Browsing by Author "Chen, M"

Now showing 1 - 8 of 8

Catalyzed growth model of carbon nanotubes by microwave plasma chemical vapor deposition using CH4 and CO2 gas mixtures
(2003-10-01)
Chen, M
;
Chen, CM
;
Koo, HS
;
Chen, CF
;
材料科學與工程學系
Catalyst growth carbon nanotubes (CNTs) have been synthesized successfully by microwave plasma chemical vapor deposition using CH4-CO2 gas sources, and Fe, Ti, Fe/Ti as catalysts. Significant difference of morphology in the carbon deposition was observed between Fe and Ti catalyst. By adjusting growth parameters of CH4 to CO2, a high yield of vertically aligned CNTs can be found in an Fe-deposited substrate. Ti is shown to be not suitable as a catalyst in CNT production. In the present work, we investigated the effect of H-2 plasma pretreatment on the CNT growth from the viewpoint of catalyst morphology, using Fe as the catalyst. After the H-2 pretreatment, significant catalyst particle sintering was observed and resulted in a broad size distribution of catalyst particles. The diameter of CNTs was governed by the catalyst particle size. The diameter of CNTs thus increased as the H-2 plasma pretreated time increased. The CNT diameters were distributed in the range approximately 10-20 nm when Fe-deposited substrate was not pretreated. However, the diameter of CNTs increased from 30 to 300 nm when Fe-deposited substrate was pretreated from 1 to 15 min. The CNT growth model in catalysts, as a function of a gas environment of CH4-CO2 gas mixture, was investigated. (C) 2003 Elsevier Science B.V. All rights reserved.
Growth of carbon nanotubes by microwave plasma chemical vapor deposition using CH4 and CO2
(2002-12-02)
Chen, M
;
Chen, CM
;
Chen, CF
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Carbon nanotubes (CNTs) were grown vertically and aligned on Fe catalytic nanoparticles which were deposited on a Si substrate at low temperature using CH4 and CO2 gas mixtures. A dynamic form of optical emission spectroscopy was used to C detect the species in the plasma. These data show the dominant species in gas phase reaction. The composition of plasma significantly affects the reaction mechanism of CNTs growth and diamond film synthesis. The growth quality of CNTs is better C than a conventional reaction in a gas mixture of hydrogen and hydrocarbons. However, the highest yield of CNTs of approximately 70% was obtained by microwave plasma chemical vapor deposition for CH4-CO2 gas mixture at a flow rate of CH4/CO2 at 96%, power supply of 300 W, reaction time of 20 min and bias voltage of 150 V In summary, the oxygen containing in the CH4-CO2 gas system can increase the amount of C-2 In the C-2-rich plasma the excited C-2 emission at a higher intensity is beneficial to graphite deposition, which enhances CNTs synthesis on catalyst-deposited surface quality. (C) 2002 Elsevier Science B.V. All rights reserved.
High efficiency microwave digestion purification of multi-walled carbon nanotubes synthesized by thermal chemical vapor deposition
(2006-03-01)
Chen, CM
;
Chen, M
;
Peng, YW
;
Yu, HW
;
Chen, CF
Multi-walled carbon nanotubes (MWCNTs) synthesized by thermal chemical vapor deposition (Thermal CVD) have been successfully purified by high efficient microwave digestion purification method. In this paper, microwave digestion method was introduced to purify carbon nanotubes, because that acid can absorb microwave energy rapidly and dissolve metal efficiently in this method. MWCNTs were placed in 100 ml TFM (thennally resistant form of Teflon) vessels with 50 ml 5 M nitric acid. First, the temperature ramped up to 210 degrees C in 30 min and then kept for 10, 20, 30 to 120 min to digest the catalyst. The efficiency of this method was analyzed by thermogravimetric analysis (TGA) and the results showed that the catalyst content dropped from 10.39% to 1.75% only in 10-min digestion and 1.03% for 120 min treatment. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) were applied to analyze the property and structure of raw and purified MWCNTs. This study presents a highly efficient and non-destructive purification method for high purity MWCNTs which can be provided in many applications or intrinsic property studies. (c) 2005 Elsevier B.V All rights reserved.
Low-temperature series expansions for square-lattice Ising model with first and second neighbour interactions
(2002-12-20)
Kao, YM
;
Chen, M
;
Lin, KY
;
物理研究所
;
Institute of Physics
We have calculated the low-temperature series expansions of the spontaneous magnetization and the zero-field susceptibility of the square-lattice ferromagnetic Ising model with first-neighbour interaction J(1) and second-neighbour interaction J(2) to the 30th and 26th order respectively by computer. Our results extend the previous calculations by Lee and Lin to six more orders. We use the Pade approximants to estimate the critical exponents and the critical temperature for different ratios of R = J(2)/J(1). The estimated critical temperature as a function of R agrees with the estimation by Oitmaa from high-temperature series expansions.
