DISLOCATION LOOPS IN ALPHA-SILICON NITRIDE

Thin foils of fully dense, but only partially transformed, hot-pressed silicon nitride materials have been examined by high-resolution trans mission electron microscopy. Large untransformed grains are seen to co ntain high densities of nanometer dimension, round or oval, textural f eatures. Examination at high magnification shows these to be clusters of dislocation loops, which are concentrated towards grain centers. Th e loops occur most commonly in grains of materials with a high proport ion (almost-equal-to 70%) of unconverted alpha-Si3N4, as shown by quan titative X-ray diffraction analysis. They are seen less often in mater ial more fully converted to the bets-Si3N4 phase (almost-equal-to 5% a lpha-Si3N4). The loops are believed to form by condensation from super saturated concentrations of vacancies during rapid cooling from densif ication temperature. The loops are confined exclusively to the grain i nteriors, and in consequence an estimate of the nitrogen vacancy diffu sion coefficient can be made.