Vidros boratos de cálcio e sódio: preparação e propriedades

Abstract

Borate glasses in different compositions have attracted the interest of researchers in the most diverse areas of technological application due to their low cost, ease preparation, in addition to their thermal, optical, electrical properties, among others. Pure boron oxide, B2O3, is known to be a glass former with high forming ability, however, its low chemical stability makes it necessary to add modifying elements to improve the glass properties, especially regarding to water adsorption. The addition of alkaline earth metal oxides as modifiers to the network of B2O3 glass is known to improve its properties, however, a liquid-liquid phase separation makes it difficult to obtain a homogeneous liquid during the melting process, which favors processes of unwanted crystallization, named devitrification. In the present work, high quality glass matrices (free of bubbles, streaks, cracks or crystallization) were obtained in the ternary system 66.67B2O3 – (33.33 - x)CaO – xNa2O, with x ranging from 0 to 33.33 in mol %, named as CaNaBO-x. The compositions ranged from calcium tetraborate, (x = 0) CaB4O7, to sodium tetraborate, (x = 33.33) Na2B4O7. Glass samples were obtained by the melting method at 1000 °C in a Pt/Au crucible, and molding followed by annealing at 500 °C. The physicochemical properties of glasses were evaluated by density, absorption in the mid-infrared region (FTIR) and thermoluminescence (TL) measurements, in order to evaluate the potential of these matrices in radiation dosimetry. All samples showed intense thermoluminescent emission, with maximum temperature ranging from 116 °C to 176 °C. However, the CaNaBO-5 sample proved to be the most interesting for radiation dosimetry because it allows trapping a greater number of charge carriers, with high TL emission intensity and maximum temperature, which is still interesting from the TL stability point of view. The concentration of Na2O in these samples is sufficient to avoid the liquid-liquid phase separation commonly observed in binary systems containing alkaline earth metals, which is a very important factor in the process of high-quality glass preparation.