On radiation-induced fluidization (quasi-melting) of silicate glasses
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On radiation-induced fluidization (quasi-melting) of silicate glasses Michael Ojovan and Gunter Mobus Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK ABSTRACT The effect of intensive electron radiation on viscous flow in silicate glasses is analysed and shown that it can result in a many orders of magnitude decrease of viscosity and stepwise decrease of activation energy of flow. Fluidisation or quasi-melting of glasses on intensive electron irradiation is caused by bond breaking via the radiation-chemical reaction Si-O-Si + e- Si-O + Si + (e-) which weakens the silicate glass network and leads to five-fold coordination of oxygens around the silicon. An explicit equation of viscosity was obtained for irradiated glasses as well as an equation for glass transition temperature. The assessments of temperature increase by electron radiation show that radiation-induced fluidisation of glasses can occur at minimal thermal effects. Radiation-induced fluidisation of glasses can result in nanoscale patterning effects caused by surface tension forces. Changes in the viscous flow behaviour are also important in conditions of long-term irradiation for glasses used in nuclear installations as well as for nuclear waste glasses.
INTRODUCTION Long-term behaviour of materials in conditions of continuous irradiation is one of the topics of current interest. New materials and technologies are required to meet the demands of prolonged exposure to ionizing radiation and increased service life for the next generation of reactors including both fission and fusion nuclear reactors as well as to ensure a reliable immobilisation of resulting nuclear waste [1,2]. Vitreous silicate and phosphate materials are used for several decades to immobilise nuclear waste and are planned to be used at an extending scale in the forthcoming years. Vitreous silica is often used as optical fibres operating in strong radiation fields. The effect of irradiation on glasses has been studied in many works. A comprehensive overview of radiation effects in glasses used for immobilization of high-level waste and plutonium disposition has been given in [3]. Radiation-induced effects in vitreous and crystalline materials may significantly change their properties. For example irradiation of both glass and ceramic insulators causes intensification of surface diffusion processes and may result in surface instability [4]. We have earlier analysed the radiation-induced unbinding of alkalis from non-bridging oxygens in silicate glasses [5]. The unbinding of alkalis M such as radioactive Cs or nonradioactive Na occurs by radiation-chemical reaction: Si-O-M+ + irradiation Si-O + M, when a free hole resulting from the irradiation is first trapped at the Si-O-M+ site (leading to formation of an non-bridging oxygen hole centre) and then the alkali M is free to diffuse away to the site of a trapped electron where it stabilises the uncompensated charge [3]. For alumina-
containing g
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