JP5516042B2 - Clathrate compound, thermoelectric conversion material, and production method thereof - Google Patents

Description

  The present invention relates to a clathrate compound, a thermoelectric conversion material, and a production method thereof.

  Conventionally, thermoelectric conversion elements have a power generation function based on the Seebeck effect and a thermoelectric cooling function based on the opposite Peltier effect. The power generation function is obtained by causing a temperature difference between both ends of the thermoelectric conversion element, and the cooling function is obtained by energizing the thermoelectric conversion element. The thermoelectric conversion element has a structure in which, for example, a P-type thermoelectric conversion material and an N-type thermoelectric conversion material are alternately connected in series between two electrode plates.

In such a thermoelectric conversion element, the performance as the element can be obtained as a figure of merit Z of thermoelectric conversion. The figure of merit Z of thermoelectric conversion is represented by Z = S ² σ / κ. S is the Seebeck coefficient of the thermoelectric conversion material, σ is the electrical conductivity of the thermoelectric conversion material, and κ is the thermal conductivity of the thermoelectric conversion material.

As one of the thermoelectric conversion materials described above, for example, a clathrate compound has been proposed as disclosed in Patent Document 1. Among these clathrate compounds, Ba ₈ (Ga, Sn) ₄₆ clathrate has a particularly high performance in a low temperature region (see Non-Patent Document 1), and is a thermoelectric conversion material expected for applications from room temperature to around 400 ° C. is there.

Japanese Patent Laid-Open No. 13-044519

Thermoelectric Conversion Symposium 2003 Proceedings, Japan Thermoelectric Society, p. 148-149

However, although the above Ba ₈ Ga ₁₆ Sn ₃₀ thermoelectric conversion material exhibits a relatively high figure of merit (ZT) in a low temperature region, it does not reach ZT = 1.0, which is a standard for practical use, and further performance Improvement of the index (ZT) is desired. Furthermore, Ga, which is a constituent element of the Ba ₈ Ga ₁₆ Sn ₃₀ thermoelectric conversion material, is higher in cost than Ba and Sn, and it is desirable to change to an element cheaper than Ga when practical use is assumed. Further, since Ga has a large mass, for example, when this material is used for exhaust heat power generation from automobile exhaust gas, the weight of the material causes a reduction in fuel consumption. For this reason, the thermoelectric conversion material using a light element is desired.

In view of the above points, the present invention provides a high figure of merit (ZT) as compared with Ba ₈ Ga ₁₆ Sn ₃₀ thermoelectric conversion materials, and provides inexpensive and light clathrate compounds, thermoelectric conversion materials, and methods for producing them. Objective.

In order to achieve the above-mentioned object, the inventors of the present invention, Ba ₈ Ga _16-x Al _x Sn ₃₀ , in which part of Ga ₈ Ga ₁₆ Sn ₃₀ thermoelectric conversion material is replaced with Al that is lighter and cheaper than Ga. Has developed a new clathrate material with no synthesis example. _{Moreover, Ba 8 Ga 16-x Al} x Sn 30 thermoelectric conversion material obtained by substituting a part of Al and Ga _were found to exhibit high performance index (ZT) than in the Ba ₈ Ga 16 _{Sn 30} thermoelectric conversion material.

Therefore, in the invention according to claim 1, composition formula _{Ba 8 (Ga x, Sn y} , Al z) 46 ( where, x + y + z = 1 , 0 <x ≦ 1, 0 ≦ y ≦ 1,0 <z ≦ It is characterized by being represented by 1).

As described above, the structure containing Al with respect to the conventional Ba ₈ Ga ₁₆ Sn ₃₀ clathrate compound provides a large figure of merit (ZT) as compared with the conventional Ba ₈ Ga ₁₆ Sn ₃₀ clathrate compound. (See FIG. 1). In addition, since Al is contained, an inexpensive and light clathrate compound can be obtained.

  Here, Ga / (Ba + Ga + Sn + Al), which is the atomic ratio of Ga in the clathrate compound of the present invention, is preferably in the range of 0.09 to 0.28 (see FIG. 1). Further, Sn / (Ba + Ga + Sn + Al), which is the atomic ratio of Sn in the clathrate compound of the present invention, is preferably in the range of 0.55 to 0.57 (see FIG. 1). By being in the said range, manufacture of the clathrate compound which concerns on Claim 1 becomes easy.

