Retarded Elasticity of Glasses in the System of As-Se

Abstract

Creep was measured for glasses in the system of As-Se by a torsional method under a constant torque in the glass transition range. Compliance, J(t), of retarded elasticity can be normally described as
J(t)=∑iJ_i(1-e^-t/τi) (1)
where t is time and τ is retardation time. When i was even 1, a correlation coefficient obtained for each run was higher than 0.95 and thus it was considered that calculated curves (at i=1) fitted well with the measured ones.
The micro-viscosity, η_G, of retarded elasticity (Fig. 1) can be obtained by using τ and J_∞ (compliance at the final stage in each run, ≅1/G₂). The plots of log ηG agaist 1/T gave essentially a straight line for each sample as shown in Fig. 6. Thus the apparent activation energy for the process of deformation due to retarded elasticity was calculated by the following equation, ΔE_ηG=R⋅d(logη_G)/d(1/T), where R is the gas constant. The activation energy was approximately one-half of that for viscous flow as shown in Fig. 7 and was lower than the bonding energy of As-Se.
Final compliance, J_∞, showed a maximum at η≅10¹⁴ poise in every composition of As-Se glasses. The following equation expressed as a function of viscosity is proposed,
J_∞=(1-k₂η_G/η)(f₁[k₁/η]+f₂[nk₁/η]) (10)
where k₁, k₂, n, f₁ and f₂ are parameters for fitting. When [k₁/η] and [nk₁/η] exceed 1, they were always replaced by 1. This equation means that J_∞ is mainly dominated by the term, (1-k₂η_G/η), in lower viscosity range (η<10¹⁴ poise), while in higher viscosity range (η>10¹⁴ poise) it is controlled by the term, [k₁/η] or [nk₁/η]. The term (1-k₂η_G/η) formulates the competition between η_G and η, and the terms [k₁/η] and [nk₁/η] can be attributed to the interactions of layers in glass structure. The calculated curves based on Eq. (10) agreed well with the measured values as shown in Fig. 8 (a), (b) and (c).
Coefficient f₁ was found to decrease with increasing Se content and f₂ increased conversely with Se content (Fig. 10). Though k₂ should theoretically be equal to 1, it decreased with As content. In addition, it was found that f₂ and k₂, when they were plotted against the Se atom fraction, changed appreciably their slopes at Se≅80 atom% (around As₂Se₈) as shown in Figs. 9 and 10. It was therefore presumed that f₁ would depend on the ring structure of network and f₂ and k₂ would depend on the chain structure.