Showing papers on "Volume fraction published in 2001"

Optical Properties of Thin Films of Au@SiO2 Particles

Abstract: Homogeneous films of Au@SiO2 particles have been deposited on glass as a prototype 3D “artificial solid” using the LBL method. The film thickness is controlled by the number of dipping cycles and is measured by AFM. Each cycle results in approximately one monolayer of particles being deposited. The particle films are dense, but disordered. The optical properties of the resulting thin films have been analyzed as a function of the particle volume fraction, which is controlled through the silica shell thickness. We find that the surface plasmon peak position in films with volume fractions up to φ > 0.5 is accurately predicted by the Maxwell−Garnett model. The films exhibit remarkably uniform, transmitted colors and display metallic reflection at low angles of incidence, even at low volume fractions. The films can be annealed at T > 500 K to provide extremely stable, optical films.

Magnetic and X-ray diffraction measurements for the determination of retained austenite in TRIP steels

Abstract: The accurate determination of the volume fraction of retained austenite is of great importance for the optimization of transformation induced plasticity (TRIP) steels. In this work, two aluminium-containing TRIP steels are studied by means of magnetization and X-ray diffraction (XRD) measurements. By fitting the field dependence of the approach to saturation in the magnetization curves, the saturation magnetization is determined, which is linearly related to the volume fraction of retained austenite. Moreover, information with respect to the microstructure can be obtained from the fitting parameters and the demagnetizing factor for the magnetization curve. The volume fractions obtained from the magnetization measurements are compared with data from XRD measurements. A discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements.

On the magneto-elastic properties of elastomer–ferromagnet composites

Abstract: We study the macroscopic magneto-mechanical behavior of composite materials consisting of a random, statistically homogeneous distribution of ferromagnetic, rigid inclusions embedded firmly in a non-magnetic elastic matrix. Specifically, for given applied elastic and magnetic fields, we calculate the overall deformation and stress–strain relation for such a composite, correct to second order in the particle volume fraction. Our solution accounts for the fully coupled magneto-elastic interactions; the distribution of magnetization in the composite is calculated from the basic minimum energy principle of magneto-elasticity.

Tribological properties of carbon-nanotube-reinforced copper composites

Abstract: Tribological properties of carbon-nanotube-reinforced copper composites were investigated using a pin-on-disk test rig under dry conditions. The composites containing 4–16 vol% carbon nanotubes (CNTs) were fabricated by a powder-metallurgy technique. The tests were carried out at normal loads between 10 and 50 N, and the effect of volume fraction of CNTs on tribological behavior of the composites was examined. The composites revealed a low coefficient of friction compared with the copper matrix alloy. Due to the effects of the reinforcement and reduced friction, the wear rate of the composites decreased with increasing volume fraction of CNTs at low and intermediate loads. The composites with a high volume fraction of CNTs exhibited high porosity and their wear resistance decreased under high-load conditions.

High frequency properties of passive materials for ultrasonic transducers

Abstract: The acoustic properties of passive materials for ultrasonic transducers have been measured at room temperature in the frequency range from 25 to 65 MHz using ultrasonic spectroscopy. These materials include alumina/EPO-TEK 301 composites and tungsten/EPO-TEK 301 composites. Experimental results showed that the acoustic impedance of the composites monotonically increased with the volume fraction of the particle filler, which is in agreement with the Denavey model. The attenuation, however, peaked between 7 and 9% volume fraction of particle filler. For comparison, several other passive materials were also fabricated and measured. The results suggest that materials that possess a higher attenuation also appear to have a larger velocity dispersion.

Rheological properties and dispersion stability of magnetorheological (MR) suspensions

Abstract: In the present article, the rheological responses and dispersion stability of magnetorheological (MR) fluids were investigated experimentally. Suspensions of magnetite and carbonyl iron particles were prepared as model MR fluids. Under an external magnetic field (H 0) and a steady shear flow, the yield stress depends upon H 0 3/2. The Yield stress depended on the volume fraction of the particle (φ) linearly only at low concentration and increased faster at high fraction. Rheological behavior of MR fluids subjected to a small-strain oscillatory shear flow was investigated as a function of the strain amplitude, frequency, and the external magnetic field. In order to improve the stability of MR fluid, ferromagnetic Co-γ-Fe2O3 and CrO2 particles were added as the stabilizing and thickening agent in the carbonyl iron suspension. Such needle-like particles seem to play a role in the steric repulsion between the relatively large carbonyl iron particles, resulting in improved stability against rapid sedimentation of dense iron particles. Furthermore, the additive-containing MR suspensions exhibited larger yield stress, especially at higher magnetic field strength.

