X. F. Shang

Field-enhancement factor for carbon nanotube array

To estimate the apex field-enhancement factor associated with an array of carbon nanotubes (CNTs) on a planar cathode surface, the model of the floating spheres between parallel anode and cathode plates was proposed. An approximate formula for the enhancement factor was derived showing that the intertube distance of a CNT array critically affects the field emission. When the intertube distance is less than the height of the tube, the enhancement factor decreases rapidly with decreasing distance in qualitative agreement with the numerical simulation. Considering the field-emission current density, the field emission can be optimized when the intertube distance is comparable with the tube height, in accordance with the results from the experiments. Finally, the influence of the anode-cathode distance on the enhancement factor was also discussed, showing that the anode-cathode distance has little effect on the field emission from a CNT array. Thus, we can reduce the theshold voltage to some extent by decreas...

Influence of NH3 atmosphere on the growth and structures of carbon nanotubes synthesized by the arc-discharge method

Using the improved arc-discharge equipment, we selected graphite as electrode and prepared the multi-walled carbon nanotubes in an atmosphere of pure NH3 at pressure of 0.02 MPa and discharge current of 70 A by the arc-discharge method. The influence of NH3 atmosphere on the growth and structure of carbon nanotubes was studied. By analyzing transmission electron microscope and scanning electron microscope micrographs, we found that a large quantity of multi-wall carbon nanotubes can be grown on the top surface and in the inner of interlayer between central part and outer part in the cathode deposits. The results show that there is no significant difference of the shapes and the structures between NH3 atmosphere and other atmosphere such as He, H2, et al., except their positions. This study demonstrates that the arc-discharge method in NH3 atmosphere is one highly efficient method in the preparation of carbon nanotubes.

A model calculation of the tip field distribution for a carbon nanotube array and the optimum intertube distance

In a previous paper, the model of a floating top segment was proposed and used successfully to calculate the field enhancement factor as well as the field distribution on the top surface for a single carbon nanotube (CNT). In this paper, the model is extended to a CNT array. A set of modified linear charge equations has been derived for the CNT array using an approximation of multipole potential expansion. Fictitious charges inside a specific segment can be solved from these equations to calculate the field around the tips. Some numerical calculations of the field enhancement factor have been carried out, quantitatively accounting for the reduction of the factor with decreasing intertube distance. The relative field strength distribution on the top hemisphere of a CNT array is calculated too; it is observed to be close to that of a single CNT. Using the Fowler-Nordheim formula, the field emission intensity of a CNT array is maximized with varying intertube distance. The resulting optimum intertube distance to height ratio decreases gradually with increasing height to radius ratio, and is close to 2 only for height to radius ratio around 3 × 10(2).

A model calculation of the tip field distribution for a single carbon nanotube

In order to study the tip field of the carbon nanotube (CNT) or CNT array standing upright on a cathode plane, we proposed a model in which the long CNT is replaced by a floating top segment at the cathode potential. Using the fictitious charge and image method, a simple approximate formula for the field enhancement factor of a single CNT linear with the height-to-radius ratio has been deduced. This result is evidently improved compared to that from the floating sphere model. The normalized field strength distribution on the top hemisphere is also calculated and plotted, with the result depending slightly on the height-to-radius ratio. An estimation shows that the field emission intensity on the cylindrical surface of CNT is negligible compared to that on the top hemisphere.

Synthesis and characterization of inorganic solid electrolytes of Li2O-SiO2-P2O5 and Li2O-TiO2-SiO2-P2O5 systems

The lithium-ion-conducting inorganic solid electrolytes in the oxide systems Li2O-SiO2-P2O5 and Li2O-TiO2-SiO2-P2O5 were prepared by the solid-state reaction, and the electrolyte pellet made by cold-pressing method had diameter of 13 mm and was about 1 mm thick. Phase identification and surface morphology of the products were carried out by X-ray diffraction and scanning electron microscopy. Ionic conductivity of the pellets was investigated through ac impedance. The results show that the adding of other cations can improve the ionic conductivity of the solid electrolyte, and the sintering temperature and duration can influence the ionic conductivity. The maximum ionic conductivity in the samples is 9.9 × 10−4 S/cm in the Li2O-TiO2-SiO2-P2O5 system.

A novel method to produce large amount single-walled carbon nanotubes by arc discharging

A new method to produce large quantities of single-walled carbon nanotubes (SWCNTs) with high purity is developed. Using several composite graphite rods containing Y and Ni powder as the anode, and a high purity graphite hemisphere as the cathode, a cloth-like deposit could be obtained by dc arc-discharge in helium at high temperature. The deposit contained about 50% SWCNTs. In this way, more than one gram deposit could be produced in a few minutes. The structure and morphology of the SWCNTs were then examined using transmission electron microscopy, Raman spectroscopy, and thermogravimetric analysis.

The influence of ethanol on the growth and structure of carbon nanotubes

Using chemical vapor deposition to decompose ethanol at high temperature with Fe2+/zeolite as catalyst, we synthesized carbon nanotubes (CNTs) with thinner walls and open ends. Compared with CNTs synthesized from methane (CH4), the CNTs synthesized here are straighter and have larger inner diameters and fewer defects when the experimental parameters are well controlled. CNTs of such structures will have greater potential applications.

Synthesis by arc-discharge method and electrochemical lithium insertion properties of double-walled carbon nanotubes

Double-walled carbon nanotubes could be synthesized by arc-discharge method using ball-milling mixtures of iron, cobalt, nickel and sulfide as catalysts. Structures of these carbon nanotubes were characterized by TEM, HRTEM and Raman spectroscopy. Also the lithium insertion properties were examined. The results showed that the irreversible capacity of the double-walled carbon nanotube lithium ion batteries is high, which is considered to be related to the formation of SEI on the surface of the electrodes in the process of electrochemistry reaction.