Fibrillar Colloidal Boehmite; Progressive Conversion to Gamma, Theta, and Alpha Aluminas

The conversion of boehmite to transition aluminas and to alpha (corundum) was studied with ( a ) fibrillar colloidal boehmite, about 50 a.u. in diameter, ( b ) finely divided boehmite in the form of thin elongated platelets, and ( c ) macroscopic boehmite crystals. In the range 500 o to 700 o C., ( a ) was converted to fibrillar gamma, essentially unchanged in size or shape, ( b ) was dehydrated to gamma, but the platelets appeared to be partly cracked longitudinally about every 70 a.u., and ( c ) was similarly cracked into much smaller crystallites of gamma. At about 1000 o C., ( a ) yielded only rodlike theta alumina, whereas ( b ) and ( c ) appeared to give a mixture of theta and delta aluminas, characterized by round holes or pores developed from the cracks. In the range 1000 o to 1100 o C., all the aluminas were converted to alpha, at a rate which was strongly dependent on temperature. The conversion of ( a ) and ( b ) to alpha could be followed by subsequent examination with the electron microscope. The alpha crystals, once nucleated, apparently grew rapidly to 0.5 or 1.0 μ in size, since there was no evidence of smaller alpha crystals. The smaller alpha crystals appeared to grow while embedded in a glassy matrix which must have been amorphous alumina. This phase acted as a bridge between the diminishing theta particles and the growing alpha crystals and may have consisted of the alumina which was in the process of being transported.