JP2010080828A - Thermoelectric material, and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crystal orientation thermoelectric material which can be manufactured without requiring complicated and expensive processes and has high thermoelectric conversion characteristics, and to provide a method of manufacturing the same. <P>SOLUTION: The thermoelectric material is a sintered body which has crystal orientation zinc oxide whose degree of c axis orientation by the Lotgering method is ≥0.6 and <0.9 as a principal component and is a material which has high thermoelectric performance whose thermoelectric output factor at 20 to 1,000°C is ≥8.0×10<SP>-4</SP>Wm<SP>-1</SP>k<SP>-2</SP>, and the method of manufacturing the thermoelectric material is constituted by including a process of heating plate-like particles of layered zinc hydroxide containing a metal element to be a dopant. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an oxide thermoelectric material and a method for producing the same.

In recent years, thermoelectric conversion elements using metal oxides have been proposed as a technique for solving the high temperature durability / toxicity which has been a problem in conventional metal compound systems, and the research is rapidly progressing. The characteristics of a normal thermoelectric conversion element are: Seebeck coefficient α (μV · K ⁻¹ ), which is a thermoelectromotive force per unit temperature difference, conductivity σ (S · cm ⁻¹ ), and thermal conductivity κ (W · m ^{− It} is represented by several characteristic factors using ¹ · K ⁻¹ ). One of them is a thermoelectric output factor represented by α ² σ, and a performance index Z (= α ² σ / κ) obtained by dividing the thermoelectric output factor by thermal conductivity, and the performance index Z includes an absolute temperature. A dimensionless figure of merit ZT multiplied by T is used as a performance index. In general, the larger these values, the better the thermoelectric characteristics. Here, as described in Non-Patent Document 1, it is known that the crystal anisotropy of the electrical conductivity and thermal conductivity in an oxide is greatly dependent on the carrier characteristics. Whether or not can be controlled even in the bulk shape is a factor that greatly affects the improvement of the characteristics of the oxide thermoelectric conversion element.

  Moreover, since the volume of the element required in order to extract energy efficiently from a heat source becomes very large, the thermoelectric conversion element needs to be able to synthesize a large amount of oxide bulk used for the element. Therefore, in order to replace conventional thermoelectric conversion elements using expensive metal compounds with oxides, it is essential to be able to manufacture oxide bulk bodies that are the heart of thermoelectric conversion devices at low cost and in large quantities. Can be cited as a major issue.

Here, in order to produce a thermoelectric conversion element, it is required that power can be generated with high efficiency by a combination of a p-type semiconductor in which carriers are holes and an n-type semiconductor in which electrons are carriers. However, compared to the p-type in which NaCo ₂ O ₄ polycrystal is reported to have a high ZT of about 0.7, which is comparable to a metal compound, the n-type oxide system has a ZT of only about 0.3. The fact that it has not been obtained is an obstacle to the widespread use of oxide thermoelectric conversion devices. At present, there is an increasing demand for breakthroughs in thermoelectric properties of n-type oxides.

  Oxides doped with trivalent or tetravalent metal ions based on zinc oxide (ZnO) have been reported to exhibit high ZT among n-type thermoelectric materials, and are promising candidate materials for oxide thermoelectric conversion elements. Has been attracting much attention (Non-Patent Document 2).

  In addition, ZnO having a wurtzite crystal structure is known as a highly conductive surface in which the plane perpendicular to the c-axis (c-plane) is lined with Zn. Therefore, if a molded body based on ZnO having c-axis orientation such that current flows in a direction parallel to the c-plane of ZnO can be designed and manufactured, high thermoelectric conversion ability can be expected.

  As a technique for achieving crystal anisotropy with such a ZnO molded body, a complete alignment structure can be obtained by producing a bulk single crystal, but an extremely high temperature (˜2000 for growth of a metal oxide having a high melting point). ° C) and a dedicated single crystal growth apparatus are required, and there is a problem that the production is very expensive.

