host material in the absence of the particles or inclusions, resulting in a higher power factor S ² σ). In addition, or alternatively, the thermoelectric material can have an energy band (e.g., a conduction band or a valence band) for a charge carrier type that has a higher energy than the associated energy band of the host material for the corresponding charge carrier type And can be characterized by inclusions having. The thermoelectric material may optionally include a number of properties discussed herein for thermoelectric materials. For example, the thermoelectric material may exhibit a ZT value of about 1 or more, about 1.1 or more, about 1.2 or more, about 1.3 or more, about 1.4 or more, about 1.5 or more, or about 2 or more.

Certain aspects of the invention may be better understood with reference to the following drawings, which are not necessarily drawn to scale.

1A is a schematic of a plurality of grains in a thermoelectric material, wherein some of the grains include at least one settling region, consistent with some aspects of the present invention;

Figure IB is a schematic diagram of a host material having an embedded inclusion therein, consistent with certain aspects of the present invention;

FIG. 1C is a diagram of the conduction energy diagram for the material shown in FIG. 1B; FIG.

Figure 2 is an XRD pattern of p-type BiSbTe nanoparticles produced by ball milling, consistent with some embodiments;

FIG. 3A is a SEM image of the p-type BiSbTe nanoparticles of FIG. 2; FIG.

Figure 3B is a lower resolution TEM micrograph of BiSbTe nanoparticles of Figure 2;

Figure 3C is a higher resolution TEM micrograph of the BiSbTe nanoparticles shown in Figure 3B;

4 is a diagram and photograph of a DC hot press (plasma pressure or spark plasma sintering) apparatus that may be used with some aspects of the present invention;

5 is a graph showing the temperature dependence of the electrical conductivity for the bulk material of the p-type BiSbTe alloy in the state of the art and the thermoelectric material made from the particles of Fig. 2, consistent with some embodiments;

Figure 6 is a graph showing the temperature dependence of the heat transfer material made from the particles of Figure 2 and the bulk material of the p-type BiSbTe alloy in the state of the art, the temperature dependence of the Seebeck coefficient, consistent with some embodiments;

Figure 7 is a graph showing the temperature dependence of the power factor for bulk materials of thermoelectric materials made from the particles of Figure 2 and p-type BiSbTe alloys in the state of the art, consistent with some embodiments;

Figure 8 is a graph showing the temperature dependence of the thermal conductivity of a thermoelectric material made from the particles of Figure 2 and a bulk material of a p-type BiSbTe alloy in the state of the art, consistent with some embodiments;

9 is a graph showing the temperature dependence of the ZT, the performance index for the bulk material of the p-type BiSbTe alloy in the state of the art and the thermoelectric material made from the particles of FIG. 2, consistent with some embodiments;

10 is a TEM micrograph of thermoelectric material made from the particles of FIG. 2;

FIG. 11 is an enlarged TEM micrograph of the thermoelectric material made from the particles of FIG. 2, showing the nanoscale of tightly packed nanograms;

FIG. 12 is a TEM micrograph of a thermoelectric material made from the particles of FIG. 2, showing the presence of grain larger than the nanograms shown in FIG. 11;

FIG. 13 is a TEM micrograph of the thermoelectric material made from the particles of FIG. 2, showing the presence of a nanodot;

Figure 14 is a TEM micrograph of a thermoelectric material made from the particles of Figure 2 showing the presence of nanodots with small angle boundaries;

FIG. 15 is a TEM micrograph of a thermoelectric material produced from the particles of FIG. 2, showing an illustration of a diagram showing electron diffraction patterns of Te precipitates and Te precipitates; FIG.

16 is a graph showing the temperature dependence of the electrical conductivity for a thermoelectric material made from a p-type SiGe bulk starting material, consistent with some embodiments;

17 is a graph depicting the temperature dependence of the Seebeck coefficient on thermoelectric materials made from p-type SiGe bulk starting material, consistent with some embodiments;

18 is a graph showing the temperature dependence of thermal conductivity for a thermoelectric material made from a p-type SiGe bulk starting material, consistent with some embodiments;

19 is a graph showing the temperature dependence of the figure of merit, ZT, on thermoelectric materials made from p-type SiGe bulk starting material, consistent with some embodiments;

Figure 20 is a graph showing the temperature dependence of the electrical conductivity for a thermoelectric material made from an n-type SiGe bulk starting material, consistent with some embodiments;

21 is a graph depicting the temperature dependence of the Seebeck coefficient for a thermoelectric material made from an n-type SiGe bulk starting material, consistent with some embodiments;

22 is a graph showing the temperature dependence of thermal conductivity for a thermoelectric material made from an n-type SiGe bulk starting material, consistent with some embodiments;

23 is a graph depicting the temperature dependence of the figure of merit, ZT, on thermoelectric materials made from an n-type SiGe bulk starting material, consistent with some embodiments;

Figure 24 is a TEM micrograph of a ball milled sample of a SiGe bulk starting material consistent with some aspects of the present invention;

Figure 25 is a TEM micrograph of the particle of Figure 24 after high temperature compression, the illustration showing the corresponding electron diffraction pattern for the sample;

Figure 26 is a high-resolution TEM micrograph of the hot compression sample shown in Figure 25;

Figure 27 is a graph showing the temperature dependence of the electrical conductivity for a thermoelectric material made from p-type Bi _0.3 Sb _1.7 Te ₃ bulk starting material, consistent with some embodiments;

28 is a graph depicting the temperature dependence of the Seebeck coefficient on thermoelectric materials made from p-type Bi _0.3 Sb _1.7 Te ₃ bulk starting material, consistent with some embodiments;

29 is a graph showing the temperature dependence of thermal conductivity for a thermoelectric material made from a p-type Bi _0.3 Sb _1.7 Te ₃ bulk starting material, consistent with some embodiments;

Figure 30 is a graph showing the temperature dependence of the figure of merit, ZT, on thermoelectric materials made from p-type Bi _0.3 Sb _1.7 Te ₃ bulk starting material, consistent with some embodiments;

Figure 31 is a graph showing the temperature dependence of the electrical conductivity for a thermoelectric material made from a p-type Bi _0.5 Sb _1.5 Te ₃ bulk starting material, consistent with some embodiments;

32 is a graph showing the temperature dependence of the Seebeck coefficient on thermoelectric materials made from p-type Bi _0.5 Sb _1.5 Te ₃ bulk starting material, consistent with some embodiments;

33 is a graph showing the temperature dependence of thermal conductivity for a thermoelectric material made from a p-type Bi _0.5 Sb _1.5 Te ₃ bulk starting material, consistent with some embodiments;

34 is a graph showing the temperature dependence of the figure of merit, ZT, on thermoelectric materials made from p-type Bi _0.5 Sb _1.5 Te ₃ bulk starting material, consistent with some embodiments.

