JP5088116B2 - Method for manufacturing thermoelectric conversion element - Google Patents

Description

  The present invention relates to a method for manufacturing a thermoelectric conversion element containing ceramics as an insulating material, and more particularly to further stabilize the composition of a product produced by this manufacturing method and improve thermoelectric conversion characteristics.

  The thermoelectric conversion material is a material that can mutually convert heat energy and electric energy, and is a material that constitutes a thermoelectric conversion element used as a thermoelectric cooling element or a thermoelectric power generation element. This thermoelectric conversion material performs thermoelectric conversion using the Seebeck effect, and the thermoelectric conversion performance is represented by the following formula (1) called a figure of merit ZT.

ZT = α ² σT / κ (1)
(Where α is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the measured temperature)

  As is clear from the above formula (1), it can be seen that in order to increase the figure of merit of the thermoelectric conversion material, it is sufficient to increase the Seebeck coefficient α and the electrical conductivity σ of the material to be used and decrease the thermal conductivity κ. . Here, in order to reduce the thermal conductivity κ of the material, fine particles (inactive fine particles) that do not react with the base material of the thermoelectric conversion material may be added to the particles of the starting material of the thermoelectric conversion material. As a result, the inactive fine particles can scatter phonons, which are the main cause of heat conduction in the thermoelectric conversion material, thereby reducing the thermal conductivity κ.

  However, in the conventional thermoelectric conversion material, due to the uneven distribution of the inert fine particles, the influence of deterioration of other physical properties such as the electrical resistivity due to the uneven distribution of the inert fine particles is larger than the phonon scattering effect by the inert fine particles, Improvement of the performance of thermoelectric conversion materials is hindered. In order to solve this problem, for example, a thermoelectric conversion material is disclosed in which the starting material is fine particles, and fine particles such as ceramics that do not react with the base material are uniformly dispersed and sintered (for example, Patent Document 1). And 2).

Japanese Unexamined Patent Publication No. 2000-261047 JP-A-3-148879

  The above disclosed technique makes both the starting raw material and the inert fine particles fine particles, and the inert fine particles are easily dispersed throughout the base material of the thermoelectric conversion material, so that the probability of existing between the starting raw material particles increases. The crystallization of the base material particles can be prevented. In addition, since the starting material and the inert fine particles are prepared so that the particle size ratio is approximately equal to 1 in size, the inert fine particles can be uniformly distributed without being unevenly distributed in the thermoelectric conversion material. The deterioration of other physical properties such as electrical resistivity due to uneven distribution of inert fine particles can be suppressed.

  However, particles having a particle size in the nano order have a large specific surface area, so that they tend to aggregate due to van der Waals force or the like. Therefore, only by mixing thermoelectric conversion material particles and inert fine particles as in the conventional method, FIG. As shown in FIG. 2, the inert fine particles 2 are aggregated into a micro size, and the inert fine particles 2 cannot be dispersed in the nano-order in the thermoelectric conversion material 1. As a result, the interval between the inert materials becomes larger than the phonon mean free path, and the thermal conductivity cannot be sufficiently reduced.

  In the above prior art, the inert fine particles are uniformly dispersed to adjust other physical property values such as electrical resistivity that are not directly related to the above formula (1). The electrical conductivity σ and thermal conductivity κ directly related to ZT have not been studied. Therefore, the inert fine particles in the above prior art have a micron-scale particle size. Further, no precise examination has been made on the dispersion state of the inert fine particles.

  Since carriers (electrons or holes) contained in the thermoelectric conversion material can transmit both heat and electricity, the electrical conductivity σ and the thermal conductivity κ are in a proportional relationship. Furthermore, it is known that the electrical conductivity σ and the Seebeck coefficient α are in an inversely proportional relationship. Therefore, generally, even if the electrical conductivity σ is improved, the thermal conductivity κ increases and the Seebeck coefficient α decreases accordingly. In addition, since the effective mass and the mobility are in an inversely proportional relationship, the effective mass is reduced when the mobility is improved.

