JP4766004B2 - 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.

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 apparent from the above formula (1), in order to improve the thermoelectric conversion performance of the thermoelectric conversion material, it is necessary to increase the Seebeck coefficient α and the electric conductivity σ of the material to be used and to decrease the thermal conductivity κ. Recognize. 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 an excellent figure of merit.

  In order to solve the above problems, according to the present invention, ceramic particles and thermoelectric conversion material particles having an average particle diameter of 1 to 100 nm and a pH adjuster are mixed in alcohol to obtain ceramic particles and thermoelectric conversion material particles. A method for producing a thermoelectric conversion thermoelectric element is provided, which includes the steps of aggregating the ceramic particles and thermoelectric material particles and then sintering after preparing the alcohol dispersion.

  According to the present invention, by aggregating ceramic particles and thermoelectric conversion material particles in a dispersion of ceramic particles and thermoelectric conversion material particles having an average particle diameter of 1 to 100 nm, the distance between the ceramics is an average free path of phonons. Since phonon scattering becomes active at the interface with the ceramic, the lattice thermal conductivity is greatly reduced, and the performance of the thermoelectric conversion element is improved.

  First, the relationship between the figure of merit ZT and the structure of the thermoelectric conversion element will be described in detail with reference to FIG. As shown in FIG. 2, the thermal conductivity κ of the thermoelectric conversion element gradually decreases as the structure size of the thermoelectric conversion element becomes smaller than the length of the mean free path of phonons. 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 element is smaller than the phonon mean free path, the electric conductivity σ of the thermoelectric conversion element does not decrease, and the particle size is generally less than the mean free path of the carrier. Decrease in case. Thus, using the fact that the structural dimension of the thermoelectric conversion element where the thermal conductivity κ begins to decrease and the structural dimension of the thermoelectric conversion element where the electrical conductivity σ begins to decrease are different from each other, By making the structure dimension of the thermoelectric conversion element not less than the average free path of carriers and not more than the average free path of phonons so that the structure dimension of the thermoelectric conversion element has a large reduction rate of the thermal conductivity κ, The expressed figure of merit ZT can be further increased.

  Here, the structure dimension of the thermoelectric conversion element is defined by the particle diameter of ceramic particles, which is an insulating material dispersed in the thermoelectric conversion element, or the dispersion interval between the 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 present invention, first, ceramic particles and thermoelectric conversion material particles having an average particle diameter of 1 to 100 nm and a pH adjuster are mixed in alcohol to prepare an alcohol dispersion of ceramic particles and thermoelectric conversion material particles.

As the ceramic, generally used materials such as alumina, zirconia, titania, magnesia, silica, and ceria 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 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 3 W / mK, more preferably 5 W / mK or more, and further preferably 10 W / mK or more. When the thermal conductivity κ is larger than 3 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 element in the nano-order specified in the present invention tends to decrease the thermal conductivity κ as the thermoelectric conversion material having a larger thermal conductivity κ is used. In particular, when a thermoelectric conversion material having a thermal conductivity κ larger than 3 W / mK is used, the effect of reducing the thermal conductivity κ appears greatly.

  The average particle diameter of the thermoelectric conversion material particles and ceramic particles is not more than the average free path of phonons, specifically 1 to 100 nm, preferably 1 to 20 nm. When particles having such a particle size are used, the distance between the ceramics dispersed in the formed thermoelectric conversion element is less than the mean free path of phonons of the thermoelectric conversion material, and phonon scattering occurs in the thermoelectric conversion element. Since it occurs sufficiently, the thermal conductivity κ of the thermoelectric conversion element is reduced, and the figure of merit ZT is improved.

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.

Thus, 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 the dispersion interval of at least some ceramic particles is equal to or less than the mean free path of phonons 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. for the CoSb ₃ -based material, it may be less than the mean free path of phonons at 400 ° C.

  The mixing ratio of the ceramic particles to the thermoelectric conversion material particles is preferably 5 to 40 vol%.

