JP3051922B1 - Oxide members for thermoelectric conversion elements - Google Patents

Abstract

An oxide having stability in a high-temperature atmosphere, which is an advantage of an oxide, and exhibiting high thermoelectric performance even at around room temperature, and having a simple manufacturing process from a practical viewpoint. An oxide member for a conversion element is provided. SOLUTION: An oxide A _x Ni _1-x O (where 0 ≦ x ≦
0.4, A is Li, Na, and a combination thereof), and a thermoelectric performance index of about 0.2 × 10 ⁻⁴ K ⁻¹ at about 200 ° C. and 800 to ¹
An oxide member for a thermoelectric conversion element having a thermoelectric performance index of 0.7 × 10 ⁻⁴ K ⁻¹ or more at 000 ° C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a thermoelectric conversion element for oxides member or the like, more particularly A _x N
i _1-x O (where 0.01 < x ≦ 0.4, A is Li, N
a or a combination thereof) based oxide member for a thermoelectric conversion element, which has a thermoelectric performance index of about 1 × 10 ⁻⁴ K ⁻¹ in a temperature range from room temperature to about 1000 ° C. and is stable in a high-temperature atmosphere. The present invention relates to a novel oxide member for a thermoelectric conversion element whose manufacturing process is simpler than existing non-oxide thermoelectric conversion materials.

[0002]

2. Description of the Related Art At present, it is said that about 2/3 of the total supplied energy is discarded as waste heat, and if a part of these can be recovered, the energy use efficiency should be improved.
Thermoelectric conversion materials are materials that directly convert heat energy into electric energy, and thermoelectric power generation using this material is expected as a distributed energy conversion system suitable for effective use of waste heat. In order to meet these requirements, a thermoelectric conversion material having excellent characteristics is required. The figure of merit (Z) of the thermoelectric conversion material is given by the following equation. Z = σα ² / λ where α is the thermoelectromotive force (Seebeck coefficient), σ is the electrical conductivity, and λ is the thermal conductivity. As a material having high conversion efficiency, it is necessary that | α | and σ are both large (semiconductor-like) and λ is small. Σα ² is also defined as an output factor.

[0003] Of the various types of waste heat, when waste heat from a garbage incinerator, a household gas appliance, or the like is used, the temperature is 400 ° C in the atmosphere.
Since the thermoelectric generator is used in the above environment, the thermoelectric converter is required to have oxidation resistance. For this reason, when using an existing alloy-based thermoelectric conversion material, it is necessary to apply an antioxidant coating to the thermoelectric conversion material. For this reason, as a new development that meets the demand for oxidation resistance, we have discovered a new material that has performance equal to or better than existing high-temperature thermoelectric conversion materials in oxide systems that are stable in high-temperature air. Recently, attention has been paid to research into devices.

At present, through various research and development in Japan, the performance index of the same level as that of the existing non-oxide-based iron disilicide (FeSi ₂ ) -based thermoelectric conversion material has been obtained even with various oxide materials ( Norimitsu Murayama, Kunihito Kawamoto, Ceramics 3
3 (1998) 161). The oxides of particular interest among them are (BaSr) PbO and NaCo ₂ O ₄ . Both perform the best among the oxides, each with n
It is a promising thermoelectric material candidate for p-type and p-type. But 7
Stability at a high temperature of 00 ° C. or more is a problem. 1000 ° C
As a stable material in the vicinity, there is a ZnO-based material, but judging from past research reports, there is no possibility that the material system can be developed beyond the highest performance so far, and the problem that the performance decreases at low temperatures There is.

[0005]

Under these circumstances, the present inventors have studied the possibility of using an electrically conductive oxide as a thermoelectric material in view of the above prior art, Examination of metal oxides revealed that nickel oxide
Focusing on the fact that the conductivity is increased by adding an alkali element, the inventors have found that they are useful as oxide members for thermoelectric conversion elements, and have completed the present invention. An object of the present invention is to provide an existing typical thermoelectric oxide, FeS
It has the same level of thermoelectric figure of merit as i ₂ , has stability in high-temperature atmosphere, which is an advantage of oxides, and shows high thermoelectric performance near room temperature. From a practical viewpoint, Another object of the present invention is to provide an oxide member for a thermoelectric conversion element whose manufacturing process is simple.

