Dong Su

Electrochemical performance of graphene nanosheets and ceramic composites as anodes for lithium batteries

A nanocomposite anode material for lithium batteries is designed and fabricated by the insertion of graphite nanosheets (GNS) into the ceramic network of silicon oxycarbide (SiOC) ceramics for the development of structurally and electrochemically stable lithium batteries. The GNS forms a layered phase in the SiOC ceramic network from the self-assembly of graphite oxides (GO) introduced in a polysiloxane precursor through thermal transformations after pyrolysis. The composite anode (GNS/SiOC) exhibits an initial discharge capacity attaining 1141 mAh g−1. The discharging capacity decreases in the first eight cycles and stays at 364 mAh g−1 in the following cycles. This reversible discharging capacity is higher than that of a graphite reference (328 mAh g−1) and a SiOC monolithic. Correlating the discharge capacities to the material compositions and structures suggest that the interface between SiOC and GNS contributes to the enhanced capacity of the composite anode, in addition to those from GNS and SiOC. Further increasing the electrochemical performance is possible by the increase of the amount of GNS in the composite.

Super-hydrophobic hexamethyl-disilazane modified ZrO2–SiO2 aerogels with excellent thermal stability

Hexamethyl disilazane modified ZrO2–SiO2 aerogels (HMDS/ZSAs) have been prepared by the addition of HMDS into ZrO2–SiO2 gels during the aging process in order to improve their thermal stability at high temperature. FTIR analysis shows that the methyl siloxy groups ((CH3)3Si–O–) could replace the superficial hydroxy groups (–OH) through the condensation between the (CH3)3Si–OH of HMDS and –OH on the initial ZSA surface. HMDS/ZSAs could maintain their three-dimensional network structure without collapse and exhibit a remarkable thermal stability with retention of most physical characteristics up to a temperature of 1000 °C, such as a high surface area (174.4 m2 g−1), high pore volume (0.7246 cm3 g−1) and high elastic modulus (5.23 MPa). TGA and XRD analyses demonstrate that the phase transition of HMDS/ZSAs from amorphous to crystalline occurs up to 1000 °C, which is 100 °C higher than that of ZSAs. It was demonstrated that the modified inert methyl siloxy groups could form small silicon particles (3 to 5 A in diameter) to restrict the migration of grain boundaries, which was favorable to suppress the ZrO2 crystallization at elevated temperatures. Furthermore, HMDS/ZSAs exhibit super-hydrophobic properties with a contact angle of 154°. The high performance of HMDS/ZSAs paves the way for their applications in the field of thermal insulation, energy-absorbing services, environmental remediation, etc.

High-strength mullite fibers reinforced ZrO2–SiO2 aerogels fabricated by rapid gel method

A rapid gelation process is adopted to fabricate mullite fibers-reinforced ZrO2–SiO2 (M/ZrO2–SiO2) aerogels. The short-cut mullite fibers are introduced into ZrO2–SiO2 sol via aging and supercritical drying, and the epoxides are used as gelation accelerators. The as-prepared M/ZrO2–SiO2 aerogels have a three-dimensional reticulated porous structure similar to those of pure ZrO2–SiO2 aerogels observed by scanning electron microscopy, which indicates that the addition of fibers does not obviously affect the morphology of aerogels. It is observed that the mullite fibers disperse in the aerogels homogeneously, and fibers combine well with aerogels. M/ZrO2–SiO2 aerogel composites exhibit high compressive strengths up to 0.438xa0MPa, which indicates that this structure benefits the loading transfer and thus enhances their mechanical properties. Moreover, the thermal conductivity of M/ZrO2–SiO2 aerogel composites is as low as that of the pure ZrO2–SiO2 aerogels (~0.0270xa0Wxa0m−1xa0K−1).

