JPH06252451A - Thermoelectric material of doped semiconductor base and thermoelectric device or thermoelectric element couple - Google Patents

Abstract

PURPOSE:To realize thermoelectric material and a thermoelectric device both excellent in thermal resistance and large in figure of merit by a method wherein doped semiconductor base is made to contain counter dopant possessed of a compensation effect so as to be maximal in power factor. CONSTITUTION:Provided that Z denotes the characteristics of thermoelectric material, Z is represented by a formula, Z=P/K wherein P is a power factor or P=S<2> (Seebeck coefficient) Xdelta (electric conductivity), and K is thermal conductivity. Thermoelectric material is zinc oxide wherein zinc, indium, and lithium are mixed, and provided that the content ratio of zinc, indium, and lithium is represented by alpha:beta:gamma (alpha+beta+gamma=1), beta and gamma are so specified as to satisfy formulas, 0.00<beta<5.00% and 0.00<gamma<0.42%, and thermoelectric material is burned. Therefore, the thermoelectric material concerned is stable in characteristics even in an oxidizing atmosphere, so that excellent N-type thermoelectric material and a thermoelectric device or a thermoelectric element couple formed thereof can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric material and a thermoelectric element or thermoelectric element pair using a doped semiconductor substrate used for waste heat power generation, geothermal power generation, etc. The present invention also relates to a zinc oxide-based thermoelectric material and a thermoelectric element or a thermoelectric element pair, which have excellent heat resistance and can have a large figure of merit z and can be obtained by firing in air using an easy powder firing method.

[0002]

2. Description of the Related Art When two different kinds of conductors or semiconductors are joined and one junction is kept at a high temperature and the other junction is kept at a low temperature, there is a phenomenon that an electromotive force corresponding to this temperature difference is generated. This phenomenon is called the Seebeck effect.

There are thermoelectric generations such as exhaust heat utilization power generation and geothermal power generation that apply such Seebeck effect, and this thermoelectric generation and thermoelectric cooling are collectively called thermoelectric conversion.
Such heat exchange is considered as one of direct energy conversion because it does not involve moving parts.

Examples of thermoelectric materials having such a thermoelectric conversion action include Bi ₂ Te ₃ , Pb ₂ Te ₃ and Pb.
It is known that the main component is an intermetallic compound such as Te or GeTe.

However, such a thermoelectric material mainly composed of a conventionally known intermetallic compound uses a large amount of rare elements, requires high purity, and has a complicated manufacturing process. It was.

[0006] In addition, most of the conventionally known thermoelectric materials have low optimum and maximum operating temperatures, and are easily unstable at high temperatures. For example, when PbTe is used for power generation. , A closed system must be adopted to prevent deterioration of the thermoelectric element due to sublimation of Te,
Since the system is complicated and large-scale, the cost is high in this respect as well.

Further, as a thermoelectric material which has excellent heat resistance and can be used even in an oxidizing atmosphere at a high temperature, one having a composition in which indium oxide is added to zinc oxide has been developed (n-type thermoelectric element). Japanese Patent Application Laid-Open No. 63-1153
No. 88).

However, in order to sinter a material having such a composition, it must be fired in a vacuum, so that a special firing furnace is required, and the cost is also high.

[0009]

As described above, in the conventional thermoelectric material, a closed system is used in order to prevent the deterioration of the element due to the sublimation of the element or the high purity of the rare element. There is a problem in that the cost is extremely high because it has to be adopted or a special baking furnace is required because it has to be baked in a vacuum.

In the conventional material, the carrier concentration is adjusted by the amount of the dopant of the semiconductor substrate to increase the power factor to some extent, and the thermal conductivity is decreased by other additives and the structure of the structure to reduce the performance index z. The method of increasing the size is common, and the optimization of the power factor itself has not been focused on.

Therefore, in order to solve such a problem little by little, and the thermoelectric element is used for, for example, power generation using waste heat or geothermal power generation, it has excellent heat resistance up to 700 ° C. (973 K). In addition to having a large power factor p at high temperature and a better figure of merit z, it is possible to keep the raw material cost and the manufacturing cost of the thermoelectric material low and to provide them at low cost. Was becoming.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of such conventional problems, and is an inexpensive manufacturing method in which an inexpensive raw material and a general and easy powder firing method are used to perform firing in the atmosphere. Can also be obtained by 700 ℃ (973K)
And a thermoelectric material which can withstand use up to a temperature above 100 ° C., has excellent heat resistance, has a large power factor p at a high temperature of around 700 ° C., and has an increased figure of merit z. It is intended to provide a thermoelectric element or a thermoelectric element pair using the.

