slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
USAXS of Thermophoretically Sampled Oxides from Flames PowerPoint Presentation
Download Presentation
USAXS of Thermophoretically Sampled Oxides from Flames

Loading in 2 Seconds...

play fullscreen
1 / 1

USAXS of Thermophoretically Sampled Oxides from Flames - PowerPoint PPT Presentation


  • 106 Views
  • Uploaded on

Pyrolytic Synthesis of Nanopowders. USAXS instrument at UNICAT. Small-Angle X-ray Scattering. Aggregated Particles vs. Non-Aggregated. Small-Angle X-ray Scattering. USAXS instrument is too compact to photograph well. -Typically use TEM grids -Can use Al foil -We are really interested

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'USAXS of Thermophoretically Sampled Oxides from Flames' - reegan


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

Pyrolytic Synthesis of Nanopowders

USAXS instrument at UNICAT

Small-Angle X-ray Scattering

Aggregated Particles vs. Non-Aggregated

Small-Angle X-ray Scattering

USAXS instrument is too compact to photograph well

-Typically use TEM grids

-Can use Al foil

-We are really interested

in in-situ studies

0.1 s

HAB

Aggregated

And Polydisperse

Non-Aggregated

Polydisperse

I(q) ~ N ne (q)2

I(q) ~ N ne (q)2

Knudsen Number:

LAT

G2

Thermophoretic

Sampling

10 ms

G

Scattering Relates Mass (I)

To Size (1/q)

r = size scale of observation

r ~ 1/q

Rg,2

N(r) = Vscatt/V(r)

ne(r) = re V(r)

2q

Rg

df, B2

G1

Rg

Rg,1

Dimensional Scattering

1-d: I(q) ~ 1/q

2-d: I(q) ~ 1/q2

Consider a plate: Object is 2-d i.e. df = 2

B

G

0.3 – 1.0 m

0.6 m

~25 m

~25 m

sample

B1

-Flame temperature ~2000°C

at highest

-Often most growth occurs in

10 milli-seconds

Premixed

Flame

Diffusion

Flame

I0

Surface Scattering

DOA = G1/G1

-df

log(I)

B

Thermophoretic Particle Velocity, cT

UNICAT

APS

Particulate Scattering

X-rays in

log(q)

http://www.uni.aps.anl.gov/~ilavsky/sas.htm

Black:

Filter Powder

Aggregated

Polydisperse

Blue:

Growth of

Sintered Part.

Standard 1-D collimation setup*slit-smeared geometry (a.k.a. Bonse-Hart**)

Van der Waals Interactions:

Yellow:

Nucleation

USAXS at 3’rd Generation Source Allows Flux

To Observe Small Numbers of High Contrast Particles

Such as on a Thermophoretic Grid or Foil

14

Attractive Energy

Red: No Part.

x

s = x/dp

  • Post-measurement desmearing calculationJ.A. Lake; Acta Cryst 23 (1967) 191-194
  • Typical slit-length ~ 0.05 Å-1

* G. G. Long, A. J. Allen, J. Ilavsky, P. R. Jemian, and P. Zschack, in CP521, Synchrotron Radiation Instrumentation:

11th 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 Phyzik189, 151 (1966).

Filter Powder

5 mm HAB

10 mm HAB

30 mm HAB

50 mm HAB

70 mm HAB

150 mm HAB

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 :100-1000nm A)

Strongly bound cluster/cluster aggregation.

Agglomeration: Weakly bound cluster/cluster aggregation:

(1000nm-10µm)

Nanoscale from deep quench.

Variable Oxygen-Flow Rate

17 g/h SiO2 Flame

Diffusion flame (DF)

Sustained premixed flame (SPF)

Nucleation:

Gibbs-Thompson

(Ostwald-Freudlich or Hoffman-Lauritzen or Kelvin)

Equation

Using pseudo-equilibrium thermodynamics:

"Deep quench conditions give nanoparticles"

2.5

4.7

8.5

13.7

24 L/min

2.5

4.7

8.5

13.7

24 L/min

Hendrik Kammler 10/02

Supersaturated vapor:

High particle conc.:

Deep quench:

"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-31-109-ENG-38."

(Reaction rate is high due to temp.

and Laplace Eqn. )

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

13

9

Thermophoretic Sampling

5

Particle Transport:

kn >> 1

Free Molecular Regime:

Particles interact thermally with gas

D ~ dp-1/2

Self-Sharpening PSD

Self-Preserving Limit

1

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.

kn << 1

Continuum Regime:

Zero Velocity Boundary Condition

D = kT/(3phdp) Stokesian Transport

PSD

Broadens

Always

Thermophoretically Sampled Silica

On Aluminum Foil From

High Flow Rate Premixed Flame

10

6

2

INTRODUCTION

Pyrolytic Particle Growth Has a Wide Range of Conditions:

O2

O2

Fuel

O2 + Fuel

Fuel

O2

7

11

UNICAT USAXS Camera

15

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

Premixed Silica Flame

Diffusion Carbon Sooting

Flame

16

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

8

12

4