Showing papers on "Volume fraction published in 1988"

Experimental determination of the nanocrystalline volume fraction in silicon thin films from Raman spectroscopy

Abstract: The effect of crystallite sizes L smaller than 100 nm on the integrated Raman cross section Σc of the transverse optical (TO) mode of fcc silicon was studied experimentally in fully nanocrystallized thin films. The Σc/Σa (amorphous) ratio of this mode is shown to be 1 up to L=30 A, and to decay exponentially down to 0.1 at larger L. A systematic procedure taking into account both this effect and the experimental optical absorption coefficient αexp at the excitation wavelength is then proposed for the determination of the crystalline volume fraction in mixed phase (amorphous/nanocrystalline) silicon systems by Raman measurements.

Void growth and failure in notched bars

Abstract: Void growth and ductile failure in the nonuniform multiaxial stress fields of notched bars are studied numerically and experimentally. U-notched bars with different notch acuities are made from partially consolidated and sintered iron powder compacts with various residual porosities. The materials are modelled using an elastic-viscoplastic constitutive relation that accounts for strength degradation resulting from the growth of microvoids. The matrix stress-strain relation and the initial void volume fractions used in the calculations are determined experimentally. The remaining parameters in the constitutive equations are evaluated from micromechanical models. Comparisons of the calculations with experimental results indicate that the constitutive model can provide good estimates of the evolution of the void volume fraction and of the strength reduction induced by void growth under a variety of nonuniform stress histories.

Effects of sic whiskers and particles on precipitation in aluminum matrix composites

Abstract: The age-hardening precipitation reactions in aluminum matrix composites reinforced with discontinuous SiC were studied using a calorimetric technique. Composites fabricated with 2124, 2219, 6061, and 7475 alloy matrices were obtained from commercial sources along with unreinforced control materials fabricated in a similar manner. The 7475 materials were made by a casting process while the others were made by powder metallurgy: the SiC reinforcement was in the form of whiskers or particulate. It was found that the overall age-hardening sequence of the alloy was not changed by the addition of SiC, but that the volume fractions of various phases and the precipitation kinetics were substantially modified. Precipitation and dissolution kinetics were generally accelerated. A substantial portion of this acceleration was found to be due to the powder metallurgy process employed to make the composites, but the formation kinetics of some particular precipitate phases were also strongly affected by the presence of SiC. It was observed that the volume fraction of GP zones able to form in the SiC containing materials was significantly reduced. The presence of SiC particles also caused normally quench insensitive materials such as 6061 to become quench sensitive. The microstructural origins of these effects are discussed.

Preparation, structure, and properties of two‐phase co‐continuous polymer blends

Abstract: Phase continuity has been explored as a function of composition for three two-phase polymer blends produced by mixing in the melt: polystyrene/poly(methyl methacrylate), polystyrene/cis-polybutadiene, and poly(methyl methacrylate)/ethylene-propylene rubber. The condition for dual phase continuity Is the application of shear close to phase inversion and this can be predicted fairly accurately using the relation where 1 and 2 are the blend components, η is viscosity, is the shear rate in the mixing device used to produce the blend and ϕ is volume fraction. The co-continuous materials, which we call Interpenetrating polymer blends (IPBs) are non-equilibrium structures and are subject to disruption by changes in flow regime.

The analysis of the electrical conductivity of graphite conductivity of graphite powders during compaction

Abstract: The conductivity of four graphite powders was measured as a function of the volume fraction of graphite (or air) along (axial) and across (transverse) the direction of compression. The conductivity varies by between two and four orders of magnitude during these measurements. The results are shown to fit an equation which expresses the conductivity of the mixture as a function of the conductivity of the graphite and two morphology parameters. One parameters is the critical volume fraction, at which the graphite-air mixture would become insulating (percolation threshold) and the other an exponent is a combination of the effective demagnetisation coefficients of the grains and the critical volume fraction. Electron micrographs of the powder grains are shown and volume orientation percentage measurements of the graphite using X-rays were made in order to try to understand the results better.

