Dislocation Loops in a‐Silicon Nitride

Thin foils of fully dense, but only partially transformed, hot pressed silicon nitride materials have been examined by high‐resolution transmisson electron microscopy. Large untransformed grains are seen to contain high densities of nanometer dimension, round or oval, textural features. Examination at high magnification shows thest to be clusters of dislocation loops, which are concentrated towards grain centers. The loops occur most commonly in grains of materials with a high proportion (70%) of unconverted α‐Si 3 N 4 as shown by quantitative x‐ray diffraction analysis. They are seen less ofte in material more fully converted to the β‐Si 3 N 4 phase (5%α‐Si 3 N 4 ). The loops are believed to form by condensation form supersaturated concentrations of vacancies during rapid cooling from densification temperature. The loops are confined exclusively to the grain interiors, and in consequence an estimate of the nitrogen vacancy diffusion coefficient can be made