Investigation of thermometric material Ti1-xScxCoSb modeling of characteristics

Abstract

The second part of the complex research of Ti1-xScxCoSb thermometric material for the sensitive elements of thermoelectric and electro resistant thermal converters is presented. Simulation of thermodynamic, electrotechnical, energetic and structural characteristics of Ti1-xScxCoSb semiconductor thermometric material for various options of atoms placement is performed. It is determined that under the orderly variant of the crystal structure Ti1-xScxCoSb the characteristic simulation results do not correspond to the experimental research results of temperature and concentration dependences of the resistivity, thermoEMF coefficient of the Fermi εF level behavior character, etc. Thus, for the ordered structure of Ti1-xScxCoSb, the simulation showed that the substitution in the crystallographic position 4a of the TiCoSb compound of atoms Ti (3d24s2) at Sc (3d14s2) generates structural defects of the acceptor nature since the Sc atom has fewer 3 d-electrons. Adding to the TiCoSb the smallest in the experiment concentration of Sc atoms by replacing the Ti atoms radically changes the behavior of the resistivity ρ and the coefficient of thermo-EMF α Ti0.995Sc0.005CoSb. In the 80–350 K temperature range, the resistivity value ρ increases with temperature increasing, and the conductivity of Ti0.995Sc0.005CoSb is metallic. It means that the addition of the smallest in the experiment concentration of atoms Sc (x=0.005) which should generate acceptors changed the position of the Fermi level εF in a way that could only cause the appearance of donors in the semiconductor. Thus, if in TiCoSb the Fermi level εF laid in the restricted area, then the metallization of the conductivity Ti0.995Sc0.005CoSb indicates that it not only approached the conduction zone but also crossed its leakage level, and electrons remain the main carriers of electricity. This is indicated by the negative values of the thermo-EMF coefficient α Ti0.995Sc0.005CoSb, which is only possible if donors of unknown nature are generated. The metallization of the conductivity Ti0.995Sc0.005CoSb also does not match the results of the electronic structure simulation for the ordered structure variant. After all, the simulation demonstrates that at the smallest concentration of the Sc acceptor impurity, the Fermi level εF drifts from the conduction zone εC to the middle of the restricted zone εg. Therefore, in the high-temperature area of dependence ln(ρ(1/T)), there must be an activation area associated with the thermal emission of electrons from the Fermi level εF into the conduction zone εC, and the value of the electron activation energy ε1ρ should be greater than in the case of TiCoSb. To clarify the crystalline and electronic structure of the TiCoSb compound, electronic state density distribution (DOS) simulations were performed for various options of occupying crystallographic positions by atoms, as well as occupying by atoms of tetrahedral voids of structure that make up ~24 % of the unit cell volume. It is shown that structural defects of the donor and acceptor nature are present in the TiCoSb base compound as a result of the location in the tetrahedral voids of the structure of additional Co * atoms and the presence of vacancies in the crystallographic position of 4a of the Ti atoms. Introduction to TiCoSb compound of impurity Sc atoms by substitution at position 4a of Ti atoms generates structural defects of acceptor nature, and the ratio of Ti1-xScxCoSb in the concentrations of available defects of donor and acceptor nature determines the location of the Fermi level εF and mechanisms of conductivity. The obtained results allow us to predictably simulate and obtain thermometric materials Ti1- xScxCoSb for the sensitive elements of thermotransducers.