Silicon-29 Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy of Sintered Silicon Nitride Ceramics

Silicon-29 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is used to study the grain boundary phases that result during sintering of Si3N4 powders in the presence of yttria and alumina added as sintering aids. A series of five Si3N4 ceramic cylinders are examined. The densification of alpha-to beta-Si3N4 is clearly observed, and alpha/beta ratios are determined by an NMR spectral subtraction/integration procedure when incomplete conversion occurs. Y-Si-O-N phases resulting from the reaction of Y2O with the alpha-Si3N4 during high-temperature sintering are observed by the Si-29 MAS NMR technique. When compared with quantitative powder X-ray diffraction analysis of the same samples, differences in composition are observed in the sintered ceramic components that may be explained by the presence of amorphous phases as well as paramagnetic impurities. The comparison of techniques points out the range of applicability of each analytical approach and suggests that the best way to quantify grain boundary phases in sintered Si3N4 ceramic requires a multiple technique approach.