FEATURES OF SIMULATION OF CHARACTERISTICS OF THERMOMETRIC MATERIAL Lu1-xZrxNiSb

The results of modeling the thermometric characteristics of the semiconductor solid solution Lu1-xZrxNiSb, which is a promising thermometric material for the manufacture of sensitive elements of thermoelectric and electro resistive thermocouples, are presented. Modeling of the electronic structure of Lu1-xZrxNiSb was performed by the Korringa-Kohn-Rostoker (KKR) method in the approximation of coherent potential and local density and by the full-potential method of linearized plane waves (FLAPW). KKR simulations were performed using the AkaiKKR software package in the local density approximation for the exchangecorrelation potential with parameterization Moruzzi, Janak, Williams in the semi-relativistic one taking into account the spin-orbit interaction. The implementation of the method in the Elk software package was used to perform FLAPW calculations. To check the limits of the existence of the thermometric material Lu1-xZrxNiSb, both methods were used to calculate the change in the values of the period of the unit cell a(x) in the range x=0–1.0. It is shown that there is an agreement between the change in the values of a(x) Lu1-xZrxNiSb calculated by the FLAPW method and the results of experimental studies. The obtained result indicates higher accuracy of modeling of structural parameters Lu1-xZrxNiSb by the FLAPW method in comparison with the KKR method. To study the possibility of obtaining thermometric material Lu1-xZrxNiSb and to establish the limits of its existence in the form of a continuous solid solution, modeling of thermodynamic characteristics in the approximation of harmonic oscillations of atoms within the theory of DFT density functional for a hypothetical solid solution Lu1-xZrxNiSb, x=0–1.0. The change in the values of the enthalpy of mixing ΔH and the total energy E Lu1-xZrxNiSb, x=0–1.0, allows us to state that the thermometric material exists in the form of a solid substitution solution in the concentration range x=0–0.20, stratification occurs (spinoidal phase decay) and thermometric material does not exist. To understand the mechanisms of electrical conductivity of the thermometric material Lu1-xZrxNiSb, the methods of entry of impurity Zr atoms into the matrix of the basic semiconductor p-LuNiSb and their occupation of different crystallographic positions, as well as the presence of vacancies in them, were investigated. For this purpose, its electronic structure was modeled for different variants of the spatial arrangement of atoms and the presence of vacancies in crystallographic positions. It is shown that the most acceptable results of experimental studies are the model of the electronic structure of p-LuNiSb, which assumes the presence of vacancies in the crystallographic positions of 4a Lu atoms (~0.005) and 4c Ni atoms (~0.04). In this model of the spatial arrangement of atoms and the presence of vacancies at positions 4a and 4c, the LuNiSb compound is a semiconductor of the hole-type conductivity, in which the Fermi level eF is located near the level of the valence band eV. The kinetic characteristics of the semiconductor thermometric material Lu1-xZrxNiSb, in particular, the temperature dependences of the resistivity ρ(T,x) and the thermopower coefficient α(T,x) are modeled. It is established that at the lowest concentrations of impurity atoms Zr the Fermi level eF Lu1-xZrxNiSb passes from the bandgap to the conduction band eС. This is indicated by the negative values of the thermopower coefficient α(T,x) and the metallic conductivity type Lu1-xZrxNiSb. This changes the type of main current carriers from holes to electrons.

KINETIC AND ENERGETIC PERFORMANCES OF THERMOMETRIC MATERIAL TiCo1-xMnxSb: MODELLING AND EXPERIMENT

The results of a complex study of the semiconductor thermometric material TiСo1-xMnxSb, х=0.01–0.10, for the producing of sensitive elements of thermoelectric and electro resistive sensors are presented. Microprobe analysis of the concentration of atoms on the surface of TiСo1-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 TiСo1-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 TiСo1-xMnxSb, х=0.01–0.10, for different variants of spatial arrangement of atoms is performed. To model energetic and kinetic performances, particularly the behavior of the Fermi level, the band gap, 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 TiСo1-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 band gap. In a semiconductor with the composition TiCo0.99Mn0.01Sb, the Fermi level is located in the middle of the band gap, 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 TiСo1-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)) TiСo1‑xMnxSb at the lowest concentration of impurity atoms Mn, х=001, indicates the location of the Fermi level in the band gap of the semiconductor thermopower coefficient α(Т,х) 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(Т,х) TiСo0,99Mn0,01Sb 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 TiСo0,98Mn0,02Sb, the sign of the thermopower coefficient α(Т,х) remains negative, but the value of resistivity ρ(х,Т) 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(х,Т) in the region of concentrations х=0.01–0.02 shows that acceptors are generated in the TiСo1-xMnxSb semiconductor when Co atoms are replaced by Mn, which capture free electrons, reducing their concentration. However, negative values of the thermopower coefficient α(Т,х) are evidence that either the semiconductor has a significant concentration of donors, which is greater than the number of introduced acceptors (х=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 TiСo1-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 TiСo1-xMnxSb, it is shown that the introduction of impurity Mn atoms into TiCoSb can simultaneously generate in the semiconductor 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 TiСo1-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 TiСo1-xMnxSb is a promising thermometric material.

