JP2003282963A - Thermoelectric conversion material made of palladium oxide and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermoelectric conversion material which can easily control a type and a concentration of a carrier and thermoelectric characteristics in association with the type and the concentration by an element replacement and which can form both a P-type material and an N-type material without unstable element such as Na in the material usable in a wide temperature range of a liquid nitrogen temperature (-196°C) to about 300°C. <P>SOLUTION: The thermoelectric conversion material contains a palladium oxide represented by the formula: A<SB>1-x</SB>B<SB>x</SB>Pd<SB>3</SB>O<SB>4</SB>(wherein A is Ca, Sr or Ba, B is Li, Na, K, Sc, Y, La, And, Sm, Eu, Gd, Dy, Er, Ho or Yb, and x is 0<x≤1). The material is different from ACo<SB>x</SB>O<SB>y</SB>which can form only a P-type, and can separately form an N-type and the P-type according to the type of the component B of the formula. Excellent thermoelectric characteristics in which an absolute value of a thermoelectromotive force at the ambient temperature is 50 μV/K or more or a power factor of 0.5 μW/cmK<SP>2</SP>or more can be obtained by selecting the components A and B and a value of x. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a thermoelectric conversion material using a palladium metal oxide and a method for producing the same, and more specifically, to a liquid nitrogen temperature (−196 ° C.) or higher to 3 or higher.
The present invention relates to a thermoelectric conversion material composed of a palladium metal oxide that can be used over a wide temperature range up to about 00 ° C., and a method for producing the thermoelectric conversion material.

[0002]

2. Description of the Related Art Thermoelectric power generation using a thermoelectric conversion material (thermoelectric power generation) is a Seebeck effect, that is, two different metals or different thermoelectric conversion materials such as a p-type semiconductor and an n-type semiconductor are thermally arranged in parallel. It is a technology that directly converts thermal energy into electric power by utilizing the thermoelectric effect in which thermoelectromotive force is generated at both ends when they are electrically connected in series and a temperature difference is applied between the joints. Since a current flows through the circuit by connecting a load to form a closed circuit and electric power can be taken out, it is partially put to practical use as a remote power source, a space power source, a military power source, and the like.

Up to now, various materials have been synthesized as candidates for thermoelectric conversion materials, but none have been found that greatly exceeds the dimensionless figure of merit ZT = 1. In particular, any thermoelectric conversion material effective in a low temperature region, that is, a temperature region near room temperature has a problem that the performance index has a large temperature dependency. For example, p-Bi ₂ Te ₃ (55) + Sb
₂ Te ₃ (45) is an excellent thermoelectric conversion material, but the temperature range showing good characteristics is very narrow, around 300K.

So far, the Z value is the maximum, and a typical thermoelectric conversion material used for industrial applications is a Bi ₂ Te ₃ system material, but this material has a low melting point and an effective temperature range of 3
Since it is around 00K, it cannot be used in a high temperature range of 300 ° C or higher. For this reason, there is a problem that the temperature difference, which is the driving force that causes the Seebeck effect, cannot be made large, and the thermoelectric conversion efficiency is limited to 5 to 6%.

Further, the price of Te, which is a constituent element, is rather high, and moreover, a toxic element such as Sb is required as a dopant thereof, so that attention must be paid to toxicity in production and use thereof. In addition, there is a problem that it is not preferable from the point of view of environmental impact when the product is discarded after use.

[0006] Therefore, the present inventors have
Eliminates the human toxicity and cost problems described above
As a thermoelectric conversion material having an improved Z value,
ACo_xO_y(In the formula, A is Li, Na or K, and x
Is 1 ≦ x ≦ 2, and y is 2 ≦ y ≦ 4)
Thermoelectric conversion material consisting of substance and elemental composition formula (A _ZB
_1-Z) Co_xO_y[In the formula, A is Li, Na or K, B
Is Mg, Ca, Sr, Ba, Sc, Y, Bi or Te
Where z is in the range 0 <z <1, and x is 1 ≦ x ≦
2, y is 2 ≦ y ≦ 4]
A thermoelectric conversion material was developed (JP-A-9-321346)
News).

