| Fourier transform infrared (FTIR) spectroscopy was used to determine the surface and bulk structural relaxation kinetics of high purity vitreous silica. The fundamental asymmetric stretching vibration located at ∼1120 cm−1 was monitored in reflection mode to determine the surface structural relaxation times, while the first overtone of that band, near 2260 cm−1, was monitored in transmission mode to determine the bulk structural relaxation times.; Two different sets of silica glass were investigated. Both glasses were high purity silica, the main impurity being chlorine that was added during the production of the glass to remove water, as both glasses were designed to have exceptional optical transmission properties. One set of glass, provided by the Furukawa Electric Co., was cut from a densified preform that had not yet been drawn into fiber, while the other set of glass was commercially available optical fiber. The as-received fictive temperature of the densified preform glass was measured to be approximately 1100°C while the as-received fictive temperature of the optical fiber glass was estimated to be near 1600°C. Therefore, an initial heat-treatment was needed to change the fictive temperature of each glass to the same value. It was found that despite the differences in geometry, that both surfaces relaxed at the same rate, within experimental error, as long as each type of glass had the same initial fictive temperature.; The surface structural relaxation kinetics were measured as a function of temperature, water vapor pressure, and applied stress, while the bulk structural relaxation kinetics, being immune to the presence of water vapor or an applied stress, were measured only as a function of temperature. The surface of vitreous silica was found to relax at a faster rate than the bulk. Furthermore, the presence of water vapor or an applied tensile stress accelerated the kinetics of surface structural relaxation, while an applied compressive stress decelerated the rate of surface structural relaxation.; An equation was developed that estimates the surface structural relaxation time of high purity silica for any temperature, water vapor pressure, and applied tensile stress. This equation suggests that although the traditional relaxation times of bulk silica are extremely long at room temperature, the surface can relax significantly faster, and under certain conditions may have an appreciable rate of structural relaxation at room temperature. For example, an applied stress of 1.5 GPa and a water vapor pressure of 526 Torr at room temperature has an estimated surface relaxation time of approximately three years, some 47 orders of magnitude faster than bulk relaxation at the same temperature! Thus, the common belief that silica glass cannot relax at room temperature is not entirely correct. It is true that the bulk structural relaxation is so slow that it can be considered nil at room temperature. However, it is suggested that this is not necessarily the case for the surface. Therefore, as technology continues to decrease the size of devices, structural relaxation of the surface of vitreous silica at low temperatures, i.e. far below the glass transition temperature, may need to be considered as a potential source of change for the properties and performance of a device during its service lifetime. |
