High aspect ratio crystals

In theory, the combination of ceramic powders (A) with components (BuB2,B3) that form ceramic whiskers (B) at elevated temperatures should make it possible to produce ceramics with in situ whiskers. After the whisker formation is completed, the greenware is densified at a higher temperature. The disadvantages of this approach are potential reactions between A, Bu B2,..B the difficulties in removing the metal catalysts required to grow whiskers and the space constraints that limit the ability of the whiskers to form high aspect ratio crystals. 

Other platelet crystals with a high aspect ratio are reported for phtalocyanine [5.245], graphite [5.246] and 1,4-diketopyrrolopyrroles [5.247],... 

The crystal quality can be varied by careful adjustment of bismuth concentration, temperature, pH, pressure, reactor geometry, and by addition of surfactants. The usually tetragonal bipyramidal structure can be flattened to platelets with a high aspect ratio. Products with an aspect ratio of 10-15 show low luster and a very good skin feel and are used as fillers in cosmetics. Crystals with higher aspect ratios show an exceptional luster and are mainly used for nail polish [5.122, 5.123, 5.128, 5.130]. 

If the crystals produced are high aspect ratio needles or flat plates, seed crystals should be prepared separately, as discussed in Section 5.5. 

The dimensions of the silicalite which is crystallized are strongly influenced by changes in reactant concentrations (Fig. 2 and 3, Table 3). These differences are most pronounced in the aspect ratios (i.e. length/width) of crystals synthesized. Aspect ratios were found to differ considerably for the S- and T-series where the alkalinity was varied over an extensive range. For constant alkalinity, a rather constant aspect ratio is observed (B-series). Constant values of aspect ratios were observed for batches with identical alkalinities but where the source of silica was changed from precipitated silica to Ludox. High aspect ratios have been reported previously for silicalite crystallized at low alkalinities (19,20). 

Its topography reveals the unique structure consisting of aligned prisms or rods with 5pm diameter that extent approximately perpendicular from the dentin-enamel junction towards the tooth surface. Each rod consists of tightly packed carbonated hydroxyapatite crystals with very high aspect ratio. Nano-indentation studies revealed a pronounced anisotropy as the stiffness differs parallel and perpendicular to the rod extension. Even so, different fibre orientation on a micro level as shown in Figure 3.4b account for a quasi-isotropic behaviour. 

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