Preparation and characterization of continuous SiZrOC fibers by polyvinyl pyrrolidone-assisted sol-gel process

Continuous PZSO–PVP gel fibers have been drawn from polyzirconosiloxane (PZSO) solution consisting of two-component silicon alkoxides (tetraethoxysilane and dimethyldiethoxylsilane) and ZrOCl2, utilizing polyvinyl pyrrolidone (PVP) as spinning reagent and ethanol as solvent. The PVP addition significantly improves the solution spinnability and stability by adjusting the solution viscosity, the coordination with Zr atom, and the steric hindrance effect. The PZSO–PVP fibers are composed of smooth dense surface and elliptical-shaped cross section due to nonhomogeneous shrinkage during gelation and drying. Fourier transform infrared and X-ray photoelectron spectroscopy analysis on the PZSO–PVP fibers suggest a structure of mixed SiO4 and SiC2O2 units with incorporated Zr in the form of Zr–O–Zr and Zr–O–Si. SiZrOC ceramic fibers are obtained through subsequent drying and pyrolysis treatment, with good thermal stability up to 1500xa0°C. The fibers are of mixed silicon oxycarbide phase embraced with ZrO2 phase and free carbon but no SiC phase.

Improvement of thermal stability of ZrO2–SiO2 aerogels by an inorganic–organic synergetic surface modification

The thermal stability of ZrO2–SiO2 aerogels was significantly improved by inorganic–organic synergetic surface modifications: inorganic ions [Fe(III)] surface modification and hexamethyldisilazane gas phase modification. The replacement of Hs from surface hydroxyl groups on the aerogel by Fe(III) ions and silyl groups played a critical role in isolating the hydrous particles of ZrO2–SiO2 aerogels. So the particle growth caused by the condensation of hydroxyl groups upon firing was inhibited. Meanwhile, the decomposition of the silyl groups upon heat treatment produced SiO2 particles, which could serve as pining particle to inhibit the crystallization of ZrO2. Hence, the porous microstructure of the modified aerogels was still well preserved up to 1000xa0°C, with a high specific surface area of 203.5xa0m2/g, and a considerable pore volume of 0.721xa0cc/g. These characteristics of the modified aerogels suggest that it has great potential on ultrahigh-temperature applications in the fields of thermal insulation, catalysis, and catalyst support, etc.

Continuous sol–gel derived SiOC/HfO2 fibers with high strength

This study presents the fabrication and characterization of continuous SiOC/HfO2 fibers with high strength by the sol–gel process. Continuous polyhafnosiloxane (PHfSO) gel fibers are spun from the solutions of silicon alkoxides and hafnium dichloride using polyvinyl pyrrolidone as a spinning reagent, and then transform into dense SiOC/HfO2 fibers with homogeneous shrinkage by subsequent drying and pyrolysis treatment. Fourier transform infrared and X-ray photoelectron spectra together with X-ray diffraction analysis indicate that the amorphous SiOC/HfO2 fibers consist of mixed silicon oxycarbide (SiOxC4−x, x = 1–4) and tetravalent hafnium–oxygen units embraced with a certain free-carbon phase. Scanning electron microscopy and transmission electron microscopy observations reveal that the SiOC/HfO2 fibers with homogenous Hf distribution exhibit a circular-shaped or an elliptical-shaped cross-section depending on their thickness when employed as gel fibers. Mechanical testing shows that the SiOC/HfO2 fibers exhibit good mechanical property with the maximum tensile strength of 1.5 GPa arising from the incorporation of Hf in the SiOC network.

