THE TRANSFER OF METAL-IONS ACROSS THE WATER-NITROBENZENE INTERFACE FACILITATED BY NEUTRAL CARRIER ETH-129

The transfer of H+, Li+, Na+, Zn2+, Mg2+ and Cu2+ facilitated by ionophore ETH 129 (N, N, N', N'-tetracyolohexyl-3-oxapentanediamide) across water/nitrobenzene interface has been studied by the cyclic voltammetry. The mechanism of the transfer process has been discussed. The diffusion coefficients and the stability constants of the complexes formed in the nitrobenzene phase have been determined.

CYCLIC VOLTAMMETRIC STUDY ON THE TRANSFER MECHANISM OF ALKALI-METAL IONS ACROSS LIQUID LIQUID INTERFACE FACILITATED BY TRITON X-100

The transfer behavior of alkali motal ions K~+ and Na~+ across the interfaces of water/nitrobenzene and water/1, 2-dichloroethane facilitated by Triton X-100 is investigated by cyclic voltammetry with four electrodes. The equations of interfacial half-wave potential derived in terms of the mechanism proposed isverified by the experimental data and consistent with the practical △_0~wφ_p-pM curves.

Material response and failure mechanism of unidirectional metal matrix composites under impulsive shear loading

The material response and failure mechanism of unidirectional metal matrix composite under impulsive shear loading are investigated in this paper. Both experimental and analytical studies were performed. The shear strength of unidirectional C-f/A356.0 composite and A356.0 aluminum alloy at high strain rate were measured with a modified split Hopkinson torsional bar technique. The results indicated that the carbon fibers did not improve the shear strength of aluminum matrix if the fiber orientation aligned with the shear loading axis. The microscopic inspection of the fractured surface showed a multi-scale zigzag feature which implied a complicated shear failure mechanism in the composite. In addition to testing, the micromechanical stress field in the composite was analyzed by the generalized Eshelby equivalent method (GEEM). The influence of cracking in matrix on the micromechanical stress field was investigated as well. The results showed that the stress distribution in the composite is quite nonhomogeneous and very high shear stress concentrations are found in some regions in the matrix. The high shear stress concentration in the matrix induces tensile cracking at 45 degrees to the shear direction. This in turn aggravates the stress concentration at the fiber/matrix interface and finally leads to a catastrophic failure in the composite. From the correlation between the analysis and experimental results, the shear failure mechanism of unidirectional C-f/A356.0 composite can be elucidated qualitatively.

An elasto-plastic constitutive relation of whisker-reinforced metal-matrix composite

A material model for whisker-reinforced metal-matrix composites is constructed that consists of three kinds of essential elements: elastic medium, equivalent slip system, and fiber-bundle. The heterogeneity of material constituents in position is averaged, while the orientation distribution of whiskers and slip systems is considered in the structure of the material model. Crystal and interface sliding criteria are addressed. Based on the stress-strain response of the model material, an elasto-plastic constitutive relation is derived to discuss the initial and deformation induced anisotropy as well as other fundamental features. Predictions of the present theory for unidirectional-fiber-reinforced aluminum matrix composites are favorably compared with FEM results.

Particulate Size Effects in the Particle-Reinforced Metal-Matrix Composites

The influences of I,article size on the mechanical properties of the particulate metal matrix composite;are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted.

Failure of SiC particulate-reinforced metal matrix composites induced by laser thermal shock

Thermal failure of SiC particulate-reinforced 6061 aluminum alloy composites induced by both laser thermal shock and mechanical load has been investigated. The specimens with a single-edge notch were mechanically polished to 0.25 mm in thickness. The notched-tip region of the specimen is subjected to laser beam rapid heating. In the test, a pulsed Nd:glass laser beam is used with duration 1.0 ms or 250 mu s, intensity 15 or 70 kW/cm(2), and spot size 5.0 mm in diameter. Threshold intensity was tested and fracture behavior was studied. The crack-tip process zone development and the microcrack formation were macroscopically and microscopically observed. It was found that in these materials, the initial crack occurred in the notched-tip region, wherein the initial crack was induced by either void nucleation, growth, and subsequent coalescence of the matrix materials or separation of the SiC particulate-matrix interface. It was further found that the process of the crack propagation occurred by the fracture of the SiC particulates.

