Novel Continuous Carbon and Ceramic Nanofibers and Nanocomposites
Yongkui Wen - Ph.D. Dissertation Defense
Advisor: Dr. Yuris Dzenis
Date: Friday, April 23, 2004
Time: 11:30 a.m.
Place: 205N Walter Scott Engineering Center
Advanced composite materials are finding wide use in a variety of military and civilian applications. Advanced fibers are key component of advanced composite materials. Properties of advanced fibers are expected to further substantially improve with reduction of their diameter. However, conventional advanced fiber manufacturing methods can’t produce fibers smaller than about two microns. One-dimensional nanomaterials attract rapidly growing interest and are expected to find broad applications in nanotechnology. Carbon nanotubes have been recently shown to have the highest mechanical and other unusual properties. However, carbon nanotubes are discontinuous in nature that limits their application in advanced composites. Continuous nanofibers have substantial advantages for composite and other applications. Recent studies in our group showed revolutionary improvements in fracture properties of specially designed nano-micro composites reinforced with polymer nanofibers. The objective of this dissertation was to develop and demonstrate novel continuous carbon and ceramic nanofibers, nanocomposites, and processes for their manufacture. Electrospinning process was used to manufacture continuous nanofiber precursors that were then converted to carbon or ceramic nanofibers by appropriate thermal treatments. The resulting nanofibers were characterized by SEM, FE-SEM, TEM, AFM, TGA, DSC, IR and XRD techniques and initial studies on their application in composites were conducted.
Manufacturing of carbon nanofibers from PAN precursor is described in Chapter 2 of the dissertation. The electrospun nanofibers were continuous, uniform in diameter, and the samples didn’t contain impurities, unlike carbon nanotubes or vapor grown carbon fibers. XRD studies on the carbon nanofibers fired at different temperatures showed that higher temperature resulted in better nanostructure.
Toughening effects of as-spun PAN, stabilized PAN, and carbon nanofibers on Mode I and Mode II interlaminar fracture of advanced graphite-epoxy composites were examined by DCB and ENF tests respectively in Chapter 3. The results showed that the interlaminar fracture toughness increased substantially with carbon nanofiber reinforcement. 200% improvement in Mode I fracture toughness and 60% in Mode II fracture toughness were achieved with a minimum increase of weight. SEM fractographic analysis showed nanofiber pullout and crack bridging as the dominant nanomechanisms of toughening.
Chapter 4 describes manufacturing of aligned carbon nanofibers and nanocomposites by a modified electrospinning technique. Analysis showed that mechanical properties of nanofibers and nanocomposites improved with alignment and stretch-stabilization of carbon nanofibers. Nanofiber properties were back-calculated from composite properties.
Nanofabrication of ceramic 3Al2O3-2SiO2, SiO2-TiO2 and ZrO2 nanofibers by a novel combination of sol-gel and electrospinning techniques invented recently at UNL is described in Chapters 5 and 6. The ceramic nanofibers were continuous, with a few exceptions, and had diameters as small as 80 nm. Effects of the process parameters on their geometry and structure were studied. The possibility of nanomanufacturing of nanocrystalline continuous nanofibers was demonstrated. The results of this dissertation will have an impact in the field of high performance fibers and nanocomposites. This study can pave way for consideration of continuous electrospun nanofibers as reinforcement in the next generation nanocomposites.
