Features of bismuth telluride based ternary alloys for thermoelectric applications

. The presented article provides a comparative analysis of the methods of manufacturing technology for semiconductor branches of p-and n-type conductivity. For comparative analysis, a method is considered for obtaining ternary alloys based on the Bi 2 Te 3 base material by pressing and zone melting. The sequence of the manufacturing process, the principles of operation and design of devices used to obtain alloyed substances, as well as methods for studying and measuring the electrophysical parameters of the thermoelement legs included in the assembled thermopile are described. The principles of automatic temperature control of an electric furnace by a two-position method are indicated. The advantages of the zone melting method are determined and the best values of the figure of merit of a thermoelectric material are shown. It is proved that the original values of the parameters of the branches are preserved after they are assembled into a battery.


Introduction
Global energy consumption is growing every year, while fossil fuel reserves are depleting [1].Moreover, the issue of global climate change, the greenhouse effect, atmospheric pollution and other environmental and climate problems is very acute.At the same time, the understanding of environmental problems arising from high energy demand has grown, and the understanding of the connection between energy and environmental safety has changed.The world has increased the pace of search for some strategies that can compensate for the use of fuel and reduce the level of pollution crisis [1].Alternatives such as heat recovery [2,3], energy management [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and renewable energy sources [6][7][8] have emerged in order to reduce energy costs, as well as solve environmental problems.The use of renewable energy technologies, such as solar, wind, geothermal, fuel cells etc., since clean energy production has increased significantly in many countries.In this context, the creation and introduction of alternative energy sources becomes a prerequisite for sustainable development.Renewable energy technologies such as solar, thermal, wind, hydroelectric, biofuels, fuel cells, etc., are becoming more advanced and more fashionable to develop.Literally before our eyes, not only solar energy has developed, but a new branch of agrophotovoltaic has emerged.The use of thermoelectric generators is justified for various high-tech applications related to space and military applications, the development of applications for unmanned aerial vehicles, for example, in agriculture in hard-to-reach places.But the most important argument for the development of this type of electricity production is that it does not contribute to the depletion of natural resources and environmental degradation, it is environmentally friendly energy production.At the same time, it is necessary that this parameter has the maximum possible efficiency (efficiency) values without noticeably reducing the long-term effect.This is a very difficult scientific and technical task, the solution of which is currently quite relevant.To increase the efficiency of using thermoelectric power generators, the development of hybrid energy methods, the sharing of thermoelectric devices with devices of the same agrophotovoltaic, or wind energy can be relevant.The increase in conversion efficiency is mainly due to an increase in the so-called thermoelectric material efficiency parameter (ZT).
where S, σ, T are Seebeck coefficient, electrical conductivity, and absolute temperature, respectively.And the parameter κ stands for the total thermal conductivity that is comprised of the lattice thermal conductivity, the electronic thermal conductivity, and the so-called bipolar thermal conductivity.The more ZT, the higher the efficiency of thermoelectric conversion.Equation (1) shows that thermoelectric materials with lower thermal conductivity, higher electrical conductivity and higher Seebeck coefficient will exhibit better thermoelectric performance.This parameter was introduced by the Great Soviet physicist Abram Joffe in 1949 [5].A.F. Joffe demonstrated that the most suitable thermoelectric results are to be expected in semiconductor materials In insulators, each free electron is very effective (high Seebeck coefficient), but there are few (< 10 15 cm -3 ) and the total efficiency of charge transfer is negligible.In metals, the amount of carriers is huge (~10 22 cm -3 ), but their efficiency is very low.The optimal combination of heat and electrophysical properties is achieved in highly doped semiconductors with a charge carrier concentration of the order of 10 19 -10 20 cm -3 .In this case, it turned out that despite the relatively low electrical conductivity, semiconductors may have significantly higher Seebeck coefficient values and lower thermal conductivity values.One of the widely used methods of increasing Z is the development of technology and the search for new materials, together with the improvement of known materials.Solids Bi2Te3−Bi2Se3 and Bi2Te3−Sb2Te3 are heavily used as the most effective thermoelectric materials to produce thermoelectricity at temperatures below 300−350℃.A diamond-like compound is a typical example agreeing with the concept of the pseudo cubic structure by tuning its lattice distortion parameter to unity.However, besides the band structure, optimized carrier concentration and reduced lattice thermal conductivity are also required for a high thermoelectric figure of merit (ZT).
A solid solution based on bismuth telluride (Bi2Te3−Sb2Te3).Bi2Te3 has a number of advantages.First, it has high thermoelectric parameters.Second, alloying based on it can produce materials with different conductivity types of both n -type and p -type.Third, telluride bismuth is easiest to produce in the form of fairly perfect monocrystals.
Many properties of such materials are important from the point of view of application, are determined by structural defects and related conditions.Some defects may be involved in the transformation of material structure when thermally activated processes, affecting the pattern of charge distribution and changes in interatomic distances.Practical interest may be caused by the type of defects at which a high charge transfer density is achieved at the maximum dispersion of phonon, which makes it possible to increase the thermoelectric quality of the general view of horizontal zone melting plant material by virtue of decrease in thermal conductivity.Structural construction features strongly influence the mechanical parameters of crystals, which is important for obtaining toughened materials.

Fig. 1. General view of horizontal zone melting plant 2 Materials and methods
Before choosing a technology for manufacturing semiconductor samples of p-and n-legs of a thermoelement (TE), a thorough study of the methods developed and implemented by previous applicants for thermoelectric energy was carried out [18][19][20][21][22][23][24][25].The influence of atmospheric gases on the properties of substances, the volatilization of components at high temperatures of the working chamber, and the like were also analyzed.Preliminary computational and theoretical results turned out to be encouraging to some extent.Therefore, the method of zone melting of a thermoelectric material of a triple compound based on (Вi, Sb)2(Te, Se)3 was chosen with the implementation of the technology in an laboratory audience using an electric resistance furnace (EPS) of indirect heating with a tubular chamber version [21].This choice was made on the basis of the following factors and considerations.EPS, after being connected to a source of electrical energy, enters the mode in a period of a fairly short duration.The existing furnace had the ability to tightly close the covers of the working chamber, consisting of asbestos with a metal casing, and was equipped with an automatic power regulator of the heating elements.The temperature of the working chamber made it possible to reach values of 1400ºС.This temperature was not used in our studies, since the melting temperature of the components of the substance was classified as low-temperature.
Zone melting also has advantages in that samples obtained by melting along temperature zones during crystallization are well formed into a lattice.In addition, the formation of defects such as cracks, deformations and uneven distribution of alloying impurities is practically not observed.All this can be eliminated only with the correct choice of the mode temperature, the speed of passage of the ampoule with the substance along the working chamber, as well as a clear account of the time spent by the starting materials in the corresponding temperature zones.There is another factor that cannot be neglected.This is a strict attitude towards obtaining a homogeneous substance.Homogeneity, as shown by the authors of [26][27][28][29], to some extent depends on the configuration of the ampoule.This is explained by the fact that the fraction of the melt crystallized in the lower part of the ampoule is small and the accumulation of tellurium in the melt does not lead to a stronger crystal inhomogeneity.However, such a solution to the issue of heterogeneity has the disadvantage of consuming a large amount of the source material.The study of this issue shows the presence of such a disadvantage occurs in the vertical melting of substances.Based on this, we considered the Champness method [20].The essence of this method lies in the implementation of the technological process for obtaining ternary alloys using a horizontal zone melting unit.The installation for obtaining a substance from a ternary compound by the horizontal zone melting method provided the maximum heating temperature up to 1200ºС (Fig. 1 and 2).At the same time, depending on the need, it was possible to set other operating temperatures with the movement of the arrow designed to fix the temperature limit.The arrow is mounted on a thermometer located on the side of the furnace casing.Automatic regulation of the furnace operation, with parallel regulation of the temperature of the working chamber, is carried out using a temperature sensor made of a chromel-aluminum thermocouple.Signals from thermocouples are fed to a millivoltmeter measuring device, the reading of which is automatically converted to degrees Celsius using graduated data stored in the memory of a digital measuring object.The temperature control of the zones was carried out by a two-position method in each zone.The temperature of the zones is pre-programmed based on theoretical assumptions and entered into the memory of the temperature controller.A simplified diagram of on-off temperature control is shown in figure 2. The principle of operation