First-principles studies on structural properties of β-cristobalite

Journal ArticleThe structure of β-cristobalite has been studied through a first-principles total-energy minimization in the local-density approximation using a Car-Parrinello-type algorithm combined with the Vanderbilt ultrasoft pseudopotential scheme. It was found that the hypothetical ordered structure proposed by Wright and Leadbetter is favored and the resulting structural parameters agree very well with experiment. Also, correlated relaxation of Si and O atoms toward a-cristobalite positions in the vicinity of domain boundaries is indicated

Liu et al reply

Journal ArticleIn our Letter [1], we have demonstrated the plausibility of a structure for ?-cristobalite SiC2 consisting of domains of I Aid symmetry, and provided strong evidence against other proposed models, based on first-principles total energy and lattice dynamics calculations. We are pleased that the Author's of the preceding Comment [2] agree with us on the major findings of our study, namely, the incorrectness of previous models, and the fact that the structure is derived by rotating the tetrahedra about the average Fd3m structure with dynamic orientational disorder

Multilayer distortion in the reconstructed (110) surface of Au

A new LEED intensity analysis of the reconstructed Au(110)-(1×2) surface results in a modification of the missing row model with considerable distortions which are at least three layers deep. The top layer spacing is contracted by about 20%, the second layer exhibits a lateral pairing displacement of 0.07 Å and the third layer is buckled by 0.24 Å. Distortions in deeper layers seem to be probable but have not been considered in this analysis. The inter-atomic distances in the distorted surface region show both an expansion and a contraction compared to the bulk value and range from 5% contraction to about 4% expansion

First-principles study of crystalline silica

Journal ArticleWe have investigated the structural properties of five different crystalline forms of Si02 using a first-principles approach. An ultrasoft Vanderbilt pseudopotential is generated for oxygen which enables us to use a small plane-wave cutoff of 25 Ry. The relative stability, the equation of state, and pressure-dependent structural parameters of all five polymorphs have been calculated and found to be in very good agreement with available experimental results

Micromechanics analysis of thermal expansion and thermal pressurization of a hardened cement paste

The results of a macro-scale experimental study of the effect of heating on a fluid-saturated hardened cement paste are analysed using a multi-scale homogenization model. The analysis of the experimental results revealed that the thermal expansion coefficient of the cement paste pore fluid is anomalously higher than the one of pure bulk water. The micromechanics model is calibrated using the results of drained and undrained heating tests and permits the extrapolation of the experimentally evaluated thermal expansion and thermal pressurization parameters to cement pastes with different water-to-cement ratios. It permits also to calculate the pore volume thermal expansion coefficient f a which is difficult to evaluate experimentally. The anomalous pore fluid thermal expansion is also analysed using the micromechanics model

Structural Ceramics by Fused Deposition of Ceramics

Fused Deposition of Ceramics (FDC) is a SFF technique, based on FDMTM technology, for fabrication of advanced structural ceramics from powderlbinder filaments. In this study, in-situ reinforced (ISR) Si3N4 powder and polymer/wax based binder systems were used as filament material for FDC processing using a commercially available FDMTM system, 3D Modeler. Powderlbinder feedstocks were mixed using a torque rheometer and filaments were fabricated using a capillary rheometer and twin screw extruder. Green FDC components were built from these filaments and then characterized for inter-road and inter-layer bonding. Binder removal procedures were established for FDC green components to yield brown parts without distortion or shape change. Brown FDC parts were characterized for carbon residue, pore distribution and dimensional changes. Brown FDC parts were then sintered and the sintered density, microstructure, and shrinkage anisotropy were studied.Mechanical Engineerin

Structure determination of the (1×2) and (1×3) reconstructions of Pt(110) by low-energy electron diffraction

The atomic geometry of the (1×2) and (1×3) structures of the Pt(100) surface has been determined from a low-energy electron-diffraction intensity analysis. Both structures are found to be of the missing-row type, consisting of (111) microfacets, and with similar relaxations in the subsurface layers. In both reconstructions the top-layer spacing is contracted by approximately 20% together with a buckling of about 0.17 Å in the third layer and a small lateral shift of about 0.04 Å in the second layer. Further relaxations down to the fourth layer were detectable. The surface relaxations correspond to a variation of interatomic distances, ranging from -7% to +4%, where in general a contraction of approximately 3% for the distances parallel to the surface occurs. The Pendry and Zanazzi-Jona R factors were used in the analysis, resulting in a minimum value of RP=0.36 and RZJ=0.26 for 12 beams at normal incidence for the (1×2) structure, and similar agreement for 19 beams of the (1×3) structure. The (1×3) structure has been reproducibly obtained after heating the crystal in an oxygen atmosphere of 5×10-6 mbar at 1200 K for about 30 min and could be removed by annealing at 1800 K for 45 min after which the (1×2) structure appeared again. Both reconstructed surfaces are clean within the detection limits of the Auger spectrometer. CO adsorption lifts the reconstruction in both structures. After desorption at 500 K the initial structures appear again, indicating that at least one of the reconstructions does not represent the equilibrium structure of the clean surface and may be stabilized by impurities