Carbon Nanotubes: Synthesis, Properties and Applications in Modern Electronic Devices

ISBN-10
0549925481
ISBN-13
9780549925484
Series
Carbon Nanotubes
Category
Carbon
Language
English
Published
2008
Publisher
ProQuest
Author
Charan Masarapu

Description

This dissertation covers several diverse and novel aspects of carbon nanotubes (CNTs) related to synthesis, properties and perspective applications in devices. Growth of CNTs on electrically conductive substrates is promising for many applications. However, the lack of complete knowledge of the substrate effects on the CNT growth poses many technical challenges. The mechanism for the growth of aligned multiwall carbon nanotubes (MWNTs) directly on chemically treated stainless steel (type 304) using a chemical vapor deposition process (CVD) is unveiled through detailed X-ray photoelectron spectroscopy (XPS) analysis. It is found that the CNTs prefer to grow on the enriched surface of iron oxides obtained by the chemical treatment rather than on the passive chromium oxide films generally present on the surface of the as-received stainless steel substrates. According to the XPS peak fitting results, to obtain high density aligned MWNTs, the ratio of iron oxide to chromium oxide on the surface of stainless steel should be much greater than one. Based on this growth mechanism, for the first time, site selective MWNT synthesis on stainless substrate is demonstrated. This kind of controlled CNT growth on conducting substrate results in very promising, diverse applications of CNTs in futuristic nanodevices. Understanding the physical properties of MWNTs, such as the specific heat at low temperature, leads to developing more efficient CNT based temperature sensors and thermal devices. In this regard, the specific heat of an aligned bulk MWNT sample peeled from SiO 2 substrate is measured from 300 K to 1.8 K. The dimensional behavior of the specific heat curve at different temperature ranges is accurately analyzed. Interestingly, a T 2 dependence of the specific heat is observed below 5 K, suggesting a nuclear hyperfine component pertaining to the Schottky effect. This effect arises from the magnetic impurities in the MWNTs, whose presence is confirmed by thermogravimetric analysis and microscopy observations. The final goal of synthesizing the CNTs is for a purposeful application, where the properties of the CNTs can be effectively utilized. CNTs are excellent field emitters and have good high temperature stability. A field emission diode for high temperature operation with aligned MWNTs synthesized on SiO 2 substrate is demonstrated for the first time. The diode showed stable performance from 25 °C all the way up to 300 °C, without any significant change in the diode parameters both in the forward and reverse bias. Such kind of diode is very useful in electronic devices employed in extreme temperature environments for down-hole drilling, seismographic and space applications. CNTs have attracted a lot of attention as electrode materials in electrochemical energy storage devices due to their porous structure and high surface area. The success of the aligned MWNT synthesis directly on the conducting substrate stainless steel provided the opportunity to test the MWNTs as anode material in rechargeable lithium ion battery. The assembled cell showed amazing performance at a high charge-discharge rate (1C rate) with a marked increase in the specific capacitance with continuous charge-discharge cycling. The increase in the specific capacitance with cycling is attributed to the combined effect of the gradual MWNT structural changes and the contribution of the stainless steel substrate which is actually acting as a current collector in Li ion intercalation process. An increase in the specific capacitance with cycling means the lifetime of the battery increases with usage which is extraordinary for any electronic device powered by rechargeable lithium ion batteries. Electrochemical power sources that offer high energy and power densities and can also withstand a harsh temperature environment have become extremely desirable in applications ranging from hybrid electric vehicles to military weapons. In this regard, a supercapacitor coin cell with singlewall carbon nanotube (SWNT) film electrodes and polypropylene carbonate organic electrolyte is assembled and tested in the temperature range from 25 °C to 100 °C. Repeated electrochemical measurements - cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge discharge, performed at 25 °C and 100 °C with continuous heating and cooling showed very consistent performance without any cell degradation. Cyclic voltammograms displayed ideal capacitive behavior even at a significant scan rate of 1000 mV/s at 25 °C and 100 °C. The ability of the cell to undergo several tens of thousands of galvanostatic charge-discharge cycles at 100 °C and still work normally when cooled back to 25 °C is highly exceptional. Also, the cell is able to withstand current densities as high as 100 A/g during the galvanostatic charge-discharge yielding eminent power densities. The charge-discharge study at 25 °C with a current density of 20 A/g up to 264700 cycles showed excellent capacitance stability. The usage of such a supercapacitor potentially enables far-reaching advances in back-up energy storage and high pulse power applications. (Abstract shortened by UMI.).

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