Factors influencing the texture and stability of maghemite obtained from the thermal decomposition of lepidocrocite

The thermal decomposition of a series of lepidocrocite samples has been mon itored by means of the Constant Rate Thermal Analysis (CRTA). This method a llows controlling the reaction temperature in such a way that both the reac tion rate and the partial pressure of the water vapor generated in the reac tion are maintained constant at values that call be arbitrarily selected by the user. The final products obtained from the thermal dehydroxylalion of the lepidocrocite precursors under residual pressures of water vapor rangin g from 5 x 10(-5) mbar to 10 mbar have been characterized by TEM, XRD, and Mosbauer spectroscopy. It has been concluded that they preserve the shape a nd size of the corresponding precursors. The precise control of the partial pressure of the water vapor self-generated in the above reaction allows ta iloring the internal porosity of the maghemite-hematite mixture obtained as final product. It has been shown that the larger the particle size of the starting lepidocrocite is, the larger the porosity of the iron oxide obtain ed. The ratio gamma-Fe2O3/alpha-Fe2O3 is strongly dependent on both the tex ture of the starting lepidocrocite and the partial pressure of water vapor selected for performing its dehydration. Thus, the percentage! of maghemite increases by decreasing the particle size of the stal ting lepidocrocite a nd by increasing the residual pressure of water vapor. An interpretation of this behavior is given.