JP2011243824A - Anisotropic thermoelectric material, and radiation detector and power generation device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an anisotropic thermoelectric material which has anisotropy of a large Seebeck coefficient and can obtain large crystals along a surface having the anisotropy so as to realize high radiation detectability and high electromotive voltage in electric power generation.SOLUTION: The anisotropic thermoelectric material is expressed by a chemical formula SrLaNbO, has a layered perovskite type crystal structure of which block layers formed of octahedrons of NbOof four layers are periodically stacked in a c axial direction, and has an absolute value of the difference between a Seebeck coefficient in an a axial direction and a Seebeck coefficient in a direction perpendicular to an a axis of 100 μV/K or more. Thereby the anisotropic thermoelectric material can realize the high radiation detectability and high electromotive voltage in the electric power generation.

Description

  The present invention relates to a material that converts thermal energy into electrical energy, and a thermoelectric conversion technology that performs radiation detection and power generation.

  When a temperature difference occurs between both ends of the thermoelectric material, an electromotive voltage is generated in proportion to the temperature difference. This effect is known as the Seebeck effect for converting thermal energy into electrical energy, and the generated electromotive voltage V is expressed by V = S (−ΔT) by the temperature difference ΔT and the material-specific Seebeck coefficient S. The performance of the thermoelectric material is evaluated by the electrical resistivity ρ and the thermal conductivity κ in addition to the Seebeck coefficient. By combining these, the quality of the thermoelectric material is determined, and the electric power can be taken out. The conversion efficiency at that time is represented by an index called ZT.

As described above, in thermoelectric materials that generally exhibit isotropic physical properties, an electromotive voltage generated by the Seebeck effect appears only in the same direction as the temperature difference applied to the material. On the other hand, in a thermoelectric material exhibiting anisotropy in electrotransport properties, as shown in FIG. 1, when a system in which the crystal axis abc is tilted with respect to the space axis xyz is defined, the given temperature difference is orthogonal In the direction, generation of an electromotive voltage due to the temperature difference is observed. That is, in the system as shown in FIG. 1, when a temperature difference ΔT _z is given in the _z- axis direction, an electromotive voltage V _x is generated in the x-axis direction as shown in the following equation (2).

  Where l is the width of the sample, d is the thickness of the sample, α is the inclination angle of the ab plane relative to the sample surface, ΔS is the difference (anisotropy) between S in the c-axis direction and S in the ab plane direction . As described above, the effect of generating an electromotive force in a direction different from the heat flow in the inclined arrangement of the anisotropic thermoelectric material is called an anisotropic thermoelectric effect or non-diagonal thermoelectric effect. The electromotive voltage generated by the anisotropic thermoelectric effect is proportional to the anisotropy ΔS of the Seebeck coefficient, the sample aspect ratio l / d, and the sine value sin2α that is twice the stacking inclination angle from the equation (2).

Utilizing this anisotropic thermoelectric effect, a radiation detector consisting of a YBa ₂ Cu ₃ O _7-δ (YBCO) graded thin film [Patent Document 1] and a radiation detector consisting of a Ca _x CoO ₂ graded thin film [ Non-Patent Document 1] has been disclosed so far.

For example, Ca _x CoO ₂ has an anisotropic crystal structure in which conductive CoO ₂ layers and insulating Ca layers are alternately stacked along the c-axis direction. Therefore, in a Ca _x CoO ₂ thin film in which the c-axis is inclined and laminated on an appropriate substrate surface, the CoO ₂ surface corresponds to the layered surface in FIG. 1, and the same system as in FIG. 1 is established. When the electromagnetic waves on the surface of Ca _x CoO ₂ thin film is inclined stacking is incident, a temperature difference occurs in the normal direction of the Ca _x CoO ₂ thin film surface, as a result, Ca _x CoO ₂ thin film by anisotropic thermoelectric effect described above An electromotive force is generated in a direction parallel to the surface of the substrate. Thus, by reading the electromotive voltage generated in the direction parallel to the surface of the thin film with the incidence of the electromagnetic wave, the Ca _x CoO ₂ gradient laminated thin film can detect the electromagnetic wave with a sensitivity of about 600 mV / K.

In YBCO, ΔS is at most 10 μV / K, and in Ca _x CoO ₂ , ΔS is about 35 μV / K [Non-Patent Document 1]. In order to achieve higher detection sensitivity, a larger ΔS is required.

In the layered materials such as YBCO and Ca _x CoO ₂ described above, there is a large anisotropy of thermoelectric characteristics between two directions, ie, a direction parallel to the layer (layer parallel direction) and a direction perpendicular to the layer (layer vertical direction). Compared to this, there is almost no anisotropy in different directions in the layer. In addition, the crystal of these layered materials is easy to grow in the direction parallel to the layer and difficult to grow in the direction perpendicular to the layer. Therefore, in the flux method and the floating zone method generally used in the production of single crystals, Only very thin crystals of about 100 μm in the direction, typically several μm to several tens of μm, can be obtained. In order to produce a device using the anisotropic thermoelectric effect, the thin single crystal must be further obliquely cut out to form an inclined laminate. Therefore, the finally obtained inclined laminated body remains at most several tens of μm square, which is about the same as the thickness of the single crystal. For these reasons, it is necessary to grow a graded alignment thin film of layered material by vapor phase growth using a single crystal substrate as a template in order to realize a large-area device that utilizes the anisotropic thermoelectric effect in conventional layered materials. However, in this case, only a thickness of at most several μm can be obtained. For this reason, it is difficult to efficiently create a temperature difference with respect to the device by means other than light absorption, and it has been limited to use in sensor applications such as radiation detection. In order to use in power generation applications, it is necessary to obtain an inclined laminated body having a thickness of at least several tens of μm or more in a larger area in order to efficiently make a temperature difference by solid contact or the like.

