D. R. Uhlmann

A kinetic treatment of glass formation

Abstract A kinetic treatment of glass formation is presented. This treatment is based on the construction of time-temperature-transformation curves corresponding to some barely detectable degree of crystallinity. From such curves, the minimum cooling rates required to form glasses of various materials are estimated. The most important factors determining the glass-forming abilities of different materials are suggested to be the magnitude of the viscosity at the melting point and the rate of increase in viscosity with falling temperature below the melting point.

Applications of thermoanalytical techniques to the study of crystallization kinetics in glass-forming liquids, part I: Theory☆

Abstract A critical review is presented of mathematical methods advanced over the last 20 years for the analysis of data obtained from non-isothermal thermoanalytical studies of crystallization of glass-forming liquids. Methods proposed by Ozawa, Piloyan and Borchardt, Coats, Redfern and Sestak, Ozawa and Chen, Takhor, Kissinger, and Augis and Bennett are examined in detail. All these methods are based on the Avrami treatment of transformation kinetics and define an effective crystallization rate coefficient having an Arrhenian temperature dependence. Several different ways of mathematically treating the data have been proposed. Most are shown to be based on an incorrect neglect of the temperature dependence of the rate coefficient. By taking proper account of the temperature dependence of the crystallization rate coefficient, all the methods are shown to lead to similar conclusions. In detail, it is shown that the effective activation energy of the overall crystallization process can be calculated from the slope of the line obtained by plotting 1n[ Q /( T P − T 0 )] versus 1/ T P is the temperature of maximum crystallization rate, T O is an initial temperature and Q is the heating rate employed in the experiment. It is further argued that in general the overall crystallization rate coefficient is not Arrhenian in character. Thus, in general, non-isothermal transformation cannot be treated analytically. A detailed description of nonisothermal transformation can, however, be obtained by numerical methods. Such a method is described and its uses in obtaining kinetic data from thermoanalytical studies are demonstrated.

Fast ion transport in oxide glasses

The discovery of fast ion transport in solids has in recent years stimulated much interest in the scientific community both with regard to obtaining improved understanding of the phenomenon and applying such materials in advanced battery systems. More recently the phenomenon of fast ion transport has also been observed to occur in increasing numbers of amorphous systems. In this paper we review recent transport data obtained in over 100 glasses which appear to exhibit exceptionally high Ag, Li, Na and F ion conductivities at temperatures far from the melting points. We also include results recently obtained in our laboratory for glasses in the lithium borate system. Common characteristics of these glasses are summarized and are compared with predictions of classical diffusion theory. Relatively low and composition-independent values of σ0 indicate poor agreement with a simple isolated-ion diffusion model, while composition-dependent activation energies are related to structural changes. Some glasses share similar properties with their crystalline counterparts while others do not. This is discussed in terms of the relative disorder already existing within the crystals. The need for improved characterization of glasses is also discussed.

On the nature of crystal growth from the melt

Abstract The theory of interface motion as applied to crystal growth by Cahn and his coworkers is examined in detail. This theory, as derived, applied to systems which can have a second order phase transformation but not to liquid-solid or vapor-solid phase transformations which are first order. In this paper, the formalism of the theory is applied to these first order phase transformations. Reasonable agreement with experiment still cannot be obtained. This is because the molecular configuration of the interface is averaged out in the theory by considering a diffuse interface, rather than taking it into account properly in calculating the growth rate. Experimental data on crystal growth have been accumulated and analyzed. It is concluded from this analysis that the theory of Jackson on interface roughness qualitatively predicts crystal growth morphology. Most of the quantitative data on crystal growth are, however, of questionable reliability or are not sufficiently complete for detailed comparison with theory. None of the existent theories of crystal growth can account for these data.

Gel technology in ceramics

Abstract The use of sol-gel techniques to prepare ceramic materials ranks high among those areas in ceramic science and technology which are changing most rapidly and which offer the greatest promise for outstanding improvements in both understanding and applications. This paper reviews the important theoretical considerations, processing techniques and applications related to sol-gel derived ceramics. The gelation behavior of colloidal and polymeric gels, in general, and the influence of such variables as solvent type and concentration, pH, catalyst concentration, temperature, etc. on several oxide systems including silica are considered. The importance of capillary stresses and the various techniques used to minimize them during drying are discussed as are the relevant theories which describe the sintering and firing behavior of the dried gels. To produce ceramics containing more than one oxide, appropriate chemical techniques must be employed to obtain the desired homogeneity. The addition of salts, partial hydrolysis, and alkoxide complexation are among the more widely employed techniques reviewed. A review of current and potential applications of sol-gel derived ceramics, which includes specific examples from the areas of novel glasses, fibers, abrasives, thin films and coatings, is included as well. The paper closes with recommendations and suggestions for future work especially in areas which would benefit from the expertise of physicists and chemists.

