JP2013211378A - Thermoelectric material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermoelectric material properly having excellent thermoelectric conversion performance while suppressing the manufacturing cost.SOLUTION: A thermoelectric material comprises a sintered body obtained by mixing and sintering a base material particle composed of a thermoelectric conversion material and a guest particle composed of the thermoelectric conversion material. The base material particle and the guest particle are composed of the same kind of a material, and thermal conductivity of the guest particle is smaller than that of the base material particle. For example, into a Bi-Sb-Te based base material, the guest particle is dispersed which is obtained by doping a small amount of heterogeneous elements such as In to the same Bi-Sb-Te based material. The mixed ratio between the base material particle and the guest particle is the base material particle:the guest particle=9:1 to 6:4 on a weight basis.

Description

  The present invention relates to a thermoelectric material comprising a sintered body obtained by mixing and sintering base material particles and guest particles.

  Thermoelectric conversion refers to the mutual conversion of thermal energy and electrical energy using the Seebeck effect or the Peltier effect. If thermoelectric conversion is used, it is possible to generate an endothermic phenomenon or an exothermic phenomenon by taking out electric power from the heat flow using the Seebeck effect or by passing an electric current through the material using the Peltier effect. Since thermoelectric conversion is direct conversion, no extra waste is discharged during energy conversion, exhaust heat can be used effectively, and there are no moving parts such as motors and turbines, so maintenance is free. It has the advantages such as.

  Currently, using the above advantages, it is also used in fields that require precise temperature control such as sensor elements, optical elements, semiconductor circuits such as LSI substrates, laser diodes, refrigerators, wine cellars, automobiles, etc. . In addition, with the recent seriousness of energy and environmental problems, a wide range of areas such as aviation, space, construction, geology and meteorological observation, medical hygiene, microelectronics, and waste heat utilization in the petrochemical, metallurgy, and power industries It is also expected to be put to practical use. Examples of such materials for thermoelectric conversion include Bi-Te, Mg-Si, Fe-Si, Si-Ge, Pb-Te, Fe-V-Al, chalcogenide, and skutter. Development of semiconductors and ceramics such as dieto-based, filled skutterudite-based, and boron carbide-based materials is under development. Bi-Te-based thermoelectric materials that can be used in the room temperature range that have been put to practical use so far This is limited to degenerate semiconductors with high mobility.

A performance index Z (K ⁻¹ ) or a dimensionless performance index ZT is used for performance evaluation of a thermoelectric material (thermoelectric conversion element). The figure of merit Z is determined by the equation Z = S ² σ / κ. S represents the Seebeck coefficient, σ represents electrical conductivity (electrical conductivity), and κ represents thermal conductivity (thermal conductivity). The dimensionless figure of merit ZT is a value obtained by multiplying the figure of merit Z by the absolute temperature T. The higher the figure of merit Z or the dimensionless figure of merit ZT, the higher the thermoelectric conversion performance. Therefore, in order to obtain good thermoelectric conversion performance, it is most effective that the Seebeck coefficient S and the electrical conductivity σ are high and the thermal conductivity κ is low.

  As one means for improving the thermoelectric conversion performance, there is a method of reducing the thermal conductivity by mixing nano-sized guest particles as phonon scattering particles in a thermoelectric material. In this case, each guest particle needs to be uniformly dispersed in a range in which the distance between guest particles is smaller than the phonon mean free path. However, when producing a thermoelectric material by mixing and sintering base material particles and guest particles, it is difficult to uniformly disperse nano-sized guest particles.

  Therefore, in Patent Document 1, a base particle made of a Bi-Te-based thermoelectric material and a core-shell type composite particle made of a material different from the base material particle, the insulating guest particles having the same specific gravity as the base material particle And sintered together. Thus, by making the specific gravity of the base material particles and the guest particles equal, even if the base material particles and the guest particles are mixed and sintered, the guest particles can be uniformly dispersed. Moreover, in patent document 1, in order to set it as the above guest particles, the guest particle is produced by the synthesis method.