The pH adjusting material is used to suppress aggregation of nanoparticles and the like in the slurry, and a known material can be appropriately applied. For example, nitric acid, aqueous ammonia, sodium borohydride (NaBH ₄ ), etc. Can be used.

  Although alcohol will not be restrict | limited especially if the said thermoelectric conversion material particle | grains and ceramic particle | grains can be disperse | distributed, It is suitable to use ethanol.

  As pH of this dispersion liquid, it is preferable to adjust to 3-6 or 8-11, and it is more preferable that it is 4-6 or 8-10.

After preparing an alcohol dispersion of ceramic particles and thermoelectric conversion material particles in this manner, the ceramic particles and thermoelectric conversion material particles are aggregated. This aggregation is performed, for example, by changing the pH of the alcohol dispersion so that the ceramic particles and the thermoelectric conversion material particles are aggregated. Specifically, when silica or alumina is used as ceramic particles and skutterudite (CoSb ₃ ) is used as thermoelectric conversion material particles, the isoelectric point of silica is pH 3, and the isoelectric point of alumina is pH 6-8. And the isoelectric point of CoSb ₃ is considered to be pH 4-7. Accordingly, the dispersion of CoSb ₃ / alumina or CoSb ₃ / silica is maintained in a dispersed state by keeping the pH at 10, and HCl is added to this dispersion to bring the pH to around 5 to 7 to make CoSb ₃ and alumina or CoSb _3. And silica can be uniformly agglomerated.

  Moreover, the alcohol as a dispersion medium of the alcohol dispersion is volatile, and therefore the ceramic particles and the thermoelectric conversion material particles may be aggregated by evaporating the alcohol from the alcohol dispersion.

  The thus obtained aggregate of ceramic particles and thermoelectric conversion material particles is washed and dried as necessary, and then subjected to SPS sintering at, for example, 580 ° C. by a general sintering method, thereby obtaining a thermoelectric conversion element. It is done.

  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. 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.

Example 1
1.7 g of Co _0.94 Ni _0.06 Sb ₃ particles having an average particle diameter of 5 to ₂₀ nm and 0.26 g of SiO ₂ particles having an average particle diameter of 5 to 10 nm were added to 100 mL of ethanol, mixed, and the pH was adjusted to 10 with ammonia. HCl was added to the ethanol dispersion thus obtained to adjust the pH to 7. Then, Co _0.94 Ni _0.06 Sb ₃ particles and SiO ₂ particles aggregated. Here, the volume fraction of Co _0.94 Ni _0.06 Sb ₃ particles and SiO ₂ particles was Co _0.94 Ni _0.06 Sb ₃ : SiO ₂ = 7: 3. The HAADF image and EDX measurement result of this aggregate are shown in FIG. 3 and FIG. The aggregate was then washed with 300 mL of water + ethanol solution and dried to recover the powder. Thereafter, SPS sintering was performed at 500 ° C. to 600 ° C. to obtain a bulk sintered body of about φ10 × 1 to 2 mm. A TEM image of this sintered body is shown in FIG.

Example 2
Instead of SiO ₂ particles, 0.58 g of Al ₂ O ₃ particles having an average particle size of 20 to ₃₀ nm were used, and φ10 was applied in the same manner as in Example 1 except that HCl was added to the ethanol dispersion to adjust the pH to 3. A bulk sintered body of about 1 to 2 mm was obtained. The volume fraction of CoSB ₃ particles and Al ₂ O ₃ particles in the aggregate was Co _0.94 Ni _0.06 Sb ₃ : Al ₂ O ₃ = 6: 4. A TEM image of the aggregate is shown in FIG. 6, and a TEM image of the sintered body is shown in FIG.

