J.J. Kai

Numerical simulation modeling on the effects of grain boundary misorientation on radiation-induced solute segregation in 304 austenitic stainless steels

The purpose of this study is to develop a model to describe the effects of the grain boundary misorientation on the radiation-induced solute segregation (RIS) in 304 stainless steels. A simple rate equation model with modified boundary conditions, which included the fluxes of defects diffusing along the grain boundaries to the grain boundary dislocations, was developed for RIS at boundaries with different Σ values. The results of the model calculations were compared to the experimental results previously reported by us. It was found that the model could clearly predict the same trends that the Cr depletion levels at special boundaries in irradiated 304 stainless steels were increasing with Σ. The model calculations also showed that the widths of the segregation cusps was decreasing with increasing Σ.

Observations of Al segregation around dislocations in AlGaN

Transmission electron microscopy has been used to observe Al segregation around the threading dislocations in Al0.1Ga0.9N and Al0.3Ga0.7N grown by metalorganic chemical vapor deposition on 6H–SiC. Dislocation lines were found to have up to 70% more Al concentration than those regions free of dislocations in the matrix. The Al-depleted regions around the dislocations are shown to be within a few nanometers from the dislocation lines. The results also show that more Al segregate to edge dislocations than to screw ones.

Segregation of Bismuth to Triple Junctions in Copper

Bismuth segregation in copper has been studied using energy-dispersive X-ray spectrometry (EDX) in a JEOL 2010F transmission electron microscope. In addition to the expected solute enrichment at grain boundaries, we have observed extremely high concentrations of bismuth at certain triple junctions, with significantly greater enrichment factors than in the adjacent grain boundaries. It is shown here that the triple junction segregation is a function of the parameters of the grain boundaries at the triple line, and existence of this type of segregation implies that the affected triple junctions embody excess free energy. At least one of the observed triple junctions may not obey the usual X-product rule, as a result of deviations from the exact coincidence misorientations.

Room-temperature ferromagnetism in self-assembled (In, Mn)As quantum dots

Self-assembled In1−xMnxAs quantum dots (0.19⩽x⩽0.45) have been grown on GaAs (100) substrates by low-temperature molecular beam epitaxy. The microstructure analysis revealed that the uniformly distributed In1−xMnxAs dots have a zinc blende structure as x⩽0.38. Furthermore, all samples exhibit ferromagnetic state at 5K, and their Curie temperatures range from 260to340K varying with x. These (In, Mn)As quantum dots are promising for room-temperature spintronic devices.

Comparison of stress corrosion cracking susceptibility of thermally-sensitized and proton-irradiated 304 stainless steel using electrochemical noise techniques

Abstract The effects of irradiation on the stress corrosion cracking are examined through C-ring stress corrosion tests on thermally-sensitized and sensitized plus irradiated 304 stainless steels using the electrochemical noise technique. All of the C-ring specimens were bolt-loaded to 30 MPa m and subjected to an environment of high purity, oxygen-saturated water at 288°C. During testing the current signal remained unchanged on the thermally-sensitized 304SS, whereas the current signal of the irradiated specimen steadily increased. The increased current was attributed to the advancement of intergranular stress corrosion cracks which was subsequently verified with scanning electron microscopy. The field emission gun – scanning transmission electron microscopic measurements and radiation-induced segregation model calculations were also carried out in an attempt to correlate grain boundary composition with IASCC resistance. It appears that the pre-existing chromium depletion profiles were not significantly altered by proton irradiation below 0.1 dpa. The increased IASCC susceptibility was probably due to irradiation hardening.

Toward Understanding Polycrystalline Aggregate Structure: Analysis of a Twin Intersection and the Interactions Between Interfaces in Diamond

We provide a detailed description and discussion of the interactionsbetween two narrow twins in diamond. It is shown that latticedisplacements result from such interactions in many cases, and fornarrow twins these can correspond to large strains that alter thebalance of energies leading to the equilibrium interfacialstructure. The accommodation of these strains and the modes ofrelaxation of the incompatible interfacial structures are discussed. It is shown that there are several modes of relaxation of the triplejunctions that can interact with each other in a rich variety ofways, leading to several structural degrees of freedom.

Cross-sectional shape modulation of physical properties in ZnO and Zn1−xCoxO nanowires

We have prepared comparable-diameter ZnO and Zn1- xCoxO nanowires with both circular and hexagonal cross-sections. The average diameters are ~113 and ~134 nm for cylindrical and hexagonal nanowires, respectively. The as-grown nanowires have been characterized via structure, electrical conductivity and photoluminescence (PL) spectrum measurements. Pure ZnO nanowires were Co-ion implanted to make magnetic Zn1- xCoxO nanowires for magnetization studies. Bumpier edge surfaces on a nanometre scale, higher densities of stacking faults and a bending feature along the growth direction have been found in cylindrical ZnO nanowires. As compared with hexagonal nanowires, we have observed relatively higher conductivities in cylindrical nanowires, which implied large numbers of shallow donors existing in the latter nanowires. The cylindrical ZnO nanowires also displayed intensified green defect emission and considerably more stacking faults in the crystalline structure. In addition, we have found increased magnetization and stronger ferromagnetic ordering in cylindrical than in hexagonal Zn1-xCoxO nanowires, and have experimentally identified that the point defects of either Zn interstitials or O vacancies played governing roles in ferromagnetism. We conclude that the cross-sectional shape effect originating a varied point defect density can profoundly modulate the structural, electrical, optical as well as magnetic properties of ZnO and Zn1-xCoxO nanowires.

Synthesis and Microstructural Study of Fe-doped Zn1-xCuxO Diluted Magnetic Semiconductor Nanowires

Diluted magnetic semiconductors (DMSs) have drawn much attention and interesting in recent years because of the possibility involving charge and spin degrees of freedom in a single substance [1]. However, as dimension closed to nanoscale, the quantum effect will also be even more apparent. One-dimensional (1-D) nanowire heterostructures are potentially functionalities feasible for nanoscale electronics and optoelectronics [2, 3]. Hence, synthesis of DMS nanowire heterostructure is of particular interest in nanoscale spintronics. Moreover, it is necessary to understand the correlation of magnetic properties and microstructure in DMSs with spatial resolution of near or even better than nanometer scale.

Observations of segregation of Al in AlGaN alloys

Transmission electron microscopy has been used to characterize Al segregation in Al 0.1 Ga 0.9 N and Al 0.3 Ga 0.7 N alloys grown by metal organic chemical vapor deposition on 6H-SiC. It has been found that an interlayer of AlGaN alloy with much higher Al content was formed at first, followed by normal growth of nominal composition of AlGaN alloy. In Al 0.1 Ga 0.9 N and Al 0.3 Ga 0.7 N films, dislocation lines were also found to have more Al segregated than those regions free of dislocations in the matrix. Furthermore, it shows that more Al atoms segregate to an edge dislocation than to a screw one.

Orbital susceptibilities of PbSe quantum dots.

Different sizes of three-dimensional PbSe quantum dots have been synthesized for the study of orbital magnetic susceptibilities. Two types of orbital susceptibilities have been found, including the Curie susceptibility and finite-size corrections to the Landau susceptibility. The Curie term of a quantum dot manifests itself in the temperature dependence of magnetic susceptibility at low temperatures, while the field dependence of differential susceptibility at high temperatures shows finite-size corrections to the Landau susceptibility. Both of the two kinds of orbital susceptibility, estimated per quantum dot, show linear dependence on the size.