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The thermoelectric conversion material described in Japanese Patent No. 346 has a performance index Z
It has a relatively high value, and has high thermoelectric conversion characteristics over a wide temperature range from liquid nitrogen temperature to 650 ° C or higher, and it can be used stably, and the physical property values in that temperature range are also high. Although it is almost constant and has excellent physical properties, it is difficult to control its thermoelectric properties by element substitution, etc., and non-uniformity of Na in the Na layer and solid solution with other elements impede electrical conduction and deteriorate thermoelectric properties. I will end up. Further, the above-mentioned material is a P-type material showing a positive thermoelectromotive force, and an N-type material cannot be made even by using this system.

Therefore, the present invention is based on the liquid nitrogen temperature (-
It is a thermoelectric conversion material that can be used over a wide temperature range from 196 ° C.) to about 300 ° C., and the type and concentration of carriers and the thermoelectric properties associated therewith can be easily controlled by element substitution, and it is possible to use such materials as Na It is an object of the present invention to provide a material which does not have a stable element and which can be made into both a P-type material and an N-type material.

[0009]

In order to produce both a P-type material and an N-type material, a degeneration having an energy gap with a band gap of 0.2 electron volt or less, as in conventional thermoelectric conversion materials. Semiconductors must be realized with oxides. Further, it is necessary that both the valence band in which P carriers conduct and the conduction band in which N carriers conduct are relatively wide. As a result of various trial and error, the inventors of the present invention have tried to realize such a material by the formula CaPd.
It was found that a complex palladium oxide represented by ₃ O ₄ , SrPd ₃ O ₄ , or BaPd ₃ O ₄ is a material that meets such conditions.

That is, the present invention relates to the composition formula A _1-x B _x
Pd ₃ O ₄ (A is Ca, Sr, or Ba,
B is Li, Na, K, Sc, Y, La, Nd, Sm,
Eu, Gd, Dy, Er, Ho, or Yb, and x
Is a palladium oxide represented by 0 <x ≦ 1), which is a thermoelectric conversion material.

The present invention also provides a composition formula A _1-x B _x Pd.
_{In 3} O ₄ (where x is 0 <x ≦ 1), A is C
a or Sr, B is Li, Na, or K, and is a P-type material, which is a thermoelectric conversion material. Further, the present invention is a composition formula _{A 1-x B x Pd 3} O 4 ( here, x is, 0 <x ≦ 1) in, A is Ca or S
r, B are Sc, Y, La, Nd, Sm, Eu, Gd, D
The thermoelectric conversion material is y, Er, Ho, or Yb, and is an N-type material.

Further, the present invention provides the compositional formula A by firing.
_{1-x B x Pd 3 O} 4 ( provided that, A is, Ca, Sr or Ba,, B is, Li, Na, K, Sc , Y, La, N
d, Sm, Eu, Gd, Dy, Er, Ho, or Yb
Where x is a raw material in which components are mixed so that 0 <x ≦ 1), the flux is mixed and calcined, then the flux is removed, and firing is performed at 830 ° C. to 900 ° C. And a method for producing a P-type thermoelectric conversion material. Further, in the present invention, the flux is an alkali halide,
The method for producing a thermoelectric conversion material is characterized in that the calcination temperature is 800 to 830 ° C.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION A thermoelectric conversion material comprising a composite oxide of the present invention has a composition formula A _1-x B _x Pd ₃ O ₄ (provided that
A is Ca, Sr, or Ba, B is Li, Na,
K, Sc, Y, La, Nd, Sm, Eu, Gd, Dy,
Er, Ho, or Yb, and x is represented by 0 <x ≦ 1). This composite oxide has a NaPt ₃ O ₄ type crystal structure.

In the composition formula, A component is replaced with Li and N of B component.
A P-type material can be made by substituting a or K. In addition, the A component is Sc, Y, La, N of the B component.
d, Sm, Eu, Gd, Dy, Er, Ho, or Yb
An N-type material can be produced by substituting with. In the above general formula, the value of x is 1 or less, but both the resistivity and the thermoelectromotive force decrease as the value of x increases, so x is more preferably 0.2 or more and 0.5 or less.

