JP6809852B2 - Thermoelectric conversion element and thermoelectric conversion module - Google Patents

Description

The present invention relates to a thermoelectric conversion element using a Si clathrate compound which is a thermoelectric conversion material, and a thermoelectric conversion module using the thermoelectric conversion element.

Conventionally, a combination of a thermoelectric conversion material unit and an electrode layer has been known as a thermoelectric conversion element, and in particular, a thermoelectric conversion material unit in which a plurality of thermoelectric conversion material units are electrically arranged is used as a thermoelectric conversion module.
A thermoelectric conversion module utilizing the Seebeck effect makes it possible to convert thermal energy into electrical energy. In the actual thermoelectric conversion, the p-type thermoelectric conversion material unit and the n-type thermoelectric conversion material unit are used to alternately electrically connect them in series. By utilizing the properties of the thermoelectric conversion module, it is possible to convert the exhaust heat discharged from industrial / consumer processes and mobile objects into effective electric power, so thermoelectric conversion is attracting attention as an energy-saving technology that considers environmental issues. There is.

Therefore, there is a demand for a thermoelectric conversion material used in a high temperature range of about 400 to 800 ° C. such as waste heat power generation. Clathrate compounds are attracting attention as one of such new thermoelectric conversion materials. Although several types of promising clathrate compounds have been reported, Ba-Ga-Al-Si and Ba-Ga-Al-Ge clathrate compounds are attracting attention from the viewpoint of cost.
Regarding Si clathrate compounds, some compositions and synthetic methods of clathrate compounds composed of Ba, Ga, Al and Si have already been disclosed. For example, in Patent Document 1, x (10.8 ≦ x ≦ 12.2) Si atoms per unit cell are replaced with either Al atom or Ga atom, and Ba ₈ (Al, Ga) _x. A single crystal of Si _{46-x and} a method for producing the same are disclosed.

By the way, in a thermoelectric conversion module using a clathrate compound, it is necessary to join the thermoelectric conversion material portion and the electrode at a high temperature portion and a low temperature portion. For example, in a thermoelectric conversion module using a Bi-Te clathrate compound used in the temperature range of room temperature to 250 ° C., these bonding uses solder, brazing material, etc. with almost no consideration of the influence of heat. It is realized by a relatively easy method.
However, since thermoelectric conversion modules using Si clathrate compounds are used in the temperature range of room temperature to 900 ° C., it is necessary to consider thermal measures including heat resistance of the joint portion between the thermoelectric conversion material portion and the electrode in the high temperature portion. There is. At a high temperature of about 400 to 800 ° C. such as waste heat power generation, a compound layer may be formed at the bonding interface due to mutual diffusion of elements between the thermoelectric conversion material and the electrode. For example, in the case of a Si clathrate compound, the class Silicide, which is a compound of Si constituting the clathrate compound and the electrode material, may be formed. Such silicides generally have a high melting point and have a coefficient of linear expansion different from that of non-silicide electrode materials, which can cause cracks and cracks due to thermal cycles. Further, in the case of the Ag electrode, VDD is not formed, but diffusion is rapid and there is a risk of characteristic deterioration.

Therefore, at the time of module fabrication, it is necessary to electrically connect the p and n-type thermoelectric conversion materials in series, not to form an intermetallic compound by mutual diffusion, and to reduce the contact resistance. As one of the solutions, it is conceivable to use, for example, a U-shaped element as an element without an electrode.
As the U-shaped element, for example, in Patent Document 2, an iron-silicon thermoelectric conversion element is used in order to improve heat dissipation on the low temperature side and locally and efficiently heat the joint portion on the high temperature side. It has been proposed to join a p-type material and an n-type material on the high temperature side. Further, Patent Document 3 discloses one of the methods for manufacturing a U-shaped element in order to improve the power generation efficiency without increasing the manufacturing cost. Patent Document 4 discloses that a p-type thermoelectric conversion material having a layered perovskite structure and an n-type thermoelectric conversion material are used to directly join a part thereof. Further, Patent Document 5 discloses an element that can withstand a high temperature by directly joining a part of a p-type material and an n-type material using a scatterdite-based thermoelectric conversion material.

