We have explored the effect of alloying an unreactive metal, Sn, on the dynamics of D2 dissociative chemisorption at Pt(111). By comparing D2 sticking and recombinative desorption on Pt(111) with that on the ordered p(2×2) Sn/Pt(111) and (∛×∛)R30° Sn/Pt(111) surface alloys, we examine the influence of the local surface composition on reactivity. The energy dependence of D2 sticking S(E) has been measured for all three surfaces using a hyperthermal beam. We find that the activation barrier for dissociative chemisorption is low on the p(2×2) alloy, but the sticking probability is reduced, compared to Pt(111), by an increase in the steric constraint on the dissociation site. Sticking on the (∛×∛)R30° alloy is inefficient at thermal energies with a threshold of ∼280 meV, below which the sticking probability falls exponentially. The increase in the barrier to D2 dissociation occurs as the stable, high coordination Pt3–D binding sites are lost by formation of the (∛×∛)R30° alloy. Despite the large activation barrier, sticking is dominated by the vibrational ground state with the barrier occurring in the entrance channel, before the D2 bond has stretched. Departures from a normal energy scaling indicate that the dissociation site is localized in the unit cell and we suggest favorable dissociation sites on the alloy surfaces. Estimates for the heats of adsorption, obtained by comparing activation energies to adsorption and desorption, indicate an abrupt decrease in the D binding energy as the Pt3 sites are lost. We show that sticking and desorption parameters are consistent with an increasing steric constraint for adsorption/desorption on the alloy surfaces as the Sn content is increased and an increase in the barrier to dissociation as the stable Pt3 sites are lost by alloying.
2008) "Comparison of the mechanical and physical properties of a carbon fibre epoxy composite manufactured by resin transfer moulding using conventional and microwave heating". Composites Science and Technology,, [1854][1855][1856][1857][1858][1859][1860][1861] Comparison of the mechanical and physical properties of a carbon fibre epoxy composite manufactured by resin transfer moulding using conventional and microwave heating AbstractMicrowave processing holds great potential for improving current composite manufacturing techniques, substantially reducing cure cycle times, energy requirements and operational costs. In this paper, microwave heating was incorporated into the resin transfer moulding technique. Through the use of microwave heating, a 50% cure cycle time reduction was achieved. The mechanical and physical properties of the produced carbon fibre/epoxy composites were compared to those manufactured by conventional resin transfer moulding. Mechanical testing showed similar values of flexural moduli and flexural strength for the two types of composites after normalisation of the corresponding data to a common fibre volume fraction. A 9% increase of the interlaminar shear strength (ILSS) was observed for the microwave cured composites. This enhancement in ILSS is attributed to a lowering of resin viscosity in the initial stage of the curing process, which was also confirmed via scanning electron microscopy by means of improved fibre wetting and less fibre pull-out. Furthermore, both types of composites yielded minimal void content ( properties of the produced carbon fibre/epoxy composites were compared to those manufactured by conventional resin transfer moulding. Mechanical testing showed similar values of flexural moduli and flexural strength for the two types of composites after normalisation of the corresponding data to a common fibre volume fraction. A 9% increase of the interlaminar shear strength (ILSS) was observed for the microwave cured composites. This enhancement in ILSS is attributed to a lowering of resin viscosity in the initial stage of the curing process, which was also confirmed via scanning electron microscopy by means of improved fibre wetting and less fibre pull-out. Furthermore, both types of composites yielded minimal void content (<2%). Dynamic mechanical thermal analysis revealed comparable glass transition temperatures for composites produced by both methods. A 15 °C shift in the position of the ȕ-transition peak was observed between thermally and microwave cured composites, suggesting an alteration in the cross-linking path followed.
Initial sticking probabilities for D2 dissociative chemisorption at a Ag(111) surface have been measured for translational energies in the range Ei=220–500 meV, as a function of incident angle θi and gas temperature, using seeded molecular beams. Sticking probabilities are dependent on the D2 internal state distribution and scale with the normal component of the translational energy. The data has been fit by assuming that dissociation is independent of molecular rotation, being the sum of contributions from molecules in different vibrational states v with a sticking function S0(Ei,θi,v)=A/2{1+tanh[Ei cos2 θi−E0(v)]/w(v)}, in a manner similar to the behavior on copper. Sticking parameters E0, the translational energy required for S0 to reach half of its maximum value, are determined with good precision (±25 meV) for levels v=3 (328 meV) and v=4 (170 meV) with width parameters w=54 and 63 meV, respectively, while the barriers for levels v=1 and 2 are close to the upper limit of the sticking data and E0 is estimated as 700±100 and 510±70 meV, respectively. Parameters for the vibrational ground state (v=0) could not be obtained, since sticking of this state is negligible at translational energies less than 500 meV. No dissociation could be observed at thermal energies Ei⩽70 meV either on a flat or a defective surface.
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