In one aspect, the invention relates to thermoelectric materials having high ZT values and methods of making such materials. Generally, such thermoelectric materials typically comprise a plurality of grains. Such grains may be present in the form of nano-sized grains, which may be obtained, for example, from bulk materials such as starting materials for thermoelectric materials. Generally, thermoelectric materials consistent with aspects of the present invention may include grains of various sizes. For example, a thermoelectric material may have some grains larger than 1 탆 and a grain smaller than 1 탆. In some embodiments, the thermoelectric material is substantially free of grains that can adversely affect the ZT value of the thermoelectric material (e.g., the ZT value of the total material is greater than about 5%, greater than about 10%, greater than about 15 , Not more than about 20%, not more than about 25%, not more than about 30%, not more than about 40%, or not more than about 50%). Some embodiments relate to thermoelectric materials having a plurality of grains having an average grain size of about microns (e.g., greater than about 1 micron). In some cases, the thermoelectric material can not substantially contain large grains. Non-limiting examples include grains larger than about 5000 nm, grains larger than 1000 nm, grains larger than 300 nm, grains larger than 100 nm, grains larger than 50 nm, grains larger than 20 nm, or grains larger than 10 nm, Includes what does not exist. In many cases, such grains may optionally include one or more precipitation regions or other types of inclusions having an average size in the range of, for example, about 50 nm. In some preferred embodiments, at least some, and preferably substantially all, of the grains comprise a precipitation zone, nanoparticles and / or other types of inclusions; These various inclusions can be formed by chemical reactions in the in situ reaction system and / or by the insertion of such inclusions. A further aspect relates to a material having a plurality of grain sizes (e.g., at least some of which are nano-sized grains and some of which are greater than 1 micron), wherein some of the grains optionally include a precipitate region or other form of inclusions can do. In other words, the thermoelectric materials of the present invention can be used in a variety of applications including, but not limited to, sub-micron sized grains in the presence or absence of a precipitation zone, grains greater than 1 micron in the presence or absence of a precipitation zone (e.g., using modulation doping) Or any combination of sub-micron grains in the presence or absence of precipitation regions and grains of greater than 1 micron. Some of these grains can be formed by a number of mechanisms including, but not limited to, formation of a precipitation zone during material compression, particle insertion into the host matrix, and / or formation by solid state chemical reaction.

The ZT value of the thermoelectric material of the present invention may have various values. For example, the peak ZT value or the average ZT value of a thermoelectric material can be calculated from the peak ZT value of the corresponding starting material forming the thermoelectric material by compressing the nanoparticles at elevated temperature under pressure after converting the starting material into nanoparticles or May be greater than the average ZT value. For example, the ZT value of the thermoelectric material can be about 25 to about 1000% greater than the ZT value of the starting material. As another example, the ZT value of the thermoelectric material may be substantially greater than 1000% of the ZT value of the starting material. The starting material may have a range of ZT values. In some embodiments, the thermoelectric material formed has a ZT value of at least about 0.8, at least about 0.9, at least about 1, at least about 1.1, at least about 1.2, at least about 1.3. About 1.4 or more, about 1.5 or more, about 1.6 or more, about 1.7 or more, about 1.8 or more, about 1.9 or more, or about 2 or more. In some embodiments, the thermoelectric material may exhibit a ZT value ranging from a lower limit to one of the ZT values and an upper limit to a value of about 4, about 5, or about 6.

While these enhanced ZT values can be ascertained without being limited to temperature, in some embodiments the thermoelectric material can exhibit the improved ZT value at a particular temperature or within a certain temperature range. For example, thermoelectric materials can exhibit improved ZT values at temperatures below about 2000 ° C, below about 1000 ° C, below about 800 ° C, below about 600 ° C, or below about 400 ° C. As another example, the thermoelectric material may be heated to a temperature within the range of room temperature or including room temperature (e.g., less than about 200 占 폚, less than about 150 占 폚, less than about 100 占 폚, less than about 60 占 폚, Or less than about < RTI ID = 0.0 > 20 C). ≪ / RTI > As another example, thermoelectric materials can exhibit improved ZT values in cryogenic temperatures or in temperature ranges that include cryogenic temperatures (e.g., less than about 0 ° C, less than about -50 ° C, or less than about -100 ° C) , And such materials may be useful for special cooling applications such as air conditioners, refrigerators or superconductors. In some embodiments, the temperature range exhibiting an improved ZT value may depend on the composition of the thermoelectric material. In some non-limiting examples, the boron carbide-based composition may, in some embodiments, exhibit an improved ZT value below about 2000 ° C and the SiGe-based composition may, in some embodiments, exhibit an improved ZT value below about 1000 ° C , The PbTe-based composition may, in some embodiments, exhibit an improved ZT value below about 600 ° C and / or the Bi ₂ Te ₃ -based composition may, in some embodiments, exhibit an improved ZT value below about 200 ° C. In another non-limiting example, the thermoelectric material comprises Bi _x Sb _1-x and exhibits improved ZT at room temperature (e.g., less than about 20 캜).

The high ZT value of such a thermoelectric material, although not necessarily associated with a particular theory, may be the result of a change in any combination of thermal conductivity, Seebeck coefficient, and electrical conductivity. Thermal conductivity has two contributing factors: the lattice contributing factor and the electron contributing factor. In single crystal or polycrystalline samples with large grains, the lattice thermal conductivity is fixed for a particular material. However, if the bulk material consists of nano-sized grains and / or nanoparticles encapsulated in a grain larger than the nanoparticles, then three effects produced from the nanoparticles and / or encapsulated nanoparticles can be considered. First, the lattice part of the thermal conductivity degrades due to phonon interference scattering. Second, the Seebeck coefficient can be increased due to the carrier filtering effect (typically the low energy electron / hole scattering increases the Seebeck coefficient), and third, the electrical conductivity can be increased due to the modulation doping effect Particles act as carrier (electron and hole) contributing factors, thereby reducing impurity scattering compared to homogeneously doped conventional materials. The contribution of electrons to thermal conductivity can be potentially reduced by the interfacial barrier scattering of electrons, in particular by the dipole contribution to thermal conductivity, which causes the barrier to preferentially scatter one type of charge (electrons or holes) And may not substantially affect other types of carriers. In addition, the quantum size effect may further affect the Seebeck coefficient and electrical conductivity such that S ² ? Is increased. Accordingly, some aspects of the present invention provide a method of making a dense sample (e.g., from about 90% to about 100% of the theoretical density value) by a hot press, a unidirectional hot press, an isostatic hot press comprising P ² C For the preparation, nanoparticles prepared by ball milling the starting material can be used. These hot compressed samples typically exhibit lower thermal conductivity than bulk counterparts, thereby improving the ZT value; If the ZT obtained due to the deterioration of the thermal conductivity is sufficient, the power factor may be lowered, but the power factor is usually maintained or improved.

In some embodiments, the thermoelectric material may comprise a grain produced from a bulk starting material (e.g., a bulk thermoelectric material). Examples include bulk starting materials with high power factor and / or starting materials with good ZT values (e.g., ZT values greater than about 0.1). For example, the ZT of the starting material may be greater than about 0.05, greater than about 0.1, greater than about 0.2, greater than about 0.3, greater than about 0.4, greater than about 0.5, or even higher have. In some non-limiting cases, the starting material has a ZT value of less than about 0.8, less than about 0.9, less than about 1, less than about 1.1, less than about 1.2, less than about 1.3, 1.4, lower than about 1.5, or lower than about 2. As another example, the starting material of the thermoelectric material may have a high thermal conductivity (for example, S ² ?) Of not less than 20? / Cm ² , preferably not more than 40? / Cm ² K ² , Greater than 2 W / mK). Such bulk thermoelectric materials may be specially prepared or commercially available materials may be used. Although many bulk starting materials are solids that can be ground to produce grains, bulk starting materials may be produced from other thermodynamic conditions, such as gases, when producing grains from gaseous condensation, and produce grains by chemical methods A bulk starting material can be produced from other thermodynamic conditions such as liquids. It is also understood that the grain can be produced from one or more forms of bulk starting materials or a mixture of materials having different thermodynamic phases (e.g., a mixture of liquid and gas).

