JP2007165662A - Thermoelectric conversion material and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoelectric conversion material of a polycrystal object with excellent thermoelectric conversion efficiency, and to provide its manufacturing method. <P>SOLUTION: In the thermoelectric conversion material, the ab surface of crystal grain of hexagonal system compound defined by the chemical formula of A<SB>X</SB>BO<SB>2</SB>, in which the element A shows one kind or more than two kinds among Na, K and Li, and the element B shows one kind or more than two kinds among Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag and In while showing X by a numerical value within the range of 0.45-0.55 is oriented in one direction so as to satisfy the degree of orientation F≥0.3 operated by F=(P-P<SB>0</SB>)/(1-P<SB>0</SB>). The thickness of an A-depleted phase whose X is not more than 0.5 in the surface of the crystal grain is not more than 100 nm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a thermoelectric conversion material having excellent thermoelectric conversion efficiency and a method for producing the same.

In recent years, as energy consumption increases on a global scale, problems related to the global environment such as global warming and environmental pollution have been studied as important issues from various viewpoints. In particular, fossil fuels generate not only CO ₂ but also various harmful substances when burned, and therefore reduction of the amount of use is required. In addition, leakage of radioactive materials and disposal of radioactive waste remain as unsolved issues for nuclear fuel.

  Therefore, as one of energy sources that replace fossil fuels and nuclear fuels, various technologies using waste heat generated in various plants and factories have been studied. For example, a technique of generating power by supplying waste heat to a boiler to recover thermal energy and rotating a turbine with the obtained water vapor has been widely used. However, since this power generation technology requires a large-scale power generation facility equipped with a boiler, a turbine, etc., it is difficult to introduce the power generation technology into a plant or factory that is restricted by location conditions.

On the other hand, the technology that generates electricity by directly converting thermal energy into electrical energy (hereinafter referred to as thermoelectric conversion) does not require large-scale facilities, so it is not limited by location conditions. It can be introduced not only in plants but also in small factories and plants.
In order to efficiently perform thermoelectric conversion, a material having excellent energy conversion efficiency and excellent heat resistance is required. As an index for evaluating the energy conversion efficiency of thermoelectric conversion, a ZT value calculated by the following equation (2) is known. It means that it has the outstanding energy conversion efficiency, so that this ZT value is large.

ZT = (TS ² ) / (ρκ) (2)
T: Temperature (K)
S: Seebeck coefficient (VK ^-1 )
ρ: Specific resistance (Ωm)
κ: Thermal conductivity (Wm ⁻¹ K ⁻¹ )
Materials that convert thermal energy into electrical energy (hereinafter referred to as thermoelectric conversion materials)
(A) single crystal,
(B) Broadly divided into polycrystals in which crystals are arranged in a certain direction. For example, the βFeSi ₂ polycrystal used as a thermoelectric conversion material has a ZT value (530 ° C.) of about 0.5. In addition, various thermoelectric conversion materials have been developed, but few have a ZT value exceeding 0.6.

Among the thermoelectric conversion materials that have already been developed, it has been reported that the Na _0.5 CoO ₂ single crystal has a ZT value exceeding 1. However, since Na _0.5 CoO ₂ single crystals can only be produced as small as a few millimeters at present, it is difficult to adopt them for power generation facilities using thermoelectric conversion. Therefore, attempts have been made to produce a large-sized thermoelectric conversion material composed of Na _0.5 CoO ₂ polycrystal by powder metallurgy technology, but it has not yet been put into practical use.

Patent Document 1 discloses a technique for forming a polycrystalline thermoelectric conversion material by forming a metal oxide powder, performing discharge plasma sintering, and further performing hot plasma sintering. However, the ZT value of the thermoelectric conversion material obtained by this technique is about 0.2 to 0.3, which is insufficient for adoption in power generation equipment by thermoelectric conversion.
JP 2003-95741 A

  An object of the present invention is to solve the above problems and to provide a polycrystalline thermoelectric conversion material having excellent thermoelectric conversion efficiency and a method for producing the same.

