M. Kamal Akhtar

Formation of agglomerate particles by coagulation and sintering—Part II. The evolution of the morphology of aerosol-made titania, silica and silica-doped titania powders

Abstract A two-dimensional particle size distribution model is used to describe gas phase synthesis of titania and silica powders. By incorporating a coalescence term along with a particle sintering rate in the governing population balance equation, the model traces the evolution of both the volume and surface area of aerosol particles. The simultaneous calculation of particle volume and surface area distribution leads to a direct characterization of the primary particles (grains) as well as particle aggregates. The effects of residence time, temperature and particle material properties on powder morphology are investigated. Effective sintering rates for pure titania, silica and silica-doped titania are deduced by comparing grain sizes obtained by model simulations and those measured experimentally from specific surface area and microscopic analyses.

In Situ Coating of Flame-Made TiO2 Particles with Nanothin SiO2 Films

Rutile TiO2 particles made by flame spray pyrolysis (FSP) were coated in a single step with SiO2 layers in an enclosed flame reactor. This in situ particle coating was accomplished by a hollow ring delivering hexamethyldisiloxane (HMDSO) vapor (precursor to SiO2) through multiple jets in swirl cross-flow to Al-doped nanostructured rutile TiO2 aerosol freshly made by FSP of a solution of titanium tetraisopropoxide and aluminum sec-butoxide in xylene. The as-prepared powders were characterized by (scanning) transmission electron microscopy (STEM and TEM), energy dispersive X-ray analysis, X-ray diffraction, nitrogen adsorption, electrophoretic mobility, DC plasma optical emission (DCP-OES), and Fourier transform infrared (FT-IR) spectroscopy. The coating quality was assessed further by the photocatalytic oxidation of isopropyl alcohol to acetone. The effect of HMDSO injection point and vapor concentration on product particle morphology was investigated. The titania particles were uniformly SiO2-coated with controlled and uniform thickness at a production rate of about 30 g h(-1) and exhibited limited, if any, photoactivity. In contrast, spraying and combusting equivalent mixtures of the above Si/Al/Ti precursors in the above reactor (without delivering HMDSO through the hollow ring) resulted in particles segregated in amorphous (SiO2) and crystalline (TiO2) domains which exhibited high photocatalytic activity.

Monte Carlo Simulation of Particle Coagulation and Sintering

A Monte Carlo simulation has been developed to describe the gas phase coagulation and sintering of nan-oclusters. The cluster-cluster aggregation model is modified to include a finite interparticle binding energy. Particle restructuring and densification (sintering) are incorporated into the model by modifying Kadanoffs algorithm for random particle walks on the surface of the cluster. The effect of sintering on aggregate size distribution and fractal dimension has been investigated in simulations of two-dimensional clusters. The binding energy and the relative rates of aggregation and sintering are the primary variables affecting particle structure. In the initial stages, the sintering process results in aggregates which are compact on small length scales. As time progresses and the aggregates become larger, the sintering process slows down and the fractal dimension of the aggregates decreases. The model is able to track the effect of reactor residence time and temperature on the specific surface area a...

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.

Thermal Aerosol Processes

Gas phase powder synthesis refers to the production of particles by gas phase (aerosol) processes. These processes have received increased attention in materials processing since they do not involve the tedious steps and large, environmentally burdensome, liquid volumes of wet chemical processes. In addition, the time scales of aerosol processes are much shorter than those involving solid-solid reactions. Furthermore, these processes can produce materials of high purity at high yields. Industry can provide several success stories of implementing aerosol processes in production of particulate commodities (carbon blacks, pigmentary titania, fumed silica) and optical fibers (Pratsinis and Mastrangelo, 1989). Though few of these processes are currently used for non-oxide powder synthesis, there is a strong likelihood that aerosol processes will also be used in large scale manufacture of non-oxides.

Synthesis of Titania Powder by Titanium Tetrachloride Oxidation in an Aerosol Flow Reactor

Vapor phase synthesis of titania particles by oxidation of titanium tetrachloride (TiCI 4 ) was studied in an aerosol reactor between 1200 K and 1723 K. The effect of process variables (reactor residence time, temperature, reactant concentration) on powder size and phase characteristics was investigated using the differential mobility particle sizer, scanning electron microscopy and X-ray diffraction. The morphology of the particles remained unchanged under the process conditions investigated; titania particles were primarily anatase though the rutile weight fraction increased with increase in reactor temperature. The geometric number average diameter of the particles was between 0.13 µm and 0.35 [m and the geometric standard deviation of the particle size distribution was about 1.4. The average particle size increased with increasing temperature, TiCI 4 concentration and residence time. The observed changes in the particle size distribution were compared with those predicted by solving the aerosol dynamic equation by a sectional method and accounting for coagulation and first order chemical reaction.