Thermoelectric properties of silicon oxide

. The article describes the results obtained from the study of the thermal conductivity of granulated silicon particles covered with a layer of silicon oxide. The results of the study show that the thermal conductivity of granulated silicon increases from  1,12 W/mK to  2,74 W/mK at T  300÷600 K, which is consistent with the results of silicon oxide (  SiO2  1 W/mK). It was also observed that the electrical conductivity changes in the range of  0,0038÷0,017 (Ohm  sm) -1 . The results of the study were explained based on the formation of a layer of silicon oxide on the surface of granulated silicon particles obtained by powder technology. Temperature-induced thermal voltaic effects are observed in the silicon oxide layer. This process depends on the formation of electron-hole pairs in cases with localized access, which leads to an increase in parameters  and  .


Introduction
Recently, there has been an increased interest in the method of preparing thermoelectric material and studying its characteristics by pressing together silicon particles covered with silicon oxide with powder technology (see, for example, [1] and references therein).According to the authors, the main characteristics of the thermoelectric material depend on the tunnel contacts and local energy levels formed in the silicon oxide layer.However, the research results were found to be inconsistent with those of silicon (Si148 W/mK) or silicon oxide (SiO21 W/mK).That is, at 400 K, of granulated silicon thermoelectric material the Seebeck coefficient is 500 microvolts/degree, thermal conductivity is 10÷15 W/mK.In order to clarify this imbalance, it is interesting to study the formation of silicon oxide layer on the surface of silicon particles prepared by powder technology and their main thermoelectric parameters.In connection with these, in this work, the results of the study of the formation of a silicon oxide layer on the surface of silicon particles prepared by powder technology and their thermoelectric parameters are discussed.

Materials and methods
It is known that the process of charge transfer in semiconductor particles prepared by powder technology and their thermoelectric properties depend on the method and structure of granular semiconductor preparation (see, for example, [2÷6] and references therein).Silicon with r-type electrical conductivity 1 (Omsm) -1 was selected for research [5÷13].The silicon content was ground to a powder in an oxygen atmosphere.Then, the main thermoelectric parameters of silicon particles (,  and ) were studied by the method of Egor and Disselkhorsta (Fig. 1) [14].It should be noted that this method was used in the research of Mg3Sb2 and ZnSb particles (see, for example, [2÷4] and references therein).Researches were carried out in the process of temperature change T=300-700 K.The composition of silicon particles was studied using a scanning electron microscope.
Figure 1 shows a simplified scheme of samples using the Egor and Disselhorsta method.ZnSb particles (1) are placed in a pipe-shaped dielectric (2).Two of the particles (A and В) are pressed by copper contacts (МА and МВ), as shown in Figure 1.The pressure force is Р30÷50 kilograms [14].In this case, the sample can be imagined in the form of a stem.Based on the method of Egor and Disselkhorsta, when Q heat is applied, the charges generated in area A move to area В, and an electric current is generated due to the temperature difference in contacts МА and МВ.The temperature difference is monitored using ТА and ТВ thermocouples.It should be noted that all studies were conducted during the processes of increasing and decreasing the temperature of heat treatment and time intervals.Between each heat treatment, the sample was cooled for 1 h, then reexamined.

Results and discussion
Figure 2 shows the dependence of  on temperature.It can be seen from Figure 2 that the thermal conductivity increases from 1,12 W/mK to 2.74 W/mK at Т300÷600 К.This corresponds to the results of silicon oxide (SiO21 W/mK), which is fundamentally different from the results obtained in [4].
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, it suddenly increases at T≤400 K and decreases at Т400÷450 K, jump changes were observed at the next stages of temperature increase.
It is known that the efficiency of thermoelectric devices is explained by the  =  2   indicator of the thermoelectric material.That is, an increase in  and  and  a decrease in  lead to an increase in the efficiency of thermoelectric devices.In our case, it was observed that the parameters  and  increase with temperature (figures 2 and 3, respectively).Figure 3 shows the dependence of electrical conductivity () on temperature.It can be seen from the figure that T≤383 K increases by one and changes steadily at T400÷600 K.This is consistent with the results of .In our opinion, the increase of  and  parameters with temperature may depend on the physical properties of the areas of the interparticle boundaries (areas 3 and 4, Fig. 1a) between two adjacent granular silicon particles.In our case, this interparticle boundary (areas 3 and 4, Fig. 1a) consists of a field of silicon oxide.To clarify this, the chemical composition of granulated silica particles was studied.Figures 4 and 5 show micrographs and X-ray spectral characteristics of granulated silica particles, respectively.The size of silicon particles is from 400 nanometers to 30 micrometers, and their surface is different, and it was found that their surface consists of unevenness (Figure 3).The composition of the particles is 94% pure silicon, and the rest is silicon dioxide compound (SiO2) (Figure 4).That is, the core of the particle is pure silicon, and its surface consists of silicon dioxide compound (SiO2).It was found that silicon dioxide is located on the surface of the particle, and its amount is distributed decreasing towards the core of the particle.So, the surface of silicon particles consists of a layer of silicon dioxide.They form a layer of silicon oxide in the areas of the inter particle boundary (areas 3 and 4, Fig. 1a) when placed in a dielectric (2) body in the form of a tube (1).
It is known that oxygen atoms diffuse in silicon with an increase in temperature [6÷13] and this leads to the formation of SiO2 or SixOу precipitates on the surface.As the concentration of oxygen atoms increases, the pore permeability between the silicon granules increases (Fig. 3).That is, with an increase in temperature, the localized traps with Ein energy level are ionized in the areas of the inter particle boundary (areas 3 and 4, Fig. 1a) consisting of a layer of silicon dioxide (Fig. 1c) [6÷13].With the formation of electron-hole pairs in them, thermal voltaic effects appear [7÷9].It should be noted that the results of current and voltage measurements confirmed the manifestation of thermovoltaic effects.As the temperature increases, the charges that appear in the A area move along the Ein energy levels to the В area with a relatively low temperature [15].As a result, at T≤400 K, the total  increases by one at the same time as the heat transfer in the areas of the inter particle boundary (areas 3 and 4, Fig. 1a).Such a process occurs that at the next stages of temperature increase, new localized traps with energy level Ein appear in the areas of the inter particle boundary (areas 3 and 4, Fig. 1a).The capture of charge carriers in them leads to a steady change in .In this case, the electrical conductivity and the potential difference can be brought to the same standard, causing a decrease in  at T400÷450 K, and a jump change at the next stages of temperature increase.

Conclusions
Thus, the results of the formation of silicon dioxide on the surface of granulated silicon particles, its thermal conductivity and electrical conductivity are different from the results of the thermoelectric material obtained based on granulated silicon presented in [4], and correspond to the results of silicon oxide.Research shows that granulated silicon particles with a surface layer of silicon dioxide can expand the possibility of creating a variety of semiconductor devices and thermoelectric materials that are cost-effective.Mechanisms to explain charge transfer processes and thermal conductivity may be important in explaining the physical processes occurring in thermoelectric materials whose surfaces are coated with coatings such as silicon dioxide.