Ceramic powder preparation by spray pyrolysis

Vita.Spray pyrolysis for ceramic powder preparation was studied, theoretically and experimentally. The forming mechanism for derived particles using air or argon as carrier was established based on the experimental results obtained. This mechanism was used to develop a mathematical model for spray pyrolysis. Mass, momentum, and heat transfers inside and around the aerosol droplets were, for the first time, considered simultaneously. The model was verified and used to study the effects of operating parameters on the quality of the final product. The simulated and experimental results were in agreement and proved that: (a) raising the pyrolysis temperature increased the size and size distribution of resultant particles, and decreased the specific surface area and the bulk density; (b) lowering the starting solution concentration decreased the size and size distribution of resultant particles, and increased the specific surface area and bulk density. Accordingly, to produce the powder with much smaller sizes, narrower size distribution, and minimum cavity, the process should start with a dilute solution and have a two-stage heating procedure (a low processing temperature followed by a high pyrolysis temperature). This study also indicated that the production of hollow spheres is inevitable. Trials to add a combustible material to produce active porous crust which consolidates at high temperature were unsuccessful. To eliminate the production of hollow spheres, NH3 was employed into the process to react with the aerosols containing Ni(II) and Fe(III) nitrate in the stoichiometric ratio to form NiFe2O4. The resultant particles were characterized and the feasibility of using this ammonia process was evaluated. In addition, the forming steps and reaction mechanism for aerosol droplets involved in this process were proposed. Using a dilute starting solution, spherical submicron solid NiFe20 4 particles with narrow size distribution were produced in one step at pyrolysis temperatures as low as 823 K. When a concentrated starting solution was used, the oxides were reduced to nitrides and then to metals at high pyrolysis temperature (1223 K). The fine solid powder so obtained was active and on thermally treating these particles at 1073 K in air, the powder oxidized to form NiFe2O4