JP3523600B2 - Thermoelectric element manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a thermoelectric semiconductor element, in which a thermoelectric semiconductor material is sintered by microwave irradiation.

[0002]

2. Description of the Related Art Conventionally, a Peltier semiconductor in which a P-type semiconductor and an N-type semiconductor are joined via a metal to form a PN junction pair, one end of which heat is generated and the other end of which is cooled by the direction of a current flowing through the joint, is called a Peltier. Thermoelectric semiconductor elements that utilize the effect are compact and simple in structure, and can be applied to various devices such as CFC-less cooling technology, photodetection elements, electronic cooling elements such as semiconductor manufacturing equipment, and temperature control devices such as temperature control of laser diodes. Is expected to be widely used.

For these thermoelectric semiconductor elements, Bi ₂ Te _{3 is} used as the thermoelectric semiconductor material having the best performance near room temperature.
Chabogen compounds such as Sb ₂ Te ₃ and Bi ₂ Se ₃ and solid solutions thereof are mainly used.

In order to improve the performance of these thermoelectric elements, it is very important to reduce the amount of oxygen. Therefore, a predetermined amount of each element of high purity is weighed, vacuum sealed in a glass tube, and melted.
A method of producing a raw material ingot by stirring and then cooling, crushing and classifying the ingot, performing hydrogen reduction, and then hot pressing in an inert atmosphere to obtain a sintered body can be mentioned. For example, it is preferable to melt and pulverize a mixed powder of a main component and an additive of three or four elements consisting of bismuth, tellurium, selenium, and antimony, and calcine the obtained alloy powder by a hot pressing method. Kaihei 01-1064
No. 78 publication.

Further, a molten and crushed alloy powder is filled in a die and extruded by a cylinder while being heated, and a die outlet is formed into one thermoelectric element cross-sectional shape to prepare a long thermoelectric semiconductor material, and a predetermined shape is prepared. The hot extrusion method of producing a thermoelectric semiconductor element by cutting at a length of 1 can be mentioned. For example, insert the extruded material of thermoelectric material into an extrusion die, and extrude the material with a cylinder while heating,
A method for obtaining a thermoelectric semiconductor element by cutting it at a constant length is described in JP-A-08-186299.

[0006]

However, in the method using a hot press as in the above-mentioned Japanese Patent Laid-Open No. 01-106478, although a dense body can be obtained, it is necessary to cut it into a desired element size with a cutter or the like after the sintering step. Therefore, there is a problem that cutting chips are generated, material costs are wasted, and product costs rise.

Further, in this method, the thermoelectric performance of the element differs depending on the part to be cut, resulting in a large variation in performance.
There was a problem that many defects occurred.

In the method disclosed in Japanese Unexamined Patent Publication No. 08-186299, the time required for extrusion is long because the thermoelectric elements are cut one by one, and the extrusion process is performed at a high temperature of about 400 ° C. Will be exposed to high temperatures for a long time. For this reason, there is a problem in that the elements extruded in the initial stage and the elements extruded in the final stage are exposed to high temperatures for different times, so that the characteristics of the thermoelectric elements vary widely.

Further, there is a problem that it is difficult to obtain a dense body by a simple and low-cost sintering method such as normal pressure firing, and the performance is inferior.

Therefore, an object of the present invention is to provide a method for manufacturing a thermoelectric element which is dense and has no characteristic variation at low cost.

[0011]

The present invention is capable of simplifying the manufacturing process of a thermoelectric material and obtaining a uniform dense body by firing a molded body having a substantially same shape as a product by firing by microwave heating. It was made based on the knowledge that it is possible to achieve cost reduction and uniform element performance.

That is, the method of manufacturing a thermoelectric element of the present invention is
It is characterized in that a raw material powder of a thermoelectric material consisting of at least two kinds selected from i, Te, Sb and Se is molded and then fired by using microwave heating.

According to this method, it is possible to obtain a dense body, which is hard to be obtained by the conventional sintering method, at a low temperature for a short time by firing by heating by irradiation of microwaves. The evaporation amount of tellurium or the like can be suppressed, and the composition change near the surface can be prevented. As a result, composition shift,
Alternatively, it is not necessary to add an excessive amount of selenium or tellurium, and since the molded body is heated uniformly, a material having uniform characteristics can be obtained and variation in characteristics can be suppressed.

