JP2000077730A - Porous thermoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To contrive to raise the strength of a thermoelectric conversion element consisting of porous semiconductor layers by a method wherein an inorganic functional fiber, which is used as an insulator, is provided between the P-type porous semiconductor layer and the N-type porous semiconductor layer. SOLUTION: An insulation between a P-type porous semiconductor layer 9 of a conductive form and an N-type porous semiconductor layer 10 of a conductive form is made by an inorganic functional fiber 8, which is used as an insulator, to form a P-N junction part 3. One thermoelectric conversion element is formed of the layers 9 and 10, the insulator 8 and an electrode 6 is installed on the element on the uppermost part on the low-temperature contact part side 5 of a branch end on the opposite side to the high-temperature contact part side 4 of the P-N junction part 3, and the electrode 6 is made to connect with the outside. Moreover, elements, which are formed into the entirely same constitution and respectively consist of semiconductor layers 13 and 14 and an insulator 8, are put separately from each other by the same inorganic functional fibers 15 as one used for the insulation between both semiconductor layers of the element under this one thermoelectric conversion element, and are respectively connected with the upper elements through conductors 12 at the low-temperature contact parts of the branch ends.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric conversion element having a porous structure for use in gas-fired thermoelectric power generation.

[0002]

2. Description of the Related Art In recent years, various energy conversion technologies have been developed from the viewpoint of effective use of energy in relation to environmental problems. Among them, there is thermoelectric power generation as one of the means capable of effectively converting the temperature difference into electric power. Thermoelectricity is a phenomenon in which different types of electrical conductors are joined together, such as thermocouples used for temperature measurement, and current is generated when one of the junctions is set to high temperature and the other end is set to low temperature. Thermocouples of p-type and n-type semiconductors having higher current generation efficiency have been used in various forms from space development to consumer appliances.

The use of this thermoelectric phenomenon for power generation is thermoelectric power generation, and various thermoelectric conversion elements have been studied. FIG. 1 schematically shows the structure of a thermoelectric conversion element as a columnar shape.
(Type) semiconductor 1 and an n-type (or p-type) semiconductor 2 are pn-joined by a junction surface 3. Here, joining surface 3
When the right side surface 4 orthogonal to the above is heated and the left side surface 5 serving as a branch end is cooled, a thermoelectromotive force is generated and current can be taken out by the electrode 6. The right surface 4 is a high-temperature contact portion, and the left surface 5 is a low-temperature contact portion. In this case, the high-temperature contact portion is a flat surface and the legs of the p-type and n-type semiconductors hang from the surface, and thus are also called π-type thermoelectric conversion elements.

The temperature difference between the high-temperature contact portion and the low-temperature contact portion must be increased in view of the power generation efficiency. However, in order to avoid a decrease in the temperature difference due to heat conduction, the length L of FIG. is there. On the other hand, since the bonding surface 3 is orthogonal to the heating surface, T in FIG. 1 cannot be increased to keep the average temperature of the pn bonding surface high. When used for power generation or the like, it is desirable that the semiconductor has excellent heat resistance. Therefore, a silicide is selected for the semiconductor. However, such a material is usually extremely brittle, and the π-shaped shape shown in FIG. The stress concentrates on 3 and is destroyed by a slight deformation during use or in the manufacturing process of the element. Therefore, p
An insulator is inserted into the gap 7 between the type and n-type semiconductors to avoid this.

