PSEUDOMORPHIC TRANSFORMATIONS OF EUHEDRAL CRYSTALS OF GAMMA-ALOOH INTO ALUMINAS

The thermal transformation of synthetic well-shaped hexagonal laths of boehmite-AlOOH was studied between 200-degrees-C and 1500-degrees-C by transmission electron microscopy (TEM), selected area-electron diffraction (SAED) and X-ray diffraction (XRD), to characterize the transition aluminas that appear before the formation of alpha-Al2O3. It was observed in the single crystals that dehydroxylation produces, between 400-degrees-C and 500-degrees-C, pores regularly organized and parallel to the larger diameter of the hexagons which have the same direction (100) as the original boehmite crystal; this gives a striated appearance to the pseudomorphs, which is maintained until after heating at 900-degrees-C, when the transformation from the gamma phase to the theta phase occurs. At 1000-degrees-C, pseudomorphs appear and the theta and alpha phases coexist; the pores have lost their orderly disposition and larger pores or openings are now irregularly distributed in the mixed single crystals, whose crystalline structure can be observed in the lattice image by the use of high-resolution electron microscopy. The alpha-alumina pseudomorphs exist as platy elongated single crystals at 1100-degrees-C and coalesce at 1200-degrees-C; from 1300-degrees-C to 1500-degrees-C the pseudomorphs coalesce into round polygonal particles of alpha-alumina, whose crystalline structure could be demonstrated by SAED, XRD or lattice images.