JP4380606B2 - N-type thermoelectric conversion material and thermoelectric conversion element - Google Patents

Description

  The present invention relates to a thermoelectric conversion material and a thermoelectric conversion element used for a thermoelectric power generation device using exhaust heat, and more particularly to a thermoelectric conversion material and a thermoelectric conversion element used for an n-type semiconductor.

  Currently, only a part of the enormous amount of heat energy generated from factories and automobiles is used, and most of it is discarded as waste heat. In response to growing environmental problems, various means for efficiently using various types of energy have been studied. One of them is the efficient use of exhaust heat.

  Conventionally, exhaust heat from large-scale facilities has been used, but in recent years, thermoelectric power generation that can efficiently use exhaust heat even on a small scale has attracted attention. Thermoelectric power generation is the Seebeck effect, that is, two different types of metals and different thermoelectric power generation materials such as p-type semiconductor and n-type semiconductor are placed in parallel and electrically connected in series, This is a technology that directly converts thermal energy into electric power by utilizing the thermoelectric effect that thermoelectromotive force is generated at both ends when a temperature difference is given. By connecting a load to a thermoelectric conversion element used for such thermoelectric power generation to form a closed circuit, a current flows through the circuit and electric power can be taken out. Conventionally, various constituent materials have been studied for improving the performance of such thermoelectric conversion elements.

In Patent Document 1, a zinc oxide-based composite oxide (Zn _1-x Al _x ) O ₁ ± δ (provided that a part of zinc oxide zinc is replaced with aluminum is a thermoelectric power generation material composed of a composite oxide of zinc oxide and alumina. The aluminum substitution amount x is 1>x> 0, and δ is a minute value). By replacing a part of zinc oxide with aluminum, in a wide temperature range of 0 to 1000 ° C., a high conductivity of 1000 S / cm, a high Seebeck coefficient of 100 to 200 μV / ° C. (n-type semiconductor thermoelectric conversion material) In this case, it is said that a thermoelectric power generation material showing the Seebeck coefficient becomes negative) is obtained.

Patent Document 2 relates to a semiconductor ceramic composition for producing a thermoelectric element for use in thermoelectric power generation, a high-temperature n-type thermoelectric element composition Zn _1-XY A _X B _Y O (based on ultrafine zinc oxide particles). However, A is a typical group 13 metal, B is lanthanum or nickel, X is 0.005 ≦ X <0.05, and Y is 0.005 ≦ Y <0.03). The performance of a thermoelectric element used for power generation using the Seebeck effect increases as the dimensionless figure of merit increases, so it can be said that the higher the Seebeck coefficient and electrical conductivity, the lower the thermal conductivity, the better. Among these dimensionless figure of merit factors, zinc oxide ultrafine particles having an average particle size of 200 nm or less that can be used at high temperatures are used for the base material in order to lower the thermal conductivity. It is said that a high-temperature n-type thermoelectric element composition having a large output factor can be obtained by adding a lanthanum compound or a nickel compound as a secondary additive to a system in which a group 13 typical metal compound is added as an impurity.

In addition to using fine particles as a base material, there is a method of increasing phonon scattering by precipitating a heterogeneous phase at the grain boundary.
JP-A-8-186293 JP 2001-284661 A

  In the first place, a thermoelectric conversion material made of a zinc oxide-based composite oxide has a problem that the thermal conductivity is high and the dimensionless figure of merit of the thermoelectric semiconductor is small.

  In the thermoelectric power generation material described in Patent Document 1, it is described that the Seebeck coefficient and the electrical conductivity are improved to increase the output factor, but the thermal conductivity is not reduced.

  On the other hand, the high-temperature n-type thermoelectric element composition described in Patent Document 2 uses an ultrafine zinc oxide particle to increase the crystal grain interface, thereby increasing phonon scattering and reducing thermal conductivity. It has been. However, Patent Document 2 does not show the measurement result of thermal conductivity, and even if other factors are referred to, the absolute value of the Seebeck coefficient is 100 to 150 μV / K at most, and the dimensionless figure of merit is sufficiently improved. I can't say that.

