JP2007243070A - Oriented zinc oxide based thermoelectric conversion material and thermoelectric conversion device using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oriented zinc oxide based thermoelectric conversion material useful as a thermoelectric conversion material, a thermoelectric element and a thermoelectric conversion device or the like. <P>SOLUTION: A thermoelectric zinc oxide based sintered body comprising a zinc oxide as a principal component wherein a c-axis orientation by a Lotgering method is 0.9 or larger and relative density is 95% or larger at an X-ray diffraction peak on the particular face of the sintered body, and an excellent oriented thermoelectric conversion material and an excellent thermoelectric element with high electric conductivity or the like manufactured by applying a magnetic field to slurry including a zinc oxide based thermoelectric material powder, orienting the material powder while rotating the slurry, solidifying the slurry to manufacture a compact and thereafter sintering the compact can be provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a zinc oxide-based thermoelectric conversion material and a method for producing the same, and more specifically, an oriented thermoelectric material having crystal orientation, a method for producing the same, a thermoelectric element and a thermoelectric produced using these oriented thermoelectric materials. It relates to a conversion module and the like. The present invention makes it possible to produce an oriented zinc oxide sintered body that achieves high orientation and high density, which was difficult to produce with the prior art, and thereby allows the direction of current flow. The c-axis orientation of the plane parallel to the surface is 0.9 or more, the relative density is 95% or more, the thermoelectric figure of merit is 2.5 × 10 ⁻⁴ / K or more, and the thermoelectric zinc oxide has high electrical conductivity. Products such as a sintered body and its thermoelectric conversion element are provided.

  Thermoelectric materials that can directly convert thermal energy into electrical energy are attracting attention. Among thermoelectric materials, zinc oxide thermoelectric materials are relatively inexpensive and abundant, do not contain toxin elements, and can be used in high-temperature oxidizing atmospheres. Suitable for use. The thermoelectric characteristic peak of the zinc oxide-based thermoelectric material is ZT = 0.3 at 1273K (non-oriented bulk body), which is only about 30% of ZT = 1, which is a standard for practical use.

  The main cause is low electrical conductivity. The conventional conductive oxide sintered body has remarkably low electric conductivity as compared with the single crystal material. This is presumably because the electrical conduction mechanism of oxide is highly anisotropic and easily affected by grain boundary scattering. In particular, since zinc oxide crystals have anisotropy of various characteristics depending on the crystal axis direction (crystal plane), it is expected that various characteristics are improved by orienting a specific crystal plane.

  The prior art document describes using acicular zinc oxide particles as a starting material and orienting zinc oxide crystals in the c-axis direction by an extrusion molding method to produce a zinc oxide-based sintered body having varistor characteristics. (Patent Document 1). Furthermore, another prior art document describes that an oriented zinc oxide sintered body is produced by a doctor blade method using ZnO having a shape anisotropy as a starting material or a precursor powder material thereof ( Patent Document 2).

However, when shape anisotropic particles are used as a starting material, the manufacturing cost is increased, and the production of an oriented sintered body by the doctor blade method is poor in productivity. In the above method, the degree of c-axis orientation is as low as 0.86 and the relative density is as low as 90%. As a result, the thermoelectric power factor is 3.0 × 10 ⁻⁴ (W / mK ² ) @ 800 ° C. Stayed in.

Japanese Patent Publication No. 4-48746 JP 2002-16297 A

  In such a situation, in view of the above-described conventional technology, the present inventors have developed a zinc oxide thermoelectric material having a high degree of c-axis orientation, a high relative density, and a high electrical conductivity at a low production cost and high cost. As a result of intensive research aimed at developing new technologies that can be produced with productivity, orientation with high electrical conductivity and high degree of orientation and density, which was difficult to produce with conventional technology The inventors have found that a zinc oxide-based thermoelectric conversion material can be produced, and have completed the present invention. The present invention relates to an oriented zinc oxide thermoelectric conversion material having a high degree of crystal orientation and density, a production method thereof, a thermoelectric element having excellent thermoelectric performance produced using the oriented zinc oxide thermoelectric conversion material, a thermoelectric conversion device, and a thermoelectric The object is to provide a conversion module.

