Showing papers on "Volume fraction published in 1996"

Microwave permittivity of dry sand

Abstract: The relative dielectric constant, or relative permittivity, /spl epsiv/ of dry snow, is independent of frequency from about 1 MHz up to the microwave range of at least 10 GHz. New measurements of with improved accuracy were made with a specially designed resonator operating near 1 GHz. The coaxial sensor accurately defines the sample volume whose actual mass can be determined to give the density of the snow sample. A special electronic instrument, called a resometer, enabled accurate and rapid measurements under field conditions. Some 90 measurements of different kinds of dry snow (fresh, old, wind-pressed snow, depth hear, and refrozen crusts) were made at test sites in the Swiss and Austrian Alps. The data indicate that /spl epsiv/ is a function of snow density only, given that the standard deviation of 0.006 from the fitted curve is just due to the expected measurement errors. The interpretation of these data in terms of physical mixing theory favors the effective medium formula of Polder and van Santen (1946). The data allow to relate the average axial ratio X as a function of ice volume fraction. Both prolate and oblate spheroids can explain the data. Independent reasoning gives preference to oblate particles. In both cases, the axial ratio increases with increasing fraction up to a critical value of 0.33, followed by a decrease at still higher fractions. The destructive metamorphism of slowly compacting snow explains the increase of X, while the following decrease might be due to sintering. So far, no effect on /spl epsiv/ by a liquid-like surface layer on the ice grains at temperatures between -10/spl deg/C and 0/spl deg/C has been observed.

Microwave dielectric characterization of binary mixtures of water, methanol, and ethanol

Abstract: The complex dielectric constants of binary mixtures of water–methanol and water–ethanol in the frequency range from 45 MHz to 26.5 GHz, and binary mixtures of methanol–ethanol in the range from 200 MHz to 26.5 GHz have been measured with various volume fractions around room temperature by means of an open‐ended coaxial sensor and a network analyzer. Methanol–ethanol mixtures display a near‐Debye dispersion while water–alcohol mixtures show a Cole–Davidson dispersion. The logarithm of relaxation time log τ and dielectric decrement Δe for methanol–ethanol mixtures show a good linear relation with the volume fraction of methanol, while log τ and Δe extracted with the Debye function for water–alcohol mixtures display a near‐linear relation with volume fraction of water. Two simple formulas are proposed for identifying the volume fractions of the components in binary mixtures of alcohol–alcohol and water–alcohol from a knowledge of τ and Δe for the pure liquids and the mixtures. The validity of these formulas has been demonstrated with three blind tests. The relation between the mole fraction of water and log τ for water–methanol and water–ethanol mixtures extracted by the use of a Cole–Davidson function clearly shows two linear regions, which implies a change of relaxation mechanism with mole fraction.

Dependence of microwave absorbing property on ferrite volume fraction in MnZn ferrite-rubber composites

Abstract: The effective complex permeability and permittivity spectra in composites with various ferrite volume fractions were calculated using the Bruggeman effective medium theory. The matching conditions for the maximum microwave absorption were determined by substituting the complex permeability and permittivity in the equation for the impedance matching condition. The minimum reflection loss increases from -5 to -40 dB as the ferrite volume fraction increases for /spl nu//sub f/<0.26, where single dip of minimum reflection loss was observed. However, two matching conditions were observed for v/sub f//spl ges/0.26, and two minimum reflection losses shown to be about -40 dB. The results indicate that a microwave absorber with reflection loss of about -40 dB can be designed by controlling the ferrite volume fraction in MnZn ferrite-rubber composites in the frequency range 0.8-12 GHz.

Acoustic Spectroscopy for Concentrated Polydisperse Colloids with High Density Contrast

Abstract: The mechanisms of sound attenuation are different in dispersions with low and high density contrast. “Viscous losses” are dominant in high density contrast dispersions whereas “thermal losses” predominate in dispersions with low density contrast. The rutile dispersion is chosen as an instance of the high density contrast system. The dispersion of the neoprene latex is an instance of the low density contrast system. The dilution experiment performed with both systems shows that the role of the particle−particle interaction is quite different in these two dispersions. The measured spectra show that attenuation remains a linear function of the volume fraction in the latex dispersion even at 30 vol %. At the same time, attenuation exhibits a nonlinear dependence on the volume fraction for the rutile dispersion even at 10 vol %. This difference means that particle−particle interaction contributes more to the “viscous losses” than to the “thermal losses”. We associate this effect with the difference between “vi...

