MULTIPLE FEATURES OF A POROUS STRUCTURE AS ASSESSED FROM THE HYSTERESIS OF NITROGEN ADSORPTION-DESORPTION - CASE-STUDY OF THE FORMATION OF UO3 FROM UO2(NO3)(2)CENTER-DOT-6H(2)O

The porous structure of intermediates in the thermolysis of hexahydrat ed uranyl nitrate was found to provide an interesting case study for t he use of nitrogen adsorption-desorption isotherms : nonrigid sheetlik e pore structures, rigid mesopores, interconnected structures, and cle arly bimodal pore size distributions were the situations encountered. The use of sample controlled thermal analysis (SCTA) made it possible to compare two highly reproducible and well-defined thermolysis routes (under residual pressures of 10(-1) and 20 mbar, respectively) leadin g to very different porous UO3. Adsorption studies, with the help of S EM. investigations and prior X-ray studies, permit a proposition for t he mechanism of the formation of the different porous structures.