Effect of Processing Parameters on the Density and Microstructure of Direct Laser Sintered Al-12Si Powders

The effect of processing parameters on the sintering behaviour of gas atomised Al-12Si powders has been investigated. Laser power, scanning rate, scan spacing and layer thickness are found to control the densification and the resultant microstructural characteristics of the laser sintered parts. It was found that sintered density increased as the energy density increased reaching a maximum of 80.2% at an energy input per unit volume of 67 J mm-3. For parts produced with a slightly lower power density (50 J mm-3), the microstructure consisted of fine dendrites with interconnected porosity while parts fabricated with a slightly higher power density (100 J mm-3) were noted to have a preponderance of coarse dendrites with a discontinuous network of irregular shaped pores surrounded by a fully dense aluminium-silicon matrix.Mechanical Engineerin

Selective Laser Sintering of Polymer Nanocomposites

This paper describes the fabrication and characterization of polymer nanocomposite (PNC) materials for use in the selective laser sintering (SLS) process. PNC materials are of great interest generally because of their excellent physical properties, and offer excellent potential in rapid manufacturing of structural polymeric parts. Three different nano additive materials have been used: cerium oxide IV, yttrium stabilized zirconia, and layered Hectorite clay. These materials have been used to reinforce PA6 polymer using solution blending and spray drying to create powder with particle sizes in the range of 5-40 µm. The mechanical properties and microstructure of the PNC materials have been evaluated and the results compared to those of unfilled polymer.Mechanical Engineerin

The Effect of Model Formulation on the Comparative Performance of Artificial Neural Networks and Regression

Multiple linear regression techniques have been traditionally used to construct predictive statistical models, relating one or more independent variables (inputs) to a dependent variable (output). Artificial neural networks can also be constructed and trained to learn these complex relationships, and have been shown to perform at least as well as linear regression on the same data sets. Research on the use of neural network models as alternatives to multivariate linear regression has focused predominantly on the effects of sample size, noise, and input vector size on the comparative performance of these two modeling techniques. However, research has also shown that a mis-specified regression model or an incorrect neural network architecture also contributes significantly to poor model performance. This dissertation compares the effects on model performance of various formulations of regression and neural network models, measuring performance in terms of mean squared error and variance. A factorial experiment is conducted in which model parameters are varied. Simulated data from three different functions are used to generate training and testing data sets. Statistical tests are used to determine differences in performance as well as the degree of model robustness, or the degree to which model performance is insensitive to changes in model formulation. Based on the experimental results and conclusions, a predictive modeling methodology is proposed that capitalizes on the advantages of both neural network and regression approaches and assists practitioners in constructing accurate and robust predictive models

Uptake of systematic reviews and meta-analyses based on individual participant data in clinical practice guidelines: descriptive study.

To establish the extent to which systematic reviews and meta-analyses of individual participant data (IPD) are being used to inform the recommendations included in published clinical guidelines

Influence of school community and fitness on prevalence of overweight in Australian school children

AbstractThe study objectives were (1) to determine the variation in prevalence of overweight between school communities, (2) to evaluate the relationship between cardiorespiratory fitness and the probability of being overweight among different school communities, and (3) to test whether this relationship varies between school communities. Using a repeated cross-sectional design, data from 31,424 (15,298 girls, 16,126 boys) Australian school children who had objective assessments of body composition and physical performance were used. Ninety-one schools located across 5 states and territories were included. Independent samples were taken across 12 school years (2000–2011). Analysis used generalised linear mixed models in R with a two-level hierarchical structure—children, nested within school communities. Predictor variables considered were: level 1—gender, age, cardiorespiratory fitness and year of measurement; level 2—school community. A total of 24.6% of the children were overweight and 69% were of low fitness. Variation in the prevalence of overweight between school communities was significant, ranging from 19% to 34%. The probability of being overweight was negatively associated with increasing cardiorespiratory fitness. The relationship was steepest at low fitness and varied markedly between school communities. Children of low fitness had probabilities of being overweight ranging between 26% and 75% depending on school community, whereas those of high fitness had probabilities of <2%. Our findings suggest that most might be gained from a public health perspective by focusing intervention on the least fit children in the worst-performing communities