Therefore, as a technique for producing a crystalline orientation polycrystalline ZnO thermoelectric material, a polycrystalline orientation structure using a reactive templated grain growth (RTGG) method described in Patent Document 1 is described. Body manufacturing methods are known. The polycrystalline ZnO oriented structure using this RTGG method is a doctor blade which is one of tape molding methods using a dispersion slurry obtained by wet mixing basic zinc sulfate plate crystals and Al ₂ O ₃ particles. It is disclosed that the green sheet produced by the method can be cut, stacked several tens of layers, laminated and pressure-bonded, degreased and fired by heating, further subjected to isostatic pressing (CIP) treatment, and then sintered. The degree of c-axis orientation by the Lottgering method of the Al ³⁺ doped zinc oxide oriented body produced by this RTGG method is 0.82 to 0.86, indicating that it has high crystal orientation. However, in this manufacturing method, although a c-axis oriented ZnO sintered body doped with Al ³⁺ can be obtained, sulfuric acid remains as an impurity even after firing at a high temperature because a sulfate that does not volatilize at a high temperature is used as a raw material. In addition, since a structure in which large flaky crystals are integrated and the sinterability is lowered, there is a concern that the conductivity and further the thermoelectric properties may be adversely affected. In addition, since Al ₂ O ₃ particles are used as a raw material, there is a possibility that the solid solution of Al ^{3+ in} the crystal of ZnO occurs unevenly. Actually, although the value of the Seebeck coefficient is not disclosed, the thermoelectric output factor at 800 ° C. disclosed as an index of thermoelectric conversion performance is 3.0 × 10 ⁻⁴ (W · m ⁻¹ · K ⁻¹ ), Remains low. Furthermore, since a very complicated process is performed as a production method, it is considered difficult to produce a ZnO crystal oriented molded body at a low cost expected for producing a polycrystal.

Also the slurry containing ZnO and Al ₂ O ₃ particles, high while applying a rotating magnetic field (~10T) solidifies and thermoelectric properties of sintered ZnO c-axis oriented fabricated is disclosed by sintering (Patent Document 2). The c-axis orientation degree by the lotgering method of the zinc oxide oriented body doped with Al produced by this rotating magnetic field application method is 0.90 or more, and has a very high crystal orientation. However, also in this method, similar to the above-described method for producing a molded body by the RTGG method, Al ₂ O ₃ particles are used as a raw material, and therefore, the solid solution of Al in ZnO crystals may occur unevenly. There is. The absolute value of the Seebeck coefficient, which is the thermoelectromotive force at 800 ° C., remains as low as 190 μV · K ⁻¹ or less. The absolute value of the Seebeck coefficient of the conventional ZnO sintered body doped with Al ³⁺ is approximately 200 μV · K ^{−1 in} a wide temperature range from room temperature to about 1000 ° C. as described in Non-Patent Document 1. It is known that: This low Seebeck coefficient has a problem that it causes a decrease in electromotive force per single element as well as a decrease in various thermoelectric characteristics (output factor, figure of merit, dimensionless figure of merit). In addition to these, there is a concern that the size of the bulk molded body that can be produced in order to use the rotating magnetic field device is limited, and a large-scale facility is required, resulting in high costs.

"Monthly Ceramics", 161-165 pages, March 1998, published by the Ceramic Society of Japan “Journal of Materials Chemistry”, p. 85-90, 1996. JP 2003-095741 A JP 2007-243070 A

  In view of the above problems, the present invention aims to provide a crystal-oriented thermoelectric material that can be produced without requiring a complicated and expensive process and has high thermoelectric conversion characteristics, and a method for producing the same. To do.

As a result of intensive studies, the inventors have found that a sintered body mainly composed of crystallographically oriented zinc oxide having a c-axis orientation degree of 0.6 or more and less than 0.9 by the Lottgering method is 20 ° C. or more and 1000 ° C. The present inventors have found that the thermoelectric output factor at a temperature equal to or lower than 0 ° C. is 8.0 × 10 ⁻⁴ W · m ⁻¹ · K ⁻² or more and exhibits a high thermoelectric conversion ability, and have reached the present invention.

That is, the thermoelectric material of the present invention is a sintered body mainly composed of zinc oxide, and has a c-axis orientation degree of 0.6 or more and less than 0.9 by the Lottgering method, and is a thermoelectric material at 20 ° C. or more and 1000 ° C. or less. The output factor is 8.0 × 10 ⁻⁴ W · m ⁻¹ · K ⁻² or more. Moreover, the manufacturing method of the thermoelectric material of this invention comprises the process of heating the plate-like particle | grains of the layered zinc hydroxide salt containing the metal element used as a dopant.

