JP2003037300A - Liquid-metal jointed thermoelectric conversion module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To further improve the power generating efficiency and service life of a thermoelectric conversion module. SOLUTION: In order to eliminate factors hindering the improvement of the power generating efficiency of the thermoelectric conversion module, the electrodes of the module are formed to have gentle projecting curved surfaces. Consequently, the projecting sections of the electrodes improve the thermal resistances and electrical resistances of the electrodes, while producing very thin liquid-metal surfaces, by pushing the liquid metal taking charge of bond to the side sections of the electrodes. In addition, it has been confirmed that when the thickness of the liquid metal applied to the electrodes is adjusted to <=20 μm, the service life of the module is improved.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a thermoelectric conversion module using a thermoelectric conversion element. These thermoelectric conversion modules are also used for factory waste heat recovery or automobile waste heat recovery.

[0002]

2. Description of the Related Art Thermoelectric conversion modules are of two types, p-type and n-type.
When two types of semiconductor thermoelectric conversion elements are electrically connected in series and thermally connected in parallel and a temperature difference is applied between the joints, electromotive force is generated, and when a load is connected to the outside, a current flows. The electric output can be obtained. In this way, the principle of converting thermal energy into electrical energy using a thermoelectric conversion element is well known.

In a conventional thermoelectric conversion module (hereinafter simply referred to as "module"), thermoelectric conversion elements are two-dimensionally arranged and each thermoelectric conversion element is fixed to an electrically insulating flat plate by a method such as soldering. It had a modular structure.

The performance of the module depends on the material of the thermoelectric element, the size and shape of the element, the number of element pairs forming the power generation module, and further on the size and shape of the electrodes and the method of joining the elements to the electrodes. .

For example, as the thermoelectric material, a bismuth tellurium type thermoelectric semiconductor element is used, and the size of the element is 3.2 × 3.2 × 1.
72mm in size, the electrode is a copper piece of 10.8 x 3.8 x 0.5mm. The number of element pairs is 18, and when the module size is 50 x 50 mm, the power generation conversion efficiency is about 3% when soldered.

[0006]

One of the factors that hinder the improvement of power generation efficiency is soldering, and the thickness of the solder causes heat resistance, which makes heat transfer and heat conduction difficult, and increases electrical resistance, resulting in high efficiency. Is decreasing.

[0007] The goodness and badness of the "wettability" in soldering are one of the criteria for joining evaluation, but the best method is still being pursued. Further, when voids (holes) are generated in the solder of the connecting portion, the thermal resistance and the electric resistance that hinder the improvement of the power generation efficiency are increased. However, at present, no method has been found to prevent these occurrences.

On the other hand, it is possible to use liquid metal instead of solder, but since liquid metal is a liquid at room temperature as compared with solid bonding such as solder, it has a large chemical activity and diffuses into the thermoelectric element. It may form an alloy and cause an increase in heat resistance and electric resistance.

As a countermeasure against this, the thermoelectric element is provided with a protective film formed by plating metal such as nickel. However, since this plating step is performed on the wafer-shaped element, if the element is cut out after plating, the side surface of the element, which is the cut-out surface, is not plated.

Then, the liquid metal diffuses into the thermoelectric element from the non-plated side surface to form an alloy. Therefore, power generation efficiency begins to decrease after several tens to one hundred and several tens of hours,
There was a problem that a long life could not be maintained.

[0011]

According to the present invention, in order to eliminate the factor that hinders the improvement of power generation efficiency, the electrode has a gentle convex curved surface. As a result, the liquid metal that is responsible for bonding is pushed to the side by the electrode of the convex portion, and an extremely thin liquid metal surface is created, improving thermal resistance and electric resistance.

At the same time, when the liquid metal is extruded, the action of letting bubbles escape to the side surface is suppressed, the generation of voids is suppressed, and the thermal resistance and electric resistance are improved.

As a method of forming a copper electrode having a gently convex curved surface, press punching is adopted when processing a copper plate into a predetermined size, but a curved surface generated at the time of manufacture may be used.

An example of a gently convex curved surface generated by press punching is shown in FIGS.