  Therefore, an object of the present invention is to solve the above conventional problems and provide a method for manufacturing a thermoelectric conversion element having a better performance index.

In order to solve the above problems, according to the first invention,
A strongly alkaline first solution containing a reducing agent and ceramic particles is dropped into a strongly acidic second solution containing a salt of an element constituting the thermoelectric conversion material, and the resulting particulate matter is dried. A step of obtaining composite particles; and a step of sintering the composite particles to obtain a thermoelectric conversion element;
The manufacturing method of the thermoelectric conversion element characterized by including is provided.

Moreover, according to 2nd invention in order to solve the said problem,
The strongly alkaline first solution containing the reducing agent is dropped into the strongly acidic second solution containing the salt of the element constituting the thermoelectric conversion material and the ceramic particles, and the obtained particulate matter is dried. A step of obtaining composite particles; and a step of sintering the composite particles to obtain a thermoelectric conversion element;
The manufacturing method of the thermoelectric conversion element characterized by including is provided.

  In the present invention, a thermoelectric conversion element having a better figure of merit can be obtained.

  First, the relationship between the figure of merit ZT and the structure of the thermoelectric conversion material will be described in detail with reference to FIG. As shown in FIG. 2, the thermal conductivity κ of the thermoelectric conversion material gradually decreases as the structure size of the thermoelectric conversion material becomes smaller starting from the length of the phonon mean free path. Therefore, the figure of merit ZT is improved when the structure size is designed to be smaller than the mean free path of phonons.

  On the other hand, even if the structure size of the thermoelectric conversion material is smaller than the average free path of phonons, the electrical conductivity σ of the thermoelectric conversion material does not decrease, and the particle size is generally less than the average free path of carriers. Decrease in case. Thus, using the fact that the structural dimension of the thermoelectric conversion material where the thermal conductivity κ begins to decrease and the structural dimension of the thermoelectric conversion material where the electrical conductivity σ begins to decrease are different from each other, By making the structure dimension of the thermoelectric conversion material not less than the average free path of the carrier and not more than the average free path of the phonon so that the structure dimension of the thermoelectric conversion material has a large reduction rate of the thermal conductivity κ, The expressed figure of merit ZT can be further increased.

  Here, the size of the thermoelectric conversion material is defined by the particle size of ceramic particles, which are insulating materials dispersed in the thermoelectric conversion material, or the dispersion interval between ceramics. Therefore, in the present invention, the dispersion interval between the ceramics is controlled so as to obtain the above effect.

  That is, in the thermoelectric conversion element obtained by the method of the present invention, the interval between the ceramic particles dispersed in the thermoelectric conversion material is equal to or less than the phonon mean free path of the thermoelectric conversion material.

Here, the mean free path (MFP) is calculated using the following equation.
Carrier MFP = (mobility × effective mass × carrier velocity) / elementary charge phonon MFP = 3 × lattice thermal conductivity / specific heat / sound velocity In the above equation, each value is converted from the literature value and the approximate equation of the temperature characteristic, Only measured values are used for specific heat.

Here, the results of the carrier MFP and the phonon MFP calculated for Co _0.94 Ni _0.06 Sb ₃ and CoSb ₃ are shown below.

As described above, the carrier MFP and the phonon MFP are determined by the material and the temperature. In the thermoelectric conversion element obtained by the present invention, it is sufficient that at least a part of the structure dimension is equal to or less than the phonon mean free path when the power factor (α ² σ) of the thermoelectric conversion material is the maximum output. Since the power factor (α ² σ) shows the maximum output at 400 ° C. in the CoSb ₃ system, it may be equal to or less than the phonon mean free path at 400 ° C. Specifically, this interval is preferably 1 nm or more and 100 nm or less, and more preferably 10 nm or more and 100 nm or less.

  Based on the above examination, in the present invention, first, a strong alkaline first solution containing a reducing agent is prepared. Here, strong alkalinity means that the pH is 11 or more.

Any reducing agent may be used as long as it can reduce the ions of the elements constituting the thermoelectric conversion material. For example, NaBH ₄ , hydrazine, or the like can be used.