For example, Japanese Patent Publication No. 54-41315 discloses FIG.
Discloses an invention of a method for manufacturing an element in which an insulator is inserted into the gap 7 of the element as shown in FIG. First, a semiconductor compound powder corresponding to 2 in FIG. 1 is charged into a mold for sintering, and an insulating powder corresponding to a gap and a bonding portion having the same thickness as the insulating powder layer are placed thereon. A heat-generating compound compound powder corresponding to the above is charged, and a conductive compound semiconductor powder different from 2 corresponding to 1 in FIG. 1 is further charged, and these are sintered by pressure and integrated. It is. Japanese Patent Laid-Open Publication No. Hei 3-293783 discloses that a p-type iron silicide (FeSi ₂ ) is used as a semiconductor.
And an n-type iron silicide, and an invention using forsterite (Mg ₂ SiO ₄ ) to which a small amount of B ₂ O ₃ is added as an insulator. This is because the thermal expansion coefficient of the iron silicide of the semiconductor and that of the insulator are almost equal, so that the generation of stress due to heat during use such as heating and cooling is suppressed, and the element is hardly damaged due to the filling of voids. Can be done. It is stated that since the thermal expansion coefficients are equal, even in a manufacturing process of hot pressing and heat-treating the raw material powder of the semiconductor and the insulator, a good element can be obtained without cracking due to thermal strain.

The change in thermoelectromotive force due to the temperature of the thermoelectric conversion element, that is, the thermoelectric power is determined by the material to be used. Therefore, it is necessary to use a material having as high a thermoelectric power as possible. For this reason, various efforts have been made to develop a material having a large thermoelectric power. However, it seems that the material has been improved to almost the limit at present.

However, recently, even when the same thermoelectric conversion material is used, by increasing the temperature difference between the high-temperature junction and the low-temperature junction and making the temperature gradient as steep as possible, the electric output and the power generation efficiency can be significantly improved. Sex was revealed. According to this, the semiconductor of the element is a porous sintered body through which gas can flow from a dense solid. For example, in the case of an element as shown in FIG. 2, a low-temperature fuel mixed gas is applied to the porous body from the left side of the column. And burn on the right side surface. Then, the low-temperature contact side on the left side is cooled by the mixed gas, and the high-temperature contact side on the right side is directly heated by the flame, and there may be a porous body with poor thermal conductivity between them. This makes it possible to significantly increase the temperature gradient between the power generation section and the power generation section, thereby greatly improving the power generation efficiency. By using a thermoelectric conversion element having such a structure, it is expected to produce a highly efficient gas-fired thermoelectric generator.

In order to conduct gas sufficiently, the porosity must be 50%.
It is necessary. The method of manufacturing such a porous body is to first prepare semiconductor particles having a certain size,
These particles are sintered under pressure. However, if the porosity increases, the area of the adhesion portion between the particles decreases, and the strength of the porous body decreases. When a silicide is used as a semiconductor that withstands high heat, a brittle material is used as a porous body, so that the strength of this thermoelectric conversion element is greatly reduced as compared with the case of using a dense solid. In particular, if a thermoelectric conversion element having a shape as shown in FIG. 1 is to be manufactured, the joint is broken by a slight stress. For this reason, it is necessary to pay more attention to the structure, the material of the insulator, the method of using the same, the method of manufacturing the element, and the like than in the case of the thermoelectric conversion element composed of a dense solid.

[0009] There is an experimental example in which a thermoelectric conversion element of this porous body was manufactured using chromel and alumel metal particles used for a thermocouple. In this case, for example, in a cylindrical thermoelectric conversion element made of a porous body having a structure as shown in FIG. 2, the upper half is made of chromel and the lower half is made of alumel. A porous sintered body is prepared, and only the portion of the thickness T serving as the thermocouple junction end at the right end of the cylinder is left, and the junction interface is removed by electric discharge machining or a diamond cutter to form a void.

