JP2006310361A - Thermoelement and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoelement having a high Seebeck coefficient, a large performance index, excellent impact resistance, superior thermal strain resistance, and superb moldability. <P>SOLUTION: The thermoelement is made of a compound. The compound is expressed by a general expression R<SB>x</SB>(M<SB>y</SB>Si<SB>1-y</SB>)<SB>(100-x)</SB>. In this case: R is at least one kind selected from rare earth elements containing Y; M is at least one kind selected from B, C, Al, P, Zn, As, Se, In, Sn, Sb, Te, Pb, and Bi; x is atom.%; y is an atom ratio; and 20≤x≤50 and 0.002≤y≤0.5 are met. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is mainly used in a temperature range of −50 ° C. to 100 ° C., generally in the vicinity of room temperature, a cooling device using the Peltier effect, a temperature control device, and a power generation device that generates power using a temperature difference by the Seebeck effect. The present invention relates to a thermoelectric material used for a thermocouple, a temperature sensor or the like using a thermoelectromotive force, and a manufacturing method thereof.

  When different types of semiconductors are joined to form an electric circuit and a direct current is passed, heat is generated at one junction and an endothermic phenomenon occurs at the other junction. This phenomenon is called the Peltier effect. Electronically cooling an object using the Peltier effect is called thermoelectric cooling, and a device configured for these purposes is called a thermoelectric cooling element, or generally a Peltier element. Further, when a temperature difference is generated between the two junctions, an electromotive force proportional to the temperature difference is generated. This phenomenon is called the Seebeck effect, and power generation using the generated electromotive force is called thermoelectric power generation. Further, a sensor that knows a temperature difference between two junctions by joining different kinds of metals to form an electric circuit and measuring a thermoelectromotive force generated between the two junctions is called a thermocouple.

Various sensors referred to in the present invention using the Seebeck effect are not only a thermocouple, but also a change in intensity (intensity variable) corresponding to temperature one-to-one by detecting a temperature difference by a potential difference, A device, module, or system intended to be fed back to various functions.
Elements having a basic structure obtained by bonding different kinds of metals or semiconductors as described above are collectively referred to as thermoelectric elements, and metals or semiconductors having high thermoelectric performance used therefor are referred to as thermoelectric materials.
Since thermoelectric cooling is cooling by a solid element, there is no need to use harmful refrigerant gas, no noise is generated, and local cooling is also possible. Furthermore, since the heating by the Peltier effect is possible by switching the current direction, precise temperature control can be performed. Applications that take advantage of these features include storage of electronic components, such as cooling and precision temperature control, and wine coolers that are important for temperature control. If thermoelectric materials with high performance at low temperatures below room temperature are used, chlorofluorocarbon, etc. It is also possible to realize a refrigerator, a refrigerator, and a vehicle-mounted seat cooler that do not use any harmful gas.

On the other hand, thermoelectric power generation includes wearable devices such as power generation using waste heat from heat engines such as factories, power plants and automobiles, power generation using abundant solar energy, or thermoelectric power watches using the temperature difference between body temperature and outside air, etc. , Enabling effective use of energy. Furthermore, a thermoelectric material having a large thermoelectromotive force and a small resistance has high utility value as a highly sensitive temperature sensor such as a thermocouple.
The high performance of thermoelectric elements usually means that the thermoelectromotive force (V), Seebeck coefficient (α), Peltier coefficient (π), Thomson coefficient (τ), Nernst coefficient (Q), Etchingshausen coefficient (P), The electrical conductivity (σ), the power factor (PF), the figure of merit (Z), the dimensionless figure of merit (ZT) is high, the thermal conductivity (κ), the Lorentz number (L), the electrical resistivity (ρ ) Is low. The performance of these thermoelectric elements is referred to as various thermoelectric performances. The Seebeck coefficient is also called thermoelectric power.

