Showing papers on "Volume fraction published in 1985"

Hard sphere colloidal dispersions: Viscosity as a function of shear rate and volume fraction

Abstract: The viscosities of suspensions of sterically stabilized (hard) silica spheres in cyclohexane are reported as a function of shear rate (γ) and volume fraction (6×10−4<φ<0.6). The shear thinning scales according to (ηr−η∞)/(η0−η∞) =1/(1+1.31ηγa3/kT) with limiting low and high shear viscosities described up to φ∼0.35 by η0=1+5/2φ+(4±2)φ2+(42±10)φ3 , η∞=1+5/2φ+(4±2)φ2+(25±7)φ3 . At higher volume fractions the viscosity becomes more sensitive to φ and diverges at φm=0.63±0.02 (γ→0) , φm=0.70±0.02 (γ→∞) . The experimental results compare well with existing hard sphere theories and the data of Krieger (1972) for aqueous lattices. Even at the highest volume fraction neither yield stresses nor shear thickening are observed.

Jute-reinforced polyester composites

Abstract: Raw jute fibre has been incorporated in a polyester resin matrix to form uniaxially reinforced composites containing up to 60 vol% fibre. The tensile strength and Young's modulus, work of fracture determined by Charpy impact and inter-laminar shear strength have been measured as a function of fibre volume fraction. These properties all follow a Rule of Mixtures relationship with the volume fraction of jute. Derived fibre strength and Young's modulus were calculated as 442 MN m−2 and 55.5 GN m−2 respectively. Polyester resin forms an intimate bond with jute fibres up to a volume fraction of 0.6, above which the quantity of resin is insufficient to wet fibres completely. At this volume fraction the Young's modulus of the composite is approximately 35 GN m−2, the tensile strength is 250 MN m−2, the work of fracture is 22 kJ m−2 and the inter-laminar shear strength is 24 MN m−2. The properties of jute and glass fibres are compared, and on a weight and cost basis jute fibres are seen in many respects to be superior to glass fibres as a composite reinforcement.

Tailoring the Properties of Composite Piezoelectric Materials for Medical Ultrasonic Transducers

Abstract: Transducers for medical ultrasonic imaging have been made from composite piezoelectric materials. This paper describes a simple physical model for the material properties which govern the thickness-mode oscillations in thin plates of PZT-rod/polymer composites. We consider the case where the lateral periodicity of the rods is much smaller than all relevant acoustic wavelengths. Expressions are derived for the effective material parameters in terms of the properties of the constituents. The composites' properties can then be tailored to device requirements by choosing appropriate components and by varying the volume fraction of piezoceramic. Our analysis reveals the need for a trade-off between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced. The predictions of this model are in good agreement with measurements made on rod-composite plates containing 5 to 35 percent PZT.

The contiguity of liquid phase sintered microstructures

Abstract: Contiguity is a measure of the dispersed phase contact area in a composite microstructure and is particularly important to the properties of liquid phase sintered materials. For an assumed spherical geometry, the contiguity is calculated for various interfacial energies, grain size ratios, and volume fractions of solid phase. The volume fraction solid is linked to the dihedral angle to indicate conditions where grain shape accommodation is necessary. The effect of a distribution in solid phase grain sizes is to lower slightly the contiguity from the monosized grain value. Coalescence is linked to the occurrence of curved boundaries at intergrain contacts involving differing grain sizes. The results are correlated to prior observations on cemented carbides.

Plastic flow in dispersion hardened materials

Abstract: In this review consideration is given to the sequence of processes which can occur in dispersion strengthened materials during plastic deformation. Consideration is given to the role of elastic back stresses and their influence on the magnitude of the Bauschinger effect and on dimensional stability. The formation and stability of the plastic zones around second phase particles is described and the competitive processes of void formation and diffusional relaxation are considered. Attention is given to the description of the microstructural features of dispersion hardened materials in terms of the local volume fraction of clustered particles rather than the average size and volume fraction. Evidence is presented to show that a number of important properties such as fracture behavior and recrystallization need to be described in terms of local volume fractions. In the final section of the review emphasis is placed on the relationship between basic concepts of dispersion hardening and the development of new processing methods capable of producing synthetic microstructures with specific properties.

