The vector fields admitting one-parameter spatial symmetry group and their reduction

For a n-dimensional vector fields preserving some n-form, the following conclusion is reached by the method of Lie group. That is, if it admits an one-parameter, n-form preserving symmetry group, a transformation independent of the vector field is constructed explicitly, which can reduce not only dimesion of the vector field by one, but also make the reduced vector field preserve the corresponding ( n - 1)-form. In partic ular, while n = 3, an important result can be directly got which is given by Me,ie and Wiggins in 1994.

Synthesis of thick Ni66Cr5Mo4Zr6P15B4 amorphous alloy coating and large glass-forming ability by laser cladding

A thick amorphous alloy (a-alloy) coating was synthesized by laser cladding. The a-alloy had a multicomponent chemistry, i.e., Ni66Cr5MO4Zr6P15B4 (in atom%). The maximum thickness of the coating is 0.8 mm. The a-alloy coating had large glass-forming ability (GFA) with wide supercooled liquid region (SLR) ranging from 52 to 61 K through the coating. The reason for high GFA in the a-alloy coating was discussed. (C) 2002 Published by Elsevier Science B.V.

Properties of high k gate dielectric gadolinium oxide deposited on Si(100) by dual ion beam deposition (DIBD)

Gadolinium oxide thin films have been prepared on silicon (100) substrates with a low-energy dual ion-beam epitaxial technique. Substrate temperature was an important factor to affect the crystal structures and textures in an ion energy range of 100-500 eV. The films had a monoclinic Gd2O3 structure with preferred orientation ((4) over bar 02) at low substrate temperatures. When the substrate temperature was increased, the orientation turned to (202), and finally, the cubic structure appeared at the substrate temperature of 700 degreesC, which disagreed with the previous report because of the ion energy. The AES studies found that Gadolinium oxide shared Gd2O3 structures, although there were a lot of oxygen deficiencies in the films, and the XPS results confirmed this. AFM was also used to investigate the surface images of the samples. Finally, the electrical properties were presented. (C) 2004 Elsevier B.V. All rights reserved.

Mn implanted GaAs by low energy ion beam deposition

High dose Mn was implanted into semi-insulating GaAs substrate to fabricate embedded ferromagnetic Mn-Ga binary particles by mass-analyzed dual ion beam deposit system at room temperature. The properties of as-implanted and annealed samples were measured with X-ray diffraction, high-resolution X-ray diffraction to characterize the structural changes. New phase formed after high temperature annealing. Sample surface image was observed with atomic force microscopy. All the samples showed ferromagnetic behaviour at room temperature. There were some differences between the hysteresis loops of as-implanted and annealed samples as well as the cluster size of the latter was much larger than that of the former through the surface morphology. (C) 2004 Elsevier B.V. All rights reserved.

FexSi grown with mass-analyzed low-energy dual ion beam deposition

Heavily iron-implanted silicon was prepared by mass-analyzed low-energy dual ion beam deposition technique. Auger electron spectroscopy depth profiles indicate that iron ions are shallowly implanted into the single-crystal silicon substrate and formed 35 nm thick FexSi films. X-ray diffraction measurements show that as-implanted sample is amorphous and the structure of crystal is partially restored after as-implanted sample was annealed at 400degreesC. There are no new phases formed. Carrier concentration depth profile of annealed sample was measured by Electrochemical C-V method and indicated that FexSi film shows n-type conductivity while silicon substrate is p-type. The p-n junction is formed between FexSi film and silicon substrate showing rectifying effect. (C) 2003 Elsevier B.V. All rights reserved.

(Ga, Gd, As) film growth on GaAs substrate by low-energy ion-beam deposit

(Ga, Gd, As) film was fabricated by the mass-analyzed dual ion-beam epitaxy system with the energy of 1000 eV at room temperature. There was no new peak found except GaAs substrate peaks (0 0 2) and (0 0 4) by X-ray diffraction. Rocking curves were measured for symmetric (0 0 4) reflections to further yield the lattice mismatch information by employing double-crystal X-ray diffraction. The element distributions vary so much due to the ion dose difference from AES depth profiles. The sample surface morphology indicates oxidizing layer roughness is also relative to the Gd ion dose, which leads to islandlike feature appearing on the high-dose sample. One sample shows ferromagnetic behavior at room temperature. (C) 2003 Elsevier B.V. All rights reserved.

NUMERICAL ANALYSIS OF THE FLOWFIELD IN A SUPERSONIC COIL WITH AN INTERLEAVED JET CONFIGURATION AND ITS EFFECT ON THE GAIN DISTRIBUTION

In a supersonic chemical oxygen-iodine laser (COIL) operating without primary buffer gas, the features of flowfield have significant effects on the Laser efficiency and beam quality. In this paper three-dimensional, multi-species, chemically reactive CFD technology was used to study the flowfield in mixing nozzle implemented with a supersonic interleaving jet configuration. The features of the flowfield as well as its effect on the spatial distribution of small signal gain were analyzed.

Snap-through and pull-in instabilities of an arch-shaped beam under an electrostatic loading

The snap-through and pull-in instabilities of the micromachined arch-shaped beams under an electrostatic loading are studied both theoretically and experimentally. The pull-in instability that results in a system collision with an electrode substrate may lead to a system failure and, thus, limits the system maximum displacement. The beam/plate structure with a flat initial configuration under an electrostatic loading can only experience the pull-in instability. With the different arch configurations, the structure may experience either only the pull-in instability or the snap-through and pull-in instabilities together. As shown in our computation and experiment, those arch-shaped beams with the snap-through instability have the larger maximum displacement compared with the arch-shaped beams with only the pull-in stability and those with the flat initial configuration. The snap-through occurs by exerting a fixed load, and the structure experiences a discontinuous displacement jump without consuming power. Furthermore, after the snap-through jump, the structures are demonstrated to have the capacity to withstand further electrostatic loading without pull-in. Those properties of consuming no power and increasing the structure deflection range without pull-in is very useful in microelectromechanical systems design, which can offer better sensitivity and tuning range.

