Fundamental Study and Development of 0-3 Dielectric Composites with High Dielectric Constant
Type of Degreedissertation
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Dielectric composites are promising materials for many applications. Polymer-based 0-3 composites with a high dielectric constant represent an important category of dielectric composites being studied in research. Different conductors were used as filler for 0-3 dielectric composites. By combining solution casting and hot-pressed processing, two nanocomposites with nano-sized Ni particles embedded into copolymer poly (vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)70/30 mol%] and poly (vinylidene fluoride-co-chlorotrifluoroethylene) [P(VDF-TrFE) 88/12 mol%] have been prepared. Uniform dispersion of nanoparticles in nanocomposite materials was achieved by improvement of processing conditions. Dielectric constants of more than 1000 with relatively low loss were obtained in both systems. Both composites had a high percolation threshold (φc>50 vol.%), making the material reproducible for practical applications. Novel all-organic dielectric composites based on polypyrrole (PPy) nanoclips were prepared by the same process as Ni-polymer composites. Due to their 2D structure and large surface area, the composites were highly flexible with a high dielectric constant and a low φc (φc<8 wt.%). Dielectric constant more than 1000 was obtained in PPy-P(VDF-TrFE) composites. The dielectric constant of a composite with 8 wt.% of PPy is more than 100 times higher than that of the P(VDF-TrFE)70/30 matrix, rising to 1200 at room temperature. From study of Ni-polymer composites and PPy-polymer composites, a new dielectric process was observed in both composites, which was a relaxation process with a very low relaxation frequency. There are three mechanisms in this conductor-polymer composite: 1) the dielectric relaxation process from the polymer matrix, 2) the new dielectric relaxation process from the composite, and 3) the conductivity of the conducting filler. From study of Ni-polymer composites and PPy-polymer composites, the φc and critical value (s) of composite are different with the selecting data at different frequency and temperature. This conclusion was confirmed using six reported systems with different conducting fillers from literature. High length-to-width ratio may cause much of the difference in φc and s with different frequencies. The mechanism behind the dielectric properties of conductor-dielectric composites was explained in this dissertation by introducing the dielectric loss (or conductivity). In this research, the contribution of the loss to the effective dielectric constant (εeff) is dependent on the microstructure of the composite. Three different models were studied: the series model is for 2-2 composite, Maxwell model is for 0-3 composite and Lichtenecker's logarithmic mixing model leads itself to 0-0 composite. The conductivity of matrix and filler both have much effect on the dielectric constant and loss. A way to develop composites with high dielectric constant and low loss was introduced.
- Lin Zhang's dissertation.pdf