Glasses are disordered materials that lack the periodicity of crystals but behave mechanically like solids. The most common way of making a glass is by cooling a viscous liquid fast enough to avoid crystallization. Although this route to the vitreous state-supercooling-has been known for millennia, the molecular processes by which liquids acquire amorphous rigidity upon cooling are not fully understood. Here we discuss current theoretical knowledge of the manner in which intermolecular forces give rise to complex behaviour in supercooled liquids and glasses. An intriguing aspect of this behaviour is the apparent connection between dynamics and thermodynamics. The multidimensional potential energy surface as a function of particle coordinates (the energy landscape) offers a convenient viewpoint for the analysis and interpretation of supercooling and glass-formation phenomena. That much of this analysis is at present largely qualitative reflects the fact that precise computations of how viscous liquids sample their landscape have become possible only recently.

Supercooled liquids and the glass transition

La theorie presentee permet d'obtenir une formulation explicite de l'influence des rugosites ainsi que des variations locales de constante dielectrique n2 (dues par exemple a une modification de composition ou de compacite) sur la reflexion rasante d'un faisceau de rayons X monochromatique, dans la mesure ou les rugosites relevent d'une distribution gaussienne et a condition que n2 ne depende que de la profondeur Z par rapport au plan moyen de la surface eclairee. L'analyse des verres silicates polis mecaniquement sur polissoir en poix, a l'aide de suspensions aqueuses d'oxydes divers, revele que la couche de polissage se compose en realite de deux zones bien distinctes. La premiere, tout a fait superficielle, l'epaisseur ne depassant pas quelques dizaines d'angstroms, presente une densite toujours inferieure a celle du coeur de l'echantillon et semble imputable au fluage plastique et a l'hydrolyse de la surface pendant le polissage. La seconde, sous-jacente, s'etend au contraire sur plusieurs centaines d'angstroms et met en jeu un processus soit de densification (silice pure, alumino-silicate) soit de lacunisation (verres a assez forte teneur en ions alcalins). Nous examinons egalement l'influence de la duree du polissage, du type d'oxyde utilise, et (ou) des traitements thermiques effectues apres polissage, sur les divers parametres qui caracterisent ces couches.

https://hal.archives-ouvertes.fr/jpa-00244786/document

Caractérisation des surfaces par réflexion rasante de rayons X. Application à l'étude du polissage de quelques verres silicates

This review is concerned with the properties and structure of silica glass. The following topics are treated: Types of silica glasses; The vitreous state of silica glasses: thermodynamical approach, atomistic approach; Optical properties; absorption and fluorescence, refractive index and homogeneity; Mechanical and thermal properties: specific volume, volume relaxation, volume and pressure, elastic and internal friction behaviour, heat capacity and heat conduction, strength, crystallization.

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Properties and structure of vitreous silica. I

Structural transformations in non-metallic solids induced by energetic heavy ions at intermediate (∼1013−1016 ions/cm2) and high (∼ 1017 ions/cm2) doses normally take the form of amorphization, crystallization, or stoichiometry changes. Using the results of 72 substances both a temperature-ratio and a bond-type criterion are shown to be successful in predicting the occurrence of amorphization or crystallization. The former, based on a physical model involving thermal spikes, states that amorphization should occur whenever the ratio (crystallization temperature)/(melting point) exceeds 0.30. The latter, of more or less empirical origin, states that amorphization should occur whenever the ionicity is ≤0.47. Stoichiometry changes are basically different from amorphization and crystallization and this is reflected in the criterion. It is shown that whether or not a substance (such as AgBr or TiO2) loses a component (such as Br or O) during bombardment correlates with the heat of atomization showing a...

Criteria for bombardment-induced structural changes in non-metallic solids

The rate‐theory approach of Eyring and the free‐volume theory of Cohen and Turnbull for liquid viscosities have been reconsidered. The rate theory has been reformulated using the Cohen and Turnbull expression for the probability of finding a vacant site and yields an equation of the form η=A0exp[Ev*/RT+γV0/Vf]. This result has been applied to many liquids ranging from fused silica to liquid argon and including polyatomic van der Waals as well as hydrogen‐bonded liquids.Consistent fits to both temperature and pressure dependence of viscosity were obtained. A significant conclusion is that the free volume of liquids is considerably higher than that suggested by the WLF and Cohen and Turnbull equations.

On the Relative Roles of Free Volume and Activation Energy in the Viscosity of Liquids

Densification of silica glass was carried out at IS kb and 2000°C and of boron oxide glass at 20 kb and 900° C. The subsequent annealing of the densified glasses was examined at atmospheric pressure. The annealing behavior of glasses densified in the nonrigid condition is shown to be drastically different from that of the rigidly densified materials. The annealing kinetics are discussed in terms of two different processes of volume flow.

High-pressure Effects on Oxide Glasses: III, Densification in Nonrigid State

The annealing or volume variation with time of both silica and boron oxide glasses densified in the rigid state was studied. Appreciable annealing of silica glass was observed at temperatures as low as 200°C and of boron oxide glass at even room temperature. This anomalous volume flow depended on temperature, time, and specific volume and was characterized by small apparent relaxation times and activation energies. The differences between this type of volume relaxation and other properties such as delayed elasticity, cold flow, and stabilization are discussed. Devitrification to α-cristobalite observed at temperatures as low as 500°C is attributed to the presence of shear stresses.

High-Pressure Effects on Oxide Glasses: II, Subsequent Heat Treatment

Crystalline B2O3 was readily obtained from the glassy phase at high pressures and temperatures. Single crystals 0.3 mm. in dimension of both α-B2O2 (ordinary hexagonal) and β-B2O3 (dense monoclinic) were prepared. The difference in the density of the two forms was about 20%. Crystals of β-B2O3 were only slowly attacked by H2O and dilute HF. At atmospheric pressure, transitions between the α-, β-, and liquid phases were extremely sluggish. A tentative phase diagram was constructed up to 1100°C. and 90,000 atmospheres, the probable melting curve being calculated by the use of the Simon melting equation.

Crystallization and Phase Relations of Boron Trioxide at High Pressures

A general principle for the preparation of semiconducting oxide glasses is presented. Some borates, phosphates, and germanates were prepared. The mechanism of electrical conduction was demonstrated to be electronic rather than ionic by (a) the absence of electrolysis, (b) the absence of electromotive force when the glass was used as an electrolyte, and (c) the magnitude of the activation energies for conduction.

Semiconducting Oxide Glasses: General Principle for Preparation

The melting point of quartz is established to be below 1450°C. and possibly above 1400°C. The product of fusion is a liquid of high viscosity which undergoes slow transformation to give the commonly encountered vitreous silica. At temperatures between 1450deg; and 1700°C., both cristobalite and glass are formed when quartz is heated in air for periods in excess of 15 minutes. The quartz-cristobalite transformation is sensitive to the ambient atmosphere. After heating to temperatures above 1730°C., inhomogeneities are still present in both fused quartz and fused cristobalite in the form of unmelted microcrystals. Their presence is indicated by X-ray diffraction, metallographic, infrared, and hardness measurements.

J. D. Mackenzie

Papers

High-pressure Effects on Oxide Glasses: III, Densification in Nonrigid State

High-Pressure Effects on Oxide Glasses: II, Subsequent Heat Treatment

Crystallization and Phase Relations of Boron Trioxide at High Pressures

Semiconducting Oxide Glasses: General Principle for Preparation

Fusion of Quartz and Cristobalite