Optical properties and microstructure of gold-fluorocarbon-polymer composite films

Abstract: The optical transmittance of plasma-deposited composite gold\char21{}fluorocarbon-polymer thin films is analyzed in terms of effective-medium theories. The effect of increasing gold volume fraction is shown to be correctly described by the Sheng model, provided the gold-cluster and polymer-inclusion sizes and shapes analyzed by transmission electron microscopy are properly taken into account. Moreover, good consistency is obtained between the present results and our previous study of the electrical conductivity variation at and above the percolation threshold. An extension of the Sheng model has been developed to deal with three-phase systems and has been applied in this work to test the possible existence of an interfacial carbon layer at the gold-polymer interface. The effects of topological disorder and higher-order multipole interactions between the clusters are discussed. Annealing of the films above the glass transition temperature of the polymer results in a drastic change of the film optical properties which is attributed to an increase of gold volume fraction, due to the collapse of the polymer phase, and above all to a modification of the cluster sizes and shapes by sintering and coalescence of gold particles as confirmed by transmission electron microscopy.

Time-dependent Deformation of Composite Resins — Compositional Considerations:

Abstract: The compressive yield strength and creep of 21 dental composite resins were evaluated and correlated with filler volume percent and extent of cure in the resin. In general, yield and creep of composites were more dependent upon filler volume fraction than on extent of cure. However, the types of monomers and fillers used in the composite formulation appeared to play a major role in determining the compressive characteristics of the materials.

Flow of interdendritic liquid and permeability in pb-20 Wt Pct Sn alloys

Abstract: Directionally solidified Pb-20 wt pct Sn alloys of uniform microstructures were produced with various primary and secondary dendrite arm spacings. Permeabilities of these alloys were investigated with approximately 0.19 and 0.29 volume fraction liquid, for flow parallel to the direction of primary dendrite arms. The permeabilities of the samples with approximately 0.19 volume fraction liquid were also obtained for flow normal to the primary dendrite arms. It was found that for flow parallel to the primary dendrite arms, permeability varied with d 1 2 and g L 2 (d1 is the primary arm spacing and gL is the volume fraction of liquid). There appears to be no relation between permeability for this parallel flow and the secondary dendrite arm spacing. For flow perpendicular to the primary dendrite arms, permeability is approximately 0.06 to 0.20 that for parallel flow, and in this case the permeability appears to be strongly dependent upon the secondary dendrite arm spacing.

Percolation conductivity in Nafion membranes

Abstract: This article deals with the method of determination of a threshold volume fraction of the conductive phase within perfluorosulfonic acid ionomer Nafion. Experiments have been performed with the commercial Nafion-120 and Nafion-427 membranes equilibrated with concentrated sodium chloride and sodium hydroxide solutions at 353 K. It has been stated that the insulator-to-conductor transition in membranes occurs at the critical volume fraction of the conductive phase (Vc) equal to 0.1. The same Vc has been estimated for a geometrical cluster-network model. Lower than the theoretical Vc for a classical dense-packed-hard-sphere model (Vc = 0.15), the volume fraction for the membranes is caused mainly by channels connecting the ionic clusters. The critical exponent t has been calculated for both membranes and found to be equal to 1.6 for Nafion-120 and 1.5 for Nafion-427. Both these constants correspond to those theoretically predicted for 3D systems. The ratios of sodium ion mobility in the internal membrane solution to its mobility in the equilibrating NaCl or NaOH solutions (u+/u+) are below unity, and they are dependent on the nature and concentration of the electrolyte.