  Non-Patent Document 2, Patent Document 2 and Non-Patent Document 3 include devices that perform cooling and power generation by utilizing material anisotropy and anisotropic thermoelectric effect generated by forming an inclined laminated structure made of different materials. It is disclosed. These are artificially laminated two types of materials with different characteristics, and the anisotropic ΔS of the Seebeck coefficient can be increased to several hundred μV / K by combining the materials, but the thickness should be 1 mm or less. It is difficult to obtain a large electromotive voltage because the aspect ratio l / d of the device cannot be increased. For this reason, it is necessary to flow a large current at a low voltage during cooling or power generation, which causes a problem that the power supply for operation and the circuit design of the inverter become complicated.

  From the background described above, in order to realize excellent radiation detection sensitivity, the anisotropy of the Seebeck coefficient is large, that is, ΔS of 35 μV / K or more and the thickness when the inclined laminate is 1 mm or less. Necessary. In order to obtain a high voltage while efficiently generating a temperature difference during power generation, it is necessary that ΔS of 35 μV / K or more and the thickness of the inclined laminated body be 10 μm or more and 1 mm or less.

On the other hand, Non-Patent Document 4 discloses Sr _2-x La _x Nb ₂ O ₇ as a material of a layered substance having anisotropy in directions in which the layer parallel directions are different (Non-Patent Document 4). Non-Patent Document 4 discloses a crystal synthesis method, a lattice constant, an electric transport property, and a magnetic property of this material.

In Non-Patent Document 4, as shown in FIG. 2, Sr _2-x La _x Nb ₂ O ₇ is a layered perovskite in which block layers composed of four NbO ₆ octahedrons are periodically stacked in the c-axis direction. It has a structure. According to Non-Patent Document 4, the electrical resistivity of Sr _2-x La _x Nb ₂ O ₇ differs in the axial direction. The electrical resistivity in the b-axis direction of Sr _2-x La _x Nb ₂ O ₇ is 10 times the electrical resistivity in the a-axis direction, and the electrical resistivity in the c-axis direction is the electrical resistivity in the a-axis direction. The value is 100 times that of. Further, there is a large anisotropy between the electrical resistivity in the a-axis direction, which is the layer parallel direction, and the electrical resistivity in the b-axis direction. Single crystals of Sr _2-x La _x Nb ₂ O ₇ are produced by a general method such as the Czochralski method or the floating zone method, with a size of several mm in the layer parallel direction and about 1 mm in the layer vertical direction. it can.

So far, there is no document that discloses the anisotropy of the Seebeck coefficient of Sr _2-x La _x Nb ₂ O ₇ , and it is not disclosed in Non-Patent Document 4 and the like described above. Further, there is no literature suggesting the anisotropy of the Seebeck coefficient of Sr _2-x La _x Nb ₂ O ₇ , and it is not disclosed in the above Non-Patent Document 4 or the like. This is because even if there is anisotropy in electrical resistivity, the difference ΔS in the Seebeck coefficient directly related to the electromotive voltage due to the anisotropic thermoelectric effect does not always increase. This can also be seen from the fact that ΔS is small in YBCO where the ratio of the electrical resistivity in the layer parallel direction to the layer vertical direction is several tens of times at room temperature (Non-patent Document 5). Therefore, it is difficult to immediately find the anisotropy of the Seebeck coefficient from the disclosure content of Non-Patent Document 4 and the like.

JP-A-8-247851 Japanese Patent No. 4078392

Applied Physics Letters 95, 051913 (2009). Applied Physics Letters 89, 192103 (2006). Applied Physics Letters 94, 061917 (2009). Progress in Solid State Chemistry 29, 1-70 (2001). Physical Review B 50, 6534-6537 (1994).

As described above, in order to realize high radiation detection sensitivity by the anisotropic thermoelectric effect, it is necessary to increase the anisotropy ΔS of the Seebeck coefficient and the device aspect ratio l / d. Specifically, ΔS is preferably 35 μV / K or more and the device thickness is 1 mm or less. In addition, when used for power generation, it is necessary to add a temperature difference efficiently, and it is necessary that ΔS is 35 μV / K or more, and that the thickness when the inclined laminate is 10 μm or more and 1 mm or less. Such a device cannot be realized when a conventional layered material such as Ca _x CoO ₂ or an inclined laminated structure made of different materials is used.

  The present invention solves the above-described conventional problems, and provides an anisotropic thermoelectric material that realizes high radiation detection sensitivity and high operating voltage during power generation, and a radiation detector and power generation device using the same. Objective.

In order to solve the above-mentioned conventional problems, the present inventors have conducted research on the anisotropic thermoelectric properties of thermoelectric materials, and as a result, have a layered perovskite structure represented by the chemical formula Sr _2-x La _x Nb ₂ O ₇ It is shown that the Seebeck coefficient shows large anisotropy not only between the layer parallel direction and the layer vertical direction but also between the a-axis direction and the b-axis direction, which are the layer parallel directions where crystal growth is easy. Based on the finding and this finding, the present invention has been reached.

