| Perovskite SrTiO3 based thermoelectric material is an important research field for thermoelectric material,which is a type of oxide thermoelectric materials with advantages of low costs for raw materials,temperature and chemical stability and non-toxic elements.Beside these general advantages,SrTiO3 has a well-doping property,i.e.,it can keep its perovskite structure under heavy doping level.However,the problems for SrTiO3 materials are the complex preparation process,energy-extensive consumption and low thermoelectric properties.In this thesis,we have realized the preparation of several types of element doped SrTiO3 nanopowders with a simple process of high-energy ball milling,and the corresponding bulk samples were prepared by carbon burial sintering process.According to the penetrating analysis for the thermoelectric properties of the bulk samples,we have obtained the doped SrTiO3 samples with ultra-high electrical conductivity and low lattice thermal conductivity,which provides an idea for the industrial production of high-performance oxide thermoelectric materials.To be specific,we aim at addressing the preparation of powder and bulk sample and the enhancement of thermoelectric properties of single-,dual-and multi-element doped SrTiO3 thermoelectric materials,and divide this thesis into three parts as follows:Firstly,we explored the essential conditions for the preparation of nanopowders of element-doped SrTiO3 by changing the milling speed and raw materials,and summarized the phase transition process.La-doped SrTiO3 nanopowders was successfully synthesized via this method,and the corresponding bulk samples were prepared using the carbon burial sintering process.With the increase of the La content,there was a peak power factor value of 0.63 mW·m-1·K-2 at the high temperature for Sr0.92La0.08TiO3 sample.However,La doping has little effect on the thermal conductivity,and this sample also had the maximum figure-of-merit ZT of 0.2@1050K.After the preparation of La-doped SrTiO3 samples,the Nb-doped SrTiO3 powder samples were synthesized with trace amount of TiO2 raw materials.The sintering process was adjusted to enhance the sintering temperature higher than eutectic temperature of SrTiO3-TiO2,and TiO2 vanished after sintering.When the sintering enhanced 100 K,the figure-of-merit ZT increase by 2.7 times than that of the sample sintered at the low temperature for 5 mol%Nb-doped SrTiO3 samples.By means of element-doping and the introduction of vacancies on Sr site,the maximum ZT value for Nb-doped SrTiO3 samples was 0.22 for Sr0.9Ti0.9Nb0.10O3 samples.Secondly,the methods of powder synthesis and bulk sample preparation were also successfully transplanted to the preparation of La-Nb and Dy-Nb codoped SrTiO3 samples.The effect of three elements on the thermoelectric properties were studied by the change of the ratio of La/Nb and the ratio of Dy/Nb.In the La-Nb codoped samples,La and Nb tend to moderate the thermoelectric properties separately.Sr0.95La0.05Ti0.9Nb0.1O3 had the maximum ZT value of 0.21,which fell in between the La-and Nb-doped samples.However,the Dy-Nb codoped samples exhibited the interactive relationship between Dy and Nb,which made the Sr0.9Dy0.1Ti0.9Nbo0.13 sample the lowest lattice thermal conductivity of 1.45 W·m-1·K-1,and the corresponding ZT of 0.27@1100 K.According to the analysis of the thermoelectric properties of the single and dual doped SrTiO3 bulk samples,we found that(1)La can modify the electrical properties effectively,and proper La doping can obtain a maximum ZT;(2)Dy can effectively decrease the lattice thermal conductivity and keep the electrical properties and thus enhance the ZT;(3)Nb can enhance the ZT for the enhancement of the electrical conductivity and meanwhile the decrease of the lattice thermal conductivity.Finally,we combined the three elements for the production of high thermoelectric property samples.And the sintering process was also optimized by the introduction of a temperature platform,which can cause the shell-vesicular architectures.Using this sintering process,Sr0.9La0.05Dy0.05Ti0.8Nb0.2O3 had a carrier mobility 5.68 cm2·V-1·s-1,the ultra-high electrical conductivity 5200 S·cm-1 at 300 K,and a low lattice thermal conductivity 1.6 W·m-1K-1 from 700 to 1100 K.The electronic thermal conductivity is 60%of the total thermal conductivity,which is the main factor to influence the thermal conductivity.The corresponding maximum ZT is 0.27@1100 K.By further optimizing the Nb content,Sr0.9La0.05Dy0.05Ti0.95Nb0.05O3 had a relative high power factor of 0.75 mW·m-1·K-2 and low total thermal conductivity of 2.95 W-m-1·K-1,and the maximum ZT of 0.28@1100 K,which is also the maximum value among the doped samples in this thesis. |
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