GRAIN-GROWTH AND DENSIFICATION IN PALLADIUM OXIDE PARTICLES DURING SPRAY-PYROLYSIS

A model for aerosol-phase densification of particles during their synt hesis by spray pyrolysis is presented. The model was used to describe the evolution of the specific surface area of PdO powders synthesized at temperatures between 400 to 800 degrees C (residence times 27.9 to 22.7 s). Surface areas and grain sizes ranged between 56 m(2)/g and 4 nm at 400 degrees C to 3.2 m(2)/g and 40 nm at 800 degrees C. The char acteristic coalescence time was determined as: tau = 1.7 x 10(13) T d( 3) exp(1.3 x 10(5)/RT) [for lattice diffusion] and tau = 2.7 x 10(19) T d(4)exp (1.6 x 10(5)/RT) [for grain boundary diffusion] (tau is in s econds, T in degrees Kelvin, R is 8.314 J/(mol.K) and d is in meters), but the data and model did not allow identification of the specific m echanism. The model provides a general approach for correlating change s in particle surface area with reactor operating conditions that is a pplicable to a wide variety of materials.