Density Range (density + range)

Distribution by Scientific Domains


Selected Abstracts


Heat Transfer in Polypropylene-Based Foams Produced Using Different Foaming Processes,

ADVANCED ENGINEERING MATERIALS, Issue 10 2009
Marcelo Antunes
This paper presents the characterization of the cellular structure and thermal conduction behaviour of polypropylene foams produced using different foaming processes, with the aim of selecting the best possible PP foam thermal insulator. Thermal conductivity results have shown that the global heat transfer behaviour is controlled by the relative density. For relative densities higher than 0.2, thermal conductivity differences were insignificant, the data being predicted by the mixture's rule and Russell's model. In the low density range, all of the proposed models underestimated the overall conductivity, the effect of the processing method being more significant, slight differences being observed between foams produced by extrusion and those produced by gas dissolution with higher cell sizes and anisotropies. Foams with finer cellular structures showed to be better insulating materials. [source]


Bacterivorous grazers facilitate organic matter decomposition: a stoichiometric modeling approach

FEMS MICROBIOLOGY ECOLOGY, Issue 2 2009
Hao Wang
Abstract There is widespread empirical evidence that protist grazing on bacteria reduces bacterial abundances but increases bacteria-mediated decomposition of organic matter. This paradox has been noted repeatedly in the microbiology literature but lacks a generally accepted mechanistic explanation. To explain this paradox quantitatively, we develop a bacteria-grazer model of organic matter decomposition that incorporates protozoa-driven nutrient recycling and stoichiometry. Unlike previous efforts, the current model includes explicit limitation, via Liebig's law of minimum, by two possible factors, nutrient and carbon densities, as well as their relative ratios in bacteria and grazers. Our model shows two principal results: (1) when the environment is carbon limiting, organic matter can always be decomposed completely, regardless of the presence/absence of grazers; (2) when the environment is nutrient (such as nitrogen) limiting, it is possible for organic matter to be completely decomposed in the presence, but not absence, of grazers. Grazers facilitate decomposition by releasing nutrients back into the environment, which would otherwise be limiting, while preying upon bacteria. Model analysis reveals that facilitation of organic matter decomposition by grazers is positively related to the stoichiometric difference between bacteria and grazers. In addition, we predict the existence of an optimal density range of introduced grazers, which maximally facilitate the decomposition of organic matter in a fixed time period. This optimal range reflects a trade-off between grazer-induced nutrient recycling and grazer-induced mortality of bacteria. [source]


Exergetic performance analysis of a PEM fuel cell

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 5 2006
M. Ay
Abstract In this paper we investigate the effects of thermodynamic irreversibilities on the exergetic performance of proton exchange membrane (PEM) fuel cells as a function of cell operating temperature, pressures of anode and cathode, current density, and membrane thickness. The practical operating conditions are selected to be 3,5 atm for anode and cathode pressures, and 323,353 K for the cell temperatures, respectively. In addition, the membrane thicknesses are chosen as 0.016, 0.018 and 0.02 cm, respectively. Moreover, the current density range of the PEM fuel cell is selected to be 0.01,2.0 A cm,2. It is concluded that exergy efficiency of PEM fuel cell decreases with a rise in membrane thickness and current density, and increases with a rise of cell operating pressure and with a decrease of current density for the same membrane thickness. Thus, it can be said that, in order to increase the exergetic performance of PEM fuel cell, the lower membrane thickness, the lower current density and the higher cell operating pressure should be selected in case PEM fuel cell is operated at constant cell temperature. Copyright © 2005 John Wiley & Sons, Ltd. [source]


Quantum transport in high mobility AlGaN/GaN 2DEGs and nanostructures

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2006
S. Schmult
Abstract High mobility two-dimensional electron systems in GaN/AlGaN heterostructures have been realized by plasma assisted molecular beam epitaxy on GaN templates. In the density range of 1011 cm,2 to 1012 cm,2, mobility values exceeding 160000 cm2/Vs have been achieved. Scattering mechanisms that presently limit the production of higher mobility samples are discussed. We present results of a systematic study of the weak localization and antilocalization corrections to the classical conductivity at very low magnetic fields. The unambiguous observation of a conductivity maximum at B = 0 suggests that spin,orbit scattering is not negligible in GaN heterostructures as one might expect for a wide-bandgap system. We have recently realized electron transport through GaN nanostructures. We report on the transport properties of the first quantum point contacts (QPCs) in GaN. These devices are used to study one-dimensional transport in the Nitride system. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Variation of the Fermi level in n-type microcrystalline silicon by electron bombardment and successive annealing: ESR and conductivity studies

