Radiation Effect on the Viscosity of the Simple B 2 O 3 Glasses
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extremely large (750 eV/nm) and localised along the path of the ions. We proposed a model based on the fact that at such a very large dE/dx, the glass must be extremely fluid during a very short time in the wake of the ion (thermal spike) [2]. The purpose of this paper is to report new results about the effect of 2.5 MeV electron irradiations on the creep velocity of B 20 3 glass fibers. With electrons, dE/dx is very small (0.29 eV/nm) compared to swift ions and results in interactions with matrix electrons or atoms with small energy transfers. The electron flux is such that the mean energy deposition per unit volume and per unit time (0.15 eV/nm 3 s) has the same order of magnitude as during Ar swift ion irradiations (0.52eV/nm 3s). However in contrast to swift ions where the energy deposition takes place along the path of the ions in a cylinder of radius approximately equal to 20nm, the energy deposition under electron irradiations is homogeneous and the defect created can be considered as isolated. A study of the viscosity of the B20 3 was already reported [3, 4] but it was at very high electron flux giving a mean energy deposition per unit volume and per unit time five orders of magnitude larger (2 104 eV/nm 3 s). Futhermore, the method was very indirect (study of the coagulation of lead particles performed in situ in transmission electron microscopes) and the viscosity was very low compared to the viscosity obtained under swift ion irradiation. In this paper we report also an attempt to measure the viscosity of a borosilicate glass under irradiation. 117
Mat. Res. Soc. Symp. Proc. Vol. 504 © 1998 Materials Research Society
EXPERIMENTAL B20 3 glass fibers have been drawn out from the melt. Their diameter d was 1.5 mm. The specific experimental apparatus built for high energy heavy ion irradiation [2] has been fixed at the end of one of the line of our 2.5 MeV electron Van de Graaff accelerator. It is able to measure viscosity between 109 and 1014 Ns/m 2 . A 12 I.tm stainless steel window separates the experimental device, in which a 1 bar Helium atmosphere is introduced, from the vacuum of the accelerator line. Only 3 cm of the 10 cm of the fibers was irradiated. The heating of the irradiated part is only due to the energy loss of electrons within the glass. With the chosen diameters of the fibers,
the energy is mainly dissipated by convection at the surface and not by conduction along the fiber; the temperature is then almost constant on the 3 cm irradiated. The total loss of energy throughout the fiber does not exceed 0.3MeV. In this experiment, the temperature of irradiation depends on the electron flux that varies from 6.2 1012 e-cm-2s- 1 to 4.2 1014 e-cm- 2 s-1. A first evaluation of the temperature in the irradiated area is carried out through a sapphire window by means of a radiometric infrared microscope assuming both a transmittance of the window and an emissivity of the sample equal to the unity. The flux is measured using a Faraday cup. RESULTS AND DISCUSSION We have shown previously that, under
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