			Home About us Contact

Uniform Density (uniform + density)

Distribution by Scientific Domains

Physics and Astronomy	50%

Selected Abstracts

Effect of Interparticle Potential on Forming Solid, Spherical Agglomerates during Drying

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2004
Geoff E. Fair
The effect of the interparticle potential on the shapes of the agglomerates obtained by drying slurry droplets has been investigated using aqueous alumina slurries formulated in the dispersed and weakly attractive (dispersed + added salt) states. For the dispersed slurry, the droplets dried to irregular shapes with hollow centers. When just the right amount of salt was added to produce an attractive, but nontouching, particle network, the droplets dried as solid spheres. These results are discussed by relating both the nature of the particle network (repulsive or weakly attractive), the slurry rheology, and the consolidation mechanics of the networks to the requirements for maintaining a spherical geometry and uniform density during droplet drying. [source]

Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild-2

METEORITICS & PLANETARY SCIENCE, Issue 2 2006
A. T. Kearsley
The ability of soft aluminum alloy to record hypervelocity impacts as bowl-shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild-2, with a known and constant spacecraft-particle relative velocity and effective surface-perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well-documented particle size range from 10 ,m to nearly 100 ,m. Light gas gun buckshot firings of these particles at approximately 6 km s,1 onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild-2, independent of the active impact detector instruments aboard the Stardust spacecraft. [source]

GaN based light emitters fabricated on bulk GaN substrates.

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2004
New class of low dislocation density devices
Abstract In this work we demonstrate the quality of laser diodes fabricated on bulk GaN substrates. These substrates were obtained via high-temperature, high-pressure synthesis and are characterized by a dislocation density as low as 102 cm,2. The epitaxial structure was deposited using a combination of MOVPE and MBE methods. Thanks to such a low and uniform density of dislocations it was possible to realize high power, pulsed current laser diodes with a total output power exceeding 2.5 W. Interestingly enough, the MBE growth method can produce very satisfying results once the growth is resumed on bulk GaN substrates. This contradicts the common believe concerning the poor applicability of the MBE method for the growth of nitride violet laser structures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

On testing for multivariate ARCH effects in vector time series models

THE CANADIAN JOURNAL OF STATISTICS, Issue 3 2003
Pierre Duchesne
Abstract Using a spectral approach, the authors propose tests to detect multivariate ARCH effects in the residuals from a multivariate regression model. The tests are based on a comparison, via a quadratic norm, between the uniform density and a kernel-based spectral density estimator of the squared residuals and cross products of residuals. The proposed tests are consistent under an arbitrary fixed alternative. The authors present a new application of the test due to Hosking (1980) which is seen to be a special case of their approach involving the truncated uniform kernel. However, they typically obtain more powerful procedures when using a different weighting. The authors consider especially the procedure of Robinson (1991) for choosing the smoothing parameter of the spectral density estimator. They also introduce a generalized version of the test for ARCH effects due to Ling & Li (1997). They investigate the finite-sample performance of their tests and compare them to existing tests including those of Ling & Li (1997) and the residual-based diagnostics of Tse (2002).Finally, they present a financial application. Adoptant une approche spectrale, les auteurs proposent des tests permettant de détecter des effets ARCH multivariés dans les résidus d'un modèle de régression multivarié. Leurs tests reposent sur une comparaison en norme quadratique de la densité spectrale uniforme et d'un estimateur à noyau de la densité spectrale des résidus carrés et des produits croisés des résidus. Ces tests sont convergents sous une contre-hypothèse fixe quelconque. Les auteurs présentent une nouvelle application du test de Hosking (1980) qui correspond dans leur approche au choix particulier d'un noyau uniforme tronqué. Cependant, l'emploi d'autres pondérations leur permet d'obtenir des test encore plus puissants. Les auteurs étudient notamment la procédure de Robinson (1991) pour le choix du paramètre de lissage de l'estimateur de la densité spectrale. Os proposent aussi une version généralisée du test pour effets ARCH de Ling & Li (1997). Ils examinent le comportement de leurs tests dans de petits échantillons par voie de simulation et les comparent aux tests de Ling & Li (1997) et aux diagnostiques de Tse (2002) fondés sur les résidus, us présentent en outre une application financière. [source]