Browsing by Author "Rokomanyuk Mariya"

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Kinetic and energetic performances of thermometric material TiCo1-xMnxSb: modelling and experiment
(Lviv Politechnic Publishing House, 2021) Krayovskyy Volodymyr; Pashkevych Volodymyr; Rokomanyuk Mariya; Haranuk Petro; Romaka Volodymyr; Stadnyk Yuriy; Romaka Lyubov; Horyn Andriy
The results of a complex study of the semiconductor thermometric material TiCo1-xMnxSb, x=0.01–0.10, for the production of sensitive elements of thermoelectric and electro resistive sensors are presented. Microprobe analysis of the concentration of atoms on the surface of TiCo1-xMnxSb samples established their correspondence to the initial compositions of the charge, and X-ray phase analysis showed the absence of traces of extraneous phases on their diffractograms. The produced structural studies of the thermometric material TiCo1-xMnxSb allow to speak about the ordering of its crystal structure, and the substitution of Co atoms on Mn at the 4c position generate structural defects of acceptor nature. The obtained results testify to the homogeneity of the samples and their suitability for the study of electrokinetic performances and the manufacture of sensitive elements of thermocouples. Modeling of structural, electrokinetic, and energetic performances of TiCo1-xMnxSb, x=0.01–0.10, for different variants of the spatial arrangement of atoms is performed. To model energetic and kinetic performances, particularly the behavior of the Fermi level , the bandgap , the density of states (DOS) distribution was calculated for an ordered variant of the structure in which Co atoms at position 4c are replaced by Mn atoms. Substitution of Co atoms (3d74s2) by Mn (3d54s2) generates structural defects of acceptor nature in the TiCo1-xMnxSb semiconductor (the Mn atom contains fewer 3d- electrons than Co). This, at the lowest concentrations of impurity atoms Mn, leads to the movement of the Fermi level from the conduction band to the depth of the bandgap . In a semiconductor with the composition TiCo0.99Mn0.01Sb, the Fermi level is located in the middle of the bandgap , indicating its maximum compensation when the concentrations of ionized acceptors and donors are close. At higher concentrations of impurity Mn atoms, the number of generated acceptors will exceed the concentration of donors, and the concentration of free holes will exceed the concentration of electrons. Under these conditions, the Fermi level approach, and then the level of the valence band TiCo1-xMnxSb cross: the dielectric-metal conductivity transition take place. The presence of a high-temperature activation region on the temperature dependence of the resistivity ln(ρ(1/T)) TiCo1-xMnxSb at the lowest concentration of impurity atoms Mn, x=001, indicates the location of the Fermi level in the bandgap of the semiconductor thermopower coefficient α(T,x) at these temperatures specify its position - at a distance of ~ 6 meV from the level of the conduction band . In this case, electrons are the main carriers of current. The absence of a low temperature activation region on this dependence indicates the absence of the jumping mechanism conductivity. Negative values of the thermopower coefficient α(T,x) TiCo0,99Mn0,01Sb at all temperatures, when according to DOS calculations the concentrations of acceptors and donors are close, and the semiconductor is maximally compensated, can be explained by the higher concentration of uncontrolled donors. However, even at higher concentrations of impurity Mn atoms in TiCo0,98Mn0,02Sb, the sign of the thermopower coefficient α(T,x) remains negative, but the value of resistivity ρ(x, T) increases rapidly, and the Fermi level deepens into the forbidden zone at a distance of ~ 30 meV. The rapid increase in the values of the resistivity ρ(x, T) in the region of concentrations x=0.01–0.02 shows that acceptors are generated in the TiCo1-xMnxSb semiconductor when Co atoms are replaced by Mn, which capture free electrons, reducing their concentration. However, negative values of the thermopower coefficient α(T,x) are evidence that either the semiconductor has a significant concentration of donors, which is greater than the number of introduced acceptors (x=0.02), or the crystal simultaneously generates defects of acceptor and donor nature. The obtained result does not agree with the calculations of the electronic structure of the TiCo1-xMnxSb semiconductor. It is concluded that more complex structural changes occur in the semiconductor than the linear substitution of Co atoms by Mn, which simultaneously generate structural defects of acceptor and donor nature by different mechanisms, but the concentration of donors exceeds the concentration of generated acceptors. Based on a comprehensive study of the electronic structure, kinetic and energetic performances of the thermosensitive material TiCo1-xMnxSb, it is shown that the introduction of impurity Mn atoms into TiCoSb can simultaneously generate an acceptor zone (substitution of Co atoms for Mn) and donor zones and of different nature. The ratio of the concentrations of ionized acceptors and donors generated in TiCo1-xMnxSb will determine the position of the Fermi level and the mechanisms of electrical conductivity. However, this issue requires additional research, in particular structural and modeling of the electronic structure of a semiconductor solid solution under different conditions of entry into the structure of impurity Mn atoms. The investigated solid solution TiCo1-xMnxSb is a promising thermometric material. Key words: Electronic structure; Resistivity; Thermopower.