Permeability, Diffusion and Solubility of Gases in Polyethylene, Polyamide 11 and Poly (Vinylidene Fluoride)

Abstract: The gases transport coefficients, permeability, diffusion and solubility, are determined by the time lag method on a specific permeation cell. Three semicrystalline polymers, polyethylene (PE), polyamide 11 (PA11) and poly(vinylidene fluoride) (PVF2), are studied in the presence of helium (He), argon (Ar), nitrogen (N2), methane (CH4) and carbon dioxide (CO2) for temperatures ranging from 40 to 80°C in the case of PE, and from 70 to 130°C for both other materials. The applied pressures are, in the majority of tests, of 10 MPa for He, Ar, N2 and CH4, and of 4 MPa for CO2, except in some particular cases where the influence of pressure was studied. In the case of PE, the influence of the volume fraction of the amorphous phase, ranging from 0. 21 to 0. 70, the influence of temperature and the influence of the nature of the gas on the transport processes are investigated. Also, the independence of these phenomena related to pressure and sample thickness, between 0. 5 and 6 mm, is shown. For PA11, after determining the influence of temperature and of the nature of the gas used, the effect of the plasticizer incorporation in this polymer was studied. Regarding PVF2, apart the classic parameters that are temperature and the kind of gas used, we compare the coefficients of transport of CH4 and CO2 in PVF2 made up by extrusion or by compression moulding. For each polymer, it is shown that permeability, diffusion and solubility depend on temperature following Arrhenius' laws. It also seems that diffusion is directly related to the gases molecule size and that the solubility coefficient can be linked to the epsilon/K gases parameter. The comparison of the results obtained with the available data in the literature seems satisfactory.

Fracture and impact behaviours of hollow micro-sphere/epoxy resin composites

Abstract: Fracture and impact behaviours of hollow-glass micro-sphere/epoxy resin composites are studied in terms of fracture toughness, fractography, flexural properties and impact force. Volume fraction of micro-spheres for the composites was varied up to 0.65. The addition of micro-spheres did not enhance the specific fracture toughness of the composites despite the presence of a pinning mechanism at relatively low volume fractions. Performance in reducing the impact force was enhanced as the content of micro-spheres increased, but at the expense of other properties such as specific fracture toughness and specific flexural strength, while specific flexural modulus marginally increased at some high volume fractions of micro-spheres.

Fabrication process and electrical behavior of novel pressure-sensitive composites

Abstract: A novel route was developed to fabricate a new pressure-sensitive composite by dispersing homogeneously conductive carbon particles in an insulating silicone rubber matrix. The composites showed a gradual change in electrical resistivity with applied pressure within percolation threshold region at a constant temperature. This type of gradual fall of resistivity with applied pressure is very important to fabricate pressure sensors. Various amounts of carbon particles were dispersed in a rubber matrix to understand the effect of volume fraction of conductive filler with applying external pressure on resistivity. A quantitative general effective media (GEM) theory was used to understand the resistivity of carbon–rubber composites system over a large range of volume fraction of carbon with applied pressure. The use of two different sizes of silicon rubber particles showed a significant effect in gradual fall of resistivity with applied pressure in the narrow range of percolation threshold. However, a large variation in resistivity from 1st measuring to 10th measuring was observed. A significant improvement in successive measuring of resistivity variation from 1st measuring to 10th measuring was observed when composites were fabricated in hexane solvent media. Finally, nano-sized Al2O3 was dispersed to control the resistivity variation upon successive measurement and to improve the mechanical properties of the composites. The material was suggested to use as unique materials as pressure sensors in practical applications mainly for robots.

Angle of repose and segregation in cohesive granular matter.

