RU2794354C1 - Method for obtaining nanostructure thermoelectric materials - Google Patents

Abstract

FIELD: powder metallurgy.

SUBSTANCE: invention is related in particular to manufacturing nanostructured thermoelectric materials (TEM). It can be used to make thermoelements. TEM based on Bi₂Te₃, or Sb₂Te₃, or PbTe, or GeTe, or SiGe is synthesized by direct fusion of the main components, taken in a stoichiometric ratio, with addition of alloying components. Synthesized TEM is ground using a high-energy planetary ball mill with a grinding ball diameter of 5 mm and a mass ratio of balls to TEM of 10:1 to obtain a nanodispersed powder with a predominance of particles from 10 to 100 nm. Powder compaction is carried out by spark plasma sintering.

EFFECT: increase of thermoelectric quality, mechanical strength of TEM, expansion of operating temperature range.

1 cl, 8 dwg, 6 tbl

Description

The invention relates to the field of thermoelectric energy conversion, in particular, to the production of thermoelectric materials (TEM), from which thermoelements operating on the Seebeck effect are made. Such thermoelements are the basis of thermoelectric generators designed to convert thermal energy into electrical energy.

The efficiency of thermoelements is mainly determined by the thermoelectric figure of merit (Z) of the TEM, calculated by the formula:

where s - thermoEMF; σ - electrical conductivity; κ - thermal conductivity.

It should be noted that in order to characterize the efficiency of TEM at a certain temperature, the dimensionless parameter ZT (T, thermodynamic temperature in K) is often used.

второй - снижение теплопроводности ТЭМ.There are two ways to increase Z: the first is to increase the power factor

the second is a decrease in the thermal conductivity of the TEM.

However, it has not yet been possible to significantly increase the power factor for a number of reasons. In the last two decades, there has been a significant increase in the activity of scientific research with the aim of reducing thermal conductivity. In this direction, the creation of nanostructured TEMs /1-3/ is promising.

электронной

и биполярной

составляющими:The thermal conductivity (k) of TEMs, which are semiconductor materials, is determined by: phonon

electronic

and bipolar

components:

In the operating temperature range of the thermoelement, phonon and electronic heat transfer takes place. The bipolar component of thermal conductivity appears at temperatures above the maximum temperature of the TEM operating interval. The presence of bipolar heat transfer significantly reduces the quality factor of the TEM.

The thermoelement is a thermocouple consisting of legs connected in series and made from TEMs of n- and p-types of conductivity. TEMs, in most cases, are doped solid solutions: for use at temperatures up to 150°C (low-temperature TEMs) based on Bi ₂ Te ₃ (n-type) and Sb ₂ Te ₃ (p-type); for use at temperatures up to 300°C (medium temperature TEM), also based on Bi ₂ Te ₃ and Sb ₂ Te ₃ , but with a different ratio of components and doping; for temperatures up to 600°C (medium temperature TEMs) based on PbTe (n-type) and GeTe (p-type). The medium temperature range extends from 150 to 600°C. To create thermoelements operating at temperatures up to 900°C, high-temperature materials based on SiGe (n-type and p-types) /3/ are used.

TEMs are manufactured by direct fusion of components taken in a ratio determined by a stoichiometric formula (TEM synthesis). After synthesis, in order to create an optimal structure to obtain maximum Z values, TEMs are subjected to directional crystallization, as a rule, by the following methods: zone melting; extrusion; hot pressing or Bridgman processing. These are the classical methods of obtaining TEM, which allow reaching ZT values of the order of 1.0 /3/.

As mentioned above, a promising way to increase ZT is the creation of nanostructured TEMs. The essence of this method is as follows. Synthesized TEMs are subjected to grinding to a particle size not exceeding, as a rule, 100 nanometers. Then the resulting fine powder is compacted to obtain bulk samples, mainly by hot pressing or spark plasma sintering. The grain sizes in the obtained bulk TEMs correlate with the size of the powder particles from which the TEMs are compacted. An increase in Z in nanostructured TEMs occurs due to a decrease in κ _f and, accordingly, the total thermal conductivity (κ). The phonon thermal conductivity decreases due to intense scattering of phonons with a wavelength commensurate with the grain sizes in nanostructured TEMs.

