JP6082663B2 - Thermoelectric conversion material and method for producing the same - Google Patents

Description

  The present invention relates to a clathrate compound, and particularly relates to a thermoelectric conversion material using the compound and a method for producing the same.

Thermoelectric conversion elements using the Seebeck effect can convert thermal energy into electrical energy. Utilizing this property, it is possible to convert exhaust heat exhausted from industrial / consumer processes and mobile objects into effective electric power, and thermoelectric conversion elements are attracting attention as energy-saving technologies in consideration of environmental problems.
The dimensionless figure of merit ZT of the thermoelectric conversion element using the Seebeck effect can be expressed by the following formula (1).
ZT = S ² T / ρκ (1)
In the formula (1), S, ρ, κ, and T represent the Seebeck coefficient, electrical resistivity, thermal conductivity, and measurement temperature, respectively.

As is clear from equation (1), in order to improve the performance of the thermoelectric conversion element, it is important to increase the Seebeck coefficient of the element, to decrease the electrical resistivity, and to decrease the thermal conductivity. .
Conventionally, thermoelectric conversion elements using bismuth / tellurium-based materials, silicon / germanium-based materials, lead / tellurium-based materials, and the like are known as thermoelectric conversion materials exhibiting a high performance index.

By the way, the conventional thermoelectric conversion element has the problem which should be solved, respectively.
For example, bismuth / tellurium-based materials have a large ZT value at room temperature, but when the temperature exceeds 100 ° C., the ZT value decreases rapidly, and is used as a thermoelectric conversion material at about 200 ° C. to 600 ° C. such as waste heat power generation. become unable. Bismuth / tellurium and lead / tellurium contain lead and tellurium, which are environmentally hazardous substances.

Therefore, there is a demand for new thermoelectric conversion materials that have good thermoelectric performance, low environmental impact, and light weight. And a clathrate compound attracts attention as one of such new thermoelectric conversion materials.
The composition and synthesis method of the clathrate compound composed of Ba, Ga, Al, and Si have already been disclosed, and Patent Document 1 discloses x (10.8 ≦ x ≦ 12.2) Si atoms per unit cell. Has disclosed a single crystal of Ba ₈ (Al, Ga) _x Si _46-x substituted with either an Al atom or a Ga atom and a method for producing the same. Patent Document 2 discloses that in a P-type Ba—Al—Si clathrate compound, the ZT at 700 K is 1.01.
Further, Patent Document 3 discloses that the Ba—Ga—Al—Si clathrate compound has a ZT at 800 ° C. of 0.4 or more. Composition ratio _Ba H _Ga Patent Document _{4 I Al J Si K Pd L} (7 ≦ H ≦ 8,9 ≦ I ≦ 12,0 ≦ J ≦ 2,33 ≦ K ≦ 35,0 <L ≦ 2, H + I + J + K + L = It is disclosed that the clathrate compound having 54) and the thermoelectric conversion material using the same have a higher ZT than a similar material containing no Pd in a temperature range of room temperature to 600 ° C.

JP 2004-67425 A Japanese Patent No. 4413323 (paragraph 0048, etc.) JP 2012-033867 A JP 2012-256759 A

However, these clathrate compounds have the following problems.
That is, in the technique described in Patent Document 1, ZT is not clear and there is a concern that the performance is low. In the technique described in Patent Document 2, the p-type is disclosed, but the ZT for the n-type is not clear and there is a concern that the performance is low.
In the technique described in Patent Document 3, ZT at 600 ° C. is not clear, and further performance improvement is desired, for example, it can be used for waste heat power generation. In the technique described in Patent Document 4, Pd which is a rare element is used, and there is a concern that it is expensive.

  Therefore, a main object of the present invention is a novel thermoelectric conversion material that is suitable for a thermoelectric conversion element in a temperature range of 600 ° C. that is an inexpensive material that does not contain harmful elements and can be used for heat generation, and its manufacture. It is to provide a method.