That is, the anisotropic thermoelectric material of the present invention is represented by the chemical formula Sr _2-x La _x Nb ₂ O ₇ , and is a layered structure in which block layers composed of five NbO ₆ octahedrons are periodically stacked in the c-axis direction. It has a perovskite crystal structure and has a large anisotropy such that the absolute value of the difference between the a-axis direction and the Seebeck coefficient in the direction perpendicular to the a-axis is 100 μV / K or more. Furthermore, among the crystal axes having anisotropy, the a-axis and the b-axis are oriented in the layer parallel direction of a layered structure in which crystal growth is easy, and a crystal of several mm square can be easily obtained by a general crystal growth method. be able to.

  The anisotropic thermoelectric material of the present invention has anisotropy with a large Seebeck coefficient with respect to different directions in the layer parallel direction in which crystal growth is easy.

Illustration of anisotropic thermoelectric effect The figure which showed the crystal structure of anisotropic thermoelectric material Sr _2-x La _x Nb ₂ O ₇ of the present invention The figure which showed the structure of the crystal orientation thin film of Sr _2-x La _x Nb ₂ O ₇ on the single crystal substrate described in the second embodiment The figure which showed arrangement | positioning at the time of cutting out an inclination crystal piece from the plate-shaped crystal | crystallization described in Embodiment 3. The figure which showed the structure of the radiation detector as described in Embodiment 3. The figure which showed the structure of the radiation detector as described in Embodiment 4. The figure which showed the structure of the electric power generation device as described in Embodiment 5. The figure which showed the structure of the radiation detector as described in Embodiment 6. Top view showing the configuration of the radiation detector according to the sixth embodiment Shows the X-ray diffraction pattern of _{Sr 2-x La x Nb 2} O 7 as described in Example 1 View showing a change in c-axis length of _{Sr 2-x La x Nb 2} O 7 as described in Example 1 The figure which showed the measurement principle of Seebeck coefficient and thermal conductivity as described in Example 1 Shows thermal conductivity of Sr _1.8 La _0.2 Nb ₂ O ₇ according kappa, Seebeck coefficient S, the temperature dependence of the electrical resistivity ρ in Example 2

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
In Embodiment 1, the production of a bulk single crystal in the anisotropic thermoelectric material of the present invention will be described.

The anisotropic thermoelectric material of the present invention is an electrically conductive carrier (electrically conductive) in an oxide material having a crystal structure as shown in FIG. 2 represented by chemical formulas Sr ₂ Nb ₂ O ₇ , Sr ₄ Nb ₄ O ₁₄ , SrNbO _{3.5, and the} like. Carrier) and is used in the form of a bulk or thin film made of a single crystal or a crystal-oriented sintered body. The directions of the a, b, and c axes of the Sr ₂ Nb ₂ O ₇ crystal are as described in FIG.

In this embodiment mode, a method for manufacturing a single crystal bulk of the anisotropic thermoelectric material Sr _2-x La _x Nb ₂ O ₇ of the present invention will be described. The Sr _2-x La _x Nb ₂ O ₇ single crystal is obtained by melting an oxide containing strontium, lanthanum, and niobium and performing crystal growth. At this time, strontium carbonate (SrCO ₃ ), dilanthanum trioxide (La ₂ O ₃ ), niobium pentoxide (Nb ₂ O ₅ ), and the like can be used as raw materials. These raw materials are weighed and mixed so as to have a desired molar ratio, and fired to obtain a starting material for single crystal growth. As a condition in the firing process, synthesis by a solid phase reaction method at 1000 to 1400 ° C. is preferable, and as a firing atmosphere, an inert atmosphere gas such as air, oxygen, or argon is introduced. In order to obtain a desired structure in a later process, it is desirable to sinter and synthesize by introducing an inert gas such as argon or a reducing gas such as argon gas containing hydrogen. In addition, since the role of lanthanum in the present invention is to inject electric conductive carriers by doping into the crystal, other rare earth elements, bismuth thallium, etc. may be used as long as the electric conductive carriers can be injected. In addition, other methods for injecting electrically conductive carriers include depleting oxygen in the material and doping with a Group 6 transition metal element such as molybdenum instead of niobium.

  Next, crystal synthesis is performed using the obtained starting material. Examples of the crystal synthesis method include a self-flux method, a floating zone method, a pulling method represented by the Czochralski method, and the like. As a method for obtaining crystals relatively easily, there is an infrared concentrated heating type floating zone method. When the floating zone method is used, it is necessary to form the starting material into a rod shape prior to crystal growth. Therefore, the starting material obtained by firing the raw material is molded into a rod shape (φ5 × 100 mm) with a hydraulic hand press and fired. This calcination is preferably performed immediately below the melting point in the vicinity of 1300 to 1500 ° C., and it is desirable to obtain a baked product with sufficient strength. Further, it is desirable to flow in an inert atmosphere gas such as argon as the firing atmosphere. In order to obtain a desired structure in a later process, it is desirable to flow in an oxygen reducing gas such as an argon gas containing hydrogen.

  When the floating zone method is used, the rod-shaped starting material is moved at a speed at which the melt moves stably, and crystals are grown. Specifically, it is desirable that the rod-shaped starting material is moved downward at a speed of about 5 to 15 mm / Hr.

  Moreover, impurities and element defects in the crystal may be contained to the extent that electrical conductivity is not hindered.

Above, when showed an example of a method for manufacturing a single crystal of _{Sr 2-x La x Nb 2} O 7 , to produce a _{Sr 2-x La x Nb 2} O 7 of the present invention, the self-flux method, Chukuraru In addition to using the ski method, it can be created by other commonly used methods.

(Embodiment 2)
In Embodiment 2, preparation of a thin film single crystal in the anisotropic thermoelectric material of the present invention will be described.

A method of producing the Sr _2-x La _x Nb ₂ O ₇ crystalline orientation thin film 31 of the present invention will be described with reference to FIG. The Sr _2-x La _x Nb ₂ O ₇ thin film in the present embodiment is formed on the single crystal substrate 32.