However, none of these prior arts relate to U-shaped elements using Si clathrate compounds. In the Si clathrate, the mutual diffusion of transition metal elements (for example, Fe, Ni, Co, Cu) causes the transition metal silicide to be formed near the bonding interface, which has a large effect on cracking and chipping on the bonding surface and element resistance. There's a problem. In particular, in a general Si clathrate containing three or more kinds of elements, in addition to the transition metal silicide, another compound is often formed at the same time. This is because the composition near the interface deviates from the narrow solid solution region of the Si clathrate due to the formation of the transition metal silicide. In this case, cracks and chips on the joint surface and the influence on the element resistance are further large. Therefore, even with the formation of the U-shaped element, the problem of silicide being formed on the joint surface and cracking and cracking due to the thermal cycle still remains.

Japanese Unexamined Patent Publication No. 2004-6742 Japanese Unexamined Patent Publication No. 1-144501 JP-A-2010-206024 Japanese Unexamined Patent Publication No. 2010-27631 Japanese Patent No. 2000-25526

A main object of the present invention is that when a U-shaped device using a clathrate compound is formed, a compound containing a transition metal silicide is not or is not easily formed at an interface, and cycle durability at a high temperature is high. It is an object of the present invention to provide a good thermoelectric conversion element.

As a result of diligent research to solve the above problems, the present inventors have found that a p-type thermoelectric conversion material section composed of a p-type Si clathrate compound and an n-type thermoelectric conversion material section composed of an n-type Si clathrate compound. We have found a combination in which transition metal silicide is not formed or is difficult to form during direct bonding.
That is, the present invention has a p-type thermoelectric conversion material portion made of a p-type Si clasprate compound and an n-type thermoelectric conversion material portion made of an n-type Si clasprate compound, and the p-type thermoelectric conversion material portion has joined the n-type thermoelectric conversion material unit directly, the p-type Si clathrate compound and the n-type Si clathrate compound, _both having a composition _{a x B y Si Z X w} , a is, Ba Or Sr, B is one or more elements selected from the group consisting of Ga, Al, Cu, Ni, Au and Pt, and X is one or two of B (boron) or Pd. It has been found that the above problems can be solved by using a thermoelectric conversion element which is an optional additive element selected from the above. Here, x + y + z + w = 54, which is determined by the number of atoms on the lattice of the Si clathrate compound. Also, w contains 0, which indicates that X is an optional additive element.

According to the present invention, the thermoelectric conversion material portion made of an n-type Si clathrate compound and the thermoelectric conversion material portion made of a p-type Si clathrate compound are directly bonded on the high temperature side, and cracks or cracks occur at the interface thereof. Etc. are not detected, and it is possible to provide a thermoelectric conversion element that is well bonded both electrically and physically.

It is an external view of the U-shaped thermoelectric conversion element. It is a figure which shows typically the structure of the thermoelectric conversion element module of this invention, (a) is the perspective view which shows typically the internal structure of the thermoelectric conversion element module, (b) is the cross section of the thermoelectric conversion element module. It is sectional drawing which shows typically. It is a figure which shows the change of the resistance value in the vicinity of the interface of the thermoelectric conversion element of an Example.

As described above, in the vicinity of the connection surface of the Si clasprate compound, transition metal elements such as Fe, Ni, Co, and Cu are mutually diffused to form a transition metal silicide in the vicinity, which causes cracks and cracks due to a thermal cycle. There's a problem. The reason for this is that when a hard intermetallic compound with a high melting point is generally formed, such as a transition metal silicide, it is used in a high temperature environment such as 800 ° C. due to the difference in linear expansion coefficient from the Si clathrate material. It has been found that the thermal cycle reduces durability. Further, transition metal silicide having higher resistance than electrode materials such as Cu and Ni leads to an increase in element resistance. In particular, it is difficult to maintain the performance at a high temperature with a Si clathrate compound having a relatively high efficiency and being easily affected by contamination, unlike a material having low efficiency and not easily affected by contamination such as iron silicide.

