Improved thermoelectric properties of layered Ti_1−xNb_xS_2−ySe_y solid solutions

The thermoelectric properties of a series of the polycrystalline samples of titanium dichalcogenides with partial substitution of Ti for Nb and S for Se were investigated. It was found that sintering of the samples improved the thermoelectric efficiency (ZT), and the maximum ZT was achieved at sintering temperature of 600°C. A further increase in the sintering temperature (850°C and 950°C) led to the recrystallization of the samples, as a result, the Seebeck coefficient sharply decreased and electrical conductivity dramatically increased. The temperature dependences of electrical conductivity σ(T) in the temperature range from 4.2 to 300 K and Seebeck coefficient S(T) in the temperature range from 77 to 300 K were investigated in order to determine the nature of the observed improvements in thermoelectric properties due to double substitutions and sintering. Two-dimensionalization of electron transport properties of Ti_1−xNb_xS_2−ySe_y solid solutions was found. The Fermi energy E_F2D was estimated using the temperature dependences of Seebeck coefficient. The relationship between the Fermi energy E_F2D and figure of merit ZT was established. The effect of sintering on parameters σ(T), S(T), charge carrier concentration (n_2D), mobility (µ), and thermal conductivity (k) was found. The optimal value of Fermi energy E_F2D in terms of figure of merit ZT = 0.31 at room temperature (T = 300 K) was found for Ti_0.98Nb_0.02S_1.3Se_0.7 sample sintered at 600°C.