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In this paper, we present our study on multi-frequency scatter-broadening observations of a large sample of pulsars, made using the Ooty Radio Telescope (ORT) and the Giant Metrewave Radio Telescope (GMRT). For each pulsar, the scatter-broadening time scales (τsc) have been estimated at different observing frequencies and the dependence of τsc with the observing frequency, i.e., the frequency scaling index (α) has been obtained. We report estimates of α for a set of 39 pulsars, of which 31 are completely new and provide the first-time measurement on about 50% of the sample. This enhanced sample suggests that almost 65% of the pulsars have an α much lower than the conventional value of 4.4 for a Kolmogorov type turbulence spectrum, and a considerably large scattering strength. An increase in scattering strength is observed with the distance to the pulsar in the Galaxy.
We have achieved what we believe to be the first atomic resolution STM images for a uranium compound taken at room temperature. The a, b, and c lattice parameters in the images confirm that the USb2 crystals cleave on the (001) basal plane as expected. The a and b dimensions were equal, with the atoms arranged in a cubic pattern. Our calculations indicate a symmetric cut between Sb planes to be the most favorable cleavage plane and U atoms to be responsible for most of the DOS measured by STM. Some strange features observed in the STM will be discussed in conjunction with ab initio calculations.
According to parallel-flow theory based on the Orr–Sommerfeld equation, a mixing layer is unstable at all Reynolds numbers. However this is untenable from energy considerations, which demand that there exist a non-zero Reynolds number below which disturbances cannot extract net energy from the mean flow. It is shown here that a linear stability analysis of similarity solutions of the plane mixing layer, including the effects of flow non-parallelism using the minimal composite theory and the properties of adjoints, following Govindarajan & Narasimha (Theor. Comput. Fluid Dyn. vol. 19, 2005, p. 229) resolves the issue by yielding a non-zero critical Reynolds number for co-flowing streams of any velocity ratio. The critical Reynolds number for the total disturbance kinetic energy is found to be close to 30 for all velocity ratios in the range from zero to unity.
A novel magnetic pair-making interaction between Mn and Ru shows a strong correlation between the magnetic ordering and electronic transport, which was well exemplified in the investigation of bulk polycrystalline samples of La0.7Pb0.3Mn1⊟xRuxO3 and La0.6Pb0.4Mn1⊟xRuxO3, where 0.0 ≤ x ≤ 0.4. The metal-insulator transition (Tρ) complementing the Curie temperature (Tc) was observed up to 30% of Ru in La0.7Pb0.3Mn1⊟xRuxO3, and extended up to 40% of Ru in La0.6Pb0.4Mn1⊟xRuxO3, showing a unique double-exchange ferromagnetic exchange interaction between Mn and Ru ions. An upturn in resistance due to charge carrier localization at low temperatures (T<0.5 Tc) for more than 20% Ru doping was due to a dominant hole carrier density contribution rather than to grain boundary effects as inferred from the scanning electron microscopy and energy dispersive x-ray studies of the samples sintered at 1200 and 1400 °C. The charge localization effect of the eg electrons was removed by tuning the hole carrier density as demonstrated in the La0.6Pb0.4Mn1⊟xRuxO3 samples. Long range correlations between magnetism and transport in this series was attributed to the presence of mixed valence Ru(IV/V) and Mn(III)/(IV) pair, which shows a unique double exchange mediated interaction.
When radio waves propagate through a irregular medium, scattering by the random refractive index inhomogeneities can lead to a wide variety of phenomena, which include intensity scintillation. The observed scattering can be interpreted to gain information about the random medium and such inversion studies are valuable when the accessibility of the medium becomes difficult. This paper briefly describes the intensity scintillation of celestial radio sources caused by the turbulence in the solar wind and summarizes the salient features of the method employed in mapping the structure of disturbances leaving the Sun out to ∼1 AU.
