JP2006062951A - Thermoelectric conversion material and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an n-type thermoelectric conversion material comprising an oxide, especially having a smaller heat conductivity value (κ). <P>SOLUTION: The thermoelectric conversion material is a compound containing an alkaline earth metal (Ae) and titanium (Ti) and oxygen wherein the molar ratio (Ti/Ae) of the titanium and the alkaline earth metal is two or more and at least part of titanium atoms are trivalent titanium ions. The method for manufacturing the thermoelectric conversion material comprises a step of sintering a metal compound mixture into the thermoelectric conversion material. The metal compound mixture contains the alkaline earth metal (Ae) and the titanium (Ti) at the molar ratio (Ti/Ae) of two or more. The metal compound mixture is held and sintered in an inert gas atmosphere or in a reducing atmosphere at a temperature of 900°C-1,700°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thermoelectric conversion material. More specifically, the present invention relates to an n-type thermoelectric conversion material made of an oxide.

  Thermoelectric conversion power generation is power generation by converting thermal energy into electrical energy using the Seebeck effect in which thermoelectromotive force is generated by applying a temperature difference to the thermoelectric conversion material. Thermoelectric power generation is expected as an environmentally-friendly power generation that can be put to practical use because it can use various exhaust heat such as geothermal heat and incinerator heat as a heat source.

  The energy conversion efficiency of the thermoelectric conversion material depends on the value (Z) of the figure of merit of the thermoelectric conversion material. The value of the figure of merit (Z) is a value obtained by the following equation (1) using the value of the Seebeck coefficient (α), the value of electrical conductivity (σ) and the value of thermal conductivity (κ) of the material. The thermoelectric conversion material having a larger value of performance index (Z) is considered to be a thermoelectric conversion material having better energy conversion efficiency.

Z = α ² × σ / κ (1)

  The thermoelectric conversion materials include a p-type thermoelectric conversion material having a positive Seebeck coefficient and an n-type thermoelectric conversion material having a negative Seebeck coefficient. Usually, a thermoelectric conversion element in which a p-type thermoelectric conversion material and an n-type thermoelectric conversion material are electrically connected in series is used for thermoelectric conversion power generation. Since the energy conversion efficiency of the thermoelectric conversion element depends on the value (Z) of the performance index of the p-type thermoelectric conversion material and the n-type thermoelectric conversion material, in order to obtain a thermoelectric conversion element with good energy conversion efficiency, the performance index A p-type thermoelectric conversion material and an n-type thermoelectric conversion material having a large value (Z) are required.

Further, as an n-type thermoelectric conversion material made of an oxide, a thermoelectric conversion material made of Zn _0.98 Al _0.02 O (see, for example, Patent Document 1) or a thermoelectric conversion material made of SrTiO ₃ having a perovskite crystal structure (for example, a patent) Reference 2) is proposed.

JP-A-8-186293 JP-A-8-231223

  However, in the conventional n-type thermoelectric conversion material made of an oxide, the thermal conductivity value (κ) has not been sufficient. An object of the present invention is to provide an n-type thermoelectric conversion material made of an oxide and having a smaller thermal conductivity value (κ).

As a result of intensive studies aimed at solving the above problems, the present inventors have completed the present invention.

