Thermoelectric properties of granular Mg3Sb2 particles

. This article presents the results obtained in the study of thermoelectric properties of granulated Mg3Sb2 particles. The results of the study show that the thermoelectric properties of granulated Mg3Sb2 particles mainly depend on the physical processes occurring in the interparticle boundary areas. As the temperature increases, the localized traps in the interparticle boundary areas are ionized, and the capture of charge carriers in them leads to a decrease in electrical conductivity (  ). The Seebeck coefficient (  ) increases as the temperature difference occurs due to the potential difference and phonon absorption. Also, impurity thermal-voltaic effects appear with the formation of electron-hole pairs in impurity states with an energy level of Ein in the interparticle boundary regions. As a result, the total  increases at the same time as the thermal conductivity of the two adjacent areas. The convergence of electrical conductivity and potential difference leads to a relatively stable change of  . These processes lead to a change in the ZT index from  0.021 to  1.3 at T=300-700 K.


Introduction
In the field of world energy, the creation of new types of energy sources that do not emit harmful substances, ecologically clean, and the development of methods of their use is one of the urgent problems of today.In this field, the direct conversion of thermal energy into electrical energy with the help of semiconductor thermoelectric materials or thermoelectric devices takes the leading place.The efficiency of thermoelectric material is ZТ= 2 Т/, and its main parameters are electrical conductivity () and Seebeck coefficient (), on the other hand, electrical resistance () and thermal conductivity () are required to be low [1][2][3][4][5][6][7].Among semiconductor materials, Mg3Sb2 type materials have low thermal conductivity, and interest in studying its electrophysical and thermoelectric properties has increased.For example, in works [1][2][3][4][5][6][7], it was shown that the main thermoelectric parameters of the Mg3Sb2 semiconductor depend on the structure of the material, temperature, and Mg atoms.It was observed that with the increase in temperature, the formation of electron-hole pairs in its forbidden zone increases  and , and on the contrary, the phonon movement in the crystal lattice decreases , which in turn leads to an increase in ZT.Theoretical and practical studies show that it is recognized that such a result can be achieved by controlling the structure of the material and the effect of an additional Mg atom on it, as well as the method of obtaining Mg3Sb2.For example, the thermoelectric parameters of Mg3Sb2 have been improved by introducing Mg atoms [1][2][3][4][5][6][7].However, the thermoelectric properties of granular Mg3Sb2 particles are one of the unexplored areas.
In [8], we studied the temperature dependence of the specific resistance () of granulated Mg3Sb2 particles.Research results show that  increases with temperature, while  decreases.It is interesting to study how this condition affects the thermoelectric properties of granulated Mg3Sb2 particles.

Materials and Methods
Mg3Sb2 material was selected for research.Based on powder technology, Mg3Sb2 was pulverized to a particle state [8].Then, a mixture of granulated Mg3Sb2 particles was prepared using ethyl alcohol.The mixture is placed on a heat-resistant, for example, tubeshaped ceramic base and pressed with a force of 30-50 kg.It is heated from the outside using a heating furnace (Q).As a result, the aggregate of Mg3Sb2 particles granulated inside the ceramic substrate takes the form of stergen.Egor and Disselkhorsta's method can be used to determine the thermal conductivity of stergen heated by electric current [8][9][10][11][12][13].The novelty of the research is that, for the first time, the thermoelectric properties of granulated Mg3Sb2 particles during the temperature change of T=300-700 K were studied based on the method of Egor and Disselkhorsta [8,9].
Figure 1 shows a simplified scheme of the sample using the Egor and Disselhorsta method.According to the method of Egor and Disselkhorsta, when Q heat is applied to the sample, an electric driving force appears due to the temperature difference in the MA and MВ contacts.The temperature difference was controlled using TA and TВ thermocouples.It should be noted that all studies were conducted in the process of temperature increase and decrease.depending on the mobility () and concentration (n) of charge carriers as follows.Under the influence of temperature, ionization of inpurity and thermal oscillation of the crystal lattice occur, as a result of which the free path of carriers decreases.In this case, the reduction of  and n leads to a corresponding reduction of  (Figure 2, case a-b).The steady change of  in the later stages of temperature increase is explained as dependent on the increase of n.However, it was observed that the results of this study are significantly different from the results obtained by other scientists on Mg3Sb2 material.For example, in works [1][2][3][4][5], it was observed that electrical conductivity takes values of MOm and increases with temperature.The reason for this is the technology of Mg3Sb2 material preparation.That is, the results obtained in works [1][2][3][4][5] refer to the Mg3Sb2 material prepared by pressing under vacuum conditions, and it was observed that the decrease in the amount of Mg atoms leads to a decrease in  [1-7].The temperature dependence of , like , can be conditionally divided into two parts, at T563 K (a-b) and the next state (b-c) (Fig. 2, line 2).The dependence of  on temperature corresponds to the results obtained in [1,2,4,5].That is, when Q heat is given, a temperature gradient (TA and TВ, Fig. 1a) and a potential difference appear in the A and В (xA and xA) areas.Such a process occurs that a large number of localized traps are ionized in two adjacent areas (areas 3 and 4, Fig. 1a) between the granules located in area A of the sample [8,[10][11][12][13][14][15][16].As a result, the potential difference increases with the capture of charges in ionized traps, and a temperature gradient occurs due to the absorption of phonons in localized traps.This, in turn, may lead to an increase in  at T563 K (Figure 2, line 2, case a-b).In the next stages of temperature increase, thermal energy also increases in area V.As a result, temperature gradient and potential difference are relatively reduced.This, in turn, may lead to a relatively stable variation of  at T≥550 K.

