Diffusivity

Distribution by Scientific Domains
Distribution within Chemistry

Kinds of Diffusivity

  • effective diffusivity
  • magnetic diffusivity
  • mean diffusivity
  • oxygen diffusivity
  • thermal diffusivity

  • Terms modified by Diffusivity

  • diffusivity value

  • Selected Abstracts


    Silicon-Doped LiFePO4 Single Crystals: Growth, Conductivity Behavior, and Diffusivity

    ADVANCED FUNCTIONAL MATERIALS, Issue 11 2009
    Ruhul Amin
    Abstract Single crystals of silicon doped LiFePO4 with a silicon content of 1% are grown successfully by the floating zone technique and characterized by single-crystal and powder X-ray diffraction, secondary ion mass spectroscopy, and chemical analysis. Electron paramagnetic resonance demonstrates the presence of only Fe2+; no traces of Fe3+ are found. Impedance spectroscopy as well as step-function polarization/depolarization (DC) measurements are carried out using the cells Ti/LiFe(Si)PO4/Ti and LiAl/LiI/LiFe(Si)PO4/LiI/LiAl. The electronic and ionic conductivities as well as the Li-diffusivity of the sample in the major crystallographic directions ([h00], [0k0], and [00l]) are determined. Within experimental error the transport properties along the b- and c-axes are found to be the same but differ significantly from the a-axis, which exhibits lower values. Compared to undoped LiFePO4, Si-doping leads to an increase of the ionic conductivity while the electronic conductivity decreases, which is in agreement with a donor effect. The activation energies of conductivities and diffusivities are interpreted in terms of defect chemistry and relevant Brouwer diagrams are given. [source]


    Microwave and convective dehydration of ethanol treated and frozen apple , physical properties and drying kinetics

    INTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 6 2002
    Tomas Funebo
    The objective of this study was to examine how the drying kinetics and physical properties of apples are affected by pre-treatment with 95% ethanol or freezing at ,18 °C before microwave-assisted air dehydration at 50, 60 and 70 °C. Microwave heating was used to obtain these temperatures in the centre of the apple cubes. After dehydration the shrinkage and rehydration capacity were measured. The texture of dehydrated and rehydrated samples was analysed with a puncture test in a texture analyser. Samples were also analysed with confocal laser scanning microscopy to determine the correlation between physical and microstructural properties. Diffusivity in the different dehydration processes was calculated. Ethanol-treated apples showed both high rehydration and high effective rehydration capacity compared with the other samples. Freezing before dehydration increased the diffusivity and reduced the firmness of rehydrated apples compared with no pre-treatment. [source]


    Effect of Aluminum Oxide Addition on the Flexural Strength and Thermal Diffusivity of Heat-Polymerized Acrylic Resin

    JOURNAL OF PROSTHODONTICS, Issue 6 2008
    Ayman E. Ellakwa BDS
    Abstract Purpose: This work was undertaken to investigate the effect of adding from 5% to 20% by weight aluminum oxide powder on the flexural strength and thermal diffusivity of heat-polymerized acrylic resin. Materials and Methods: Seventy-five specimens of heat-polymerized acrylic resin were fabricated. The specimens were divided into five groups (n = 15) coded A to E. Group A was the control group (i.e., unmodified acrylic resin specimens). The specimens of the remaining four groups were reinforced with aluminum oxide (Al2O3) powder to achieve loadings of 5%, 10%, 15%, and 20% by weight. Specimens were stored in distilled water at 37°C for 1 week before flexural strength testing to failure (5 mm/min crosshead speed) in a universal testing machine. Results were analyzed by one-way analysis of variance and post hoc Tukey paired group comparison tests (p < 0.05). Weibull analysis was used to calculate the Weibull modulus, characteristic strength, and the required stress for 1% and 5% probabilities of failure. Cylindrical test specimens (5 specimens/group) containing an embedded thermocouple were used to determine thermal diffusivity over a physiologic temperature range (0 to 70°C). Results: The mean flexural strength values of the heat-polymerized acrylic resin were (in MPa) 99.45, 119.92, 121.19, 130.08, and 127.60 for groups A, B, C, D, and E, respectively. The flexural strength increased significantly after incorporation of 10% Al2O3. The mean thermal diffusivity values of the heat-polymerized acrylic resin (in m2/sec) were 6.8, 7.2, 8.0, 8.5, and 9.3 for groups A, B, C, D, and E, respectively. Thermal diffusivities of the composites were found to be significantly higher than the unmodified acrylic resin. Thermal diffusivity was found to increase in proportion to the weight percentage of alumina filler, which suggested that the proper distribution of alumina powders through the insulating polymer matrix might form a pathway for heat conduction. Conclusion: Al2O3 fillers have potential as added components in denture bases to provide increased flexural strength and thermal diffusivity. Increasing the flexural strength and heat transfer characteristics of the acrylic resin base material could lead to more patient satisfaction. [source]