In particular, the dimensionless figure of merit (ZT) is expressed as ZT = α ² σT / κ (where T is the absolute temperature), and determines the thermoelectric conversion energy efficiency such as coefficient of performance in thermoelectric cooling and conversion efficiency in thermoelectric power generation. It is an important element to do. Therefore, the efficiency of cooling and power generation can be increased by producing a thermoelectric element using a thermoelectric material having a large value of the figure of merit (Z = α ² σ / κ).
That is, as the thermoelectric material, a material having a large Seebeck coefficient (α), a large electric conductivity (σ), a large output factor (PF = α ² σ), and a low thermal conductivity (κ) is preferable. In other words, a material having a large Seebeck coefficient (α) and a large ratio of electrical conductivity to thermal conductivity σ / κ (= 1 / TL) is preferable.
In thermoelectric power generation applications, materials with high output factors as well as performance indexes may be required. The figure of merit (Z) is a value obtained by dividing the output factor (PF = α ² σ) by the thermal conductivity (κ). When the value of κ is small, the figure of merit becomes large even if the output factor is the same. However, if κ is too small, an element is inserted in a portion having a temperature difference, so that the thermal resistance increases. Due to this, there is a problem that the whole system becomes large and the capital cost and the operating cost become large.

  An excellent thermoelectric material used as various sensors using thermoelectromotive force due to temperature difference is a combination of a thermoelectric material having a positive Seebeck coefficient and a thermoelectric material having a negative Seebeck coefficient in order to increase detection sensitivity and accuracy. It is advantageous to be used. The thermoelectromotive force per 1 ° C. (= absolute value of relative thermoelectric power) is required to be as high as 50 μV / K or more at room temperature, and therefore the Seebeck coefficient (= absolute thermoelectric power) of the positive and negative thermoelectric materials. ) Is required to be at least 25 μV / K or more. Copper-constantan (thermal electromotive force per 1 ° C. near room temperature to 50 μV / K), alumel-chromel (thermal electromotive force per 1 ° C. near room temperature to 41 μV / K), platinum-platinum rhodium (1 ° C. near room temperature) Metal thermoelectric materials such as thermoelectromotive force per unit (up to 7μV / K) are commonly used, but they are composed of precious metals only or use multi-component alloys to maintain material costs and stable performance. There were problems such as high manufacturing costs.

As described above, in thermoelectric materials, it is first necessary to achieve a high Seebeck coefficient with an absolute value of 25 μV / K or higher, preferably 50 μV / K or higher, and a high output factor that increases accordingly, thereby improving the figure of merit. However, other than that, impact resistance, heat distortion resistance and molding characteristics are also required.
The thermoelectric power generation element generates power using the temperature difference between the high temperature side and the low temperature side, and the thermoelectric cooling element functions by transferring the amount of heat from the low temperature side to the high temperature side by current. Then, an element is inserted into a portion having a temperature difference. Therefore, a difference in thermal expansion occurs between the low temperature side and the high temperature side, and a thermal shear stress is generated in the element.

In addition, Bi ₂ Te ₃ semiconductors are currently used as thermoelectric cooling elements. However, when solder is used for electrical joining on the high temperature side, coarsening of the solder structure occurs, resulting in uneven thermal shear stress. Occurs in the device. Due to the generation of these shear stresses, the life of the thermoelectric semiconductor element due to the thermal cycle is extremely deteriorated.
Conventionally, using a thermoelectric semiconductor material such as a melted polycrystalline body or powder sintered body that is relatively strong in shear stress and not cleaved at the expense of various thermoelectric performance, depending on the application, soft electrode materials such as liquid metals Various element structures for stress relaxation such as a skeleton structure have been devised and proposed by using them to relax the shear stress. However, these proposals have a problem that the cost performance is inferior because the process and structure are complicated. The appearance of a thermoelectric material that can withstand the above-described thermal shear stress is desired.

These problems are caused by the fact that most of the materials that have been put to practical use as thermoelectric elements are multi-element semiconductor materials doped with various substances in the crystal structure of Bi ₂ Te ₃ series, which is cleaved and brittle. It has occurred.
The problems caused by the brittleness of the Bi ₂ Te ₃ material that has been put to practical use include, in addition to the above-mentioned weakness to thermal shear stress, lack of impact resistance, and machinability such as cutting. There are some points that are poor.
On the other hand, a material called a 4f electron strongly correlated material containing rare earth elements such as YbAl ₃ , CePd ₃ , and CeRhAs has been studied as a thermoelectric material that changes to the brittle material. A material showing high thermoelectric properties in a region lower than room temperature has been found, and practical use is expected as a thermoelectric material functioning particularly in a low temperature region of 10 to 200 K. However, good properties have been obtained near room temperature. Absent. Also, the inclusion of expensive elements such as precious metals is one factor that hinders practical application.

