Title

Processing of Carbon-Silicon Carbide Hybrid Fibers

Date of Award

1-1-2019

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical, Materials and Bioengineering

Advisor/Chair

Advisor: Khalid Lafdi

Abstract

Two processing methods were used to fabricate carbon-SiC hybrid fibers. The first approach utilized various ratios of a silicone polymer (polydimethylsiloxane) as a silicon source and polyacrylonitrile (PAN) as a carbon source. The second approach used a mixture of silicon nanoparticles at various concentrations in PAN. The two formulations were independently converted into fiber form using an electrospinning process. Nanofibers with several hundred of nanometer diameters were successfully fabricated and subsequently stabilized and carbonized at 1000░C. In the first approach, three phases were found to be present: nanocrystalline SiC, turbostratic carbon and SiOC. The resulting fibers showed a core-skin structure with the skin rich in carbon and a core dominated by silicon-based phases in the form of SiC or SiOC phases. A significant improvement in both tensile strength and elastic modulus was observed for C-SiC hybrid fibers as compared to SiC-free carbon fibers produced in this study.In the second approach, one of the key issues identified for the study was particle distribution during the electrospinning. The analysis revealed that large silicon particles were located in the skin and the smaller ones were located at the core. The migration rate from the core was the fastest for large particles and was diminished as the particles became smaller in size due to inertial effects. The threshold for the Stokes number was found to be around 2.2 x 10-4 with a critical particle size of 1.0 x 10-7m in diameter. The current results are very promising, as it demonstrated a novel way for the fabrication of PAN/SiC ceramic nanofiber with a gradient of particle size and properties from the skin to the core.In addition, the ratio of Si to carbon precursor and heat treatment procedure were optimized to process hybrid nanofibers with high oxidation resistance. After pyrolysis to 1250░C, the nanofibers showed two-dimensional ordered carbon and SiC nano phases. Samples with 90 wt%PAN/10 wt% Si showed approximately four-time improvement in char yield as compared with 100 wt% PAN. It was concluded that the SiC phase played a major role in ordering the carbon phase. The carbon and SiC crystallinities had a great impact on improvements in the mechanical properties and the oxidation resistance, respectively. The SiC grain growth was predicted using Scherrer formula, and its exponent was found to be around n = 4 with activation energy around 35 KJ/mol.K. For such growth, the dominant grain growth mechanism was concluded to be grain boundary diffusion.Furthermore, a complete transformation of Si to SiC occurred at 1250░C. However, for heat treatments below 1000░C, three phases, including Si, C, and SiC were present. The effect of microstructural changes due to the heat treatment on oxidation resistance was determined using thermogravimetric analysis. The char yield showed linear increasing growth as the carbonization temperature ranged from 850░C to 1250░C. Increasing the holding times at higher temperatures produced a significant increase in thermal stability because of SiC grain growth. At long holding times, the SiC phase was observed to function as both an antioxidation coating and a mechanical reinforcing phase. Such structural changes lead to changes in fiber mechanical properties. The tensile strength was the highest for fibers carbonized at 850░C, while the modulus increased monotonically with increasing carbonization temperature

Keywords

Materials Science, Nanoscience, carbon-SiC hybrid fibers, polyacrylonitrile, polydimethylsiloxane, pyrolysis

Rights Statement

Copyright 2019, author

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