Abstract: We study the effect of fluids on the angle of repose and the segregation of granular matter poured into a silo. The experiments are conducted in two regimes where: (i) the volume fraction of the fluid (liquid) is small and it forms liquid bridges between particles thus giving rise to cohesive forces, and (ii) the particles are completely immersed in the fluid. The data is obtained by imaging the pile formed inside a quasi-two-dimensional silo through the transparent glass side walls and using color-coded particles. In the first series of experiments, the angle of repose is observed to increase sharply with the volume fraction of the fluid and then saturates at a value that depends on the size of the particles. We systematically study the effect of viscosity by using water-glycerol mixtures to vary it over at least three orders of magnitude while keeping the surface tension almost constant. Besides surface tension, the viscosity of the fluid is observed to have an effect on the angle of repose and the extent of segregation. In case of bidisperse particles, segregation is observed to decrease and finally saturate depending on the size ratio of the particles and the viscosity of the fluid. The sharp initial change and the subsequent saturation in the extent of segregation and angle of repose occurs over similar volume fraction of the fluid. Preferential clumping of small particles causes layering to occur when the size of the clumps of small particles exceeds the size of large particles. We calculate the azimuthal correlation function of particle density inside the pile to characterize the extent of layering. In the second series of experiments, particles are poured into a container filled with a fluid. Although the angle of repose is observed to be unchanged, segregation is observed to decrease with an increase in the viscosity of the fluid. The viscosity at which segregation decreases to zero depends on the size ratio of the particles.

Effects of Si on the aging behaviour and formability of aluminium alloys based on AA6016

Abstract: The heat treatable 6xxx series (Al–Mg–Si–(Cu)) aluminium alloys are finding increasing use in automotive skin panel applications where relatively high formability and in-service strength for dent resistance are major requirements In Europe, the alloy of choice for such applications is currently the low Cu-containing alloy AA6016, which typically contains approximately 04 wt% Mg and 10 wt% Si, and which derives its strength from the precipitation hardening phase, Mg2Si The volume fraction of Mg2Si is, in turn, affected primarily through the level of Mg within the alloy, although the Si content is also important The level of Si within the alloy influences the solution heat treated (T4) strength and the subsequent aging response of the 6xxx series alloys, again predominantly through its effect on the volume fraction of Mg2Si In this paper, the effects of Si content on the aging behaviour and mechanical properties (formability) of alloys based on the AA6016 composition are described

Small-angle x-ray and neutron scattering studies of the volume phase transition in thermosensitive core-shell colloids

Abstract: The volume transition in thermosensitive colloidal core–shell particles is investigated by small-angle x-ray scattering (SAXS), small-angle Neutron scattering (SANS), and dynamic light scattering (DLS). The latex particles are dispersed in water and consist of a solid poly(styrene) core with a diameter of 100 nm. The thermosensitive shell is made up of poly(N-isopropylacrylamide) (PNIPA) chains crosslinked by 2.5 mol % N,N ’-methylenbisacrylamide (BIS). Water is a good solvent for PNIPA at room temperature but becomes a poor solvent above 32 °C. The PNIPA network of the shell undergoes a volume transition at this temperature. As a result the diameter of the particle shrinks. The scattering intensities of the particles measured by SAXS and SANS as a function of temperature may be decomposed into a part deriving from the overall structure and a part originating from the fluctuations within the network. The analysis of the overall structure leads to the volume fraction of the swollen network at different temperatures. SANS in conjunction with contrast variation demonstrates that the network is confined in a well-defined shell. SAXS and SANS data therefore allow the phase diagram of the network in the shell of the particles to be derived, i.e., the average volume fraction of the network in the shell can be determined as a function of temperature. DLS corroborates this result but demonstrates that there is a small fraction of chains exceeding the outer radius derived from SAXS and SANS. The static intensity caused by the fluctuations of the network becomes the leading contribution at high scattering angles. SAXS data show that this part can be described by a Lorentzian both below and above the volume transition. The analysis demonstrates that critical fluctuations of the network around the transition temperature are fully suppressed. This finding is explained by the strong steric constraint of the network by its confinement within a shell of colloidal dimension. The swelling and shrinking can only take place along the radial direction and the chains are bound to the solid surface of the cores which remains constant during the transition.