  The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and reduce the thermal conductivity while maintaining a sufficiently reduced resistivity (reciprocal of conductivity), thereby reducing the thermoelectricity having a high dimensionless figure of merit. It is to provide a conversion material.

The n-type thermoelectric conversion material according to the present invention is a thermoelectric conversion material represented by a composition formula (Zn _{1 -xy} Al _x Fe _y ) O, where x = 0.020 to 0.040, y = 0.005. The dimensionless figure of merit at 890 ° C. is 0.09 or more.

  A thermoelectric conversion element according to the present invention includes an n-type semiconductor element containing the n-type thermoelectric conversion material, a p-type semiconductor element, a common electrode to which one end of the n-type semiconductor element and one end of the p-type semiconductor are connected, and n And an electrode independently connected to the other end of the p-type semiconductor and the other end of the p-type semiconductor.

  In the thermoelectric conversion material of the present invention, Al is dissolved in Zn sites of ZnO to reduce resistivity, and Fe is further dissolved in Zn sites to increase phonon scattering, thereby reducing lattice thermal conductivity. Thereby, the dimensionless figure of merit can be improved.

  Moreover, the thermoelectric conversion element which can perform a thermoelectric power generation efficiently is obtained by comprising an n-type semiconductor element using this thermoelectric conversion material.

  Hereinafter, the present invention will be specifically described based on examples.

First, ZnO powder having an average particle size of 280 nm, α-Fe ₂ O ₃ powder, and colloidal γ-Al ₂ O ₃ were prepared as starting materials for (Zn _{1 -xy} Al _x Fe _y ) O. Each starting material was weighed and prepared so that the contents of Zn, Al, and Fe shown in FIG.

  The blended material was put into a pure water solvent, pulverized with a ball mill for 16 hours, wet mixed, and evaporated to dryness to obtain a mixed powder. The mixed powder was preformed with a press and further molded with an isostatic hydrostatic press of 250 MPa to obtain a molded body. The molded body was fired at 1400 ° C. for 1 hour in the air to obtain a sintered body of an n-type semiconductor element.

  The thermoelectric conversion element is configured by combining this n-type semiconductor element with a p-type semiconductor element. A common electrode is connected to one end of the n-type semiconductor element and one end of the p-type semiconductor element. On the other hand, independent electrodes are connected to the other end of the n-type semiconductor element and the other end of the p-type semiconductor element, respectively. In such a thermoelectric conversion element, when the common electrode is heated or the independent electrode at the other end of the n-type semiconductor element and the p-type semiconductor element is cooled, the p-type semiconductor element becomes n by the thermally excited carriers between the electrodes. The potential is higher than that of the type semiconductor element. When a load is connected between the electrode at the other end of the n-type semiconductor element and the electrode at the other end of the p-type semiconductor element, a current flows from the p-type semiconductor element to the n-type semiconductor element.

[Evaluation methods]
The resistivity, Seebeck coefficient, and thermal conductivity of the n-type semiconductor element made of the thermoelectric material (Zn _1-xy Al _x Fe _y ) O were measured at a predetermined temperature.

  The sample size is 3mm in length x 3mm in width x 12mm in length, and R thermocouple for temperature measurement is placed at 2.5mm from both ends of the sample (distance between terminals for temperature and voltage measurement is 7mm). The negative side (Pt line) of the R thermocouple was used for voltage measurement, and the R thermocouple was shared for temperature and voltage measurement. And the Pt electrode for electricity supply was arrange | positioned at the both ends of the sample. The sample was placed in a temperature bath set at 100 ° C. to 890 ° C., and a constant current of −5.0 A to +5. By the direct current four-terminal method at each measurement temperature with a thermoelectric property measuring apparatus RZ2001i-DM of Ozawa Scientific Co., Ltd. The voltage was measured in steps of 2.5 A up to 0 A, and the resistivity was calculated according to the following formula. ρ = (V / I) × (ab / c) (ρ: resistivity, V: voltage, I: current, a: length, b: width, c: distance between terminals).

