Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations.Owing to the unstable stacking energy,the [01ˉ1] superdislocation is dissociated into partial dislocations separated by a stacking fault.The simulation results show that when the crack speed is larger than a critical speed,the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip;subsequently the super intrinsic stacking faults are formed in adjacent {111} planes,meanwhile the super extrinsic stacking faults and twinning also occur.Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning,which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.     

蓝宝石衬底上RF-MBE生长的GaN中的极性控制和螺旋位错的降低

近年来人们报道了用MBE方法生长GaN的飞速进展,利用RF-MBE方法可以获得高的GAN生长速率和高的电子迁移率.本文讨论了用RF-MBE方法在蓝宝石衬底上生长GaN过程中的极性控制和螺旋位错的降低.在充分氮化的蓝宝石衬底上直接生长GaN,使GaN的极性控制为N-极性,并用高温生长的AlN核化层实现GaN的Ga-极性.对于N-和Ga-极性的GaN这两种情况,高温生长的AlN中间迭层的引入,可以有效地抑制螺旋位错的扩散.位错的降低使GaN的室温电子迁移率得到提高,对于Ga-极性的GaN,其值为332cm²/V·s;而对于N-极性的GaN,其值为688cm²/V·s.     

平板马兰各尼流动中数的影响

本文提出了边界层充分发展情况下平板马兰各尼流动动量方程和能量方程的相似解,分析了流动与传热随Pr数的变化特征;由于在核沸腾中蒸汽气泡的一般直径大于估算边界层厚度,因而可以忽略表面张力影响,将这一结果用于气泡周界马兰各尼流动效应的初步分析.     

位错对阳极氧化铝多孔薄膜纳米孔阵列结构的影响

多孔铝氧化物薄膜;电解质强度;位错对阳极氧化铝多孔薄膜纳米孔阵列结构的影响     

高温闪烁晶体Ce:YAP的化学腐蚀形貌

本文报导了Ce:YAP晶体位错蚀坑的腐蚀条件;同时报导了几个主要晶面的位错蚀坑形貌,(100)面呈扁豆形,(010)面呈菱形,(101)面呈椭圆形,这些形状与各自晶面的对称性一致.通过蚀坑形貌在晶体横截面内观察到了小面生长核心区和熔质尾迹等缺陷,并分析了其成因.     

Surface-interface analysis of InxGa1-xAs/InP heterostructure in positive and negative mismatch system

The change of In content in the InxGa1-xAs/InP system leads to the variation of the lattice constant and thereby to the negative mismatch between the base InP and the positive mismatch. Here, we studied the surface morphology and dislocation relationship of In_xGa_1-xAs/InP (100) in the positive and negative mismatch system by different characterization techniques. Under the same mismatch, the surface morphology and mass effect of negative mismatch were greater than those of positive mismatch. The reason was that in the negative mismatch system, during the film growth, the disorder degree at the interface increases, leading to an increase in dislocation density, meanwhile, the dislocation in the substrate more easily moved into the film, thus increasing the film and the dislocation density in it. Moreover, the mechanism of the buffer layer was also clarified. The addition of the buffer layer first limited the dislocation movement in the substrate, and secondly reduced the mismatch between the epitaxial layer and the substrate, thereby reducing mismatch dislocation.     

A constitutive model coupling irradiation with two-phase lithiation for lithium-ion battery electrodes

When lithium-ion batteries serve in extreme environments like space, severe irradiation might induce significant decay of the electrochemical performances and mechanical properties. In this paper, an electrochemical-irradiated constitutive model is proposed to explore the evolution of dislocation, defect and stress in electrodes during a two-phase lithiation process. The results from the analytic formulation and finite difference calculations show that, as Li intercalation proceeding, the hoop stress in the surface of a spherical particle converts from compression into tension because the large lithiation strain and plastic yielding at the front pushes out the material behind it. And the plastic flow resistance continuously increases with increasing irradiation dose result from the impediment of a defect to dislocation glide. There is a clear peak in the distribution of stress at yielding locations due to the competition between dislocations multiplication and defects annihilation. The model is meaningful for thoroughly understanding the micro-mechanism of lithiation deformation and provides a guideline for predicting their mechanical behaviours of lithium-ion batteries in unconventional environment.     

Energetics and electronic structure of refractory elements in the dislocation of NiAl

Using DMol and the discrete variational method within the framework of the density functional theory,we study the alloying effects of Nb,Ti,and V in the [100](010) edge dislocation core of NiAl.We find that when Nb(Ti,V) is substituted for Al in the center-Al,the binding energy of the system reduces 3.00 eV(2.98 eV,2.66 eV).When Nb(Ti,V) is substituted for Ni in the center-Ni,the binding energy of the system reduces only 0.47 eV(0.16 eV,0.09 eV).This shows that Nb(Ti,V) exhibits a strong Al site preference,which agrees with the experimental and other theoretical results.The analyses of the charge distribution,the interatomic energy and the partial density of states show that some charge accumulations appear between the impurity atom and Ni atoms,and the strong bonding states are formed between impurity atom and neighbouring host atoms due mainly to the hybridization of 4d5s(3d4s) orbitals of impurity atoms and 3d4s4p orbitals of host Ni atoms.The impurity induces a strong pinning effect on the [100](010) edge dislocation motion in NiAl,which is related to the mechanical properties of the NiAl alloy.     

Interaction of two edge dislocations in free-space propagation

This paper studies in detail the interaction of two edge dislocations nested in a Gaussian beam propagating in free space. It shows that in free-space propagation the edge dislocations are unstable and vanish, and two noncanonical vortices with opposite topological charge take place when off-axis distances c₁ and c₂ of two edge dislocations are non-zero, and the condition k²w₀⁸+32c₁c₂(w₀²-2c₁c₂)z²>0 is fulfilled (k-wave number, w₀-waist width). A noncanonical vortex appears when one off-axis distance is zero. However, one edge dislocation is stable when two edge dislocations are perpendicular and one off-axis distance is zero. Two perpendicular edge dislocations both with zero off-axis distance are also stable. The analytical results are illustrated by numerical examples.     

电化学法沉积氧化锌晶须

应用阳极氧化法沉积氧化锌(ZnO)松针状晶须,SEM和XRD观察、表征样品形貌与结构.结果表明,电沉积氧化锌晶须直径为20～50 nm、长度200～300 nm、长径比10～15,纯度高,结晶度好,属六方晶系纤锌矿(JCPDS 36-1451).循环伏安法研究了该氧化锌结晶过程,并考察了电解液的锌酸盐[Zn(OH)4]2-浓度和基底表面氧化膜对晶须生成的影响.