Computational Design Of Novel HALF-HEUSLER Based Thermoelectric Materials And Its Thermoelectric Effect

In order to further improve the electrical conductivity, the Seebeck coefficient and the thermal conductivity of thermoelectric materials, in the present thesis, a novel Half-Heusler based thermoelectric compounds materials system was designed by the calculation method. Basing on the calculation, the optimized material system was prepared and the related experimental characterization has been performed. The calculation method is verified by the good agreement with the experimental results.Firstly, the crystal structure and electronic structure of the 19 species of Half-Heusler compounds were calculated by the CASTEP module of Material Studio4.0 software, which is based on the first-principles theory. And the TiFeSb was adopted as the matrix of the new designed Half-Heusler compounds according to the comparing and analyzing. Furthermore, in order to study the influences of the doped elements, the atoms space occupying, elastic properties, electronic transport properties and phonon transport properties of 8.33at% doped system for 46 kinds of elements were calculated. And chose one of the doped system (Ti_0.75Mn_0.25)FeSb to validate the computational model.Basing on the result of the optimal calculation, Mn doped TiFeSb system was studied by experiments. The TiFeSb and (Ti_0.75Mn_0.25)FeSb bulk material were prepared by solid state reaction and spark plasma sintering method, respectively. Their microstructures were characterized by optical microscopy, scanning electron microscopy and X-ray diffraction tests. The material resistivity and Seebeck coefficient were measured by ZEM-3 thermoelectric properties device. The thermal conductivity coefficient was measured by LFA-427 laser thermal conductivity analyzer. Because there were no related standard powder diffraction cards about TiFeSb phase and (Ti_0.75Mn_0.25)FeSb phase, the X-ray diffraction spectrum of these two phases were calculated by Material Studio4.0 software REFLEX module.Experimental results indicate that: TiFeSb and (Ti_0.75Mn_0.25)FeSb bulk materials can be obtained by the process of solid-state reaction at 850℃, 144h, and spark plasma sintering under the 40MPa precursor at 800℃. The XRD patterns of the experimentally synthesized TiFeSb and (Ti_0.75Mn_0.25)FeSb bulk materials could get a good correspondence with the theoretical calculation results. And the relative errors of lattice constant between the experimental data and calculated were less than 0.3%. Those results indicate that the model building and parameter selecting are reasonable. The SEM observation shows that TiFeSb and (Ti_0.75Mn_0.25)FeSb phases have lath morphology. Further EDS analyzing indicates that the chemical composition of lath is the same as the designed. Thermoelectric properties results showed that, the Seebeck coefficient values of TiFeSb and (Ti_0.75Mn_0.25)FeSb are positive, which means that they were p-type thermoelectric material. The ZT value of (Ti_0.75Mn_0.25)FeSb reached 0.047 at 50℃, which was 4.27 times improved comparing with the undoped matrix (0.011). The theoretical calculation results are confirmed by experimental characterization and the goal of designing a new Half-Heusler compound thermoelectric materials system was reached. Those results indicate that the calculation method to design a new Half-Heusler compounds materials system is feasible.