JP2011181725A - Anisotropic thermoelectric material, radiation detector using the same, and power generation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anisotropic thermoelectric material having an anisotropy having a large Seebeck coefficient and obtaining a large crystal along a surface having the anisotropy for achieving a high radiation detection sensibility and a high electric-generating voltage in power generation. <P>SOLUTION: The anisotropic thermoelectric material has a stratified perovskite type crystal structure shown by chemical formula Sr<SB>5</SB>Nb<SB>5</SB>O<SB>17</SB>and periodically laminating block layers consisting of NbO<SB>6</SB>octahedrons in five layers in the (c) axial direction, and has an absolute value of 40 μV/K or more in terms of a difference between the Seebeck coefficients in the (a) axial direction and the vertical direction to the (a) axis. The high radiation detection sensibility and the high electric-generating voltage in the power generation are achieved by such an anisotropic thermoelectric material. <P>COPYRIGHT: (C)2011,JPO&INPIT

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, and the quality of the thermoelectric material is determined by combining these.

In general, in a thermoelectric material exhibiting 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 (1).

  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 aspect ratio l / d of the sample, and the sine value sin2α that is twice the stacking inclination angle from the equation (1).

Using 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, Sr ₅ Nb ₅ O ₁₇ is disclosed as a material that is a layered substance and has anisotropy with respect to different directions in the layer parallel direction (Non-patent Document 4). As shown in FIG. 2, Sr ₅ Nb ₅ O ₁₇ has a layered perovskite structure in which block layers composed of five NbO ₆ octahedrons are periodically stacked in the c-axis direction. According to Non-Patent Document 4, the electrical resistivity of Sr ₅ Nb ₅ O ₁₇ is 100 times different in the b-axis direction and 10000 times different in the c-axis direction compared to the a-axis direction, and is in the layer parallel direction There is a large anisotropy in terms of electrical resistivity between the a-axis direction and the b-axis direction. A single crystal of Sr ₅ Nb ₅ O ₁₇ can be produced by a general method such as a flux method or a floating zone method with a size of several mm in the layer parallel direction and about 1 mm in the layer vertical direction.

So far, the anisotropy of the Seebeck coefficient of Sr ₅ Nb ₅ O ₁₇ has not been disclosed. 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 and the layer vertical direction is several tens of times at room temperature (Non-patent Document 5).

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 that the anisotropy ΔS of the Seebeck coefficient and the device aspect ratio l / d be large. 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 conventional problem, the present inventors have conducted research on the anisotropic thermoelectric properties of thermoelectric materials, and as a result, in crystals of materials having a layered perovskite structure represented by the chemical formula Sr ₅ Nb ₅ O ₁₇ , We found 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 b-axis direction, which are the layer parallel directions where crystal growth is easy. Based on this, the present invention has been reached.

That is, the anisotropic thermoelectric material of the present invention is represented by the chemical formula Sr ₅ Nb ₅ O ₁₇ , and is a layered perovskite crystal in which block layers made of five NbO ₆ octahedrons are periodically stacked in the c-axis direction It has a 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 40 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. Therefore, by using a general crystal growth method, ΔS is 40 μV / K or more and the device thickness is 1 mm or less, a highly sensitive radiation detector and ΔS is 40 μV / K or more, and the device thickness is 10 μm or more and 1 mm or less. A high-performance power generation device can be realized.

Illustration of anisotropic thermoelectric effect Illustrates the crystal structure of the anisotropic thermal conductive material Sr ₅ Nb ₅ O ₁₇ of the present invention It illustrates the structure of the crystal orientation film of Sr ₅ Nb ₅ O ₁₇ on a single crystal substrate according to 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. It shows the X-ray diffraction pattern of Sr ₅ Nb ₅ O ₁₇ according to Example 1 The thermal conductivity κ of Example 1 Sr ₅ Nb ₅ O ₁₇ according a diagram showing temperature dependence of the Seebeck coefficient S, electric resistivity ρ

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

(Embodiment 1)
The anisotropic thermoelectric material of the present invention is an oxide material having a crystal structure as shown in FIG. 2 represented by the chemical formula Sr ₅ Nb ₅ O ₁₇ or SrNbO _3.4 , and is composed of a single crystal or a crystal-oriented sintered body. Used in bulk or thin film form. The directions of the a, b, and c axes of the Sr ₅ Nb ₅ O ₁₇ crystal are as described in FIG.

