Direct Wolf summation of a polarizable force field for silica

We extend the Wolf direct, pairwise r^(-1) summation method with spherical truncation to dipolar interactions in silica. The Tangney-Scandolo interatomic force field for silica takes regard of polarizable oxygen atoms whose dipole moments are determined by iteration to a self-consistent solution. With Wolf summation, the computational effort scales linearly in the system size and can easily be distributed among many processors, thus making large-scale simulations of dipoles possible. The details of the implementation are explained. The approach is validated by estimations of the error term and simulations of microstructural and thermodynamic properties of silica.Comment: See http://link.aip.org/link/?JCP/132/194109 - 8 pages, 6 figures. Changes in v3: Copyright notice added, minor typographical changes. Changes in v2: 1. Inserted Paragraph in Sec. IV B describing the limitations of the TS potential. 2. We corrected transcription errors in Tab. II, and adjusted the deviation percentages mentioned in Sec. IV B, first paragraph, accordingl

Effect of Process Variables on Supercritical Fluid Impregnation of Composites with Tebuconazole

This study examines the effects of pressure, temperature, and treatment time on supercritical fluid impregnation of such composites as plywood, particleboard, flakeboard, and medium-density fiber-board. Carbon dioxide with methanol as a cosolvent was used as the supercritical fluid, with tebuconazole as the biocide. Biocide distribution, as measured by extraction and analysis, generally increased with pressure, temperature, and treatment time, although the retentions sometimes decreased at the highest pressure tested (4500 psig). In general, biocide retentions were far above those required for fungal protection, and the distribution was more uniform than that found with conventional pressure treatments. The results suggest that supercritical fluid impregnation represents a simple method for impregnating composites with biocides without the permanent damage typical of other treatment systems

Internal Pressure Measurement Techniques and Pressure Response in Wood During Treating Processes

The development of pressure inside wood during preservative impregnation was studied using Douglas-fir heartwood and ponderosa pine sapwood. Pressure sensors mounted on sample holders provided the most reliable measurements. As expected, pressure equilibrated most rapidly with air as the treatment medium and ponderosa pine as the test species. Pressure changes were relatively slow in Douglas-fir heartwood, suggesting that process conditions involving relatively rapid changes in pressure conditions will have little effect on fluid penetration away from the wood surface

Effect of Wood Characteristics on Pressure Responses During Supercritical Carbon Dioxide Treatment

The potential for using the anatomical properties of wood to predict response to supercritical fluid impregnation was investigated using an array of hardwood and softwood species. Longitudinal resin canals were a reasonable predictor of softwood response to pressure application, while radial gas permeability and/or fiber dimensions were useful for the same predictions in hardwoods. Most other anatomical characteristics were poorly correlated with pressure response. The results suggest that there is some ability to use limited anatomical measurements to predict the receptivity of a given species to supercritical fluid impregnation, thereby reducing the need for iterative treatment trials to assess suitability of a species for use in this process

Internal Pressure Development During Supercritical Fluid Impregnation of Wood

Supercritical fluid impregnation has tremendous potential for effectively impregnating a variety of species, but little is known about the pressure response in wood during this process. Pressure response was studied in a number of wood species using specially designed high pressure probes, which allowed in-situ monitoring of the treatment process. Pressure response was relatively rapid in permeable species such as pine, but tended to lag in less permeable species. In some cases, the differences between surface and internal pressure exceeded the material properties of the wood, and crushing or fractures resulted. The results indicate that the rates of pressure application and release can be tailored to control pressure differentials to avoid wood damage

Effects of Supercritical Fluid Treatments on Physical Properties of Wood-Based Composites

The effects of supercritical fluid (SCF) impregnation on physical properties of composites were investigated at various pressures, temperatures, and treatment times with carbon dioxide as the fluid and tebuconazole as a biocide. In general, within the ranges tested, SCF treatment had no negative effects on modulus of rupture, (MOR), modulus of elasticity, (MOE), or dimensional stability of particleboard, flakeboard, or medium-density fiberboard; in some instances, SCF treatment appeared to be associated with slight property improvements in some panel types. Shear tests of plywood suggested a similar lack of treatment effect. Results indicate that, despite the elevated pressure employed during the process, SCF impregnation does not adversely affect the mechanical properties of various wood-based composites

Two-level laser light statistics

The statistics of the light emitted by two-level lasers is evaluated on the basis of generalized rate equations. According to that approach, all fluctuations are interpreted as being caused by the jumps that occur in active and detecting atoms. The intra-cavity Fano factor and the photo-current spectral density are obtained analytically for Poissonian and quiet pumps. The algebra is simple and the formulas hold for small as well as large pumping rates. Lasers exhibit excess noise at low pumping levels.Comment: 9 pages, 3 figures, in Optics Communication format (elsevier

Hydrogenation of CO on a silica surface: an embedded cluster approach

The sequential addition of H atoms to CO adsorbed on a siliceous edingtonite surface is studied with an embedded cluster approach, using density functional theory for the quantum mechanical (QM) cluster and a molecular force field for the molecular mechanical (MM) cluster. With this setup, calculated QM/MM adsorption energies are in agreement with previous calculations employing periodic boundary conditions. The catalytic effect of the siliceous edingtonite (100) surface on CO hydrogenation is assessed because of its relevance to astrochemistry. While adsorption of CO on a silanol group on the hydroxylated surface did not reduce the activation energy for the reaction with a H atom, a negatively charged defect on the surface is found to reduce the gas phase barriers for the hydrogenation of both CO and H2C = O. The embedded cluster approach is shown to be a useful and flexible tool for studying reactions on (semi-)ionic surfaces and specific defects thereon. The methodology presented here could easily be applied to study reactions on silica surfaces that are of relevance to other scientific areas, such as biotoxicity of silica dust and geochemistry

Electronic properties of silica nanowires

Thin nanowires of silicon oxide were studied by pseudopotential density functional electronic structure calculations using the generalized gradient approximation. Infinite linear and zigzag Si-O chains were investigated. A wire composed of three-dimensional periodically repeated Si4O8 units was also optimized, but this structure was found to be of limited stability. The geometry, electronic structure, and Hirshfeld charges of these silicon oxide nanowires were computed. The results show that the Si-O chain is metallic, whereas the zigzag chain and the Si4O8 nanowire are insulators

Ab initio studies of phonon softening and high pressure phase transitions of alpha-quartz SiO2

Density functional perturbation theory calculations of alpha-quartz using extended norm conserving pseudopotentials have been used to study the elastic properties and phonon dispersion relations along various high symmetry directions as a function of bulk, uniaxial and non-hydrostatic pressure. The computed equation of state, elastic constants and phonon frequencies are found to be in good agreement with available experimental data. A zone boundary (1/3, 1/3, 0) K-point phonon mode becomes soft for pressures above P=32 GPa. Around the same pressure, studies of the Born stability criteria reveal that the structure is mechanically unstable. The phonon and elastic softening are related to the high pressure phase transitions and amorphization of quartz and these studies suggest that the mean transition pressure is lowered under non-hydrostatic conditions. Application of uniaxial pressure, results in a post-quartz crystalline monoclinic C2 structural transition in the vicinity of the K-point instability. This structure, intermediate between quartz and stishovite has two-thirds of the silicon atoms in octahedral coordination while the remaining silicon atoms remain tetrahedrally coordinated. This novel monoclinic C2 polymorph of silica, which is found to be metastable under ambient conditions, is possibly one of the several competing dense forms of silica containing octahedrally coordinated silicon. The possible role of high pressure ferroelastic phases in causing pressure induced amorphization in silica are discussed.Comment: 17 pages, 8 figs., 8 Table