Date of Award

8-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Materials Science and Engineering

Advisor

Peng, Fei

Committee Member

Kennedy , Molly S

Committee Member

Kornev , Konstantin

Abstract

Silicon carbide (SiC) is widely used in many fields due to its unique properties. Bulk SiC normally has a flexural strength of 500 - 550 MPa, a Vickers hardness of ~27 GPa, a Young's modulus of 380 - 430 GPa, and a thermal conductivity of approximately 120 W/mK. SiC fibers are of great interest since they are the good candidates for reinforcing ceramic matrix composites (CMCs) because of the weavability and high temperature strength of about two to three GPa at about 1000 °C. Silicon carbide fibers have been synthesized from polycarbosilane (PCS) with ~25 μm diameter using the melt-spinning method, followed by the curing and pyrolysis. In order to fabricate SiC fibers with small diameters, electrospinning method has been studied. The electrospinning technique is notable in that the fiber diameters can be controlled over a scale of nanometers to micrometers by controlling the processing parameters. However, there have only been limited studies of synthesis of silicon carbide fibers from polycarbosilane by electrospinning method. Moreover, there is no previous report for tensile strength testing of SiC fibers synthesized by electrospinning. The main objectives of this thesis are to study these problems. In this study, SiC fibers were obtained from polycarbosilane solutions using electrospinning method. In these solutions, dimethylformamide (DMF) and xylene were used as the solvents. The spinnability of the solutions was studied at different polycarbosilane concentrations, as were the ratios between DMF and xylene. The influence of electrospinning parameters such as voltage, flow rate and volume ratio of solvent on fiber diameter were studied. It was found that a minimal DMF content was ii required for the solutions to be spinnable for each PCS concentration. However, DMF content could not exceed 40% of the solvent volume, otherwise PCS could not be dissolved. The fiber diameters increased with increasing flow rate, and slightly decreased with increasing applied voltage. It was observed that pores on fiber surfaces, which have not been reported before. A higher density of pores resulted from higher environment humidity. The pore formation mechanism is hypothesized to be caused by the water vapor diffusing into the jet surface since DMF and water are miscible. A phase separation was caused by this diffusion process, since PCS could not be dissolved in either DMF or water. The obtained green fibers were then thermally cured in air and pyrolysized under argon. It was found in this thesis that the optimal curing temperature was between 250 200 to 350 ºC. Below 200 ºC, oxygen could not effectively react with the green fibers, while above 350 ºC, the green fibers began to decompose. The pyrolysis process was studied from 1000 to 1600 °C. The SiC fibers had the microstructure of dense inner core with porous outer surface. The pyrolysis has an optimal temperature range of ~1000 to 1100 °C. Above 1100 °C, the surface pores became more evident due to the decomposition of SiCxOy phase. Above 1500 °C, the SiCxOy phase substantially decomposed, resulting in exaggerated SiC grains on the fiber surface. The mechanical properties of the SiC fibers were characterized using the single filament tensile test. The tensile strength of the obtained fibers showed ~ 1.2 GPa between 1000 and 1100 ºC.

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