Showing papers on "Volume fraction published in 1997"

Piezoelectric Fiber Composites with Interdigitated Electrodes

Abstract: Piezoelectric Fiber Composites were previously introduced as an alternative to monolithic piezoceramic wafers for structural actuation applications. This manuscript was an investigation into the improvement of piezoelectric fiber composite performance through a nonconventional electroding scheme. Two microelectromechanical models were developed that predict the composite properties. These models were used to examine the trends of composite properties versus fiber volume fraction for various constituent materials. Several etched electrode PZT fiber composites, with fiber volume fractions ranging from 7% to 58%, were manufactured and tested. Experimental measurements showed excellent agreement with both the trends and magnitude of model predicted values. The maximum fiber volume fraction composites demonstrated a capacitance (E3 /Eo) of 550, piezoelectric free strain constants (d33 and d31) of 150 pm/V and-70 pm/V, and piezoelectric clamped stress (e33) of 5 C/m2, showing a substantial improvement over prev...

Viscoelastic properties of colloidal gels

Abstract: The microstructure and scaling of mechanical properties of dense colloidal gels were investigated as a function of volume fraction and strength of interparticle attraction. Gels were reversibly formed by cooling suspensions of octadecyl silica particles in decalin or tetradecane. Shear history independence of mechanical properties was ensured by preshearing the suspensions in the gelled state. Gelation resulted in suspensions with apparent fractal dimensions of 1.4. Shear densification resulted in an apparent fractal dimension of 2.5 for structures containing many particles. When the gelled suspension was presheared, just after the shear rate was set to zero, elastic moduli were small. Over time, the moduli recovered to a time independent value, G∞′, at a rate α. When measured over a wide range of volume fraction, φ, and strength of interparticle attraction, G∞′, α, and the strain limiting the extent of linear response to oscillatory deformations, γM, fell onto master curves when plotted as a function of ...

Effect of hydrogen dilution on the structure of amorphous silicon alloys

Abstract: We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.

Yield stress thixotropic clay suspension : investigations of structure by light, neutron, and x-ray scattering

Abstract: The characteristic length scales of the structure and fractal behavior of a thixotropic colloidal suspension of synthetic clay were studied by using a combination of small-angle neutron and x-ray scattering and static light scattering. At the same time, macroscopic mechanical behavior at rest was characterized by means of rheometric measurements. Two characteristic length scales were detected in these yield stress suspensions of discotic texture. The first, measuring several tens of nanometres, is linked to a fractal dimension of 3. The second, of the order of 1 mm, is linked to a fractal behavior of dimension D that increases with the particle volume fraction. Consequently, it is suggested that the structure of the dispersions at rest is composed of subunits measuring a few tens of nanometers that combine to form dense aggregates measuring about 1 mm. At larger length scales, these micrometer-sized aggregates are rearranged to form a continuous three-dimensional isotropic structure that has a fractal behavior of dimension D, which gives the gels their texture. The increase of this fractal dimension with the particle volume fraction, the ionic strength, and the gelation time is correlated to a hardening of the mechanical properties of the gels at rest. The gel state is reached above a volume fraction f v for a given ionic strength and gelation time. In the gel phase, a critical volume fraction f vc separates two domains. Gels belonging to the domain f v,f v,f vc have a fractal behavior of dimension D5160.05, suggesting an alignment of the micrometer-sized aggregates that leads to the formation of a mechanically weak fibrous structure. Gels belonging f v.f vc have a fractal dimension D51.860.01, corresponding to a mechanically stronger structure consisting of zones of high and lower particle density. A scaling law enabled these fractal dimensions to be correlated with the effect of the volume fraction on the yield stress. In contrast to what is commonly assumed in relation to clay suspensions, it is suggested here that it is the large length scales, of the order of 1 mm, associated with a fractal arrangement that governs the macroscopic mechanical behavior.