    A Limited Review of Water Diffusivity and Solubility in Glasses and Melts

    JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008
    James E. Shelby
    A limited review of the literature dealing with water solubility and diffusion in oxide glasses and melts is presented, with an emphasis on simple and commercial compositions and on work during the past decade. Several methods for determination of water solubilities and diffusivities are discussed. Experimental results are presented for silicate, borate, and germanate glasses and melts. Water diffusivities always increase with increasing temperature and modifier oxide content in these melts. Variations in water solubility and diffusivity with alkali and alkaline earth identity for otherwise identical compositions are small, while variations with the identity of the glass-forming oxide are large. Water solubility increases with increasing modifier oxide content in alkali silicate melts, but decreases with increasing modifier oxide content in alkali borate and germanate melts. [source]


    Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber-Reinforced Ceramic Composite

    JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2001
    Kathleen R. McDonald
    Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of ,1 to ,0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: (i) reductions of ,20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, (ii) almost complete recovery of the longitudinal diffusivity following unloading, and (iii) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity. [source]


    Chemical Diffusivity of BaTiO3,,: IV, Acceptor-Doped Case

    JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2000
    Chang-Rock Song
    The chemical diffusivity of 1.8 mol% aluminum-doped BaTiO3,, was measured on single-crystal specimens, as a function of ambient oxygen partial pressure, in the range 10,18 atm ,PO2, 1 atm and at temperatures of 800°,T, 1100°C, via a conductivity-relaxation technique. As in the polycrystalline, undoped BaTiO3,, described in Part II of this work, the chemical diffusivity exhibited a maximum, of thermodynamic origin, approximately at the stoichiometric composition (,= 0). The measured diffusivity was analyzed, based on the defect structure proposed and Wagner's classic theory of chemical diffusion, and the mobilities of the electrons and holes, as well as all of the relevant defect-equilibrium constants, then were evaluated with no prior assumptions. The evaluated parameters were compared with those for the undoped BaTiO3,,. [source]


    Hydrogen diffusion in titanium-hydride observed by the diffraction-enhanced X-ray imaging method

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2007
    K. Mizuno
    Abstract The X-ray refraction imaging technique was applied to the quantitative study of hydrogen diffusion in titanium-hydride. Diffusivity and activation energy of hydrogen diffusion were determined by direct observation of the hydride. The hydride was formed on the surface of titanium by electrolytic-charging at 31, 63 and 98 °C. The specimens were cut into 1 mm thick slices for cross-sectional observations. The hydride layer was observed using the diffraction-enhanced X-ray imaging (DEI) method. Boundaries between titanium and the hydride were observed as thick black or white lines parallel to the specimen surface in the DEI images similar to previously reported results. Hydride distribution caused by hydrogen diffusion from the surface was calculated using an assumed hydrogen diffusivity value and from the solution of the appropriate diffusion equation. The results were converted to the intensity profiles of refraction images of the hydride using the measured rocking curve from an analyzer. The calculated intensity profiles were compared to those obtained from the experimental results and the fitted diffusivity was determined by trial and error. The activation energy of hydrogen in titanium-hydride thus obtained, 0.55 ± 0.07 eV, showed good agreement with the widely accepted value obtained by the internal friction. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Long-distance transport of gases in plants: a perspective on internal aeration and radial oxygen loss from roots