Although many starting materials may be used, in some embodiments, the bulk starting materials may be selected from any combination of bismuth-based materials, lead-based materials, and / or silicon-based materials. In some embodiments, the bulk starting material may be selected from a variety of alloys, such as a bismuth-antimony-tellurium alloy, a bismuth-selenium-tellurium alloy, a bismuth-antimony- tellurium-selenium alloy, Selenium alloy, a silicon-germanium alloy, or any combination thereof. Particular embodiments can be derived using bulk starting materials that are p-type or n-type materials. For example, such a starting material may be a parent composition, for example a formally modified form of Bi ₂ Te ₃ . By way of example, the n-type material may be formed by substituting tellurium in Bi ₂ Te ₃ with selenium so that the stoichiometry of the bulk material has the formula Bi ₂ Te _3-x Se _x where x ranges from about 0 to about 0.8. . In the case of p-type materials, for example, antimony may be used to replace bismuth with the stoichiometry of the bulk material having the formula Bi _x Sb _2-x Te ₃ (where x ranges from about 0 to about 0.8) . In a particular embodiment, the bulk starting material used is Bi _0.5 Sb _1.5 Te ₃ . Generally, the bulk starting material may be a crystalline material or a polycrystalline material (e.g., a polycrystalline with an average crystal grain size greater than about 1 micron). Other examples of starting materials include MgSi ₂ , InSb, GaAs, CoSb ₃ , Zn ₄ Sb ₃ , and the like. In some cases, the bulk starting material may be a material whose critical power factor value S ² σ is, for example, at least about 20 ㎶ / cm K ² . In this case, the bulk starting material may have a suitable ZT value (e.g., greater than about 0.1) due to the low thermal conductivity of the bulk starting material, or the power factor may be a threshold or higher, Can be low due to the relatively high thermal conductivity of the starting material.

In some embodiments, particles of thermoelectric material (e.g., nanoparticles) can be produced from bulk starting materials or elemental materials by methods other than grinding / milling one or more starting materials. The particles may be prepared by a number of methods including those known to those skilled in the art. Non-limiting examples include gas phase condensation, laser ablation, chemical synthesis (e.g., wet or dry processes), rapid atomization cooling, and the like. Thus, the scope of the present application is not limited to the specific method of making the particles discussed herein. It is understood that particle generation techniques may be combined in a manner to produce materials for consolidation. For example, some particles may be generated by ball milling (e.g., to create a host material), while other particles may be created by one or more other techniques (e.g., gas phase condensation, laser ablation, etc.) . ≪ / RTI >

The grain forming the thermoelectric material may have various properties. In some embodiments, each grain has a crystal structure. In this case, the thermoelectric material may comprise a polycrystalline-like structure that is generally lacking (e.g., randomly distributed) in predominantly oriented grains. In some cases, the grains may also exhibit dominant orientation patterns due to grain morphology, wherein the general grain orientation of the grains may be random or somewhat dominant relative to each other. Thus, this aspect is not surprising given the fact that, despite the small defects in the average crystal structure or the heterogeneity of the composition, it is known that the average crystal structure (including, for example, a superlattice structure formed as a stack of semiconductor layers) Which is significantly different from the plurality of thermoelectric materials formed.

The grains constituting the various thermoelectric materials discussed herein may have varying sizes. In some embodiments, the size is generally nanometer-scale and is generally less than 1 micron. For example, the average grain size of the grain may be less than about 500 nm, or less than about 200 nm, or less than about 100 nm, or about 50 nm, or less than about 20 nm. In this aspect, the average grain size may be greater than a somewhat lower threshold (e.g., about 1 nm). In some cases, the average grain size can be measured using a variety of methods, including those understood by those skilled in the art. For example, a transmission electron microscope (referred to herein as a "transmission electron micrograph") can be used to image the grain, and then the size of the grain can be measured and the average value determined. Since the particles are typically present in an irregular form, the size of the measured particles can be determined using a number of techniques, including those known to those skilled in the art. For example, the largest dimension of grain from an image (e.g., SEM and / or TEM image) can be used, or the effective diameter can be calculated based on surface area measurements or the effective cross-sectional area of the grain from the image.

In many aspects of the present invention, the grain of thermoelectric material can be compressed such that the final product exhibits the desired properties (e.g., improved ZT value). In some embodiments, the thermoelectric material comprises grains compacted in a structure that exhibits low porosity (e.g., the actual density of the final product is greater than the actual density of the final product used in the composition, e. G., In some embodiments, Which may be close to the theoretical density of the bulk starting material), which can help to obtain an improved ZT value. Porosity is defined as the difference between the theoretical density and the actual density of the material divided by the theoretical density. In general, the phrase "theoretical density" is known to those skilled in the art. The porosity of the thermoelectric material may be less than about 10%, less than about 5%, or less than about 4%, or less than about 3%, or less than about 2%, or less than about 1% ≪ / RTI > In some embodiments, the thermoelectric material exhibits a density up to 100% of the theoretical density. In some embodiments, the density of the thermoelectric material may be between 100% and 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, or 99.9%, or may be the respective theoretical density. Without wishing to be bound by theory, it is believed that compression can help maintain contact between the grains, which can help maintain the electrical conductivity of the thermoelectric material.

Some embodiments relate to a thermoelectric material formed from a plurality of grains, wherein the one or more grains may comprise one or more precipitation regions. By way of example, FIG. 1 schematically illustrates this thermoelectric material exhibiting a polycrystalline structure comprising a plurality of grains 110. The grain may also include one or more settling regions 120, which can improve the thermoelectric properties of the thermoelectric material. The settling region may be characterized by a heterogeneous composition such as having a composition and / or phase different from the rest of the grain. The precipitation region may also be characterized in that the precipitation region has a crystal structure similar to that of the matrix in which the precipitation region is encapsulated, although it is oriented in different crystal directions. In some embodiments, in spite of a defect due to the presence of the precipitation region, one or more of the precipitation regions may be embodied as discrete particles (e.g., nanoparticles) encapsulated in the grain, . In some embodiments, the thermoelectric material may comprise other grains that do not have a settling region. In an alternative embodiment, substantially all of the grains constituting the thermoelectric material comprise a settling region. The precipitation region typically has a size (e.g., a maximum average size) of about 10 nm or less or about 50 nm or less (e.g., in a range of about 1 to about 50 nm). The formation of the precipitation zone is described in U.S. Pat. No. 5,502,505, filed October 29, 2004, entitled " Nanocomposites with High Thermoelectric Figures of Merit ", the title of which is incorporated herein by reference in its entirety. May be accomplished in a variety of ways, including those discussed in U. S. Patent Application Publication No. US 2006/0102224 to < RTI ID = 0.0 > 10 / 977,363.

In some cases, the precipitation region is spontaneously produced, for example, through the formation of a thermoelectric material by the methods discussed herein. In other cases, the precipitation zone is created by mixing two types of nanoparticles having different melting points. For example, one type may have a lower melting point than another type. By mixing the nanoparticles and heating / consolidating them (for example, at a temperature close to the melting point of one type of nanoparticles, but below the melting point of the other type), the nanoparticles with the lower melting point are mixed with another type of nano Grain can be formed around the particles. In other words, grains formed from one type of nanoparticle can encapsulate other types of nanoparticles. Examples of ensemble materials that can be used to form such encapsulated nanoparticles include bismuth-telluride material systems, lead-telluride material systems, silicon-germanium material systems, and the like.

It should be understood that while the above discussion is directed to forming precipitates in thermoelectric materials, other materials are formed by using different types of inclusions in the matrix (e.g., using nanoparticles in a host). For example, two or more types of nanoparticles that can be mixed together to form a thermoelectric material can not contain a precipitate, but may still have useful properties (e.g., using modulation doping). Thus, disclosure of the disclosure herein for precipitation may be used for other types of inclusions, as the case may be. For example, the precipitation region or inclusion region can be formed through a solid phase chemical reaction of the particles with the hose (for example, Si and Mo in Fe, Mn, Mg, Ag, Cr, W, Ta, Ti , Cu, Ni or V metal particles react to form particles such as MoSi ₂ , FeSi ₂ , and MgSi ₂ ).

