JP6893344B2 - Copper gallium tellurium-based p-type thermoelectric semiconductor and thermoelectric power generation element using it - Google Patents

Description

The present invention relates to a copper gallium tellurium-based p-type thermoelectric semiconductor and a thermoelectric power generation device using the same.

Conventionally, thermoelectric semiconductors have been actively researched for materials in order to use energy efficiently in modern society, and great demand has been established for use in reliable and quiet cooling devices and generators. ing.
A copper gallium tellurium compound is known as one of the thermoelectric semiconductors. As shown in Patent Document 1 and Non-Patent Document 1, the copper gallium tellurium compound exhibits low electric resistance at high temperature, a large Seebeck coefficient, and a relatively low thermal conductivity, and exhibits excellent performance as a thermoelectric semiconductor in a high temperature region. Shown. However, it is a performance at a high temperature of 800 K or higher (about 500 ° C or higher). On the other hand, there is a high demand for effective use of heat in the low and medium temperature regions near room temperature. In order to meet such demand, a new thermoelectric conversion material as a thermoelectric semiconductor was required. Further, there has been a demand for a thermoelectric power generation element having high power generation efficiency in a low temperature and medium temperature region close to room temperature.

Japanese Unexamined Patent Publication No. 2013-016685

Advanced Materials, vol. 24 (2012) pp. 3622-3626 Inorganic Materials, vol. 43 (2007) pp. 12-17

An object of the present invention is to provide a thermoelectric semiconductor and a thermoelectric power generation element having high thermoelectric conversion efficiency even in a relatively low temperature region as a thermoelectric semiconductor of about 400 ° C. from around room temperature.

As a result of various studies, the present inventors have added an element having one or more magnetisms selected from the group consisting of chromium, manganese, iron, cobalt, and nickel to a copper gallium tellurium compound, thereby making a comparison for the above purpose. It has been found that a p-type thermoelectric semiconductor having high thermoelectric conversion efficiency can be provided even in a low operating temperature region. The configuration of the present invention is as follows.

(Structure 1)
A p-type thermoelectric semiconductor containing a copper gallium tellurium compound.
A p-type thermoelectric semiconductor comprising one or more elements selected from the group consisting of chromium, manganese, iron, cobalt, and nickel.

(Structure 2)
The p-type thermal semiconductor according to Configuration 1, wherein the element is contained as the total amount of the element in a molar ratio of more than 0% and less than 5% with respect to the copper gallium tellurium compound.

(Structure 3)
The p-type thermoelectric semiconductor according to Configuration 1, wherein the element is contained as a total amount of the element in a molar ratio of 1% or more and 3% or less with respect to the copper gallium tellurium compound.

(Structure 4)
The p-type thermoelectric semiconductor according to any one of configurations 1 to 3, wherein the composition is a tetragonal p-type thermoelectric semiconductor having the composition represented by the formula 1.
CuGa _1-x M _x Te ₂ (Equation 1)
(M is 1 or more consisting of Cr, Mn, Fe, Co, and Ni, and x is more than 0 and less than 1.)

(Structure 5)
The p-type thermal semiconductor according to configuration 4, wherein x is more than 0 and less than 0.05.

(Structure 6)
The p-type thermoelectric semiconductor according to the configuration 4, wherein x is 0.01 or more and 0.03 or less.

(Structure 7)
A thermoelectric power generation device in which a p-type semiconductor and an n-type semiconductor are integrated, wherein the p-type thermoelectric power generation element according to any one of configurations 1 to 6 is used as the p-type semiconductor. ..

The thermoelectric semiconductor of the present invention has the characteristics of low electrical resistance (high electrical conductivity), high Seebeck coefficient, and low thermal conductivity even in a relatively low operating temperature range such as 400 ° C from near room temperature. Therefore, high thermoelectric conversion efficiency can be obtained.
Further, the thermoelectric power generation element using the thermoelectric semiconductor of the present invention can obtain high thermoelectric power generation efficiency even in the low operating temperature region as described above. For this reason, heat can be recovered from a low temperature range as compared with the conventional case, and the range of available heat is greatly expanded.