[0006]

According to the present invention, there is provided a compound having the following structure: A _x Ni _1-x
O (however, 0.01 <x ≦ 0.4, A is Li or Na,
Or a mixture of Li and Na), from room temperature to 100
An acid having a thermoelectric performance index of about 1 × 10 ⁻⁴ K ⁻¹ at around 0 ° C.
Thermoelectric conversion element comprising a nitride member. The present invention also provides an oxide member comprising the above compound,
When the thermoelectric figure of merit is 0.2 × 10 ^-4 K ^-1 or more around ℃, 8
The thermoelectric figure of merit is 0.7 × 10 ⁻⁴ K ⁻¹ at 00 to 1000 ° C.
Thermoelectric conversion elements and components of the above oxides member, an oxide member made of the compound, the output factor 20
0 ℃ near at ^{0.54 × 10 -4 Wm -1 K -2} , the components of 2.5 × 10 ^-4 Wm ^-1 K ^-2 or more members in the vicinity of 1000 ° C.
Thermoelectric conversion elements, oxides member is to desirable embodiments the thermoelectric conversion element, of the which is a compound of the crystal structure of the rock salt structure. Furthermore, the present invention is a thermoelectric conversion element for thermoelectric generation or thermoelectric cooling, characterized by comprising a compound having the above structure as a constituent element.

[0007]

Next, the present invention will be described in more detail. Examples of compounds suitably used in the present invention include L
i _x Ni _1-x O or Na _y Li _z Ni _1-yz O (however,
x: about 0.024, y: about 0.1, z: about 0.01).

The thermoelectric properties of the A _x Ni _1-x O system have been actively studied since 1950 in order to understand the conduction mechanism of the oxide NiO.
The idea that substitutional nickel oxide is band conductive is likely. Based on the theory of band conduction, the higher the electrical conductivity, the logarithmically decreases its thermoelectromotive force, but in this system, the thermoelectromotive force at high temperatures is relatively large without decreasing with the increase in electrical conductivity. The output factors are shown. But,
This system has never been considered as an oxide member for a thermoelectric conversion element.

The oxide member for a thermoelectric conversion element of the present invention has been found to be able to increase the electric conductivity by increasing the amount of A (Li and / or Na) as an alkali element and to obtain a large output factor. It was done. By gradually increasing the amount of Li, the thermoelectric characteristics of the LiNiO-based material can be improved, and the thermoelectric characteristics on the low temperature side can be improved. However, if the amount of Li exceeds a certain amount, the conductivity is improved, but the thermoelectromotive force is reduced, and as a result, the output factor is reduced. The lower the thermal conductivity is, the better. However, even if the amount of Li exceeds this amount, the thermal conductivity does not decrease and the final value of Z becomes small. The range of x of the alkali element A specified its preferable range as described above. It is significant that a thermoelectric figure of merit of about 1 × 10 ⁻⁴ K ⁻¹ can be achieved by simply substituting a small amount of Li for nickel oxide. The oxide member for a thermoelectric conversion element of the present invention has a temperature of 1 to room temperature.
It has a thermoelectric figure of merit of about 1 × 10 ^-4 K ^-1 at around 000 ° C, is stable in a high temperature atmosphere, has a simpler manufacturing process than existing non-oxide thermoelectric materials, and can be mass-produced It has special advantages such as Further, the above-mentioned figure of merit is almost the same level as that of non-oxide type FeSi ₂ which is an existing typical thermoelectric conversion material. The oxide member for a thermoelectric conversion element of the present invention, due to the above characteristics, for example,
It is used as a thermoelectric conversion element for thermoelectric generation using the following waste heat source. Example of exhaust heat source: exhaust gas from automobiles = 300 to 900 ° C., exhaust gas from refuse incineration facility 150 to 900 ° C., fuel cell steam 15
0-900 ° C.

[0010] The oxide member for a thermoelectric conversion element of the present invention has an advantage that the process is simpler than the existing non-oxide-based manufacturing method. This is because a vacuum is required or a fine structure needs to be controlled. Further, when used at a high temperature, a treatment for preventing deterioration due to reaction with oxygen in the atmosphere is included. These are the advantages of LiNiO being an oxide, and that it can be used even at 1200K or more compared to other oxide-based thermoelectric members (1000
K or less), which are often found in natural systems called NiO,
There are merits such as being non-toxic, highly safe, and inexpensive, and thus being an industrially useful material.

[0011]

Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. Example 1 (Production of sample) Mixing nickel oxide and lithium oxide
It was calcined at 50 ° C. for 6 hours and sintered at 1250 ° C. for 4 hours.
The relative density of the sample was around 70%. Li element is Ni
By substituting O for Ni sites, the crystal lattice constant of the sample was reduced, and it was confirmed by an X-ray diffraction pattern that the sample was a single phase. Since the amount of Li substitution is greatly different from the starting composition due to evaporation of Li during sintering of the sample, the amount of Li substitution after sintering was determined by ICP-AES method (0.2 g of H
Dissolved in 10 ml of an aqueous Cl solution). The thermoelectromotive force and the electrical conductivity were measured in the air at a temperature in the range of 200 to 1000 ° C., and the samples of the same batch were measured for the thermal conductivity from room temperature to about 1000 ° C. in a vacuum of about 0.1 Pa. The conversion figure of merit was determined. Preparation and measurement were performed in the same manner in the case of Na substitution. That is, in the case of Na substitution, nickel oxide and sodium carbonate were mixed and calcined at 770 ° C. for 10 hours so that all of the mixed starting materials were dissolved. Subsequent sintering is the same as for Li at 1250 ° C for 4 hours.
Sintered for hours. When Na and Li are dissolved simultaneously,
First, nickel oxide and sodium carbonate are mixed, and 770
After calcining at 10 ° C for 10 hours,
It was calcined at 50 ° C. for 6 hours and sintered at 1250 ° C. for 4 hours.
In such a production method, the amount of solid solution of Li greatly depends on the process, but in the case of Na, the final amount of solid solution can be determined by solid solution at low temperature calcination.

