JP2006027970A - Method for manufacturing multiple oxide sintered compact - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sintered compact which is composed of an oxide having a high Seebeck coefficient as a raw material, has good electric conductivity even in a polycrystalline sintered compact and can exhibit high thermoelectric conversion efficiency, and a method for manufacturing the same. <P>SOLUTION: The method for manufacturing the multiple oxide sintered compact includes molding a multiple oxide crystalline powder satisfying following conditions (i) and (ii), then sintering the molding: (i) The sintered compact has the composition expressed by general formula: Ca<SB>a</SB>A<SP>1</SP><SB>b</SB>Co<SB>c</SB>A<SP>2</SP><SB>d</SB>O<SB>e</SB>(where A<SP>1</SP>is one or ≥2 kinds of the elements selected from the group composed of Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Pb, Sr, Ba, Al, Bi, Ag, Y, and lanthanoids; A<SP>2</SP>is one or ≥2 kinds of the elements selected from the group composed of Ti, V, Cr, Mn, Fe, Ni, Cu, Mo, W, Nb, Ta, Bi and Ag; and 2.2≤a≤3.6; 0≤b≤0.8; 2≤c≤4.5; 0≤d≤2; 8≤e≤10) and (ii) the sintered compact contains ≥90% crystal of ≥3 μm in the length on the longest side, wherein the average length of the length on the longest side is 5 to 100 μm and the ratio of the crystal present within a range of the average value ±50% is ≥50%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a composite oxide sintered body, a composite oxide sintered body obtained by this method, a p-type thermoelectric conversion material comprising the composite oxide sintered body, and a thermoelectric power generation module.

In Japan, the effective energy yield from primary supply energy is only about 30%.
About 70% of the energy is eventually discarded into the atmosphere as heat. Also, most of the heat generated by combustion in factories and garbage incinerators is discarded into the atmosphere without being converted into other energy. In this way, we humans are wasting a great deal of heat energy and gaining little energy from actions such as the burning of limited fossil fuels.

  In order to improve the energy yield, it is effective to be able to use the thermal energy discarded in the atmosphere. One effective technical means for this purpose is thermoelectric conversion that directly converts thermal energy into electrical energy. This thermoelectric conversion uses the Seebeck effect and is an energy conversion method in which a potential difference is generated by generating a temperature difference at both ends of a thermoelectric conversion material to generate electric power. In power generation using such thermoelectric conversion, that is, thermoelectric power generation, one end of the thermoelectric conversion material is arranged in a high temperature part generated by waste heat, and the other end is arranged in the atmosphere (room temperature part), and both ends are arranged. Since electricity can be obtained simply by connecting a conducting wire to the motor, a movable device such as a motor or a turbine necessary for general power generation is unnecessary. For this reason, equipment cost is also low, there is no discharge | emission of gas by combustion etc., and it can generate electric power continuously until the thermoelectric conversion material deteriorates. In addition, since thermoelectric power generation can generate power with a high output density, the power generator (module) itself can be reduced in size and weight, and can also be used as a mobile power source for mobile phones, laptop computers, and the like.

  In this way, thermoelectric power generation is expected as a technology that will play a part in the solution to the serious problem of depletion of energy resources that will be predicted in the future, but in order to realize thermoelectric power generation, high thermoelectric conversion efficiency is expected. Therefore, it is necessary to supply a large amount of thermoelectric conversion materials having excellent heat resistance and chemical durability.

  At present, intermetallic compounds are known as substances having high thermoelectric conversion efficiency. However, the thermoelectric conversion efficiency of intermetallic compounds is at most about 10%, and can only be used in air at a temperature of about 300 ° C. or lower. Some types of intermetallic compounds include toxic elements and rare elements as constituent elements.

  For this reason, thermoelectric conversion using waste heat has not yet been put to practical use, is composed of elements that are less toxic and have a large amount of existing elements, has excellent heat resistance, chemical durability, etc., and has high thermoelectric conversion efficiency. Development of materials is expected.

In recent years, Co-based composite oxides containing Ca, Bi, Sr, Na, and the like have been reported as materials having high thermoelectric conversion efficiency (Non-patent Document 1). However, although these composite oxides can exhibit high thermoelectric conversion efficiency in a single crystal state, the thermoelectric conversion efficiency is reduced to about 1/3 as compared with a single crystal when a polycrystalline sintered body is used. Such a decrease in performance is considered to be caused by a higher electrical resistance in the polycrystalline sintered body than in the case of a single crystal.

In order to put the thermoelectric conversion material into practical use, it is desired to use a polycrystalline sintered body that is relatively easy to manufacture. For this reason, there is a strong demand for the development of a material having good electrical conductivity even in a polycrystalline sintered body and having excellent thermoelectric conversion efficiency.
Xu et al., Applied Physics Letters vol. 80, pp. 3760-3762 (2002)

  The present invention has been made in view of the current state of the prior art described above, and its main purpose is to have good electrical conductivity even in a polycrystalline sintered body using an oxide having a high Seebeck coefficient as a raw material. And it is providing the sintered compact which can exhibit high thermoelectric conversion efficiency, and its manufacturing method.

