Structure And Transport Features Of (Hf,Zr) NiSn Based Half-Heusler Thermoelectric Materials

Thermoelectric (TE) materials can directly realize the reversible conversion of electricity and thermal energy based on the electron and phonon transport process in materials, and are highly potential for the collection of waste heat or electric cooling. The designing of TE devices is based on Seebeck effect and Peltier effect. With the advantages such as simple structure, abrasion free, no moving parts, quite, small, safe, maintenance free and etc., they have extensive application prospects. The development of TE materials that can adapt to the high temperature of waste heat is of significant urgency.In the present work, (Hf,Zr)NiSn half-Heusler TE were studied. Phase-pure sampes were prepared by levitation melting followed by spark plasma sintering. The disorder and eletron transport process were modeled and the origin of the high powerfactor in this system was investigated. Isoelectronic substitution was adopted to enhance phonon scattering and suppress lattice thermal conductivity, in order to improve TE efficiency. The main results are listed as below:1) High-resolution synchrotron radiation powder X-ray diffraction was adopted to study the intrinsic disorder in ZrNiSn system.Rietveld refinements were carried out for both unannealed and annealed ZrNiSn samples, revealing the existence of interstitial fractional occupancy of Ni on the vacant site, which can not be eliminated by annealing.The Fourier map also showed extra charge density at 4b site.The electrical conductivity, Seebeck coefficient and thermal conductivity from 300 to 900 K of the unannealed and annealed ZrNiSn samples displayed no obvious distinction, implying no obvious Zr/Sn disorder transition during the annealing procedure. Above all, interstitial fractional occupancy of Ni on the vacant site was the most possible intrinsic disorder in ZrNiSn system.2) The eletron and phonon transport in ZrNiSn material and the influences of intrinsic disorder on charge carriers were studied. The carrier concentration was tuned by Sb doping at Sn sublattice, and the density of effective mass m-～2.8 ±0.2me was obtained based on SKB(Single Kane Band) model. The optimal carrier concentration was 3-5×1020 cm-3 and the highest zT～0.9 was achieved at 1000 K for ZrNiSn0.99Sb0.01.The electron mobility of unalloyed ZrNiSn samples followed μ∝ T-0.5 relationship, which was typical for alloy scattering dominated charge transport process. Alloy scattering, acoustic phonon scattering and optical polar scattering were taken into account for the analysis of mobility and the results indicated that the intrinsic disorder in ZrNiSn system imposed significant influences on the charge transport process. Low defomation potential Edef～5 eV and alloy potential Eal ～0.5 eV were obtained, which compensated the deterioration of mobility caused by large m* and led to the high powerfactor. The influence of polar scattering was negligible at high carrier concentration (>1020 cm-3). The intrinsic disorder did not have obvious influences on the phonon transport process and Umklapp process was the dominant phonon transport mechanism.3) Isoelectronic substitution of Hf at Zr site was adopted to enhance phonon scattering and hence reduce the lattice thermal conductivity. In most cases, when solid solution forms, the mobility will decrease due to the additional carrier scattering mechanism caused by the introduced atoms. However, the Hf/Zr alloy did not cause the deterioration of mobility in ZrNiSn system, which mainly resulted from:a) HfNiSn possessed a smaller m*～2.4 me and with increasing content of Hf, m* of the system graduately decreased, which was beneficial for higher mobility; b) The obtained Eal～ 0.1 eV for Hf/Zr disorder was far smaller than that of the intrinsic disorder. Since the Ni/vacant disorder possessed more disctinct contrast of mass and atom radius, the influences of Hf/Zr disorder on charge transport might be screened. Meanwhile, the Hf/Zr disorder significantly reduced the lattice thermal conductivity by enhanced phonon scattering. Highest zT～1.1 was achieved in Zr0.2Hf0.8NiSn0.98Sb0.015 at 1000K.4) Isoelectronic substitution of Pt on the Ni sublattice was adoped based on the Hf/Zr alloyed solid solution to further suppress lattice thermal conductivity. Ni/Pt disorder effectively enhanced the phonon scattering process and reduced the lattice thermal conductivity. Only 5% of Pt in Hf0.65Zr0.35NiSn0.9sSb0.02 would cause～40% reduction of lattice thermal conductivity. Meanwhle, the Pt/Ni disorder caused decrease on carrier mobility and Eal～0.5 eV was obtained based on SPB(Single Parabolic Band) model. The electron thermal conductivity decreased as a consequence of the reduced carrier mobility. The TE efficiency was improved since the total lattice thermal conductivity decreased faster than the carrier mobility.5) Exploration on the p type ZrNiSn and ZrCoSb based half-Heusler compounds. The room temperature Seebeck coefficient turned to be positive with 4% Co doping at Ni site in ZrNiSn system. The electrical conductivity saturated when the Y doping content reached 14% in ZrNi0.98Co0.12Sn. The optimal carrier concentration could not be reached by either Y or Co doping in ZrNiSn system. In ZrCoSb system, Sn doping at Sb site could tune the carrier concentration effectively and the optimal doping level was about 10% Sn at Sb site.