Polytypes, electron microscopy

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of sihcon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-siUcon carbide formed by transformation from the P-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

The analysis of siUcon carbide involves identification, chemical analysis, and physical testing. For identification, x-ray diffraction, optical microscopy, and electron microscopy are used (136). Refinement of x-ray data by Rietveld analysis allows more precise deterrnination of polytype levels (137). 

Daulton, T. L., Bematowicz, T. J., Lewis, R. S. et al. (2003) Polytype distribution in circumstellar silicon carbide Microstructural characterization by transmission electron microscopy. Geochimica et Cosmochimica Acta, 67, 4743-4767. 

The surface morphology of grains has been studied by secondary electron microscopy (SEM) (Hoppe et al., 1995). Such studies have been especially useful for pristine SiC grains that have not been subjected to any chemical treatment (Bernatowicz et al., 2003). Finally, the transmission electron microscope (TEM) played an important role in the discovery of presolar SiC (Bernatowicz et al., 1987) and internal TiC and other subgrains in graphite (Bernatowicz et al., 1991). It has also been successfully applied to the study of diamonds (Daulton et al., 1996) and of polytypes of SiC (Daulton et al., 2002, 2003). 

Kogure T, Nespolo M (2001) Atomic stractures of planar defects in oxybiotite. Am Mineral 86 336-340 Konishi H, Akai J (1990) HRTEM observation of new complex polytype of biotite from dacites in Higashiyama hills, Niigata, central Japan. Clay Sci 8 25-30 Mauguin MCH (1928) Etude de Micas au moyen du rayons X. Bull Soc fr Mineral Cristallogr 51 285-332 Menter JW (1956) The direct study by electron microscopy of crystal lattices and their imperfections. Proc R Soc London A236 119... 

Transmission electron microscopy using high resolution lattice imaging techniques allows polytype analysis within single grains in the microstructure of dense SiC bodies [27],... 

The microstructure of p-BN during the a-BN p-BN phase transition [13 to 15] and the formation of multilayer polytypes of p-BN (deformation-induced polytypism) [16] have been studied by electron microscopy. Besides the phase transformation, structure compaction, plastic deformation, and structural relaxation stages have been detected. The chemical composition of the surface of p-BN, as studied by X-ray photoelectron spectroscopy (XPS), has a higher content of oxygen than the original a-BN annealing of p-BN at 800°C results in the depletion of N and the enrichment of oxygen and carbon in the surface layer [17]. 

The isotherm of H-Beta is reported in figure 4 and exhibits an hyterisis loop indicating that mesopores are present, the closer point of the hyterisis loop is found to be close to P/Po=0.42. Catastrophic desorption of mesopores is seen as generally found for dealuminated HY zeolites (ref. 8), one can discuss about the origin of this phenomenon. It is certainly due to mesopores, evidenced in transmission electron microscopy, which are not directly connected to the exterior of the crystals. The formation of these mesopores is probably related to the presence of faults in the stacking sequence of polytypes as mentioned in ref. 9. 

A study from Van Dyck et al. (1976a,b), by electron diffraction and microscopy, confirms these arrangements, and shows other polytypes having the following arrangements ... 

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