  The present inventor has intensively studied to achieve the above-described object. As a result, the present inventors have found that a plate-like Co-based composite oxide having a specific composition has electrical anisotropy and good electrical conductivity in a certain crystal axis direction. For such a complex oxide, when the oxide crystal powder having the longest side of the crystal grains within a certain range and having a well-aligned grain size is used as a raw material, it is molded and sintered. It has been found that a polycrystalline sintered body with a well-aligned crystal axis and a low electrical resistivity can be obtained by bonding. Furthermore, when applying a method of aligning the crystal axis direction before forming the oxide crystal powder or a method of uniaxial pressing during sintering, the electrical resistivity is even lower and excellent thermoelectric conversion performance. The present inventors have found that an oxide sintered body having the above can be obtained and completed the present invention.

That is, the present invention provides the following method for producing a composite oxide sintered body, a composite oxide sintered body, a p-type thermoelectric conversion material, and a thermoelectric power generation module.
1. A method for producing a composite oxide sintered body, characterized in that a composite oxide crystal powder that satisfies the following conditions (i) and (ii) is molded and then sintered:
(I) General formula: Ca _a A ¹ _b Co _c A ² _d O _e (where A ¹ is Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu
, Zn, Pb, Sr, Ba, Al, Bi, Ag, Y and one or more elements selected from the group consisting of lanthanoids, A ² is Ti, V, Cr, Mn, Fe, Ni , Cu, Mo, W, Nb, Ta, Bi and Ag, one or more elements selected from the group consisting of 2.2 ≦ a ≦ 3.6; 0 ≦ b ≦ 0.8; 2 ≦ c ≦ 4.5; 0 ≦ d ≦ 2; 8 ≦ e ≦ 10. Having a composition represented by
(Ii) 90% or more of crystals having the longest side length of 3 μm or more, the average value of the longest side length of the crystal being 5 to 100 μm, and the average value of ± 50% The ratio is 50% or more.
2. Item 2. The method for producing a complex oxide sintered body according to Item 1, wherein the complex oxide crystal powder contains 70% or more of crystals having the length of the longest side of the crystal 3 or more times the crystal thickness.
3. Item 3. The method according to Item 1 or 2, wherein the composite oxide crystal powder is formed and sintered after aligning the crystal axis directions.
4). The method according to item 3, wherein the method of aligning the directions of the crystal axes of the composite oxide crystal powder is any one of the following (i) to (iv):
(I) A method of pressing a crystal powder into a plate shape and press-molding the pressed plate-like crystals further,
(Ii) a method of suspending crystal powder in a liquid and filtering the suspension;
(Iii) A method in which crystal powder is suspended in a viscous fluid and then passed through a slit,
(Iv) A method of applying a magnetic field in one direction to a suspension containing crystal powder.
5. Item 5. The method according to any one of Items 1 to 4, wherein the complex oxide crystal powder is sintered under uniaxial pressure.
6). Item 6. The method according to Item 5, wherein the method of sintering under uniaxial pressure is a hot press sintering method or a method of discharge sintering under pressure.
7). A composite oxide sintered body obtained by the method according to any one of Items 1 to 6.
8). Item 8. The complex oxide sintered body according to Item 7, having an Seebeck coefficient of 100 (V / K or more) at an absolute temperature of 300K or more.
9. Item 9. The composite oxide sintered body according to Item 7 or 8, which has an electrical resistivity of 10 mΩcm or less at a temperature of 300 K or higher in absolute temperature.
10. Item 10. The complex oxide sintered body according to any one of Items 7 to 9, which has a thermal conductivity of 3 W / mK or less at an absolute temperature of 300 K or more.
11. A p-type thermoelectric conversion material comprising the composite oxide sintered body according to any one of Items 7 to 10. 12 A thermoelectric power generation module comprising the p-type thermoelectric conversion material according to Item 11.

  Hereinafter, the method for producing the composite oxide sintered body of the present invention will be specifically described.

Raw Material Powder (1) Composition In the method for producing a composite oxide sintered body of the present invention, as a raw material, a general formula: Ca _a A ¹ _b Co _c A ² _d O _e (where A ¹ is Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Pb, Sr, Ba, Al, Bi, Ag, Y and one or more elements selected from the group consisting of lanthanoids, A ² is one or more elements selected from the group consisting of Ti, V, Cr, Mn, Fe, Ni, Cu, Mo, W, Nb, Ta, Bi and Ag, and 2.2 ≦ a ≦ 3.6; 0 ≦ b ≦ 0.8; 2 ≦ c ≦ 4.5; 0 ≦ d ≦ 2; 8 ≦ e ≦ 10)) is used. In the above general formula, examples of lanthanoid elements include La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu.

  The composite oxide represented by the above general formula has a positive Seebeck coefficient, and when a temperature difference is generated between both ends of the composite oxide material, the potential generated by the Seebeck coefficient This shows the characteristics as a p-type thermoelectric conversion material that is lower than the low temperature side.

The complex oxide crystal represented by the above general formula is composed of a layer having a rock salt structure with a composition ratio of Ca ₂ CoO ₃ composed of Ca, Co and O, and six 0's in an octahedral arrangement on one Co. And a structure in which CoO ₂ layers arranged two-dimensionally so that their octahedrons share sides with each other are alternately stacked, and a part of Ca is replaced by A ¹ , and this layer A part of Co and a part of Co in the CoO ₂ layer are replaced by A ² . The complex oxide crystal having such a structure is an oxide having an extremely low electric resistance in the plane direction of the layered structure, that is, the ab plane direction.

