29Si MAS-NMR Study of the Short-Range Order in Potassium Borosilicate Glasses

Potassium borosilicate glasses were prepared in families having the general formula of RK2O·B2O3·NSiO2, where R is the ratio of potassium oxide to boron oxide and N is the ratio of silicon dioxide to boron oxide. The glasses were prepared for values of R ranging from 0 to 7.0 in the families N= 0.5, 1.0, 2.0, and 4.0. 29Si MAS‐NMR measurements were performed on these glasses to determine the short‐range order around the silicon atom. A model of proportional sharing of the added potassium oxide between the silicate and the borate groups was suggested. This model was tested against other suggested models where proportional sharing begins after a minimum amount of potassium oxide, R0, and was observed to provide a better fit to the 29Si chemical shifts obtained. As was observed in the 29Si MAS‐NMR studies of the RLi2O·B2O3·NSiO2 glasses, the proportional sharing model with R0= 0 is in stark disagreement with that proposed by the 11B NMR studies of the alkali borosilicate glasses. This problem is as yet not understood. Since K2CO3 was used as the starting material for K2O, it was observed that at large R values, R > RCO2, where RCO2= 2.3 for N= 1, RCO2= 4.0 for N= 2, and RCO2= 5.0 for N= 4, CO2 was retained in the melt in the fashion similar to that observed for other high‐alkali borate and silicate glasses. The N= 0.5 family did not exhibit retention at the compositions studied. 29Si MAS‐NMR could be used to determine where CO2 retention began in composition and the proportion of K2O/K2CO3 in the melt (glass).