The b-axis and c-axis direction Seebeck coefficients of the Sr _2-x La _x Nb ₂ O ₇ a-axis direction Seebeck coefficient show large anisotropy. Therefore, in order to increase the electromotive voltage due to the anisotropic thermoelectric effect, the a-axis direction 33 of the Sr _2-x La _x Nb ₂ O ₇ crystalline orientation thin film 31 is inclined with respect to the substrate surface 34 as shown in FIG. It is preferable to perform crystal growth. Hereinafter, an inclination angle formed between the a-axis of Sr _2-x La _x Nb ₂ O ₇ and the substrate surface 34 is defined as α.

The single crystal substrate 32 used is preferably a single crystal material such as SrTiO ₃ , MgO, Al ₂ O ₃ , LaAlO ₃ , NdGaO ₃ , YAlO ₃ , LaSrAlO ₄ , or Si. The inclination angle a formed by the a-axis direction 33 of Sr _2-x La _x Nb ₂ O ₇ and the substrate surface 34 can be controlled by changing the inclination angle of the single crystal substrate 32 from the low index plane. The low index plane means (100), (010), (001), (110), (011), (101), (111) and equivalent planes. In the case of Al ₂ O ₃ , these are (0001), (11-20), (1-100), (10-12), (11-23), (10-11) and equivalent surfaces.

The inclination angle α formed by the a-axis direction 33 of Sr _2-x La _x Nb ₂ O ₇ and the substrate surface 34 is preferably between 10 ° and 80 °, more preferably 30 ° to 60 °. . This is because the electromotive voltage generated by the anisotropic thermoelectric effect is proportional to sin2α, and α is maximized at 45 °, as shown in equation (1).

The film thickness of the crystal orientation thin film 31 of Sr _2-x La _x Nb ₂ O ₇ is not particularly limited as long as the a-axis direction 33 maintains a constant angle α with respect to the substrate surface 34, In order to efficiently create a temperature difference in the film thickness direction of the Sr _2-x La _x Nb ₂ O ₇ crystal orientation thin film 31 by light absorption, it is preferably 30 nm or more.

When the crystal orientation thin film 31 of Sr _2-x La _x Nb ₂ O ₇ is produced by sputtering, an oxide in which the molar ratio of Sr, La and Nb is 2-x: x: 2 is targeted. Sputtering is preferably performed in pure argon or a mixed gas of argon and hydrogen. The pressure during sputtering is preferably in the range of 0.1 Pa to 10 Pa. The substrate temperature during the production of the thin film is preferably maintained between 400 ° C. and 900 ° C.

Above, an example of a method for manufacturing a Sr _2-x La _x Nb ₂ crystal orientation film 31 O _7, a direction 33 of the _{Sr 2-x La x Nb 2} O 7 is with respect to the substrate surface 34 A manufacturing method is not particularly limited as long as an inclined structure can be realized. In addition to sputtering, the thin film is formed using various methods such as vapor deposition such as vapor deposition, laser ablation, and chemical vapor deposition, or growth from a liquid phase or a solid phase. Things are possible.

(Embodiment 3)
In Embodiment 3, a radiation detector using the bulk single crystal described in Embodiment 1 is described.

A configuration of a radiation detector using Sr _2-x La _x Nb ₂ O ₇ manufactured by the method described in Embodiment 1 will be described with reference to FIGS. 4 and 5. FIG.

First, a plate-like crystal 41 obtained by crystal growth as shown in FIG. 4 is cleaved and cut with a blade to cut out an inclined crystal piece 42. At this time, as shown in FIG. 4, the a-axis of Sr _2−x La _x Nb ₂ O ₇ is cut out so as to be inclined at an angle α with respect to the longitudinal outer surface of the inclined crystal piece 42.

  Next, as shown in FIG. 5, the first electrode 51 and the second electrode 52 are arranged on both end faces on the short side of the inclined crystal piece 42 so as to face the longitudinal direction of the inclined crystal piece 42.

  Since the distance from the center position of the first electrode 51 to the center position of the second electrode 52 with respect to the thickness of the tilted crystal piece 42 corresponds to the aspect ratio l / d in equation (1), a large electromotive voltage is obtained. Therefore, it is preferable that the tilted crystal piece 42 is thin. Specifically, the thickness of the tilted crystal piece 42 is preferably 1 mm or less.

It is preferable to use a material having excellent conductivity for the first electrode and the second electrode. For example, metals such as Cu, Ag, Mo, W, Al, Ti, Cr, Au, Pt, In, or nitrides or oxides such as TiN, tin-added indium oxide (ITO), SnO ₂ may be used. Good. In addition, solder, silver solder, conductive paste, or the like is used as the electrode. Further, Al, Ti, Ta, or the like may be provided as an intermediate layer in order to reduce the contact resistance between the first electrode and the second electrode and Sr _2-x La _x Nb ₂ O ₇ .

A radiation detector can be configured by connecting the tilted crystal piece provided with the electrodes as described above to the radiator 53. At this time, the connection surface between the radiator 53 and the inclined crystal piece 42 has an angle of α with the a-axis of Sr _2−x La _x Nb ₂ O ₇ . It is preferable that the connection portion between the radiator 53 and the inclined crystal piece 42 is electrically insulated.

  The radiator 53 can be made of ceramic or single crystal made of electrically insulating alumina, aluminum nitride, boron nitride or the like. Further, copper, aluminum, stainless steel, titanium or the like coated with an insulator can also be used.

