Nuclear magnetic resonance studies of 0.139MO (or M′2O)•0.673SiO2•(0.188-x)Al2O3•xB2O3 (M = Mg, Ca, Sr and Ba, M′= Na and K) glasses

The B-11, Al-27 and Si-29 magic angle spinning (MAS) NMR spectra of 0.139MO (or M-2'O)(.)0.673SiO(2)(.) (0.188 - x)Al2O3 (.) xB(2)O(3) - (M = Mg, Ca, Sr and Ba, M' = Na and K) glasses were examined. When three network forming oxides such as SiO2, Al2O3 and B2O3 coexist, Al2O3 reacts preferably with network modifier such as M-2'O or MO. The reactivity of residual network modifier with SiO2 was high compared with B2O3 in the order of K2O < Na2O < BaO < SrO < CaO < MgO. The apparent equilibrium constants, K-app, of the reactions, Si(Q(3)) + B(Q(3)) = Si(Q(4)) + B(Q(4)), were determined. Here, the Si(Q(n)) or B(Q(3)) and B(Q(4)) represent the SiO2 structural units or three- and four-coordinated boron atoms, respectively. The subscript are the number of bridging oxygen atoms. The K.pp values indicate the reactivity of MO or M-2'O with B2O3 and SiO2 in the MO(M-2'O) (.) SiO2 (.) Al2O3 (.) B2O3 glasses. The reaction proceeds to the left direction in the order of MgO > CaO > SrO > BaO > Na2O > K2O. The order of the reactivity is the same as that of the electronegativities among alkali and alkaline earth metals. (C) 2003 Elsevier B.V. All rights reserved.