JP2018160541A - Thermoelectric conversion element - Google Patents

Abstract

An object of the present invention is to provide a highly reliable device. The element L (represents one or more elements selected from the group consisting of In, Yb, Eu, Ce, La, Nd, Ga and Sr), the element M (Co, Rh, Ir, Fe, Ni, representing one or more elements selected from the group consisting of Pt, Pd, Ru and Os), and the element Pn (representing one or more elements selected from the group consisting of Sb, As, P, Te, Sn, Bi, Ge, Se and Si) a thermoelectric conversion member composed of a skutterudite-type material containing one or more elements), and an L layer containing the element L and an oxygen element disposed on at least one of a top surface and a bottom surface of the thermoelectric conversion member. and an adhesion layer containing an element Q (representing one or more elements selected from the group consisting of Ti, Mo, Co, and Ta) arranged on the L layer, wherein the average oxygen concentration of the L layer is higher than the average oxygen concentration of the adhesion layer, the thermoelectric conversion element. [Selection drawing] Fig. 1

Description

  The present disclosure relates to a thermoelectric conversion element and a manufacturing method thereof.

  Thermoelectric conversion materials and thermoelectric conversion modules using the same are used in cooling or power generation applications as devices that convert heat and power. For example, when a direct current is passed through the thermoelectric conversion material, heat transfer occurs from one surface to the other surface, and an endothermic surface and a heat generating surface are generated. This phenomenon is called the Peltier effect, and the object can be cooled without providing a movable part by modularizing the thermoelectric conversion material and bringing the endothermic surface into contact with the object to be cooled. On the other hand, when a temperature difference is given to both ends of the thermoelectric conversion material, an electromotive voltage proportional to the temperature difference is generated. This phenomenon is called the Seebeck effect. For example, it is possible to convert thermal energy into electrical energy by bringing one surface of the module into contact with a target that is waste heat of excess heat energy and cooling the other surface by air cooling or water cooling. That is, waste heat energy can be recovered as electric energy. Thermoelectric conversion modules using the Seebeck effect have been attracting attention as power generation devices in recent years, and development has become active as a new method for using thermoelectric conversion modules.

  A bismuth-tellurium-based material is most known as a material that effectively generates the thermoelectric conversion phenomenon. Modules using bismuth-tellurium-based materials have already been put into practical use for cooling applications utilizing the Peltier effect, and are also used for laser diode temperature adjustment applications for optical communications. For this reason, the use of bismuth-tellurium-based materials for power generation is also being studied. However, since the power generation efficiency of the thermoelectric conversion material (bismuth-tellurium-based material) has temperature dependence, it has not been widely used in power generation applications.

This point will be described in detail. As a physical property value representing the characteristics of the thermoelectric conversion material, there is a Seebeck coefficient S (unit: V / K). This is a numerical value that represents the magnitude of the electromotive voltage associated with the temperature difference, and is a numerical value that represents the voltage per unit temperature difference. This Seebeck coefficient takes a positive or negative value depending on the thermoelectric conversion material. This is determined depending on whether the carrier in the thermoelectric conversion material is a hole or an electron. When the Seebeck coefficient is a positive value, it is called P-type, and when it is negative, it is called N-type. Is common. Another physical property value representing the physical properties of the thermoelectric conversion material is electrical resistivity ρ (unit: Ω · m). When an electromotive voltage associated with the Seebeck effect is generated, electricity flows through the thermoelectric conversion material, but the electric power that can be extracted in the power generation application is proportional to the product of this voltage and current. Therefore, when the electrical resistivity is low, the electric power that can be extracted increases. That is, the two physical property values directly affect the power generation capacity of the thermoelectric conversion material, and the power factor PF (unit: W / mK ² ) (hereinafter, referred to as “equation (1)”)
It is simply expressed as a numerical value “PF”).

  Further, although it is not a physical property value that directly affects power generation, the thermal conductivity κ (unit: W / m · K) is also a value that represents the characteristics of the thermoelectric conversion material. This is because when the Seebeck effect is caused with respect to a certain thermal energy, if the thermal conductivity of the thermoelectric conversion material is too large, a temperature difference in the material hardly occurs. For this reason, a material having a low thermal conductivity can increase the temperature difference, and as a result, the amount of power generation can be increased. As an index combining the Seebeck coefficient S, the electrical resistivity ρ, and the thermal conductivity κ, there is a dimensionless figure of merit ZT represented by the following formula (2).

  The reason why the absolute temperature T (K) is included in the dimensionless figure of merit ZT is that each numerical value has temperature dependency. However, as described above, since PF represents the power generation amount itself, ZT is used as a standard representing the thermoelectric conversion performance. That is, when the thermal conductivity κ is extremely small, ZT may show a large value, but the power generation amount does not increase unless PF is also large at the same time. For example, bismuth-tellurium-based materials have the highest PF around room temperature, and tend to decrease as the temperature increases. Bismuth-tellurium type materials are therefore not suitable for use at high temperatures.

  Here, when a large electric power is to be obtained using the thermoelectric conversion material, it is necessary to make a large temperature difference. In recent years, attempts have been made to convert heat from about 300 ° C. discharged from prime movers such as plants and automobiles into electricity for effective use. However, in the bismuth-tellurium-based material, as described above, the PF decreases as the temperature rises against the aim of increasing the temperature difference and increasing the power generation amount. In other words, it is difficult to increase the amount of power generation due to its temperature dependence, and it is essential to study new materials.

