JP5664998B2 - Thermoelectric conversion material, thermoelectric power generating element and Peltier cooling element using the same - Google Patents

Description

  The present invention relates to a thermoelectric conversion material, a thermoelectric power generating element and a Peltier cooling element using the thermoelectric conversion material, and more particularly to a rare earth sulfide, a thermoelectric power generating element and a Peltier cooling element using the rare earth sulfide.

  Thermoelectric power generation and Peltier cooling are well known as energy conversion technologies using thermoelectric conversion. Thermoelectric power generation uses conversion from thermal energy to electrical energy by the Seebeck effect, and this technology is particularly attracting attention as an energy-saving technology that can recover waste thermal energy (waste heat) as electrical energy. . Peltier cooling is the reverse of thermoelectric power generation and uses conversion from electrical energy to thermal energy by the Peltier effect, and is a cooling technique using electrical energy. However, the efficiency of thermoelectric conversion is low, so the practical use of these technologies remains in a limited field.

Thermoelectric conversion uses physical phenomena related to materials such as Seebeck effect and Peltier effect. That is, in order to improve the efficiency of thermoelectric conversion, it is necessary to develop a material with high performance (thermoelectric conversion material). The performance of the thermoelectric conversion material is evaluated by the thermoelectric figure of merit (ZT) shown in the following Equation 1.
Equation 1: ZT = S ² T / ρκ
Here, S is the Seebeck coefficient, T is the temperature, ρ is the electrical resistivity, and κ is the thermal conductivity. In order to expand the application field of thermoelectric conversion materials, it is necessary to improve this thermoelectric figure of merit.

  There are bismuth tellurium, lead tellurium, silicon / germanium, rare earth borides, rare earth sulfides, etc. as thermoelectric conversion materials that are currently in use or are being considered for use. It cannot be said that the value is very high.

Rare earth sulfide is a known material, and its composition formula is R _z S ₃ (R is Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb. And at least one selected from the group of rare earth metals of Lu, and the range of z can be expressed as 1.80 or more and 2.50 or less. Its crystal structure is cubic Th ₃ P ₄ type. Since the melting point is as high as about 2000 ° C., it is expected as an n-type thermoelectric conversion material that can be used up to a high temperature. Here, since the thermal conductivity generally decreases as the number of elements constituting the material increases, the rare earth sulfide expected as a high-temperature thermoelectric conversion material contains two or more rare earth metals in R.
In addition, sulfur is an element that is very abundant on the earth, and there is no worry of exhausting it as a resource. For this reason, rare earth sulfides have received much attention from the viewpoint of resources.

In the prior art, the thermoelectric figure of merit of a multi-element rare earth sulfide containing two or more rare earth metals in R is improved by utilizing the metal conductive behavior (Patent Document 1, Non-Patent Document 1). For example, according to Patent Document 1, Sm _x La _y S ₃ , Eu _x La _y S ₃ , Yb _x La _y S ₃ , (x ranges from 0.11 to 0.60, and y ranges from 1.65 or more and 2.14 or less, and a value obtained by adding x and y is 2.25) indicates a metal conductive behavior. Non-Patent Document 1 describes that Nd _x Gd _y S ₃ (x is 1.00, y is in the range of 1.00 to 1.08) also exhibits a metal conductive behavior. .
In metal conduction, both the absolute value of the Seebeck coefficient and the electrical resistivity increase linearly as the temperature increases. As a result, as the temperature rises, the thermoelectric figure of merit defined by Equation 1 increases slowly. Since the degree of this increase is low, in this conventional technique, the thermoelectric figure of merit of the multi-element rare earth sulfide does not reach a sufficiently high value at high temperatures.
Similarly, most existing thermoelectric conversion materials such as bismuth / tellurium, lead / tellurium, silicon / germanium described in [0004] improve the thermoelectric figure of merit using metal conductive behavior. Not high enough.

As described above, since the multi-element rare earth sulfide utilizes the metal conductive behavior, it has been considered that the high temperature thermoelectric figure of merit does not reach a sufficiently high value.
On the other hand, it is theoretically known that there is a possibility that a high temperature thermoelectric figure of merit can be improved by utilizing hopping conduction. According to Non-Patent Document 2, for example, in the variable region hopping conduction, as the temperature T increases, the absolute value of the Seebeck coefficient increases in proportion to T ^1/2 , while the electrical resistivity is expressed as exp (C / T It decreases in proportion to ^1/4 ). Here, C is a constant. As a result of these, as the temperature rises, the thermoelectric figure of merit defined by Equation 1 increases significantly. Furthermore, a high thermoelectric figure of merit can be obtained at high temperatures by adjusting the hopping length, activation energy, and concentration of hopping carriers.

