RU2475333C1 - Method of extruding thermoelectric material based on bismuth and stibium chalcogenides - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to production of thermoelectric material based on bismuth and stibium chalcogenides by hot extrusion. Said material may be used for thermoelectric conversion of energy. Formed thermoelectric material billet is heated to 370-470°C by variable unipolar pulsed electric current at the pressure of 26-36 MPa.

EFFECT: ten- to twenty-fold decrease in extrusion pressure, improved material properties.

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Description

The invention relates to the field of thermoelectric energy conversion, in particular to the production of a thermoelectric material based on bismuth and antimony chalcogenides by hot extrusion.

The method of hot extrusion of thermoelectric materials is widely used in their production (Thermoelectric cooling. Edited by Bulat L.P., SPb., SPbGUNiPT, 2002; Sabo E.P. Thermoelectricity. No. 1, p.45, 2006). The method consists in forcing a thermoelectric material through a hole in a heated mold. The main advantage of this method is associated with an improvement in the strength characteristics of the material, while the thermoelectric characteristics can remain both at the level of the characteristics of the material obtained by crystallization from the melt and may be somewhat lower.

The main parameters that determine the extrusion process are the process temperature, pressure and the associated extrusion rate. For thermoelectric materials based on bismuth and antimony chalcogenides, the recommended extrusion temperatures for p-type are 420 ± 20 ° C, and for n-type 450 ± 20 ° C (Sabo E.P. Thermoelectricity. No. 1, p. 45, 2006) . The lower temperature limit is associated with the condition of uniform outflow of material from the hole, the upper one - with the chemical resistance of the thermoelectric material to oxidation processes and its interaction with the mold material. In the literature, there are indications of a slightly larger temperature range - 350-520 ° C (D. Vasilevsky, S. Turenne, R. Masut. Proc. Of 6 ^th European Conference on Thermoelectrics, July 2-4, 2008, Paris, France, pI-04-1). The pressure range of the process is also indicated there - 120-760 MPa. The extrusion pressure depends both on the process temperature and on the material extraction coefficient k, which is equal to the ratio of the areas of the billet and the outlet for extrusion. However, since pressure is proportional to ln (k) (Sabo EP Thermoelectricity. No. 1, p. 45, 2006), then the dependence on k is weak, which makes it possible to compare the process pressure regardless of the value of k.

As the production of thermoelectric products develops, there is a continuous increase in the diameter of the extruded material, since this reduces the amount of waste in the production process. The main obstacle to the increase in diameter is the high extrusion pressures, leading to the use in production of increasingly powerful and expensive presses and more bulky and expensive molds, in which the thickness of the walls of the cage increases with increasing pressure. Therefore, the technical task of this invention is to reduce the pressure at which the extrusion process occurs.

The most commonly used method for producing thermoelectric material by extrusion is known. In it, a pre-made tablet (press blank) of thermoelectric material is placed in a pre-heated (to a temperature equal to ≈0.8 melting point of the material) matrix of a special mold, the material is closed with a punch to which pressure is applied. Under pressure, material flows out through the die. Mold heating is carried out by an external electric heater. This method produces a polycrystal having a pronounced texture. Mold temperatures for Bi _0.5 Sb _1.5 Te ₃ are 370-410 ° C. Pressure - 20-30 t / cm ² . The efficiency of p-type Z materials up to 3 · 10 ^-3 K ^{-1 of} n-type Z materials is not more than 2.7 · 10 ^-3 K ^-1 . (I. A. Drabkin “Methods for the production of thermoelectric materials based on bismuth telluride, their comparative characteristics.” Bulat L.P. ed., Thermoelectric cooling. St. Petersburg, St. Petersburg State University of Physics and Technology, pp. 81-82. 2002; Sabo E.P. Thermoelectricity . No. 1, p. 45, 2006).

A modification of the extrusion method is known for small drawing ratios, in which a step die is used to improve the quality of the extruded material (Jean-Pierreard, Dmitry Vasilevskiy, Jacques L'Eeuyer. US patent 6596226 B1, Jul. 22, 2003).

