SURFACE FRACTAL DIMENSIONS OF ALUMINA AND ALUMINUM BORATE FROM NITROGEN ISOTHERMS

The effects of calcination temperature on the surface roughness of alu mina and aluminum berate samples, prepared by the coprecipitation meth od and low-water sol-gel processes, were investigated by fractal analy sis of their nitrogen adsorption isotherms. The surface fractal dimens ion D was calculated from their nitrogen isotherms using both the frac tal isotherm equations derived from the FHH (Frenkel-Halsay-Hill) theo ry and the recently proposed Neimark equation based on thermodynamics. It was found that for samples calcined from 500 to 1100 degrees C, th e D values evaluated using fractal FHH equation were nearly constant ( 2.5-2.6) despite a large reduction of the BET surface area and pore vo lume. Only after a 1250 degrees C calcination, when the alpha-alumina crystalline phase starts to sinter, was there a slightly larger change of the surface fractal dimension. On the other hand, Neimark's method suggested that thermal effects induce an enormous change on surface r oughness of examined samples. The discrepancy of D values obtained wit h the above two methods was discussed.