According to Non-Patent Document 3, one sulfide samarium Sm Z _{S 3} is a rare earth sulfide (range _z is 2.31 or less 2.00 or higher) has been reported to exhibit a hopping conduction behavior. However, because of the binary rare earth sulfide containing only Sm in R, its thermal conductivity is high. As a result, the thermoelectric figure of merit remains as low as about 0.15 even at a high temperature of 727 ° C. Furthermore, since R contains only Sm, it is difficult to adjust the hopping length, activation energy, and concentration of hopping carriers, and no further improvement in the thermoelectric performance index can be expected.
Further, according to Non-Patent Document 4, it is reported that Sm _0.75 Eu _0.75 Gd _0.75 S _3, which is a kind of multi-element rare earth sulfide, exhibits a hopping conductive behavior. However, it is greatly influenced by Eu ions whose valence is stable at 2+, and shows p-type and very inferior thermoelectric conversion characteristics different from general rare earth sulfides.
In other reported thermoelectric conversion materials, even when hopping conduction is used, the thermoelectric figure of merit has not reached a sufficiently high value. For example, according to Patent Document 2, the conduction mechanism of terbium polyboride, which is a kind of rare earth boride, follows the hopping law, but its thermoelectric figure of merit remains as low as about 0.06 even at a high temperature of 727 ° C.

United States Patent, Reg. No. H197 Patent No. 4081547

Michihiro OHTA and Shinji HIRAI, “Thermoelectric Properties of NdGd1 + xS3 Prepared by CS2 Sulfurization”, Journal of Electronic Materials. 38, no. 7, pp. 1287-1292, January 28, 2009. Neville MOTT, "Conduction in Non-Crystalline Materials" (Oxford Science Publications), pp. 18-38, 1987. A. V. GOLUBKOV, M.M. M.M. KAZANIN, V. V. KAMINSKII, V.K. V. SOKOLOV, S. M.M. SOLV'EV, L. N. TRUSHNIKOVA, “Thermoelectric Properties of SmSx (x = 0.8-1.5)”, Inorganic Materials, Vol. 39, no. 12, pp. 1251-1256, January 8, 2003. Michihiro OHTA, Toshihiro KUZUYA, Hideto SASAKI, Taku KAWASAKI, Shinji HIRAI, "Synthesis of Multinary Rare-earth Sulfides PrGdS3, NdGdS3, and SmEuGdS4, and Investigation of their Thermoelectric Properties", Journal of Alloys and Compounds, Vol. 484, Issues 1-2, pp. 268-272, April 23, 2009.

  Multi-element rare earth sulfides are known materials and are candidates for n-type high-temperature thermoelectric conversion materials without resource limitations. However, in the conventional technology, since the design as a thermoelectric conversion material is performed by utilizing a metal conductive behavior, the thermoelectric performance index of the multi-element rare earth sulfide does not reach a sufficiently high value. On the other hand, it is generally known that the thermoelectric figure of merit can be greatly improved at high temperatures by utilizing hopping conduction.

Therefore, in order to solve these problems, the present invention includes a rare earth metal A that exists in ions in a valence fluctuation state and a rare earth metal B that exists in ions that are not in a valence fluctuation state. Composition formula (A _x B _y ) S ₃ (the range of x is 0.80 or more and 1.20 or less, the range of y is 0.80 or more and 1.30 or less, and a value obtained by adding x and y Range of 1.80 or more and 2.50 or less), producing a ternary rare earth sulfide that causes effective hopping conduction behavior to improve thermoelectric figure of merit and high thermoelectric figure of merit at high temperature It aims at providing the thermoelectric conversion material which implement | achieved. It is another object of the present invention to provide a thermoelectric power generation element and a Peltier cooling element using the thermoelectric conversion material.