A thermoelectric material with excellent workability, impact resistance, and life against thermal cycles not found in existing thermoelectric semiconductors because it uses inexpensive elements and has metallic properties such as high electrical conductivity. Appearance is expected.
Chairman Tsujikawa, IEEJ Technical Report No. 624, 1997, IEEJ, p. 35

  The subject of the present invention is that when used mainly in the temperature range of −50 ° C. to 100 ° C., high performance as a thermoelectric element can be expected, that is, a thermoelectric material having a high Seebeck coefficient (α) and a large output factor (PF). And the manufacturing method thereof, and at the same time, the above problems caused by the brittleness of the thermoelectric semiconductor, for example, the poor machinability, and having a high figure of merit (Z) and high electrical conductivity, A thermoelectric material particularly suitable for a thermoelectric element for thermoelectric power generation or thermoelectric cooling and a method for producing the thermoelectric material.

In order to solve the above-mentioned problems, the present inventors have studied the relationship between the composition of the material mainly composed of 4f element and Si and the thermoelectric characteristics, and as a result, the composition range of the ternary material having excellent thermoelectric characteristics and The present invention has achieved the object of the present invention by finding a composition and a range showing metallic properties excellent in crystal structure and molding processing characteristics. At the same time, the inventors have found a method for producing a material having high thermoelectric properties by performing pressure sintering, and further performing pressure sintering after fine pulverization.
That is, the present invention is as follows.

(1) General formula _{R x (M y Si 1-} y) (100-x) ( where, R represents at least one, M is selected from rare earth elements including Y, B, C, Al, P, Zn , As, Se, In, Sn, Sb, Te, Pb and Bi, x is atomic%, y is an atomic ratio, 20 ≦ x ≦ 50, 0.002 ≦ y ≦ 0.5 ) A thermoelectric material comprising the compound represented by ₎ .
(2) The thermoelectric material according to (1), wherein the main phase has a tetragonal or orthorhombic crystal structure.
(3) The thermoelectric material according to (1) or (2), wherein the electrical conductivity is 0.2 MS / m or more.
(4) The method for producing a thermoelectric material according to any one of (1) to (3), wherein R, M, and Si metal are alloyed and pressure-sintered at 0.01 to 10 GPa.
(5) The above (1) to (3), wherein R, M and Si metals are alloyed, ground to an average particle size of 0.1 to 10 μm, and then pressure sintered at 0.01 to 10 GPa. The manufacturing method of the thermoelectric material in any one of.

  According to the present invention, a thermoelectric material having a high Seebeck coefficient (α), a large output factor (PF), a high figure of merit (Z), and a high electrical conductivity mainly in a temperature range of −50 ° C. to 100 ° C. can do.

The thermoelectric material of the present invention has a general formula R _x (M _y Si _1-y ) _(100-x) (where R is at least one selected from rare earth elements including Y, M is B, C, Al , P, Zn, As, Se, In, Sn, Sb, Te, Pb and Bi, x is atomic%, y is an atomic ratio, 20 ≦ x ≦ 50, 0.002 ≦ y ≦ 0.5).
R in the general formula is a rare earth element containing Y, that is, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and It refers to at least one selected from Lu. Among these, at least one selected from Ce, Sm and Yb is preferable, and at least one selected from Ce and Yb is more preferable.

In the compound of the present invention, each element of Ce, Sm, and Yb is usually a trivalent ion, and the number of electrons in the 4f orbit at that time is 1, 5, and 13, respectively. However, the energy levels of Ce ³⁺ 4f ¹ and Yb ³⁺ 4f ¹³ are close to the Fermi level, and the energy of the electronic states of Sm ³⁺ and Sm ²⁺ are almost equal. Compared to the state, the trivalent ion states of these three elements are energetically unstable. Accordingly, the 4f state of Ce ³⁺ , Yb ³⁺ , and Sm ³⁺ tends to be mixed with conduction electrons, and Ce is trivalent and tetravalent, and Sm and Yb is slightly mixed with trivalent and divalent. This cf hybrid effect induces a giant thermoelectric power peculiar to strongly correlated materials due to the effect of losing the magnetic moment of rare earth ions through the Kondo effect.

