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Chapter 1 Introduction Cold Cathode Materials for Pseudospark Switches by Shaomao Li A thesis submitted to the Graduate Faculty of Auburn University in partial fulfillment of the requirements for the Degree of Master of Science Auburn, Alabama May 14, 2010 Keywords: pseudospark switch, carbon nanotubes, pulse power, trigger, field emission, breakdown Copyright 2009 by Shaomao Li Approved by Hulya Kirkici, Chair, Associate Professor of Electrical and Computer Engineering Mark Nelms, Professor of Electrical and Computer Engineering Michael Baginski, Associate Professor of Electrical and Computer Engineering Abstract In general, for plasma switches, the initiation of the plasma is critical and this is usually achieved by a “trigger” scheme. The seed electrons needed to initiate a breakdown can be generated by several means such as thermionic, field or optical emission. While the thermionic emission has been used mainly in vacuum tubes and is a mature technology, field emission (or cold-cathode electron emission) has been the subject of recent studies and technology. The efficiency of these seed electron emission determines how well the plasma switch can close or open. It is known that the carbon nano tubes (CNTs) have excellent field emission characteristic under high vacuum pressure of 10-7 to10-6 Torr and they have been considered as prima candidates as cold-cathode electron emitters. On the other hand, most plasma switches operate at mTorr pressure ranges, which is relatively higher than the vacuum pressure levels mentioned above. Therefore, the electron emission characteristics of CNTs in elevated pressures higher than vacuum need to be known to evaluate possible use of them as trigger electrode. The purpose of this research is to observe the field emission characteristics of several different kinds of CNT samples and other nano materials in background pressures ranging from 10-3 Torr to 1 Torr. ii Acknowledgments First of all, I would like to thank my advisor, Dr. Kirkici, for offering me the opportunity to explore the wonderful research world. Her constant guidance and encouragement is indispensable for me to carry out this work. Secondly, I want to thank Dr.Koppisetty and Dr.Chen, who graduated from Auburn University. The knowledge and the way of conducting scientific research I learned from them are invaluable and priceless. Thirdly, I thank my mother Xiaofen Chen, and my father Qifu Li, for their love, encouragement and supports. Finally, but not the least importantly, I want to thank my colleagues and collaborators: Mark L.Lipham, Haitao Zhao, Esin Sozer, Ramesh Bokka. I also thank the Technical Support: Linda Barresi and Calvin Cutshaw and the other people who gave their precious help. Without their efforts, this work would be impossible iii Table of Contents Abstract ……………………………………………………………………………..…………….ii Acknowledgments………………………………………………………………………………..iii List of Tables ............................................................................................................................ vi List of Figures…………………………………………………………………………................vii Chapter 1 Introduction ................................................................................................................1 Chapter 2 Background .................................................................................................................3 2.1 Pulsed Power Engineering......................................................................................3 2.2 Switches ................................................................................................................6 2.2.1 Closing Switches ...........................................................................................8 2.2.2 Opening Switches........................................................................................ 13 2.3 Pseudospark Switches .......................................................................................... 15 2.3.1 Hollow Cathode Discharge .......................................................................... 15 2.3.2 Pseudospark Discharge ................................................................................ 16 2.4 Triggering Mechanisms…………………………………………………………...18 2.4.1 Pulsed Low-Current Glow Discharge: ......................................................... 19 2.4.2 Surface Discharge Triggering: ..................................................................... 19 2.4.3 Optical Triggering ....................................................................................... 20 2.5 Field Emission Materials Literature Review………………………………………22 Chapter 3 Carbon Nanotubes ..................................................................................................... 25 3.1 Carbon Nanotubes and Structure…………………………………………………..25 3.2 Properties of Carbon Nanotubes ........................................................................... 29 iv 3.3 Synthesis of CNTs ............................................................................................... 30 3.3.1 Growth Mechanism ..................................................................................... 30 3.3.2 Arc Discharge Method ................................................................................ 31 3.3.3 Laser Ablation Method ................................................................................ 33 3.3.4 Chemical Vapor Deposition ......................................................................... 34 Chapter 4 Field Emission Experiment ....................................................................................... 37 4.1 Research Objective………………………………………………………………...37 4.2 Experiment Setup…………………………………………………………………38 4.3 Field Emission Characteristics of CNTs…………………………………………..40 4.3.1 SEM Images of CNTs .................................................................................. 40 4.3.2 Field Emission Intensity .............................................................................. 43 4.4 Field Emission of Zinc Oxide (ZnO) .................................................................... 48 4.5 Field Emission of Oxygen-free Copper ............................................................... 50 4.6 Field Emission of Nanocrystalline Diamond……………………………………..53 Chapter 5 Summary .................................................................................................................. 56 References ................................................................................................................................ 59 v List of Tables Table 2.1 Summary of properties of closing switch................................................................. 14 Table 4.1 SEM image of CNT before experiment ...................................................................... 41 Table 4.2 SEM image of CNT after experiment ......................................................................... 42 Table 5.1 Comparison of different materials field emission ......................................................... 57 vi List of Figures Figure 2.1 Pulse Shape…………………………………………………………………………….4 Figure 2.2 General scheme of a pulse power generator ................................................................5 Figure 2.3 Generator with capacitive energy storage and closing switch ......................................5 Figure 2.4 Generator with inductive energy storage and opening switch ......................................6 Figure 2.5 Components of a typical closing switch ...........................................................................9 Figure 2.6 Rang of gas pressures and operating voltage for gas switches ................................. 10 Figure 2.7 Evolution of voltage, current and power loss in gas switches .................................... 10 Figure 2.8 Structure of thyristor and two transistors equivalent circuit ....................................... 11 Figure 2.9 A four-stage repetitive thyristor switch ..................................................................... 12 Figure 2.10 Basic circuit with capacitive energy and a ferromagnetic switch ............................. 13 Figure 2.11 Electric field configuration of typical pseudospark geometry .................................. 16 Figure 2.12 Cross sectional view of an electrical triggered pseudospark switch ......................... 17 Figure 2.13 Schematic drawing of pseudospark switch with pulsed glow discharge trigger........ 19 Figure 2.14 Schematic drawing of pseudospark switch triggered by surface discharge............... 20 Figure 3.1 Buckyball structure ..................................................................................................... 26 Figure 3.2 Classification of CNTs: (a) armchair, (b) zigzag, and (c) chiral nanotubes ................ 27 Figure 3.3 Structures of MWCNTs ............................................................................................ 28 Figure 3.4 Schematics of the growth mechanism: (a) root growth and (b) tip growth .......................... 31 Figure 3.5 Schematic diagram of the arc apparatus ......................................................................... 32 vii Figure 3.6 Schematics of experimental setup of laser ablation technique ........................................... 33 Figure 3.7 Schematic diagram of a PECVD setup for carbon nanotubes growth ........................ 35 Figure 4.1 Circuit diagram of field emission intensity measurement set-up ................................ 39 Figure 4. 2 SEM image of Aligned MWCNT with a tilted
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