ema5001 lecture 24 transformation kinetics for sic …...ema 5001 physical properties of materials...
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© 2016 by Zhe Cheng
EMA5001 Lecture 24
Transformation Kinetics for SiC
Formation
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
Kinetics for the Formation of Nano SiC
via Carbothermal Reduction Reaction
Overall Reaction: SiO2 + 3C = SiC + 2CO
2
Nano-scale SiO2-C Mixture
Cheng, Z., MS Thesis, 2004
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
Confirmation of SiO &
the Two-Step Reaction
3
EDX
XRD of Vapor Deposits
SiO = Si + SiO2
Cheng, Z., MS Thesis, 2004
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
Reaction Kinetics from Weight Change
Best fit to Interface-controlled “Grow in”/”Shrinking Core” model
4
Weight Loss Data Fitting to Interface-controlled
“shrinking-core” model
Cheng, Z., MS Thesis, 2004
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
Comparison with Other Kinetic Models
Good fit also to Nucleation – Growth model
5
Nucleation – Growth Model Diffusion-Controlled “Grow In”
Model – Jander’s Equation
Site saturation, 1D growth
Constant nucleation - no growth Cheng, Z., MS Thesis, 2004
Also good fit, n =1
Very poor fit
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
Microstructure Changes to Confirm
Reaction Model
6
Essentially very small
growth of SiC
crystallites during
CTR reaction
at intermediate range
Growth of porosity
matching
expectation
Cheng, Z., MS Thesis, 2004
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
Rate-Limiting Step & Activation Energy
Reaction is limited by sub-step of
SiO + C = SiC + 2CO
For the following reasons
Always have SiO fomartion
Shape of SiC dependent largely
on C size and not SiO2 size
7
Cheng, Z., MS Thesis, 2004
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
TEM
8
BEFORE Reaction
f = 35%
BEFORE Reaction
f = 63%
Cheng, Z., MS Thesis, 2004 Little grain growth during reaction
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EMA 5001 Physical Properties of Materials Zhe Cheng (2016) 24 Transform Kinetics – SiC Formation
SEM
9
BEFORE CTR Reaction AFTER CTR Reaction
f = 0%
Almost fully dense
f = 95%
Highly porous