Vaporization of alkali borosilicate glasses

Abstract

The vaporization behavior of glasses in the R 2O·B 2O 3·3SiO 2, Na 2O·0.15R′ 2O·B 2O 3·3SiO 2 and 0.85Na 2O·0.15R′ 2O·B 2O 3·3SiO 2 systems has been studied by a mass-spectrometric Knudsen effusion method in the temperature range 806–1172 K. Here, R is an alkali metal and R′ an alkali metal other than Na. Vapor species of RBO 2(g) and R 2(BO 2) 2(g) have been identified over the glasses in the R 2O·B 2O 3·3SiO 2 system and those of NaBO 2(g), R′BO 2, Na 2(BO 2) 2(g) and NaR′ (BO 2) 2(g), except for NaLi(BO 2) 2(g), over the glasses in the other two systems. The vapor of Na(g) has been further observed in the initial stage of heating over the glasses in the Na 2O·0.15R′ 2O·B 2O 3·3SiO 2 system, while no vaporization of R′(g) occurs. Chemical activities of RBO 2 and R 2(BO 2) 2 pseudo components in the glasses become small on going Li to Cs, that is, the vaporization of CsBO 2(g) and Cs 2(BO 2) 2(g) is most suppressed thermochemically from the glasses. Enthalpies of formation and dissociation energies for R 2(BO 2) 2(g) and NaR′(BO 2) 2(g) are not so different from one another, except for D 0°(Li 2(BO 2) 2).