JP2007116156A - Compound thermoelectric material and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a PbTe-based compound thermoelectric material having an enhanced thermoelectric property beyond the past limit, and a method of manufacturing the thermoelectric material that eliminates defects of the conventional compound thermoelectric materials. <P>SOLUTION: A PbTe (lead telluride)-based compound thermoelectric material with a rock salt structure has a chemical composition of Ag<SB>1-x</SB>Pb<SB>18+y</SB>SbTe<SB>20</SB>(when a stoichiometric composition of AgPb<SB>18</SB>SbTe<SB>20</SB>, in which Ag and Sb substitute for part of Pb in PbTe, is regarded as a reference, x is the amount of shortage relative to the stoichiometric value of Ag, y is the amount of excess relative to the stoichiometric value of Pb, x=0.2 to 0.6, and y=3 to 5), and is characterized in that the thermoelectric property is a dimensionless performance index ZT greater than 1. A method of manufacturing the compound thermoelectric material comprises: a step of mixing powders of constituent elements at the measurement ratio corresponding to the chemical composition; a step of producing compound powder having the chemical composition by subjecting the powder mixture to a high-energy ball milling process or mechanical alloying; and a step of subjecting the produced compound powder to spark plasma sintering to harden and form the powder. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a new energy material and a method for producing the same, and more particularly to a compound thermoelectric material using a thermoelectric semiconductor compound having a high temperature differential electromotive effect and a method for producing the compound thermoelectric material.

  Thermoelectric materials, also called temperature differential electromotive materials, are based on two basic thermoelectric effects--the Seebeck effect and the Peltier effect, which provide a conversion between thermal energy and electrical energy. Refers to energy materials.

  Compared to conventional power generation technology, thermoelectric power generation devices manufactured from thermoelectric materials have a simple structure, robustness and durability, no moving parts, easy microfabrication, and no maintenance required It has high reliability, long life, no noise, no pollution, and low temperature waste heat can be used.

  Compared with conventional compression cooling technology, thermoelectric cooling devices manufactured with thermoelectric materials do not require chlorofluorocarbon, do not cause contamination, are easy to downsize, have no moving parts, and do not generate noise. With the advantages of

  Therefore, as the energy and environmental issues have become more and more serious in recent years, the application of thermoelectric devices has been spreading day by day, such as aviation / space, national defense construction, geological and meteorological observation, medical hygiene, microelectronics, etc. There is a wide range of prospects for application in all areas such as chemicals, metallurgy, and waste heat utilization in the electric power industry.

For the thermoelectric material having the application prospects as described above, when its Seebeck coefficient S, conductivity σ, and thermal conductivity K are the main performance parameters, its output factor P = S ² σ and dimensionless figure of merit ZT = (S ² σ / K) T is the most commonly used figure of merit when evaluating thermoelectric materials, and good thermoelectric materials have a high Seebeck coefficient S, a high conductivity σ, and a low heat transfer coefficient K. Is required.

Thermoelectric materials that are currently under active research are Bi ₂ Te ₃ (Bismuth Telluride) and its solid solution alloys applied to low temperature regions, PbTe (Lead Telluride) and its alloys applied to moderate temperature regions, (Skutterudite) structure, SiGe alloy applied to high temperature region, clathrate structure, and others such as half-Heysler alloy, high boron solid, oxide and polymer thermoelectric material.

Among them, the application of Bi ₂ Te ₃ and its solid solution alloy is relatively mature, and its intrinsic performance index ZT exceeds 1. However, since the use temperature is relatively low, the material is mainly used for electronic cooling.

PbTe-based thermoelectric compounds are suitable for temperature difference power generation, but their performance is expected to improve. The dimensionless figure of merit ZT of the conventional block PbTe-based thermoelectric material is less than 1. According to a report in a recent document (Non-patent Document 1), the PbTe-based compound block body material has a thermoelectric performance superior to that of the Bi ₂ Te ₃ alloy. However, the production technique is relatively special, and the detailed process is unknown. Currently, some research institutions are also conducting experiments related to the material system, but the dimensionless figure of merit ZT obtained there is far below the value reported by K. F Hsu et al., Of which ZT is the highest 1.07.

  In another aspect, at present, thermoelectric materials are produced using powder metallurgy techniques such as grinding and heat-compression, mainly after synthesizing compounds using a melting method. This type of production technology involves complex processes, complex equipment, long production cycles, high energy consumption, and long exposure times of materials to high temperatures, so the volatilization of components is critical. There are disadvantages.

