The Influence of Structural Relaxation upon the Low-Temperature Thermal Conductivity of Ancient Natural Glasses

It has been observed that the experimental low -temperature (<10K) properties of glasses depend upon a sample's thermal history. Apparently, the intrinsic glassy excitations (TLS) which dominate the low -temperature properties of glasses are affected by the structural relaxation which occurs in a glass ample in order to move that sample toward the equilibrium configuration for its current temperature. Investigations of structural relaxation in glasses show that relaxation processes involve a broad spectrum of relaxation rates and that the rates of the processes which prevail in a given experiment decrease dramatically with decreasing temperatures. Previous studies of low-temperature glassy properties on samples subjected to various thermal schedules have employed heat treatments at high temperatures, near or above the glass transition temperature, T_{rm g}, for relatively short annealing periods (<10 ^{-2} years). The present investigation studies relaxation behavior and its effects on the low-temperature TLS on the time scale of 10 ^4-10^7 years. These unusually long annealing times are made accessible to the laboratory by studying ancient natural glasses--amber, a fossil resin and obsidian, a volcanic glass--which have been annealed in the earth over geologic times. The low -temperature TLS behavior of the as-received glass was recorded via thermal conductivity, kappa, measurements (0.07K < T < 10K), and subsequently, the sample was heated above its T_{rm g} and then quenched to change the structural state of the sample. kappa was measured again and compared to the as-received measurements. The obsidian kappa showed no significant change after heat treatment above T_{rm g} . A possible explanation for this result could be the existence of a finite lower bound for the temperature range within which structural relaxation can occur. Such a temperature range is seen in polymers and the geologic annealing temperature for obsidian ~0.3T _{rm g} is outside the ranges commonly seen in polymers. The amber kappa showed a ~5.5% reduction in magnitude after heat treatment above T _{rm g}, similar to the results seen for metallic glasses annealed for ~ 10^{-3} years. This suggests that the long-time relaxation processes which prevail in the amber experiment and the short-time relaxation processes which prevail in the glassy metal experiments have a common origin. At this time, no theoretical model appears to be able to explain the relationship between structural relaxation and the low-temperature TLS for all materials measured. Finally, if in fact 10^6 years is a long enough annealing time for amber to reach its equilibrium configuration at 295K, then some minimum density of low-temperature TLS must be included in the equilibrium state of a glass.