JP4900569B2 - Method for producing aluminum-containing zinc oxide sintered body - Google Patents

Description

The present invention relates to a method for producing an aluminum-containing zinc oxide sintered body . More specifically, the present invention relates to a method for producing an aluminum-containing zinc oxide sintered body that is useful as a thermoelectric conversion material and has characteristics of low thermal conductivity and high conductivity.

Conventionally, p-type and n-type are known as thermoelectric conversion materials. A thermoelectric conversion material is a functional material that can extract electricity based on a temperature difference between materials, and its performance is generally indicated by a figure of merit Z. Specifically, the figure of merit Z is given by the following formula (1)
Z = S ² × σ / κ (1)
(However, S is the Seebeck coefficient, σ is the conductivity, and κ is the thermal conductivity.)
Indicated by. Here, (S ² × σ) in the formula is particularly called an “output factor”.

  In order to improve the performance of the thermoelectric conversion material (increase Z), considering from the equation (1), it is important to increase the output factor and decrease the thermal conductivity.

Metal oxides are well known as thermoelectric conversion materials. In particular, zinc oxide (Zn _1-x Al _x O) in which a part of zinc is substituted with aluminum is known as an n-type thermoelectric conversion material, and is expected to be used in a high temperature range such as waste heat power generation. For example, Patent Documents 1 to 4 describe a thermoelectric conversion material composed of zinc oxide obtained by substituting a part of zinc with aluminum (hereinafter also referred to as “aluminum-containing zinc oxide”) and uses thereof.

  The aluminum-containing zinc oxide has the following characteristics. In other words, it can be manufactured inexpensively without containing rare elements, has low harmfulness to the human body, and has a larger output factor than other n-type thermoelectric conversion materials. On the other hand, it also has a feature that its thermal conductivity is extremely large compared to other n-type thermoelectric conversion materials. Therefore, there is a problem that the figure of merit Z cannot be sufficiently increased because of the large thermal conductivity despite having a large output factor.

  In order to improve the above problem, for example, pores are introduced into the material to lower the relative density, thereby reducing the thermal conductivity κ. That is, it is a decrease in thermal conductivity due to porosity. In this case, the relationship between the relative density and the thermal conductivity can be estimated by Maxwell's formula. That is, assuming that the thermal conductivity of ZnO is 30 W / mK and the thermal conductivity of pores (air) is 0.024 W / mK, the relative density is reduced to 70% by weight (introduction of 30% by weight of pores). The thermal conductivity can be reduced to about half with 16 W / mK.

However, even if the above method is adopted, the figure of merit Z has not been sufficiently increased. The reason is that, due to the decrease in the relative density, the conductive path is also decreased, so that the conductivity σ is also decreased at the same time. Note that the relationship between the relative density and the electrical conductivity is not limited, but generally the electrical conductivity significantly decreases when the relative density is less than 90%. Based on these reasons, the conventional method is characterized by low thermal conductivity and high conductivity by any manufacturing method such as atmospheric pressure sintering, pressure sintering, vacuum sintering, and discharge plasma sintering. Thus, an aluminum-containing zinc oxide sintered body that combines the above is not obtained.
Japanese Patent Laid-Open No. 62-132380 JP-A-8-186293 JP 2001-44520 A JP 2002-118300 A

An object of this invention is to provide the manufacturing method of the aluminum containing zinc oxide sintered compact which has the characteristic of low thermal conductivity and high conductivity useful as a thermoelectric conversion material.

  As a result of intensive studies to achieve the above object, the present inventor is able to achieve the above object when employing discharge plasma sintering under specific conditions as a method for producing an aluminum-containing zinc oxide sintered body. The headline and the present invention were completed.

That is, this invention relates to the manufacturing method of the following aluminum containing zinc oxide sintered compact .