Without wishing to be bound by any particular theory, it is believed that the precipitate region or other types of inclusions can improve phonon scattering in the thermoelectric material, which can lead to a reduction in the thermal conductivity of the thermoelectric material. In addition, the n-doped or p-doped regions can enhance the electrical conductivity of the thermoelectric material, for example, through a modulation doping mechanism. In this case, some or all of the charge carriers (electrons and holes) may be donated by the deposition region or other inclusions enclosed in a larger grain. Since the distance between the inclusion regions can be greater than the distance between atomic dopants in the homogeneously doped material, the impurity scattering of the charge carriers is reduced relative to the impurity scattering of the homogeneously doped material. Such a modulation doping-like mechanism can increase the electrical conductivity through the improvement of the carrier mobility. In some cases, these precipitation areas or other inclusions may improve the Seebeck coefficient by further scattering lower energy carriers than higher energy carriers. Thus, the precipitation zone or other inclusions can improve the ZT of the thermoelectric material.

In other embodiments, the precipitate region, or grain region, or other inclusions may be preferentially doped. In this case, the carriers in these regions can fall into the surrounding host medium when they are present at higher potential energy. For example, in the case of modulation doping, the doping in the host material is correspondingly reduced or completely eliminated, thereby reducing the scattering of the ionized impurities and improving the electron mobility in the host.

Embodiments that include a precipitation zone or other inclusions in the grain may exhibit multiple particle sizes. In some embodiments, the grain size is generally consistent with the size described herein for grains smaller than microns. For example, the average grain size may be less than about 500 nm, less than about 200 nm, less than about 100 nm, less than about 50 nm, or less than about 20 nm. Alternatively or additionally, the average grain size may be greater than about 1 nm. In other embodiments having one or more inclusions, the grain size may be greater than microns. For example, the plurality of grains can have an average size of about 2 microns, about 5 microns, or about 10 microns. In a particular embodiment, the plurality of grains have an average size in the range of about 1 to about 10 microns, in the range of about 1 to about 5 microns, or in the range of about 1 to about 2 microns.

The size of the deposit area or inclusions may vary. For example, the size of the settling region can be limited by the size of the grain in which the settled region is enclosed. In many embodiments, the precipitate region or inclusions may have an average size ranging preferably from about 1 to about 50 nm, or from about 1 to about 20 nm. In another example, for example, where a modulation doping mechanism is used to increase electronic performance, the deposition region or inclusions may have a larger size, for example, a size of 1 nm to 10 microns, Reduction of the phonon thermal conductivity of the region is achieved by alloying or nanogramming.

Some aspects relate to thermoelectric materials that are fabricated to exhibit modulated doping to achieve an improved figure of merit. In some embodiments, the thermoelectric material can comprise particles (e.g., nanoparticles) or other inclusions that are encapsulated in a host material, wherein the inclusions are formed by donating a charge carrier (e. G., Electrons or holes) , And increases the carrier mobility at the host. This can advantageously improve the electrical conductivity of the entire thermoelectric material, and thus improve the thermoelectric performance, which is characterized by the thermoelectric performance of the thermoelectric material, for example, the ZT value of the thermoelectric material. In many such cases, the host is selected to have an n-type or p-type doping level (typically, a spatially substantially uniform doping level) that is not initially doped or is less than the typical doping value for thermoelectric materials. For example, the initial doping level of a host can be 1.5 times, 2 times, 5 times, 10 times, 100 times, or 1000 times less than a conventional thermoelectric material. Also, the enclosed inclusions (e.g., the settling region or individual particles) can be formed of a doped or undoped material.

By way of example, FIG. 1B schematically illustrates a thermoelectric material comprising a host 130 in which a plurality of particles 140 are encapsulated, wherein the particles act as inclusions. In this case, the host comprises a plurality of grains 135, e.g., a plurality of crystalline grains, which in some cases are less than about 1 micron, for example, in the range of about 500 nm to less than about 1 micron Size (e. G., The maximum particle size at all dimensions). In other cases, the grain size may be larger, for example, in the range of about 1 to about 20 microns. Further, in some cases, the particles may have a size in the range of less than about 1 micron, for example, in the range of about 1 to about 200 nm, or in the range of about 2 to about 100 nm (e.g., the maximum size in all directions) While in other cases the particle size can be in the range of 1 micron or more, for example, from about 1 to about 10 microns. The inclusions 140 can be formed in a variety of ways. For example, they may be formed as precipitation areas using appropriate techniques, including those discussed herein with respect to other aspects. In other cases, these may be formed from a material different from that of the host using, for example, the technique discussed in the above referenced patent application entitled " Nanocomposite with High Thermoelectric Performance Index ". In yet another case, the particles can be formed, for example, through a solid phase chemical reaction during the consolidation step.

Without loss of generality, in this example, the host 130 is assumed to be a SiGe alloy having a plurality of grains 135 of micron and / or nano size, and the particles 140 are MoSi ₂ Molybdenum). This thermoelectric material can be formed, for example, in the following manner: molybdenum is added to SiGe, the material is melted, and the material is cooled (e.g., in the manner discussed above) to produce the ingot , Which can be ground and compressed as needed. In this approach, MoSi ₂ particles is, for example, during the cooling step is formed through the Mo and Si in the solid state chemical reactions. The SiGe host may be doped p-type, for example, although the SiGe host is otherwise unduly doped in this example, for example, the SiGe host may be doped p, Lt; / RTI > In addition, holes can be generated by the presence of MoSi ₂ . This hole donation to the host improves the hole mobility in the thermoelectric material, thus improving the electrical conductivity and consequently improving the thermoelectric performance of the thermoelectric material. In other cases, the particles are obtained by grinding particles of FeSi ₂ , MgSi ₂ or the like by grinding Si element and Ge element or SiGe crystalline alloy with Fe, Mn, Mg, Cr, W, Ta, Ti, Cu, Or by solid state chemical reaction of Si in the host (e.g., SiGe) by grinding together the Si element and the Ge element or the SiGe alloy with each of the silicides. Some of these can be applied to n-type, while others can be applied to p-type materials. Other nanoparticles (e.g., metallic nanoparticles and / or semiconductor nanoparticles) that do not react with Si may be used to produce modulation doping, such as Ag, as inclusions.

For a further illustration of this donation of charge carriers from particle to host, although not to be construed as a particular theory, FIG. 1C illustrates a portion 151, 152, 153, corresponding to the host material, (For example, the SiGe-based material having MoSi ₂ grains sealed therein) showing the portions 161 and 162 of the SiGe-based materials 161 and 162, respectively. The diagram is schematic and is presented for illustrative purposes only. The charge carriers (e.g., electrons or holes) in the energy bands (e.g., the conduction band or the valence band) of the particles 161 and 162 may be either the conduction band or the valence band of the host 151, It can have higher energy than the energy in the energy band. Thus, a number of charge carriers in a particle, which may be due to further doping in the particles or due to a large electron density (such as in a metal or semimetal) inherent to the particles, Can be moved. Such charge carrier transport from particle to host can advantageously increase carrier mobility, for example, by reducing the dopant in the host material and thus reducing the scattering of ionized impurities. In this manner, higher electrical conduction can be achieved. In some cases, even though a higher overall electron mobility is not achieved, grain boundary boundaries still scatter electrons, so this modulation doping is still beneficial by compensating for the reduction in mobility due to electron grain boundary scattering. The particles used in the modulation doping may potentially induce higher Gebeck coefficients, as they can scatter low energy carriers, and may also potentially induce reduced thermal conductivity of both phonons and electrons. In other cases, the particles can donate holes to the host rather than donating electrons to the host. Furthermore, while not wishing to be bound by any particular theory, it is believed that the mechanism for this donation of charge carriers from particle to host is to move from the higher energy level of the valence band of the particle to the lower energy level of the host valence band Some of the holes, or electrons in the host band of the host, can be pulled into the particles to create more holes in the host, or they can be based on some holes in the host, And may be based on generating more holes in the host.