  The crystal-oriented thermoelectric material and the method for producing the same according to the present invention are excellent in productivity because they can be produced without requiring complicated and expensive processes. Furthermore, since the thermoelectric material of the present invention is a bulk molded body and has crystal anisotropy, it can exhibit high conductivity and thermoelectric conversion characteristics. Therefore, application to a highly efficient thermoelectric conversion element becomes possible.

  Hereinafter, the present invention will be described in detail.

  The thermoelectric material of the present invention is a sintered body containing zinc oxide as a main component, and has a c-axis orientation degree of 0.6 or more and less than 0.9 by the Lottgering method, and exhibits high c-axis orientation. In the thermoelectric material of the present invention, by having such a high degree of c-axis orientation, when a temperature difference is given to both ends of the material, an anisotropic high conductivity is expected in a direction perpendicular to the c-axis. it can.

Here, the crystal orientation of the sintered body in the present invention is determined by an X-ray diffraction measurement apparatus (for example, “Xpert Pro” manufactured by Panalical, X-ray source: CuKα, wavelength: 1.54 angstrom, applied voltage: 45 kV). You can investigate. Further, it is possible to quantitatively calculate the degree of orientation by analyzing the peak intensity obtained by the X-ray diffraction measurement by the Lottgering method. In the present invention, the degree of c-axis orientation by the Lottgering method is to obtain I (h k l) peak intensities obtained by X-ray diffraction, and the ratio of I (0 0 l) to the sum of these peak intensities. And is calculated by f 1 given by the following equation.
_{f = (P - P 0)} / (1 - P 0)
Here, P is a peak intensity expressed by P = ΣI (0 0 l) / ΣI (h k l) and obtained from the oriented sample. P ₀ is represented by P ₀ = ΣI ₀ (0 0 l) / ΣI ₀ (h k l), and is a peak intensity obtained from an unoriented sample.

The thermoelectric material of the present invention has a thermoelectric output factor of 8.0 × 10 ⁻⁴ W · m ⁻¹ · K ⁻² or more at 20 ° C. or more and 1000 ° C. or less. Such excellent thermoelectric properties can exhibit both high conductivity and Seebeck coefficient at a high level by the above-mentioned high c-axis orientation and uniform solid solution of Al ^{3+ in} the sintered body. Yes, a high thermoelectric power factor can be maintained from a low temperature of 20 ° C. or higher to a high temperature of 1000 ° C. or lower. This makes it possible to function stably as a thermoelectric power generation element in a wide temperature range, and in particular, stable thermoelectric power generation is expected in a medium to high temperature range of 500 ° C. or higher and 1000 ° C. or lower, which is expected for oxide thermoelectric elements. it can.

Furthermore, the thermoelectric material of the present invention exhibits a high Seebeck coefficient of 200 μV · K ⁻¹ or more in a wide temperature range of 100 ° C. or more and 1000 ° C. or less. As described in the background art described above, the absolute value of the Seebeck coefficient of a conventional ZnO sintered body doped with Al ³⁺ is from room temperature to about 1000 ° C. as described in Non-Patent Document 1. It is known that it is approximately 200 μV · K ⁻¹ or less in a wide temperature range. The cause of the high Seebeck coefficient in the conventional thermoelectric material of the present invention is unknown at present, but is considered to be due to high c-axis crystal orientation and uniform solid solution of Al ³⁺ . This high Seebeck coefficient contributes to an increase in output voltage as an element when modularized. For example, when a temperature difference of 500 ° C. is applied to one element using the thermoelectric material of the present invention, it is 0.1 V or more. An electromotive force can be generated.

The relative density of the thermoelectric material of the present invention is 85% or more and 95% or less. Usually, with relative density in this range, there is a concern about the decrease in conductivity due to the decrease in the conductive path in the sintered body. However, in the thermoelectric material of the present invention, high c-axis orientation and uniform solid solution of Al ³⁺ By exhibiting high conductivity in the direction perpendicular to the c-axis, it is considered that the remarkable decrease in conductivity is suppressed, and that excellent thermoelectric properties can be expressed by exhibiting a high Seebeck coefficient. .

