Dislocation Sites Research Articles

Evaporation Spirals on {111} and {001} Surfaces of MgAl2O4 Spinel

Abstract Processes at dislocation sites are frequently predominant mechanisms of surface evaporation or growth and thus determine resulting surface morphologies. Dislocation-induced evaporation patterns are also a convenient object for observation of dynamics of step motion on crystal surfaces, as such patterns have “fixed” origins, being pinned at the point of dislocation emergence. In the present work, observations of evaporation spirals formed on ﹛111﹜ and ﹛001﹜ surfaces of MgAl2O4 spinel are reported. 2x2x1 mm samples of single-crystal MgAl2O4 spinel of ﹛111﹜ and ﹛001﹜ orientation were annealed in a spinel crucible in vacuum (10-4—10-5 Torr) in a Centorr™ furnace at 1800°C for 8 hours. SPM images were collected in air in contact mode on a Nanoscope III (Digital Instruments, Santa Barbara, CA), using Si3N4 cantilevers (Ultralevers, Park Inst., Sunnyvale, CA) with a nominal spring constant of 0.12 N/m. Selected sample surfaces were examined for impurities by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoemission spectroscopy (XPS).

Growth and microstructure of In xGa 1− xN films grown on SiC substrates via low pressure metalorganic vapor phase epitaxy

The growth and microstructures of In x Ga 1− x N films ( x≤0.23) grown on α(6H)–SiC(0001) wafer/AIN buffer layer/GaN heterostructures by low pressure metalorganic vapor phase epitaxy have been investigated. The system deposition pressure limited the InN content in these films. The maximum InN contents achievable at the deposition pressures of 45 and 90 torr were ∼13 and ∼23%, respectively. Kinetic phenomena based on the rates of adsorption and desorption of the In growth species off the growth surface are presented to explain the film composition dependence on the system pressure. The surface morphologies and microstructures of the In x Ga 1− x N films were analyzed using several techniques, and the formation of pinhole defects in the films was investigated. Most of the pinhole defects were associated with threading dislocations with a c-component Burgers vector. Edge-type dislocations were never observed to terminate in pinholes in the samples observed here. Indium segregation to areas around the defect areas was observed, as was an In compositional gradient in the growth direction. Based on experimental observations, the strain field around dislocations with a c-component Burgers vector could result in the increase of In atoms at the dislocation sites in the film, which result in a change to the local growth mode of the film and causes the pinhole defects to form.

Crystal growth and defect study of BiSbTe3

BiSbTe3 single crystals have been grown by the Bridgman technique. Microscopic observations of the as-grown crystals reveal typical features, such as striations on the top free surface, which are attributed to the effect of growth conditions. A nitric-acid-based reagent capable of revealing dislocations has been developed and tested. Etch pits are produced at the dislocation sites, but some discrepancies have been observed on matched cleavage surfaces. The structural difference between the matched cleavage surfaces is discussed.

In Situ Observation of the Surface Processes Involved in Dissolution from the (010) Surface of Potassium Ferrocyanide Trihydrate in Aqueous Solution Using an Integrated Electrochemical−Atomic Force Microscope

The surface processes accompanying dissolution from the (010) surface of potassium ferrocyanide trihydrate in aqueous solution have been determined using an integrated electrochemical−atomic force microscope (IE-AFM). This instrument employs a Pt-coated AFM tip that functions as an electrode, as well as a conventional topographical imaging device. The dissolution process was induced by oxidizing ferrocyanide at the tip, from an initially saturated solution, thereby creating a local undersaturation at the crystal−solution interface. The subsequent dissolution behavior was imaged in real time as a function of the diffusional driving force. At low driving force, the dissolution process proceeds mainly by the retreat of preexisiting steps aligned in the dominant [102] direction. With increasing driving force, dissolution occurs via the unwinding of steps from screw dislocation sites, leading to the formation of macroscopic etch pits, with an outline morphology determined by the [102] and [201] directions. At ...