In this embodiment mode, a method for manufacturing a single crystal bulk of the anisotropic thermoelectric material Sr ₅ Nb ₅ O ₁₇ of the present invention will be described. The Sr ₅ Nb ₅ O ₁₇ single crystal is obtained by melting an oxide containing strontium and niobium and performing crystal growth. At this time, strontium carbonate (SrCO ₃ ), niobium pentoxide (Nb ₂ O ₅ ), and the like can be used as raw materials. These raw materials are weighed and mixed so that the molar ratio of niobium to strontium is 1, and are fired to be used as starting materials 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.

  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 heating 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. For firing in this process, it is desirable to perform firing immediately below the melting point in the vicinity of 1300-1500 ° C., and it is desirable to obtain sufficient strength, and the firing atmosphere is an inert atmosphere gas such as argon or later in the later process. In order to obtain a desired structure in the 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, crystals grow by moving the rod-shaped starting material at a speed at which the melt moves stably. Specifically, it is desirable that the rod-shaped starting material moves downward at about 5 to 15 mm / Hr.

Above, an example of a method for manufacturing a single crystal of Sr ₅ Nb ₅ O _17, in fabricating the Sr ₅ Nb ₅ O ₁₇ of the present invention, the method is not of course limited to the above.

(Embodiment 2)
A method for producing the Sr ₅ Nb ₅ O ₁₇ crystal orientation thin film 31 of the present invention will be described with reference to FIG. The Sr ₅ Nb ₅ O ₁₇ thin film in the present embodiment is formed on the single crystal substrate 32.

Although details will be described in the examples described later, since the Seebeck coefficient in the b-axis and c-axis direction shows large anisotropy with respect to the Seebeck coefficient in the a-axis direction of Sr ₅ Nb ₅ O ₁₇ , In order to increase the electromotive voltage, it is preferable to perform crystal growth so that the a-axis direction 33 of the crystal orientation thin film 31 of Sr ₅ Nb ₅ O ₁₇ is inclined with respect to the substrate surface 34 as shown in FIG. Hereinafter, an inclination angle formed between the a-axis of Sr ₅ Nb ₅ O ₁₇ and the substrate surface 34 is 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 ₅ Nb ₅ O ₁₇ 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 , the low index plane is (0001), (11-20), (1-100), (10-12), (11-23), (10-11) and their equivalents. Surface.

The inclination angle α formed between the a-axis direction 33 of Sr ₅ Nb ₅ O ₁₇ and the substrate surface 34 is preferably between 10 ° and 80 °, and more preferably 30 ° to 60 °. This is because the electromotive voltage generated by the anisotropic thermoelectric effect is proportional to sin2α and becomes maximum at 45 °, as shown in equation (1).

The film thickness of the crystal orientation thin film 31 of Sr ₅ Nb ₅ O ₁₇ 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 set the temperature difference in the film thickness direction of the crystal orientation thin film 31 of Sr ₅ Nb ₅ O ₁₇ , it is preferably 30 nm or more.

When the crystal orientation thin film 31 of Sr ₅ Nb ₅ O ₁₇ is produced by sputtering, an oxide whose molar ratio of Sr and Nb is 1: 1 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 thin film production is preferably maintained between 400 ° C. and 800 ° C.

Above, Sr ₅ Nb ₅ showed an example of a method for manufacturing a crystal orientation film 31 O _17, a manufacturing method as long realized the structure a-axis direction 33 of Sr ₅ Nb ₅ O ₁₇ is inclined with respect to the substrate surface 34 Is not particularly limited. In addition to the sputtering method, various methods such as vapor deposition such as vapor deposition, laser ablation, and chemical vapor deposition, and growth from a liquid phase and a solid phase are possible.

