Thermoelectric properties of Co doped TiNiCo<i><sub>x</sub></i>Sn alloys fabricated by melt spinning

Although TiNiSn-based half-Heusler thermoelectric materials obtain high power factors, their high lattice thermal conductivity greatly hinders the improvement of thermoelectric properties. In this paper, TiNiCo_xSn (x=0~0.05) samples were prepared by melt spinning combined with spark plasma sintering method and their phase, microstructure and thermoelectric properties are studied. The XRD results show that the main phase of all samples is TiNiSn phase, and no other impurity phases are found, indicating that the high purity single phase can be prepared by rapid quenching process combined with SPS process. During the solidification process, the large cooling rate (10⁵-10⁶ K/s) is conducive to obtaining the uniform nanocrystalline structure. The grains are closely packed with a grain size of 200-600 nm. The grain size decrease to 50-400 nm for the Co-doping samples, which indicates that Co doping can reduce the grain size. For the x=0 sample, the thermal conductivity of the rapid quenching sample is significantly lower than that of bulk sample, with an average decrease of about 17.8%. Compared with the TiNiSn matrix, the thermal conductivity of the Co-doping samples are significantly reduced, and the maximum decrease is about 38.9%. The minimum value of lattice thermal conductivity of TiNiCo_xSn samples is 3.19 W/mK. Therefore, Co doping can significantly reduce the к_l of TiNiCo_xSn (x=0.01~0.05) samples. With the increase of Co doping amount x, n/p transition is observed in the TiNiCo_xSn samples, resulting in a gradually decrease of the conductivity and the power factor, and finally the deterioration of the electrical transport performance. Among them, the TiNiSn sample obtains the highest power factor of 29.56 W/mK² at 700 K. The zT value decreases with the Co doping amount x, and the maximum zT value of TiNiSn sample at 900 K is 0.48. This work shows that the thermal conductivity of TiNiSn can be effectively reduced by using the melt spinning process and magnetic Co doping.