  In order to use the thermoelectric conversion member in a region exceeding 300 ° C., there is a problem that it is difficult to maintain not only the power generation performance but also the quality of the thermoelectric conversion module. That is, the higher the temperature is, the more the element material diffuses into the bonding layer that bonds the thermoelectric conversion element and the electrode. In the part where the diffusion of the element material in the bonding layer proceeds, the mechanical strength decreases and the electrical resistivity increases, leading to a decrease in power generation efficiency as a whole module.

  Here, Patent Document 1 relates to a thermoelectric conversion module. This document shows that the diffusion and oxidation of a metal element on a joint surface between a thermoelectric conversion element (thermoelectric conversion member) and an electrode are suppressed by disposing a diffusion prevention layer. FIG. 6 shows a schematic cross-sectional view of the thermoelectric conversion element of the thermoelectric conversion module disclosed in Patent Document 1 and its joint. As shown in FIG. 6, in the thermoelectric conversion module, the P-type thermoelectric conversion element 101 and the N-type thermoelectric conversion element 102 are joined to the high temperature side electrode 108 covered with the electrode protective layer 107 via the solder layer 106. Yes. Further, a first diffusion prevention layer 103 and a second diffusion prevention layer 104 for preventing metal diffusion are disposed between the thermoelectric conversion elements 101 and 102 and the solder layer 106, and the solder layer 106. A solder bonding layer 105 for bonding to is also disposed. Patent Document 1 describes that by adopting such a configuration, metal diffusion and oxidation at the joint portion of thermoelectric conversion elements 101 and 102 can be suppressed.

International Publication No. 2015/030218

  However, the technique of Patent Document 1 has the following problems. In Patent Document 1, a bismuth-tellurium-based material is used as a material constituting the thermoelectric conversion element, and the use temperature range is about 250 to 300 ° C. In recent years, it has been studied to use a cobalt-antimony material as a constituent material of a thermoelectric conversion element. Cobalt-antimony materials exhibit high performance in a temperature range of 300 ° C. or higher. However, one of the constituent elements, Sb, has a feature that the diffusion coefficient is very high. Furthermore, metal diffusion generally proceeds faster as the ambient temperature is higher. That is, in the thermoelectric conversion module of Patent Document 1 and the thermoelectric conversion module that has been studied in the past, the power generation efficiency in a high temperature region of 300 ° C. or higher is not sufficient, or diffusion prevention of element material is not sufficiently considered, Thermoelectric conversion characteristics could not be improved due to metal diffusion of the element material into the bonding layer with the electrode.

  The present disclosure has been made in view of the above-described problems, and provides a highly reliable thermoelectric conversion element that suppresses a decrease in power generation efficiency of a module due to metal diffusion of an element material into a bonding layer at a high temperature. The purpose is to do.

  In order to achieve the above object, a thermoelectric conversion element of the present disclosure has the following configuration. Element L (represents one or more elements selected from the group consisting of In, Yb, Eu, Ce, La, Nd, Ga and Sr), element M (Co, Rh, Ir, Fe, Ni, Pt, Pd, One or more elements selected from the group consisting of Ru and Os), and one or more elements selected from the group consisting of elements Pn (Sb, As, P, Te, Sn, Bi, Ge, Se and Si) And an L layer containing the element L and oxygen disposed on at least one of the top and bottom surfaces of the thermoelectric conversion member, and the L An adhesion layer including an element Q (representing one or more elements selected from the group consisting of Ti, Mo, Co, and Ta) disposed on the layer, and the average oxygen concentration of the L layer is It is larger than the average oxygen concentration of the adhesion layer.

  As described above, the present disclosure provides a thermoelectric conversion element that can retain thermoelectric conversion characteristics even when used at high temperatures.

Schematic sectional view of a thermoelectric conversion element according to an embodiment of the present disclosure 2A is an electron microscopic image of a cross section of an element according to an embodiment of the present disclosure, FIG. 2B is an elemental analysis spectrum of an adhesion layer, FIG. 2C is an elemental analysis spectrum of a metal layer, and FIG. 2D is an elemental analysis of a thermoelectric conversion member. Spectrum, FIG. 2E is the atomic number concentration at each point in FIG. 2A FIG. 3A is a schematic diagram illustrating a process of melting the thermoelectric conversion material in the manufacturing method according to the embodiment of the present disclosure, and FIG. 3B illustrates a process of filling the insulator with the molten thermoelectric conversion material in the manufacturing method. FIG. 3C is a schematic diagram illustrating a step of cutting the insulator and the thermoelectric conversion member in the manufacturing method, and FIG. 3D is a schematic diagram illustrating a step of forming an adhesion layer on the thermoelectric conversion member in the manufacturing method. Schematic sectional view of a thermoelectric conversion module according to an embodiment of the present disclosure 5A is an electron microscope image of a cross section after a durability test of an element in which the metal layer of the embodiment of the present disclosure does not exist, FIG. 5B is an atomic number concentration at each point in FIG. 5A, and FIG. 5C is an embodiment of the present disclosure. 5D is an electron microscopic image of a cross-section after an endurance test of an element having a metal layer of FIG. Schematic sectional view of a conventional thermoelectric conversion element

  Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

(Thermoelectric conversion element)
FIG. 1 is a schematic cross-sectional view showing a thermoelectric conversion element 1 according to an embodiment of the present disclosure. In the present embodiment, among the thermoelectric conversion members, both end surfaces that are bonded to the electrode via an adhesion layer, a bonding member, or the like are referred to as an upper surface and a bottom surface, respectively. Moreover, the outer periphery of the area | region pinched | interposed into the upper surface and the bottom face is called a side surface. As shown in FIG. 1, the thermoelectric conversion element 1 of the present embodiment includes a thermoelectric conversion member 2, an insulator 4 that covers the side surface of the thermoelectric conversion member 2, and an upper surface and a lower surface of a cylindrical thermoelectric conversion member 2. The metal layer 3 is disposed, and the adhesion layer 5 is disposed outside the metal layer 3. Even if the insulator 4 is not provided, it is possible to obtain the effect of preventing diffusion. However, it is better to provide the insulator 4 from the viewpoint of suppressing oxidation deterioration. In the present embodiment, the thermoelectric conversion member 2 has a cylindrical shape and the insulator 4 covers the side surfaces thereof, but the shapes of the thermoelectric conversion member 2 and the insulator 4 are not particularly limited. For example, the thermoelectric conversion member 2 can have an arbitrary shape such as a prismatic shape. However, regardless of the shape of the thermoelectric conversion member 2, the surface other than the top surface and the bottom surface of the thermoelectric conversion member 2 on which the adhesion layer 5 is disposed is covered with the insulator 4. It is preferable from the viewpoint of suppressing oxidation deterioration of the member 2.

Here, the thermoelectric conversion member 2 is composed of a skutterudite-type material containing a specific element. Hereinafter, the skutterudite type material will be described. As a thermoelectric conversion material suitable for a temperature range exceeding 300 ° C., there is a material having a crystal structure called a skutterudite type. The skutterudite-type material includes a group VIII element M and a Pn selected from a group IVB element, a group VB element, and a group VIB element in the short periodic table, and has a composition represented by the general formula M ₄ Pn ₁₂ It is a crystalline solid solution. Examples of M in the above general formula include elements such as Co, Rh, Ir, Fe, Ni, Pt, Pd, Ru, and Os. Examples of Pn in the above general formula include Sb, As, P, Te, Sn, Bi. , Ge, Se, and Si.

Further, the crystal lattice of the skutterudite type material has a vacancy at a rate of one per M ₄ Pn ₁₂ , and a rare earth element such as La, Ce, or Yb is present in all or part of the vacancy. Or, alkaline earth elements such as Ba and Ca, and earth metal elements such as Tl, In, and Sn can be filled. These are called filled skutterudites and are included in skutterudite-type materials. The filled skutterudite type material is represented by a general formula L _x M ₄ Pn ₁₂ material (L _x represents the element introduced into the vacancies, and 0 <x ≦ 1).

Conventionally, as a skutterudite-type thermoelectric conversion material, for example, a cobalt-antimony-based material is known, and the composition formula of the material is represented by Co ₄ Sb ₁₂ . Co ₄ Sb ₁₂ alone is an N-type thermoelectric conversion material and exhibits a good Seebeck coefficient. However, the electrical resistivity is as high as about 1 × 10 ⁻⁴ at room temperature, and the thermal conductivity is as high as about 10 W / mK at room temperature. For this reason, both PF and ZT, which are indexes indicating the power generation capability of the thermoelectric conversion material, are low. However, as described above, the crystal has a relatively large void in the crystal lattice, and the thermoelectric conversion characteristics are improved by adding other elements (filled skutterudite type). For example, when a rare earth element such as Yb (ytterbium) is added to the Co ₄ Sb ₁₂ , the electrical resistivity and the thermal conductivity can be reduced. In particular, the thermal conductivity is effectively reduced by the presence of another element. Such an effect is called a rattling effect. This is because the added element (for example, Yb) enters the voids of the basic skeleton Co ₄ Sb ₁₂ and causes thermal vibration independent of the Co ₄ Sb ₁₂ to cause phonons (lattice vibrations) of the basic skeleton Co ₄ Sb _12. It is for suppressing.

Here, the thermoelectric conversion member 2 of the thermoelectric conversion element 1 of the present embodiment represents one or more elements selected from the group consisting of the elements L (In, Yb, Eu, Ce, La, Nd, Ga, and Sr). ), Element M (representing one or more elements selected from the group consisting of Co, Rh, Ir, Fe, Ni, Pt, Pd, Ru and Os), and element Pn (Sb, As, P, Te, Sn) , Representing one or more elements selected from the group consisting of Bi, Ge, Se and Si). Hereinafter, when the element L is In, the element M is Co, and the element Pn is Sb, that is, the thermoelectric conversion member 2 is a filled skutter in which In is added to a material represented by the composition formula Co ₄ Sb ₁₂ This will be described in detail by taking as an example a case of being made of a die-type material.

  As described above, the thermoelectric conversion element 1 of the present embodiment includes the thermoelectric conversion member 2, the metal layers 3 formed on both end surfaces of the thermoelectric conversion member 2, and the side surfaces of the thermoelectric conversion member 2 and the metal layer 3. The formed insulator 4 and the adhesion layer 5 formed over the upper surface of the metal layer 3 and the end surface of the insulator 4 are included. More specifically, the thermoelectric conversion member 2 has a column shape, and its side surface is covered with the insulator 4. The end surfaces (upper surface and bottom surface) of the thermoelectric conversion member 2 are not covered with the insulator 4. The metal layer 3 and the adhesion layer 5 are disposed on these end faces.