Schematic diagram of the crystal structure of the semiconductor material of the present invention. The powder X-ray diffraction pattern figure of a copper gallium tellurium compound sample. The thermoelectric characteristic diagram which investigated the electric conductivity characteristic of a copper gallium tellurium compound. The thermoelectric characteristic diagram which investigated the Seebeck coefficient characteristic of a copper gallium tellurium compound. The thermoelectric characteristic diagram which investigated the output factor characteristic of a copper gallium tellurium compound. The thermoelectric characteristic diagram which investigated the thermal conductivity characteristic of a copper gallium tellurium compound. The thermoelectric characteristic diagram which investigated the dimensionless figure of merit ZT characteristic of a copper gallium tellurium compound. The thermoelectric characteristic diagram which investigated the output factor characteristic of a copper gallium tellurium compound. The thermoelectric characteristic diagram which investigated the dimensionless figure of merit ZT characteristic of a copper gallium tellurium compound. The thermoelectric characteristic diagram which investigated the output factor characteristic of the copper gallium tellurium compound to which iron was added. The cross-sectional schematic diagram which shows the thermoelectric power generation element of Example 3. FIG.

(Thermoelectric semiconductor)
First, the method for manufacturing a thermal semiconductor of the present invention will be described.
As a raw material, it is selected from the group consisting of copper (Cu), manganese (Mn), tellurium (Te), and chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni). The above magnetic element M is prepared. Here, the case where manganese is prepared as the magnetic element M will be mainly described, but a manganese replaced with chromium, iron, cobalt, or nickel may be used as the magnetic element M. Further, two or more selected from manganese, chromium, iron, cobalt and nickel may be used as the magnetic element M.

Next, Cu, Ga, M and Te are weighed so as to have a molar ratio of 1: 1-x: x: 2. Here, when the magnetic element M is composed of the above-mentioned plurality of elements, it is weighed so that the total amount thereof becomes x in terms of molar ratio. It is preferable that x is more than 0 and less than 0.05. Here, the meaning of "beyond 0" means that the magnetic element M is substantially contained, and is not included when, for example, it is unintentionally contained in a very small amount as an impurity. If it is unintentionally contained in a very small amount, the amount and distribution are likely to vary, and the thermoelectric characteristics are likely to vary.
It is particularly preferable that x is 0.01 or more and 0.03 or less because an impurity phase is not generated, the thermoelectric conversion efficiency is high, and the thermoelectric characteristics are stable.
After mixing the weighed elements, the mixture is heated to a temperature at which Cu, Ga, M and Te are sufficiently melted, and then the atoms move, although at a temperature such as 600 ° C. to form a solid phase. Slowly cool so that homogeneous crystal growth proceeds to a temperature at which homogeneous crystals are likely to form.
Then, the mixture is cooled to room temperature to obtain an ingot in which the magnetic element M is added to the copper gallium tellurium compound.
Next, this ingot is pulverized using, for example, a ball mill, a sintered body is obtained by a discharge plasma sintering method, a hot press method, or the like, and a predetermined amount of a predetermined amount of magnetic element M is added to the copper gallium tellurium compound CuGa _1. producing p-type thermoelectric semiconductor formed of _-x M _x Te _2.

FIG. 1 shows a schematic diagram of the crystal structure (chalcopyrite structure) of _{the copper gallium tellurium compound CuGa 1-x} Mn _x Te ₂ to which manganese (Mn) is added as the magnetic element M. Since manganese is added in a molar ratio of only a few percent, it shows that only one gallium (Ga) site out of several unit cells is replaced with manganese (Mn). The same applies when the other magnetic element M described above is added in place of manganese.