However, the thermoelectric conversion element of chromel-alumel has a small thermoelectromotive force and cannot be used for a practical thermoelectric generator. From the viewpoint of the magnitude of the thermoelectromotive force and its cost, the material for the element is considered to be optimally iron silicide. However, this material is extremely brittle, and is similar to the case of chromel-alumel which is a metal alloy mainly composed of Ni. Processing such as cutting is impossible at first. To cope with such a problem, for example, Japanese Patent Application Laid-Open No. 9-237920 discloses that in a thermoelectric conversion element using a porous p-type semiconductor and an n-type semiconductor, a ceramic-based material is used in a gap between semiconductors or in an insulating portion. An invention for filling with an adhesive has been proposed. In this case, if it is described as a cylindrical element in FIG.
-Type or n-type semiconductor powder, predetermined length (LT)
Block 9 or 1 in the shape of a half-column
0, and disc-shaped blocks 11 of the same diameter including a pn junction, in which both semi-circular semiconductors corresponding to the layer thickness T of the junction are abutted with powders of both semiconductors, and manufactured by sintering. I do. P-type 9 and n-type 10 in the shape of a half cylinder
A ceramic adhesive as an insulator is applied to and bonded to the cylinder half-split surface portion of the block to form a cylinder. The ceramic adhesive becomes the insulator 8 by heating. The disk 11 is overlaid on the bottom surface of the cylinder so that the pn junction surface is parallel to the bonding surface, and the conductivity type of the cylinder disk matches.
The element is completed by performing sintering for close adhesion. According to this method, a porous body made of a brittle material is used to obtain a heat conversion element that can sufficiently withstand use. Further, the ceramic adhesive preferably has a coefficient of thermal expansion approximately equal to that of the porous semiconductor.

As described above, a thermoelectric conversion element using a porous semiconductor is inevitably made to have a structure having a weak portion in shape using a brittle material. Therefore, the structure requires a more reliable element. A simpler manufacturing method is desired.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a thermoelectric conversion element using a porous semiconductor having a structurally sufficient strength and excellent performance, and a method for easily manufacturing the same. It is in.

[0013]

The semiconductor used for the thermoelectric conversion element for power generation must be made of a material having high heat resistance that can withstand high-temperature use in order to increase the efficiency of power generation, and gas can flow therethrough. It must be porous. Then, in order to burn the passed gas on the surface and raise the temperature to a high temperature, gas conduction must be good and the porosity must be 50% or more.

As a semiconductor that can withstand high temperatures, silicides such as FeSi ₂ are used. However, these materials are brittle in material, and if sintered into a porous material, machining such as cutting is almost impossible. Since the element has a shape in which stress tends to concentrate on the bonding surface as described above, it is necessary to reinforce the element by embedding an insulator between semiconductors. In the case of FeSi _2, the semiconductor changes to p-type or n-type depending on the element added in a small amount.
Although there is no difference in the coefficient of thermal expansion between the semiconductors, if the coefficient of thermal expansion of the insulator between the semiconductors is not the same as that of the semiconductor, stress is generated due to temperature change and the element is destroyed. Will be. That is, the obtained element must not be destroyed by a slight impact.

The present inventors have conducted various studies on such various requirements for thermoelectric conversion elements, paying particular attention to an insulator inserted between semiconductors. When manufacturing a thermoelectric conversion element, as in the method of Japanese Patent Publication No. 54-41315 described above, the raw material powder is placed in a sintering mold, and the semiconductor layer, the insulating layer, the layer of the bonding portion, and the Charge with one semiconductor layer,
In the method of integrating these by pressure sintering, in the case of a porous body, there is anxiety about formation of the insulating layer and the bonding layer. In particular, when it is desired to reduce the thickness of the insulating layer, the formation of the insulating layer becomes insufficient, and the insulating failure often occurs. If each semiconductor layer, insulator, etc. are preliminarily sintered into blocks each having a required shape in advance and assembled to form a final structure and subjected to pressure main sintering, such anxiety disappears, but sufficient porosity is secured. If this is attempted, the block after the preliminary sintering is extremely unstable, and it is difficult to obtain a good product. In addition, this method greatly increases the manufacturing steps. As a result of such studies, it has been found that it is preferable to use inorganic functional fibers for the insulator.

