Composites containing percolated networks and/or core-shell structures useful for microsystems applications
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Polymer matrix composites (PMCs) can have percolated networks of fillers or uniform core-shell structures depending on the fabrication process used. PMCs with percolated networks of electrically conductive fillers can be useful for various applications, including anti-static materials, electromagnetic interference (EMI) shielding, electronic-nose devices, sensors and conductors, while PMCs with uniform core-shell structures of dielectric and/or conductive fillers can be useful for applications such as embedded capacitors, gate dielectrics, energy storage devices and electromechanical transducers. In this research, experimental and computational investigations have been conducted on both types of polymer matrix composite systems. First, electrical percolation in phase segregated polymer matrix conductive composites consisting of poly(methyl methacrylate) (PMMA) microspheres and antimony tin oxide (ATO) nanoparticles was investigated. The nanocomposites were fabricated by mechanical blending combined with compression molding. As a result of this process, the matrix PMMA was transformed into space filling polyhedra and the ATO nanoparticles were primarily distributed at the interfaces between matrix-rich regions, forming a 3D interconnected network. Percolation was achieved at a very low ATO content (0.18 vol %) when using monosize spheres. The effects of the average size and size distribution of the matrix polymer microspheres were investigated and the correlation between processing, electrical conductivity and microstructure was investigated using impedance spectroscopy and SEM. A parametric finite element approach was chosen for simulating this unique microstructure-driven percolation behavior. Good agreement was obtained between the experimental and the simulation results. The simulation model developed is applicable to many different kinds of phase segregated insulator-conductor composite systems to predict the electrical percolation behavior. Second, an investigation was conducted on polymer matrix dielectric composites with uniform core-shell microstructure containing poly(vinylidene fluoride) (PVDF), barium titanate (BaTiO3) and/or multiwall carbon nanotubes (MWCNT). Composites with high dielectric permittivity and low dielectric loss were obtained by a simple phase separation method combined with compression molding to evenly distribute the nanofillers in the polymer matrix at all length-scales. The effects of nanofiller type, size and distribution in the dielectric PMCs were investigated as a way to improve the dielectric properties. Impedance spectroscopy was used to examine the dielectric properties of PMCs and to reveal interesting dependences on the size and shape of the nanofillers used. The maximum relative real permittivity achieved was above 70 with very low loss (<0.045 at 1kHz). For quantitative analysis of the dielectric polarization mechanism in the dielectric PMCs, an equivalent circuit model consisting of a modified Debye circuit and a parallel RC circuit in series was introduced to describe the dielectric responses of the multi-phase nanocomposites. The finite element approach was also used to simulate the dielectric properties of PMCs with one or more types of uniformly distributed fillers. It was found that the interfaces between the matrix and the filler play a significant role in improving the dielectric properties. The geometry-based 3D finite element simulation model can be applicable to different composite systems with fillers of different shapes and properties.