Low-temperature synthesis multiwalled carbon nanotubes by microwave plasma chemical vapor deposition using CH4-CO2 gas mixture
(2003-02-01)
Chen, M
;
Chen, CM
;
Shi, SC
;
Chen, CF
;
材料科學與工程學系
Vertically aligned carbon nanotubes (CNTs) with multiwalled structures were successfully grown on a Fe-deposited Si substrate at low temperatures below 300 degreesC by microwave plasma chemical vapor deposition using of a CH4-CO2 gas mixture. The low temperature would be beneficial for reducing the diameter of CNTs but it will also decrease the growth rate on the substrate. The low-temperature growth of CNTs was achieved by decreasing both the microwave power and the total gas pressure. The constituents of the gas mixture are also key factors in chemical vapor deposition. After deposition, the microstructure morphology of CNTs was observed by using scanning electron microscope and high-resolution transmission microscope. The results showed that even at low temperature a high yield of carbon nanotubes with a high emission current density and a small diameter is obtained. Vertically aligned multiwalled CNTs with a diameter about 12 nm were investigated by scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM).
Microwave digestion and acidic treatment procedures for the purification of multi-walled carbon nanotubes
(2005-03-01)
Chen, CM
;
Chen, M
;
Peng, YW
;
Lin, CH
;
Chang, LW
Multi-walled carbon nanotubes (MWCNTs) synthesized by thermal chemical vapor deposition have been successfully purified by using efficient microwave digestion method. Recently, various purification methods have been provided to eliminate metal catalysts and most were acid treatment procedures that take long time and damage the structure of nanotubes. Comparing to these methods, microwave digestion takes a tremendous advantage because the acid in this approach can absorb microwave energy rapidly and dissolve metal efficiently without damaging the wall structure and the processing time could be the shortest. The ability of microwave digestion method was studied by various experimental parameters. The efficient purification proceeded for 90 min and at 210 degrees C with 5 M nitric acid treatment in the microwave digestion system to dissolve the metal catalyst. High-resolution transition electron microscopy and Raman spectra were used to characterize the specimen. Purities were characterized by thermogravimetric analysis (TGA). Results showed that the amount of catalyst decreased with increasing acid treatment time and concentration. Purity of MWCNTs achieved as high as about 99.9%. This study presents a highly efficient purification method for carbon nanotubes with high purity and no structure destruction. In addition, carbon nanotubes synthesized by various methods were also purified and the results were comprehensively discussed. (c) 2004 Elsevier B.V. All rights reserved.
Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapor deposition at low temperature
(2002-09-01)
Chen, M
;
Chen, CM
;
Chen, CF
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Vertically-aligned carbon nanotubes(CNTs) with multi-walled structure were successfully grown on a Fe-deposited Si substrate at low temperature below 330degreesC by using the microwave plasma chemical vapor deposition of methane and carbon dioxide gas mixture. This is apparently different from the conventional reaction in gas mixtures of hydrogen and methane, hydrogen and acetylene, and hydrogen and benzene ... etc. High quality carbon nanotubes were grown at lower temperature with CO2 and CH4 gas mixture than those used by the previous. After deposition, the microstructure morphology of carbon nanotubes was observed with scanning electron microscope and high-resolution transmission electron microscope. The characteristics of carbon nanotubes were analyzed by laser Raman spectroscopy. The results showed the variation of the flow rate ratio of CH4/CO2 from 28.5 sccm/30 sccm to 30/30 sccm and the DC bias voltage from -150 V to -200 V, at 300 W microwave power, 1.3-2.0 kPa range of total gas pressure, and substrate temperatures between 300degreesC and 350degreesC. Vertically aligned carbon nanotubes with the diameter of about 15 nm and multi-walled structure were illustrated by SEM and HRTEM. However, the highest yield of carbon nanotubes of about 50% was obtained at low temperature below 330degreesC by MPCVD for the CH4/CO2 gas mixture with properly controlled parameters. (C) 2002 Kluwer Academic Publishers.
Purification of multi-walled carbon nanotubes by microwave digestion method
(2004-04-01)
Chen, CM
;
Chen, M
;
Leu, FC
;
Hsu, SY
;
Wang, SC
Here developed is an efficient method for purification of multi-walled carbon nanotubes synthesized by electron cyclotron resonance chemical vapor deposition (ECRCVD). Carbon nanotubes may find their limited use for some applications as they contain a small fraction of metal catalysts in the tubes and tend to have defects along the graphene tube wall. Defects within the multi-walled carbon nanotubes would reduce electrical and structure properties. A two-step acidic treatment in microwave digestion system is used to dissolve metal catalysts. HNO3 or HCl can rapidly absorb microwave heat and energy and completely dissolve metal, which reside in carbon nanotubes without damage. The processing time of the two-step microwave-assisted and acid-treated time to dissolve metal in the MWCNTs is below 1 h. After purification, the amount of residual catalyst metals in samples is estimated by using the thermogravimetric analysis (TGA). The results show that a high-yield of multi-walled carbon nanotubes with approximately 5% of metal is obtained. (C) 2003 Elsevier B.V. All rights reserved.