of the controller is as follows.Two temperature values are fixed: "upper" and "lower".This is indicated in the figure 3 as, T1 and T2.When the "Start" button is pressed, the MS magnetic starter receives power and closes its contacts located in the input circuit of the electric furnace.Heating starts, or switches to the next temperature heating mode.When the set temperature is reached, in order to maintain the initial base material for a predetermined time, the regulator, by switching on and off, maintains the set temperature for as long as necessary.Turning off the oven is as follows.When the upper set temperature is reached, the arrow-pointer, made of metal material, closes contacts T2, as a result of which power is supplied to the trip relay TR.The relay opens its contact in the magnetic starter circuit and de-energizes the electric furnace.During the period of switching on and switching off, temperature deviations from the set values were measured due to the thermal inertia of the material of the heating element and the admissibility of such a deviation was established.Since small deviations from the given temperature do not strongly affect the properties of the electrophysical parameters of the substance, changes in the chamber temperature within ±0.5% were allowed.Figure 4 shows a chart for controlling the chamber's operating temperature.As can be seen from the figure, the calculated data from the operational indicators, temperature changes differ by about 3÷4%.This is due to the influence of the thermal inertia of the heating element.The material of the heating element is Fechral, which was recommended by the standardization committee specifically for the operating temperature range up to 1400ºС.In the process of melting, the absence of the influence of thermal inertia on the properties of the substance was established.parameters strongly depends on the method of their manufacture.In addition, the method of pressing by the method of directional crystallization, due to the orientation of the grains, leads to a decrease in the thermoelectric figure of merit Z. Anisotropy of the mechanical properties of ternary alloys based on Bi2Te3-Sb2Te3 was also revealed.This is due to the fact that the resulting crushed substance in the form of metal powder has the shape of a polyhedron or needle-shaped chips.When they are pressed on planes perpendicular to the pressure force of the particles, a partial orientation of the grains occurs.This, to a certain extent, contributes to a change in the anisotropy of the material properties.
Based on the above experiments and the results of the work, we adopted the zone melting method for the manufacture of thermoelement legs.First, the stoichiometric composition of the material is established.A simple mathematical method determines the number of components of the melt.To obtain an n-type alloy, we selected 85 atm.%Bi2Te3 and 15 atm.%.Bi2 Se3 doped by 0,04 TeI4, of the following composition Bi-55,186 wt.%, Te-38,663 wt.%, Se-6,151 atm.%.To obtain a p-type alloy, the composition of the charge was determined, corresponding to 70 atm.%Sb2Te3, 30 atm.% Bi2Te3 doped by 0,25 Cd, by the method of proportion of the following amount: Bi -15,67% wt., Te -57,18% wt., Sb -27,15% weight.
The starting materials are placed in a quartz crucible and introduced into the melting zone.According to the temperature regime, the zone consists of five parts.The third zone corresponds to the highest temperature, since at this stage it is necessary to achieve good melting of the melt components, mixing them and forming the desired structure.In contrast to [27][28][29][30], the movement of the quartz substrate with the base material along the chamber of the tubular heater was carried out on a ribbon specially pulled up at the two ends of the chamber using the shafts of asynchronous motors with a power of 0.22 W. The speed of movement of the substrate was within 5 mm/h.The temperature of the material to be melted was measured with a thermocouple attached to the bottom of the substrate with a bead inside a drilled hole 0.18 mm in diameter.To obtain good thermal contact and to accurately measure the temperature, the thermocouple was lubricated with paste.This paste has good thermal conductivity.The thermocouple wires were insulated and well protected from heating by the heating elements.After obtaining the melt, its partial slow cooling was also carried out inside the chamber, by a gradual decrease in temperature.The last procedure was performed in two other zones, where an exponential temperature gradient is maintained along the chamber.Then a sufficiently hot ingot was removed from the chamber and subjected to cutting.The substance was cut on an electric spark cutting unit.The operation was carried out in a special bath, where the coolant, in addition to reducing the temperature of the alloy heated after the contact of the cutting wire with the material, also served to extinguish the spark.A current, about 1,5 A, was passed through the cutting wires, which gave the expected results, i.e. cut material smoothly with minimal mechanical defects.Half-legs of thermoelements were cut with dimensions of 4x4x7 mm 3 .The second batch of branches were made in the following sizes: 4x4x14 mm 3 .
