Anomalous behavior of hydrodynamic modes in the two dimensional shear flow of a granular material

The growth rates of the hydrodynamic modes in the homogeneous sheared state of a granular material are determined by solving the Boltzmann equation. The steady velocity distribution is considered to be the product of the Maxwell Boltzmann distribution and a Hermite polynomial expansion in the velocity components; this form is inserted into them Boltzmann equation and solved to obtain the coeificients of the terms in the expansion. The solution is obtained using an expansion in the parameter epsilon =(1 - e)(1/2), and terms correct to epsilon(4) are retained to obtain an approximate solution; the error due to the neglect of higher terms is estimated at about 5% for e = 0.7. A small perturbation is placed on the distribution function in the form of a Hermite polynomial expansion for the velocity variations and a Fourier expansion in the spatial coordinates: this is inserted into the Boltzmann equation and the growth rate of the Fourier modes is determined. It is found that in the hydrodynamic limit, the growth rates of the hydrodynamic modes in the flow direction have unusual characteristics. The growth rate of the momentum diffusion mode is positive, indicating that density variations are unstable in the limit k--> 0, and the growth rate increases proportional to kslash} k kslash}(2/3) in the limit k --> 0 (in contrast to the k(2) increase in elastic systems), where k is the wave vector in the flow direction. The real and imaginary parts of the growth rate corresponding to the propagating also increase proportional to kslash k kslash(2/3) (in contrast to the k(2) and k increase in elastic systems). The energy mode is damped due to inelastic collisions between particles. The scaling of the growth rates of the hydrodynamic modes with the wave vector I in the gradient direction is similar to that in elastic systems. (C) 2000 Elsevier Science B.V. All rights reserved.

Positional dependence of material flow in friction stir welding: analysis of joint line remnant and its relevance to dissimilar metal welding

Understanding material flow in friction stir welding is important for production of sound dissimilar metal welding that control the intermixing of two alloys being welded and consequent formation of new constituents which influences the weld properties. In the present experimental investigation material flow patterns are visualised using dissimilar and similar aluminium alloys using a simple innovative ,experiment. The experimental results reveal that only a portion of material transported from the leading edge undergoes chaotic flow and the remaining is deposited systematically in the trailing edge of the weld. Using this information it is shown that the formation of a friction stir welding defect, joint line remnant, does not occur only when the weld interface is on the advancing side. The material flow visualisation study has been utilised to analyse the mechanism of weld formation and its usefulness in improving fatigue properties and for dissimilar metal welds.

Coupling of surface acoustic waves propagating in two separated piezoelectric media

The coupling of surface acoustic waves propagating in two separated piezoelectric media is studied using the perturbation theory of Auld. The results of the analysis are applied to two configurations using Bi12GeO20 and CdS crystals. It is found that the loss due to coupling is about 7 dB at 50 MHz in the cases of (111)-cut, [110]-prop. Bi12GeO20 and Y-cut, 60°-X prop. CdS combination. On étudie le couplage des ondes acoustiques de surface se propageant sur deux milieux piezo-eléctriques par la théorie de perturbation de Auld. Les resultats d'analyse sont appliqué's aux deux configurations des cristanx Bi12GeO20 et CdS. On trouve que la perte par couplage est environ de 7 dB a 50 MHz dans le cas de combination de (111)-coupe, [110]-prop. Bi12GeO20 et Y-coupe, 60°-X prop. CdS.

Microwave driven hydrothermal synthesis of LiMn2O4 nanoparticles as cathode material for Li-ion batteries

Among the various cathode materials studied for Li-ion batteries over the past many years, spinet LiMn2O4 is found to be one of the most attractive materials. Nanoparticles of the electrode materials sustain high rate capability due to large surface to volume ratio and small diffusion path length. Nanoparticles of spinel LiMn2O4 have been synthesized by microwave hydrothermal technique using prior synthesized amorphous MnO2 and LiOH. The phase and purity of spinel LiMn2O4 are confirmed by powder X-ray diffraction. The morphological studies have been investigated using field emission scanning electron microscopy and high-resolution transmission electron microscopy. The electrochemical performances of the material for Li insertion/extraction are evaluated by cyclic voltammetry, galvanostatic charge-discharge cycling and AC impedance studies. The initial discharge capacity is found to be about 89 mAh g(-1) at current density of 21 mA g(-1). (C) 2010 Elsevier B.V. All rights reserved.

