Zhanjun Liu

Growth of carbon nanotubes on graphene by chemical vapor deposition

Abstract Graphite oxide powder impregnated with an Ni(NO3)2 solution was freeze-dried, thermally-reduced at 300 oC and then chemical-vapor deposition at 1000 oC was used to grow carbon nanotubes (CNTs) on graphene using methane as the carbon source. The morphology and electrical conductivities of the graphite oxide, reduced graphene and CNT-graphene hybrids were characterized. Results indicate that the density of CNTs on the graphene and the electrical conductivity of the hybrids increase with increasing Ni(NO3)2 concentration in the solution. Isolated graphene lamellae were connected by CNTs, giving rise to a 3D conducting network that provided conducting channels for electron transport.

Double Core–Shell Si@C@SiO2 for Anode Material of Lithium-Ion Batteries with Excellent Cycling Stability

Lithium-ion batteries (LIBs) composed of silicon (Si) anodes suffer from severe capacity decay because of the volume expansion deriving from the formation of Li15 Si4 alloy. In this study, we prepared a double core-shell Si@C@SiO2 nanostructure by the modified Stöber method. In the process of Si lithiation, the carbon layer alleviates the large pressure slightly then the silica shell restricts the lithiation degree of Si. The combination of carbon interlayer and silica shell guarantees structural integrity and avoids further decay of capacity because of the formation of stable solid-electrolyte interphase (SEI) films. The resultant Si@C@SiO2 presents remarkable cycling stability with capacity decay of averagely 0.03 % per cycle over 305 cycles at 200 mA g-1 , an improvement on Si@C (0.22 %) by more than a factor of 7. This encouraging result demonstrates that the designation involved in this work is effective for mitigating the capacity decay of Si-based anodes for LIBs.

Influence of filler type on the performance and microstructure of a carbon/graphite material

Abstract Carbon/graphite material was prepared by subjecting a mixture of coal-tar pitch binder and a relevant filler to uniaxial compression at 150 MPa for 10 min, followed by calcination at 1 300 °C for 1 h, and graphitization at 2 300 °C in an induction furnace. Four fillers, carbon black (CB), petroleum-coke powder, needle-coke powder, and natural graphite powder (NG), were used. The effect of filler type on the performance and microstructure of the material was investigated. Results reveal that the CB-based material has excellent flexural and compressive strength, with the highest values of 88.0 and 173.2 MPa, respectively, but poor thermal and electrical conductivity. The thermal conductivity of the NG-based material has the highest value of 278 W/m·K, but the flexural and compressive strength are limited to 51.1 and 90.2 MPa, respectively. Microstructural analysis showed that the NG-based material has the largest crystallite size, as well as the most perfect orientation of graphite layers.

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.

Microstructure and molten salt impregnation characteristics of a micro-fine grain graphite for use in molten salt reactors

Abstract The microstructure and molten salt impregnation characteristics of a micro-fine grain isotropic graphite ZXF-5Q from Poco Inc. was investigated. The microstructural characteristics of the pores caused by gas evolution, calcination cracks, Mrozowski cracks, and the crystal structure were characterized by optical microscopy, mercury porosimetry, helium pycnometry, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Results show that the ZXF-5Q has uniformly-distributed pores caused by gas evolution with very small entrance diameters (∼0.4 μm), and numerous lenticular Mrozowski cracks. Molten salt impregnation with a molten eutectic fluoride salt at 650 °C and 1, 3 and 5 atm, indicate that ZXF-5Q could not be infiltrated even at 5 atm due to its very small pore entrance diameter. Some scattered global salt particles found inside the ZXF-5Q are possibly formed by condensation of the fluoride salt steam during cooling.

Electrochemical properties of a silicon nanoparticle/hollow graphite fiber/carbon coating composite as an anode for lithium-ion batteries

Herein, a double strategy to modify the cycling performance of pure silicon nanoparticles (SiPs) was applied. Hollow graphite fibers (HGFs) with a good graphite structure could improve the electrical conductivity of the electrode. The SiP/HGF composite maintained a discharge capacity of 556.2 mA h g−1 and a charge capacity of 548.6 mA h g−1 after 50 cycles at the current density of 50 mA g−1, which obviously promoted the lifetime as compared to that of the anode of pure SiPs. Carbon coating could minimize the direct contact between the SiPs and electrolyte and buffer the volume changes during cycling. The silicon nanoparticle/hollow graphite fiber/carbon-coated (SiP/HGF/C) composite delivered the initial discharge and charge capacities of 1327.2 and 936.6 mA h g−1, respectively, at a current density of 50 mA g−1. After the first cycle, the charge capacity began to steadily increase to 1122.7 mA h g−1 until the thirty-first cycle. The double strategy effectively buffered the volume changes, enhanced the intensity of the electrode, and improved the overall electrical conductivity during discharge–charge cycles. The low-cost SiP/HGF/C composite showed an optimized electrochemical performance as compared to the pure SiP anode.

Hierarchically Multiporous Carbon Nanotube/Co3O4 Composite as an Anode Material for High-Performance Lithium-Ion Batteries

Carbon nanotubes (CNTs) and porous Co3 O4 nanorod (Co3 O4 p-NR) composites are self-assembled to form a hierarchical porous structure through a facile hydrothermal method to meet the requirements of long cycle life, high capacity, and excellent rate capability for next-generation lithium-ion batteries. CNTs are embedded in Co3 O4 p-NR clusters to form a 3D conductivity network, which reduces the transportation resistance of electrons and ions. Co3 O4 p-NRs are assembled from nanoparticles, which enlarge the contact area between electrode and electrolyte to provide more space to buffer the large volumetric changes associated with repeated electrochemical reactions and maintain the structural integrity. The obtained samples exhibit a high reversible capacity (1083 mA h g-1 after 140 cycles at 0.5 Ag-1 ), superior rate capability (521 mA h g-1 at 8 Ag-1 ), and excellent cyclic stability, with a capacity decay of 0.57 mA h g-1 per cycle at a high current of 1 Ag-1 over 200 cycles. The specific heterodimensional structure gives rise to a new approach to exploit high-performance electrode materials.

The structure of an in-situ formed titanium-boron-carbon coating on a graphite substrate

Abstract A titanium-boron-carbon coating was fabricated on a graphite substrate by heating TiB2 powder on a graphite surface above the eutectic temperature. The coating consisted of a pure graphite layer on the outer surface and a TiB2-C alloy layer inside. The graphite layer had many wrinkles due to the difference in the thermal expansion coefficients of TiB2 and graphite. The TiB2-C alloy layer had a continuous three-dimensional interpenetrating network microstructure. The d002 value of the graphite in the alloy layer was 0.335 6 nm, which was quite close to that of single crystal graphite (0.335 4 nm). Raman and X-ray photoelectron spectroscopy indicated that the graphite in both layers was doped substitutionally with boron atoms. A water quench thermal shock test verified a high adhesion strength between the coating and the substrate. This method is promising for the fabrication of thermal barrier coatings on carbon materials.