Fabrication of transparent, conductive phase-segregated ITO/PC composites
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Fabrication of transparent, conductive phase-segregated ITO/PC composites Celeste Mason and Rosario A. Gerhardt* School of Materials Science and Engineering Georgia Institute of Technology 771 Ferst Drive, Atlanta, GA 30332-0245 *[email protected] ABSTRACT Fabrication of transparent, conductive phase-segregated ITO/PC composites was accomplished by compression molding of dry mixed constituents. Mixtures of 50nm indium tin oxide particles in a polycarbonate matrix of compositions ranging from phr = 1.0 to 0.001 were measured by optical and electrical methods. Measured transmittance ranged from 5% to 68% at 400nm, 15% to 80% at 700nm, and averaged 10% to 75% across the visible spectrum. Resistance was measured by impedance spectroscopy. Resistivities of the different composites spanned 10 orders of magnitude. Electrical percolation was found to occur near phr = 0.15. INTRODUCTION Percolation in Segregated Composites In order to create a composite material with high optical transmittance and acceptable electrical conductivity, it is necessary to maintain a high proportion of the more transparent matrix constituent and introduce only the minimum amount of electrically conductive filler necessary. In this case, percolation, or joining of particles of the filler material across the bulk of the composite in the directions of interest, must occur. For composite systems, a heterogeneous mixture of constituents produces the most ideal balance of materials and properties. When each material is partially segregated to allow one or more to form a network, electrical percolation may occur at a lower filler loading [1,2]. In this manner, a composite composed of a highly transparent, insulative material as the matrix (major constituent) and a minimal amount of conductive particle filler should be able to perform to the proposed specifications. It should be noted that even if the conductive filaments do not completely connect (leaving nanometer or micron scale gaps), tunneling of electrons may occur through the insulative region at sufficient voltage levels to allow electron transport in some materials. Materials Used The matrix material used for this project was Lexan 121, a form of polycarbonate (PC) that comes as cylindrical pellets of approximately 2-3mm length and 2mm diameter. Optical transmissivity of pure polycarbonate can be as high as 85%. PC is very insulating and has a refractive index of ~1.585. The Vicat Softening Point is 146 °C and its glass transition temperature is 147 °C [3]. Indium tin oxide (ITO) is a transparent, conductive ceramic normally formed as thin films that are used as transparent electrodes in the manufacture of numerous products such as displays and solar cells [4]. The properties of ITO films greatly depend on the method of deposition and deposition parameters used, but a typical film may have a refractive index of 1.95 and can be optically transparent in the visible spectrum to ~90% [5]. The ITO nanoparticle powder used in these experiments was obtained from Sigma Aldrich
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