JP 2011-134989 A

  In Patent Document 1, since the guest particles are uniformly dispersed in a range in which the distance between guest particles is smaller than the mean free path of phonons, it is effective for lowering the thermal conductivity. However, in Patent Document 1, it is essential that the guest particles have a nano size of 5 to 10 nm. When the guest particles are made finer in this way, the thermal conductivity decreases, but the electrical conductivity tends to decrease. In addition, since the insulating guest particles made of a material different from the base material particles are dispersed, the electrical conductivity is significantly reduced. This is because the guest particles exist as electric resistors in the thermoelectric material, and the resistance is typically arranged in series in the thermoelectric material. With this, the thermoelectric conversion performance improvement effect is limited, and a thermoelectric material having excellent thermoelectric conversion performance cannot be obtained. Moreover, in Patent Document 1, since it is necessary to produce guest particles by a synthesis method, the manufacturing cost increases.

  Therefore, the present invention solves the above-described problems, and an object of the present invention is to provide a thermoelectric material that accurately has excellent thermoelectric conversion performance while suppressing manufacturing cost.

  As a means for that, the thermoelectric material of the present invention is composed of a sintered body obtained by mixing and sintering base particles made of a thermoelectric conversion material and guest particles made of a thermoelectric conversion material, and the guest particles are It is a material of the same kind as the base material particle including the same material as that constituting the base material particle, and has a thermal conductivity smaller than that of the base material particle.

  The guest particles and the base material particles are "same type of material". The guest particles are made of the same material (element) as the material (elements) constituting the base material particles, but the thermal conductivity is made smaller than that of the base material particles. Therefore, when the composition is slightly different from the base material particles, the thermal conductivity is higher than that of the base material particles while the guest particles are mainly composed of the same material (element) as the material (element) constituting the base material particles. In some cases, a different element is slightly doped (added) in order to reduce the thickness.

  For example, if the base material particle is a Bi-X-Te-based semiconductor (X is one or more selected from the group consisting of Sb, Se, SbI, and AgBr), the guest particles are In order to make the thermal conductivity smaller than that of the base material particles, the guest particles are doped with Bi, X, Te, which are the materials constituting the base material particles, and also with different elements different from these. can do. In this case, the doping amount of the different element is 0.5 to 5 mol%. In addition, the heterogeneous element is one or more selected from the group consisting of In, Ge, Sn, Al, Ga, C, Si, Pb, B, Tl, Be, Mg, Ca, Sr, and Ba. In particular, In is preferable.

  Moreover, after making both the base material particles and guest particles semiconductors of Bi-X-Te system (X is one or more selected from the group consisting of Sb, Se, SbI, AgBr), In order to make the guest particles have a lower thermal conductivity than the base material particles, the X content in the guest particles can be made different (increase or decrease) from the base material particles.

  The mixing ratio of the base material particles and the guest particles is base material particles: guest particles = 9: 1 to 6: 4 on a weight basis.

  In the present invention, “◯◯ ˜XX” indicating a numerical range means a range including the lower limit and the upper limit. Therefore, if it is expressed accurately, it will be “XX or more and XX or less”.

  According to the present invention, since the base material particles and the guest particles are made of the same kind of material, the specific gravity of the both becomes inevitably substantially equal, and the guest particles can be uniformly dispersed in the thermoelectric material. In addition, the specific gravity relationship between the base material particles and the guest particles does not need to be strictly considered as in Patent Document 1, so that the degree of freedom in design is increased. If the guest particles can be uniformly dispersed in the thermoelectric material, the sinterability and strength of the thermoelectric material are also advantageous. Although referred to as “guest particles” for convenience of explanation, since the guest particles are made of the same type of thermoelectric conversion material as the base material particles, in the present invention, the guest particles and the base material of the thermoelectric material are used together with the base material particles. Become.

