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 11B, 27Al and 29Si magic angle spinning (MAS) NMR spectra of 0.139MO (or M′2O) · 0.673SiO2 · (0.188−x)Al2O3 · xB2O3 · (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′2O 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, Kapp, of the reactions, Si(Q3) + B(Q3)=Si(Q4) + B(Q4), were determined. Here, the Si(Qn) or B(Q3) and B(Q4) represent the SiO2 structural units or three- and four-coordinated boron atoms, respectively. The subscript are the number of bridging oxygen atoms. The Kapp values indicate the reactivity of MO or M′2O with B2O3 and SiO2 in the MO(M′2O) · 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.