  When the incident light 54 is incident on the surface of the radiation detector manufactured in this way on the side where the radiator is not disposed, a temperature difference is caused in the thickness direction of the inclined crystal piece 42, and the first electrode is caused by the anisotropic thermoelectric effect. An electromotive voltage is generated between 51 and the second electrode 52. The incident light 54 can be detected by reading this electromotive voltage.

(Embodiment 4)
In the fourth embodiment, a radiation detector using the thin film single crystal described in the second embodiment will be described.

  The configuration of the radiation detector according to the present embodiment will be described with reference to FIG.

In the radiation detector of the present embodiment, a crystal orientation thin film 61 of Sr _2-x La _x Nb ₂ O ₇ manufactured by the method described in Embodiment 2 is disposed on a single crystal substrate 62, and tilted orientation is achieved. The first electrode 63 and the second electrode 64 are disposed on the thin film 61.

For the first electrode 63 and the second electrode 64, it is preferable to use a material having excellent conductivity. For example, metals such as Cu, Ag, Mo, W, Al, Ti, Cr, Au, Pt, In, or nitrides or oxides such as TiN, tin-added indium oxide (ITO), SnO ₂ may be used. Good. In addition, solder, silver solder, conductive paste, or the like may be used as the electrode. Further, Al, Ti, Ta, or the like may be provided as an intermediate layer in order to reduce the contact resistance between the first electrode and the second electrode and Sr _2-x La _x Nb ₂ O ₇ .

  Incident light 65 applied to the surface of the radiation detector manufactured in this way on the side of the tilted alignment thin film 61 is absorbed by the tilted alignment thin film 61, causing a temperature difference in the film thickness direction, and an anisotropic thermoelectric effect. Thus, an electromotive voltage is generated between the first electrode 63 and the second electrode 64. The incident light 65 can be detected by reading this electromotive voltage.

(Embodiment 5)
As shown in FIG. 7, a power generation device can be configured by connecting a heat collector 75 to the same configuration as that of the radiation detector described in the third embodiment.

  The radiator 74 and the heat collector 75 in the present embodiment may be thermally connected in a state of being electrically insulated at the joint surface with the inclined crystal piece 71, like the radiator 53 in the third embodiment. preferable.

  For the radiator 74 and the heat collector 75, electrically insulating alumina, aluminum nitride, boron nitride, or the like can be used. Further, copper, aluminum, stainless steel, titanium or the like coated with an insulator can also be used.

  In order to increase the aspect ratio, which is the distance from the end of the first electrode 72 to the end of the second electrode 73, with respect to the thickness of the inclined crystal piece 71, the inclined crystal piece 42 is preferably thin. Specifically, the thickness of the tilted crystal piece 42 is preferably 1 mm or less.

  On the other hand, during power generation, heat transfer between the heat source and the heat collection body 75 and heat transfer between the cooling medium and the heat dissipation body 74 are various, such as solid contact, contact by fluid, or contact with gas. Since the method is used, it is difficult to efficiently apply a temperature difference to the tilted crystal piece 71 when the thickness of the tilted crystal piece is too thin. Therefore, in order to efficiently generate power, the thickness of the tilted crystal piece 71 is preferably 10 μm or more.

(Embodiment 6)
In Embodiment 6, radiation detectors having different electrode arrangements in the material of the present invention will be described.

  The configuration of the radiation detector according to the present embodiment will be described with reference to FIGS. 8 (a) and 8 (b).

A single crystal of Sr _2-x La _x Nb ₂ O ₇ is prepared by the method described in Embodiment 1, and this is cleaved and cut with a blade to cut out a plate-like crystal piece 81. At this time, it is preferable that the widest surface of the crystal piece 81 is substantially perpendicular to the c-axis of Sr _2-x La _x Nb ₂ O ₇ . 8 (a) and 8 (b), the crystal piece 81 has a rectangular parallelepiped shape, but may be a flat plate having a surface substantially perpendicular to the c-axis. Various shapes may be used.

Next, an electrode is formed on the surface of the crystal piece 81. At this time, the direction in which the electrode 82a and the electrode 82b arranged so as to face each other with respect to the central portion of the crystal piece 81 or the direction in which the electrode 83a and the electrode 83b face each other is Sr _2-x La _x Nb ₂ O ₇ It is inclined with respect to the a-axis and b-axis (α in FIG. 8 (b)). The inclination angle is preferably 30 ° or more and 60 ° or less, and most preferably 45 °. This is because the electromotive voltage generated by the anisotropic thermoelectric effect is proportional to sin2α, and α is maximized at 45 °, as shown in equation (1).

The light to be detected is applied to the light irradiation region 84 near the center of the crystal piece 81, but the Seebeck coefficient and thermal conductivity of Sr _2-x La _x Nb ₂ O ₇ are anisotropic with respect to the a-axis and b-axis. Therefore, an anisotropic thermoelectric effect occurs in the ab plane. As a result, an electromotive voltage is generated between the opposing electrodes, and radiation detection can be performed.

It is preferable to use a material having excellent conductivity for the electrode. For example, metals such as Cu, Ag, Mo, W, Al, Ti, Cr, Au, Pt, In, or nitrides or oxides such as TiN, tin-added indium oxide (ITO), SnO ₂ may be used. Good. In addition, solder, silver solder, conductive paste, or the like is used as the electrode. Further, Al as the intermediate layer in order to reduce the contact resistance between the electrode and _{Sr 2-x La x Nb 2} O 7, Ti, or the like may be provided Ta.

  Hereinafter, more specific examples of the present invention will be described.

