JP2012231025A - Thermoelectric conversion module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermoelectric conversion module whose bonding interface is prevented from oxidizing to reduce loss of output power.SOLUTION: The thermoelectric conversion module comprises: thermoelectric conversion elements (5, 6) which convert a temperature difference into power and are made of a metal oxide; an electrode member (3) for extracting power converted by the thermoelectric conversion elements; and a conductive bonding layer (4) bonding the thermoelectric conversion elements to the electrode member. The bonding layer has oxygen intrusion prevention means (11) for preventing oxygen from intruding into the bonding layer.

Description

  The present invention relates to a thermoelectric conversion module, and relates to joining of a thermoelectric conversion material using an oxide and an electrode metal.

Thermoelectric conversion materials are widely known as materials that can directly convert heat into electricity. When one end of the thermoelectric conversion material is set to a high temperature and the other end is set to a low temperature, an electromotive force is generated between the end portions according to the temperature difference between the high temperature portion and the low temperature portion. An electromotive force generated between the end portions of this material is called a thermoelectromotive force, and such an effect is called a Seebeck effect. The Seebeck effect reverses the direction of the current by increasing or decreasing the temperature of the junction of the thermoelectric conversion element. In addition, when a current is passed from the other end of the thermoelectric conversion material, a phenomenon in which a temperature difference occurs between both ends is called a Peltier effect, and an element having these effects is called a thermoelectric conversion element. In general, n-type semiconductors in which carriers excited by heat are electrons and p-type semiconductors in which holes are holes are known as thermoelectric conversion materials. A configuration in which these materials are alternately joined via electrodes to form a π-type module is well known. This π-type module is configured by joining one end of a p-type semiconductor and one end of an n-type semiconductor with an electrode. When this junction is at a high temperature and the other end of the p-type semiconductor and the n-type semiconductor is at a low temperature, the temperature is high. The electromotive force generated between the end portions of the p-type semiconductor and the n-type semiconductor can be efficiently extracted according to the temperature difference between the portion and the low temperature portion. Moreover, a larger electromotive force can be obtained as the temperature difference increases.

There are various thermoelectric conversion materials, and bismuth tellurium (BiTe), lead tellurium (PbTe), silicon germanium (SiGe), iron silicide (FeSi), and the like, which are mainly composed of metals, are known. These metal-based thermoelectric conversion materials have been reported to have high thermoelectric conversion performance. For example, when used at a high temperature of 400 ° C. or higher, oxidation of the material proceeds in an atmosphere containing oxygen such as in the air. Therefore, it is not suitable for use in the atmosphere.

On the other hand, oxide materials such as zinc oxide (ZnO), strontium titanate (SrTiO ₃ ), and sodium cobaltate (NaCo ₂ O ₄ ) whose main component is oxide are also widely known in addition to the above materials. Since these oxide-based thermoelectric conversion materials are stable in the atmosphere, high thermoelectric performance can be exhibited at high temperatures where metal-based materials cannot be used.

A thermoelectric conversion material mainly composed of zinc oxide is a high dimensionless figure of merit ZT (= S ² σT / κ: S is a Seebeck coefficient) among n-type thermoelectric conversion materials by doping trivalent or tetravalent metal ions. , Σ is a conductivity, T is an absolute temperature, and κ is a thermal conductivity). As ZT is evaluated as an index representing the performance of the thermoelectric conversion material, the larger the value, the better the thermoelectric performance. Since the metal ion has more valence electrons than zinc (Zn), it substitutes for a zinc site to generate free electrons serving as carriers. Preferably, at least one selected from Al ³⁺ , Ca ³⁺ , In ³⁺ , Y ³⁺ , Ti ⁴⁺ , Sn ⁴⁺ , Zr ⁴⁺ , Si ⁴⁺ , Ce ⁴⁺ and La ³⁺ is included.

As for the joints when these materials are modularized, both mechanically and electrically favorable joints are required for both the joints that are heated and the joints that are cooled. When modularizing metal materials, various means such as soldering, brazing, and thermal spraying are used. Since the metal-based thermoelectric conversion material has a relatively low operating temperature range of several hundred degrees to exhibit performance and is a metal, it is relatively easy to bond to the electrode.

