Scaling-up the Production of Nanosized SiO2-particles in a Double Diffusion Flame Aerosol Reactor

Synthesis of nanophase silica (SiO₂) from hexamethyldisiloxane (HMDS) oxidation in a co-flow diffusion flame reactor at atmospheric pressure is investigated focusing on high production rates of powder. A new experimental set-up is introduced, including a diffusion burner which is operated with a ring-shape double diffusion flame. Significantly high HMDS concentrations are used resulting in SiO₂ production rates of up to 130g/h. Deposition of silica powder on the burner face is eliminated by the design of a special diffusion burner and higher collection rates are achieved using a baghouse filter. The specific surface area and the product powder composition are analyzed. Carbon black coated silica particles were produced at high production rates (130g/h) at low oxygen flow rates or using a mixture of nitrogen and oxygen as oxidant. The size of the product particles was controlled in the range of 15–170nm.     

Anharmonic lattice dynamics in germanium measured with ultrafast x-ray diffraction

Damping of impulsively generated coherent acoustic oscillations in a femtosecond laser-heated thin germanium film is measured as a function of fluence by means of ultrafast x-ray diffraction. By simultaneously measuring picosecond strain dynamics in the film and in the unexcited silicon substrate, we separate anharmonic damping from acoustic transmission through the buried interface. The measured damping rate and its dependence on the calculated temperature of the thermal bath is consistent with estimated four-body, elastic dephasing times (T2) for 7-GHz longitudinal acoustic phonons in germanium.     

A perturbation analysis of the intrinsic conditioning of an approximate null vector computed with a SVD

Let A be an m ×n real matrix with singular values σ₁ ? ··· ? σ_n?1 ? σ_n ? 0. In cases where σ_n ? 0, the corresponding right singular vector υ_n is a natural choice to use for an approximate null vector ofA. Using an elementary perturbation analysis, we show that κ = σ₁/(σ_n?1 ? σ_n) provides a quantitative measure of the intrinsic conditioning of the computation of υ_n from A.     

Approximation with sums of exponentials in Lp[0, ∞)

We consider the problem of approximating a given f from L_p [0, ∞) by means of the family V_n(S) of exponential sums; V_n(S) denotes the set of all possible solutions of all possible nth order linear homogeneous differential equations with constant coefficients for which the roots of the corresponding characteristic polynomials all lie in the set S. We establish the existence of best approximations, show that the distance from a given f to V_n(S) decreases to zero as n becomes infinite, and characterize such best approximations with a first-order necessary condition. In so doing we extend previously known results that apply in L_p[0, 1].     

Formation of scandium carbides and scandium oxycarbide from the elements at high-(P, T) conditions

Synchrotron diffraction experiments with in situ laser heated diamond anvil cells and multi-anvil press synthesis experiments have been performed in order to investigate the reaction of scandium and carbon from the elements at high-(P,T) conditions. It is shown that the reaction is very sensitive to the presence of oxygen. In an oxygen-rich environment the most stable phase is ScO_xC_y, where for these experiments x=0.39 and y=0.50-0.56. If only a small oxygen contamination is present, we have observed the formation of Sc₃C₄, Sc₄C₃ and a new orthorhombic ScC_x phase. All the phases formed at high pressures and temperatures are quenchable. Experimentally determined elastic properties of the scandium carbides are compared to values obtained by density functional theory based calculations.     

A hot-atom reaction kinetic model for H abstraction from solid surfaces

Measurements of the abstraction reaction kinetics in the interaction of gaseous H atoms with D adsorbed on metal and semiconductor surfaces, H(g)+D(ad)/S→ products, have shown that the kinetics of the HD products are at variance with the expectations drawn from the operation of Eley–Rideal mechanisms. Furthermore, in addition to HD product molecules, D₂ products were observed which are not expected in an Eley–Rideal scenario. Products and kinetics of abstraction reactions on Ni(100), Pt(111), and Cu(111) surfaces were recently explained by a random-walk model based solely on the operation of hot-atom mechanistic steps. Based on the same reaction scenario, the present work provides numerical solutions of the appropriate kinetic equations in the limit of the steady-state approximation for hot-atom species. It is shown that the HD and D₂ product kinetics derived from global kinetic rate constants are the same as those obtained from local probabilities in the random walk model.

The rate constants of the hot-atom kinetics provide a background for the interpretation of measured data, which was missing up to now. Assuming that reconstruction affects the competition between hot-atom sticking and hot-atom reaction, the application of the present model at D abstraction from Cu(100) surfaces reproduces the essential characteristics of the experimentally determined kinetics.     

Less strain energy despite fewer misfit dislocations: the impact of ordering

The average strain state of Ge films grown on Si(111) by surfactant mediated epitaxy has been compared to the ordering of the interfacial misfit dislocation network. Surprisingly, a smaller degree of average lattice relaxation was found in films grown at higher temperature. On the other hand, these films exhibit a better ordered dislocation network. This effect energetically compensates the higher strain at higher growth temperature, leading to the conclusion that, apart from the formation of misfit dislocations, their ordering represents an important channel for lattice-strain energy relaxation.