There are phonons with a short free path, medium length and long-wavelength phonons. All TEMs are substitutional solid solutions, in which substitutional atoms in the crystal lattice scatter short-wavelength phonons - this is the main principle for creating TEMs with low thermal conductivity. The main task of obtaining nanostructured TEMs with grain sizes up to 200 nm is the scattering of medium-wavelength phonons, which account for up to 40% of the transferred thermal energy /4/. Thus, the phonon component of the thermal conductivity and, accordingly, the overall thermal conductivity of the TEM is significantly reduced, which leads to an increase in Z (formula 1) and efficiency (ZT) of the TEM in the range of operating temperatures.

In this case, the electrical conductivity slightly decreases due to the scattering of charge carriers on nanostructures, however, this scattering is significantly less than the scattering of phonons due to electron tunneling /5/.

A technical solution /6/ is known, in which to increase the scattering of phonons (reduce κ _f and increase ZT), nanostructured low-temperature TEM based on low-temperature bismuth telluride add nanodispersed powders of metal or silicon oxides, or fullerenes or carbon nanotubes. The disadvantage of this invention is not a high thermoelectric figure of merit of the TEM.

Inclusions from nanosized powders reduce electrical conductivity. In addition, it is impossible to obtain a uniform distribution of nanodispersed powders in TEM.

A technical solution /7/ is known, in which for low-temperature materials based on Bi ₂ Te ₃ and Sb ₂ Te ₃ obtained by direct alloying of components, an increase in the ZT parameter, exceeding the value of 1.0, is achieved due to nanostructuring and due to the use in the composition of TEM , as a grinding agent, compounds of a layered structure based on Mo or W, or BN, or graphite sulfides, which prevent the formation of agglomerates in the preparation of nanodispersed TEM powders and create additional centers for phonon scattering. The disadvantage of this solution is that the presence of a grinding agent in the TEM structure significantly reduces its electrical conductivity. There are also problems associated with the uniform distribution of the grinding agent in the TEM, i.e. creating a homogeneous material. The annealing of materials used in the technology for uniform distribution of impurities will not give a positive effect in this case. In addition, the presence of layers of grinding agents that are not continuous, according to the authors, reduces the mechanical strength of the TEM.

A technical solution /8/ is known, in which, in order to increase Z, diamond particles 3-5 nm in size are added to a nanostructured low-temperature TEM - Bi _x Sb _2-x Te ₃ p - type of conductivity, to increase the scattering of phonons. Thus, ZT≥1.0 is achieved. The concentration of diamond particles reaches 15% of the TEM volume. However, the presence of diamond particles, an ideal dielectric material, reduces the electrical conductivity of the TEM, which negatively affects the thermoelectric figure of merit. In this regard, there is no evidence in the literature for an increase in the ZT of such materials.

The closest technical solution adopted for the prototype is the "Method for producing thermoelectric materials based on bismuth and antimony tellurides" /9/. The technical result of the claimed invention is the production of highly efficient nanostructured TEMs with high mechanical and thermoelectric properties and the expansion of their scope. The technical result is achieved by direct synthesis of TEMs followed by zone melting. After that, the TEM is ground to obtain a nanodispersed powder with a particle size of not more than 200 nm. Then the powder is briquetted and compacted by hot pressing. This is followed by hot extrusion. This technical solution is low-tech, as it is difficult to manufacture, contains a large number of technological operations. It makes no sense to carry out directional crystallization (zone melting), after which the material is ground to a nanodispersed powder. Compacting the powder for a day at high temperature leads to its recrystallization, i.e., the formation of large grains in the compacted TEM, which is further aggravated by hot extrusion. All of the above leads to the loss of the nanostructure, which provides the effect of phonon scattering and a decrease in the phonon component of thermal conductivity, leading to an increase in the TEM efficiency.