In metal conduction, both the absolute value of the Seebeck coefficient and the electrical resistivity increase as the temperature increases. As a result, as the temperature rises, the thermoelectric figure of merit defined by Equation 1 increases gradually. Therefore, a large increase in the thermoelectric figure of merit due to the temperature rise cannot be expected.
In contrast, in hopping conduction, the absolute value of the Seebeck coefficient increases as the temperature increases, while the electrical resistivity decreases. As a result, the thermoelectric figure of merit increases significantly as the temperature increases. In other words, hopping conduction can lead to a high thermoelectric figure of merit at high temperatures.

  In the valence fluctuation state, ions in the solid oscillate in different valence states. Carrier conduction (electrons and holes carrying charge) hops between ions of different valences to cause electrical conduction, resulting in a high thermoelectric figure of merit at high temperatures.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that, in the rare earth sulfide R _z S ₃ , which is abundant in resources, R exists in the valence fluctuation state of Sm and valence fluctuation. It has been found that when Gd present in ions that are not in a dynamic state is included, an effective hopping conduction behavior is caused to improve the thermoelectric figure of merit, resulting in a high thermoelectric figure of merit at a high temperature of 500 ° C. or higher. . This composition formula is (Sm _x Gd _y ) S ₃ (where x ranges from 0.95 to 1.10, y ranges from 0.95 to 1.15, and x plus y) The range is 1.80 or more and 2.50 or less). Sm ions are oscillated by Sm2 ⁺ and Sm3 ⁺ . Further, (Sm _x Gd _y ) S ₃ is a multi-element rare earth sulfide, and its thermal conductivity shows a low value.

It is a known fact that not only Sm but also Yb ions are likely to cause a valence fluctuation state. Therefore, it has been concluded that the same effect can be obtained when the rare earth metal present in the ions in the valence fluctuation state is at least one selected from Sm and Yb. Yb ions are likely to swing with Yb ²⁺ and Yb ³⁺ . When these ions are included at the same time, the carrier hops between ions of the same rare earth metal as well as between the ions of different rare earth metals. For example, between the ^{Sm 2+} and ^{Sm 3+,} of course, is also the carrier hops between ^{Sm 2+} and ^{Yb 3+.}

On the other hand, not only Gd but also all rare earth metal ions can be in a state where the valence fluctuation state is not present. That is, it was concluded that the same effect can be obtained when the rare earth metal present in ions that are not in a valence fluctuation state is at least one selected from La, Ce, Pr, Nd, and Gd. As a result of further investigation, the rare earth metal existing in ions that are not in the valence fluctuation state is at least one selected from Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu. It was concluded that the same effect can be obtained by using seeds.
Regardless of the type of rare earth metal, the valence of all ions is stable at 3+, and they are not directly involved in the behavior of hopping conduction. Regardless of the type of rare earth metal, it is important that both ions in the valence fluctuation state and ions not in the valence fluctuation state exist. The presence of ions that are not in a valence fluctuation state between ions in a valence fluctuation state changes the hopping length and activation energy. In addition, the concentration of hopping carriers is determined by the composition ratio of the rare earth metal present in ions in the valence fluctuation state and the rare earth metals present in ions not in the valence fluctuation state. That is, since the hopping length, activation energy, and concentration of hopping carriers can be optimized, the high temperature thermoelectric performance index can be improved.

In the thermoelectric conversion material of the present invention, the composition of x is 0.95 or more and 1.10 or less, and y is 0.95 or more and 1.15 or less. It was concluded that the same effect can be obtained when the range of x is 0.80 or more and 1.20 or less and the range of y is 0.80 or more and 1.30 or less.
As described in the above [0006], according to Patent Document 1, when the range of x is 0.11 or more and 0.60 or less and the range of y is 1.65 or more and 2.14 or less, hopping conductive However, it is said that the metal-conducting behavior is exhibited and the thermoelectric figure of merit is improved by utilizing the metal-conducting behavior. On the other hand, the present invention uses hopping conduction behavior and is not restricted by the technique of Patent Document 1 as described above. That is, in a range that deviates from the values of x and y described in Patent Document 1, the thermoelectric figure of merit is improved by utilizing hopping conduction behavior. From this meaning, the present invention is different from the prior art. It is clearly different.
Further, as described in [0008], in Non-Patent Document 4, Sm _0.75 Eu _0.75 Gd _0.75 S _3, which is a kind of multi-element rare earth sulfide, exhibits a hopping conductive behavior. It has been reported. However, since both the values of x and y are too small, they are greatly influenced by Eu and cannot cause hopping conductive behavior that is effective in improving the thermoelectric figure of merit. As a result, Sm _0.75 Eu _0.75 Gd _0.75 S ₃ exhibits p-type thermoelectric conversion characteristics that are very poor.

