Yumeng Zhao

Transformation of onion-like carbon from nanodiamond by annealing

Onion-like carbon (OLC) was synthesized by annealing nanodiamond in low vacuum (1 Pa) at the temperatures from 500 to 11400°C. The high-resolution transmission electron microscope images, X-ray diffraction patterns and Raman spectra showed that, when the annealing temperatures were lower than 900°C, there was no OLC fabricated. The amorphous carbon and the nanodiamond coexisted. The graphitization started from the surfaces of the nanodiamond particles. When the annealing temperatures were higher than 900°C, the OLC was fabricated. At 900°C, OLC began appearing and the size of the OLC particles was smaller than 5 nm. At the annealing temperature of 1400°C all the nanodiamond particles were transformed into OLC. The OLC particles exhibited similarity to the original nanodiamond particles in shape. Based on these results, a mechanism for the OLC synthesis by the method of annealing in vacuum was provided.

HRTEM and Raman characterisation of the onion-like carbon synthesised by annealing detonation nanodiamond at lower temperature and vacuum

Onion-like carbon (OLC) was synthesised by annealing detonation nanodiamond for 1.5 h at temperatures from 500 to 1400°C and at a vacuum of 1 Pa. The results showed that the nanodiamond was transformed into the amorphous carbon (a-C) at first and then the a-C was transformed into the OLC gradually with the increase in annealing temperature. Moreover, at the annealing temperature of 600°C, the nanodiamond started transforming into a-C from the edge of the nanodiamond particle (1 1 1) crystal plane. At the annealing temperature of 750°C, the nanodiamond was transformed into the a-C completely. At the annealing temperature of 850°C, the a-C began transforming into the OLC at the edge area. At the annealing temperature of 1000°C, the OLC particle with a size smaller than 5 nm was synthesised. However, in the centre of the OLC particle, untransformed a-C coexisted. At the annealing temperature of 1100°C, the microstructure of the OLC particle with a size smaller than 5 nm became optimised. At the annealing temperature of 1200°C, the OLC particle with a size larger than 5 nm was fabricated. There was also untransformed a-C coexisting in the centre of the OLC particle. At the annealing temperature of 1350°C, all the a-C was transformed into the OLC. The average size of the OLC was approximately 5 nm, which was the same as that of the nanodiamond. The layers of the OLC were varied from several to 12.

Generation of microplasma using multiwall carbon nanotubes cathode

Microplasma was produced in argon gas in a scanning electron microscope at near-atmospheric pressure using a multi-wall carbon nanotube (CNT) film as a cathode. It is demonstrated that with the CNT film used as a cathode, the breakdown voltage was much lower than the breakdown voltage when the conventional cathode made of flat metal film was used and the discharge was highly reproducible. These features of the gas discharge are defined by the field emission from the CNT cathode.

Field emission from carbon nanotubes in air

The properties of the field emission (FE) from multi-walled carbon nanotubes (CNTs) in air used to generate the microplasma at near-atmospheric pressure were investigated in a removable gas cell built into a scanning electron microscope. The gaps between the electrodes were adjusted from 5 to 100 μm and the pressure was changed from 0 to 100 kPa. The obtained results have shown that the FE properties of the CNTs at 10 kPa and lower pressures were the same as those in vacuum. At a pressure more than 10 kPa, the FE threshold voltage in air was higher than those in vacuum, and increased with increasing atmospheric pressure. When the FE threshold voltage became higher than that of the gas breakdown, the microplasma was ignited before the FE initiation. Thus, the FE properties of the CNTs in air were stable when the FE potential was lower than the voltage of conventional gas discharge with CNT cathode.

Hydrogen absorption–desorption characteristics of a Gd2Co7-type Sm1.6Mg0.4Ni7 compound

In this paper, we report a new Gd2Co7-type Sm1.6Mg0.4Ni7 compound as a hydrogen storage material with a special hydrogen absorption/desorption process and good hydrogen storage ability. The Gd2Co7-type Sm1.6Mg0.4Ni7 compound absorbs 1.88 wt% H2 within 17 min at 298 K under 10 MPa H2. Meanwhile, the hydrogen absorption speed accelerates to 3.4 min after 20 hydrogenation/dehydrogenation cycles with a 1.44 wt% H2 under 3 MPa H2. Especially, the capacity retention of the compound is 99.3% at the 100th cycle. We found the hydrogen absorption/desorption of the compound undergoes two equilibrium stages, relating to the transformation of H2 between H-solid solution phase and hydride phase with a lower rate and higher enthalpy change at the lower concentration H2 stage, and the direct conversion between H2 and the hydride phase with a higher rate and lower enthalpy change at the higher concentration H2 stage. The two step mode lowers the inner-molecular strain and mismatch in subunit volumes of the compound in hydrogen absorption/desorption, caused by the transformation of H2 at the lower concentration of H2 stage, thus leading to good structural stability and excellent cycling stability. The new insights are expected to provide viable intermetallic materials as high-pressure tank materials for hydrogen storage with nice hydrogen storage properties.

Enhanced electrochemical properties of LaFeO3 with Ni modification for MH–Ni batteries

In this work, we synthesized LaFeO3–xwt%Ni (x = 0, 5, 10, 15) composites via a solid-state reaction method by adding Ni to the reactants, La2O3 and Fe2O3. Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) results revealed that Ni powders evenly dispersed among the LaFeO3 particles and apparently reduced their aggregation, which imparted the composites with a loose structure. Moreover, the Ni formed a conductive network, thus improving the conductivity of the composites. The maximum discharge capacity of the LaFeO3 electrodes remarkably increased from 266.8 mAh·g–1 (x = 0) to 339.7 mAh·g–1 (x = 10). In particular, the high-rate dischargeability of the LaFeO3–10wt%Ni electrode at a discharge current density of 1500 mA·g−1 reached 54.6%, which was approximately 1.5 times higher than that of the pure LaFeO3. Such a Ni-modified loose structure not only increased the charge transfer rate on the surface of the LaFeO3 particles but also enhanced the hydrogen diffusion rate in the bulk LaFeO3.

Effects of Annealing Temperature on Microstructure and Electrochemical Properties of Perovskite-type Oxide LaFeO 3 as Negative Electrode for Metal Hydride/Nickel(MH/Ni) Batteries

We reported the effects of annealing temperatures on microstructure and electrochemical properties of perovskite-type oxide LaFeO3 prepared by stearic acid combustion method. X-Ray diffraction(XRD) patterns show that the annealed LaFeO3 powder has orthorhombic structure. Scanning electron microscopy(SEM) and transmission electron microscopy(TEM) images show the presence of homogeneously dispersed, less aggregated, and small crystals(30―40 nm) at annealing temperatures of 500 and 600 °C. However, as the annealing temperature was increased to 700 and 800 °C, the crystals began to combine with each other and grew into further larger crystals(90―100 nm). The electrochemical performance of the annealed oxides was measured at 60 °C using chronopotentiometry, potentiodynamic polarization, and cyclic voltammetry. As the annealing temperature increased, the discharge capacity and anti-corrosion ability of the oxide electrode first increased and then decreased, reaching the optimum values at 600 °C, with a maximum discharge capacity of 563 mA·h/g. The better electrochemical performance of LaFeO3 annealed at 600 °C could be ascribed to their smaller and more homogeneous crystals.