Generally, the type of starting material, the temperature at which the enhanced ZT is measured, the grain components, the forming method, and other characteristics and processes that may be associated with these aspects include all features and methods discussed in this application , Which are consistent with the grain size, precipitation zone and / or other inclusions described. For example, the grain may be formed from a suitable thermoelectric material, as discussed above, and may further comprise an n-type or p-type dopant. In another example, the formed thermoelectric material has a ZT value of at least about 1.0, at least about 1.5, at least about 2, or from about 1 to about 5. As another example, the thermoelectric material formed may have a ZT value (e.g., improved compared to the starting material) at an operating temperature of less than about 2000 占 폚, less than about 1000 占 폚, less than about 600 占 폚, less than about 200 占 폚, . As another example, the grain of the thermoelectric material may comprise at least one of a bismuth-based material (e.g., Bi ₂ Te ₃ and / or an associated alloy thereof), a silicon-based material, and a lead-based material. In connection with the fabrication of such thermoelectric materials, the method of forming nanoparticles from bulk starting materials or elemental materials can be performed by using parameters (e.g., grinding speed, duration, and / or temperature) to obtain the desired nanoparticles for compression Including cryogenic temperatures), but may be applied as discussed above in this disclosure. This adjustment of the nanoparticle size can also be used to obtain the desired grain size (e.g., less than 1 micron, or greater than 1 micron to less than 10 microns) in the final thermoelectric material. The compression method can also be applied as discussed herein and as applied by those skilled in the art.

Another aspect of the present invention relates to a method of manufacturing a thermoelectric material. In this way, a plurality of nanoparticles are generated from the thermoelectric material. The nanoparticles can be consolidated under elevated pressure under pressure to form a thermoelectric material. Types of thermoelectric initiators that can be used to produce nanoparticles include, but are not limited to, any of the bulk materials described herein and others known to those skilled in the art. Accordingly, embodiments may include a thermoelectric material having a ZT value of at least about 1 (e.g., at a temperature of less than about 2000 C). Alternatively or alternatively, the methods can use n-doped or p-doped starting materials (e.g., bulk thermoelectric materials, which are elements and / or alloys).

Various techniques can be used to produce nanoparticles from thermoelectric materials. In some embodiments, nanoparticles are prepared by grinding thermoelectric materials. Grinding can be performed using a mill, for example, a ball mill using a planetary motion, a figure-eight-like motion, or other motion. When producing nanoparticles, some techniques, such as grinding techniques, generate significant heat, which can affect nanoparticle size and properties (e.g., causing particle agglomeration). Thus, in some embodiments, cooling of the thermoelectric material can be performed while grinding the thermoelectric material. This cooling can make the thermoelectric material more susceptible to fracture, and can facilitate the production of nanoparticles. Cooling and particle generation can be accomplished by wet milling and / or cryomilling (e.g., in the presence of dry-ice or liquid nitrogen surrounding the mill). Aspects of the present invention may also employ other methods for forming nanoparticles. Such methods may include gas phase condensation, wet chemical methods, high velocity radiation of molten material and other suitable techniques.

Consolidation of nanoparticles at elevated temperature under pressure can be performed in a variety of ways under a variety of conditions. A process such as a hot press (also referred to herein as "P ² C"), also known as spark plasma sintering, can be used to impose the desired pressure and temperature during consolidation. Apparatus for carrying out the present process and for carrying out the present process is described in US patent application Ser. No. 10 / 977,363, filed October 29, 2004, which is incorporated herein by reference in its entirety, U.S. Patent No. 2006/0102224 ≪ / RTI >

The pressure used is usually the superatmospheric pressure, which allows the use of low temperatures to achieve consolidation of the nanoparticles. Generally, the pressure used may range from about 10 to about 900 MPa. In some embodiments, the pressure ranges from about 40 to about 300 MPa. In other embodiments, the pressure ranges from about 60 to about 200 MPa.

Regarding the above-mentioned temperature rise, a certain range of temperatures can be used. In general, the temperature is typically in the range of about 200 캜 to about the melting point of the thermoelectric material. In some exemplary embodiments, the temperature ranges from about 400 to about 2000 ° C, from about 400 to about 1200 ° C, from about 400 to about 600 ° C, from about 400 to about 550 ° C. In the case of some exemplary n-doped materials, the temperature ranges from about 450 to about 550 DEG C, while for some exemplary p-doped materials, the range is some higher (e.g., about 475 DEG C to about 580 DEG C). Other temperature ranges may also be used in connection with the processing of n-type materials and p-type materials. These particular pressure and temperature ranges can be used for any material, although they can preferably be applied to materials such as BiSbTe alloys and BiSeTe alloys.

The pressure and temperature can be maintained for a time sufficient to allow consolidation of the nanoparticles. In some embodiments, the time ranges from about 1 second to about 10 hours.

Other consolidation conditions may be used to form the thermoelectric materials described herein. For example, nanoparticles can collide with other particles at high speed to achieve low temperature compression. A subsequent heat treatment may optionally be used to form the thermoelectric material. Other consolidation processes can use annealing of particles (e.g., nanoparticles) with little or no pressure to consolidate the particles. In this case, the temperature may be selected to induce annealing of the particles whatever the sample is held at at any pressure during annealing. In other cases, the particles can be consolidated at a relatively low temperature and a high pressure to form a consolidated material, such as a material close to 100% theoretical density. Subsequently, the thermoelectric material can be formed by annealing the consolidated material at an elevated temperature. Thus, the consolidation technique need not be limited to P ² C or hot compression.

In an exemplary embodiment, nanopowders made of various materials from commercially available materials can be produced by high energy ball milling to obtain nanoparticles having a particle size as small as 1 nm. In some cases, dry milling can be combined with wet milling and / or creo-milling to inhibit aggregated particles from agglomerating into larger sized particles due to heat generated during milling. In this way, more dispersed particles can be obtained. These powders can be compressed into solid samples by hot pressing, including P ² C technology. In many embodiments, a theoretical approximately 100% density can be achieved in this manner within a short period of time (typically about 1 to about 10 minutes per sample). The hot compressed sample produced by this method can be reduced to a very small fraction of its original value in both n-type and p-type, while keeping the power factor comparable to bulk counterparts, Significantly improved.

For example, for a p-type commercial material of Bi _x Sb _2-x Te ₃ (where x can range from about 0 to about 0.8), the commercial material has a maximum ZT value of about 1 After ball milling and high temperature compression, the ZT value of the commercially available material may be 1.4 or higher. This improvement is mainly due to the reduced thermal conductivity due to the presence of nanostructures in the sample.