As shown in Example 1 below, the copper gallium tellurium compound CuGa _1-x Mn _x Te ₂ to which less than 5 mol% of manganese is added becomes a homogeneous crystal having a chalcopyrite structure, which is one of the tetragonal systems. When 5 mol% of manganese is added, an impurity phase appears in the crystal.
When an impurity phase appears, it becomes difficult to stabilize the thermoelectric properties. Therefore, it is not preferable that the copper gallium tellurium compound contains manganese as the magnetic element M in a molar ratio of 5% or more. As shown in this example, from the viewpoint of obtaining crystals of sufficient purity, in the copper gallium tellurium compound CuGa _1-x M _x Te _{2 to} which the magnetic element M, which is the p-type thermoelectric semiconductor of the present invention, is added, x Is preferably less than 0.05.

Next, the thermoelectric characteristics of the thermoelectric semiconductor composed of the copper gallium tellurium compound to which the magnetic element M is added in a predetermined amount will be described.
The thermoelectric characteristics are measured, for example, as follows.
Electric conductivity (σ), Zebeck coefficient (S), thermal diffusivity (α), thermal capacity ( _Cp ), and sample density (s) of a thermoelectric semiconductor made of a copper gallium tellurium compound to which a predetermined amount of magnetic element M is added. d) is measured, and the output factor (PW), thermal conductivity (κ), and dimensionless performance index (ZT) are calculated from the measurement results. Here, the output factor PW can be calculated by the formula of ^{PW = S 2} σ, the thermal conductivity κ _{can be calculated by the formula of κ = α · C p} · d, and the dimensionless figure of merit ZT ^{can be calculated by the formula of ZT = S 2 σT / κ.} Here, T is the absolute temperature.

As shown in Example 1 below, by adding, for example, manganese as the magnetic element M, the electrical conductivity σ is increased by an order of magnitude. This is thought to be mainly due to the increased carrier concentration.
When the carrier concentration increases significantly, the Seebeck coefficient S usually decreases significantly, and the ^{output factor PW calculated by S 2} σ does not simply increase. However, as shown in Example 1, the Seebeck coefficient S is not significantly reduced in the sample to which manganese is added, and is reduced by only about half at the maximum. Further, the thermal conductivity κ is lower than that when manganese is not added, and has preferable characteristics as a thermoelectric semiconductor.
From these facts, as shown in Example 1, the addition of manganese doubles the output factor PW and the dimensionless figure of merit ZT by about 3 times in a relatively low temperature region of about 400 ° C. from around room temperature.

Such a large increase in the output factor PW is due to the fact that the carriers (electrons and holes) responsible for electrical conduction in the semiconductor interact with the added magnetic element (magnetic ion) to increase the effective mass, resulting in an increase in effective mass. It is presumed that the large Seebeck coefficient was maintained despite the increase in carrier density.
To support this, the effective mass of carriers was calculated from the Hall coefficient measurement for the manganese-added sample. As a result, the copper gallium telluride without the addition of manganese, effective mass of electrons was calculated from the Hall coefficient measurement and Seebeck coefficient value was 0.22 m o _{(m o} is the free electron mass). In contrast, 1 mol% of manganese, 2 mol%, and the effective electron mass at 3 mol% added sample, respectively 0.62 meters _o, 1.0 m _o, was 0.92 m _o. Therefore, it is considered that the interaction between the magnetic ions and the carriers in the semiconductor and the increase in the effective mass brought about the improvement of the thermoelectric characteristics. Since the same interaction works when similar magnetic ions are added, the same effect can be obtained with chromium, manganese, iron, cobalt, and nickel as transition metal elements exhibiting ferromagnetism or antiferromagnetism.
On the other hand, for rare earth elements exhibiting magnetism (neodymium (Nd), gadolinium (Gd), dysprosium (Dy), etc.), the permissible content in the copper gallium tellurium compound is on the order of ppm, so the effect is small.

In Non-Patent Document 2, an experiment of adding manganese to a copper gallium tellurium compound is carried out, but only the magnetism is measured there. Its purpose is to search for new ferromagnets and apply spintronics, not to thermoelectric semiconductors. No measurements of electrical conductivity σ above room temperature, Seebeck coefficient S, and thermal conductivity κ have been found, and it is difficult to infer from Non-Patent Document 2 how the addition of manganese affects thermoelectric properties. ..