The thermoelectric conversion material Sr _2-x La _x Nb ₂ O ₇ of the present invention was produced by an infrared concentrated heating type floating zone method. First, strontium carbonate, dilanthanum trioxide and niobium pentoxide, which are raw materials in a stoichiometric ratio, are weighed and mixed so as to have a desired composition, and fired at 1400 ° C. for 20 hours in an argon stream. went. Next, the obtained powder was molded into a cylindrical rod shape and again fired at 1400 ° C. for 20 hours in an argon stream. Furthermore, using the obtained raw material rod, the melt was moved at 8 mm / Hr in an argon stream containing hydrogen (0.75%) by the floating zone method, 20 mm in the a-axis direction, 4 mm in the b-axis direction, and c-axis direction. A 1 mm crystal was prepared. A part of the obtained crystal was pulverized into a powder and subjected to structural evaluation by X-ray diffraction. As a result, a peak pattern as shown in FIG. 9 was observed in Sr _1.8 La _0.2 Nb ₂ O ₇ . Moreover, indexing was possible for all peaks, and it was confirmed that the sample was a single phase. Further, as shown in FIG. 10, the c-axis length changed in proportion to the lanthanum amount x according to the Vegard law. Therefore, it was found that a desired amount of La was doped. It was also found that samples with x = 0.5 or more contained impurities rather than a single-phase form.

The thermal and electrical conductivity characteristics of the samples prepared for the above method were evaluated. The 4-terminal method was used for measuring the electrical resistivity. Measurement was performed using a sample cut and molded by a precision cutting machine so that the distance was 4 mm in the a-axis and b-axis directions and 0.75 mm in the c-axis direction. A gold electrode using chromium as an adhesive layer was used as the electrode on the sample. This electrode was prepared using a sputtering method. A stationary method was used to measure the Seebeck coefficient and thermal conductivity. The measurement principle will be described with reference to FIG. A sample cut and molded by a precision cutting machine was used as the measurement sample 111. Gold electrodes 112 having aluminum as an adhesive layer were formed on the upper and lower surfaces of the measurement sample in order to extract electrical and thermal signals. Thereafter, the sample was fixed to the copper lead bar 114 using a silver paste 113 that hardened at room temperature. A heater 115 and a high temperature side thermometer 116 are connected to the upper lead bar 114, and a low temperature side thermometer 118 and a heat bath 119 are connected to the lower lead bar 117. A measuring jig provided so that the heat generated by the heater was absorbed by the lowest heat bath 119 was used. Specifically, the heat generated by the heater flows into the sample 111 through the upper lead bar 114, and is absorbed into the lower heat bath 119 by flowing into the lower lead bar 117 from the sample 111. A measuring jig was used. The temperature difference was constantly monitored by reading two different temperatures between the top and bottom of the sample with a thermometer attached to the lead bar. If the electromotive force and temperature difference near the thermometer are known, the Seebeck coefficient can be measured. Thermal conductivity is a physical quantity that is proportional to the cross-sectional area and temperature gradient and inversely proportional to the length of the direction in which heat flows. Therefore, a heat flow was generated using a heater, and the thermal conductivity was normalized by the cross-sectional area, length, and temperature difference of the sample between thermometers. When thermal radiation from the measurement jig and the sample through the surrounding gas increases, accurate measurement of thermal conductivity cannot be performed. Therefore, the measurement was performed in a vacuum of about 10 ⁻² Pa so that the heat quantity did not escape from the measurement jig / sample surface as much as possible.

Table 1 shows the characteristics measured for each sample in the a-axis direction at room temperature. There was a tendency for the electric resistivity and Seebeck coefficient to decrease monotonously as the amount of lanthanum increased. On the other hand, there was no significant change in thermal conductivity. The maximum conversion efficiency that can be estimated from these results was about 4.6 × 10 ⁻² . This value was about 200 times that of the material not doped with lanthanum.

Sr _1.8 La _0.2 Nb ₂ O ₇ was produced in the same manner as in Example 1. FIG. 12 shows the temperature dependence of the thermal conductivity κ, Seebeck coefficient S, and electrical resistivity ρ measured in the crystal axis a, b, and c axis directions at room temperature or lower. It was confirmed that all the characteristics showed large anisotropy depending on the crystal axis. It was confirmed that the Seebeck coefficient first revealed in the present example also showed large anisotropy. Specifically, it was found that it was about −210 μV / K in the a-axis direction near room temperature, -60 μV / K in the b-axis direction, and -15 μV / K in the c-axis direction. In other words, the anisotropy of the Seebeck coefficient is ΔS to 150 μV / K between the a-axis direction and the b-axis direction, and ΔS to 100 μV / K between the a-axis direction and the c-axis direction. There was found. Further, the thermal conductivity and electrical resistivity also showed large anisotropy similar to the Seebeck coefficient. From the temperature dependence of electrical resistivity and Seebeck coefficient, Sr _1.8 La _0.2 Nb ₂ O ₇ was found to have semiconducting properties. From the temperature dependence of thermal conductivity, it is considered that La doping disturbs the uniformity in the crystal and becomes the phonon scattering center responsible for thermal conduction.

A gradient oriented thin film of the thermoelectric conversion material Sr _1.8 La _0.2 Nb ₂ O ₇ of the present invention was produced by sputtering.

The substrate was a 10 mm square, 500 μm thick SrTiO ₃ single crystal, and the plane orientation was 10 ° inclined from the (001) plane in consideration of the crystallinity of the thin film. A thin film was formed on this substrate by RF magnetron sputtering using a 4-inch diameter Sr _1.8 La _0.2 Nb ₂ O ₇ sintered body target.

