Copper antenna
Chapter 8. Summary and future work
The work described in this dissertation focused mainly on the development and characterization of vertically oriented graphene electrical double layer capacitors with fast frequency response and high capacitance. Research into VOGN has been ongoing for several years at College of William and Mary and this work is the first extensive research into utilizing VOGN for EDLCs. Chapter 4 starts this discussion by introducing VOGN grown using acetylene (C2H2) feedstock on Ni substrates. The use of C2H2 showed that it provides higher capacitance, faster growth rate, better verticality and sheet height uniformity compared with previous growths done using methane (CH4) feedstock. Ni substrates provided a base for the VOGN with ~ 0.05Ω equivalent series resistance and the highly conductive graphene sheets enabled EDLCs to have electrical responses similar to electrolytic capacitors. A maximum capacitance of ~160 µF/cm2 was observed for a 10-min growth at a temperature 750°C and ~ 265 µF/cm2 for a temperature of 850ºC, for uncoated VOGN coin cells at 120 Hz. The phase angle behavior for these EDLCs was close to -85 degrees which is adequate for filtering applications. The data shown in chapter 4 shows that the phase angle and capacitance values remain stable up to ~10kHz frequency which shows good frequency responsiveness of VOGN EDLCs.
The growth of VOGN on Al was achieved for the first time and used to make functional EDLCs. The major difficulty of Al to be a viable substrate is the stability of the stable native oxide Al2O3. This inhibits good ohmic contact to be made between the Al and the VOGN films. A novel method of thinning the Al oxide by first RF sputtering the surface with Ar/H2
at a substrate temperature of 620ºC, very near the melting point of pure Al (Tm = 660ºC).
This substrate temperature reduced the native oxide thickness by CO desorption and O dissolution into the Al bulk. Introducing acetylene simultaneously to prevent further
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development of the oxide layer, coated the surface quickly and prevented reformation of the oxide. For growth on Al, only pure acetylene was used since introduction of H2 reduced the VOGN growth. Although Ni is a good substrate material for EDLCs, when comparing costs for component manufacture, Al is more cost effective. Al is currently being used for commercial electrolytic capacitors. Initial data have shown the VOGN on Al provided a maximum of ~ 80 µF /cm2 for 9 sccm flow rate for acetylene and ESR of ~0.6Ω. The phase angle behavior maintained ~ -86º for 120 Hz. Initial results have shown that this type of EDLC can be a future replacement for electrolytic capacitors following further in-depth characterization and optimization. An EDLC capacitor replacement of an electrolytic would not have the polarity requirement and high failure rate of present day electrolytics.
The next step to advance EDLCs as fast response capacitors capable of AC filtering (see chapter 6) was to try carbon black (CB) coatings on VOGN to substantially increase the surface area and, therefore, the specific capacitance. The EDLCs for CB coatings on both a 10 minute and 20 minute VOGN/Ni architecture resulted in specific capacitance levels up to 2.3 mF/cm2 at 120 Hz with a phase angle at -80º or better. This specific capacitance is the highest for EDLCs with efficient 120 Hz response, reported to date. These results were achieved with a non-uniform coating of the carbon black where most of the CB was at the upper edges of the VOGN in the form of a crust. It remains a future objective to perfect the coating method to uniformly coat the side walls and valleys as shown in Figure 6.3. Since most of the surface area of the VOGN was not coated, the experimental data suggest that a uniform coating of ~100 nm thick CB on VOGN/Ni 10 µm high over the entire surface area of the underlying architecture should provide a specific capacitance approaching ~0.1 F/cm2 with good frequency response at 120 Hz.
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The interdigitated design project was a unique approach in trying to commercialize VOGN EDLC capacitors. In collaboration with the U.S. Army Research Laboratory and capacitors manufacturer Cornell Dubilier, a prototype was developed that could be commercialized in the future. Further optimization in VOGN and electrolyte effects in capacitance related variables and production engineering, needs to be addressed before mass scale production is considered. EDLC cells operate at low voltage thus, cells must be connected in series to create a high-voltage capacitor. Figure 8.1 shows one approach for interconnecting planar EDLC cells and thus create high-voltage EDLCs. This involves stacking the planar cells then interconnecting them on opposite edges. Volumetric efficiency and frequency response of the planar design is maintained in this stack.
Figure 8.1: Stacking of interdigitated cells to create high voltage EDLC
Another method would be to interconnect the planar interdigitated EDLC cells using metallization on the substrate as seen in figure 8.2. interdigitated design is laser ablated through the VOGN grown on a Ni coated alumina substrate to create multiple electrically-isolated regions. The gaps are then individually covered with an electrolyte, making sure that each electrolyte band does not touch its neighbor. This is a “bipolar” design in two dimensions, with the substrate metallization serving as the bipolar plate which connects the
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separate EDLC cells in series and if each cell is rated at a voltage V then for x no of separate cells, the total voltage rating of the capacitor would be X*V. This type of cell could be used in a “rolled-up” type of capacitor design.
Figure 8.2: Interdigitated EDLC design connected in a plane. The VOGN is grown on Ni coated substrate and laser ablated to create separate isolated regions.
Another approach would be to grow VOGN sheets on both sides of the substrates that would double the volumetric efficiency. This would require a modified RF-PECVD growth technique that would allow growth on both sides simultaneously. If this can be achieved, then the final packaging method can be stacking or, rolling, as is done for electrolytic capacitors. Carbon black coating on VOGN/Al systems can be used to increase capacitance by over an order of magnitude and has the potential of replacing present day electrolytic capacitors.
In collaboration with Cornell Dubilier capacitor manufacturers, we were able to produce a computational model [70] to predict the actual behavior of capacitance and phase angle for a VOGN EDLC with a uniform carbon black coating. This was done using the modelling software COMSOL. These results show that a uniform coating of CB can take the specific capacitance well into the mF/cm2 range. From this model, the highest capacitance that was predicted came for 0.15µm pore size (300 nm sheet separation) and 10µm VOGN
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height coated with 100nm carbon black, was ~42mF/cm2. This model doesn’t consider any effects from the VOGN undergrowth that contains shorter sheets that has stinted growths. These sheets have considerable spread across the substrate thus suggesting a significant contribution to the overall capacitance.
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Vita
Dilshan Viraj Premathilake was born in Kandy, Sri Lanka on September 1983. He graduated from Trinity College-Kandy in 2002 after sitting for the island wide advanced level examination and became eligible for university studies in physical science at the University of Peradeniya. In 2004, he started his Bachelor’s degree in physics and advanced mathematics. Due to exemplary work, in 2006 he was chosen to do a special degree in physics minoring in advanced mathematics and graduated in 2008. After a year of working as a teaching assistant at the University of Peradeniya, he was selected in August 2009 for graduate studies by the Department of Physics at Old Dominion University. In 2011, he was awarded a Master of Science in Physics. In August 2012, he was accepted into the Department of Applied Science Ph.D. program at the College of William and Mary and began his graduate studies. In January 2013, he began working with Professor R.A. Outlaw on the VOGN capacitor project. This project included working with Dr. J.R. Miller of Case Western Reserve University and Mr. S.G Parler of Cornell Dubilier Electronics.