The objective of the claimed invention is to increase the thermoelectric figure of merit (Z) and, accordingly, the efficiency (ZT) of medium-temperature TEMs based on Bi ₂ Te ₃ or Sb ₂ Te ₃ or PbTe or GeTe and high-temperature TEMs based on SiGe, intended for the manufacture of generator thermoelements operating on Seebeck effect at temperatures from 150 to 900°C.

To achieve this task, a method is proposed for obtaining nanostructured thermoelectric materials (TEM), including the synthesis of TEM, grinding the synthesized TEM in an inert medium to obtain a powder and compacting said powder to form a nanostructured TEM, characterized in that TEM is synthesized based on Bi ₂ Te ₃ or Sb ₂ Te ₃ or PbTe or GeTe or SiGe by direct fusion of the main components, taken in a stoichiometric ratio, with the addition of alloying components, the synthesized TEM is ground using a high-energy planetary ball mill with a grinding ball diameter of 5 mm and a mass ratio of balls and TEM of 10:1 to obtaining a nanodispersed powder with a predominance of particles from 10 to 100 nm, and the compaction of the powder is carried out by spark plasma sintering (SPS).

As a result of spark plasma sintering of the powder, with a predominance of particles from 10 to 100 nm, bulk nanostructured TEM samples with the appropriate grain size are obtained. These grain sizes effectively scatter phonons with a mean free path, which account for up to 40% of the transferred thermal energy, which leads to an increase in the thermoelectric figure of merit (Z) of the TEM.

Determination of powder dispersion and structure (grain size) of bulk nanostructured TEMs was carried out using transmission electron microscopy. A JEM 2100 high-resolution transmission electron microscope (JEOL, Japan) with an accelerating voltage of 200 kV was used for the studies.

The elemental composition of powders and bulk nanostructured TEM samples obtained after SPS was determined using a JSM-6480LV scanning electron microscope (JEOL, Japan) with an INCA DRY COOL energy-dispersive spectrometry attachment (OXFORD INSTRUMENTS, Great Britain).

To determine the electrical conductivity, thermopower and thermal conductivity of nanostructured TEMs, the technique presented in /10/ was used. The thermal conductivity was determined by the absolute stationary method. Based on the data obtained, Z and ZT were calculated.

The density of the resulting bulk nanostructured TEM was determined by hydrostatic weighing.

Vickers microhardness was measured using a PMT-3M microhardness tester using the reconstructed indenter imprint.

To explain the essence of the invention, the following figures are presented:

In FIG. Figure 1 shows the dependence of the particle size of the SiGe powder (2.2 wt.% P) on the grinding time when using grinding balls of different diameters.

In FIG. Figure 2 shows the dependence of the particle size of the SiGe powder (2.2 wt. % P) on the grinding time at various ratios of the masses of the balls and TEM.

In FIG. Figure 3 shows the dependence of the particle size of the SiGe powder (2.2 wt.% P) on the grinding time at various speeds of rotation of the planetary disk of the ball mill.

In FIG. Figure 4 shows the dependence of the particle sizes of TEM powders on the grinding time under optimal grinding conditions.

In FIG. Figure 5 shows the dependence of Z on the temperature of spark plasma sintering for TEM: 1) Si _0.8 Ge _0.2 (doped with 2.2 wt.% P); 2) Si _0.8 Ge _0.2 (doped with 0.78 wt% B); 3) PbTe (doped with 0.2 wt.% PbI ₂ and 0.3 wt.% Ni); 4) Ge _0.96 Bi _0.04 Te; 5) Bi ₂ Te _2.85 Se _0.15 (doped with 0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ); 6) Bi _0.5 Sb _1.5 Te _2.92 Se _0.08 (doped _with 3 wt% Te, 0.3 wt% Pb and 1.7 wt% Se).