That is, this invention provides the following thermoelectric conversion material and its manufacturing method.
<1> A compound containing alkaline earth metal (Ae), titanium (Ti) and oxygen, wherein the molar ratio of titanium to alkaline earth metal (Ti / Ae) is 2 or more, and titanium A thermoelectric conversion material, wherein at least some of the atoms are trivalent titanium ions.
<2> One or more compounds selected from the group consisting of Ae ₂ Ti ₁₃ O ₂₂ , Ae _x Ti ₈ O ₁₆ (where x is 0.8 or more and 2 or less) and Ae ₂ Ti ₆ O ₁₃ The said thermoelectric conversion material to contain.
<3> A compound having a crystal structure having a one-dimensional chain in which octahedrons formed by titanium atoms and six oxygen atoms around the titanium atoms share points and / or edges and / or part of faces. The thermoelectric conversion material according to any one of the above.
<4> The thermoelectric conversion material according to any one of the above, wherein the ratio of trivalent titanium ions to all titanium atoms is 10% or more.
<5> The ratio according to any one of the above, wherein the ratio of the distance from the titanium atom to the nearest alkaline earth metal atom to the distance between the nearest titanium atoms in the compound is in the range of 0.5 or more and less than 1.0. Thermoelectric conversion material.
<6> A part of Ae is substituted with one or more elements selected from the group consisting of Li, K, Na, Zn, Y, La, Ce, Pr, Nd, Pm, Sm, Bi, and Pb The thermoelectric conversion material in any one.
<7> Any of the above, wherein a part of Ti is substituted with one or more elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb and W The thermoelectric conversion material described in 1.
<8> atom M (where M is one or more elements selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb and W) and At least four one-dimensional chains in which MO ₆ octahedrons formed by the _six oxygen atoms around them share a part of the ridge and are joined together share a part of the top of the MO ₆ octahedron. A thermoelectric conversion material comprising a compound having a one-dimensional tunnel-type crystal structure, which is a crystal structure having a tunnel space surrounded by at least four one-dimensional chains.
<9> One or more types of structures in which the one-dimensional tunnel-type crystal structure is selected from the group consisting of a pyrolsite-type crystal structure, a ramsdellite-type crystal structure, a hollandite-type crystal structure, a romanesite-type crystal structure, and a todorokite-type crystal structure Said thermoelectric conversion material.
<10> The thermoelectric conversion material according to any one of the above, wherein the relative density is 60% or more.
<11> The thermoelectric conversion material according to any one of the above, which is coated with an oxygen-impermeable film.
<12> A thermoelectric conversion element comprising the thermoelectric conversion material according to any one of the above.
<13> A method for producing a thermoelectric conversion material by sintering a metal compound mixture that becomes a thermoelectric conversion material by sintering, wherein the metal compound mixture contains alkaline earth metal (Ae) and titanium (Ti), Ti / Ae is contained so as to have a value of 2 or more, and the metal compound mixture is held at a temperature in the range of 900 ° C. to 1700 ° C. in an inert gas atmosphere or a reducing atmosphere and sintered. The manufacturing method of the thermoelectric conversion material characterized by the above-mentioned.
<14> A method for producing a thermoelectric conversion material by sintering a metal compound mixture that becomes a thermoelectric conversion material by sintering, wherein the metal compound mixture contains alkaline earth metal (Ae) and titanium (Ti), Ti / Ae is contained so as to have a value of 2 or more, and the metal compound mixture is fired while being held in a reducing atmosphere at a temperature in the range of 600 ° C. to 1100 ° C., and the resulting fired product is molded. A method for producing a thermoelectric conversion material, wherein the obtained molded body is sintered while being held at a temperature in the range of 1100 ° C. to 1700 ° C. in an inert gas atmosphere or a reducing atmosphere.
<15> A portion of Ae is substituted with one or more elements selected from the group consisting of Li, K, Na, Y, La, Ce, Pr, Nd, Pm, Sm, Bi, and Pb The manufacturing method in any one of the said.
<16> A part of Ti is substituted with one or more elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb and W. The manufacturing method according to claim 1.

  The thermoelectric conversion material of the present invention is an n-type thermoelectric conversion material composed of an inexpensive Ti-containing oxide, has a smaller thermal conductivity value (κ) and a better figure of merit (Z). Since the thermoelectric conversion material of the present invention has good energy conversion efficiency, the present invention is extremely useful industrially.

  The thermoelectric conversion material of the present invention is a compound containing alkaline earth metal (Ae), titanium (Ti) and oxygen, and the molar ratio of titanium to alkaline earth metal (Ti / Ae) is 2 or more. And at least part of the titanium atoms are trivalent titanium ions. If Ti / Ae is less than 2, the thermal conductivity tends to increase, which is not preferable. Further, Ti / Ae is preferably 3 or more and more preferably 4 or more in order to increase the Seebeck coefficient and electric conductivity. The upper limit of Ti / Ae is usually about 20, and if it exceeds 20, the figure of merit (Z) tends to decrease. Further, Ti / Ae is more preferably 4 or more and 7 or less.

The thermoelectric conversion material of the present _{invention, Ae 2 Ti 13 O 22,} Ae x Ti 8 O 16 ( here, x is 0.8 to 2.), Consisting AeTi ₇ O ₁₄ and Ae ₂ Ti ₆ O ₁₃ It is preferable to contain one or more compounds selected from the group, and it is more preferable to contain Ae _x Ti ₈ O ₁₆ (where x is 0.8 or more and 2 or less).