In order to put a polycrystalline thermoelectric conversion material into practical use, the present inventor conducted intensive studies on the thermoelectric conversion efficiency. as a result,
(A) The thermoelectric conversion material has a small specific resistance of the ab plane of the crystal grains. For example, Na _x CoO ₂ crystals have a specific resistance (room temperature) of the ab plane of 0.2 to 0.3 mΩcm, whereas the c axis direction Specific resistance is 8 mΩcm, anisotropy is observed,
(B) By arranging the crystal grains of Na _x CoO ₂ so that current flows in the ab plane direction having a small specific resistance, a polycrystalline body having a small specific resistance can be obtained.
(C) By reducing the specific resistance of the grain boundary of the crystal grains constituting the polycrystal, a polycrystal having a small specific resistance can be obtained.
(D) By increasing the relative density of the polycrystal, a polycrystal having a small specific resistance can be obtained.
I got the knowledge. The present invention has been made based on these findings.

That is, according to the present invention, one or more of Na, K and Li is element A, and one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag and In are used. Is the element B, X is a numerical value in the range of 0.45 to 0.55, and the ab plane of the crystal grains of the hexagonal compound defined by the chemical formula A _X BO ₂ is the degree of orientation F calculated by the following formula (1) The thermoelectric conversion material is oriented in a certain direction so as to satisfy ≧ 0.3, and the thickness of the A-deficient phase with X of less than 0.5 existing on the surface of the crystal grains is 100 nm (nanometers) or less.

F = (P−P ₀ ) / (1−P ₀ ) (1)
P: ΣI (hk0) / ΣI (hk1) in oriented crystal grains
P ₀ : ΣI (hk0) / ΣI (hk1) in non-oriented crystal grains
ΣI (hk0): Sum of X-ray diffraction intensities from (hk0) plane ΣI (hk1): Sum of X-ray diffraction intensities from all planes In the thermoelectric conversion material of the present invention, the relative density of the hexagonal compound is 98%. The above is preferable.

Moreover, this invention is a method of obtaining the thermoelectric conversion material of this invention mentioned above by the powder mixing method or the spray pyrolysis method, and the latter is more suitable. In the powder mixing method, for example, A carbonate and B oxide are mixed, cultured, and pulverized by a ball mill. A method such as a ball mill or a jet mill can be applied to the pulverization, but the method is not limited thereto. On the other hand, the thermal decomposition method is a method for producing an A _x BO ₂ raw material powder by spray pyrolysis of an aqueous solution containing A and B in a temperature range of 750 to 950 ° C. In any powder production method, a classification step can be added as necessary. The obtained raw material powder is a method for producing a thermoelectric conversion material that is shaped while being oriented in a magnetic field of 5 T or more and then fired.

In the method for producing a thermoelectric conversion material of the present invention, it is preferable that 50% by mass or more of powder particles having an aspect ratio of 1 to 2 are present in the fine raw material powder. Moreover, it is preferable to perform a HIP process after baking.
The 50% particle size refers to a 50% particle size of a normal distribution by laser diffraction particle size distribution. The aspect ratio refers to a value calculated by the average length in the ab plane direction / length in the c-axis direction of the fine-grained raw material powder (that is, single crystal). HIP processing refers to hot isostatic pressing.

  According to the present invention, a polycrystalline thermoelectric conversion material having excellent thermoelectric conversion efficiency can be obtained.

In the present invention, as an element effective for improving the thermoelectric conversion efficiency, one or more selected from Na, K, Li among alkali metal elements is element A, and Cr, Mn, Fe among metal elements are used. , Co, Ni, Cu, Zn, Ag, In, or one or more selected from elemental element B.
In producing the thermoelectric conversion material in the present invention, in the spray decomposition method, an aqueous solution containing the ions of the element A and the ions of the element B is subjected to spray pyrolysis in a temperature range of 750 to 950 ° C. to become a raw material of the thermoelectric conversion material. A powder (hereinafter referred to as raw material powder) is produced.

If the temperature at which the aqueous solution is subjected to spray pyrolysis is less than 750 ° C., the crystals of the hexagonal compound A _X BO ₂ do not grow sufficiently. On the other hand, when it exceeds 950 ° C., another compound (for example, Co oxide or the like) is generated. Therefore, the temperature at which spray pyrolysis is performed needs to satisfy the range of 750 to 950 ° C. X is a numerical value within the range of 0.45 to 0.55.
The raw material powder thus produced is classified to select fine powder having a 50% particle size of 10 μm or less (hereinafter referred to as fine raw material powder). The reason is that the fine raw material particles with a 50% particle size of 10 μm or less are single crystals of the hexagonal compound A _X BO ₂ , whereas if the 50% particle size exceeds 10 μm, the polycrystalline fine particles This is because the raw material powder is mixed. If the particles of the fine raw material powder are single crystals, it is possible to arrange the crystal structure in a certain direction by applying a magnetic field when performing molding described later. On the other hand, if the 50% particle size is less than 0.1 μm, the fine raw material particles are excessively refined, so that the torque during orientation decreases and the orientation deteriorates, or when molding described later is performed. Problems such as scattering around the molding apparatus and clogging of the conveying device occur. Therefore, it is preferable to select a raw material powder having a 50% particle size of 0.1 to 10 μm and use it as a fine-grain raw material powder. The 50% particle size refers to a 50% particle size of a normal distribution by laser diffraction particle size distribution.