Etch pit study of different crystallographic faces of L-arginine hydrobromide monohydrate (LAHBr) in alcohols

Chemical etchants based on simple alcohols were successfully applied for the first time to reveal dislocation sites on the polished {1 0 0}, {0 1 0}, {0 0 1} and {1 1 0} faces of L-arginine hydrobromide monohydrate. Fast dissolving etchants could produce etch pits only on the `F' faces but they have no effect on the `S' face. Selective behaviour of the etchants for revealing inclined dislocations and cooperating spirals has been demonstrated. Presence of growth spirals on {0 0 1} and {1 1 0} faces reveal that growth of these faces is governed by screw dislocation mechanism.

A hydrogen selective gas sensor from highly oriented films of carbon, obtained by fracturing charcoal

Abstract Single layer carbon film has been produced to fabricate a highly selective hydrogen sensor by depositing a partially crystalline carbon film from a single layer suspension onto an alumina substrate. The deposited carbon film, which has a hexagonal-turbostratic structure with an interlayer spacing of (d002) 3.58 A, show high orientation with intense (00 l ) Bragg diffraction lines in their X-ray diffraction patterns. When the deposited films were promoted with catalysts selected from Pt-group metals, a covalent Pt–C bond is formed between highly dispersed Pt particles and the unsaturated dangling carbon bonds at the edges and/or at dislocation sites. These catalyst particles are responsible for the dissociation of hydrogen molecules and subsequent electron donation to the conduction band, resulting in high sensitivity and selectivity hydrogen gas sensors. At a temperature of 110°C, the sensor's response to hydrogen is linear in concentration up to 103 ppm, with a 15–30 s response time and a 20–60 s recovery time. The lower operating temperature enhances safety when dealing with hydrogen gas.

In Situ Optical Interferometric Studies of the Growth and Dissolution Behavior of Paracetamol (Acetaminophen) Crystals. 3. Influence of Growth in the Presence of p-Acetoxyacetanilide

The influence of p-acetoxyacetanilide (PAA) as an additive in the surrounding paracetamol solution was investigated during growth and dissolution of paracetamol crystals. The different crystal faces of paracetamol, {110}, {201̄}, and {001}, exhibited two different mechanisms of interaction with the additive. According to the first mechanism, PAA molecules are adsorbed onto the crystal surfaces, i.e. {110} faces, and decrease both the step free energy and the step velocity functions. The second mechanism, which is characteristic for the {001} crystal faces, consists of a selective adsorption of PAA onto the active dislocation sites and suppression of more powerful growth centers in favor of elemental dislocations. The character of adsorption is defined by the affinity between the molecular structure of the crystal surface and the additive molecules, which permits incorporation into the {110} growth sectors. Lattice strain produced by incorporation of PAA exhibits a maximum followed by a relaxation process. This relaxation is caused by formation of cleavage defects and corresponds to an increase of the enthalpy of fusion and decrease of the crystal density as functions of PAA concentration.

Photoplastic effect and Vickers microhardness in III–V and II–VI semiconductor compounds

The influence of light illumination on the dislocation behaviour in GaAs and ZnS has been investigated by room temperature indentation tests in darkness and under illumination. It is shown that the photoplastic effect (PPE) can be evidenced by this technique providing: (i) small loads are applied to the indentor; and (ii) the illumination wavelength is close to the absorption edge of the semiconductor. Transmission electron microscopy studies indicate that: (i) in GaAs the negative PPE originates in an illumination induced increase of the mobility of α dislocations, due to non radiative recombination of excited carriers at dislocation sites; and (ii) in ZnS the positive PPE originates in an illumination induced increase of the Peierls stress.

Dislocation diffusion mechanism in Pd/Ag crystals

In epitaxial Pd/Ag crystals, dislocations in the palladium film have a large influence on the effective diffusion coefficient. In silver the diffusion coefficient hardly depends on the dislocation density. With a dislocation density of 1010m−2 the diffusion coefficient in silver is larger than in the palladium, and at 773 K it is 10−18 m2/s. At a dislocation density of 3·1012 m−2, the diffusion coefficient in the palladium becomes larger than in silver, and at 773 K it is 3·10−18 m2/s. It is most likely that diffusion in silver takes place via the lattice, while in palladium it occurs at mobile dislocation sites.