In the thermoelectric conversion material of the present invention, the hopping length, activation energy, and hopping are adjusted by adjusting the composition ratio of the rare earth metal existing in the valence fluctuation state ion and the rare earth metal existing in the valence fluctuation state ion. We have succeeded in improving the high-temperature thermoelectric figure of merit by optimizing the concentration of the carriers that are being used.
As described in [0008] above, Sm _Z S ₃ containing only rare earth metals present in valence fluctuation ions reported in Non-Patent Document ₃ (z range is 2.00 or more and 2.31). In the case of the following), since it does not include rare earth metals present in ions that are not in a valence fluctuation state, the hopping length, activation energy, and concentration of hopping carriers cannot be adjusted through adjustment of the composition ratio. An improvement in the thermoelectric figure of merit cannot be expected. Sm _Z S ₃ is a binary rare earth sulfide, and its high thermal conductivity is also a fatal defect.

  Further, the thermoelectric conversion material of the present invention uses the same kind of rare earth metal having both functions of a rare earth metal present in ions in a valence fluctuation state and a rare earth metal present in ions not in a valence fluctuation state. Therefore, there is a great advantage that reaction (formation of a plurality of compounds) or segregation that is likely to occur between different metals can be suppressed.

  Furthermore, the present inventors have found that a thermoelectric conversion material made of rare earth sulfide that realizes a high thermoelectric figure of merit using hopping conduction can be used for a thermoelectric power generation element and a Peltier cooling element.

  Multi-element rare earth sulfide is expected as an n-type high-temperature thermoelectric conversion material without resource constraints, but the conventional technology uses metal-conducting behavior, so it has sufficiently high thermoelectric performance at high temperatures. The index has not been reached. On the other hand, hopping conduction has the potential to provide a high thermoelectric figure of merit at high temperatures, but there are no examples of full utilization at present. According to the present invention, it is possible to cause a hopping conductive behavior that is effective in improving the thermoelectric figure of merit in a multi-element rare earth sulfide, and furthermore, it has a hopping conductive behavior with a rare earth sulfide. Can effectively enhance each other's benefits and improve the thermoelectric figure of merit.

  Furthermore, the invented thermoelectric conversion material composed of rare earth sulfides exhibiting hopping conductive behavior can be used for thermoelectric power generation elements and Peltier cooling elements.

(A) Seebeck coefficient and (b) electrical resistivity of Sm _x Gd _y S ₃ (x is 1.00 and y is 1.00 or more and 1.06 or less) produced in Example 1 of this invention It is a figure which shows temperature dependence. Sm _x Gd _y S ₃ (x is 1.00, y is 1.00 or more and 1.06 or less) and La _x Gd _y S ₃ (x is _x ) prepared in Example 1 and Comparative Example 1 of the present invention. It is a figure which shows the temperature dependence of the thermoelectric figure of merit of 1.00 and y being 1.00 or more and 1.03 or less. (A) Seebeck coefficient and (b) electrical resistivity of La _x Gd _y S ₃ (x is 1.00 and y is 1.00 to 1.03) produced in Comparative Example 1 of the present invention It is a figure which shows temperature dependence.

The thermoelectric conversion material of the present invention is based on a multi-element rare earth sulfide that can be expressed by a composition formula (A _x B _y ) S ₃ (the range of a value obtained by adding x and y is 1.80 or more and 2.50 or less). Yes. The thermal conductivity of multi-element rare earth sulfides is low.

In the present invention, in the composition formula (A _x B _y ) S ₃ , A includes at least one selected from Sm and Yb present in ions in a valence fluctuation state in order to cause hopping conduction behavior. Preferably, A includes Sm present in ions in a valence fluctuation state.

  On the other hand, B includes Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu which are not in a valence fluctuation state, preferably B is not in a valence fluctuation state It is characterized in that it contains at least one selected from La, Ce, Pr, Nd, and Gd present in ions, and more preferably contains Bd in ions that are not in a valence fluctuation state in B.

In the present invention, in the composition formula (A _x B _y ) S ₃ , the range of x is 0.8 or more and 1.2 or less, and the range of y is 0.8 or more and 1.3 or less, preferably x Is in the range of 0.95 to 1.10 and y is in the range of 0.95 to 1.15.