From the above, Ce, Sm, and Yb have a remarkable effect of increasing the Seebeck coefficient by the effect of 4f electrons, compared to other elements, and in particular, there is one Ce and one hole in the 4f orbit. If Yb is contained, cf hybridization is easily realized and the highest thermoelectric performance is exhibited. Accordingly, preferred R components are Ce, Sm and Yb, and more preferred components are Ce and Yb. Of these, inexpensive Ce is most preferable.
The content of R in the thermoelectric material of the present invention is 20 atomic% or more and 50 atomic% or less. If the R content is less than 20 atomic%, the Seebeck coefficient (α) is small in the temperature range centered at room temperature. When the R content exceeds 50 atomic%, the Seebeck coefficient is greatly reduced, and in particular, the oxidative deterioration of a thermoelectric material containing La and Ce becomes remarkable, resulting in a material with poor durability. The particularly preferable range of the R content is 30 atomic% or more and 40 atomic% or less.

The M component is at least one selected from B, C, Al, P, Zn, As, Se, In, Sn, Sb, Te, Pb, and Bi.
The content of the M component in the thermoelectric material of the present invention is always less than or equal to the Si content. The range of the content of the M component relative to the whole is obtained by calculating the range of the formula y (100−x) when 20 ≦ x ≦ 50 and 0.002 ≦ y ≦ 0.5, and the value is 0.1 to 40 atomic%.
The M component is included mainly for the purpose of lowering the thermal conductivity in order to improve the figure of merit (Z), but if it exceeds the Si content, the electrical conductivity may increase, but the Seebeck coefficient will decrease. To do. Further, when the M component is randomly substituted at the Si site, the thermal conductivity is particularly lowered, and the thermoelectric performance including the figure of merit (Z) is improved. If the M component is less than 0.1 atomic% of the whole, there is no effect of addition, and if it exceeds 40 atomic%, the Seebeck coefficient decreases and the figure of merit (Z) decreases.

The thermoelectric material of the present invention containing each M component excluding the four elements of Bi, Te, Pb and Sn, which has a very high affinity with solder, has a thermal cycle exceeding 100 ° C. in a module using solder for bonding. It can be particularly preferably used in severe applications. In addition, if it is 10 atomic% or less in M component, metal elements, such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ag, Cu, Hf, Ta, W, etc. May be contained. These elements can be added in a range that does not adversely affect for the purpose of increasing electrical conductivity.
The range of the Si content with respect to the entire thermoelectric material of the present invention is to calculate the range of the formula (1-y) (100-x) when 20 ≦ x ≦ 50 and 0.002 ≦ y ≦ 0.5. And the value is 25 atomic% or more and 79.84 atomic% or less. When the Si content is less than 25 atomic%, the Seebeck coefficient (α) is extremely deteriorated. If the Si content exceeds 79.84 atomic%, the electrical conductivity and the power factor decrease. A particularly preferable Si content range is 30 atomic% or more and 70 atomic% or less.

The total content of the M component and Si is 50 to 80 atomic%, preferably 60 to 70 atomic%. When the total content of the M component and Si is less than 50 atomic%, the oxidation resistance is poor, and when it exceeds 80 atomic%, the electrical conductivity is extremely lowered.
In the present invention, in order to obtain a higher Seebeck coefficient and oxidation resistance, the crystal structure of the main phase of the thermoelectric material is preferably tetragonal or orthorhombic. Here, the main phase refers to a portion that occupies 50% by volume or more of the total thermoelectric material of the present invention. In determining the crystal structure of the main phase, an X-ray diffraction method is used. In order to know the quantitative quantity ratio of the main phase, an X-ray analysis method and EPMA may be combined.

For example, the material of Ce ₃₃ (Al _0.05 Si _0.95 ) ₆₇ composition ((100−x) = 67) of the present invention is almost 100% by volume when melted under a normal method, for example, under normal pressure. Is a CeSi ₂ type tetragonal thermoelectric material of Ce (Al _0.05 Si _0.95 ) ₂ composition. In this case, when (100−x)> 67, cubic Si, Al, or (Si + Al) subphase is mixed and the Seebeck coefficient increases, but the electrical conductivity decreases. When the subphase exceeds 50% by volume or (100−x) exceeds 80, the figure of merit is extremely lowered. Further, in the case of melting by the above method, if (100−x) <67, an orthorhombic Ce (Al, Si) subphase is generated, but the thermoelectric characteristics do not change greatly. However, when (100-x) is less than 50, the oxidation resistance is inferior.