Sonochemical synthesis of Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta nanocrystallites: oxygen storage material, CO oxidation and water gas shift catalyst

Nanocrystalline Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta of similar to 4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe3+ ion in CeO2, lattice oxygen is activated and 33% Fe substituted CeO2 i.e. Ce0.67Fe0.33O1.835 reversibly releases 0.31O] up to 600 degrees C which is higher or comparable to the oxygen storage capacity of CeO2-ZrO2 based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd2+/0(0.89 V) and Fe3+/2+ (0.77 V) with Ce4+/3+ (1.61 V), Pd ion accelerates the electron transfer from Fe2+ to Ce4+ in Ce0.65Fe0.33Pd0.02O1.815, making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce0.65Fe0.33Pd0.02O1.815 is found to be as low as 38 kJ mol(-1). Ce0.67Fe0.33O1.835 and Ce0.65Fe0.33Pd0.02O1.815 have also shown high activity for the water gas shift reaction. CO conversion to CO2 is 100% H-2 specific with these catalysts and conversion rate was found to be as high 27.2 mu moles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce0.65Fe0.33Pd0.02O1.815.

Evaluation of Erosion Resistance of Metallic Materials and the Role of Material Properties in Correlations

A study of the correlations between material properties and normalized erosion resistance (inverse of erosion rates) of various materials tested in the rotating disk and the flow venturi at various intensities indicates that different individual properties influence different stages of erosion. At high and low intensities of erosion, energy properties predominate the phenomenon, whereas at intermediate intensities strength and acoustic properties become more significant. However, both strength and energy properties are significant in the correlations for the entire spectrum of erosion when extensive cavitation and liquid impingement data from several laboratories involving different intensities and hydrodynamic conditions are considered. The use of true material properties improved the statistical parameters by 3 to 37%, depending on the intensity of erosion. It is possible to evaluate qualitatively the erosion resistances of materials based on the true stress-true strain curves.

Platinum particles supported on titanium nitride: an efficient electrode material for the oxidation of methanol in alkaline media

In the present study, titanium nitride which shows exceptional stability, extreme corrosion resistance, good electronic conductivity and adhesion behaviour is used to support platinum particles and then used for methanol oxidation in an alkaline medium. The catalyst shows very good CO tolerance for the electrochemical oxidation of methanol. In situ infrared spectroelectrochemical data show the remarkable ability of TiN to decompose water at low over potentials leading to -OH type functional groups on its surface which in turn help in alleviating the carbon monoxide poisoning associated with methanol oxidation. TiN supported catalysts are found to be very good in terms of long term stability, exchange current density and stable currents at low over voltages. Supporting evidence from X-ray photoelectron spectroscopic data and cyclic voltammetry clearly demonstrates the usefulness of TiN supported Pt catalysts for efficient methanol oxidation in alkaline media.

Mechanical Properties of Al-Mica Particulate Composite Material

Cast aluminium alloy mica particle composites of varying mica content were tested in tension, compression, and impact. With 2.2 percent mica (size range 40µm – 120µm) the tensile and compression strengths of aluminium alloy decreased by 56 and 22 percent, respectively. The corresponding decreases in percent elongation and percent reduction are 49 and 39 percent. Previous work [2] shows that despite this decrease in strength the composite with 2.5 percent mica and having an UTS of 15 kg/mm2 and compression strength of 28 kg/mm2 performs well as a bearing material under severe running conditions. The differences in strength characteristics of cast aluminium-mica particle composites between tension and compression suggests that, as in cast iron, expansion of voids at the matrix particle interface may be the guiding mechanism of the deformation. SEM studies show that on the tensile fractured specimen surface, there are large voids at the particle matrix interface.