(Embodiment 3)
A configuration of a radiation detector using Sr ₅ Nb ₅ O ₁₇ 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 into inclined crystal pieces 42. At this time, as shown in FIG. 4, the a-axis of Sr ₅ Nb ₅ O ₁₇ is set to have an angle α with respect to the longitudinal outer surface of the tilted crystal piece 42.

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

  Since the distance between the first electrode 51 and the second electrode 52 divided by the thickness of the tilted crystal piece 42 corresponds to the aspect ratio l / d in the equation (1), in order to obtain a large electromotive voltage, the tilted crystal piece The thickness of 42 is preferably 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 contact resistance between the first electrode and the second electrode and Sr ₅ Nb ₅ O ₁₇ .

A radiation detector can be configured by connecting the heat dissipating body 53 to the tilted crystal piece provided with the electrodes as described above. At this time, the connection surface between the radiator 53 and the tilted crystal piece 42 is a surface having an angle α with the a-axis of Sr ₅ Nb ₅ O ₁₇ . 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)
The configuration of the radiation detector according to the present embodiment will be described with reference to FIG.

The radiation detector of the present embodiment includes a first electrode 63 and a second electrode on the crystal orientation thin film 61 of Sr ₅ Nb ₅ O ₁₇ manufactured on the single crystal substrate 62 by the method described in the second embodiment. It can be configured by producing the electrode 64.

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 is used as the electrode. Further, Al, Ti, Ta, or the like may be provided as an intermediate layer in order to reduce contact resistance between the first electrode and the second electrode and Sr ₅ Nb ₅ O ₁₇ .

  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 obtained by dividing the distance between the first electrode 72 and the second electrode 73 by 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 can be performed by various methods such as solid contact, contact by fluid, or contact with gas. Therefore, when the thickness of the tilted crystal piece is too thin, it becomes difficult to efficiently add a temperature difference to the tilted crystal piece 71. Therefore, in order to efficiently generate power, the thickness of the tilted crystal piece 71 is preferably 10 μm or more.

(Embodiment 6)
The configuration of the radiation detector according to the present embodiment will be described with reference to FIG.

A single crystal of Sr ₅ Nb ₅ O ₁₇ is prepared by the same method as 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, the widest surface of the crystal piece 81 is preferably substantially perpendicular to the c-axis of Sr ₅ Nb ₅ O ₁₇ . Further, in FIG. 8, the crystal piece 81 has a rectangular parallelepiped shape, but may be a flat plate having a surface substantially perpendicular to the c-axis, so it is not limited to a rectangular parallelepiped and may have various shapes such as a circular plate and a polygonal plate. be able to.

Next, an electrode is formed on the surface of the crystal piece 81. At this time, the facing direction of the electrode 82a and the electrode 82b or the electrode 83a and the electrode 83b arranged to face each other with respect to the central portion of the crystal piece 81 is inclined with respect to the a axis and the b axis of Sr ₅ Nb ₅ O _17. ing. 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 becomes maximum at 45 °, as shown in equation (1).

The light to be detected is irradiated to the light irradiation region 84 near the center of the crystal piece 81, but the Seebeck coefficient and thermal conductivity of Sr ₅ Nb ₅ O ₁₇ are anisotropic with respect to the a axis and the b axis. 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, Ti, Ta, or the like may be provided as an intermediate layer in order to reduce the contact resistance between the electrode and Sr ₅ Nb ₅ O ₁₇ .

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

The thermoelectric conversion material Sr ₅ Nb ₅ O ₁₇ of the present invention was produced by an infrared concentrated heating type floating zone method. First, strontium carbonate and niobium pentoxide as raw materials in a stoichiometric ratio were weighed and mixed so as to have a ratio of 1: 1, and baked at 1400 ° C. for 20 hours in an argon stream. Next, the obtained powder was molded into a cylindrical bar shape and fired again at 1400 ° C. for 20 hours in an argon stream. Next, using the obtained raw material rod, the melt was moved at 8 mm / Hr in an argon stream containing hydrogen (3%) 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 of Sr ₅ Nb ₅ O ₁₇ as shown in FIG. 9 was observed. Moreover, indexing was possible for all peaks, and it was confirmed that the sample was a single phase.