  In addition, in the sense of increasing the electrical conductivity, the thermoelectric conversion material of the present invention is such that the octahedrons formed by the titanium atoms and the six surrounding oxygen atoms form points and / or edges and / or part of the faces. It is preferable to contain a compound having a crystal structure having a one-dimensional chain bonded and bonded, and among the compounds, the octahedron has a crystal structure having a one-dimensional chain bonded by sharing a part of a ridge. More preferred are compounds.

In the thermoelectric conversion material of the present invention, the ratio of trivalent titanium ions to all titanium atoms may be 10% or more. When the ratio of trivalent titanium ions is small, the figure of merit may be lowered due to a decrease in electrical conductivity of the n-type thermoelectric conversion material. Moreover, although the upper limit of the ratio of a trivalent titanium ion may be a trivalent titanium ion for all the titanium atoms, it is more preferable that it is 50% or less. For example, the ratio of trivalent titanium ions to all titanium atoms in the above compound is 92% in Ae ₂ Ti ₁₃ O ₂₂ and Ae _x Ti ₈ O ₁₆ (where x is 0.8 or more and 2 or less. In the case of x = 0.8, it is 20%, x = 2 is 50%, and Ae ₂ Ti ₆ O ₁₃ is 33%. In applications where the operating temperature is low, oxygen vacancies are generated in these crystal structures, and the ratio of trivalent titanium ions can be controlled. Of these, preferable Ae _x Ti ₈ O ₁₆ has a hollandite type crystal structure, and depending on the constituent elements, x can be maintained at 2 or less. In Ae _x Ti ₈ O ₁₆ , the amount of trivalent titanium ions can be controlled by the amount of Ae (x).

  In the thermoelectric conversion material of the present invention, a part of the alkaline earth metal (Ae) and / or titanium (Ti) may be substituted with elements having different ionic radii and / or valences. By this substitution, the figure of merit can be improved by improving the electrical conductivity of the thermoelectric conversion material of the present invention or reducing the thermal conductivity.

  Specific examples of the element that substitutes Ae include one or more elements selected from the group consisting of Li, K, Na, Zn, Y, La, Ce, Pr, Nd, Pm, Sm, Bi, and Pb. Na, K, Bi is preferable, and Bi is more preferable. Moreover, as substitution amount, it is 0.001 mol-0.5 mol normally in 1 mol of Ae.

  Specific examples of the element replacing Ti include one or more elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb and W. Preferably, V and Mn are used. Moreover, as substitution amount, it is 0.001 mol-0.5 mol normally in 1 mol of Ti.

  The alkaline earth metal (Ae) in the present invention means at least one selected from the group consisting of Mg, Ca, Sr and Ba. Ae is preferably Ba and / or Sr.

In addition, when the crystal structure of the thermoelectric conversion material of the present invention is an arbitrary titanium atom, the ratio of the distance between the nearest titanium atoms and the distance to the nearest alkaline earth metal atom ((Ti -Ti) / (Ti-Ae)) is preferably in the range of 0.5 or more and less than 1.0. If (Ti-Ti) / (Ti-Ae) is less than 1.0, the distance between Ti atoms and Ti atoms is close, and the n-type thermoelectric conversion material tends to have high electrical conductivity and exhibits a high figure of merit. It tends to be a thermoelectric conversion material. The value of (Ti—Ti) / (Ti—Ae) is, for example, 1.14 which is not preferable for BaTiO _3. On the other hand, for example, Ba _x Ti ₈ O ₁₆ (where x is 0.8 or more and 2 or less). Hereinafter, the description of the range of the value of x is the same.), Ba ₂ Ti ₁₃ O ₂₂ and Ba ₂ Ti ₆ O ₁₃ are 0.81, 0.83, and 0.76, respectively, and are preferably small. . On the other hand, the lower limit value of (Ti-Ti) / (Ti-Ae) is usually about 0.5. In addition, the distance between atoms, such as Ti-Ae, can be calculated from, for example, the atomic coordinates and lattice constants of each atom obtained by Rietveld analysis.