In the invention according to claim 2, the composition formula is Ba ₈ (Ga _x , Sn _y , Al _z ) ₄₆ (where x + y + z = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 <z ≦ 1). in a clathrate compound represented, wherein the Al / an atomic ratio of Al in the clathrate compound (Ba + Ga + Sn + Al ) is 0.20 or less greater than 0.

  Thus, when the ratio of Al / (Ba + Ga + Sn + Al) is larger than 0, the figure of merit (ZT) of the clathrate compound is improved, and the clathrate compound can be obtained at a low raw material cost and light weight. The inventors have also found that a clathrate compound having Al / (Ba + Ga + Sn + Al) exceeding 0.20 cannot be produced. Therefore, when Al / (Ba + Ga + Sn + Al) is 0.20 or less, the clathrate compound according to the present invention can be produced.

  The invention according to claim 3 is characterized by comprising the clathrate compound according to claim 1 or 2.

  As described above, since a large figure of merit (ZT) is obtained when the clathrate compound contains Al, a high figure of merit (ZT) can be obtained by constructing a thermoelectric conversion material with a clathrate compound containing Al. Can be obtained.

  Such a thermoelectric conversion material may be the clathrate compound itself according to claim 1 or 2 or may contain other components. The thermoelectric conversion material of the present invention may be p-type or n-type.

The invention according to claim 4, a method for producing a clathrate compound according to claim 2, have a step of a melt by melting constituent elements of clathrate compound, the constituent elements of the clathrate compound In the step of melting the material to form a melt, the atomic ratio of Al in the melt is set to be greater than 0 and 0.17 or less .

  Thus, by making the ratio of Al in the melt larger than 0, the clathrate compound to be produced can contain Al. Moreover, the clathrate compound whose Al / (Ba + Ga + Sn + Al) is 0.20 or less can be manufactured by making the atomic ratio of Al in a melt into 0.17 or less (refer FIG. 1).

Then, as in the invention described in claim 5 , by using the clathrate compound manufacturing method described in claim 4 , a thermoelectric conversion material containing the clathrate compound itself or other components can be manufactured. .

It is the figure which showed the table | surface of the preparation ratio of a clathrate compound which concerns on 1st Embodiment of this invention, a composition ratio, a Seebeck coefficient, a specific resistance, and a figure of merit. It is the figure which showed the temperature pattern of the typical electric furnace when synthesize | combining a clathrate compound. It is explanatory drawing showing the sample state immediately after the synthesis | combination of a clathrate compound. It is the figure which showed an example of the crystal structure of the clathrate compound which concerns on this embodiment. It is the figure which showed the X-ray-diffraction chart of sample S4 of FIG. It is the figure which showed the figure of merit of each sample with respect to temperature.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, the same or equivalent parts are denoted by the same reference numerals in the drawings.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. First, a method for producing a clathrate compound according to the present invention will be described.

  First, Ba, Ga, Sn, and Al having a purity of 99% or more, which are constituent elements of the clathrate compound, were weighed so as to have a charging ratio (atomic ratio) shown in the table of FIG. For example, the atomic ratio of Al is expressed by Al / (Ba + Ga + Sn + Al). The same applies to Ba, Ga, and Sn.

There are seven types of samples, S1 to S7, with different charge ratios, and a clathrate compound is produced for each. Here, the sample S0 is a conventional product (Ba ₈ Ga ₁₆ Sn ₃₀ , type-VIII) that does not contain Al.

  The table shown in FIG. 1 shows the composition ratio (at%) of the single crystal obtained with respect to the raw material charge ratio (at%) of each of the samples S1 to S7, as well as the Seebeck coefficient (μV / K), specific resistance at 500K (Ωm), and figure of merit (ZT) at 500K, respectively. Further, the composition ratio of the samples S0 and S5 and the Seebeck coefficient and specific resistance of the samples S1 and S7 are not measured.

