Manfred Geier, R. Patrick Earhart, Terry Parker Division of Engineering Colorado School of Mines

1. Synthesis of Silica Nanoparticles using a Multi-Element Diffusion Burner: Effects of Precursor Concentration on Particle Size and Morphology Manfred Geier, R. Patrick Earhart, Terry Parker Division of Engineering Colorado School of Mines Golden, CO 80401 Presented at: Third Joint Meeting of the US Sections of the Combustion Institute March 16-19

2. Flame synthesis is an attractive way of manufacturing powder materials • Vapor Phase Combustion Synthesis : • High potential for application in industrial production • Flexible in operation • Several parameters to control product quality • Multi-Element Diffusion Burner: • Simple scale-up for production of large quantities • Complex interaction of fluid mechanics, chemical kinetics, and thermodynamics • Controlling the properties of nanosized powder: • Understanding of determining mechanisms • Reaction models

3. Powder growth depends on a range of physical mechanisms Influential factors for particle growth: • Kinetics of gas-to-particle conversion • Collision frequencies and efficiencies • Coalescence characteristics

4. Critical radius is a function of temperature and partial pressure • Critical radius (classical nucleation theory) • Nucleation rRate (classical nucleation theory) • Nucleation rate (collision controlled nucleation) • Critical radius < monomer radius • No thermodynamic barrier to nucleation • Newly formed molecules serve as stable nuclei • Nucleation rate = rate of formation of monomers (chemical reaction)

5. Coagulational growth (collision controlled) Particle growth can be due to collisions or surface reactions • General dynamic equation for aerosols: • Collision controlled growth rate in free molecular Regime: • Complete conversion of precursor to powder assumed • Spread of particle size distribution neglected • Surface-reaction dominated growth (precursor or intermediate consumption): Nucleation Surface growth (Pratsinis, 1998) (Pratsinis, 1998)

6. Particle morphology is determined by coalescence and collision times • Characteristic collision time: • Collision frequency function for free molecular regime • For monodisperse particulate of spherical particles • Characteristic coalescence (sintering) time: • Dominate time sets particle morphology (Bensberg, 1999) (Kodas, 1999)

7. Henken burner (38.1 x 38.1 mm) CH4/O2/N2 flame: Equivalence ratio 0.5 Ad. flame temperature 2461 K Total flow rate 15 SLPM Coflow, 6.35 mm Fuel Line, ID 0.51mm Honeycomb structure, 0.813 mm cells Experiments rely on a multi-element diffusion burner • SiCl4 as precursor (5% in CH4/SiCl4 mixture) • Individual mass flow controllers for each gas flow

8. Non-intrusive measurements are based on multiple scattering measurements • Sizing: Multiple measurement of scattered light: 2 wavelengths, 4 angles, 3 polarization states • Beam waists 0.55 mm (both wavelengths) • 8000 measurements for single data point (2 kHz sampling frequency) recorded to allow for analysis of temporal fluctuations • Temperature: fiber optic coupled measurement of CO2 emission at l= 4.388 mm

9. Particle properties by LMS-fitting of the over-determined system • 3 unknowns (Volume Median Diameter (VMD), Geometric Standard deviation (sg) and Volume Fraction), 5 simultaneous measurements • LMS-fitting to Mie-solutions assuming lognormal particle size distribution • Solution includes error bars • Error weighted averaging • Characteristic change in signal composition indicates onset of agglomeration (Earhart, Parker, Applied Optics 41:4421 - 4431, 2002)

10. Flame structure causes significant variation in particle flow field: Complicates measurements Offers further opportunities to control particle properties Quasi-homogeneous particle flow for current experimental conditions characterized by velocity ratio VR ~ 9.8. Stable pattern with Surrounding coflow Aluminum chimney Multi-element diffusion burners do NOT produce a homogeneous particle field Images for 100 mm above burner surface

11. Particle growth is initially linear in time • Linear growth rate at early stages – nucleation/surface reaction dominated growth • Asymptotic approach of limit value – collision dominated growth • Onset of agglomeration observed at later residence times – collision dominated growth • Delayed particle growth for smallest concentration – collision limited nucleation?

12. Volume fraction increases robustly at early times • Increase up to high residence times: • Rapid mass addition in linear growth region – nucleation/surface growth • Slow increase in later stages – nucleation / surface reaction / coagulation • Onset of agglomeration for higher seed concentrations • Assumption of instantaneous nucleation invalid? • Significant quantity below detection limit?

13. Size distribution narrows with increasing residence time • Wide size distribution at early stages – nucleation/surface growth • Strong narrowing for higher seed concentrations – effective scavenging (predicted by Zachariah, 1990) • Near-monodisperse size distribution at high residence times

14. Temperature falls slowly with increasing height above burner (but more accurate measurements needed) Agglomeration below 1950 K for 50 nm particle – consistency with findings Results indicate a transition from coalescence to agglomeration 200 nm 100 nm 50 nm 25 nm

15. Conclusions • Confirmed: • Increase of particle growth rates and volume fraction with initial seed concentration • Asymptotic value for particle size increases with initial seed concentration • Agglomeration in lower temperature environment (collision controlled growth, large coalescence times) • Unexpected: • Linear dependence of asymptotic value for particle size on initial seed concentration (0.033% to 0.33%) • Linear growth in particle size at early residence times • Continued increase in volume fraction at later times

16. Conclusions • Possible explanation for linear growth: • Solid phase below detection limit at early time – modify diagnostics to decrease detection limit • Continued growth: • Incomplete scavenging: particles below detection limit added to detectable ones 0.033% Monomer size assumed 0.33% • Possible transition in growth regime over concentration range studied

17. QUESTIONS?