Studies on improving thermoelectric properties by adding elements with different valences to semiconductors have been conducted in various places. This utilizes the fact that "optimization of carrier density" occurs, and this method is well known. However, the present invention is not merely "optimization of carrier density", but utilizes a completely new mechanism of enhancing effect by interaction between magnetism and carriers.

The fact that the present invention is not based on "optimization of carrier density" has been confirmed by experiments in which a part of gallium (Ga) is replaced with iron (Fe) having no difference in formal valence. Both Ga and Fe are trivalent ions, and the number of carriers does not change. The details are shown in Example 2 below.

(Thermoelectric power generation element)
It is known that about 3/4 of the energy is wasted as waste heat. Thermoelectric conversion is a method that can recover thermal energy and directly convert it into useful electrical energy, and has merits such as ease of maintenance and scalability due to the absence of moving parts. In order to use it for such power generation, it is necessary to form an element with p-type and n-type thermoelectric materials having high thermoelectric conversion efficiency.

As shown in FIG. 11, in the thermoelectric power generation element 31, the n-type semiconductor 32 and the p-type semiconductor 33 are electrically connected between the electrode 35 on the low temperature side and the electrode 34 on the high temperature side via these electrodes. It is an element having a structure arranged in series with each other.
As the p-type semiconductor 33, a copper gallium tellurium compound to which the magnetic element M of the present invention is added in a predetermined amount is used. As a result, heat can be recovered from the low temperature region as compared with the conventional case, the range of available heat can be greatly expanded, and energy can be recovered from the low temperature range, which has been difficult in the past.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to these Examples.

(Example 1)
Example 1 shows a p-type thermoelectric semiconductor when manganese (Mn) of 1%, 2% and 3% in molar ratio is added to a copper gallium tellurium compound.

((Manufacturing of thermoelectric semiconductor))
First, as raw materials, copper (Cu) (powder, Furuuchi Kagaku Co., Ltd., 99.99%), gallium (Ga) (granular, Furuuchi Kagaku Co., Ltd., 99.999%), manganese (Mn) (powder). , Furuuchi Chemical Co., Ltd., 99.99%), Tellur (Te) (granular, High Purity Chemistry Laboratory Co., Ltd., 99.99999%) were prepared.
Next, Cu, Ga, Mn and Te were weighed using an electronic balance so that the molar ratio was 1: 0.99: 0.01: 2.
Then, it was heated to 900 ° C. for 3 days in a vacuum quartz tube, held for 2 hours, and then cooled to 600 ° C. Then, the mixture was cooled to room temperature for 6 hours to obtain an ingot of _{CuGa 0.99} Mn _0.01 Te _2.
Next, this ingot was crushed in a dairy pot, filled in a graphite die (inner diameter 10 mm), obtained by a discharge plasma sintering method, and composed of a copper gallium tellurium compound to which 1 mol% of manganese was added. A p-type thermoelectric semiconductor was manufactured. Here, for sintering, Dr. Fuji Radio Industrial Co., Ltd. Using SINTER, the sintering conditions were a pressure of 50 MPa, a temperature of 600 ° C., and 5 minutes.

Further, Cu, Ga, Mn and Te are weighed so as to have a molar ratio of 1: 0.98: 0.02: 2 and 1: 0.97: 0.03: 2, and the same step is performed. Then, a p-type thermoelectric semiconductor made of a copper gallium tellurium compound to which 2 mol% and 3 mol% of manganese was added was prepared.

((Characteristic evaluation of thermoelectric semiconductor))
The powder X-ray diffraction pattern of the copper gallium tellurium compound sample to which 1 mol%, 2 mol% and 3 mol% of manganese was added is shown in FIG. In this figure, x represents the molar ratio of manganese. Further, in the figure, the case where x = 0 without adding manganese (Comparative Example 1) and the case where x = 0.05 with 5% of manganese added in molar ratio are also shown (Comparative Example 2). There is.

