Property Improvement And P-type Doping Of MNiSn-based (M=Hf,Zr,Ti) Thermoelectric Materials

Thermoelectric (TE) materials have promising applications in refrigeration and power generation for the ability of direct thermal-electrical energy conversion. The half-Heusler (HH) intermetallic alloys constitute an important class of TE materials that hold promise in high temperature power generation and it is noteworthy that the constituent elements of HH alloys are ecologically friendly. Thus, in this thesis the study on the MNiSn-based (M=Hf, Zr, Ti) half-Heusler alloys about the compositions, micro-structure and the improvement of the thermoelectric properties are coverd. The highlights of the results are as follows.Levitation melting was adopted to fabricate the half-Heusler materials and it has been proved that the pure half-Heusler phases can be rapidly obtained by this method. It overcame the fabrication problem that introduced by the very different melting points and vapor pressures of the single elemental starting materials.One big challenge of half-Heusler alloys is their relatively high thermal conductivity compared with other state-of-the-art thermoelectric materials. Nanostructuring, site substitution or doping and the grain size controlling are effective ways to reduce the thermal conductivity and improve the thermoelectric properties. Some attempts of Y-Sb co-doping on (Hfo.6Zro.4)NiSn0.98Sb0.02 materials have been done. Although the introduced electrons reduced the electrical conductivity, it improved the power factor for the improvement of the Seebeck coefficient. The Pisarenko plot indicated that the change of the Seebeck coefficient was affected by the carrier concentration. The final TE properties were also improved after Y doping and the best ZT value reached about 0.76 for the 1% Y doped samles. The thermal conductivities of (Hf,Zr)NiSn-based materials were reduced by substituting Ti on M site and grain size controlling methods, the lowest thermal conductivity about 3.7Wm-1K-1 was obtained when the content of Ti was 40%. From the results of the theoretical calculation, the mass fluctuation scattering is dominant compared with the stress field fluctuation scattering in the alloy scatterinig that introduced by Ti substitution. Also the best substituting content of Ti indicated from the calculation is about 40%, which is consistant with results of the experiment.In this thesis, p-type doping on MNiSn-based (M=Hf, Zr, Ti) materials has been done. As the holes introduced increasing, the sign of the Seebeck coefficient turned from negative to positive, which indicated that the materials began to show p-type properties. So it is an effective way to obtain p-type half-Heusler materials by holes doping. The (Hfo.6Zro.4)NiSn and ZrNiSn materials turned to be p-type materials when the doping element(Y and Yb) reached about 10%.Ti doping on Y site of the YNiSb materials has also been done in this thesis. The value of Seebeck coefficient at high temperature and the final thermoelectric properties were improved after Ti doping. The ZT maximum of 1.5%Ti doped materials improved about 60% compared with that of the undoped materials.