Study of Percolation in PMMA / Indium Tin Oxide Composites
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Study of Percolation in PMMA / Indium Tin Oxide Composites Charles J. Capozzi1, Sandra Shackelford, Runqing Ou and Rosario A. Gerhardt Georgia Institute of Technology, School of Materials Science and Engineering Atlanta, GA 30332-0245, U.S.A. 1
SURF student from Rutgers University, School of Engineering Piscataway, NJ 08854-8065, U.S.A. ABSTRACT Knowledge of percolation in binary composites is critical to the development of new materials with specific electrical and optical properties. This report investigates the detection of percolation in novel two-phase composites consisting of poly(methyl) methacrylate(PMMA) and indium tin oxide (ITO). ITO is a filler of particular interest primarily for possessing optical clarity consistent with PMMA in the visible light range. AC impedance measurements were performed on specimens with varying concentrations of ITO particles to determine the percolation threshold. Percolation was observed when specimens contained 2-3% vol. of nanosized ITO and 6%-8% vol. of coarse-sized ITO. Thus, the percolation threshold appeared significantly decreased with reduced particle size of the filler. It is speculated that minor agglomerates in the bulk of the specimens may have prevented percolation from occurring at even lower volume fractions of the filler phase. INTRODUCTION Long before the fabrication of man-made composite materials, the percolation phenomenon has existed in nature. The concept of percolation is relevant to the paths of fires which ravage through forests, or to waters that find the ocean across land. However, the occurrence of percolation in two-phase composites, specifically, has been of significant interest to the scientific community due to the wide practical applications that can utilize the unique properties of conductor-insulator systems. In a two-phase composite, percolation describes the point at which the phase of lesser quantity forms a continuous network throughout the phase of greater quantity. Figure 1 illustrates an un-percolated and a percolated two-phase system where the dark particles represent the filler phase.
a)
b)
Figure 1. Example of development of percolation with increasing filler concentration: low concentration (a), high concentration (b). Research involving insulating phases as hosts for electrical conductors is becoming vital to applications that require materials with diverse properties. Many properties of the composite,
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electrical conductivity, in particular, can depend heavily on the percentage of conducting phase present. For example, slight variation in the filler volume fraction around the percolation threshold region can result in a change of the electrical conductivity over many orders of magnitude. The general effective media (GEM) is an empirical equation derived from mean-field theory and percolation theory, which may be used to help predict the conductivity of the composite as a function of its composition. The GEM equation (Eqn. 1) holds particular value considering the increasing attention being given to conduct
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