In FIG. 6 shows a diffraction pattern from a sample of Si _0.8 Ge _0.2 (2.2 wt.% P).

In FIG. 7 shows a diffraction pattern from a sample of Si _0.8 Ge _0.2 (0.78 wt.% B).

In FIG. Figure 8 shows the dependence of the ZT parameter of nanostructured TEMs: 1) Si _0.8 Ge _0.2 (doped with 2.2 wt % P); 2) Si _0.8 Ge _0.2 (doped with 0.78 wt% B); 3) PbTe (doped with 0.2 wt.% PbI ₂ and 0.3 wt.% Ni); 4) Ge _0.96 Bi _0.04 Te; 5) Bi ₂ Te _2.85 Se _0.15 (doped with 0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ); 6) Bi _0.5 Sb _1.5 Te _2.92 Se _0.08 (doped _with 3 wt% Te, 0.3 wt% Pb and 1.7 wt% Se).

The proposed method is carried out as follows.

Synthesis of TEMs: based on Bi ₂ Te ₃ or Sb ₂ Te ₃ or PbTe or GeTe was carried out in a rocking muffle furnace, and based on SiGe, in an Indutherm VTC 200 V vacuum casting machine by direct alloying of the main components, taken in a stoichiometric ratio, with the addition of alloying components. After synthesis, TEMs were crushed using a ShchD-6 crusher and an IKA A11 knife-type mill in an inert medium in a Plas-Labs glove box to a particle size of about 250 μm. The resulting powder was placed in stainless steel beakers with metal grinding balls 5 mm in diameter and additionally ground using a Retsch PM400 MA high-energy planetary ball mill to obtain a nanodispersed powder with a predominance of particles in the range from 10 to 100 nm. The ratio of the mass of balls and TEM was 10:1, the speed of rotation of the planetary disk of the ball mill 400 rpm Grinding was carried out for 60 minutes.

The indicated dispersion of powders (from 10 to 100 nm) is optimal for the following reasons. The grain size in the compacted (bulk) material correlates with the fineness of the initial TEM powder. At the same time, it has been established that when obtaining powders with a dispersion of less than ten nanometers, during the grinding process, they are actively combined into large agglomerates, from hundreds of nanometers to microns. There is also a hardening effect, which increases the particle size. In addition, in the SPS process occurring at elevated temperatures, with the predominance of powders of minimum size (less than 10 nm) with a high specific surface area, the probability of recrystallization increases with the formation of large grains in compacted samples. During spark plasma sintering of powders with a predominance of particles from the range of 10 to 100 nm, bulk nanostructured TEM samples with corresponding grain sizes in the TEM structure were obtained (Table 1). Grains of this size effectively scatter medium-wavelength phonons, which account for up to 40% of the transferred energy. With an increase in the proportion of particles in the compacted powder with sizes from 50 to 150 nm, the grain sizes increased to 300 nm. When compacting powder with a particle size of 80 to 200 nm, grains up to 500 nm in size dominated in TEM (Table 1). Grains of such sizes do not scatter phonons with an average long free path.

Experimental data for various technological parameters, as an example, are presented for TEM - SiGe (doped with 2.2 wt.% P), n-type. For TEMs based on Bi ₂ Te ₃ , Sb ₂ Te ₃ , PbTe, GeTe and SiGe (doped with 0.78 wt.% B), similar dependences were obtained with some deviations in the average sizes of powder particles, which will be presented in Fig. 4.

Thus, an increase in the grain size in the TEM structure reduces the efficiency of scattering of phonons with an average long free path, which negatively affects the decrease in the phonon component of thermal conductivity.