The present invention for solving the above-described problems comprises the following technical means.
(1) A thermoelectric zinc oxide sintered body containing zinc oxide as a main component, and 1) an x-ray diffraction peak on a specific surface of the sintered body has a c-axis orientation degree of 0.90 or more by the Lottgering method 2) A thermoelectric zinc oxide sintered body characterized by having a relative density of 95% or more.
(2) The thermoelectric zinc oxide sintered body according to (1), wherein the thermoelectric performance index of the sintered body is 2.5 × 10 ⁻⁴ / K or more.
(3) A magnetic field is applied to the slurry containing the zinc oxide-based thermoelectric material powder, the material powder is oriented while rotating the slurry, and the slurry is solidified to produce a molded body, and then the molding is performed. A method for producing a thermoelectric zinc oxide sintered body characterized by sintering the body.
(4) The molar ratio of ZnO and Al ₂ O _{3 in} the raw material powder is ZnO: Al ₂ O ₃ = 100−2 * x: x (0.1 ≦ x ≦ 5), as described in (3) above A method for producing a thermoelectric zinc oxide sintered body.
(5) The method for producing a thermoelectric zinc oxide sintered body according to (3), wherein the number of revolutions of the slurry is 1 to 100 revolutions / minute.
(6) The method for producing a thermoelectric zinc oxide sintered body according to (3), wherein a magnetic field of at least 8 Tesla is applied to rotate the slurry in a horizontal plane.
(7) A thermoelectric conversion device, wherein the thermoelectric zinc oxide sintered body according to any one of (1) to (3) is used as a thermoelectric element.
(8) A thermoelectric conversion module, wherein the thermoelectric zinc oxide sintered body according to any one of (1) to (3) is used as a thermoelectric element.

Next, the present invention will be described in more detail.
The present invention is a thermoelectric zinc oxide sintered body containing zinc oxide as a main component, and an x-ray diffraction peak on a specific surface of the sintered body has a c-axis orientation degree of 0.90 or more by the Lottgering method. The relative density is 95% or more. In this invention, it is set as the preferable embodiment that the thermoelectric performance index of the said sintered compact is 2.5 * 10 < ^-4 > / K or more.

Further, the present invention is a method for producing the thermoelectric zinc oxide sintered body, wherein a magnetic field is applied to the slurry containing the zinc oxide thermoelectric material powder, and the material powder is oriented while rotating the slurry. In addition, after the slurry is solidified to produce a molded body, the molded body is sintered. In the present invention, in the above method, the molar ratio of the raw material ZnO and Al ₂ O ₃ is ZnO: Al ₂ O ₃ = 100−2 * x: x (0.1 ≦ x ≦ 5), The preferred embodiment is that the number of revolutions of the slurry is 1 to 100 revolutions / minute, and a magnetic field of at least 8 Tesla is applied to rotate the slurry in a horizontal plane.

  Further, the present invention provides a thermoelectric conversion module characterized in that the thermoelectric zinc oxide sintered body is used as a thermoelectric element, and the thermoelectric zinc oxide sintered body is used as a thermoelectric element. It is characterized by the point of the thermoelectric conversion device characterized by the above.

Next, the manufacturing method of the thermoelectric zinc oxide sintered compact of this invention is demonstrated. In the method for producing a zinc oxide electrolyte thermoelectric material of the present invention, the molar ratio of ZnO and _Al 2 _{O 3} is _{ZnO: Al 2 O 3 = 100-2} * x: x between (0.1 ≦ x ≦ 5) A powder having a composition as a main component is used as a raw material. In this case, in particular, a molar ratio of ZnO: Al ₂ O ₃ = 98: 1 is preferable in order to improve thermoelectric characteristics.