  The Seebeck coefficient of the same sample as above was measured with the same measuring device. The sample was placed in a temperature bath set to 100 ° C. to 890 ° C., and a part of the sample was cooled so that a difference of 0 to 5 ° C. was obtained between the high temperature part and the low temperature part. The temperature of the high temperature part and the low temperature part was measured with an R thermocouple (Pt-Rh), and the electromotive force therebetween was measured with the Pt line of the R thermocouple. The Seebeck coefficient was calculated from S = V / (Th−Tl) (S: Seebeck coefficient, V: voltage, Th: high temperature part temperature, Tl: low temperature part temperature).

  The thermal conductivity was measured using an Nd glass laser and an R thermocouple (Pt-PtRh) with a laser flash method thermal constant measuring device TC7000 manufactured by ULVAC-RIKO. A sample having a diameter of 8 mm and a thickness of 2 mm was placed in a temperature bath set at 100 ° C. to 890 ° C., and specific heat and thermal diffusivity were measured. Thermal conductivity was calculated from κ = α · Cp · d (κ: thermal conductivity, α: thermal diffusivity, Cp: specific heat, d: density).

The output factor was calculated from the measured Seebeck coefficient and resistivity by P = S ² / ρ (P: output factor, S: Seebeck coefficient, ρ: resistivity).

The dimensionless figure of merit was calculated from the measured Seebeck coefficient, resistivity, thermal conductivity and temperature from ZT = (S ² T) / (ρκ) (ZT: dimensionless figure of merit, S: Seebeck coefficient, T: absolute temperature) , Ρ: resistivity, κ: thermal conductivity).

[result]
Table 1 and FIGS. 1 to 5 show the temperature characteristics of the resistivity, Seebeck coefficient, thermal conductivity, output factor, and dimensionless figure of merit of the prepared samples, respectively. Table 1 shows the thermoelectric characteristics of the manufactured sample at 890 ° C.

From Table 1, the sample numbers 9 and 16 of the comparative examples are (Zn _{1 -xy} Al _x Fe _y ) O thermoelectric conversion elements and do not contain Fe. The thermal conductivity is greater than 7 W / Km and the dimensionless figure of merit is less than 0.08. In contrast, Sample Nos. 12, 13, and 18 in which the Zn sites were replaced with Fe had a thermal conductivity of 7 W / Km or less, and a dimensionless figure of merit of 0.09 or more. These sample numbers 12, 13, and 18 have a lower dimensionless figure of merit due to lower thermal conductivity and lower resistivity compared to sample numbers 9 and 16 to which no Fe is added. Yes.

  FIG. 1 shows the relationship between the temperature and resistivity of a sample in which 0.020 mol of Al is substituted at the Zn site of ZnO and Fe is not added, and a sample in which Fe is substituted by 0.005 mol to 0.015 mol. A sample in which only Al is substituted at the Zn site has a low resistivity on the low temperature side, but the resistivity increases as the temperature increases. On the other hand, the sample in which Al and Fe are substituted at the Zn site shows a tendency that the resistivity decreases as the temperature increases.

FIG. 6 shows a powder X-ray diffraction diagram (XRD analysis) of the same sample as FIG. The peaks of ZnO and (Zn, Fe) Al ₂ O ₄ were confirmed. The peak of (Zn, Fe) Al ₂ O ₄ approached the peak of FeAl ₂ O ₄ as the amount of substitution of Fe increased, and the strength of the different phase became weaker. From the tendency of the resistivity of the sample substituted with Fe to decrease and the tendency of the diffraction peak to shift to FeAl ₂ O ₄ due to the substitution of Fe, it is estimated that a part of Fe is dissolved in Zn sites and acts as a donor .

  FIG. 2 shows the relationship between temperature and Seebeck coefficient for the same sample as FIG.