Morphologies of microphase-separated A2B simple graft copolymers

Abstract: The morphological behavior of a series of well-defined A2B simple graft or “Y” architecture block copolymers is characterized via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). This model architecture is formed by grafting a polystyrene block onto the center of a polyisoprene backbone. The volume fraction windows in which specific strongly segregated microphase-separated morphologies are observed are shifted to higher volume fractions of the PS graft material than in the corresponding linear diblock copolymers. These findings are in good agreement with recently calculated theoretical phase behavior for simple graft, A2B, block copolymers. However, a new morphology, not found in neat linear diblock copolymers, is also observed. This A2B material is microphase separated into wormlike micelles but not ordered on a lattice. This morphology is found at high PS graft volume fraction (φs = 0.81), where the two PI chains per molecule are initially forced to the concave side of the...

The continuum field approach to modeling microstructural evolution

Abstract: The majority of advanced engineering materials contain multiphase and/or multidomain structures. Their physical and mechanical properties depend strongly on the number of phases present and their mutual arrangement; the volume fraction of each phase; and the shape, size, and size distribution of domains (or grains). This article describes a continuum diffuse-interface field approach to modeling microstructural evolution and its application to a number of different processes, including precipitation reactions through nucleation and growth, structural transformations involving symmetry changes, and curvature-driven grain growth.

Observations on the formation of β-Al5FeSi phase in 319 type Al-Si alloys

Abstract: In Al-Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification The β-Al5FeSi iron intermetallic phase, in particular, is known for its detrimental effect on the properties, and is controlled variously by the iron content and the melt/solidification conditions of the alloy The formation of the iron intermetallics has been observed in commercial 319 alloy end-chilled castings, obtained from non-treated and treated melts, where the effects of cooling rate, strontium modification and grain refinement have been studied The volume fraction of β-phase formed was seen to increase with the decrease in cooling rate (ie with increasing distance from the chill) in the untreated alloy Sympathetic (preferential) nucleation of the β-iron needles was also observed, in which the branching of β-needles from a parent needle resulted in the formation of large β-needle entities that can cover distances of ∼ 1200 μm across the matrix surface The beneficial effect of modification, ie strontium addition to the melt, was manifested through its influence on the fragmentation and dissolution of the β-needles The strontium “poisons” the sites where sympathetic nucleation takes place Dissolution was accelerated with increasing strontium content, the optimum level being ∼ 300 ppm Grain refining, on the other hand, negated the beneficial effect of modification, in that the β-needles underwent thickening and the sympathetic nucleation/branching also occurred The modified alloy was found to possess the lowest volume fraction of β-Al5FeSi phase among the unmodified, modified, grainrefined, and modified/grain-refined alloys

Hindered diffusion of spherical macromolecules through dilute fibrous media

Abstract: Results are presented for the effect of solute–fiber hydrodynamic interactions on the hindered diffusion of a spherical macromolecule in random media comprised of cylindrical fibers Hydrodynamic interactions are calculated by representing the sphere as a collection of point singularities and accounting for the fibers by using a numerical version of slender‐body theory Electrostatic and other nonhydrodynamic interactions are neglected The calculations show that the hydrodynamic mobility of the solute decreases in an exponential‐like fashion as the fiber volume fraction is increased Also, at a given volume fraction, a medium of thinner fibers hinders solute transport more than a medium of thicker fibers The results compare well with experimental data, both for protein diffusion in solutions of the polysaccharide Dextran and for protein diffusion in cross‐linked agarose gels

Particle‐Inhibited Grain Growth in AI2O3‐SiC: I, Experimental Results

Abstract: An extensive experimental study has been conducted to examine the influence of ultraflne SiC particles on grain growth in A12O3. Grain growth has been studied as a function of annealing time, particle volume fraction, and annealing temperature. The SiC particles reduce the grain-growth rate of A12O3 by >3 orders of magnitude, resulting in final grain sizes that decrease with increasing particle volume fraction. The fraction of SiC particles on grain boundaries, φ, has been measured as a function of annealing time, particle volume fraction, and annealing temperature. φ decreases during grain-growth anneals, implying that grain boundaries are able to break free from particles and this is true to a greater extent for smaller volume fractions of particles. Further, it has been discovered that φ and average grain size, G, are strongly correlated, such that any increase in G results in a predictable decrease in pH. This new finding has revealed a path for microstructure development in this system that proves to be critical in the equilibrium and kinetic analyses described in a companion paper.