Here, the inorganic functional fiber is formed into a sheet or a blanket and has a heat resistant temperature.
Heat resistant ceramic fiber of 1200 ° C or higher. Using this sheet, for example, when manufacturing a thermoelectric conversion element of the form shown in FIG. 2, the raw material powder of one semiconductor is charged into a mold,
On top of this, an inorganic functional fiber sheet having a required thickness with the pn junction remaining is placed, and then another semiconductor raw material powder is charged so that a porous body is obtained. Perform pressure sintering. This makes it possible to obtain a porous electrothermal conversion element which has sufficient strength in use and is not likely to be damaged by thermal shock such as heating and cooling during production and use.

When the inorganic functional fiber is used as an insulator of a thermoelectric conversion element, it is needless to say that it has sufficient electric insulation, but it has air permeability equal to or higher than that of a porous semiconductor. Since there is no gas conduction hindrance and some deformation is possible, the expansion and contraction of the porous semiconductor at the time of heating and cooling can be absorbed, and there is no risk of breakage due to the difference in the coefficient of thermal expansion from the insulator. In addition, mechanical bonding is also obtained because the semiconductor particles sink into the fibers during pressure sintering, and a slight deformation at that time also has the effect of ensuring the pn junction at the bonding surface of the two semiconductors. it was thought.

As described above, by using the inorganic functional fiber as the insulator, the strength of the thermoelectric conversion element made of the porous semiconductor can be improved, and the element can be manufactured by one sintering step. Will be able to do it. Thus, using this technique, an attempt was made to fabricate a laminated element in which a plurality of thermoelectric conversion elements were connected in series, as schematically shown in FIG. This has a structure in which p-type or n-type semiconductors having mutually different conductivity types are alternately stacked in order with an insulator made of inorganic functional fiber interposed therebetween, and a pn junction, A conductor joint is provided on the low-temperature side, and the high-temperature contact section and the low-temperature contact section are connected in series with the same direction. The figure shows a case in which five elements are connected in series. However, if it is desired to increase the number of serially connected elements in order to increase the obtained voltage, they may be stacked in the same manner. In this way, a plurality of elements are stacked in a sintering mold to form a series connection by laminating materials such as semiconductor particle powder, inorganic functional fiber sheet, and conductor powder. By sintering, it was confirmed that integrated sintering was possible at once.

In this laminated element, since the insulator used in and between the elements has air permeability, the laminated element as a whole has low resistance to gas conduction, and is suitable for gas-fired thermal power generation. It was also found that the laminate did not break due to heating and cooling during the manufacturing process or during use, and had sufficient strength.

The gist of the present invention based on the above study results is as follows.

(1) A thermoelectric conversion element having a junction between a p-type porous semiconductor and an n-type porous semiconductor, wherein an insulator provided between the semiconductors is made of an inorganic functional fiber. Characteristic porous thermoelectric conversion element.

(2) A porous thermoelectric conversion element, wherein a plurality of the thermoelectric conversion elements of the above (1) are stacked with an inorganic functional fiber interposed therebetween as an insulator between adjacent elements.

(3) The material is characterized in that p-type and n-type porous semiconductor materials and sheet-like inorganic functional fibers are placed in a laminated state in a mold and integrated sintering is performed. The method for producing a porous thermoelectric conversion element according to the above (1) or (2).

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to specific examples.

The type of semiconductor used for the thermoelectric conversion element is not particularly limited, but it is preferable to use a silicide in terms of high thermoelectric power and enduring high temperature.
The use of FeSi ₂ is desirable in terms of cost advantage. In the case of FeSi ₂ , p-type and n-type semiconductors can be obtained by replacing part of Fe with Co and Mn, respectively, as usual. Materials having these compositions are melted, and powder of spherical particles as a raw material for sintering is produced by a gas atomizing method or the like. The particle size may be selected according to the required strength and porosity of the sintered body.