The thermoelectric conversion material of the present invention can be obtained by mixing the raw materials in a predetermined mixing ratio and firing in an oxidizing atmosphere. The raw material is not particularly limited as long as it can form the target complex oxide by firing, and various compounds such as simple metal, oxide, and carbonate can be used. As the Pd source, oxides, nitrates, chlorides, hydroxides, organometallic compounds, etc. that can form Pd oxides by firing can be used. Also for the components A and B, oxides, hydroxides, chlorides, carbonates, nitrates, organic metal salts and the like can be used.

The firing means is not particularly limited, and an electric heating furnace,
Firing is performed in an oxidizing atmosphere such as an oxygen stream or air in a gas heating furnace or the like. For firing temperature and firing time,
For the N-type material, it may be normally fired at about 900 to 1100 ° C. for about 24 to 48 hours.

However, this composition formula A _1-x B _x P
In d ₃ O ₄ , L at Ca, Sr, or Ba site
If a P-type material is produced by substituting i, Na, or K, PdO of the raw material or Pd generated from the raw material is heated at a temperature higher than 900 ° C. for firing the raw material.
O is reduced, Pd is deposited, and Li and Na are hardly solid-dissolved, so that a substance having a desired composition cannot be obtained. In addition, the resistivity increases because the metal Pd is precipitated as an impurity. In order to prevent this, it is necessary to maintain the calcinability while lowering the reaction temperature.

The present inventor has found that a desired material can be fired by lowering the firing temperature by mixing the flux with the raw material and calcining. That is, as a flux, an alkali halide such as NaCl or KCl is used as a water-soluble flux, and the raw material and the flux are mixed at a weight ratio of about 1: 2.
Calcination at ~ 830 ° C. If the temperature is lower than 800 ° C, the reaction of the raw materials does not proceed. Particularly, when carbonate is used as a raw material, carbonic acid is not released. If the temperature exceeds the upper limit, the flux will evaporate and the reaction will not proceed. The flux remaining after the calcination is removed in a solvent that dissolves only the flux before the main firing. For example, the flux can be easily removed by using water as a solvent and roasting with water.

The subsequent main firing temperature is 830 ° C to 900 ° C.
Is preferred. When the temperature is lower than the lower limit temperature, the sintering does not proceed sufficiently, the mechanical strength of the sintered body decreases, the grain boundary resistance increases, and the characteristics deteriorate. Metal Pd when the upper limit temperature is exceeded
Is deposited. Thus, by preventing the precipitation of the metal Pd, which is an impurity, by the solid-phase reaction method using an alkali halide as a flux, a high-quality P-type material can be manufactured. The produced P-type material exhibits thermoelectric performance comparable to that of the oxide thermoelectric conversion material ACoxOy, which has the highest performance so far.

The thermoelectric conversion material of the present invention has a composition formula A _1-x B, unlike ACoxOy, which can only produce conventional P-type materials.
_{In x} Pd ₃ O ₄ , it is a new material in which N type and P type are made different depending on the type of B component, and the absolute value of the thermoelectromotive force at room temperature is selected by selecting A component and B component and the value of x. Value is 50μV / K or more or power factor is 0.5μW
It is possible to obtain excellent thermoelectric properties of / cmK ² or more.

[0021]

[Example] (Example 1) CaCO ₃ , L having a purity of 3N as a raw material
i ₂ CO ₃ and PdO were mixed with a mortar and a pestle for 30 minutes or more together with 5N NaCl as a flux and the raw materials were weighed so that the total weight was 4 g to match the composition ratio. I matched it. Then, it was calcined at 800 ° C for 24 hours. The calcined sample was put into pure water and roasted in hot water to remove the flux. Then, it was shaped into pellets by a pressing machine.
Finally, the main firing was performed. The firing conditions at this time were 48 hours at 950 ° C for the sample with x = 0, and 12 hours at 830 ° C for the other samples.
It was time.

In powder X-ray diffraction, an Fe tube was used as the X-ray source, the divergence slit and the scattering slit were both 0.5 deg, the light receiving slit was 0.15 mm, and the scan speed was 8 de.
The measurement was performed in the range of 2θ of 10 deg to 120 deg as g / min.
Prior to electrical measurement, the finished sample was shaped to fit a sample holder for measurement. Specifically, the shape of the disk-shaped sintered body sample was cut with a cutter and sandpaper to a width of 1.5.
mm, length 10 mm, thickness 0.5 mm.