  Patent Document 1 discloses a method in which a PbTe-based raw material powder is made into a single phase by mechanical alloying (mechanical alloying), molded and sintered, and Patent Document 2 discloses a method in which PbTe-based raw material powder is pulverized and mixed by a ball mill. On the other hand, non-patent literature 2 describes an Ag-Pb-Sb-Te thermoelectric material, and non-patent literature 3 describes an Ag-Pb-Sb-Te based material. Each thermoelectric property is disclosed. However, none of the thermoelectric characteristics could be improved beyond the above limit.

JP 2004-296473 A Japanese Patent Laid-Open No. 7-7186 K. F Hsu et al., Science, 303 (2004), 818 Proceedings of 2005 Annual Conference of Japan Ceramics Association (2005.3.) Proceedings of the 2nd Annual Meeting of the Thermoelectric Society of Japan (TSJ2005), pp. 38-39, August, (2005)

  An object of the present invention is to provide a PbTe-based compound thermoelectric material that has improved thermoelectric characteristics beyond the conventional limit, and a method for producing the thermoelectric material that eliminates the conventional drawbacks.

In order to achieve the above object, according to the present invention, a compound thermoelectric material having a rock salt structure based on lead telluride PbTe,
When the chemical composition is represented by Ag _1-x Pb _{18 + y} SbTe ₂₀ and the stoichiometric composition AgPb ₁₈ SbTe _{20 in} which a part of Pb of PbTe is substituted with Ag and Sb is used, x is the chemistry of Ag. Indicates a deficiency relative to the stoichiometric value, y indicates an excess relative to the stoichiometric value of Pb, the x value can vary between 0.2 and 0.6, and the y value ranges from 3 to 5 Can vary between
A compound thermoelectric material characterized in that the dimensionless figure of merit ZT> 1 is provided.

The present invention further relates to a method for producing the compound thermoelectric material of the present invention,
Blending the powder of each component element at a measurement ratio corresponding to the chemical composition,
The compounded powder mixture is subjected to a high energy ball mill process or mechanical alloying to produce a compound powder of the above chemical composition,
There is also provided a method for producing a compound thermoelectric material, characterized in that the produced compound powder is cured by performing discharge plasma sintering.

  The compound thermoelectric material of the present invention can obtain a material having high thermoelectric performance by adopting the above chemical composition and adjusting the electrical performance and thermoelectric performance.

  The production method of the compound thermoelectric material of the present invention adopts mechanical alloying, so that the synthesis temperature is close to room temperature, the equipment is simple, the cost is low, and the large-scale production is compared with the conventional melting process. At the same time, the alloy crystal grains obtained are fine, the conductivity is not significantly reduced, and the thermal conductivity can be lowered, so that better thermoelectric performance is obtained. Moreover, by adopting discharge plasma sintering, sintering at a low temperature can be performed in a short time, thereby improving the production efficiency and at the same time preventing an increase in the length of the sintered crystal particles.

  The present invention provides a new thermoelectric material and a method for producing the same. The thermoelectric material belongs to an Ag—Pb—Sb—Te compound, and its dimensionless figure of merit ZT reaches 1.37. For the thermoelectric material, mechanical alloying (Mechanical Alloying MA) is used to synthesize the compound powder, and then the block body is produced at a relatively low temperature using the Spark Plasma Sintering SPS technology. It is possible to sinter and synthesize materials.

  In the present invention, high-performance PbTe-based Ag—Pb—Sb—Te compound thermoelectric materials are produced by employing mechanical alloying MA and spark plasma sintering SPS techniques.

  Mechanical alloying (mechanical alloying) is a process technology in which metal powder is directly synthesized into an intermetallic compound or alloy by energy generated by collision in a high energy ball mill process. Compared with the conventional melting process, the synthesis temperature is close to room temperature, the equipment is simple, the cost is low, and it is suitable for large-scale production. The electrical conductivity does not decrease remarkably and the thermal conductivity of the material can be decreased by increasing the phonon grain boundary scattering, so that better thermoelectric performance is obtained.

  In discharge plasma sintering, in a vacuum environment, the upper and lower graphite pressing heads pressurize the sintered body and generate a discharge plasma by pulsed direct current. Joule current is generated by eddy current inside the sintered body. It is a process technology that completes sintering within a very short time by generating heat and activating the surface, which includes low sintering temperature, short sintering cycle, high production efficiency, The crystal particles are not easily elongated.

The material composition selected in the present invention is shown as Ag _1−x Pb _{18 + y} SbTe ₂₀ . The compound has the same rock salt structure as PbTe. Under the condition that the original structure of PbTe is maintained unchanged, the position of Pb is replaced with an equal amount of Ag and Sb, x indicates the amount of deviation from the equivalent replacement, and y exceeds the measurement ratio. It is possible to obtain a material with high thermoelectric performance by indicating the Pb content to be adjusted and thereby adjusting the electrical performance and thermoelectric performance.

