JPH09162448A - Thermoelectric element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lighten thermal distortion by making this thermoelectric element has such structure that a semiconductor layer, which has thermoelectric function, is joined, at the face substantially vertical to the direction of the heat flux, with the electrode layer consisting of unidirectional carbon-carbon complex material where carbon fibers are arranged in substantially parallel with the direction of heat flux at the time of being used as a thermoelectric element. SOLUTION: One side of each semiconductor layer 1 and 1' which has p-type and n-type heat function is joined, at junction face 4, with a metallic electrode 2 on high temperature side. Residual one side is joined, at junction face 5, in Y direction (vertical) with a metallic electrode 2' on low temperature side through a UDCC electrode (unidirectional carbon-carbon complex material) 3 joined at the junction face 7. The semiconductor layer 1 and 1' are isolated and insulated with a heat insulating material 6. The junction between the UDCC face and the metal is good in solder adhesion such as metallic brazing agent or the like, so the formation is easy, and the thermal expansion coefficient in Y direction of the UDCC is about middle between that of the metal and that of the semiconductor, and also the elastic modulus in Y direction is small at 10GPa, so the thermal distortion can be lightened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric element of a thermoelectric generator that directly converts thermal energy into electric power, or a Peltier element (thermoelectric element in a broad sense) of a thermoelectric cooling apparatus that produces low temperature cold heat by passing an electric current through the element. In particular, the present invention relates to a thermoelectric element useful for power conversion and recovery from waste heat of various heat engines and factories, a small generator, a cooling / heating system having a simple structure, and a refrigerator.

[0002]

2. Description of the Related Art As shown in FIG. 5, a conventional thermoelectric element has a semiconductor layer (10, 1) having p-type and n-type thermoelectric power.
0 ') with a metal electrode (20) having a much lower electrical resistivity and a higher thermal conductivity than these, and a joint surface (3).
0) and the π shape as shown in FIG.
There is a U-shape shown in FIG. 5B in which the junction surface (40) on one side of the p-type and n-type semiconductor layers (10, 10 ') is a direct junction. ′) And the metal electrode (20) must be joined. Moreover, the technique for forming this junction is one of the important problems in the production of thermoelectric elements, and this requires advanced or complicated techniques, which makes the thermoelectric elements expensive and hinders their general spread. In many cases.

Semiconductor materials for thermoelectric elements include bismuth-tellurium-based, lead-germanium-tellurium-based, and silicon-based materials.
There are germanium series, selenium series, iron silicide series,
For these, materials having a high thermoelectric power, a low electric resistivity and a low thermal conductivity in the operating temperature range are used. On the other hand, the electrode material is preferably a metal such as aluminum, copper or silver which has a low electric resistivity and a high thermal conductivity.

The bonding between these thermoelectric material semiconductors and the electrode metal is sufficiently adherent, mechanically strong, and capable of withstanding the thermal strain caused by the rapid heating / cooling thermal cycle, and at the bonding interface at high temperature. Since it is required to form a joint that is unlikely to deteriorate due to chemical reaction and diffusion between the semiconductor and the solder, it is necessary to use a high-performance product such as compression bonding with a spring, soldering with a diffusion barrier treatment, and diffusion bonding with a matched thermal expansion coefficient. Complex technologies are often used in combination.

For example, even in a bismuth-tellurium-based thermoelectric element used at a relatively low temperature of 200 ° C. or lower, the low temperature side is joined with a copper electrode and a special bismuth / tin based solder, and the high temperature side is made of metal such as aluminum. Plasma spraying or diffusion barrier treatment such as Ni plating or plasma spraying on the bonding surface of the semiconductor and the electrode metal, and then bonding with high temperature solder has been performed (Teledyne EnergySy
stems, Eng. and Appl. Manual, Aug. 1983 :).