Further, according to the present invention, it is possible to obtain a sintered body which is substantially the same as the product, so that the step of cutting the sintered body of the thermoelectric semiconductor material can be omitted. Therefore, the yield due to material loss during cutting, chipping of elements, etc. can be improved, and the material cost can be reduced.

Further, it is preferable to perform a reduction treatment by irradiating the molded body with microwaves prior to the firing.
Conventionally, the reduction treatment that takes a long time can be performed in a short time, and the firing can be performed as it is, so that the efficiency of the thermoelectric element manufacturing can be improved by the consistent process.

Further, the frequency of the microwave is 0.9
It is preferably at least GHz. By using the microwave of 0.9 GHz or more, the thermoelectric material can effectively absorb the microwave and shorten the firing time,
It is possible to further suppress the volatilization amount and make the crystal particles finer, and it is possible to improve the uniformity and thermoelectric performance. Further, since the firing temperature is low and the time is short, the process cost for forming the element can be suppressed and the final cost of the thermoelectric module can be reduced.

Furthermore, the firing by microwave heating comprises a step of raising the temperature of the molded body at a rate of 10 ° C./min or more, and a step of holding the temperature at 200 ° C. to 550 ° C. for a time of 30 minutes or less. It is preferable to include. This allows
Since the relative density of the thermoelectric element can be densified to 95% or more, and the evaporation amount of the volatile component can be reduced by the short-time treatment, it is possible to suppress the variation in characteristics and realize the fine structure in which the grain growth is further suppressed.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is based on Bi, Te, Sb, S.
The present invention relates to a thermoelectric element made of at least two kinds of thermoelectric semiconductors selected from e. First, a raw material powder made of the above thermoelectric semiconductor is prepared. A known method can be used for this raw material. For example, a glass tube is filled with the mixed powder, and the inside of the container is vacuumed or filled with argon gas to prevent the metal powder from being oxidized. Then, the temperature can be raised to a predetermined temperature by a rocking furnace, the metal powder can be melted, and the obtained alloy lump can be crushed to be a firing raw material. At this time, an element such as HgBr ₂ or SbI may be added as a dopant.

When the molten alloy is cooled, it may be cooled below the freezing point as it is, but in consideration of the crystal orientation, it is kept at a temperature above the freezing point and is cooled from one end to be unidirectionally solidified. It is preferable to control the crystal growth orientation by performing the above.

For crushing the above-mentioned alloy ingot, known methods such as a stamp mill, a ball mill and a vibration mill can be mentioned.

In order to remove oxygen in the raw material made of a thermoelectric semiconductor after pulverization, it is preferable to carry out the reduction treatment by irradiating with microwaves in a reducing atmosphere such as hydrogen gas. This reduction treatment can be carried out in an ordinary heating furnace, but in particular, microwave heating can be carried out in a short time, and the reactivity of the surface is further improved to enhance the sinterability,
The thermoelectric properties of the sintered body can be improved. Note that this reduction treatment may be performed after molding as long as it is before firing.

This firing raw material is formed by a known molding method, for example,
A molded body is produced by a uniaxial pressing method, a CIP method, a casting method, an injection molding method or the like. When a molding method using a pressing method is used, a desired shape can be obtained by molding with a molding pressure of 100 MPa, but a molded body having an element shape can be formed by forming a tape shape using a doctor blade method and stacking the molded bodies. May be produced.

At this time, it is important that the size of the molded body is substantially the same as that of the product after firing. As a result, it is not necessary to cut the element after sintering, and it is possible to suppress the generation of defective products due to material loss or chipping during cutting. Further, since the molded body is heated uniformly, it is possible to suppress variations in performance among the elements.

The obtained compact is fired by microwave heating. That is, since the compact is irradiated with microwaves and the alloy absorbs the microwaves to generate heat by itself, the energy efficiency is high, the temperature can be rapidly raised, and the cost can be reduced. Moreover, since the microwaves uniformly act on the surface of the material particles, it is possible to uniformly heat the molded body and suppress variations in characteristics.