Ostwald ripening in a system with a high volume fraction of coarsening phase

Abstract: Experiments on the coarsening behavior of two-phase mixtures in a model Pb-Sn system are reported. This system fulfills most of the assumptions of theory and has the particular advantage that all the materials parameters necessary for a comparison between the experimentally measured and theoretically predicted coarsening kinetics are known. We have examined the coarsening of Sn-rich and Pb-rich solid phases in contact with eutectic liquid in the volume fraction solid range above approximately 0.6 where the development of a solid skeletal structure inhibits sedimentation. Particle intercept distributions are measured and found to be time independent when scaled by the average intercept. This invariance is interpreted as evidence that scale factor coarsening is present. The intercept distributions are in good agreement with the predictions of theory. Measurements of average intercept diameter as a function of time establish unambiguously that the coarsening follows the theoretically predictedt 1/3 kinetics. The coarsening rate constants are measured as a function of volume fraction solid and are found to exceed the values calculated from theory using the known thermophysical properties of the Pb-Sn system by factors ranging from approximately 2 to 5.

Soot production in axisymmetric laminar diffusion flames at pressures from one to ten atmospheres

Abstract: Laminar axisymmetric ethylene-air diffusion flames were studied at pressures from 0.1 to 1.0 MPa (1–10 standard atmospheres) in order to gain insight into the processes controlling the evolution of soot from such flames. Measurements of the attenuation of laser light across the flame diameter were used to characterize the volume of soot present in the flame. Data were acquired as a function of axial position above the burner port, allowing independent identification and examination of regions of soot formation and soot oxidation. The peak integrated soot volume fraction fv across the diameter of the flameincreases with pressure p according to ∫ − ∞ + 8 f υ d r ∝ p n where n=1.2±0.1 for pressures between 0.1 and 1.0 MPa. Integral volume fraction measurements begin to fall noticeably below this trend as pressure approaches 1.0 MPa, indicating decreasing dependence of soot volume fraction on pressure at the highest pressures studied. As at atmospheric pressure, the quantity of soot emitted from a particular flame is ultimately determined by a competition between formation and oxidation processes. These experiments show that not only does the formation of soot increase strongly with pressure, but also that soot oxidation removes a decreasing fraction of the soot that is formed as pressure increases. Hence, it is increasingly difficult to prevent the emission of smoke at increasing pressure. The reciprocal of the maximum non-smoking fuel mass flow rate 1 / m ˙ fuel increases with pressure according to 1 / m ˙ fuel ∝ p 1.3

Grain growth in synthetic marbles with added mica and water

Abstract: Evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder, through primary recrystallization and normal grain growth, to a size stabilized by second phases. To form the marbles, reagent grade CaCO3 was mixed with 0, 1 and 5 volume% mica and heat-treated under pressure with added water. Densification with negligible recrystallization occurred within one hour at 500° C and 500 MPa confining pressure. Primary recrystallization occurred at 500–550° C, causing increases of grain size of factors of 2–5. Resulting samples had uniform grain size, gently curved grain boundaries, and near-equilibrium triple junctions; they were used subsequently for normal grain growth studies. Normal grain growth occurred above 550° C; at 800° C, grain size (D) increased from 7 μm (D 0) to 65 μm in 24 hours. Growth rates fit the equation, D n -D 0 =Kt, where K is a constant and n≃2.6. Minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size, D max, which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0.3 and 1. Once D max is reached, normal growth continues only if second phases are mobile or coarsen, or if new driving forces are introduced that cause unpinning of boundaries. Normal grain growth in Solnhofen limestone was significantly slower than in pure synthetic marble, suggesting that migration is also inhibited by second phases in the limestone.

Sintering of TiO2–Al2O3 Composites: A Model Experimental Investigation

Abstract: In recent theoretical work we have shown that the sintering of a composite is strongly affected by the shear deformation of the continuous phase. This phenomenon was studied experimentally in a model system consisting of a TiO2 matrix dispersed with nondensifying agglomerates of Al2O3. The results of this study are reported here. Several interesting results were obtained: (a) The experimentally obtained sintering rate of the composite could be successfully predicted by measuring the concomitant shear and densification rate of the matrix phase in a sinter-forging experiment and using the developed analysis; (b) the densification rate of the composite changed with the volume fraction of the dispersed phase, but was unaffected by the size of the dispersed phase; and (c) processing flaws appeared to form only when the size of the dispersed phase was greater than about 10 μm. A technique for measuring the parameter β, which was used to correlate theory and experiments, is described. The procedures used for preparation of powders (from alkoxides) and the green compacts are also described in detail.