Studies of thermometric material Lu1-xZrxNiSb
(Lviv Politechnic Publishing House, 2022) Pashkevych Volodymyr; Krayovskyy Volodymyr; Rokomanyuk Mariya; Haranuk Petro; Romaka Volodymyr; Stadnyk Yuriy; Romaka Lyubov; Horyn Andriy; Fruchart Daniel
The results of experimental research of perspective thermometric material Lu1-xZrxNiSb which can be used for the production of sensitive elements of thermoelectric and electroresistive thermometers are presented. Thermometric materials Lu1–xZrxNiSb, x = 0.01 – 0.10, were made by fusing a charge of components in an electric arc furnace with a tungsten electrode (cathode) in an atmosphere of purified argon under a pressure of 0.1 kPa on a copper water cooled hearth (anode). Heat treatment of alloys consisted of homogenizing annealing at a temperature of 1073 K. Annealing of samples was carried out for 720 h in vacuumed up to 1.0 Pa ampoules of quartz glass in muffle electric furnaces with temperature control with an accuracy of ±10 K. Diffraction arrays were obtained on a diffractometer DRON-4.0 (FeKα radiation), and the structural characteristics of Lu1–xZrxNiSb were calculated using the Fullprof program. The chemical and phase compositions of the samples were monitored using a scanning electron microscope (Tescan Vega 3 LMU). The study of the temperature dependences of the resistivity ρ(T, x) and the thermopower coefficient α(T, x) Lu1–xZrxNiSb was performed in the temperature range of 80÷400 K on samples in the form of rectangular parallelepipeds measuring ~1.0×1.0×5.0 mm3. Measurements of the values of the specific magnetic susceptibility χ(x) of Lu1–xZrxNiSb samples were performed by the relative Faraday method at a temperature of 273 K using a thermogravimetric installation with an electronic microbalance EM-5-ZMP in magnetic fields up to 10 kGs. Microprobe analysis of the concentration of atoms on the surface of Lu1–xZrxNiSb samples, x = 0.01–0.10, established their correspondence to the initial compositions of the charge, and X-ray phase analysis showed no traces of extraneous phases on the sample diffractograms, except for the main phase. The nonmonotonic nature of the change in the values of the unit cell period of the thermometric material an(x) Lu1–xZrxNiSb, x = 0.01 – 0.10, which differs from the results of modeling structural characteristics using software packages AkaiKKR and Elk. The nonmonotonic change in the values of the period of the unit cell a(x) Lu1–xZrxNiSb and the presence of the extremum dependence suggests that the impurity Zr atoms introduced into the matrix of the LuNiSb basic semiconductor can simultaneously occupy partially different crystallographic positions in different ratios. The temperature resistivities ρ and the thermopower coefficient α of the LuNiSb base semiconductor contain high- and low-temperature activation regions, which is characteristic of doped and compensated semiconductors. The introduction into the LuNiSb structure of the lowest concentration of impurity Zr atoms in the experiment (x = 0.01) radically changes both the behavior of the temperature dependences of the resistivity ρ and the thermopower coefficient α and the type of the main electric current carriers. The values of the resistivity ρ(T, x) Lu1–xZrxNiSb only increase with increasing temperature, which is characteristic of the metallic type of electrical conductivity and is due to the mechanisms of scattering of current carriers. This nature of the change in electrical resistance ρ(T, x) is evidence that the Fermi level εF has left the bandgap εg and is in the conduction band εC. This is indicated by the negative values of thermopower coefficient α(T, x) at all сoncentrations and temperatures. Studies of the magnetic susceptibility χ(x) showed that the samples as a basic semiconductor LuNiSb, as well as the thermometric material Lu1–xZrxNiSb, at all concentrations of impurities Zr, are Pauli paramagnetic. There is a synchronicity of the behavior of χ(x) with the dependences of the resistivity ρ(x, T) and the thermopower coefficient α(x, T), which is due to the change in the density of states at the Fermi level g(εF). The results of experimental studies of the Lu1–xZrxNiSb thermometric material completely coincide with the results of modeling its kinetic characteristics under the presence of vacancies in the crystallographic positions 4a and 4c of the Lu and Ni atoms, respectively. Such studies allow making adjustments in the structural studies of thermometric material with an accuracy that significantly exceeds the accuracy of X-ray research methods. The obtained results will allow us to clarify the spatial arrangement of atoms in the nodes of the unit cell, as well as to identify the mechanisms of electrical conductivity to determine the conditions for the synthesis of thermosensitive materials with maximum efficiency of thermal energy conversion into electricity.