Moreover, a IIA group element, a IIIB group element, 3d, 4d, 5d transition metal element, etc. can be added to a zinc oxide thermoelectric material as a dopant. Thereby, the carrier concentration of the zinc oxide-based thermoelectric material can be adjusted from 1.0 × 10 ¹⁹ to 1.0 × 10 ^21, and as a result, the thermoelectric characteristics can be enhanced.

  In the method of the present invention, the zinc oxide thermoelectric material powder is used as a main ingredient as a raw material, and a slurry such as water, a solvent such as ethanol or isopropanol, a dispersant such as ammonium polyacrylate, ammonium citrate, or polyethyleneimine is added to the slurry. Is made. In the present invention, it is important to apply a magnetic field to the slurry so that the zinc oxide thermoelectric material particles contained in the slurry are oriented by a strong magnetic field.

  In the case of the zinc oxide thermoelectric material particles, it is necessary to use a magnetic field of 8 Tesla (hereinafter referred to as T) or more. In particular, 9T or more, and more preferably 10T or more are preferable for increasing the degree of orientation.

  The average of the average particle size of the raw materials used at this time (particle size when the cumulative weight ratio is 50%) is preferably 20 μm or less, particularly 10 μm or less, and more preferably 1 μm or less by laser diffraction. This makes it possible to easily obtain a zinc oxide sintered body having a high degree of orientation and density.

  In addition, although the apparatus provided with the general superconducting magnet can be used for a magnetic field generator, in this case, it is preferable that a magnetic field is a horizontal magnetic field at the point which raises an orientation degree. Furthermore, by rotating the slurry in the same plane as the horizontal magnetic field, it is possible to orient the c-axis, which is the hard axis of magnetization of the zinc oxide thermoelectric material. The number of revolutions of the slurry at this time is preferably 1 to 100 revolutions / minute, and more preferably about 30 revolutions / minute.

  In the present invention, it is important to orient the particles in the slurry in a magnetic field and to solidify and mold the slurry. For the molding, a known molding method such as a casting molding method or a gel casting method can be used in the magnetic field generator.

  Next, in the present invention, it is important to obtain an oriented sintered body having the main crystal oriented by firing the obtained molded body. For sintering, an atmospheric pressure sintering method, hot press, hot forging, HIP sintering method, or the like can be used. In particular, the atmospheric pressure sintering method is desirable in terms of manufacturing cost. As for firing, the primary particles in the molded body grow during grain firing. At that time, if larger primary particles are oriented in a predetermined direction, the fine powder existing in the periphery also has the same orientation direction. Growth will begin to progress.

  The degree of orientation of the thermoelectric zinc oxide sintered body produced as described above is such that the slurry is rotated in a horizontal plane while applying a magnetic field whose c-axis orientation is perpendicular to the direction in which the magnetic field is applied. If the slurry is rotated in a horizontal plane while applying a magnetic field of 8T, it becomes 0.9 or more. When a zinc oxide thermoelectric material having such an orientation is used, the electrical conductivity in the plane direction perpendicular to the c-axis is high, so that the thermoelectric characteristics in the c-plane direction, that is, the figure of merit Z is high. can get.

Here, the degree of c-axis orientation is calculated by the Lottgering method, and each of the I _(hkl) peak intensities obtained by x-ray diffraction is obtained. _This indicates the ratio of _(00l) and is calculated by f given by the following equation.
f = (P−P ₀ ) / (1−P ₀ )

Here, P is represented by P = ΣI ₍ 001 ₎ / ΣI _(hkl) , and is the peak intensity obtained from the oriented sample. P ₀ is represented by P ₀ = ΣI _{0 (00l)} / ΣI _{0 (hkl)} , and is a peak intensity obtained from ICDD (non-oriented sample).

Here, the figure of merit Z is defined as Z = σS ² / k, where σ is the electrical conductivity, S is the Seebeck coefficient, and k is the thermal conductivity, and indicates the efficiency of the thermoelectric material. is there. Furthermore, in the oriented thermoelectric element of the present invention, the degree of c-axis orientation of the plane parallel to the direction of current flow is 0.9 or more and the figure of merit z is 2.7 × 10 ⁻⁴ / K or more. It becomes possible to exhibit excellent performance as a thermoelectric element.

