Cu-substitution effect on thermoelectric properties of the TiNi-based shape memory alloys

Abstract

We have studied the effects of Cu substitution on thermoelectric properties of Ti50Ni50-xCux (0 ≤ x ≤ 30 at. %) shape memory alloys by means of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements. From the electrical resistivity and Seebeck coefficient studies, it is found that the Cu substituted TiNi alloys show a metallic nature in the entire temperature range. However, thermal hysteresis behavior was observed in all the TiNi-based alloys near martensitic transition, which confirms the first order phase transition. Transformation starting temperature of B19′ martensite (Ms) is found to be decreased with increase in Cu substitution (x > 5), whereas that of B19 martensite (M′s) increased gradually with Cu content, and the thermal hysteresis behavior becomes weaker upon substitution of Cu. It is also found that the separation between B19 and B19′ phases in the 7.5% Cu doped TiNi alloy is clearly evident in the Seebeck coefficient measurement, which is not seen in the resistivity data. Finally, analysis of thermal conductivity reveals that the anomalous feature in κ at the B19 ↔ B19′ transformation for 7.5, 10, and 15% Cu-substituted TiNi alloys which can be mainly attributed to the electronic contribution, while a large anomalous peak observed at the B19 → B2 transformation in the warming process is due to change in the lattice thermal conductivity. The relative change in thermal conductivity (Δκ/κ) near martensitic transformation is found to be increased with increase in Cu content, reaches a giant value of 200% for 10% Cu-substituted TiNi alloy and then starts to decrease with further Cu substitution.