The thermoelectric conversion material of the present invention is characterized by containing a compound having a one-dimensional tunnel type crystal structure. Here, the one-dimensional tunnel type crystal structure is an atom M (where M is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb and W). And the MO ₆ octahedrons formed by the _six oxygen atoms around it form a one-dimensional chain by sharing a part of the ridge, and this one-dimensional chain is at least A crystal structure having a tunnel space surrounded by at least four one-dimensional chains by gathering four and sharing part of the vertices of the MO ₆ octahedron with each other. At this time, alkaline earth metal or the like is disposed in the tunnel space. The thermoelectric conversion material of the present invention contains a compound having this one-dimensional tunnel-type crystal structure, so that the thermal conductivity can be reduced by phonon scattering, or the thermal conductivity value (κ) is smaller. It becomes.

As the one-dimensional chain, a single chain in which MO ₆ octahedrons are bonded in one row, a double chain in which MO ₆ octahedrons are bonded in two rows, and an MO ₆ octahedron bonded in three rows One-dimensional tunnel type when the tunnel space is surrounded by four single chains (that is, when the MO ₆ octahedron is arranged in one vertical row and one horizontal row). The crystal structure is called a pyrolsite type crystal structure, and is surrounded by two single chains and two double chains (ie, when MO ₆ octahedrons are arranged in one vertical row and two horizontal rows). The one-dimensional tunnel-type crystal structure is called the ramsdellite-type crystal structure, and when it is surrounded by four double chains (that is, when the MO ₆ octahedron is arranged in two vertical rows and two horizontal rows) The one-dimensional tunnel type crystal structure is called a hollandite type crystal structure. Triple when it is surrounded by a chain called (i.e., when the MO ₆ octahedra are arranged vertically in two rows × horizontal three rows) one-dimensional tunnel-type crystal structure Romanee site type crystal structure, four triple A one-dimensional tunnel type crystal structure when surrounded by a chain (that is, when MO ₆ octahedrons are arranged in three vertical rows and three horizontal rows) is referred to as a todorokite crystal structure. Among them, a thermoelectric conversion material containing a compound having a hollandite-type crystal structure is preferable because a compound having a hollandite-type crystal structure exists stably even at high temperatures. An example of a hollandite crystal structure is shown in FIG.

  The thermoelectric conversion material of the present invention is mainly used in the form of a powder, a sintered body, and a thin film, and particularly used as a sintered body. When using the thermoelectric conversion material of this invention as a sintered compact, the shape and dimension can be used in a suitable shape as a thermoelectric conversion element. Specifically, it can be used in a shape suitable as a thermoelectric conversion element, such as a plate shape, a columnar shape, and a square shape.

  In addition, the thermoelectric conversion material of the present invention is preferably a thermoelectric conversion material with high orientation in the sense of increasing the value of electrical conductivity (σ). Examples of the form of the thermoelectric conversion material having high orientation include oriented sintered bodies and single crystals.

Next, a method for producing the thermoelectric conversion material of the present invention will be described.
Taking the case where the thermoelectric conversion material of the present invention is used in the form of a sintered body as an example, a method for producing the thermoelectric conversion material of the present invention will be described. The thermoelectric conversion material of the present invention can be produced by sintering a metal compound mixture that becomes the thermoelectric conversion material of the present invention by sintering, that is, alkaline earth metal (Ae) and titanium (Ti) are converted to Ti. It can be produced by sintering a metal compound mixture that is contained so that the molar ratio of / Ae is 2 or more. Specifically, it can be produced by weighing a compound containing a corresponding metal element so as to have a predetermined composition and sintering the resulting metal compound mixture. For example, a compound represented by Ba _1.23 Ti ₈ O _16, which is one of the preferred composition, the BaCO ₃ and TiO ₂ Ba: Ti molar ratio is 0.154: 1 and were weighed so as to, after mixing It can be produced by sintering the resulting metal compound mixture.

  Examples of the compound containing the metal element include alkaline earth metal (Ae), titanium, Li, K, Na, Y, La, Ce, Pr, Nd, Pm, Sm, Bi, Pb, V, Cr, and Mn. , Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb, W-containing compounds, such as oxides or hydroxides, carbonates, nitrates, halides, sulfates In addition, a compound such as an organic acid salt, which is decomposed and / or oxidized at high temperature to be an oxide, is used. Instead of the compound, a metal containing the above metal element may be used. Examples of the compound containing titanium include titanium oxide, metal titanium, titanyl sulfate and the like. Titanium oxide is preferably used, and the compound containing alkaline earth metal (Ae) includes carbonate, sulfate, A hydroxide etc. are mentioned, A carbonate is used preferably.