When developing the technology for obtaining powders, grinding balls with a diameter of: 3; 5 and 10 mm and mass ratio of balls and TEM: 5:1; 10:1 and 15:1. The optimal ball diameter of 5 mm and the ratio of the mass of balls and TEM 10:1 were experimentally established, which makes it possible to obtain a powder with a predominance of particles from 10 to 100 nm (Fig. 1 and 2). The use of grinding balls with a diameter of 10 mm (Fig. 1) and the ratio the mass of the balls and TEM 15:1 (Fig. 2) leads to a decrease in the time to obtain a powder (grinding time) with a minimum size. However, an increase in the size of the balls and their number leads to their intense interaction with each other and with the TEM with the release of significant thermal energy during impacts. This leads to the destruction of the balls and an increase in the temperature of the glasses in which the TEM and balls are placed, as well as to an increase in the temperature of the working drum of the mill. An increase in temperature activates the process of agglomeration and work hardening of powder particles, in connection with this, the size of the powder increases. In addition, iron fragments are observed in the powder, which reduce the efficiency of TEM.

Reducing the size of the balls to 3 mm and the ratio of the mass of the balls and TEM 5:1 leads to an increase in the grinding time (Fig. 1 and 2).

The dependence of the particle size of the powder on the speed of rotation of the planetary disk of the ball mill, on which the glasses with the TEM are fixed, is shown in Fig. 3. At speeds of 200 and 300 rpm, it is not possible to obtain a powder with a particle size of the order of 10 nm. Increasing the speed to 500 rpm reduces the time to reach the minimum particle size. However, it is not possible to obtain a powder with a particle size of the order of 10 nm. This is due to the fact that at a speed of 500 rpm, the process of formation of agglomerates and work hardening of powder particles is intensified. This is due to the fact that as the rotation speed increases, the process temperature increases. Thus, on the basis of the experiment, the optimal speed of rotation of the planetary disk is set - 400 rpm (Fig. 3)

Under optimal conditions for obtaining TEM powders based on Bi ₂ Te ₃ or Sb ₂ Te ₃ or PbTe or GeTe or SiGe (ball diameter 5 mm, ratio of balls and TEM by weight 10:1, planetary disk rotation speed 400 rpm), their intensive grinding occurs in the first 40 min (Fig. 4), and the minimum powder sizes are reached after 60 min, with a spread of 1 to 3 min for different TEMs. A further increase in the grinding time leads to an increase in the average particle size, which is associated with powder agglomeration and hardening. In this regard, the optimal grinding time was set to 60 minutes.

Thus, the following optimal conditions for obtaining nanodispersed powders for TEMs based on Bi ₂ Te ₃ or Sb ₂ Te ₃ or PbTe or GeTe or SiGe have been established: the diameter of the grinding balls is 5 mm, the mass ratio of the balls and TEM is 10:1, the rotation speed of the planetary disk 400 rpm, grinding time 60 min.

Nanostructured bulk TEM samples were obtained from their nanodispersed powders by spark plasma sintering on an SPS 511S setup as follows. Before loading TEMs into the SPS 511S installation, for TEMs based on Bi ₂ Te ₃ or Sb ₂ Te ₃ , briquettes were obtained from their powder by cold pressing on a PGL-20 press. For other TEMs, this operation was not performed. Briquetting of TEM powders based on Bi ₂ Te ₃ and Sb ₂ Te ₃ is necessary to eliminate the formation of pores in bulk nanostructured samples of these TEMs during SPS. For TEMs based on PbTe, GeTe, and SiGe, this phenomenon was not observed.

TEM powder based on PbTe or GeTe or SiGe or TEM briquettes based on Bi ₂ Te ₃ or Sb ₂ Te ₃ were placed in a cylindrical graphite mold and covered with graphite punches from both sides. The operation was performed in the "Plas-Labs" box. The mold assembled in this way was taken out of the box through the lock and placed in the SPS 511S machine. The TEM was pressed using a hydraulic press of the installation under the action of pressure on the powder. The set temperature of the SPS was controlled by the power of the electric current applied to the powder. The sintering time at a given temperature was also regulated. All parameters of the IPS technological process were controlled automatically according to a given program.

