Dielectric and thermal conductivity behavior of continuous fiber polymer-matrix structural composite materials in the through-thickness direction
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Continuous fiber polymer-matrix composites are lightweight structural materials. With fibers in the form of laminae in the plane of the laminate and their properties being different from the matrix, the composite is anisotropic, with the in-plane properties dominated by the fibers and the through-thickness properties governed by both fibers and matrix. This dissertation addresses the dielectric and thermal conduction behavior of the composites in the through-thickness direction, including modification of the composites for tailoring these properties. The polymer is epoxy. The modification involves filler or interlayer incorporation, partial prepreg resin removal, and/or curing pressure variation. Carbon fiber and glass fiber composites are separately addressed in relation to the dielectric behavior, whereas glass fiber composites are addressed in relation to the thermal conduction behavior. The dielectric behavior is relevant to energy storage and microelectronic signal propagation speed, while the thermal conductivity is relevant to heat dissipation from microelectronics. Glass fiber composites are particularly attractive for their electrical insulation ability, as needed for printed wiring boards. By interlayer incorporation at the interlaminar interface, carbon fiber composites with positive and negative permittivity have been achieved. For negative permittivity, the interlayer is cellulosic paper that contains tap water, which evaporates and leaves low-concentration ions during composite fabrication. The series combination of these composites gives extraordinary high permittivity (with the relative dielectric constant up to 78,000) in accordance with the equation for positive and negative capacitors in series. This is the first time that this equation is found to be valid for a series combination of capacitors of opposite sign. Thus, a new route for achieving very high permittivity is obtained. Continuous glass fiber epoxy-matrix composites exhibit through-thickness relative dielectric constant and through-thickness thermal conductivity that are comparable to those of the individual glass fiber (axial) when the curing pressure is high. These properties are dominated by the contribution by the laminae (as opposed to the contribution by the interlaminar interfaces). The contribution from a lamina is effectively modeled by using the Rule of Mixtures with the fibers and matrix in parallel in the through-thickness direction. This means that fiber-fiber contacts in the through-thickness direction provide an effective dielectric polarization or thermal conduction path. By filler (boron nitride nanotube) incorporation, partial prepreg resin removal, and/or curing pressure increase, electrically insulating glass fiber composites with enhanced through-thickness thermal conductivity have been obtained. Curing pressure increase and partial prepreg resin removal also cause the relative dielectric constant to increase. These thermal and dielectric effects are due to enhanced fiber-fiber contact within the laminae and the consequent increased connectivity in the through-thickness direction. Low-k dielectrics are electrical insulators with very low values of the relative dielectric constant, as needed for reducing the signal propagation delay in microelectronics. This work provides fumed-alumina-derived nanoporous alumina as a new low-k dielectric material that exhibits relative dielectric constant down to 2.0 (up to 2 MHz).