In this microwave heating, the molded body is placed in the resonator of the microwave heating apparatus and is oscillated by an oscillating tube such as a magnetron, a klystron or a gyrotron.
This is to irradiate the molded body with the microwave guided into the cavity resonator through the waveguide.

By surrounding the periphery of the molded body with a heat insulating material made of alumina fiber or the like, it is possible to suppress the heat radiation from the sample surface and heat it effectively. The sample temperature can be measured by a known measurement method, for example, a non-contact method such as a thermocouple of tungsten-rhenium or a dichroic thermometer.

The frequency of the microwave used for the microwave heating is preferably 0.9 GHz or higher, particularly 2 GHz or higher, further 10 GHz or higher, and more preferably 25 GHz or higher. By setting the frequency to 0.9 GHz or more, the powder raw material made of the thermoelectric semiconductor can effectively absorb the microwave, and the heating / sintering time can be shortened as much as possible.
Therefore, the amount of volatilization can be suppressed, the crystal grains of the thermoelectric semiconductor can be sintered without grain growth, the crystal grains can be miniaturized, and the thermal conductivity can be lowered.

Further, the microwave heating is preferably performed in an atmosphere of an inert gas such as nitrogen gas or argon in order to prevent the alloy from being oxidized.

The microwave heating includes a temperature raising step and a firing step, and the temperature raising rate is 10 ° C. in the temperature raising step.
/ Min or more is preferable. Further, in the firing step, it is preferable to maintain the temperature at 200 ° C. to 550 ° C. for a time of 30 minutes or less.

By such rapid temperature rise and short-time firing, the compositional change of the alloy due to evaporation can be suppressed, grain growth can be suppressed, and the thermal conductivity can be lowered to improve thermoelectric properties. Further, as a result of increasing the reactivity and diffusion rate of the particle surface, the relative density of the sintered body can be densified to 95% or more, particularly 98% or more. For example, when soldering the surface of the sintered body, Excellent thermoelectric properties can be exhibited without the performance of the solder penetrating and causing a deterioration in performance.

If the temperature raising rate is lower than 10 ° C./min, it takes a long time to raise the temperature, so that components such as tellurium and selenium are likely to volatilize to cause compositional deviation, which causes defects. Therefore, 10 ° C / min or more, especially 15 ° C / mi
It is preferably n or more, more preferably 20 ° C./min or more.

Similarly, in order to prevent composition deviation, 200
℃ ~ 550 ℃, especially 300 ℃ ~ 500 ℃, even 35
It is preferable to perform calcination at a temperature of 0 ° C. to 450 ° C. for 30 minutes or less, particularly 20 minutes or less, further 10 minutes or less, and more preferably 5 minutes or less.

The thermoelectric element thus obtained can be subjected to outer diameter processing as desired to uniform the dimensions. Then, for example, a plurality of the above-mentioned thermoelectric elements are arranged, sandwiched between a pair of heat exchange substrates, and the thermoelectric elements are electrically connected to produce a thermoelectric module, and electricity is supplied to the arranged thermoelectric elements. , One of the heat exchanger plates can be cooled.

[0034]

Example As starting materials, Bi ₂ Te with n-type bismuth, tellurium, and selenium having a purity of 99.99% or more was used.
_2.85 Se _0.15 was weighed, and these mixed powders were vacuum sealed in a Pyrex (registered trademark) glass tube, melted and stirred in a rocking furnace, and then cooled to prepare a thermoelectric semiconductor material ingot. Then, after roughly pulverizing using a stamp mill, a rotary mill was applied in an ethanol solvent for 24 hours.

The pulverized raw material was put into a silica tube and 2 L / m
Irradiate with microwave of 28 GHz in hydrogen gas flow of in to perform hydrogen reduction treatment, then uniaxial press 100M
1 mm length, 1 mm width, and 1.5 mm length at a molding pressure of Pa
A molded body of was produced.