On the other hand, oxide-based thermoelectric conversion materials have high thermoelectric conversion performance in a higher operating temperature range and are expected to be used in the atmosphere, and the joints are required to have heat resistance and oxidation resistance. Bonding with solder or the like is not suitable because of its low heat resistance. Further, it is generally considered that joining with an electrode made of a metal having conductivity is difficult due to poor wettability. Among them, there are some reported examples of a method for joining thermoelectric conversion modules using zinc oxide, which is known as an oxide-based thermoelectric material.

In Patent Document 1, when connecting a thermoelectric conversion material made of oxide with a low resistance and a conductive paste containing a noble metal powder and a specific composite oxide is used as a connection material for the thermoelectric conversion material, peeling of the connection portion occurs. It is difficult to maintain good thermoelectric conversion performance for a long time.

  Patent Document 2 discloses that when zinc oxide, which is a thermoelectric conversion material, is thermally sprayed and brazed in a vacuum, it is difficult to join due to low wettability to the surface or evaporation of zinc. In this method, an electrode and a thermoelectric conversion element are mechanically caulked using a part of a thermoelectric conversion material as a bonding portion by an electroless plating method and bonded without improving the electric resistance. That is, thermal spraying and electroless plating require a large thermal spraying apparatus and plating apparatus.

  Further, in Patent Document 3, since the performance of the thermoelectric conversion element may deteriorate when used at a high temperature, the surface of the thermoelectric conversion element is particularly covered and covered with silica, alumina, silicon carbide or the like. Therefore, it is possible to suppress deterioration of characteristics even by using, for example, an oxidizing gas in a high temperature atmosphere.

International Publication Number WO05 / 036661 (FIG. 5) Japanese Patent Laying-Open No. 2005-19783 (FIG. 4) JP 2009-32893 A (FIG. 9)

  Zinc oxide used as a thermoelectric conversion material contains a metal element serving as a dopant, and metal ions are substituted at zinc sites in a crystal lattice. In this crystal lattice, trivalent aluminum is substituted in place of zinc that is ionic in terms of valence, so that a difference in valence from that of divalent oxygen occurs, and thus conductivity is imparted. In addition, oxygen defects are generated in the crystal lattice, and therefore conductivity is imparted. Although it is an oxide that is stable in the atmosphere, when used in a hot atmosphere, the bulk is stable and maintains thermoelectric performance, but a slight reoxidation may occur on the surface. This reoxidation makes the zinc oxide layer less conductive. In particular, if the bonding interface with the electrode is a structure that can come into contact with the atmosphere, for example, a bonding interface that has fine pores and air may enter, the re-oxidation will also insulate the bonding interface and conduct electricity. Sexuality may get worse.

In the above Patent Document 1 and Patent Document 2, although it is said that good bonding has been made, it does not refer to the shape and state of the bonding interface, for example, when it is a bonding interface in which pores exist, It does not correspond to the performance degradation caused by the denaturation due to oxidation of the interface between the thermoelectric material and the electrode material.

  Depending on the application method, the bonding interface after bonding with a noble metal paste, for example, silver (Ag) paste and firing, may have a void at the bonding interface with the electrode, thereby forming an insulating layer. This is because when a thermoelectric conversion material is bonded using a silver paste containing a general organic solvent as a binder, pores are generated when the organic solvent is volatilized at the bonding interface between the thermoelectric conversion element and the electrode material in the drying process. Is formed. As a result, vacancies formed at the bonding interface with the electrode after firing cause oxidation of the element surface as a route for atmospheric components including oxygen, and the electrical resistance increases with time, resulting in a high electrical resistance. That is, even if the bonding is mechanically good, the bonding is poor electrically.

  Further, even if it is a silver paste, it is considered that a good bonding interface can be formed by firing in air at a high temperature at which silver particles are completely dissolved, for example, at a silver melting point of about 960 ° C. Re-oxidation easily occurs and an impurity layer may be formed, and the bonded interface after firing may exhibit high electrical resistance.

  Further, in Patent Document 3, the deterioration of thermoelectric performance due to oxidation from the surface is suppressed. However, when an oxidizing gas enters from the bonding interface between the thermoelectric conversion element and the electrode, the thermoelectric conversion results from the bonding interface. The element is oxidized and the thermoelectric characteristics are degraded.

  Therefore, an object of the present invention is to provide a thermoelectric conversion module that prevents oxidation generated at the junction interface of the thermoelectric conversion module and that has little loss of output power.