    PLANT CELL & ENVIRONMENT, Issue 1 2003
    T. D. COLMER
    ABSTRACT Internal transport of gases is crucial for vascular plants inhabiting aquatic, wetland or flood-prone environments. Diffusivity of gases in water is approximately 10 000 times slower than in air; thus direct exchange of gases between submerged tissues and the environment is strongly impeded. Aerenchyma provides a low-resistance internal pathway for gas transport between shoot and root extremities. By this pathway, O2 is supplied to the roots and rhizosphere, while CO2, ethylene, and methane move from the soil to the shoots and atmosphere. Diffusion is the mechanism by which gases move within roots of all plant species, but significant pressurized through-flow occurs in stems and rhizomes of several emergent and floating-leaved wetland plants. Through-flows can raise O2 concentrations in the rhizomes close to ambient levels. In general, rates of flow are determined by plant characteristics such as capacity to generate positive pressures in shoot tissues, and resistance to flow in the aerenchyma, as well as environmental conditions affecting leaf-to-air gradients in humidity and temperature. O2 diffusion in roots is influenced by anatomical, morphological and physiological characteristics, and environmental conditions. Roots of many (but not all) wetland species contain large volumes of aerenchyma (e.g. root porosity can reach 55%), while a barrier impermeable to radial O2 loss (ROL) often occurs in basal zones. These traits act synergistically to enhance the amount of O2 diffusing to the root apex and enable the development of an aerobic rhizosphere around the root tip, which enhances root penetration into anaerobic substrates. The barrier to ROL in roots of some species is induced by growth in stagnant conditions, whereas it is constitutive in others. An inducible change in the resistance to O2 across the hypodermis/exodermis is hypothesized to be of adaptive significance to plants inhabiting transiently waterlogged soils. Knowledge on the anatomical basis of the barrier to ROL in various species is scant. Nevertheless, it has been suggested that the barrier may also impede influx of: (i) soil-derived gases, such as CO2, methane, and ethylene; (ii) potentially toxic substances (e.g. reduced metal ions) often present in waterlogged soils; and (iii) nutrients and water. Lateral roots, that remain permeable to O2, may be the main surface for exchange of substances between the roots and rhizosphere in wetland species. Further work is required to determine whether diversity in structure and function in roots of wetland species can be related to various niche habitats. [source]


    Transport Diffusivity of CO2 in the Highly Flexible Metal,Organic Framework MIL-53(Cr),

    ANGEWANDTE CHEMIE, Issue 44 2009
    Fabrice Salles Dr.
    Der Diffusionsmechanismus von CO2 im hochflexiblen metall-organischen Gerüst MIL-53(Cr) (siehe Bild), das einen Strukturübergang zwischen einer groß- (A) und einer kleinporigen Form (B) zeigt, wurde durch quasielastische Neutronenstreuung in Kombination mit Moleküldynamiksimulationen studiert. Bei A wurde eine normale eindimensionale Diffusion beobachtet, bei B bewegen sich die Teilchen hingegen nacheinander in einer linearen Kette. [source]


    Determination of Drying Characteristics and Effective Diffusivity for Sugar Cubes

    CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 11 2004
    C. Akosman
    Abstract In this study, the drying characteristics of sugar cubes have been investigated. Drying experiments with sugar cubes were conducted in a laboratory-type tray dryer at temperatures ranging from 45 to 95,°C and different air velocities (0.43, 0.56, and 0.7,m/s). Drying rates increased with the increase in temperature and air velocity. At high temperatures, the moisture content of the solid reached equilibrium moisture value in short time periods. Experimental data were analyzed by using the analytical solution of the unsteady-state diffusion equation to determine the effective diffusivity in sugar cubes. [source]


    Electromigration diffusivity spectrometry: A way for simultaneous determination of diffusion coefficients from mixed samples

    ELECTROPHORESIS, Issue 17 2010
    Suhua Yang
    Abstract A novel method was proposed for simultaneous measurement of diffusion coefficients, (D), from mixed samples by electrophoresis and termed electromigration-based diffusivity spectrometry. After theoretical treatment, D- equation for practical use has been deduced. With a modified CE system built in laboratory, electromigration-based diffusivity spectrometry has been realized and validated to suit for fast and accurate determination of diffusivities of mixed aromatic amino acids, phenols and aromatic organic acid, giving diffusivity spectra by peak area versus D, much similar to mass spectra. The precision of the measurement was found to critically depend on pH value of running buffer, which should be so selected that the analytes and internal standards could be charged at above 0.5e. The standards have to be selected at an electric flux far from each other and from analytes. In these cases, sample and running buffer concentrations, voltage and system temperature were found to have only negligible impact on the determination. In our test, the obtained measuring precision was generally kept within 1% for five runs, and the measured values of D agreed well with those from literature, with a deviation of less than 2.2% after the right use of calibration standards. [source]


    Silicon-Doped LiFePO4 Single Crystals: Growth, Conductivity Behavior, and Diffusivity

