Improved thermoelectric properties of layered Ti(1-)(x)Nb(x)S(2-)(y)Se(y)solid solutions

Abstract

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 maximumZTwas achieved at sintering temperature of 600 degrees C. A further increase in the sintering temperature (850 degrees C and 950 degrees 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 sigma(T) in the temperature range from 4.2 to 300 K and Seebeck coefficientS(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-)(x)Nb(x)S(2-)(y)Se(y)solid solutions was found. The Fermi energyE(F2D)was estimated using the temperature dependences of Seebeck coefficient. The relationship between the Fermi energyE(F)(2)(D)and figure of meritZTwas established. The effect of sintering on parameters sigma(T),S(T), charge carrier concentration (n(2D)), mobility (mu), and thermal conductivity (k) was found. The optimal value of Fermi energyE(F2D)in terms of figure of meritZT = 0.31 at room temperature (T = 300 K) was found for Ti(0.98)Nb(0.02)S(1.3)Se(0.7)sample sintered at 600 degrees C.