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Diffusion Resistance (diffusion + resistance)
Selected AbstractsAmmonia Gas Sensor Using Polypyrrole-Coated TiO2/ZnO NanofibersELECTROANALYSIS, Issue 12 2009Ying Wang Abstract Highly porous polypyrrole (PPy)-coated TiO2/ZnO nanofibrous mat has been successfully synthesized. The core TiO2/ZnO nanofibers have an average diameter of ca. 100,nm and the shell of ultrathin PPy layer has a thickness of ca. 7,nm. The NH3 gas sensor using the as-prepared material exhibited a fast response over a wide dynamic range and high sensitivity with a detection limit of 60,ppb (S/N=3). Compared to conventional pristine PPy film, the improved performance in NH3 detection can be attributed to the free access of NH3 to PPy and a minimized gas diffusion resistance through the ultrathin PPy layer. [source] Palladium and platinum sorption on a thiocarbamoyl-derivative of chitosanJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2010A. Butewicz Abstract Immobilizing thiourea onto chitosan allowed using the polymer for the recovery of platinum groups metals (PGMs) in acidic solutions (up to 1,2M HCl concentrations). At low HCl concentration protonated amine groups may sorb chloroanionic metal species (electrostatic attraction mechanism); however, most of sorption proceeds through chelation on sulfur containing groups (less sensitive to acidic conditions). The bi-site Langmuir equation was used for fitting sorption isotherms. The sorption of PGMs was weakly affected by the composition of the solution (presence of high concentration of anions and base metals). Maximum sorption capacities for Pd(II) and Pt(IV) ranged between 274 and 330 mg g,1 in 0.25M HCl solutions and decreased to 150,198 mg g,1 in 2M HCl solutions: Pd(II) sorption was systematically higher than Pt(IV) sorption. The pseudo-second rate equation was used for modeling the uptake kinetics. Agitation speed hardly affected uptake kinetics indicating that external diffusion resistance is not the rate controlling step. Desorption yield higher than 85% were obtained using thiourea in 0.1M HCl solution. The adsorbents could be reused for at least three cycles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Dimethylsilylbis(1-indenyl) zirconium dichloride/methylaluminoxane catalyst supported on nanosized silica for propylene polymerizationJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2008Kuo-Tseng Li Abstract A dimethylsilylene-bridged metallocene complex, (CH3)2Si(Ind)2ZrCl2, was supported on a nanosized silica particle, whose surface area was mostly external. The resulting catalyst was used to catalyze the polymerization of propylene to polypropylene. Under identical reaction conditions, a nanosized catalyst exhibited much better polymerization activity than a microsized catalyst. At the optimum polymerization temperature of 55°C, the former had 80% higher activity than the latter. In addition, the nanosized catalyst produced a polymer with a greater molecular weight, a narrower molecular weight distribution, and a higher melting point in comparison with the microsized catalyst. The nanosized catalyst's superiority was ascribed to the higher monomer concentration at its external active sites (which were free from internal diffusion resistance) and was also attributed to its much larger surface area. Electron microscopy results showed that the nanosized catalyst produced polymer particles of similar sizes and shapes, indicating that each nanosized catalyst particle had uniform polymerization activity. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Role of Silica Properties in the Polymerisation of Ethylene Using Supported Metallocene CatalystsMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 1 2010Virginie F. Tisse Abstract The role of certain morphological properties of different silica used as supports for metallocene catalysts was investigated in order to quantify their eventual influence on the kinetics of ethylene polymerisation in heptane slurry, as well as on the polymer properties. It was shown that there is no clear link between porosity and observed activity with some highly porous silica producing relatively inactive supported catalysts. It was further shown that within a batch of silica, the particle size had a significant impact on observed kinetics. While it appears that there is possibly some diffusion resistance encountered in the larger particles, it was also shown that prolonged contact between the support and the alkylating agent could increase reaction rates in the larger particles. [source] Resistance to CO2 diffusion in cuticular membranes of amphibious plants and the implication for CO2 acquisitionPLANT CELL & ENVIRONMENT, Issue 1 2007HENNING FROST-CHRISTENSEN ABSTRACT Cuticular membranes (CMs) were isolated from leaves of amphibious and submerged plants and their CO2 resistances were determined as a contribution to establish quantitatively the series of resistances met by CO2 diffusing from bulk water to the chloroplasts of submerged leaves. The isolation was performed enzymatically; permeabilities were determined and converted to resistances. The range of permeance values was 3 to 43 × 10,6 m s,1 corresponding to resistance values of 23 to 295 × 103 s m,1, i.e. of the same order of magnitude as boundary layer resistances. The sum of boundary layer, CM, leaf cell and carboxylation resistances could be contained within the total diffusion resistance as determined from the photosynthetic CO2 affinity of the leaf. From the same species, the aerial leaf CM resistance was always higher than the aquatic leaf CM resistance. In a terrestrial plant, the CM resistance to CO2 diffusion was found lower in leaves developed submerged. [source] From ancient genes to modern communities: the cellular stress response and the evolution of plant strategiesFUNCTIONAL ECOLOGY, Issue 5 2005S. PIERCE Summary 1Two major plant strategy theories attempt to explain how phenotype determines community structure. Crucially, CSR plant strategy theory suggests that stress and sporadic resource availability favour conservative phenotypes, whereas the resource-ratio hypothesis views the spatial heterogeneity of resources as selecting for optimal foraging in chronically unproductive habitats. Which view is most realistic? 2The ecophysiology literature demonstrates that stress is comprised of two processes: (1) limitation of resource supply to metabolism; and (2) damage to biomembranes, proteins and genetic material (chronic stress). Thus stress is defined mechanistically as the suboptimal performance of metabolism. 3Adaptations to limitation buffer metabolism against variability in external resource supply; internal storage pools are more consistent. Chronic stress elicits the same ancient cellular stress response in all cellular life: investment in stress metabolites that preserve the integrity and compartmentalization of metabolic components in concert with molecular damage-repair mechanisms. 4The cellular stress response was augmented by morphological innovations during the Silurian,Devonian terrestrial radiation, during which nutrient limitation appears to have been a principal selection pressure (sensu CSR theory). 5The modern stress,tolerator syndrome is conservative and supports metabolism in limiting or fluctuating environmental conditions: standing resource pools with high investment/maintenance costs impose high internal diffusion resistances and limit inherent growth rate (sensu CSR theory). 6The resource-ratio hypothesis cannot account for the cellular stress response or the crucial role of ombrotrophy in primary succession. CSR theory agrees with current understanding of the cellular stress response, terrestrial radiation and modern adaptations recorded in chronically unproductive habitats, and is applicable as CSR classification. [source] | |||||||||||||