Sebastian V. R. Mastrangelo

Nomographs for vapor synthesis of ceramic powders

Abstract A dimensionless analysis is presented for production of ceramic powders by gas-phase reaction and coagulation over the entire particle size spectrum. A discrete-sectional coagulation model (based on conservation of the square of the aerosol volume) is used to simulate powder formation and growth by this process. For production of titania particles, this model is used to create nomographs for the total particle concentration, the geometric mean diameters and the standard deviation of the particle size distribution as functions of dimensionless residence time, X , and dimensionless time for chemical reaction, θ. These nomographs readily relate process variables to product powder characteristics, thus facilitating the optimal design and operation of powder manufacturing units. Practical applications of the nomographs are presented for particle production in furnace and flame reactors.

Effect of Dopants in Vapor Phase Synthesis of Titania Powders

Gas phase synthesis of titania from titanium tetrachloride (Ticl 4 ) oxidation in the presence of dopants (SiCl 4 and POCl 3 ) was systematically investigated in an aerosol reactor as a function of temperature (1300–1700 K) and dopant concentration (0–15 mole % of TiCl 4 ). The particle morphology was dramatically altered in the presence of dopants from polyhedral to spherical. Energy dispersive analysis indicated that the powders were homogeneous and that the dopants were not segregated at the surface or at the grain boundaries. Lattice parameter measurements from X-ray diffraction indicated that the dopant oxide was present in solid solution in titania. While titania synthesized in the absence of dopants was ∼80% anatase, the introduction of Si 4+ and P 5+ resulted in greater than 98 % anatase. The effects of foreign ions on titania phase composition, aggregate size and gas phase coalescence are explained by the creation of oxygen vacancies and reduction/enhancement of the titania sintering rates.

The effect of ionic additives on aerosol coagulation

Abstract The effect of ionic additives on particle size characteristics during gas phase production of finepowders is investigated. In aerosol manufacture of particulate commodities, electrolytes are sprayed into the process to control the phase and size characteristics of the product powders. These electrolytes dissociate into their constitutive ions, some of which preferentially adsorb onto the particle surface and give rise to electrostatic repulsive forces between particles that affect particle-particle interactions, and hence, particle growth by coagulation. The size distribution of these particles depends on the strength of charging at the particle surface, which is related to the ionization of the electrolytes. A thermochemical equilibrium calculation is performed to estimate the charging ability of alkali metals (Na and K). The effects of ionic additives on the rate of aerosol coagulation are numerically evaluated using a sectional solution for the aerosol dynamics equation. It is found that ionic additives can result in narrow size distributions and small average particle sizes, in agreement with experimental observations in the literature. In addition, these distributions are narrower than those predicted by the self-preserving theory for coagulation.