CARBON NANOTUBE BASED TRANSPARENT CONDUCTIVE COATINGS Paul J Glatkowski Eikos Inc. 2 Master Drive, Franklin, MA. 02038 ABSTRACT The use of carbon nanotube to impart electrical conductivity to polymeric films and coatings while maintaining excellent optical transparency is presented. Examples and data are provided for nanotube composite films and coatings exhibiting optical transparency useful for electrostatic dissipation and for relatively high conductivity electrodes in consumer electronic applications. Coating with optical transparency of 90%T and electrical resistivity of 200 Ω/□ are formed using simple wet coating processes. This technology is compared to competitive coating materials. The properties and processing advantages of Nanoshieldtm technology are finding use in commercial and military applications such as touch screens, large area displays; and next generation flexible displays and solar voltaic collectors. Keywords: Nanocomposite, Coatings, Thin Films 1. INTRODUCTION Eikos, Inc. has demonstrated the use of its Nanoshield technology, to impart electrical conductivity while maintaining high optical transparency in a variety of polymeric films and coatings. In this paper are presented two examples of this technology. The first application is to impart ElectroStatic Dissipation (ESD) protection to films and coatings. The second example demonstrates Nanoshield’s application to low resistivity optical coatings suitable for use as electrode layers in displays, touch screens, and in EMI shielding. However, first is provided some background on the Nanoshield technology and relationship to other competitive transparent conductive technologies. Nanoshield technology is based on the use of carbon nanotubes as electrically conductive particles. The unique quality of this form of carbon is simple in that the individual particles possess the attributes of high electrical conductivity (3x10+4 S/cm), high aspect ration (>103), and the unique capability of forming ropes of individual particles. This combination of properties allows for the formation of conductive networks through a host material with tunable electrical resistivity and excellent optical transparence. Eikos has demonstrated nanotube loaded materials with electrical properties ranging from <1Ω through >1012 Ω, although not optically transparent through this entire range. In this paper the focus is on transparent coating and films utilizing nanotubes to modify the electrical properties of the polymer without significantly altering the other characteristics of the materials. Although only first widely reported in 1991,i,ii carbon nanotubes are now readily synthesized in gram quantities in the laboratories all over the world, and are also being offered commercially. Carbon nanotubes are essentially single graphite layers wrapped into tubes, either single walled (SWnT) or multi walled (MWnT) formed into several concentric layers,iii as shown in Figure 1. Carbon nanotubes can be synthesized in lengths up to 100 microns with nanometer scale diameters. Figure 1: Schematic depicting various form of carbon Single walled carbon nanotubes are particularly good candidates to impart conductivity to insulating resins. Depending on the tube diameter and angle of roll with respect to the graphite structure NTs exhibit metallic conductivityiv. Nanotubes can be either electrically conductive or semiconductive, depending on their helicity, leading to nanoscale wires and electrical components. These one-dimensional fibers exhibit electrical conductivity as high as copper, thermal conductivity as high as diamond, strength 100 times greater than steel at one sixth the weight, and high strain to failure. If utilized to its promising potential, the field of nanotechnology will revolutionize next generation materials for a wide range of applications. 1.1 Shortcomings of Current Materials There exist a limited number of low temperature techniques to impart electrical conductivity to an insulating layer, they are: • Apply a thin metallic coating like gold, silver, copper • Apply a metal oxide coating like InSnO2 • Layer a conducting polymer or like material under a very thin protective coating • Fill a hard coating with conducting powder like metals, carbon, and doped conducting polymer • Admix to a coating at the molecular level, a doped polymer or carbon nanotubes. Figure 2: Conductive coating maintains electrical properties (180 Ω/□) after Creasing. Note ITO coatings fail during this test. Vacuum deposited indium tin oxide (ITO) is the industry standard material to provide optically transparent electrical conductivity to glass and polymeric films. However, the performance of ITO suffers when applied to plastic. These thin coatings are fragile and are readily damaged during bending or other stress inducing conditions see Figure 2. Furthermore, the process of vacuum deposition is not conducive to forming patterns and circuits. This results in the need for expensive photolithographic