Preparation And Optimization Of Half-Heusler Alloys And Zintl Phase Thermoelectric Materials

Development of new energy source is more imperative due to the crisis of energy resources. A major research is to develop new energy conversion forms. Thermoelectric (TE) materials can be used for direct conversion between heat energy and electricity. But the low efficiency restrain it's widely utilization. Investigation and development of new TE compounds is an important way to the TE research. Half-Heusler alloys and Zintl phase are two kinds of new TE materials for high temperature application. In the present work, ZrNiSn-based half-Heusler alloys and YbZn2Sb2 and Yb14MnSb11 Zintl phase compound were studied. The preparation, microstructures and TE properties of these materials were investigated. The main results are as follow.1. Rapid preparation of ZrNiSn-based alloys and Yb14MnSb11-based compounds. Single phased ZrNiSn-based half-Heusler alloys with homogenous microstructure were prepared by levitation meting and spark plasma sintering. Yb14MnSb11 Zintl compound was prepared by modified induction melting. The raw materials were melting at 2200℃. The extreme high temperature made the content of secondary phases decrease.2. The carrier concentration of ZrNiSn-based alloys was optimized by Sb doping. Sample with 2% Sb doping had the best thermoelectric properties. ZrNiSn-based alloys with Zr substituted by Ti and Hf were prepared. The lattice thermal conductivity decreased due to mass fluctuation scattering and strain filed fluctuation scattering. Single phased solid solutions were got for samples with Zr substituted by Hf and the maximum ZT reached 1.0 at 1000 K for Hf0.6Zr0.4NiSn0.98Sb0.02 sample.3. ZrNiSn-based half-Heusler alloys with refined grain size were prepared by melt spinning to decrease the lattice thermal conductivity. The submicron grains were observed and many nano particles were found distributed in the submicron grains. EDS showed that the nano particles were metallic (Hf+Zr)Ni2Sn. The samples prepared by levitation melting and melt spinning had the same composition. The atom ratio is (Hf+Zr):Ni:(Sn+Sb)≈1:1.06:1. The carrier concentration for the MS samples was higher than the LM samples with the same composition due to the nano metallic particles introduced in the melt spinning process. It was found that the lattice thermal conductivity of MS ZrNiSn-based half-Heusler alloys decreased by alloy scattering and boundary scattering. The lattice thermal conductivity decreased more than 20% below 100 K. The decrease of lattice thermal conductivity by boundary scattering is about 5%～20% above 150 K. The effect still exists at temperature 900 K. The maximum ZT reached 0.9 and 0.85 for MS Hfo.6Zro.4NiSno.98Sb0.02 and Hf0.5Zr0.5NiSn0.98Sb0.02 samples with Sb doping and Zr substituted by Hf.4. The electrical conductivity and carrier concentration decrease with Mn content increase in the YbZn2-xMnxSb2 solid solutions. The total thermal conductivity decreased due to the decrease of electron thermal conductivity. Sample with Mn content of 0.1 had the best ZT 0.65 at 726 K,34% higher than the matrix. Carrier concentration and electrical conductivity decreased for YbZn2-zAlzSb2 solid solutions with Al content increase. The Seebeck coefficient increased at room temperature and thermal conductivity was the same with matrix. Doping with Te and Bi increased the ZT of YbZn2Sb2.5. Nonstiochimetric Yb14MnSb11 Zintl compounds with excess Mn or Sb were prepared to optimize the carrier concentration. Excess Mn can restrain the formation of Yb4Sb3 secondary phase. The maximum ZT reached 0.35 at 700 K for Yb14Mn1.05Sb11 sample and 40% higher than the matrix. Lu doping increased the carrier concentration and electrical conductivity, and decreased the Seebeck coefficient of Yb14Mn1.05Sb11. The point defect scattering introduced by Lu doping decreased the lattice thermal conductivity. The maximum ZT reached 0.45 at 673 K for Yb14Mn1.05Sb11 sample and 28% higher than the matrix.