Showing papers in "Journal of Non-crystalline Solids in 1968"

Conduction in glasses containing transition metal ions

Abstract: In a discussion of conduction in glasses containing transition metal ions, the following points are stressed: 1. (a) The process is similar to “impurity conduction” in doped and compensated semi-conductors. 2. (b) There should be two terms in the activation energy, the Miller-Abrahams term and a polaron hopping term. 3. (c) Both terms should tend to zero, giving a decreasing slope of the ln p versus 1/T curve, as T → 0. 4. (d) The Heikes-Ure formula for the thermopower is discussed and a tentative explanation given of the difference between vanadium- and iron-containing glasses.

Primary and secondary phase separation of sodium silicate glasses

Abstract: The immiscibility curve in the system Na2OSiO2 shows that the amount of Na2O content in the silica rich phase is small at 500°C and hardly changes with temperature up to 800°C but increases sharply in the interval 800–865°C The silica content of the alkali-rich phase increases continuously with temperature The curve is thus extremely asymmetric The electron microscope photographs show that the separate phases (primary phase separation) separate again into two phases during cooling or during a second heating of the glass at a lower temperature This secondary phase separation does not depend upon the duration of the initial heating of the glass It is connected with the asymmetry of the immiscibility curve The alkali-rich phase separates more easily but with the help of a repeated treatment the silica-rich phase can also be caused to phase separate This secondary phase separation is a general phenomenon and must be given attention when discussing the number of phases in the glass Certainly the existence of triple-phase glasses cannot be excluded Phase separation of higher orders can be achieved with the help of heat treatments in several steps

Measurements of electrical conductivity and optical absorption in chalcogenide glasses

Abstract: Glasses in the systems As2S3As2Se3 and As2Se3As2Te3 have been studied. Optical absorption at room temperature and electrical conductivity at temperatures between 30°C and 150°C have been measured. It is concluded that no single model of conductivity is applicable to all materials. The results are consistent with the suggestion that the conductivity may be controlled by potential barriers arising from structural and compositional fluctuations.

Fracture phenomena and strength properties of chemically and physically strengthened glass: I. General survey of strength and fracture behaviour of strengthened glass

Abstract: One of the methods most frequently used for reinforcing glass objects is to introduce compressive stresses at the surface Such stresses can be produced physically, as in the thermal toughening process, or chemically, by applying a film of a lower coefficient of thermal expansion than the glass body itself, or by producing a compressive surface layer making use of the “ion stuffing” technique The advantages and disadvantages of the different strengthening methods are discussed with relation to the observed strength values (measured either by quasi-static loading or by impact loading) and the fracture behaviour of reinforced glass objects

The structure and strength of glass fibers

Abstract: The strength and structure of flawless and commercial glass fibers are discussed and compared with those of bulk lasses. Particular attention is directed towards the nature of the surface layer of glass fibers which is responsible for the high strength. From a consideration of structure and defect types, it is concluded that glass strengths can be categorized into four levels. The magnitude and nature of these four strenght levels are discussed.

Luminescence centers in lead silicate glasses

Abstract: Absorption spectra and luminescence spectra of lead silicate glasses excited by the 2537 A Hg line have been studied at liquid nitrogen temperature. The samples studied were three lead glasses of the system K 2 OPbOSiO 2 having a PbO content of 1.5, 13.5 and 28.5 mol%, PbCl 2 and BaCl 2 (Pb). The glassy samples having 1.5 and 13.5 PbO mol% show three emission maxima at about 2.7, 3.4 and 3.9 eV respectively; BaCl 2 (Pb) at 2.78 3.50 and 3.94eV. PbCl 2 and the glass containing 28.5 mol% of PbO show a single luminescence maximum at 2.78 and 2.54 eV respectively. The relative efficiency (η) and the activation energy of thermal quenching ( E s ) have been measured. The Stokes shift (Δe) has been evaluated from emission and absorption data. The relation between, η, E s and Δe is discussed. Experimental results suggest that: a) Emission of lead silicate glasses is not related to Pb 0 atoms but to Pb ++ ion levels; b) The visible emission, both in vitreous and in crystalline compounds, is related to the 3 P 0 → 1 S 0 transition; c) The thermal dependence curves of luminescence efficiency agree with a localized center both in glass and in crystals; d) The Stokes shift, activation energy of thermal quenching and the luminous efficiency can be related by means of multiphonon theory. The results confirm the similarity of emitting centers in glasses and in crystals.

Glass formation in alkali molybdate systems

Abstract: Glass formation in alkali molybdate systems is investigated through measurements of the critical cooling rate, i.e. the cooling rate necessary to prevent crystallization entirely. Similarities and differences with alkali tungstate systems are noted, and the latter are explained by postulating a more continuous transition of the molybdenum ion from tetrahedral to octahedral coordination with decreasing alkali oxide content of the systems as compared with the behavior of tungstates, where tetrahedral coordination is found in all glasses. In addition, disproportionation of dimolybdate ions into monomers and higher polymeric units is assumed to be less pronounced in molybdate systems than the disproportionation of ditungstate ions in the corresponding tungstate systems. Density and thermal expansion data on molten alkali molybdate systems indicate that glass forming melts have a relatively spacious structure, in accordance with the idea that glass formation is facilitated by the formation of chains of (distorted) tetrahedra.