  The dopant element in the present invention is a metal element whose conductivity is improved by substituting Zn element existing in the crystal lattice of zinc oxide, and preferably B, Al, Ga, In, Fe, Co , Y, Sc, V, Eu, Si, Ge, Ti, Zr, and Hf. More preferably, it is at least one selected from B, Al, Ga and In which are trivalent metal ions. Since these dopant elements have more valence electrons (3 or 4) than zinc, they dissolve in zinc sites in zinc oxide to generate free electrons serving as carriers. Therefore, the conductivity of zinc oxide doped with these metal elements can be expected to improve dramatically. Since these metal elements have an ionic radius relatively close to that of zinc ions, they can be stably dissolved in the crystal of zinc oxide without causing distortion in the crystal structure. ("New development of transparent conductive film II", 31-40 pages, October 2002, CMC Publishing)

  In the present invention, a more preferable metal element is Al. Since Al has a small distortion when dissolved in ZnO and can be doped into a ZnO crystal at a molar ratio of up to about 0.1, as a result, high conductivity and thermoelectric conversion characteristics can be expected.

  The molar ratio of the dopant element to zinc contained in the zinc oxide sintered body in the present invention is preferably 0.001 or more and 0.2 or less. Here, the molar ratio of the dopant element is the ratio of the molar amount of the dopant element to the molar amount of Zn contained in the zinc oxide sintered body. When the dopant is contained at such a ratio, the zinc oxide sintered body can realize high conductivity by increasing free electrons serving as carriers.

  Below, it describes regarding the manufacturing method of the thermoelectric material of this invention.

The thermoelectric material of the present invention is produced using plate-like particles of layered zinc hydroxide salt containing a metal element as a dopant as a raw material. Here, the layered zinc hydroxide salt in the present invention is a layered metal hydroxide salt whose basic structure is a brucite structure containing Zn ^{2+ in the} crystal layer as a main component. A layered metal hydroxide salt having such a brucite structure as a basic structure is classified into a layered double hydroxide and a layered metal hydroxide (depending on the type and number of metal ions contained in the crystal layer). Also referred to as Layered Metal Hydroxide, or Double Hydroxide Salt (Hydroxy Double Salt).
Organic anions such as ions and inorganic anions such as nitrate ions are preferred.

  In the layered zinc hydroxide salt used in the present invention, the molar ratio of the dopant element to zinc is preferably 0.001 or more and 0.2 or less. Here, the molar ratio of the metal element serving as the dopant is the ratio of the molar amount of the metal element serving as the dopant to the molar amount of Zn contained in the layered metal hydroxide. When the dopant is contained in the layered zinc hydroxide salt in such a ratio, the zinc oxide sintered body after heating can realize high conductivity. In addition, the said metal / zinc ratio maintains the substantially same molar ratio also in the zinc oxide sintered compact after a heating.

The existence state of the dopant contained in the layered zinc hydroxide salt is a state in which it is intruded between the lattices even if it is substituted and dissolved at the Zn ²⁺ site in the brucite sheet of the layered zinc hydroxide salt It may be.

  In the present invention, the plate-like particles of the layered zinc hydroxide salt containing the dopant element can be produced, for example, by a wet process using a hydrolysis reaction of the raw material metal salt in an aqueous solution. Methods such as a hydrothermal synthesis method and a uniform precipitation method can be suitably used.

  The layered zinc hydroxide salt used in the present invention has a brucite structure as a basic structure as described above. Since the brucite layer is cationic, an anion is contained between the layers in order to maintain the electrical neutrality of the crystal.

