152 research outputs found
Influence of substrate off-cut on the defect structure in relaxed graded Si-Ge/Si layers
No full textRelaxed graded Si-Ge/Si layers can be used in a variety of micro-electronics applications such as templates for III-V/Si integration, in high speed field effect transistor (FET) structures and as detectors in optical communication Each of these applications requires a different final Ge concentration in the graded Si-Ge layer. With increasing Ge content in the graded layer, some of the materials concerns that need to be addressed are- (i) a high surface roughness, (ii) the formation of dislocation pile-ups, and (iii) an increase in the threading dislocation density. We have shown that there is a substantial improvement in the surface roughness and the dislocation pile-up density of the graded Si-Ge layers by depositing on (001) 6 degrees off-cut substrates. The substrate miscut also facilitates favorable intersections of {111} planes that aid reactions between the 60 degrees dislocations to form edge dislocations with Burgers vectors of the type 1/2 and resulting in a novel hexagonal dislocation structure. Such reactions occurred more readily in the Ge-rich regions of the graded layers where the growth temperature was high enough to aid dislocation climb. The edge dislocations with in-plane Burgers vectors lack a tilt component and the decreased rate of tilting in the Ge-rich regions is confirmed by triple crystal X-ray reciprocal space maps. This novel dislocation structure offers opportunities to explore new processes which may eliminate spatially variant strain fields in relaxed epitaxial layers
Modelling and optimisation of rapid prototyping
No full textThis paper proposes a Virtual Reality (VR) system for modelling and optimisation of Rapid Prototyping (RP) processes. The system aims to reduce the manufacturing risks of prototypes early in a product development cycle, and hence, reduces the number of costly design-build-test cycles. It involves modelling and simulation of RP in a virtual system, which facilitates visualisation and testing the effects of process parameters on the part quality. Modelling of RP is based on quantifying the measures of part quality, which includes accuracy, build-time and efficiency with orientation, layer thickness and hatch distance. A mathematical model has been developed to estimate the build-time of the Selective Laser Sintering (SLS) process. The model incorporates various process parameters like layer thickness, hatch space, bed temperatures, laser power and sinter factor, etc. It has been integrated with the virtual simulation system to provide a test-bed to optimise the process parameters. © 2002 Elsevier Science B.V. All rights reserved.link_to_subscribed_fulltex
Visualisation of rapid prototyping
No full textA virtual prototyping (VP) system developed for simulation of rapid prototyping processes is presented. The visualization capability of the system is demonstrated by comparing virtual parts with the corresponding physical prototypes.link_to_subscribed_fulltex
Al-Si and Al-Si-Mg Cast Alloys Shrinkage Porosity Estimation
No full textUS A356 and US 413 cast aluminium alloys shrinkage characteristic have been discussed in the present study. Specific volume reduction leads to shrinkage in castings and it can be envisaged as a casting defect. Finite difference based casting process simulation software has been used to study the shrinkage characteristic and it is quantified using mathematical formulae. The three dimensional model of the shrinkage defect has been constructed using CAD application software. Shrinkage characteristic has also been quantified through experimental validation studies and compared well with casting process simulation. Shrinkage characteristic study and control is essential for producing defect free castings. Influence of casting shape on the shrinkage characteristic has been studied in this paper
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An Interactive Virtual System for Simulation and Optimization of Rapid Prototyping
No full textThe paper describes the development of a computer system for simulation and optimization of
rapid prototyping (RP) processes. The system provides a test-bed for virtual prototyping by
integrating product design and RP with simulation and realistic visualization techniques. It
enhances the dimensional accuracy and reduces the build-time of product prototypes. The
virtually fabricated parts may be exported in VRML format over the Internet for effective
communication between the manufacturer and the customer. The designer may use the system
to design-build-break as many parts as required at a relatively low cost andin a short period
of time. Therefore, virtual simulation of RP processes facilitates tuning of the control
parameters according to the requirements, and hence reduces the number of physical
prototypes needed to produce a part.Mechanical Engineerin
Measuring the effect of enhanced cleaning in a UK hospital : a prospective cross-over study
No full textIncreasing hospital-acquired infections have generated much attention over the last decade. There is evidence that hygienic cleaning has a role in the control of hospital-acquired infections. This study aimed to evaluate the potential impact of one additional cleaner by using microbiological standards based on aerobic colony counts and the presence of Staphylococcus aureus including meticillin-resistant S. aureus. We introduced an additional cleaner into two matched wards from Monday to Friday, with each ward receiving enhanced cleaning for six months in a cross-over design. Ten hand-touch sites on both wards were screened weekly using standardised methods and patients were monitored for meticillin-resistant S. aureus infection throughout the year-long study. Patient and environmental meticillin-resistant S. aureus isolates were characterised using molecular methods in order to investigate temporal and clonal relationships. Enhanced cleaning was associated with a 32.5% reduction in levels of microbial contamination at handtouch sites when wards received enhanced cleaning (P < 0.0001: 95% CI 20.2%, 42.9%). Near-patient sites (lockers, overbed tables and beds) were more frequently contaminated with meticillin-resistant S. aureus/S. aureus than sites further from the patient (P = 0.065). Genotyping identified indistinguishable strains from both handtouch sites and patients. There was a 26.6% reduction in new meticillin-resistant S. aureus infections on the wards receiving extra cleaning, despite higher meticillin-resistant S. aureus patient-days and bed occupancy rates during enhanced cleaning periods (P = 0.032: 95% CI 7.7%, 92.3%). Adjusting for meticillin-resistant S. aureus patient-days and based upon nine new meticillin-resistant S. aureus infections seen during routine cleaning, we expected 13 new infections during enhanced cleaning periods rather than the four that actually occurred. Clusters of new meticillin-resistant S. aureus infections were identified 2 to 4 weeks after the cleaner left both wards. Enhanced cleaning saved the hospital £30,000 to £70,000.Introducing one extra cleaner produced a measurable effect on the clinical environment, with apparent benefit to patients regarding meticillin-resistant S. aureus infection. Molecular epidemiological methods supported the possibility that patients acquired meticillin-resistant S. aureus from environmental sources. These findings suggest that additional research is warranted to further clarify the environmental, clinical and economic impact of enhanced hygienic cleaning as a component in the control of hospital-acquired infection
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