Structural and vibrational relaxations during aging of an ultralow-expansion glass

Abstract

We accurately measured the aging of an ultralow-expansion (ULE) glass for 1250 days using a diode laser resonated in the glass cavity at −3.3 °C, where the glass has zero thermal expansion coefficient. We observed that the glass shows exponential and linear decreases in length ΔL(d)/L(0)=A0[exp(−d/τ0)−1]−K0×d, where d is the day, τ0 is the relaxation time, and A0 and K0 are constants. Equation shows the structural relaxation with a spontaneous linear contraction after the termination of a classical exponential contraction. It was observed that the rate of linear contraction K0 becomes slightly smaller after an extremely long aging time. These results indicate that after prolonged aging, the length of the glass spontaneously decreases at a constant rate with respect to time, and that its linear contraction becomes weaker due to glass densification.In addition, it is theoretically shown that glass aging is a nonlinear irreversible process with the spontaneous relaxations of a linear contraction and a dissipated heat. The origin for the linear contraction is an osmotic pressure which is spontaneously generated during glass aging, and the origin for the dissipated heat is the irreversibility on thermal vibrations of molecules. The result obtained is a verification of the Glansdorff–Prigogine evolutional criterion, which is first applied to a structural relaxation phenomenon in this paper. We conclude that a glass reaches a steady nonequilibrium state, which has the dissipative structure both in a steady configurational state and in steady irreversible processes.