  The mixing of the compound containing the metal element may be performed by either a dry mixing method or a wet mixing method, but preferably by a method capable of more uniformly mixing the compound containing the metal element. Examples thereof include a ball mill, a V-type mixer, a vibration mill, an attritor, a dyno mill, and a dynamic mill.

  Depending on the composition, the thermoelectric conversion material of the present invention can be obtained by holding and sintering the metal compound mixture at a temperature in the range of 900 ° C. to 1700 ° C. for 0.5 to 48 hours, for example. it can. The sintering temperature is preferably in the range of 1100 ° C. to 1400 ° C., and more preferably 1200 ° C. to 1350 ° C. When the sintering temperature is less than 900 ° C., it is difficult to sinter, and depending on the composition of the thermoelectric conversion material, the value of electrical conductivity (σ) may decrease. When the sintering temperature exceeds 1700 ° C., the performance index value (Z) of the thermoelectric conversion material of the present invention may be lowered due to abnormal grain growth or melting depending on the composition of the thermoelectric conversion material. Although the sintering atmosphere depends on the composition, when reducing tetravalent titanium ions to trivalent titanium ions, an inert gas atmosphere or a reducing atmosphere can be used in a vacuum. preferable. An example of the reducing atmosphere is an atmosphere containing hydrogen. Examples of the inert gas atmosphere include a nitrogen atmosphere and a rare gas atmosphere.

  Moreover, it is preferable to bake the metal compound mixture before the sintering. By baking to produce a fired product, the metal compound mixture can be decomposed and / or oxidized into oxides such as hydroxide, carbonate, nitrate, halide, organic acid salt, etc. at high temperature. When it is contained, it is possible to convert it into an oxide, carbon dioxide gas, and water of crystallization, and to improve the uniformity of the composition of the sintered body and the structure of the sintered body obtained during sintering. It is possible to improve or suppress deformation of the sintered body.

  Although the firing conditions depend on the composition, for example, if the metal compound mixture contains carbonate, the firing temperature is 600 ° C. to 1100 ° C., and the firing atmosphere is an inert gas. It is performed in an atmosphere or a reducing atmosphere, and the firing time is 0.5 to 24 hours. Further, the fired product can be pulverized to produce a pulverized product. This pulverization can be performed by a pulverizer which is usually used industrially, such as a ball mill, a vibration mill, an attritor, a dyno mill, and a dynamic mill. In addition, when firing is performed before sintering, the sintering is performed at a temperature in the range of 1100 ° C. to 1700 ° C. in an inert gas atmosphere or a reducing atmosphere.

  Moreover, it is preferable to shape | mold about the said metal compound mixture, the said calcined product, or the said ground product before the said sintering, and to manufacture a molded object. Moreover, it is more preferable to perform this molding on the pulverized product. Moreover, you may perform shaping | molding and sintering simultaneously. The molded body may be manufactured so as to have an appropriate shape as a thermoelectric conversion element such as a plate shape, a square shape, and a cylindrical shape. Examples of the molding method include uniaxial press, cold isostatic press (CIP), mechanical It can be performed by pressing, hot pressing, hot isostatic pressing (HIP) or the like. In addition, the metal compound mixture, the calcined product, or the pulverized product may contain a binder, a dispersant, a release agent, and the like.

  The method for producing the thermoelectric conversion material of the present invention described above is the method for producing the thermoelectric conversion material of the present invention when the thermoelectric conversion material of the present invention is used in the form of a sintered body. In order to ensure the strength of the body, the density of the sintered body is preferably 60% or more in relative density, more preferably 90% or more, and still more preferably 95% or more. Further, if the relative density is less than 60%, the electric conductivity value (σ) tends to be small. The density of the sintered body is controlled by the particle size of the metal compound mixture, the particle size of the calcined product or the particle size of the pulverized product, the molding pressure when producing the compact, the sintering temperature, the sintering time, etc. can do. In addition, the sintered body obtained by the above-described sintering may be pulverized to produce a sintered product, and the sintered product may be re-sintered.