    ADVANCED FUNCTIONAL MATERIALS, Issue 11 2009
    Ruhul Amin
    Abstract Single crystals of silicon doped LiFePO4 with a silicon content of 1% are grown successfully by the floating zone technique and characterized by single-crystal and powder X-ray diffraction, secondary ion mass spectroscopy, and chemical analysis. Electron paramagnetic resonance demonstrates the presence of only Fe2+; no traces of Fe3+ are found. Impedance spectroscopy as well as step-function polarization/depolarization (DC) measurements are carried out using the cells Ti/LiFe(Si)PO4/Ti and LiAl/LiI/LiFe(Si)PO4/LiI/LiAl. The electronic and ionic conductivities as well as the Li-diffusivity of the sample in the major crystallographic directions ([h00], [0k0], and [00l]) are determined. Within experimental error the transport properties along the b- and c-axes are found to be the same but differ significantly from the a-axis, which exhibits lower values. Compared to undoped LiFePO4, Si-doping leads to an increase of the ionic conductivity while the electronic conductivity decreases, which is in agreement with a donor effect. The activation energies of conductivities and diffusivities are interpreted in terms of defect chemistry and relevant Brouwer diagrams are given. [source]


    Contaminant Transport in Fractured Chalk: Laboratory and Field Experiments

    GROUND WATER, Issue 6 2003
    K. Witthüser
    Laboratory experiments were performed on chalk samples from Denmark and Israel to determine diffusion and distribution coefficients. Batch tests were used to define sorption isotherms for naphthalene and o-xylene. Linear sorption isotherms were observed and described with Henry-isotherms. Because of the high purity and low contents of clay minerals and organic carbon, Danish and white Israeli chalk generally have low retardation capacities. Con-trarily, gray Israeli chalk, with organic carbon fractions as high as 1.092%, remarkably retards organic contaminants. The Koc concept is not applicable to predicting distribution coefficients based on the organic carbon content in the chalk samples. Effective diffusivities of o -xylene, naphthalene, and several artificial tracers were determined using through-diffusion experiments. Based on measured diffusion coefficients and available literature values, a chalk specific exponent of 2.36 for Archie's law was derived, allowing a satisfactory estimate of relative diffusivities in chalk. A field-scale tracer test with uranine and lithium was performed in the Negev desert (Israel) to examine the transfer-ability of diffusivities determined on small rock samples in the laboratory. Due to low recovery rates of the tracer, a modified single fissure dispersion model was used for inverse modeling of the breakthrough curves. Resulting diffusivities deviate insignificantly from the laboratory values, which are considered to be representative for the investigated part of the aquifer and applicable in transport models. [source]


    Cover Picture: Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals (Adv. Funct.

    ADVANCED FUNCTIONAL MATERIALS, Issue 11 2006
    Mater.
    Abstract Hollow nanocrystals have been synthesized through a mechanism analogous to the Kirkendall Effect. When a cobalt nanocrystal reacts with sulfur in solution, the outward diffusion of cobalt atoms is faster than the inward diffusion of sulfur atoms through the sulfide shell. The dominating outward diffusion of cobalt cations produces vacancies that can condense into a single void in the center of the nanocrystal at high temperatures. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds and is described in the Full Paper by A.,P. Alivisatos and co-workers on p.,1389. Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It is found that performing the reaction at >,120,°C leads to fast formation of a single void inside each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature-dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, the occurrence of significant inward transport of sulfur anions can be inferred as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells, implying significant porosity and/or a defective structure. Indirect evidence for fracture of shells during growth at lower temperatures was observed in shell-size statistics and transmission electron microscopy images of as-grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system. [source]


    Response of saturated porous media to cyclic thermal loading

    INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 11 2003
    E. Blond
    Abstract The response of a semi-infinite saturated porous medium subjected to a harmonic thermal loading on its free face is studied herein. The pressure diffusion equation that governs the fluctuation of the interstitial pressure is established. It allows us to obtain prevalent parameters, i.e. the thermal and fluid mass diffusivities and the coefficient of relative bulk variation. Closed-form solutions of the maximum fluid pressure Pmax and its location xcr are derived. It is shown that the location xcr of Pmax is localized and depends on the diffusivity ratio and the frequency of the thermal loading while the magnitude of Pmax depends on the diffusivity ratio and the thermal amplitude. Master curves for xcr and Pmax versus diffusivity ratio are built. It follows that three regimes can be distinguished: namely, thin spalling, thick spalling or in-depth cracking and no cracking. Copyright © 2003 John Wiley & Sons, Ltd. [source]


    Original article: Apparent thermal diffusivity estimation for the heat transfer modelling of pork loin under air/steam cooking treatments

    INTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 9 2010
    Massimiliano Rinaldi
    Summary Apparent thermal diffusivity linear functions vs. product temperature were estimated for pork cooked under two different treatments (forced convection, FC and forced convection/steam combined, FC/S) at 100, 110, 120 and 140 °C by means of experimental time,temperature data and a developed finite-difference algorithm. Slope and intercept of each function were employed to calculate apparent thermal diffusivity at 40, 55 and 70 °C. Generally, FC/S treatments gave significantly higher apparent thermal diffusivities in comparison with FC conditions. Apparent thermal diffusivities were used to develop a model for cooking time and final core temperature prediction on the basis of oven setting. The model was validated by means of additional cooking tests performed at different temperatures of those employed for model development. Root mean square error values lower than 3.8 °C were obtained comparing predicted and experimental temperature profiles. Percentage errors lower than 3.1% and 3.5% were, respectively, obtained for cooking times and final core temperatures. [source]


    Computational analysis of an instantaneous chemical reaction in a T-microreactor

    AICHE JOURNAL, Issue 6 2010
    Dieter Bothe
    Abstract We extend and apply a method for the numerical computation of convective and diffusive mixing in liquid systems with very fast irreversible chemical reaction to the case of unequal diffusivities. This approach circumvents the solution of stiff differential equations and, hence, facilitates the direct numerical simulation of reactive flows with quasi-instantaneous reactions. The method is validated by means of a neutralization reaction which is studied in a T-shaped micromixer and compared with existing experimental LIF-data. Because of their large are-to-volume ratio, microreactors are well suited for fast chemical reactions which are seriously affected by the slow diffusive transport in aqueous media. Numerical computations for different reactor dimensions reveal the fact that, in a dimensionless setting, the obtained conversion is independent of the reactor size, if the flow conditions are the same. This corresponds to an increase of space-time-yield proportional to the square of the inverse scale factor. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]


    Tapered element oscillating microbalance (TEOM) studies of isobutane, n-butane and propane sorption in ,- and Y-zeolites

    AICHE JOURNAL, Issue 5 2010
    Kening Gong
    Abstract A TEOM is used to elucidate the adsorption/desorption characteristics of alkylation reactants on USY- and ,-zeolites. Equilibrium adsorption isotherms were obtained on USY- and ,-zeolites using n-butane, isobutane and propane as proxy reactant molecules (T = 303,398 K, adsorbate partial pressure 0,1.2 bar). Analysis of the transient adsorption/desorption profiles of these molecules from either a bed of the zeolite or pelletized particles of the crystals (with mean size < 1 ,m) demonstrate that diffusion in the secondary meso-/macroporous structure formed in the packing or the pellets controls the overall sorption rates. The experimental adsorption/desorption profiles from the pelletized zeolites were regressed with available mathematical models to obtain effective meso-/macropore diffusivities for reactant molecules, and nearly perfect fits of the experimental and the modeled profiles. Taking into account the dead volume in the system, a criterion for reliable measurements of either micropore or mesopore diffusivities by the TEOM technique is derived. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source]


    The effect of mixer properties and fill level on granular flow in a bladed mixer

    AICHE JOURNAL, Issue 2 2010
    Brenda Remy
    Abstract The discrete element method was used to study the effect of mixer properties and fill level on the granular flow of monodisperse, cohesionless spheres in a bladed mixer. For fill levels just covering the span of the blades, a three-dimensional (3-D) recirculation zone develops in front of the blades, which promotes vertical and radial mixing. Increasing fill level reduces the size of the recirculation zone, decreases bed dilation and hinders particle diffusivities. However, above a critical fill level, the behavior of the particles within the span of the blade is found to be invariant of fill level. At low-fill levels, the pressure within the particle bed varies linearly with bed height and can be approximated by hydrostatics. At higher fill levels, a constant pressure region develops within the span of the blades due to the angled pitch of the blades. Cylinder wall friction is shown to significantly influence granular behavior in bladed mixers. At low-wall friction, the 3-D recirculation zone observed for high-wall friction conditions does not develop. High-wall friction leads to an increase in convective and diffusive particle mixing. Shear stresses are shown to be a function of wall friction. Blade position along the vertical axis is shown to influence flow patterns, granular temperature and stress. The effect of increasing the mixer diameter at a constant particle diameter was also studied. When the mixer diameter is larger than a critical size such that wall effects are minimized, the observed granular behavior follows simple scaling relations. Particle velocities and diffusivities scale linearly with mixer size and blade speed. Normal and shear stress profiles are found to scale linearly with the total weight of the particle bed. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source]


