Features of thermoelectric properties of some magnetic semiconductors

,


Introduction
The search for environmentally friendly, compact, durable, maintenance-free commercial methods of generating electricity is becoming an increasingly priority.Comparatively speaking, TEPG has been recognized as one of the main energy conversion technologies, mainly because of the advantages mentioned above.These are advantages such as compact size, relatively light weight, noiselessness and zero emissions.This method is interesting because it can provide power generation in terms of safety, reliability and maintainability.Obviously, this method of generating energy can generate heat or electricity for long periods of time, measured in decades.In addition, it must be said that over the past few years, new industries have appeared, such as 5G communication, the Internet of Things network and wearable electronics, which have demonstrated a great demand for miniature cooling and power generation based on highly efficient thermoelectric devices at room temperature [1][2][3][4][5][6][7][8].And the most important thing is that TEPG devices are also cost-effective when the environment is taken into account.This makes the method attractive for power generation [1][2][3][4][5][6][7][8][9][10] in addition to its excellent characteristics for use in high-tech fields, including specialized medicine, space and military applications.The European Union is already investing in low-energy electricity generation and sensors.At the same time, China is making progress in the theory of cooling and the production of thermoelectric semiconductors.[9][10][11][12].
The key parameter, "dimensionless thermoelectric efficiency ZT", defined as ZT = S 2 σT/κ, is a measure of the conversion efficiency of the thermoelectric device.Here S, σ, T is the Seebeck coefficient, electrical conductivity and absolute temperature, respectively.In addition to the low value of thermal conductivity, for the successful operation of the thermoelectric devices, it is required that the power factor thermoelectricity (PF = S 2 σ) is large.At the moment, Bi2Te3-based materials remain the most effective and most widely studied thermal insulation materials at room temperature, and in their bulk form they there are quite a lot of methods for obtaining semiconductor compounds, developed in order to obtain good thermoelectric materials for thermoelectric energy converters (TC) [5,[12][13][14][15].The existing methods are based on the dependence of electrophysical parameters, and, consequently, thermoelectric figure of merit Z, on the technology of manufacturing thermoelement legs.Today, there are many materials that, according to most researchers, are considered promising substances with intriguing thermoelectric properties.At the beginning of the development of thermoelectric power engineering, it was shown [5] that a good working material should have a high electrical conductivity σ and a thermal emf coefficient α.In addition, the lower the thermal conductivity of the material, the higher its quality factor.Most studies are aimed at introducing dopants into the composition of base materials.Moreover, due to the fact that the amount of the alloying substance makes it possible to change, and to some extent regulate the values of the parameters of the material, many works have been published in this direction.
Chronological observation and analysis of existing scientific literature shows the dominant indicators of the use of a semiconductor compound based on bismuth telluride.These substances have not bad values of quality factor.
There are several ways to improve the thermoelectric figure of merit.The main thing at the moment seems to be the use of spatially inhomogeneous materials with inhomogeneities, the dimensions of which are comparable with the characteristic wavelengths of electrons and phonons, i.e. lie in the nanometer region.However, some options for obtaining good substances related to the technical and economic indicators of devices, simplification of technological aspects have not yet been deeply considered.In this regard, in this work, an attempt was made to simplify the technology for obtaining semiconductor materials of a ternary compound in ordinary laboratory conditions.dominate commercial thermoelectric applications.[5,[15][16][17][18] However, we must admit that the search for new materials and also the search for the most effective technologies remain relevant today.
Experts suggest that the use of magnetic semiconductors may be another way to achieve a factor of high power.We chose CuFeS2, a natural magnetic semiconductor known as chalcopyrite, for research.CuFeS2 has a tetragonal structure with the space group I 42d.This structure is derived from zinc blende.The modification of the structure occurs due to the alternate distribution of Cu and Fe ions in the cation centre, which leads to a doubling of the c axis.