Results and Discussion
Figures 3 and 4 show the temperature dependence of parameters  and ZT, respectively.It is known that  is mainly explained by crystal lattice conductivity of thermoelectric material and phonon migration.For example, a thermoelectric material with a polycrystalline structure leads to a decrease in  due to the phonon migration in the intergranular boundary regions and a decrease in the conductivity of the crystal lattice.However, in our case, no reduction of  was observed.On the contrary, at T≤560 K, a sudden increase and then a steady change were found (Fig. 3).The studied sample consists of two contiguous border areas (areas 3 and 4, Fig. 1a).In our opinion, the increase of  with temperature may depend on the physical properties of the areas of two adjacent boundaries (areas 3 and 4, Fig. 1a) between granulated Mg3Sb2 particles.That is, with an increase in temperature, localized traps with Ein energy level ionize in the areas of two adjacent boundaries (areas 3 and 4, Fig. 1a) (Fig. 1b) [11][12][13][14][15][16][17][18][19][20][21][22][23].With the formation of electron-hole pairs in them, Impurity Thermal-voltaic Effects appear [15][16][17][18][19][20][21][22][23].It should be noted that the results of current and voltage measurements confirmed the manifestation of Impurity Thermal-voltaic Effects.As the temperature increases, the charges that appear in the A area move along the Ein energy levels the B area with a relatively low temperature [15].As a result, the total  increases at the same time as the conductivity of the two adjacent areas.Such a process occurs that in the next stages of temperature increase, the temperature of areas A and B becomes stable.In this case, the convergence of electrical conductivity and potential difference leads to a relatively stable change of  (case T≥560 K).It was observed that ZT is also significantly affected by the physical processes that increase with temperature in the areas of two adjacent boundaries (areas 3 and 4, Fig. 1a) between granular Mg3Sb2 particles (Fig. 4).For example, at T=300-700 K, the ZT indicator changes from 0.021 to 1.3 (Fig. 4).
It is known that the best (ZT) index in high-temperature thermoelectric materials today belongs to the high quantum dot lattice, and its value has been determined to be close to 3.5.American experts have shown that the ZT index is greater than 2 in the solid substance obtained on the basis of nanostructures.The ZT index of Mg3Sb2 thermoelectric materials is 0.23 [1].This is much less than our research results.In our opinion, it is possible to change the ZT index up to 1.3 by controlling the physical processes manifested in the areas of two adjacent boundaries between granular Mg3Sb2 particles (areas 3 and 4, Fig. 1a).

Conclusions
Thus, the thermoelectric properties of Mg3Sb2 particles depend on the physical processes manifested in the interparticle boundary areas.For example, with an increase in temperature, impurity thermal-voltaic effects appear with the formation of electron-hole pairs in impurity states in two boundary regions.As a result, the total  increases at the same time as the thermal conductivity of the two adjacent areas.In other words, it is possible to change the ZT index by controlling the physical processes manifested in two adjacent boundary areas between granulated Mg3Sb2 particles.The results of the research can expand the possibility of creating relatively cheap various semiconductor devices and thermoelectric materials based on granulated Mg3Sb2.

Fig. 1 .
Fig. 1.A simplified scheme of sample measurement using Egor and Disselkhorsta method (a), temperature difference (b).Here, 1 -Mg3Sb2 particles, 2heat-resistant dielectric case, 3 and 4interparticle boundary area, ohmic contacts and thermocouples in A and V areas, respectively, MA and MВ and TA and TВ.

Figure 2
Figure2shows the dependence of  and  on temperature.The dependence of  on temperature can be conditionally divided into two parts (a-b) and (b-c) (Fig.2, line 1).At T375 K, it decreases sharply (case a-b), then changes steadily (case b-c).It should be noted that in[8] we explained the temperature dependence of  (specific resistance)