Ultrafast Synthesis and Thermoelectric Properties of Mn1+ xTe Compounds.

MnTe compounds show great potential for thermoelectric applications in the intermediate temperature range (500-800 K) because of their large Seebeck coefficient and intrinsically low thermal conductivity. So far, the existing methods for the synthesis of MnTe compounds remain constrained to multistep processes that are time- and energy-intensive. Herein, we demonstrate ultrafast synthesis of high-density bulk MnTe compounds using a combination of self-propagating high-temperature synthesis (SHS) and plasma activated sintering. The entire synthesis and processing procedure takes less than 1 h. The thermodynamic consideration suggests that the SHS process includes two steps: (1) Mn + 2Te → MnTe2 + Q1 and (2) MnTe2 → MnTe + Te. With the heat released by step (1), the process moved in cycle and finished in a rather short time. The effect of extra Mn content on the structure and thermoelectric properties was investigated. There is some solubility limit of extra Mn in the Mn1+ xTe compound. The extra Mn occupy interstitial sites, leading to a decrease of carrier concentration while enhancing Seebeck coefficient and decreasing thermal conductivity. Low-temperature heat capacity data indicates that the Mn1.06Te compound has a high effective mass of 8.34 m0 and a low Debye temperature of 186 K, which are beneficial for the large Seebeck coefficient and low thermal conductivity. Therefore, the maximum ZT value reaches 0.57 at 850 K for the Mn1.06Te compound.

Thermoelectric Properties of Ga/Ag Codoped Type-III Ba24Ge100 Clathrates with in Situ Nanostructures.

Because of the low thermal conductivity and high electrical conductivity, type-III Ba24Ge100 clathrates are potentially of interest as power generation thermoelectric materials for midto-high temperature operations. Unfortunately, their too high intrinsic carrier concentration results in a quite low Seebeck coefficient. To reduce the carrier concentration, we prepared a series of Ga/Ag codoped type-III Ba24Ge100 clathrate specimens by vacuum melting and subsequently compacted by spark plasma sintering (SPS). Doping Ga-Ag on the sites of Ge reduces the concentration of electrons and, at higher concentrations, also leads to the in situ formation of BaGe2 nanoprecipitates detected by the microstructural analysis. As a result of doping, the Seebeck coefficient increases, the thermal conductivity decreases, and the dimensionless figure of merit ZT reaches a value of 0.34 at 873 K, more than three times the value obtained with undoped Ba24Ge100.