  Here, the metal layer 3 includes an element L and an oxygen element (in this specification, the metal layer 3 is also referred to as an “L layer”). The metal layer 3 exhibits a high effect in ensuring quality when the thermoelectric conversion element 1 including the thermoelectric conversion member 2 made of a skutterudite type material is used in a high temperature environment. In particular, it has an effect of suppressing metal diffusion of the element material constituting the thermoelectric conversion member 2 at a high temperature. Here, the element L contained in the metal layer 3 and the element L in the filled skutterudite type material in the thermoelectric conversion member 2 are usually the same. Therefore, in the present embodiment, since the element L in the thermoelectric conversion member 2 is In, the metal layer 3 is composed of an oxide containing In. Further, the average oxygen concentration in the metal layer 3 is desirably 25 atm% or more and 65 atm% or less, as will be described later. The average oxygen concentration of the metal layer 3 can be specified by analysis by energy dispersive X-ray spectroscopy described later.

  The insulator 4 can be a member made of a material having high heat resistance such as quartz (glass), silica glass, or ceramic such as alumina, but is preferably made of quartz (glass). The thickness of the insulator 4 is preferably 0.2 mm or greater and 5.0 mm or less. When the insulator 4 is thin, the mechanical strength obtained by disposing the insulator 4 is lowered. On the other hand, if the thickness of the insulator 4 is too thick, the ratio of the volume of the thermoelectric conversion member 2 to the volume of the thermoelectric conversion element 1 is relatively lowered. As a result, the electrical resistance increases, leading to a decrease in the amount of power generation.

  The adhesion layer 5 serves as a metal diffusion prevention film for suppressing metal diffusion generated by exposing the thermoelectric conversion element 1 to a high temperature, and a bonding film for mounting the thermoelectric conversion element 1 on a wiring board. As described above, the adhesion layer 5 is required to have two roles of metal diffusion suppression and bonding. Therefore, two layers or three layers including a film for suppressing metal diffusion and a film for bonding to the wiring board. It is preferable to be comprised from the above films | membranes. When the adhesion layer 5 is composed of a plurality of films, a film for suppressing metal diffusion is disposed on the surface in contact with the metal layer 3.

  The film for suppressing metal diffusion preferably contains the element Q (one or more elements selected from the group consisting of Ti, Mo, Co, and Ta), and the film is made of, for example, only these metals It may be made of these oxides and nitrides. Among these, Ti is particularly preferable because it has a property of easily reacting with oxygen. When the adhesion layer 5 contains Ti, as will be described later, Ti easily takes oxygen from the metal layer 3 when the adhesion layer 5 is formed, and the adhesion layer 5 having desired mechanical strength and electrical conductivity is easily obtained. Hereinafter, the adhesion layer 5 will be described as including Ti. On the other hand, metals such as Ag, Au, and Cu can be used as the film for bonding to the wiring board. The thickness of each film can be appropriately selected according to the shape of the thermoelectric conversion element 1 and the required performance.

  The average oxygen concentration of the adhesion layer 5 is lower than the average oxygen concentration of the metal layer 3. The average oxygen concentration of the adhesion layer 5 is preferably 0 atm% or more and 40 atm% or less, as will be described later. The average oxygen concentration of the adhesion layer 5 can also be specified by analysis using energy dispersive X-ray spectroscopy described later. Further, the oxygen concentration in the adhesion layer 5 preferably has a gradient in the thickness direction, and the oxygen concentration on the surface on the metal layer 3 side (also referred to as “first surface”) is bonded to the bonding portion or the like. It is preferably higher than the oxygen concentration on the side surface (also referred to as “second surface”). The presence or absence of an oxygen concentration gradient and the difference in oxygen concentration between the first surface and the second surface can also be confirmed by analysis using energy dispersive X-ray spectroscopy or the like.

  Here, there is no restriction | limiting in the shape of the thermoelectric conversion element 1 of this indication, Although it can be set as arbitrary shapes, such as a column shape and a prism shape, it is a column shape especially, and the outer diameter (phi) is 0.3 mm or more and 10. It is preferable that it is 0 mm or less. This is because, although other columnar shapes such as a prismatic shape serve as a thermoelectric conversion element, the cylindrical shape can alleviate stress concentration when the thermoelectric conversion element 1 is exposed to a high temperature. Further, when the outer diameter φ is 0.3 mm or less, the absolute mechanical strength of the thermoelectric conversion element 1 tends to be lowered, and the handling at the time of manufacture and the strength reliability when modularized are likely to be lowered. On the other hand, when the outer diameter φ is 10.0 mm or more, the number of elements that can be arranged per unit area decreases. When a thermoelectric conversion module is used for power generation, the power is determined by the generated voltage and current. However, when an element having a large outer diameter is used, the module has a low voltage and a high current because the number of elements is small. This is because even if the elements have the same performance, the voltage as a module is determined by the number of elements, and the current is determined by the total electric resistance of the elements.

  Moreover, although the height of the thermoelectric conversion element 1 can be arbitrarily set according to the magnitude | size of a desired thermoelectric conversion module, it is preferable that they are 0.3 mm or more and 5.0 mm or less. The “height of the thermoelectric conversion element 1” in the present embodiment refers to the length of the thermoelectric conversion element 1 in the axial direction, that is, the length of the thermoelectric conversion element 1 perpendicular to the adhesion layer 5. As the thermoelectric conversion element 1 is thinner, that is, the height of the thermoelectric conversion element 1 is lower, the electric resistance is lowered and the amount of power generation is increased. However, when the height of the thermoelectric conversion element 1 is lowered, it becomes difficult to create a temperature difference between the end faces of the thermoelectric conversion element 1 (between both electrodes). For this reason, the electrical resistance is lowered, and the amount of power generation is offset by the increase in current and the decrease in temperature difference. On the other hand, when the thermoelectric conversion element 1 is too thick, a temperature difference between the end faces is easily provided, but the electric resistance is increased. Therefore, the opposite phenomenon occurs when the thermoelectric conversion element 1 is thin. Therefore, it is necessary to set the height of the thermoelectric conversion element 1 according to the temperature environment in which the thermoelectric conversion module is installed.