When manganese is not added and when the amount of manganese added is 1 mol%, 2 mol% and 3 mol% (x = 0 to 0.03), all the samples are consistent with the calculation assuming the chalcopyrite structure. , Indicates that a uniform sample could be prepared. On the other hand, in the case of x = 0.05 in which manganese was added at a molar ratio of 5%, a small but impurity phase peak was observed in the portion indicated by the arrow, and a completely uniform sample could not be obtained. ..
When the impurity phase appears, it may be difficult to stabilize the thermoelectric properties such as a decrease in electrical conductivity. Therefore, it may not be preferable that the copper gallium tellurium compound contains manganese in a molar ratio of 5% or more.

Next, the sintered body was cut out in a rod shape, the electrical conductivity (σ) and the Seebeck coefficient (S) were measured, and the output factor (PW) was calculated by the formula ^{PW = S 2 σ.} In addition, the thermal diffusivity (α) was measured using a sample cut out on a disk. Moreover, using a small test piece was measured heat capacity _{C p} by DSC (Differential Scanning Calorimetry) method. Then, the thermal conductivity (κ) was calculated by _{κ = α · C p} · d from the thermal diffusivity (α), the heat capacity (C _p ), and the density (d) of the sample. The dimensionless figure of merit (ZT) was then calculated by ^{ZT = S 2 σT / κ.} Here, T is the absolute temperature.

The relationship between the electrical conductivity σ, the Seebeck coefficient S, the output factor PW, the thermal conductivity κ, and the dimensionless performance index ZT with respect to the temperature from 324K to 669K is shown in FIG. It is shown in FIG.

By adding manganese, the electrical conductivity σ is significantly improved, and especially when manganese is added in an amount of 3 mol% (x = 0.03), the improvement rate reaches about 10 times (see FIG. 3). This is thought to be mainly due to the increased carrier concentration.
When the carrier concentration increases significantly, the Seebeck coefficient S usually decreases significantly, and the ^{output factor PW calculated by S 2} σ does not simply increase. However, as shown in FIG. 4, the Seebeck coefficient S does not decrease so much even in the sample to which manganese is added, and the decrease is limited to 50% at the maximum.
From this, as shown in FIG. 5, the output factor PW was greatly increased by the addition of manganese. In particular, the rate of increase in the low temperature range from room temperature to 500 K is large, and in particular, when manganese is added in an amount of 2 mol% (x = 0.02) or more, the rate of increase exceeds 100%.

As shown in FIG. 6, the temperature dependence of the thermal conductivity κ of the copper gallium tellurium compound sample to which manganese is added is lower than that when manganese is not added, and has preferable characteristics as a thermoelectric semiconductor.

From these facts, as shown in FIG. 7, the ^{dimensionless figure of merit ZT calculated by ZT = S 2} σT / κ is obtained by adding manganese to the copper gallium tellurium compound from a low temperature region close to room temperature. A value almost twice as high as that when manganese was not added was obtained.
The changes in the output factor PW and the dimensionless figure of merit ZT with respect to the manganese addition concentration are shown in FIGS. 8 and 9, respectively. By adding manganese, thermoelectric characteristics such as output factor PW and dimensionless figure of merit ZT were sharply improved.

(Example 2)
Example 2 is an example in which it is confirmed that the improvement in thermoelectric characteristics described in Example 1 is not due to "optimization of carrier density".

The experimental result of adding iron (Fe) to the copper gallium tellurium compound is shown in FIG. Fe is added as _{CuGa 1-x} Fe _x Te _{2 in} a form that replaces a part of Ga. Here, CGFT0, CGFT1, CGFT2 and CGFT3 described in FIG. 10 _{indicate the case where x of CuGa 1-x} Fe x Te ₂ is 0, 0.01, 0.02 and 0.03, respectively. That is, the case where iron is added to the copper gallium tellurium compound in an amount of 0 mol%, 1 mol%, 2 mol% and 3 mol%, respectively, is shown. The sample was prepared by substituting manganese with iron in the same process as in Example 1.
Since both Ga and Fe are trivalent ions and there is no change in the formal value, there is no change in the number of carriers. Therefore, it can be said that the second embodiment does not correspond to the “optimization of carrier density”.

