Zhaoju Yu

Single-source-precursor synthesis of high temperature stable SiC/C/Fe nanocomposites from a processable hyperbranched polyferrocenylcarbosilane with high ceramic yield

Hydrosilylation of vinyl ferrocene with allylhydridopolycarbosilane was used to synthesize a processable hyperbranched polyferrocenylcarbosilane (HBPFCS), which was characterized by combination of gel permeation chromatography, Fourier transform infrared (FT-IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. The polymer-to-ceramic transformation of the HBPFCSs was then investigated by FT-IR and 13C MAS NMR spectroscopy as well as by thermal gravimetric analysis (TGA). A self-catalytic effect of ferrocenyl units in the HBPFCS skeleton on dehydrocoupling was found during a curing process at 170 °C resulting in a high ceramic yield of ca. 80% at 1200 °C in Ar. Finally, microstructures and magnetic properties of the final ceramics were studied by techniques such as X-ray diffraction, energy dispersive spectroscopy, Raman spectroscopy, transmission electron microscopy and vibrating sample magnetometry. The final ceramic (pyrolysis temperature ≥900 °C) is characterized by a microstructure comprised of a SiC/C/Fe nanocomposite. Turbostratic carbon layers located at the segregated α-Fe crystal boundary avoid interdiffusion and explain the exclusive existence of α-Fe in a SiC/C matrix even at 1300 °C. Variations of the iron content in the HBPFCSs and of the pyrolysis conditions facilitate the control of the composition and ceramic micro/nanostructure, influencing in particular magnetic properties of the final SiC/C/Fe nanocomposite ceramic.

Versatile fabrication of arbitrarily shaped multi-membrane hydrogels suitable for biomedical applications

In this work, arbitrarily shaped multi-membrane hydrogels were successfully fabricated from gel-core templates using the layer-by-layer (LbL) method. Namely, the first gel membrane layer was formed around a gel-core template when the crosslinker loaded gel-core was soaked in a polysaccharide solution, and it was then ripened in a crosslinker solution, in which the crosslinker was loaded for the fabrication of the following layer. The formation and control of the gel membrane layer were studied in detail. The results indicated that a reasonably rapid crosslinking of the polysaccharide was essential for the successful preparation of a multi-membrane hydrogel, irrespective of chemical or physical crosslinking. The formation of a gel membrane layer was found to be controlled by the diffusion of the crosslinker. The chemically and the physically crosslinked multi-membrane hydrogels were characterized, and the chemically crosslinked chitosan multi-membrane hydrogel exhibited a unique sub-layered microstructure. The chitosan multi-membrane hydrogel which was sensitive to pH was fabricated using terephthalaldehyde as the crosslinker, and the hydrogel displayed LbL disintegration in acidic medium. Chondrocytes were cultivated in the presence of the multi-membrane hydrogel, and they could be easily attached to proliferate quickly. Because of the arbitrary shape, solid or hollow structure, pH sensitivity and biocompatibility, the polysaccharide multi-membrane hydrogels are promising materials for biomedical applications.

SiC/HfyTa1−yCxN1−x/C ceramic nanocomposites with HfyTa1−yCxN1−x-carbon core–shell nanostructure and the influence of the carbon-shell thickness on electrical properties

Dense monolithic SiC/HfyTa1−yCxN1−x/C (y = 0, 0.2 and 0.7) ceramic nanocomposites were prepared upon spark plasma sintering of amorphous SiHfTaC(N) ceramic powders which were synthesized from single-source-precursors. The microstructural evolution of the ceramic powders was investigated using elemental analysis, X-ray diffraction, Raman spectroscopy and transmission electron microscopy (TEM). The results reveal that the powdered and dense monoliths of SiC/HfyTa1−yCxN1−x/C ceramic nanocomposites annealed at T ≥ 1700 °C and at 2200 °C, respectively, are characterized by the presence of a homogeneous dispersion of HfyTa1−yCxN1−x-carbon core–shell nanoparticles within a β-SiC matrix. Hf/Ta atomic ratios (or y values) of the in situ generated HfyTa1−yCxN1−x-carbon core–shell nanoparticles can be controlled precisely by molecular tailoring of the preceramic precursors, which further tunes the thickness of the in situ formed carbon shell. Interestingly, with increasing the value y the thickness of the carbon shell increases, while the electrical conductivity of the dense monolithic SiC/HfyTa1−yCxN1−x/C (y = 0, 0.2 and 0.7) nanocomposites significantly reduces. The unique HfyTa1−yCxN1−x-carbon core–shell nanostructure opens a new strategy towards tailoring the electrical conductivity of SiC/HfyTa1−yCxN1−x/C nanocomposites for potential electromagnetic applications in harsh environments.

Single-source-precursor synthesis and electromagnetic properties of novel RGO–SiCN ceramic nanocomposites

Single-source-precursors (SSPs) have been synthesized through chemical modification of poly(methylvinyl)silazane (HTT 1800) with graphene oxide (GO) via an amidation reaction catalyzed by ZnCl2. With the formation of an SSP, the restacking of GO was effectively prevented by the HTT 1800 grafted at the surface of GO. After pyrolysis of warm-pressed green bodies comprising the SSP, GO-HTT 1800, monolithic silicon carbonitride (SiCN) ceramic nanocomposites containing in situ thermally reduced graphene oxide (RGO), namely RGO–SiCN, were successfully prepared. The resultant RGO–SiCN nanocomposites possess versatile electromagnetic (EM) properties ranging from EM absorbing to shielding behavior. With 2.5 wt% GO in the feed, the final RGO–SiCN nanocomposite exhibits an outstanding minimal reflection coefficient (RCmin) of −62.1 dB at 9.0 GHz, and the effective absorption bandwidth reaches 3.0 GHz with a sample thickness of 2.10 mm. With the same GO content, the resultant RGO–SiCN nanocomposite prepared by mechanical blending exhibits a far inferior RCmin of −8.2 dB. This finding strongly supports the advantage of the developed SSP route suitable for the fabrication of RGO–SiCN nanocomposites with significantly enhanced EM properties. With 12.0 wt% GO content in the feed, the obtained RGO–SiCN nanocomposite reveals an excellent shielding effectiveness of 41.2 dB with a sample thickness of 2.00 mm.