Effect Of Screw Dislocation Research Articles

The effect of screw dislocation on Helium Bubble growth in Tungsten: molecular dynamic simulation study

The effect of a 1/2[111] screw dislocation on the growth of helium bubble in tungsten is investigated by molecular dynamics simulation method. When helium bubble grows at sites less than 3 nm away from the dislocation, tungsten self-interstitial atoms (SIAs) pushed out by the growing bubble are absorbed by and then migrate rapidly along the dislocation line, inducing the screw dislocation to evolve into a helical configuration. The existence of screw dislocation can facilitate the growth of helium bubble. The interaction energies between the screw dislocation and point defects (helium, SIA and vacancy) are calculated. The interaction energies are all negative, indicating that the screw dislocation can attract and serve as a sink for these point defects. Further investigation shows that the interactions between the screw dislocation and these point defects are local, and stem from the special stress distribution pattern of the screw dislocation.

Molecular dynamics study of self-diffusion in the core of a screw dislocation in face centered cubic crystals

Self-diffusion along the screw dislocation core in aluminum, nickel, copper and silver in the absence of any pre-existing point defects in the structure is studied using molecular dynamics. Simulation results show that the effect of screw dislocation on enhancing self-diffusion is more remarkable in Al and Ni than in Cu, and no significant enhancement of self-diffusion has been observed in the core regions of Ag. This behavior has been related to higher stacking fault energies and resultant smaller dissociation distance of partial dislocations in Al and Ni, compared to Cu and Ag.

Synthesis and properties of novel liquid-medicine-filter shaped ZnO nanostructures

Liquid-medicine-filter shaped ZnO nanostructures have been synthesized on Al2O3-coated Si (111) substrates by chemical vapor deposition method (CVD) at 1050°C. Every liquid-medicine-filter shaped ZnO nanostructure is made up of one nanorod and two nanowires at the ends. The liquid-medicine-filter shaped ZnO nanostructures are characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectrometer (EDS) photoluminescence (PL). The results indicate that the liquid-medicine-filter shaped ZnO nanostructures are wurtzite hexagonal structure and the growth direction is [0001]. The liquid-medicine-filter shaped ZnO nanostructures became the new member of ZnO nanostructures for the novel configuration. PL reveals ultraviolet (UV) emission at 384nm and a broad emission peak at 540nm. These novel liquid-medicine-filter shaped ZnO nanostructures will provide an improvement for electronic and optical devices. The pre-prepared Al2O3 film on the Si (111) substrate solves the troublesome lattice mismatch problem between the Si substrate and ZnO, and makes the growth of liquid-medicine-filter shaped ZnO nanostructures more effective. In addition, the effect of screw dislocation and polar surfaces in understanding crystal growth mechanisms in nanometer scale were also provided.

Nanopipes in Gallium Nitride Nanowires and Rods

Gallium nitride nanowires and rods synthesized by a catalyst-free vapor-solid growth method were analyzed with cross section high-resolution transmission electron microscopy. The cross section studies revealed hollow core screw dislocations, or nanopipes, in the nanowires and rods. The hollow cores were located at or near the center of the nanowires and rods, along the axis of a screw dislocation. The formation of the hollow cores is consistent with effect of screw dislocations with giant Burgers vector predicted by Frank.

Effect of Screw Dislocations on the Roughening Transition

The behaviour of the interface of a crystal which is threaded with parallel screw dislocations is analysed in the limits of weak and strong coupling of the interface to the underlying lattice. If the dislocations are distributed as closely spaced pairs of opposite sign the interface is faceted at low temperatures, and undergoes the roughening transition in the weak coupling limit. In contrast, if the dislocations are distributed entirely at random the interface seems to have a glassy, non-faceted low temperature state, and undergoes the super-roughening transition in the weak coupling limit.

Enhancement of critical current densities by heavy-ion irradiation in YBa2Cu3O7- delta observed using the high-resolution Faraday effect.