Further, in an example of a lead-germanium-tellurium-based thermoelectric element, as shown in FIG. 6, the low temperature side of the p-type and n-type columnar semiconductors (10, 10 ') are respectively columnar. Ni-PbTe eutectic alloy is welded to the Ni cap (50) of No. 3, and the outer peripheral surface of the Ni cap is covered with insulating ceramics. The Ni cap portion is heated from the low temperature side to the high temperature side by the spring (60). Press down on
By adopting a method of pressure-bonding the semiconductor (10) on the high temperature side and the electrode part (20), the thermal strain due to heating and cooling on the high temperature side is relaxed (G. Guzzoni, Proc. 1st ICTEC, Ar
lington, p136, 1976 :). In addition, in FIG.
0 is an insulating material, 80 is a heat dissipation bed, 90 is a hot wall, 100
Indicates a radiation fin, respectively.

Further, in the example of the thermoelectric element for the power source of the silicon-germanium-based planetary explorer Voyager, the high temperature junction is a silicon-germanium-based p-type or n-type semiconductor,
A plate made of an Si-Mo eutectic alloy that is an electrode is diffusion bonded by hot pressing. The Si-Mo alloy has poor thermal conductivity and electrical resistance characteristics, but these are sacrificed. Avoids thermal distortion. Also, for low temperature side joining,
In order to avoid deterioration due to thermal distortion of the semiconductor and electrode, it is welded or diffusion bonded to a tungsten plate and then bonded to copper (PA O'Rioda, Proc. 4th IECEC, Arlington, p1
5, 1982: see).

As can be understood from these examples, in the prior art, alloys of aluminum, copper, silver and the like having low electric resistance and high thermal conductivity are preferable as electrode materials, but these metals and semiconductors are used. The joining method of is difficult, especially in the high temperature part, the part where heating and cooling are repeated, since the difference in the thermal expansion coefficient of these metals and the semiconductor is large, cracking due to thermal strain, deterioration such as peeling easily occurs, There is a problem that these metals cannot be directly bonded to the semiconductor. Further, in a high temperature portion (200 ° C. or higher), a solder component may chemically react with or diffuse into the semiconductor layer, which may deteriorate the characteristics of the semiconductor layer. The layers had to be pre-formed and then soldered.

[0009]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to facilitate a method for joining a semiconductor and an electrode in a practical technique of a thermoelectric element, and to join a semiconductor and an electrode.
It is an object of the present invention to provide a thermoelectric element having a novel structure which can avoid the problem of deterioration due to thermal strain, or the problem of deterioration due to reaction between semiconductor and solder or diffusion and diffusion.

[0010]

According to the present invention, a unidirectional carbon in which a semiconductor layer having thermoelectric power has carbon fibers arranged substantially parallel to the direction of heat flux when used as a thermoelectric element. There is provided a thermoelectric element having a structure in which an electrode layer made of a carbon composite material is bonded on a surface substantially perpendicular to the direction of each heat flux.

The present inventors have found that unidirectional carbon-carbon composite material (unidirectional carbon fiber reinforced carbon matrix composite material, hereinafter UDC).
C is sometimes abbreviated as a material having excellent heat resistance, and the thermal conductivity in the carbon fiber array direction (hereinafter sometimes abbreviated as X direction) is very high, and is about 700 W / (m ·
k) What can be done (aluminum: about 240 W / (m
・ K), copper: about 400W / (m ・ k), silver: about 430W
/ (M · k)}, and the electrical resistivity is 2 × 10 ⁻⁴ Ω · cm
Is sufficiently small, and the coefficient of thermal expansion is 5 in the direction perpendicular to the carbon fiber array direction (hereinafter sometimes abbreviated as Y direction).
10 × 10 ^-6 / K, which is relatively small, about halfway between semiconductor materials and metals, and has an elastic modulus in the Y direction of about 10 GPa.
It is very small and has elasticity in the Y direction, so it has the ability to relax thermal strain, and also has good wettability and adhesion to high temperature solder such as silver solder and copper solder, and plasma spraying at high temperature with semiconductor materials. Alternatively, the present invention has been completed as a result of researching a material having characteristics such as having a bondability by hot pressing and the like, and earnestly researching application of this material to a thermoelectric element.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
The thermoelectric element of the present invention is characterized by having a structure as shown in FIG. 1 or 2, for example. That is, FIG. 1 shows the case where the UDCC is used only on one side of the semiconductor layer, and FIG.
(B) uses UDCC only on one side on the high temperature side. The structure of FIG. 2 is a case where UDCC is used on both surfaces of the semiconductor layer.