  In order to make the thermal conductivity of the guest particles smaller than that of the base material particles, it is only necessary to dope the guest particles with a very small amount of different elements or to slightly change the composition. Is easy to set. Moreover, since it is not necessary to produce guest particles by a synthesis method, manufacturing costs can be reduced.

  The guest particles are prepared such that the thermal conductivity is lower than that of the base material particles, so that the thermal conductivity of the thermoelectric material can be reliably reduced. On the other hand, it is not always necessary to make the guest particles nano-sized for the purpose of reducing the thermal conductivity. In addition, since the guest particles are made of the same kind of material as the base material particles, the guest particles do not become an active electrical resistor in the thermoelectric material. Therefore, a decrease in electrical conductivity can be avoided.

  As described above, in the present invention, only the thermal conductivity can be accurately reduced while avoiding a decrease in the electric conductivity of the thermoelectric material. Accordingly, the figure of merit Z or the dimensionless figure of merit ZT of the thermoelectric material, that is, the thermoelectric conversion performance can be reliably improved.

It is a graph which shows the tendency of thermal conductivity. It is a graph which shows the tendency of a dimensionless figure of merit. It is a graph which shows the tendency of Seebeck coefficient. It is a graph which shows the tendency of electrical conductivity. It is a graph which shows the tendency of carrier concentration. It is a graph which shows the tendency of the thermal conductivity which the electron used as a carrier contributes. It is a graph which shows the tendency of the thermal conductivity which phonon contributes. 2 is a structural photograph of Comparative Example 1. 6 is a structural photograph of Comparative Example 2. 2 is a structural photograph of Example 1. 3 is a structure photograph of Example 2.

  The present invention will be described in detail below. The thermoelectric material of the present invention is composed of a sintered body obtained by mixing and sintering base material particles and guest particles, both of which are composed of the same type of thermoelectric conversion material but having slightly different compositions.

(Base material particles)
As the base material particles, basically known materials conventionally used as thermoelectric conversion materials can be used without particular limitation. Specifically, Bi-Te, Mg-Si, Fe-Si, Si-Ge, Pb-Te, Fe-V-Al, chalcogenide, skutterudite, filled skutterudite Examples thereof include boron carbide-based semiconductors and ceramics. Among these, a Bi-Te based semiconductor is preferable. Among the thermoelectric materials currently in practical use, it has excellent thermoelectric conversion performance in a low temperature range of room temperature (about 20 ° C.) to 200 ° C., and has a high performance index Z or dimensionless performance index ZT. Because it can be expected.

Bi-Te based semiconductors become intermetallic compounds A ₂ B ₃ type, expressed as Bi-X-Te _{system, (Bi a X 1-a} ) 2 Te 3 (a is greater than 0 and less than 1), namely Bi _a X _2-a Te ₃ (a is greater than 0 and less than 2). Examples of X herein include one or more selected from the group consisting of Sb, Se, SbI, and AgBr. By appropriately selecting the type of X, the Bi-Te based semiconductor becomes a P-type semiconductor or an N-type semiconductor. In the present invention, both the P-type semiconductor and the N-type semiconductor are similarly improved in thermoelectric performance. Further, in order to efficiently convert an intermetallic compound into an N-type semiconductor, a halogen element such as I, Cl, or Br can be added as a dopant. For example, in the manufacturing process of a thermoelectric conversion material, by adding one or more powders selected from AgI, CuBr, SbI ₃ , SbCl ₃ , SbBr ₃ , HgBr _2, etc. to the raw material powder, it is efficiently N-type It can be a thermoelectric conversion material.