For the resistivity measurement, the 4-terminal method was used so that the contact resistance, the resistance of the measurement line, etc. would not be superimposed on the measured resistance. Copper wires were used for the terminals. At that time, silver paste was used for electrical contact between the terminal and the sample. The measurement was performed from 4.2 K to 300 K. In the experiment, the sample mounted on a dedicated sample holder was cooled in a liquid helium cryostat. At that time, a current of 1 mA was passed from the constant current source to the sample, the voltage at this time was read with a nanovolt meter, the direction of the current was reversed, and the voltage was read again. A cell Knox thermometer was used to measure the temperature of the sample, and the measurement was performed at a temperature interval of 0.5 K.

The thermoelectromotive force is 4.2 to 300 K according to the stationary method.
Was measured up to. A rectangular parallelepiped sample was attached between two copper plates facing each other with silver paste (Dupont 4922N), and one of the copper plates was heated to a sheet resistance to give a temperature difference of 0.5-1K. The temperature difference was measured using a copper-constantan differential thermocouple, and the temperature of the sample was measured using a Cellnox thermometer at a temperature interval of 2-3 K. The output voltage of the thermocouple and the thermoelectromotive force of the sample were read with a nanovoltmeter.

FIG. 1 shows the temperature dependence of the resistivity ρ and the thermoelectromotive force (Seebeck coefficient) S of the obtained Ca _1-x Li _x Pd ₃ O ₄ .
It can be seen that S and ρ decrease as the value of x increases, and carriers are injected together with Li. FIG. 2 shows the temperature dependence of the power factor of the obtained Ca _1-x Li _x Pd ₃ O ₄ . The power factor S ² / ρ reaches 1 μW / cmK ² at room temperature, which is comparable to that of Na-Co-O. Figure 3 shows the obtained Ca _1-x Li _x Pd ₃ O.
₄ The X-ray diffraction pattern (B) of the thermoelectric material was prepared by the usual solid-phase reaction method shown in Comparative Example 1 below, and was Ca _1-x Li _x Pd ₃ O ₄ (A).
It is a graph shown in comparison with. As shown in FIG. 3B, in the sample using the flux, metal Pd which is an impurity
(Indicated by * in FIG. 3A) is decreasing.

Comparative Example 1 In Example 1, the flux was not used and the calcination conditions were set to 95.
The synthesis was carried out at 0 ° C for 36 hours and main firing at 950 ° C for 36 hours.
FIG. 4 shows the temperature change of the resistivity of the obtained Ca _1-x Li _x Pd ₃ O ₄ . It can be seen that the metal Pd was deposited and the resistivity increased.

Example 2 Firing was carried out under the same conditions as in Example 1 except that Na ₂ CO _{3 was} used instead of Li ₂ CO _{3 which} was the raw material of Example 1. FIG. 5 shows the temperature dependence of the resistivity ρ and the thermoelectromotive force (Seebeck coefficient) S of the obtained Ca _1-x Na _x Pd ₃ O ₄ . It can be seen that similar characteristics can be obtained by using Na instead of Li.

Example 3 Firing was carried out under the same conditions as in Example 1 except that SrCO ₃ was used as a raw material for Sr instead of CaCO ₃ . FIG. 6 shows the temperature dependence of the thermoelectromotive force (Seebeck coefficient) S of the obtained Ca _0.8 Sr _0.2 Pd ₃ O ₄ in comparison with CaPd ₃ O ₄ . Basically similar characteristics were obtained even when Sr was used instead of Ca.

Example 4 Firing was carried out under the same conditions as in Example 1 except that Sc ₂ O ₃ , Y ₂ O ₃ or La ₂ O ₃ was used in place of Li ₂ CO _{3 which} was the raw material of Example 1. did. Fig. 7 shows the obtained Ca _0.8 X.
The temperature change of the resistivity of _0.2 Pd ₃ O ₄ is shown. Sc, Y, or La
It can be seen that the temperature change of the resistivity is suppressed by the substitution of trivalent ions such as and electrons are doped.