Toughness Of Ni/Al2O3 Interfaces As Dependent On Micron-Scale Plasticity And Atomistic-Scale Separation

Ceramic/metal interfaces were studied that fail by atomistic separation accompanied by plastic dissipation in the metal. The macroscopic toughness of the specific Ni alloy/Al2O3 interface considered is typically on the order of ten times the atomistic work of separation in mode I and even higher if combinations of mode I and mode II act on the interface. Inputs to the computational model of interface toughness are: (i) strain gradient plasticity applied to the Ni alloy with a length parameter determined by an indentation test, and (ii) a potential characterizing mixed mode separation of the interface fit to atomistic results. The roles of the several length parameters in the strain gradient plasticity are determined for indentation and crack growth. One of the parameters is shown to be of dominant importance, thus establishing that indentation can be used to measure the relevant length parameter. Recent results for separation of Ni/Al2O3 interfaces computed by atomistic methods are reviewed, including a set of results computed for mixed mode separation. An approximate potential fit to these results is characterized by the work of separation, the peak separation stress for normal separation and the traction-displacement relation in pure shearing of the interface. With these inputs, the model for steady-state crack growth is used to compute the toughness of the interface under mode I and under the full range of mode mix. The effect of interface strength and the work of separation on macroscopic toughness is computed. Fundamental implications for plasticity-enhanced toughness emerge.

Determination Ofinterfacial Mechanical Parameters For An Al/Epoxy/Al2O3 System Byusing Peel Test Simulations

Peel test measurements and simulations of the interfacial mechanical parameters for the Al/Epoxy/Al2O3 system are performed in the present investigation. A series of Al film thicknesses between 20 and 250 microns and three peel angles of 90, 135 and 180 degrees are considered. Two types of epoxy adhesives are adopted to obtain both strong and weak interface adhesions. A finite element model with cohesive zone elements is used to identify the interfacial parameters and simulate the peel test process. By simulating and recording normal stress near the crack tip, the separation strength is obtained. Furthermore, the cohesive energy is identified by comparing the simulated steady-state peel force and the experimental result. It is found from the research that both the cohesive energy and the separation strength can be taken as the intrinsic interfacial parameters which are dependent on the thickness of the adhesive layer and independent of the film thickness and peel angle.

Surface-induced melting of metal nanoclusters

We investigate the size effect on melting of metal nanoclusters by molecular dynamics simulation and thermo dynamic theory based on Kofman's melt model. By the minimization of the free energy of metal nanoclusters with respect to the thickness of the surface liquid layer, it has been found that the nanoclusters of the same metal have the same premelting temperature T-pre = T-0 - T-0(gamma(su) - gamma(lv) - gamma(sl))/(rhoLxi) (T-0 is the melting point of bulk metal, gamma(sv) the solid-vapour interfacial free energy, gamma(sl) the liquid-vapour interfacial free energy, gamma(sl),l the solid-liquid interfacial free energy, p the density of metal, L the latent heat of bulk metal, and xi the characteristic length of surface-interface interaction) to be independent of the size of nanoclusters, so that the characteristic length of a metal can be obtained easily by T-pre, which can be obtained by experiments or molecular dynamics (MD) simulations. The premelting temperature T-pre of Cu is obtained by AID simulations, then xi is obtained. The melting point T-cm is further predicted by free energy analysis and is in good agreement with the result of our MD simulations. We also predict the maximum premelting-liquid width of Cu nanoclusters with various sizes and the critical size, below which there is no premelting.

Finite element analysis on stresses field of normalized layer thickness within ceramic coating on aluminized steel

Multilayer ceramic coatings were fabricated on steel substrate using a combined technique of hot dipping aluminum(HDA) and plasma electrolytic oxidation(PEO). A triangle of normalized layer thickness was created for describing thickness ratios of HDA/PEO coatings. Then, the effect of thickness ratio on stresses field of HDA/PEO coatings subjected to uniform normal contact load was investigated by finite element method. Results show that the surface tensile stress is mainly affected by the thickness ratio of Al layer when the total thickness of coating is unchanged. With the increase of A] layer thickness, the surface tensile stress rises quickly. When Al2O3 layer thickness increases, surface tensile stress is diminished. 'Meanwhile, the maximum shear stress moves rapidly towards internal part of HDA/PEO coatings. Shear stress at the Al2O3/Al interface is minimal when Al2O3 layer and Al layer have the same thickness.

