Development of a flashlamp-pumped Cr:LiSAF laser operating at 30 Hz

Cr3+:LiSrAlF6 crystals are an interesting laser medium because of their spectroscopic characteristics: They present a broad emission band in the near infrared and can be pumped either by a flashlamp or by diodes. Up to now, their limitation has been mostly due to their poor thermal properties that limit the laser performance either in the repetition rate in a pulsed system or output power in cw systems. We have designed and constructed a flashlamp-pumped laser using a standard rod pumping cavity that avoids most of the heat generated in the pumping process and allows operation at a fairly high repetition rate of 30 Hz with a high average power of 20 W in a conservative operation mode.     

Roughening transition of grain boundaries in metals and oxides

Extensive theoretical analysis and experimental observations show surface roughening transitions of crystals. The surface roughening is characterized by step free energy, which gradually decreases to 0 at the roughening transition temperature. For a crystal of finite size, the surface roughening transition is manifested by gradual increase of the curved edge and corner areas. In alloys, the interfaces between the solid and the liquid phases can be either singular, partially rough, or completely rough at different temperatures. Their thermally induced roughening transitions are similar to those of the solid-vapor interfaces. The interface roughening and the reverse transition to singular structures can also be induced by additives. The grain boundaries of any misorientation angles in oxides and metals also show roughening transitions. The singular grain boundaries have either flat, hill-and-valley, or kinked shapes, and with temperature increase or composition changes, they become defaceted to curved shapes. These defaceted grain boundaries are rough. It is thus possible to produce either singular or rough grain boundaries by heat-treatment or additives to vary their properties.     

Stress relaxation in bending of AISI 316 at 773 K

In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hours is characterized. Samples were cut either parallel or transverse to the rolling direction and treated at different temperatures prior to the bending tests. The mechanical behavior shown by the longitudinal samples is quite different from that of the transverse samples and so we conclude there must be differences at the level of their microstructures. However, the presence of sigma phase precipitates in both cases is the consequence of a stress-assisted process. Besides, intragranular carbide density in either sample proved to be a function of the relaxation time.     

x-index: Identifying core competency and thematic research strengths of institutions using an NLP and network based ranking framework

Scientometrics - The currently prevailing international ranking systems for institutions are limited in their assessment as they only provide assessments either at an overall level or at very broad...     

The effects of deforming knitted glass fabrics on the basic composite mechanical properties

The effects of deforming knitted fabrics on the tensile and compressive properties of their composites have been investigated for the weft-knit Milano rib fibre architecture. The properties have been studied for both the course and wale directions for composites with fabrics deformed in either of the two directions. It was found that any change in the mechanical properties of the deformed composites with respect to their undeformed counterpart is strongly related to the changes in the knit structure brought about by the induced deformation to the knitted fabric. Deformation in the knitted fabric also affects the tensile fracture mode whereby increased deformation, be it wale- or course-wise, transforms transverse fracture to shear fracture in either loading axis. On the contrary, the compressive fracture mode is insensitive to fabric deformation. Fractographic studies using stereo-optical and scanning electron microscopy have further revealed that tensile failure is caused by fibre breakages occurring at two locations of the knitted loops—one, at the leg components and, two, at fibre crossover points, whilst compression failure is controlled by Euler buckling of the looped fibres of the knitted composite. All these characteristics were revealed to be related to the microstructure of the knitted composite laminates.     

Phase inversion of particle-stabilized materials from foams to dry water

Small particles attached to liquid surfaces arise in many products and processes, including crude-oil emulsions and food foams and in flotation, and there is a revival of interest in studying their behaviour. Colloidal particles of suitable wettability adsorb strongly to liquid-liquid and liquid-vapour interfaces, and can be sole stabilizers of emulsions and foams, respectively. New materials, including colloidosomes, anisotropic particles and porous solids, have been prepared by assembling particles at such interfaces. Phase inversion of particle-stabilized emulsions from oil in water to water in oil can be achieved either by variation of the particle hydrophobicity (transitional) or by variation of the oil/water ratio (catastrophic). Here we describe the phase inversion of particle-stabilized air-water systems, from air-in-water foams to water-in-air powders and vice versa. This inversion can be driven either by a progressive change in silica-particle hydrophobicity at constant air/water ratio or by changing the air/water ratio at fixed particle wettability, and has not been observed in the corresponding systems stabilized by surfactants. The simplicity of the work is that this novel inversion is achieved in a single system. The resultant materials in which either air or water become encapsulated have potential applications in the food, pharmaceutical and cosmetics industries.     

Escape trajectories are deflected when fish larvae intercept their own C-start wake

Fish larvae may intercept their own wake during sharp turns, which might affect their escape performance. We analysed C-starts of larval zebrafish (Danio rerio, Hamilton, 1822) using a computational fluid dynamics approach that simulates free swimming (swimming trajectory is determined by fluid forces) by coupling hydrodynamics and body dynamics. The simulations show that fish may intercept their own wake when they turn by 100–180°. During stage 1 of a C-start, the fish generates a strong jet at the tail that is shed into the wake. During stage 2, the fish intercepts this wake. Counterfactual simulations showed that wake interception increased the lateral force on the fish and reduced the fish''s turning angle by more than 5°. Wake interception caused no significant acceleration tangential to the trajectory of the fish and did not affect total power output. While experimental and simulation evidence suggests that fish larvae can either undershoot or intercept but not overshoot their wake, our simulations show that larger fish might be able to avoid intercepting their wake by either under- or overshooting. As intercepting its own wake modifies the fish''s escape trajectory, fish should account for this effect when planning their escape route.     