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. For the molding of the sample, a precision cutting machine was used to cut it to 4 mm in the a-axis and b-axis directions and to 0.75 mm in the c-axis direction and measured. A gold electrode using chromium as an adhesive layer was selected as the electrode on the sample, and was prepared using a sputtering method. A stationary method was used to measure the Seebeck coefficient and thermal conductivity. A sample cut and molded by a precision cutting machine was used as a measurement sample, and gold electrodes having aluminum as an adhesive layer were formed on the top and bottom surfaces of the measurement sample in order to take out electrical and thermal signals. Thereafter, the sample was fixed on a copper flat plate with a silver paste that hardened at room temperature. A heater and a high temperature side thermometer are connected to the upper terminal, and a heat bath and a low temperature side thermometer are connected to the lower terminal. The measuring jig is configured such that heat generated by the heater flows into the sample through the uppermost lead bar and is similarly absorbed by the lowermost heat bath. Two different temperatures between the top and bottom of the sample can be read with a thermometer attached to a flat plate, and the temperature difference can be monitored constantly. In order to measure the Seebeck coefficient, if the electromotive force and the temperature difference near the thermometer are known, the measurement can be performed. 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, the heat conductivity is normalized by the cross-sectional area, length, and temperature difference of the sample between thermometers by generating heat flow using a heater. In the measurement of thermal conductivity, accurate measurement cannot be performed if thermal radiation from the measurement jig and the sample through the surrounding gas increases. Therefore, the heat quantity was measured in a vacuum of about 10 ^-2 Pa so as not to escape from the measuring jig / sample surface as much as possible.

  FIG. 10 shows the temperature dependence of the measured thermal conductivity κ, Seebeck coefficient S, and electrical resistivity ρ below room temperature. It was confirmed that all the characteristics showed large anisotropy depending on the crystal axis. The temperature dependence of the electrical resistivity is qualitatively similar to the data disclosed in Non-Patent Document 4. The Seebeck coefficient first revealed in the present example also shows a large anisotropy, which is about -10 μV / K in the a-axis direction near room temperature, while -55 μV / K in the b-axis direction and -70 μV in the c-axis direction. Turned out to be / K. That is, the anisotropy of the Seebeck coefficient was as large as ΔS˜45 μV / K between the a-axis direction and the b-axis direction and ΔS˜60 μV / K between the a-axis direction and the c-axis direction.

A crystal orientation thin film of the thermoelectric conversion material Sr ₅ Nb ₅ O ₁₇ of the present invention was produced by sputtering.

The substrate was a 10 mm square, 500 μm thick SrTiO ₃ single crystal with a plane orientation inclined 10 ° from the (001) plane. A thin film was formed on this substrate by RF magnetron sputtering using a Sr ₅ Nb ₅ O ₁₇ sintered compact target having a diameter of 4 inches.

Pre-sputtering is performed for 1 hour at an output of 100 W with an atmosphere gas of 97% Ar and 3% H ₂ , 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 of Sr and Nb in the thin film was approximately 1: 1. Further, pole figure measurement by X-ray diffraction revealed that the a-axis of Sr ₅ Nb ₅ O ₁₇ was inclined by about 12 ° with respect to the substrate surface.

A radiation detector was produced using the Sr ₅ Nb ₅ O ₁₇ crystal produced in Example 1.

First, a crystal of Sr ₅ Nb ₅ O ₁₇ was cut out with a precision cutting machine as shown in FIG. 4 to obtain an inclined crystal piece having an in-plane of 4 mm × 0.5 mm, 1 mm in the c-axis direction, and an inclination angle α of 45 °. . 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 constant value of about 1.8 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 3.8 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, comparing the radiation detector consisting of this Sr ₅ Nb ₅ O ₁₇ element with the conventional radiation detector consisting of Ca _x CoO ₂ thin film element, the radiation detection consisting of this Sr ₅ Nb ₅ O ₁₇ element This means that the sensitivity has been improved by about 16 times.

In Example 2, an electrode was provided on the surface of the Sr ₅ Nb ₅ O ₁₇ thin film formed to a thickness of 1 μm on the 10 mm square SrTiO ₃ single crystal substrate, and a radiation detector having the same configuration as FIG. 6 was manufactured.