  The thermoelectric conversion material of the present invention can be used at 500 ° C. or higher.

  The element for thermoelectric generation and the element for Peltier cooling of the present invention are characterized by being composed of the thermoelectric conversion material as a thermoelectric conversion material.

  Next, an Example and a comparative example are shown and this invention is demonstrated in detail. In addition, description of the Example shown below and a comparative example is for making an understanding of invention easy, and this invention is not restrict | limited by description of these embodiment. In other words, modifications and other implementation conditions based on the technical idea of the present invention are naturally included in the present invention.

Example 1
As Example 1, Sm _x Gd _y S ₃ containing Sm present in ions in a valence fluctuation state and Gd existing in ions not in a valence fluctuation state (x is 1.00 and y is 1.00). Thus, the thermoelectric conversion characteristics were evaluated. As starting materials, samarium oxide Sm ₂ O ₃ and gadolinium oxide Gd ₂ O ₃ were prepared. These oxides were separately placed on a quartz boat, and samarium sulfide Sm ₂ S ₃ and gadolinium sulfide Gd ₂ S ₃ were synthesized by sulfiding with a carbon disulfide gas at a temperature of 850 ° C. for 8 hours.

Next, the synthesized Sm ₂ S ₃ and Gd ₂ S ₃ powders, and further GdH ₃ powders are mixed in order to adjust the amount of Gd, placed on a graphite boat, and reacted at a temperature of 1300 ° C. for 2 hours in a vacuum. Thus, Sm _x Gd _y S ₃ was produced. Here, GdH ₃ was used because it is easy to handle in the atmosphere and decomposes at about several hundred degrees to release unnecessary hydrogen as a gas in the final product.
Finally, this synthetic powder is filled in a graphite mold and sintered under pressure at a temperature of 1200 ° C. and a pressure of 30 MPa for 2 hours in a vacuum to obtain a dense structure having a cubic Th ₃ P ₄ type crystal structure. A sintered body was produced.

The lattice constant of the produced sintered body was evaluated using an X-ray diffraction method. The expected lattice constant is about 0.839 nm when Sm ions are present only in Sm ^{3+ and} not in a valence fluctuation state. However, the actual lattice constant is about 0.844 nm, which is larger than the expected value. This result means that the valence of the Sm ion is lower than 3+, that is, it is suggested that the valence fluctuation state of Sm ²⁺ and Sm ³⁺ exists.

The thermoelectric conversion characteristics of the produced sintered body were evaluated in a temperature range from room temperature to 680 ° C. In all the produced sintered bodies, the Seebeck coefficient showed a negative value (n-type). As shown in FIG. 1, as the temperature increases, the absolute value of the Seebeck coefficient increases while the electrical resistivity decreases. That is, Sm _x Gd _y S ₃ exhibited a hopping conductive behavior. From the Arrhenius plot of the electrical resistivity, the activation energy of the hopping conductive behavior was found to be 0.14 eV to 0.19 eV.
The thermal conductivity showed values from 0.5 W / Km to 0.8 W / Km in the measured temperature range.

As a result, as shown in FIG. 2, the thermoelectric figure of merit that can be calculated using Equation 1 was significantly increased. In the case of SmGd _1.02 S ₃ , the thermoelectric figure of merit increases significantly from 0.0003 at room temperature to 0.3 at 680 ° C. (1000 times the value at room temperature) as the temperature increases. This significant improvement in the thermoelectric figure of merit means that in rare earth sulfides, it has succeeded in causing an effective hopping conductive behavior that improves the thermoelectric figure of merit. Considering that the melting point of the rare earth sulfide is about 2000 ° C., it can be easily understood that this thermoelectric figure of merit reaches a value that surpasses other thermoelectric conversion materials at higher temperatures.

(Comparative Example 1)
As Comparative Example 1, La _x Gd _y S ₃ (x is 1.00 and y is 1.00 or more and 1.03 or less) that does not contain any rare earth metal present in the valence fluctuation state of ions. The thermoelectric conversion characteristics were evaluated. As starting materials, lanthanum oxide La ₂ O ₃ and gadolinium oxide Gd ₂ O ₃ were prepared. Lanthanum sulfide La ₂ S ₃ and gadolinium sulfide Gd ₂ S ₃ were synthesized by placing these oxides separately on a quartz boat and sulfiding with a carbon disulfide gas at a temperature of 850 ° C. for 8 hours.