In order to solve the above problems, according to one aspect of the present invention,
Mainly clathrate compound,
There is provided a thermoelectric conversion material in which a phase composed of Sm or Bi is dispersed in a parent phase of the clathrate compound.

Preferably, the thermoelectric conversion material is
Mainly a BaGaAlSi-based clathrate compound,
The clathrate compound has the chemical formula Ba _a Ga _b Al _c Si _d X _e (X = Sm or Bi, 7 ≦ a ≦ 9, 7 ≦ b ≦ 15, 1 ≦ c ≦ 8, 28 ≦ d ≦ 33, 0 ≦ e. ≦ 1, a + b + c + d + e = 54), and the density n (1 / mm ² ) of the Sm or Bi dispersion is more than 0 and less than 1000, and is an n-type thermoelectric conversion material.

According to another aspect of the invention,
A manufacturing method for manufacturing the thermoelectric conversion material,
A preparation step of preparing a clathrate compound having a predetermined composition by mixing, melting and solidifying Ba, Ga, Al, Si, Sm or Bi as raw materials;
A crushing step of crushing the clathrate compound into fine particles;
A sintering step of sintering the fine particles;
The manufacturing method of the thermoelectric conversion material which has this is provided.

ADVANTAGE OF THE INVENTION According to this invention, the thermoelectric conversion material which has the outstanding thermoelectric conversion characteristic, and its manufacturing method can be provided.
In particular, a novel thermoelectric conversion material suitable for an n-type thermoelectric conversion element in a temperature range of 600 ° C., which is an inexpensive material and does not contain harmful elements and can be used for waste heat power generation, and further the thermoelectric conversion material The manufacturing method of can be provided.

It is an element mapping image by EPMA concerning the Example of a thermoelectric conversion material. It is the figure which represented roughly the relationship between the density n of the dispersed particle in the thermoelectric conversion material (Sm is included) and the absolute value | S | of Seebeck coefficient. In the figure, “◯” is an example and “×” is a comparative example. It is the figure which represented roughly the relationship between the density n of the dispersed particle in the thermoelectric conversion material (Bi is included) and the absolute value | S | of Seebeck coefficient. In the figure, “◯” is an example and “×” is a comparative example.

  Hereinafter, preferred embodiments of the present invention will be described.

(A) clathrate compound and preferred embodiment according clathrate compound in the form of a thermoelectric conversion material present invention, the composition ratio _{Ba a Ga b Al c Si d} X e (X = Sm or Bi, 7 ≦ a ≦ 9,7 ≦ b ≦ 15, 1 ≦ c ≦ 8, 28 ≦ d ≦ 33, 0 ≦ e ≦ 1, a + b + c + d + e = 54), and is mainly composed of a compound containing at least Ba, Ga, Al and Si at the same time. Alternatively, it is an n-type thermoelectric conversion material in which the second phase containing Bi is dispersed.

In the clathrate compound according to the present embodiment, a basic lattice is mainly composed of a Si clathrate lattice, a Ba element is included therein, and a part of atoms constituting the clathrate lattice is Ga, Al. It has the structure substituted by.
The “clathrate compound” according to the present embodiment has an Si clathrate phase as a main component (parent phase) and has another phase (second phase) made of Sm or Bi that does not correspond to the Si clathrate phase. . Regarding the condition of “0 ≦ e” in the composition ratio Ba _a Ga _b Al _c Si _d X _e , “e = 0” means that the Si clathrate phase does not contain Sm or Bi, and “0 <E ”means that Sm or Bi is contained (is dissolved) in the Si clathrate phase. In the clathrate compound according to the present embodiment, Sm or Bi may be dissolved in the Si clathrate phase (the presence or absence of the solid solution does not matter) but is dispersed as the second phase. is necessary.

