سجى مظهر نبات العجرش

Lignin macromolecules are potential precursor materials for producing electrospun nanofibers for composite applications. However, little is known about the effect of lignin type and blend ratios with synthetic polymers. This study analyzed blends of poly(acrylonitrile-co-methyl acrylate) (PAN-MA) with two types of commercially available lignin, low sulfonate (LSL) and alkali, kraft lignin (AL), in DMF solvent. The electrospinning and polymer blend solution conditions were optimized to produce thermally stable, smooth lignin-based nanofibers with total polymer content of up to 20 wt % in solution and a 50/50 blend weight ratio. Microscopy studies revealed that AL blends possess good solubility, miscibility, and dispersibility compared to LSL blends. Despite the lignin content or type, rheological studies demonstrated that PAN-MA concentration in solution dictated the blend’s viscosity. Smooth electrospun nanofibers were fabricated using AL depending upon the total polymer content and blend ratio. AL’s addition to PAN-MA did not affect the glass transition or degradation temperatures of the nanofibers compared to neat PAN-MA. We confirmed the presence of each lignin type within PAN-MA nanofibers through infrared spectroscopy. PAN-MA/AL nanofibers possessed similar morphological and thermal properties as PAN-MA; thus, these lignin-based nanofibers can replace PAN in future applications, including production of carbon fibers and supercapacitors.

Three preceramic polymer formulations for potential use in additive manufacturing technologies were investigated. The polymeric precursors include an allyl hydrido polycarbosilane (SMP-10), a mixture of SMP-10 with a reactive ester (1,6-hexanediol diacrylate, HDDA), and a polydimethylsiloxane (4690A/B). The SMP-10/HDDA proved to have outstanding photo-curing properties, high-resolution printing, and the ability to easily transform into the silicon carbide phase. The same polymeric mixture showed the lowest viscosity value which is preferred in vat additive manufacturing. Thermogravimetric analysis showed that, after pyrolysis to 1350 °C, the polydimethylsiloxane polymer showed the highest onset decomposition temperature and the lowest retained weight (52 wt%) while the allyl hydrido polycarbosilane showed the lowest onset decomposition temperature and highest retained weight (71.7 wt%). In terms of crystallography, X-ray diffraction and microstructural results showed that the ceramic matrix composites contained both silicon carbide and silicon oxycarbide. Overall, the results are very promising for the fabrication of ceramic materials using additive manufacturing technologies.

The properties of ceramic materials are dependent on crystal size and their distribution. These parameters can be controlled using electrospinning of the two/phase mixed system. The pre/ ceramic solution consists of silicon nano/particles and polyacrylonitrile (PAN) polymer mixture. Particles distribution during the electrospinning technique was characterized using TEM microscopy and modeled using Finite Element Method (FEM). The experimental and numerical results were in agreement. Large silicon particles were located in the skin and the smaller ones were located at the core. This illustrated by the migration rate from the core which was the fastest for large particles and it diminished as the particles become smaller in size. The threshold for Stokes number was found to be around 2.2E-4 with a critical particle size of 1.0E7 m in diameter. The current results are very promising, as it demonstrated a novel way for the fabrication of PAN/Si ceramic nano/fibers with a gradient of particle size and properties from the skin to the core.

In this study, electrospinning was successfully used to fabricate hybrid Carbon-Silicon Carbide (C-SiC) fibers using polyacrylonitrile (PAN) and silicon (Si) nano-particles as precursors. Silicon-to-carbon precursor ratios and heat treatment were optimized to fabricate hybrid nano-fibers with high oxidation resistance. After heat treatment, the nano-fibers showed two-dimensional ordered carbons and SiC nano-phases formation. As a result, a noticeable improvement in mechanical and thermal properties was achieved. Samples with 90 wt% PAN/10 wt% Si showed about four-time improvements in char yield as compared with 100 wt% PAN. In addition, after heat treatment of the fiber at 1250 °C, SiC was formed. The SiC phase played a major role in ordering the carbon phase. The carbon and SiC crystallinities had a great impact on the oxidation and mechanical property improvement of hybrid nanofibers. The determination of crystal size was carried out using X-ray results obtained at different temperatures and holding times. The SiC grain growth was predicted using kinetic model and its exponent was found to be around n = 4 with an activation energy around 35 KJ/mol.K. For such growth, the dominant grain growth mechanism was grain boundary diffusion.

PDMS/PAN nanofibers

Nanofibers with several hundred of nanometers were successfully fabricated using electrospinning process and a mixture of two types of polymers which are: polydimethylsiloxane and polyacrylonitrile as precursors. After stabilization and carbonization at 1000 °C, three phases which are: silicon carbide (SiC), carbon, and oxy-SiC were presented. Spectroscopic and microscopic techniques had confirmed the presence of nanocrystalline SiC and turbostratic carbons. These phases formed an intertwined network at the nanometric scale. In addition, the resulted fibers showed a core-skin effect with skin richer in carbon and a core mainly dominated by silicon-based phases in the form SiC or SiOC ceramics. A significant improvement was observed in both tensile strength and elastic modulus in these hybrid fibers. In term of crystallography, these nanofibers seem to exhibit similar microstructure that was observed in Nicalon fiber. However, it was difficult to determine the ratio of these phases and their influence on the physical properties of these hybrid fibers.

تم اختيار الدكتورة سجى كمقوم علمي لعده مجلات عالمية