  In the case where the thermoelectric conversion material of the present invention contains trivalent titanium ions, the surface of the thermoelectric conversion material may be oxygenated if there is a risk that the trivalent titanium ions may be oxidized and deteriorated by long-term use. The film may be coated with an oxygen-impermeable film that hardly permeates. Specific examples of the material for the oxygen-impermeable film include alumina, titania, zirconia, silicon carbide, and the like. Examples of the method for coating the thermoelectric conversion material with the material include an aerosol deposition method, a thermal spraying method, and a CVD method. (Chemical vapor deposition).

  The thermoelectric conversion material of the present invention can be produced by the above-described method, but other production methods include a method including a coprecipitation step, a method including a hydrothermal step, a method including a dry-up step, and a sputtering step. A method including a step by CVD, a method including a step by CVD, a step including a sol-gel step, a method including a FZ (floating zone melting method) step, a method including a step by TSCG (template type single crystal growth method), and the like.

Moreover, the thermoelectric conversion element which has the thermoelectric conversion material of this invention is demonstrated. As the thermoelectric conversion element, for example, a known technique as disclosed in JP-A-5-315657 can be used. The thermoelectric conversion material of the present invention is an n-type thermoelectric conversion material. In this case, a p-type thermoelectric conversion material is used in combination. A known technique can be used as the p-type thermoelectric conversion material, and examples thereof include NaCo ₂ O ₄ and Ca ₃ Co ₄ O ₉ (see JP-A-9-321346 and JP-A-2001-64021). .

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by these. In addition, the method shown below was used for the evaluation of the thermoelectric characteristics and structure of the thermoelectric conversion material.
1. Electrical conductivity A sintered body sample was processed into a prism shape, a platinum wire was attached with a silver paste, and the measurement was performed by a DC four-terminal method. The measurement was performed while changing the temperature in the range of room temperature to 773 K in a nitrogen gas flow.
2. Seebeck coefficient An R thermocouple and a platinum wire were attached with silver paste to both ends of a sintered body sample processed into the same shape as in the measurement of electrical conductivity, and the temperature and thermoelectromotive force of the sintered body sample were measured. The measurement was performed while changing the temperature in the range of room temperature to 773 K in a nitrogen gas flow. A glass tube into which air has flowed is brought into contact with one side of the sintered body sample to create a low temperature portion, and the temperature at both ends of the sintered body sample is measured with an R thermocouple. Thermoelectromotive force (ΔV) was also measured. The temperature difference (ΔT) at both ends of the sintered body sample was controlled in the range of 1 to 10 ° C. by controlling the air flow rate, and the value (α) of the Seebeck coefficient was calculated from the slopes of ΔT and ΔV.
3. Thermal conductivity The specific heat and thermal diffusivity of the sintered body sample were measured by a laser flash method in vacuum at room temperature. For the measurement, a laser flash method thermal conductivity measuring device TC-7000 type manufactured by Vacuum Riko Co., Ltd. was used.
4). Structure and Composition Analysis The crystal structure of the powder sample and the sintered body sample was analyzed by a powder X-ray diffraction method using a Rigaku Corporation X-ray diffraction measurement apparatus RINT2500TTR type. The composition of the sample was measured using a fluorescent X-ray apparatus PW1480 manufactured by Philips. Regarding the ratio of trivalent titanium ions, the amount of weight increase when the sample was oxidized in the atmosphere at 1200 ° C. was the amount of oxygen required when the trivalent titanium ions were oxidized to tetravalent titanium ions. Calculated by assuming.
5. Density of Sintered Body The relative density of the sintered body sample was calculated by the Archimedes method.

Example 1
Titanium oxide (Ishihara Techno Co., Ltd., PT-401M (trade name)) and barium carbonate (Nippon Chemical Industry Co., Ltd., LC-1 (trade name)) were used as the raw material powder. Titanium oxide and barium carbonate were weighed so that the molar ratio was Ba: Ti = 0.154: 1, and then plastic balls were used as a mixing medium and mixed for 6 hours by a dry ball mill. The mixed powder was calcined by flowing nitrogen containing 2% by volume at 1000 ° C. for 6 hours to obtain a calcined product. The fired product was dry pulverized by a ball mill using zirconia balls as a pulverizing medium to obtain a pulverized product. The pulverized product was molded into a disk shape by a uniaxial press (molding pressure was 200 kg / cm ² ) and an isostatic press (molding pressure was 1500 kg / cm ² ), and the resulting molded body was 100%. Thermoelectric conversion material 1 was obtained by sintering at 1300 ° C. for 12 hours in a sintering furnace in which hydrogen gas was passed.