Of composition ratio of the chemical formula _{Ba a Ga b Al c Si d} X e, Ga, Al, Si, each composition ratio of X b, c, d, e are generally has the following relationship.
b + c + d + e = 46
If such a relationship is satisfied, the clathrate compound can be realized mainly with a Si clathrate phase and can have an ideal crystal structure.
The thermoelectric conversion material according to this embodiment has an absolute value | S | of the Seebeck coefficient at 600 ° C. of 50 or more.
Furthermore, in the thermoelectric conversion material according to the present embodiment, when an element mapping image by EPMA is acquired, when the number of dispersed particles (Sm or Bi) larger than 1 μm is measured on an image having a field of view of 450 μm × 450 μm or more, dispersion is performed. The particle density n (1 / mm ² ) is greater than 0 and less than 1000 (0 <n <1000), preferably 200 <n <500, more preferably 250 <n <450.
In addition, the thermoelectric conversion material concerning this embodiment has the said clathrate compound as a main component, and may contain a small amount of other additives.

(B) Manufacturing method The manufacturing method of the thermoelectric conversion material concerning preferable embodiment of this invention,
(1) a preparation step of preparing a clathrate compound having a predetermined composition by mixing, melting, and solidifying Ba, Ga, Al, Si, Sm or Bi as a raw material;
(2) a pulverizing step of pulverizing the clathrate compound into fine particles;
(3) a sintering step of sintering the fine particles;
Have
By passing through these steps, there is an advantage that a material having a predetermined composition and few pores (voids) can be obtained.

(1) Preparation Step In the preparation step, an ingot of a clathrate compound having a predetermined composition and a uniform composition is produced.
First, a predetermined amount of raw material (Ba, Ga, Al, Si, Sm or Bi) is weighed and mixed so as to obtain a desired clathrate compound composition. The raw material may be a simple substance, an alloy or a compound, and the shape thereof may be powder, flakes or lumps.

  As the melting time, a time required for all raw materials to be homogeneously mixed in a liquid state is required, but considering the energy required for production, it is desirable that the melting time be as short as possible. Therefore, the melting time is preferably 1 to 100 minutes, more preferably 1 to 10 minutes, and particularly preferably 1 to 5 minutes.

The method for melting the powder composed of the raw material mixture is not particularly limited, and various methods can be used.
Examples of the melting method include heating with a resistance heating element, high frequency induction melting, arc melting, plasma melting, and electron beam melting.
As the crucible, graphite, alumina, cold crucible or the like is appropriately used according to the heating method.
When melting, it is preferably performed in an inert gas atmosphere or a vacuum atmosphere in order to prevent oxidation of the material.
In order to obtain a homogeneously mixed state in a short time, it is preferable that fine powdery raw materials are mixed. However, Ba preferably has a lump shape in order to prevent oxidation. It is also preferable to add mechanical stirring or electromagnetic stirring at the time of melting.

After melting, ingots may be cast using a mold or solidified in a crucible. In order to homogenize the completed ingot, an annealing treatment may be performed after melting.
The annealing treatment time is preferably as short as possible in consideration of energy saving during manufacturing, but a long time is required in consideration of the annealing effect. The treatment time for the annealing treatment is preferably 1 hour or longer, more preferably 1 to 10 hours.

  The treatment temperature for the annealing treatment is preferably 700 to 950 ° C, more preferably 850 to 930 ° C. When the processing temperature is less than 700 ° C., there is a problem that homogenization becomes insufficient. When the processing temperature exceeds 950 ° C., concentration segregation due to remelting occurs.

(2) Pulverization step In the pulverization step, the ingot obtained in the preparation step can be pulverized using a ball mill or the like to obtain a fine particle clathrate compound. The fine particles obtained are desired to have a fine particle size in order to improve the sinterability. In the present embodiment, the particle diameter of the fine particles is preferably 150 μm or less, more preferably 1 μm or more and 75 μm or less.

  In order to obtain fine particles having a desired particle size, the particle size is prepared after the ingot is pulverized by a ball mill or the like. The particle size may be adjusted by sieving using a ISO 3310-1 standard Lecce test sieve and a Lecce sieve shaker AS200 digit. In addition, it can replace with this grinding | pulverization process, and can also manufacture fine powder using various atomizing methods, such as a gas atomizing method, and a flowing gas evaporation method.