Next, the synthesized La ₂ S ₃ , Gd ₂ S ₃ and GdH ₃ powders are mixed and placed on a graphite boat, and La _x Gd _y S ₃ is reacted at a temperature of 1300 ° C. for 2 hours in a vacuum. Produced. Finally, this synthetic powder is filled in a graphite mold and sintered under pressure at a temperature of 1200 ° C. and a pressure of 30 MPa for 2 hours in a vacuum to obtain a dense structure having a cubic Th ₃ P ₄ type crystal structure. A sintered body was produced.

The lattice constant of the produced sintered body was evaluated using an X-ray diffraction method. The lattice constant agreed with the value expected when no rare earth metal present in ions in the valence fluctuation state was included. That is, this result suggests that the produced La _x Gd _y S ₃ does not contain any rare earth metal present in ions in a valence fluctuation state.

The thermoelectric conversion characteristics of the produced sintered body were evaluated in a temperature range from room temperature to 680 ° C. In all the produced sintered bodies, the Seebeck coefficient showed a negative value (n-type). As shown in FIG. 3, as the temperature increased, both the absolute value of the Seebeck coefficient and the electrical resistivity increased. That is, La _x Gd _y S ₃ exhibited a metal conductive behavior.
The thermal conductivity showed values from 0.5 W / Km to 1.3 W / Km in the measured temperature range.

From the above results, as shown in FIG. 2, the thermoelectric figure of merit increases gradually. In the case of LaGd _1.00 S ₃ , the thermoelectric figure of merit gradually increases from 0.04 at room temperature to 0.2 at 680 ° C. (5 times the value at room temperature) as the temperature increases. Compared to Example 1, the thermoelectric figure of merit up to this temperature (low temperature) is high. However, as shown in FIG. 3, when the temperature rises further, the electrical resistivity only increases and cannot be lowered. Therefore, as shown in FIG. A large increase in the figure of merit cannot be expected, and it can be said that it is difficult to use it as a thermoelectric conversion material.

On the other hand, in Example 1 using the rare earth metal existing in the valence fluctuation state and the rare earth metal existing in the valence fluctuation state, the thermoelectric figure of merit at a low temperature is low, but 680 When it reaches ° C., the value reaches 1.5 times as much as the thermoelectric figure of merit of Comparative Example 1. From this result, it can be said that the thermoelectric conversion material of the present invention is useful at a high temperature.
Further, when the temperature rises, it is strongly estimated that the electrical resistivity is further lowered, and thus it is useful as a thermoelectric conversion material. This tendency can be said to be a common characteristic when the rare earth metal existing in the valence fluctuation state and the rare earth metal existing in the valence fluctuation state are used in the above composition range. Furthermore, even if the thermoelectric figure of merit at low temperature is low, the thermoelectric conversion material of the present invention in which the thermoelectric figure of merit at a high temperature region rises rapidly increases the electrical resistivity when the temperature rises, and the thermoelectric figure of merit at high temperature It can be said that it is far more useful than conventional thermoelectric conversion materials that cannot be increased.

(Comparative Example 2)
By using cerium oxide CeO ₂ , praseodymium oxide Pr ₆ O ₁₁ , neodymium oxide Nd ₂ O ₃ , and gadolinium oxide Gd ₂ O ₃ as starting materials, these oxides are sulfided with carbon disulfide gas at a temperature of 850 ° C. for 8 hours. Cerium sulfide Ce ₂ S ₃ , praseodymium sulfide Pr ₂ S ₃ , neodymium sulfide Nd ₂ S ₃ , and gadolinium sulfide Gd ₂ S ₃ were synthesized. Further, these binary rare earth sulfides and GdH ₃ powder are mixed and reacted at a temperature of 1300 ° C. to 1500 ° C. for 1 hour to 2 hours in a vacuum to obtain CeGdS ₃ , PrGdS ₃ , Nd _x Gd _y S ₃ (x is 1.00 and y is 1.00 or more and 1.08 or less) was produced. Finally, these synthetic powders are pressure-sintered in vacuum at a temperature of 1200 ° C. to 1400 ° C. and a pressure of 30 MPa for 1 to 2 hours to obtain a dense sintered body having a cubic Th ₃ P ₄ type crystal structure. A ligature was prepared.

