| With rapid development of the science and technology,material science is closely integrated with different disciplines.Macroscopic properties of the material are more and more dependent on microstructure properties,with unceasing progress in nanoengineering technology.However,the miniaturization of electronic and aerospace components,and integrated circuits with rapid development put forward exacting requirements for material performance.The growth and performance of crystals,transport properties and synthesis of electronic devices with high performance,are related to the defect hehavior.The defects in materials(point defects,interfaces and grain boundaries),can engineer the material performance(including mechanical and electrical properties),for example,alloy solution hardening and carrier concentration regulation in semiconductor materials.Therefore,it is of great importance to study the defect heaviour of materials to optimize their performance.In this dissertation,we have studied the influence of plane defects(interface and grain boundary)on mechanical properties and point defect behavior of materials by first-principles method.By exploring the physical mechanism behind it,we have proposed a scheme of manipulating the functional properties by defect engineering to improve the material macro properties.Followings are our brief study descriptions.1.Investigating the solute segregation behavior at different Al/Al2Cu interfaces:Aiming at the contradiction of experimental observations and DFT calculations of segregation behavior of solute atoms(X=Si,Mg,Zn)at the coherent and semi-coherent interfaces of Al/Al2Cu interface predicted by previous studies,the segregation behavior of different solutes has been studied by first-principles calculations.The results show that there is a non-negligible local structural distortion at the Al/Al2Cu semi-coherent interface,which has vital influence on the solute segregation behavior.On this basis,the solute segregation energy at the interface by theoretical calculation can well explain the variation trend of solute concentration measured by experiments.For example,the concentration of solute Si at the Al/Al2Cu semi-coherent interface is higher than that at the coherent interface.Furthermore,the lattice mismatch also has a great influence on solute segregation,which can qualitatively explain the experimental results.Our work solves the problem that the solute segregation behavior at the Al/Al2Cu interface predicted by theoretical calculation is inconsistent with the experimental observation,and provides a guidance for accurately predicting the interface structure and properties in the future,lays a foundation for further investigation on the behavior of complex semiconductor interfaces as well.2.Investigating the effect of interface on defect behavior in PbTe based thermoelectric materials:Introducing the precipitated phase into PbTe materials is one of the important methods to optimize the thermoelectric properties of PbTe based materials using interface engineering.However,the interface influence mechanism on the defect behavior is still unclear.We studied the PbTe/PbXTe/XTe(X=Ca,Sr,Ba)interface influence on the formation energy and thermoelectric properties of defects(intrinsic and extrinsic Na doping)by using the first-principles method.We found that:(1)Napb1-and Nasr1-defects in the solid solution interface PbSrTe have the lowest formation energy among PbTe/PbSrTe/SrTe interface systems.This suggests Na dopant is easy to gather to PbSrTe solid solution interface,which is consistent with the experimental observation;(2)There is a novel low formation energy defect NaTe3+occurring at the PbSrTe solid solution interface.These low energy defects and interfaces can improve the electrical properties(for example,Seebeck coefficient)and decrease the thermal conductivity of PbTe materials.Our work proposes that the interface engineering can provide a novel idea for regulating the thermoelectric properties of PbTe based compounds.3.Investigating the influence of defect segregation at grain boundary on mechanical properties for PbTe materials:Although PbTe has superior thermoelectric properties,its mechanical properties(too soft)limit its wide applications.By constructing PbTe(111)<112>/PbTe(111)<112>grain boundaries in PbTe,we have studied the effects of segregation of different defects(Li,Na,K,Ga,In,Tl,As,Sb,Bi,Br,I)on the mechanical properties.We found that:(1)on the one hand,the segregation of low energy defects on the PbTe(111)<112>/PbTe(111)<112>grain boundary can improve the electronic structure,and contribute to the improvement of the thermoelectric properties of PbTe;(2)On the other hand,different defects segregating at the PbTe grain boundaries have different infulence on its’ mechanical properties:V pb2-,Lipb1-,Napb1-,Gapb1-,Inpb1-,Tlpb1-,Aspb1-,AsTe1+,SbPb1+,SbTe1-and BiTe1+ defects segregating at the PbTe grain boundaries can softening of Pb-Te bonds,which is favorable for reducing the lattice thermal conductivity;VTe2+,TePb2+,KPb1-,BiPb1+,BrTe1+,ITe1+ defects segregating at the PbTe grain boundary can increase the structural stiffness of grain boundary and improve the application range of PbTe based materials.Our work provides the theoretical guidance for improving the performance of PbTe materials(including electronic properties and mechanical properties)by grain boundary engineering.4.Manipulating the thermoelectric properties of SnTe by defect engineering:Although SnTe is a sister of PbTe,it has poor thermoelectric properties due to the high hole concentration(Np～1021 cm-3),small band gap(0.18 eV 300 K)and large energy difference between the light and heavy bands(ΔE(L-∑)=0.35 eV),which results in the poor thermoelectric performance(zT～0.4).Combining the TB(tight binding)model with DFT calculations,we have predicted the effective dopants Ge-As from a variety of doped atoms that can improve the electrical properties of SnTe.Because of manipulating band structure under Ge-As co-doped SnTe,the Eg and the energy difference ΔE(L-∑)of SnTe is enlarged to～0.20 eV and deceasing to 0.12 eV,respectively,which effectively inhibits the bipolar effects,facilitates the band degeneracy and improves the Seebeck coefficient.Additionally,we have measured the thermoelectric properties of Ge-As codoping SnTe by experiments.Additionally,the weighted mobility and conversion efficiency of Ge-As codoped SnTe is superior to that of monocrystal SnTe.Our work provides the guiding strategies for optimizing the thermoelectric performance of SnTe with defect engineering. |