Mechanism of Inclusion Damage in Laser Glass

The mechanism of inclusion damage in laser glass is associated with the temperature rise of particles, or surface regions of particles, relative to the surrounding glass. The particles of greatest concern are metallic, although at very high‐power levels ceramic inclusions containing large concentrations of highly absorbing ions can likewise result in failure. Solutions to the heat‐flow problems of a perfectly conducting sphere in a medium of finite conductivity and of the infinite composite solid indicate that temperatures of metal particles subject to a 20‐J/cm−2, 30‐nsec laser pulse can exceed 10 000°K for a range of particle sizes. These high temperatures produce stresses in the glass adjacent to the particles which can exceed the theoretical strength of the glass, and result in failure. The effects on the breakdown condition of flux level and pulse time, as well as the size, shape, thermal expansivity, and spectral emissivity of the particle, and the heat capacity and thermal conductivity of particle ...

Sol—gel derived coatings on glass

Abstract Sol—gel processing is a versatile method for depositing oxide based coatings on a variety of substrates in an economical manner. The present review will consider coatings deposited on glass, and will discuss a number of representative applications for such coatings, namely anti-reflection coatings, transparent conducting coatings, anti-static coatings, fluorinated coatings, coatings incorporating active dye molecules and ferroelectric coatings.

Insights into the structure of alkali borate glasses

Abstract In the light of extensive new glass transition temperature (Tg) and density (ϱ) data obtained for the ternary glass system Li2O(LiCl)2B2O3 we have re-examined the well accepted structural models developed for borate glasses by Krogh-Moe and co-workers. We find that, in contrast to the conclusions of NMR, Raman and IR spectroscopy, glasses in the binary and in the ternary systems with common O/B ratios do not exhibit identical boron-oxygen networks. Rather, we find that as LiCl in the ternary is increased the network is systematically weakened and expanded to accommodate the large Cl− anion. On the basis of these observations we question the ability of spectroscopic techniques such as Raman and NMR to confirm the uniqueness of structural models based on combinations of intermediate-range units. We propose a tentative model which is qualitatively in agreement with the observed changes in Tg and ϱ upon addition of LiCl, and explains how the large Cl− ions can be accommodated without large increases in energy due to strain and electrostatic effects while maintaining BO4/BO3 constant. Recent transport results on some fast ion conducting glasses in this system are also discussed in terms of the proposed model.

The flow of glass at high stress levels: I. Non-Newtonian behavior of homogeneous 0.08 Rb2O·0.92 SiO2 glasses

Abstract A homogeneous 0.08 Rb2O·0.92 SiO2 glass has been tested at temperatures between 480 and 555°C (low-stress viscosities between 1017.1 and 1013.5 poise) and at stress levels up to 109.5 dyne/cm2 or more. At low stress levels, Newtonian behavior was observed; at stress levels above some critical stress, the viscosity was observed to decrease with increasing stress. This critical stress for non-Newtonian flow was found to be about 109.1 dyne/cm2, approximately independent of temperature. The variation of the viscosity with stress in the high-stress region can be described by absolute rate theory only by assuming a flow volume which increases with increasing stress.

High‐temperature flow behavior of glass‐forming liquids: A free‐volume interpretation

High‐temperature viscous flow data are reported for three relatively simple organic liquids: o‐terphenyl, salol, and α‐phenyl‐o‐cresol. The results are combined with previous flow data on the same liquids to obtain viscosity‐temperature relations over some 15 orders of magnitude. For each liquid, as well as for tri‐α‐naphthylbenzene, the free‐volume theory of Turnbull and Cohen is found to provide a close representation of the data for viscosities between 10−2 P (the high‐temperature limit of the data) and about 104 P. Over this range, even the pre‐exponential factors derived from fitting the experimental results are in close agreement with those predicted by the theory. At lower temperatures, the free‐volume expression overestimates the difficulty of flow. For all four organic liquids, the fractional free volumes at the points of departure, TB, of the experimental data from the free volume predictions are close to 0.015. An empirical high‐temperature flow relation of the free‐volume form is suggested as ...