In order to achieve the above object, in the thermoelectric conversion module, the present invention provides a thermoelectric conversion element made of a metal oxide for converting a temperature difference into electric power, and for extracting electric power converted by the thermoelectric conversion element. Oxygen intrusion prevention means for preventing the invasion of oxygen into the joining layer, comprising an electrode member and a conductive joining layer for joining the thermoelectric conversion element and the electrode member. It is characterized by having.

  In the present invention configured as described above, oxygen in the atmosphere does not enter the bonding layer during use of the thermoelectric conversion module. For this reason, it can prevent that the oxygen which penetrate | invaded into the joining layer reaches | attains a thermoelectric conversion element and an electrode member, oxidizes these, and increases an electrical resistance.

In the present invention, preferably, the oxygen intrusion prevention means is a dense film formed so as to cover the entire outer peripheral surface of the bonding layer.
According to the present invention configured as described above,
Since the entire outer peripheral surface of the bonding layer is covered with a dense film and is not exposed to the atmosphere, oxygen can be reliably prevented from entering the bonding layer.

In the present invention, preferably, the dense film is made of an amorphous oxide containing silicate as a main component.
According to the present invention configured as described above, since the dense film formed on the outer periphery of the bonding layer is a silicate oxide having an amorphous structure, the bonding layer does not form pores that allow oxygen to enter. Are not exposed to the atmosphere.

In this invention, Preferably, an electrode member has a recessed part corresponding to the joint surface of a thermoelectric conversion element.
According to the present invention configured as described above, since the bonding layer is accommodated in the recess, the distance between the portion of the dense film exposed to the outside air and the bonding layer can be increased. As a result, even when a defect occurs in a part of the bonding layer, the possibility of oxygen reaching the bonding layer can be reduced.

ADVANTAGE OF THE INVENTION According to this invention, the oxidation which arises in the joining interface of a thermoelectric conversion module can be prevented, and the thermoelectric conversion module with little loss of output electric power can be provided.

It is a schematic diagram which shows the structure and embodiment of the uni couple (thermoelectric conversion module using a pair of thermoelectric conversion elements) which is the 1st Embodiment of this invention. It is a schematic diagram which shows the structure and embodiment of the unicouple which is the 2nd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(N-type element material)
Any n-type oxide thermoelectric material may be used as long as electrons function as carriers and function as the thermoelectric material. Specifically, those containing zinc oxide, strontium titanate (SrTiO ₃ ), and calcium manganate (CaMnO ₃ ) as main components are preferable. Most preferably, the main component is zinc oxide, which contains at least one selected from Al, Ga, In, and rare earth metals (Y, Sc, lanthanoid) as a dopant, and is 100 S / at room temperature (20 ° C.). Those having conductivity of cm or more can be used.

(P-type element material)
Any p-type oxide thermoelectric material may be used as long as holes serve as carriers and function as the thermoelectric material. Specifically, what has a layered oxide containing cobalt as a main component is preferable. Most preferred is a material containing sodium cobaltate (Na _x CoO ₂ (x = 0 to 1)) or calcium cobaltate ([Ca ₂ CoO ₉ ] _x CoO ₂ ).

(Module manufacturing method)
The schematic diagram used as the structure of the typical thermoelectric conversion module in this invention is shown in FIG. The high temperature side insulating substrate 1 and the electrode 3 are bonded by an insulating bonding layer 2. The electrode 3 and the n-type thermoelectric conversion element 5 are joined by the conductive bonding layer 4 to form an n-type thermoelectric conversion element-conductive bonding layer bonding interface 7a. The electrode 3 and the p-type thermoelectric conversion element 6 are joined by the conductive bonding layer 4 to form a p-type thermoelectric conversion element-conductive bonding layer bonding interface 7b. The low temperature side insulating substrate 10 and the electrode 3 are joined by the insulating joining layer 2.

(Temperature difference provided)
The high temperature-side insulating substrate 1, a high temperature to join the heat quantity Q _in are in contact high temperature heat source 8. On the other hand, the low-temperature side insulating substrate 10 is in contact with the low-temperature cooling source 9, and the amount of heat Q _out flows out, resulting in a low temperature. Therefore, the thermoelectric conversion module A and the high temperature corresponding to the heat quantity Q _in, the temperature difference becomes low in accordance with the leaked heat Q _out is given.