In order to prolong the life, it is necessary to prevent the liquid metal from coming into direct contact with the portion without the protective film of the thermoelectric element. The excessively applied liquid metal will squeeze out to the side surface where the protective film of the thermoelectric element cannot be applied.

If the coating amount is kept to a necessary minimum, only the function of the liquid metal, that is, the bonding between the thermoelectric element and the electrode, is performed, and excess liquid metal that shortens the life is not generated.

The thickness of the liquid metal may be 20 μm or less, preferably 17 μm or less.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention and is an exploded perspective view of a thermoelectric conversion module before assembly.

FIG. 1 shows the entire flat plate type module,
Reference numeral 22 is a plate-like electrically insulating high thermal conductive substrate (hereinafter referred to as “substrate”), 23 is a thermoelectric conversion element, 24, 25,
Is an electrode, and 26 and 27 are grid-shaped electrode holders, through holes of which are formed in a grid, respectively.
4, 25 are loosely fitted and held at fixed positions so as to be movable within a minute range. Reference numeral 30 denotes a lattice-shaped element holder, in which through holes are formed in a lattice shape, and the thermoelectric conversion elements 23 are loosely fitted in these through holes and are held at fixed positions movably within a minute range.

Before assembly, the electrodes 24, 2
A liquid metal (not shown) that is liquid at room temperature is applied to the surface of No. 5 facing the thermoelectric conversion element 23 using a cotton swab or the like. Liquid metal is similarly applied to both surfaces of the thermoelectric conversion element 23. The grid-shaped electrode holder 27 is placed on the substrate 22, and the electrodes 25 are loosely fitted in the through holes.

Next, the grid-like element holder 30 is placed thereon, the thermoelectric conversion elements 23 are loosely fitted in the through holes, and the grid-like electrode holder 26 is placed thereon. The electrodes 24 are loosely fitted in the respective through holes, and the substrates are further stacked.
Thermal conductive grease is applied to the inner surface of the substrate as needed.

Finally, the bolt is used as a substrate and each holder 26, 3 is
Insert the nuts (not shown) through the through holes at the four corners of 0 and 27.
Etc., or tap screws are formed in the holes at the four corners of the substrate 22 and screwed into the tap screws to fix the whole. At this time, the spacer is placed on the high temperature side of the grid electrode holder 2
6 and the lattice-shaped element holder 30. The spacer may be interposed between the grid electrode holder 27 and the grid element holder 30 on the low temperature side, or both.

If the spacer is placed on the high temperature side, the grid electrode holder 26 on the high temperature side can be prevented from falling. Further, if it is placed on the low temperature side, the lattice element holder 30 is prevented from falling.

Here, the liquid metal used in the present invention which is liquid at room temperature will be described. Examples of the liquid metal used include mercury and gallium (liquid at 30 ° C.) in which indium (solid at room temperature) is dissolved. In the experiment in which the characteristics shown in FIGS. 2 and 3 described later were obtained, gallium: indium mixed at a weight ratio of 3: 1 was used. It is liquid at room temperature at this mixing ratio. As described above, when the amount of gallium is further large, it is a liquid, but conversely, when the amount of indium is increased, a solid is deposited, but it is melted when the temperature is raised.

The electrodes 24 and 25 and the thermoelectric conversion element 23
However, according to the result of the experiment, the power generation operation did not occur because of poor contact when the liquid metal was not used.

The substrate may be made of a material having a high thermal conductivity, such as aluminum, which has been subjected to an insulating coating process by anodic oxidation and sealing treatment. The thermoelectric conversion element 23 is preferably a bismuth tellurium-based thermoelectric semiconductor element having excellent thermoelectric conversion efficiency up to 250 ° C., and p-type elements and n-type elements are alternately arranged and electrically connected in series. The electrodes 24 and 25 (materials: copper and copper alloys, etc.) are made of liquid metal (material: indium: gallium in a weight ratio of 3: 1).
Are electrically connected in series and the final electrical output is obtained from the electrical output terminal. The grid electrode holders 26 and 27 and the grid element holder 30 (material: Bakelite etc.)
4, 25 and the thermoelectric conversion element 23 are put in place.