(3) Sintering step In the sintering step, the finely divided clathrate compound obtained in the pulverization step is sintered to obtain a solid having a uniform shape with a small number of voids. As the sintering method, a discharge plasma sintering method, a hot press sintering method, a hot isostatic pressing method, or the like can be used.

  When using the discharge plasma sintering method, the sintering temperature, which is one condition for the sintering, is preferably 600 to 1000 ° C, more preferably 900 to 1000 ° C. The sintering time is preferably 1 to 10 minutes, more preferably 3 to 7 minutes. The pressure is preferably 40 to 80 MPa, more preferably 50 to 70 MPa.

When the sintering temperature is less than 600 ° C., the material is not sintered, and when the sintering temperature is 1100 ° C. or more, it is dissolved. When the sintering time is less than 1 minute, the density is low, and when the sintering time exceeds 10 minutes, the sintering is completed and saturated, and it is considered that there is no significance in taking more time.
In particular, in the sintering step, the finely divided clathrate compound is heated to the sintering temperature and held at the temperature for the sintering time, and then the clathrate compound is cooled to a temperature before heating. In this case, the process of heating the clathrate compound in the form of fine powder to the sintering temperature and the process of holding at that temperature are brought into a pressurized state, and then the pressurized state is released in the process of cooling the clathrate compound. .
According to such pressure operation, it is possible to suppress cracking in the sintering step of the fine powder clathrate compound.

(C) Confirmation of production of clathrate compound phase Whether or not a clathrate compound has been produced by the above production method can be confirmed by powder X-ray diffraction (XRD). Specifically, the sintered sample is pulverized again and subjected to powder X-ray diffraction measurement. If the obtained peak indicates a type 1 clathrate phase (Pm-3n, No. 223), the type 1 class is used. It can be confirmed that the rate compound was synthesized.

  However, since there are actually a type 1 clathrate phase alone and an impurity phase, an impurity peak is also observed. The strongest peak ratio of the Si clathrate compound phase in the clathrate compound according to this embodiment is 85% or more, preferably 90% or more, and more preferably 95% or more.

The strongest peak ratio means the strongest peak (IHS) of the Si clathrate compound phase measured in powder X-ray diffraction measurement, and the impurity phase A (BaGa _{4 -Y} (Al, Si) _Y (0 ≦ Y ≦ 4). strongest peak intensity) (IA), than the strongest peak intensity of the impurity phase B (such as BaAl ₂ Si ₂₎ (IB), is defined by the following equation (2).
“Strongest peak ratio” = IHS / (IHS + IA + IB) × 100 (%) (2)

(D) Characteristic Evaluation Test Next, characteristic evaluation for calculating the dimensionless figure of merit ZT of the thermoelectric conversion material manufactured by the above method will be described.
The characteristic evaluation items are Seebeck coefficient S, electrical resistivity ρ, and thermal conductivity κ.
In the characteristic evaluation test, composition analysis, microstructure observation, and sintering density measurement are performed using an electron beam microanalyzer (EPMA-1610 manufactured by Shimadzu Corporation). Various characteristic evaluation samples are cut out and shaped from a cylindrical sintered body of 20 mmφ (diameter 20 mm) × 5 to 20 mm (height 5 to 20 mm).

“Seebeck coefficient S” and “electric resistivity ρ” are measured by a four-terminal method using a thermoelectric property evaluation apparatus ZEM-3 manufactured by ULVAC-RIKO.
“Thermal conductivity κ” is calculated from the measurement results of specific heat c, density δ, and thermal diffusivity α according to the following equation (3).
κ = cδα (3)
The “specific heat c” is measured by a DSC (Differential Scanning Calorimetry) method. As a measuring device, a differential scanning calorimeter EXSTAR6000DSC manufactured by SII Nano Technology Co., Ltd. is used.
“Density δ” is measured by the Archimedes method. As a measuring apparatus, a precision electronic balance LIBBROR AEG-320 manufactured by Shimadzu Corporation is used.
“Thermal diffusivity α” is measured by a laser flash method. As a measuring device, a thermal constant measuring device TC-7000 manufactured by ULVAC-RIKO Co., Ltd. is used.