In the SPS process, the lower the sintering temperature and time, the lower the probability of collective recrystallization and, therefore, there will be more grains in the compacted material, commensurate with the dispersion of the initial TEM powder particles. However, at low temperature and insufficient time, incomplete sintering of the powder particles is possible. In this case, a non-uniform, multi-phase TEM is obtained. Therefore, in the process of SPS, the sintering temperature and time are optimized for each TEM separately, taking into account the data of the study of the structure, phase analysis of the nanostructured TEM, as well as their thermoelectric figure of merit (Z).

The optimal sintering temperature was determined taking into account the melting temperatures of each TEM. Studies were carried out at the following intervals of its values, for TEM based on: Bi ₂ Te ₃ and Sb ₂ Te ₃ at temperatures from 300 to 500°C; PbTe, GeTe from 350 to 600°С; SiGe at temperatures from 800 to 1200°C. In FIG. 5 shows the results of the dependence of Z on sintering temperatures. The Z values are given for the following TEM composition: Bi ₂ Te _2.85 Se _0.15 (doped with 0.4 wt % Bi ₁₁ Se ₁₂ Cl ₉ ); Bi _0.5 Sb _1.5 Te _2.92 Se _0.08 (doped with 3 wt% _Teg , 0.3 wt% Pb and 1.7 wt% Se); PbTe (doped with 0.2 wt.% Pb ₂ and 0.3 wt.% Ni); Ge _0.96 Bi _0.04 Te; Si _0.8 Ge _0.2 (doped with 2.2 wt. %) P); Si _0.8 Ge _0.2 (doped with 0.78 wt.% B).

At temperatures below optimal, complete consolidation of the powder is not achieved; non-single-phase TEMs and, accordingly, with a low Z value. An increase in temperature above the optimum leads to repeated recrystallization, accompanied by an increase in grains and, accordingly, a decrease in the effect of phonon scattering, which is reflected in the Z values.

To determine the effect of sintering time on Z, the process was carried out for: 3, 5, 10 and 15 minutes at optimal sintering temperatures. The influence of the TEM sintering time on Z is the same as when the process is carried out at different SPS temperatures. At 3 minutes of TEM sintering (for SiGe at 3 and 5 minutes), complete consolidation of the powder is not achieved. In the study of the phase composition of TEM, regions are observed in which no sintering of the powder occurred. With the duration of the sintering process from 5 to 15 min, homogeneous single-phase TEM samples are obtained. However, with an increase in the TEM sintering time of more than 5 min (for SiGe, more than 10 min), grain recrystallization processes are activated and grain sizes increase significantly, which leads to a decrease in Z.

Thus, taking into account the data on Z, the optimal values of temperature and sintering time are determined (Table 2)

The pressure value determines the density of the resulting bulk nanostructured TEM. As a result of the research, it was found that the maximum density (98%) of the theoretically possible one is achieved at the following pressure values (Table 3)

Thus, the optimal pressure values were determined equal to 50 MPa for TEMs based on: Bi ₂ Te ₃ or Sb ₂ Te ₃ or GeTe or SiGe and 80 MPa for TEMs based on PbTe. It should also be noted that the density significantly affects the electrical conductivity of materials, which increases with increasing density.

As a result of experimental studies, it was found that the doping of these TEMs with impurities in order to obtain maximum Z values did not affect the modes of powder production and the modes of obtaining nanostructured bulk TEMs by spark plasma sintering.