Example 3
1.7 g of Co _0.94 Ni _0.06 Sb ₃ particles having an average particle diameter of 5 to ₂₀ nm and 0.26 g of SiO ₂ particles having an average particle diameter of 5 to 10 nm were added to 100 mL of ethanol, mixed, and the pH was adjusted to 10 with ammonia. The ethanol dispersion thus obtained was dried at room temperature in an N ₂ inert gas flow for ₂ to 3 hours while applying ultrasonic vibration. At this time, Co _0.94 Ni _0.06 Sb ₃ particles and SiO ₂ particles are aggregated as the liquid level of the solvent is lowered, and ethanol is completely evaporated to coagulate Co _0.94 Ni _0.06 Sb ₃ particles and SiO ₂ particles. A collection was obtained. Here, the volume fraction of Co _0.94 Ni _0.06 Sb ₃ particles and SiO ₂ particles was Co _0.94 Ni _0.06 Sb ₃ : SiO ₂ = 7: 3. The HAADF image and EDX measurement result of this aggregate are shown in FIGS. Subsequently, this aggregate was subjected to SPS sintering at 500 ° C. to 600 ° C., thereby obtaining a bulk sintered body having a diameter of about 10 × 1 to 2 mm. A TEM image of this sintered body is shown in FIG.

Example 4
A bulk sintered body having a diameter of about 10 × 1 to 2 mm was obtained in the same manner as in Example 1 except that 0.58 g of Al ₂ O ₃ particles having an average particle size of 20 to ₃₀ nm were used in place of the SiO ₂ particles. The volume fraction of Co _0.94 Ni _0.06 Sb ₃ particles and Al ₂ O ₃ particles in the aggregate was Co _0.94 Ni _0.06 Sb ₃ : Al ₂ O ₃ = 6: 4. A TEM image of the aggregate is shown in FIG. 11, and a TEM image of the sintered body is shown in FIG.

  The performance of the thermoelectric conversion element which is the obtained sintered body was measured, and the results are shown in the following table.

As is clear from the above results, the figure of merit ZT is improved in the device obtained by the method of the present invention as compared with the comparative material produced only from Co _0.94 Ni _0.06 Sb ₃ particles.

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 | tissue dimension of a thermoelectric conversion material, Seebeck coefficient (alpha), electrical conductivity (sigma), or thermal conductivity (kappa). It is a HAADF image of the aggregate of Co _0.94 Ni _0.06 Sb ₃ particles and SiO ₂ particles. It is a graph which shows the EDX measurement result of the aggregate shown in FIG. 2 is a TEM image of a sintered body in Example 1. It is a TEM image of the aggregate of Co _0.94 Ni _0.06 Sb ₃ particles and Al ₂ O ₃ particles. 3 is a TEM image of a sintered body in Example 2. It is a HAADF image of the aggregate of Co _0.94 Ni _0.06 Sb ₃ particles and SiO ₂ particles. It is a graph which shows the EDX measurement result of the aggregate shown in FIG. 4 is a TEM image of a sintered body in Example 3. It is a TEM image of the aggregate of Co _0.94 Ni _0.06 Sb ₃ particles and Al ₂ O ₃ particles. 10 is a TEM image of a sintered body in Example 4.

Claims (5)

  Ceramic particles and thermoelectric conversion material particles having an average particle diameter of 1 to 100 nm and a pH adjuster are mixed in alcohol to prepare an alcohol dispersion of the ceramic particles and thermoelectric conversion material particles, and then the ceramic particles and thermoelectric material A method for producing a thermoelectric conversion thermoelectric element, comprising the steps of aggregating particles and then sintering.   The method for producing a thermoelectric conversion thermoelectric element according to claim 1, wherein the ceramic particles and the thermoelectric material particles are aggregated by changing the pH of the alcohol dispersion.   The method for producing a thermoelectric conversion thermoelectric element according to claim 1, wherein the ceramic particles and the thermoelectric material particles are aggregated by evaporating alcohol from the alcohol dispersion. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the thermoelectric conversion material is a CoSb ₃ system or a Bi ₂ Te ₃ system. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the pH adjusting material is NaBH ₄ , NaOH, or NH ₃ .