【作者】 刘娟；

【导师】 陈中钧；

【作者基本信息】 电子科技大学 ， 物理学， 2020， 硕士

【摘要】 自Seebeck效应和Peltier效应被发现起,热电材料就被广泛关注。热电材料在应用过程中都有热电转换效率的问题,我们通常用热电优值ZT来评估热电材料的热电转换效率,材料的ZT值越高,说明它的热电转换效率越高。因此,提高材料的热电优值是科研工作者努力的方向。SrAl₂Si₂具有许多吸引人的物理性质,但是在室温下它的热电优值较低,这严重限制了它的热电应用。此外,有相关实验指出,掺杂Y原子可以改善SrAl₂Si₂材料的热电性能,基于此,我们对SrAl₂Si₂进行了掺杂Y的研究,主要研究了Sr_1-xY_xAl₂Si₂（x=0,0.25,0.5和0.75）的晶体结构、电子性质和光学性质。对于三元铝硅合物AAl₂Si₂,它具有双层结构,稳定性很好,为探究三元铝硅合物中电子价化合物和非电子价化合物的区别,本文又对比研究了电子价化合物SrAl₂Si₂和非电子价化合物YAl₂Si₂的晶体结构、电子性质和光学性质。首先,采用GGA方法计算了Sr_1-xY_xAl₂Si₂（x=0,0.25,0.5和0.75）的结构参数、Mulliken布居、能带结构、态密度和一些光学常数。研究发现掺杂Y的SrAl₂Si₂晶体结构发生了明显的压缩,Sr_0.5Y_0.5Al₂Si₂发生了相变,由三方晶系转变为单斜晶系。此外,能带结构的计算表明,SrAl₂Si₂是一种半金属,在导带和价带之间有很小的重叠;掺杂Y原子后,SrAl₂Si₂从半金属向金属转变,并且,随着掺杂浓度从x=0.25,0.5到0.75,体系的金属性逐渐增强,这与态密度（DOS）的计算结果一致。这些结果表明,通过提高Y原子的掺杂浓度,SrAl₂Si₂基合金的热电性能很可能得到进一步的改善。最后,计算并分析了Sr_1-xY_xAl₂Si₂（x=0,0.25,0.5和0.75）晶体的介电函数、吸收谱、能量损失谱和反射谱,得出掺杂Y后的SrAl₂Si₂的静态介电常数明显增大,Sr_1-x-x Y_xAl₂Si₂是一种前景较好的介电材料,并且在18-30 eV能量范围内是良好的紫外透光材料。其次,我们计算了纯相YAl₂Si₂的结构参数、Mulliken布居、能带结构、态密度和一些光学常数。研究发现SrAl₂Si₂、YAl₂Si₂的晶格参数c逐渐减小,但是晶格常数a则变化不明显。能带结构的计算表明,与SrAl₂Si₂相比,YAl₂Si₂在价带和导带之间的重叠部分更多,层间耦合作用也更强,所以金属性更强,这意味着YAl₂Si₂的热电性能优于SrAl₂Si₂;态密度的计算结果得出,YAl₂Si₂在费米能级处的态密度比SrAl₂Si₂的大得多,这与能带结构的计算结果相吻合,也对它们的超导性有很大影响。最后,光学常数的计算得出,YAl₂Si₂的静态介电常数更大,说明YAl₂Si₂材料是相对良好的介电材料;并且在22 eV²7.8 eV范围内,YAl₂Si₂的光透性更好,这对发现并制备出高透射率的材料具有一定的指导价值。掺杂能修正材料的能带结构,其传输特性也会随之改变,这意味着好的掺杂可以改善材料的热电性能。掺杂改性的研究为寻找高性能的热电材料提供了新思路,具有一定的理论指导作用。更多还原

【Abstract】 Since the discovery of the Seebeck effect and the Peltier effect,thermoelectric materials have been widely concerned.Thermoelectric conversion efficiency is a standard to measure the quality of a thermoelectric device.We usually use the thermoelectric figure of merit ZT to evaluate the material’s thermoelectric conversion efficiency.The larger the ZT value of the material,the higer the thermoelectric conversion efficiency.Therefore,improving the ZT value of the material is the direction for researchers.SrAl₂Si₂ is a silicide with pseudogaps at the Fermi level and has many attractive physical properties,but it has low thermoelectric figure of merit at room temperature which hampered its thermoelectric performance.In addition,some literatures demonstrated experimentally that substituting Y onto the Sr sites could improve the thermoelectric properties of SrAl₂Si₂ materials.Based on these,we present a series of first-principles density function calculations on the structural,electronic and optical properties of Sr_1-x-x Y_xAl₂Si₂（x=0,0.25,0.5 and 0.75）materials.For the ternary aluminum silicide AAl₂Si₂,it is very stable because of its double-layer structure.In order to explore the difference between valence compounds and non-electron valence compounds in ternary aluminum silicides,we investigated the difference in crystal structure,electronic and optical properties between the electron valence compound SrAl₂Si₂ and the non-electron valence compound YAl₂Si₂.First,the first-principles methods are used to study the structural,electronic and optical properties of the Y-doped Sr_1-xY_xAl₂Si₂（x=0,0.25,0.5 and 0.75）.Indeed,the structure was compressed evidently for Y-doped SrAl₂Si₂,and a structural transition was observed from trigonal to monoclinic configuration for Sr_0.5Y_0.5Al₂Si₂.Band structure calculations revealed that SrAl₂Si₂ undergo semimetal to metal-like transition and the metallic characteristics was enhanced with increasing Y content from x=0.25,0.5 to 0.75,which is consistent with the density of states（DOS）.Finally,the dielectric function,absorption spectrum,energy-loss spectrum and reflectivity were calculated and analyzed for Y-doped SrAl₂Si₂ crystals,which shows that it is a promising dielectric material and UV-transparent material around the range（18-30 eV）.Secondly,we calculated the structural parameters,Mulliken population,energy band structure,density of states and some optical constants of pure YAl₂Si₂.The study result shows that from SrAl₂Si₂ to YAl₂Si₂ the lattice parameter c gradually decreased,but the lattice constant a did not change significantly.The calculation of the energy band structure shows that compared with SrAl₂Si₂ compound,YAl₂Si₂ has more overlap between the valence bands and the conduction bands,and the inter-layer coupling is also more stronger,so YAl₂Si₂ is more metallic,which means that the thermoelectric performance of YAl₂Si₂ is better.Density of states calculatiosn revealed that the density of states of YAl₂Si₂ at the Fermi level is much larger than that of SrAl₂Si₂,which is consistent with the calculation results of the energy band structure and it also has a great influence on their superconductivity.Finally,the calculation of optical constants shows that the static dielectric constant of YAl₂Si₂ is larger,which indicating that YAl₂Si₂material is a good dielectric material;and in the range of 22 eV²7.8 eV,YAl₂Si₂ has better light transparent,which has guiding values for the discovery and preparation of high transparent materials.Doping can modify the energy band structure,and so affects the electrical transport properties of these materials,which means that good doping can improve the thermoelectric performance of the material.The research on doping modification provides new ideas and theoretical guidance for finding high-performance thermoelectric materials.更多还原