In some embodiments, rather than compressing the thermoelectric starting material into nanoparticles (or using other particle generation techniques) to compress the nanoparticles, the nanoparticles may be separated from two or more elemental materials (e.g., element Bi and element Te) (For example, by grinding). The nanoparticles are then mixed and compressed at elevated temperature (e.g., the pressure and temperature discussed above) under pressure to provide a ZT value of greater than or equal to about 1, preferably greater than or equal to about 1.2, or greater than or equal to about 1.5, (E.g., a thermoelectric material having a polycrystalline structure having a grain size of less than about 500 nm, and preferably in a range of about 1 to about 100 nm).

In an alternative embodiment, two or more bulk materials can be simultaneously grinded to produce various nanoparticles having different compositions. The grinding process can be used to "mechanically alloy" the nanoparticles. Mechanical alloying may also be performed by separately producing two or more different particles, then mixing the particles together, grinding them further to form an alloy, and reducing the size of these particles to form alloyed nanoparticles . These particles may be consolidated to form thermoelectric materials having one or more of the properties discussed herein.

In another embodiment, different types of nanoparticles are separately produced using any of the techniques discussed herein (e.g., grinding of bulk element materials (e.g., bismuth or tellurium)) and then mixed together And the thermoelectric material can be formed by consolidation. Any additional grinding of the mixture may be applied before consolidation. The final consolidated material formed by any of these processes may be any of the compositional characteristics described herein, for example, Bi ₂ Te _3-x Se _x ( Where x ranges from about 0 to about 0.8: Bi ₂ Te _2.8 Se _0.2 ) or Bi _x Sb _2-x Te ₃ where x ranges from about 0 to about 0.8: for example, , Bi _0.5 Sb _1.5 Te ₃ ).

Other aspects of forming a thermoelectric material employ one or more iterations of the steps used to form the thermoelectric material as discussed herein. For example, particles (e. G., Nanoparticles) can be generated and consolidated into a material structure from one or more starting materials (e. G., Starting materials or elemental materials of bulk thermoelectric materials). The resulting structure can then be used to produce new multiple particles (e.g., by grinding the material structure), and these new particles can subsequently be consolidated to form other material structures. This process can be repeated several times to form the final thermoelectric material. Such a process can help in the production of a thoroughly mixed small grain size.

In some embodiments, it may be advantageous to protect the particles resulting from oxidation (e.g. during the ball milling process). A non-limiting example of a protective technique involves exposing generated particles (e.g., the environment in which the grinding of the material occurs) to an oxygen-depleted environment, such as a relatively vacuum, or an environment with low oxygen content relative to atmospheric pressure. The resulting particles may also be exposed to some form of chemical coating to reduce oxygen exposure of the surface; The coating can be optionally removed later in the thermoelectric material manufacturing process. Thus, the protection scheme may include a number of suitable techniques, including those known to those skilled in the art.

The following Examples section provides a further explanation of various aspects of the invention and a description of the feasibility of the inventive method for producing thermoelectric materials exhibiting improved thermoelectric properties. However, the following examples are provided for illustrative purposes only and do not represent the best results achievable by practicing the methods of the present invention.

Example 1: Nanocrystalline bulk p-type Bi _x Sb _2-x Te ₃ material

A commercially available material (p-type BiSbTe alloy ingot) is pulverized and introduced into a zirconia jar inside a glove box under an argon atmosphere to avoid oxidation. A few zirconia balls (5 to 15 mm in size) are also applied and sealed. The seal is placed in a ball mill and milled at a speed of 100 to 2000 rpm for a total time of about 0.5 to 50 hours. The characteristics of the powder are confirmed using a scanning electron microscope (SEM), a transmission electron microscope (TEM) and x-ray diffraction (XRD).

Figure 3 shows the x-ray diffraction (XRD) pattern of the nano powder after ball milling. The XRD pattern demonstrates that the powder is single phase and corresponds well to the pattern of Bi _0.5 Sb _1.5 Te ₃ . The enlarged diffraction peak indicates that the particles are small. This small size is confirmed by a scanning electron microscope (SEM) image of the nano powder shown in FIG. 2A and a low magnification transmission electron microscope (TEM) image of the powder shown in FIG. 2B. The low-resolution TEM image of FIG. 2B clearly shows that the number of nanoparticle sizes is about 50 nm and the average size is about 20 nm. The high resolution TEM image shown in Figure 2C identifies the good crystallinity and clean particle surface of the nanoparticles, which is desirable for good thermoelectric properties. The illustration of Figure 2C also indicates that some nanoparticles are even smaller than 5 nm.

Once a powder is obtained, the powder sample is processed such that the nano powder is introduced into a 1/2 "diameter die and hot pressed to a bulk disk sample having a diameter of 1/2" and a thickness of 2-12 mm. After milling, the powder (stored inside the glove box to prevent oxidation) is introduced into the graphite die and compressed into pellets using a DC hot press technique (see FIG. 4). The parameters for high temperature compression conditions are 40 to 160 MPa and 450 to 600 deg. The density is close to 100% of the theoretical value of all compositions. For measurement of electrical conductivity and thermal conductivity and Seebeck coefficient using both DC and AC methods, a disk with a diameter of 1/2 "and a thickness of 2 mm and a bar of about 2 x 2 x 12" Cut and polish.

Typically, in the production of a hot compressed sample, the powder is exposed to a predetermined pressure and the apparatus is activated at the set heating rate. Upon reaching the predetermined elevated temperature, the sample is heated for a period of about 0 to about 60 minutes, preferably about 0 to about 30 minutes, about 0 to about 10 minutes, or less than about 0 to 5 minutes (e.g., for 2 minutes) Temperature and pressure. Then, cooling is started. However, it should be appreciated that the pressure may be imposed while the sample reaches the elevated temperature or after the elevated temperature has been reached.

Figures 5 to 9 compare the temperature dependence of various properties of a high temperature compacted nanocrystalline material (represented by BP0572 in the figure) and a commercially available material (represented by a commercial ingot in the figure) p-type BiSbTe alloy ingot. In Figures 5 to 9 all properties are measured from the same sample. The discs of cylinder-type thickness are compressed at high temperature, cut along the compression direction and perpendicular to the compression direction, and then measured. To test the temperature stability of the nanocrystalline bulk sample, the same sample is repeatedly measured to 250 캜. No substantial degradation of properties is observed.

Figure 5 compares the temperature dependence of the electrical conductivity of nanocrystalline samples and commercial samples. Electrical conductivity is measured by a 4-point current-switching technique. The electrical conductivity of the nanocrystalline bulk sample is somewhat higher than the electrical conductivity of the commercial ingot.

FIG. 6 compares the temperature dependence of the Jecke coefficient of the nanocrystalline sample and the commercial sample, while FIG. 7 compares the temperature dependence of the power factor (S ² ) of the sample. The Zebeck coefficients were calculated using a commercially available device (ZEM-3, Ulvac, Inc.) for the same bar-shaped sample with a cross-sectional area of 2 x 2 mm 2 and a length of 12 mm cut along the disk surface It is measured by the electrostatic DC method based on the slope. These properties are also measured for the homomade system for the same sample. The two sets of measurements are within 5% of each other. The Seebeck coefficient of the nanocrystalline sample is somewhat lower or higher than that of the temperature-dependent ingot, which allows the power factor of the nanocrystalline sample to be comparable to the power factor of the commercial ingot below 75 ° C, Is higher than the power factor of the commercial ingot at a temperature higher than 75 캜.

8 shows the temperature dependence of the thermal conductivity of the nanocrystalline sample and the commercially available sample. The thermal conductivity is derived from the measurement of the thermal diffusivity and heat capacity of the sample. The thermal diffusivity is measured by a laser-flash method on the disk along the disk axis direction using a commercially available system (Netzsch Instruments, Inc.). After the laser-flash measurement, the bars are cut from the disk and their thermal diffusivity is measured along the bar (disk-face) direction using the Angstrom method in a home-built system. The thermal diffusivity values of the bars and disks are within 5%.