(2) Particle size In the production method of the present invention, it is necessary to use a compound oxide crystal powder represented by the above general formula containing 90% or more of crystals having a longest side length of 3 μm or more. There is preferably used one containing 90% or more of crystals having a longest side length of 5 μm or more. Further, the composite oxide crystal powder has an average value of the length of the longest side of the crystal of 5 to 100 μm, preferably 5 to 50 μm, more preferably 7 to 25 μm, and the average value is within a range of ± 50%. It is necessary that the ratio of a certain crystal is 50% or more.

  In the present specification, the length of the longest side of a crystal is measured from a crystal image observed with a scanning electron microscope, and is evaluated based on the measurement results of 100 or more crystals.

  The complex oxide crystal that satisfies the above-mentioned conditions is a state in which the crystal grains grow moderately and the grain diameters are almost uniform. You can get a body.

In particular, the composite oxide crystal powder used in the present invention preferably includes 70% or more of plate-like crystals having a longest side length of 3 times or more with respect to a thickness perpendicular to the longest side, 90% % Or more is more preferable. Such a complex oxide crystal is a crystal that grows well in a plate shape,
The orientation is particularly good.

(3) Preparation method of raw material powder As long as the composite oxide crystal powder used in the present invention satisfies the above-mentioned conditions, the production method is not particularly limited. Can be obtained, and a complex oxide with a well-aligned crystal grain size can be obtained.

  First, as a raw material for producing a complex oxide crystal powder that satisfies the above-mentioned conditions, 60% of microcrystals having the same composition as the target complex oxide crystal and having a longest diameter less than 3 μm are used. The fine powdery complex oxide contained above is used.

Such a finely powdered complex oxide can be produced, for example, by mixing raw materials and firing so as to have an element component ratio similar to the element component ratio of the target complex oxide. The raw material is not particularly limited as long as it can form an oxide by firing, and elemental simple substance, oxide, various compounds (such as carbonates) and the like can be used. For example, calcium sources include calcium (Ca), calcium oxide (CaO), calcium peroxide (CaO ₂ ), calcium carbonate (CaCO ₃ ), calcium nitrate (Ca (NO ₃ ) ₂ ), calcium hydroxide (Ca ( OH) _2), calcium chloride (CaCl ₂₎ and its hydrates, alkoxide compounds (dimethoxy calcium (Ca (OCH _{3) 2),} diethoxy calcium _{(Ca (OC 2 H 5)} 2), dipropoxy calcium (Ca (OC ₃ H ₇ ) ₂ )) can be used, and Co sources include cobalt (Co), cobalt oxide (CoO, Co ₂ O ₃ , Co ₃ O ₄ ), cobalt carbonate (CoCO ₃ ), cobalt nitrate (Co (NO ₃ ) ₂ ), cobalt hydroxide (Co (OH) ₂ ), cobalt chloride (CoCl ₂ ), alkoxide compounds (dipropoxycobalt (Co (OC ₃ H ₇ ) ₂ etc.), etc. can be used. In the same manner, elemental elements, oxides, chlorides, carbonates, nitrates, hydroxides, alkoxide compounds, etc. can be used for these elements. A compound containing two or more constituent elements of the composite oxide of the present invention may be used, and the above-mentioned raw materials may be used singly or in combination of two or more of the element source materials.

The firing temperature and firing time may be the conditions under which the target composite oxide is formed, and are not particularly limited. For example, in the temperature range of about 800 to 1200 ° C., the firing may be performed for about 20 hours to 40 hours. good. In the case where carbonates, organic compounds, or the like are used as the raw material, it is preferable to pre-fire before firing to decompose the raw material, and then fire to form the desired composite oxide. For example, when carbonate is used as the raw material, it may be calcined at about 600 to 800 ° C. for about 10 hours and then fired under the above-described conditions.

  The firing means is not particularly limited, and any means such as an electric heating furnace or a gas heating furnace can be adopted. The firing atmosphere may be usually an oxidizing atmosphere such as in an oxygen stream or in the air. However, if the source material contains a sufficient amount of oxygen, for example, it may be fired in an inert atmosphere. . The amount of oxygen in the produced composite oxide can be controlled by the oxygen partial pressure during firing, the firing temperature, the firing time, and the like. The higher the oxygen partial pressure, the higher the oxygen ratio in the above general formula. .

  By the above method, a composite oxide having the same composition as the target composite oxide crystal can be obtained. The composite oxide thus obtained may be further pulverized as necessary to form a fine powder composite oxide containing 60% or more of microcrystals having a longest diameter of less than 3 μm.

Using the fine powdery composite oxide thus obtained, the composite oxide crystal powder satisfying the above conditions, that is, containing 90% or more of crystals having a longest side length of 3 μm or more, To produce a composite oxide crystal powder satisfying the condition that the average value of the side length is 5 to 100 μm and the ratio of crystals within the range of the average value ± 50% is 50% or more First, the fine powdered composite oxide has a lower melting point than the composite oxide and can dissolve the fine powdered composite oxide in the melt (hereinafter referred to as “flux component”). Is mixed). The flux component is a compound that satisfies the above-described conditions, and can be used without particular limitation as long as it is a substance that does not inhibit the properties of the target composite oxide. In particular, the flux component is preferably a substance that is soluble in a solvent that does not readily dissolve the target composite oxide, such as water or ethanol. By using such a substance, it is possible to easily separate and remove the flux component by a method such as washing after producing the target composite oxide crystal powder.