A known method of producing thermoelectric material by extrusion, in which the heating of the mold and the billet is carried out by passing a constant electric current through them. In this case, the mold was heated to temperatures of 300-400 ° C, the extrusion pressure was 100-200 MPa. (Byong-Gue Min, Kyung-Wook Jang, Dow-Bin Hyun, Dong-Hi Lee. Proceeding od 16 ^th International Conference on Thermoelectrics, Dresden, pp 76-80, 1997). The method adopted for the prototype.

The disadvantages of the known extrusion methods include the need for high pressures, which requires the use of powerful press equipment for large diameters of the extrudable material (Sabo E.P. Thermoelectricity. No. 1, p. 45, 2006; D. Vasilevsky, S. Turenne, R. Masut . Proc. Of 6 ^th European Conference on Thermoelectrics, July 2-4, 2008, Paris, France, p.1-04-1).

The technical result of the invention is to simplify the process by reducing the extrusion pressure by 10-20 times and the possibility of using less powerful presses for extrusion, as well as improving the thermoelectric, mechanical and structural properties of the material.

The technical result is achieved by the fact that in the method of extrusion of a thermoelectric material based on bismuth and antimony chalcogenides, comprising heating a press blank by passing electric current through them and applying a force to the press blank, according to the invention, the heating is carried out by passing an alternating unipolar pulse current through the press blank, and the magnitude of the applied force is 26-36 MPa.

The essence of the invention lies in the fact that during the extrusion of a thermoelectric material based on antimony and bismuth chalcogenides, the mold and the billet are heated by passing through them an alternating unipolar pulsed current, while the magnitude of the applied to the press decreases significantly pressure blank, which was 26-36 MPa. It was found that at such a pressure range, the extrusion rate falls into the optimal speed range equal to 1-5 mm / min for low-temperature and medium-temperature thermoelectric materials (Sabo EP Thermoelectricity. No. 1, p. 45, 2006). With such a decrease in pressure, the extrusion is carried out at the same temperatures (370-470 ° C), which are characteristic of the usual process of extrusion of a thermoelectric material based on bismuth and antimony chalcogenides. During the passage of the current pulse, the main heat is released at the grain boundaries due to the contact grain resistance, the temperature in this region can significantly exceed the average temperature of the sample, while individual grains appear to be covered with a shell at a higher temperature than the internal temperature of the grain. In the period between pulses, the temperatures of the outer shell and grain are equalized. The outer shell of grains, heated to higher temperatures during the passage of an electric current pulse, plays the role of a lubricant around the grain and facilitates its orientational ordering during extrusion, which leads to an improvement in the texture of the extruded material. An increase in the degree of texturing does not significantly affect the thermoelectric figure of merit of the p-type material due to the lack of anisotropy in it, but it improves the efficiency of the thermoelectric material of the n-type.

It is known to use pulsed current in the process of spark plasma sintering (SPS method). The impact of the pulsed current in this process leads to an improvement in sintering processes and a decrease in the porosity of the sample (JRGrosa, A.Zavaliangos. Rev. Adv. Mater. Sci. 5, p.24, 2003), while during extrusion this effect facilitates the process expiration of the workpiece through the die, improving the texture of the resulting sample.

A diagram of the extrusion process under the influence of a unipolar pulse current is shown in Figure 1.

Examples of the method

Example 1

In a mold consisting of a holder (5, Figure 1), a die (4) and a punch (2) made of graphite or other material suitable for electrical and strength characteristics, they were placed in a billet (3) of a thermoelectric material based on p-type solid solution Bi _0.4 Sb _1.6 Te _{3 with a} diameter of 20 mm The billet was closed with a punch (2) of the mold. The entire assembly was fixed between the movable and fixed plungers (1, 6) of the hydraulic press. They were supplied with a pulsed electric current with a frequency of 300 Hz, a duty cycle of 0.5, and with an effective value of 400-440 A, which heats the press blank to temperatures adopted during the extrusion of thermoelectric materials based on bismuth and antimony chalcogenides (in this example, 420 ° C) . Then a pressure of 26-36 MPa was applied to the punches and the extrusion process was started. The speed of the extrusion process is in the range of 1-5 mm / min, which is consistent with the speeds adopted in the normal extrusion process. The degree of drawing (depending on the design of the die) ranged from 6.25 to 44. At the end of the extrusion process, the electric current was turned off, the load was removed, the mold was cooled and the extruded sample was removed, which was subjected to subsequent annealing at 340 ° C to relieve mechanical stresses in 24 hours.