The thermoelectric material of the present invention preferably has an electric conductivity of 0.2 MS / m or more, more preferably 0.5 MS / m or more. The upper limit is 30 MS / m. Depending on the M component, the electrical conductivity of Ce may greatly exceed 1.3 MS / m. In this case, the material is more preferable from the viewpoint of moldability.
The thermoelectric material of Ce-M-Si composition of the present invention having metallic properties has transport properties different from those of conventional Bi-Te based thermoelectric materials. Bi-Te-based thermoelectric materials have drawbacks in impact resistance, thermal strain resistance, and molding characteristics, and further, the life of thermoelectric semiconductor elements due to thermal cycles deteriorates due to the generation of shear stress and reaction with solder. These problems are all rooted in the fact that the Bi-Te thermoelectric material is a semiconductor material. Since Bi-Te is a semiconductor material having an electric conductivity of less than 0.2 MS / m, the impact resistance, heat distortion resistance, molding process characteristics, etc. required for the thermoelectric material are insufficient.

When the thermoelectric material of the present invention has an electric conductivity of 0.2 MS / m or more, the thermoelectric material that falls within the composition range of the present invention is a metallic material. Maintain sufficient ductility. The above problems can be drastically solved by using a thermoelectric element using the thermoelectric material of the present invention, which does not contain Bi and Te, which are highly compatible with solder, as a main component and is not a brittle semiconductor but metallic. .
Next, although the manufacturing method of the thermoelectric material of this invention is demonstrated, it does not specifically limit to these. As a manufacturing process used in a manufacturing method of a typical R-M-Si material of the present invention, (1) alloying, (2) pulverization / classification, (3) molding, (4) cutting / plastic processing, This will be described below. The material of the present invention is manufactured through, for example, steps (1) → (4), (1) → (3) → (4), (1) → (2) → (3) → (4).

(1) Alloying As a manufacturing method of RM-Si alloy, R, M, and Si metal are melted by high frequency, high frequency melting method in which the metal is cast into a mold, etc., metal components are charged in a boat such as copper, and arc discharge Arc melting method that melts by melt, drop cast method that melts the melted arc melt into a water-cooled mold at once, super rapid quenching method or roll rotation that blows or pours molten metal melted on a rotating copper roll to obtain a ribbon-like alloy Starting from oxide raw materials for all or part of the constituent elements, solid phase diffusion or reaction while reducing oxides. R / D method for producing an alloy, mechanical alloying method in which each metal component and / or alloy is reacted while being finely pulverized with a ball mill, etc. The obtained alloy is heated in a hydrogen atmosphere, once decomposed into R and / or other constituent element hydrides and an alloy phase, and then recombined with low hydrogen gas partial pressure at low temperatures to expel hydrogen. Any of the HDDR methods can be used.

  Depending on the type of M component and the production method, the composition ratio of the preparation may be different from the composition ratio of the finished product. For example, a method including a step of using Bi, Sb, Zn, Te or the like having a high vapor pressure as an M component and dissolving by exposure to a high temperature in a large melting chamber in a manufacturing method such as an arc melting method or a high frequency melting method. If M is selected, the M component is vaporized and dissipated from the alloy. Therefore, it is necessary to make adjustments such as adding a large amount of the M component in advance. In addition to the loss due to vaporization due to high vapor pressure, rare earth elements may be lost due to reaction with melting vessels such as crucibles or with oxygen in the system. The charge composition ratio must be determined above.

When the high frequency melting method or arc melting method is used, the produced alloy has a larger crystal grain size and better crystallinity than the case of using the ultra-quenching method, mechanical alloying method, etc. Depending on the combination of elements and the composition ratio, the secondary phase is likely to be phase-separated, and a long-time heat treatment for homogenization may be required after the alloy is produced.
In addition, when a rapid quenching method or a roll rotation method is used, fine crystal grains can be obtained, and submicron particles can be prepared depending on conditions. However, when the cooling rate is high, the alloy may become amorphous, and the thermoelectric characteristics may deteriorate. Also in this case, annealing after the alloy preparation is effective.
The drop cast method is a method that can obtain an alloy having intermediate properties and microstructure between the two. Depending on the alloy composition, the heat treatment time can be extremely shortened, and the method has excellent productivity. It is one of.

An alloy obtained by the gas atomization method often takes a spherical form and can be prepared from a fine powder to a coarse powder. Also in this case, it is necessary to perform annealing to improve the crystallinity depending on the conditions.
Alloys prepared by the R / D method, mechanical alloying method, and HDDR method have fine crystal grains, and in order to obtain a compositionally uniform nitrided powder, the area of the interface per unit volume is increased. In order to suppress thermal conductivity and improve thermoelectric properties, it is particularly effective to use these manufacturing methods.