Using the high-resolution Faraday technique, domain patterns of flux penetration in heavy-ion-irradiated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films are obtained. To compare the influence of different projectiles, the samples were irradiated with 25-MeV oxygen ions, 230-MeV nickel ions, and 1-GeV lead ions with various fluences perpendicular to the sample surface and hence parallel to the c axis of the materials. The irradiation with $^{16}\mathrm{O}$ produces point-defect cascades mainly perpendicular to the path of the projectile whereas the irradiation using $^{58}\mathrm{Ni}$ ions creates spherical regions of amorphized material along the path. The $^{208}\mathrm{Pb}$ ions create columnar tracks along the c axis of the material. For a simultaneous observation of irradiated and unirradiated regions of a single epitaxial thin film, only one-half of each sample was exposed to the irradiation. To obtain data for the acting-volume pinning forces and the local critical current densities, a numerical calculation also regarding the curvature of the flux lines is carried out. Oxygen and nickel irradiation lead to an enhancement of the critical current density by a factor of 2. By lead irradiation, an enhancement of the critical current density up to a factor of 5 is obtained. This critical current density at the largest fluence used reaches the theoretical predictions for an enhancement of the critical current density by means of irradiation-induced columnar tracks. The effects of the irradiation-induced defects are compared to those of the pinning sites present in the background pinning. The density of the irradiation-induced columnar defects and their influence on the pinning are compared to the density and effect of screw dislocations. Our results clearly indicate that screw dislocations do not act as effective pinning centers because of their small density.

Effect of screw dislocation on the propagation of waves in an elastic cylinder

Effect of screw dislocation on the propagation of waves in an elastic cylinder

The effect of screw-dislocations on the energy loss of channeled ions

Abstract By the presence of dislocations the channels in a single crystal are distorted. The distortion modifies the motion in planar channels, and the extent to which the modified motion influences energy loss spectra of ions transmitted through thin single crystals with screw-dislocations is examined. In the calculation the stopping power model of Robinson1 is used, and the motion of the ions in the distorted channels is solved approximately. Within the model used it is concluded that the presence of dislocations will be hard to detect using only energy loss measurements.

Surface processes in electrocrystallization

Abstract A short review of the theories of crystal growth of perfect and imperfect planeS is given. Experimental evidence of two-dimensional nucleation mechanism of growth of perfect crystal planes is provided for the first time and some crystallographic parameters of this process are determined. A quantitative verification of Frank's theory is given by investigating the growth phenomena on nearly perfect planes. The effect of screw dislocations on crystal growth is studied and some phenomena concerning the influence of surface active impurities on crystal growth and dissolution are reported. The theory of heterogeneous phase formation is briefly discussed and an experimental verification of the basic relation following this theory is given. The effect of the crystallographic structure upon nucleation is closely studied. A method permitting the determination of the activity distribution curve of nucleation sites on the basis of their critical supersaturation is also given.

Surface sensitivity of rectification

The diffusion coefficient of sulphur into PbS, FeS 2 and other sulphide semiconductors has been studied as a function of temperature by employing the etching and light profile microscope techniques. The penetration of sulphur into an n-type crystal brings about various changes in the electrical parameters of the sample. It is possible to obtain by controlled treatment layers, with the requisite carrier concentration for the study of photovoltaic effect and transistor action. It has been possible to observe in PbS photovoltaic effect and transistor action with current gain equal to 0.83 and voltage gain of about 4. With treatment of natural and artificial n-type crystals, a definite improvement in rectifying characteristics has been observed. The slope α of the semilog plot of such treated samples is nearly 40 V -1 at room temperature as expected by the theory. The slope α is usually found to depend on φ which in turn varies from point to point on the crystal surface and from layer to layer in the treated samples. This point-sensitivity as observed on the surfaces of a number of crystals is attributed to the crystal imperfections which terminate on the surface. Similar results have been observed employing a single crystal of germanium grown by Marconi Wireless and Co. This crystal surface (111) is seen to be covered with a mosaic structure, the figures being hexagonal in shape. These figures can be interpreted as a row or a lattice of dislocations as pointed out by Burgers. The effect of screw dislocations on the electrical properties has been investigated using SiC crystals. An interesting correlation of the effect of dislocation on the rectifying characteristics has been indicated.

Dislocation spiral kinetics of crystal evaporation

The effect of screw dislocations on the kinetics of evaporation of otherwise perfect metal crystals is considered. A criterion for departure from the limiting law of evaporation of perfect crystals in which the monatomic ledges emanate from crystal edges, i.e. evaporation coefficient α = 1 3 + 2 3 ( p p e ) , is given in terms of concentration of screw dislocations and temperature. At large undersaturations, high concentrations of screw dislocations and low temperatures tend to favor departure from the limiting law. At small undersaturations, the limiting law should apply regardless of temperature and concentration of screw dislocations. A criterion is given for the degree of undersaturation required to produce appreciable pitting at screw dislocations.