Local Structure of Amorphous (PbO)x[(B2O3)1-z(Al2O3)z]y(SiO2)y Dielectric Materials by Multinuclear Solid State NMR

Abstract

Structural speciation of glasses in the systems PbO−B2O3−SiO2, PbO−B2O3−Al2O3−SiO2, and PbO−Al2O3−SiO2 were studied using solid state 29Si, 27Al, 11B, and 207Pb nuclear magnetic resonance (NMR) and Raman spectroscopy. Application of these methods provided insight into the role of Al2O3 incorporation in the lead borosilicate glass networks. The general composition range studied was (PbO)x[(B2O3)1-z(Al2O3)z]y(SiO2)y where x = 0.35, 0.5, and 0.65, y = (1 − x)/2 and z = 0.0, 0.5, and 1.0. Additional insight was obtained via 27Al 2D-3QMAS experiments. The 207Pb spin−echo mapping spectra showed a transition from ionic (Pb2+) to covalently bound lead species with increased PbO contents in the borosilicate glasses. The addition of aluminum to the glass network further enhanced the lead species transition, resulting in a higher relative amount of covalent lead bonding in the high PbO content alumino-borosilicate glass. The number of BO4 units present in the 11B MAS NMR decreased with increasing PbO contents for both the borosilicate and the alumino-borosilicate glass systems, with the addition of aluminum further promoting the BO3 species. A deshielding of the 11B chemical shifts and the 27Al chemical shifts with increased lead contents (independent of changes in the quadrupolar coupling) was also observed for both glass systems and was attributed to an increasingly homogeneous glass structure. The 29Si spectra of the borosilicate and alumino-borosilicate glasses showed a downfield shift with increased PbO concentrations representing incorporation of Pb into the silicate and aluminosilicate networks. The Raman spectra were characterized by increased intensities of Si−O−Pb peaks and decreased intensities of Q3 peaks with increased PbO contents and showed no evidence of BO3 or BO4 ring species. Both the NMR and the Raman data point toward systems where lead is increasingly incorporated into the B2O3−SiO2 and the B2O3−SiO2−Al2O3 networks at high PbO concentrations, with the addition of Al2O3 enhancing the trend.