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part II. calculation of flow curves

Abstract: The role of metastable retained austenite(γ R), its volume fraction, and mechanical stability on the flow characteristics of a dual phase steel containing 20 vol pct of ‘as quenched’ martensite in a ferrite matrix has been examined in this paper employing the flow curve expressions derived in Part I of this paper. It has been found that for a given volume fraction ofγ R, its mechanical stability plays a crucial role in enhancing the ductility. Whereas highly stableγ R does not contribute either to strength or ductility of the steel, highly unstableγ R which causes an increase in the strength is detrimental to ductility. Aγ R which is moderately stable and undergoesγ R → α′ transformation over a larger strain range is beneficial to enhanced ductility. Increasing amounts of moderately stableγ R significantly increase both the strength and the ductility of dual-phase steels through a sustained work-hardening due toγ R → α′ transformation. Load transfer which is determined by a parameterq has a significant contribution to work-hardening. A value of ∣|q∣|= 4500 MPa has been found to partition realistically the stress and strain in these steels.

Transformation toughening and grain size control in??-Al2O3/ZrO2 composites

Abstract: In order to fabricate transformation-toughenedβ″-Al2O3 and optimize its mechanical and electrical properties it was found to be necessary to carefully control the particle size distribution of the starting powders and their mixing. The ionic resistivity of the composites depended primarily on the volume fraction of ZrO2. Additions between 10 and 20 vol% produced materials with ionic resistivities (300° C) between 6 and 10 Ωcm and eliminated exaggerated grain growth of theβ″-Al2O3. Comparison ofβ″-Al2O3 composites containing either tetragonal (t-) ZrO2 or cubic (c-) ZrO2 with the single phase material showed that the major strengthening mechanism is the reduction in critical flaw size. This occurred by the elimination of the flaw population associated with abnormally large grains. For maximum increases in fracture toughness and strength, however, the use of t-ZrO2 (transformation toughening) as a second phase is preferred.

Stress dependence of the coefficient of moisture diffusion in composite materials

Abstract: A model for the effect of external loading on diffusion into the bulk resin matrix of unidirectional composite materials is proposed. The model attributes the effect to a change in the free volume of the resin matrix, which is equal to its volume strain. The latter is calculated by substracting the volume strain of the fibers from that of the composite. The ratio of the diffusion coefficients in the stressed and unstressed states is expressed as a function of the stress level, the volume fraction of the fibers, and the angle between the applied stress and the fiber direction. Calculations for a glass-fiber-reinforced epoxy composites are presented.

A study of the gas barrier properties of highly oriented polyethylene

Abstract: Measurements of permeability and diffusivity have been undertaken for the gases helium and oxygen in a range of highly oriented polyethylene films. The solubilities deduced for both gases are proportional to the amorphous volume fraction, showing that the noncrystalline regions are the transport medium in all instances. The changes in diffusion coefficient are more complex. A large reduction in diffusion coefficient is observed with increasing draw ratio, and this is particularly marked in the case of the larger oxygen molecule, where significant differences are also observed for different grades of polymer and different drawing conditions. These changes in diffusion coefficient are discussed in the light of previous studies of diffusion in polymers and present knowledge of the changes in structure produced by drawing.

Heat transfer between wall and solid-water suspension flow in horizontal pipes

Abstract: Apparatus made of copper and acrylic pipes of 14 mm, 19 mm and 25 mm i.d. were constructed to study the fundamental characteristics of heat transfer from the wall to water suspensions of glass beads (d=0.06 to 1.0mm) or ion exchange resin (d= 0.8 mm) flowing through horizontal pipes. The operating range of Reynolds numbers and volume fraction of solid is from 3000 to 50, 000 and from 0 to 0.1, respectively. In the case of particle sizes larger than 0.35 mm in diameter, the heat transfer coefficient is always larger than that of a simple water flow and is not strongly affected by particle size, pipe diameter or volume fraction of the solid in the asymmetric suspension flow. In the case of particle sizes smaller than 0.15 mm in diameter, on the other hand, heat transfer reduction is found to occur in the range of Reynolds numbers from 6000 to 15, 000. The temperature profiles of fluids in the horizontal radial direction are symmetric with respect to the pipe axis even at low Reynolds numbers. In the vertical radial direction perpendicular to the pipe axis, the temperature profiles of fluids are asymmetric with respect to the pipe axis.