Next, the contact between the electrodes 24 and 25 and the thermoelectric element 23 will be described. FIG. 4 shows a cross section of a short side of a rectangular electrode, and FIG. 5 shows a thermoelectric element sandwiched between two electrodes. In this way, when the surface of the electrode on the high temperature side that contacts the thermoelectric element is formed in a convex shape, the excess liquid metal on the contact surface is extruded toward the side, and at the contact portion, only a very thin film-shaped liquid metal is formed. , Electrical resistance and thermal resistance are reduced.

A specific method of forming the electrode is to prepare a male / female mold and punch it with a press machine. Therefore, although the shape is almost flat, a curved surface formed when punching by a press machine is formed in the end cross section of the entire rectangular corner. Since the electrode material is copper, it is "sticky", so the cross section of the cut is not sharp. The electrodes used in the experiment are 10.8 × 3.8 mm.

FIG. 2 shows two output characteristics of a thermoelectric conversion module 20 by soldering, a thermoelectric conversion module 20 using a liquid metal of indium gallium (weight ratio 3: 1), and conversion using a convex electrode. Show efficiency. Liquid metal 1
2 and 2 have the same size and material, and the composition of the liquid metal, and show data variations. Thermoelectric conversion element 2
Bismuth tellurium was used as 3. The “conversion efficiency” shown in the figure is the ratio of the electric output obtained from the thermoelectric conversion module to the thermal energy given to the thermoelectric module. The "high temperature side temperature" represents the temperature on the high temperature side when a temperature difference is applied to the modules, and the cooling side was cooled with cold water at 10 ° C to perform the experiment. According to the figure, it has been found that the use of the convex electrode improves the conversion efficiency by about 0.5% as compared with the conventional soldering and the liquid metal with the concave electrode.

Next, FIG. 3 shows the decreasing tendency of the conversion efficiency with the passage of time due to the difference in the coating amount of the liquid metal. As is clear from this figure, it was found that if the thickness of the liquid metal applied to the electrode is 20 μm or less, the long-term conversion efficiency does not significantly decrease.

[0031]

EFFECTS OF THE INVENTION In the thermoelectric conversion module, the shape of the electrode piece that contacts the thermoelectric conversion element is formed to be convex on the side facing the thermoelectric conversion element, so that the liquid metal on the contact surface becomes extremely thin and the electrical resistance and heat The resistance has decreased. In addition, when the thickness of the liquid metal applied to the electrodes was reduced, the life was significantly improved compared with the conventional one.

[Brief description of drawings]

FIG. 1 is an exploded perspective view of a liquid metal junction thermoelectric module according to the present invention.

FIG. 2 Comparison diagram of conversion efficiency

[Figure 3] Life comparison chart

FIG. 4 is an enlarged view of an electrode according to the present invention.

FIG. 5 is a schematic diagram of a contact state of an electrode and a thermoelectric conversion element according to the present invention.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshitaka Ota             1-1-1 Higashi 1-1-1 Tsukuba City, Ibaraki Prefecture             Inside the Tsukuba Center, National Institute of Advanced Industrial Science and Technology

Claims (4)

[Claims] 1. A thermoelectric conversion element, comprising: a plurality of thermoelectric conversion elements; and an electrode for connecting the thermoelectric conversion elements to each other, wherein the thermoelectric conversion element and the electrode are arranged so as to be movable relative to each other. In the thermoelectric conversion module in which the electrical connection is performed by a liquid metal that is liquid at room temperature, the electrode on the high temperature side is
A thermoelectric conversion module, which is formed in a shape that is convex with respect to the thermoelectric conversion element. 2. The thermoelectric conversion module according to claim 1, wherein the electrode is made of copper and an alloy containing copper as a main component. 3. The thermoelectric conversion module according to claim 1, wherein the convex shape of the electrode is formed by punching. 4. A thermoelectric conversion element, comprising: a plurality of thermoelectric conversion elements; and an electrode for connecting the thermoelectric conversion elements to each other, wherein the thermoelectric conversion element and the electrode are arranged so as to be relatively movable, and electrically. In the thermoelectric conversion module in which the physical connection is made by a liquid metal that is liquid at room temperature, the thickness of the liquid metal applied to the surface of the high temperature side electrode that contacts the thermoelectric conversion element is 20 μm or less. .