Long-time self-diffusion coefficient and zero-frequency viscosity of dilute suspensions of spherical Brownian particles

Abstract: We study the long‐time self‐diffusion coefficient and the zero‐frequency effective viscosity for a suspension of spherical Brownian particles. The correction to first order in the volume fraction to the self‐diffusion coefficient and the correction to second order in the volume fraction to the effective viscosity are given by integral expressions derived by Batchelor. Using exact series expansions of the hydrodynamic interaction functions in powers of the inverse distance between centers of a pair of particles we obtain accurate values for the correction terms. We consider hard spheres with mixed slip‐stick boundary conditions as well as spherical liquid droplets with a viscosity different from the bulk.

Viscoelastic properties of aqueous concentrated polystyrene latex dispersions containing grafted poly(ethylene oxide) chains

Abstract: The viscoelastic properties of aqueous concentrated polystyrene latex dispersions containing grafted poly(ethylene oxide) (PEO) chains were investigated using oscillatory and steady-state shear stress-shear rate measurements. The relative viscosity-effective volume fraction results were fitted to the Dougherty-Krieger equation for hard spheres by adjusting the value for the adsorbed layer thickness Δ. The latter was found to decrease with an increase in volume fraction of the dispersion and near to close-packing considerable compression of the chains occurred. From the oscillatory measurements, the complex modules G ∗ , storage modules G′, and loss modulus G″ were obtained as a function of frequency at various latex volume fractions. The results showed that the dispersion changes from being more viscous (G″ > G′) to more elastic (G′ > G″) over a narrow range of volume fraction φ of the dispersion, i.e., when φ is increased from 0.465 to 0.5. Within the range of increase of φ, it is likely that the chains undergo some compression and interpenetration of the peripheries. When φ increased significantly above 0.5, the system becomes predominantly elastic and significant interpenetration and compression of the chains occur. Indeed when φ is further increased to 0.585 and 0.62, the moduli increase by several orders of magnitude and so does the dynamic viscosity. Under these conditions, the latex behaves as an elastic “gel.”

Internal Stress Superplasticity in 2024 Al-SiC Whisker Reinforced Composites

Abstract: The isothermal and thermal cycling creep behavior of SiC whisker reinforced aluminum composites was investigated. Compression creep tests were performed for two volume fraction composites (10% SiC ...

Inclusion size and sintering of composite powders

Abstract: Ceramic powders containing dispersed inert particulates often exhibit dramatically lower densification rates than the pure powder. This decrease results from the shrinkage incompatibilities of the matrix and the inclusion during densification. The analyses of this phenomenon all predict that the shrinkage rate is only dependent on the overall volume fraction of the inert second phase, and not on the particle size. Experiments on ZnO containing dispersed SiC particles of different size showed that the retarding effect of the second phase increases significantly with decreasing particle size, at constant SiC volume fraction.

Preparation and Magnetic Properties of Bi1.5Pb0.5Sr2Ca2Cu3Ox Superconductors

Abstract: New high-Tc superconductors were prepared by the partial substitution of Pb for Bi in Bi2Sr2Ca2Cu3Ox. The X-ray diffraction analysis of Bi1.5Pb0.5Sr2Ca2Cu3Ox showed that most of the sintered sample was in the 105 K phase. The lattice parameters were a=5.410 A and c=37.18 A. The transition temperature was 105 K. The lower critical field, Hc1, was 6 Oe at 77 K which was much smaller than that of YBa2Cu3O7-y (52 Oe). The volume fraction of the 105 K phase was estimated to be about 80%.