Structure and mechanical properties of ceramic coatings fabricated by plasma electrolytic oxidation on aluminized steel

Ceramic coatings were formed by plasma electrolytic oxidation (PEO) on aluminized steel. Characteristics of the average anodic voltages versus treatment time were observed during the PEO process. The micrographs, compositions and mechanical properties of ceramic coatings were investigated. The results show that the anodic voltage profile for processing of aluminized steel is similar to that for processing bulk Al alloy during early PEO stages and that the thickness of ceramic coating increases approximately linearly with the Al layer consumption. Once the Al layer is completely transformed, the FeAl intermetallic layer begins to participate in the PEO process. At this point, the anodic voltage of aluminized steel descends, and the thickness of ceramic coating grows more slowly. At the same time, some micro-cracks are observed at the Al2O3/FeAl interface. The final ceramic coating mainly consists of gamma-Al2O3, mullite, and alpha-Al2O3 phases. PEO ceramic coatings have excellent elastic recovery and high load supporting performance. Nanohardness of ceramic coating reaches about 19.6 GPa. (c) 2007 Elsevier B. V. All rights reserved.

Radially polarized and pulsed output from passively Q-switched Nd:YAG ceramic microchip laser

For the first time, to the best of our knowledge, a radially polarized laser pulse was produced from a passively Q-switched Nd:YAG ceramic microchip laser with a piece of Cr4+:YAG crystal as the saturable absorber and multilayer concentric subwavelength grating as the polarization-selective output coupler. The averaged laser power reached 450 mW with a slope efficiency of 30.2%. The laser pulse had a maximum peak power of 759 W, a minimum pulse duration of 86 ns, and a 6.7 kHz repetition rate at 3.7 W absorbed pump power. The polarization degree of the radially polarized pulse was measured to be as high as 97.4%. Such a radially polarized laser pulse with a high peak power and a short width is important to numerous applications such as metal cutting. (C) 2008 Optical Society of America

Photoelectric characteristics of metal/InGaN/GaN heterojunction structure

A heterojunction structure photodetector was fabricated by evaporating a semitransparent Ni/Au metal film oil the InGaN/GaN structure. The photocurrent (PC) spectra show that both the Schottky junction (NiAu/InGaN) and the InGaN/GaN isotype heterojunction contribute to the PC signal which suggests that two junctions are connected in series and result in a broader spectral response of the device. Secondary electron, cathodoluminescence and electron-beam-induced current images measured from the same area of the edge surface clearly reveal the profile of the layer structure and distribution of the built-in electric field around the two junctions. A band diagram of the device is drawn based oil the consideration of the polarization effect at the InGaN/GaN interface. The analysis is consistent with the physical mechanism of a tandem structure of two junctions connected in series.

The effect of growth temperature on structural quality of GaN/AlN quantum wells by metal-organic chemical vapour deposition

The effect of the growth temperature on the surface and interface quality for the GaN/AlN multiquantum well (MQW) layer grown by metal-organic vapour chemical deposition is investigated. The obtained GaN/AlN MQW structure is almost coherent to the underlying AlGaN layer at improved growth conditions. With a relatively low growth temperature, the GaN/AlN MQW growth rate increases, the surface roughness reduces considerably and no macro steps are observed, resulting in a better periodicity of MQW.

InN nanoflowers grown by metal organic chemical vapor deposition

Hexangular indium nitride nanoflower pattern is observed from scanning electron microscopy and atomic force microscopy. The sample is grown on c-plane (0001) sapphire by metal organic chemical vapor deposition with intentional introduction of hydrogen gas. With the aid of hydrogen, a stable existence of metallic indium is achieved. This will induce the growth of InN nanoflowers via self-catalysis vapor-liquid-solid (VLS) process. It is found that the VLS process is modulated by the interface kinetics and thermodynamics among the sapphire substrate, indium, and InN, which leads to the special morphology of the authors' InN nanoflower pattern. (c) 2006 American Institute of Physics.