STUDY OF THERMOMETRIC MATERIAL Er1-xScxNiSb. II. EXPERIMENTAL RESULTS

The results of a comprehensive study of the crystal and electronic structures, kinetic and energetic performances of the semiconductor thermometric material Er1-xScxNiSb, (x=0–0.1) are presented. Microprobe analysis of the concentration of atoms on the surface of Er1-xScxNiSb samples established their correspondence to the initial compositions of the charge, and the diffractograms of the samples are indexed in the structural type of MgAgAs. Because the atomic radius Sc (rSc=0.164 nm) is smaller than that of Er (rEr=0.176 nm), it is logical to reduce the values of the unit cell's period a(x) Er1-xScxNiSb, which correlate with the results of mathematical modeling. The temperature dependences of the resistivity ln(ρ(1/T)) contain high- and low-temperature activation regions, which are specific for semiconductors and indicate the location of the Fermi level in the bandgap, and positive values of the thermopower coefficient a(x, T) specify its position – near the valence band . This result does not agree with the results of modeling the electronic structure for its ordered version. The presence of a low-temperature activation region on the ln(ρ(1/T)) p-ErNiSb dependence with an activation energy =0.4 meV indicates the compensation of the sample provided by acceptors and donors of unknown origin. A decrease in the values of the resistivity ρ(x, T) and the thermopower coefficient a(x, T) points to an increase in the concentration of holes in p-Er1- xScxNiSb in the area of concentrations x=0–0.03. This is possible in a p-type semiconductor only by increasing the concentration of the main current carriers, which are holes. The fact of increasing the concentration of acceptors in Er1-xScxNiSb at insignificant concentrations of impurity atoms is also indicated by the nature of the change in the values of the activation energy of holes from the Fermi level to the valence band . Consequently, if in p-ErNiSb the Fermi level was at a distance of 45.4 meV from the level of the valence band , then at the concentration Er1-xScxNiSb, x=0.01, the Fermi level shifted towards the valence band and was located at a distance of 13.6. Since the Fermi level reflects the ratio of ionized acceptors and donors in the semiconductor, its movement by x=0.01 to the valence band is possible either with an increase in the number of acceptors or a rapid decrease in the concentration of ionized donors. At even higher concentrations of Sc impurity in p-Er1-xScxNiSb, x≥0.03, low-temperature activation sites appear on the ln(ρ(1/T)) dependences, which is a sign of compensation and evidence of the simultaneous generation of acceptor and donor structural defects in the crystal nature. This is also indicated by the change in the position of the Fermi level in the bandgap of the semiconductor Er1-xScxNiSb, which is almost linearly removed from the level of the valence band : (x=0.05)=58.6 meV and (x=0.10)=88.1 meV. Such a movement of the Fermi level during doping of a p-type semiconductor is possible only if donors of unknown origin are generated. For a p-type semiconductor, this is possible only if the concentration of the main current carriers, which are free holes, is reduced, and donors are generated that compensate for the acceptor states. This conclusion is also confirmed by the behavior of the thermopower coefficient a(x, T) at concentrations x≥0.03. The results of structural, kinetic, and energy studies of the thermometric material Er1-xScxNiSb allow us to speak about a complex mechanism of simultaneous generation of structural defects of acceptor and donor nature. However, the obtained array of experimental information does not allow us to unambiguously prove the existence of a mechanism for generating donors and acceptors. The research article offers a solution to this problem. Having the experimental results of the drift rate of the Fermi level as the activation energy (x) from the Fermi level to the valence band by calculating the distribution of the density of electronic states (DOS) sought the degree of compensation, which sets the direction and velocity of the Fermi level as close as possible to the experimental results. DOS calculations are performed for all variants of the location of atoms in the nodes of the unit cell, and the degree of occupancy of all positions by their own and/or foreign atoms. It turned out that for ErNiSb the most acceptable option is one that assumes the presence of vacancies in positions 4a and 4c of the Er and Ni atoms, respectively. Moreover, the number of vacancies in the position Er (4a) is twice less than the number of vacancies in the position Ni (4c). This proportion is maintained for Er1-xScxNiSb. Vacancies in the positions of Er (4a) and Ni (4c) atoms Er1-xScxNiSb are structural defects of acceptor nature, which generate two acceptor zones and in the semiconductor. The introduction of impurity Sc atoms into the ErNiSb structure by substituting Er atoms in position 4a is also accompanied by the occupation of vacancies by Sc atoms and a reduction in their number. Occupying a vacancy, the Sc atom participates in the formation of the valence band and the conduction band of the semiconductor Er1-xScxNiSb, acting as a source of free electrons. We can also assume that the introduction of Sc atoms into the structure of the compound ErNiSb is accompanied by a process of ordering the structure of Er1-xScxNiSb and Ni atoms occupy vacancies in position 4c. This process also, however, 2 times slower, leads to a decrease in the concentration of structural defects of acceptor nature. In this case, Ni, giving valence electrons, now act as donors.