Example 2. The extrusion method was carried out as in example 1, but the thermoelectric material of the n-type Bi _1.9 Sb _0.1 Te _2.85 Se _{0.15 was} taken as the material of the billet. In this case, the mold and the billet were heated by a pulsed electric current with a frequency of 300 Hz, a duty cycle of 0.5, and with an effective value of 400-440 A to a temperature of 420 ° C.

Table 1 shows the extrusion results of samples of the p-type solid solution Bi _0.4 Sb _1.6 Te ₃ and n-type Bi _0.9 Sb _0.1 Te _2.85 Se _0.15 . The particle size distribution of the powder for the billet is 250-40 microns. The diameter of the billet is 20 mm. The degree of drawing (depending on the design of the die) ranged from 6.25 to 44. A unipolar pulsed electric current with a frequency of 300 Hz, a duty cycle of 0.5 and an effective value of 400-440 A was passed through the sample, the extrusion temperature was 420 ° C, pressure 26 -36 MPa, the extrusion rate at these temperatures and pressure of 1-5 mm / min.

After extrusion, the samples were annealed at a temperature of 340 ° C for 24 hours.

The thermoelectric, mechanical, and structural (texturedness) properties of the materials obtained are shown in Table 1.

Table 1. Thermoelectric (electrical conductivity σ, thermoEMF α, thermal conductivity κ, efficiency Z), mechanical (tensile strength) and structural (degree of texturing) properties of extruded thermoelectric materials based on bismuth and antimony chalcogenides at room temperature.

Table 1 Material composition and type of conductivity α, μV / K σ, Ohm ^-1 cm ^-1 κ, W / mK Thermoelectric effective. Z * 1000, K ^-1 Mechanic durable tensile, MPa Degree textured
% Bi _0.4 Sb _1.6 Te ₃ 230-225 750-855 1.20-1.24 3.3-3.5 120-140 60-65 p-type Bi _0.9 Sb _0.1 Te _2.85 Se _0.15 202-190 1010-1240 1.30-1.36 3.1-3.3 100-120 65-70 n-type

For comparison, Table 2 shows the material characteristics we obtained for a conventional extrusion process with external heating of the billet after annealing at a temperature of 340 ° C for 24 hours.

Table 2. Parameters of the extrusion process and thermoelectric properties of Bi _0.4 Sb _1.6 Te ₃ samples obtained by extrusion by the prototype method.

Table 2 Process parameters Hood ratio 16,0 18.1 44,4 Diameter of the billet, mm 10 85 10 Process temperature, ° C 420 420 420 Process pressure, MPa 750 247 560 Characteristics of the obtained samples σ, Ohm ^-1 cm ^-1 700-878 909-960 685-862 α, μV / K 211-229 210-216 223-232 κ, W / mK 1.21-1.26 1.27-1.41 1.06-1.40 Z * 1000, K ^-1 3.05-3.14 3.09-3.24 3.13-3.25 The degree of texturing,% 45 fifty fifty

The data shown in table 2, the data show that the pressure during extrusion in a known manner is 10-20 times higher than with the claimed method. Moreover, the p-type material extruded in a known manner has a thermoelectric figure of merit at the level of (3.05-3.25) · 10 ^-3 K ^-1 . Thus, the extrusion process carried out by the claimed method allows to reduce the pressure by 10-20 times, thereby simplifying the process by using less power for extrusion of presses, and to obtain p- and n-type material with improved thermoelectric (thermoelectric efficiency), mechanical (strength) and structural (textured) properties.

Claims (1)

A method of extrusion of a thermoelectric material based on bismuth and antimony chalcogenides, comprising heating a billet to an extrusion temperature of 370-470 ° C by passing electric current through it and applying pressure, characterized in that the heating of the billet is carried out by passing an alternating unipolar pulsed electric current through it , and the magnitude of the applied pressure is 26-36 MPa.

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