Micromechanical modelling of cemented sands under low amplitude oscillations

This is the published version, made available with the permission of the publisher.Microstructural continuum modelling is applied to study the mechanical properties of cemented sands under low amplitude oscillatory loading. The effect of cement in sand is accounted for by considering adhesion in addition to friction at the inter-particle contact. Under the assumption that cemented sands are statistically isotropic random packings of equal spheres, a closed form relationship is derived for the initial shear modulus of cemented sands under isotropic confining stress. Also, the secant moduli and damping ratios are computed numerically for various strain amplitudes. The influence of degree of cementation, initial confining stress, particle size, void ratio and strain amplitude on the mechanical properties of cemented sands are investigated. Comparisons of the theoretically predicted results are made with experimental measurements

Current status of hepatocellular carcinoma detection: screening strategies and novel biomarkers.

Hepatocellular carcinoma (HCC) remains a major cause of cancer-related mortality worldwide. Delayed diagnosis is a major factor responsible for the poor prognosis of HCC. Several advances have been made in the field of liver imaging with the use of novel imaging contrasts, improving current imaging techniques with contrast-enhanced computed tomography (CT) and magnetic resonance imaging (MRI), introduction of new technologies such as contrast liver ultrasound, and development of novel biomarkers with the goal of early detection of HCC and improving outcomes of patients with HCC. This review focuses on current surveillance strategies and development of biomarkers with the goal of early detection of HCC

The Influence of Transient Thermal Gradients and Substrate Constraint on Delamination of Thermal Barrier Coatings

A systematic study of factors affecting the delamination energy release rate and mode mix of a thermal barrier coating attached to a substrate is presented accounting for the influence of thermal gradients combined with rapid hot surface cooling. Transient thermal gradients induce stress gradients through the coating and substrate, which produce overall bending if the substrate is not very thick and if it is not constrained. Due to their influences on the coating stresses, substrate thickness and constraint are important aspects of the mechanics of delamination of coating-substrate systems, which must be considered when laboratory tests are designed and for lifetime assessment under in-service conditions. Temperature gradients in the hot state combined with rapid cooling give rise to a maximum energy release rate for delamination that occurs in the early stage of cooling and that can be considerably larger than the driving force for delamination in the cold state. The rates of cooling that give rise to a large early stage energy release rate are identified.Engineering and Applied Science

Quantized Orbits and Resonant Transport

A tight binding representation of the kicked Harper model is used to obtain an integrable semiclassical Hamiltonian consisting of degenerate "quantized" orbits. New orbits appear when renormalized Harper parameters cross integer multiples of $\pi/2$. Commensurability relations between the orbit frequencies are shown to correlate with the emergence of accelerator modes in the classical phase space of the original kicked problem. The signature of this resonant transport is seen in both classical and quantum behavior. An important feature of our analysis is the emergence of a natural scaling relating classical and quantum couplings which is necessary for establishing correspondence.Comment: REVTEX document - 8 pages + 3 postscript figures. Submitted to Phys.Rev.Let

Advanced photovoltaic power systems using tandem GaAs/GaSb concentrator modules

In 1989, Boeing announced the fabrication of a tandem gallium concentrator solar cell with an energy conversion efficiency of 30 percent. This research breakthrough has now led to panels which are significantly smaller, lighter, more radiation resistant, and potentially less expensive than the traditional silicon flat plate electric power supply. The new Boeing tandem concentrator (BTC) module uses an array of lightweight silicone Fresnel lenses mounted on the front side of a light weight aluminum honeycomb structure to focus sunlight onto small area solar cells mounted on a thin back plane. This module design is shown schematically. The tandem solar cell in this new module consists of a gallium arsenide light sensitive cell with a 24 percent energy conversion efficiency stacked on top of a gallium antimonide infrared sensitive cell with a conversion efficiency of 6 percent. This gives a total efficiency 30 percent for the cell-stack. The lens optical efficiency is typically 85 percent. Discounting for efficiency losses associated with lens packing, cell wiring, and cell operating temperature still allows for a module efficiency of 22 percent which leads to a module power density of 300 Watts/sq. m. This performance provides more than twice the power density available from a single crystal silicon flat plate module and at least four times the power density available from amorphous silicon modules. The fact that the lenses are only 0.010 ft. thick and the aluminum foil back plane is only 0.003 ft. thick leads to a very lightweight module. Although the cells are an easy to handle thickness of 0.020 ft., the fact that they are small, occupying one-twenty-fifth of the module area, means that they add little to the module weight. After summing all the module weights and given the high module power, we find that we are able to fabricate BTC modules with specific power of 100 watts/kg

On the origin of the 1/f noise in shallow germanium p(+)-n junctions

The low-frequency noise of shallow germanium p(+)-n junctions is studied, for diodes with or without a nickel-germanide Ohmic contact. It is shown that the application of NiGe not only reduces the series resistance, resulting in a higher forward current, but also results in a lower 1/f noise at forward bias. From the observed geometry dependence, it is concluded that germanidation suppresses the 1/f noise generated in the series resistance, leaving surface-state-assisted generation-recombination at the junction perimeter as the dominant flicker noise source

Current trends in the development and applications of superconducting materials

The discovery of the phenomenon of superconductivity by Kamerlingh On nes in 1911 was the first indication of the possibility of electrical conduction without any associated Joule loss. The technological application of the property (which was essentially manifested at liquid helium temperatures) had to await the development of stable superconducting materials capable of withstanding high currents and large magnetic fields. Although many materials - elements, alloys, ternary chalcogenides, and recently oxides - have been found to be superconducting, only a few of them have received attention for significant applications. This is based on three important parameters namely Tc, the transition temperature, Hc2, the upper critical field and Jc, the critical current density. Tc and Hc2 are considered intrinsic to the material, while Jc is influenced by the microstructure, and has to be optimised during fabrication of the material in the useful form. On these considerations, Nb-Ti, Nb3Sn and V3Ga have emerged as proven materials for significant applications while PbMo6S8 is still under development. Despite the fact that all these materials have to be used only at liquid helium temperatures on account of their low Tc , major developments have taken place in harnessing particularly the niobium alloys to produce superconducting magnets. Towards the end of 1986, a break-through has been achieved in the direction of raising the Tc. Many ceramic oxides, notably Y1Ba2Cu3O7, have exhibited Tc in the vicinity of 100 K. These materials have also been shown to have high Hc2, about 180 Tesla. Attempts are now being made to realise a high Jc. It is too early to say whether such materials can be fabricated in suitable forms capable of carrying high currents. Among the major areas in which superconducting materials have so far been used, mention should be made of superconducting magnets for high energy particle accelerators, magnetohydrodynamic power generation, magnetic resonance imaging, and fusion research programmes. In other potential applications such as motors and magnetically levitated transportation, economic break-even has not been achieved, mostly on account of the need to use liquid helium. The discovery of the high temperature superconductors capable of operating at liquid nitrogen temperatures thus promises a revolution in electrical technology. The paper reviews the development and applications of superconducting materials, with reference to work being done in India