RESEARCH OF SENSITIVE ELEMENTS OF THERMOELECTRIC CONVERTERS BASED ON Hf1-xNbxNiSn

The results of modeling and experimental studies of the structural, magnetic, electrokinetic and energy properties of the thermometric material Hf1-xNbxNiSn, as well as the conversion functions of the sensitive elements of a thermoelectric thermometer based on it at temperatures of 4.2–1000 K are presented. For the case of an ordered variant of the crystal structure of the thermometric material, the simultaneous generation of donor and acceptor states in the forbidden band εg of the semiconductor is established. The dependence between the spatial arrangement of atoms in the nodes of the Hf1-xNbxNiSn unit cell and the mechanisms of electrical conductivity is revealed, which allows us to determine the conditions for the synthesis of materials with the maximum efficiency of converting thermal energy into electrical energy. It is shown that at Hf1-xNbxNiSn concentrations, x=0–0.02, the substitution of Hf atoms (5d26s2) by Nb atoms (4d45s1) in position 4a mainly occurs, which generates donor states. In the concentration range Hf1- xNbxNiSn, x=0.02–0.05, the substitution of Ni atoms (3d84s2) in position 4a by Nb atoms mainly occurs, which generates acceptor states, and at concentrations 0.05<x, the substitution of Hf atoms by Ni atoms generates additional donor states. The studied thermometric material Hf1-xNbxNiSn is promising for the manufacture of sensitive elements of thermoelectric thermometers. The transformation functions of the thermoelectric pair Hf0.99Nb0.01NiSn-(PtRh 13), the thermoelectrodes of which are made of the studied thermometric material and platinoid (PtRh 13) (positive branch), were simulated.

Thermometric materials Hf1-xNbxNiSn, x=0.01–0.10, for the manufacture of sensitive elements of temperature transducers were obtained by fusing the 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 cooled bottom (anode). Pre-fused sponge titanium was used as a getter. Heat treatment of Hf1-xNbxNiSn alloys consisted of homogenizing annealing at a temperature of 1073 K. Annealing of samples was carried out for 720 h in evacuated quartz ampoules (up to 1.0 Pa) in muffle electric furnaces with temperature control with an accuracy of ±10

K. Arrays of diffraction data were obtained on a STOE STADI-P powder X-ray diffractometer Cu Kα1-radiation). The structural characteristics of the Hf1-xNbxNiSn samples were calculated using the Fullprof software package. The chemical and phase compositions of the samples were monitored using metallographic analysis (Tescan Vega 3 LMU scanning electron microscope).

To optimize the parameters of the crystal and electronic structures, energy and kinetic properties of thermometric materials Hf1-xNbxNiSn, calculations were performed within the framework of the density functional theory (DFT) using the Vienna Ab initio Simulation Package VASP v. 5.4.4 with PAW-type potentials. The calculation of electronic kinetic coefficients was performed using the Exciting code (FLAPW method – Full Potential Linearized Augmented Plane Waves) by solving the linearized Boltzmann equation in the constant relaxation time approximation. The distribution of the density of electronic states (DOS) was simulated using the Korringa-Kohn-Rostoker (KKR) method. The transformation functions of the thermoelectric thermometer at temperatures of 4.2– 1000 K were simulated using the FLAPW methodмand the results of experimental measurements served as reference currents.

X-ray structural studies of the crystal structure of samples of the thermometric material Hf1-xNbxNiSn, x=0–0.10, established that the diffractograms are indexed in the structural type MgAgAs and there are no reflections of other phases on them. Based on the obtained diffraction patterns, the change in the period of the unit cell a(x) Hf1-xNbxNiSn, x=0–0.10 was calculated. It was expected that the period of the cell a(x) would decrease with increasing concentration of Nb atoms (rNb=0.146 nm), since its atomic radius is smaller than that of the Hf atom (rHf=0.158 nm). However, the change in the period of the cell a(x) Hf1-xNbxNiSn, x=0–0.10, is far from the expected and is of a complex nature. Thus, at concentrations of Nb atoms, x=0–0.02, the values of the cell period a(x), as predicted, rapidly decrease. However, at concentrations x=0.02–0.05, we observe the same rapid growth of the dependence a(x), which in the region of concentration x≈0.05 passes through a maximum and then decreases again. The obtained results of the change in the cell period a(x) Hf1-xNbxNiSn, x=0–0.10, indicate complex structural transformations, which are a consequence of simultaneous changes in several crystallographic positions of the semiconductor thermometric material, which will cause a redistribution of the density of electronic states.

Simulation of the density of states (DOS) distribution for the ordered variant of the crystal structure Hf1-xNbxNiSn, x=0–0.10, shows the position of the Fermi level εF and the width of the band gap εg. Thus, if in n-HfNiSn the Fermi level εF is located in the band gap εg near the edge of the conduction band εC, then at the lowest concentration of Nb atoms, x=0.005, the Fermi level εF will cross the edge of the conduction band εC: a dielectric-metal transition of conductivity will occur.

Studies of the specific magnetic susceptibility χ(x) of the thermometric material Hf1-xNbxNiSn, x=0–0.10, showed that the basic semiconductor n-HfNiSn is a weak diamagnet, as indicated by the negative values of the specific magnetic susceptibility χ at room temperature. Doping n-HfNiSn with Nb atoms makes the semiconductor Hf1-xNbxNiSn a Pauli paramagnet, in which the specific magnetic susceptibility is determined exclusively by the electron gas and is proportional to the density of states at the Fermi level g(εF)(x). The studies established that in the concentration range x=0–0.02 there is a rapid increase in the values of χ(x) of Hf1- xNbxNiSn, caused by the generation of donor states and an increase in the concentration of free electrons when replacing Hf atoms with Nb atoms. At higher concentrations of Nb atoms, the rate of change of the magnetic susceptibility χ(x) Hf1-xNbxNiSn decreases due to the appearance and increase in the concentration of acceptor states, which capture free electrons, reducing their concentration. The obtained results show that an external magnetic field with a strength of H≤10 kG does not affect the thermometric characteristics of thermal converters made of this material.

The nature of the change in the values of the specific electrical resistance ρ and the thermopower coefficient α of Hf1-xNbxNiSn with both temperature and concentration of Nb atoms is consistent with the conclusions made on the basis of structural studies and modeling of the electronic structure. Since the electrical resistance ρ(T,x) of Hf1-xNbxNiSn increases almost linearly with increasing temperature, which is the result of metallization of electrical conductivity, this material is unsuitable for obtaining sensitive elements of resistance thermometers.

The temperature dependences of the thermopower coefficient α(T,x) Hf1-xNbxNiSn show that the main carriers of the electric current of the thermometric material at all temperatures are free electrons. This is indicated by the negative values of the thermopower coefficient α at all concentrations and studied temperatures. It is shown that with an increase in the concentration of Nb atoms, the absolute change in the values of the thermopower coefficient α(T,x) Hf1-xNbxNiSn decreases. The reason for this is the decrease in the width of the forbidden band εg of the thermometric material. According to the nature of the behavior of the thermopower coefficient α(T,x) Hf1-xNbxNiSn, it is possible to establish the concentration of Nb atoms at which the change in the values of the thermopower coefficient α(T,x) will be the largest. In this case, this condition is achieved with the concentration of the semiconductor thermometric material Hf0.99Nb0.01NiSn, which became the basis for forming the electrode (negative leg) of the thermoelectric thermometer.