  Here, FIG. 2A shows an electron microscope image showing that the metal layer 3 is present in the thermoelectric conversion element 1 of the present embodiment actually produced by the method described later. FIG. 2A is an observation of a cross section when cut in parallel with the central axis of the thermoelectric conversion element 1 produced by the manufacturing method described later. In FIG. 2A, there is one dark layer above. Further, a boundary line is seen around the center of FIG. 2A, and a light-colored layer exists between the boundary line and the dark-colored layer. Here, when elemental analysis was performed on each of the points (1) to (5) in FIG. 2A, Ti and O were mainly detected in the dark layer. Further, In and O were mainly detected in the light-colored layer. FIG. 2B, FIG. 2C, and FIG. 2D show elemental analysis spectra at point (2), point (4), and point (5) in FIG. 2A, respectively.

  2B to 2D are measurement results obtained by energy dispersive X-ray spectroscopy, respectively. In this method, characteristic X-rays peculiar to an element generated by irradiating an object with an electron beam are detected, and the element is identified by spectroscopically analyzing it. The horizontal axis in FIGS. 2B, 2C, and 2D represents energy specific to an element when irradiated with an electron beam, and the peak position changes depending on the element. On the other hand, the vertical axis represents the intensity of detected energy. In addition, Pt detected in FIGS. 2B to 2D is a conductive treatment agent for SEM observation, and is a substance previously applied to the surface by sputtering. As is apparent from the measurement results, the composition differs at point (2), point (4), and point (5) in FIG. 2A. In FIG. 2D (point (5)), Co and Sb are detected, and it can be seen that the region below the boundary line is the thermoelectric conversion member 2. On the other hand, in FIG. 2B (point (2)), Ti is detected, and it can be seen that the dark layer is the adhesion layer 5. In FIG. 2C (point (4)), In and O are detected, and it can be seen that the light-colored layer is the metal layer 3. Here, it is considered that In detected in the metal layer 3 is precipitated from the thermoelectric conversion member 2. In addition, it is considered that O contained in the metal layer 3 was detected because In reacted with oxygen when In contacted with air and its surface was oxidized. As analytical instruments, SU-70 scanning electron microscope manufactured by Hitachi High Technology and IncaX-act manufactured by Oxford Instruments were used.

  FIG. 2E is a measurement result by the above-mentioned energy dispersive X-ray spectroscopy, and the vertical axis in FIG. 2E indicates the measurement location (points (1) to (5)) in FIG. 2A, and the horizontal axis is It represents the atomic concentration of each element at each measurement location. Focusing on the oxygen concentration in FIG. 2E, the oxygen concentration gradually increases from point (1) to point (4), that is, from the end surface (second surface) of the adhesion layer 5 to the metal layer 3. This is because the In in the metal layer 3 was oxidized by contact with air at the same time as the precipitation, and further, the adhesion layer 5 was formed on the metal layer 3 by sputtering or the like, so that Ti in the adhesion layer 5 became a metal layer. It is considered that oxygen was taken from 3 and oxidized. The sputtered film of the adhesion layer 5 starts to be formed from the metal layer 3 side and increases in thickness. Therefore, in the adhesion layer 5, the end face (from the interface (first surface) side with the metal layer 3 ( It is considered that the oxygen concentration gradually decreased toward the second surface) side.

(Method for manufacturing thermoelectric conversion element)
Hereinafter, the manufacturing method of the thermoelectric conversion element 1 of the above-described embodiment will be described with reference to FIG. The thermoelectric conversion element 1 includes a step of melting a thermoelectric conversion material 2a prepared in a desired composition in a crucible (see FIG. 3A), and an insulator (hereinafter also referred to as “insulating tube”) 4 in the crucible 6. Is inserted, and the melted thermoelectric conversion material 2a is sucked up (see FIG. 3B), the insulating tube 4 after the heat treatment is cut into a desired length (see FIG. 3C), and the insulating tube 4 is filled. Performing a step of heat-treating the thermoelectric conversion material 2a (not shown) and a step of forming the adhesion layer 5 on the end face of the thermoelectric conversion body 2 filled in the insulating tube 4 after the heat treatment (see FIG. 3D). Can be manufactured. Hereinafter, the manufacturing method will be described by taking as an example a case where a material obtained by adding In to Co ₄ Sb ₁₂ is used as the thermoelectric conversion material 2a. However, if a skutterudite material is used as the thermoelectric conversion material 2a, it can be produced by the same method even if the elements and the mixing ratio are arbitrarily adjusted.

First, a thermoelectric conversion material 2a prepared in advance, for example, In _x Co ₄ Sb ₁₂ (0 <x ≦ 1) is prepared (see FIG. 3A). The thermoelectric conversion material 2a may be in the form of a powder or an ingot. In addition, the addition amount of In may satisfy 0 <x ≦ 1 in the above composition formula, but 0.1 ≦ x ≦ 0.5 is particularly preferable. If the amount of In added is small, the thermoelectric conversion characteristics of the obtained thermoelectric conversion member 2 may not be sufficiently improved. On the other hand, if the amount of In added is too large, compounds other than skutterudite are formed, and in this case, the thermoelectric conversion characteristics also deteriorate. The thermoelectric conversion material 2 a is heated to 1100 to 1200 ° C. in the crucible 6 and melted. The crucible 6 is preferably heat-resistant, has little reaction with the thermoelectric conversion material 2a, and is low in cost. For example, a carbon crucible is preferably used. Further, heating is preferably performed in a vacuum or in the presence of an inert gas such as nitrogen from the viewpoint that oxidation of the thermoelectric conversion material 2a can be suppressed.