More specifically, FIG. 1A shows a semiconductor layer (1, 1 ') having p-type and n-type thermoelectric power.
However, one surface is bonded to the high temperature side metal electrode (2) at the bonding surface (4), and the other surface is bonded at the bonding surface (7).
It is bonded to the low temperature side metal electrode (2 ') at the bonding surface (5) through the CC electrode (3), and the semiconductor layers (1) and (1') are isolated by a heat insulating material (6). It shows the case where it has a different structure. Further, FIG. 1B shows that the respective semiconductor layers (1, 1 ′) having p-type and n-type thermoelectric powers are
The case where it differs from FIG. 1A only in that it is joined to the high temperature side metal electrode (2) through the UDCC electrode (3). In addition, FIG. 2 shows that both sides of the respective semiconductor layers (1, 1 ′) having p-type and n-type thermoelectric powers have a high temperature side metal electrode (2) and a low temperature side metal via the UDCC electrode (3). Only the point of being joined to the electrode (2 ′) is different from that in FIG. 1.

The UDCC used has a thermal conductivity in the X direction of 400 to 700 W / (m · k) at room temperature and 300 to 500 W / (m · k) at a temperature of 500 K, and has a matrix structure. A material that is sufficiently dense and capable of precise cutting is preferable. For example, a material obtained by a technique such as Japanese Patent Application No. 6-323507, Japanese Patent Application No. 2-44029, and Japanese Patent Application No. 3-234166 may be used. 1-5 mm with a rotating saw in the direction
A plate-like object (the X direction is the thickness direction) cut into a thickness and, if necessary, a polished cut surface is suitable.

Since such a plate-shaped material of UDCC has a very high heat resistance, a semiconductor layer (1) as shown in FIG. 1 ') is bonded to the UDCC plate-like body (3) to easily prepare a unit (8). As a method for manufacturing such a unit (8), the method of depositing the semiconductor layer (1) on the UDCC substrate (3) is not limited to the plasma spraying method, and various methods can be adopted. For example, when a very thin semiconductor layer having a thickness of 1 to several μm is formed, plasma CVD is used.
Method, sputtering, ion plating method, or the like can be used. In addition, the semiconductor powder is dispersed in a thermally decomposable viscous liquid to form a paste or paint,
By applying or printing this on the UDCC substrate (3) with a constant thickness, firing and sintering,
On the substrate (3), a semiconductor (1,
You can get a unit (8) with 1 ').

A large number of such units (8) are manufactured in advance for semiconductor p-type and n-type, and the UDCC surface of the pair of p-type and n-type units is attached to the low temperature side electrode plate (2) such as a copper plate. ') With high temperature solder such as copper solder, fill the gap between the p-type and n-type units with a heat insulating material such as quartz powder,
Then, the high temperature side electrode (2) is formed by plasma spraying of an appropriate metal, whereby the thermoelectric element having the structure of FIG. 1 (a) is manufactured.

The element having the structure shown in FIG. 1B is basically the same as the structure shown in FIG. 1A except that the high temperature side and the low temperature side are reversed. When the low temperature side electrode (2 ′) is formed by plasma spraying of metal, there is no problem since it is exactly the same as in FIG. 1A, but when this is bonded to a semiconductor as a metal plate, it depends on the difference in thermal expansion coefficient. It is necessary to select the joining method in consideration of the problem of thermal strain, the solder element diffusion reaction at the interface, and the like depending on the operating temperature. However, even in this case, since the joining is on the low temperature side, the problem is considerably mitigated, and it is also possible to perform brazing directly to a copper plate or the like.

In many cases, the semiconductor layer of the thermoelectric element can be formed by agglomerating and sintering a semiconductor powder whose composition has been adjusted in advance by a hot pressing method, and the UDCC plate-like material described above has a heat resistance. Since it is high and the compressive strength is sufficiently high, a unit (8 ′) as shown in FIG. 4 can be produced by sandwiching a certain amount of semiconductor powder from above and below with this plate-like material and hot pressing. In this case, some semiconductor layers cannot be bonded to the UDCC plate by hot pressing, but the semiconductor layer is preformed by cold pressing in advance, and the semiconductor layer and the UDCC plate are not formed. The unit (8 ′) can be prepared by applying an appropriate adhesion aid on the interface and then hot pressing. When the semiconductor constituent element slightly reacts at the interface at a certain temperature to form a strong bond, as in the case where the semiconductor layer is a Si-Ge system, the bonding aid is not necessary.