(substrate)
The insulating substrate is used for the purpose of maintaining electrical insulation, improving thermal uniformity and mechanical strength. Originally, the material of the substrate is not particularly limited, but a material having high thermal conductivity that does not cause melting or breakage at high temperatures, is chemically stable, and does not react with thermoelectric conversion materials, bonding materials, etc. Considering it, it is preferable to use an insulating material. By using a substrate having high thermal conductivity, the temperature of the high temperature portion of the element can be brought close to the temperature of the high temperature heat source, and the thermal electromotive voltage can be increased. In addition, since the thermoelectric conversion material used in the present invention is an oxide, it is preferable to use oxide ceramics such as alumina as the substrate material in consideration of the coefficient of thermal expansion.

(Joining method)
When the thermoelectric conversion element is bonded to the electrode, it is preferable to use a bonding agent that can be connected with low resistance. For example, a noble metal paste such as silver (Ag), gold (Au), platinum (Pt), silver solder, solder, or the like can be suitably used. In the present invention, it is preferable to use a silver paste that can easily obtain mechanical and electrical good bonding by firing and is easy to handle. A dense coating composed mainly of amorphous oxide was formed so as to cover the entire outer peripheral surface of the bonding layer thus formed.

(Baking temperature)
As a baking temperature of the silver paste of this invention, 500-800 degreeC is preferable. By baking at this temperature, the silver paste used in the present invention is dissolved and reliable bonding is possible. In addition, since the surface oxidation of zinc oxide does not occur at around 400 ° C. in the atmosphere, it is considered that the interface has good conductivity. Inappropriate.

(Production of n-type thermoelectric elements)
The manufacturing method of an n-type thermoelectric conversion element is shown. Zinc oxide powder (made by High Purity Chemical Laboratory, particle size of about 1 μm), γ-alumina powder (made by High Purity Chemical Laboratory, particle size of about 2 to 3 μm), yttria powder (made by Shin-Etsu Chemical, particle size of about 0.2 μm) are available These raw materials were weighed so as to have a predetermined molar ratio of Zn: Al: Y = 95: 3: 2. This was put into a polyethylene bottle, and a 10 mm diameter iron ball ball coated with nylon was added so as to be 10 times the weight of the powder, followed by a dry ball mill treatment for 15 hours. After separating the powder with a metal mesh sieve, it was pressed by uniaxial press molding and further subjected to cold isostatic pressing (CIP) to produce a disk-shaped pellet having a diameter of about 25 mm and a thickness of about 7 mm. This disk-shaped pellet was sintered and produced by firing at 1400 ° C. in the atmosphere for about 10 hours. The n thermoelectric conversion material produced in this way was used as a 4 × 4 × 10 mm ³ columnar element using a diamond cutter (Ibet 4000 manufactured by Buehler).

(Production of p-type thermoelectric element)
The manufacturing method of a p-type thermoelectric conversion element is shown. Cobalt oxide powder (manufactured by High Purity Chemical Laboratory) and sodium carbonate (manufactured by Wako Pure Chemical Industries) were prepared, and these raw materials were weighed so as to have a predetermined molar ratio of Na: Co = 1.1: 2. Sodium carbonate powder is desirably added in an excess of about 10% because Na is scattered during the firing process and a sintered body having a stoichiometric ratio cannot be obtained. This was put into a polyethylene bottle, and a 10 mm diameter iron ball ball coated with nylon was added so as to be 10 times the weight of the powder, followed by a dry ball mill treatment for 24 hours. The obtained powder was collected and then fired at 800 ° C. for 12 hours in the air. A 10% excess of sodium carbonate was further added to the obtained powder, followed by a dry ball mill treatment as described above, followed by firing at 800 ° C. for 12 hours in the air. This was pressed by uniaxial press molding, and further subjected to cold isostatic pressing (CIP) to produce a disk-shaped pellet having a diameter of about 25 mm and a thickness of about 7 mm. This disk-shaped pellet was sintered and produced by firing at 900 ° C. for about 12 hours in an air atmosphere. The p thermoelectric conversion material produced in this way was used as a 4 × 4 × 10 mm ³ columnar element using a diamond cutter (Ibet made by Buehler).