processing to form patterns. In Figure 3 all the primary competitive coating technologies are compared. CNT dispersions Sputtered ITO ITO Nano Metal ICP dispersions dispersions dispersions Transparency { { { Conductivity { z { Cost { { z Color { z { Printing capability { z z { Flexibility / durability { z z { { Environmental stability { { { { z { z Excellent Good Poor Figure 3: Comparison of competitive transparent conductive coating technology How does carbon nanotube have and advantage over the use of other conducting particles to make composite coatings? Since conduction is by electrical charge percolation from particle to particle through the coating, typically these materials must be filled with high loading levels (>>5%) of the conductive media to reach significant electrical conductivity. The high loading levels result in poor mechanical and optical properties. Carbon nanotube composites reach the electrical percolation threshold at loading levels of only 0.04% wt, therein this low loading does not affect the other properties of the matrix material. Conducting polymers represent the most investigated alternative to ITO coatings. However, after over a decade of research and development conducting polymers still can not match the optical and electrical performance of ITO. Additionally, conducting polymer suffer from thermal and environmental stability problems preventing their widespread use in commercial applications. 2. ESD COATINGS Eikos, Inc. demonstrated the use of its Nanoshield technology in ESD protection in a variety of polymers. These coatings exhibited visible a wide range of light transmittance (20-99%T), with electrical resistivity designed to be primarily in the Mega-Ohm or higher range (suitable for ESD applications). Eikos formed transparent SWnT nanocomposite ESD films using colorless space durable polyimides, LaRC -CP1, LaRC - CP2, and TOR-NC, a polyimide based on triphenyl phosphine oxide derivatives available from Triton Systems Inc. The resulting composite films are transparent and environmentally stable, having all the mechanical, thermal, and optical characteristics of the virgin polyimides, but with the added capability of ESD. In addition, the films will be inherently bulk conducting, environmentally stable, lighter weight, and manufacturable. Considering all these factors and current film systems, a significantly reduced final cost for deployed films is anticipated. All these aspects and advantages are graphically presented in Figure 4. Figure 4: Advantages for NanoShield-ESD™ when used to impart electrical properties to films The use of carbon nanotubes in this work is to impart electrical conductivity and therefore granting the films inherent ESD properties without secondary coatings or treatments. Additionally, these ESD films do not suffer from inherent temperature dependent electrical characteristics of other conducting polymeric coatings and films. Finally, since the ESD properties are provided by the nanotubes dispersed throughout the polymer matrix, the ESD properties will not deteriorate with surface degradation due to erosion or matrix breakdown. This is a significant advantage over conventional metallized or CVD coatings, which are susceptible to damage from space environment. This technology is based on Eikos’ inventions (patent #6265466) called Nanoshield™, for the use of carbon nanotubes for imparting electromagnetic shielding (EMS) to polymers. Work is currently being conducted at Eikos on nanotube enhanced electromagnetic shielding for the US Army. The EMI shielding polymers can be used to shield electrical component enclosures and shelters from the deleterious effects of external radio, microwave, and millimeter-wave interference/damage. 2.1 Summary of ESD Coating Results The proof of this concept is in our demonstration that Eikos can impart electrical conductivity to a resin system without adversely affecting the other physical properties. This summary data presented in this section demonstrate this concept using three polyimides; CP1, CP2 (both from SRS Technologies), and TOR-NC (Triton Systems Inc). Similar results to those presented below, have been collected on other resins and are expected from most other polymer resins useful for film forming and coatings applications. The key issues for SWNT successful incorporation into an ESD films and coatings are listed here with summary of results obtained: I. Electrical resistivity; concentration, and thickness of nanotube filled films a. Resistivity easily adjusted from 102 to 1012 at any thickness greater than 1 micron b. Resistivity through bulk or surface of films demonstrated with very high optical clarity and low haze II. Thermal effect on conductivity a. Resistivity insensitive to temperature and humidity from at least -78 to +300C b. Resistivity lowers with increasing voltage c. Resistivity insensitive to temperature cycling and soak III. Optical transparency of SWNT filled matrix for window and lens applications