Specific examples of the flux component include alkali metal compounds and boron-containing compounds. Among these, specific examples of alkali metal compounds include alkali metal chlorides such as lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl), and hydrates thereof; lithium carbonate (Li ₂ CO ₃ ), Alkali metal carbonates such as sodium carbonate (Na ₂ CO ₃ ) and potassium carbonate (K ₂ CO ₃ ). Specific examples of the boron-containing compound include boric acid (B ₂ O ₃ ). A flux component can be used individually by 1 type or in mixture of 2 or more types.

  The amount of the flux component used varies depending on the type of flux component used, the composition of the target composite oxide, the heating temperature, etc., so it cannot be specified unconditionally. The amount is such that only a part of the oxide is dissolved. In particular, the amount of the flux component used is such that the amount of the fine powdered complex oxide dissolved in the melt of the flux component is less than 50%, that is, 50% or more of the fine powdered complex oxide is dissolved. It is preferable that the amount remains undissolved.

  The heating temperature of the mixture of the fine powdered complex oxide and the flux component is equal to or higher than the melting temperature of the flux component, and only a part of the fine powdered complex oxide is dissolved in the melt of the flux component. Preferably, the temperature is such that the dissolved amount of the fine powdered complex oxide in the melt of the flux component is less than 50%.

  At this time, in order to avoid contamination from the melting container and prevent evaporation of the blended components, for example, when using an alumina crucible, the heating temperature is usually about 700 ° C. to 950 ° C., preferably What is necessary is just to be about 800 to 900 degreeC.

  The heating time is not particularly limited, and may be a time required for the fine powdered complex oxide to dissolve in the melt of the flux component. Usually, it may be about 1 to 50 hours. The heating means is not particularly limited, and known means such as an electric heating furnace and a gas heating furnace can be appropriately employed.

  The atmosphere during heating may be an oxygen-containing atmosphere such as air or an oxygen stream, but when the mixture contains a sufficient amount of oxygen, the atmosphere may be heated in an inert atmosphere.

  By heating by the above-described method, in the melt of the flux component, a component having a small particle size, for example, a small crystal of about 1 μm or less is dissolved in the fine powdered composite oxide, and a relatively large crystal is It remains without dissolving. By cooling the mixture in which such a small crystal component was dissolved and a relatively large crystal component remained, it was dissolved in the melt of the flux component on the relatively large crystal that remained solid. The complex oxide component tends to recrystallize and grow into larger crystals. By such a method, a crystal containing 90% or more of crystals having a longest side length of 3 μm or more, an average value of the longest side length of 5 to 100 μm, and being within a range of an average value ± 50% Thus, a composite oxide crystal satisfying the condition that the ratio of the ratio of 50% or more is obtained.

The specific size of the formed complex oxide crystal may vary depending on the type of flux component, the amount of flux added, the heating time, the heating temperature, the cooling rate, and the like. For example, 100 parts by weight of fine powdered complex oxide As a flux component, 16 parts by weight of K ₂ CO ₃ and KCl
4 parts by weight and after heating at 850 ° C. for 20 hours and cooling at 300 ° C. per hour,
A crystal containing 93% of crystals having a longest side length of 3 μm or more, an average value of the longest side length of 7 μm, and a longest side length in the range of 3.5 to 10.5 μm is 60%. It is possible to obtain a composite oxide crystal powder occupying a percentage.

Sintering method According to the present invention, a composite oxide crystal powder satisfying the above-described conditions is formed into a predetermined shape and then sintered to obtain a target composite oxide sintered body. be able to. According to such a method, since the composite oxide crystal as a raw material is a plate-like crystal that has grown moderately, when forming and sintering, the directions of the crystal axes are easily aligned, and excellent orientation is achieved. A sintered body having the same can be obtained. The obtained sintered body has ab surfaces with low electric resistance well aligned, and becomes a sintered body with a very low electric resistance in the direction parallel to the ab surface.

(1) Orientation treatment In the present invention, it is particularly preferable to align the crystal axis directions of the crystal particles before molding the composite oxide crystal powder, and thereafter to mold and sinter.

Although there is no particular limitation on the method for aligning the crystal axis directions, for example, the following method can be exemplified as a method that enables effective orientation.
(I) A method in which the crystal powder is pressed into a plate shape, and such pressed plate-like crystals are further stacked and pressed.
(Ii) A method of orienting the well-grown surface of the plate crystal parallel to the filter paper or the filter surface by suspending the crystal powder in a liquid and filtering the suspension.
(Iii) A method of orienting crystals by suspending crystal powder in a viscous fluid and passing through a slit by a method generally called a doctor blade method.
(Iv) A well-grown surface of a plate-like crystal by applying a magnetic field of about 1 to 10 Tesla in one direction to the suspension by utilizing the magnetic anisotropy of the crystal by a method generally called a magnetic orientation method. Orienting and then removing the liquid.