    Dependence of reaction kinetics on H2O activity as inferred from rates of intergranular diffusion of aluminium

    JOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2010
    W. D. CARLSON
    Abstract Quantitative constraints on the accelerative effects of H2O on the kinetics of metamorphic reactions arise from a comparison of rates of intergranular diffusion of Al in natural systems that are fluid-saturated, hydrous but fluid-undersaturated, and nearly anhydrous. Widths of symplectitic reaction coronas around partially resorbed garnet crystals in the contact aureole of the Makhavinekh Lake Pluton, northern Labrador, combined with time,temperature histories from conductive thermal models, yield intergranular diffusivities for Al from ,700,900 °C under nearly anhydrous conditions. Those rates, when extrapolated down temperature, are approximately three orders of magnitude slower than rates derived from re-analysis of garnet resorption coronas formed under hydrous but fluid-undersaturated conditions near 575 °C in rocks of the Llano Uplift of central Texas, which are in turn approximately four orders of magnitude slower than rates at comparable temperatures derived from numerical simulations of prograde garnet growth in fluid-saturated conditions in rocks from the Picuris Range of north-central New Mexico. Thus, even at constant temperature, rates of intergranular diffusion of Al , and corresponding length scales and timescales of metamorphic reaction and equilibration , may vary by as much as seven orders of magnitude across the range of H2O activities found in nature. [source]


    Diffusion-controlled growth of wollastonite rims between quartz and calcite: comparison between nature and experiment

    JOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2002
    R. Milke
    Abstract Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion-controlled. From the rate constants, the D,,-values of the rate-limiting species were derived, i.e. the product of grain boundary diffusion coefficient D, and the effective grain boundary width, ,. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that (D,CaO,)/(D,SiO2,),1.33, and that SiO2 diffusion controls rim growth. D,SiO2, increases from about 10,25 to 10,23 m3 s,1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol,1. In runs at 1 GPa, performed in a piston-cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion-advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO2 diffusion is rate-limiting. D,, increases from about 10,21 to 10,20 m3 s,1 as temperature increases from 1100 to 1200 °C. D,SiO2, or D,CaO, in rims on calcite is c. 10 times larger than D,SiO2, in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact-metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine-infiltrated metacherts to water-bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact-metamorphism as compared to our experimental results. [source]


    Theoretical analysis and experimental study on SO2 adsorption onto pistachio-nut-shell activated carbon

    AICHE JOURNAL, Issue 2 2009
    Aik Chong Lua
    Abstract The adsorption study of SO2 onto the activated carbon prepared from pistachio-nut shell was studied theoretically and experimentally. A single-particle sorption model known as concentration-dependent surface diffusivity micropore, surface and macropore diffusion control model incorporating micropore, macropore and surface diffusions, together with a nonlinear isotherm at the micropore mouth, has been derived and solved by a finite difference method. The effects of different types of nonlinear isotherms and concentration dependent surface diffusivities have been thoroughly studied. The effects of adsorbate concentration and temperature on adsorption were studied experimentally. Good agreement was found between the model predictions and the experimental results. The value of the tortuosity factor and the extracted diffusion coefficients obtained are consistent with their corresponding values reported. © 2008 American Institute of Chemical Engineers AIChE J 2009 [source]


    Reverse osmosis of nonaqueous solutions through porous silica-zirconia membranes

    AICHE JOURNAL, Issue 2 2006
    Toshinori Tsuru
    Abstract Porous silica-zirconia membranes with pore diameters from 0.8 to 2 nm were prepared by a sol-gel process, and applied to the separation of alcohols (hexanol, octanol, decanol) and alkanes (hexane, decane, tetradecane) in ethanol solutions by reverse osmosis over the temperature range from 25 to 60° C. A silica-zirconia membrane with a pore diameter of 1 nm showed a molecular weight-cut-off (MWCO) of 200 in ethanol solutions. Rejection increased with the applied pressure, for both alcohol and alkane solutes. However, the rejection of alcohols was found to decrease with temperature, while that for alkanes remained nearly constant. The separation characteristics were examined for the following membrane parameters: solvent permeability, Lp, reflection coefficient, ,, and solute permeability, P, based on the Spiegler-Kedem equation. The viscosity of solutions and the diffusivity of alkanes and alcohol solutes in nano-sized pores were found to show a larger temperature dependency than in bulk. The diffusivity of alkane solutes showed the same temperature dependency as the viscosity of ethanol in nano-sized pores, while the diffusivity of alcohol solutes showed a larger temperature dependency than the viscosity of ethanol, probably because of a larger interaction between alcohol solutes and the hydrophilic surface of silica-zirconia membranes. Diffusion experiments were carried out to confirm the temperature dependency of the diffusivities in nano-sized pores. A bilayer model verified that solute permeabilities by reverse osmosis and diffusion experiments were consistent with each other. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source]