  Conventionally, producing a zinc oxide-based sintered body in which zinc oxide crystals are oriented includes, for example, using acicular zinc oxide particles as a starting material, or zinc oxide having a shape anisotropy or a precursor thereof. Various attempts have been made by using materials and utilizing the doctor blade method. However, the conventional method has a limit in improving the degree of orientation and the relative density, and it has been difficult to produce an excellent oriented thermoelectric conversion material having high electrical conductivity. In contrast, in the present invention, an oriented zinc oxide sintered body having both a high degree of orientation and a high density is obtained by applying a magnetic field to the slurry containing raw material powder and orienting the slurry while rotating. This makes it possible to produce an excellent thermoelectric conversion material having high electrical conductivity that could not be provided by a conventional method.

The following effects are exhibited by the present invention.
(1) A new zinc oxide thermoelectric conversion material having high crystal orientation and high density and high electrical conductivity can be provided.
(2) A thermoelectric element made of the thermoelectric conversion material, a thermoelectric conversion device using the thermoelectric conversion material as a thermoelectric element, and a thermoelectric conversion module can be provided.
(3) By applying a magnetic field to the slurry containing the raw material powder, a zinc oxide sintered body having a high degree of crystal orientation and high density can be produced and provided.
(4) A highly oriented thermoelectric zinc oxide sintered body containing zinc oxide as a main component and a thermoelectric element comprising the sintered body can be provided.
(5) A thermoelectric conversion device and a thermoelectric conversion module using the thermoelectric element can be provided.
(6) According to the present invention, it becomes possible to produce a sintered body of a zinc oxide-based thermoelectric material having a high degree of orientation and density, which has been difficult with the prior art, and has an excellent electrical conductivity due to reduction of grain boundary scattering. It is possible to provide an oriented thermoelectric conversion material.
(7) According to the present invention, it is possible to provide an excellent oriented thermoelectric conversion material having high electrical conductivity while maintaining a high absolute value Seebeck coefficient and low thermal conductivity, which was difficult with the prior art. became.

  Next, the present invention will be specifically described based on examples, but the present invention is not limited to these examples.

As a raw material powder, a molar ratio ZnO: Al ₂ O is used: a ZnO powder having an average particle diameter of 0.5 μm and a purity of 99.99% or more and an Al ₂ O ₃ powder having an average particle diameter of 0.2 μm or less and a purity of 99.999 or more. Weighed at ₃ = 98: 1. These raw material powders were wet mixed using a ball mill for 24 hours. The obtained mixed powder was synthesized at 1100 ° C. for 12 hours. The obtained synthetic powder was put into an alumina container together with a solvent and alumina balls, and pulverized for 4 to 10 hours using a planetary ball mill. Ethanol was used as the solvent. After the obtained slurry was dried, a fine synthetic powder was obtained.

As a result of obtaining an average particle size of the obtained powder by a laser diffraction method, it was D ₅₀ = 0.7-2.0 μm. In the fine synthetic powder, ethanol is used as a solvent, and in order to improve dispersibility, 0.5% by weight of a dispersant is added to the powder, and the mixture is mixed so that the solid content is 30% by volume. Produced. 5 cc of the slurry was poured into a Teflon (registered trademark) mold having an inner diameter of 30 mm and a height of 20 mm, and the slurry was dried while rotating the slurry at 30 rpm while applying a magnetic field. A Teflon (registered trademark) mold was placed in the apparatus as shown in FIG. 1 with respect to a magnetic field, and an experiment was performed under predetermined conditions. Note that the rate of change of the magnetic force according to the distance from the center of the magnet was measured in advance, and was changed depending on the position of the Teflon (registered trademark) mold. Table 1 shows characteristics such as experimental conditions and thermoelectric figure of merit.