Influence of T-stress, cohesive strength and yield strength on the competition between decohesion and plastic flow in a crack edge vicinity

The discussion is based on the cell model of materials. Each cell contains one dominant kernel of micro-separation, for instance a particle. A cell is either in a cohesive or a decohesive state, the latter implying instability at load control. The process region consists of cells which have reached the decohesive state. The cells are characterized by their linear size and their cohesion-decohesion relation. The process region develops either in an elastic or in a plastic environment. In the latter case, it may be more or less deeply embedded in the plastic region. In some cases there will be no process region, only plastic flow. The ratio between the cohesive strength and the yield strength is an important parameter for describing this competition between decohesion and plastic flow, but the T-stress also plays a part. The fracture toughness depends on the area under the cohesion-decohesion curve and on the embedment of the process region.     

Study of Instructional Modes for Introductory Computing

The School of Engineering at Vanderbilt University requires all engineering students in their first semester to take a course that introduces computers in engineering. Two questions arise: is the best setting in which to teach this type of material either a combination of lecture and laboratory or all‐laboratory; and does a student‐owned laptop computer contribute more to learning? For two years the course was organized with these two different modes of instruction. For a third year the students learned in the all‐laboratory environment and 30 percent of the students used their own laptop computer. At the end of the semester an in‐depth questionnaire with quantitative ratings was given to the students to determine if there were differences in their learning preferences. Contingency tables were used to compare responses. There were several statistically significant differences in student responses favoring the all‐laboratory mode and students using laptop computers. Several of them are: the students are much more comfortable with computers at the end of the semester; either laboratory or working by oneself were the preferred settings for learning; and lecture was not a preferred setting for learning any topic.     

Growth and decay in generalized thermoelasticity

A theory of linear thermoelasticity has been proposed by Green and Lindsay [J. Elasticity 2 (1972) 1-7]. Using their theory we treat the thermoelastic problem for a semi-infinite cylinder where the lateral surface of the cylinder is held either at zero temperature and zero displacement, or at zero heat flux and zero traction. Growth and decay bounds for the energy are derived.     

Statistical analysis of excitonic transitions in single,free-standing GaN nanowires: Probing impurity incorporation in the poissonian limit

Single, free-standing GaN nanowires grown by plasma-assisted molecular-beam epitaxy have been investigated with low temperature micro-photoluminescence. The quantitative analysis of the luminescence spectra of around 100 nanowires revealed that each nanowire exhibits its own individual spectrum. A significant fraction of nanowires exclusively emits at energies corresponding to either surface-donor-bound or free excitons, demonstrating that optical properties of individual nanowires are determined by a few impurity atoms alone. The number of impurities per nanowire and their location within the nanowires varies according to Poissonian statistics.     

Biomolecule-compatible support structures for biomolecule coupling to physical measuring principle surfaces

As part of studies on biomolecule-compatible interfacial structures for practice-relevant biosensor and biochip developments, new film-forming aminocelluloses of the 'P-CH2-NH-(X)-NH2' type (P = cellulose) with spacer structures (X = special oligoamine residues) at C6 and solubilising groups (S = tosylate or carbanilate) at C2C3 of the anhydroglucose unit (AGU) were synthesised and their film properties and covalent coupling with enzyme protein examined. Depending on the nature and degree of substitution (DS(S)) of the ester groups (S) at C2C3, the new aminocellulose derivatives are soluble either in DMA and DMSO (with S = carbanilate) or in water (with S = tosylate). The aminocellulose derivatives form transparent films from their solutions. AFM investigations of the film surfaces have either shown very flat (topography <1 nm) films or tubular topographies of nanostructure size, depending on structural and environment-induced factors of influence. Especially in the case of films from water-soluble aminocelluloses with oligoamine residues at C6, inter alia, enzyme-specific pH values and different positive charge distributions can be adjusted by partial protonation of the NH2 end groups. By means of the covalent coupling of the new aminocelluloses with glucose oxidase (GOD) it was shown that the enzyme coupling efficiency can be decisively optimised by the interplay of aminocellulose structure, coupling structure and enzyme protein.     

Effect of environmental factors on solar UV measurements

For a correct understanding and interpretation of solar UV data sampled at ground level, several practical and theoretical problems of measurement are shown. In particular, the comparison between either different sites or instruments is thoroughly analysed, and the effects due to O3 level, albedo and cloudiness are discerned. The impact of clouds and their position with respect to the sun on the observation carried out both by spectral and broad-band instruments are also examined.     