1 . A method for producing an aluminum-containing zinc oxide sintered body having the following steps in order:
(1) Step 1 of preparing a raw material mixture containing zinc oxide and alumina, the alumina content being 2% by weight or less of the total amount of zinc oxide weight and alumina weight,
(2) Step 2 of heating the green compact made of the raw material mixture to 800 to 1100 ° C. at a temperature rising rate of 50 ° C./min or more,
(3) Discharge plasma in 10 minutes or less while pressurizing the heated green compact at 40 MPa or less in an inert gas having a gas pressure of 0.05 to 0.13 MPa and maintaining the temperature. Step 3 for sintering, and (4) Step 4 for lowering the temperature of the sintered body obtained by the discharge plasma sintering at a temperature lowering rate of 50 ° C./min or more until at least 600 ° C.
2. Item 2. The production method according to Item 1, wherein the alumina content in the raw material mixture is 1 to 2 wt% of the total amount of zinc oxide weight and alumina weight.
3. Item 3. The method according to Item 1 or 2, wherein the zinc oxide is a powder, and the average particle size of the powder is 2 µm or less.
4). Item 4. The production method according to any one of Items 1 to 3, wherein the alumina is a powder, and the average particle size of the powder is 5 nm to 2 µm.

Hereinafter, the manufacturing method of the aluminum containing zinc oxide sintered compact of this invention is demonstrated.

Aluminum-containing zinc oxide sintered body The aluminum-containing zinc oxide sintered body of the present invention has a general composition formula: Zn _1-y Al _x O _{1 + δ}
[However, 0 <x ≦ 0.035, 0 ≦ y ≦ x, 1 + δ represents the number of oxygen atoms]
Indicated by. The aluminum content in the sintered body is not limited. For example, a so-called substitutional solid solution in which some zinc atoms of zinc oxide are substituted with aluminum atoms, aluminum atoms have entered between zinc oxide lattices. There are so-called interstitial solid solutions.

  Although x which shows content of aluminum should just be 0 <x <= 0.035, 0.015 <= x <= 0.035 is more preferable.

  The zinc content is represented by 1-y. The value that y can take varies depending on, for example, the above-described aluminum-containing mode. The value that y can take is not limited, but y may be 0 ≦ y ≦ x.

  The number of oxygen atoms is represented by 1 + δ. The range in which δ can be taken is not limited, but is preferably in the range of 0 ≦ δ ≦ (3x−2y) / 2.

  In the aluminum-containing zinc oxide sintered body of the present invention, the decrease in the conductivity σ is suppressed even when the relative density is low as compared with the conventional product. Specifically, when the relative density is 70 to 90%, the conductivity at 20 ° C. is 250 S / cm or more. Further, when the relative density is 80 to 90%, the conductivity at 20 ° C. is 500 S / cm or more.

  In addition, the relative density in this specification is a density calculated by the following formula. That is, the measured density of the bulk body including pores and defects is defined as the bulk density (actual density), the theoretically calculated density without including pores and defects is defined as the theoretical density, and both densities are expressed as [bulk density / theoretical density × 100 (%)] is a density calculated by substitution.

  The average crystal particle diameter of the aluminum-containing zinc oxide sintered body of the present invention is not limited, but is preferably 2 μm or less, and more preferably 500 nm or less. This value is the arithmetic average of the measured diameter values obtained by the intercept method or the image analysis method for 100 or more crystal particles randomly selected by observation with an electron microscope.

  In the aluminum-containing zinc oxide sintered body of the present invention, the thermal conductivity κ decreases as the relative density decreases. This is an effect based on porosity due to a decrease in relative density, and such correlation substantially matches the correlation curve theoretically specified by the maxwell equation. In addition, when the relative density is fixed, the thermal conductivity κ decreases as the measurement temperature of the sintered body increases. Therefore, the thermal conductivity κ is lowered as the material is operated in a high temperature environment. Can be used in

  As described above, when the relative density is lowered, the aluminum-containing zinc oxide sintered body of the present invention can lower the thermal conductivity κ, and keep the decrease in the conductivity σ high as if the conventional common sense is overturned. can do. Since it has such characteristics, the aluminum-containing zinc oxide sintered body of the present invention has a high performance index Z of the thermoelectric conversion material. In addition, when used with a material having a low relative density, it contributes to weight reduction of thermoelectric conversion materials, improvement of thermal shock resistance, cost reduction, and the like.

