Showing papers on "Volume fraction published in 2005"

Freeform Fabrication of Ceramics via Stereolithography

Abstract: Ceramic green bodies can be created using stereolithography methods where a ceramic suspension consisting of 0.40–0.55 volume fraction ceramic powder is dispersed within an ultraviolet-curable solution. Three ceramic materials were investigated: silica for investment casting purposes, and alumina and silicon nitride for structural parts. After mixing the powders in the curable solution, the ceramic suspension is photocured, layer by layer, fabricating a three-dimensional ceramic green body. Subsequent binder removal results in a sintered ceramic part. Three-dimensional objects have been fabricated from a 0.50 volume fraction silica suspension.

Thermal conductivity and interfacial resistance in single-wall carbon nanotube epoxy composites

Abstract: We report thermal conductivity measurements of purified single-wall carbon nanotube (SWNT) epoxy composites prepared using suspensions of SWNTs in N-N-Dimethylformamide (DMF) and surfactant stabilized aqueous SWNT suspensions. Thermal conductivity enhancement is observed in both types of composites. DMF-processed composites show an advantage at SWNT volume fractions between ϕ∼0.001 to 0.005. Surfactant processed samples, however, permit greater SWNT loading and exhibit larger overall enhancement (64±9)% at ϕ∼0.1. The enhancement differences are attributed to a ten-fold larger SWNT/solid-composite interfacial thermal resistance in the surfactant-processed composites compared to DMF-processed composites. The interfacial resistance is extracted from the volume fraction dependence of the thermal conductivity data using effective medium theory. [C. W. Nan, G. Liu, Y. Lin, and M. Li, Appl. Phys. Lett. 85, 3549 (2004)].

The effect of silica nano particles and rubber particles on the toughness of multiphase thermosetting epoxy polymers

Abstract: The substantial increase in toughness achieved when nano-SiO2 particles were dispersed in a hot-cured single-part epoxy polymer was investigated. The synergistic effect of having a multiphase structure is based upon both nano-SiO2 and rubbery particles. The modulus, of the rubber-particulate epoxy polymer increases steadily as the wt.% of the silica nanophase was increased. The results suggested that the volume fraction of the rubbery-particulate phase which was formed was independent of the concentration of the nano-silicate phase present.

Oxide ceramic coatings on aluminium alloys produced by a pulsed bipolar plasma electrolytic oxidation process

Abstract: In the paper, a modified Plasma Electrolytic Oxidation process (PEO) to produce ceramic coatings on Al alloys, suitable for tribological applications, is discussed. The process utilises bipolar current pulses in the kHz frequency range, providing better control over plasma discharges occurring at the sample surface. The coatings, formed on a 2024 series Al alloy, are characterized by means of optical microscopy, SEM, EDX, XRD and surface profilometry. Microhardness and scratch adhesion tests are performed to evaluate the coating mechanical performance. The effects of current pulse frequency on both the layer growth kinetics and the process energy efficiency are discussed. It is found that in the 1 to 3 kHz frequency range, the layer growth rate can be increased from 0.5 to 1 to 1.6 to 3.2 μm min−1 and the volume fraction of the porous outer layer can be reduced from 25 to 20% to 15 to 10% of the total layer thickness, compared to the conventional 50 Hz AC PEO process. The inner layer, despite a slight increase in porosity, preserves a relatively high hardness of 1200 to 1500 HK25 and good adhesion (LC2=60 N), which should be sufficient for many tribological applications.

Deformation Behavior of Bimodal Nanostructured 5083 Al Alloys

Abstract: Cryomilled 5083 Al alloys blended with volume fractions of 15, 30, and 50 pct unmilled 5083 Al were produced by consolidation of a mixture of cryomilled 5083 Al and unmilled 5083 Al powders. A bimodal grain size was achieved in the as-extruded alloys in which nanostructured regions had a grain size of 200 nm and coarse-grained regions had a grain size of 1 µm. Compression loading in the longitudinal direction resulted in elastic-perfectly plastic deformation behavior. An enhanced tensile elongation associated with the occurrence of a Luders band was observed in the bimodal alloys. As the volume fraction of coarse grains was increased, tensile ductility increased and strength decreased. Enhanced tensile ductility was attributed to the occurrence of crack bridging as well as delamination between nanostructured and coarse-grained regions during plastic deformation.