Crack-like processes governing the onset of frictional slip

We perform real-time measurements of the net contact area between two blocks of like material at the onset of frictional slip. We show that the process of interface detachment, which immediately precedes the inception of frictional sliding, is governed by three different types of detachment fronts. These crack-like detachment fronts differ by both their propagation velocities and by the amount of net contact surface reduction caused by their passage. The most rapid fronts propagate at intersonic velocities but generate a negligible reduction in contact area across the interface. Sub-Rayleigh fronts are crack-like modes which propagate at velocities up to the Rayleigh wave speed, V _R, and give rise to an approximate 10% reduction in net contact area. The most efficient contact area reduction (~20%) is precipitated by the passage of ‘slow detachment fronts’. These fronts propagate at ‘anomalously’ slow velocities, which are over an order of magnitude lower than V _R yet orders of magnitude higher than other characteristic velocity scales such as either slip or loading velocities. Slow fronts are generated, in conjunction with intersonic fronts, by the sudden arrest of sub-Rayleigh fronts. No overall sliding of the interface occurs until either of the slower two fronts traverses the entire interface, and motion at the leading edge of the interface is initiated. Slip at the trailing edge of the interface accompanies the motion of both the slow and sub-Rayleigh fronts. We might expect these modes to be important in both fault nucleation and earthquake dynamics.     

International facilities location: A heuristic procedure for the dynamic uncapacitated problem

It is now well known that most major companies no longer operate in a single market. To penetrate global markets and obtain their benefits, companies are under tremendous pressure to reduce the price of their products, and thus their production and material costs. When a foreign location is used, the components of a product are produced there and final assembly takes place either at the foreign location or at the parent domestic plant. This paper first presents the issues related to international facilities location (IFL) problems, and provides the framework required to deal with such problems. It then presents a heuristic algorithm for solving the IFL problem. Extensive computational experience was gained by solving a variety of IFL problems of different sizes.     

General route to vertical ZnO nanowire arrays using textured ZnO seeds

A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented. A approximately 10 nm-thick layer of textured ZnO nanocrystals with their c axes normal to the substrate is formed by the decomposition of zinc acetate at 200-350 degrees C to provide nucleation sites for vertical nanowire growth. The nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered nanorod-polymer solar cells.     

Optical properties of disordered and amorphous metal films

Structural disorder may be introduced into thin films by quench condensation and at liquid He temperatures such films have either an amorphous or a microcrystalline structure. We have studied the optical properties of a variety of such quench-condensed metal films and their annealing properties. The electronic structure of free-electron-like metals is generally strongly influenced by disorder, and in cases where the films are truly amorphous the spectrum is remarkably similar to that of the corresponding liquid. We shall present the results of these measurements and some models which explain new features in the spectra will be discussed.     

The role of C–H⋯O hydrogen-bonding interactions in the solid-state packing of squaraine dyes

The role of C–H⋯O hydrogen-bonding interactions in the solid-state packing networks of squaraine dyes has been investigated by re-examining the nine previously published X-ray crystal structures of these materials. In addition, two new compounds, 2,4-bis(1-hexyl-3,3-dimethyl-2-indolinylidenemethyl)cyclobutenebis(ylium)-1,3-diolate and 2,4-bis(1-(ethan-2-ol)-3,3-dimethyl-2-indolinylidenemethyl)cyclobutenebis(ylium)-1,3-diolate, have had their structures determined and lattice packings discussed with respect to their hydrogen-bonding networks. Analysis of the eleven structures shows, in seven cases, two emergent packing motifs. All structures exhibit molecules in stacked columns, but the two differing patterns arise from specific intermolecular C–H⋯O associations which are directed at the molecules of either an adjacent parallel column or a column of opposing direction. In the four structures which do not exhibit either of these two packing arrangements, hydrogen-bonding associations are still important in the analysis of the overall lattice network.     

测量复合材料应变分布的两种云纹法

对各向异性、非均质的复合材料,由于理论分析上的困难,利用实验应力分析的方法测定其应力应变分布,比均质材料更为重要。云纹法的基本原理是利用两个栅片之间的干涉得出云纹图。其中一块栅片贴到试件上,跟随试体一起变形,这块栅称为试件栅。另一块印制在透明材料上,它不随试件变形,称为分析栅或基准栅。变形后的试件栅和分析栅重叠在一起相干涉,得到的图形叫云纹图。      

The Silicon Valley-Hsinchu Connection: Technical Communities and Industrial Upgrading

Silicon Valley in California and the Hsinchu-Taipei region ofTaiwan are among the most frequently cited ‘miracles’of the information technology era. The dominant accounts ofthese successes treat them in isolation, focusing either onfree markets, multinationals or the role of the state. Thispaper argues that the dynamism of these regional economies isattributable to their increasing interdependencies. A communityof US-educated Taiwanese engineers has coordinated a decentralizedprocess of reciprocal industrial upgrading by transferring capital,skill and know-how to Taiwan, and by facilitating collaborationsbetween specialist producers in the two regions. This case underscoresthe significance of technical communities and their institutionsin transferring technology and organizing production at theglobal as well as the local level.