  When the above-mentioned various methods are employed to align the crystal axis directions of the crystal grains, the degree of the treatment is not particularly limited, and may be appropriately determined according to the degree of orientation of the target sintered body. .

  Hereinafter, an example of a method for evaluating the orientation of a sintered body obtained by the method of the present invention will be described with reference to FIG. 1 schematically showing an outline of a measuring method.

First, an arbitrary axis (χ axis) is determined in a plane parallel to the surface of the sample (sintered body). The sample is irradiated with X-rays or neutrons in the plane including the χ axis. At this time, strong diffraction is obtained at a specific θ angle depending on the crystal structure of the substance constituting the sample. The sintered body obtained by the method of the present invention has strong diffraction from the ab plane, and θ at this time is defined as θ _ab . Usually, a diffraction peak in the form of a normal distribution is obtained within the range of θ _ab ± 1 ° centering on this θ _ab . The diffraction area intensity is obtained by integrating the diffraction peaks.

Such X-ray or neutron diffraction measurement within the range of θ _ab ± 1 ° is performed in the range of χ = 0 ° to 90 ° by rotating the sample around the χ axis. At this time, χ = 0 ° is when the sample surface and the θ axis are perpendicular to each other. The diffraction integrated intensity at a certain chi square and I (χ), the average value I _r at χ = 0 ° ~90 ° for I (chi) using the following equation.

Then, when χ indicating the strongest diffraction area intensity is χ _max and the diffraction area intensity at that time is I _max (= I (χ _max )), the value obtained by the following formula quantifies the orientation of the sample. The converted value DD _max .

When a sample having no orientation at all and crystal grain orientation is completely random is evaluated using this method, the DDmax value obtained is 1. On the other hand, the higher the sample orientation, the larger the value. For example, in the sample of Example 1 described later, the DDmax value is 11, and the DDmax of the comparative example with poor orientation is
The value was 4.

In order for the sintered body obtained by the method of the present invention to have a sufficiently low electrical resistivity, the DDmax value obtained by the above method is preferably about 6 or more, more preferably about 8 or more, More preferably, it is about 10 or more. Accordingly, when the methods (i) to (iv) are adopted to align the crystal axis directions, it is preferable to perform the treatment so that the DDmax value of the obtained sintered body becomes a sufficiently high value. In particular, it is preferable to carry out the treatment to an extent of about 8 or more.

  In the present invention, by adopting the above-mentioned various methods to align the crystal direction, forming, and then firing, to obtain a polycrystalline sintered body having a much lower electric resistivity and excellent thermoelectric conversion efficiency. Can do.

(2) Molding and Sintering Method There is no particular limitation on the molding method, and a method such as uniaxial pressure molding can be applied as appropriate according to the shape of the objective sintered body.

  There is no particular limitation on the sintering temperature and the sintering time, and the sintering temperature and the sintering time may be the conditions under which the intended sintered body is formed. For example, it may be fired until a desired sintered body is obtained in a temperature range of about 700 to 1200 ° C. The atmosphere during sintering may be usually an oxidizing atmosphere such as the air or an oxygen stream, but when it contains a sufficient amount of oxygen, it may be heated in an inert atmosphere.

  In particular, in the present invention, it is preferable to sinter in a state where uniaxial pressure is applied in a direction perpendicular to the well-grown plane, that is, the ab plane. By sintering under uniaxial pressure, it becomes possible to control the grain growth direction of the crystal powder, so that the crystal grains grow with good orientation and the orientation of the crystal grains is well aligned. You can get a body.

As a method of sintering in a uniaxially pressurized state, a hot press sintering method, a discharge plasma sintering method (SPS) under pressure, or the like can be employed. As conditions for hot press sintering, for example, under a pressure of about 3 to 100 MPa, a heating temperature of about 700 to 950 ° C., preferably 80
The heating time may be about 0 to 900 ° C. and the heating time may be about 1 to 40 hours. As conditions for discharge plasma sintering under pressure, for example, under a pressure of about 3 to 100 MPa, a heating temperature of 70
The heating time may be about 0 to 950 ° C, preferably about 800 ° C to 900 ° C, and the heating time may be about 10 minutes to 1 hour. According to these methods, it is possible to obtain a sintered body in which the ab surfaces of the crystal grains are well aligned perpendicular to the pressure axis.

Composite oxide sintered body FIG. 2 shows a surface perpendicular to the pressure axis of the sintered body obtained in Example 1 described later as an example of a sintered body sintered under uniaxial pressure. The X-ray diffraction pattern (a) and the X-ray diffraction pattern (b) of the plane parallel to the pressure axis are shown. In the X-ray diffraction pattern (a) of the surface perpendicular to the pressure axis, a diffraction peak indexed by (00l) appears strongly, and in the X-ray diffraction pattern (b) of the surface parallel to the pressure axis, (00l ) The diffraction intensity of peaks other than) increased. This result means that in the composite oxide sintered body obtained by the method of the present invention, the ab surfaces of the crystal grains are aligned perpendicular to the pressure axis.

