18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS INTERFACIAL EVALUATION OF TRANSPARENT AND CONDUCTIVE CNT AND ITO COATINGS ON PET SUBSTRATES WITH NANO-STRUCTURAL ASPECTS Z. J. Wang 1 , D. J. Kwon 1 , G. Y. Gu 1 , K. L. DeVries 2 , J. M. Park 1,2 * 1 School of Materials Science and Engineering, Engineering Research Institute, Gyeongsang National University, Jinju, Korea 2 Department of Mechanical Engineering, The University of Utah, Salt Lake City, U. S. A. * Corresponding author ( jmpark@gnu.ac.kr) Keywords : CNT coating, surface resistance, four-point method, transmittance, wettability Contact angle measurement is widely used for 1 Introduction investigating surface characteristics on various Recently, carbon nanomaterials (CNMs) have materials. The surface free energy of a material attracted with considerable attention in the research controls its adhesion, adsorption, lubrication, joint and industrial field due to their unique mechanical strength, wettability, etc. The wettability of solid and electrical properties for multi-functional purpose surfaces is an important problem in surface science [1]. Carbon nanocomposites have high stiffness, and practical applications. Young’s equation is used strength and good electrical conductivity at to describe the wettability, where 90 ◦ acts as the relatively low concentrations of reinforcing CNMs. critical contact angle (CA) to divide the concept of Conductivity and high aspect ratio of carbon hydrophobic and hydrophilic properties. The nanotube (CNT) are attractive properties for macroscopic Young-Dupre equation correlates the producing conductive composites with a minimum contact angle to the surface and interfacial tensions. of material. The electrical conductivity of carbon In the research reported here, transparent and nanotube is very high. Films made of CNTs also conductive carbon nanotube coatings were possess a low sheet resistance and exhibit an optical fabricated, by a dip-coating method on polyethylene transmittance in the visible spectrum comparable to terephthalate (PET) substrates. The changes of that of commercial indium tin oxide (ITO). This is electrical and optical properties of these CNT surprising as it has been demonstrated that SWCNT coatings depended mainly on the number of dip- films can exhibit conductivity and transmittance coatings and the CNT concentration. Interfacial values comparable to conventional ITO. In particular, properties were investigated for CNT and ITO transparent conducting CNT coatings on flexible coatings, on PET substrates, by measurement of substrates such as polyethylene terephthalate (PET) electrical resistance on specimens under cyclic can be used as ITO/PET electrodes in chemical and loading [3]. mechanical stability and exhibit a wider electrochemical window. 2 Experimental The feature can be used for coatings to get 2.1 Materials transparent and conductive networks. For this, various techniques have been investigated such as, Multi-wall carbon nanotube (MWCNT, IlJin dip-coating, spraying, spin-coating, vacuum Nanotech Co., Korea) and ITO (125R, Mijitech, filtration, Langmuir-Blodgett deposition, and Korea) as coating materials was used. To prepare electrophoretic deposition (EPD). Among them, the CNT solution in coating process, 2-propanol was dip coating provides CNT thickness control easily used as dispersion solvents of CNT. and it offers a low cost, simple process, and uniform 2.2 Electrical resistance measurement deposition. However, it takes a number of iterative The CNT coated PET films were washed using dip-coating steps to ensure sufficient electrical conductivity [2]. distilled water after the dip-coating process and then
dried in an oven to more completely remove the instrument measures the intensity of light after it solvent in the coating. The surface resistance of the passing through a specimen ( I ) and compares it to CNT coated PET was determined using a four-point the intensity of light before it passes through the method. The surface resistance was then calculated specimen ( I o ). The ratio I/I o ( T ), called the using a dual configuration method, and compared transmittance, is usually expressed as a percentage: with the results of a single configuration method, to eliminate edge effects and to obtain more uniform I = I × 100 % (4) T electrical resistances. This dual configuration 0 method is based on the equations: 3 Results and Discussion = × (1) R k R s a a 3.1 Morphology of CNT coating by dip-coating 2 CNT coating networks were observed using SEM ⎛ ⎞ ⎛ ⎞ R R = − + ⎜ ⎟ − ⎜ ⎟ 14 . 696 25 . 173 7 . 872 (2) a a k ⎜ ⎟ ⎜ ⎟ technique. Figure 1 shows the FE-SEM photos for a ⎝ R ⎠ ⎝ R ⎠ b b surface morphology of CNT coated PET films with: (a) one cyclic coating, (b) dual cyclic coating, and 2.3 Fabrication of CNT coating (c) triple cyclic coating. The concentration of CNT Acid-treated CNT was dispersed in 2-propanol dispersed solution is 0.05 wt%, PET films were solvent for 12 hours under sonication. 0.05 wt% washed using ethanol solvent under sonication CNT solution was used and the CNT coated layer before the dip-coating process. In the PET film with was fabricated with different dipping cycles. Pre- one cyclic coating case, CNT was coated on the PET cleaned PET film in ethanol solvent was used as a film just partially, the density of CNT networks was substrate and sonicated in double distilled water, and much lower than others. Moreover, dual cyclic then finally rinsed intensively. After the dip-coating coated specimen exhibited the medium state, density process, the specimen was dried in the clean-oven at of CNT networks increased compared with first 60 ˚ C to evaporate remaining water completely. specimen. On the other hands, in the triple cyclic coating case, the PET film was coated with CNT 2.4 Wettability measurement completely and more densely than the others. Since Tensiometric method is the most versatile and surfactant did not use in the CNT networks, the suitable way for static and dynamic contact angle CNTs contact each other directly, which greatly measurement on flat plate and filament. Dynamic enhances the electrical conductivity. contact angles of PET substrate with CNT coating were measured using Wilhelmy plate technique (Sigma 70, KSV Co., Finland). Four dipping liquids double purified water, formamide, ethylene glycol and diiodomethane were used. Dynamic contact angle, surface energies, donor and acceptor components, polar and dispersive free energy terms Fig. 1. FE-SEM photos of CNT coatings with different of PET substrates with different cyclic coating CNT cycles by dip-coating coating were measured. The basic equation for 3.2 Transparent properties Wilhelmy plate method is as follow Transparent properties were measured using UV = + γ θ − cos (3) F mg P F spectrum test after specimen was dried again. Figure b LV 2(a) exhibits the correlation between transmittance where F is total force, m is the mass of plate, g is versus wavelength of PET substrate with different acceleration of gravity, F b is buoyancy force, P is cyclic coating. The neat PET film was set as fiber perimeter. standard for 100% transparence. As expected, 2.5 Transmittance measurement transmittance decreased with increasing the cyclic coating. The transmittance of CNT coated PET films Transmittance of the coatings was measured using were in the range of 78 to 90%. As the dipping an Ultraviolet-Visible (UV) Spectrophotometer. This
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