The unstable thermoelectric effect in non-stoichiometric Cu.sub.2Se during the non-equilibrium phase transition

The superionic [alpha] â [beta] phase transition in Cu.sub.1.96Se thermoelectric material is investigated by means of thermal analysis (DSC) and measurements of Seebeck coefficient and electrical conductivity. Results of the DSC measurements with 1-10 K/min heating and cooling rates show that the material is close to the equilibrium phase composition during the transformation. However, the kinetic limitation of the process exists, which is indicated by supercooling. At the beginning of the [beta] [right arrow] [alpha] transition, the most significant kinetic delay was attributed to the nucleation of the [alpha] phase. During the phase transformation, the Seebeck coefficient was lower than in a stabilised material (measured with 0.1 K/min heating/cooling rate). During cooling, a decrease from 130 [mu]V/K (in a stabilised measurement) to 7 [mu]V/K (5 K/min cooling rate) was observed. The deviation from the expected values of the Seebeck coefficient was correlated with the difference between the actual and equilibrium phase compositions.