Structure and pertinent length scale of a discotic clay gel.

Abstract: A characteristic length scale of the structure of a thixotropic colloidal clay suspension of discotic texture was demonstrated by static light scattering. The length scale, of the order of a micron, corresponds to a fractal dimension that increases from 1 to 1.8 close to a critical volume fraction. Simultaneously, the change in yield stress with the volume fraction was studied by the vane method. A scaling law enabled these fractal dimensions to be correlated with the effect of the volume fraction on the yield stress.

Collisional viscosity of FCC particles in a CFB

Abstract: Particle velocity distributions were measured for a flow of 75-{micro}m FCC (fluidized catalytic cracking) particles in a CFB (circulating fluidized bed) using a new video-digital camera technique. From the spread of particle histograms, a random oscillating particle velocity was determined. This random velocity was used to compute the powder viscosity with the help of dense-phase kinetic theory of granular flow. There was excellent agreement between this kinetic-theory measurement and previous macroscopic viscosity measurements. The dimensionless group kinematic viscosity divided by the oscillating velocity and particle diameter correlated all available data as a function of particle volume fraction.

Effects of SiCp size and volume fraction on the high cycle fatigue behavior of AZ91D magnesium alloy composites

Abstract: High cycle fatigue tests (i.e., stress-controlled, axial) were conducted on monolithic AZ91D and AZ91D magnesinm alloy composites processed via squeeze casting and extrusion to contain either 15 gm or 52 gm size SiC particles, at both the 20% and 25% volume fraction reinforcement level. The effects of changes in SiC particle size and volume fraction on the high cycle fatigue behavior have been determined. In addition, the number of cracked particles on the fatigue fracture surfaces, as well as the level of damage beneath the fatigue fracture surfaces were quantified in order to determine the effects of particle size on the evolution of damage during fatigue and during overload failure. Commercial purity Mg specimens containing a large grain size were also tested in fatigue for comparison with the alloy and composite data.

Influence of reinforcement volume fraction and size on the microstructure and abrasion wear resistance of hot isostatic pressed white iron matrix composites

Abstract: The changes in the microstructure and wear resistance of a powder metallurgical high-Cr white iron after the incorporation of TiC particles were studied in the present work. Various reinforcement volume fractions and sizes were used in order to examine their influence on the three-body abrasion wear resistance. The experiments were carried out at three different austenitizing temperatures. The most important observation after a microstructural examination was the increased amount of martensite in the composites subjected to identical heat treatment procedures with the unreinforced alloy. The austenite-to-martensite transformation in the composites increased with the TiC volume fraction and with the austenitizing temperature. This indicates that the two parameters have a key role in the transformation mechanism, which seems to be mechanically induced. The increasing of martensitic transformation with the TiC content in the composites enhanced continuously the supporting ability of the iron alloy matrix to the TiC particles, which in turn increased the wear resistance of the composites. The abrasion wear resistance increased with the TiC volume fraction until the onset of spalling. However, in composites containing coarse reinforcements, spalling occurred earlier in the wear process. This decreased wear resistance significantly because spalled TiC particles contributed additionally to wear.

Computational modeling of gas/particle flow in a riser

Abstract: Axial solid velocity, solid volume fraction, and solid shear viscosity were computed in the riser of a circulating fluidized-bed reactor using a two-phase 2-D computational fluid dynamic model. The time-averaged model predictions agree well with the experimental data of Miller and Gidaspow (1992). The model predicts a core-annulus flow in the riser, similar to that found experimentally. The maximum velocity in the core agrees well with the measurements, but the downflow in the annulus is somewhat overpredicted. The solid volume fractions profiles agree well in both core and annulus, with discrepancy in the core at the level close to the inlet. The radial profile of solid shear viscosity computed by the turbulent kinetic energy model is ten times lower in the core than that found experimentally, but with a linear function of solid volume fraction in the measurement, the computed profile agrees well with experiments.