  Specific steps according to a desirable mode of the method for producing the compound thermoelectric material of the present invention are as follows.

1. Based on the chemical formula Ag _1-x Pb _{18 + y} SbTe ₂₀ , a single powder of Ag, Pb, Sb and Te is used as a raw material. A small amount (0.1% by volume) of ethanol was added to the tank. The role of ethanol is to prevent the powder from sticking onto the tank wall and affecting the subsequent ball mill effect.

  2. After vacuum evacuation in advance, high purity argon gas (Ar) as a ball mill protective gas was flowed in, and then a ball mill tank was installed in a planetary ball mill machine to perform a dry mill. The rotation speed of the ball mill was determined based on the model number of the ball mill machine and the material and size of the mill ball and ball mill tank. In the present invention, a QM-2 type planetary ball mill machine produced by Nanjing University ceremonial bowl, a 250 mL stainless steel ball mill tank, and a stainless steel mill ball having a diameter of 10 mm are employed. When the rotation speed is 300 rpm, the required ball mill time is 5 hours.

  3. The crushed powder material was taken out, loaded into a φ20 mm graphite mold, consolidated with a graphite pressing head, placed in an SPS machine, and sintered under vacuum (about 6 Pa) conditions. The temperature rising rate of SPS was 50 ° C./min, the maximum heat retention temperature was 300 to 500 ° C., the SPS pressure was 50 MPa, and the temperature of the furnace was cooled to room temperature after the heat retention.

  4). After the sample was taken out, the surface of the sample was polished with sandpaper, then physical phase analysis and microscopic structure analysis were performed, and a measurement test of thermoelectric performance was performed.

  When a technique (abbreviation: MA + SPS technique) in which the above-mentioned mechanical alloying and discharge plasma sintering are mutually coupled is used for the production of a thermoelectric material, the following advantages are provided.

  (1) Since the process flow is short and the efficiency is high, it is suitable for industrialized large-scale production.

  (2) Less energy is consumed because high temperature melting and long time heat compression are not required.

  (3) Since the volatilization of harmful elements is reduced, the environmental impact of the manufacturing process is relatively small.

  (4) Since the crystal grains of the obtained material are fine, the thermoelectric performance is more excellent.

[Example 1]
Silver (Ag) powder, antimony (Sb) powder, lead (Pb) powder, tellurium (Te) powder as a raw material, weighing the powder of the total meter 20g with a metering ratio of Ag _0.8 Pb ₂₂ SbTe _20, stainless steel ball mill tank (A volume of 250 mL) and a stainless steel mill ball having a diameter of 10 mm (weight ratio of mill ball to powder: 18: 1) were added. The ball mill tank is filled with Ar as a protective gas, and the planetary ball mill (QM-2 type, Nanjing University Gikijo) implements the planetary ball mill for 5 hours (rotation speed: 300 r / min) before mechanical alloying ( MA) reaction produced Ag _0.8 Pb ₂₂ SbTe ₂₀ compound powder. As shown in FIG. 1 (a), after the MA treatment, the powder material obtained there becomes a single phase and has a cubic structure, and each diffraction peak is identified based on the PbTe phase. Can do. When the obtained powder material was sintered under a pressure of 50 MPa and 400 ° C. for 2 minutes, the relative density was 98% or more (see the SEM photograph shown in FIG. 2). As shown in FIG. 1 (b), the phase structure basically coincides with the powder before SPS sintering, and no change has occurred. FIG. 2 shows (1) an SEM photograph and (2) a TEM photograph of an SPS sintered body having an Ag _0.8 Pb ₂₂ SbTe ₂₀ composition, and an Ag _0.8 Pb ₂₂ SbTe ₂₀ compound produced by MA and SPS. Thermoelectric materials have been shown to have high density, fine grains (average grain size of about 5 μm) and relatively uniform.

[Example 2]
Ag _0.8 Pb ₂₁ SbTe ₂₀ , Ag _0.8 Pb ₂₂ SbTe ₂₀ , Ag _0.8 Pb ₂₃ SbTe _{20 based on} silver (Ag) powder, antimony (Sb) powder, lead (Pb) powder and tellurium (Te) powder as raw materials A total amount of 20 g of powder was weighed to produce three sets of samples with different Pb contents. The powder synthesis and SPS sintering conditions were the same as in Example 1. 3 and 4 show the relationship between the resistivity (ρ) and Seebeck coefficient (S) of the SPS sintered samples of three types of compositions Ag _1-x Pb _{18 + y} SbTe ₂₀ compounds and temperature, respectively. Compared with the other two sets, Ag _0.8 Pb ₂₂ SbTe ₂₀ has the lowest resistivity, and the magnitude of the absolute value of its Seebeck coefficient is located between the other two. FIG. 5 shows the relationship between the output factor (S ² / ρ) calculated using the data shown in FIGS. 3 and 4 and the temperature. As shown in FIG. 5, the composition of Ag _0.8 Pb ₂₂ SbTe ₂₀ has the highest output factor, reaching 1766 μW / mK at 650K. FIG. 6 shows the relationship between the dimensionless figure of merit ZT of the composition (Ag _0.8 Pb ₂₂ SbTe ₂₀ ) and the temperature, and ZT reaches 1.37 at the highest measured temperature (673 K).