The fine raw material powder thus selected can maximize the ZT value when particles having an aspect ratio in the range of 1 to 2 are present in an amount of 50% by mass or more. In addition, the fine raw material powder having an aspect ratio in the range of 1 to 2 is less than 50% by mass of the fine raw material powder (that is, a single crystal of hexagonal compound A _X BO ₂ ) when performing molding described later. The arrangement becomes non-uniform, and the thermoelectric conversion efficiency when used as a thermoelectric conversion material is lowered. The aspect ratio refers to a value calculated by the average length in the ab plane direction / length in the c-axis direction of the fine-grain raw material powder.

  Next, it shape | molds, applying a magnetic field to the selected fine raw material powder. For forming the fine-grained raw material powder, a technique called press molding in which a fine-shaped raw material powder is filled in a form having a predetermined shape and pressed is suitable. The magnetic field applied when forming the fine raw material powder is 5T or more. By forming while applying a magnetic field, the crystal structure of the fine-grain raw material powder that is a single crystal is arranged in a certain direction (hereinafter referred to as orientation). If the magnetic field is less than 5T, the effect of orienting the fine-grain raw material powder cannot be obtained, so that the thermoelectric conversion efficiency when used as a thermoelectric conversion material is lowered. On the other hand, in order to apply a magnetic field exceeding 20 T, not only a great amount of electric power is consumed, but also the equipment becomes expensive, and the manufacturing cost of the thermoelectric conversion material increases. Therefore, the magnetic field applied when molding the fine raw material powder is 5T or more, more preferably 5 to 20T.

By forming while applying a magnetic field, the fine-grain raw material powder (that is, a single crystal of hexagonal compound A _X BO ₂ ) is oriented in one direction to constitute a formed body. When the degree of orientation of the molded body is evaluated using the index F (that is, the degree of orientation) calculated by the following equation (1), by molding while applying a magnetic field of 5T or more, the A _X BO ₂ single crystal The degree of orientation F of the resulting molded body is 0.3 or more. When the F value is 0.3 or more, sufficient thermoelectric conversion efficiency can be obtained when used as a thermoelectric conversion material. In addition, it means that the ratio of the crystal grain arranged in a fixed direction (namely, orientation) is so high that F value of a polycrystal is large.

F = (P−P ₀ ) / (1−P ₀ ) (1)
P: ΣI (hk0) / ΣI (hk1) in oriented crystal grains
P ₀ : ΣI (hk0) / ΣI (hk1) in non-oriented crystal grains
ΣI (hk0): Sum of X-ray diffraction intensities from (hk0) plane ΣI (hk1): Sum of X-ray diffraction intensities from all planes Next, the compact of the raw material powder is charged into a firing furnace and fired. Thus, a thermoelectric conversion material having a predetermined shape (that is, a polycrystal of a hexagonal compound A _X BO ₂ ) is obtained. At this time, a phase with a low concentration of element A (the phase where X is less than 0.5, hereinafter referred to as an A-deficient phase) is formed on the crystal grain surface of the thermoelectric conversion material. The thickness of the A-deficient phase is 100 nm or less. It is preferable.

When the thickness of the A-deficient phase exceeds 100 nm, the index ZT value calculated by the following equation (2) decreases and the specific resistance of the thermoelectric conversion material increases. However, since the thickness of the A-deficient phase is reduced to 100 nm or less by firing under the above-described conditions, the specific resistance of the thermoelectric conversion material is reduced and the thermoelectric conversion efficiency is improved.
ZT = (TS ² ) / (ρκ) (2)
T: Temperature (K)
S: Seebeck coefficient (VK ^-1 )
ρ: Specific resistance (Ωm)
κ: Thermal conductivity (Wm ⁻¹ K ⁻¹ )
Furthermore, it is preferable to perform a hot isostatic pressing method (so-called HIP treatment) after firing the compact of the raw material powder. The reason is that the relative density of the thermoelectric conversion material (about 100 MPa) can be increased to 98% or more by HIP treatment. For example, it is preferable to perform the HIP treatment in pure O ₂ at 1000 atm and 750 ° C. for 2 hours, but it is not limited to this condition. If the relative density of the thermoelectric conversion material is 98% or more, the specific resistance of the thermoelectric conversion material can be significantly reduced, and the thermoelectric conversion efficiency can be further increased.