Pre-sputtering is performed for 1 hour at an output of 100 W with an atmosphere gas of 99% Ar and 1% H ₂ at 1.0 Pa, and then deposited on a substrate heated to 700 ° C. for 5 hours under the same conditions as pre-sputtering. As a result of cooling to room temperature over 2 hours, a thin film having a thickness of 1 μm was obtained.

It was confirmed by energy dispersive X-ray fluorescence analysis that the composition ratio in the thin film was almost the desired composition. In addition, pole figure measurement by X-ray diffraction revealed that the a-axis of Sr _1.8 La _0.2 Nb ₂ O ₇ was inclined about 12 ° with respect to the substrate surface.

Using the Sr _1.8 La _0.2 Nb ₂ O ₇ crystal produced in Example 2, a radiation detector was produced.

First, as shown in Fig. 4, by cutting the crystal of Sr _1.8 La _0.2 Nb ₂ O ₇ with a precision cutting machine, it is 4 mm × 0.5 mm in the ab plane, 1 mm in the c-axis direction, and the inclination angle α is 45 ° A crystal piece was obtained. Electrodes were prepared on the two sides of this inclined crystal piece of 0.5 mm × 1 mm by sputtering in the order of titanium and gold.

  Next, the tilted crystal piece was fixed on an aluminum nitride plate having a thickness of 500 μm using Apiezon grease, and a radiation detector as shown in FIG. 5 was produced.

  Electromagnetic waves generated from an infrared lamp (wavelength 800-2000 nm, power 480 mW) are incident on the center of the sample from the same direction as the incident light 54 in FIG. 5 (spot diameter 3 mm) on the surface of the radiation detector. The radiation detector was evaluated by measuring the electromotive voltage generated between the electrodes.

  When the electromagnetic wave from the lamp was not incident on the device, no electromotive voltage was generated between the electrodes, but when the electromagnetic wave was incident on the device, it rapidly increased and showed a steady value of about 3.6 mV. When the incidence of the lamp was stopped, no electromotive voltage was generated. From the above experimental results, the sensitivity of the radiation detector of this example was estimated to be 7.5 mV / W.

In the Ca _x CoO ₂ thin film disclosed in Non-Patent Document 1, it has been confirmed that an electromotive force of 110 mV is generated for an electromagnetic wave of 480 mW, and the sensitivity is 0.23 mV / W. Therefore, the radiation detector consisting of this Sr _2-x La _x Nb ₂ O ₇ element achieves a sensitivity improvement of about 32 times compared to the conventional radiation detector consisting of Ca _x CoO ₂ thin film element. That's right.

In Example 3, an electrode was provided on the surface of a Sr _1.8 La _0.2 Nb ₂ O ₇ thin film produced with a film thickness of 1 μm on a 10 mm square SrTiO ₃ single crystal substrate, and a radiation detector having the same configuration as that of FIG. 6 was produced. .

The two electrodes were produced by sputtering in the order of tantalum and platinum. The opposing direction of the two electrodes was arranged to be parallel to the direction when the a-axis of Sr _1.8 La _0.2 Nb ₂ O ₇ was projected onto the thin film surface as shown in FIG. The size of the electrode was 1 mm square, and the distance between the two electrodes was 6 mm.

  Electromagnetic waves generated from an infrared lamp (wavelength 800-2000 nm, power 480 mW) are incident on the center of the sample from the same direction as the incident light 65 in FIG. 6 (spot diameter 5 mm) on the surface of the radiation detector. The radiation detector was evaluated by measuring the electromotive voltage generated between the electrodes.

When the electromagnetic wave from the lamp was not incident on the radiation detector, no electromotive voltage was generated between the electrodes, but when the electromagnetic wave was incident, the electromotive voltage increased rapidly and showed a constant value of about 0.4 mV. . When the incidence of the lamp was stopped, no electromotive voltage was generated. From the above experimental results, it was estimated that the sensitivity of the radiation detector of this example was 0.83 mV / W. This was about three times as sensitive as the radiation detector composed of the Ca _x CoO ₂ thin film element of Non-Patent Document 1.

A power generation device was produced using the Sr _1.8 La _0.2 Nb ₂ O ₇ sample produced in Example 1.

First, as shown in Fig. 4, by cutting out Sr _1.8 La _0.2 Nb ₂ O ₇ crystal with a precision cutting machine, it is 4mm x 0.8mm in the ab plane, 1mm in the c-axis direction, tilt angle α is 10 ° A crystal piece was obtained. Electrodes were produced on two 1 mm square surfaces of the tilted crystal piece by sputtering in the order of titanium and gold.

  Using two copper heat sinks coated with 1 μm thick aluminum nitride, the inclined laminate was sandwiched and fixed with Apiezon grease to produce a power generation device as shown in FIG. A copper pipe is embedded inside the copper heat sink. The heat sink can be cooled by flowing cold water through the pipe, and the heat sink can be heated by flowing hot water through the pipe.

  By supplying 15 ° C cold water to one of the copper heat sinks and flowing 65 ° C hot water to the other, a temperature difference was created in the power generation device. As a result, a maximum power of 0.4 mW could be extracted from between the electrodes. This value is about 30 times that of a material not doped with lanthanum.