Fatigue of rubber-modified epoxies: effect of particle size and volume fraction

Abstract: A change in crack-tip plastic zone/rubber particle interactions induces a transition in the fatigue crack propagation (FCP) behaviour of rubber-modified epoxy polymers. The transition occurs at a specific K level, K T, which corresponds to the condition where the size of the plastic zone is of the order of the size of the rubber particles. At ΔK>ΔK T, rubber-modified epoxies exhibit improved FCP resistance compared to the unmodified epoxy. This is because the size of the plastic zone becomes large compared to the size of the rubber particles and, consequently, rubber cavitation/shear banding and plastic void growth mechanisms become active. At ΔK>ΔK T, both neat and rubber-modified epoxies exhibit similar FCP resistance because the plastic zone size is smaller than the size of the rubber particles and hence, the rubber cavitation/shear banding and plastic void growth mechanisms are not operating. As a result of these interactions, the use of smaller 0.2 μm rubber particles in place of 1.5 μm rubber particles results in about one order of magnitude improvement in FCP resistance of the rubber-modified system, particularly near the threshold regime. Such mechanistic understanding of FCP behaviour was employed to model the FCP behaviour of rubber-modified epoxies. It is shown that the near threshold FCP behaviour is affected by the rubber particle size and blend morphology but not by the volume fraction of the modifiers. On the other hand, the slope of the Paris-Erdogan power law depends on the volume fraction of the modifiers and not on the particle size or blend morphology.

Shear modulus and yield stress measurements of attractive alumina particle networks in aqueous slurries

Abstract: The shear modulus and yield stress of attractive alumina particle networks in aqueous slurries was determined as a function of volume fraction (01 to 05), pH (2, 4, 5, 6, and 9), and salt (NH4l) concentration (025M to 234) using both vane and couette rheological tools Consistent with previous observations concerning the relative strength of attractive particle networks, the shear modulus increased to a plateau value with salt concentration In this work we have shown that the salt concentration at which this plateau value is achieved is a function of the pH, and thus, the surface charge density The values of the shear modulus [G′], yield stress [τy], and yield strain [γy] of the attractive networks can be described with power law functions for particle volume fraction [φ] (G′∝φ475, τy∝φ36, and γy∝φ−11) and salt concentration [c] (G′∝ [c]20, τ, ∝ [c]115, and γy∝ [c]−085)

The effect of particle distribution on damage formation in particulate reinforced metal matrix composites deformed in compression

Abstract: Image analysis results are reported on the generation of damage in particulate reinforced metal matrix composites during compressive deformation. The technique allows the automated collection of data on the incidence of particle fracture and void formation in the matrix as a function of important microstructural parameters such as local particle volume fraction and particle size. There is a strong relationship between damage and the local volume fraction of the reinforcement proving that damage formation is accentuated in regions of particle clustering. With the SiC reinforced materials examined, there was observed to be a change in dominance of damage mechanism from particle fracture at low local volume fractions to void formation in the matrix within strongly clustered regions. The results are compared with finite element (FE) modelling of the compressive deformation of clustered particles using a simple cluster of equi-spaced particles. The FE results suggest that plastic flow is generally inhibited in clustered regions. In certain highly clustered configurations shielding is such that flow does not occur in the heart of the cluster even at high levels of average plastic strain. The modelling suggests that the change in dominance of damage mechanism is related to the dramatic increase in tensile hydrostatic stresses in the matrix with higher levels of particle clustering.

Phase separation in aqueous solutions of lens gamma-crystallins: special role of gamma s.

Abstract: We have studied liquid-liquid phase separation in aqueous ternary solutions of calf lens gamma-crystallin proteins. Specifically, we have examined two ternary systems containing gamma s--namely, gamma IVa with gamma s in water and gamma II with gamma s in water. For each system, the phase-separation temperatures (Tph (phi)) alpha as a function of the overall protein volume fraction phi at various fixed compositions alpha (the "cloud-point curves") were measured. For the gamma IVa, gamma s, and water ternary solution, a binodal curve composed of pairs of coexisting points, (phi I, alpha 1) and (phi II, alpha II), at a fixed temperature (20 degrees C) was also determined. We observe that on the cloud-point curve the critical point is at a higher volume fraction than the maximum phase-separation temperature point. We also find that typically the difference in composition between the coexisting phases is at least as significant as the difference in volume fraction. We show that the asymmetric shape of the cloud-point curve is a consequence of this significant composition difference. Our observation that the phase-separation temperature of the mixtures in the high volume fraction region is strongly suppressed suggests that gamma s-crystallin may play an important role in maintaining the transparency of the lens.