Electrical properties of iron powder-based composites encapsulated by the oxide coating

The electrical characteristics of composite materials based on iron powders ASC 100.29 (Sweden) were investigated. The surface of powders is encapsulated by an insulating ferrite coating. The conductivity and the thermopower were studied, the cores were made by the pressing method, and then the samples were cut from the cores. The results show that the decrease in the electrical resistance with increasing temperature is due to a change in the contact between the grains in the sample. A change in the boundaries between grains (or domains) creates the conditions for formation of a new magnon, which affects the thermopower coefficient value. The research results can be used in the synthesis of composites with specified electromagnetic characteristics for practical applications.

Investigation of Ti1-xMoxCoSb Semiconducting Solid Solution

The effect of doping of the TiCoSb compound (MgAgAs structure type) by Mo atoms on the features of the structural characteristics and behavior of the electrokinetic, energetic and magnetic properties of the Ti1-xMoxCoSb semiconducting solid solution (х = 0 - 0.06) in the temperature interval 80 - 400 K was studied. It was shown that including of Mo atoms (rМо= 0.140 nm) in the ToCoSb structure by substitution of Ti atoms (rТі= 0.146 нм) in 4a position is accompanied with non-monotonous variation of the lattice parameter values а(х), indicating unpredictable structural changes. Based on analysis of the variation of the electric resistivity values, thermopower coefficient, magnetic susceptibility and energetic characteristics, it was concluded that simultaneous generation in the crystal of the structural defects of the donor and acceptor nature (donor-acceptor pairs), which generate corresponding energy levels in the band gap of semiconductor and determine its electrical conductivity.

Термоэлектрический механизм полевой эмиссии из углеродных наноструктур

A model describing the features of field emission from carbon nanomaterials is considered. The model is based on taking into account the effect of electron drag by ballistic phonons in the region of the temperature gradient inside the emission center. The model does not require any additional assumptions about the special energy structure of the emission center. Quantitative estimates of the thermopower coefficient, made on the basis of the emission model, are in good agreement with the experimental results.

Study of electrokinetic and magnetic properties of ZrNi1-xRhxSn semiconductive solid solution

The features of electrokinetic, energy state and magnetic characteristics of ZrNi1-xRhxSn semiconductive solid solution were investigated in the ranges: T = 80-400 K, x = 0-0.10. It was shown that substitution of Ni atoms (3d84s2) by Rh atoms (4d85s1) in the structure of ZrNiSn compound generated the structural defects with acceptor nature, and holes became the main charge carriers in the ZrNi1-xRhxSn at low temperature. Based on analysis of the motion rate of the Fermi level ΔεF/Δх in ZrNi1-xRhxSn to the valence band and change of sign of thermopower coefficient from positive to negative it was suggested that the structural defects of acceptor and donor natures were generated simultaneously (donor-acceptor pairs), and deep donor band ɛD2 was formed.

Features of Structural, Electrokinetic and Energy State Characteristics of V x+yCo1-ySb3 Skutterudite

The structure characteristics, temperature and concentration dependences of the electrical resistivity and the thermopower coefficient for the Vx+yCo1-ySb3 skutterudite were investigated in the ranges: T = 80 - 400 K, x = 0.02 - 0.20. It was shown that the introduction of V atoms into the structure of CoSb3 thermoelectric material was accompanied by an increase in the efficiency of conversion of thermal energy into electrical energy. It was established that the inversion in the sign of the thermopower coefficient for Vx+yCo1-ySb3 not observed for these concentrations of V impurity atoms. Based on the analysis of electrokinetic and energy state characteristics of Vx+yCo1-ySb3, it was assumed that the V (3d34s2) impurity atoms simultaneously replaced Co (3d74s2)atoms, generating structural defects of acceptor nature, and were located in the icosahedral voids of the crystal structure and generated donors.