  As the kind of anion, an anion that is hardly decomposed or volatilized by heat treatment at a high temperature of the molded body, and hardly remains as an impurity in the molded body is preferable. That is, since hardly volatile anions such as sulfate ions and halide ions are difficult to decompose or volatilize by heat treatment, there is a concern that they may remain in the zinc oxide crystal, resulting in adverse effects on the performance as a thermoelectric conversion element. This is because there is a risk of effect. Here, the hardly volatile anion means that it is adsorbed or dissolved in the surface of the oxide after firing without being dissipated in the atmosphere by firing at about 1000 ° C. in the atmosphere. Anion species remaining at 0.01 mol or more are shown. The remaining anionic species is indicated. Therefore, the interlayer anion in the present invention is not particularly limited as long as it is rapidly decomposed or volatilized by heat treatment at a high temperature. For example, acetate ions, lactate ions, butyrate ions, amino acid ions, etc. Organic anions and inorganic anions such as nitrate ions are preferred.

  The layered zinc hydroxide salt used in the present invention has an average plate surface diameter of 100 nm or more and 30000 nm or less, and an average plate thickness of 10 nm or more and 1000 nm or less. The average aspect ratio represented by (average plate surface diameter / average plate thickness) of the plate-like particles of the layered zinc hydroxide salt used in the present invention is 10 or more and 1000 or less.

  When the average plate surface diameter, average plate thickness, and average aspect ratio of the layered zinc hydroxide salt are outside the above ranges, the orientation of the plate-like particles by the molding process becomes insufficient, and the molded body has a random crystal orientation state. There is a risk of becoming. Here, the average plate surface diameter and the average plate thickness in the present invention are each a plate of 50 particles observed in a field of view of a magnification of 10,000 times of a scanning electron microscope (for example, “S-800” manufactured by Hitachi, Ltd.). It can be measured by averaging the surface diameter and the plate thickness.

  In another aspect of the present invention, the method includes a step of molding plate-like particles of a layered zinc hydroxide salt containing a dopant element. The molded body is derived from the plate-like shape of the layer-purified zinc hydroxide salt, and has crystal orientation preferentially oriented in an orientation (c axis) perpendicular to the sheet constituting the layered zinc hydroxide layer. Furthermore, a polycrystalline zinc oxide sintered body can be obtained by heating the c-axis oriented layered zinc hydroxide salt molded body. This zinc oxide sintered body is oriented along the c-axis of the wurtzite structure of zinc oxide. Also, in the present invention, a layered zinc hydroxide salt raw material powder is calcined and phase-transformed into zinc oxide, and then a molded body is produced and heat-treated to obtain a c-axis oriented zinc oxide sintered body. It is possible to obtain. In this case, the calcining temperature is preferably 300 to 1500 ° C., as long as it is a temperature at which phase transition to zinc oxide occurs. As described above, in the present invention, it is important that a phase transition to crystal orientation ZnO occurs due to a change in crystal structure in a topotactic manner by firing the layered zinc hydroxide salt.

  As a method for producing a layered zinc hydroxide salt or a calcined body molded body used in the present invention, either a dry molding method or a wet molding method can be suitably used. Examples of the dry molding method include a uniaxial press molding method, a hot press method, and a hot forge method. Examples of the wet molding method include an injection molding method, a cast molding method, an extrusion molding method, a pressure molding method, and a centrifugal molding method. In the present invention, in order to produce a zinc oxide sintered body having crystal orientation at low cost, it is the simplest method, and a uniaxial press molding method capable of mass production can be used more suitably. . Moreover, in order to improve the filling density of the molded object shape | molded by the said shaping | molding method, you may perform a hydrostatic pressure press (CIP) process.

  In this invention, the heating temperature for manufacturing a zinc oxide sintered compact should just be a temperature which ZnO sinters and a dopant is solid-solved in a ZnO crystal lattice, and 1000 to 1500 degreeC is preferable. In order to improve the sinterability, a spark plasma sintering (SPS) method or the like may be used.

  Since the zinc oxide sintered body is preliminarily contained in the layered zinc hydroxide salt as a raw material alone as a dopant, the dopant is uniformly doped in the crystal lattice of ZnO after firing. Conductivity and thermoelectric conversion characteristics can be realized. Therefore, it can be used as a thermoelectric material.