The obtained molded body was placed in an alumina heat insulating material in a cavity resonator of a microwave heating furnace, and was irradiated with microwaves to be fired. Here, the microwave source of the microwave heating furnace has a frequency of 0.915 GHz and an output of 3 k.
W, 2.45 GHz, output 2 kW magnetron,
A gyrotron with a frequency of 28 GHz and an output of 10 kW was used. Then, the reduction treatment is performed by microwave heating in a hydrogen flow of 2 L / min under the conditions of Table 1,
The firing was performed under the conditions shown in Table 1 by microwave heating in a 2 L / min argon gas stream.

Sample No. 13 is a pressure of 300 Kg / cm ^{2 at} a temperature of 500 ° C. in the Ar atmosphere of the pulverized raw material.
Was hot-pressed for 10 minutes to prepare a sintered body having a diameter of 200 mm, and then cut into a piece having a length of 0.9 mm, a width of 0.9 mm and a length of 1.2 mm. In addition, the sample No. No. 14 is a material having a length of 0.9 mm, a width of 0.9 mm, and a length of 500 mm, which is fired while extruding the crushed raw material in an Ar atmosphere. The length is 0.9 mm, the width is 0.9 mm, and the length is 1.2 mm. Disconnected.

The surface of the obtained sintered body was ground, the specific gravity was measured by the Archimedes method, and the relative density was measured from the theoretical density. Further, the electrical conductivity σ at 25 ° C. was measured by the 4-terminal method, and the specific resistance ρ was calculated from σ = 1 / ρ.

Further, the thermal conductivity was measured by a laser flash method of JISR1611 by separately preparing a test piece having a diameter of 10 mm and a thickness of 1 mm. The Seebeck coefficient was measured at 25 ° C. by using a thermoelectric power evaluation device manufactured by Vacuum Riko Co., Ltd. to prepare a prismatic sample having a length of 4 mm, a width of 4 mm, and a length of 15 mm. The performance index Z was calculated from Z = S ² / ρk (S is Seebeck coefficient, ρ is resistivity, and k is thermal conductivity).

Further, in order to evaluate the characteristic variation, the variation (defective rate) of the figure of merit was calculated based on 100 samples. Furthermore, the loss of the raw material from the molded body to the sample preparation was measured from the weight change. The results are shown in Table 1.

[0041]

[Table 1]

Sample No. of the present invention. Nos. 1 to 12 have a relative density of 95% or more, and all the performance indexes Z are 3 × 10 ⁻³ / K.
It was an element excellent in thermoelectric performance. In addition, the performance index of 100 thermoelectric elements produced under the same conditions had a small variation of 2% or less.

On the other hand, the sample No. manufactured by the hot press method and out of the range of the present invention. 13 has a figure of merit Z of 2.41 × 10
^-3 / K, figure of merit variation 6%, raw material loss rate 46
%Met.

Further, sample No. manufactured by the extrusion firing method and outside the scope of the present invention. 14 has a thermoelectric performance Z of 2.52 × 10 ⁻³ /
K, 12% dispersion in performance index, 33% raw material loss rate
Met.

[0045]

According to the method of manufacturing a thermoelectric element of the present invention, a dense body can be obtained at a low temperature in a short time without applying pressure, and a cutting step can be omitted. As a result, there is no waste of material and thermoelectric performance is improved. An excellent thermoelectric semiconductor element can be realized.

Claims (4)

(57) [Claims] 1. A method for producing a thermoelectric element, which comprises forming a raw material powder of a thermoelectric material consisting of at least two kinds selected from Bi, Te, Sb and Se, and then firing it by using microwave heating. 2. The method for manufacturing a thermoelectric element according to claim 1, wherein the compact is irradiated with microwaves to carry out a reduction treatment prior to the firing. 3. The method of manufacturing a thermoelectric element according to claim 1, wherein the microwave has a frequency of 0.9 GHz or higher. 4. A step of raising the temperature of the molded body at a rate of 10 ° C./min or more by firing by the microwave heating;
4. The method for manufacturing a thermoelectric element according to claim 1, further comprising a step of holding the temperature at 00 ° C. to 550 ° C. for 30 minutes or less.