(Example 5) In Example 4, calcination was performed for 36 hours at a temperature of 950 ° C without using flux, and main firing was performed for 9 hours.
It was carried out at 50 ° C for 36 hours. Other conditions were the same as in Example 4. FIG. 8 shows the temperature dependence of the thermoelectromotive force of the obtained Ca _0.8 X _0.2 Pd ₃ O ₄ . Especially for the composition of X = La, it is −50 μV at room temperature.
It was found to have a thermoelectromotive force of / K or more, and an N-type material with an absolute value of thermoelectromotive force of 50 μV / K or more was obtained.

[0031]

The composition formula of the present invention A _1-x B _x Pd ₃ O ₄
According to the composite palladium oxide represented by, it is possible to easily control the type and concentration of the carrier and the thermoelectric characteristics associated therewith by element substitution, do not have an unstable element such as Na, and
It is possible to provide a material that can be used as both a mold material and an N-type material, and the liquid nitrogen temperature (-196 ° C) or higher to 3
It is possible to provide a thermoelectric conversion material that can be used over a wide temperature range up to about 00 ° C.

[Brief description of drawings]

FIG. 1 is a graph showing the temperature dependence of resistivity ρ and thermoelectromotive force (Seebeck coefficient) S of the Ca _1-x Li _x Pd ₃ O ₄ thermoelectric conversion material obtained in Example 1.

FIG. 2 is a graph showing the temperature dependence of the power factor of the Ca _1-x Li _x Pd ₃ O ₄ thermoelectric conversion material obtained in Example 1.

FIG. 3 compares the X-ray diffraction pattern (B) of the Ca _1-x Li _x Pd ₃ O ₄ thermoelectric conversion material obtained in Example 1 with a sample (A) prepared by a usual solid-phase reaction method. It is a graph shown.

FIG. 4 is a graph showing the temperature change of the resistivity of Ca _1-x Li _x Pd ₃ O ₄ obtained in Comparative Example 1.

5 is a graph showing the temperature dependence of the resistivity ρ and thermoelectromotive force (Seebeck coefficient) S of the Ca _1-x Na _x Pd ₃ O ₄ thermoelectric conversion material obtained in Example 2. FIG.

FIG. 6 is a graph showing temperature dependence of thermoelectromotive force of thermoelectric conversion materials of Ca _0.8 Sr _0.2 Pd ₃ O ₄ obtained in Example 3 and CaPd ₃ O ₄ obtained in Example 1.

FIG. 7 is a graph showing the temperature change of the resistivity of the thermoelectric conversion material of Ca _0.8 X _0.2 Pd ₃ O ₄ obtained in Example 4.

FIG. 8 is a graph showing the temperature dependence of the thermoelectromotive force (Seebeck coefficient) S of the Ca _0.8 X _0.2 Pd ₃ O ₄ thermoelectric conversion material obtained in Example 5.

Claims (5)

[Claims] 1. A composition formula A _1-x B _x Pd ₃ O ₄ (where A is Ca, Sr, or Ba, and B is Li, N).
a, K, Sc, Y, La, Nd, Sm, Eu, Gd, D
y, Er, Ho, or Yb, and x is 0 <x ≦
A thermoelectric conversion material comprising a palladium oxide represented by 1). 2. A is Ca or Sr, B is Li, Na,
The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material is K or P-type material. 3. A is Ca or Sr, B is Sc, Y, L
The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material is a, Nd, Sm, Eu, Gd, Dy, Er, Ho, or Yb and is an N-type material. 4. The composition formula A _1-x B _x Pd ₃ O is obtained by firing.
₄ (However, A is Ca, Sr, or Ba, B is L
i, Na, K, Sc, Y, La, Nd, Sm, Eu, G
d, Dy, Er, Ho, or Yb, and x is 0 <
x is mixed with a raw material in which the components are mixed so that x ≦ 1), and the mixture is calcined and then the flux is removed.
The method for producing a thermoelectric conversion material according to claim 2, wherein the firing is performed at 0 ° C or higher and 900 ° C or lower. 5. The flux is an alkali halide,
The method for producing a thermoelectric conversion material according to claim 4, wherein the calcination temperature is 800 to 830 ° C.