A time to search: finding the meaning of variable activation energy

This review discusses an approach to linking the variable activation energy of a process to the kinetic parameters of its individual steps.

Modgil-Virk Formulation of Single Activation Energy Model of Radiation Damage Annealing in SSNTDs: A Critical Appraisal

Passage of heavy ions produces radiation-damage trails known as latent tracks in a variety of solid-state nuclear-track detectors (SSNTDs). These tracks are made visible in an optical microscope by a simple process known as chemical etching. It is a well-known fact that latent tracks are radiation damage trails in SSNTDs, which can be annealed by thermal heating. Modgil-Virk formulation of single-activation-energy model of radiation damage annealing was proposed as an empirical approach for explaining the thermal fading of nuclear tracks in SSNTDs. The empirical formulation of this model is based on track annealing data collected from both isothermal and isochronal experiments performed on different types of SSNTDs using a variety of heavy ion beams and fission fragments. The main objective of this empirical model was to resolve some contradictions of variable activation energy derived by using Arrhenius plots to study annealing in mineral SSNTDs. Some equivalent versions of the Modgil-Virk model have been proposed but the concept of single activation energy is vindicated in all empirical formulations. The model always yields a unique value of activation energy independent of the nature of the ion beam used and the degree of annealing. The anisotropy of the mineral SSNTDs is revealed by variation in activation energy along different crystal planes and even with different orientations of the ion beam on the same plane. Some recent experiments are a pointer to the successful exploitation of this model for future cosmic-rays studies using SSNTDs.

About activation energy of viscous flow of glasses and melts

ABSTRACTData on a viscous flow model based on network defects – broken bonds termed configurons – were analysed. An universal equation has been derived for the variable activation energy of viscous flow Q(T) of the generic Frenkel equation of viscosity η(T)=A∙exp(Q/RT) which is known to have two constant asymptotes – high QH at low temperatures and low QL at high temperatures. The defect model of flow used by e.g. Doremus, Mott, Nemilov, Sanditov states that higher the concentration of defects (e.g. configurons) the lower the viscosity. We have used the configuron percolation theory (CPT) which treats glass–liquid transition as a percolation-type phase transition. Additionally the CPT results in a continuous temperature relationship for viscosity valid for both glassy and liquid amorphous materials. We show that a particular result of CPT is the universal temperature relationship for the activation energy of viscous flow: Q(T)=QL+RT∙ln[1+exp(-Sd/R) exp((QH-QL)/RT)] which depends on asymptotic energies QL (for the liquid phase) and QH (for the glassy phase), and on entropy of configurons Sd. This equation has two asymptotes, namely Q(T<<Tg) = QH, and Q(T>>Tg) = QL. Moreover we demonstrate that the equation for Q(T) practically coincides in the transition range of temperatures with known Sanditov equation.