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010
Oleksandr Astakhov
Abstract ESR and conductivity studies have been preformed on ,c-Si:H exposed to 2 MeV electron bombardment and successive annealing in order to investigate the influence of the defect density on the electronic properties of n-type ,c-Si:H. With this approach one can vary the defect density in one and the same sample and directly deduce its influence on the electronic properties. The defect density is varied by 2 orders of magnitude with strong influence on the dark conductivity and electron spin resonance (ESR) properties. The relationship of ESR and conductivity data obtained over the whole defect density range is in agreement with the data obtained on the sets of samples deposited with different doping level. The results indicate that the Fermi level position in ,c-Si:H is defined by a balance of defect and donor states densities regardless of which of these quantities is varied. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Effect of Gas Density on the Hydrodynamics of Bubble Columns and Three-Phase Fluidized Beds

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003
Arturo Macchi
Abstract Experiments were performed at ambient temperature and pressure in a 127 mm inner diameter column with a 55% wt. aqueous glycerol solution, 6-mm spherical borosilicate beads and four gases , helium, air, carbon dioxide and sulphur hexafluoride , giving a 35-fold gas density range. The dispersed bubble flow regime was sustained to higher gas velocities and gas holdups for denser gases. This finding appears to be due to the reduction of the maximum stable bubble size (i.e. enhanced bubble break-up), rather than to formation of smaller bubbles at the distributor with increasing gas density. The effect of gas density was significant both with and without the particles present, with gas holdup increasing, bed voidage increasing and liquid holdup decreasing with increasing gas density. The holdup correlations of Han et al. (1990) have been modified to incorporate the effect of gas density. On a mené des expériences à la température et à la pression ambiante dans une colonne de 127 mm de diamètre intérieur avec une solution de glycérol aqueux de 55 % en poids, des billes de verre de borosilicate sphériques de 6 mm et quatre gaz , hélium, air, gaz carbonique et hexafluorure de soufre , donnant une gamme de densité gazeuse multipliée par 35. Le régime d'écoulement bouillonnant dispersé est maintenu jusqu'à des vitesses et des rétentions de gaz supérieures pour les gaz plus denses. Ce résultat semble être dû à la réduction de la taille de bulle stable maximum (c.-à-d. une rupture de bulles améliorée), plutôt qu'à la formation de bulles plus petites au distributeur avec l'augmentation de la densité gazeuse. L'effet de la densité gazeuse est significatif avec ou sans la présence des particules, avec l'augmentation de la rétention de gaz, l'augmentation du vide du lit et la diminution de la rétention de liquide avec l'augmentation de la densité gazeuse. Les corrélations de rétention de Han et al. (1990) sont modifiées de sorte à introduire l'effet de la densité gazeuse. [source]


The influence of algal ration and larval density on growth and survival of blacklip pearl oyster Pinctada margaritifera (L.) larvae

AQUACULTURE RESEARCH, Issue 8-9 2000
M S Doroudi
This paper reports on two factorial design experiments conducted to examine the combined effects of algal ration and larval density on growth and survival of black lip pearl oyster Pinctada margaritifera (L.) larvae. In the first experiment, 1-day-old larvae were cultured for 7 days at densities of 1, 2 and 5 mL,1 and fed algal rations of 0, 2.5, 5 or 12.5 × 103 cells mL,1. In the second experiment, 13-day-old larvae were cultured for 7 days at the above densities and fed algal rations of 0, 7.5, 15 and 37.5 × 103 cells mL,1. Algal rations in both experiments were composed of a 1:1 mixture of the flagellates Isochrysis aff. galbana clone T.ISO and Pavlova salina. Response surface contour diagrams were generated from growth and survival data to estimate optimal culture conditions. Results showed that, within a suitable larval density range, growth of P. margaritifera larvae was significantly (P <,0.01) influenced by algal ration. At the end of the first experiment, greatest survival occurred at a ration of 4.5,11.5 × 103 cells mL,1 and at a larval density of ,,3 mL,1. Greatest antero-posterior shell length was shown by larvae fed between 4.5 × 103 and 11.5 × 103 algal cells mL,1. In the second experiment, greatest antero-posterior shell length occurred within an algal ration range of 15,32 × 103 cells mL,1. The greatest survival occurred below an algal ration of <,2.5 × 103 cells mL,1; however, this algal ration supported poor algal growth. The results indicate that, to maximize growth and survival, P. margaritifera larvae should be fed at an algal ration of ,,8 × 103 cells mL,1 and cultured at a density of three larvae mL,1 up to 8 days of age. Older (13- to 20-day-old) larvae should be cultured at a density of <,2 mL,1 and fed an algal ration of ,,25 × 103 cells mL,1. [source]