More specifically, the figure of merit Z is given by the following formula (1)
Z = S ² × σ / κ (1)
(Where S is the Seebeck coefficient, σ is conductivity: S / cm, κ is thermal conductivity: W / mK), and the relative density is 65 to 90%, The figure of merit Z at 20 ° C. is 0.2 × 10 ⁻⁵ / K or more. When the relative density is 80 to 90%, the figure of merit Z at 20 ° C. is 0.5 × 10 ⁻⁵ / K or more.

  The thermoelectric conversion characteristics evaluated by the above figure of merit are clearly superior to conventional products, that is, the aluminum-containing zinc oxide sintered body of the present invention is an n-type thermoelectric conversion material such as waste heat power generation, geothermal power generation, This is advantageous for application development in high-temperature areas such as solar thermal power generation. In addition, an n-type thermoelectric conversion material can take out an electric current because an electron flows into the low temperature end from the high temperature end inside a material (the direction of an electric current is reverse).

Method for Producing Aluminum-Containing Zinc Oxide Sintered Body The aluminum-containing zinc oxide sintered body of the present invention can be suitably produced by a production method using a discharge plasma sintering method having the following steps in order:
(1) Step 1 of preparing a raw material mixture containing zinc oxide and alumina, the alumina content being 2% by weight or less of the total amount of zinc oxide weight and alumina weight,
(2) Step 2 of heating the green compact made of the raw material mixture to 800 to 1100 ° C. at a temperature rising rate of 50 ° C./min or more,
(3) Discharge plasma in 10 minutes or less while pressurizing the heated green compact at 40 MPa or less in an inert gas having a gas pressure of 0.05 to 0.13 MPa and maintaining the temperature. Step 3 for sintering, and (4) Step 4 for lowering the temperature of the sintered body obtained by the discharge plasma sintering at a temperature lowering rate of 50 ° C./min or more until at least 600 ° C.

  As described above, in the production method of the present invention, zinc oxide and alumina are used as raw materials, and a green compact made of a raw material mixture is subjected to discharge plasma sintering under specific conditions. Although the discharge plasma sintering can be performed by a known sintering machine, the aluminum-containing zinc oxide sintered body of the present invention can be obtained by performing the discharge plasma sintering under specific conditions.

  An example of the discharge plasma sintering machine is shown in FIG. In this discharge plasma sintering machine, a pair of electrodes are arranged at the upper end and the lower end, and a direct current pulse current is applied between the electrodes during the discharge plasma sintering. Between the upper and lower carbon plates, there is a graphite die that contains a sample (raw material), and the sample contained in the graphite die is sandwiched between a pair of graphite punches and subjected to discharge plasma sintering in a pressure applied state.

Hereafter, each said process is demonstrated.
≪Process 1≫
In step 1, a raw material mixture containing zinc oxide and alumina is prepared, the alumina content being 2% by weight or less of the total amount of zinc oxide weight and alumina weight.

  The shapes of zinc oxide and alumina are not limited, and those in various states such as particles, powder, whiskers and the like can be widely used. As for the type of alumina, various crystal forms such as γ-type, α-type, δ-type, η-type, θ-type and amorphous type can be used, and aluminum hydroxide can also be used. In the present invention, it is particularly preferable to use γ alumina (so-called activated alumina).

  The size of zinc oxide is not limited, but when it is a powder, the average particle size is preferably 2 μm or less, more preferably about 100 to 500 nm.

  The size of alumina is not limited, but in the case of powder, the average particle size is preferably about 5 nm to 2 μm, more preferably about 20 nm to 0.5 μm.

