Electro-polymers show great impact on many modern technologies such as flat panel display, sensors and actuators, energy storage, and energy harvesting. Two important families of electro-polymers: aromatic polyurea and polyvinylidene fluoride (PVDF) based polymer, are studied in this dissertation for: dielectric and piezoelectric applications.
A chemical vapor deposition (CVD) system was designed and developed. Making use of it, aromatic polyurea thin films were fabricated. The high quality films exhibit many advantages for energy storage applications, especially for high temperature capacitors. The relative high dielectric constant (∼4.2) and high breakdown strength (800MV/m) yield the released energy density as high as 12J/cm3 at room temperature. TGA and dielectric measurement show the material has a good thermal stability up to 200°C. High temperature characterizations demonstrate the material has a high breakdown strength (>500MV/m) and high released energy density (>6 J/cm3) even at high temperatures such as 180°C. The study also establishes that in high energy density capacitor design, one can not rely on the low field capacitance data even for “linear” dielectric materials. Various high field conduction processes can cause dielectric losses at high fields much higher than those deduced from the low field data.
This study also reveals that the properties of aromatic polyurea films highly depend on the composition ratio between two monomers, diphenylmethane-diisocyanate (MDI) and diphenylmethane-diamino (MDA). MDA-rich polyurea has been reported to exhibit a dielectric constant as high as 15 with low loss to temperatures above 150°C. Surface morphology study with SEM and AFM revealed that the apparent high-k in MDA-rich polyurea films originates from the non-uniformity of its thickness and erroneous interpretation of the experimental data rather than from its intrinsic properties.
This dissertation also extends the earlier works of improving the electromechanical response of defects modification of PVDF based films to the piezoelectric properties. PVDF was modified by including bulky group, hexafluoropropylene (HFP), as defects in the molecular structure. The new P(VDF-HFP) 10 wt% copolymer exhibits a high transverse piezoelectric response with both high piezoelectric d31 (d31=43.1pm/V) and electromechanical coupling k 31 coefficients (k31=0.187) under quasi-static condition. The phase change nature also results in a large frequency dispersion of the piezoelectric response and a smaller d31 (=20.5 pm/V) at 50 kHz. Electromechanical resonance measurement shows the thickness coupling factor of P(VDF-HFP) is about kt=0.13 around frequency 70MHz.
For many device applications such as energy harvesting, it is highly desirable to operate the active materials under high strain and stress. The material responses under high field conditions such as stress and electrical field are not well known. The direct piezoelectric response of PVDF was investigated under high mechanical stress. It was found nonlinear piezoelectric effect occurs in PVDF when the strain is beyond 0.3%. Both the coupling factor k31 and piezoelectric coefficient d 31 increase with strain and stress. The piezoelectric polymer can withstand high strain (∼3%) without degrading the piezoelectric responses, which is very attractive for energy harvesting applications.
Through an optimized system design including high strain active material selection, mechanical amplification design and active electronic circuit control, an experimental platform for the energy harvesting from ocean waves was developed. Experiment results show great improvement in energy conversion efficiency through active energy harvesting approach. The active piezoelectric energy harvesting system can generate 22mW power with the polymer film 4x4cmx20cmx24μm under strain ∼1%, which is approximately 3∼5 times of the passive approach at the same conditions.