However, as a combination of p-type and n-type Si clathrate compound, Both _have a composition _{A x B y Si Z X w} , wherein a x + y + z + w = 54, w comprises 0, A is Ba or Sr, B is one or more elements selected from the group consisting of Ga, Al, Cu, Ni, Au and Pt, and X is one or two of B (boron) or Pd. It is an optional additive element selected from the species. By using a thermoelectric conversion element having such a composition, it is possible to prevent the formation of a compound containing a transition metal silicide at the interface.

(Thermoelectric conversion element)
The composition _A x _B y _Si Z _{X A} in _w of Si clathrate compound constituting the thermoelectric conversion element may be Ba. Further, B in the composition _A x _B y _Si Z _{X w} may be Ga and Al. The composition _A x _B y _Si Z _{X w} in B may be Cu or Ni.
Further, B in the composition _A x _B y _Si Z _{X w} of Si clathrate compound constituting the p-type thermoelectric conversion material unit, if it is possible to form a p-type thermoelectric conversion material portion, Ga, Al, Cu, It may be one kind or two or more kinds of elements selected from the group consisting of Ni, Au and Pt. Generally, when B is Au or Pt, it tends to form a p-type thermoelectric conversion material portion.

And Si clathrate compound constituting the p-type and n-type thermoelectric conversion material of an embodiment of the present invention, to select the Ba as the A in the composition _{A x B y Si Z X w} , Ga and as the B Examples thereof include Ba-Ga-Al-Si clathrate compounds in which Al is selected. In the Ba-Ga-Al-Si clathrate compound, the basic lattice is mainly composed of a Si clathrate lattice, the Ba element is contained therein, and a part of the atoms constituting the clathrate lattice is Ga. , Al-substituted structure. This clathrate compound is a compound containing Ba, Ga, Si, and Al at the same time.

Chemical formulas of clathrate compound of an embodiment of the present invention, for example of the formula of clathrate compound _Ba a _Ga b _Al composition ratio of _{c Si d, Ba, Ga,} Al, the composition ratio of Si a, b, c , D generally have the following relationship [1]. Further, the composition ratios b, c, and d of Ga, Al, and Si generally have the following relationship [2]. If these relationships are satisfied, the clathrate compound can be realized as having a Si clathrate phase as a main component, and can have an ideal crystal structure.
a + b + c + d = 54 ... [1]
b + c + d = 46 ... [2]
The thermoelectric conversion material parts 11 and 12 may contain a Si clathrate compound as a main component and a small amount of other additives and impurities.

As a Si clathrate compound, there is a Si clathrate compound that can be substituted with an element other than the above Ga and Al. For _{example, Ba a Ga b Al c Cu} d Ni e Au f Pt g Si of composition ratio of _{h, Ba, Ga, Al,} Cu, Ni, Au, Pt, each composition ratio of Si a, b, c, d , E, f, g, and h generally have the following relationship [3]. Further, the composition ratios b, c, d, e, f, g, and h of Ga, Al, Cu, Ni, Au, Pt, and Si generally have the following relationship [4]. If these relationships are satisfied, the clathrate compound can be realized as having a Si clathrate phase as a main component, and can have an ideal crystal structure.
a + b + c + d + e + f + g + h = 54 ... [3]
b + c + d + e + f + g + h = 46 ... [4]

For example, if the composition for producing a Si clathrate compound is the chemical formula Ba _a Ga _b Al _c Si _h , then 7 ≦ a ≦ 9,0 ≦ b ≦ 15,0 ≦ c ≦ 15, 27 ≦ h ≦ 35. For example, if the composition for producing a Si clathrate compound is the chemical formula Ba _a Cu _{d S h} , then 7 ≦ a ≦ 9,2 ≦ d ≦ 10, 36 ≦ h ≦ 44. For example, if the composition for producing a Si clathrate compound is the chemical formula Ba _a Ni _e Si _h , then 7 ≦ a ≦ 9,1 ≦ e ≦ 7,39 ≦ h ≦ 45. For example a composition to create a Si clathrate compound, if formula _Ba a _Au f Si _h, the 7 ≦ a ≦ 9,2 ≦ f ≦ 10,36 ≦ h ≦ 44. For example, if the composition for producing a Si clathrate compound is the chemical formula Ba _a Pt _{g S h} , then 7 ≦ a ≦ 9,1 ≦ g ≦ 7,39 ≦ h ≦ 45. Further, as the Si clathrate compound, a clathrate compound containing a small amount of other additives and impurities in the Ba-Ga-Al-Si clathrate compound may be used. That is, the Si clathrate compound may be a Ba-Ga-Al-Si-X (X = B (boron), Pd) -based clathrate compound. B and Pd may be useful for increasing the Seebeck coefficient.

