Roger J. Araujo

Selective polarization of light due to absorption by small elongated silver particles in glass.

Polarizing glasses were made by stretching glass containing colloidal silver. The position of the absorption bands depends strongly on the elongation. This dependence seems to be consistent with the predictions of the theory of light absorption by small metal particles based on a free electron model.

Statistical mechanics of chemical disorder: application to alkali borate glasses

Abstract An improvement on the formalism suggested by Araujo for studying chemical disorder is discussed. The formalism is used to calculate the fraction of boron atoms which are in tetrahedral coordination in alkali-borate glasses as a function of composition and temperature.

Colorless glasses containing ion-exchanged silver

Introduction of silver into a silicate glass often leads to reduction and coloration. The electron required for silver reduction is extracted from a nonbridging oxygen atom. The use of high-field-strength ions limits the number of nonbridging oxygens and silver reduction.

Statistical mechanical model of boron coordination

Abstract A statistical mechanical model is used for calculating the fraction of boron atoms in tetrahedral coordination in alkali aluminoborosilicate glasses. The results are compared with nuclear magnetic resonance measurements. The calculated structure of certain glasses as a function of temperature is correlated with physical properties. A model for the calculation of halogen solubility is also proposed. The calculated changes of halogen solubility accompanying changes of temperature are correlated with observed changes in nickel color and certain characteristics of photochromic glasses. Phase separation in alkali borates is predicted at low temperatures but not at high ones.

Ion exchange equilibria between glass and molten salts

Abstract It is shown that interaction between the ions undergoing ion exchange leads to a sigmoidal equilibrium curve. A negative interaction in the glass causes the ratio of concentration of the two ions to be more nearly equal to unity in the glass than it is in the molten salt. It is further shown that the Rothmund–Kornfeld parameter is a measure of the non-ideality of the system. A value of the parameter greater than unity indicates a negative interaction energy.

Transient absorption in excimer-exposed silica

The transient absorption produced in high-purity fused silica by exposure to a 193-nm excimer laser is investigated as a function of exposure, dissolved molecular hydrogen content, and hydrogen-related processing. Long-term recovery of transmittance was found to correlate with the dissolved molecular hydrogen concentration, whereas short-term fade was due to geminate recombination of an E? center with an H radical. The redarkening process was shown to be the result of photolysis of SiH, which regenerates color centers. When the silica was processed in a hydrogen atmosphere at high temperature and subsequently exposed at 193 nm, the glass was found to produce an absorption spike, a fast, recoverable decrease in transmittance. The origin of the spike was linked to the creation of a precursor produced in the thermal reaction of silica with hydrogen. The precursor can be identified by its signal in the Raman spectrum. It is suggested that the precursor has absorption at 193 nm.

Kinetics of Bleaching of Photochromic Glass

Glasses containing small particles of silver halide are caused to darken by the action of uv light. Fading occurs when the exciting light source is removed and is often enhanced by irradiation with visible light. It is shown that simultaneous optical and thermal bleaching can be separated mathematically and that optical bleaching is a first order process. A model for thermal bleaching based on diffusion is consistent with rate studies and qualitatively explains the dependence of the fading rate on the darkening conditions.

A statistical mechanical model for chemical disorder in interacting systems: Application to phase separation in alkali silicates, alkaline earth silicates and alkaline earth aluminosilicates

Abstract The present model provides a formalism for studying the effects of chemical disorder in a system of interacting atoms in which the number of neighbors of each kind of atom need not be the same. When they are the same, this model reduces to the quasi-chemical approximation. It is applied to a model for phase separation in alkali silicates and alkaline earth silicates which involves a mixture of fourfold and sixfold coordinated species. The sixfold coordinated species represents a pseudo-atom comprised of two silicon atoms each bonded to non-bridging oxygen atoms which are strongly associated. A miscibility dome similar to that observed in sodium silicates and barium silicates is predicted. The suppression of phase separation by alumina is predicted.

Opthalmic glass particularly photochromic glass

Abstract Photolysis of certain glasses containing minute silver halide droplets leads to visible absorption. The temperature dependence of the induced absorption is strongly correlated with the rate at which the glass reverts to its clear state upon the cessation of radiation. The factors which influence the compromises in fade rates and temperature dependence will be discussed.

Sodium redistribution between oxide phases

Alkali transport between a glass substrate and an oxide over layer occurs by a mechanism of ion exchange between the alkali and protons. Thermodynamics requires that the difference in chemical potential of the alkali and the proton be the same in both phases at equilibrium. The influence of chemical composition of the glass on the chemical potential of alkali ions is discussed. Good agreement between predictions of the direction of alkali transport and observed transport is obtained.