Numerical analysis of spatial evolution of the small signal gain in a chemical oxygen-iodine laser operating without primary buffer gas

A chemical oxygen iodine laser (COIL) that operates without primary buffer gas has become a new way of facilitating the compact integration of laser systems. To clarify the properties of spatial gain distribution, three-dimensional (3-D) computational fluid dynamics (CFD) technology was used to study the mixing and reactive flow in a COIL nozzle with an interleaving jet configuration in the supersonic section. The results show that the molecular iodine fraction in the secondary flow has a notable effect on the spatial distribution of the small signal gain. The rich iodine condition produces some negative gain regions along the jet trajectory, while the lean iodine condition slows down the development of the gain in the streamwise direction. It is also found that the new configuration of an interleaving jet helps form a reasonable gain field under appropriate operation conditions. (c) 2007 Elsevier Ltd. All rights reserved.

Thermal stress wave and spallation induced by an electron beam

Thermal stress wave and spallation in aluminium alloy exposed to a high fluency and low energy electron beams are studied theoretically. A simple model for the study of energy deposition of electrons in materials is presented on the basis of some empirical formulae. Under the stress wave induced by energy deposition, microcracks and/or microvoids may appear in target materials, and in this case, the inelastic volume deformation should not vanish. The viscoplastic model proposed by Bodner and Partom with corresponding Gurson's yield function requires modification for this situation. The new constitutive model contains a scalar field variable description of the material damage which is taken as the void volume fraction of the polycrystalline material. Incorporation of the damage parameter permits description of rate-dependent, compressible, inelastic deformation and ductile fracture. The melting phenomenon has been observed in the experiment, therefore one needs to take into account the melting process in the intermediate energy deposition range. A three-phase equation of state used in the paper provides a more detailed and thermodynamical description of metals, particularly, in the melting region. The computational results based on the suggested model are compared with the experimental test for aluminium alloy, which is subjected to a pulsed electron beam with high fluency and low energy. (C) 1997 Elsevier Science Ltd.

Dynamic analysis of arrest of buckle propagation on a beam on a nonlinear elastic foundation by FEM

Based on the dynamic governing equation of propagating buckle on a beam on a nonlinear elastic foundation, this paper deals with an important problem of buckle arrest by combining the FEM with a time integration technique. A new conclusion completely different from that by the quasi-static analysis about the buckle arrestor design is drawn. This shows that the inertia of the beam cannot be ignored in the analysis under consideration, especially when the buckle propagation is suddenly stopped by the arrestors.

Internal resonance of an L-shaped beam with a limit stop .1. Free vibration

Real-life structures often possess piecewise stiffness because of clearances or interference between subassemblies. Such an aspect can alter a system's fundamental free vibration response and leads to complex mode interaction. The free vibration behaviour of an L-shaped beam with a limit stop is analyzed by using the frequency response function and the incremental harmonic balance method. The presence of multiple internal resonances, which involve interactions among the first five modes and are extremely complex, have been discovered by including higher harmonics in the analysis. The results show that mode interaction may occur if the higher harmonics of a vibration mode are close to the natural frequency of a higher mode. The conditions for the existence of internal resonance are explored, and it is shown that a prerequisite is the presence of bifurcation points in the form of intersecting backbone curves. A method to compute such intersections by using only one harmonic in the free vibration solution is proposed. (C) 1996 Academic Press Limited

Internal resonance of an L-shaped beam with a limit stop .2. Forced vibration

A limit stop is placed at the elbow of an L-shaped beam whose linear natural frequencies are nearly commensurable. As a result of this hardening device the non-linear system exhibits multiple internal resonances, which involve various degree of coupling between the first five modes of the beam in free vibration. A point load is so placed as to excite several modes and the resulting forced vibration is examined. In the undamped case, three in-phase and two out-of-phase solution branches have been found. The resonance curve is extremely complicated, with multiple branches and interactions between the first four modes. The amplitudes of the higher harmonics are highly influenced by damping, the presence of which can effectively attenuate internal resonances. Consequently parts of the resonance curve may be eliminated, with the resulting response comprising different distinctive branches. (C) 1996 Academic Press Limited

Circular brass foil failed by plugging opposite to direction of incident laser beam

The peripheries of circular foils of 30 mm in diameter and 0.1 mm thick are fixed while their surfaces are subjected to a long pulsed laser over a central region that may vary from 2 mm to 6 mm in diameter. Failure is observed and classified into three stages; they are referred to as thermal bulging, localized shear deformation, and perforation by plugging. A distinct feature of the failure mode is that bulging and plugging occurred in the direction opposite to the incident laser beam. Such a phenomenon can be expected to occur for a laser intensity threshold value of about 0.61 x 10(6) W/cm(2) beyond which local melting of the material begins to take place.

On the velocity of buckle propagation in a beam on a nonlinear elastic-foundation

The paper revisits a simple beam model used by Chater et al. (1983, Proc. IUTAM Symp. Collapse, Cambridge University Press) to examine the dynamics of propagating buckles on it. It was found that, if a buckle is initiated at a constant pressure higher than the propagation pressure of the model (P-p), the buckle accelerates and gradually reaches a constant velocity which depends upon the pressure, while if it is initiated at P-p, the buckle propagates at a velocity which depends upon the initial imperfection. The causes for the difference are also investigated.