Aligned Carbon Nanotube Carpets on Carbon Substrates for High Power Electronic Applications A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy By Betty Tun-Huan Quinton M.S. Egr., Wright State University, 2008 B.S., University of Illinois at Chicago, 2006 2016 Wright State University WRIGHT STATE UNIVERSITY GRADUATE SCHOOL April 25, 2016 I HEREBY RECOMMEND THAT THE DISSERTATION PREPARED UNDER MY SUPERVISION BY Betty Tun-Huan Quinton. ENTITLED Aligned Carbon Nanotube Carpets On Carbon Substrates For High Power Electronic Applications. BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy. Sharmila M. Mukhopadhyay, Ph.D. Dissertation Director Dr. Frank W. Ciarallo, Ph.D. Director, Ph.D. in Engineering Program Robert E. W. Fyffe, Ph.D. Vice President for Research and Dean of the Graduate School Committee on Final Examination Sharmila M. Mukhopadhyay, Ph.D. Hong Huang, Ph.D. P. Terrence Murray, Ph.D. James D. Scofield, Ph.D. Henry Daniel Young, Ph.D. Abstract Quinton, Betty Tun-Huan. Ph.D., Engineering Ph.D. Program, Wright State University, 2016. Aligned Carbon Nanotube Carpets On Carbon Substrates for High Power Electronic Applications. One of the driving forces behind nanotechnology research is the miniaturization of electronic devices. Electrical and thermal transport properties of device materials at micrometer and nanometer scales become very important in such applications. Carbon materials, especially carbon nanotubes (CNTs), have exceptionally low density and superior electrical, thermal, and mechanical properties. Vertically aligned CNTs attached to lightweight carbon substrates may hold the key to fully use these outstanding properties. However, the majority of studies reported to date involve either loosely unattached CNTs or CNTs attached to traditional electronic grade silicon, which have limited use in lightweight electronic components. Studies of CNT arrays attached to carbon substrates are extremely scarce, but if successful, such a composition could lead to unprecedented lightweight electronic devices with superior electrical and thermal transport properties. This dissertation is aimed at performing detailed investigation of such structures. This work investigates the synthesis-structure-property relationships of CNT arrays attached to carbon surfaces relevant to power electronic applications. Several detailed investigations were performed to achieve the goal of creating multiscale combination materials and to test their feasibility as high power electronic devices. Background iii studies were piloted to determine the most practical growth technique and growth parameters in order to achieve dense CNT growth. Floating catalyst chemical vapor deposition was determined to be the most effective, scalable, and reliable growth method. In addition, an oxide buffer layer was deemed necessary for dense CNTs growth on carbon substrate. Several oxides were compared in order to determine the most suitable for CNTs growth while providing superior thermal properties. Among the buffer oxides investigated in this study, the ALD Al2O3 buffer layer provided the fastest CNT nucleation and most uniform size distribution. However, Al2O3 buffer layer was plagued by adhesion issues, which may limit future applications. Plasma SiO2 offers a slower initial nucleation rate, but yields the tallest carpet height in identical growth conditions, and also appears to be the most stable and repeatable. Thermal properties investigations were conducted on the final products, which consisted of aligned CNTs arrays of different carpet heights on carbon substrates. Observations show that the thermal resistance of the CNT array varies linearly with CNT carpet height, as expected. This variation was used to estimate the thermal conductivity of multi-walled nanotube in the carpet, and found to be approximately 35W/m-K. This value shows promise that such lightweight structure can replace current commercially available products. This dissertation will reveal key results and discuss the investigations from the following areas: comparing chemical vapor deposition growth techniques, the importance of oxide buffer layer on carbon substrates, the effect of buffer layer composition, structure and iv thickness on CNT growth, and the feasibility of such lightweight structures for power electronics through thermal analysis investigations. v Table of Contents Abstract .......................................................................................................................... iii Table of Contents ........................................................................................................... vi List of Figures .............................................................................................................. xiii List of Tables ............................................................................................................... xvii List of Equations ......................................................................................................... xvii Acknowledgements .................................................................................................... xviii Dedication ..................................................................................................................... xx Section I: Introduction and Background ................................................................... xxvi CHAPTER 1 Introduction ............................................................................................... 1 1.1 Introduction ........................................................................................................... 1 1.2 Motivation ............................................................................................................. 3 1.3 Scope and Objectives ............................................................................................. 6 CHAPTER 2 Background ............................................................................................... 8 2.1 Background and literature reviews ........................................................................ 8 2.2 Significance of Carbon and Its Allotropes ............................................................ 9 2.3 Diamond versus Graphite .................................................................................... 11 2.4 Carbon Nanotube ................................................................................................. 13 2.4.1 Physical Properties of CNTs ......................................................................... 15 2.4.1.1 Electrical properties: .............................................................................. 15 2.4.1.2 Thermal properties: ................................................................................ 16 2.4.1.3 Mechanical properties: ........................................................................... 16 2.5 Possible Applications with CNTs ........................................................................ 17 2.6 Carbon Materials for High Power Electronics Applications ............................... 18 2.6.1 sp3 Carbon Diamond ................................................................................ 18 2.6.2 sp2 carbon Graphitic and Vitreous ........................................................... 19 2.7 History of Buffer Layer Usage for CNT Growth ................................................ 20 2.7.1 Background: Evolution of buffer layers ....................................................... 20 2.7.2 Pure Metallic buffer: ..................................................................................... 21 2.7.3 Alloy Metallic buffer: ................................................................................... 21 2.7.4 Non-metallic buffer:...................................................................................... 23 vi 2.7.5 Critical buffer layer thickness: ...................................................................... 23 CHAPTER 3: Materials and Techniques Used In This Investigation ........................... 25 3.1 Materials used as substrates ................................................................................. 25 3.1.1 Dense carbon (sp3) Diamond Substrates: ................................................ 25 3.1.2 Lightweight carbon (sp2) materials graphite, CNT, carbon foams .......... 27 3.1.2.1 Pressed Graphite:.................................................................................... 27 3.1.2.2 RVC foam: ............................................................................................. 28 3.1.2.3 Highly Oriented Pyrolytic Graphic (HOPG) from SPI, Inc: .................. 30 3.1.2.4 Model carbon samples ............................................................................ 32 3.2 Metals Catalysts Materials: ................................................................................. 33 3.3 Oxide Buffer Layer Materials and Comparison: ................................................. 34 3.3.1 Oxide Materials for the Investigation ........................................................... 34 3.3.1.1 Alumina (Al2O3):.................................................................................... 35 3.3.1.2 Silica (SiO2): .......................................................................................... 35 3.3.2 Formation of Oxide Buffer Layer Materials ................................................