Validation of mixture equations for dielectric‐magnetic composites

Abstract: Mixture equations relate the effective macroscopic electromagnetic properties (dielectric constant and magnetic permeability) of a composite material to the intrinsic microscopic electromagnetic properties of its constituents. The predictions of mixture equations in popular use generally agree with each other when particles are widely separated (small volume fraction), but begin to diverge when the volume fraction of particles in the composite is greater than 10%. This paper reviews the assumptions and characteristics of different representative mixture equations. The validity of these mixture equations for application to magnetic composites was tested by comparing their predictions of macroscopic electromagnetic properties to VHF and UHF measurements of these properties for a series of well‐characterized dielectric‐magnetic composite samples comprised of ferrite particles in an epoxy binder. The investigation emphasized the parameter regimes where large disparities between the predictions of the various equations exist, namely, large volume fraction of particles, complex dielectric constant and magnetic permeability (lossy material), and large contrast ratio between particle and binder properties. Correlation and lack of correlation between the mixture equation predictions and experimental data are shown.

Particle size dependence of the magnetic properties of ultrafine granular films

Abstract: Magnetic granular solids consist of ultrafine metal granules of nanometer sizes embedded in an insulating medium. The magnetic properties are dictated by the microstructure such as particle size, metal volume fraction, which are process controlled. We report the results of a series of granular Fe60(SiO2)40 films in which the particle size has been systematically varied. The magnetic coercivity, ranging from 500 to 2200 Oe, increases with particle size. The ferromagnetic‐superparamagnetic transition had been studied by SQUID magnetometry and Mossbauer spectroscopy. The value of the magnetic anisotropy energy is found to be much larger than that due to magnetocrystalline anisotropy.

Enhanced magnetic coercivity in magnetic granular solids

Abstract: We have fabricated magnetic granular Fe‐SiO2 solids over a wide metal volume fraction range 15%–100%. The microstructure of the samples has been characterized by using transmission electron spectroscopy, electron diffraction, and Mossbauer spectroscopy. Giant magnetic coercivity, as high as 2500 Oe, has been observed in samples with nanometer particle size. The behaviors of coercivity across the metal volume fraction range are attributed to the change of particle size and percolation effect.

Structured polymer blends

Abstract: The influence of viscosity ratio and volume fraction on the morphology of a blend of two incompatible polymers is shown for the model system polyethylene/polystyrene (PE/PS). An attempt has been made to fixate the morphology by crosslinking the dispersed (PE) phase using electron beam or gamma irradiation. Due to shielding of the PS matrix, the crosslinking in the PE phase is less effective up to now, to achieve complete fixation of the morphology.

Formation of propagation pattern in two-phase flow systems with application to volcanic eruptions

Abstract: SUMMARY The hydrodynamic behaviour of a two-phase system with different densities and viscosities is investigated, assuming that both phases are continuous phases and incompressible and that no phase change occurs. Mass and momentum conservation equations describe the relative motion of the two phases under the gravitational force when the inertia term is not negligible. Using the derived equations the stability of a spatially constant structure along gravity is examined in the one-dimensional case for an infinite system without boundaries, where the basic state is taken as constant velocity and volume fractions. Two special cases are considered: the first is a gas-liquid system where the density of one of the phases is neglected; the second is a deformable solid-liquid system where the densities of the two phases are taken to be the same except for the calculation of the buoyancy force. The linear stability analysis shows that a small perturbation always generates a propagating wave which grows unstably with a characteristic wavelength. Since the spatially uniform structure of a two-phase system is always unstable, the growth pattern should be observed in nature if the growth rate and spatial scale of the system are suitable. The dependence of the wavelength and growth rate on parameters is divided intc two regimes. In one regime the wavelength, which equals the compaction length, and growth rate depend on the typical pore size. In the other regime, on the other hand, they do not depend on the typical pore size but only on the kinematic viscosity of the liquid phase and the volume fraction, given the frictional law. Which regime a real system chooses depends on a nondimensional parameter. The first case (gas-liquid system) is applied to magma effusion in an eruption-style volcano. The system consisting of a silicate liquid phase and a gas phase can have a wavelength from metres to tens of metres and a growth rate from the order of less than a second to 1 week depending on viscosities, typical pore size and volume fraction. As a result, the rhythm and discreteness of magma effusion observed at volcanic eruptions may be explained by the mechanism presented here. The theory provides a relation between the observed period of the magma effusion rate, the mixture exit velocity of gas plus magma and the volume fraction of magma, given the frictional law. Using the relation, the mixture exit velocity is estimated from the observed period for the 1986 Izu-Oshima eruption. The applicability to partially molten systems in the mantle and the core and basic assumptions are discussed.

