Study On Thermoelectric Properties Of P-type Ti₂FeNiSb₂ And N-type ScNbNi₂Sn₂ Based Double Half-heusler Compounds

Thermoelectric materials can achieve reversible conversion of thermal energy and electrical energy,which plays an important role in solving energy crisis and environmental pollution problems.Half-Heusler compounds have gradually become a research hotspot in recent years as promising medium-to high-temperature thermoelectric materials because of their excellent electrical properties,good mechanical properties and thermal stability.However,high thermal conductivity has become the main factor limiting their applications.Due to the intrinsic disordered structure and small phonon group velocity,the double half-Heusler compounds have become a new type of half-Heusler compound with intrinsic low lattice thermal conductivity,which has attracted widespread attention.In order to improve the ZT value of p-type Ti₂Fe Ni Sb₂ and n-type Sc Nb Ni₂Sn₂ based double half-Heusler compounds,a series of performance optimization methods were designed and studies on the thermoelectric performance were carried out.Since the intrinsic Ti₂Fe Ni Sb₂ has an extremely low electrical conductivity,we doped the sample with Sn on the Sb site and the doping effect was explored.On this basis,the thermoelectric properties of Hf alloying on the Ti sites were studied.An enhanced room temperature Hall carrier concentration～4.8×10²¹ cm^-3 was achieved by doping Sn on the Sb site,which effectively improves the electrical conductivity.Hf alloying can significantly enhance phonon scattering by introducing point defects.A reduced lattice thermal conductivity of 1.95 m W^-1 K^-1and a peak ZT of 0.52 at 923 K were obtained in Ti_1.6Hf_0.4Fe Ni Sb_1.7Sn_0.3,which realizes the improvement of thermoelectric performance.The Ti（Fe,Co,Ni）Sb pseudo-ternary phase diagram was designed to realize the control of the defect concentration based on the 18-electron valence equilibrium line,and the influence of Co doping on the Ni sites and Hf alloying on the electrical and thermal properties were respectively studied.Co doping can not only increase the carrier concentration,but also introduce alloying scattering,which contributes to the enhanced power factor of 2.21 m W m^-1 K^-2 and reduced lattice thermal conductivity of 2.8 m W^-1 K^-1 for Ti₂Fe Co_0.3Ni_0.7Sb₂ at 973 K.Combined with the mass and strain fluctuations introduced by Hf alloying,the lattice thermal conductivity of 2.5 m W^-1 K^-1 and a peak ZT of 0.85 in Ti_1.6Hf_0.4Fe Co_0.3Ni_0.7Sb₂were achieved at 973 K,which is of great significance for the thermoelectric power generation applications.The effect of Sc doping and Hf alloying on thermoelectric properties was studied.The effect of p-type doping by different elements on different sites of Ti₂Fe Ni Sb₂ was compared,and the optimized carrier concentration range was proposed.The doping effect of Sc is poorer than that of Co and Sn,which results in the limited improvement of the electrical conductivity and the bipolar diffusion appeared in higher temperature.Combined with the reduction of the thermal conductivity by Hf alloying,a peak ZT of 0.58 was achieved in Ti_1.3Hf_0.5Sc_0.2Fe Ni Sb₂.The effect of Co doping is the best,not only can obtain higher electrical conductivity,but also have proper Seebeck coefficient,and the optimized room temperature Hall carrier concentration is about 4.0×10²¹cm^-3.In view of the thermoelectric properties of Sc Nb Ni₂Sn₂-based materials,the effect of donor doping by different elements was explored.The optimal doping concentration of Zr element was determined,and the alloying of V and Ta was tried.Doping Zr on Sc sites and Nb sites can respectively achieve n-type and p-type semiconductors.An enhanced room-temperature Hall carrier concentration～1.05×10²¹ cm^-3 and a peak ZT of 0.4 at 923 K were achieved by 20 at.%Zr doping on the Sc sites.V or Ta alloying does not enhance the thermoelectric performance.