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SYNOPSISRuthenium tetroxide solution was prepared in situ by oxidation of ruthenium compounds at lower oxidation states with an excess of sodium periodate. The solution was able to stain saturated polymer POM in the vapor phase and it remained effective for up to 2 weeks. The melting behavior of POM samples stained for different lengths of time was studied with DSC and the staining process was analyzed. The results indicated that ruthenium tetroxide affected both the amorphous region and the crystals. During the early stage of the staining process, the tetroxide reacted preferentially with the amorphous material near the specimen surface. Much of this affected material was washed away after the rinsing process, thus resulting in an apparent increase in crystallinity. Prolonged staining would cause more crystals to degrade; hence, the crystallinity would drop. A two-step vapor staining technique was developed to improve the contrast between the amorphous and crystalline regions of ultrathin POM sections. The lamellar structure of POM spherulites was revealed and examined under TEM. The results showed that POM spherulites possess a category 2 structure and the thickness of the lamellae is about 7 nm. 0

This paper explores the polytype distribution and polytypic transformations occurring during the reaction-bonding of a commercial silicon carbide, REFEL. The initially formed polytype is found to be almost exclusively β-SiC, which either deposits epitaxially on the original α-SiC grits or as fine material, in situ in the silicon. The exothermic nature of the reaction causes the epitaxially deposited material to undergo the β→α solid state transformation, usually going almost to completion in a few minutes. Two types of epitaxial material are distinguished ; type I has its stacking sequence directly seeded by the original grain, whilst type II has no direct contact with the original α stack, but can be seeded by 'sideways' nucleation. The fine (type III) material is never found to transform, either because of nucleation problems or because it is formed at a lower temperature. The kinetics β→α transformation indicate a peak local temperature of ~ 2000°C and the rapidity of the transformation is sufficient to rule out any large-scale diffusion. Other microstructural features are noted, including the unexpected presence of some 2H-SiC in certain 'nodules' in the compact, associated with high impurity levels and unusual reaction conditions. The observed behaviour emphasizes the importance of local temperature, impurities and nucleation effects in determining polytype stability. Further, the preferential initial growth of β-SiC confirms it as the high-symmetry, rapidly-forming polytype.

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