Dynamic growth of tensile cracks by ductile and brittle fracture mechanisms in a viscoplastic material

In this paper, a finite element analysis of steady-state dynamic crack growth under Mode I, plane strain, small-scale yielding conditions is performed in a rate dependent plastic material characterized by the over-stress model. The main objective of the paper is to obtain theoretically the dependence of dynamic fracture toughness on crack speed. Crack propagation due to a ductile (micro-void) mechanism or a brittle (cleavage) mechanism, as well as transition from one mode to another are considered. The conversion from ductile to brittle has been observed experimentally but has received very little attention using analytical methods. Local fracture criteria based on strains and stresses are used to describe ductile and brittle fracture mechanisms. The results obtained in this paper are in general agreement with micro-structural observations of mode conversion during fracture initiation. Finally, the particular roles played by material rate sensitivity and inertia are examined in some detail.

Interferometry with a novel composite material

An interesting application of optical phase conjugation is phase conjugate interferometry. We report here a new approach to real-time interferometry which combines the high phase conjugate efficiency of photorefractive crystals and the fast response times offered by dye-doped polymer films by using a composite structure. The ability of this material to generate two independent but overlapping phase conjugate waves. which can interfere to reveal the phase changes in a test object, is discussed and demonstrated with a specific example.

Structural refinement using high-resolution powder X-ray diffraction data of Ca0.5Ti2P3O12, a low-thermal-expansion material

The structures of Ca0.5Ti2P3O12 and Sr0.5Ti2P3O12, low-thermal-expansion materials, have been refined by the Rietveld method using high-resolution powder X-ray diffraction (XRD) data. The assignment of space group R[3 with combining macron] to NASICON-type compounds containing divalent cations is confirmed. 31P magic-angle spinning nuclear magnetic resonance (MASNMR) data are presented as supporting data. A comparison of changes in the polyhedral network resulting from the cation distribution, is made with NaTi2P3O12 and Nb2P3O12. Factors that may govern thermal expansion in this family of compounds are discussed.

Coupled electrostatic-elastic analysis for topology optimization using material interpolation

In this paper, we present a novel analytical formulation for the coupled partial differential equations governing electrostatically actuated constrained elastic structures of inhomogeneous material composition. We also present a computationally efficient numerical framework for solving the coupled equations over a reference domain with a fixed finite-element mesh. This serves two purposes: (i) a series of problems with varying geometries and piece-wise homogeneous and/or inhomogeneous material distribution can be solved with a single pre-processing step, (ii) topology optimization methods can be easily implemented by interpolating the material at each point in the reference domain from a void to a dielectric or a conductor. This is attained by considering the steady-state electrical current conduction equation with a `leaky capacitor' model instead of the usual electrostatic equation. This formulation is amenable for both static and transient problems in the elastic domain coupled with the quasi-electrostatic electric field. The procedure is numerically implemented on the COMSOL Multiphysics (R) platform using the weak variational form of the governing equations. Examples have been presented to show the accuracy and versatility of the scheme. The accuracy of the scheme is validated for the special case of piece-wise homogeneous material in the limit of the leaky-capacitor model approaching the ideal case.

Performability of automated manufacturing systems with centralized material handling

We present a framework for performance evaluation of manufacturing systems subject to failure and repair. In particular, we determine the mean and variance of accumulated production over a specified time frame and show the usefulness of these results in system design and in evaluating operational policies for manufacturing systems. We extend this analysis for lead time as well. A detailed performability study is carried out for the generic model of a manufacturing system with centralized material handling. Several numerical results are presented, and the relevance of performability analysis in resolving system design issues is highlighted. Specific problems addressed include computing the distribution of total production over a shift period, determining the shift length necessary to deliver a given production target with a desired probability, and obtaining the distribution of Manufacturing Lead Time, all in the face of potential subsystem failures.

Stabilized alpha-Ni(OH)2 as electrode material for for alkaline secondary cells

Hydrotalcite-like compounds of formula Ni1-xAl(x)(OH)2(CO3)x/2 . nH2O (x = 0.1 to 0.25), having the same structure as that of alpha-Ni(OH)2, have been synthesized by substituting nickel hydroxide with aluminum. Of these, the compounds of compositions x greater-than-or-equal-to 0.2 are found to have prolonged stability in strong alkaline medium. The electrodes comprising stabilized alpha-Ni(OH)2 of x = 0.2 composition are rechargeable with discharge-capacity values of 240 (+/- 15) mAh-g-1 and are attractive for applications in various alkaline secondary cells employing nickel-positive electrodes.