  The solvent of the first solution is not particularly limited as long as it can disperse the reducing agent and the ceramic particles that may be contained in some cases, but it is preferable to use alcohol, particularly ethanol.

  Next, a strongly acidic second solution containing a salt of an element constituting the thermoelectric conversion material is prepared. Here, strong acid means that the pH is 3 or less.

The thermoelectric conversion material may be P-type or N-type. The material of the P-type thermoelectric conversion material is not particularly limited, and for example, Bi ₂ Te ₃ system, PbTe system, Zn ₄ Sb ₃ system, CoSb ₃ system, half-Heusler system, full Heusler system, SiGe system, etc. can be used. . As the material of the N-type thermoelectric conversion material, a known material can be applied without particular limitation. For example, Bi ₂ Te ₃ system, PbTe system, Zn ₄ Sb ₃ system, CoSb ₃ system, half-Heusler system, full Heusler system SiGe, Mg ₂ Si, Mg ₂ Sn, CoSi, or the like can be used.

Thermoelectric conversion material used in the present invention, it is preferable power factor is greater than 1 mW / K ^2, more preferably 2 mW / K ² or more, more preferably 3 mW / K ² or more. When the output factor is 1 mW / K ² or less, a great performance improvement cannot be expected. Further, the thermal conductivity κ of the thermoelectric conversion material is preferably larger than 5 W / mK, more preferably 7 W / mK or more, and further preferably 10 W / mK or more. When the thermal conductivity κ is larger than 5 W / mK, the effect of the present invention is particularly remarkable. In other words, the effect of controlling the microstructure dimensions of the thermoelectric conversion material at the nano-order specified in the present invention tends to decrease the thermal conductivity κ more significantly as the thermoelectric conversion material having a higher thermal conductivity κ is used. In particular, when a thermoelectric conversion material having a thermal conductivity κ greater than 5 W / mK is used, the effect of reducing the thermal conductivity κ is significant.

For example, when the thermoelectric conversion material is CoSb ₃ , the salt of an element constituting such a thermoelectric conversion material is cobalt chloride hydrate and antimony chloride, and in the case of Co _1-x Ni _x Sb ₃ , It means cobalt chloride hydrate, nickel chloride and antimony chloride. In the case of Bi ₂ Te ₃ , it means bismuth chloride and tellurium chloride.

  The solvent of the second solution is not particularly limited as long as it can disperse the salt of the element constituting the thermoelectric conversion material and the ceramic particles that may be included in some cases, but it is preferable to use alcohol, particularly ethanol. is there.

  In the second solution containing the salt of the element constituting the thermoelectric conversion material, raw material ions of the thermoelectric conversion material, such as Co ions and Sb ions, are present. Therefore, when the first solution containing the reducing agent is dropped into the second solution, these ions are reduced, and particles of elements constituting the thermoelectric conversion material, such as Co particles and Sb particles, are precipitated. .

  The dropping of the strongly alkaline first solution into the strongly acidic second solution can be terminated until the strongly acidic second solution becomes neutral.

A pH adjuster can be added to adjust the pH of the first and second solutions. As the pH adjusting material, known materials can be appropriately applied. For example, hydrochloric acid, acetic acid, nitric acid, aqueous ammonia, sodium hydroxide, sodium borohydride (NaBH ₄ ) and the like can be used.

The first solution or the second solution contains ceramic particles. As the ceramic, generally used materials such as alumina, zirconia, titania, magnesia, silica, and the like can be used. Among these, silica, zirconia, and titania are preferable from the viewpoint of low thermal conductivity. Moreover, the kind of ceramic particle to be used may be a single kind or a combination of two or more kinds. The specific resistance of the ceramic is preferably larger than 1000 μΩm, more preferably 10 ⁶ μΩm or more, and still more preferably 10 ¹⁰ μΩm or more. When the specific resistance is 1000 μΩm or less, the heat conduction is high, which may hinder the improvement of ZT.

