Performance Optimization And Module Assembly Of NbFeSb Based Thermoelectric Materials

Half-Heusler alloys,which have high mechanical strength,good thermal stability and abundantly available constituent elements,have been widely studied as promising high temperature thermoelectric?TE?materials in recent years.Compared to traditional p-type MCoSb?M=Ti,Zr,Hf?and SiGe alloys in high temperature TE application,p-type RFeSb?R=V,Nb,Ta?shows better thermoelectric performance and lower price.However,the lattice thermal conductivity of RFeSb is still high,which restricts the improvement of its thermoelectric performance.And the carrier and phonon transport mechanisms are unclear.Meanwhile,the RFeSb based devices for thermoelectric power generation or other thermoelectric application are rarely studied.In this work,a comprehensive study of RFeSb based alloy was carried out.Synthesis process improvement and composition optimazation were used to enhance the TE performance of RFeSb.Single parabolic band model and Debye model were applied to analyze the transport performance of RFeSb.A research on RFeSb based TE module was also carried out.The main conclusions are listed as below.1)(Nb_1-xTa_x)_0.8Ti_0.2FeSb samples with high purity and elemental homogeneity were successfully prepared.NbFeSb and TaFeSb shew great similarity in their valence band structure,and hence Nb/Ta alloying nearly didn't change the density effective mass of carrier.Nb/Ta alloying also had little effect on carrier mobility because the covalent radius between Nb and Ta were very close and thus the alloy scattering potential was very low.However,the strong point defect scattering,caused by the large mass fluctuation among Nb,Ta and Ti,significantly suppressed the phonon transport process and reduced the lattice thermal conductivity of the RFeSb.Finally,the zT was enhanced due to the maintenance of electrical properties and the optimization of thermal properties.(Nb_0.64Ta_0.36)_0.8Ti_0.2FeSb and(Nb_0.6Ta_0.4)_0.8Ti_0.2FeSb shew the highest zT of¹.6 at 1200 K.Compared to the unalloyed sample,the peak zT was enhanced by 45%,and the zT_device was enhanced by 36%.2)In thermal transport,the electron phonon scattering would reduce the lattice thermal conductivity of heavily doped NbFeSb.In electrical transport,the influence of ionized impurity scattering was not significant.The results of theoretical calculation shew that the effect of ionized impurity scattering on carrier mobility could be neglected above 200 K.The ratio of effective Bohr radius to screening radius was also large,indicating that the effect of ionized impurity scattering on carrier transport would not be significant.At low temperature,the mobility of NbFeSb samples with different carrier concentrations had different sensitivity to grain refinement.When the carrier concentration was low,the mobility around room temperature could be reduced by grain refinement,but when the carrier concentration was high,even if the average grain size was reduced to 250 nm,the mobility almost kept the same.3)Zr doped NbFeSb had large power factor of 70×10^-4 Wm^-1K^-2 as well as high thermal conductivity of 12.7 Wm^-1K^-1 at room temperature,and could serve as the p-type material in active cooling device.The lattice thermal conductivity and power factor of NbFeSb could be further improved by increasing the grain size.In addition,the doping efficiency of dopant was higher and the grain size of sample was larger when 7.5 at.%Zr or Hf was doped.The cause of this phenomenon remains to be explored.4)Large bulks with?12.7 mm×2 mm of n-type Hf_0.5Zr_0.5NiSn_0.98Sb_0.02 and p-type(Nb_0.8Ta_0.2)_0.8Ti_0.2FeSb HH alloys were successfully prepared.The microstructure characterization and TE property measurements shew that the HH bulks had good phase purity and compositional homogeneity.Both n-type and p-type samples reached the comparable zT of¹.0 at around 1073K.The assembled single stage HH module,whose dimensions were optimized by a three-dimensional full-parameter finite element model,presented a high maximum power density of 2.11W/cm² and a high maximum conversion efficiency of 8.3%when the hot and cold side temperatures were 997 K and 342 K.Compared to the previous RFeSb based TE module,the conversion efficiency was enhanced by 33%,while the power density was almost the same.