Figure 9 shows the change in the figure of merit ZT as a function of the temperature of the nanocrystalline sample and the commercial sample. This leads to a significantly improved ZT in the temperature range of 20 to 250 DEG C, because the thermal conductivity of the nanocrystalline bulk sample is significantly lower than the thermal conductivity of the commercial ingot and, more importantly, the difference is further increased as the temperature is increased. Figure 9 also shows that the peak ZT value shifts to a higher temperature (100 占 폚). The peak ZT of the nanocrystalline bulk sample is about 1.4 at 100 캜, which is significantly higher than the peak ZT of the commercial Bi ₂ Te ₃ -based alloy. The ZT value of the commercial ingot starts to drop at a temperature of 75 캜 or higher, and falls to 0.25 or less at 250 캜. By comparison, the nanocrystalline bulk sample exhibits a ZT higher than 0.8 at 250 ° C. Such ZT characteristics are much more desirable for power generation applications because no superior materials with high ZT in the temperature range can be obtained at this time.

A detailed microstructure test is performed on the nanocrystalline bulk sample using a transmission electron microscope (TEM). A TEM specimen is prepared by cutting, polishing and ion milling the bulk nanocrystalline sample. The hot compressed nanocrystalline bulk pellets are cut into 2x3x1 mm blocks and ground to 2x3x0.002 mm using a mechanical tripod grinder. The sample is glued to the copper grid and milled for 30 minutes with an incident energy of 3.2 kV and a beam current of 15 μA at an incident angle of 3.5 ° using a Precision Ion Polishing System (Gatan Inc.). Figures 10 to 15 represent representative TEM micrographs, which show the major structural features observed.

Generally, as shown in Figures 10 and 11, most of the grains are nanoscale. Moreover, the nanograms are highly crystalline and are randomly oriented (with a large angle between the lattice planes) with very clear boundaries. As shown in Figure 11, the nanograms can be intimately packed and correspond to a density measurement that suggests a fully dense sample. There are also larger particles as shown in Fig. As shown in Fig. 13, the high resolution TEM brown rice technique shows that these grains are composed of nano dots of 2 to 10 nm in size without boundaries. These nanodots are typically in the Sb-rich state with an exemplary composition approaching Bi: Sb: Te = 8:44:48 and Sb displaces Te. Some nano dots have no boundary with the matrix as shown in Fig. 13, but other nano dots observed include a matrix and a small angle boundary as shown in Fig. As shown in Fig. 15, a pure Te precipitate having a size in the range of 5 to 30 nm is also observed. The selected-area electron diffraction pattern shown in the illustration of FIG. 15 identifies the Te phase. Generally speaking, nano dots can be found within an area of 50 nm diameter each.

While not wishing to be bound by any particular theory, it is believed that these nanodots can be formed during the hot-press heating and cooling process. As shown in Fig. 12, larger grain containing nano-dots can cause non-uniform milling of the ingot during ball milling. These large grains can grow much larger during hot-press compression through Oswald Ripening. If there is a large population of nano interface features (e.g., nanograms) in the materials of the present invention, nanodots can not be the only reason for strong phonon scattering.

Example  2: nanocrystalline SiGe  matter

Silicon element materials and germanium element materials, both p-type and n-type, are used as starting materials and are milled using a ball mill to form nanoparticles of about 1 to about 200 nm in size. These elemental materials may in some cases have a ZT of less than about 0.01. It is also understood that SiGe alloys may be used to form the particles, possibly leading to further improvement in the final fabricated material. The sample is hot pressed at a pressure of about 40 to about 200 MPa and at a temperature of about 900 to 1300 DEG C to form a thermoelectric material sample.

Figures 16-19 show graphs depicting the temperature dependence of the various properties of the hot pressed nanocrystalline material formed from a p-type SiGe ball milled bulk material. The characteristics are measured using the same technique as described above for Figs. 5-9. Figure 16 shows the temperature dependence of the electrical conductivity of a nanocrystalline p-type SiGe sample. 17 shows the temperature dependence of the Seebeck coefficient of a nanocrystalline p-type SiGe sample. Figure 18 shows the temperature dependence of thermal conductivity for a p-type SiGe sample. 19 shows the change in the figure of merit ZT of a nanocrystalline p-type SiGe sample as a function of temperature.

Figures 20-23 show graphs depicting the temperature dependence of the various properties of the hot pressed nanocrystalline material formed from an n-type SiGe ball milled bulk material. 20 shows the temperature dependence of the electrical conductivity of the nanocrystalline n-type SiGe sample. 17 shows the temperature dependence of the Seebeck coefficient of a nanocrystalline n-type SiGe sample. Figure 18 shows the temperature dependence of the thermal conductivity for an n-type SiGe sample. Figure 19 shows the change in the figure of merit ZT of a nanocrystalline n-type SiGe sample as a function of temperature.

Figures 24-26 illustrate TEM micrographs of p-type SiGe materials associated with nanocrystalline materials. Figure 24 shows a TEM micrograph of a ball milled powder sample of a SiGe bulk material, showing nanosized microparticles of milled microparticles. 25 shows a TEM micrograph of a SiGe powder sample after hot compression. The micrograph shows a plurality of grains in the tightly packed nanoscale range of the hot compressed material. The illustration of Figure 25 shows the selected-area electron diffraction pattern taken for the sample. Figure 26 shows a high-resolution TEM of a hot compressed SiGe sample showing the nano-size of a plurality of grains of a sample, which represents a plurality of grain boundaries set for phonon scattering.

Example 3: Temperature tailoring of nanocrystalline p-type BiSbTe materials [

To illustrate how the performance index ZT can be customized for various temperature conditions, a sample of nanocrystalline p-type BiSbTe alloy material was prepared. In particular, materials of the type Bi _x Sb _2-x Te ₃ can be prepared with a variety of stoichiometries, depending on the desired x value. Two special illustrative types of samples are prepared: a p-type nanocrystalline, high temperature compacted material with a stoichiometry of Bi _0.3 Sb _1.7 Te ₃ , and a p-type nanocrystalline material with a stoichiometry of Bi _0.5 Sb _1.5 Te ₃ , High temperature compressed material. A suitable bulk starting material is ground with a ball mill to form a nanoparticle sample. The sample is compressed at 40 to 160 MPa and at 450 to 600 DEG C for about 5 minutes or less.

27 to 30 show the electrical conductivity, the Seebeck coefficient, the thermal conductivity and the temperature dependence of the ZT for the nanocrystalline Bi _0.3 Sb _1.7 Te ₃ sample, respectively, while FIGS. 31 to 34 show the Bi _0.5 Sb _1.5 Te ₃ sample The heat conductivity, and the temperature dependency of ZT, respectively. The measurement was carried out as described in Example 1. 30 and 34, the peak ZT values for the Bi _0.3 Sb _1.7 Te ₃ sample were measured at about 150 ° C, while the peak ZT values for the Bi _0.5 Sb _1.5 Te ₃ sample were measured at about 75 ° C Respectively.

Thus, the results indicate that the peak ZT of the nanocrystalline material can be tailored for special temperature range applications. For example, low temperature peak materials can be used in applications that are adjusted to be closer to room temperature use, such as cooling, while high temperature peak materials can be used for applications such as power generation.