The unit (8 ') shown in FIG. 4 is also manufactured by hot pressing the two units (8) shown in FIG. 3 manufactured by the plasma spraying method mentioned above on the semiconductor side. can do. This method is suitable for producing a so-called split junction type thermoelectric element. That is, 78 atom% Si-G with good performance in the high temperature region on the high temperature side
e, 63.5 atom% Si- with good performance in the low temperature side in the middle temperature range
As Ge, the unit (8) shown in FIG. 3 is produced, and then two of them are combined and hot pressed to produce the unit (8 ′) of FIG.

Such a unit (8 ') is a semiconductor p
Type and n-type are manufactured in advance, and one side of the UDCC of the pair of p-type and n-type units is bonded to the low-temperature side electrode plate (2 ′) such as a copper plate with high-temperature solder such as copper solder to form the p-type. n
Fill the gap between the mold units with a heat insulating insulating material such as quartz powder,
After that, the high temperature side electrode (2) is formed by plasma spraying of an appropriate metal, or the high temperature side electrode is adhered to the UDCC surface by a high temperature solder such as copper solder in the same manner as the low temperature side electrode. A thermoelectric device having a structure of 2 is manufactured.

[0021]

As shown in FIGS. 1 (a), 1 (b) and 2, that is, the semiconductor layer having thermoelectric power has carbon fibers arranged substantially parallel to the direction of heat flux when used as a thermoelectric element. The thermoelectric element of the present invention has a structure in which it is bonded to the electrode layer made of UDCC on the surface substantially perpendicular to the direction of each heat flux. Since it has good solder adhesion, it is easy to form a joint, and the coefficient of thermal expansion in the Y direction of UDCC is about halfway between that of metals and semiconductors, and at the same time, the elastic modulus in the Y direction is as small as 10 GPa. In addition, since a thermal strain relaxation effect is exhibited, it is possible to bond a metal plate having a large coefficient of thermal expansion (17 to 20 × 10 ⁻⁶ / k) such as copper through a UDCC to a semiconductor layer even at a high temperature side. It will be possible. Most of the constituent elements of UDCC are carbon, and the diffusion chemical reaction between carbon and the semiconductor constituent elements is unlikely to occur, so that the deterioration due to the chemical reaction of the semiconductor is extremely small.

Further, the thermal conductivity of UDCC in the X direction is very large, larger than that of copper or silver at a low temperature, and almost the same as that of copper or silver even at a high temperature of 500 K or more. It is possible to provide a highly efficient thermoelectric element that exerts a reducing effect. Further, the electrical resistivity of the UDCC in the X direction is about 2 × 10 ⁻⁴ Ω · cm, and even if the thickness is 3 mm, the resistance per unit area in the X direction is 6 ×.
1 / for the resistance of a semiconductor layer with the same thickness as 10 ^-5 Ω
Since it is sufficiently small as about 10 and its internal resistance loss is small, a highly efficient thermoelectric element can be provided.

Further, since the heat resistance of UDCC is extremely high and it has the effect of relaxing the thermal strain, the UDCC plate is used as a substrate and the semiconductor is plasma sprayed or U
The basic unit of the thermoelectric element can be manufactured with high productivity by hot pressing with the semiconductor sandwiched between the DCC plates. According to this method, the thickness of the basic unit of the thermoelectric element can be increased as compared with the area which has not been seen in the past. It is possible to manufacture a thin layer type thermoelectric element having a semiconductor layer having a small thickness.

The value of the thermoelectric element is not simply determined by the conversion efficiency, but if the conversion efficiency is the same, the manufacturing cost per unit area has an important value. The molding cost of the semiconductor layer for the thermoelectric element is very high, and the thin layer thermoelectric element can reduce the semiconductor layer cost per area.
Even if the board cost is added, the cost can be reduced. UDCC
Has a thermal conductivity in the X direction of 300 to 700 W / (m · k)
Is very large, but that in the Y direction is 10 to 30 W / (m
Since it is characterized by being very small as (k), the heat loss due to the heat flow in the Y direction is small, and even if the semiconductor layer is sufficiently thin in the element of the structure of the present invention, the heat loss due to the heat flow bypassing the semiconductor layer The conversion efficiency of the device can be kept high as a result.

