Albert Napolitano

Inadequacies of Viscosity Theories for B2O3

The viscosity of B2O3 glass was measured from 1010–1014 P by the fiber‐elongation method and found to be Arrhenius with an activation energy of 94 kcal/mol. These new data were combined with previously reported rotation data (101–1010 P) and gave a smooth plot. The complete viscosity data were used to test the equations of the best‐known viscosity theories but no reasonable fits were found. Therefore, an examination of the validity of the basic assumptions underlying these theories was made. As a result of this study, it was found that the temperature dependence of the viscosity is not controlled by structural effects such as free volume, configuration entropy, etc., but by activation energy effects in the viscous flow process. The onset of the non‐Arrhenius region is a direct consequence of the appearance of a symmetric distribution of relaxation times and/or a distribution of activation energies which cannot be explained by existing viscosity theories. Finally, all theories are in error in predicting th...

Supercritical Viscosity Anomaly in Oxide Mixtures

The viscosity anomaly, previously detected above the critical point of some fluid binary mixtures, is investigated in the sodium–borosilicate oxide system. At the sodium–borosilicate dome, the critical composition is selected with small CaO and Al2O3 doping additions to obtain samples whose critical temperatures range over 190°C and have minimum high‐temperature structural differences. Viscosities between 102 and 109 P (10−1 N·sec/m2) are measured. A fractional excess viscosity is defined as the difference between the viscosity of each melt and a normal viscosity function, divided by the normal viscosity. The normal viscosity function is taken to have the form of the viscosity curve from the sample with the lowest critical temperature. The fractional excess viscosity then shows an anomalous increase near the critical point. The fractional excess viscosity, Δη / η0, attains a value of 2 near the consolute temperature but appears to remain finite at the critical point. This critical‐point effect can be dete...

Interaction of microstructure development with viscous flow processes in glass

Abstract Measurements in glasses undergoing phase separation showed a large increase of viscosity with heat-treatment time. A correlation of this effect with electron micrographs of the structure indicated that the change in viscosity was related to an increase in size of the microstructure. A theoretical analysis is presented which is based on a model relating point-to-point variations in molecular environments to viscous flow processes in glass.

Investigation of liquid-liquid phase transitions in oxide melts by viscosity measurements

Results from viscosity measurements conducted both above and below the liquid-liquid phase transition of a series of molten oxide glasses are reported in order to analyze the effect of supercritical composition fluctuations on viscous flow, and to investigate the mechanisms of phase separation. Measurements of four oxide mixtures with similar high temperature structures and widely different critical temperatures, revealed an anomalous increase in viscosity at temperatures above the critical point. The anomalous increase occurs when large composition fluctuations characterizing the critical point are present. The effect is explained in terms of an interaction between viscous flow and the supercritical fluctuations through the structural relaxation process. An analysis of this interaction is presented.Measurements conducted at temperatures slightly below the critical point of one of these glasses indicate that the microstructure resulting from the phase separation is highly sensitive to the preceding heat-treatment. Phase separation by the formation of isolated spheres of the silica-rich component is identified a few degrees below the critical point. Further measurements of viscosity by a fibre elongation method, conducted far below the critical temperature, are reported in order to analyze the growth mechanisms occurring in the separated phases. In this case, the rearrangement stage of phase separation is characterized by a growing interconnected structure.