The inorganic functional fiber is usually called a heat-resistant ceramic fiber, and is obtained by blending widely used high-purity alumina and silica in a weight ratio of about half and melt-spinning the fiber into a sheet. Is used. It suffices if the heat resistance temperature is 1200 ° C. or more, the bulk density is 0.30 g / cc or less, and the linear shrinkage rate by heating is 2.0% or less (1100 ° C., heating for 2 hours).

A columnar element having an outer diameter of 30 mm and a height of 20 mm having the shape shown in FIG. 2 was produced. Approximate diameter in carbon mold for sintering
500 μm p-type or n-type FeSi ₂ semiconductors 9, 10
(In this case, there is no boundary with the disk 11 and it is integrated)
Particle powder and inorganic functional fiber sheet of 30 mm width, 17 mm length and 2 mm thickness (manufactured by Toshiba Monoflux Co., Ltd., FIBERFRAX
-Model number: Paper # 300) 8 is placed and the pressure is 50k
Sintering was performed at gf / cm ² , a heating temperature of 780 ° C., and a heating time of 5 minutes. The area of the pn junction 3 is 30 mm wide and 3 mm long
It is. As a result of taking out the element after sintering from the mold and observing it, no crack, breakage or the like was found at all. An electrode 6 was formed by placing an Ag ribbon on the branch end side and sintering at the same time. To measure the electromotive force, the pn contact side of the obtained device was heated with a hot plate, and the branch end side was brought into contact with a container cooled with liquid nitrogen. FIG. 4 shows the results of measuring the temperatures of the high-temperature contact portion and the low-temperature contact portion of the device.
At 0 ° C, the temperature in the low temperature part reached almost temperature equilibrium at -30 ° C. FIG. 5 shows the measurement result of the temperature difference between the high temperature part and the low temperature part and the thermoelectromotive force at that time. The electromotive force when the temperature difference almost reaches equilibrium is
328 μV / ° C. Since the voltage value increases in response to the temperature difference and the thermoelectromotive force well corresponds to the Seebeck coefficient (thermoelectric power) of 350 μV / ° C of FeSi ₂ , this inorganic functional fiber sheet is used as an insulator. It was clear that the thermoelectric performance of the thermoelectric conversion element having the above structure was sufficiently satisfactory.

Next, an element having a structure in which a plurality of these thermoelectric conversion elements are connected in series and stacked will be described. FIG.
Fig. 2 schematically shows the shape. In this figure, reference numerals 9 and 10 denote p-type or n-type porous semiconductors having different conductive types, which are insulated by the inorganic functional fibers 8 and form a pn junction at 3. Therefore, the semiconductors 9, 1
0 and the insulator 8 to form one thermoelectric conversion element, and p
An electrode 6 is provided on the uppermost element on the low-temperature contact portion side 5 at the branch end opposite to the high-temperature contact portion side 4 of the -n junction and connected to the outside. Below this, semiconductors 13, 14 of exactly the same configuration,
And the element composed of the insulator 8 are separated by the same inorganic functional fiber 15 used for insulating both semiconductors of the element, and connected to the upper element by the conductor 12 at the low-temperature contact portion at the branch end. I do. Thereby, two thermoelectric conversion elements can be connected in series such that the directions of the high-temperature contact portion and the low-temperature contact portion are aligned. By repeating this, a larger number of laminated elements can be obtained.

An element having the structure shown in FIG. 3 and having three elements connected in series was prototyped, and the electromotive force was measured. The width of the element was 30 mm, the length was 30 mm, the thickness of each semiconductor was 5 mm, and the width of the pn junction 3 was 3 mm. Therefore, the inorganic functional fiber sheet 8 inserted as an insulator has a thickness of 2 mm, a width of 30 mm, and a length of 27 mm. The same sheet was used for the insulator 15 with the adjacent element, and the Ag conductor 12 was inserted at the branch end opposite to the pn junction 3. That is, the size of the conductor for connecting the adjacent elements in series is 3 mm in width and 30 mm in length. Further, the height of the obtained laminated element is 40 mm in a composite of three elements.

