Hanqing Liu

Oxidation behavior of silicon nitride fibers obtained from polycarbosilane fibers via electron beam irradiation curing

Near-stoichiometric silicon nitride (Si3N4) fibers, which were successfully prepared from polycarbosilane fibers via electron beam irradiation curing, were heat-treated at elevated temperature for 2 h in the air atmosphere. The compositions and microstructures of silicon nitride fibers before and after heat treatment were investigated by XRD, XPS, NMR, SEM and TEM analyses. Tensile properties of the untreated and heat-treated fibers were also studied. The results show that the untreated fibers were mainly composed of amorphous silicon nitride and a few Si2N2O phases. During the heat treatment process, the oxidation of the fibers from silicon nitride to silicon dioxide initiated at 1100 °C. As the treatment temperature increased to 1400 °C, an oxidation layer with a thickness of ∼2 μm was formed on the fiber surface. Besides, when the treatment temperature was up to 1200 °C, the strength retention of the fibers still was 50.29%, which indicates that the fiber might possess a high serving life at a temperature lower than 1200 °C.

A novel high-content CNT-reinforced SiC matrix composite-fiber by precursor infiltration and pyrolysis process

A novel carbon nanotube (CNT)-reinforced SiC matrix composite-fiber with excellent mechanical, electrical, and thermal resistant properties was fabricated by filling the macroscopic CNT fibers with a polycarbosilane precursor-derived SiC matrix at 1000 °C. The SiC matrix had a nanoporous and amorphous microstructure, uniformly distributed among the CNTs. The CNT experienced some compressive residual stresses due to the matrix shrinkage during processing, leading to densification of the fiber, increase in the hopping channel number, and strong CNT/SiC interfacial interaction. The composite fiber displayed a brittle-fracture response during tensile deformation, with the tensile modulus and strength ∼1.5 times higher those of pure fiber. The dominant fracture mechanism was a bridging effect of SiC on the CNTs that favored better load transfer during tensile deformation. Unlike that for the pure fiber, the mechanical performance of the composite fiber was well maintained after harsh treatment at 1000 °C in Ar, evidencing its excellent thermal resistant property. In addition, the electrical conductivity of the composite-fiber was ∼1 time higher than that of the pure fiber and 4–5 times higher than that of the PAN-based carbon fiber, mainly due to the denser fiber microstructure after SiC infiltration. Finally, the composite-fiber showed a better oxidation resistant property, demonstrating promising applicability under high temperature and oxidation conditions.

Tunable Coupled-Resonator-Induced Transparency in a Photonic Crystal System Based on a Multilayer-Insulator Graphene Stack

We achieve the effective modulation of coupled-resonator-induced transparency (CRIT) in a photonic crystal system which consists of photonic crystal waveguide (PCW), defect cavities, and a multilayer graphene-insulator stack (MGIS). Simulation results show that the wavelength of transparency window can be effectively tuned through varying the chemical potential of graphene in MGIS. The peak value of the CRIT effect is closely related to the structural parameters of our proposed system. Tunable Multipeak CRIT is also realized in the four-resonator-coupled photonic crystal system by modulating the chemical potentials of MGISs in different cavity units. This system paves a novel way toward multichannel-selective filters, optical sensors, and nonlinear devices.