Furthermore, the Ba-Ga-Al-Si- X based clathrate compound, formula _{Ba a Ga b Al c Si d} X of the composition ratio of _x, Ba, Ga, Al, Si, each composition ratio of X a, b , C, d, and x generally have the following relationship [5].
a + b + c + d + x = 54 ... [5]
The Ba-Ga-Al-Si-X-based clathrate compound may also contain a small amount of other additives and impurities.

(U-shaped thermoelectric conversion element)
As shown in FIG. 1, the U-shaped thermoelectric conversion element basically includes thermoelectric conversion material units 11 and 12. The thermoelectric conversion material unit 11 is composed of a thermoelectric conversion material containing a Si clathrate compound as a main component, and exhibits n-type thermoelectric characteristics. Further, the thermoelectric conversion material unit 12 is composed of a thermoelectric conversion material containing a Si clathrate compound as a main component, and exhibits p-type thermoelectric characteristics. The p-type and n-type thermoelectric conversion material parts are electrically connected, and these are incorporated as a module 60 as shown in FIG. 2 (a).

It is preferable that the directly bonded side of the U-shaped thermoelectric conversion element is the high temperature side. In this case, the side where the n-type thermoelectric conversion material portion and the p-type thermoelectric conversion material portion are separated is the low temperature side.
On the low temperature side of the U-shaped thermoelectric conversion element, the electrodes and wiring can be easily connected by a conventional joining method such as solder or metal paste.
On the high temperature side where bonding is extremely difficult, a good bonding state can be maintained even in a thermal cycle at a high temperature by using a U-shaped thermodynamic conversion element. Further, a U-shaped thermoelectric conversion element may be connected to the low temperature side. In this case, since the number of bonding interfaces on the low temperature side is reduced, there is an advantage that the element resistance is reduced.

(Thermoelectric conversion module)
The thermoelectric conversion module is a module having a function of converting the thermal energy applied to the thermoelectric conversion element into electrical energy.
As shown in FIG. 2A, the thermoelectric conversion module 60 is mainly composed of a U-shaped thermoelectric conversion element, a low temperature side wiring 42, a high temperature side insulating substrate 51, and a low temperature side and an insulating substrate.
As shown in FIG. 2B, in the thermoelectric conversion module 60, the U-shaped thermoelectric conversion element is the low temperature side of the p-type thermoelectric conversion material unit 12 and the n-type thermoelectric conversion material unit of another U-shaped thermoelectric conversion element. The low temperature side of 11 has a configuration in which the low temperature side is electrically arranged in series via the low temperature side wiring.

The high-temperature side wiring and the low-temperature side wiring have a function of electrically connecting the n-type thermoelectric conversion material unit 11 and the p-type thermoelectric conversion material unit 12 in series.
As the material of the low temperature side wiring 42, any conductive metal may be used, and Cu, Ag, Al, or the like can be used.
The high temperature side insulating substrate 51 and the low temperature side insulating substrate 52 have a function of fixing the n-type thermoelectric conversion material portion 11 and the p-type thermoelectric conversion material portion 12 and the low temperature side wiring 42, and the thermoelectric conversion module 60 is uniform. It has a function to receive heat.

The material of the high temperature side insulating substrate 51 may be any material having a melting point equal to or higher than the upper limit temperature to be used (for example, 800 ° C.) and insulated from the high temperature side of the U-shaped thermoelectric conversion element, for example, alumina. You can. Further, the material of the low temperature side insulating substrate 52 may be the same as or different from that of the high temperature side insulating substrate 51, but it needs to be a material that is insulated from the low temperature side wiring 42.
The thermoelectric conversion module 60 does not have to have the high temperature side insulating substrate 51.
In this case, the connection between the high temperature side and the high temperature side insulating substrate 51 of the U-shaped thermoelectric conversion element is eliminated, the thermal stress applied to the U-shaped thermoelectric conversion element is relaxed, and the reliability of the thermoelectric conversion module 60 at high temperature is improved. ..
Such a thermoelectric module may be mounted on a moving body such as an automobile. At that time, it can be used to obtain electricity by utilizing the waste heat of the moving body.