The lattice constant of the produced sintered body was evaluated using an X-ray diffraction method. All lattice constants agreed with the values expected when no rare earth metals present in ions in the valence fluctuation state were included. That is, this result suggests that all of the produced CeGdS ₃ , PrGdS ₃ , and Nd _x Gd _y S ₃ do not contain any rare earth metal present in the valence fluctuation state ions.

The Seebeck coefficient and electrical resistivity of the produced Nd _x Gd _y S ₃ sintered body were evaluated in the temperature range from room temperature to 680 ° C. In all the produced sintered bodies, the Seebeck coefficient showed a negative value (n-type). Furthermore, as the temperature increased, both the absolute value of the Seebeck coefficient and the electrical resistivity increased. That is, Nd _x Gd _y S ₃ exhibits a metal conductive behavior, and a high thermoelectric figure of merit cannot be expected at high temperatures.

Although the present invention has been described mainly with reference to the materials of the above-described examples, the materials listed above, that is, the composition formula (A _x B _y ) S ₃ (A is at least one selected from Sm and Yb, and B is , Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu, and the range of x is 0.80 or more and 1.20 or less The range of y is 0.80 or more and 1.30 or less, and the range of the sum of x and y is 1.80 or more and 2.50 or less. it can. That is, under the above composition conditions, A contains at least one rare earth metal present in ions in a valence fluctuation state in the range of x from 0.80 to 1.20, and B in a valence fluctuation state. It is important to contain at least one rare earth metal present in the range of ions in the range of y from 0.80 to 1.30. When these conditions are satisfied, an excellent thermoelectric conversion material can be obtained. It becomes possible.

  Although not specifically shown as an example, a rare earth metal existing under the same or similar condition as that shown in the example, that is, a rare earth metal existing in a valence fluctuation state ion and an ion not in a valence fluctuation state All can be implemented by arbitrarily setting the type and composition ratio within the above range.

  Furthermore, although not specifically shown as an Example, it cannot be overemphasized that the thermoelectric conversion material of this invention can be utilized for the element for thermoelectric power generation, and the element for Peltier cooling.

The rare earth sulfide of the present invention has a composition formula (A _x B _y ) S ₃ (x) including a rare earth metal A present in ions in a valence fluctuation state and a rare earth metal B present in ions not in a valence fluctuation state. Is in the range of 0.80 to 1.20, y is in the range of 0.80 to 1.30, and x and y are in the range of 1.80 to 2.50. It is possible to cause a hopping conductive behavior that is effective in improving the thermoelectric figure of merit in this multi-element rare earth sulfide, and the hopping conductive behavior of the rare earth sulfide. It is useful as a thermoelectric conversion material because it has the great effect that it can effectively enhance each other's advantages and improve the high temperature thermoelectric figure of merit, and it is also useful for thermoelectric power generation elements and Peltier cooling It is useful to the child.

Claims (7)

Formula _{(A x B y) S 3} (A is, Sm, at least one selected from Yb, B is, Sc, Y, La, Ce , Pr, Nd, Gd, Tb, Dy, Ho, Er , Tm, and Lu. The range of x is 0.80 or more and 1.20 or less, and the range of y is 0.80 or more and 1.30 or less. Range of 1.80 to 2.50), A contains at least one rare earth metal present in ions in the valence fluctuation state, and B in the valence fluctuation state. A thermoelectric conversion material comprising at least one rare earth metal present in the form of no ions. Formula (A _x B _y) thermoelectric conversion material according to claim 1, wherein the A of S ₃ is Sm present in the valence fluctuation state ions.   3. The thermoelectric conversion material according to claim 1, wherein B is at least one selected from La, Ce, Pr, Nd, and Gd existing in ions that are not in a valence fluctuation state.   The thermoelectric conversion material according to any one of claims 1 to 3, wherein B is Gd present in ions that are not in a valence fluctuation state. Range of x composition formula (A _x B _y) S ₃ is 1.10 or less than 0.95, according to claim 1 y ranges is characterized in that 1.15 or less than 0.95 5 to 4. The thermoelectric conversion material according to any one of items 1 to 4. The element for thermoelectric power generation using the thermoelectric conversion material in any one of Claim 1 to 5 . The element for Peltier cooling using the thermoelectric conversion material in any one of Claim 1 to 5 .