Abstract
The present study was performed in order to study the phase transformation of alkoxy-derived PbO, two polymorphic modifications of litharge (tetragonal) and massicot (orthorhombic), caused by heating and grinding. Techniques employed are DTA, X-ray analysis and electron microscopy.
Transformation isotherms of litharge (A) having a tetragonal plate-like form were characterized by sigmoidal shape, and kinetics were best described by the Avrami equation, ln(1-α)=-ktn(n=2), where a, t, k, and n are the fraction of transformation, time, rate constant and proportionality constant, respectively. In the case of litharge (B) obtained by grinding, n in the Avrami equation was 3. The above results are explained by the difference on particle forms. The values of the activation energies calculated from the Arrhenius plots are about 50 kcal/mol and 35kcal/mol, respectively. These activation energies represent those of the nucleation growth of massicot.
The transformation from massicot to litharge was completed by grinding for 10min in vacuum, and the ground product was only litharge through long times. The line profiles of the X-ray powder diffraction patterns of the ground litharge showed the decrease of height and the broadening of breadth with increasing time. The size of crystallites became rapidly small by grinding for 20 h, and it was hardly influenced by the subsequent grinding. On the other hand, the lattice distortion increased gradually with increasing time. When litharge ground for long times was heated in air, PbCO3·PbO and 4PbCO3·3PbO were formed by the reaction with CO2 in the heating process. This results show that PbO has a tendency to absorb CO2.