(Guest particles)
The guest particles are made of the same material as the base material particles, but have lower thermal conductivity than the base material particles. For example, if the base particle is a Bi-X-Te-based semiconductor (X is one or more selected from the group consisting of Sb, Se, SbI, and AgBr), the guest particle is also the base particle. In the same manner as above, it is basically made of a Bi-X-Te-based semiconductor (X is one or more selected from the group consisting of Sb, Se, SbI, and AgBr). In addition, the composition is slightly changed in order to lower the thermal conductivity than the base material particles. In order to make the thermal conductivity of the guest particles smaller than that of the base material particles, a different element different from Bi, X, and Te may be doped (added) in a small amount. If X is Sb, SbI, or AgBr, the thermal conductivity of the guest particles can be made smaller than that of the base material particles by making the X content smaller than the composition of the base material particles. When the content of X is made smaller than the composition of the base material particles, the value of a in the composition formula represented by Bi _a X _2−a Te ₃ may be made larger than the composition of the base material particles. If X is Se, the X content may be greater than the composition of the base material particles. That is, in the composition formula represented by Bi ₂ Te _3-a Se _a , the value of a may be made larger than the composition of the base material particles.

On the other hand, when doping a different element, the different element is selected from the group consisting of In, Ge, Sn, Al, Ga, C, Si, Pb, B, Tl, Be, Mg, Ca, Sr, and Ba. Species or two or more can be mentioned. At this time, the doping amount of the different element is set to 0.5 to 5 mol%. If the doping amount of the different element is less than 0.5 mol%, the difference between the thermal conductivity of the guest particles and the thermal conductivity of the base material particles is too small, and the effect of lowering the thermal conductivity of the thermoelectric material cannot be obtained. On the other hand, when the doping amount of the different element exceeds 5 mol%, the abundance of the different element serving as an impurity becomes excessive from the viewpoint of the original functional properties of the thermoelectric material, thereby degrading the thermoelectric conversion performance. Formula when doped with different _{element, (Bi a X 1-a} ) 2-b A b Te 3 (a: 0 than and less than 1, b: 0.01~0.1, A is different element) It can be expressed as. Or Bi ₍₂₎ (Te _(a) X _(1-a) ) _(3-b) A _(b) (a: greater than 0 and less than 1, b: 0.01 to 0.1, A is a different element ).

(Production method)
The thermoelectric material of the present invention can be obtained by mixing base material particles and guest particles and then sintering. At this time, the mixing ratio of the base material particles and the guest particles is base material particles: guest particles = 9: 1 to 6: 4 on a weight basis. When the mixing ratio of the guest particles is too small, the effect of improving the thermoelectric conversion performance by the guest particles cannot be obtained. On the other hand, even if the mixing ratio of guest particles is too large, the thermoelectric conversion performance tends to decrease.

  Base material particles and guest particles are obtained by pulverizing an ingot prepared to have a predetermined composition. Specifically, each raw material is mixed so as to have a predetermined composition, and then an ingot alloyed by high-frequency melting or arc melting is obtained. Next, the obtained ingot is pulverized, classified as necessary, and sintered to a predetermined shape to obtain a nanocomposite sintered body (thermoelectric conversion element).

  The average particle diameter of the base material particles may be about the same as the conventional one. Specifically, the average particle diameter of the base material particles may be about 0.1 to 200 μm. Conventionally, it has been preferable that the base material particles before sintering should be as small as possible. This is because the base crystal grain size after sintering becomes fine, which is effective in reducing the thermal conductivity. However, in the present invention, since the crystal grain size tends to be small by mixing the same kind of guest particles, it is not necessary to make the base material particles too fine.

  On the other hand, when sintering mixed particles of base material particles and guest particles, conventionally, the average particle size of guest particles is preferably set to a nanosize of about 10 to 100 nm so that phonons can be scattered. However, in the present invention, the guest particles are prepared so that the thermal conductivity is smaller than that of the base material particles, and the guest particles basically assume the base material of the thermoelectric material. The particles do not necessarily need to be refined to a nano size, and the average particle size before sintering may be about 0.1 to 200 μm.