(Production method)
In the preparation step, an ingot of a clathrate compound having a predetermined composition and a uniform composition is produced. First, a predetermined amount of raw materials (Ba, Ga, Al, Si, etc.) are weighed and mixed so as to have a desired clathrate compound composition. The raw material may be a simple substance, an alloy or a compound, and its shape may be powder, flaky or lumpy. Further, it is preferable to use a mother alloy of Al—Si instead of simple substance Si as a raw material of Si because the melting point is lowered.

As the melting time, it is necessary that all the raw materials are uniformly mixed in a liquid state, but considering the energy required for production, it is desirable that the melting time is as short as possible. Therefore, the melting time may be 1 to 100 minutes, further may be 1 to 10 minutes, and may be particularly 1 to 5 minutes.
The method for melting the powder composed of the raw material mixture is not particularly limited, and various methods can be used. Examples of the melting method include heating with a resistance heating element, high-frequency induction melting, arc melting, plasma melting, electron beam melting, and the like. As the crucible, graphite, alumina, cold crucible or the like is used depending on the heating method. The melting is preferably carried out in an inert gas atmosphere or a vacuum atmosphere in order to prevent oxidation of the material.

In order to obtain a homogeneously mixed state in a short time, preferably fine powdered raw materials are mixed. However, Ba is preferably in the form of a lump in order to prevent oxidation. It is also preferable to add mechanical or electromagnetic stirring during melting.
In order to make an ingot after melting, it may be cast using a mold or solidified in a crucible. In order to homogenize the finished ingot, annealing treatment may be performed after melting.
The ingot obtained in the preparation step can be pulverized using a ball mill or the like to obtain a fine particle clathrate compound. The obtained fine particles are desired to have a fine particle size in order to improve the sinterability. The particle size of the fine particles is preferably 100 μm or less, and more preferably 1 μm or more and 75 μm or less.

In order to obtain fine particles having a desired particle size, the ingot is crushed with a ball mill or the like, and then the particle size is adjusted. Examples of the method for preparing the particle size include a test sieve manufactured by Lecce Co., Ltd. and a sieving machine AS200 digit manufactured by Lecce Co., Ltd., which are ISO3310-1 standards. Fine powder may be produced by changing the sieving to various atomization methods such as a gas atomization method or a flowing gas evaporation method.

(Example 1)
(Preparation of thermoelectric conversion element sample)
High-purity Ba having a purity of 2N or more and high-purity Al, Ga, Si, Au, Cu, Ni, and Pt having a purity of 3N or more are weighed at the blending ratios shown in Table 1 (blending amount (g)). The raw material mixture was prepared.

In Table 1, Samples A to I are raw material mixtures of the thermoelectric conversion material of Si clathrate compounds. A step of arc-melting this raw material mixture on a water-cooled copper hearth with a current of 300 A for 1 minute in an Ar (argon) atmosphere, then inverting the ingot to eliminate non-uniformity of the raw materials, and performing arc melting again. Was repeated 5 times and cooled to room temperature on a water-cooled copper hearth as it was to obtain an ingot having a clathrate compound. Then, in order to improve the uniformity of the ingot, an annealing treatment was performed at 900 ° C. for 6 hours in an argon atmosphere.

The obtained ingot was pulverized using an agate planetary ball mill to obtain fine particles. At this time, the particle size was adjusted using an ISO3310-1 standard Lecce test sieve and a Lecce sieve shaker AS200 digit so that the particle size of the obtained particles was 75 μm or less. In order to confirm the polarity of the obtained fine particles for sintering of each sample, a sintered body for character evaluation was prepared. Each fine particle was filled in a sintering mold, and sintering was performed using a discharge plasma sintering method (SPS method). At the time of sintering, the pressure was increased to 50 MPa and then heated. Although the sintering was performed in a vacuum atmosphere, it may be in an inert atmosphere such as Ar gas. By measuring the temperature of the surface of the sintered mold, it was heated to about 900 to 1050 ° C., sintered at that temperature for 5 minutes, released from the pressurized state, and cooled to room temperature.

