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Shraddha Shekar, Markus Sander,Alastair Smith, Markus Kraft17 September, 2010
A novel pathway for flame synthesis of silica nanoparticles
Shraddha [email protected]
Silica nanoparticlesSilica Nanoparticles: network of Si-O bonds such that Si:O = 1:2
Applications:
•Support material for functional/composite nanoparticles, catalysis
•Bio-medical applications, drug delivery
•Optics, optoelectronics, photoelectronics
•Fabrics, clothes
Shraddha [email protected]
Physical systemPrecursor(TEOS) Flame reactor
P ≥ 1 atm
T ≈ 1100 - 1500 K
Silica nanoparticles
Macroscopic level questions:•Optimal process conditions?
•Final product properties?•Final particle size
distribution?Indu
stria
l Sc
ale
Mol
ecul
ar
Scal
e
Answers from molecular level studies:•How to determine the thermochemistry of the system?•What happens in the gas-phase?•How do gas-phase precursors form the particles?•How to describe the overall system from first-principles?
Shraddha [email protected]
Methods: Ab initio modelling
Quantum Chemistrycalculations
Statistical Mechanics
H(T) S(T) Cp(T)
Thermochemistrycalculation
ChemicalKinetics
Equilibriumcalculation
Overall Model
Species generation
Population Balance Model
Ref: W. Phadungsukanan, S. Shekar, R. Shirley, M. Sander, R. H. West, and M. Kraft. First-principles thermochemistry for silicon species in the decomposition of tetraethoxysilane. J. Phys. Chem. A, 113, 9041–9049, 2009
Shraddha [email protected]
Equilibrium Plot
Ref: W. Phadungsukanan, S. Shekar, R. Shirley, M. Sander, R. H. West, and M. Kraft. First-principles thermochemistry for silicon species in the decomposition of tetraethoxysilane. J. Phys. Chem. A, 113, 9041–9049, 2009
Shraddha [email protected]
Reaction kinetics
• Equilibrium– Hints towards the
existence of stable intermediates & products.
– Intermediates Si(OH)x(OCH3)4-x Si(OH)y(OC2H5)4-y
– Main Product Si(OH)4
• Kinetics– Reaction set generated
to include all intermediates and products from equilbrium.
– Reactions obey Arrhenius law rate constant k = ATβe-Ea/RT
– Rate parameters (A, β, Ea) fitted to experimental vaues (a)
(a) J. Herzler, J. A. Manion, and W. Tsang. Single-Pulse Shock Tube Study of the decomposition of tetraethoxysilane and Related Compounds. J. Phys. Chem. A, 101,5500-5508, 1997
Shraddha [email protected]
Reaction Mechanism• Heuristic reaction mechanism
C8H20O4SI = C6H16O4SI + C2H4 C6H16O4SI = C4H12O4SI + C2H4 C4H12O4SI = C2H8O4SI + C2H4 C2H8O4SI = H4O4SI + C2H4 C8H20O4SI = C7H17O4SI + CH3 C7H17O4SI = C2H4 + C5H13O4SI C5H13O4SI = C2H4 + C3H9O4SI C3H9O4SI = C2H4 + CH5O4SI CH5O4SI = H4O4SI + CH C7H17O4SI = C7H16O4SI + H C7H16O4SI = C6H13O4SI + CH3 C6H13O4SI = C6H12O4SI + H C6H12O4SI = C4H10O4SI + C2H2 C4H10O4SI = C2H8O4SI + C2H2 C8H20O4SI = C2H4 + C6H16O4SI2
C6H16O4SI2 = C2H4 + C4H12O4SI2C4H12O4SI2 = C2H4 + C2H8O4SI2 C2H8O4SI2 = C2H4 + H4O4SI C8H20O4SI = C2H5 + C6H15O4SI2 C6H15O4SI2 + H = C6H16O4SI2 C6H16O4SI2 = C2H5 + C4H11O4SI2 C4H11O4SI2 + H = C4H12O4SI2 C4H12O4SI2 = C2H5 + C2H7O4SI2 C2H7O4SI2 + H = C2H8O4SI2 C2H7O4SI2 = H3O4SI + C2H4 H3O4SI = H2O3SI + OH H2O3SI = SIO2 + H2O C6H15O4SI2 = C2H3 + C4H12O4SI2 C4H11O4SI2 = C2H3 + C2H8O4SI2 C2H8O4SI2 = H3O4SI + C2H3
Shraddha [email protected]
Experimental validation
(a) J. Herzler, J. A. Manion, and W. Tsang. Single-Pulse Shock Tube Study of the decomposition of tetraethoxysilane and Related Compounds. J. Phys. Chem. A, 101,5500-5508, 1997
Shraddha [email protected]
Optimisation Theory• Experimental data
• Model response with paramaters x
• and
xBAx For a simple linear model, K=1
cxBAcx 0,,
000 xBAcxEx ,, 220 cBcxc ,,Var
expexp0
exp
cxx 0
x Kxx ,...,1xwith
with uncertainty factor c andstandard normally distributed
A. Braumann, P. L. W. Man, and M. Kraft. Statistical approximation of the inverseproblem in multivariate population balance modelling. Ind. Eng. Chem. Res., 49:428–438, 2010. doi:10.1021/ie901230u
Shraddha [email protected]
Model OptimisationThe rate parameters have been fitted to shock-tube experimental data provided by Herzler et al(a)
(a) J. Herzler, J. A. Manion, and W. Tsang. Single-Pulse Shock Tube Study of the decomposition of tetraethoxysilane and Related Compounds. J. Phys. Chem. A, 101,5500-5508, 1997
Step 2: Sensitivity AnalysisTo identify the 4 most sensitive parameters
Step 1: Low discrepancy seriesTo perform a pre-scan of parameters for 18 Si reactions.