The thermoelectric conversion material 1 was black and the relative density was 99%. FIG. 2 shows an X-ray diffraction pattern of the sintered body obtained. The thermoelectric conversion material 1 is Ba _1.23 Ti ₈ O ₁₆ single phase, it was confirmed that the hollandite-type crystal structure is a one-dimensional tunnel-type crystal structure. As a result of the composition analysis, the molar ratio (Ti / Ba) of the sample was 6.49, which is the same as the composition at the time of weighing. Further, the ratio of trivalent titanium ions to all titanium atoms was 30.8%, and it was confirmed that the thermoelectric conversion material 1 was a compound having the intended composition, that is, Ba _1.23 Ti ₈ O ₁₆ .

3 shows a graph of the temperature dependence of the Seebeck coefficient of the thermoelectric conversion material 1, FIG. 4 shows a graph of the temperature dependence of the electrical conductivity of the thermoelectric conversion material 1, and FIG. 5 shows the temperature of the thermal conductivity of the thermoelectric conversion material 1. A dependency graph is shown. The electric conductivity of the thermoelectric conversion material 1 is 4.66 × 10 ³ S / m at 100 ° C. and 11.2 × 10 ³ S / m at 500 ° C., and the semiconductor conductivity increases with increasing temperature. Showed a good behavior. The Seebeck coefficient was −136 μV / K at 100 ° C. and −156 μV / K at 500 ° C., and the Seebeck coefficient showed a negative value in all measurement temperature ranges, and the absolute value showed a high value exceeding 100 μV / K. . Thermoelectric conversion material 1 was a stable compound in the air at 500 ° C. or lower. The thermal conductivity was 2.10 W / mK at 100 ° C. and 2.38 W / mK at 500 ° C., and the magnitude was sufficiently smaller than that of BaTiO ₃ which is also a Ba—Ti—O-based oxide.

Examples 2-6
The starting materials were weighed and mixed while changing the molar ratio (Ti / Ba) so as to be the compounds represented by the composition formulas of Examples 2 to 6 shown in Table 1, and baked under the same conditions as in Example 1. A ligation sample was prepared. When Ti / Ba is 6.15 or more and 8 or less, the crystal phase of the sample is a Ba _x Ti ₈ O ₁₆ single phase having a hollandite structure, and the relative densities of the thermoelectric conversion materials of Examples 2 to 6 are all 95. % Or more.

  Table 2 shows the electrical conductivity, Seebeck coefficient, and thermal conductivity of Examples 2 to 6 at 100 ° C and 500 ° C. The thermoelectric characteristics of the obtained sintered body were evaluated in the same manner as in Example 1. All Seebeck coefficients of Examples 2 to 6 showed negative values, and the absolute values thereof were high values exceeding 100 μV / K. The thermal conductivities of Examples 2 to 6 were sufficiently small values as thermoelectric conversion materials.

Examples 7-10
The starting materials were weighed and mixed so as to be the compounds represented by the composition formulas of Examples 7 to 10 shown in Table 1, and sintered body samples were produced under the same conditions as in Example 1. The crystal phases of the samples of Examples 7 to 10 were compound single phases having a hollandite structure, and the relative densities of the thermoelectric conversion materials of Examples 7 to 10 were all 95% or more.

  Table 2 shows the electrical conductivity, Seebeck coefficient, and thermal conductivity of Examples 7 to 10 at 100 ° C. and 500 ° C. Substituting Ba with the elements Sr and Bi is effective for improving the thermoelectric characteristics, and particularly substituting with the Bi element is effective for improving the electrical conductivity and reducing the thermal conductivity. FIG. 6 shows the temperature dependence of the dimensionless figure of merit ZT of the thermoelectric conversion material in Examples 1, 3, and 10.

The schematic diagram which shows an example of a hollandite type crystal structure. The figure which shows the X-ray-diffraction figure of the thermoelectric conversion material of Example 1. FIG. The figure which shows the temperature change of the value of the Seebeck coefficient of the thermoelectric conversion material of Example 1. FIG. The figure which shows the temperature change of the value of the electrical conductivity of the thermoelectric conversion material of Example 1. FIG. The figure which shows the temperature change of the value of the thermal conductivity of the thermoelectric conversion material of Example 1. FIG. The figure which shows the temperature change of the dimensionless figure of merit of the thermoelectric conversion material of Examples 1, 3, and 10.

