Huiqi Wang

SiC-ZrO2(3Y)-Al2O3 nanocomposites superfast densified by spark plasma sintering

Abstract Heterogeneous precipitation method has been used to produce 5 wt% SiC-15 wt% ZrO2-Al2O3 powder, from aqueous suspension of nano-SiC, aqueous solutions of zirconium oxychloride, yttrium nitrate and aluminium chloride and ammonia. The resulting gel was calcined at 1100 °C. Nano-SiC-ZrO2(3Y)-Al2O3 composites were superfast densified using spark plasma sintering (SPS) process by heating to a sintering temperature in the range of 1350 to 1600 °C, at a heating rate of 600 °C/min, with no holding time, and then fast cooled to 600 °C within 2–3 minutes. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 wt% SiC-15 wt% ZrO2(3Y)-Al2O3 superfast densified by SPS at 1450 °C reached as high as 1.2 GPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al2O3 grains and the fracture mode of the nanocomposites was transgranular fracture.

Fabrication of nano Y–TZP materials by superhigh pressure compaction

Abstract Y–TZP nanoceramics have been fabricated by superhigh pressure compacting and pressureless sintering. It was found that, by applying superhigh pressure of about 3GPa, the green compacts with a relative density of about 60% could be obtained, which was 12% higher than that by a normal cold isostatic pressure of 450 MPa. The obvious advantage of the higher relative density of the green compact is that the sintering temperature could be decreased to as low as 1050°C. The reason for the good sintering behavior is believed to be both the increase in contact points between the particles and the decrease in pore size. Nano Y–TZP ceramics with grain size of about 80 nm could be obtained when sintered at 1050°C for 5 h.

Highly microporous graphite-like BCxO3-x/C nanospheres for anode materials of lithium-ion batteries

In the present work, we report Li storage in a spherical graphite-like BCxO3−x/C (g-BCO/C) nanostructure with an average diameter of ca. 75 nm. Highly microporous g-BCO/C nanospheres with a BET specific surface area of 493.35 m2 g−1 were fabricated via a simple hydrothermal carbonization process combined with one-step annealing. The microstructure of the g-BCO/C nanospheres was characterized by means of XRD, TEM and Raman spectroscopy. XPS measurements reveal the formation of a boron solid-solution phase such as BC3 (x = 3), BC2O (x = 2) and BCO2 (x = 1), as well as the concentration of substitutional boron which is around 1.75 at.%. Further, 11B-NMR spectra confirmed the formation of BC3. Benefiting from the unique structural features and boron doping, the g-BCO/C nanospheres exhibit excellent electrochemical performances as an anode material for Li-ion batteries. A high initial reversible capacity (591 mA h g−1, 200 mA g−1) and high-rate capacity (262 mA h g−1, 1000 mA g−1), as well as stable capacity (446 mA h g−1, 200 mA g−1) after 100 cycles are delivered. The improved Li storage performance is attributed to three merits of g-BCO/C nanospheres: the large number of edge defects induced by boron, the micropores in the material and the large spacing of the lattice fringes (d002) which can provide extra Li storage regions.

Structural evolution of rayon-based carbon fibers induced by doping boron

In the present work, we provide a systematic analysis of the structural evolution of rayon-based carbon fibers (RCFs) induced by doping boron using scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Raman spectroscopy. For the first time, boron-doped RCFs with tunable amounts of boron were fabricated by exposing the RCFs to a vapor of boron by the decomposition of boron carbide (B4C). SEM and XRD results indicate that at the higher temperatures the strong erosion of boron vapor not only changed the original structure of RCFs, but also produced some flaws. Interestingly, when the temperature of doping boron is higher than 2200 °C, the graphite basal planes of RCFs are perpendicular to the fiber axis. Raman spectra also confirmed the presence of disorders and flaws in graphitic layers because of the displacement and solid solution of boron in the carbon lattice. Further, the chemical environment of boron species was ascertained by 11B nuclear magnetic resonance, indicating that boron atoms exist in three chemical environments, including the substitutional boron (BC3), boron clusters and B4C. Moreover, the TGA data indicated that doping boron greatly improved the oxidation inhibition of RCFs, and is superior to increasing the heat treatment temperature for improving the oxidation resistance. Such a systematic analysis of the structural evolution and oxidation resistance of RCFs induced by doping boron thus provides industrial potential for preparing RCFs with higher oxidation resistance at up to 800 °C.

The reaction behavior of carbon fibers and TaC at high temperatures

Chopped carbon fibers (CFs) and TaC particles were dispersed uniformly and then sintered at temperatures of 1773, 2123, 2298, 2473 and 2823 K, respectively. The effect of sintering temperature on the microstructure of CFs was investigated. The results showed that the reaction between CFs and TaC particles was controlled by solid diffusion. When the temperature was lower than 1773 K, mainly carbon diffused into TaC. However, Ta diffused into the CFs while carbon diffused into TaC particles and then precipitated as graphite when the sintering temperature was above 2123 K. During the process, the structure of TaC was not influenced. The preferential crystalline orientation of the graphite precipitated from TaC particles increased with the increase of sintering temperature. The R value determined by Raman spectra decreased from 0.12 to 0 as the temperature increased from 2123 to 2823 K, meaning the formation of perfect graphite crystallites.

Carbon-Dot-Based Heterojunction for Engineering Band-Edge Position and Photocatalytic Performance

A photocatalytic reaction is always governed by energy band configuration of the catalyst, but its modulation is challenging. Here, the adjustment of band-edge positions of Ag3 PO4 through fluorescent carbon dots is reported for the first time. Both Ag3 PO4 and carbon dots which keep the similar sizes constitute a heterojunction. Such heterostructure not only promotes visible light absorption, photogenerated charge separation, and transfer, but it also transforms photocatalytic activity of Ag3 PO4 from photooxidation to photoreduction due to the huge changes of band-edge positions. In addition, the heterojunction structure of carbon dots and Ag3 PO4 exhibits unique temperature-responsive photocatalytic activities and higher photocatalytic stability than the pure Ag3 PO4 . According to the contrast experiments and related characterizations, the possible mechanism of photocatalytic reaction is proposed. Therefore, this work offers an alternative route for adjusting energy band positions or tuning photocatalytic performance as well as for preparing novel carbon-dot-based heterostructure.

Cross-linked Nanohybrid Polymer Electrolytes with POSS Cross-linker for Solid-state Lithium Ion Batteries

A new class of freestanding cross-linked hybrid polymer electrolytes (HPEs) with POSS as the cross-linker was prepared by a one-step free radical polymerization reaction. Octavinyl octasilsesquioxane (OV-POSS) with eight functional corner groups was used to provide cross-linking sites for the connection of polymer segments and the required mechanical strength to separate the cathode and anode. The unique cross-linked structure offers additional free volume for the motion of EO chains and provides fast and continuously interconnected ion-conducting channels along the nanoparticles/polymer matrix interface. The HPE exhibits the highest ionic conductivity of 1.39 × 10−3 S cm−1, as well as excellent interfacial compatibility with the Li electrode at 80°C. In particular, LiFePO4/Li cells based on the HPE deliver good rate capability and long-term cycling performance with an initial discharge capacity of 152.1 mAh g−1 and a capacity retention ratio of 88% after 150 cycles with a current density of 0.5 C at 80°C, demonstrating great potential application in high-performance LIBs at elevated temperatures.