Ultrafast Synthesis and Thermoelectric Properties of MnTe 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 → MnTe + Q and (2) MnTe → 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 MnTe 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 MnTe compound has a high effective mass of 8.34 m 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 MnTe compound.