  Mixing of zinc oxide and alumina is performed using, for example, a known ball mill or mixer. Mixing is performed so that the alumina content is 2% by weight or less of the total amount of zinc oxide weight and alumina weight. Among these, the range in which the alumina content is 1 to 2% by weight is preferable.

The raw material mixture may be essentially only two components of zinc oxide and alumina, but is selected from, for example, indium, tin, rare earth, etc., in order to improve the discharge plasma sintering characteristics, the characteristics of the sintered body, etc. At least one kind may be further included as an additive (third component). The content of such additives is not limited, but is preferably 0.2% by weight or less in the raw material mixture.
≪Process 2≫
In step 2, the green compact made of the raw material mixture is heated to 800-1100 ° C. at a temperature rising rate of 50 ° C./min or more.

  The green compact can be produced, for example, by storing the raw material mixture in a graphite die that stores a sample of a discharge plasma sintering machine and compressing it with a pair of attached graphite punches. The green compact as used herein means one that is molded to a certain degree.

The green compact is heated to 800-1100 ° C. at a temperature rising rate of 50 ° C./min or more. Here, the heating rate is preferably about 50 to 100 ° C./min. The temperature is raised until the temperature of the green compact reaches a specified temperature in the range of 800 to 1100 ° C.
≪Process 3≫
In step 3, the heated green compact is discharged in an inert gas having a gas pressure of 0.05 to 0.13 MPa at a pressure of 40 MPa or less and maintained at the temperature for 10 minutes or less. Plasma sintering.

  The green compact whose temperature has been raised is subjected to spark plasma sintering while being pressurized at 40 MPa or less (preferably 30 MPa or less) and maintaining the temperature (800 to 1100 ° C.). Although the lower limit of pressurization is not limited, it is about 10 MPa.

  The time for spark plasma sintering is 10 minutes or less, but it is preferable if it is completed in 5 minutes or less. The lower limit of the sintering time is not limited, but is at least about 1 minute.

The atmosphere of the discharge plasma sintering is an inert gas atmosphere having a gas pressure of 0.05 to 0.13 MPa, preferably 0.08 to 0.11 MPa. In general, a vacuum atmosphere is known as an inert atmosphere. However, a vacuum atmosphere is not preferable because zinc oxide is easily sublimated, and an inert gas atmosphere is particularly used in the present invention. Examples of the inert gas include an atmosphere that does not contain a reactive gas such as oxygen, such as nitrogen, argon, or helium.
≪Process 4≫
In step 4, the temperature of the sintered body obtained by the discharge plasma sintering is decreased at a temperature decrease rate of 50 ° C./min or more until at least 600 ° C. Here, the temperature lowering rate is preferably faster, and preferably 100 ° C./min or more. The upper limit of the temperature lowering rate is not limited, but is about 200 ° C./min.

  When the relative density is lowered, the aluminum-containing zinc oxide sintered body of the present invention can lower the thermal conductivity κ, and can maintain the decrease in the conductivity σ as high as if the conventional common sense is overturned. . Since it has such characteristics, the aluminum-containing zinc oxide sintered body of the present invention has a high performance index Z of the thermoelectric conversion material. In addition, when used with a material having a low relative density, it contributes to weight reduction of thermoelectric conversion materials, improvement of thermal shock resistance, cost reduction, and the like.

More specifically, when the relative density is 65 to 90%, the figure of merit Z at 20 ° C. is 0.2 × 10 ⁻⁵ / K or more. When the relative density is 80 to 90%, the figure of merit Z at 20 ° C. is 0.5 × 10 ⁻⁵ / K or more.

  The thermoelectric conversion characteristics evaluated by the above figure of merit are clearly superior to conventional products, that is, the aluminum-containing zinc oxide sintered body of the present invention is an n-type thermoelectric conversion material such as waste heat power generation, geothermal power generation, This is advantageous for application development in high-temperature areas such as solar thermal power generation.

  According to the manufacturing method of the present invention, the sintered body of the present invention having the above characteristics can be easily manufactured.

