Browsing by Subject "dielectric properties"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Electric Field Alignment of Cellulose Based-Polymer Nanocomposites(2012-07-16) Kalidindi, Sanjay VarmaCellulose whiskers (CWs) obtained from naturally occuring cellulose are nano-inclusions which show a lot of promise as mechanical reinforcements in polymers. Typically, a relatively high content is added to realize improvement in effective mechanical behavior. This enhancement in modulus is usually followed by a modest increase in strength but generally the ductility and toughness decrease. Our approach is to use small concentrations of CWs so as not to detrimentally affect processability, toughness and ductility. By aligning the small concentrations, we target the same kind of improvement in modulus and strength as reported in the literature, but at much smaller volume contents. In this work, we investigate the effect of AC electric field on the alignment of dispersed nanoscale CW in a polymer. Polyvinyl acetate (PVAc) is used as the model polymer because of the good interaction between CWs and PVAc. A low concentration of 0.4wt% was used for the study. Two dispersion methods, namely basic and modified, were developed. The basic method led to micron scale dispersion. Using the modified method, CWs were individually dispersed in PVAc with average lengths and diameters of 260 nm and 8 nm respectively yielding an aspect ratio of approximately 30. The behavior of CWs (alignment and chain formation) under an applied electric field was found to be a function of applied electric field magnitude, frequency and duration. Following alignment, the CW/PVAc nanocomposites are thermally dried in the presence of electric field to maintain the aligned microstructure. Improvements in dielectric constant and mechanical properties were observed for the aligned cases as compared to random case and pure PVAc. The optimal electric field magnitude, frequency and duration for the alignment and chain formation were found to be 200Vpp/mm, 50 KHz for duration of 20 minutes for the microcomposite and 250Vpp/mm, 10KHz for a duration of 1hr for the nanocomposite. At 0.4wt% concentration, 21% increase in dielectric constant for the optimal nanocomposite case. Above Tg, a 680% improvement in elastic modulus at 0.4wt% concentration for the optimal nanocomposite case. The reason for the significant reinforcement is attributed to alignment (rotation and chain formation) and chain-chain interaction (3D network formation and hydrogen bonding).Item Optimization of Polymer-based Nanocomposites for High Energy Density Applications(2012-07-16) Barhoumi Ep Meddeb, AmiraMonolithic materials are not meeting the increasing demand for flexible, lightweight and compact high energy density dielectrics. This limitation in performance is due to the trade-off between dielectric constant and dielectric breakdown. Insulating polymers are of interest owing to their high inherent electrical resistance, low dielectric loss, flexibility, light weight, and low cost; however, capacitors produced with dielectric polymers are limited to an energy density of ~1-2 J/cc. Polymer nanocomposites, i.e., high dielectric particles embedded into a high dielectric breakdown polymer, are promising candidates to overcome the limitations of monolithic materials for energy storage applications. The main objective of this dissertation is to simultaneously increase the dielectric permittivity and dielectric breakdown without increasing the loss, resulting in a significant enhancement in the energy density over the unmodified polymer. The key is maintaining a low volume content to ensure a high inter-particle distance, effectively minimizing the effect of local field on the composite's dielectric breakdown. The first step is studying the particle size and aspect ratio effects on the dielectric properties to ensure a judicious choice in order to obtain the highest enhancement. The best results, as a combination of dielectric constant, loss and dielectric breakdown, were with the particles with the highest aspect ratio. Further improvement in the dielectric behavior is observed when the nanoparticles surface is chemically tailored to tune transport properties. The particles treatment leads to better dispersion, planar distribution and stronger interaction with the polymer matrix. The planar distribution of the high aspect ratio particles is essential to limit the enhancement of local fields, where minimum local fields result in higher dielectric breakdown in the composite. The most significant improvement in the dielectric properties is achieved with chemically-treated nano TiO2 with an aspect ratio of 14 at a low 4.6 vol% loading, where the energy density increased by 500% compared to pure PVDF. At this loading, simultaneous enhancement in the dielectric constant and dielectric breakdown occurs while the dielectric loss remains in the same range as that of the pristine polymer.