Crystallization kinetics of suspensions of hard colloidal spheres

Abstract: The crystallization of suspensions of sterically stabilized polymer particles, with hard-sphere-like interactions, is studied by laser light scattering. Over the range of volume fractions examined, from just below melting to the glass transition, crystallization occurs by homogeneous nucleation. After the suspensions are shear melted, the intensity, position, and width of the main interlayer Bragg reflection are measured as functions of time. From these the amount of crystal, the average linear crystal dimension, the number of crystals, and the volume fraction of the crystal phase are obtained. No assumptions are made concerning the functional time dependence of nucleation or growth processes. Below the melting concentration the observed crystallization process is compatible with the classical picture of sequential nucleation and growth of isolated crystals. However, when the melting concentration is exceeded, nucleation events are correlated, nucleation is accelerated, and high nucleation rate densities suppress crystal growth. Above the melting concentration we infer, with the aid of the equations of state for the hard-sphere fluid and solid, that the first identifiable crystals are in mechanical equilibrium with the embedding fluid and, consequently, strongly compressed by it. Ensuing nucleation lags expansion of the crystal lattice.

Shear and compressive rheology of aggregated alumina suspensions

Abstract: The shear and compressive properties of aggregated alumina particles are determined as functions of volume fraction and the strength of the interparticle attraction. Over a range of volume fractions, yield stresses, τy, elastic moduli, the strain delimiting the extent of the linear elastic response, and compressive yield stress, Py, are well described by power-law functions of volume fraction, while the role of interparticle attractions can be accounted for by expressing these mechanical properties as (ϕ/ϕg − 1)n, where ϕg captures the strength of particle attractions and n the microstructure. The links between compressive and shear properties are well described by linear elastic models where the Py and τy are a function of Poisson's ratio which, for the suspensions investigated, has a value near 0.49.

Soot volume fraction and particle size measurements with laser-induced incandescence.

Abstract: Laser-induced incandescence from soot was analyzed with a time-dependent, numerical model of particle heating and cooling processes that includes spatial and temporal intensity profiles associated with laser sheet illumination. For volume fraction measurements, substantial errors result primarily from changes in gas temperature and primary soot particle size. The errors can be reduced with the proper choice of detection wavelength, prompt gating, and high laser intensities. Two techniques for primary particle size measurements, based on ratios of laser-induced incandescence signals from a single laser pulse, were also examined. Compared with the ratio of two integration times, the newly proposed ratio of two detection wavelengths is better suited for simultaneous volume fraction and size measurements, because it is less temperature sensitive and produces stronger signals with, however, a lower sensitivity to size changes.

Effects of lamellar spacing, volume fraction and grain size on creep strength of fully lamellar TiAl alloys

Abstract: Lamellar spacing, volume fraction of constituent phases and grain size are the major structural factors to be controlled in alloy design of fully lamellar TiAl alloys. Creep tests were carried out on binary TiAl alloys with fully lamellar structure, to elucidate effects of the three structural factors on their creep strength. Lamellar alloys give better creep strength than the constituent single phases at higher stresses, due to their higher yield stress. Creep rate of lamellar alloys decreases with decreasing lamellar spacing, suggesting that a fine lamellar structure is effective in improving creep strength. The decrease is remarkable at high stresses. At low stresses however, the difference in creep rate diminishes, due to the increasing contribution of dynamic recrystallization and interface sliding. Creep rate is insensitive to volume fraction of the constituent phases and aluminum concentration is free in alloy design of TiAl alloys as far as creep strength is concerned. Creep rate is also independent of grain size d when d>100 μm. Because of easy dynamic recrystallization in the lamellar alloys, creep rate may increase when d

Dielectric and microstructure properties of polymer carbon black composites

Abstract: Dielectric and physicochemical properties of a composite material prepared by incorporating carbon black particles into a polymer matrix were investigated. Two types of carbon blacks, having very different structures of aggregates, were used. The volume fraction of the carbon blacks ranged from 0.2% to 7%, i.e. below and above the percolation threshold concentration observed from the measurements of dc conductivity. The composite samples were characterized in terms of: swelling by a compatible solvent, electron paramagnetic resonance (EPR) response, and frequency variation of permittivity. First, the article attempts to evaluate the diffusion coefficient of an appropriate solvent in these materials. Sorption kinetics experiments with toluene indicate that the initial uptake of solvent exhibits a square root dependence in time as a consequence of Fick’s law and permit to evaluate the effective diffusion coefficient in the range 10−11–10−12 m2 s−1 depending on the volume fraction of the carbon black in the ...