Example 2 (Results) FIG. 1 shows the temperature dependence of the electric conductivity and the thermoelectromotive force of the sintered body. Pure NiO is an insulator by nature, but Li
And the like can be replaced with Ni to increase conductivity. However, according to the conventional semiconductor theory, the higher the electric conductivity, the logarithmically decreases the thermoelectromotive force.In this sample, the thermoelectromotive force increases with the temperature, despite the higher electric conductivity. The result was almost constant, so that a high output factor was obtained. FIG. 2 shows the temperature dependence of the thermal conductivity and the figure of merit. The value of thermal conductivity was 2-3 times lower than the value of NiO, and decreased with increasing temperature. The figure of merit exceeded 0.7 × 10 ⁻⁴ K ⁻¹ at temperatures above 800 ° C.
The same effect was obtained in the case of the sample in which Na was replaced. N
FIG. 3 shows the temperature dependence of the electric conductivity and the thermoelectromotive force of the sintered body sample to which Li was added simultaneously with the substitution of a. 0.25 × 10 ⁻⁴ near 1000 ° C., which is larger than the case where only Li is substituted.
An output factor of Wm ^-1 K ^-2 was obtained. It was confirmed that the thermoelectric conversion performance can be significantly improved only by controlling the alkali element of A.

The effects of the A substitution can be summarized into three parts: 1) a large output factor was obtained because the thermal electromotive force did not decrease while the electric conductivity was greatly increased. 2) The amount of A increased. Stable output is possible in a wide temperature range. 3) The thermal conductivity is reduced by substitution and the figure of merit can be increased.

[0014]

As described in detail above, the present invention relates to oxide A
x Ni1-x O (where 0.01 <x ≦ 0.4, A is Li
And / or Na) for a thermoelectric conversion element, which according to the present invention has a thermoelectric performance index of about 1 × 10 ⁻⁴ K ⁻¹ from room temperature to around 1000 ° C. and is stable in a high-temperature atmosphere. Therefore, an oxide member for a thermoelectric conversion element can be provided which has a simpler manufacturing process than existing oxide thermoelectric conversion materials and can be mass-produced. The above-mentioned oxide member for a thermoelectric conversion element has a figure of merit at the same level as FeSi ₂ which is an existing surface thermoelectric conversion material, and is useful as a thermoelectric conversion element for thermoelectric generation and a thermoelectric conversion element for thermoelectric cooling. It is.

[Brief description of the drawings]

FIG. 1 shows the temperature dependence of the electric conductivity and the thermoelectromotive force of a Li _x Ni _1-x O (x = 0.024) sintered body.

FIG. 2 shows the temperature dependence of the thermal conductivity and the figure of merit of a Li _x Ni _1-x O (x = 0.024) sintered body.

FIG. 3 shows Na _x Li _y Ni _1-xy O (x: about 0.1,
y: about 0.01) Indicates the temperature dependence of the electric conductivity and the thermoelectromotive force of the sintered body.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-170856 (JP, A) JP-A-7-272729 (JP, A) JP-A-6-333772 (JP, A) JP-A-2- 260581 (JP, A) JP-A-4-285052 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C01G 53/00 H01L 35/22 CA (STN)

Claims (5)

(57) [Claims] An oxide A _x Ni _1-x O (0.01 to 0.01)
<X ≦ 0.4, A is Li, Na or a combination thereof) , and a thermoelectric conversion element having an oxide member as a constituent element. 2. An oxide member comprising the compound according to claim 1, wherein the thermoelectric figure of merit at about 200 ° C. is 0.2 × 1.
An oxide member having a thermoelectric performance index of 0.7 × 10 ⁻⁴ K ⁻¹ or more at 800 to 1000 ° C. which is 0 ^-4 K ⁻¹ or more is a constituent element.
Thermoelectric conversion element. 3. An oxide member comprising the compound according to claim 1, wherein the power factor is 0.54 × 10 4 at around 200 ° C.
^-4 Wm ^-1 K ^-2 , 2.5 × 10 ^-4 Wm ^-1 around 1000 ° C
A thermoelectric conversion element comprising K- ² or more members . 4. oxide member thermoelectric conversion device according to any one <br/> of claims 1 to 3, characterized in that a compound of the crystal structure of rock salt structure. 5. A thermoelectric conversion element for thermoelectric generation or thermoelectric cooling, comprising the member according to claim 1 as a constituent element.