Effect of carbon content and ferrite grain size on the tensile flow stress of ferritic spheroidal graphite cast iron

Abstract: The effects of varying carbon content and ferrite grain size on the tensile flow stress of ferritic spheroidal graphite cast iron have been investigated. The flow stress of this material can be expressed by the following equation as a function of graphite volume fraction (Vg) and ferrite grain size (df) without an influence of graphite nodule diameter: Φ = K3(l - k1Vg)({Φ0} + k2df). The relationship between tensile stress (Φ) and the volume fraction of graphite (Vg) obtained by the analysis of plastic deformation can be approximated by the above equation. It is apparent from the results that the flow stress of ferritic spheroidal graphite cast iron is influenced appreciably by ferrite grain size and the triaxial stress field developed in the ferrite matrix between graphite nodules.

Liquid‐liquid interfacial areas formed by turbine impellers in baffled, cylindrical mixing tanks

Abstract: An extensive experimental study on the formation of liquid-liquid interfacial areas was conducted using flat-blade turbine impellers in standard mixing tank geometry. The interfacial areas were measured with a light probe for different combinations of volume fraction, continuous and dispersed phase physical properties, and mechanical mixing conditions.

Parameterizations of scattering intensities and values of the angularities and of the interphase surfaces for three-component amorphous samples.

Abstract: Different functional parameterizations of the radiation intensity scattered by an N-component amorphous sample are considered. Each parameterization is such that (i) it depends only on the areas and on the angularities of the samples' interphase surfaces, (ii) it fulfils all the known physical constraints and (iii) it yields a rather simple algebraic expression both for the correlation function and for the scattered intensity. The parameterizations have been used for analysing the scattering data relevant to some three-component catalysts. Provided the volume fraction of the metal is not very small, the best fit yields satisfactory results only for some of the considered parameterizations. In this way, the determination of both the areas and the angularities of catalysts appears possible.

On the Origin of Negative Normal Stresses in Sheared or Lyotropic Liquid Crystals

Abstract: In steady shear flow, sustained negative primary normal‐stress differences have been reported for several liquid crystalline systems, in most detail for m‐cresol solutions of helicoidal polypeptides and for copolyesters. A theoretical model is herewith proposed in which “dumbbell” molecules are constrained from rotating freely in the shear flow by the intermolecular potential that produces liquid crystalline order. In steady state, the molecules are imperfectly aligned, corresponding to a shortening or buckling of molecular layers. The tendency for the layers to straighten toward perfect order causes a compressive force along the streamlines, corresponding to a negative normal stress. For this actually to occur, a dimensionless group (containing the molecular axial ratio, shear rate, solute volume fraction and intermolecular potential) is predicted to be within a restricted range. The experimentally observed range for the polypeptide solutions is wider, probably because of polydispersity (imperfect monodispersity), and the theory underestimates the magnitude of observed negative normal stress. Although the copolyesters must be characterized by different independent variables, again one can identify a dimensionless group such that all the negative normal stresses occur within a restricted range. This simple model identifies dimensionless variables that will be important in any detailed microrheological theory based on calculating the distribution of molecular orientations.

Instability of the CsCl structure in ionic solids at high pressures

Abstract: It is shown by a simple model calculation that the ionic solids in the CsCl phase transform discontinuously to a tetragonal phase under high pressures at a fractional volume in the neighborhood of 0.5. This general result follows as a consequence of competing Madelung energy and repulsive energy terms. The $\frac{c}{a}$ variation with pressure and transition volume fraction are in approximate agreement with the recent ultrahigh pressure experiments on cesium halides.