Study On Preparation And Thermoelectric Performance Of Half-Heusler Alloys Based On Phase Diagram Design

Thermoelectric（TE）materials can realize the direct mutual conversion between thermal energy and electricity,which is of strategic significance for alleviating the current global energy tension.Half-Heusler（HH）alloys are a class of medium-to-high temperature TE materials with many advantages required for large-scale commercial applications,such as high TE power factor,strong mechanical properties,and high thermal stability.The electron and phonon transport properties of TE materials are closely related to their phase compositions,microstructures,and phase stabilities.Phase diagram is an important basis for material development,research and production,which has a crucial impact on the preparation and performance of TE materials.Therefore,this thesis aims to establish a thermodynamic database of half-Heusler system TE materials,design high-quality base TE materials,and further optimize the TE properties of materials by doping,alloying,etc.The main research results are as follows:Several isothermal sections of five half-Heusler thermoelectric materials（ABX）were constructed by the equilibrium alloy method.These isothermal sections include the Zr-Ni-Sn ternary system at 973 K and 1173 K,the Hf-Ni-Sn ternary system at1173 K,the Nb-Co-Sn ternary system at 1173 K,the Nb-Co-Sb ternary system at 1173K and 1323 K,and the Nb-Fe-Sb ternary system at 1173 K.Meanwhile,the phase equilibrium diagram of the Nb-Co-Sb system in the whole temperature range was obtained by thermodynamic calculation for the first time.The solubility limits of B-site or A-site for different systems at the corresponding temperature were determined by phase diagrams.Under the temperature of 973 K and1173 K,the solubility range of the Ni site in Zr Ni_1+xSn is 0（27）x（27）0.07 and 0.01（27）x（27）0.13,respectively.Under the temperature of 1173 K,the solubility range of the Ni site in Hf Ni_1+xSn is 0.04（27）x（27）0.16,the solubility of the Co site in Nb Co_1+xSn is 0.01（27）x（27）0.07,the solubility of the Fe site in Nb Fe_1+xSb is 0.01（27）x（27）0.13.Under the temperature of 1173 K and 1323 K,the vacancy concentrations of Nb_1-xCo Sb are 0.17≤x≤0.22 and 0.17≤x≤0.20,respectively.The calculation of phase diagram showed that the vacancy concentration range in Nb_1-xCo Sb increased firstly and then decreased with temperature increasing.At 1173±20 K,there was a maximum solution range of（35）x=0.042.The lattice occupancy of extra B-site atoms and their effects on electrical transport in different systems are revealed.In the solid solubility range defined by the phase diagram,TEM analysis confirmed that additional B-site atoms occupy the lattice 4d（3/4,3/4,3/4）interstitial sites,forming an intermediate-Heusler（IH）phase.An energy filtering effect is formed at the interface between HH and IH,which can filter low-energy carriers,thereby reducing the carrier concentration at low temperature in Zr Ni Sn,Hf Ni Sn and Nb Co Sn systems,and improving the mobility and Seebeck coefficient.The B-rich atoms will generate an impurity state in the forbidden band,reducing the band gap,while relieving the bipolar diffusion effect caused by the increased carrier concentration with increasing temperature.At last,the electrical properties of Zr Ni Sn and Nb Co Sn were improved.Creating a Ni-poor environment can weaken the impurity state and expand the band gap,in which the band gap of Zr Ni_0.93Sn is close to the eigenvalue～0.5 e V.The introduction of Fe-rich atoms into p-type Nb Fe Sb-based alloys will neutralize hole and reduce mobility,which is detrimental to electrical performance.The effects of additional B-site atoms on thermal transport in different systems are revealed.In the solid solution range defined by the phase diagram,the B-rich atoms in Zr Ni Sn and Nb Co Sn will generate localized“rattling mode”at low frequency,where the low-frequency optical branch and the acoustic branch will avoid crossing and the acoustic branch will soften,reducing the phonon group velocity and the Debye temperature,thereby reducing the lattice thermal conductivity.Among that,the room temperature lattice thermal conductivity of Zr Ni_1+xSn_1+z is reduced from 8.4W m^-1 K^-1（x=0.02）to 6.0 W m^-1 K^-1（x=0.11）,while the value of Nb Co_1+xSn_1+z is reduced from 8.5 W m^-1 K^-1（x=0.03）to 7.4 W m^-1 K^-1（x=0.06）.In contrast,the Ni-rich environment in Hf Ni_1+xSn_1+z and the Fe-rich environment in Nb Fe_1+xSb has little effect on phonon transport.The Ta-alloyed Nb_1-yTa_yCo_1.06Sn_0.9Sb_0.1 and Nb_0.8-xTa_xCo Sb compounds were designed,and the average figure of merit of Nb_1-xTi_xFe Sb was optimized by direct ball milling.Alloying can effectively enhance the phonon scattering and then reduce the lattice thermal conductivity,but also can decrease the effective mass of the density of states and the carrier mobility.Finally,the z T_max of Nb_0.9Ta_0.1Co_1.06Sn_0.9Sb_0.1 and Nb_0.7Ta_0.1Co Sb was 0.8 at 973 K.The synthesis method of direct ball milling is beneficial to improve the TE performance of Nb Fe Sb-based materials at low temperature.In this case,the optimal doping amount of Ti at the Nb site is x=0.25,the carrier concentration is 2.6×10²¹ cm^-3,the z T_max is 1.0（973 K）,and the z T_avg is0.64（RT-973 K）.