  In order to efficiently pulverize the base material particles and guest particles, it is preferable to coarsely pulverize the ingot and then finely pulverize it. The ingot is roughly pulverized by a jaw crusher, a hammer, a stamp mill, a rotor mill, a pin mill, a cutter mill, a coffee mill, a mortar and the like. The fine pulverization after the coarse pulverization can be performed by a rotating ball mill, a vibration ball mill, a planetary ball mill, a wet mill, a jet mill or the like.

  As a sintering method, a normal pressure sintering method, a pressure sintering method, a hot press sintering method, a high temperature isostatic pressing (HIP) sintering method, or the like can be employed. In this case, it is also preferable to form the raw material powder into a predetermined shape by uniaxial press molding, tape molding method, hot extrusion method or the like before sintering. Moreover, it can also sinter by discharge plasma sintering (SPS sintering). With spark plasma sintering, a powder filled in a hollow cylindrical mold (die) is pressed from above and below by two upper and lower pressing members (punch) in a vacuum environment (inert atmosphere). By generating pulsed DC current using the upper and lower punches as electrodes to generate discharge plasma, Joule heat is generated by eddy current inside the powder and the surface is activated, so that sintering can be performed in a short time. Technology. In this case, there are features such that sintering can be performed at a lower temperature than conventional sintering methods, productivity is high, and crystal grains of the sintered body are difficult to be coarsened. Hot press sintering and SPS sintering also have an effect of suppressing reduction in electrical conductivity due to orientation of crystal grains.

  As the base material particles and guest particles, a Bi-Sb-Te based semiconductor having the composition shown in Table 1 was used, and a mixed powder obtained by mixing these at a ratio shown in Table 1 was subjected to SPS sintering under the conditions shown in Table 1. Thus, a thermoelectric material (Example) for an evaluation test was obtained. Moreover, as a comparative example, only the base material particles used in the examples and the thermoelectric material consisting only of guest particles were obtained by sintering under the same conditions as in each example. The base material particles and the guest particles were obtained by pulverizing ingots prepared by using respective raw materials in an electric melting furnace with a mortar until the average particle diameter was about 90 μm.

For each of the obtained Examples and Comparative Examples, electrical resistance value ρ, carrier concentration n, mobility μ, electrical conductivity (electric conductivity) σ, Seebeck coefficient S, thermal conductivity (thermal conductivity) κ, lattice vibration The heat conduction κ _p caused by (phonon) and the dimensionless figure of merit ZT were measured. The results are shown in Table 2, and graphs showing the tendency of some physical properties are also shown in FIGS. Finally, structure photographs of each example and comparative example were observed. The structure | tissue photograph is shown to FIGS. For measurement of the electrical resistance value ρ, carrier concentration n, mobility μ, electrical conductivity σ, and Seebeck coefficient S, a Hall measuring device (ResiTest 8300) manufactured by Toyo Technica was used. Further, the thermal conductivity kappa, the measurement of the lattice thermal conductivity kappa _p was used by ULVAC Inc. of laser flash apparatus (Tc7300).

  From the results of FIG. 1, the base material particles and the guest particles are the same type of material, but the thermal conductivity of Comparative Example 2 consisting only of guest particles is smaller than that of Comparative Example 1 consisting only of base material particles. It was. Thus, it was confirmed that the thermal conductivity can be lowered by doping a different kind of element into the same kind of material as the base material particles. However, as is apparent from the results of FIG. 2, it can be seen that there is a limit to the effect of improving the thermoelectric conversion performance simply by doping with a different element. On the other hand, in Examples 1 to 3, by mixing the base material particles and the guest particles, the Seebeck coefficient S is improved as shown in the result of FIG. 3 and the heat as shown in the result of FIG. While suppressing the decrease in the conductivity σ, the thermal conductivity κ was effectively decreased as shown in the results of FIG. Therefore, as is clear from the results of FIG. 2, in Examples 1 to 3, the thermoelectric conversion performance was greatly improved. The theory is examined.