    Multicomponent mass transport model for the sorption of metal ions on bone char

    AICHE JOURNAL, Issue 9 2004
    Danny C. K. Ko
    Abstract The sorption of binary mixtures of copper/cadmium and copper/zinc ions onto bone char was studied in fixed beds. The effects of solution flow rate, initial dye concentration, and bone char particle size range were investigated. A mass transport model based on film-surface diffusion and the IAS model for the equilibrium relationship was used to develop theoretical fixed-bed breakthrough curves. The model incorporates the Sips isotherm for the first time in fixed-bed predictions, since this isotherm gives an excellent correlation of the experimental equilibrium data. The model was used to determine the optimum surface diffusivities as 7.37 × 10,10 and 2.73 × 10,9 cm2/s for copper and cadmium in the Cu/Cd system; and 1.61 × 10,9 and 2.43 × 10,9 cm2/s for copper and zinc in the Cu/Zn system. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2130,2141, 2004 [source]


    Analysis of multicomponent adsorption kinetics on activated carbon

    AICHE JOURNAL, Issue 4 2003
    L. P. Ding
    An integrated mathematical model for the kinetics of multicomponent adsorption on microporous carbon was developed. Transport in this bidisperse solid is represented by balance equations in the macropore and micropore phases, in which gas-phase diffusion dominates the mass transfer in the macropores, with the phenomenological diffusivities represented by the generalized Maxwell,Stefan (GMS) formulation. Viscous flow also contributes to the macropore fluxes and is included in the MS expressions. Diffusion of the adsorbed phase controls the mass transfer in the micropore phase, which is also described in a similar way by the MS method. The adsorption isotherms are represented by a new heterogeneous modified vacancy solution theory formulation of adsorption, which has proved to be a robust method for adsorption on activated carbons. The model is applied to the coadsorption and codesorption of C2H6 and C3H8 on Ajax and Norit carbon, as well as the displacement on Ajax carbon. The effect of the viscous flow in the macropore phase is not significant for the cases studied. The model accurately predicts the overshoot behavior and rollup of C2H6 during coadsorption. The prediction for the heavier compound C3H8 is always satisfactory, though at higher C3H8 mole fraction, the overshoot extent of C2H6 is overpredicted, possibly due to neglect of heat effects. [source]


    Separation of mono- and dibranched hydrocarbons on silicalite

    AICHE JOURNAL, Issue 9 2002
    E. Jolimaitre
    Breakthrough curves for mixtures of C5 and C6 hydrocarbons with different degrees of branching were obtained experimentally on a silicalite molecular sieve by fixed-bed experiments. The kinetic separation of di- from monobranched hydrocarbons was feasible on this type of zeolite: dimethyl molecules enter silicalite crystals very slowly, whereas monomethyl molecules are quickly adsorbed. Experimental results were compared to a theoretical isothermal model, considering the variation of diffusivity with concentration according to the Maxwell,Stefan theory. The parameters of the model (adsorption equilibria and diffusivities) were determined from single-component breakthrough curves. Experimental curves were generally well represented by the model, but the variation of diffusivity with concentration, as predicted by the Maxwell,Stefan theory, did not significantly improve the model prediction of experimental breakthrough curves by using a mean value of diffusivity. [source]


    Measuring anisotropic thermal conduction in polyisobutylene following step shear strains

    AICHE JOURNAL, Issue 3 2000
    Hadjira Iddir
    The connection between polymer chain orientation and several macroscopic properties in a polymer melt was studied using mechanical and optical techniques. Anisotropic thermal conductivity following shear deformation was measured using forced Rayleigh light scattering, the refractive index tensor is followed using birefringence measurements, and the stress was measured mechanically in a parallel-plate rheometer. The thermal diffusivity measured in the flow and neutral directions increased and decreased, respectively, immediately following the deformation. These quantities then relaxed to the equilibrium value on the time-scale of the stress-relaxation memory. Comparison of the difference between measured flow and neutral direction thermal diffusivities with the analogous flow-induced birefringence in the same deformation provided indirect evidence for a linear relation between stress and thermal diffusivity at two different values of strain. Mechanical measurements were used to characterize the memory of the fluid. [source]


    Epoxy-functionalized, low-glass-transition-temperature latex.

    JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2002

    Abstract This article describes the results of experiments examining the competition between the polymer diffusion rate and the crosslinking rate in low-glass-transition-temperature, epoxy-containing latex films in the presence of a diamine. We examined films formed from donor- and acceptor-labeled poly(butyl acrylate- co -methyl methacrylate- co -glycidyl methacrylate) copolymer latex and studied the influence of several parameters on the growth rate of gel content and the rate of polymer diffusion. These factors include the molecular weight of the latex polymer, the presence or absence of a diamine crosslinking agent, and the cure protocol. The results were compared to the predictions of a recent theory of the competition between crosslinking and polymer diffusion across interfaces. In the initially formed films, polymer diffusion occurs more rapidly than the chemical reaction rate. Therefore, these films fall into the fast-diffusion category of this model. In our system (unlike in the model), the latex polymer has a broad distribution of molecular weights and a distribution of diffusivities. The shortest chains contribute to the early time diffusion that we measure. At later stages of our experiment, slower diffusing species contribute to the signal that we measure. The diffusion time decreases substantially, and we observe a crossover to a regime in which the chemical reaction dominates. The increases in chain branching and gel formation bring polymer diffusion to a halt. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4098,4116, 2002 [source]


    Effect of Aluminum Oxide Addition on the Flexural Strength and Thermal Diffusivity of Heat-Polymerized Acrylic Resin

    JOURNAL OF PROSTHODONTICS, Issue 6 2008
    Ayman E. Ellakwa BDS
    Abstract Purpose: This work was undertaken to investigate the effect of adding from 5% to 20% by weight aluminum oxide powder on the flexural strength and thermal diffusivity of heat-polymerized acrylic resin. Materials and Methods: Seventy-five specimens of heat-polymerized acrylic resin were fabricated. The specimens were divided into five groups (n = 15) coded A to E. Group A was the control group (i.e., unmodified acrylic resin specimens). The specimens of the remaining four groups were reinforced with aluminum oxide (Al2O3) powder to achieve loadings of 5%, 10%, 15%, and 20% by weight. Specimens were stored in distilled water at 37°C for 1 week before flexural strength testing to failure (5 mm/min crosshead speed) in a universal testing machine. Results were analyzed by one-way analysis of variance and post hoc Tukey paired group comparison tests (p < 0.05). Weibull analysis was used to calculate the Weibull modulus, characteristic strength, and the required stress for 1% and 5% probabilities of failure. Cylindrical test specimens (5 specimens/group) containing an embedded thermocouple were used to determine thermal diffusivity over a physiologic temperature range (0 to 70°C). Results: The mean flexural strength values of the heat-polymerized acrylic resin were (in MPa) 99.45, 119.92, 121.19, 130.08, and 127.60 for groups A, B, C, D, and E, respectively. The flexural strength increased significantly after incorporation of 10% Al2O3. The mean thermal diffusivity values of the heat-polymerized acrylic resin (in m2/sec) were 6.8, 7.2, 8.0, 8.5, and 9.3 for groups A, B, C, D, and E, respectively. Thermal diffusivities of the composites were found to be significantly higher than the unmodified acrylic resin. Thermal diffusivity was found to increase in proportion to the weight percentage of alumina filler, which suggested that the proper distribution of alumina powders through the insulating polymer matrix might form a pathway for heat conduction. Conclusion: Al2O3 fillers have potential as added components in denture bases to provide increased flexural strength and thermal diffusivity. Increasing the flexural strength and heat transfer characteristics of the acrylic resin base material could lead to more patient satisfaction. [source]


    A Limited Review of Water Diffusivity and Solubility in Glasses and Melts

    JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008
    James E. Shelby
    A limited review of the literature dealing with water solubility and diffusion in oxide glasses and melts is presented, with an emphasis on simple and commercial compositions and on work during the past decade. Several methods for determination of water solubilities and diffusivities are discussed. Experimental results are presented for silicate, borate, and germanate glasses and melts. Water diffusivities always increase with increasing temperature and modifier oxide content in these melts. Variations in water solubility and diffusivity with alkali and alkaline earth identity for otherwise identical compositions are small, while variations with the identity of the glass-forming oxide are large. Water solubility increases with increasing modifier oxide content in alkali silicate melts, but decreases with increasing modifier oxide content in alkali borate and germanate melts. [source]