Step 3: Response Surface MethodologyTo estimate model uncertainties
Uncertainties in model parameters for reactions R1 and R15
Shraddha [email protected]
Gas-phase mechanism
Shraddha [email protected]
Reactor Plot
Conclusion from kinetic model:
Si(OH)4 is the predominant gas-phase precursor
Shraddha [email protected]
Main reaction pathway
Si
O
O
OO
H2C
CH2
CH2
H2C
-C2H4
H3C
H2C
CH3
H
Si
O
O
OO
H2C
CH2
CH3
H3CH3C
CH3
Si
O
O
OO
H3C
H3C
CH3
H3C
Si
OH
HO
HO
OH
H2CH
-C2H4
Si
O
O
OO
H3C
H3C
H
H
-C2H4
Si
O
O
OO
H2C
CH2
CH2
H2C
-C2H4
H3C
H2C
CH3
H
CH3-C2H4
Si
O
O
OO
H2C
CH2
CH2
HH3C
CH3
CH3-C2H4
Si
O
O
OO
H2C
CH2
H
HH3C
CH3
Si
O
O
OO
H
CH2
H
HH3C
-C2H4
-C2H4
Reaction Pathway 1
Reaction Pathway 2
Shraddha [email protected]
Particle Model
Si
O
O
O
O
H
H
H
H
Si
O
O
O
O
H
H
H
H
-H2O Si
O
O
O
O
H
HH
Si
O
O
O
H
HH
INCEPTION
Si
O
O
O
OSi
O
O
Si
-nH2OSURFACEGROWTH
OSi
SiSi
Si SiO
n(-O-Si-O-Si-)
Si(OH)4 molecules in gas-phase undergo inception to form a dimer (-Si-O-Si). This dimer is considered to be the first particle.Particle growth then proceeds by subsequent removal of hydroxyl groups.
Shraddha [email protected]
Particle Processes
[1]: M. Sander, R. H. West, M. S. Celnik, and M. Kraft. A Detailed Model for the Sintering of Polydispersed Nanoparticle Agglomerates, Aerosol Sci. Tech., 43, 978-989, 2009
New inception and surface growth steps have been incorporated in a previously developed stochastic particle model developed by Sander et al. [1].
Particle ineption
Surface growthCoagulation
Particle rounding due to surface growth
P = P(p1, p2, .....pn, C, I, S)
p = p(vi)
SinteringSurface reactionCondensationInception
Shraddha [email protected]
Individual Processes3. Coagulation
Pi
Pj Pk
+
pi
pj
pi
pj pk
No Sintering Partial Sintering Complete Sintering
4. Sintering
Shraddha [email protected]
Experimental Setup of Seto et al.
Ref: T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech., 27, 422-438, 1997
Reactor 1: T = 900oC
Particle formation zone
Reactor 2: T > 900oC
Particle heating zone
Shraddha [email protected]
Model Optimisation
Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech., 27, 422-438, 1997
Material dependent sintering parameters are optimised
Optimisation method: LD series and RSM
Primary diameter (dp) and collision diameters (dc) fitted to experimental values at different temperatures.
Shraddha [email protected]
Effect of temperature
•Sintering increases as temperature increases reducing the collision diameters.
•At a temperature of about 2000 C, the collision diameter and primary diameter become identical and particles become spherical.
Shraddha [email protected]
Particle size distribution
Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech., 27, 422-438, 1997
Solid lines: ModelCircles: Experiments (a)
Shraddha [email protected]
Model produced TEM-like images
Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech., 27, 422-438, 1997
Shraddha [email protected]
Overall mechanism for particle formation
Si
O
O
O
O
H
H
H
H
Si
O
O
O
O
H
H
H
H
Si
O
O
O
O
H
HH
Si
O
O
O
H
HH
OHH
Si
O
O
O
O
H2C
CH2
CH2
H2CCH3
CH3
CH3
H3C
Si
O
O
O
O
H3C
H3C
CH3
H3C
Si
O
O
O
O
H
H
H
H
Si
O
O
O
O
H
H
H
Si
O
O
O
H
HH
Si
O
O
O
O
H
H
H
H
Si
O
O
O
O
H
HH
H
SiO
O
O
OH
H
H
H
Si
O
O
O
OH
H
H
H
Si
O
O
O
OSi
O
O
SiOSi
SiSi
Si SiO
Gas
-pha
sere
actio
nsPa
rticl
efo
rmat
i on
Parti
cle
g row
th
-2C2H4 -2C2H4
-2H2O
-nH2O
[monomer]
[primary particle]
(-O-Si-O-)n [Silica particle]
The gas-phase and particle model described above are coupled using an operator splitting technique to generate the overall model.
Shraddha [email protected]
Conclusion
1. New kinetic model proposed which postulates silicic acid Si(OH)4 as the main product of TEOS decomposition.
2. A novel pathway proposed for the formation of silica nanoparticles via the interaction of silicic acid monomers.
3. Feasibility of using first-principles to gather a deeper understanding of complex particle synthesis processes.