A study of the mechanical properties of randomly oriented short banana and sisal hybrid fiber reinforced polyester composites

Abstract: The mechanical performance of short randomly oriented banana and sisal hybrid fiber reinforced polyester composites was investigated with reference to the relative volume fraction of the two fibers at a constant total fiber loading of 0.40 volume fraction (Vf), keeping banana as the skin material and sisal as the core material. A positive hybrid effect is observed in the flexural strength and flexural modulus of the hybrid composites. The tensile strength of the composites showed a positive hybrid effect when the relative volume fraction of the two fibers was varied, and maximum tensile strength was found to be in the hybrid composite having a ratio of banana and sisal 4 : 1. The impact strength of the composites was increased with increasing volume fraction of sisal. However, a negative hybrid effect is observed when the impact strength of the composites is considered. Keeping the relative volume fraction of the two fibers constant, that is, banana : sisal = 0.32 : 0.08 (i.e., 4 : 1), the fiber loading was optimized and different layering patterns were investigated. The impact strength of the composites was increased with fiber loading. Tensile and flexural properties were found to be better at 0.40 Vf. In the case of different layering patterns, the highest flexural strength was observed for the bilayer composites. Compared to other composites, the tensile properties were slightly higher for the composite having banana as the skin material and sisal as the core material. Scanning electron micrographs of the tensile and impact fracture surfaces of the hybrid composites having volume fraction 0.20 and 0.40 Vf were studied. The experimental tensile strength and tensile modulus of hybrid composites were compared with those of theoretical predictions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1699–1709, 2005

Polyimide/BaTiO3 composites with controllable dielectric properties

Abstract: The polyimide/barium titanate (BaTiO3) composites were successfully synthesized through a colloidal process. In this process, the preparing suspension of fine BaTiO3 particles in poly(amic acid) solution, the film casting of the suspension, and the imidization with heat treatment are involved in sequence. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), thermal-gravimetric analyses (TGA) and electrometer were used to characterize the structure and properties of the obtained composites. It was found that the BaTiO3 particles in the size of 100 nm were dispersed homogeneously in the polyimide matrix without aggregation. Thermal stability of the composites could be improved with the incorporation of BaTiO3. The dielectric constant (e) and the dielectric loss (tan δ) of these composites increased with the increase of the volume fraction of BaTiO3 particles. e, tan δ was 35 and 0.0082, respectively at 10 kHz as the composite contains 50 vol% BaTiO3. Also, the dielectric properties of the composites displayed good stability within a wide range of temperature or frequency.

The rheology and microstructure of acicular precipitated calcium carbonate colloidal suspensions through the shear thickening transition

Abstract: The shear rheology and shear-induced microstructure of poly(ethylene glycol) (PEG)-based suspensions of acicular precipitated calcium carbonate (PCC) particles of varying particle aspect ratio (nominal L∕D∼2, 4, 7) are reported. These anisotropic particle suspensions demonstrate both continuous and discontinuous reversible shear thickening with increasing applied shear rate or stress similar to that observed for suspensions of spherical colloidal particles. The critical volume fraction for the onset of discontinuous shear thickening decreases as the average particle aspect ratio is increased. However, the critical stress for shear thickening is found to be independent of particle anisotropy and volume fraction. Rather, it can be predicted based on the minor axis diameter of the particles and is found to agree with values for near hard-sphere suspensions. Small angle neutron scattering during shear flow (Rheo-SANS) demonstrates that long-axis particle alignment with the flow direction is maintained through...

