Effects of fast neutron and gamma irradiation on electrical conductivity of some borate glasses

Abstract

Electrical conductivity of samples of Li₂O–B₂O₃ binary glass system containing Al₂O₃, PbO, Fe₂O₃, TiO₂ or V₂O₅ was measured at temperatures ranging between 30 and 200 °C before and after irradiation with fast neutrons or γ-rays. Base and Al₂O₃-containing glasses showed an initial rise in conductivity with the increasing temperature, followed by a steep drop, then a more gradual increase. Glass samples containing lead or one of the transition metal oxides showed a linear pattern of electrical conductivity in response to heating. In these glasses activation energy varied depending on the coordination number of the transition metal ion involved. These changes in electrical conductivity in response to temperature are ascribed to changes in the internal structure of the lithium borate glass, which is also affected by the presence of aluminum, lead or transition metals. The effects of exposing the studied glasses to irradiation were attributed to irradiation-induced changes in the configuration of the glass network, including the formation of matrix defects.

Introduction

In many applications, glass may serve as an electrical insulator or conductor, requiring an understanding of its electrical conductivity. Ionic transport is also taken into account in manufacturing glass for other applications since ionic motion in the amorphous silica layers of silica devices can interfere with their performance and must often be suppressed.

In most oxide glasses, electrical conductivity results from ionic motion. In certain glass compositions containing multivalent oxides, such as vanadium pentoxide or iron oxide, conduction is electronic. Most chalcogenide glasses, which contain pure or combined sulfur, selenium or tellurium, are also electronic conductors. The semiconducting and switching properties of these glasses have excited a great interest in their use in the electronics industries. The `salt' type glasses of halides, nitrates, sulfates, and aqueous solutions are ionic conductors. Organic glasses can be either electronic conductors or show ionic conduction resulting from impurities.

Ionic conductivity of virtually all oxide glasses results from the transport of monovalent cations. In most commercial glasses the conducting ion is sodium. Faraday's law is found to hold in these glasses and a number of electrolysis experiments [1] have established the ionic nature of the conduction process. It has also been shown that lithium ions are also quite mobile in oxide glasses [1]. In addition, potassium and hydrogen ions are known [1] to sometimes carry current although their mobility is usually lower than that of Na⁺ and Li⁺.

Even in glasses with no nominal addition of monovalent ions, conductivity results from the transport of monovalent cations. In fused silica, electrolysis experiments show that sodium and lithium ions are the conducting species even though they are present only in quantities of few parts per million [2].

Electrical conductivity of a solid conductor can be measured with either direct or alternating currents. In direct current (dc) measurement a space charge is often set up in the glass because of partial blocking of the ionic current by the electrodes. Then the current decreases rapidly with time, and its value must be extrapolated to zero time if an accurate value of conductivity is desired. To avoid this electrode polarization, alternating current of a frequency from 10³ to 10⁶ Hz is usually used. Silicate glasses show dielectric losses at these frequencies and, therefore, care must be taken to make the measurement over a wide frequency range. If a constant sample resistance is found over several decades of frequency, one can be reasonably certain that an accurate value of conductivity is being measured. Such constancy is often not achieved, making many of the experimental measurements of electrical conductivity of glasses that have been reported in the literature unreliable [2]. The type of electrode and preparation of the glass surface can influence measured conductivity, especially at frequencies below 10 Hz [3]. Accordingly, polishing the glass surface drastically improves the accuracy of conductivity measurements.

Numerous studies [4], [5], [6] have been carried out on Li⁺ ion glasses because of interest in developing high energy density batteries. The mixing of two glass formers, however, has been found to yield glasses with higher electrical conductivity and better thermal stability compared with the corresponding single former-glasses.

It is generally believed [7] that the electrical conduction in transition metal oxide (TMO) glasses is due to the hopping of polarons between sites. Mott [8] suggested that the electrical conductivity in alkali borate glasses is due to mobile alkali ions, with conductivity of these glasses being about three orders of magnitude higher than that of barium borate glasses. These materials are, therefore, most suitable for the studying ionic conduction.

Ichinose et al. [9] studied the dc conductivities of glasses in the V₂O₅–SrO–B₂O₃ system. They suggested that dc conductivity is dependent on the amount of B₂O₃, increasing with the increase in B₂O₃ content and decreasing with the increase in SrO content.

Electrical conductivity measurements [8] of glasses containing vanadium at temperatures higher than room temperature suggest small polaron hopping conduction by the transfer of electrons between V⁴⁺ and V⁵⁺. At temperatures lower than room temperature, conductivity data can be readily fitted to a Mott's variable range hopping model [9].

The effect of irradiation on glasses is believed to depend on the type and energy of irradiation, glass composition and sample parameters such as temperature [10]. It is well established that radiation damage in glass leads to active defects. These defects can be introduced by ionization or atomic displacement mechanisms or via the activation of the preexisting defects [11].

The presence of impurities, such as alkali, alkaline earth and transition metals, in the glass increases radiation-induced defects [12]. These defects may be either permanent or temporary. Defect recovery mechanisms, such as optical bleaching, control the rate of recovery during and after irradiation [13]. Conductivity changes due to irradiation can, therefore, be sensitive to glass composition and temperature, as well as both the magnitude and rate of irradiation dose [14].

The aim of this paper is to study electrical conductivity of lithium borate glass and to investigate how it is affected by the presence of Al₂O₃ or PbO. The effect of introducing small amounts (2%) of a TMO, which can simultaneously exhibit ionic and electronic conduction, is also examined. We also investigate electrical conductivity of these glasses after being exposed to different radiation doses of γ rays or fast neutrons.