When the cooling temperature is 500 ° C. or higher, it is preferably maintained in a vacuum atmosphere, but when it is lower than 500 ° C., it may be maintained in an atmospheric atmosphere.
Since each sample has different components, the appropriate sintering temperature is different although the Si clathrate is the same. If the sintering temperature is particularly low, the sintered body has a low density and may cause cracks. Further, if the sintering temperature is particularly high, the sample will melt. Therefore, it is necessary to select an appropriate sintering temperature while checking the temperature and the progress of sintering.

(Evaluation of thermoelectric conversion material)
In order to confirm that the samples A to I were clathrate compounds, a central portion of the ingot was cut out and analyzed by powder X-ray diffraction using an X-ray diffractometer (Geigerflex manufactured by Rigaku Corporation). As a result, it was confirmed that the type 1 clathrate phase was generated in all the samples. When the strongest peak ratio was calculated based on the formula [4] from the obtained results, it was confirmed that the strongest peak ratio was 95% or more.
Furthermore, the composition of the thermoelectric conversion material was analyzed with an electron probe microanalyzer (EPMA-1610 manufactured by Shimadzu Corporation). Result, in the sample A~I in Table 2, the compound of desired composition _{Ba a Ga b Al c Si d} (a + b + c + d = 54), with a compound of _{Ba a Au e Si d (a} + b + c + e = 54) was obtained.

In addition, the center of the sintered body was cut out and the Seebeck coefficient was measured. The results of the Seebeck coefficient at 800 ° C. are shown in Table 2. Samples A to G have characteristics as an n-type thermoelectric conversion material unit, and samples H and I have characteristics as a p-type thermoelectric conversion material unit. In this way, the n-type and p-type thermoelectric conversion material parts of the Si clathrate compound were produced.
The "Seebeck coefficient S" and "electric resistivity ρ" were measured by the four-terminal method using a thermoelectric characteristic evaluation device ZEM-3 manufactured by ULVAC Riko Co., Ltd.

(Manufacturing of U-shaped thermoelectric conversion element)
In order to produce a U-shaped thermoelectric conversion element, in order to join the n-type thermoelectric conversion material and the p-type thermoelectric conversion material portion, the obtained sintering particles were placed in a sintering mold and sintered. .. Here, as shown in Table 3, sample A was used as the n-type thermoelectric conversion material unit, and sample H was used as the p-type thermoelectric conversion material unit. The sintered mold is first filled with the particles of sample A, and then filled with the particles of sample H. At this time, it is desirable that the sample A is placed horizontally as much as possible and the boundary between the samples A and H is horizontal. Further, particles obtained by mixing sample A and sample H may be filled between the sample A and the sample H. In this case, at the boundary between sample A and sample H, solid phase diffusion can be further promoted and a stronger bonding interface is provided.

Sintering was performed by using a discharge plasma sintering method (SPS method), pressurizing to a pressure of 50 MPa, and then heating. Although the sintering was performed in a vacuum atmosphere, it may be in an inert atmosphere such as Ar gas. By measuring the temperature of the surface of the sintered mold, the surface was heated to 900 ° C., then sintered at 900 ° C. for 5 minutes, the pressurized state was released, and the mixture was cooled from 900 ° C. to room temperature.
At this time, it is desirable that both Sample A and Sample H are densely sintered, and 900 ° C., which is higher than the sintering temperature of both, was selected.
When the cooling temperature is 500 ° C. or higher, it is preferably maintained in a vacuum atmosphere, but when it is lower than 500 ° C., it may be maintained in an atmospheric atmosphere.

Since it is sufficient that the sample A and the sample H can be bonded by direct solid phase diffusion, another production method will be described. The particles of sample A were first filled in a sintered mold to prepare a sintered body of sample A. Further, the obtained sintered body of sample A is placed in a sintered mold, and the particles of sample H are filled. By sintering this at a lower temperature than that of sample A, a sintered body in which samples A and B are directly bonded can be obtained. This is because, as described above, the sintering temperature differs for each sample, and sample A has a higher sintering temperature than sample H. Further, the sample A and the sample H may be sintered separately. In that case, energization bonding is performed with the sintered body of sample A and sample H in contact with each other. Samples A and H are joined by solid phase diffusion.