When the zinc oxide sintered body described above is used as a thermoelectric material for a thermoelectric conversion element, it exhibits excellent thermoelectric conversion characteristics because it has a feature in which a plane parallel to the highly conductive c-plane is aligned. be able to. The thermoelectric conversion characteristics of the thermoelectric material in the present invention can be measured with a thermoelectric characteristic measuring device (for example, “RZ2001i” manufactured by Ozawa Science). Thus, using a measurement sample cut into a prismatic shape of about 5 mm × about 5 mm × about 15 mm, the conductivity (σ) when a temperature difference is given to both ends of the sample in each temperature range (for example, 0 to 1000 ° C.). ) And Seebeck coefficient (α) can be measured, and thereby the thermoelectric output factor (α ² σ) can be obtained. Further, the thermal conductivity κ can be obtained by a laser flash thermophysical property measuring apparatus (for example, “LFA-502” manufactured by Kyoto Electronics Industry Co., Ltd.). Specifically, when the surface of a measurement sample cut into a plate of about 5 mm x about 5 mm x about 1 mm is irradiated with a laser beam with a uniform energy density in a pulse shape and heated uniformly, the heat is applied to the back of the sample. The thermal diffusivity can be determined by detecting the time and temperature change to diffuse, and the thermal conductivity can be obtained from this and the sample density. From the above measurement, ZT (= α ² σ / κ) can be obtained from the conductivity, Seebeck coefficient, and thermal conductivity in various temperature ranges.

Example 1
Layered hydroxide zinc salt containing manufacturing dopant layered hydroxide zinc salt containing dopant, was synthesized by the following procedure.
Zinc acetate (manufactured by Wako Pure Chemical Industries) 0.098 mol and Al 2 ⁺ ion source aluminum acetate n hydrate (manufactured by Wako Pure Chemical Industries) 0.002 mol as a dopant are dissolved in 1000 ml of distilled water, and it is about 1 hour at room temperature. Stir. To the prepared aqueous metal salt solution, 1000 ml of a 0.1 M aqueous sodium hydroxide solution was added dropwise over 1 hour while stirring at room temperature, and then stirred for about 20 hours. After the stirring, the white gel was collected by centrifuging, and the white gel was obtained by repeating washing with distilled water and centrifuging until the pH of the supernatant reached about 7. Furthermore, the obtained white gel was subjected to suction filtration, washed with 100 ml of ethanol, and then dried at 60 ° C. for 2 hours to obtain a white thin plate.

  X-ray diffraction (XRD) measurement and crystal structure identification of this thin plate-like body using an X-ray diffraction measurement apparatus (Panalytic “Xpert Pro”, X-ray source: CuKα, wavelength: 1.54 Å, applied voltage: 45 kV) Was assigned to a single phase of layered zinc hydroxide acetate having a brucite structure. The c-axis of the layered zinc hydroxide acetate was strongly oriented perpendicular to the surface of the thin plate, and the degree of c-axis orientation by the Lottgering method was about 0.8. FIG. 1 shows the result of observation of this thin plate-like body with a scanning electron microscope (SEM, manufactured by Hitachi, Ltd., “S-800”). As can be seen from FIG. 1, the plate surface diameter is 100 to 10,000 nm, the plate thickness is 20 to 100 nm, and the thin plate-like particles having an aspect ratio of 10 to 200 are stacked in parallel to the plate surface. I understood. The average plate surface diameter, average plate thickness, and average aspect ratio determined by SEM observation were 5000 nm, 30 nm, and 100, respectively.

Preparation of crystal-oriented ZnO sintered body by calcining and sintering of layered zinc hydroxide acetate Next, the thin plate-like body obtained above is fired at 700 ° C. for 5 hours and crushed in a mortar. As a result of measuring the XRD pattern of the white powder obtained in Step 1, it was found to be wurtzite type ZnO oriented in the c-axis. The c-axis oriented ZnO white powder was pressed with a uniaxial press molding machine, and further subjected to isostatic pressing (CIP) to produce a disk-shaped pellet having a diameter of about 25 mm and a thickness of about 7 mm.

  This disk-shaped pellet was sintered by firing at 1000 ° C. for 10 hours and further at 1400 ° C. for 10 hours (ZnO—Al 2% sintered body). After polishing the surface of the sintered body with sandpaper and measuring the XRD pattern, the c-axis of ZnO is strongly oriented perpendicular to the circular surface of the sintered body, and the degree of c-axis orientation by the Lottgering method is It was 0.89. Moreover, when the energy dispersive X-ray analysis (EDX) of the powder which grind | pulverized this sintered compact was measured, and the substance amount ratio of metal conversion was calculated | required, Al contained about 2% with respect to Zn, and It was confirmed that elements other than Zn, Al and O were not contained as impurities. Moreover, as a result of calculating the relative density of a ZnO-Al2% sintered compact by the Archimedes method, it was 94%.

