ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03722.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03723.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A correlation exists between the width of the glass transition region measured using DTA or DSC during heating and the activation enthalpy ΔHη*for the shear viscosity of high‐Tginorganic glasses. This is demonstrated by the constancy of a dimensionless parameterC= (ΔHη*/R)(1/Tg – 1/Tg′), where Tg and Tg′ are temperatures marking the extrapolated onset and finish of the transition region on the DTA or DSC trace. The logarithm of the shear viscosity at Tg, log η (Tg), is also approximately constant for high‐Tg glasses. Using the universal values ofCand log η (Tg), fairly reliable estimates of shear viscosities from the transition range through the working range can be made from the Tg and Tg′ values obtained

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03724.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Following the ideas of Zachariasen and of Cooper, it is argued that the glass‐forming ability of a system can be rationalized in terms of the degrees of freedom available to form an infinitely large topologically disordered network composed of rigid structural units connected at vertices, edges, or faces. Expressions are derived for the degrees of freedom for regular and irregular networks composed of regular or distorted polytopes sharing vertices, edges, or faces. The condition that degrees of freedom must be nonnegative for such a disordered network to exist is in agreement with the empirical results on glass‐forming abilities of a variety of covalent, ionic, and metallic systems. The present approach can be used to screen the proposed models of structure of a glass and to predict the boundaries of glass‐forming regions in composition s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03725.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The propagation of indentation radial cracks in soda—lime silicate glass is measured as a function of time after indentation. Rapid lift‐off of the indenter from the specimen surface causes a step‐function perturbation in the radial crack mechanical energy release rate, thus providing access to a large range of observable crack velocities in the indentation stress field. Analysis of the data shows distinct threshold, reaction‐limited, and transport‐limited behavior in the crack velocity responses, in agreement with measurements made using macroscopic crack geometries. Atomistic models of fracture kinetics in reactive environments are fit to the data and are deconvoluted to yield the underlying atomic‐scale, bond‐rupture parameters. These latter are used to calculate potential functions for activated fracture and predict crack velocity responses as a function of temperature

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03726.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Silica and germania–silica glasses are most appropriate for use in making lightguide fibers because of their special optical and mechanical properties. They can be prepared easily in a highly pure state and thus exhibit very low optical absorption losses. In addition, because of their high bond strength and complete three‐dimensional bonding, they have a very high theoretical (and practical) mechanical strength. The two most serious drawbacks from a mechanical point of view are brittleness and reactivity with water. Their brittleness allows them to be easily damaged, while their reactivity with water gives rise to stress‐accelerated reaction or inhomogeneous corrosion in the presence (fatigue) and absence (aging) of stress. By the development of suitable polymeric and metallic coatings, both problems can be largely ove

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03727.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The theory of A. R. Cooper, Jr., and other viscoelastic theories of thermal stresses in glass are reviewed. The role of structural relaxation in the evolution of the free strain and stress relaxation times is examined. An explanation of the hysteresis loop observed on thermal contraction/expansion curves in the glass transition region is offered. Single‐exponential models are reviewed on the merit of the insight they provide into the physics of stress development. Their numerical performance is compared with that of the more comprehensive theorie

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03728.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Standard numerical techniques are used to model the development of tracer profiles in “double‐oxidation” experiments involving silicon or silicon‐based ceramics. These models are more generally applicable than previously developed analytical models. In particular, the numerical model allows the relative contribution to oxidation by network and interstitial oxygen diffusion through the silica scale to be varied, and can accommodate a position‐dependent diffusion coefficient. The results of calculations are compared to previously published

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03729.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A complete description of the first‐order Raman and Brillouin spectra of single‐crystal aluminum nitride is provided. The frequency has been measured and the symmetry identified of each of the Raman‐active zone‐center optic modes, and the five independent elastic constants have been measured. The widths of the principal Raman modes increase with increasing oxygen content up to a critical value of approximately 1 at.%, after which they are constant. This behavior supports the hypothesis of a change in the nature of the oxygen defect at that concen

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03730.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A comparative study was performed on two TiO2(110) surfaces: one annealed in vacuum, the other annealed in hydrogen. Local geometric and electronic structures were characterized using scanning tunneling microscopy and spectroscopy, in conjunction with conventional surface analytical techniques. Each surface exhibited a distinct pattern of electronic and geometric inhomogeneity. The origin of the defect structure resulting from the two treatments is discussed in terms of reduction mechanisms, defect formation thermodynamics, and chemical reactivity. The implication of local variation in the defect structure to the TiO2interaction with reactive species is also addressed.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03731.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY