Understanding the combustion process for the synthesis of mechanically robust SnSe thermoelectrics

Recently, very good thermoelectric performance in both n-type and p-type SnSe has been achieved but was accompanied by poor mechanical properties. Moreover, the synthesis routes used, either the Bridgeman method for single crystal growth or conventional melting methods for polycrystalline samples, are highly energy and time intensive. In the present study, single phase SnSe was synthesized by the Self-Propagating High Temperature Synthesis (SHS) combined with Spark Plasma Sintering (SPS) in a very short time of less than 1 h, and with minimum consumed energy. The thermodynamic and kinetic parameters, as well as the role of a pre-synthesized powder of SnSe added to the elemental Se and Sn powders prior to the SHS process, were systematically investigated for the first time. The presence of the pre-synthesized SnSe powder in the mixture of elemental Se and Sn powders promotes multipoint nucleation and inhibits the grain growth, resulting in a weak preferential orientation after SHS that is maintained in the bulk samples after SPS. Thermoelectric and mechanical properties of such samples are superior in comparison to the samples that do not contain the pre-synthesized powder of SnSe in the starting material. The compressive strength, bending strength and fracture toughness of the sample containing 30% of the pre-synthesized SnSe were 74.7 MPa, 40.6 MPa and 0.32 MPa m^1/2, respectively. A maximum ZT value of 0.5 was achieved for this sample at 773 K, which is comparable with polycrystalline samples synthesized by the conventional melting method. This work shows that the SHS reaction combined with SPS densification is an exceptionally rapid and cost effective synthesis process that yields mechanically robust bulk SnSe, a crucial step for large scale commercial applications of this thermoelectric material.