As a result of the experiment, good electrical conductivity (electrical resistance) and a large Seebeck coefficient were observed even in the compound to which Fe was added, and as shown in FIG. 10, the output factor PW increased near room temperature. It was confirmed. This result indicates that the improvement in thermoelectric properties is not due to "optimization of carrier density".

(Example 3)
Example 3 is an example in which the thermoelectric power generation element 31 is manufactured (see FIG. 11).
In the manufactured thermoelectric power generation element 31, an n-type semiconductor 32, which is a thermoelectric material chip, is bonded to an electrode 35 on the low temperature side by soldering, and an electrode 34 on the opposite side of the n-type semiconductor 32 and a high temperature side. Are also joined by solder. Further, the same electrode 34 and the p-type semiconductor 33 which is a thermoelectric material chip are bonded, and the opposite end portion of the p-type semiconductor 33 is bonded to another electrode 35 to which another n-type semiconductor 32 is bonded. With such a configuration, the thermoelectric power generation element 31 is electrically connected in series.
As the p-type semiconductor 33, the copper gallium tellurium compound to which the magnetic ion of the present invention was added was used. As a result, heat can be recovered from a lower temperature range than before, and the range of available heat has been greatly expanded.

(Comparative Example 1)
Comparative Example 1 is an example of evaluating a copper gallium tellurium compound to which manganese is not added, and the method for producing the sample was the same as that of Example 1 except that manganese was not added.
Although it has already been mentioned, the evaluation results are shown in FIGS. 2 to 9 together with the results of the examples.
This copper gallium tellurium compound also exhibits a chalcopyrite structure (see FIG. 2), but as already described, the output factor PW of Comparative Example 1 is also dimensionless from the low temperature region near room temperature as compared with the case where manganese is added. The figure of merit ZT was also significantly smaller.

(Comparative Example 2)
Comparative Example 2 is an example in which the structure of a powder sample prepared by adding 5 mol% of manganese to a copper gallium tellurium compound was evaluated by X-ray diffraction, and the method for producing the sample was to add 5 mol% of manganese. The process was the same as that of Example 1 except that the above was used.
The evaluation results are shown in FIG. 2 together with the results of the examples.
The copper gallium tellurium compound to which 5 mol% of manganese is added has a chalcopyrite structure as a basic skeleton, but as described above, an impurity phase was observed.

As described above, the p-type thermoelectric semiconductor of the present invention in which one or more elements selected from the group of chromium, manganese, iron, cobalt, and nickel are added to the copper gallium tellurium compound has a temperature of about 400 ° C. from around room temperature. As a thermoelectric semiconductor, it is a thermoelectric semiconductor that exhibits high thermoelectric conversion efficiency even in a relatively low temperature region. Further, the thermoelectric power generation element having this thermoelectric semiconductor can provide high power generation efficiency even in a relatively low operating temperature region such as 500 ° C. or lower. For this reason, heat can be recovered from a low temperature range as compared with the conventional case, and the range of available heat is greatly expanded.

31 Thermoelectric power generation element 32 n-type semiconductor 33 p-type semiconductor 34 electrode 35 electrode 36 current

Claims (2)

A p-type thermoelectric semiconductor containing a copper gallium tellurium compound.
The copper gallium telluride compound contains chromium, manganese, iron, cobalt, and one or more elements selected from the group consisting of nickel,
A p-type thermoelectric semiconductor whose composition is a tetragonal p-type thermoelectric semiconductor having the composition shown in Formula 1 .
CuGa _1-x M _x Te ₂ (Equation 1)
(M is any one or more of Cr, Mn, Fe, Co, and Ni, and x is 0.01 or more and 0.03 or less.) A thermoelectric power generating element p-type semiconductor and the n-type semiconductor is formed by integrated thermoelectric power generation device characterized by using a p-type thermoelectric semiconductor according to claim 1 Symbol placement as the p-type semiconductor.

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