From the above measurement results, the dimensionless figure of merit ZT, which is an index for evaluating the performance of the thermoelectric conversion material, can be calculated using Equation (1). From the calculated dimensionless figure of merit, the characteristics of the thermoelectric conversion material can be evaluated.
Of course, as shown in the equation (1), the dimensionless figure of merit ZT also depends on the value of the Seebeck coefficient S. Therefore, an increase in the value of the Seebeck coefficient S leads to an increase in the dimensionless figure of merit ZT. This leads to improvement of the properties of the conversion material. In the thermoelectric conversion material according to the present embodiment, the absolute value | S | of the Seebeck coefficient at 600 ° C. is 50 or more.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail using an Example, this invention is not limited by the following Example.

(1) Preparation of sample Weighed high-purity Ba and Sm or Bi with a purity of 2N or higher, high-purity Al and Ga with a purity of 3N or higher, and high-purity Si with a purity of 3N or higher at a predetermined mixing ratio. A raw material mixture was prepared (see Tables 1 and 3).

  The arc mixture is melted in an Ar (argon) atmosphere on a water-cooled copper hearth at a current of 300 A for 1 minute, and then the ingot is inverted in order to eliminate the unevenness of the raw material, and arc melting is performed again. Was repeated 5 times and cooled to room temperature on a water-cooled copper hearth to obtain an ingot having a clathrate compound.

  Thereafter, in order to improve the uniformity of the ingot, annealing treatment was performed at 900 ° C. for 6 hours in an argon atmosphere. The obtained ingot was pulverized using an agate planetary ball mill to obtain fine particles. At this time, the particle size was adjusted using the ISO 3310-1 standard Lecce test sieve and the Lecce sieve shaker AS200 digit so that the average particle size of the obtained particles was 75 μm or less.

  The obtained particles for sintering were pressurized to a pressure of 50 MPa using a discharge plasma sintering method (SPS method), heated to 1000 ° C., and then sintered at 1000 ° C. for 5 minutes. After the sintering was completed, the pressurized state was released, and cooling was performed from 1000 ° C. to room temperature.

  In addition, after the sintering of the particles for sintering was finished, when the pressure state was kept and cooling was performed, cracking occurred, but the pressure state was released after sintering as described above. When cooling from 1000 ° C. to room temperature, such cracks could be suppressed. Considering the deterioration of the sample and the die obtained, it is preferable to hold in a vacuum atmosphere at a cooling temperature of 500 ° C. or higher, but it may be held in an air atmosphere at less than 500 ° C.

  The sample sintered body thus obtained was subjected to composition analysis, X-ray diffraction of “(C) Confirmation of formation of clathrate compound” and “(D) characteristic evaluation test” It was used for.

(2) Sample Evaluation (2.1) Composition Analysis Tables 2 and 4 show the results of composition analysis.
Table 2 and Table 4, in the sample of Example 1-1～1-9,2-1～2-9, desired composition _{Ba a Ga b Al c Si d} X e (X = Sm or Bi, 7 ≦ a ≦ 9, 7 ≦ b ≦ 15, 1 ≦ c ≦ 8, 28 ≦ d ≦ 33, 0 ≦ e ≦ 1, a + b + c + d + e = 54).

(2.2) X-ray diffraction analysis The obtained sample was analyzed by powder X-ray diffraction.
From the obtained results, the strongest peak ratio was calculated based on the formula (2), and it was confirmed that it was 90% or more for all samples.

(2.3) Characteristic Evaluation The Seebeck coefficient at 600 ° C. was measured for the obtained sample as described in the above “(D) characteristic evaluation test”. As a result of the measurement, the Seebeck coefficient was negative in all the samples, and it was found that each sample was n-type.