It should be understood that the various aspects discussed herein, along with the above experimental results, disclose only a representative variety of methods and materials within the scope of the present invention. Indeed, those skilled in the art will readily appreciate that many other modifications to the methods and materials described herein may be made. All such modifications represent related aspects which also fall within the scope of the present invention. It is also to be understood that all numerical values expressing quantities of ingredients and reaction conditions, etc. used in the specification and claims are to be understood to be changed by the term "about" in all cases. Accordingly, unless otherwise indicated, numerical parameters set forth in this specification and the appended claims are approximations that may vary depending on the desired properties.

Claims (108)

A method of manufacturing a semiconductor alloy thermoelectric material, The method comprising: providing two or more different elemental powders; Mechanically alloying the at least two different elemental powders to form a semiconductor alloy powder comprising semiconductor alloy nanoparticles; And compressing the semiconductor alloy powder at an elevated temperature under pressure to form a semiconductor alloy thermoelectric material, Wherein the semiconductor alloy thermoelectric material has one or more dimensionless figure of merit (ZT) values at one or more temperatures in the range of room temperature to 2000 < 0 > C, Wherein the semiconductor alloy thermoelectric material comprises randomly oriented grains having an average grain size of less than 5000 nm and at least a portion of the grains comprises at least one precipitation region having an average size of 1 to 100 nm. ≪ / RTI > The method according to claim 1, Wherein the mechanically alloying comprises milling the at least two different elemental powders; The average size of the semiconductor alloy nanoparticles is 1 to 200 nm; A current passes through the semiconductor alloy powder during the compressing step; Wherein the semiconductor alloy thermoelectric material has a ZT value of from 1 to 3 at one or more temperatures ranging from room temperature to 2000 < 0 > C. The method according to claim 1, Wherein the semiconductor alloy thermoelectric material comprises a bismuth telluride based material or a bismuth antimony telluride based material; Wherein the semiconductor alloy thermoelectric material has a ZT value of at least 1.2 at one or more temperatures in the range of room temperature to 300 캜; Wherein providing the at least two different elemental powders comprises providing at least bismuth elemental and tellurium elemental powders or providing at least bismuth elemental, antimonic elemental, and tellurium elemental powders. Gt; The method according to claim 1, Wherein the semiconductor alloy thermoelectric material comprises a silicon germanium-based material; Wherein the semiconductor alloy thermoelectric material has a ZT value of at least 1.2 at one or more temperatures in the range of 400 to 1200 占 폚; Wherein providing the at least two different element powders comprises providing at least a silicon element powder and a germanium element powder. A method of manufacturing a semiconductor alloy thermoelectric material, Providing a mixture of two or more different elemental powders each comprising the elemental nanoparticles; And compressing the mixture under elevated pressure under pressure to alloy the elemental powders to form a semiconductor alloy thermoelectric material, Wherein the semiconductor alloy thermoelectric material has one or more dimensionless figure of merit (ZT) values at one or more temperatures in the range of room temperature to 2000 < 0 > C, Wherein the semiconductor alloy thermoelectric material comprises randomly oriented grains having an average grain size of less than 5000 nm and at least a portion of the grains comprises at least one precipitation region having an average size of 1 to 100 nm. ≪ / RTI > 6. The method of claim 5, The average size of the element nano-particles is 1 to 200 nm; Current passes through the mixture during the compression step; Wherein the semiconductor alloy thermoelectric material has a ZT value of from 1 to 3 at one or more temperatures ranging from room temperature to 2000 < 0 > C. 6. The method of claim 5, Wherein the semiconductor alloy thermoelectric material comprises a bismuth telluride based material or a bismuth antimony telluride based material; Wherein the semiconductor alloy thermoelectric material has a ZT value of at least 1.2 at one or more temperatures in the range of room temperature to 300 캜; Wherein providing the mixture of two or more different elemental powders comprises providing a mixture comprising at least a bismuth elemental powder and a tellurium elemental powder or a mixture comprising at least a bismuth elemental powder, an antimony elemental powder and a tellurium elemental powder ≪ / RTI > 6. The method of claim 5, Wherein the semiconductor alloy thermoelectric material comprises a silicon germanium-based material; Wherein the semiconductor alloy thermoelectric material has a ZT value of at least 1.2 at one or more temperatures in the range of 400 to 1200 占 폚; Wherein the step of providing a mixture of two or more different elemental powders comprises providing a mixture of at least a silicon element powder and a germanium element powder. As a thermoelectric material, Wherein the thermoelectric material comprises a semiconductor material structure comprising a plurality of randomly oriented grains having an average size of 1 nm to 10 microns; Wherein at least a portion of the grains comprise at least one precipitation region having an average size in the range of 1 to 100 nm, Wherein the thermoelectric material has a ZT value of at least one at one or more temperatures in the range of room temperature to 2000 < 0 > C. 10. The method of claim 9, Wherein the grain and the precipitation region are based on the same semiconductor alloy; Wherein the thermoelectric material has a ZT value of 1.2 to 3 at one or more temperatures in the range of room temperature to 2000 < 0 >C; Wherein the average grain size of the grain is less than 5000 nm and the average size of the precipitation region is 1 to 50 nm. 10. The method of claim 9, Wherein the thermoelectric material comprises a bismuth antimony telluride based material; Wherein the thermoelectric material has a ZT value of at least 1.2 at one or more temperatures in the range of room temperature to 300 < 0 >C; Wherein the precipitation zone comprises an antimony rich precipitation zone or a tellurium zone. 10. The method of claim 9, Wherein the thermoelectric material comprises a silicon germanium-based material; The thermoelectric material has a ZT value of at least 1.2 at one or more temperatures in the range of 400 to 1200 占 폚; Wherein the precipitate region comprises a metal silicide region. 10. The thermoelectric material according to claim 9, wherein the precipitation region is modulation doped. 10. The thermoelectric material according to claim 9, wherein when the thermoelectric material is used in an operating thermoelectric device, the precipitate region donates a charge carrier to the grain. A method of forming a semiconductor alloy thermoelectric material, The method comprising: providing a semiconductor alloy powder comprising semiconductor alloy nanoparticles; And compressing the semiconductor alloy powder at elevated temperature under pressure to produce a semiconductor alloy thermoelectric material having a ZT value of at least one at a temperature ranging from room temperature to 2000 占 폚; Wherein the semiconductor alloy thermoelectric material comprises randomly oriented grains having an average grain size of less than 5000 nm and at least some of the grains comprise at least one precipitation region having an average size of 1 to 100 nm. A method of forming a material. 16. The method of claim 15, The average size of the semiconductor alloy nanoparticles is 1 to 200 nm; Wherein the semiconductor alloy thermoelectric material has a ZT value of from 1 to 3 at one or more temperatures ranging from room temperature to 2000 < 0 > C. 17. The method of claim 16, Wherein the semiconductor alloy thermoelectric material comprises a bismuth telluride based material or a bismuth antimony telluride based material; Wherein the semiconductor alloy thermoelectric material has a ZT value of at least 1.2 at one or more temperatures ranging from room temperature to 300 < 0 > C. 17. The method of claim 16, Wherein the semiconductor alloy thermoelectric material comprises a silicon germanium-based material; Wherein the semiconductor alloy thermoelectric material has a ZT value of 1.2 or more at one or more temperatures falling within the range of 400 to 1200 占 폚. 16. The method of claim 15, wherein the forming method further comprises forming a settling region in the semiconductor alloy thermoelectric material. 20. The method of claim 19, wherein the precipitation region is formed by a solid state chemical reaction. 16. The method of claim 15, wherein the semiconductor alloy thermoelectric material has a ZT value of at least 1 at room temperature. 16. The method of claim 15, wherein the forming method further comprises passing an electrical current through the semiconductor powder during the step of compressing the semiconductor alloy powder at elevated temperature under pressure. delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete

Images