In the above process, after the thermoelectric conversion material 2a is sufficiently melted, the insulating tube 4 is inserted into the crucible 6, the melted thermoelectric conversion material 2a is sucked up, and the thermoelectric conversion material 2a is filled in the insulating tube 4. Perform (FIG. 3B). The insulating tube 4 corresponds to the insulator 4 of the thermoelectric conversion element 1 described above. In the manufacturing method of the present embodiment, the outer diameter and shape of the obtained thermoelectric conversion member 2 are determined by the inner diameter and shape of the insulating tube 4. Here, the skutterudite-type thermoelectric conversion material 2a generally has a high melting point. For example, since the melting point of Co ₄ Sb ₁₂ is around 1000 ° C., it is necessary to use a heat-resistant insulator 4. For this reason, it is preferable to use glass, particularly quartz glass having a high softening temperature. Since quartz glass is an oxide and has little reaction with the thermoelectric conversion material 2a, it is suitable as the insulating tube 4 in the manufacturing method of the present disclosure. The method of sucking up the thermoelectric conversion material 2a into the insulating tube 4 is not particularly limited. For example, one of the insulating tubes 4 is inserted into the molten thermoelectric conversion material 2a and sucked from the other using a cylinder or the like. can do. The suction amount at this time can be arbitrarily selected from the inner diameter, length, etc. of the insulating tube 4.

  Next, the insulating tube 4 filled with the thermoelectric conversion member 2 is cut into a desired length (FIG. 3C). As a method of cutting the insulating tube 4 and the thermoelectric conversion member 2, cutting with a blade or cutting with a wire cut can be adopted, but wire cutting capable of batch processing is more desirable from the viewpoint of productivity. When the insulating tube 4 is made of quartz, the thermoelectric conversion member 2 that is a metal and the hard quartz (glass) are simultaneously cut. Therefore, as conditions for wire cutting, it is possible to reduce the cracking of quartz (glass) and the application of the thermoelectric conversion member 2 by reducing the abrasive grains and slowing the wire speed.

After the cutting process described above, a step of heat-treating the insulator 4 filled with the thermoelectric conversion material 2a is performed (not shown). The heat treatment can be performed using a general electric furnace, and is performed between 500 ° C. and 800 ° C. for about 30 hours to 200 hours. Appropriate heating conditions vary depending on the composition. For example, if In _x Co ₄ Sb ₁₂ (0 <x ≦ 1), the thermoelectric conversion member 2 having good thermoelectric conversion characteristics is obtained by performing the heating at 600 ° C. for about 60 hours. Can do. Basically, the thermoelectric conversion member 2 having stable characteristics can be obtained by performing the heat treatment for a long time. Before the heat treatment in this step, the thermoelectric conversion material 2a does not constitute a skutterudite-type crystal, but the crystal structure is changed by heating. As a result, a Co ₄ Sb ₁₂ crystal filled with In can be obtained. And according to the thermoelectric conversion element 1 containing the thermoelectric conversion member 2 comprised from the said composition, sufficient thermoelectric conversion characteristics are exhibited.

  The heat treatment is preferably performed in a vacuum or in an inert gas atmosphere in order to suppress oxidation of the thermoelectric conversion material 2a and the thermoelectric conversion member 2 obtained by the heat treatment. The metal layer 3 is generated in a process in which the atomic arrangement of the thermoelectric conversion material 2a is switched to form a skutterudite structure. Of the In contained in the thermoelectric conversion material 2a, In that has not been incorporated into the skutterudite crystal is discharged outside the crystal. In is oxidized simultaneously with the precipitation, and a metal layer 3 containing In and oxygen is formed on the end face of the thermoelectric conversion member 2.

In addition, the thermoelectric conversion element 1 which has the metal layer 3 is brought about by the process of filling the thermoelectric conversion material 2a in the insulating tube 4, and the process of heat-processing at the temperature different from the temperature at the time of filling. For example, when the composition of the thermoelectric conversion material 2a is In _x Co ₄ Sb ₁₂ (0 <x ≦ 1), the temperature of the thermoelectric conversion material 2a when filling the insulating tube 4 is 1100 ° C. to 1200 ° C. The temperature of the thermoelectric conversion material 2a when heat-treating after filling the tube 4 is 500 ° C to 800 ° C. When the insulating tube 4 is filled, the thermoelectric conversion material 2a (skutterudite material) needs to be melted, so that the temperature must exceed the melting point. In addition, the heat treatment needs to be performed at a temperature sufficiently lower than the melting point in order to promote movement of atoms in a solid state, that is, in a crystalline state.

  After the heat treatment step, the adhesion layer 5 is formed on the cut surface of the thermoelectric conversion member 2. As described above, the adhesion layer 5 plays two kinds of roles: metal diffusion suppression and bonding to the wiring board. Therefore, the case where the adhesion layer 5 has a two-layer structure will be described as an example. In the present embodiment, when Ti is used as a film for suppressing metal diffusion and Ag is used as a film for bonding to a wiring board, a sputtering method is given as a method for forming each film constituting the adhesion layer 5. It is done. Although a plating method or the like can be considered as a method for forming each film, since an element such as Ti cannot be plated and the plating method is a wet method, the insulator 4 and the thermoelectric conversion member 2 are formed when the adhesion layer 5 is formed. There is a possibility that these materials enter the gaps and form a film in an unnecessary place. Although there is a film forming method by thermal spraying, it is not preferable in terms of suppressing metal diffusion because it is difficult to control the thickness of the film and the film quality tends to be relatively sparse. On the other hand, such a problem can be avoided if the film is formed by sputtering. As the thickness of the film, for example, if the film made of Ti is about 1 μm and the film made of Ag is about 400 nm, the adhesion layer 5 can have functions of suppressing metal diffusion and bonding.

