Aug 28, 2014

Neutron diffraction measurement of the internal stresses following heat treatment of a plastically deformed Al/SiC particulate metal–matrix composite

The generation of internal misfit stresses between matrix and reinforcement has been studied in an Al/SiC particulate-reinforced metal–matrix composite. Bars of the composite were deformed in bending to introduce a varying residual stress field, and subsequently heat treated at different temperatures and for different times to study the evolution of the macroscopic stress field and the interphase stresses. The results show that the shape misfit stresses between the matrix and reinforcement, which arise from the difference in thermal expansion coefficient between the two phases, are reduced by the plastic deformation, but is re-generated by heat treatment.

Higher temperatures and longer times increase the degree to which the shape misfit stresses return to their initial values. The re-generation of the shape misfit stresses is accompanied by a reduction in the variation of the macrostress field induced by the plastic bend.

Keywords:Metal–matrix composite; Internal stress; Residual stress; Neutron diffraction

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Kinetics of heterogeneous catalytic reactions

A wide variety of techniques are usually employed in investigations of heterogeneous catalysis. These investigations typically involve one or more of the following experimental approaches: (i) synthesis and testing of catalytic materials, (ii) characterization of bulk and surface properties, (iii) evaluation of surface adsorptive properties and chemical reactivity, and (iv) assessment of catalyst performance. The recent advances in quantum chemical techniques, combined with improved computer performance, make it possible to conduct quantum chemical calculations to represent more realistic models of active sites and more complex reaction schemes. Since these experimental and theoretical investigations are conducted under different conditions and on a variety of related materials, it is useful to conduct analyses of appropriate reaction schemes to consolidate the results. An important consequence of conducting reaction kinetics analysis in conjunction with results from quantum chemical calculations and experimental studies is that quantitative knowledge about the catalytic process is built at the molecular level into the kinetic model. This process of extracting fundamental knowledge provides a molecular-level basis for comparisons between catalyst systems and provides unifying principles for the design of new catalyst systems. This review provides an introduction to methods for analyses of reaction schemes that describes the results from both experimental and theoretical investigations. Several case studies are provided to illustrate different analysis methods.

Kinetics of heterogeneous catalytic reactions: Analysis of reaction schemes

ai, thermodynamic activity of component i; Ai, preexponential factor for rate constant i; AI, chemical affinity for the step I; C°, concentration at standard state conditions; Csites, surface concentration of sites; CSTR, continuous-flow stirred tank reactor; Ei, activation energy; F, molecular flow rate of feed to reactor; Fs,i, molecular site velocity for species i in a flow reactor; Fi′′, number of gas-phase molecule colliding with a surface per unit area per unit time; Floc, fraction of gaseous local entropy; ΔG, change of Gibbs free energy; h, Plancks constant; ΔH, change of enthalpy; I1, I2, I3, moments of inertia about the principal axes; kB, Boltzmann constant; ki,for, ki, forward rate constant for reaction i; ki,rev, k−i, reverse rate constant for reaction i; k, Ki,eq, equilibrium constant for reaction i; m, mass of a molecule; NC, number of carbon atoms in surface species; Ni, number of molecules of species i in the reactor; Ng, number of gas-phase molecules; Nsat, number of adsorbed molecules at saturation; Nsite, number of catalytic sites; Ns,i, number of gaseous molecules for species i per site in a batch reactor; Pi, partial pressure of species i; PFR, plug flow reactor; Ptot, total pressure; ri, rate of the chemical reaction i or elementary step i; ri′′′, rate per unit reactor volume; , forward rate of elementary step i; Ri, rate of production of species i; R, gas constant; SA, surface area of active sites; ΔS, change of entropy; Strans,3D°,translational entropy of species with three degrees of translation freedom; Strans,2D°, translational entropy of species with two degrees of translation freedom;

Srot, rotational entropy; Svib, vibrational entropy; SR, number of catalytic sites in the reactor; XRC,i, degree of rate control for step i; V, volume; VR, volume of reactor; Zi, reversibility of step i; αH, linear variation of the adsorption heat with carbon number; φi, dimensionless sensitivity of the overall rate with respects to ki; ϱi,tot, total sensitivity with respect to ki; γ, activity coefficient; νi, vibrational frequency; ν‡, frequency factor; νij, stoichiometric coefficient for the j reactant and i elementary step; σr, rotational symmetry number; σ, sticking coefficient, symmetry number of a molecule; σi, stoichiometric coefficient of the linear combination of step i that leads to an overall stoichiometric reaction; τ, space time; , turnover frequency of reaction i; Ω, number of distinguishable configurations of a compound; °, standard state, degrees, inlet condition; ‡, activated complex.

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Aug 14, 2014

Structures and electron affinities of silicon hydrides Si4Hn (n=2–10)

The silicon hydrides clusters structures and electron affinities of the Si4Hn/Si4Hn− (n=2–10) species have been examined using three density functional theory (DFT) methods. The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Three different types of energy separations presented in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The most reliable EAad, obtained at the B3LYP and BPW91 levels of theory, are 1.53 or 1.54 eV (Si4H2), 2.45 or 2.47 eV (Si4H3 for Cs←C1) and 2.50 eV for C1←C1 of Si4H3, 1.73 or 1.79 eV (Si4H4), 2.43 or 2.38 eV (Si4H5), 1.61 or 1.55 eV (Si4H6), 2.20 or 2.23 eV (Si4H7), and 2.34 or 2.37 eV (Si4H9). For Si4H8 and Si4H10, there are no reliable EAad but there are reliable VDE. The values of VDE are from 1.29 eV (BPW91) to 1.63 eV (B3LYP) for Si4H8, and from 0.55 eV (B3LYP) to 0.69  eV (BPW91) for Si4H10.

Aug 7, 2014

Conclusion and Future look about sic4h

1.Good C‐V characteristics characteristics in SiC MOS capacitors capacitors have been demonstrated demonstratedsimply by oxidation in dryO2 at 800oC, on the basis of hermodynamic and kinetic consideration.

2.High‐k dielectric films will be applicable for SiC gate stacks by using stable
interfacial SiO2 layer.

3.SiC interface research is old but will bea hot topic.

4.Si-face is much better than C-face due to a considerably lower oxidationrate in the present method.

5.MOSFET fabrication and characterization will be the next challenge.