Improved thermoelectric properties of layered Ti_1−xNb_xS_2−ySe_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 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 Ti1−xNbxS2−ySey solid solutions was found. The Fermi energy EF2D was estimated using the temperature dependences of Seebeck coefficient. The relationship between the Fermi energy EF2D and figure of merit ZT was established. The effect of sintering on parameters σ(T), S(T), charge carrier concentration (n2D), mobility (µ), and thermal conductivity (k) was found. The optimal value of Fermi energy EF2D in terms of figure of merit ZT = 0.31 at room temperature (T = 300 K) was found for Ti0.98Nb0.02S1.3Se0.7 sample sintered at 600°C.

Enhanced thermoelectric properties of polycrystalline CuCrS2−x Se x (x = 0, 0.5, 1.0, 1.5, 2) samples by replacing chalcogens and sintering

“…Moreover, the ratio of the charge carriers concentrations n 2D /n 3D after sintering increases in samples with a high selenium content (CuCrSe 2 , CuCrS 0.5 Se 1.5 , and CuCrSSe), which indicates the two-dimensionalization of electron transport. From the table 1, it can be seen that with an increase in the selenium content in CuCrS 2−x Se x , the charge carriers concentration n 2D in the samples before sintering increases similarly to that observed by us earlier in the Ti 1−x Nb x S 2−y Se y [39] and W 1−x Nb x S 2−y Se y compounds [29]. In the samples after sintering, n 2D passes through a maximum in the CuCrS 1.5 Se 0.5 sample and again decreases in the CuCrSSe, CuCrS 0.5 Se 1.5 , and CuCrSe 2 samples with an increase in the selenium content.…”

“…This is due to the dominance of spin-orbit scattering [67] caused by the suppression of the magnetic phase transition in the vicinity of 50-100 K [25,44] by the sintering of the initial sample (figure 4). The improvement of thermoelectric properties occurs as a result of an increase in spin-orbit interaction which leads to the carrier filtering effect [61,68,69] We observed antilocalization earlier in the compound Ti 0.98 Nb 0.02 S 0.7 Se 1.3 after sintering [39] and in doped polycrystals Bi 2 Te 3 [3].…”

“…In our recent articles [3,29,[32][33][34]39], we demonstrated that the low-temperature behaviour of the conductivity in layered WSe 2−x S x and TiSe 2−x S x with S and Se substitutions, and Bi 2 Te 3 doped by CuI and Pb had clear features, typical for the conductive systems with local disorder. Substitutions of the S atom for Se with a large atomic radius in the CuCrS 2−x Se x samples (x = 0, 0.5, 1.0, 1.5, 2) led to local disorder in the crystal lattice.…”

“…According to experimental data and preliminary theoretical calculations [4,[28][29][30][31][32][33][34][35][36][37][38] layered transition metal dichalcogenides are the excellent candidates for efficient thermoelectric energy converters. Our investigations of layered chalcogenides have demonstrated that effective ways to improve thermoelectric properties are substitutions in sublattices and, as a result, the changes in the morphology of crystallites in bulk sample [3,29,[32][33][34]39].…”

“…The aim of our investigation was to trace the change in the thermoelectric parameters of the CuCrS 2−x Se x (x = 0, 0.5, 1.0, 1.5, 2) compounds at temperatures below room temperature after replacement selenium to sulfur and sintering. Studies at temperatures below room temperature make it possible to elucidate the mechanisms of the observed processes due to the possibility of separating the contributions from different mechanisms to thermoelectric properties [3,4,29,[32][33][34]39]. This is due to the fact that at low temperatures the contributions of a number of mechanisms significantly decrease, while other contributions increase and dominate.…”