  The sample was fired under atmospheric pressure and atmospheric conditions under the conditions shown in Table 1 to produce a sintered body. A rectangular parallelepiped sample having a length of 15 mm, a width of 3 mm, and a thickness of 3 mm is produced from the sintered body in the direction of application of the magnetic field. The measurement was performed under conditions of room temperature to 800 ° C. Further, for the measurement of thermal conductivity, a disk-shaped sample having a diameter of 10 mm and a thickness of 2 mm was prepared from the sintered body, and measured by a laser flash method under conditions of room temperature to 800 ° C.

The thermoelectric performance index Z was calculated from the formula Z = σS ² / k. Furthermore, for the measurement of the degree of c-axis orientation f, the above rectangular solid sample was used, and the crystal orientation was determined from the obtained peak intensity by the Lottgering method. The results are shown in Table 1. The sample of the present invention showed a very large value with an orientation degree of 0.9 or more, a relative density of 99% or more, and a figure of merit of 2.7 × 10 ⁻⁴ / K or more.

Comparative Example 1
A magnetic field was applied from 0T to 10T to the slurry prepared in the same manner as in the above Example, and the slurry was dried to prepare a molded body. As a sintering condition, the sintering temperature was 1200 ° C. to 1400 ° C. The method for evaluating the thermoelectric characteristics is the same as in the examples. Table 2 shows characteristics such as experimental conditions and thermoelectric figure of merit. The obtained sample numbers 4 to 7 showed high relative density, but due to the low degree of orientation, the figure of merit showed a low value of less than 2.5 × 10 ⁻⁴ / K. On the other hand, Sample No. 8 showed a high degree of orientation, but because the density was as low as about 92%, the figure of merit also showed a low value of 2.4 × 10 ⁻⁴ / K. Sample Nos. 9 to 13 were prepared using a powder having an average particle diameter of 2 μm as a starting material, but the figure of merit was lowered due to the low degree of orientation or low density.

  As described in detail above, the present invention relates to an oriented zinc oxide thermoelectric conversion material and a thermoelectric conversion device using the same, and according to the present invention, a zinc oxide thermoelectric material sintered body having a high degree of orientation and density. It is possible to produce and provide a thermoelectric conversion material having reduced grain boundary scattering and high electrical conductivity resulting therefrom. Power generation using thermoelectric conversion can directly convert even relatively low-quality heat into electricity, so it is a technology that can recover the current unused waste heat, and the raw materials are relatively inexpensive and abundant. However, the present invention is useful as a new thermoelectric conversion material that can be widely used in the technical field. .

A schematic diagram of the experimental apparatus is shown.

Claims (8)

  A thermoelectric zinc oxide sintered body containing zinc oxide as a main component, wherein (1) an x-ray diffraction peak on a specific surface of the sintered body has a c-axis orientation degree of 0.90 or more by the Lottgering method (2) A thermoelectric zinc oxide sintered body characterized by having a relative density of 95% or more. The thermoelectric zinc oxide sintered body according to claim 1, wherein the sintered body has a thermoelectric performance index of 2.5 × 10 ⁻⁴ / K or more.   A magnetic field is applied to the slurry containing the zinc oxide thermoelectric material powder, the material powder is oriented while rotating the slurry, and the slurry is solidified to produce a molded body. A method for producing a thermoelectric zinc oxide sintered body characterized by comprising the steps of: The thermoelectric zinc oxide according to claim _{3, wherein} the molar ratio of the raw material ZnO to Al ₂ O ₃ is ZnO: Al ₂ O ₃ = 100-2 * x: x (0.1 ≦ x ≦ 5). A method for producing a sintered body.   The manufacturing method of the thermoelectric zinc oxide sintered compact of Claim 3 whose rotation speed of the said slurry is 1-100 rotations / min.   The method for producing a thermoelectric zinc oxide sintered body according to claim 3, wherein a magnetic field of at least 8 Tesla is applied to rotate the slurry in a horizontal plane.   A thermoelectric conversion device, wherein the thermoelectric zinc oxide sintered body according to any one of claims 1 to 3 is used as a thermoelectric element.   A thermoelectric conversion module, wherein the thermoelectric zinc oxide sintered body according to any one of claims 1 to 3 is used as a thermoelectric element.