Computer simulation of rheological phenomena in dense colloidal suspensions with dissipative particle dynamics

Abstract: The rheological properties of colloidal suspensions of spheres and rods have been studied using dissipative particle dynamics (DPD). We have measured the viscosity as a function of shear rate and volume fraction of the suspended particles. The viscosity of a 30 vol% suspension of spheres displays characteristic shear-thinning behaviour as a function of shear rate. The values for the low- and high-shear viscosity are in good agreement with experimental data. For higher particulate densities, good results are obtained for the high-shear viscosity, although the viscosity at low shear rates shows a dependence on the size of the suspended spheres. Dilute suspensions of rods show an intrinsic viscosity which is in excellent agreement with theoretical results. For concentrated rod suspensions, the viscosity increases with the third power of the volume fraction. We find the same scaling behaviour as Doi and Edwards for the semidilute regime, although the explanation is unclear. The DPD simulation technique therefore emerges as a useful tool for studying the rheology of particulate suspensions.

Rheological Properties of Concentrated, Nonaqueous Silicon Nitride Suspensions

Abstract: The rheological properties of nonaqueous silicon nitride powder suspensions have been investigated using steady shear and viscoelastic measurements. The polymeric dispersant, Hypermer KD-3, adsorbed strongly on the powder surfaces, and colloidally stable, fluid suspensions up to a volume fraction of {Phi} = 0.50 could be prepared. The concentrated suspensions all displayed a shear thinning behavior which could be modeled using the high shear form of the Cross equation. The viscoelastic response at high concentrations was dominated by particle interactions, probably due to interpenetration of the adsorbed polymer layers, and a thickness of the adsorbed Hypermer KD-3 layer, {Delta} {approx} 10 nm, was estimated. The volume fraction dependences of the high shear viscosity of three different silicon nitride powders were compared and the differences, analyzed by using a modified Krieger-Dougherty model, were related to effective volume effects and the physical characteristics of the powders. The significantly lower maximum volume fraction, {Phi}{sub m} = 0.47, of the SN E-10 powder was referred to the narrow particle size distribution and the possibility of an unfavorable particle morphology.

Monte Carlo Simulations of Conductivity of Composite Electrodes for Solid Oxide Fuel Cells

Abstract: Composite electrodes for solid oxide fuel cells were modeled by three-dimensional resistor networks. The networks were generated on a computer by identifying neighbors in either cubic lattices randomly occupied by electrolyte or electrode particles, or in random packings generated by sequential deposition of such particles in random order. The resistance between any two particle sites i and j were taken to be functions of the conductivities of the particles residing in site i and j and the periphery of the necks formed between them. An emphasis was put on parameters believed to be relevant for cermets of Ni and yttria-stabilized zirconia. The conductivity of the networks were calculated numerically, and the results of the model are in good agreement with experimental findings. A sharp transition from low to high conductivity occurs at approximately 30 volume percent of electrode material for cubic lattices and for the random packings with uniform particle radii. In the bimodal random packings, this percolation threshold increases with increasing electrode-particle radius relative to the electrolyte-particle radius. This is suggested as a possible explanation for cermet deactivation under operation, since upon aggregation of electrode particles the percolation threshold may increase past the given volume fraction of electrode materialmore » in the composite.« less

Thermodynamics of tetragonal zirconia formation in a nanolaminate film

Abstract: Zirconia–alumina transformation‐toughening nanolaminates were fabricated by reactive sputter deposition. The average crystallite size and volume fraction of each zirconia polymorph were determined by x‐ray diffraction. The volume fraction of tetragonal zirconia, the phase necessary for transformation toughening, was found to strongly depend upon the zirconia layer thickness. An end‐point thermodynamics model involving hemispherical cap zirconia crystallites was developed to explain this phenomenon. In excellent agreement with experimental results, the model predicts that unity volume fraction of tetragonal zirconia is produced in the nanolaminate when the zirconia layer thickness is less than the radius at which a growing zirconia crystallite spontaneously transforms to the monoclinic phase.