The obtained samples were subjected to the measurement of their parameters.To study the properties of the branches, well-known methods for measuring electrical conductivity, thermal conductivity, and thermoelectromotive force coefficient were used, and the quality factor of the obtained samples was calculated [31][32][33][34].Our method for measuring the electrophysical parameters of a thermoelectric material differed from other methods [35][36][37][38] in that in this case, half-branches were cut from the substance and a thermoelectric converter in the form of a battery was immediately assembled.It consisted of eight thermoelements.The choice of this measurement method is also justified by the fact that in real cases it is planned to assemble a battery from our substances and use it as a current source in industrial enterprises in order to obtain direct current.In such cases, it is naturally expected that the hot end of the battery will be attached to a heat source, i.e., connected to other structures.In this E3S Web of Conferences 458, 01030 (2023) EMMFT-2023 https://doi.org/10.1051/e3sconf/202345801030case, it is necessary to check the influence of the contact of the material with the heat source, and, in addition, to study the possibilities of cascading thermoelements.Since the thermopile mounting method belongs to other authors, without giving in to details, we will describe the sequence of measurements.Note that the legs of the thermoelements were made of a semiconductor material based on ternary alloys BiTeSe (n -type) and BiSbTe (p -type), obtained by us by the described method.The geometric dimensions of the thermoelement legs were 14x4x4 mm 3 .Iron was used as the upper connection plate, while the lower connection plate was made of nickel.The very top, that is, the junction, which was calculated for heating, was covered with a ceramic plate based on beryllium oxide BeO.This material has good thermal conductivity and practically very close coefficient of thermal expansion to iron.The measurement and study of the energy and electrophysical parameters of the TB were carried out using several methods: heating the TEC surface by illuminating the light flux, switching to various modes (for example, idle, maximum power and short circuit modes) and the Harman method.

Results and discussion
According to the experiment, it was found (fig.5.) that in the branches of both types of conductivity, the electrical conductivity decreases exponentially with increasing temperature.Moreover, the decrease in σ for a material with electronic conductivity (curve 1) is much slower than for a substance with hole conductivity (curve 2).The temperature dependence of these properties was studied up to 420 K.At the same time, in almost the same way, the Seebeck coefficient increases (fig.6).In contrast to the n-type, this coefficient in the p-type material grows faster.These indicators are very close to the values given in [38][39].Moreover, different values of ΔТ, up to 200 K (at Тh=393 K and Тc=193 K), were obtained by two methods: illumination and heating of thermopile hot junctions.In order to achieve a good light absorption effect, the surface of the ceramic plate was coated with black paint.According to the Stefan-Boltzmann law, with such surfaces, absorption occurs one hundred percent.Its cold junctions were cooled by nitrogen vapor.The results did not differ significantly, which indicates the unimportance of the type of heat source.The decrease in the thermal conductivity coefficient is observed for both types of conductivity (fig.7).When comparing the values of the coefficients, the largest indicators χ correspond to the branches with electronic conductivity [40][41][42][43][44].However, with the achievement of T≈360 K, this parameter increases.The point of change in the course of thermal conductivity for both types of substance is the same.

Conclusions
Technological developments and a comprehensive study of the electrophysical properties of the legs of a thermoelement and a thermoelectric battery have shown that thermoelectric substances based on a triple compound based on Bi2Te3-Sb2Te3 and Bi2Te3-Se2Te3 obtained by zone melting are one of the best materials for the creation and operation of thermoelectric batteries.The thermoelectric figure of merit of the material, without complicating the technology of its manufacture, reaches values over 3•10 -1 К -1 , which gives good prospects for this technology and the creation of autonomous power sources in the field of thermoelectric power engineering.The use of thermopiles in the small-scale power industry makes it possible to additionally effectively use the waste of thermal energy from industrial enterprises.This contributes to solving one of the problems of energy saving and power supply to energy consumers in hard-to-reach areas of the planet.The possibility of operation of thermopile without energy losses on light fluxes, expands its areas of use.In addition, these sources of electrical energy, in the latter case, work as environmentally friendly alternative energy supply devices, which is a very important indicator of modern energy.

Fig. 4 .
Fig. 4. Scheme of automatic on-off temperature control of an electric resistance furnace.The technological work carried out on the manufacture of p-and n-type thermoelement legs by pressing by the authors of this work showed that the anisotropy of electrophysical