(2) Pulverization / Classification The material melted in (1) above, such as high-frequency melting method, arc melting method, drop casting method, etc. can be used as a thermoelectric material by cutting or plastic working after it is heat-treated. Although the grains are large and the mechanical strength is higher than that of the existing Bi ₂ Te ₃ series material, further improvement in impact resistance may be required. For this reason, it is preferable to sinter after first grinding and / or classification. After grinding and / or classification, it is effective to improve the mechanical strength by sintering and introducing many grain boundaries per unit volume. Further, when the grain boundary thus introduced into the material functions as a fanon scattering center, Z is improved because the thermal conductivity is lowered.

The material of the present invention is pulverized mainly using a jaw crusher, a hammer, a stamp mill, a rotor mill, a pin mill, a cutter mill, a coffee mill, etc. A pin mill, manual or automatic mortar is mainly used for fine pulverization, but usually these pulverizers are often used in combination.
Further, after pulverization, classification using a sieve, a vibration or sonic classifier, a cyclone or the like and appropriate particle size adjustment is also effective for forming a more homogeneous microstructure. It is preferable to arrange the particle diameter within 0.05 to 100 μm. When the particle diameter exceeds 100 μm, the grain boundary effect is small, and when it is less than 0.05 μm, the electric conductivity tends to decrease. In particular, when finely pulverized to an average particle size of 0.1 to 10 μm, many grain boundaries can be introduced per unit volume, the thermal conductivity can be reduced, the crystallinity is maintained well, and the electrical conductivity is also good. Since it does not deteriorate, various thermoelectric performances are improved, which is preferable.

The average particle diameter is determined by measuring the volume equivalent diameter distribution using a laser diffraction particle size distribution meter and determining the median diameter obtained from the distribution curve. In order to improve density and electrical conductivity, it may be better to have a certain particle size distribution, but even in that case, the particle size distribution is in the range of 0.05 to 100 μm. It is desirable. After pulverization and classification, annealing in an inert gas or hydrogen can remove structural defects and is effective in some cases.
Even when the powder material is obtained directly using the mechanical alloying method or the R / D method, it can be adjusted to an appropriate particle size by the pulverization and / or classification as described above.

(3) Molding The powder material obtained as described above is put into a mold and compacted in a cold state and used as it is, or subsequently subjected to cold rolling, forging, shock wave compression molding, etc. There are methods for molding, but in many cases, sintering is performed while heat treatment at a temperature of 50 ° C. or higher. The heat treatment atmosphere is preferably a non-oxidizing atmosphere, and the heat treatment is preferably performed in an inert gas such as a rare gas such as argon or helium or nitrogen gas, or in a reducing gas containing hydrogen gas. A temperature condition of 500 ° C. or lower is possible in the air. Sintering at normal pressure or under pressure may be used, and further, sintering in a vacuum may be used.

In the present invention, this heat treatment is preferably performed simultaneously with the compacting. As a method of performing the heat treatment and the compacting simultaneously, a pressure sintering method such as a hot press method, a HIP (hot isostatic press) method, an SPS (discharge plasma sintering) method, or the like can be used. In addition, in order to make a pressurization effect remarkable, 0.01-10 GPa is preferable in the heat-sintering process, 0.1-10 GPa is more preferable, and 2-10 GPa is the most preferable. If the applied pressure is less than 0.01 GPa, the effect of pressurization is poor and there is almost no difference between atmospheric sintering and thermoelectric properties. When pressure sintering is performed, the lattice constant changes or the crystal system changes. Sometimes. In the case of a strongly correlated material, the relative position of the 4f element, such as Ce, and the adjacent non-4f electron element is slightly different, so the electron cloud overlap changes even in the same composition, and the cf hybridization method changes. The thermoelectric properties, particularly the Seebeck coefficient, may be improved.

For example, in Ce ₃₃ Al ₆ Si ₆₁ , the lattice constants of the melted material and the hot-pressed material are different, and the Seebeck coefficient is higher in the material hot-pressed at 4 GPa. At this time, when the lattice volumes of the two are compared, the hot-pressed material is about 1% smaller.
By applying pressure sintering, for example, a high-pressure phase different from the phase predicted from the conventionally known Ce-Si binary phase diagram has been observed, but the crystal structure of the material with high thermoelectric performance is , Orthorhombic or tetragonal materials. In general, the shorter the distance between the 4f element and the adjacent non-4f element, the higher the correlation, and the Seebeck coefficient improves. Therefore, it is preferable to perform pressure sintering because the thermoelectric performance is often improved.