Example 3
Silver (Ag) powder, antimony (Sb) powder, lead (Pb) powder, tellurium (Te) powder as raw materials, Ag _0.4 Pb ₂₂ SbTe ₂₀ , Ag _0.6 Pb ₂₂ SbTe ₂₀ , Ag _0.8 Pb ₂₂ SbTe ₂₀ A total amount of 20 g of powder was weighed to produce three sets of samples with different Ag contents. The powder synthesis and SPS sintering conditions were the same as in Example 1. The room temperature resistivity of the samples having a silver content of 0.4, 0.6, and 0.8 is 0.125 Ωm, 0.185 Ωm, and 0.175 × 10 ⁻³ Ωm, respectively. Both samples with a silver content of 0.4 and 0.6 have excessively high resistivity, and a sample with an Ag content of about 0.8 shows a resistivity significantly lower than both other compositions. Therefore, the overall thermoelectric performance is high, and the dimensionless figure of merit ZT reaches 1.37.

The present invention discloses a method for producing a PbTe-based high-performance thermoelectric material using a method combining mechanical alloying (MA) and spark plasma sintering (SPS), and the composition of the material is Ag _1-x. Pb _{18 + y} SbTe ₂₀ is displayed, in which x indicates the amount deviating from the equivalent substitution of Ag, and y indicates the Pb content exceeding the measurement ratio in the raw material powder. Compared with conventional powder metallurgy technology, the MA + SPS technology according to the present invention has advantages such as short process flow, high efficiency, low energy consumption, and suitable for industrialized large-scale production. The temperature difference electromotive material obtained has better thermoelectric performance. The dimensionless figure of merit ZT of Ag _1-x Pb _{18 + y} SbTe ₂₀ thermoelectric material produced using MA and SPS reaches 1.37 at 450 ° C.

FIG. 1 shows _{X of} Ag _1-x Pb _{18 + y} SbTe ₂₀ (x = 0.2, y = 4) compound powder synthesized by mechanical alloying and its SPS sintered block (sintering temperature = 400 ° C.). It is a line diffraction spectrum figure. FIG. 2 shows (1) scanning electron microscope (SEM) and (1) of Ag _1-x Pb _{18 + y} SbTe ₂₀ (x = 0.2, y = 4) compound block body sample sintered at 400 ° C. 2) A transmission electron microscope (TEM) photograph. FIG. 3 is a graph showing the relationship between the resistivity and temperature of SPS sintered body samples of Ag _1-x Pb _{18 + y} SbTe ₂₀ compounds having different compositions. FIG. 4 is a graph showing the relationship between the Seebeck coefficient and the temperature of SPS sintered body samples of Ag _1-x Pb _{18 + y} SbTe ₂₀ compounds having different compositions. FIG. 5 is a graph showing the relationship between the output factor and temperature of SPS sintered compact samples of Ag _1-x Pb _{18 + y} SbTe ₂₀ compounds having different compositions. FIG. 6 is a graph showing the relationship between the dimensionless figure of merit ZT and temperature of the SPS sintered material corresponding to the composition having the highest output factor (Ag _0.8 Pb ₂₂ SbTe ₂₀ ).

Claims (2)

A compound thermoelectric material having a rock salt structure based on lead telluride PbTe,
When the chemical composition is represented by Ag _1-x Pb _{18 + y} SbTe ₂₀ and the stoichiometric composition AgPb ₁₈ SbTe _{20 in} which a part of Pb of PbTe is substituted with Ag and Sb is used, x is the chemistry of Ag. Indicates a deficiency relative to the stoichiometric value, y indicates an excess relative to the stoichiometric value of Pb, the x value can vary between 0.2 and 0.6, and the y value ranges from 3 to 5 Can vary between
A dimensionless figure of merit, ZT> 1, a compound thermoelectric material. It is a manufacturing method of the compound thermoelectric material of Claim 1, Comprising:
Blending the powder of each component element at a measurement ratio corresponding to the chemical composition,
The compounded powder mixture is subjected to a high energy ball mill process or mechanical alloying to produce a compound powder of the above chemical composition,
A method for producing a compound thermoelectric material, characterized in that the produced compound powder is cured by performing discharge plasma sintering.