Formation of a Silicate L3 Phase with Continuously Adjustable Pore Sizes

Abstract: The lyotropic L3 phase was used as a template to form nanoporous monolithic silicates with continuously adjustable pore sizes. The monolith was optically isotropic and transparent with a nonperiodic network. The pore size was adjusted by a change in the solvent volume fraction rather than by a change of the surfactant. Unlike other silicates, the bicontinuous pores were water-filled; removal of surfactant was not necessary to access the pores. Measured characteristic dimensions were from six to more than 35 nanometers. For a given solvent fraction, x-ray scattering indicated little variation of pore widths, in marked contrast to the polydisperse pores of aerogels.

Investigation of draping and its effects on the mold filling process during manufacturing of a compound curved composite part

Abstract: Resin Transfer Molding (RTM) is a composite material manufacturing process during which resin is injected into a mold cavity filled with a fibrous reinforcing preform. Application of woven and stitched fiber mats in fabricating preforms for RTM is a highly viable means of manufacturing affordable composites. Draping, in this paper, refers to the act of bringing a flat workpiece into contact with an arbitrary tool surface. As a result, the draping of woven and stitched mats tends to cause the mat to deform to the tool geometry. This paper details an experimental study designed to demonstrate the effect of draping preforms on mold filling, and final structure of the composite product. A mold was built providing a conical mold cavity. Flow visualization experiments were performed, as well as shearing angle and preform fiber volume fraction measurements from manufactured cone parts. Experimental results show that draping does significantly change fiber orientation, fiber volume fraction, permeability and hence the injection pressure or flow rate required to fill a mold. This data was compared with numerical prediction codes for preform deformation, permeability calculation, and mold filling. Preform deformation predictions were found to overpredict in areas of high deformation, due to the assumptions made by the numerical algorithm used. Permeability components calculated are reasonable, but are affected by overpredicted preform deformation, and interpolation methods used. Prediction of injection pressure and flow rate histories are good, while predicted flow front shapes fail to capture some experimental features. Difficulties in predicting flow front shapes are attributed to three mechanisms; a decrease in permeability magnitudes due to increased volume fraction, a smaller volume of fluid required due to increased volume fraction, and reorientation of fiber tows changing the direction of principle permeability components.

Tunneling giant magnetoresistance in heterogeneous F e − S i O 2 granular films

Abstract: Magnetic and transport properties are examined for heterogeneous ${\mathrm{F}\mathrm{e}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ granular films prepared on glass substrates by rf sputtering. The superparamagnetic nature and the tunneling giant magnetoresistance are observed in the films having Fe volume fractions smaller than 45%. The magnetization and magnetoresistance (MR) curves are systematically analyzed by the summation of two Langevin functions. The dependence of the MR ratio on the Fe volume fraction is explained phenomenologically by the distance between the nearest-neighbor grain surfaces. The temperature dependence of the MR ratio is also discussed.

Viscoelastic material functions of noncolloidal suspensions with spherical particles

Abstract: We have characterized various steady and time-dependent material functions of suspensions of a non-Newtonian binder, poly(dimethyl siloxane), incorporated with 10%–60% by volume of hollow and spherical glass beads The material functions included storage and the loss moduli, shear stress and first normal stress difference growth and relaxation, relaxation modulus upon step strain and creep and recovery behavior Both constant shear stress and shear rate experiments were carried out using multiple rheometers over a broad temperature range (−35 to 40 °C) while following sample fracture and wall slip effects With increasing volume fraction, φ, of the noncolloidal particles, the strain range, over which linear viscoelastic behavior is observed, became narrower and the relaxation time of the suspension increased Increasing solid content gave rise to the development of the yield stress and the dependence of large amplitude oscillatory shear properties on time and deformation history The yield stress values i

Mathematical Models of Flavor Release from Liquid Emulsions

Abstract: The penetration theory of interfacial mass transfer was used to model flavor release from liquid emulsions. The model was used to predict the rates of release and partitioning properties of two volatiles, one hydrophilic (diacetyl) and the other hydrophobic (heptan-2-one), as a function of the oil volume fraction. In general the initial rates of release were faster for emulsions of lower oil content, whereas the equilibrium concentrations depended on the nature of the flavor compound and the volume fraction of oil in the emulsion. Experimental in vitro results suggested that the rate limiting step for flavor release was the resistance to mass transport across the emulsion-gas interface.