Measurement of Thermoelectric Properties of Crystalline Oriented ZnO Sintered Body The c-axis oriented ZnO-Al2% sintered body produced as described above is 5 mm × 5 mm with a diamond cutter so that the longitudinal direction is in the direction perpendicular to the c-axis. A thermoelectric property measurement sample was prepared by cutting out to a size of 15 mm and polishing all surfaces with sandpaper.

  Thermoelectric properties at room temperature (20 ° C.) to 1000 ° C. of the measurement sample of the ZnO sintered body were evaluated using a thermoelectric property measuring apparatus (“RZ2001i” manufactured by Ozawa Kagaku). Data of conductivity, Seebeck coefficient and output factor at each temperature are shown in FIG.

The conductivity is 100 S · cm ⁻¹ or more in the entire temperature range of 20 ° C. or more and 1000 ° C. or less. Further, the conductivity tends to decrease slightly as the temperature rises, and the ZnO—Al 2% sintered body is metallic. Since it shows electron conduction, it shows that Al ³⁺ is solid-solved in the ZnO crystal as a dopant. The absolute value of the Seebeck coefficient is 200 μV · K ⁻¹ or more in a temperature range of 100 ° C. or more and 1000 ° C. or less. Moreover, the thermoelectric output factor shows a very high value of 8.0 × 10 ⁻⁴ W · m ⁻¹ · K ⁻² or more in the whole temperature range of 20 ° C. or more and 1000 ° C. or less. It can be seen that

  Furthermore, from a room temperature to a 5 mm × 5 mm × 1 mm thick plate sample cut out with a laser flash thermophysical property measuring device (“LFA-502” manufactured by Kyoto Electronics Industry Co., Ltd.) so that the plate thickness direction is perpendicular to the c-axis direction. The thermal conductivity κ up to 1000 ° C. was measured. The graph which calculated | required the dimensionless figure of merit ZT from these results is shown in FIG. The ZT at 1000 ° C. was 0.35, indicating high thermoelectric properties.

It is a figure which shows the SEM image of the layered zinc hydroxide acetate containing Al3 ⁺ in an Example. It is a figure which shows the thermoelectric property of the ZnO-Al2% sintered compact in an Example. It is a figure which shows the temperature dependence of ZT of the ZnO-Al2% sintered compact in an Example.

Claims (8)

A sintered body containing zinc oxide as a main component, having a c-axis orientation degree of 0.6 or more and less than 0.9 by the Lottgering method, and a thermoelectric power factor at 20 ° C. or more and 1000 ° C. or less of 8.0 × 10 ⁻ Thermoelectric material characterized by being ⁴ W · m ⁻¹ · K ⁻² or more. 2. The thermoelectric material according to claim 1, wherein the absolute value of the Seebeck coefficient at 100 ° C. to 1000 ° C. of the thermoelectric material is 200 μV · K ⁻¹ or more.   The thermoelectric material according to claim 1 or 2, wherein the relative density of the thermoelectric material is 85% or more and 95% or less.   The thermoelectric material according to claim 1, wherein the thermoelectric material is a zinc oxide sintered body that is carrier-doped by solid solution of a dopant element.   The thermoelectric material according to claim 1, wherein the dopant element is at least one selected from B, Al, Ga, and In.   The thermoelectric material according to claim 1, wherein a molar ratio of a trivalent metal element contained in the zinc oxide sintered body to zinc is 0.001 or more and 0.2 or less.   The manufacturing method of the thermoelectric material of Claims 1-6 which comprises the process of heating the plate-like particle | grains of the layered zinc hydroxide salt containing the metal element used as a dopant.   The manufacturing method of Claim 7 which comprises the process of shape | molding the said plate-shaped particle | grain, and the process of heating at 1000 degreeC or more and 1500 degrees C or less.

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