It is a figure which shows the relationship between the relative density of the sintered compact produced in Examples 1-14, and electrical conductivity (sigma). It is a figure which shows the relationship between the relative density of the sintered compact produced in Examples 1-14, and thermal conductivity (kappa). It is a figure which shows the relationship between the relative density of the sintered compact produced in Comparative Examples 1-30, and electrical conductivity (sigma). It is a figure which shows the relationship between the relative density of the sintered compact produced in Comparative Examples 1-30, and thermal conductivity (kappa). It is a figure which shows the relationship between the relative density of the sintered compact produced in Comparative Examples 1-30, and the figure of merit. It is a figure which shows the relationship between the relative density of the sintered compact produced in Examples 1-14, and the figure of merit. It is an example of the cross-sectional conceptual diagram of a discharge plasma sintering machine.

Explanation of symbols

1. 1. Pulse current Electrode 3. Carbon plate 4. 4. Graphite punch Sample (space to accommodate)
6). Graphite die 7. electrode

  The present invention will be specifically described below with reference to examples and comparative examples. However, the present invention is not limited to the examples.

Examples 1-14
≪Preparation of raw material mixture≫
Zinc oxide powder (average particle size: 0.2 μm, manufactured by HakuSitec) and γ alumina powder (average primary particle size: 0.02 μm, manufactured by Asahi Kasei) were prepared.

A raw material mixture was prepared by wet-mixing zinc oxide powder and γ-alumina powder so as to have the composition shown in Table 1 (Al ₂ O ₃ amount), drying, and further mixing with a dry ball mill.
≪Discharge plasma sintering≫
The prepared raw material mixture was housed in a graphite die left by discharge plasma (Dr. Sinter, manufactured by Sumitomo Coal Mining Co., Ltd.) to obtain a green compact. Also, the internal atmosphere gas was replaced with argon.

  Next, the raw material mixture was heated to 800-1100 ° C. (individually shown in Table 1) at a heating rate of 50 ° C./min by applying a direct current pulse current under pressure at 30 MPa, and the temperature was maintained. However, the discharge plasma sintering was performed for the time shown in Table 1 below (1 to 5 minutes), and then cooled at a rate of temperature decrease of 100 ° C./min until the temperature became below 600 ° C.

  By grinding and mirror polishing the surface of the obtained sintered body “referred to as SPS-ZnO (Al)”, 14 types of test bodies were obtained.

[In Table 1, sintering method SPS (S) indicates discharge plasma sintering]
Comparative Examples 1-30
≪Preparation of raw material mixture≫
A raw material mixture was prepared in the same manner as in the Examples except that the formulation shown in Table 2 was used.
≪Normal pressure sintering≫
The green compact of the raw material mixture (the same green compact as in the example) was placed in an atmospheric furnace, the sintering temperature was 1000 to 1400 ° C. (shown individually in Table 2), and the sintering time was as shown in Table 2 below. (2 to 24 hours), and a sintered body "PL-ZnO (Al)" was produced.

  Thirty kinds of test bodies were obtained by grinding and mirror polishing the surface of the obtained sintered body.

[In Table 2, normal pressure (P) indicates normal pressure sintering]
Test example 1
About each test body produced by the Example and the comparative example, the crystal phase was identified by XRD (the Rigaku Electric make, RINT2500VHF), and the relative density was computed by the Archimedes method. Moreover, the fine structure was observed with an electron microscope (manufactured by Hitachi, Ltd., model S-4200), and the average particle size was calculated.

  In order to evaluate the thermoelectric characteristics of each specimen, the electrical conductivity σ was measured by a direct current four-terminal method (manufactured by Mitsubishi Chemical Co., Ltd., MCP-T600). Further, the thermal conductivity κ was measured by a laser flash method (manufactured by ULVAC-RIKO, TC-7000 type), and a figure of merit Z was calculated.

