R. W. Carpenter

A comparative high-resolution study of interface chemistry in silicon-nitride-based ceramic matrix composites reinforced with silicon carbide whiskers

Abstract Silicon nitride/silicon carbide(w) ceramic matrix composites synthesized from common starting materials, except for the whiskers themselves, under the same processing conditions have been investigated. High-resolution electron microscopy and high-spatial-resolution electron energy loss nanospectroscopy were used to characterize the whisker/matrix interfaces in these composites both structurally and chemically. The presence of a discontinuous oxygen-rich amorphous layer at the two kinds of whisker/matrix interfaces examined in this paper appeared to be a general phenomenon. The wide difference (a factor of 10) between the structural image and chemical widths (oxygen distributions) of the whisker/matrix interfaces was attributed to oxygen solution into the silicon nitride matrix. The oxygen sources were the sintering aids and surface impurities.

High resolution interface analysis

Abstract The structure and composition of interfaces in composites and other structural solids has important effects on their properties and is therefore of great interest. HREM and microspectroscopy are two of the most useful methods for examination of interfaces. HREM methods are the most advanced, and microspectroscopy methods are improving rapidly. The current image resolution limit is ∼ 1.6 A which is sufficient for most interface analysis purposes. The corresponding spatial resolution limit for microspectroscopy is determined by the current in small probes and temporal stability of the probe/specimen. Calculations of probe current vs. size are given and it is shown that current spatial resolution for EELS, for edges containing several thousand counts, is about 5 nm. The effect of chemical bonding on electron energy loss absorption edges of silicon in its important ceramic compounds is presented and briefly discussed. Examples of high resolution analysis in several composites and a multiphase ceramic are presented.

Structure and precipitation on a Σ = 13 tilt grain boundary in silicon

The structure of a Σ=13 (510), [001] pure tilt grain boundary in a Czochralski-grown silicon bicrystal was characterized by high-resolution electron microscopy. The observed boundary exhibited a coincident site lattice periodicity but had an aperiodic interface dislocation core structure. Discrete impurity precipitate particles were observed at the boundary. High-spatial-resolution electron-energy-loss-spectroscopy analysis showed them to be oxide particles of the form SiOx, with 0<x<1. The role of oxygen as an intrinsic impurity in the bicrystal on the structure of the boundary and on the oxide precipitation at the boundary is briefly discussed.

Factors limiting the spatial resolution and sensitivity of EELS microanalysis in a STEM

Abstract The experimental parameters which limit the spatial resolution and sensitivity of parallel electron-energy-loss microanalysis have been reviewed in order to determine the limiting factors over a range of experimental conditions. The effects of the signal-detector characteristics and electron optics on the performance of the microanalytical technique have been quantified. The importance of statistical considerations (signal-to-noise ratios) and the specimen itself are considered in order to develop a non-arbitrary definition of microanalytical performance based not only on the electron beam dimensions, but also incorporating other experimental limitations.

A Microstructural Study of Reaction-Bonded Silicon Carbide

Interfaces in Reaction Bonded Silicon Carbide (RBSC) have been characterized by Analytical and High Resolution Electron Microscopy. Both Si/SiC and SiC/SiC interfaces were free of any oxygen impurity segregation, but contained metallic impurity precipitates. Oxygen was detected in the second phase particles in the SiC grains. A model is presented to explain the evolution of these second phase particles in the SiC grains.