However, if excessive pressure is applied to the crystal lattice in which the cf bond is generated and the lattice volume is made smaller than necessary, the 4f electrons are coupled through the RKKY interaction, and a magnetic order is generated, resulting in a giant thermoelectric In this case, care should be taken because the overall thermoelectric performance is degraded. Like the cf interaction, which has the effect of increasing the Seebeck coefficient, the competitive RKKY interaction is also a function of the lattice constant, so that the composition becomes an optimum lattice for various performances of the target thermoelectric material. It is desirable to select the crystal structure, and in the case of the RM-Si based material, it is necessary to control it within the scope of the present invention.
Among the hot press methods, the ultra-high pressure HP method, in which a green compact is charged in a capsule that undergoes compositional deformation and subjected to heat treatment while applying a large pressure from the uniaxial to triaxial directions, is a uniaxial compressor. Unlike the hot press method in which pressure heat treatment is performed in a cemented carbide or carbon mold, a pressure of 2 GPa or more, which is difficult even when using a tungsten carbide cemented mold, can be applied to the material without problems such as damage to the mold. In addition, since the capsule is plastically deformed by pressure and the inside is sealed, it can be molded without being exposed to the atmosphere, so that impurities such as oxygen can be prevented from being mixed and transpiration of volatile components can be suppressed. According to the ultra-high pressure HP method, it is attractive that a material having a composition and structure that is difficult to synthesize by other methods can be produced.

(4) Cutting / Plastic Processing The sintered sample and melted material produced as described above can be cut into arbitrary shapes and used as various thermoelectric elements. In particular, a metallic thermoelectric material of the present invention is not a brittle semiconductor material, and therefore can be easily processed into an arbitrary shape by a normal processing machine by cutting and / or plastic processing. In particular, it is a great feature that it can be easily processed into a prismatic shape, a cylindrical shape, a ring shape, a disc shape or a flat plate shape having high industrial utility value.

Once these shapes are processed, they can be further processed by cutting or the like to form tiles or square pillars having an arbitrary bottom shape. That is, from any shape, it can be easily formed by cutting and / or plastic working any shape surrounded by a curved surface and a plane including a cylindrical surface.
The cutting process mentioned here is a general metal material cutting process, which is a machining process using a saw, a lathe, a milling machine, a drilling machine, a grindstone, a diamond cutter, or the like. Rolling, forging, explosive forming, etc. Moreover, in order to remove the strain after cold working, heat treatment such as annealing at a temperature lower than the melting point of the thermoelectric material of the present invention can be performed. Depending on the composition of the thermoelectric material, anisotropy such as thermal conductivity can be imparted or strengthened by thermomechanical properties such as Seebeck coefficient and electrical conductivity by plastic working, and combined with heat treatment. It is also possible to adjust thermoelectric properties that greatly affect not only the properties but also the microstructure. Of course, after this, finishing processing corresponding to the required level can be performed by surface processing such as mechanical polishing such as buffing, electrolytic polishing, chemical cleaning, plating, coating, or the like.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.
The evaluation method of the thermoelectric material of the present invention is as follows.
The Seebeck coefficient (α) was measured by the ΔT method.
The electrical conductivity (σ) was measured by a four-terminal method, and the thermal conductivity (κ) was measured by a laser flash method.
The average particle diameter of the powder was the median diameter obtained from the distribution curve by measuring the volume equivalent diameter distribution using a laser diffraction particle size distribution analyzer.

[Example 1]
Ce having a purity of 99.9%, Al and Si having a purity of 99.999% were charged on a copper hearth so as to have an atomic ratio of 10: 1: 19, and arc-dissolved in an Ar atmosphere to obtain Ce (Al _{0. 05} Si _0.95) was smelted material having a _second composition. In addition, the arc melting was homogenized by turning the button-shaped material cooled after melting over a copper hearth and repeating the melting three times. The material was then subjected to a homogenization heat treatment at 1080 ° C. for 100 hours. Furthermore, this material is pulverized to a particle size of about 50 μm in a mortar, compacted into a 7 mmφ × 5 mm cylindrical shape at a pressure of 0.2 GPa, and pressure-sintered by an ultrahigh pressure HP method. , Ce ₃₃ (Al _0.05 Si _0.95 ) ₆₇ thermoelectric material was prepared. The pressure sintering conditions were 1250 ° C., 4 GPa, and 300 seconds. As a result of analyzing this crystal structure by an X-ray diffraction method, it was found that the main phase was a ThSi ₂ type tetragonal material. The thermoelectric properties of this material at room temperature are as follows:
The Seebeck coefficient (α) was −93 μV / K, the electrical conductivity (σ) was 1.2 MS / m, the thermal conductivity (κ) was 13 W / m · K, and the dimensionless figure of merit was 0.24. .

