On the self-pinning character of synchro-Shockley dislocations in a Laves phase during strain rate cyclical compressions

Strain rate cyclical tests in compression, between 1350 and 1500 degrees C, have been employed to study the self-pinning character of thermally activated synchro-Shockley dislocations in the C15 Cr2Nb Laves phase. An average minimum effective (pinning) stress was calculated to be necessary for their propagation. The dislocation velocity cannot respond instantly to the strain rate changes and requires variations in the mobile dislocation density because the synchro-Shockleys can be pinned if the cooperating motion of their two Shockley components is hindered. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Influence of simultaneous doping of Sb & Pb on phase formation, superconducting and microstructural characteristics of HgBa2Ca2Cu3O8+\delta

We report systematic studies of structural, microstructural and transport properties of (Hg_0.80 Sb_0.2-x Pb_x)Ba_2 Ca_2 Cu_3O_8+\delta (where x = 0.0, 0.05, 0.1, 0.15, 0.2) compounds. Bulk polycrystalline samples have been prepared by two-step solid-state reaction route at ambient pressure. It has been observed that simultaneous substitution of Sb and Pb at Hg site in oxygen deficient HgO_\delta layer of HgBa2Ca2Cu3O8+\delta cuprate high-Tc superconductor leads to the formation of Hg-1223 as the dominant phase. Microstructural investigations of the as grown samples employing scanning electron microscopy reveal single crystal like large grains embodying spiral like features. Superconducting properties particularly transport current density (Jct) have been found to be sensitive to these microstructural features. As for example (Hg0.80Sb0.05Pb0.15)Ba2Ca2Cu3O8+\delta compound which exhibits single crystal like large grains (~ 50 micrometer) and appears to result through spiral growth mechanism, shows highest Jct (~ 1.85 x 103 A/cm2) at 77K. A possible mechanism for the generation of spiral like features and correlation between microstructural features and superconducting properties have been put forward.Comment: 16 pages, 6 figures. Accepted in Physica

Characteristics of a laser-generated acoustic source in solids

This thesis describes a combined experimental and theoretical study of acoustic generation in solids by laser irradiation.Two basic types of laser acoustic source are identified, namely, the thermoelastic and the ablation source. Both sources are described in detail in metals and to a lesser extent in non-metals. The emphasis is on generation in the ultrasonic region. The thermoelastic source operates at comparatively low incident laser intensities, typically < 3 x 1012 Wm-2, and is driven by dynamic thermal expansion due to transient surface heating. The second source takes over at higher intensities, and results from the effects of surface vaporisation and plasma formation. The resultant acoustic waveforms (bulk and surface) have been studied over a wide range of experimental conditions using a pulsed Nd:YAG laser and both resonant and broad bandwidth detection probes. The surface forces produced under these various conditions are discussed in detail,and using acoustic theory, are related to the acoustic waveforms.In a wider context, thermoelastic generation is related to the photoacoustic effect in a generalised ID model

The mechanical properties and the deformation microstructures of the C15 Laves phase Cr2Nb at high temperatures

Compression tests between 1250 and 1550 degrees C and 10(-5) and 5 x 10(-3) s(-1) and transmission electron microscopy have been employed to investigate the high temperature mechanical properties and the deformation mechanisms of the C15 Cr2Nb Laves phase. The stress-peaks in the compression curves during yielding were explained using a mechanism similar to strain aging combined with a low initial density of mobile dislocations. The primary deformation mechanism is slip by extended dislocations with Burgers vector 1/2 <110 >, whereas twinning is more frequent at 10(-4) s(-1). Schmid factor analysis indicated that twinning is more probable in grains oriented so as to have two co-planar twinning systems with high and comparable resolved shear stresses. Twinning produced very anisotropic microstructures. This may be due to synchroshear: a self-pinning mechanism which requires co-operative motion of zonal dislocations. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Ceramic Joining by Gas Phase Pulsed Laser Processing

The method of Selective Area Laser Deposition (SALD) and Vapor Infiltration (SALDVI) has been successfully used to fabricate small three-dimensional SiC/SiC and SiC/metal powder parts. Ceramic joints made by this technique have been limited by the throwing power of the laser resulting in incomplete joint penetration. Studies were performed to show the effectiveness of a fiber laser, with a wavelength of 1070 nm, for a joining process. The ability of the laser to penetrate a powder bed was utilized in the joint fabrication. The combination of powder fill, and deep laser penetration into the powder bed shows potential in the field of ceramic joining.Mechanical Engineerin

Crystallographically Determined Etching and Its Relevance to the Metal-Assisted Catalytic Etching (MACE) of Silicon Powders

Metal-assisted catalytic etching (MACE) using Ag nanoparticles as catalysts and H2O2 as oxidant has been performed on single-crystal Si wafers, single-crystal electronics grade Si powders, and polycrystalline metallurgical grade Si powders. The temperature dependence of the etch kinetics has been measured over the range 5–37◦C. Etching is found to proceed preferentially in a h001i direction with an activation energy of ∼0.4 eV on substrates with (001), (110), and (111) orientations. A quantitative model to explain the preference for etching in the h001i direction is developed and found to be consistent with the measured activation energies. Etching of metallurgical grade powders produces particles, the surfaces of which are covered primarily with porous silicon (por-Si) in the form of interconnected ridges. Silicon nanowires (SiNW) and bundles of SiNW can be harvested from these porous particles by ultrasonic agitation. Analysis of the forces acting between the metal nanoparticle catalyst and the Si particle demonstrates that strongly attractive electrostatic and van der Waals interactions ensure that the metal nanoparticles remain in intimate contact with the Si particles throughout the etch process. These attractive forces draw the catalyst toward the interior of the particle and explain why the powder particles are etched equivalently on all the exposed faces

Regenerative Electroless Etching of Silicon

Regenerative electroless etching (ReEtching), described herein for the first time, is a method of producing nanostructured semiconductors in which an oxidant (Ox1) is used as a catalytic agent to facilitate the reaction between a semiconductor and a second oxidant (Ox2) that would be unreactive in the primary reaction. Ox2 is used to regenerate Ox1, which is capable of initiating etching by injecting holes into the semiconductor valence band. Therefore, the extent of reaction is controlled by the amount of Ox2 added, and the rate of reaction is controlled by the injection rate of Ox2. This general strategy is demonstrated specifically for the production of highly luminescent, nanocrystalline porous Si from the reaction of V2O5 in HF(aq) as Ox1 and H2O2(aq) as Ox2 with Si powder and wafers

Nanoindentation properties of shock-compressed single crystal Magnesium

The residual mechanical properties acquired from shock-compressed solids are often times dramatically different from those received under quasi-static conditions. This suggests that the deformation mechanisms present during shock compression may be significantly different than those seen under quasi-static conditions. Please click Additional Files below to see the full abstract