Waves in liquids with vapour bubbles

Abstract: An investigation of wave processes in liquids with vapour bubbles with interphase heat and mass transfer is presented. A single-velocity two-pressure model is used which takes into account both the liquid radial inertia due to medium volume changes, and the temperature distribution around the bubbles. An analysis of the microscopic fields of physical parameters is aimed at closing the system of equations for averaged characteristics. The original system of differential equations of the model is modified to a form suitable for numerical integration. An elliptic equation is obtained to determine the field of the mixture average pressure at an arbitrary time through the known fields of the remaining quantities. The existence of the steady structure of shock waves, either monotonic or oscillatory, is proved. The effect of the initial conditions, shock strength, volume fraction, and dispersity of the vapour phase and of the thermophysical properties of the phases on shock-wave structure and relaxation time is studied. The influence of nonlinear, dispersion and dissipative effects on the wave evolution is also investigated. The shock adiabat for reflected waves is analysed. The results obtained have proved that the interphase heat and mass transfer determined by the thermal diffusivity of the liquid greatly influences the wave structure. The possible enhancement of disturbances in the region of their initiation is shown. The model has been tested for suitability and the results of calculations have been compared with experimental data.

Dynamic-mechanical properties of interfacially modified glass sphere filled polyethylene

Abstract: Model composites of spherical glass particles dispersed in a matrix of high density polyethylene were prepared both with and without interfacial modification by an azidofunctional trialkoxysilane Dynamic mechanical measurements of the composites in the melt state were recorded The unmodified composites behave as theoretically predicted and the effect of particle—particle interaction at high volume fractions can be measured The composites with a modified interfacial region have greater shear moduli due to the effect of a region surrounding the particle modified by the silane The material in this region is largely bound to the glass surface and was examined by Fourier transform infrared spectroscopy after extraction of the bulk matrix Theoretical calculations are shown to be useful in calculating the mechanical properties and volume fraction of the interfacial region

Superconducting Au‐YBa2Cu3O7 composites

Abstract: Superconducting Au‐YBa2Cu3O7 composites have been fabricated over a Au volume fraction range of 0–100%. Microstructure measurement indicates that Au and YBa2Cu3O7 form well‐separated phases. The superconducting transition temperature Tc was found unaffected by the presence of Au. The composites exhibit low normal state resistivity and much improved ductility. The results suggest that Au is an excellent metal host for making ultrafine YBa2Cu3O7 granular solids.

Microstructure, chemistry, elastic properties and internal friction of Silceram glass-ceramics

Abstract: The temperature dependence of both Young's modulus (E) and internal friction (Q−1) from room temperature to ≈700° C has been determined by Forster's forced-resonance method for three Silceram glass-ceramics, produced by the direct controlled cooling of glass melts in the quaternary system CaO-MgO-Al2O3-SiO2. These results are correlated with microstructural and phase chemistry data as well as calculated viscosity against temperature data. In particular, the viscosity of the residual glass is shown to predominate over its volume fraction in deter mining the temperature dependence ofE andQ−1 for a given Silceram. A simple model which enables the residual glass content of Silceram glass-ceramics to be estimated from a know ledge of the proportions of silicon, iron and magnesium in the corresponding glass melts is also proposed. Furthermore, the room-temperature bulk modulus (K) and Poisson's ratio of two Silceram glass-ceramics are calculated using experimentalE and shear modulus (G) values obtained using both Forster's method and another forced-vibration technique.

Electric Birefringence of a Dispersion of Electrically Charged Anisotropic Particles

Abstract: We have measured the static and dynamic birefringence induced by an electric field on a dispersion of PTFE rods having length of about 0.33 ?m and diameter of 0.15 ?m. The rods are electrically charged and have a crystalline structure. By using a pulse consisting of a train of square waves of variable frequency, we have been able to separate the contribution of the counterion cloud from that due only to form and intrinsic anisotropy of the rods. We find that the optical anisotropy of the material is n|| - n??=?0.04. At volume fractions above 0.1% we find an important collective effect in the measured Kerr constant. Such an effect disappears upon addition of a few mM NaCl. The dynamics of electric birefringence appears to be controlled by the rotational diffusion constant of the individual rod, irrespective of the volume fraction and the added salt concentration.