  First, when electrical conduction is due to band electrons (or holes), both electrical conductivity σ and thermal conductivity κ depend on the carrier concentration n. This corresponds to the carrier concentration of a semiconductor having a relatively large density of electrons or holes (referred to as a degenerate semiconductor because it forms a Fermi distribution with a degenerate energy distribution). The semiconductor has a characteristic that the electrical conductivity σ decreases as the carrier concentration (density) n decreases.

Meanwhile, heat is not only the conduction electrons, also carried by the lattice vibrations (phonons), the thermal conductivity kappa is the sum of the contributions kappa _p and contribution kappa _c of the carrier phonon. Since the above embodiment is a P-type semiconductor, κ _c = electron contribution κ _e . Therefore, it is expressed as κ = κ _p + κ _e . kappa _e is determined by the carrier concentration n is, kappa _p depends on the constituent elements and structures of the material. Further, κ _c (κ _e ) has a relationship of κ _c (κ _e ) = LσT among the electrical conductivity σ, the Lorentz number L, and the temperature T. In general, since the Lorentz number L is defined by 1.6 to 2.4, κ _c (κ _e ) is obtained by measuring the electrical conductivity σ, so that it can be considered separately from κ _p. it can.

On the premise of this, in the present invention, since the same kind of guest particles are mixed with the base material particles, there is almost no fluctuation of the carrier concentration n as shown in the result of FIG. As a result, it is considered that the decrease in electrical conductivity σ is suppressed. Moreover, as is clear from the results of FIGS. 4 and 6, from the relationship κ _{e =} LσT, κ _e it is seen that shows the same behavior (trend) and electrical conductivity sigma. Thus, it can be seen that in the present invention, the variation of the thermal conductivity κ is more influenced by κ _p depending on the constituent elements and structure of the material than κ _e depending on the carrier concentration n. Furthermore, when the result of FIG. 7 is seen, in Examples 1-3, it is confirmed that (kappa) _p has fallen because the same kind guest particle is mixed with base material particle | grains. Thus, it is considered that only the thermal conductivity κ can be greatly reduced while suppressing the decrease in the electrical conductivity σ.

In addition, as is clear from the results of FIGS. 8 and 9, in Comparative Example 1 consisting only of base particles and Comparative Example 2 consisting only of guest particles, the crystal grain size of the thermoelectric material was relatively large. In Examples 1 and 2 in which the same kind of guest particles were mixed, the crystal grain size of the thermoelectric material was relatively fine, as is apparent from the results of FIGS. This point is also considered to contribute to a decrease in thermal conductivity κ.

Claims (7)

It consists of a sintered body obtained by mixing and sintering base particles made of thermoelectric conversion material and guest particles made of thermoelectric conversion material,
The guest particle is a thermoelectric material that is the same kind of material as the base material particle including the same material as that forming the base material particle, and has a lower thermal conductivity than the base material particle. The base material particles are Bi-X-Te-based semiconductors (X is one or more selected from the group consisting of Sb, Se, SbI, and AgBr),
The thermoelectric material according to claim 1, wherein the guest particles are formed by doping a material constituting the base material particles with a different element different from the Bi, X, and Te.   The thermoelectric material according to claim 2, wherein the guest particles are doped with 0.5 to 5 mol% of the different element.   The heterogeneous element is one or more selected from the group consisting of In, Ge, Sn, Al, Ga, C, Si, Pb, B, Tl, Be, Mg, Ca, Sr, and Ba. The thermoelectric material according to claim 2 or claim 3.   The thermoelectric material according to claim 4, wherein the different element is In. The base material particles and guest particles are Bi-X-Te-based semiconductors (X is one or more selected from the group consisting of Sb, Se, SbI, and AgBr),
The thermoelectric material according to claim 1, wherein the guest particles have a content of X different from that of the base material particles. The thermoelectric material according to claim 1, wherein a mixing ratio of the base material particles and the guest particles is base material particles: guest particles = 9: 1 to 6: 4 on a weight basis.