The complex oxide sintered body, which is an object of the present invention, has a structure in which two different types of sub-lattices are alternately laminated in the c-axis direction, so that the ab plane grows well. Uniaxial pressurization during sintering limits the crystal orientation to a plane perpendicular to the pressurization axis, and the crystal powder has a large grain size, a longest side to a large thickness ratio, and a uniform grain size. This amplifies the pressurizing effect, which promotes the orientation of the grains in the pressurizing plane. As a result, by adopting a method of sintering under uniaxial pressure, it is possible to obtain a sintered body in which the orientation of crystal axes is very well aligned,
FIG. 3 shows a scanning electron microscope (SEM) photograph of a plane parallel to the pressure axis of the composite oxide sintered body obtained in Example 1 described later. From FIG. 3, it can be seen that the plate-like crystal grains are stacked in the direction of the pressure axis, and the well-grown ab surface spreads from the front to the back.

  The complex oxide sintered body obtained by the method of the present invention has a high Seebeck coefficient and can be effectively used as a p-type thermoelectric conversion material.

The composite oxide sintered body obtained by the above-described method has a low electrical resistivity because the direction of the ab plane having a low electrical resistance is well aligned with many crystal grains, and at a temperature of 300K or higher,
It has a low electrical resistivity of about 10 mΩcm or less. In particular, when sintered under uniaxial pressure, the orientation of the crystal grains becomes very high, and further, by increasing the density, the electrical resistivity becomes very low, and at a temperature of 300K or higher, It can have a very low electrical resistivity of about 5 mΩcm or less.

  As described above, the composite oxide sintered body obtained by the method of the present invention has a high Seebeck coefficient and good electrical conductivity at the same time, and is a material having high thermoelectric conversion efficiency. For this reason, the composite oxide sintered body can be effectively used as a p-type thermoelectric conversion material used at a high temperature in the air using such characteristics.

FIG. 4 shows a schematic diagram of an example of a thermoelectric power generation module using the thermoelectric conversion material made of the composite oxide sintered body of the present invention as a p-type thermoelectric conversion element. The structure of the thermoelectric power generation module is the same as that of a known thermoelectric power generation module, and is composed of a high-temperature part substrate, a low-temperature part substrate, a p-type thermoelectric conversion material, an n-type thermoelectric conversion material, an electrode, a conductor, and the like. It is a module, and the composite oxide of the present invention is used as a p-type thermoelectric conversion material.

  As described above, the composite oxide sintered body obtained by the method of the present invention has a positive Seebeck coefficient and low electrical resistivity, and is a polycrystalline sintered body excellent in heat resistance, chemical durability, and the like. It is.

The composite oxide sintered body can be effectively used as a p-type thermoelectric conversion material used in high-temperature air, which is impossible with conventional intermetallic compounds, using such characteristics. High thermoelectric conversion efficiency can be exhibited.

Therefore, by incorporating the composite oxide sintered body into the system as a p-type thermoelectric conversion element of a thermoelectric power generation module, it is possible to effectively use the thermal energy that has been discarded up to now.

  Examples are shown below to further clarify the features of the present invention.

Example 1
By the manufacturing method <br/> the following method of the powder of composite oxide crystal, average composition was prepared powder of the complex oxide crystal represented by Ca ₃ Co ₄ O _9.

First, CaCO ₃ and Co ₃ O ₄ raw material powders are mixed so that Ca: Co = 3: 4, heated in air at 800 ° C. for 10 hours, cooled, and the fired product is then used with a mortar and pestle Crush and mix. Then further 880 ° C
And heated in oxygen for 20 hours to produce a calcined product of Ca ₃ Co ₄ O ₉ .

  The fired product was pulverized by an automatic pulverizer to obtain a sufficiently fine powder. In this powder, the ratio of crystals having the longest side length of 3 μm or more was 20%.

Next, 20 parts by weight of a powder obtained by mixing K ₂ CO ₃ and KCl at a molar ratio of 4: 1 was mixed with 100 parts by weight of the Ca ₃ Co ₄ O ₉ powder obtained by the above method, and the mixture was heated at 850 ° C. After heating for 20 hours, it was cooled at a rate of 300 ° C./hour.

The resulting product was ultrasonically washed with distilled water to wash away K ₂ CO ₃ and KCl. Thereafter, by evaporating the water and dried to give the Ca ₃ Co ₄ O ₉ powder.

A scanning electron microscope (SEM) photograph of the obtained Ca ₃ Co ₄ O ₉ powder is as shown in FIG. When about 50 SEM photographs were used and the size of the crystal grains was measured for about 1000 crystal grains, the number of crystal grains with the longest side length of 3 μm or more was 93%. The average length of the longest side was 7 μm, and the number of crystals in the range of 7 μm ± 50% was 60%.

Method for manufacturing sintered body Using the above-mentioned composite oxide crystal powder as a raw material, it is pressed into a disk shape having a thickness of about 0.3 mm and a diameter of 15 mm at a pressure of 200 MPa and processed into such a cylindrical shape. 30 sheets of raw materials (total of about 10 g) were stacked and further pressed at a pressure of 250 MPa, and spread to a diameter of 20 mm. During the spreading process, the ab surfaces of the plate-like crystal grains were aligned so as to be parallel to the pressure surface, and the orientation of the pressure-formed product was improved.

Next, the obtained molded body was heated at 880 ° C. for 20 hours while being uniaxially pressed at 5 MPa to be subjected to pot press sintering. The size of the obtained sintered body was 25 mm in diameter and 4 mm in thickness.