The Dislocation Movement in White Tin Single Crystals

The \beta-Sn single crystals used in this study were melted and grown by the modified Bridgman technique under a 10^{-4} torr vacuum, and oriented by the back-reflection Laue method. Single crystal plates were stretched in an etching solution produced hillocks at the sites of dislocations and trails of hillocks along the loci of moved dislocations. Two kinds of dislocation motion could be distinquished by the shapes of hillocks trails. The one corresponds to the movement of a dislocation which terminates at the surface, and the other, the movement of a dislocation which emerges by stressing from the interior.

Interstitial precipitation in a Fe25Cr5Al alloy

Ferritic stainless steels containing intermediate to h igh interstitial contents (greater than 200 wppm total C + N ) exhibit severe embritt!ement after heating to 950°C and cooling rapidly to room temperature [1, 2]. The underlying cause in type 446 stainless steel is the rapid precipitation of Cr carbides and nitrides at grain boundaries and dislocation sites during cooling from temperatures above 950 °C, where the interstitial atoms are in solution [2]. The complex nature of carbonitride precipitation in Fe-Cr alloys has been the subject of several previous studies of 475 °C embrittlement reaction [1, 3 6]; formation of Cr2N was reported within the first few hours of ageing [3, 4], Fe and C may substitute partially for Cr and N in Cr2N (M2X) [4], and the metastable M2X precipitates coarsen with prolonged ageing and transition to Cr23X6 occurs [5]. Spear et al. proposed that the sequence of interstitial precipitation processes in FeCrA1 alloys is MX--~ M2X---~,M23X 6 [1]. In this work, interstitial preCipitation in a Fe25Cr5A1 alloy has been indentified during ageing at 600°C and thermal cycling treatment processes. The ingot of Fe25Cr5A1 alloy was prepared by induction melting. After homogenizing, the alloy was rolled into rod 8 mm in diameter with intermediate anneals. The altoy chemical composition was (wt %) Cr 24.61, A1 4.91, Mn 0.11, Si 0.35, C 0.038, N 0.007, S 0.002, P 0.015, Ti 0.3, mixed rare earth 0.05, and Fe balance. All alloy specimens were cut from the rod and were solution heated at 1200 °C in air before quenching in water. The programme of thermai cycling treatment used involved heating in air for 3 min at 1200 °C and cooling in air at a rate of 2.8 °C/s for a period which allowed all specimens to cool to ambient temperature. A small amount of residue extracted from the specimens was examined by X-ray diffraction. Thin foils of the alloy specimens for transmission electron microscopy (TEM) studies were prepared by standard procedures and the foils examined in a Philips 400 TEM with an electron accelerating voltage Of 100kV and an incident electron probe diameter of approximately 40 nm; energy-dipersive X,ray spectroscopy (EDS) was used to enable X-ray microanalysis of the specimens. TEM investigation indicates that there is no precipitation at grain boundaries in the alloy quenched from 1200 °C as shown in Fig. 1. A small amount of intragranular blocky TiN is present in the alloy. The matrix of the alloy has a bcc structure (a-

Multiple trauma (Part I); evaluation of 200 patients with more than one major fracture

In order to compare the patients who have more than one major fracture, with the other multiple trauma patients, the records of 200 multiple traumatized patients, treated between January 1988-June 1995, were reviewed retrospectively. Existing orthopaedic injuries, treatment modalities and developing complications were analyzed. The average age, hospital stay and Modified Multiple Injury Severity Scale score of the patients were determined as follows respectively; 31.5 years (range, 5-84), 24.5 days (range, 2-116) and 27.6 points (range, 17-66). The most frequent sites of dislocations were hip, sacroiliac and elbow joints. There were also mostly tibial, femoral, costal and pelvic fractures, and 17.1% of the extremity fractures were open. So, most of the obtained ratios and features, were found to be compatible with other studies. However, the rates of major vascular trauma (16%) type III open fractures (54.4%), and fractures per patient (4.7) were quite high. In addition, our mortality rate was relatively low (%7.5), and 61% of the late deaths were due to fat embolism. As a result, although there were some differences because of the abundance of orthopaedic injuries in our patients, the most significant finding, was the late deaths occurred generally due to fat embolism, rather than multiple organ system failure or sepsis.

