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Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB 2 and B 4 C. TiB 2 and B 4 C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiO x C y . After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C). [ABSTRACT FROM AUTHOR]

Effects of oxidation cross-linking and sintering temperature on the microstructure evolution, thermal conductivity and electrical resistivity of continuous freestanding polymer-derived SiC films were investigated. The as-received films consisting of β-SiC nanocrystals embedded in amorphous SiO x C y and free carbon nanosheets were fabricated via melt spinning of polycarbosilane (PCS) precursors and cured for 3 h/10 h followed by pyrolysis from 900 °C to 1200 °C. Results reveal that nanoscale structure (β-SiC/SiO x C y /C free ) provides an ingenious strategy for constructing highly thermal conductive, highly insulating and highly flexible complexes. In particular, the 3 h-cured films sintered at 1200 °C with satisfying thermal conductivity (46.8 W m −1 K −1 ) and electrical resistivity (2.1 × 10 8 Ω m) are suitable for the realization of high-performance substrates. A remarkable synergistic effect (lattice vibration of β-SiC nanocrystals and close-packed SiO x C y , free-electron heat conduction of β-SiC and free carbon, and supporting role of oxygen vacancy) contributing to thermal conductivity improvement is proposed based on the analysis of microstructure, intrinsic properties and simulations. Eventually, the SiC films without additional dielectric layers are directly silk-screen printed with high-temperature silver paste and used as heat dissipation substrates for high-power LED devices via chip-on-board (COB) package. The final devices can emit bright light with low-junction temperature (52.6 °C) and good flexibility owing to the mono-layer SiC substrate with low thermal resistance and desirable mechanical properties. This work offers an effective approach to design and fabricate flexible heat dissipation ceramic substrates for thermal management in advanced electronic packaging fields. [ABSTRACT FROM AUTHOR]

A careful approach to the optimization of magnetic and dielectric losses in nanomaterials can improve the electromagnetic wave absorption loss performance for certain microwave absorption applications. In this study we prepared dual core (Fe/TiCN) coated with nitrogen (N) doped carbon shell nanocomposite by arc-discharge method under mix atmospheres of working gases and with varying elemental compositions. Among all nanocomposites, (Fe/TiC 0.7 N 0.3)@N–C dual-core@ N- doped shell nanocomposite exhibits enhanced microwave absorption. Owing to the novel dual-core@ N-doped shell structure and numerous defects induced by doping N in carbon shells, an improved dielectric relaxation in composite is observed and the minimum reflection loss was reached −44.36 dB at 5.3 GHz for 4.8 mm thickness. [ABSTRACT FROM AUTHOR]

Herein, low-threshold green and red random lasing actions were investigated in inorganic caesium lead bromide (CsPbBr 3) and caesium lead iodide (CsPbI 3) perovskite microcrystals, which were grown using metal organic chemical vapor deposition. Sharp random lasing emission peaks were visible on the right side of the broad photoluminescence emission spectra. Interferential patterned sapphire substrates and novel metal nanoparticles were used to enhance the lasing emission intensity and reduce the lasing emission thresholds. Fast-decayed random lasing oscillation dynamics and backscattering angular distribution of light confirmed that the multi-scattering of light was important in the reduction of the threshold in thin films on patterned sapphire substrates. Plasmonic enhancement between gold or silver nanoparticles and the perovskite microcrystals contributed similarly to low threshold random lasing emission, which was verified using the simulation result from the finite difference time domain (FDTD) method. This work has a wide range of applications in sensing, speckle-free imaging, illumination, and medical diagnosis. [ABSTRACT FROM AUTHOR]

The Simplistic formation, advantageous configuration, non-colossal magnetoresistance and broadband absorption are important parameters for microwave absorbent materials. In this study, a core-shell nanocomposite comprising of Sn-filled carbon nanotubes (Sn/CNTs) was prepared by arc discharge method. The microstructure, morphology and surface composition of Sn/CNTs-based core-shell nanocomposites were characterized in detail. Sn/CNTs nanocomposite showed a magnetic signal due to the broken bonds and defects at interfaces in Sn/CNTs. The weak ferromagnetism was found to be helpful in improving magnetic permeability in the Sn/CNTs which confirms its role as a magnetic loss material under incident electromagnetic wave. Sn-filled CNTs revealed an appropriate value of dielectric constant, which plays an important role in impedance matching upon incident electromagnetic wave. The composite of Sn-CNTs and paraffin with a 50 wt % loading showed the lowest reflection loss (RL) of −43.87 dB at 10 GHz, with a wide effective absorption band (RL ≤ −10 dB) of 3 GHz in thickness of 2.3 mm. This enhanced performance is attributed to the combined effect of the conduction loss in one-dimensional core-shell architecture, the interfacial loss Sn-CNT interface, the magnetic loss due to defects-induced ferromagnetism in Sn shell, and in the carbon-containing atomic layers of CNTs. [ABSTRACT FROM AUTHOR]

In this contribution, a technique based on molding polycarbosilane-vinyltriethoxysilane-graphene oxide (PCS-VTES-GO) precursors synthesized by chemical modification was introduced to fabricate monolithic rGO/SiC(O) nanocomposite ceramics. Results reveal that the monoliths pyrolyzed at different temperatures (1100–1500 °C) consist of β-SiC nanocrystals homogeneously embedded in amorphous SiO x C y /C free , and multilayered rGO nanosheets composited with β-SiC/SiO x C y /C free . Their nanocomposite structure provides an ingenious strategy for constructing complexes with good integrity, high ceramic yield and low linear shrinkage. This improvement is primarily attributed to the presence of GO (as graft polymers and sintering additives) which enlarges the cross-linking structure of precursors and inhibits the growth of β-SiC nanocrystals during the pyrolysis process. The final rGO/SiC(O) monoliths sintered at 1200 °C with higher hardness (19.81 GPa), satisfying ceramic yield (80.98%) and linear shrinkage (25.23%) promise to be candidates for making complex-shaped ceramic components in a simple and economical manner. [ABSTRACT FROM AUTHOR]