As in the case of the above-described single element, this laminated element is manufactured by using p-type or n-type FeSi ₂ semiconductor particle powder having a diameter of about 500 μm and the above-mentioned inorganic functional fiber sheet in a carbon mold. Was stacked and charged in the state shown in FIG. An Ag ribbon was used as the material of the conductor. These were sintered in the same manner as in the case of the simple substance, with a pressing force of 50 kgf / cm ² , a heating temperature of 780 ° C., and a heating time of 5 minutes. After sintering, the laminated element was removed from the mold and examined for cracks, breakage, and the like, but none of them was recognized.

FIG. 6 shows the results of measuring the thermoelectromotive force by applying a temperature difference to this laminated element using a hot plate and a cooling vessel, as in the case of the single element. The electromotive force increased with an increase in the temperature difference, and the thermoelectromotive force at a temperature difference of 250 ° C. was 0.25 V. The Seebeck coefficient per thermoelectric conversion element determined from this is 333 μV / ° C., and it can be seen that all elements operate without any problem.

As described above, by using an inorganic functional fiber sheet as an insulator, a large number of thermoelectric conversion elements can be arranged in a fixed volume, and furthermore, the thermoelectric conversion elements can be integrated and manufactured in one sintering step. Can be.

[0033]

According to the present invention, conventionally, the production thereof has been difficult,
It is another object of the present invention to improve the strength of a thermoelectric conversion element made of a porous semiconductor which is liable to be damaged during use, and to manufacture the thermoelectric conversion element with fewer sintering steps. The configuration of this element has an effect of facilitating the formation of a laminated element in which a plurality of elements are integrated, and has an effect of increasing the electromotive force per unit volume of the thermoelectric generator, which is significant in promoting the practical use of thermoelectric generation.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating the structure of a thermoelectric conversion element.

FIG. 2 is a schematic diagram of a thermoelectric conversion element using a porous semiconductor.

FIG. 3 is a schematic diagram of a stacked thermoelectric conversion element including a plurality of elements using a porous semiconductor.

FIG. 4 is a diagram showing a change in temperature between a high-temperature contact portion and a low-temperature contact portion during a test of a thermoelectric conversion element made of a trial porous semiconductor.

FIG. 5 is a diagram showing a temperature difference and an electromotive force at high and low temperature portions of a thermoelectric conversion element made of a porous semiconductor.

FIG. 6 is a diagram showing a temperature difference and an electromotive force at high and low temperature portions of a laminated thermoelectric conversion element using three thermoelectric conversion elements.

[Explanation of symbols]

Reference Signs List 1 p-type (or n-type) semiconductor 2 n-type (or p-type) semiconductor 3 pn junction surface 4 high-temperature contact surface 5 low-temperature contact surface 6 electrode 7 gap between semiconductors 8 insulator 9 p-type (or n-type) Porous semiconductor 10 n-type (or p-type) porous semiconductor 11 Disk formed by abutting semicircular p-type and n-type porous semiconductors having a pn junction surface at the center 12 Conductor 13 p-type (or n) Type) porous semiconductor 14 n-type (or p-type) porous semiconductor 15 insulator

Claims (3)

[Claims] 1. A thermoelectric conversion element having a junction between a p-type porous semiconductor and an n-type porous semiconductor, wherein an insulator provided between the semiconductors is made of an inorganic functional fiber. Porous thermoelectric conversion element. 2. A porous thermoelectric conversion element, wherein a plurality of the thermoelectric conversion elements according to claim 1 are laminated with an inorganic functional fiber interposed therebetween as an insulator between adjacent elements. 3. A p-type and n-type porous semiconductor material,
The method for producing a porous thermoelectric conversion element according to claim 1, wherein the porous thermoelectric conversion element is arranged in a mold in a state where the inorganic functional fiber in a sheet form is laminated, and integrated sintering is performed.