Hydrogen passivation and its effects on carrier trapping by dislocations in InP/GaAs heterostructures

In previous work we reported on the stable passivation of dislocations in InP/GaAs heterostructures by plasma hydrogenation (Chatterjee et al., Appl. Phys. Lett. vol. 65, p. 58, 1994). In this article we investigate and compare the trapping kinetics and general trapping properties of dislocations in strain relaxed p-InP grown on GaAs by metalorganic chemical vapor deposition prior to and after hydrogen passivation using deep level transient spectroscopy (DLTS) and current-voltage-temperature (I-V/T) measurements to determine the complete role of hydrogen passivation in these heterostructures. Three hole traps, T1A, T1B, and T2, were detected and attributed to dislocations in heteroepitaxial p-InP which displayed the logarithmic capture kinetics, extended dependence on fill pulse time, and broadened DLTS features expected for dislocation related traps. Quantitative analysis of the DLTS characteristics revealed progressive asymmetry in DLTS peak shape, an increase in characteristic peak width, and a decrease in activation energy as fill pulse time is increased until saturation values were reached. These observations are explained on the basis of a distribution or band of energy states for each trap resulting from the interaction of electrically active sites either between closely spaced dislocations or along dislocation cores within the strain-relaxed InP. For fill pulse times increasing from 1 μs to 10 ms, activation energies for T1A decreased monotonically from 0.80 to 0.65 eV, for T1B from 0.56 to 0.45 eV, and for T2 from 0.45 to 0.35 eV, with saturation occurring at the upper and lower limits for each trap, which indicates a qualitative measure of the energy spread for each trap. Plasma hydrogenation was not only found to passivate dislocations by reducing the trap concentration from ∼6×1014 to ∼3×1012 cm−3 for a 2 h exposure, but also strikingly altered their basic trapping properties. The qualitative measure of energy spread for the T1A and T2 traps were narrowed from ∼100 to 150 meV to ∼20 to 30 meV after a 2 h hydrogen exposure, whereas T1B was not detected after passivation. In addition, a simultaneous reduction in fill pulse saturation time, DLTS peak broadening, and peak shift as a function of hydrogen exposure time were observed. These observations suggest that hydrogen passivation modifies the dislocation trapping characteristics toward a more point defectlike behavior due to an increase in the average spacing between electrically active dislocation sites. This in turn reduces the interactions between these sites and narrows the distribution of states within each defect band. Further, reverse bias I-V/T measurements revealed that the near midgap trap T1A, which was found to dominate the space charge generation current prior to passivation, is no longer dominant after hydrogen passivation. Instead a 2 h hydrogen treatment shifted the dominant center to an activation energy which more closely matches the shallow T2 level.

Cluster/dislocation interactions in dilute aluminum-based solid solutions

The influence of single solute atoms and solute clusters on an extended edge dislocation dipole in Al was studied by atomistic simulation. Single Cu and Ag solute/dislocation interaction energy calculations showed that Cu interacts strongly with an Al extended dislocation and prefers sites in the compressive region, in agreement with elasticity theory predictions. Single Ag atoms, however, are strongly repelled by an Al extended dislocation, in contrast with elasticity theory predictions. Monte Carlo simulations of Al: 1% Cu, Al: 2% Cu, Al: 1% Ag, Al: 0.5% Cu, 0.5% Ag, and Al: 0.75% Cu, 0.25% Ag were carried out in the presence of an extended dislocation dipole at 600 K allowing for solute segregation. Cu atoms in the binary alloys were observed to segregate to the compressive regions of the extended dislocation dipole, forming widespread “atmospheres” over the width of both extended dislocations which did not affect the partial dislocation spacing. Ag in the binary alloy formed small Ag zones which also had little influence on the spacing between the partials. The ternary systems, however, exhibited highly localized solute clusters that had a large impact on the extended dislocation dipole structure, increasing the separation between the partial dislocations. The resulting cluster structures are discussed along with their influence on the apparent stacking fault energy of the alloy systems.

Reactivity at Line Defects in Antimony in Etchants with Dextro‐ and Levo Tartaric Acid

AbstractThe (111) cleavage faces of antimony single crystals were etched in aqueous mixtures of fuming nitric acid plus dextro tartaric acid and fuming nitric acid plus levo tartaric acid. Triangular, crystallographically oriented etch pits were observed at sites of dislocations. The activation energies for lateral motion of ledges and the frequency factors were calculated for both types of etchants which were found to be higher in levo tartaric acid containing etchants than in dextro tartaric acid containing etchants. The results are explained in terms of antimony complexes and the difference in the shape of dextro tartaric acid and levo tartaric acid molecular structures.