Characteristics of the sintered body The X-ray diffraction pattern of the obtained sintered body is as shown in FIG. 2, and the diffraction indexed by (00l) in the plane (a) perpendicular to the pressure axis. The peak appears strongly, and it can be seen that the ab surfaces of the crystal grains are aligned perpendicular to the pressure axis.

FIG. 3 is a scanning electron microscope (SEM) photograph of a plane parallel to the pressure axis of the obtained sintered body, and it is observed that plate-like crystal grains are stacked in the direction of the pressure axis.

  In FIG. 3, as a comparative example, the average value of the longest side is 2 μm, and the ratio of the crystal having the longest side length of 3 μm or more is 20%. About the complex oxide sintered compact produced like the method, the SEM photograph of the surface parallel to a pressure axis is shown. As is clear from this photograph, the sintered body of the comparative example obtained using the complex oxide crystal powder having a small crystal grain size clearly has an orientation as compared with the sintered body obtained in Example 1. It was inferior. FIG. 5 shows a scanning electron microscope (SEM) photograph of the composite oxide powder used for producing the sintered body of the comparative example. Compared with the composite oxide powder used for the production of the sintered body of Example 1, it can be confirmed that the crystal grains are fine.

In addition, regarding the sintered body obtained in Example 1, the DD _max value measured by the above-described method is 11, whereas the DD _max value of the sintered body of the comparative example is 4, and from this point Moreover, it has confirmed that the orientation of the sintered compact obtained by this invention method was favorable.

  FIG. 6 is a graph showing the temperature dependence of the Seebeck coefficient (S) from 300K to 1073K for the obtained sintered body. FIG. 6 also shows the measured value of the Seebeck coefficient for the sintered body of the comparative example described above.

  7 to 9 to be described later, as a comparative example, measured values for the sintered body obtained in the same manner are shown.

From FIG. 6, it can be confirmed that the composite oxide of Example 1 is a p-type thermoelectric conversion material having a positive Seebeck coefficient of 100 μV / K or higher at a temperature of 300 K or higher and having a low potential on the high temperature side. It was. In all other examples described later, the Seebeck coefficient showed a high value of 100 μV / K or higher at 300 K or higher, indicating the same tendency as in Example 1.

A graph showing the temperature dependence of the electrical resistivity of the sintered body from 300K to 1073K is shown in FIG. From FIG. 7, it can be seen that the sintered body obtained in Example 1 has a low electrical resistivity of about 5 mΩcm at a temperature of 300 K or higher. On the other hand, even if the sintered body of the comparative example using the complex oxide powder having a small crystal grain size as the raw material is press-molded by the same method as in Example 1, the electrical resistivity is 10 mΩcm. The electrical conductivity was inferior. In all other examples described later, the electrical resistivity of the obtained sintered body showed a low value of 10 mΩcm or less at 300 K or more, indicating the same tendency as in Example 1.

FIG. 8 is a graph showing the temperature dependence of the thermal conductivity of the sintered body of Example 1 from 300K to 1073K. FIG. 8 shows that the thermal conductivity of the sintered body obtained in Example 1 is a low value of 3 W / mK or less at a temperature of 300 K or more. In all other examples, the thermal conductivity was as low as 3 W / mK or less at 300 K or higher, and the same tendency as in Example 1 was shown.

Further, FIG. 9 shows a graph showing the temperature dependence of the thermoelectric dimensionless figure of merit from 300K to 1073K for the sintered body of Example 1 and the sintered body of the comparative example.

  Here, ZT is defined by the following equation, and indicates the thermoelectric conversion efficiency of the material. The higher this value, the higher the conversion efficiency.

ZT = S ² T / ρκ
S: Seebeck coefficient, T: absolute temperature, ρ: electrical resistivity, κ: thermal conductivity In the sintered body of Example 1, the thermoelectric dimensionless figure of merit increased with temperature and exceeded 0.4 at 1073K. In all other examples, the thermoelectric dimensionless figure of merit exceeded 0.4 at 1073K.

Examples 2-210
Element ratio of composite oxide crystal powder used as raw material for sintered body production, average particle size, ratio of particles with longest side length of 3 μm or more, within average value of crystal particle size ± 50% Each sintered body was produced in the same manner as in Example 1 except that the ratio of the crystals in the above was the values shown in Tables 1 to 12 below.

  Each table also shows the crystal orientation method during the production of each sintered body. In the section of the orientation method in the table, the lamination method is oriented by the same method as in Example 1, and the slit method is that the composite oxide crystal powder is stirred and mixed in Florinert. The crystal is oriented by passing through a slit having a width of 1 to 3 mm.

  In Example 3, a process for increasing the orientation of the crystal grains was not performed when the molded body was produced, and the sample was pressed at a pressure of 250 MPa to obtain a pellet-like sample having a thickness of about 5 mm and a diameter of about 20 mm. Produced. The subsequent sintering process was performed under atmospheric pressure. Other conditions such as temperature were the same as in Example 1.

For Example 4, as in Example 3, when the molded body was produced, no treatment for increasing the orientation of the crystal grains was performed, and pressing was performed at a pressure of 250 MPa to obtain a thickness of about 5 mm and a diameter of about 20 mm. A pellet-shaped sample was prepared. The subsequent sintering process was performed under uniaxial pressure as in Example 1. Other conditions such as temperature were the same as in Example 1.