An example of the implementation of the method for obtaining nanostructured TEM for the following compositions: Bi ₂ Te _2.85 Se _0.15 (doped with 0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ), n-type; Bi _0.5 Sb _1.5 Te _2.92 Se _0.08 (doped _with 3 wt. % Te, 0.3 wt. % Pb and 1.7 wt. %) Se), p-type; PbTe (doped with 0.2 wt.% PbI ₂ and 0.3 wt.% Ni), n-type; Ge _0.96 Bi _0.04 Te, p-type; Si _0.8 Ge ₀ , ₂ (doped with 2.2 wt.% P), n-type; Si _0.8 Ge _0.2 (doped with 0.78 wt.% B), p-type.

These materials were synthesized in an Indutherm VTC 200 V vacuum injection molding machine by direct alloying of the components taken in a stoichiometric ratio with the addition of the necessary dopants. The resulting TEMs were crushed using a ShchD-6 crusher and an IKA AN knife-type mill in an inert medium in a Plas-Labs glove box to a particle size of about 250 μm. Then TEM was placed in ball mill beakers, in the amount of 100 grams. After loading the TEM, crushing balls with a diameter of 5 mm were loaded into the glasses in a ratio of 1:10 by weight (10 grams of balls accounted for 1 gram of TEM). The operations were carried out in a box in an inert environment. Up to 4 jars can be installed in the ball mill at the same time. The masses of loaded glasses should differ from each other by no more than 1 gram. After loading the powder, the cups were sealed and moved from the working volume of the box to its sluice. In the process of preparing glasses with TEM for a ball mill, they were rocked for at least 1 minute to evenly distribute the balls and TEM over the volume of the glasses. After that, they were installed in the working volume of the ball mill "Retsch PM400 MA" on the planetary disk, where they were carefully fixed. and their natural cooling to room temperature.The cooled beakers were placed in a glove box, where the beakers were depressurized and the resulting TEM powder was taken for SPS.

The particle size of the TEM powders was determined from the broadening of the peaks in the X-ray diffraction pattern (FIGS. 6 and 7). The particle sizes were determined with an accuracy of 0.5 nm. The calculation of powder particle sizes, the determination of the proportion of particles of each size in the powder volume, and the determination of the average powder size were carried out using the Outset program. The composition of the powders was determined from data on the parameters of the crystal lattice. The research results, taking into account doping, are presented in Table 4.

As an example, to determine the composition and dispersion of powders under optimal conditions for their preparation, X-ray diffraction patterns for TEM are presented: SiGe (2.2 wt.% P) and Si _0.8 Ge _0.2 (0.78 wt.% B) ( Fig. 6 and 7).

Before loading the TEM into the plasma sintering unit, from Bi ₂ Te _2.85 Se _0.15 powder (0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ) and Bi _0.5 Sb _1.5 Te _2.92 Se _{0, 08} (3 wt. % _Te , 0.3 wt. %) Pb and 1.7 wt. % Se) briquettes were obtained by cold pressing on a PGL-20 press.

TEM powders: PbTe (0.2 wt.% PbI ₂ and 0.3 wt.% Ni) or Ge _0.96 Bi _0.04 Te, or Si _0.8 Ge _0.2 (2.2 wt.% P ) or Si _0.8 Ge _0.2 (0.78 wt.% B), or briquettes Bi ₂ Te _2.85 Se _0.15 (0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ), or Bi _{0 .5} Sb _1.5 Te _2.92 Se _0.08 (3 wt. % _Te , 0.3 wt. %) Pb and 1.7 wt. %) Se) was placed in a cylindrical graphite mold and covered with graphite punches from both sides. The operation was carried out in a sealed box. The mold assembled in this way was taken out of the box through the lock and placed in the SPS 511S machine.

TEM pressing during spark plasma sintering was carried out using a hydraulic press at pressures equal to: 50 MPa for Bi ₂ Te _2.85 Se _0.15 (0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ), Bi _0.5 Sb _1.5 Te _2.92 Se _0.08 (3 wt. % _Te , 0.3 wt. % Pb and 1.7 wt. % Se), Ge _0.96 Bi _0.04 Te, Si _{0, 8} Ge _0.2 (2.2 wt.% P) Si _0.8 Ge _0.2 (0.78 wt.% B) and 80 MPa for PbTe (0.2 wt.% PbI ₂ and 0.3 wt. .% Ni).