  Next, the weighed raw materials are placed in a quartz tube, the inside of the quartz tube is evacuated by a vacuum pump, and then the inside of the quartz tube is filled with Ar gas and evacuated several times. Finally, the inside of the quartz tube is evacuated. The quartz tube was sealed. Subsequently, according to the temperature pattern shown in FIG. 2, the quartz tube charged with the raw material is heated and melted (about 490 ° C.) in an electric furnace to form a melt, and the melt is further cooled to 390 ° C. over 100 hours. A single crystal was grown.

  At this stage, the single crystal is in a state of being mixed with surplus material (flux). Therefore, the inside of the quartz tube was cooled to 50 ° C. or lower using a centrifuge, and the single crystal and the excess material were separated.

  Specifically, the single crystal is grown in the quartz tube as described above, but as shown in FIG. 3, a depression 11 is provided in the quartz tube 10, and the quartz tube 10 is divided into two pieces with the depression 11 as a boundary. It is divided into rooms 12 and 13. Further, a glass wool 20 is disposed in the vicinity of the depression 11 in one chamber 12 of the quartz tube 10. Then, by placing the other chamber 13 of the quartz tube 10 outside the one chamber 12 with respect to the center of rotation of the centrifugal separation, the glass wool 20 is caught in the recess 11 of the quartz tube 10 by the centrifugal force, and the crystal body 30 is caught in the glass wool 20 and is left in the one chamber 12 of the quartz tube 10, and the surplus substance 40 which is heated and becomes liquid is separated into the other chamber 13 through the glass wool 20. In this way, the crystal body 30 was separated from the surplus substance 40, and the crystal body 30 having a metallic luster was taken out from the quartz tube 10.

  By such a method, samples S1 to S7 corresponding to the charging ratios of Ba, Ga, Al, and Sn shown in the table of FIG. 1 were manufactured. That is, the Al preparation ratio of the sample S1 was set to the smallest, and the samples S2 to S7 were set so that the Al preparation ratio increased in order.

  Then, a part of the crystal body 30 taken out from the quartz tube 10 was pulverized with a mortar made of straw and measured with an X-ray diffraction apparatus. As a result, it was confirmed that all of the samples S1 to S7 had the type-VIII clathrate structure shown in FIG. As shown in FIG. 4, the clathrate compound has a structure in which (Ga, Sn, Al) forms a cage structure and Ba is included in the cage structure.

  FIG. 5 shows an X-ray diffraction chart of sample S4. As shown in this figure, the peak indicated by the arrow is a peak indicating the crystal structure of type-VIII. Such a peak of type-VIII was the same result for other samples. Note that the X-ray diffraction result of FIG. 5 includes a peak due to the surplus substance 40 attached to the crystal surface of the crystal 30.

  Further, the result of polishing the crystal 30 and examining the composition ratio of the crystal 30 by EPMA (Electron Probe Micro Analysis) is shown in the column of “composition ratio” in the table of FIG. Al was detected in all the samples S1 to S7, and it was found that Al was substituted in the clathrate structure.

  Here, when the crystal 30 of the clathrate compound is grown as described above, the Al composition ratio of the crystal 30 is 19. As shown in FIG. It was 5 at%. The inventors tried to grow a single crystal of a clathrate compound with an Al charging ratio higher than 17 at%, but the single crystal did not grow. From these results, when the constituent elements of the clathrate compound are melted to obtain a melt, it is preferable that the atomic ratio of Al in the melt is 0.17 or less. The atomic ratio of Al in the clathrate compound obtained by such an atomic ratio is 0.20 or less. Therefore, by making the atomic ratio of Al in the melt greater than 0 and 0.17 or less, the atomic ratio of Al in the resulting clathrate compound is greater than 0 and 0.20 or less.

  The inventors cut and polished the crystal 30 and measured its specific resistance and Seebeck coefficient with a Seebeck Measurement System (manufactured by MMR Technologies). As a result, as shown in the table of FIG. 1, the specific resistance of the samples S2 to S6 is lower than that of the conventional sample S0 to which Al is not added, and the specific resistance is a conventional product. It is reduced compared to the sample S0. Further, the absolute values of the Seebeck coefficients of the samples S2 to S6 were substantially the same or higher than those of the conventional sample S0. All measured samples S2 to S6 showed negative Seebeck coefficients, and it was found that these samples S2 to S6 were all n-type thermoelectric elements. Although FIG. 1 shows the specific resistance and Seebeck coefficient at 500 K, similar results were obtained at other temperatures.