These test results are also shown in Tables 1 and 2. Further, Table 1 and the identification code shown in the right end of Table ₂ (Al 2 O ₃ amount and sintering time are the same ones the same reference numerals) with the respective characteristics (sigma, kappa, Z) to FIGS As shown in the figure.

Discussion of Results of Test Example 1 The relationship between the relative density of SPS-ZnO (Al) and the conductivity σ is shown in FIG. Further, the relationship between the relative density and the thermal conductivity κ is shown in FIG. It can be seen that in both cases where the addition amount of γ-alumina is 1% by weight and 2% by weight, it has a high conductivity σ in the range of a relative density of 70 to 100%. Moreover, it turns out that thermal conductivity (kappa) decreases with the fall of a relative density, and reduces to about 1/3 of dense zinc oxide in the lowest case. These results indicate that the discharge plasma sintering method can achieve both low thermal conductivity and high conductivity.

  The relationship between the relative density of PL-ZnO (Al) and the conductivity σ is shown in FIG. Moreover, the relationship between relative density and thermal conductivity (kappa) is shown by FIG. It can be seen that the normal pressure sintering method shows high conductivity when the relative density is near 100%, but no conductivity is obtained when the relative density is 90% or less. On the other hand, it has been found that the thermal conductivity decreases monotonously with a decrease in relative density, and is almost halved when the relative density reaches 80%. These results indicate that the normal pressure sintering method cannot achieve both low thermal conductivity and high conductivity.

  Further, as apparent from the comparison between FIG. 5 (Example) and FIG. 6 (Comparative Example), the sintered body of the present invention gives a clearly high figure of merit Z when the relative density is 90% or less. In particular, a raw material mixture is prepared so that the alumina content is substantially 1% by weight, and the green compact of the raw material mixture is subjected to spark plasma sintering at a sintering time of 900 to 950 ° C. for 5 minutes (4 to 5 minutes). It can be seen that a very good figure of merit Z is obtained.

  The following mechanism can be considered as the reason why the spark plasma sintering method shows high conductivity (despite low thermal conductivity). That is, in the aluminum-containing zinc oxide, the added alumina reacts with the zinc oxide to produce “spinel”. In normal pressure sintering, almost the same amount of spinel is generated in both low temperature sintering (1000 ° C.) and high temperature sintering (1400 ° C.), and most of the added alumina is used to generate spinel as an insulator. The amount of solid solution of alumina in zinc oxide that is consumed and contributes to conductivity is limited. On the other hand, in the spark plasma sintering method, the formation of spinel is suppressed in the region where the sintering temperature is low, and the added alumina is effectively dissolved in zinc oxide, so that the densification is not sufficiently advanced. High conductivity even in the state. Such a phenomenon is considered to occur because the discharge plasma sintering can be performed in a high-pressure environment and can be heated and cooled at an ultra-high speed.

Claims (4)

A method for producing an aluminum-containing zinc oxide sintered body having the following steps in order:
(1) Step 1 of preparing a raw material mixture containing zinc oxide and alumina, the alumina content being 2% by weight or less of the total amount of zinc oxide weight and alumina weight,
(2) Step 2 of heating the green compact made of the raw material mixture to 800 to 1100 ° C. at a temperature rising rate of 50 ° C./min or more,
(3) Discharge plasma in 10 minutes or less while pressurizing the heated green compact at 40 MPa or less in an inert gas having a gas pressure of 0.05 to 0.13 MPa and maintaining the temperature. Step 3 for sintering, and (4) Step 4 for lowering the temperature of the sintered body obtained by the discharge plasma sintering at a temperature lowering rate of 50 ° C./min or more until at least 600 ° C. Alumina content of the raw material mixture is 1-2% by weight of the total amount of zinc by weight alumina by weight oxide, the production method according to claim 1. The manufacturing method of Claim 1 or 2 whose zinc oxide is a powder and whose average particle diameter of the said powder is 2 micrometers or less. Alumina is a powder, the average particle diameter of the powder is 5 nm to 2 [mu] m, method according to any one of claims 1-3.

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