[0013]

A thermoelectric material using a doped semiconductor substrate according to the present invention is provided with a counter-dopant having a compensating effect on a doped semiconductor substrate as a power factor (Seebeck coefficient S squared × electric conductivity σ). ) Is maximized, the zinc oxide-based thermoelectric material according to the embodiment of the present invention comprises an n-type zinc oxide doped with indium, which is an n-type impurity, as a semiconductor substrate. It is characterized in that lithium, which is a p-type impurity, is added as a counterdopant so that the power factor is maximized. Further, in this embodiment,
When the content ratio of zinc, indium and lithium in zinc oxide is α: β: γ (where α + β + γ = 1), β and γ are 0.00 <β <5.00% 0.00, respectively. <Γ <0.42% of the composition is characterized in that the invention relates to a thermoelectric material using a doped semiconductor substrate having such a composition and a thermoelectric element or thermoelectric element pair using the same. The structure is used as a means for solving the above-mentioned conventional problems.

[0014]

In general, the characteristics of thermoelectric materials have a figure of merit z.
The material having a large figure of merit z is excellent in thermoelectric properties.

The performance index z is expressed by z = p / k (where p is the power factor, that is, the square of the Seebeck coefficient S × electrical conductivity σ, and k is the thermal conductivity).

As is clear from this equation, the larger the power factor p and the smaller the thermal conductivity k, the larger the figure of merit z, and the better the thermoelectric material.

Further, it is found that a material having a large Seebeck coefficient S and a large electric conductivity σ is suitable for increasing the power factor p.

By the way, although oxides are generally excellent in heat resistance, those having a large Seebeck coefficient S have a small electric conductivity σ, and those having a large electric conductivity σ have a Seebeck coefficient. Since S was small, it was not possible to obtain one having a large power factor p in any case.

The present inventor has focused on a zinc oxide-based material having a relatively large Seebeck coefficient in order to obtain a material having a large power factor p, and also has an optimum power factor which has not been noticed by the conventional method. As a result of investigating the effect of various additives by paying attention to the formation of the same, by adding lithium (Li) which is a p-type impurity as a counter dopant to zinc oxide doped with indium (In) which is an n-type impurity, It has been found that the power factor p of the oxide can be increased.

Further, when zinc oxide contains only indium, it was necessary to perform firing in vacuum. However, by containing lithium together with indium, firing in the atmosphere becomes possible, and the manufacturing cost is increased. It has been found that can also be significantly reduced.

Therefore, in the thermoelectric material according to the present invention, when the content ratio of zinc, indium and lithium in zinc oxide is α: β: γ (where α + β + γ = 1),
β and γ were regulated to be in the range of 0.00 <β <5.00% and 0.00 <γ <0.42%, respectively.

In this case, when β is 0.00%, the electric conductivity σ becomes small and the power factor p cannot be made large. Therefore, it is necessary to set 0.00% <β.
Further, when β becomes 5.00% or more, the Seebeck coefficient S becomes small and the power factor p cannot be made large as well. Therefore, it is necessary to set β <5.00%.

On the other hand, if γ is 0.00%, the sinterability deteriorates, and it is necessary to fire in a vacuum, and the electrical conductivity σ becomes small, so that the power factor p cannot be made large. , 0.00% <γ, and when γ is 0.42% or more, lithium forms a deep level in the energy band structure of zinc oxide, so that the carrier concentration decreases and the electric conduction decreases. Since the power factor p cannot be increased due to the decrease in the degree σ, it is necessary to set γ <0.42%.

In producing the zinc oxide-based thermoelectric material according to the present invention, an ordinary powder firing method carried out in the air can be employed without depending on the expensive powder firing method in a vacuum atmosphere. .

Therefore, for example, indium oxide and lithium carbonate powder are added to zinc oxide powder, sufficiently mixed, pressure-molded, and fired in the air to obtain a sintered body.

The firing temperature at this time is 1000 to 135.
It is preferably in the range of 0 ° C., and when it is lower than 1000 ° C., the electric conductivity σ is lowered and the power factor p tends to be small, and when it is higher than 1350 ° C., zinc oxide starts to sublime. It is not preferable. The firing time is preferably about 18 hours.

The sintered body thus obtained exhibits stable performance even in an oxidizing atmosphere, and becomes an excellent n-type thermoelectric material and a thermoelectric element or a thermoelectric element pair using the same.

Further, since the thermoelectric material and the thermoelectric element or the thermoelectric element pair according to the present invention mainly uses inexpensive zinc oxide as a raw material, it is cheaper than the conventional thermoelectric material which uses many rare elements, and further, Since it can be easily manufactured by a normal powder firing process, the manufacturing cost will be significantly reduced.