The Sr _1.8 La _0.2 Nb ₂ O ₇ sample produced in Example 1 was molded into a size of 6 mm (a axis) × 6 mm (b axis) × 1 mm (c axis). Two pairs of electrodes (4 in total) were prepared on the surface of this sample in the order of chromium and gold by sputtering. The prepared electrodes are arranged as shown in FIGS. 8 (a) and 8 (b), and the opposing direction of the electrode pair (for example, 82a and 82b) facing through the center of the sample is relative to the a axis on the sample surface. It is inclined 45 °. Thus, a radiation detector similar to that shown in FIGS. 8A and 8B was obtained. The distance between the centers of the two electrode placement positions was 5 mm. However, the distance between the electrode pairs can be optimized according to the application and installation location, and is not limited to 5 mm.

Laser light (wavelength 800 nm, power 225 mW) was incident on the center of the surface of the radiation detector (spot diameter 5 mm), and the electromotive force generated between the electrodes was measured. For either of the two pairs of electrodes, no electromotive force is observed when the laser beam is not incident on the sample, and when the laser beam is incident, the electromotive force increases rapidly and is constantly about 0.7 mV. showed that. When the sensitivity of the Sr _1.8 La _0.2 Nb ₂ O ₇ radiation detector was estimated based on this result, 3.1 mV / W was obtained. This was about 13 times more sensitive than the radiation detector consisting of the Ca _x CoO ₂ thin film element of Non-Patent Document 1.

  An anisotropic thermoelectric material according to the present invention has a large Seebeck coefficient anisotropy in a direction along a layered structure in which crystal growth is relatively easy, and a highly sensitive radiation detector and a power generation device having a high electromotive voltage It is useful as a material.

31 Crystalline orientation thin film 32 Single crystal substrate 33 A-axis direction 34 Substrate surface 41 Plate-like crystal 42 Inclined crystal piece 51 First electrode 52 Second electrode 53 Heat radiator 54 Incident light 61 Crystal orientation thin film 62 Single crystal substrate 63 First electrode 64 Second electrode 65 Incident light 71 Inclined crystal piece 72 First electrode 73 Second electrode 74 Radiator 75 Heat collector 81 Crystal piece 82a Electrode 82b Electrode 83a Electrode 83b Electrode 84 Light irradiation area 111 Measurement sample 112 Gold electrode 113 Silver paste 114 Upper lead bar 115 Heater 116 Upper thermometer 117 Lower lead bar 118 Lower thermometer 119 Heat bath

Claims (11)

thermoelectric material containing _{Sr 2-x La x Nb 2} O 7 the block layer made of NbO ₆ octahedra four layers in the c-axis direction has a crystal structure of the layered perovskite type which are stacked (x is 0.4 or less). The thermoelectric material according to claim 1, wherein the absolute value of the difference between the Seebeck coefficient in the a-axis direction and the Seebeck coefficient in the direction perpendicular to the a-axis is 100 µV / K or more. A substrate,
A thermoelectric material of Sr _2-x La _x Nb ₂ O ₇ (x is 0.4 or less) disposed on the substrate and having an a-axis direction inclined with respect to the substrate surface;
A thermoelectric device comprising: A radiator,
A thermoelectric material including Sr _2-x La _x Nb ₂ O ₇ (x is 0.4 or less) disposed on the radiator and having an a-axis direction inclined with respect to the substrate surface;
In a direction perpendicular to the c-axis direction of the thermoelectric material and in a direction parallel to the substrate surface, a first electrode and a second electrode disposed to face each other so as to be sandwiched between the thermoelectric materials;
A radiation detector comprising: The radiation detector according to claim 4, wherein an electromotive force is generated between the first electrode and the second electrode by an electromagnetic wave incident on the thermoelectric material. The radiation detector according to claim 4, wherein an average thickness of the thermoelectric material is 1 mm or less. a thermoelectric material containing Sr _2-x La _x Nb ₂ O ₇ (x is 0.4 or less) whose a-axis direction is inclined with respect to the substrate surface;
In a direction perpendicular to the c-axis direction of the thermoelectric material and in a direction parallel to the substrate surface, a first electrode and a second electrode disposed to face each other so as to be sandwiched between the thermoelectric materials;
A heat collector and a radiator disposed so as to be perpendicular to a direction in which the first electrode and the second electrode face each other and in a direction perpendicular to a c-axis direction of the thermoelectric material;
A power generation device comprising: The power generation device according to claim 7, wherein an average thickness of the thermoelectric material is 10 μm or more and 1 mm or less. A thermoelectric material containing Sr _2-x La _x Nb ₂ O ₇ (x is 0.4 or less);
The thermoelectric material is disposed in a direction facing each other through the center of the thermoelectric material, and the facing direction and the a-axis of the thermoelectric material are disposed at an angle between 30 degrees and 60 degrees. A first electrode and a second electrode;
A radiation detector comprising: A thermoelectric material including a substrate, Sr _2-x La _x Nb ₂ O ₇ (x is 0.4 or less) disposed on the substrate and having an a-axis direction inclined with respect to the substrate surface; and a radiation detector comprising: a first electrode and a second electrode disposed opposite to each other so as to be sandwiched between the thermoelectric materials in a direction perpendicular to a c-axis direction and parallel to the substrate surface; Make electromagnetic waves incident on the thermoelectric material,
A radiation detection method for detecting the electromagnetic wave by detecting an electromotive force between the first electrode and the second electrode. A thermoelectric material containing Sr _2-x La _x Nb ₂ O ₇ (x is 0.4 or less);
The thermoelectric material is disposed in a direction facing each other through the center of the thermoelectric material, and the facing direction and the a-axis of the thermoelectric material are disposed at an angle between 30 degrees and 60 degrees. Incident electromagnetic waves to the thermoelectric material of the radiation detector comprising the first electrode and the second electrode,
A radiation detection method for detecting the electromagnetic wave by detecting an electromotive force between the first electrode and the second electrode.