  The average particle diameter of the ceramic particles is equal to or less than the average free path of phonons, specifically 1 to 100 nm. When particles having such a particle size are used, the distance between the ceramics dispersed in the thermoelectric conversion material to be formed becomes less than the mean free path of the phonons of the ceramic, and phonon scattering is sufficiently performed in the thermoelectric conversion material. As a result, the thermal conductivity κ of the thermoelectric conversion material is reduced, and the figure of merit ZT is improved.

  In the thermoelectric conversion element obtained by the method of the present invention, the mixing ratio of the ceramic particles to the thermoelectric conversion material particles is preferably 5 to 40 vol%.

  As described above, when the element ions constituting the thermoelectric conversion material are precipitated by reduction of the raw material ions, ceramic particles having a particle diameter of 1 to 100 nm are also present in the reaction field.

  Therefore, the particulate matter obtained in the second solution to which the first solution is dropped includes ceramic particles having a particle size of 1 to 100 nm and particles of elements constituting the thermoelectric conversion material. It exists in a uniformly dispersed state.

In this reduction, by-products such as NaCl and NaBO ₃ are generated in addition to Co particles and Sb particles. Filtration is preferably performed to remove this by-product. Furthermore, after filtration, it is preferable to add alcohol or water to wash away by-products.

The particulate matter obtained in the second solution in which the first solution is dropped is dried to obtain dispersed composite particles in which ceramic particles and thermoelectric conversion material particles are uniformly dispersed and mixed. The dispersion type composite particles are washed and dried as necessary, and then subjected to thermoelectricity by SPS sintering at a temperature of 400 to 800 ° C., preferably 450 to 650 ° C. in the case of CoSb ₃ by a general sintering method. A thermoelectric conversion element is obtained in which ceramic particles are dispersed in the continuous phase of the conversion material to form a dispersed phase.

  The manufacturing method of the thermoelectric conversion element of the present invention enables control of the structure dimension (particle size of insulating material and dispersion interval between insulating materials) in nano order. That is, by preparing a uniformly distributed aggregate of ceramic particles and thermoelectric conversion material particles having an average particle diameter of 1 to 100 nm, the structure size of the thermoelectric conversion element (dispersion interval between ceramics) is an average free of phonons. Less than the stroke, preferably more than the mean free path of carriers and less than the mean free path of phonons, phonon scattering sufficiently occurs in the thermoelectric conversion element, and the thermal conductivity κ can be reduced. Further, according to the present invention, when the thermoelectric conversion material particles are precipitated, the precipitated particles are prevented from redissolving and missing, and the composition of the resulting thermoelectric conversion element is stabilized. As a result, a thermoelectric conversion element having a large figure of merit ZT represented by the formula (1) is obtained. Thus, according to the method for manufacturing a thermoelectric conversion element of the present invention, an excellent thermoelectric conversion element exhibiting a high figure of merit ZT, which has a figure of merit ZT exceeding 2 that has been difficult to produce in the past. An element can also be obtained.

Comparative Example 1
After adding and dissolving 1.0 g of cobalt chloride and 2.88 g of antimony chloride in 100 mL of ethanol, 0.2 g of alumina particles having an average particle diameter of 30 nm was added to this solution to prepare a dispersion. The pH of this dispersion was 1. This dispersion was added dropwise to a reducing agent solution prepared by dissolving 2.0 g of sodium borohydride in 100 mL of ethanol. The pH of this reducing agent solution was 14. Next, impurities were removed by washing with a mixed solution of ethanol and water. Then for 24 hours drying at 240 ° C., to form a CoSb ₃ compound is thermoelectric conversion material. The composite particles were filled and SPS sintering was performed at 600 ° C. to obtain a thermoelectric conversion element.

