Tianxiang Li

Growth mechanism of carbon nanotubes in methane diffusion flames

Abstract Straight and bamboo-like carbon nanotubes were synthesized in a methane diffusion flame using a Ni–Cr–Fe wire as a substrate. The catalyst particles were nickel and iron oxides formed on the wire surface inside the flame. When the wire was oxidized using nitric acid more nanotubes could be produced. Most nanotubes grew by a base growth mode. The formation of bamboo-like nanotubes was related to the shape of the catalyst particles. A segregation growth mechanism of bamboo-like nanotubes is proposed.

Synthesis of multiwalled carbon nanotubes using methane/air diffusion flames

An experimental investigation has been conducted on the synthesis of multiwalled carbon nanotubes (MWCNTs) using methane/air laminar diffusion flames, with an emphasis on the effects of flame temperature and catalyst particle size on nanotube growth. The yield of MWCNTs occurred on a substrate consisting of a 0.4 mm Ni−Cr (60% Ni, 26% Cr, and 14% Fe) catalyst wire connected to a stainless steel grid. The self-formed catalyst particles in the flame were analyzed by energy dispersive X-ray to be mostly nickel oxide with some iron oxide. A high-resolution transmission electron microscope analysis indicates well-graphitized layers of MWCNTs consisting of continuous lattice fringes along the cylindrical section of the nanotube. We studied the effects of flame temperature on nanotube growth by changing the sampling location along the flame height and diluting methane with nitrogen. High temperature was found to favor the growth of large-diameter nanotubes, and nanotube growth was influenced by chemical species environment in the flame. Nitric acid treated Ni−Cr wire produced smaller sized catalyst particles that can be encapsulated within the nanotubes. The growth of well-aligned, bean-sprout-like bundles of nanotubes with catalyst particles pushed up at the tube top was found to occur: the mechanism of this unique synthesis is unknown and needs further study.

A modified Ortega method to evaluate the activation energies of solid state reactions

The application of the average linear integral isoconversional method developed by Ortega for evaluating the activation energies of solid state reactions may be hindered by experimental noise and the uncertainties associated with selecting appropriate reaction segments. This paper suggests a procedure, called the modified Ortega method, which can avoid or minimize these hindrances. By applying the modified Ortega method to the kinetic analyses of both simulated and experimental data, a more consistent dependence of the activation energy on the extent of reaction conversion was found with those calculated from the modified Vyazovkin method and the Friedman method.

Comprehensive method based on model free method and IKP method for evaluating kinetic parameters of solid state reactions.

This article presents, firstly, a short review of methods for evaluating kinetic parameters of solid state reactions and a critical analysis of the isoconversional principle of model free methods. It shows theoretically that the activation energy for complex reactions is not only a function of the reaction degree but also of heating programs, and points out that any method that attempts to extract the dependences of activation energy on conversion degree without considering the dependences of heating programs is problematic. Then an analysis is given of the invariant kinetic parameters (IKP) method and recommends an incremental version of it. Based on the incremental IKP method and model free method, a comprehensive method is proposed that predicts the degree of the dependences of activation energy on heating programs, selects reliable values of activation energy and extracts the values of variable pre‐exponential factor. This comprehensive method is tested using both simulation data and experimental data, the results of which show it can not only give reliable values of kinetic parameters but also be helpful in explaining inconsistencies of kinetic results in solid state reactions.

A Horizontally Aligned One-Dimensional Carbon Nanotube Array on a Si Substrate

A one-dimensional array of carbon nanotubes horizontally aligned on silicon substrates was successfully grown using a flame synthesis method based on the template of a one-dimensional anodic aluminum oxide nanopore array. The diameter and length of nanotubes are controlled by the geometry of nanopores of the aluminum oxide template. This one-dimensional carbon nanotube array may have great potential for fabrication of nanoelectronic devices and nanoelectromechanic systems (NEMS) compatible with the planar processing technology.

Growth of Horizontally Aligned One-Dimensional Carbon Nanotubes Array on a Si Substrate

A one-dimensional (1-D) array of carbon nanotubes horizontally aligned on silicon substrates was successfully grown using a flame synthesis method based on the template of a one-dimensional anodic aluminum oxide nanopore array. The diameter and length of nanotubes are controlled by the geometry of nanopores of the aluminum oxide template. This one-dimensional carbon nanotube array may have great potential for fabrication of nanoelectronic devices and nano-electromechanic systems (NEMS) compatible with the planar processing technology.