The international risk-sharing puzzle is at business-cycle and lower frequency

We decompose the correlation between relative consumption and the real exchange rate into its dynamic components at different frequencies. Using multivariate spectral analysis techniques we show that, at odds with a high degree of risk-sharing, in most OECD countries the dynamic correlation tends to be quite negative, and signifi cantly so, at frequencies lower than two years —the appropriate frequencies for assessing the performance of international business cycle models. Theoretically, we show that the dynamic correlation over different frequencies predicted by standard open-economy models is the sum of two terms: a term constant across frequencies, which can be negative when uninsurable risk is largeand a term variable across frequencies, which in bond economies is necessarily positive, refl ecting the insurance that intertemporal trade provides against forecastable contingencies. Numerical analysis suggests that leading mechanisms proposed by the literature to account for the puzzle are consistent with the evidence across the spectrumDescomponemos la correlación entre el consumo relativo y el tipo de cambio real en sus componentes dinámicos a diferentes frecuencias. Utilizando técnicas de análisis espectral multivariado mostramos que, en contradicción con un alto grado de diversifi cación del riesgo, en la mayoría de los países de la OCDE la correlación dinámica tiende a ser bastante negativa, y signifi cativamente negativa a frecuencias inferiores a dos años —las frecuencias apropiadas para evaluar el desempeño de los modelos internacionales del ciclo económico—. En teoría mostramos que la correlación dinámica a diferentes frecuencias predicha por modelos estándar de economía abierta, es la suma de dos términos: un término constante en cada frecuencia, que puede ser negativo cuando el riesgo no asegurable es grandey un término que varia con la frecuencia, que en economías con bonos es necesariamente positivo y que refl eja la cobertura de riesgo contra contingencias predecibles proporcionada por el comercio intertemporal. El análisis numérico sugiere que los mecanismos principales propuestos por la literatura para dar cuenta de la anomalía, son consistentes con la evidencia empírica a diferentes frecuencias del espectr

Disappearing galaxies: the orientation dependence of JWST-bright, HST-dark, star-forming galaxy selection

Galaxies that are invisible in deep optical-NIR imaging but detected at longer wavelengths have been the focus of several recent observational studies, with speculation that they could constitute a substantial missing population and even dominate the cosmic star formation rate density at $z\gtrsim4$. The depths now achievable with JWST at the longest wavelengths probed by HST, coupled with the transformative resolution at longer wavelengths, are already enabling detailed, spatially-resolved characterisation of sources that were invisible to HST, often known as `HST-dark' galaxies. However, until now, there has been little theoretical work to compare against. We present the first simulation-based study of this population, using highly-resolved galaxies from the Feedback in Realistic Environments (FIRE) project, with multi-wavelength images along several lines of sight forward-modelled using radiative transfer. We naturally recover a population of modelled sources that meet commonly-used selection criteria ($H_{\rm{AB}}>27\,\rm{mag}$ and $H_{\rm{AB}}-\rm{F444W}>2.3$). These simulated HST-dark galaxies lie at high redshifts ($z=4-7$), have high levels of dust attenuation ($A_{V}=2-4$), and display compact recent star formation ($R_{1/2,\,\rm{4.4\,\mu\rm{m}}}\lesssim1\,\rm{kpc}$). Orientation is very important: for all but one of the 17 simulated galaxy snapshots with HST-dark sightlines, there exist other sightlines that do not meet the criteria. This result has important implications for comparisons between observations and models that do not resolve the detailed star-dust geometry, such as semi-analytic models or coarsely-resolved hydrodynamical simulations. Critically, we demonstrate that HST-dark sources are not an unexpected or exotic population, but a subset of high-redshift, highly-dust-attenuated sources viewed along certain lines of sight.Comment: 12 pages, 8 figures. Accepted for publication in Ap

Electrically driven photon emission from individual atomic defects in monolayer WS2.

Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources

Data Sharing: How Much Doesn't Get Submitted to GenBank?

Funding agencies and journals require researchers to deposit DNA sequences in public databases such as GenBank when the paper is published, but how often do authors fail to do so