Example 1
1.8 g of sodium borohydride (NaBH ₄ ) as a reducing agent and 0.26 g of powdered silica (SiO ₂ ) particles having an average particle size of 30 nm are added to 100 mL of ethanol, and a strongly alkaline (pH = 11) first solution is added. Prepared. Addition of cobalt nitrate hexahydrate _{(CoC1 2 · 6H 2 O)} 0.895g chloride, antimony chloride (SbCl ₃₎ 2.74 g and nickel chloride hexahydrate _{(NiCl 2 · 6H 2 O)} 0.057g of ethanol 100mL A second solution with strong acidity (pH = 3) was prepared. The strongly alkaline first solution is added dropwise until the strongly acidic second solution is neutral, and the reaction field is always kept acidic or neutral, and never becomes alkaline (pH> 10). Thus, composite particles were produced. Next, impurities were removed by washing the composite particles with a mixed solution of ethanol and water. Next, drying was performed at 240 ° C. for 24 hours to form dispersed composite particles composed of SiO _{2 as} ceramic particles and (Co, Ni) Sb _{3 as} thermoelectric conversion material. The composite particles were filled and SPS sintering was performed at 400 to 500 ° C. to obtain the thermoelectric conversion element of the present invention.

Example 2
A strong alkaline (pH = 11) first solution was prepared by adding 1.8 g of sodium borohydride (NaBH ₄ ) as a reducing agent to 100 mL of ethanol. Cobalt chloride hexahydrate (CoC1 ₂ · 6H ₂ O) 0.895 g, antimony chloride (SbCl ₃ ) 2.74 g, nickel chloride hexahydrate (NiCl ₂ · 6H ₂ O) 0.057 g and average particle size 30 nm A powdery silica (SiO ₂ ) particle (0.26 g) was added to 100 mL of ethanol to prepare a strongly acidic (pH = 3) second solution. The strongly alkaline first solution is added dropwise until the strongly acidic second solution is neutral, and the reaction field is always kept acidic or neutral, and never becomes alkaline (pH> 10). Thus, composite particles were produced. Next, impurities were removed by washing the composite particles with a mixed solution of ethanol and water. Next, drying was performed at 240 ° C. for 24 hours to form dispersed composite particles composed of SiO _{2 as} ceramic particles and (Co, Ni) Sb _{3 as} thermoelectric conversion material. The composite particles were filled and SPS sintering was performed at 400 to 500 ° C. to obtain the thermoelectric conversion element of the present invention.

The XRD of the thermoelectric conversion element of Example 1 obtained in this way is shown in FIG. This XRD result shows that the thermoelectric conversion element according to the present invention can be produced with a single CoSb ₃ phase. Thereby, the composition of the thermoelectric conversion element is further stabilized as compared with the conventional one, and the thermoelectric conversion characteristics are improved.

It is the schematic which shows the manufacturing process of the thermoelectric conversion material by the conventional method. It is a graph which shows the relationship between the structure dimension of a thermoelectric conversion material, Seebeck coefficient (alpha), electrical conductivity (sigma), or thermal conductivity (kappa). It is XRD of the thermoelectric conversion element of this invention.

Claims (5)

The strongly alkaline first solution containing the reducing agent is dropped into the strongly acidic second solution containing the salt of the element constituting the thermoelectric conversion material and the ceramic particles, and the obtained particulate matter is dried. Obtaining composite particles; and sintering the composite particles to obtain a thermoelectric conversion element;
The manufacturing method of the thermoelectric conversion element characterized by including this. A strongly alkaline first solution containing a reducing agent and ceramic particles is dropped into a strongly acidic second solution containing a salt of an element constituting the thermoelectric conversion material, and the resulting particulate matter is dried. Obtaining composite particles by; and
Sintering the composite particles to obtain a thermoelectric conversion element;
Including
A method for producing a thermoelectric conversion element, characterized in that a reaction field generated by the dropping is always in an acidic or neutral region. The method for producing a thermoelectric conversion element according to claim 1 or 2 , wherein the salt of the element constituting the thermoelectric conversion material is selected from cobalt chloride, antimony chloride, bismuth chloride, and tellurium chloride. The thermoelectric conversion material is a ₃ system or Bi ₂ Te ₃ system CoSb, method for manufacturing a thermoelectric conversion element according to any one of claims 1 to 3. The manufacturing method of the thermoelectric conversion element of any one of Claims 1-4 whose said reducing agent is sodium borohydride.

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