Further, the number of dispersed particles larger than 1 μm was measured on an image having a field of view of 450 μm × 450 μm or more, and thereby the density n (1 / mm ² ) of the dispersed particles was calculated.
FIG. 1 shows an element mapping image by EPMA of Example 1-5 as an example of the dispersion state of Sm. In FIG. 1, the phase shown in white is a phase composed of Sm, and the Sm phase is dispersed in the matrix phase.

Tables 1 to 4 show the blended amounts and composition ratios (a, b, c, d, e) of each sample, the density n of dispersed particles, and the measured values of the Seebeck coefficient.
In addition, FIG. 2 shows the relationship between the density n of the dispersed particles of the samples of Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-3 and the absolute value | S | of the measured value of the Seebeck coefficient. The graph to represent is shown. FIG. 3 is a graph showing the relationship between the density n of the dispersed particles of the samples of Examples 2-1 to 2-9 and Comparative Examples 2-1 to 2-3 and the absolute value | S | of the measured value of the Seebeck coefficient. Show. “◯” shown in the figure is each example, and “×” is a comparative example.

  As shown in Table 2 and Table 4, Comparative Example 1-1 (2-1) and Examples 1-1 to 1-3 (2-1 to 2-3) each have substantially the same composition ratio. In the same manner, Comparative Example 1-2 (2-2) and Examples 1-4 to 1-6 (2-4 to 2-6) and Comparative Example 1-3 (2-3) were carried out. Examples 1-7 to 1-9 (2-7 to 2-9) also have almost the same composition ratio. In order to investigate these tendencies, an approximate curve (broken line) between the dispersion density value of the second phase and the value of the Seebeck coefficient is shown in FIGS. 2 and 3 for each sample group having substantially the same composition ratio. As shown in FIGS. 2 and 3, the absolute value | S | of the Seebeck coefficient is improved in the sample of the example in which the phase (second phase) composed of Sm or Bi is dispersed in the matrix phase.

(3) from Conclusion specific composition ratio _{Ba a Ga b Al c Si d} X e (X = Sm or Bi, 7 ≦ a ≦ 9,7 ≦ b ≦ 15,1 ≦ c ≦ 8,28 ≦ d ≦ 33,0 ≦ e ≦ 1, a + b + c + d + e = 54), and a thermoelectric conversion material in which a phase composed of Sm or Bi is dispersed in the parent phase exhibits n-type thermoelectric characteristics and 600 It turns out that it is useful for improving the performance in the temperature range of ° C.

Claims (7)

Mainly clathrate compound,
A thermoelectric conversion material characterized in that a phase composed of Sm or Bi is dispersed in a parent phase of the clathrate compound.   The thermoelectric conversion material according to claim 1, wherein the clathrate compound is a BaGaAlSi-based clathrate compound. The clathrate compound has the chemical formula Ba _a Ga _b Al _c Si _d X _e (X = Sm or Bi, 7 ≦ a ≦ 9, 7 ≦ b ≦ 15, 1 ≦ c ≦ 8, 28 ≦ d ≦ 33, 0 ≦ The thermoelectric conversion material according to claim 1 or 2, wherein e ≦ 1, a + b + c + d + e = 54). The thermoelectric conversion material according to claim 1, wherein the density n (1 / mm ² ) of the dispersion of Sm or Bi is more than 0 and less than 1000.   The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material has n-type thermoelectric conversion characteristics. It is a manufacturing method which manufactures the thermoelectric conversion material as described in any one of Claims 1-5,
A preparation step of preparing a clathrate compound having a predetermined composition by mixing, melting and solidifying Ba, Ga, Al, Si, Sm or Bi as raw materials;
A crushing step of crushing the clathrate compound into fine particles;
A sintering step of sintering the fine particles;
The manufacturing method of the thermoelectric conversion material which has this. The sintering step includes
A heating step of heating the fine particles to a certain sintering temperature;
A temperature holding step for holding the fine particles at the sintering temperature for a certain period of time;
A cooling step of cooling the fine particles to a temperature before heating;
In the heating step and the temperature holding step, a pressurized atmosphere is used,
The method for producing a thermoelectric conversion material according to claim 6, wherein the pressurized atmosphere is released in the cooling step.