[0029]

EXAMPLE Indium oxide and lithium carbonate powder were added to zinc oxide powder and mixed with ethanol in a ball mill for 18 hours. Next, after drying this mixed powder, it is calcined at 900 to 1000 ° C. for 4 hours, and the calcined product obtained here is again mixed with ethanol in a ball mill for 1 hour.
Crushed and mixed for 8 hours.

Subsequently, the calcined powder obtained in the above step is dried, and then a small amount of binder (5 wt% polyvinyl butyral-containing ethanol solution) is added to granulate, and the granules are pressure-molded at 1 tonf / cm ^2. By doing so, a green molded body having a diameter of about 10 mm and a thickness of about 3 mm was obtained.

Next, the green molded body is subjected to 600 in the atmosphere.
After the binder was removed by holding it at 1 ° C for 1 hour, it was fired at 1200 ° C for 18 hours in the same atmosphere to obtain a zinc oxide-based sintered body.

Next, the above-mentioned sintered body was polished into a strip shape having a length of about 8 mm and a cross-sectional area of about 3 mm ² to obtain a strip-shaped sample, and JIS C 1602 R was attached to both ends of the strip-shaped sample.
A thermocouple (Pt-13% Rh, Pt) was fixed with a silver paste (DuPont 6838), and 4
Two Pt lead wires used for terminal resistance measurement were fixed with the same silver paste to prepare a measurement sample.

Next, this measurement sample was put in a measurement cell, and the electrical conductivity σ and Seebeck coefficient S were measured in the temperature range of room temperature to about 980 K (about 707 ° C.).

Of these, about 780K (about 507 ° C)
And the measurement results at about 980K (about 707 ° C) are shown in Table 1
As shown in Table 4, the indium (In) content is 0.5.
Each temperature of the power factor p (here, about 534 K (about 261 ° C.), about 725 K when the lithium content (Li content) is changed for the composition of 0%.
(3 types of about 452 ° C) and about 930K (about 657 ° C))
Fig. 1 shows the result of examining the change in the.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

As shown in Tables 1 to 4, in the examples of the present invention in which zinc oxide contained appropriate amounts of indium and lithium within the scope of the present invention, the power factor p was high at a high temperature of about 980K (about 707 ° C). Large, i.e. figure of merit z
It has been found that it is possible to obtain a thermoelectric material excellent in heat resistance and a thermoelectric element or thermoelectric element pair using the same.

On the other hand, in the comparative example in which the content ratio of indium and lithium is outside the range of the present invention, about 980.
The power factor p is small at a high temperature of K (about 707 ° C),
That is, it was confirmed that the thermoelectric material had a small figure of merit z and was not so good in heat resistance.

Further, as shown in FIG. 1, when the indium content is 0.50%, the lithium content is 0.42.
It has been confirmed that a power factor p larger than that in the case of adding indium alone (lithium content 0) is exhibited by making the content less than%.

[0042]

Since the thermoelectric material according to the present invention has the above-mentioned constitution, the raw material itself is inexpensive and easily available, and it can be fired in the air by using the usual powder firing method. Therefore, it can be manufactured at low cost.
Since the power factor p around 00 ° C has a large value and the figure of merit z is large, it can withstand use up to a temperature higher than 700 ° C, and has excellent heat resistance and thermoelectric properties. It is possible to obtain a thermoelectric element or thermoelectric element pair having a large figure of merit z and good heat resistance. When applied to geothermal power generation or the like, it has a remarkably excellent effect that it is possible to supply inexpensive electric power energy in a direct power generation type without moving parts.

[Brief description of drawings]

FIG. 1 is an example of indium (In) according to an embodiment of the present invention.
It is a graph which shows the result of having investigated the change in each temperature of a power factor when changing the lithium (Li) content about the composition whose content is 0.50%.

Front page continuation (72) Inventor Masaru Nemoto 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd.

Claims (4)

[Claims] 1. A doped semiconductor substrate, wherein a counterdopant having a compensation effect is contained in the doped semiconductor substrate so that a power factor (Seebeck coefficient S squared × electrical conductivity σ) is maximized. Thermoelectric material using. 2. A zinc oxide doped with indium which is an n-type impurity is used as a semiconductor base material, and lithium which is a p-type impurity forming a deep energy level is used as a counter dopant. Thermoelectric material according to. 3. The zinc in zinc oxide according to claim 2,
When the content ratio of indium and lithium is α: β: γ (where α + β + γ = 1), β and γ are 0.00 <β <5.00% 0.00 <γ <0.42, respectively. % Zinc oxide based thermoelectric material. 4. A thermoelectric element or a thermoelectric element pair comprising the thermoelectric material according to claim 1 and the zinc oxide-based thermoelectric material according to claims 2 and 3.