  In the sample in which only the Al is substituted at the Zn site, the Seebeck coefficient does not change much from the low temperature side to the high temperature side, and remains at a low value. On the other hand, the sample in which Al and Fe are substituted at the Zn site shows a high Seebeck coefficient from the low temperature side to the high temperature side, and it can be seen that the Seebeck coefficient is increased by the substitution of Fe.

  FIG. 3 shows the relationship between the reciprocal absolute temperature and the thermal conductivity for the same sample as FIG.

  The sample in which Al and Fe are substituted at the Zn site has a lower thermal conductivity from the low temperature side to the high temperature side than the sample in which only Al is substituted at the Zn site.

  FIG. 4 shows the relationship between temperature and output factor for the same sample as FIG.

  The sample in which Al and Fe are substituted at the Zn site does not necessarily have a larger output factor than the sample in which only Al is substituted at the Zn site. Tend to show.

  FIG. 5 shows the relationship between temperature and dimensionless figure of merit for the same sample as FIG.

  A sample in which Al and Fe are substituted at the Zn site has a dimensionless figure of merit over the entire temperature range as compared with a sample in which only Al is substituted at the Zn site. The dimensionless figure of merit is a value calculated from the Seebeck coefficient, resistivity, and thermal conductivity. From FIGS. 1 to 3, the addition of Fe increases the Seebeck coefficient and keeps the resistivity low. It can be seen that the dimensionless figure of merit has increased because of the lower.

  Consider that this result was obtained. The thermal conductivity κ is expressed as κ = κel + κph where the electronic thermal conductivity κel and the lattice thermal conductivity κph, and the electronic thermal conductivity can be reduced by decreasing the carrier concentration, but the resistivity is reduced by decreasing the carrier concentration. Is undesirable because of an increase in On the other hand, since the lattice thermal conductivity is less dependent on the carrier concentration, it is desirable to reduce the lattice thermal conductivity.

Precipitation of heterogeneous phase (Zn, Fe) Al ₂ O ₄ at the crystal grain interface seems to contribute to increase phonon scattering and decrease lattice thermal conductivity. However, it is clear from the experimental results that Fe is dissolved in the Zn site, and it is considered that the increase in phonon scattering due to crystal distortion caused by the solid solution of Fe is the main factor for reducing the lattice thermal conductivity. . Unlike solid phase precipitation, Fe solid solution also affects the Seebeck coefficient, and when a proper amount of Fe is dissolved, the dimensionless figure of merit increases due to multiple factors as well as a decrease in thermal conductivity. .

It is a figure which shows the temperature characteristic of the resistivity of the n-type thermoelectric conversion material of this invention. It is a figure which shows the temperature characteristic of the Seebeck coefficient of the n-type thermoelectric conversion material of this invention. It is a figure which shows the temperature characteristic of the thermal conductivity of the n-type thermoelectric conversion material of this invention by making a vertical axis | shaft into thermal conductivity and making a horizontal axis into the reciprocal number of absolute temperature. It is a figure which shows the temperature characteristic of the output factor of the n-type thermoelectric conversion material of this invention. It is a figure which shows the temperature characteristic of the dimensionless figure of merit of the n-type thermoelectric conversion material of the present invention. It is a powder X-ray diffraction pattern of the n-type thermoelectric conversion material of the present invention.

Claims (2)

Composition formula (Zn _1-x-y Al _x Fe _y ) A thermoelectric conversion material represented by O,
x = 0.020-0.040, y = 0.005-0.015,
An n-type thermoelectric conversion material having a dimensionless figure of merit at 890 ° C. of 0.09 or more. An n-type semiconductor element comprising the n-type thermoelectric conversion material according to claim 1 ;
a p-type semiconductor element;
A common electrode to which one end of the n-type semiconductor element and one end of the p-type semiconductor are connected;
An electrode independently connected to the other end of the n-type semiconductor and the other end of the p-type semiconductor;
The thermoelectric conversion element characterized by including.