The increasing demands on solder joints have made it imperative that they perform not only their traditional role of electrical connection but also possess good mechanical integrity. One such key mechanical property is the shear strength of the solder. A number of specimen geometries can be used to evaluate the shear strength of solders, each with its advantages and limitations. However, these geometries do not provide information about the notched fracture strength of the solder under shear. Studies on a number of ductile materials have indicated that in many of these materials shear is the preferential mode of fracture even when the imposed load is a combination of tensile and shear. This study, which is part of a larger project on the failure of materials under combined tensile-shear loading, uses a modified compact tension specimen to measure the appropriate failure criterion. It is shown that the fracture of solders under these conditions follows the general principles of a mixedmode fracture mechanism map and shear is the preferred mode of failure.
Epoxy based resins have been widely used as encapsulation materials in microelectronics components due to their good mechanical, thermal and electrical properties, outstanding adhesion to various substrates and the ability to be processed under a variety of conditions. These epoxy resins have a high glass transition temperature but low toughness and damage tolerance. As electronic devices continue to be miniaturized and the scale of integration increases, the encapsulation of these chips leads to mechanical and thermal stress related failures. Mechanical stresses could result from the mismatch in thermal expansion rates between the polymeric encapsulant, the silicon chip and the metal leadframe. The accommodation of these stresses may lead to cracking of the encapsulant or delamination. In this study, poly(oxazolidone) polymer was prepared by reacting 4,4′-Diphenyl-methanediisocyanate (MDI) with diglycidyl ether of bisphenol A (DGEBA) at elevated temperature. The mechanical properties of this polymer were investigated both as a stand-alone material as well as a bi-material sandwich with a brittle substrate. The degree of cracking in the substrate is shown to be a function of the thickness of the polymer layer. This result is analyzed based on the variation in the position of the neutral axis in this composite.
Microhardness testing is widely used for characterizing the mechanical properties of both bulk materials and thin films. Although this technique is usually associated with hardness measurements, fracture properties of brittle materials can also be studied with cracking associated with microhardness indentations. It is well known that the length of radial cracks emanating from the comers of indents made with Vickers and Berkovich indenters is related to the fracture toughness of the material. In the present study, microhardness testing has been used to follow the evolution of the mechanical properties of a 10 nm.Cu/200 nm. Ni(V)/300 nm. Al(Cu) thin film deposited on a Si substrate. Composite hardness and fracture toughness have been followed as a function of heat treatment temperatures and times and were found to be dependent on both variables. The roles of residual stresses, interdiffusion, and intermediate phase formation in the observed variation in hardness and fracture toughness are discussed.
The increasing demands on solder joints have made it imperative that they perform not only their traditional role of electrical connection but also possess good mechanical integrity. One such key mechanical property is the shear strength of the solder. A number of specimen geometries can be used to evaluate the shear strength of solders, each with its advantages and limitations. This study uses a modified double lap shear geometry to measure the shear strength of the solders as a function of strain rate. It is ahown that the shear strength measured this way is truly reflective of the complex composite formed by the copper, solder and intermetallics and may be more representative of actual conditions of use rather than measurements of the shear strengths of the bulk solder. The study also uses a modified compact tension specimen to measure the fracture of the solder under combined tensile-shear loading conditions. It is shown that the solder fracture under these conditions follows the general principles of a mixed-mode fracture mechanism map.
Metal-polymer bilayers and multilayers are being increasingly used in the microelectronics industry. In many cases, the reliability of the device or the package is determined by the mechanical behavior of these composite layers. Finite element analyses are being widely used to model these systems. An accurate experimental understanding of the mechanical behavior of these systems would further enhance the usefulness of such analyses. The present study focuses on the mechanical behavior of composite bilayers consisting of polyimide and sputtered Ti and Ti alloy thin films. Tensile tests and stretch-deformation tests have been performed on bilayers consisting of a polyimide substrate sputter-coated individually with a 50 nm and a 500 nm Ti layer, a 500 nm Ti-Cu layer, and a 500 nm Ti-Ni layer. Tensile moduli, stresses for 0.2%, 2% and 15% offset strains, and strains to failure have been obtained and compared. These values are discussed with reference to the known mechanical properties of the individual layers in the composite material.