  Furthermore, the inventors examined the figure of merit ZT of the crystal 30 with respect to temperature. The result is shown in FIG. As shown in this figure, the result that the figure of merit of all the samples S2 to S6 is higher than that of the conventional sample S0 was obtained. That is, it can be said that the samples S2 to S6 containing Al have higher thermoelectric conversion performance than the conventional sample S0 containing no Al. In FIG. 1, the figure of merit at 500K is shown.

  The clathrate compound itself containing Al obtained by the above production method can be used as a thermoelectric conversion material. Moreover, you may comprise the thermoelectric conversion material containing a clathrate compound. In this case, the thermoelectric conversion material may be n-type or p-type. Thereby, the thermoelectric conversion material whose performance index is higher than before is obtained.

(Second Embodiment)
In the present embodiment, the clathrate compound is produced by a production method different from that of the first embodiment. First, Ba, Ga, Sn, and Al having a purity of 99% or more were weighed so that the charging ratio (at%) was Ba: Ga: Sn: Al = 4: 12: 30: 4.

  Next, the raw material was melted by an arc melting method to produce an ingot. This ingot was placed on a quartz boat and placed in a quartz tube. The interior of the quartz tube was in an Ar atmosphere. The quartz tube was placed in a tubular furnace to melt the ingot, and then cooled to solidify. The temperature pattern of the tubular furnace in the process from melting to cooling is increased from 1800 ° C / hr from room temperature to 900 ° C, held at 900 ° C for 5 hours, and slowly from 10 ° C / hr from 550 ° C to 350 ° C. The temperature was set to drop.

  After cooling to 50 ° C. or lower, the sample taken out of the quartz tube had a structure in which the crystal was embedded in the solidified flux. Therefore, a part of the crystal body taken out was pulverized with a mortar of a straw and measured with an X-ray diffractometer. As a result, it was confirmed that it was a type-VIII clathrate structure. Moreover, as a result of grinding | polishing the taken-out crystal body and investigating by EPMA, it was set as the composition ratio (at%) of Ba: Ga: Sn: Al = 14.50: 23.61: 56.13: 5.76.

The obtained single crystal of about 1 mm was polished, and the specific resistance and Seebeck coefficient of the surface were measured at room temperature. As a result, the specific resistance was 3.5 × 10 ⁻⁵ Ωm. The Seebeck coefficient of the single crystal was 427 μV / K. This single crystal was p-type from the sign of the Seebeck coefficient.
(Comparative Example 1)
First, Ba, Ga, Sn, and Al having a purity of 99% or more were weighed so that the charging ratio (at%) was Ba: Ga: Sn: Al = 10.8: 0: 67.6: 21.6. . That is, Ga, which is a constituent element of the clathrate compound, is all replaced with Al.

Next, the raw material was melted by the arc melting method in the same manner as described above to produce an ingot. The temperature conditions and the like are the same as described above. The obtained crystal was pulverized with an agate mortar and measured with an X-ray diffractometer. As a result, no diffraction peak due to the clathrate structure was observed.
(Comparative Example 2)
First, Ba, Ga, and Sn having a purity of 99% or more were weighed so that the charging ratio (at%) was Ba: Ga: Sn = 8: 32: 60. That is, Al is not included in the constituent elements of the clathrate compound.

  Next, the weighed raw materials were melted in an arc melting furnace, and then cooled and solidified. The cooling rate was 1000 ° C./hr or more. The solidified ingot was placed on a quartz boat and placed in a quartz tube. The quartz tube was evacuated with a vacuum pump and then filled with Ar gas to form an Ar atmosphere. The quartz tube was then placed in an electric tubular furnace to remelt the sample, and then cooled to solidify. The temperature pattern of the tubular furnace in the melting to cooling process is to increase the temperature from room temperature to 900 ° C. at 1800 ° C./hr, hold at 900 ° C. for 5 hours, and gradually decrease the temperature from 550 ° C. to 350 ° C. at 10 ° C./hr. Set.