Materials and methods
Using a magnetic semiconductor to increase the power factor is a very simple and effective guide that is applicable to a wide range of materials.Crystals obtained by the Bridgman method, as well as sintered polycrystalline Cu1_xZnxFeS2 samples were studied in this work [5,19].The samples were synthesized by direct reaction of Cu(4N), Fe(3N) and Zn(4N) powders with S (6N).Metal powders and sulfur were sealed into evacuated silica tubes and heated at a temperature of 973 K for 24 hours, cooled and kept at a temperature of 650 K for 24 hours.The tubes were then cooled to room temperature for 24 hours.The granules were crushed and sintered by spark plasma sintering in Ar medium at a temperature of 773 K for 5 min.
The sintered granules were further annealed at a temperature of 650 K. Experimental studies of the temperature dependences of the Hall coefficient R(T) and the specific electrical conductivity σ (T) of samples with n-type conductivity were carried out by the usual probe compensation method in constant weak electric and magnetic fields in a wide temperature range of 1.5-300 K.When it was necessary, an additional calculation of the temperature dependences of the electron concentration and the mobility was carried out.

Results and discussion
Typical R(T), σ(T) and curves for CuFeS, are presented in figure 1.One can see that over a large temperature interval these values have the unusual power character R, σ, n ~ T k .In the range 1.5 К <Т<40 К the power index is equal to 3 for σ (T).The errors of measurement in this temperature interval do not enable us to construct the R(T) and n(T) curves.However, extrapolation of the n(T) value at the temperature T ~1.5 К (here the condition (Т) = const is used (see below)) gives the value n ~ 10 16 -10 17 сm -3 .In the temperature range 40K < T < 300K the power index К is equal to -3/2 for R(T) and 3/2 for σ(T) and n(T).The power dependences and the absence of the exponential dependences R(T), σ(T) and n(T) is as for the magnetic semiconductor.Therefore, taking these results and the small value of the electron concentration at low temperatures as the basis, we concluded that CuFeS2 belongs.to this class of compounds [19][20][21][22][23][24][25][26].Similar investigations of CuFeTe, also reveal the existence of the power behaviour of the R(T), o(T) and n(T) values (figure 2).However, the power indices differ from the values obtained for CuFeS 2 .The value k is equal to -1.9 for R(T), 1.15 for σ(T) and 1.9 for n(T).Concluding that CuFeTe2 also belongs to the such semiconductors one should note that the temperature dependence n(T) distinguishes this substance from classical zero-gap compounds of the first and second types, where n(T) ~ Т 3 and T 3/2 , respectively.Of special interest is the investigation of the temperature dependences of the carrier mobility (T) of both substances (fig.3).In the compound CuFeS2 this value is small,  ~ 1 cm 2 V -1 s -1 , and almost independent of temperature in the interval measured.Such behavior is characteristic of a large-radius magnetic polaron (ferron) in antiferromagnetic, when the conduction electron creates a ferromagnetic region of radius around itself [25][26][27].The results obtained are consistent with the results obtained on samples prepared using a conventional solid-phase reaction, as described earlier.13)The author noted that such samples give the Seebeck effect practically independent of temperatures above 100 K and show a large negative value at room temperature.When switching to alloying samples, as the authors note, the samples of chalcopyrite doped with a carrier are Zn0:03Cu0:97FeS2, Cu0:97-Fe1:03S2 and Cu0:95Fe1:05S2.values Cu0:97Fe1:03S2 and Cu0:95Fe1:05S2 decreases monotonically with decreasing temperature, which indicates the degenerate behaviour of the semiconductor.at 400 K, this is 3-5 microns cm for two samples.It is noted that the process of solid-phase synthesis, which was applied to real samples, turned out to be very effective for reducing the electrical resistivity.
Although the ZT of the presented samples is less than 0.1, we must emphasize that a high power factor exceeding 1 10 3 WK 2 m 1 is achieved using a new mechanism: the increased mass of the carrier is provided by a strong interaction between the carrier and the magnetic moment [27][28][29].

Conclusions
It is important to note that the properties of magnetic semiconductors, especially in their applicability to thermoelectric power, have not yet been widely studied.Evaluation of the results obtained and their comparison with the results of other authors show that using the interaction between carriers and magnetic moments can be an effective and very simple way to increaseиthe power factor.An important solution for increasing power may be to reduce thermal conductivity due to nano structuring of magnetic semiconductors.This may allow the use of these semiconductors for the manufacture of highly efficient [30] thermoelectric materials.