The issue of multiple cracks in materials and their interaction is central in understanding the overall fracture behaviour of materials. In the case of materials used in the microelectronics industry, indentation cracking has been extensively used for the measurement of fracture toughness due to its small sample size requirements as well as a relatively good correlation with values obtained from traditional fracture mechanics tests. The majority of these studies have focused on the fracture behaviour of a single indent. The present study was aimed at understanding the effect of interaction between the cracks generated on Si from a pair of sequential indents as well as a set of four sequential indents placed at the comers of a square. The distance between the indents was varied from a level comparable to the crack size to a level where interaction could be ignored. This paper discusses the changes in the nature as well as the sizes of cracks due to interaction between the stress fields of the indents.
The metallization of Si represents a important industrial process and produces a bi-layered composite of a ductile metal film on a brittle substrate. The mechanical properties of such a composite are determined by the properties of the two layers and the interface and influenced by the fact that the metallized layer, being a very thin film, possesses properties different from those of a bulk material. The fracture toughness is also influenced by the nature and distribution of defects which may be generated during use of these materials, even if the manufacturing process produces a reasonably defect free material. Indentation cracking has been extensively used for the measurement of fracture toughness due to its small sample size requirements as well as a relatively good correlation with values obtained from traditional fracture mechanics tests. The indentation process, with its associated cracks, produces permanent plastic deformation and also introduces a residual stress field. This field influences the crack pattern generated in an adjacent indent and can be used as a methodology to model the influence of multiple defect sources.
The present study was aimed at understanding the effect of a thin Ti alloy metallization layer sputtered on a Si wafer on the sizes of the cracks associated with the indents. It was also aimed at studying the interaction between cracks emanating from sequentially placed indentations. The distance between the indents which generated these cracks was varied from a level comparable to the crack size to a level where interaction could be ignored. This paper discusses the changes in the nature as well as the sizes of cracks due to the presence of the metallization layer as well as the interaction between the stress fields of the indents in this ductile thin film – brittle substrate composite and possible methodologies for delineating these effects.
Let X be a continuous random variable denoting the lifetime of a unit. Let Xk:n denote the kth order statistic based on n independent random observations on X. It has been shown that if Xk:n has decreasing failure rate (DFR) for some k, 1 ≤ k ≤ n, then X is DFR. For n ≥ 2, if Xk:n has increasing failure rate (IFR), then Xk:n–1 is also IFR, and if Xk:n is DFR, then Xk:n+1 is also DFR. The log concavity of the density, function is shown to be preserved by the kth order statistic. It has been established that if the density function of Xk:n is log convex then the density function of Xk:n+1 is also log convex. Because a k-out-of-n system of i.i.d. components each having a life distribution that of X has lifetime Xn-k+1:n the results have applications in the study of such systems.
In this paper, we consider the life distribution H(t) of a device subject to shocks governed by a finite mixture of homogeneous Poisson processes. It will be shown that if (pk), the probabilities that the device fails on the kth shock, has a discrete phase-type (DPH) distribution, then H(t) is continuous phase-type (CPH). The relationship between the mean values of (pk) and H(t) is established.
Well resolved band-edge luminescence of excitons in silicon-germanium alloy strained layers, quantum wells, and superlattices has been observed in films grown by Rapid Thermal Chemical Vapor Deposition. The signal is due to bound excitons at low temperatures and free excitons at higher temperatures, and has a strong no-phonon signal which is caused by alloy scattering. Bandgaps inferred from photoluminescence agree well with those measured by absorption spectroscopy, inferring that a no-phonon process dominates the band-edge absorption.
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