  As described above, according to the present invention, a polycrystalline thermoelectric conversion material having excellent thermoelectric conversion efficiency can be obtained.

A raw material powder of a hexagonal compound defined by the chemical formula (Na _1-Y C _Y ) _X (Co _1-Z D _Z ) O ₂ was produced by a spray pyrolysis method and a powder mixing method. The element indicated by symbol C was selected from K and Li, and the element indicated by symbol D was selected from Cr, Mn, Fe, Ni, Cu, Zn, and Ag. The combinations are as shown in Table 1. Numerical values X, Y, and Z are as shown in Table 1.

噴霧熱分解法は、NaとCoを塩化物水溶液で混合し、さらに750〜950℃で雰囲気中に噴霧することによって原料粉を製造した。
粉末混合法は、Na炭酸塩とCo酸化物を乳鉢で混合した後、850℃で仮焼し、さらに乳鉢粉砕→成形→焼結（900℃）→乳鉢粉砕→ジェットミル粉砕という手順を経て原料粉を製造した。

In the spray pyrolysis method, Na and Co were mixed with an aqueous chloride solution and further sprayed into the atmosphere at 750 to 950 ° C. to produce a raw material powder.
In the powder mixing method, Na carbonate and Co oxide are mixed in a mortar, then calcined at 850 ° C., and then mortar pulverized, molded, sintered (900 ° C.), mortar pulverized, jet mill pulverized, and then the raw material Powder was produced.

Table 1 shows the 50% particle size and aspect ratio of these raw material powders. Each particle of the raw material powder is a single crystal.
After forming the raw material powder shown in Table 1 while applying a magnetic field in a superconducting magnet, the obtained molded body was sintered at 900 ° C. (in the air) to produce a thermoelectric conversion material. Some thermoelectric conversion materials were subjected to HIP treatment (750 ° C., 1000 atm). The magnetic field applied during molding is as shown in Table 1.

Table 1 shows the degree of orientation, density, thickness of the A-deficient phase, and ZT value of the thermoelectric conversion material thus prepared. As is apparent from Table 1, it can be seen that a thermoelectric conversion material having a high ZT value was obtained.

Claims (5)

One or more of Na, K and Li is element A, and one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag and In is element B. When X is a numerical value in the range of 0.45 to 0.55, the ab plane of the crystal grains of the hexagonal compound defined by the chemical formula A _X BO ₂ satisfies the orientation degree F ≧ 0.3 calculated by the following formula (1): A thermoelectric conversion material characterized in that the thickness of the A-deficient phase, which is oriented in a certain direction and whose X is less than 0.5 on the crystal grain surface, is 100 nm or less.
F = (P−P ₀ ) / (1−P ₀ ) (1)
P: ΣI (hk0) / ΣI (hk1) in oriented crystal grains
P ₀ : ΣI (hk0) / ΣI (hk1) in non-oriented crystal grains
ΣI (hk0): Sum of X-ray diffraction intensities from (hk0) plane ΣI (hk1): Sum of X-ray diffraction intensities from all planes   The thermoelectric conversion material according to claim 1, wherein the hexagonal compound has a relative density of 98% or more.   One or more of Na, K and Li is element A, and one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag and In is element B. An aqueous solution containing the ions of the element A is spray pyrolyzed in a temperature range of 750 to 950 ° C. or a raw material powder is produced using a powder mixing method, and the raw material powder is classified to give a 50% particle size of 10 μm. A method for producing a thermoelectric conversion material, comprising: selecting the following fine raw material powder, forming the fine raw material powder while being oriented in a magnetic field of 5 T or more, and then firing the fine raw material powder.   The method for producing a thermoelectric conversion material according to claim 3, wherein 50 mass% or more of powder particles having an aspect ratio of 1 to 2 are present in the fine raw material powder.   5. The method for producing a thermoelectric conversion material according to claim 3, wherein HIP treatment is performed after the firing.