Bayerite, fl-AI(OH)3, and boehmite, 7-A1OOH, both with crystallite sizes of some tenths of a micron, were used for this investigation.The transformation of bayerite to T-alumina and of boehmite to 7-and &alumina showed a strict pseudomorphosis. 7-and 7-alumina have spinel lattices that differ in disorder: T-alumina has a strong one-dimensional disorder of the cubic close-packed stacking; for 7-alumina the oxygen sublattice is fairly well ordered, whereas the tetrahedral A1 lattice is strongly disordered.The crystallographic texture of 7-and 7-alumina could be related to their microporous texture and this in its turn to the length of the OH bonds in the hydroxides.The formation of the well ordered super spinel structure ~-A120 3 is suppressed by obstructing the diffusion of the A1 ions either by the surface of numerous pores (T-alumina from bayerite or 7-alumina from gelatinous boehmite) or by incorporating large ions such as Na ions (y-alumina from crystalline boehmite). In these cases the disordered spinel type aluminas are transformed at a high temperature to 0-and a-Al~O 3 by complete recrystallization to small particles which still have some preferred orientation.
This paper presents the modelling, design, fabrication and characterization of flow sensors based on the wind-receptor hairs of crickets. Cricket sensory hairs are highly sensitive to drag-forces exerted on the hair shaft. Artificial sensory hairs have been realized in SU-8 on suspended Si x N y membranes. The movement of the membranes is detected capacitively. Capacitance versus voltage, frequency dependence and directional sensitivity measurements have been successfully carried out on fabricated sensor arrays, showing the viability of the concept.
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