Cementing slurry and method of designing a formulation

Abstract: The present invention provides a cementing composition for an oil or analogous well, essentially constituted by a solid fraction suspended in a liquid medium The solid fraction is constituted by a coarse fraction with a packing volume fraction Φ 1 which, according to the invention, is maximised, and a fines fraction x with a packing volume fraction Φ 2 which is less than or equal to the volume fraction x 0 such that ##EQU1## The invention also provides a method of maximising the value of Φ 1 The invention can minimise fluid loss whatever the materials selected for a cementing composition

Structure characteristics of nanocrystalline element selenium with different grain sizes

Abstract: Porosity-free nanocrystalline (nc) element Se samples with mean grain sizes ranging from 13 to 70 nm were synthesized by crystallizing a melt-quenched amorphous Se solid. Microstructures of the nc-Se (with a hcp structure) samples were characterized by means of quantitative x-ray-diffraction measurements. The Bragg reflection-and the background intensities, as well as the reflection shape of the x-ray-diffraction patterns for the ne Se were analyzed according to data fitting of Me measurement results. The grain-size dependencies of the microstrain, lattice parameters, unit-cell volume, and the mean Debye-Waller parameter were determined. With a reduction of grain size, the microstrain increases significantly along [100] direction but decreases along [104] direction, and exhibits an increasing anisotropic microstrain behavior. The lattice parameter a was found to increase evidently while c decreased slightly with a decreasing gain size, resulting in a significant lattice distortion vith a dilated unit-cell volume. it agrees with the observation that the mean Debye-Waller parameter increases with a reduction of grain size, suggesting larger displacements of atoms from their ideal lattice sites in the nc-Se samples with smaller grains. The similarity of the grain-size dependencies of these structural parameters as that of the grain-boundary volume fraction implies that the intrinsic microstructure feature of ne materials is closely related to the crystallite dimension and the amount of grain boundaries.

Predicting filtration time and maximizing throughput in a pressure filter

Abstract: The modeling of pressure filtration of flocculated suspensions using compressional rheology and a knowledge of compressional yield stress P y (Φ) and a hydraulic resistance factor r(Φ) (Φ is the local volume fraction of solids) is shown to yield an initial solids loading that maximizes the throughput of the filter. The optimal initial height h 0 is such that the filtration time to reach a specified average volume fraction as output equals the handling time for the filter press. The maximum throughput of the press is then examined as a function of the remaining control parameters, the initial solids volume fraction Φ 0 , and the applied piston pressure ΔP. The dependence of filtration time on Φ 0 /Φ∞ (where Φ∞ is the volume fraction of solids at infinite time under applied pressure ΔP) enables the construction of a simple numerical model for the pressure filtration process, which accurately approximates predictions of the full compressional rheology model.

The rheology of charge stabilized silica suspensions

Abstract: The flow properties of aqueous suspensions of silica particles with diameters ranging from 117 to 780 nm were measured as a function of volume fraction, ionic strength, and continuous phase composition. Dense, aqueous suspensions of all sizes displayed similar behavior with a yield stress in the limit of low shear rate, shear thinning as the stress was raised and, above a critical volume fraction, shear thickening. Master flow curves which are weakly dependent on particle size, volume fraction, surface potential, magnitude of the decay length for the electrostatic forces, or particle size distribution are produced when stress is scaled on the suspension’s elastic modulus, G, and shear rate on G/ηc. Here ηc is the continuous phase viscosity. For suspensions which ordered at rest and of sufficiently high volume faction, shear thickening occurred as a discontinuous decrease in shear rate as the stress increased. This behavior was not observed in suspensions altered so that no order occurred at rest. When the...