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 ₅ Nb ₅ O ₁₇ 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.

  On the surface of the radiation detector, 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). ), 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 0.17 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 0.35 mV / W. This was about 1.5 times more sensitive than the radiation detector consisting of the Ca _x CoO ₂ thin film element of Non-Patent Document 1.

A power generation device was produced using the Sr ₅ Nb ₅ O ₁₇ sample produced in Example 1.

First, the Sr ₅ Nb ₅ O ₁₇ crystal was cut out with a precision cutting machine as shown in FIG. 4 to obtain a tilted crystal piece having a size of 4 mm × 0.8 mm in the ab plane, 1 mm in the c-axis direction, and a tilt angle α of 10 °. 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, and 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. .

  When a temperature difference was created in the power generation device by flowing cold water of 15 ° C on one side of the copper heat sink and hot water of 65 ° C on the other side, power of up to 1.8mW could be extracted from between the electrodes.

The Sr ₅ Nb ₅ O ₁₇ 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 produced electrodes are arranged as shown in FIG. 8, and the facing direction of the opposed electrode pair with respect to the center of the sample is inclined by 45 ° with respect to the a axis on the sample surface. In this way, a radiation detector similar to that shown in FIG. 8 was obtained. The distance between the electrode pair 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. In both cases, the electromotive force is not observed when the laser beam is not incident on the sample, but the electromotive force increases rapidly when the laser beam is incident. Indicated. Based on this result, when the sensitivity of the Sr ₅ Nb ₅ O ₁₇ radiation detection element is estimated, 1.8 mV / W is obtained. This was about 7.8 times the sensitivity of the radiation detector composed 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 Crystal Oriented Thin Film 32 Single Crystal Substrate 33 a-axis Direction 34 Substrate Surface 41v Plate-like Crystal 42 Tilted Crystal Piece 51 First Electrode 52 Second Electrode 53 Radiator 54 Incident Light 61 Crystal Oriented 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 region

Claims (6)

It is represented by the chemical formula Sr ₅ Nb ₅ O ₁₇
It has a layered perovskite type crystal structure in which block layers consisting of five layers of NbO ₆ octahedrons are periodically stacked in the c-axis direction,
An anisotropic thermoelectric material characterized in that 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 40 μV / K or more. A substrate,
A first electrode and a second electrode;
The anisotropic thermoelectric material according to claim 1 disposed on the substrate,
The a-axis direction of the anisotropic thermoelectric material is inclined with respect to the substrate surface,
The first electrode and the second electrode are arranged to face each other along the direction when the a-axis is projected onto the substrate surface,
The radiation detector, wherein the first electrode and the second electrode are electrically connected so as to sandwich the anisotropic thermoelectric material. The average thickness of the anisotropic thermoelectric material in the normal direction of the substrate is 1 mm or less,
The radiation detector according to claim 2. A heat collector and a radiator,
A first electrode and a second electrode;
The anisotropic thermoelectric material according to claim 1, which is thermally connected so as to be sandwiched between the heat collector and the heat radiating body,
The a-axis direction of the anisotropic thermoelectric material is inclined with respect to a connection surface between the anisotropic thermoelectric material and the heat collector or the heat radiator,
The first electrode and the second electrode are disposed to face each other along the direction when the a-axis is projected onto the connection surface with the heat collector or heat radiator,
The power generation device, wherein the first electrode and the second electrode are electrically connected so as to sandwich the anisotropic thermoelectric material. 5. The average thickness of the anisotropic thermoelectric material in a direction normal to a connection surface between the anisotropic thermoelectric material and the heat collector or the heat radiating body is 10 μm or more and 1 mm or less. The described power generation device. The anisotropic thermoelectric material according to claim 1 having a plate shape,
Consisting of one or more electrode pairs disposed on the surface of the anisotropic thermoelectric material,
The normal line of the flat anisotropic thermoelectric material and the c-axis direction of the anisotropic thermoelectric material are substantially parallel,
A radiation detector, wherein the opposing direction of the electrode pair is inclined with respect to the a-axis of the anisotropic thermoelectric material.

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