Bi-continuous metal matrix composites

Abstract: Bi-continuous alumina:aluminium composites were made by infiltrating an alumina preform which had the structure of a reticulated ceramic foam. The low density preforms were prepared from a polyurethane suspension of alumina powder which was pyrolysed and sintered after foaming. Higher density preforms consisted of ceramic foams with open cells. All these preforms were infiltrated with 6061 aluminium alloy using a modified squeeze caster fitted with a vacuum system and fine control of speed and pressure. The microstructure of the preform fitted an established relationship between the ratio of window diameter to cell diameter (k) and void volume fraction (Vp). Low k foams were infiltrated fully but on cooling below the solidus, interfacial debonding took place due to differential thermal contraction. This was overcome by modifying the processing conditions. High k foams which had high fractional porosity, retained sound interfacial bonding. The composites possess higher elastic modulus than conventional MMCs with a homogeneous reinforcement distribution at a given volume fraction. The loss of electrical conductivity is negligible in the lower volume fraction range because of the three dimensionally continuous aluminium phase. The experimental results are compared with a number of theoretical predictions. © 2001 Elsevier Science B.V. All rights reserved.

Hybrid effects on tensile properties of hybrid short-glass-fiber-and short-carbon-fiber-reinforced polypropylene composites

Abstract: Hybrid composites of polypropylene reinforced with short glass fibers and short carbon fibers were prepared using extrusion compounding and injection molding techniques The tensile properties of these composites were investigated taking into account the effect of the hybridization by these two types of short fibers It was noted that the tensile strength and modulus of the hybrid composites increase while the failure strain of the hybrid composites decreases with increasing the relative carbon fiber volume fraction in the mixture The hybrid effects for the tensile strength and modulus were studied by the rule of hybrid mixtures (RoHM) using the tensile strength and modulus of single-fiber composites, respectively It was observed that the strength shows a positive deviation from that predicted by the RoHM and hence exhibits a positive hybrid effect However, the values of the tensile modulus are close to those predicted by the RoHM and thus the modulus shows no existence of a hybrid effect Moreover, the failure strains of the hybrid composites were found to be higher than the failure strain of the single carbon fiber-reinforced composite, indicating that a positive hybrid effect exists Explanations for the hybrid effects on the tensile strength and failure strain were finally presented

The effect of fiber volume fraction on filament wound composite pressure vessel strength

Abstract: This paper is a continuation of previous research reported in Ref. [1]. The previous paper discussed the relationship between fiber volume fraction in filament wound composite vessels and failure pressure. This research included a design of experiment investigation of manufacturing and design variables that affect composite vessel quality and strength. Statistical analysis of the data shows that composite vessel strength was affected by the manufacturing and design variables. In general, it was found that the laminate stacking sequence, winding tension, winding-tension gradient, winding time, and the interaction between winding-tension gradient and winding time significantly affected composite strength. The mechanism responsible for increases in composite strength was related to the strong correlation between fiber volume fraction in the composite and vessel strength. Cylinders with high-fiber volume in the hoop layers tended to deliver high-fiber strength. This paper further examines the relationship between fiber volume fraction and fiber strain to failure. Data from unidirectional strand tests and additional vessel tests are presented. A computer program that is based on the thermokinetics of the resin and processing conditions is used to calculate the fiber volume fraction distribution in the filament wound vessel. The strand's strength-versus-fiber volume data together with the computer program are used to predict composite vessel burst pressure. In general, good agreement with experimental data is observed.

A nonlinear compaction model for fibrous preforms

Abstract: Based on the mechanics of porous media and physical insight gained from experimental observation, a model for predicting the nonlinear compaction of fibrous preforms in the resin transfer molding process is developed. A key physical constant — namely, preform bulk compressibility — is proposed to establish the relationship between the applied pressure and the preform bulk volume. The preform bulk compressibility is a function of fiber volume fraction and five parameters — the initial fiber volume fraction, the final (maximum attainable) fiber volume fraction, the initial pore volume compressibility, the fiber compressibility, and an empirical index. Results of compaction experiments on plain-woven fabric preforms and unidirectional non-woven materials support the validity of the model. Excellent agreement between theory and experiments has been obtained. The present model provides for fibrous preforms a nonlinear constitutive law whose coefficients can be physically interpreted.