[Comparative Example 1]
The thermoelectric properties at room temperature of the material produced by the same method as in Example 1 except that Al was not added and the amount was replaced with Si were as follows.
That is, the Seebeck coefficient (α) is −48 μV / K, the electrical conductivity (σ) is 1.1 MS / m, the thermal conductivity (κ) is 15 W / m · K, and the dimensionless figure of merit is 0.05. there were.

[Examples 2 to 4 and Comparative Examples 2 and 3]
In the same manner as in Example 1, five types of materials having the compositions shown in Table 1 were prepared, and the Seebeck coefficient (α) and electrical conductivity (σ) at room temperature were measured. The results are shown in Table 1. As a result of X-ray diffraction analysis of this crystal structure, the main phase of Example 2 is a tetragonal material, while the main phases of Examples 3 and 4 are orthorhombic materials.

[Example 5]
In the same manner as in Example 1, Ce, Al, and Si were charged into copper hearth, dissolved in Ar, and then formed into a 6 mm thick plate by a drop cast method. This was crushed to about 1 mm using a jaw crusher, coarsely pulverized with a pin mill, and then finely pulverized to 2 μm with a rotating ball mill.
This fine powder was charged into a graphite mold and sintered by an SPS method in an atmosphere of Ar _0.05 MPa under the conditions of 900 ° C. and 45 MPa for 20 minutes, and the composition was Ce ₃₃ (Al _0.05 Si _0.95 ) _67. A molded body was obtained. As a result of analyzing this crystal structure by an X-ray diffraction method, it was found to be a ThSi ₂ type tetragonal material. The 300K thermoelectric properties of this material were as follows:

The Seebeck coefficient (α) is −86 μV / K, the electrical conductivity (σ) is 1.2 MS / m, the thermal conductivity (κ) is 8.8 W / m · K, and the dimensionless figure of merit is 0.30. there were.
A molded body obtained by sintering powder having a particle size of 0.1 to 10 μm has a relatively low thermal conductivity and can realize a high figure of merit.
Since this molded body is a metal material having an electric conductivity of 0.2 MS / m or more, a machine that does not cause cracking even if it is cut to 1 mm or less with a blade speed of 1500 rpm using a diamond cutter. It had strength and processing characteristics. For comparison, an existing Bi-Te thermoelectric material having an electric conductivity of 0.1 MS / m was cut under the same conditions as described above, but it was very brittle and was sandwiched between plastics, using a low-speed diamond cutter with a blade rotation speed of 100 rpm or less. However, it was found that the material is not suitable for mechanical cutting in the range where cracks occur and productivity increases.

  As described above, according to the present invention, the thermoelectric material has a high Seebeck coefficient (α) and a large figure of merit, and is a high-performance thermoelectric element material. It is possible to provide an excellent mold processability.

Claims (5)

Formula _{R x (M y Si 1-} y) (100-x) ( where, R represents at least one selected from rare earth elements including Y, M is, B, C, Al, P , Zn, As, At least one selected from Se, In, Sn, Sb, Te, Pb and Bi, x is atomic%, y is an atomic ratio, 20 ≦ x ≦ 50, 0.002 ≦ y ≦ 0.5) ₎ A thermoelectric material comprising the compound represented.   The thermoelectric material according to claim 1, wherein the main phase has a tetragonal or orthorhombic crystal structure.   The thermoelectric material according to claim 1 or 2, wherein the electric conductivity is 0.2 MS / m or more.   The method for producing a thermoelectric material according to any one of claims 1 to 3, wherein R, M, and Si metal are alloyed and pressure-sintered at 0.01 to 10 GPa.   4. The alloy according to claim 1, wherein R, M, and Si metal are alloyed, pulverized to an average particle size of 0.1 to 10 μm, and then pressure sintered at 0.01 to 10 GPa. The manufacturing method of the thermoelectric material of description.