Newtonian Viscosity of Amorphous Silicon Carbonitride at High Temperature

Abstract: The creep viscosity of chemical-precursor-derived silicon carbonitride (SiCN), which is known to remain predominantly amorphous at temperatures below 1400°C, was measured in the temperature range 1090-1280°C. Experiments were done in uniaxial compression at constant loads in pure nitrogen atmosphere. The creep behavior exhibited three stages. In stage I the strain rate decreased rapidly with time and deformation was accompanied by densification. In stage II the samples exhibited a steady-state creep rate. In stage III, which commenced after long-term deformation, creep gradually declined to rates that were below the sensitivity of our apparatus. The relative density of the specimens during stage II and stage III remained constant at ≅2.3 g/cm3. The shear viscosity in stage II was nearly Newtonian and was measured to be 1.3 × 1013-5.0 1013 Pa·s at 1280°C, which is approximately 103 times the value for fused silica. The creep-hardened as well as uncrept specimens contained silicon nitride crystallites. The volume fraction of these crystals was variable but always less than 5%. Such a small volume fraction of crystals does not explain the dramatic creep-hardening behavior in stage III, even if it is assumed that the crystals formed during creep deformation in stage II.

Dispersion of Short Fibers in Cement

Abstract: The degree of dispersion of short microfibers in cement, as assessed by electrical resistivity measurement for the case of electrically conductive fibers at a volume fraction below the percolation threshold, is improved by the use of admixtures (namely, silica fume, acrylic particle dispersion, methylcellulose solution, and silane) and fiber surface treatment (such as ozone treatment). Acrylic particle dispersion is more effective than latex particle dispersion.

Optimization of Mo-Si-B intermetallic alloys

Abstract: Mo-Si-B intermetallics consisting of the phases Mo3Si and Mo5SiB2, and a molybdenum solid solution (“α-Mo”), have melting points on the order of 2000 °C. These alloys have potential as oxidation-resistant ultra-high-temperature structural materials. They can be designed with microstructures containing either individual α-Mo particles or a continuous α-Mo phase. A compilation of existing data shows that an increase in the volume fraction of the α-Mo phase increases the room-temperature fracture toughness at the expense of the oxidation resistance and the creep strength. If the α-Mo phase could be further ductilized, less α-Mo would be needed to achieve an adequate value of the fracture toughness, and the oxidation resistance would be improved. It is shown that microalloying of Mo-Si-B intermetallics with Zr and the addition of MgAl2O4 spinel particles to Mo both hold promise in this regard.

Convective Assembly of Antireflective Silica Coatings with Controlled Thickness and Refractive Index

Abstract: Convective assembly at high volume fraction was used to deposit silica nanoparticle coatings onto glass and silicon substrates. By allowing control of the film structure and thickness, this technique provides a means for making large-scale coatings with antireflective properties. The reflectance was reduced by 50% for silicon (at 600 nm) and by 70% for single glass/air surface. Microstructural investigations using SEM, AFM, profilometry, and ellipsometry provided good correlation to the observed macroscopic optical properties. By virtue of the coatings' uniformity, the reflectance and transmission spectra from both substrates could be modeled well by classical reflection relations, using a volume-averaged refractive index. Data analysis showed that the relatively high packing fraction in nanocoatings made from monodisperse spheres is responsible for the limit on antireflective capabilities. To overcome this restriction, low-density silica coatings were made from binary colloidal mixtures of different diam...

Fabrication and sintering effect on the morphologies and conductivity of nano-Ag particle films by the spin coating method

Abstract: By adjusting the rotating speed and colloidal concentration in the spin coating of silver nanoparticles onto a glass substrate, the coating density could be varied from 1.5 × 10−6 to 1.1 × 10−4 g cm−2, or in other words from a dispersed distribution to multi-layered close packed films. The coating density was found to be proportional to the volume fraction of silver colloids and inversely to the rotating speed to the 0.442 power. The multi-layered films were further investigated for sintering behaviour. Our results indicated that the film started to exhibit a noticeable sintering effect at about 100 °C, and at the same time it became electrically conductive. As the treatment temperature increased further, the film resistivity reached a minimum value at 250 °C and finally became insulating again when the temperature was over 400 °C due to silver coalescence into large particles and breaking up of the conductive paths. Smaller silver nanoparticles, however, would exhibit a similar phenomenon but at lower temperatures.