  Through the above steps, the thermoelectric conversion element 1 according to the embodiment of the present disclosure can be obtained.

As described above, the metal layer 3 can be deposited on the end face of the thermoelectric conversion member 2 by performing a heat treatment after the cutting process. When this order is reversed, the metal layer 3 is positively deposited on the side surface of the thermoelectric conversion member 2 and In _x Co ₄ Sb ₁₂ (0 <x ≦ 1) is exposed on the end surface of the thermoelectric conversion member 2. Become.

(Thermoelectric conversion module)
FIG. 4 is a schematic cross-sectional view illustrating an example of a thermoelectric conversion module using the thermoelectric conversion element 1 of the present disclosure. The thermoelectric conversion element 1 is arranged so that the P-type thermoelectric conversion element 1P and the N-type thermoelectric conversion element 1N are connected in series. These are sandwiched between the high temperature side substrate 7 and the low temperature side substrate 8 facing each other. A wiring electrode 9 is formed on each of the high temperature side substrate 7 and the low temperature side substrate 8, and the thermoelectric conversion element 1 and the wiring electrode 9 are electrically joined by the joint 10. Each of the P-type thermoelectric conversion element 1P and the N-type thermoelectric conversion element 1N can be constituted by an element having a thermoelectric conversion member made of a skutterudite type material. For example, the material of the N-type thermoelectric conversion member is Co ₄ Sb ₁₂ , and the material of the P-type thermoelectric conversion member is Fe _x Co ₄ -xSb _{12 in} which Co in the N-type material is partially replaced by Fe. It can be set as the material represented by. Moreover, these all contain a filling element such as In. Moreover, even if it is any composition, it is possible to form the metal layer 3 in the both end surfaces of a thermoelectric conversion member by manufacturing with the above-mentioned manufacturing method.

  Moreover, the thermoelectric conversion module of this indication combined the 1st conductivity type 1st thermoelectric conversion element and the 2nd conductivity type 2nd thermoelectric conversion element from which the said 1st thermoelectric conversion element differs in conductivity type, for example. It may be a thing. Each thermoelectric conversion element can be manufactured by the above-described manufacturing method.

  Moreover, it is preferable to use the board | substrate which consists of ceramics, such as an alumina and a silicon nitride, for the high temperature side board | substrate 7 of a thermoelectric conversion module. This is because the resin substrate is inferior in heat resistance when it is assumed to be used at a high temperature. On the other hand, the low temperature side substrate 8 is not particularly limited as long as it has sufficient heat resistance at the use temperature, and a ceramic or resin substrate can be used. The wiring electrodes 9 formed on the respective substrates are preferably made of a metal that is difficult to oxidize such as Ag in view of heat resistance. In addition, it is preferable that the joint portion 10 is not easily oxidized. For example, when Ag is used for the wiring electrode 9, it is preferable to use Ag brazing or sintered Ag in consideration of bondability.

(effect)
Hereinafter, effects of the thermoelectric conversion element and the like of the present disclosure will be described. In the thermoelectric conversion element 1 of the present disclosure, the metal layer 3 (L layer) containing the element L and oxygen constituting the material of the thermoelectric conversion member 2 exists between the thermoelectric conversion member 2 and the adhesion layer 5. Furthermore, the adhesion layer 5 usually also contains oxygen together with the element Q. Here, the element Pn such as Sb, the element M, and the like which are constituent materials of the thermoelectric conversion member 2 have a lower diffusion coefficient than oxygen. Therefore, the presence of the oxygen-containing metal layer 3 and the adhesion layer 5 between the joint 10 and the thermoelectric conversion material 2 allows the metal constituting the thermoelectric conversion member 2 to diffuse into the joint 10. It is greatly reduced. In addition, as described above, the oxygen content of the adhesion layer 5 is usually gradually reduced toward the surface. However, in such an adhesion layer 5, the oxygen content in the adhesion layer 5 as a whole is increased as compared with the case where the oxidation is progressing. The content rate becomes low. Therefore, it is considered that the required mechanical strength and electrical conductivity are possessed. In addition, it is preferable that the thickness of the metal layer 3 is 200 nm or more and 1 μm or less because both the diffusion prevention effect and the mechanical strength can be achieved.

  Here, the cross-sectional observation result of the element which performed the durability test for 100 hours in 400 degreeC environment is shown. FIG. 5A is an electron microscope image of a cross section when an element (comparative example) in which no metal layer is present is cut parallel to the central axis of the thermoelectric conversion element 1 after the durability test. FIG. 5B shows a measurement result of the element by energy dispersive X-ray spectroscopy. The vertical axis in FIG. 5B indicates the measurement locations and points (1) to (4) in FIG. 5A, and the horizontal axis indicates the atomic number concentration of each element at each measurement location. Points (1) to (3) correspond to the adhesion layer 5, and point (4) corresponds to the thermoelectric conversion member 2. When the analysis results of points (1) to (3) in the adhesion layer 5 (Ti sputter layer) are seen, Sb, In, and Co are detected, respectively, and the element material of the thermoelectric conversion member 2 to the adhesion layer 5 is detected. It can be seen that the diffusion is in progress.