a. Transmission loss of only 10-15% for 25 micron thick films with bulk conductivity b. Transmission loss of only 1-5% for thinner 2-10 micron conductive films c. Haze values typically <1% IV. Mechanical property changes to the resin and final films due to presence of nanotubes a. Tensile, modulus, and elongation to break unaffected by addition of nanotubes b. Coefficient of thermal expansion unaffected by addition of nanotubes c. No other qualitative differences between films with or without nanotubes observed V. Processing of resin and films unaffected by incorporation of nanotubes a. Viscosity, surface tension, wetting, equivalent to unfilled resin b. Casting, drying, curing, film parting, and final surface appearance identical During the development of the technology and assessment of market needs, it became apparent that higher conductivity in these coatings is highly desirable for many additional applications. Specifically, most military transparent coatings for aircraft require very low resistivity coatings on canopies to impart multifunctional characteristics to the thermoplastic substrate. Furthermore, the commercial transparent conductive coatings market also is in need of a replacement technology for Indium Tin Oxide (ITO) in the enormous flat panel display market. 3. LOW RESISTIVITY HIGH TRANSPARENCY COATINGS For this technology to advance into these broader military and commercial markets, Eikos has focused on enhancing electrical conductivity of these coatings, while maintaining high optical transparency. Our target is surface resistivity <200 Ohms/sq with >80%T @550nm, which represents ITO performance in numerous commercial applications. We have identified numerous military systems which will benefit from this technology. To that end, we have met with several military producers to discuss the application of this coating as an ITO replacement. Furthermore, we have explored the use of this technology as a replacement for ITO in flat panel displays and other applications (Architectural windows, Electroluminescent lighting systems, Touch screens, etc.). Equally important and exciting are the commercial applications of this technology. Commercial (consumer) applications of this technology are needed to dive down the cost, increase the quality, and ensure availability for the US military’s benefit. Fortunately, some very large consumer markets exist which could benefit from a new transparent conductive coating lacking the processing and handling limitations of ITO; and stability limitations of inherently conducting polymers. Since each application has its own unique set of requirements, a target of 80%T @550nm and <200 Ohm/sq resistivity, was set as a milestone leading to entry into several markets including military transparencies. This target serves as an intermediate goal whereas we intend to ultimately to exceed those which can be obtained using ITO. The relationship between our current coatings and commercially mature coatings is presented graphically in Figure 5. 1.00E+06 Target Applications EMI / LO 2 FPD 3 TS Ohms/sq 1 1.00E+05 1.00E+04 1.00E+03 3 1.00E+02 2 1.00E+01 1 1.00E+00 60.0% 70.0% 80.0% 90.0% 100.0% Transmittance at 550 nm Figure 5: Comparisons between Eikos current coating and other commercial transparent conductive coatings. Note Eikos is blue diamonds and a gold star, PEDOT conductive polymer, is red diamonds, ITO is yellow diamonds, and Gold or Silver metal is yellow squares. FPD = Flat Panel Display, TS = Touch Screen. As can be seen in Figure 5, this Nanoshield coating technology already meets the requirements for touch screen applications. Further improvements will be achieved through purification of materials and modification of coating processes. 4. COMMENTS The transparent conductive coating markets need for and ITO replacement is growing as new technologies emerge, such as large screen television, flexible displays and solar voltaic cells. Furthermore, since the electronics and military industries are always pushing technology for lower cost and higher performance, the need for these nanocomposite films becomes even more evident since only moderate improvements can be expected from ITO and conducting polymer based coatings. The most exciting aspect of this technology is that even at its infancy; it already meets or exceeds the performance of mature technologies like ITO and conducting polymers. Recent development efforts show that continuing improvements in performance are forthcoming. 5. REFERENCES i Phillip Ball, “Through the Nanotube”, New Scientist, 6 July 1996, p. 28-31 S. Iijima, Nature, 354, 56 (1991) iii B. I. Yakobson and R. E. Smalley, “Fullerene Nanotubes: C1,000,000 and Beyond”, American Scientist v.85, July-August 1997 iv R. Saito, G. Dresselhaus, and M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London (1998) M.S Dresselhaus, G. Dresselhaus, and Ph. Avouris, ed., Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Springer-Verlag Berlin, Heidelberg (2001). ii