The thermoelectric dimensionless figure of merit (ZT) and DD _max value for each sintered body are shown in Tables 1-12.

In addition, the manufacturing raw material used in each Example is shown below.
Ca source: calcium carbonate (CaCO ₃ ) However, in Example 83-100, calcium nitrate (Ca (NO ₃ ) ₂ )
Bi source: Bismuth oxide (Bi ₂ O ₃ )
Co source: Cobalt oxide (Co ₃ O ₄ ) However, in Example 83-100, cobalt nitrate (Co (NO ₃ ) ₂ )
・ Li source: Lithium carbonate (Li ₂ CO ₃ )
・ Na source: Sodium carbonate (Na ₂ CO ₃ )
・ Sr source: strontium carbonate (SrCO ₃ )
・ La source: Lanthanum nitrate (La (NO ₃ ) ₃ )
・ Ba source: Barium carbonate (BaCO ₃ )
・ Ag source: Silver oxide (Ag ₂ O)
・ Ni source: Nickel oxide (NiO)
・ Cu source: Copper oxide (CuO)
・ Pb source: Lead oxide (PbO)
・ Al source: Aluminum oxide (Al ₂ O ₃ )

The drawing which shows typically the evaluation method of the orientation of a sintered compact. The X-ray-diffraction pattern about the direction (b) parallel to the direction (a) perpendicular | vertical to the pressurization axis | shaft of the complex oxide sintered compact obtained in Example 1. FIG. The scanning electron micrograph of the surface parallel to a pressurization axis | shaft about the sintered compact of the complex oxide obtained in Example 1, and the sintered compact of a comparative example. The schematic diagram of the thermoelectric conversion module which used the complex oxide sintered compact obtained by the method of this invention as a thermoelectric conversion material. The scanning electron micrograph about the complex oxide crystal powder used in Example 1, and the powder used by the comparative example. The graph which shows the temperature dependence of Seebeck coefficient about the complex oxide sintered compact obtained in Example 1, and the sintered compact of a comparative example. The graph which shows the temperature dependence of an electrical resistivity about the complex oxide sintered compact obtained in Example 1, and the sintered compact of a comparative example. The graph which shows the temperature dependence of thermal conductivity about the complex oxide sintered compact obtained in Example 1, and the sintered compact of a comparative example. The graph which shows the temperature dependence of the thermoelectric figure of merit ZT about the complex oxide sintered compact obtained in Example 1, and the sintered compact of a comparative example.

Claims (12)

A method for producing a composite oxide sintered body, characterized in that a composite oxide crystal powder that satisfies the following conditions (i) and (ii) is molded and then sintered:
(I) General formula: Ca _a A ¹ _b Co _c A ² _d O _e (where A ¹ is Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu
, Zn, Pb, Sr, Ba, Al, Bi, Ag, Y and one or more elements selected from the group consisting of lanthanoids, A ² is Ti, V, Cr, Mn, Fe, Ni , Cu, Mo, W, Nb, Ta, Bi and Ag, one or more elements selected from the group consisting of 2.2 ≦ a ≦ 3.6; 0 ≦ b ≦ 0.8; 2 ≦ c ≦ 4.5; 0 ≦ d ≦ 2; 8 ≦ e ≦ 10. Having a composition represented by
(Ii) 90% or more of crystals having the longest side length of 3 μm or more, the average value of the longest side length of the crystal being 5 to 100 μm, and the average value of ± 50% The ratio is 50% or more. 2. The method for producing a composite oxide sintered body according to claim 1, wherein the composite oxide crystal powder contains 70% or more of crystals having a length of the longest side of the crystal 3 times or more of the thickness of the crystal. The method according to claim 1, wherein the composite oxide crystal powder is formed and sintered after aligning the directions of crystal axes of the composite oxide crystal powder. The method according to claim 3, wherein the method of aligning the direction of the crystal axes of the composite oxide crystal powder is any one of the following (i) to (iv):
(I) A method of pressing a crystal powder into a plate shape and press-molding the pressed plate-like crystals further,
(Ii) a method of suspending crystal powder in a liquid and filtering the suspension;
(Iii) A method in which crystal powder is suspended in a viscous fluid and then passed through a slit,
(Iv) A method of applying a magnetic field in one direction to a suspension containing crystal powder. The method according to claim 1, wherein the composite oxide crystal powder is sintered under uniaxial pressure. The method according to claim 5, wherein the method of sintering under uniaxial pressure is a hot press sintering method or a method of discharge sintering under pressure. A composite oxide sintered body obtained by the method according to claim 1. The composite oxide sintered body according to claim 7, which has a Seebeck coefficient of 100 (V / K or more) at an absolute temperature of 300K or more. The composite oxide sintered body according to claim 7 or 8, which has an electrical resistivity of 10 mΩcm or less at a temperature of 300 K or higher in absolute temperature. The complex oxide sintered body according to any one of claims 7 to 9, having a thermal conductivity of 3 W / mK or less at an absolute temperature of 300 K or more. A p-type thermoelectric conversion material comprising the composite oxide sintered body according to claim 7. A thermoelectric power generation module comprising the p-type thermoelectric conversion material according to claim 11.