  On the other hand, FIG. 5C is an electron microscope image of a cross section when cut in parallel to the central axis of the thermoelectric conversion element 1 after the durability test of the element in which the metal layer 3 of the embodiment of the present disclosure is present. FIG. 5D shows a measurement result of the device by energy dispersive X-ray spectroscopy. The vertical axis in FIG. 5D indicates the measurement locations and points (1) to (4) in FIG. 5C, and the horizontal axis indicates the atomic number concentration of each element at each measurement location. Points (1) and (2) correspond to the adhesion layer 5, point (3) corresponds to the metal layer 3, and point (4) corresponds to the thermoelectric conversion member 2. From FIG. 5D, Sb, Co, and In were detected at the points (1) and (2) in the adhesion layer 5 (Ti sputter layer), but compared to the points (1) to (3) in FIG. 5A, Their atomic number concentration is reduced to less than half. That is, in the thermoelectric conversion element according to the embodiment of the present disclosure, it can be confirmed that the diffusion of the element material from the thermoelectric conversion material 2a to the adhesion layer 5 in a high temperature environment is reduced. Therefore, compared with the conventional thermoelectric conversion element, it has very high reliability.

  In the thermoelectric conversion element of the present embodiment, the average oxygen concentration of the metal layer 3 (L layer) is preferably 25 atm% or more and 65 atm% or less, and the average oxygen concentration of the adhesion layer 5 is 0 atm% or more. It is desirable that it is 40 atm% or less. This is because when the average oxygen concentration of the metal layer 3 is 25 atm% or less, the improvement of the diffusion suppressing effect accompanying the oxidation of the element L constituting the metal layer 3 tends to decrease. Further, when the average oxygen concentration of the metal layer 3 is 65 atm% or more, the electrical resistivity and the mechanical strength tend to decrease. Furthermore, when the average oxygen concentration of the metal layer 3 is 65 atm% or more, Ti or the like takes away oxygen from the metal layer 3 excessively when the adhesion layer 5 is formed, and the average oxygen concentration of the adhesion layer 5 exceeds 40 atm%. It becomes easy. When the average oxygen concentration of the adhesion layer 5 is 40 atm% or more, like the metal layer 3, the electrical resistivity and the mechanical strength tend to decrease. The average oxygen concentration is a measured value in a region of 100 sqm.

  In the above description, the example in which the metal layer 3 (L layer) is provided on both the upper surface and the bottom surface of the thermoelectric conversion member 2 has been described, but the L layer is provided only on one of the upper surface or the bottom surface of the thermoelectric conversion member 2. Also good. However, in order to obtain a more remarkable diffusion preventing effect, it is preferable to dispose the L layer on both the upper surface and the bottom surface of the thermoelectric conversion member 2.

  The thermoelectric conversion material according to the present disclosure is superior in reliability of thermoelectric conversion characteristics at high temperatures as compared with conventional thermoelectric conversion elements, and can be applied to high-temperature energy recovery such as automobile and factory exhaust heat.

DESCRIPTION OF SYMBOLS 1 Thermoelectric conversion element 1P P type thermoelectric conversion element 1N N type thermoelectric conversion element 2 Thermoelectric conversion member 2a Thermoelectric conversion material 3 Metal layer 4 Insulator 5 Adhesion layer 6 Crucible 7 High temperature side substrate 8 Low temperature side substrate 9 Wiring electrode 10 Junction 101 P-type thermoelectric conversion element 102 N-type thermoelectric conversion element 103 First diffusion prevention layer 104 Second diffusion prevention layer 105 Solder bonding layer 106 Solder layer 107 Electrode protective layer 108 High temperature side electrode

Claims (7)

Element L (represents one or more elements selected from the group consisting of In, Yb, Eu, Ce, La, Nd, Ga and Sr), element M (Co, Rh, Ir, Fe, Ni, Pt, Pd, One or more elements selected from the group consisting of Ru and Os), and one or more elements selected from the group consisting of elements Pn (Sb, As, P, Te, Sn, Bi, Ge, Se and Si) A thermoelectric conversion member composed of a skutterudite-type material containing
An L layer containing the element L and oxygen, disposed on at least one of the upper surface and the bottom surface of the thermoelectric conversion member;
An adhesion layer that is disposed on the L layer and includes an element Q (representing one or more elements selected from the group consisting of Ti, Mo, Co, and Ta);
The average oxygen concentration of the L layer is greater than the average oxygen concentration of the adhesion layer.
Thermoelectric conversion element. The oxygen concentration on the first surface of the adhesion layer on the L layer side is greater than the oxygen concentration on the second surface of the adhesion layer opposite to the first surface,
The thermoelectric conversion element according to claim 1. In the adhesion layer, the oxygen concentration gradually changes in the cross section in the thickness direction.
The thermoelectric conversion element according to claim 1. The average oxygen concentration of the L layer is 25 atm% or more and 65 atm% or less,
The average oxygen concentration of the adhesion layer is 0 atm% or more and 40 atm% or less.
The thermoelectric conversion element as described in any one of Claims 1-3. An electrode is disposed on the adhesion layer;
The thermoelectric conversion element as described in any one of Claims 1-4. A joining member is disposed between the adhesion layer and the electrode.
The thermoelectric conversion element according to claim 5. The element L is In, the element M is Co, and the element Pn is Sb.
The thermoelectric conversion element as described in any one of Claims 1-6.