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The present paper considers the mechanical and fracture properties of four different epoxy polymers containing 0, 10 and 20 wt.% of well-dispersed silica nanoparticles. Firstly, it was found that, for any given epoxy polymer, their Young’s modulus steadily increased as the volume fraction, v f, of the silica nanoparticles was increased. Modelling studies showed that the measured moduli of the different silica-nanoparticle filled epoxy polymers lay between upper-bound values set by the Halpin–Tsai and the Nielsen ‘no-slip’ models, and lower-bound values set by the Nielsen ‘slip’ model; with the last model being the more accurate at relatively high values of v f . Secondly, the presence of silica nanoparticles always led to an increase in the toughness of the epoxy polymer. However, to what extent a given epoxy polymer could be so toughened was related to structure/property relationships which were governed by (a) the values of glass transition temperature, T g , and molecular weight, M c , between cross-links of the epoxy polymer, and (b) the adhesion acting at the silica nanoparticle/epoxy-polymer interface. Thirdly, the two toughening mechanisms which were operative in all the epoxy polymers containing silica nanoparticles were identified to be (a) localised shear bands initiated by the stress concentrations around the periphery of the silica nanoparticles, and (b) debonding of the silica nanoparticles followed by subsequent plastic void growth of the epoxy polymer. Finally, the toughening mechanisms have been quantitatively modelled and there was good agreement between the experimentally-measured values and the predicted values of the fracture energy, G c , for all the epoxy polymers modified by the presence of silica nanoparticles. The modelling studies have emphasised the important roles of the stress versus strain behaviour of the epoxy polymer and the silica nanoparticle/epoxy-polymer interfacial adhesion in influencing the extent of the two toughening mechanisms, and hence the overall fracture energy, G c , of the nanoparticle-filled polymers.

Experimental data from inhalation studies in rats were used to develop mathematical models of deposition, clearance, and retention kinetics for inhaled Ni compounds (high-temperature [green] NiO, Ni3S2, and NiSO4*6H2O) in the rat lung. For deposition, an updated version of an earlier model (Yu & Xu, 1986) was used in this study. Three major mechanisms of airway deposition-impaction, sedimentation, and diffusion-were considered in the deposition model. In the development of a clearance model, a single compartment model in the lung was used and a general assumption was made that the clearance of the insoluble and moderately soluble nickel compounds (high-temperature [green] NiO and Ni3S2, respectively) depends highly on the volume of retained particles in the lungs. For the highly soluble nickel compound (NiSO4 *6H2O), the clearance rate coefficient was assumed to depend on the retained particle mass and total alveolar surface. The retention half-time, however, was found to increase with the lung burden for high-temperature (green) NiO and NiSO4*6H2O particles but decrease with the lung burden for Ni3S2 particles.

A study of quantum confinement effects on the low‐dimensional electron mobility in various AlGaAs/GaAs quantum well/wire structures has been performed. The influence of the electron envelop wave function and the subband structure on the low‐dimensional electron scattering rates is evaluated. The electron transport behavior is studied through a Monte Carlo simulation. The result shows that the low‐dimensional electron mobility varies significantly with the quantum well/wire geometry. The one‐dimensional electron mobility of 9200 cm2/V s is obtained in a rectangular quantum wire with a geometry of 110 A×110 A. This value is much improved in comparison with the bulk electron mobility of 8000 cm2/V s in intrinsic GaAs and the maximum two‐dimensional electron mobility of 8600 cm2/V s in a 120 A GaAs quantum well. It is also noticed that the highest low‐dimensional electron mobility is achieved in a quantum well/wire structure where the energy separation between the first subband and the second subband is about two polar optical phonon energy.

Dielectric droplets are successfully manipulated by electropolarization forces in a parallel plate device, which extends the liquid handling capability of digital microfluidics from merely conductive droplets to dielectric ones. Actuations of silicone oil, decane, and hexadecane droplets in a 150-mum-high gap between parallel plates are demonstrated. Silicone oil droplets of different viscosities, 20 and 50 cSt, are transported and split in various gaps heights, demonstrating the manipulation capabilities of electropolarization forces. The threshold and splitting voltages are measured and compared. Moreover, the relationship between the velocity and driving voltage is studied.

The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when used as the matrix of carbon- and glass-fibre reinforced composites. The formation of ‘hybrid’ epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The fracture energy of the bulk epoxy polymer was increased from 77 to 212 J/m2 by the presence of 20 wt% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy. The largest increases in toughness observed were for the ‘hybrid’ materials. Here a maximum fracture energy of 965 J/m2 was measured for a ‘hybrid’ epoxy polymer containing 9 wt% and 15 wt% of the rubber microparticles and silica nanoparticles, respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composite materials were increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized glass particles. The latter, relatively large, glass particles were investigated to establish whether a ‘nano-effect’, with respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.