Claims (16)

  A compound containing alkaline earth metal (Ae), titanium (Ti) and oxygen, wherein the molar ratio of titanium to alkaline earth metal (Ti / Ae) is a value of 2 or more, and at least of titanium atoms A thermoelectric conversion material characterized in that a part thereof is a trivalent titanium ion. Claims containing one or more compounds selected from the group consisting of Ae ₂ Ti ₁₃ O ₂₂ , Ae _x Ti ₈ O ₁₆ (where x is 0.8 or more and 2 or less) and Ae ₂ Ti ₆ O ₁₃ Item 2. The thermoelectric conversion material according to Item 1.   A compound having a crystal structure having a one-dimensional chain in which octahedrons formed by titanium atoms and six oxygen atoms around the titanium atoms share points and / or edges and / or part of the faces. Item 3. The thermoelectric conversion material according to item 1 or 2.   The thermoelectric conversion material according to any one of claims 1 to 3, wherein a ratio of trivalent titanium ions to all titanium atoms is 10% or more.   The ratio of the distance from the titanium atom to the nearest alkaline earth metal atom to the distance between the nearest titanium atoms in the compound is in the range of 0.5 or more and less than 1.0. The thermoelectric conversion material as described.   A part of Ae is substituted with one or more elements selected from the group consisting of Li, K, Na, Zn, Y, La, Ce, Pr, Nd, Pm, Sm, Bi and Pb. The thermoelectric conversion material according to any one of 5.   A part of Ti is substituted with one or more elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb and W. The thermoelectric conversion material in any one. Atom M (where M is one or more elements selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb, and W) and surroundings thereof At least four one-dimensional chains in which MO ₆ octahedrons formed by _six oxygen atoms are bonded together by sharing a part of the ridge, and by sharing a part of the vertices of the MO ₆ octahedron, A thermoelectric conversion material containing a compound having a one-dimensional tunnel-type crystal structure, which is a crystal structure having a tunnel space surrounded by four one-dimensional chains.   The one-dimensional tunnel type crystal structure is one or more types selected from the group consisting of a pyrollsite type crystal structure, a ramsdellite type crystal structure, a hollandite type crystal structure, a romanesite type crystal structure and a todorokite type crystal structure. Item 9. The thermoelectric conversion material according to Item 8.   The thermoelectric conversion material according to any one of claims 1 to 9, having a relative density of 60% or more.   The thermoelectric conversion material according to claim 1, which is coated with an oxygen-impermeable film.   A thermoelectric conversion element comprising the thermoelectric conversion material according to claim 1.   A method for producing a thermoelectric conversion material by sintering a metal compound mixture that becomes a thermoelectric conversion material by sintering, wherein the metal compound mixture contains alkaline earth metal (Ae) and titanium (Ti), Ti / Ae It contains so that it may become a value of 2 or more by molar ratio, This metal compound mixture is hold | maintained at the temperature of the range of 900 degreeC or more and 1700 degrees C or less in inert gas atmosphere or reducing atmosphere, and is sintered. A method for producing a thermoelectric conversion material.   A method for producing a thermoelectric conversion material by sintering a metal compound mixture that becomes a thermoelectric conversion material by sintering, wherein the metal compound mixture contains alkaline earth metal (Ae) and titanium (Ti), Ti / Ae It contains so that it may become a value of 2 or more by molar ratio, this metal compound mixture is hold | maintained at the temperature of the range of 600 degreeC or more and 1100 degrees C or less in reducing environment, and the obtained baked product is shape | molded, A method for producing a thermoelectric conversion material, characterized in that the obtained molded body is held and sintered in an inert gas atmosphere or a reducing atmosphere at a temperature in the range of 1100 ° C to 1700 ° C.   14. A part of Ae is substituted with one or more elements selected from the group consisting of Li, K, Na, Y, La, Ce, Pr, Nd, Pm, Sm, Bi, and Pb. Or the manufacturing method of 14.   14. A part of Ti is substituted with one or more elements selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Hf, Sn, Nb and W. The manufacturing method in any one of -15.

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