Effective thermal conductivity of nanofluids containing spherical nanoparticles

Abstract: A theoretical model which includes considerations of the effects of an interfacial nanolayer formed by liquid molecule layering on the particle/liquid interface and of micro-convection caused by thermal motion of nanoparticles has been proposed to calculate the effective thermal conductivity of nanofluids. This model accounts for the enhancement in effective thermal conductivity of a nanofluid with respect to the suspended nanoparticle size, volume fraction, temperature and thermal conductivities of the nanoparticle and base fluid. The predicted results are in good agreement with some recently available experimental data.

Rheological behavior of water-in-oil emulsions stabilized by hydrophobic bentonite particles.

Abstract: A study of the rheological behavior of water-in-oil emulsions stabilized by hydrophobic bentonite particles is described. Concentrated emulsions were prepared and diluted at constant particle concentration to investigate the effect of drop volume fraction on the viscosity and viscoelastic response of the emulsions. The influence of the structure of the hydrophobic clay particles in the oil has also been studied by using oils in which the clay swells to very different extents. Emulsions prepared from isopropyl myristate, in which the particles do not swell, are increasingly flocculated as the drop volume fraction increases and the viscosity of the emulsions increases accordingly. The concentrated emulsions are viscoelastic and the elastic storage and viscous loss moduli also increase with increasing drop volume fraction. Emulsions prepared from toluene, in which the clay particles swell to form tactoids, are highly structured due to the formation of an integrated network of clay tactoids and drops, and the moduli of the emulsions are significantly larger than those of the emulsions prepared from isopropyl myristate.

Strain softening, yielding, and shear thinning in glassy colloidal suspensions.

Abstract: A microscopic theory for the dependence on external strain, stress, and shear rate of the transient localization length, elastic modulus, alpha relaxation time, shear viscosity, and other dynamic properties of glassy colloidal suspensions is formulated and numerically applied. The approach is built on entropic barrier hopping as the elementary physical process. The concept of an ideal glass transition plays no role, and dynamical slowing down is a continuous, albeit precipitous, process with increasing colloid volume fraction. The relative roles of mechanically driven motion versus thermally activated barrier hopping and transport have been studied. Various scaling behaviors are found for the relaxation time and shear viscosity in both the controlled stress and shear rate mode of rheological experiments. Apparent power law and/or exponential dependences of the elastic modulus and perturbative and absolute yield stresses on colloid volume fraction are predicted. A nonmonotonic dependence of the absolute yield strain on volume fraction is also found. Qualitative and quantitative comparisons of calculations with experiments on high volume fraction glassy colloidal suspensions show encouraging agreement, and multiple testable predictions are made. The theory is generalizable to treat nonlinear rheological phenomena in other soft glassy complex fluids including depletion gels.

Sliding wear resistance of metal matrix composite layers prepared by high power laser

Abstract: Two laser surface engineering techniques, Laser Cladding and Laser Melt Injection (LMI), were used to prepare three different metal matrix composite layers with a thickness of about 1 mm and approximately 25‐30% volume fraction of ceramic particles. SiC/Al‐8Si, WC/Ti‐6Al‐4Vand TiB2/Ti‐6Al‐4V layers were prepared by a Laser Melt Injection process, whereby additional material in the form of ceramic particles is injected into the molten substrate. As a result, a microstructure characterized by hard ceramic particles distributed in a metal matrix with very strong bonding is formed in the surface layer of the treated metal. A TiB/Ti‐6Al‐4V metal matrix composite layer was produced on Ti‐6Al‐4V substrates by conventional laser cladding. A mixture of TiB2/Ti powders has been used as a precursor to obtain two microstructurally distinct layers, namely eutectic and primary TiB particles dispersed in the Ti‐6Al‐4V matrix. Sliding wear properties of these metal matrix composites layers were studied at boundary lubrication conditions and compared with the wear of the substrate materials. The observed wear mechanisms are summarized and related to detailed microstructural observations. The layers have been found to show excellent interfacial bonding, coupled with dramatically improved tribological properties expressed through a relative wear resistance value ranging from 30 to 1500. D 2004 Elsevier B.V. All rights reserved.