  After cooling to 50 ° C. or lower, the sample taken out from the quartz tube had a structure in which the crystal was embedded in the flux. The taken-out crystal was pulverized with a mortar and measured with an X-ray diffractometer. As a result, it was confirmed that it had a type-I clathrate structure.

The extracted crystal was polished and examined by EPMA. As a result, the composition ratio (at%) was Ba: Ga: Sn = 14.6: 29.5: 55.9. Moreover, as a result of measuring the Seebeck coefficient of the polished crystal, it was p-type from the agreement. Furthermore, as a result of measuring the specific resistance of the crystal at room temperature (25 ° C.), it showed a high specific resistance of 1 × 10 ⁻⁴ Ωm or more.
(Comparative Example 3)
First, Ba, Ga, and Sn having a purity of 99% or more were weighed so as to have a charging ratio (at%) of Ba: Ga: Sn = 14.3: 25.0: 60.7.

  Next, the weighed raw materials were melted in an arc melting furnace, and then cooled and solidified. The cooling rate was 1000 ° C./hr or more. The sample taken out after cooling to 50 ° C. or lower was pulverized with a mortar of a straw and molded with a discharge plasma sintering apparatus. The molding conditions were pressure: 7.1 kN, holding temperature: 400 ° C., holding time: 60 min.

A part of the sintered product was pulverized with a mortar made of straw and measured with an X-ray diffractometer. As a result, it was confirmed that it was a type-VIII clathrate structure. As a result of measuring the Seebeck coefficient of a similar fired product, it was found that it was n-type from its sign. Furthermore, the specific resistance at 500K showed a high specific resistance of 7.0 × 10 ⁻⁵ Ωm or more.

  As described above, in contrast to the p-type clathrate compound obtained by the Al charging ratio according to the present embodiment, when all Ga was replaced with Al as in Comparative Example 1, no clathrate compound was obtained. . Therefore, as shown in FIG. 1, it is understood that a clathrate compound containing Al cannot be obtained unless the preparation ratio of Al is appropriate.

  Further, from the results of Comparative Examples 2 and 3, it is difficult to produce a clathrate compound (that is, a thermoelectric conversion material) having a low electrical conductivity even if the charging ratio (mixing raw material ratio) is changed in the Ba, Ga, and Sn system. It was confirmed that. Therefore, as in this embodiment, it is important to produce the clathrate compound with the Al charging ratio as shown in FIG.

(Other embodiments)
In each of the above embodiments, the samples S1 to S7 obtained by changing the charging ratio of Al have been described. However, this is an example of the clathrate compound and thermoelectric conversion material of the present invention, and in a range not departing from the present invention. It can be implemented in various ways.

DESCRIPTION OF SYMBOLS 10 Quartz tube 11 Recess of quartz tube 12 One chamber of quartz tube 13 The other chamber of quartz tube 20 Glass wool 30 Crystal body 40 Excess material

Claims (5)

The compositional formula is represented by Ba ₈ (Ga _x , Sn _y , Al _z ) ₄₆ (where x + y + z = 1, 0 <x ≦ 1 , 0 ≦ y ≦ 1, 0 <z ≦ 1) Clathrate compound. It is a clathrate compound whose composition formula is represented by Ba ₈ (Ga _x , Sn _y , Al _z ) ₄₆ (where x + y + z = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 <z ≦ 1). And
The clathrate compound is the atomic ratio of Al in the compound Al / (Ba + Ga + Sn + Al) is characterized and to torque Rasureto compound that is 0.20 or less greater than 0.   A thermoelectric conversion material comprising the clathrate compound according to claim 1. A method for producing a clathrate compound according to claim 2 ,
Have a step of a melt by melting constituent elements of the clathrate compound,
In the step of melting the constituent elements of the clathrate compound to form a melt, the atomic ratio of Al in the melt is set to be greater than 0 and 0.17 or less . A method for producing a thermoelectric conversion material, comprising the method for producing a clathrate compound according to claim 4 .

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