Thermoelectric Properties Of P-type ZrCoSb-based Half-Heusler Compounds

As a kind of functional material,thermoelectric materials can directly realize the reversible conversion of thermal and electricity energy based on the thermoelectric effect.The thermoelectric device,which is fabricated from thermoelectric materials,has extensive application prospects in the fields of power generation and solid-state refrigeration due to the advantages of long service life,environmentally friendly,wide application,etc...Although the thermoelectric device has many advantages,its conversion efficiency is low,which becomes the main factor for its large-scale application.The conversion efficiency of a thermoelectric device is determined by the thermoelectric performance(ZT value)of materials.Therefore,it is the top priority to improve the ZT value of the materials.In the present work,the p-type ZrCoSb-based half-Heusler compounds were prepared by arc melting followed by rapid direct current hot-pressing method.The ZT values of materials were optimized by improving the power factor and suppressing the lattice thermal conductivity through isoelectronic and electronic substitution on the matrix lattice.The main results were listed as below:(1)The Sb sublattice of ZrCoSb half-Heusler compounds was firstly substituted by Sn.It was found that the room temperature lattice thermal conductivity decreased sharply with the increase of Sn content,and the lattice thermal conductivity of ZrCoSb0.65Sn0.35 at room temperature was reduced to 4.9 W m-1 K-1.The further calculation results of the stress field fluctuation coefficient and the mass fluctuation coefficient showed that the stress field fluctuation coefficient was larger than the mass fluctuation coefficient,which indicated that the reduction of the lattice thermal conductivity was mainly caused by the stress field fluctuation scattering.Meanwhile,the power factor was optimized due to the different valence electrons between Sn and Sb.Finally,the ZrCoSb0.7Sn0.3 sample had the best thermoelectric performance and its ZT value reached 0.52 at 973 K.(2)The Co sublattice of ZrCoSb0.7Sn0.3 half-Heusler compounds was partially substituted by Fe.It was found that the electrical conductivity increased dramatically and the Seebeck coefficient decreased rapidly with increasing Fe substitution.The lattice thermal conductivity decreased due to the mass fluctuation and strain field fluctuation because the atomic mass and weight of Fe and Co were different.Finally,a maximum ZT value of 0.53 was obtained at 973 K for sample ZrCo0.95Fe0.05Sb0.7Sn0.3.(3)The effect of Zr substitution by Yb on the thermoelectric properties of ZrCoSb half-Heusler compounds was studied.The result showed that Yb acted as acceptor impurities in the system.The electrical conductivity of Zr1-xYbxCoSb(x ? 0.1)samples increasedgradually and the Seebeck coefficient decreased with the increase of Yb content.The power factor of Zr0.9Yb0.1CoSb sample had the highest value in the whole measured temperature range.Due to the alloying scattering effect between Zr and Yb,the lattice thermal conductivity decreased drastically.At last,the ZT value reached a maximum of 0.2 for Zr0.9Yb0.1CoSb at 873 K.(4)The Zr sublattice was partially substituted by Ti in the ZrCoSb0.7Sn0.3 half-Heusler compounds.The result showed that Ti element with small size acted as a point defect in the system,which scattered the hole carrier strongly.So the electrical conductivity decreased and the Seebeck coefficient increased with increasing Ti content.The lattice thermal conductivity was depressed owing to the alloying scattering caused by the different mass and size between Ti and Zr atoms.Finally,a peak ZT of 0.61 was achieved at 973 K for Zr0.7Ti0.3CoSb0.7Sn0.3.