In this way, a sintered body of a thermoelectric conversion material in which sample A and sample H were directly bonded was obtained. The obtained sintered body was processed and shaped into a U-shape shown in FIG. The low temperature side of samples A and H was 2 mm in length and 2 mm in width, and the joint on the high temperature side was 2 mm in length and 5 mm in width, and the height was 5 mm. At this time, on the low temperature side, Ni plating was applied so that wiring such as Cu and Al could be easily connected.
In this way, a pair of U-shaped thermoelectric conversion elements in which sample A and a part of sample B were directly bonded were obtained. Further, by electrically connecting eight pairs of U-shaped thermoelectric conversion elements in series, a thermoelectric conversion module having the same configuration as the thermoelectric conversion module 60 of FIG. 2A was produced.

Further, when the sample A-only portion and the sample B-only portion of the sintered body obtained in Example 1 were cut out and the thermoelectric characteristics were evaluated, the sample A showed n-type characteristics and the sample B showed p-type characteristics. It was.
Using an electron probe microanalyzer (EPMA-1610 manufactured by Shimadzu Corporation), the composition of the obtained U-shaped thermoelectric conversion element was mapped near the interface. At the interface between sample A (upper part) and sample B (lower part), a new compound layer is not formed and is directly bonded by element diffusion. It was found that Al and Ga existed only in sample A, and Au existed only in sample B. Ba and Si are present in samples A and B.

Further, the obtained sintered body was processed and shaped into 2 mm × 2 mm × 5 mm so as to include the interface, and the contact resistance at the interface was measured. FIG. 3 shows the results of measuring the resistance value when the measurement position was moved from sample A to sample B at a pitch of 1 μm from the end on the sample A side. As shown in FIG. 3, no gap in resistance value was confirmed at the interface. This indicates that there are no cracks or cracks at the interface, and compounds having different resistance values at the interface are not formed.
From these evaluations, in the U-shaped thermoelectric conversion element in Example 1, the sample A of the n-type thermoelectric conversion material part and the sample B of the p-type thermoelectric conversion material part are directly bonded on the high temperature side, and the interface thereof. It was found that there were no cracks or cracks in the product, and that the metal compounds including the silicon compound were not formed and the compounds were bonded well both electrically and physically by solid phase diffusion. "Crack" means that it is cracked, and "crack" means that it is cracked.

(Interface characteristics during sintering)
It was confirmed that the metal was melted from the sample B side during sintering. This means that the thermoelectric conversion material has been heated to a temperature at which the clathrate compound starts to melt (at least 900 ° C. or higher), and it can be said that sufficient element diffusion is performed. Nevertheless, no cracks or cracks were found near the interface.

(Durability after quenching)
The obtained U-shaped thermoelectric conversion element was placed on a high temperature side by direct fire (about 800 ° C.) and immersed in water. At this time, a tester was connected to each low temperature side of the n-type thermoelectric conversion material section and the p-type thermoelectric conversion material section, and the resistance value and the current value were measured. This was repeated 10 times. In appearance, no cracks or cracks occurred, and the resistance and current values were the same as before the test. Although the module is normally expected to be air-cooled at the time of mounting, it was found that it did not deteriorate even after such a rigorous test, and it was found that the durability of the thermal cycle was very high.

(Measurement of resistance at high temperature)
The resistance value of the obtained module was measured at room temperature and 700 ° C. Since no rapid increase in resistance was observed, it was found that there were no cracks or cracks at high temperatures.

(Examples 2 to 8)
In Examples 2 to 8, the n-type thermoelectric conversion material part and the p-type thermoelectric conversion material part are selected from the samples A to H according to the combinations shown in Table 3, and the U-shaped thermoelectric part is selected in the same manner as in Example 1. A conversion element was manufactured.

