Theoretical And Experimental Studies On The Electrical And Thermal Transport Properties Of Half-Heusler Thermoelectric Materials

Solid-state thermoelectric technology enables the direct conversion between heat and electricity,offering an alternative opportunity to address the environmental problems and the upcoming energy crisis.Simultaneously,the prominent advantages of thermoelectric energy conversion,for instance,without moving part,zero emission,and long working life endow thermoelectric devices with promising prospect of wide applications.However,the low conversion efficiency of a thermoelectric device currently limits its large scale application.Therefore,massive efforts have been dedicated to enhance the conversion efficiency,evaluated by the dimensionless figure-of-merit?zT?,which is proportional to the power factor?S²??and inversely proportional to the thermal conductivity???.In recent years,half-Heusler compounds have gained popularity as promising high temperature thermoelectric materials due to the excellent electrical properties,good mechanical robustness and thermal stability,wihich are conducive to large-scale commercial application.Up to now,the maximum zT of n-type half-Heusler compounds has exceeded 1.0.In order to develop high efficiency half-Heusler thermoelectric devices,the key is to develop high performance p-type counterparts,which have been a main research direction in the past years.It's reported that FeNbSb-based half-Heusler compounds,with high zT values via Ti and Hf doped at Nb site,display great potential as p-type high-temperature thermoelectric materials.However,there are many problems in the research of new FeNbSb-based half-Heusler compounds.For example,the current experimental study is only focoused on the doping at Nb site,much attention should be paid to the research on the doping at Fe and Sb sites.In addition,the further improvement of the thermoelectric performance of Fe-based half-Heusler compounds is hampered by the large band effective mass and hence low carrier mobility.Therefore,research on Fe-free half-Heusler compounds with high thermoelectric performance is also important.In this regards,the thermoelectric properties of p-type FeNbSb were investigated theoretically and experimentally.The electrical structures,phonon structures and thermoelectric properties of p-type Ru-based half-Heusler compounds with low band effective masses were also calculated systematically,the main results are listed as below:The electronic structures and thermoelectric properties of p-type FeNbSb were studied systematically by first-priciples calculations.The calculated electrical transport properties were in good agreement with the experimental ones,verifying the accuracy of our computational methods.The substitutions of Ti,Zr and Hf at Nb site have a minor effect on the band structure near the valance band maximum?VBM?.While the doping of Ce at Nb site alters the band structure near the VBM due to a strong hybridization of4f states with conduction electrons.The excellent thermoelectric performance of p-type FeNbSb is mainly due to the high band degeneracy for valence band maximum.The calculated thermoelectric properties showed that the optimal carrier concentration has been reached experimentally,while the lattice thermal conductivity can be further suppressed.Therefore,the enhancing phonon scattering is helpful to reduce the lattice thermal conductivity of bulk materials,which can be employed to further improve the thermoelectric performance of p-type FeNbSb.According to the low band effective mass near the VBM,a high power factor was gained in a wide range of carrier concentrations,which is beneficial to optimize the thermoelectric performance.The band structure near the VBM can be altered by Mn doping at Fe site,while the substitutions of Ge,Sn and Pb at Sb site have a minor effect on the band structure near the VBM,which is similar to the Ti,Zr and Hf doping at Nb site.Ru-based half-Heusler compounds with lower valence-band effective masses were investigated systematically,since Fe-based alloys possess high band effective masses.The band degeneracies of p-type RuMSb?M=V,Nb,Ta?are as high as that of p-type FeNbSb,while the carrier mobilities and hence power factors of p-type RuNbSb and RuTaSb are higher than those of p-type FeNbSb,which is mainly due to the lower band effective mass.What's more,the lower optimal carrier concentrations of p-type RuMSb are also caused by the lower band effective mass.Combined with higher Grüneisen parameters,the lower Debye temperatures of RuMSb lead to lower lattice thermal conductivities.The calculated results showed that the zT values of RuVSb and RuNbSb are limited by their low band gaps,while a high zT of 1.54 was predicted in p-type RuTaSb at 1200 K through doping.This value is even comparable to that in p-type FeNb_0.88Hf_0.12Sb,which is the state-of-the-art p-type HH compound experimentally.Accordingly,p-type RuTaSb exhibits high potential for high temperature thermoelectric power generation.P-type FeNbSb-based half-Heusler compounds were prepared by arc melting combined with spark plasma sintering.The relative densities of samples are verified to be greatly influenced by the sintering temperatures during optimizating the sintering conditions.In a certain temperature range,the relative densities and hence thermoelectric performances can be improved by the higher sintering temperatures.However,the sintering pressures have a minor effect on the relative densities.The optimal sistering temperature and sintering pressure are 1173 K and 65 MPa,respectively.FeNbSb_1-xSn_x compounds were successfully fabricated according to the optimal sintering conditions.Through Sn doping,the power factors of p-type FeNbSb were improved.What's more,the enhancing point-defect phonon scattering by Sn doping suppressed the lattice thermal conductivities successfully,which is beneficial for improved zT value.The highest zT of0.66 was obtained at 923 K for p-type FeNbSb_1-xSn_x.