(Manufacture of modules)
The thermoelectric conversion element produced by the above production method was joined to the electrode 3, and this electrode was further joined onto the alumina substrate. The electrode 3 was cut into 12 × 6 × 1 mm ³ using a silver plate (manufactured by Niraco), deburred with sandpaper, and the surface was polished. The conductive bonding layer 4 between the n-type thermoelectric conversion element 5 and the electrode 3 was prepared using a silver paste and used for bonding the electrode 3 to the high temperature side insulating substrate 1 and the low temperature side insulating substrate 10. The insulating bonding layer 2 was prepared using a ceramic bond (Ceramabond manufactured by Alemco). An appropriate amount of silver paste was applied to one surface of the device, bonded onto the electrode while applying pressure, and dried at around 100 ° C. This was baked at 800 ° C. for 2 hours. On the outer periphery of the conductive bonding layer 4 between the n-type thermoelectric conversion element 5 and the electrode 3, a glass solution (Aquarica NN310 manufactured by Clarion Co., Ltd.) is applied, dried for solvent removal, and baked at 800 ° C. for 2 hours. A dense coating 11 was formed.

  Next, the effect | action of the joining method of the thermoelectric conversion module which is embodiment of this invention is demonstrated. According to the bonding method of the present invention, the dense coating 11 becomes a bonding interface protective layer having no pores without exposing the conductive bonding layer 4 to the atmosphere. For this reason, even when used at high temperature in the atmosphere, oxygen for re-oxidizing the n-type thermoelectric conversion element-conductive bonding layer bonding interface 7a, which is the interface of zinc oxide, does not reach, so that the electric resistance is not oxidized. Since there is no increase and there is little current loss at the junction interface, the output power can be effectively improved and maintained.

  Further, if the dense coating 11 to be formed is sufficiently thick, it covers the entire outer peripheral surface of the n-type thermoelectric conversion element-conductive bonding layer bonding interface 7a, and mechanically good bonding with the conductive bonding layer 4 is achieved. Since it is formed, the thermoelectric conversion element can be mechanically excellently bonded.

  The dense film 11 formed by applying a glass solution and drying in the air is a silicate oxide having an amorphous structure. Do not expose to atmosphere.

FIG. 2 shows a second embodiment of the present invention. The second embodiment has the same configuration as the unicouple according to the first embodiment of the present invention shown in FIG. 1, and only the configuration shown in FIG. 2 is different.
The n-type thermoelectric conversion element 5 forms an electrical bond by bonding to the conductive bonding layer 4 via the n-type thermoelectric conversion element-conductive bonding layer bonding interface 7a. A concave portion corresponding to the bonding surface of the conversion element is provided, and the dense film 11 covers the n-type thermoelectric conversion element-conductive bonding layer bonding interface 7a in the concave portion. If comprised in this way, since a joining layer is accommodated in a recessed part, the distance of the part exposed to external air among dense films and a joining layer can be taken long. As a result, even when a defect occurs in a part of the bonding layer, the possibility of oxygen reaching the bonding layer can be reduced.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the invention described in the claims. Needless to say, these are also included within the scope of the present invention. For example, in the above-described embodiment, the material of the electrode may be an SUS material other than a silver plate, or an electrode obtained by screen printing a circuit shape directly on an insulating substrate with a silver paste. Moreover, although the baking process in an Example is performed in air | atmosphere, you may carry out in inert gas, such as nitrogen and argon atmosphere.

A thermoelectric conversion module 1 high temperature side insulating substrate 2 insulating bonding layer 3 electrode 4 conductive bonding layer 5 n-type thermoelectric conversion element 6 p-type thermoelectric conversion element 7a n-type thermoelectric conversion element-conductive bonding layer bonding interface 7b p-type thermoelectric Conversion element-conductive bonding layer bonding interface 8 High temperature heat source 9 Low temperature cooling source 10 Low temperature side insulating substrate 11 Dense coating Qin Heat quantity
Qout heat quantity

Claims (4)

In thermoelectric conversion module,
A thermoelectric conversion element made of a metal oxide for converting a temperature difference into electric power;
An electrode member for taking out the electric power converted by the thermoelectric conversion element;
A conductive bonding layer for bonding the thermoelectric conversion element and the electrode member;
Consists of
The thermoelectric conversion module according to claim 1, wherein the bonding layer has oxygen intrusion prevention means for preventing oxygen from entering the bonding layer. The oxygen intrusion prevention means includes
The thermoelectric conversion module according to claim 1, wherein the thermoelectric conversion module is a dense film formed to cover the entire outer peripheral surface of the bonding layer. The thermoelectric conversion module according to claim 2, wherein the dense film is made of an amorphous oxide mainly composed of silicate. The thermoelectric conversion module according to claim 3, wherein the electrode member has a recess corresponding to a joint surface of the thermoelectric conversion element.

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