Superelastic and superhydrophobic bacterial cellulose/silica aerogels with hierarchical cellular structure for oil absorption and recovery

Bacterial cellulose aerogels/silica aerogels (BCAs/SAs) are prepared using three-dimensional self-assembled BC skeleton as reinforcement and methyltriethoxysilane derived silica aerogels as filler through vacuum infiltration and freeze drying. The BCAs/SAs possess a hierarchical cellular structure giving them superelasticity and recyclable compressibility. The BCAs/SAs can bear a compressive strain up to 80% and recover their original shapes after the release of the stress. The BCAs/SAs exhibit super-hydrophobicity with a contact angle of 152° and super-oleophilicity resulting from the methyl groups on the surface of silica aerogel filler. This endows the BCAs/SAs outstanding oil absorbing capability with the quality factor Q from 8 to 14 for organic solvents and oils. Moreover, the absorbed oil can be retrieved by mechanically squeezed with a recovery of 88% related to the superelastic ability of the composites. In addition, the oil absorbing of BS/SAs could be well maintained with the quality factor Q about 11 for gasoline after harsh conditional treatment down to -200u202f°C and up to 300u202f°C. Such outstanding elastic and oleophilic properties make the BC/SAs tremendous potential for applications of oil absorbing, recovery and oil-water separation.

SiOC nanolayer wrapped 3D interconnected graphene sponge as a high-performance anode for lithium ion batteries

Silicon oxycarbides (SiOCs) are promising anode materials for high-energy LIBs because of their high theoretical capacity. However, due to their intrinsically poor electronic conductivity, the battery performance is often restricted. Herein, high performance anodes are demonstrated by designing a hierarchical 3D interconnected structure using graphene sponge as a scaffold. The graphene sponge was infiltrated with a polysiloxane precursor and further converted into porous frameworks consisting of multi-layered sandwich-like nanosheets (SiOC@graphene@SiOC) by subsequent pyrolysis. The deliberate structure not only improved the electrical conductivity, accelerated ion insertion, and shortened the ionic diffusion distance but also enabled full utilization of SiOC active sites in the anode. The 3D-GNS/SiOC anodes exhibited excellent electrochemical performance, including high initial discharge capacity (1280xa0mA h g−1 at 0.1 A g−1), high reversibility and stability (701 mA h g−1/371 μA h cm−2 after 100 cycles) and extreme rate performance (656 mA h g−1/348 μA h cm−2 at 0.5 A g−1). For full-cells, high initial charge capacity (680 mA h g−1 at 0.5 A g−1) and high stability (416 mA h g−1 at 0.5 A g−1 after 100 cycles) were obtained. Significantly, this simple and scalable method can be extended to fabricate high-rate and long-cycle SiOC or other anode materials for commercial LIBs.

Microstructure and properties of pressureless-sintered porous Si3N4 using PMMA as pore-forming agent

ABSTRACT In this study, porous β-Si3N4 ceramics containing different types of sintering additives (Yb2O3 and Lu2O3) were produced successfully by addition of PMMA (0 to 40 vol%) and pressureless sintering. The effects of PMMA content and different types of sintering additives (Yb2O3 and Lu2O3) on the microstructure and mechanical properties of the Si3N4 ceramics was studied. Porous Si3N4 ceramics with 5 wt% Yb2O3-doping had better mechanical properties than the ceramics with 5 wt% Lu2O3-doping. The flexural strength and fracture toughness of the porous Si3N4 ceramics with 5 wt% Yb2O3-doping were in ranges from 349 to 109 MPa and 6.8 to 3.8 MPa·m1/2 with porosity of 15-37%. With the increase of the porosity, the flexural strength and fracture toughness all decreased.

Fabrication of Macroporous SiCN Ceramics from Mixed Polysilazanes

Macroporous polymer-derived SiCN ceramics are fabricated directly by mixing polysilazane precursors followed with crosslinking and pyrolysis. Two kinds of polysilazanes namely polyvinylsilazane and polyhydrosilazane are mixed, crosslinked by 2, 2-Azo-bis-isobutyronitrile to form resins before pyrolyzed to form ceramics in argon flow at 1000°C. The density of the SiCN ceramic is 1.65 g/cm3 with corresponding porosity of 30 % compared to dense SiCN ceramics. SEM images show that the ceramics possess high porosity and homogeneous honeycomb-like macropores of ~2 μm. The porous SiCN exhibits good mechanical property with Vicker hardness of 11-13 GPa under a load of 0.2 kg.