Collisional particle pressure measurements in solid liquid flows

Abstract: Experiments were conducted to measure the collisional particle pressure in both cocurrent and countercurrent flows of liquid-solid mixtures. The collisional particle pressure, or granular pressure, is the additional pressure exerted on the containing walls of a particulate system due to the particle collisions. The present experiments involve both a liquid-fluidized bed using glass, plastic or steel spheres and a vertical gravity-driven flow using glass spheres. The particle pressure was measured using a high-frequency-response flush-mounted pressure transducer. Detailed recordings were made of many different particle collisions with the active face of this transducer. The solids fraction of the flowing mixtures was measured using an impedance volume fraction meter. Results show that the magnitude of the measured particle pressure increases from low concentrations (>10% solid volume fraction), reaches a maximum for intermediate values of solid fraction (30-40%), and decreases again for more concentrated mixtures (>40%). The measured collisional particle pressure appears to scale with the particle dynamic pressure based on the particle density and terminal velocity. Results were obtained and compared for a range of particle sizes, as well as for two different test section diameters. In addition, a detailed analysis of the collisions was performed that included the probability density functions for the collisoin duration and collision impulse. Two distinct contributions to the collisional particle pressure were identified: one contribution from direct contact of particles with the pressure transducer, and the second one resulting from particle collisions in the bulk that are transmitted through the liquid to the pressure transducer.

Mechanical properties of pva fiber reinforced cement composites fabricated by extrusion processing

Abstract: Discontinuous polyvinyl alcohol (PVA) fiber reinforced cement matrix composites fabricated by an extrusion process were investigated. The extruded composites have exhibited a postpeak strain hardening type of response with an enhanced composite strength. Tensile strength of extruded composites was dependent on the fiber fraction used, while the flexural strength and deflection at the peak load were influenced by both the fiber fraction and the matrix composition. Larger fiber volume fraction, longer fiber length, and higher cement content lead to a higher flexural strength, a larger deflection at peak, and a higher elastic modulus. Fracture process was studied using laser Moire interferometry and scanning electron microscopy. It was observed that sequential multiple cracking was associated with the strain hardening type of response and that the spatial distribution of fibers can control the sequence of multiple cracking. Interface between fiber and matrix was also studied using continuous, aligned fibers and with the help of Moire interferometry.

Percolation threshold of carbon black-polyethylene composites

Abstract: In this paper, the percolation threshold of the carbon black-resin composites is discussed based on the experimental results of the changes in resistivity and relative permittivity for loading carbon black, the electric field dependence of current, and the critical exponent of conductivity. It was found that a formation of infinite clusters is interrupted by a tunneling gap in the volume fraction region of carbon black loading where the change in resistivity is extremely large. It was also found that the critical exponent of conductivity for the universal law of conductivity is satisfied if the percolation threshold is estimated at the volume fraction of carbon black where a nonohmic current behavior changes into an ohmic one. It is concluded that the percolation threshold should be defined at this volume fraction of carbon black.

Synthesis of nanosized silver particles in ion-exchanged glass by electron beam irradiation

Abstract: Ag particles of 4.2 nm mean diameter have been formed inside a glass matrix, doped with silver by ion exchange, by electron beam irradiation of the glass cut into thin slices by ultramicrotomy. By this treatment, a high concentration of particles which are homogeneously arranged throughout the glass and exhibit a narrow size distribution is achieved (volume fraction of particulate silver: 3.5×10−2). The interface stress reflecting the particle/matrix interaction is comparable to that of isolated Ag particles. This new route of synthesis will allow to generate materials with strong third order nonlinear susceptibility.

Low‐Temperature Sintering and High‐Strength Pb(Zr,Ti)O3‐Matrix Composites Incorporating Silver Particles

Abstract: Lead zirconate titanate based composites containing silver particles were fabricated from commercially available PZT and Ag2O powder mixtures. Densification behavior and mechanical and dielectric properties were evaluated as a function of the volume fraction of silver. No unwanted reaction phases between the PZT matrix and silver could be detected in the X-ray diffraction analysis. The added silver particles were homogeneously dispersed in the matrix. The densification of PZT was substantially accelerated by the addition of silver particles. Mechanical properties, e.g., fracture toughness and fracture strength, were improved by the addition of silver particles. The relative dielectric constant also increased when the volume fraction of silver was increased. This observed enhancement in dielectric constant may be associated with the effective dielectric field developed by the existence of conducting silver particles.