USAXS of Thermophoretically Sampled Oxides from Flames Greg Beaucage, Nikhil Agashe, Doug Kohls- Dept. of Chemical and Materials Engineering, University of Cincinnati. Hendrik Kammler, Soritis Pratsinis- Institute of Processing Engineering, ETH, Zurich. Jan Ilavsky – Purdue University/UNICAT, Argonne "The UNICAT facility at the Advanced Photon Source (APS) is supported by the Univ. of Illinois at Urbana-Champaign, Materials Research Laboratory (U.S. DOE, the State of Illinois-IBHE-HECA, and the NSF), the Oak Ridge National Laboratory (U.S. DOE under contract with UT-Battelle LLC), the National Institute of Standards and Technology (U.S. Department of Commerce) and UOP LLC. The APS is supported by the U.S. DOE, Basic Energy Sciences, Office of Science under contract No. W ENG-38." Standard 1-D collimation setup * slit-smeared geometry (a.k.a. Bonse-Hart ** ) Post-measurement desmearing calculation J.A. Lake; Acta Cryst 23 (1967) Typical slit-length ~ 0.05 Å -1 ~25 m 0.3 – 1.0 m0.6 m * G. G. Long, A. J. Allen, J. Ilavsky, P. R. Jemian, and P. Zschack, in CP521, Synchrotron Radiation Instrumentation: 11 th US National Conference, P. Pianetta and H. Winick, Eds., AIP, College Park, 183 (2000). ** U. Bonse & M. Hart, Appl. Phys. Lett. 7, 238 (1965) and Zeit. f Phyzik 189, 151 (1966). USAXS instrument at UNICAT Abstract: Combustion of organo-metallic or halide vapors and aerosol liquid sprays can be controlled to produce enormous quantities of nano-structured powders. Such flame processes are common in the production of fumed silica, and pyrolytic titania on an industrial scale with primary particle sizes on the order of 10 nm. Pyrolytic processes can also be used with liquid phase specialty precursors through flame spray pyrolysis. Pyrolytic nano-particles are typically connected through sintering bridges, ionic bonds or van der Waals forces into ramified, mass-fractal aggregates. The study of this promising technology for nano-particle production has been hindered by the kinetics of particle growth, typically on the order of milliseconds, at high temperature, 2000°C. Using the UNICAT USAXS camera we have recently studied samples collected by shooting a TEM grid or small metal substrate through the flame at high velocity. This substrate attracts a small number of particles as it passes through the flame. Particle deposition via thermophoresis in the free molecular regime has no dependence on particle size so a true sample can be obtained. Despite the small quantity of sample, third generation synchrotron sources coupled with USAXS cameras are capable of measuring a reasonable scattering pattern on such thermophoretic samples and a particle morphological mapping as a function of distance from the burner is possible. The results of a recent study of thermophoretic samples is shown. I0I0I0I0 2222 sample X-rays in USAXS instrument is too compact to photograph well UNICAT USAXS Camera Diffusion flame (DF)Sustained premixed flame (SPF) L/min Hendrik Kammler 10/02 Variable Oxygen-Flow Rate 17 g/h SiO 2 Flame Variable Oxygen-Flow Rate 17 g/h SiO 2 Flame Premixed Silica Flame Diffusion Carbon Sooting Flame Pyrolytic Particle Growth Has a Wide Range of Conditions: O 2 + FuelFuel O2O2 O2O2 O2O2 INTRODUCTION Nucleation: Gibbs-Thompson (Ostwald-Freudlich or Hoffman-Lauritzen or Kelvin) Equation Using pseudo-equilibrium thermodynamics: "Deep quench conditions give nanoparticles" Supersaturated vapor: High particle conc.: Deep quench: (Reaction rate is high due to temp. and Laplace Eqn. ) Particle Transport: Knudsen Number: k n >> 1Free Molecular Regime: Particles interact thermally with gas D ~ d p -1/2 k n << 1Continuum Regime: Zero Velocity Boundary Condition D = kT/(3  d p ) Stokesian Transport Self-Sharpening PSD Self-Preserving Limit PSD Broadens Always x s = x/d p Attractive Energy Van der Waals Interactions: Pyrolytic Synthesis of Nanopowders Premixed Flame HAB Diffusion Flame Thermophoretic Sampling -Typically use TEM grids -Can use Al foil -We are really interested in in-situ studies -Flame temperature ~2000°C at highest -Often most growth occurs in 10 milli-seconds 10 ms 0.1 s Thermophoretic Particle Velocity, c T LAT USAXS at 3’rd Generation Source Allows Flux To Observe Small Numbers of High Contrast Particles Such as on a Thermophoretic Grid or Foil Small-Angle X-ray Scattering I(q) ~ N n e (q) 2 N(r) = V scatt /V(r) n e (r) =  e V(r) r = size scale of observation r ~ 1/q Consider a plate: Object is 2-d i.e. d f = 2 -d f log(I) log(q) Small-Angle X-ray Scattering I(q) ~ N n e (q) 2 Dimensional Scattering 1-d: I(q) ~ 1/q 2-d: I(q) ~ 1/q 2 Scattering Relates Mass (I) To Size (1/q) Surface Scattering Particulate Scattering UNICAT APS G B RgRg Non-Aggregated Polydisperse Aggregated And Polydisperse G1G1 R g,1 B1B1 G2G2 R g,2 d f, B 2 DOA = G 1 /G 1 G RgRg B Aggregated Particles vs. Non-Aggregated Thermophoretically Sampled Silica On Aluminum Foil From High Flow Rate Premixed Flame Thermophoretically Sampled Silica On Aluminum Foil From High Flow Rate Premixed Flame 14 Red: No Part. Yellow: Nucleation Blue: Growth of Sintered Part. Black: Filter Powder Aggregated Polydisperse Filter Powder 5 mm HAB 10 mm HAB 30 mm HAB 50 mm HAB 70 mm HAB 150 mm HAB Red- 5 mm above burner scattering from composition fluctuations in the gas, no particles Yellow- 10 mm HAB (≈ 20 ms) nucleation occurs ≈ 2nm nuclei form in large numbers Light Blue- 30 mm HAB 10 nm spheres with low polydispersity. This is the initial stage of coalescence. Blue- 50 mm HAB Spheres grow in size and polydispersity increases Summary: -It is possible to measure exceedingly small quantities of high-contrast nano-powders using 3’rd generation sources and USAXS (also pinhole) cameras -Through USAXS on Thermophoretic samples we have observed nucleation and coalescence -Coalescence initially leads to low polydispersity spherical and non-aggregated nano-particles -This is the first observation of such low-dispersion spherical particles in pyrolytic synthesis. -The rarity of this observation might indicate that polydispersity in primary particle size for pyrolytic powders occurs between aggregates rather than within a single aggregate 3 Stages of Nano-Particle Formation in a Flame (10 ms): Nucleation: Vapor to solid via chemical reaction (2-7nm) Coalescence/Initial Growth: monomer/monomer growth: (5-10nm) log-normal distribution in free molecular regime Sintering: Dramatic formation of spherical, low-dispersion particles (10nm) Aggregation: Lower temperature region: (10nm PP : nm A) Strongly bound cluster/cluster aggregation. Agglomeration: Weakly bound cluster/cluster aggregation: (1000nm-10µm) Nanoscale from deep quench. Thermophoretic Sampling