Atomic Force Microscopy of Hot Spot Reaction Sites in Impacted RDX and Laser Heated AP

AbstractAn atomic force microscope (AFM) has been used to reveal residual sub-micron sized decomposition sites in drop weight impacted RDX and laser irradiated AP crystals. In impacted RDX, the small and early reaction sites observed are hemispherical craters, ranging in size from 20–300 nm. The smallest reaction site encompassed about 10,000 molecules with an expected energy evolution of 2 × 10−14 J. On a somewhat larger scale hillocks of 200–800 nm were observed, their shape giving evidence of internal reaction and hot spot melting. Dislocation densities as high as 5 × 1012 per cm2 were observed in sub-ignited RDX. High resolution AFM images of the RDX lattice structure indicate molecular rotation as well as displacements at dislocation sites. In AP, after nanosecond pulsed laser irradiation, reaction sites were trumpet shaped with a smallest size of approximately 50 nm. Most sites contained a crystallographically oriented central square lid formed above the surrounding crystal surface, probably relating to the orthorhombic to cubic phase transition documented in micron scale cracking patterns observed at the laser heated sites.

Ostwald ripening in lorentz-force stabilized CuPb dispersions at low volume fractions. I—Experimental observations

We present measurements and theoretical investigations on Ostwald-ripening of Cu particles in a liquid PbCu melt with low volume content of the phase dispersed. For this purpose we used a novel technique that is able to suppress sedimentation of particles in an electrically conducting matrix. The experiments on solid-liquid CuPb dispersions in a low volume fraction range, which is not accessible by conventional experimental techniques, reveal the existence of stationary particle size distributions. The particle growth differs significantly from the growth law of the LSW-theory for diffusion-controlled Ostwald ripening, indicating that in this system the particle coarsening is not only diffusion limited. We outline a new growth model based on the assumption of preferred places for the trapping of Cu-atoms at dislocation sites emerging at the particle surface.

Helium isotope diffusion in natural diamonds

Abstract Stepped heating experiments in the range of 1200–1700°C were performed on terrestrial diamonds, some containing substantial cosmic ray-produced 3He, in order to study the diffusion coefficients and release characteristics of 3He and 4He. Cosmogenic 3He is shown to be much more retentive than 4He in the same sample, with as little as 0.3% of the total 3He, and up to 27.4% of the 4He released by 1700°C from diamonds in which cosmogenic 3He predominates (bulk 3 He 4 He = 176 R A ) . A tentative determination, from the data, of the site activation energy for cosmogenic 3He gives ~ 150 kJ/mol. The diffusion equation, with D0 = 6.1 × 10−11cm2/s, yields an extrapolated diffusivity at 1200°C of 1.9 × 10−16cm2/s. Other helium components are difficult to distinguish from each other by their apparent diffusion coefficients. In diamonds expected to have relatively low proportions of cosmogenic 3He, apparent D( 3 He) ≈ D( 4 He) , with measured and inferred diffusivities at 1200°C of 10−13 to 10−10 cm2/s. The higher apparent diffusivities of these components indicate a difference in distribution and siting relative to the homogeneous cosmogenic component. Diffusion coefficients from repeat isothermal extractions are successively lower, possibly indicating that near-surface or dislocation sites are depleted at lower temperatures. We predict that surface-correlated helium, such as that implanted from external radioactive decays, should be easily distinguishable by studies comparing grain size separates. Helium sited in isolated inclusions may be expected to have lower effective D than cosmogenic 3He, since removal from the inclusions is the ratelimiting step in low solubility systems (Trull and KURZ, 1993). However, this was not observed in our study. Bulk retention of trapped mantle helium over the age of most diamonds (> 1 Ga) would require effective diffusion coefficients at least several orders of magnitude lower than that inferred for the cosmogenic helium component at 1200°C, typical of mantle temperatures.

