Thermoelectric Transport Mechanisms Of N-type Zr-based Half-Heusler Materials

Half-Heusler compounds,which possess excellent electrical properties,robust mechanical strength,good high temperature thermal stability,have been widely investigated as high temperature thermoelectric(TE)materials in the recent years.Up to now,the maximum zT ofp-type half-Heusler compounds has exceeded 1.5.In order to develop high efficiency half-Heusler device,the key is to further optimize the TE performance of n-type half-Heusler compounds.In the present work,n-type(Hf,Zr)CoSb-and(Hf,Zr)NiSn-based half-Heusler compounds were studied.Several strategies,such as doping,band engineering,point defect engineering,were adopted to improve the zT value of these two compounds.The electron and phonon transport characteristics were analyzed by using the single parabolic band(SPB)model and Debye-Callaway model,respectively.the main results are listed as below.1)The carrier concentration of(Hf0.3Zr0.7)CoSb alloys was optimized by Nb doping.different from what has been observed in ZrNiSn-or FeNbSb-based half-Heusler materials,an ionized impurity scattering dominated charge transport was observed in(Hf0.3Zr0.7)CoSb.With increasing carrier concentration and temperature,the influence of the ionized impurity scattering on electron transport became weak,and the acoustic phonon scattering began to dominate the carrier transport at 1100 K.Due to the relatively large covalent radius difference between Nb and Zr(and Hf),high content of Nb doping induced large strain field fluctuation,leading to a further reduction in the lattice thermal conductivity.The highest zT of 0.85 was achieved at 1123 K for(Zr0.7Hf0.7)0.88Nb0.12CoSb sample.By modeling the electron transport with a single parabolic band assumption,the optimal carrier concentration was determined to be 1.3 × 1021 cm-3.2)Isoelectronic substitution of the period-6 element Hf at Zr site was adopted to further optimize the TE performance of n-type(Hf,Zr)CoSb,by considering lanthanide contraction.It was found that Hf/Zr alloying did not cause the deterioration of mobility,which is resulted from the very close covalent radii of Zr and Hf and hence weak alloy scattering of mobility.Meanwhile,the large mass difference between Zr and Hf generated strong point defect scattering of phonons and thus significantly reduced the lattice thermal conductivity.Benefiting from the largely suppressed lattice thermal conductivity and maintained carrier mobility by alloying,highest zT?1.0 was ahieved at 1173 K for n-type(Zr0.4Hf0.6)0.88Nb0.12CoSb.This result confirmed the validity of lanthanide contraction in designing high performance TE materials.3)The carrier concentrition of HfNiSn-based alloys was optimized by Sb doping at Sn site.A typical alloy scattering behavior ??T-0.5 behavior was observed in the unalloyed HfNiSn samples,consistent with the trend of ? for ZrNiSn compounds.The density of states effective mass m*?2.6me was obtained based on SPB model.The optimal carrier concentration was determined to be 4 x 1020 cm-3 at 800 K.Alloy scattering,acoustic phonon scattering and optical polar scattering were taken into account for the analysis of mobility,and a low deformation potential Edef?6 eV and alloy potential Eai?0.7 eV were obtained.The effective mass associated with the conduction-band minimum was estimated and a large band anisotropy factor K?8 was deduced.The large K value could largely compensate the deterioration of mobility caused by large m*and lead to the high power factor.4)A systematic comparison of the TE properties of n-type(Zr,Hf)CoSb and(Zr,Hf)NiSn were made.It was found that though both(Zr,Hf)NiSn and(Zr,Hf)CoSb displayed good n-type TE properties,the physical origins for their high zTs were not entirely the same:Both systems benefited from the alloy scattering;(Zr,Hf)NiSn had relatively lower m*and hence higher carrier mobility and power factor,whereas(Zr,Hf)CoSb had larger band gap and thus suppressed bipolar conduction at high temperature,contributing to lower thermal conductivity.