© 2016 by Zhe Cheng

EMA5001 Lecture 24

Transformation Kinetics for SiC

Formation

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

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

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

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

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

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

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

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