The Geography of Female Subjectivity: Ethnicity, Gender, and Diaspora

Female subjectivity forms the primary site of dislocation in Bharati Mukherjee’s stories of the making and imagining of immigrant identities in America. The experience of diaspora displaces the cultural narratives that write identity and womanhood in the home country; her stories reveal the vexed and ambivalent renarrations of “woman” produced by the dissemination of identities. This essay will examine the negotiations involved in the insertion of female subjectivity into the cultural space of “America” by focusing on the way Mukherjee’s fictions appropriate and interrogate the narratives of assimilation and feminism that provide paradigmatic texts of gender, as well as of ethnic and racial identity, in the United States and Canada.

The Geography of Female Subjectivity: Ethnicity, Gender, and Diaspora

Diaspora 3:1 1994 The Geography of Female Subjectivity: Ethnicity, Gender, and Diaspora Susan Koshy University of Louisville And I very deliberately set the story in V. S. Naipaul's birthplace because it was my "in"joke, challenging, ifyou like, Naipaul's thesis of tragedy being geographical. Naipaul's fiction seems to suggest that if you are born far from the center of the universe, you are doomed to an incomplete and worthless little life. You are bound to be, ifyou're born like a Jasmine, an Indian in the Caribbean, a comic character, you come to nothing. So I wanted to say, "Hey, look at Jasmine. She's smart, and desirous, and ambitious enough to make something of her life." Bharati Mukherjee Female subjectivity forms the primary site of dislocation in Bharati Mukherjee's stories of the making and imagining of immigrant identities in America.1 The experience of diaspora displaces the cultural narratives that write identity and womanhood in the home country; her stories reveal the vexed and ambivalent renarrations of "woman" produced by the dissemination of identities. This essay will examine the negotiations involved in the insertion of female subjectivity into the cultural space of "America" by focusing on the way Mukherjee's fictions appropriate and interrogate the narratives of assimilation and feminism that provide paradigmatic texts of gender, as well as of ethnic and racial identity, in the United States and Canada. First, I will argue that Mukherjee's celebration of assimilation is an insufficient confrontation of the historical circumstances of ethnicity and race in the United States and of the complexities of diasporic subject-formation.2 Second, while it is important to indicate the feminist concerns in Mukherjee's writing, I will try to show that such an analysis is incomplete ifit does not also address the narrativization of these concerns in her work. Since gender does not exist as a unitary text but is variously intersected by race, class, religion, and nationality, an interpretation that focuses only on the liberatory feminist motifs misses theorizing their production within other traversing relations. My interest in Mukherjee's writing is in the ways in which "woman" disrupts the homogeneities of national and 69 Diaspora 3:1 1994 ethnie identity and in the ways in which "woman" is used as a signifier to cover class discriminations. A comparison between Wife (1975), an early novel, and Jasmine (1989) will form the basis ofmy analysis, which will highlight the shifts in Mukherjee's articulation of emergent female subjectivity and the assimilationist narrative . Mukherjee's representations of female subjectivity have consistently engaged western liberal feminism's paradigmatic texts of emergent selfhood. Mukherjee has staked out a position that she claims is distanced from mainstream American and European feminism , and she has frequently indicated her disapproval of "the imperialism of the feminists" whose "tools and rhetoric" cannot be applied "wholesale and intact" to the situations of "some non-white, Asian women" (Connell 22). Despite this declaration, Mukherjee's is both a more mediated position than her statements would allow and it is one that has changed significantly in the course ofher career . She has moved since her novel Wife toward a celebration of what Norma Alarcon has described as "the most popular subject of Anglo-American feminism ... an autonomous, self-making, selfdetermining subject" (357). Thus, her work, despite its stated opposition to mainstream feminism, is also deeply complicit with some of its underlying assumptions. The difficulty can be located in Mukherjee's tendency to frame issues in terms that always claim maximum marginalization for her main characters. Insofar as they are victims, they are shown as being marked by racial, religious, or class conflicts; but Mukherjee obscures similar social relations in situations where they are part of the structures of dominance. Since her main characters are usually middle-class Indian women, class becomes the ground of greatest obfuscation and ambivalence in her fiction. Quite often, Mukherjee stages confrontations so that the gender conflicts overwrite and obscure class conflicts. One of the key moments in "The Management ofGrief" presents a confrontation between Shaila Bhave and a petty Indian customs official; he refuses to clear the coffins and Shaila abuses him in public.3...