In Examples 2 to 8, as in Example 1, the electron probe microanalyzer (EPMA-1610 manufactured by Shimadzu Corporation) was used to confirm the occurrence of cracks and cracks near the interface, and the resistance and current value for durability after quenching. When the resistance values of the modules at room temperature and 700 ° C. were measured, it was confirmed that there were no cracks or cracks at high temperatures and the thermal cycle durability was high, as in Example 1.
As described above, we have found a combination in which the transition metal silicide is not formed when the p-type thermoelectric conversion material portion and the n-type thermoelectric conversion material portion are directly bonded. When bonded with a combination of an n-type Si clathrate compound having the above-mentioned composition and a p-type Si clathrate compound, a compound containing a transition metal silicide is not formed at the interface, and a U-shape having good cycle durability at high temperatures is formed. It has been found that a clathrate conversion element can be provided.

(Comparative Examples 1 to 3)
As shown in Table 4, fine powders of Fe, Cu, and Co were integrally sintered with sample A by the SPS method instead of the n-type raw material sample H in Example 1. Let each thermoelectric conversion element be Comparative Examples 1 to 3.
In Comparative Examples 1 to 3, it was found that a silicon compound was formed when the vicinity of the interface was confirmed by an electron probe microanalyzer (EPMA-1610 manufactured by Shimadzu Corporation). In addition, cracks occurred in all of the water quenching tests.

(Comparative Examples 4 and 5)
Fine powders of CrSi ₂ and MnSi were integrally sintered with sample A by the SPS method instead of sample H in Example 1. Let these thermoelectric conversion elements be Comparative Examples 4 and 5.
In Comparative Examples 4 and 5, no new compound formation was confirmed near the bonding interface, but cracks were generated in all of them in the water quenching test. That is, it was found that it is very difficult to join the Si clathrate and the transition metal silicide. Therefore, it can be seen that joining with a transition metal in which a transition metal silicide is formed at the interface with the Si clathrate is also very difficult.

According to the present invention, a thermoelectric conversion material composed of an n-type Si clathrate compound and a p-type Si clathrate thermoelectric conversion material are directly bonded to each other, and there are no cracks or cracks at the interface thereof, and electricity is provided. It is possible to provide a thermoelectric conversion element that is well-bonded both physically and physically and a thermoelectric conversion element module using the thermoelectric conversion element, and has high industrial utility.

11 n-type thermoelectric conversion material part 12 p-type thermoelectric conversion material part 42 Low temperature side wiring 51 High temperature side insulation board 52 Low temperature side insulation board 60 Thermoelectric conversion module

Claims (8)

A p-type thermoelectric conversion material unit made of a p-type Si clathrate compound,
An n-type thermoelectric conversion material unit made of an n-type Si clathrate compound,
Have,
A U-shaped thermoelectric conversion element in which the p-type thermoelectric conversion material portion and the n-type thermoelectric conversion material portion are directly bonded on the high temperature side and separated from each other on the low temperature side.
The p-type Si clathrate compound and the n-type Si clathrate compound both have an AxBySizXw composition.
x + y + z + w = 54, w contains 0, A is Ba or Sr, and B is one or more elements selected from the group consisting of Ga, Al, Cu, Ni, Au and Pt. Yes, X is an optional additive element selected from one or two types of B (boron) or Pd, a U-shaped thermoelectric conversion element. The U-shaped thermoelectric conversion element according to claim 1, wherein A in the AxBySizXw is Ba. The U-shaped thermoelectric conversion element according to claim 1 or 2, wherein the n-type Si clathrate compound has a composition in which B in AxBySizXw is Ga and Al. The U-shaped thermoelectric conversion element according to claim 1 or 2, wherein the n-type Si clathrate compound has a composition in which B in AxBySizXw is Cu. The U-shaped thermoelectric conversion element according to claim 1 or 2, wherein the n-type Si clathrate compound has a composition in which B in AxBySizXw is Ni. The U-shaped thermoelectric conversion element according to any one of claims 1 to 5, wherein the p-type Si clathrate compound has a composition in which B in AxBySizXw is Au or Pt . A thermoelectric conversion module using the U-shaped thermoelectric conversion element according to any one of claims 1 to 6. A mobile body equipped with the thermoelectric conversion module according to claim 7.

Images