[0025]

The thermoelectric element of the present invention is a unidirectional carbon-carbon composite material in which a semiconductor layer having thermoelectric power has carbon fibers arranged substantially parallel to the direction of heat flux when used as a thermoelectric element. The electrode layer made of (UDCC) has a structure in which the electrode layers are bonded to each other in a plane substantially perpendicular to the direction of each heat flux, so that the following excellent effects are exhibited. B) The solder adhesion between the UDCC surface and the metal is good, so that it is easy to form a joint, and the coefficient of thermal expansion in the direction (Y direction) perpendicular to the arrangement direction of the carbon fibers of the UDCC is relatively small.
And since the elastic modulus is small, a thermal strain relaxation effect is expressed,
Bonding of metal plate and semiconductor layer via UDCC is possible even at high temperature. (2) Chemical reaction of semiconductor is difficult because diffusion and chemical reaction between carbon, which is the main constituent element of UDCC, and semiconductor constituent element do not occur easily. Very little deterioration due to C.)) The thermal conductivity of the UDCC carbon fiber array direction (X direction) is very large, so it exerts the effect of reducing heat loss as an electrode of the thermoelectric element. (2) UDCC X direction Has a small electric resistivity and the resistance per unit volume in the X direction is sufficiently smaller than the resistance of the semiconductor layer, so that the internal resistance loss is small. (E) UDCC has extremely high heat resistance and has a thermal strain relaxation effect. , The basic unit of the thermoelectric element can be manufactured with high productivity by plasma spraying the semiconductor using the UDCC plate as the substrate, or by hot pressing the semiconductor with the UDCC plate sandwiching the semiconductor. It is possible to manufacture, and according to this method, it is possible to manufacture a thin-layer thermoelectric element having a semiconductor layer having a thickness smaller than that which has not been seen until now.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a structural example of a thermoelectric element of the present invention when UDCC is used on one surface of a semiconductor layer, (a)
Is an example of using UDCC on the low temperature metal electrode side of the semiconductor layer, and (b) is an example of using the UDCC on the high temperature metal electrode side of the semiconductor layer.

FIG. 2 is a schematic cross-sectional view showing a structural example of a thermoelectric element of the present invention when UDCC is used on both sides of a semiconductor layer.

FIG. 3 is a schematic cross-sectional view showing a structural example of a unit in which a semiconductor layer is formed on a UDCC substrate.

FIG. 4 is a schematic cross-sectional view showing a structural example of a unit in which a semiconductor layer is sandwiched between UDCC plates.

FIG. 5 is a schematic cross-sectional view showing a basic shape of a conventional thermoelectric element, (a) showing a π-type element and (b) showing a U-type element.

FIG. 6 is a schematic cross-sectional view showing a structural example of a conventional high temperature type thermoelectric element power generator.

[Explanation of reference numerals] 1 semiconductor layer (type I) 1'semiconductor layer (type II) 2 high temperature side metal electrode 2'low temperature side metal electrode 3 UDCC electrode 4 semiconductor / metal joint surface 5 UDCC / metal joint surface 6 heat insulation Insulating material 7 Semiconductor / UDCC bonding surface 8 Semiconductor layer / UDCC substrate unit 8'UDCC plate / semiconductor layer / UDCC plate unit 10 Semiconductor layer (Type I) 10 'Semiconductor layer (Type II) 20 Metal electrode 30 Semiconductor / Metal Joining surface 50 Ni cap 60 Spring 70 Insulating material 80 Radiating bed 90 High temperature wall 100 Radiating fin

Claims (1)

[Claims] 1. A semiconductor layer having thermoelectric power is provided on an electrode layer made of a unidirectional carbon-carbon composite material in which carbon fibers are arranged substantially parallel to a direction of heat flux when used as a thermoelectric element. A thermoelectric element having a structure in which it is joined on a surface substantially perpendicular to the direction of the heat flux.