Surface fractal dimensions of alumina and aluminum borate from nitrogen isotherms

The effects of calcination temperature on the surface roughness of alumina and aluminum borate samples, prepared by the coprecipitation method and low-water sol-gel processes, were investigated by fractal analysis of their nitrogen adsorption isotherms. The surface fractal dimension D was calculated from their nitrogen isotherms using both the fractal isotherm equations derived from the FHH (Frenkel-Halsay-Hill) theory and the recently proposed Neimark equation based on thermodynamics. It was found that for samples calcined from 500 to 1100 degrees C, the 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 volume. 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 roughness of examined samples. The discrepancy of D values obtained with the above two methods was discussed.