The sintering temperature was for: Bi ₂ Te _2.85 Se _0.15 (0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ), Bi _0.5 Sb _1.5 Te _2.92 Se _0.08 (3 wt. % _Te , 0.3 wt.% Pb and 1.7 wt.% Se), PbTe (0.2 wt.% PbI ₂ and 0.3 wt.% Ni) - 450 ° C, for Ge _0.96 Bi _0.04 Te - 500 ° C, for Si _0.8 Ge _0.2 (2.2 wt.% P) Si _0.8 Ge _0.2 (0.78 wt.% B) - 1100 ° C.

Spark plasma sintering time for: Bi ₂ Te _2.85 Se _0.15 (0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ), Bi _0.5 Sb _1.5 Te _2.92 Se _0.08 (3 wt. .% _Te , 0.3 wt.% Pb and 1.7 wt.% Se), PbTe (0.2 wt.% PbI ₂ and 0.3 wt.% Ni) and Ge _0.96 Bi _{0, 04} Te was 5 min, for Si _0.8 Ge _0.2 (2.2 wt.% P) and Si _0.8 Ge _0.2 (0.78 wt.% B) - 10 min.

As a result of the implementation of this method for obtaining nanostructured TEMs, the phonon component of their thermal conductivity and, accordingly, the thermal conductivity as a whole decreases, which leads to an increase in Z and the TEM efficiency in the operating temperature range determined by the ZT parameter. The temperature dependence of ZT of nanostructured TEMs is shown in Fig. 8.

Since the parameter ZT is calculated at thermodynamic temperature, it is presented not in °C, but in K.

The increase in Z and ZT of nanostructured TEMs in relation to TEMs obtained by classical methods and not possessing a nanostructure is from 10 to 20%. The maximum ZT values and the temperature at which these values are determined are presented in Table 5.

According to the Hall-Petch law /11/, the mechanical strength of materials increases due to nanostructuring. As a result of the measurements of the TEM microhardness according to Vickers (Hv), we obtained high results for nanostructured TEMs, presented in Table 6.

It should also be noted that the creation of nanostructured medium-temperature TEMs: Bi ₂ Te _2.85 Se _0.15 (0.4 wt.% Bi ₁₁ Se ₁₂ Cl ₉ ) and Bi _0.5 Sb _1.5 Te _2.92 Se _{0, 08} (3 wt. % Te _el ., 0.3 wt. % Pb and 1.7 wt. % Se), effective in the temperature range 150 - 300 ° C, also PbTe (0.2 wt. % PbI ₂ and 0 .3 wt.% Ni) and Ge _0.96 Bi _0.04 Te, effective at temperatures of 300 - 600 ° C; and high-temperature TEMs: Si _0.8 Ge _0.2 (2.2 wt.% P) and Si _0.8 Ge _0.2 (0.78 wt.% B), effective at temperatures of 600 - 900 ° C, allows significantly expand the field of application of thermoelements made from these TEMs and, accordingly, thermoelectric generators.

Information sources

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Claims (1)

A method for producing nanostructured thermoelectric materials (TEM), including the synthesis of TEM, grinding the synthesized TEM in an inert medium to obtain a powder and compacting said powder to form a nanostructured TEM, characterized in that TEM is synthesized based on Bi ₂ Te ₃ or Sb ₂ Te ₃ or PbTe or GeTe or SiGe by direct fusion of the main components, taken in a stoichiometric ratio, with the addition of alloying components, the synthesized TEM is ground using a high-energy planetary ball mill with a grinding ball diameter of 5 mm and a mass ratio of balls and TEM of 10:1 to obtain a nanodispersed powder with a predominance of particles from 10 to 100 nm, and compaction of the powder is carried out by spark plasma sintering.

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