Electrical Conduction Through Polyvinylidene Fluoride: Exploiting Interfaces as Barriers to Charge Transport
Open Access
- Author:
- Vecchio, Michael Anthony
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 08, 2019
- Committee Members:
- Michael T Lanagan, Dissertation Advisor/Co-Advisor
Zoubeida Ounaies, Committee Chair/Co-Chair
Michael Anthony Hickner, Committee Member
Ramakrishnan Rajagopalan, Committee Member
Amira Barhoumi Ep Meddeb, Outside Member - Keywords:
- PVDF
P(VDF-TrFE)
Interface Dominated Conduction
Hopping
Schottky
Plasma Treatment
Impurity Ion
Multilayer Dielectrics - Abstract:
- Polymer capacitors exhibit a combination of unique properties including high dielectric breakdown strength, light weight, flexibility, and low-cost production that make them appealing candidates for film capacitor technologies. For high power capacitor applications, biaxially oriented polypropylene (BOPP) is considered state of the art and exhibiting breakdown strengths as high as 850 MV/m, however its low permittivity (εr = 2.2) prevents its use in high-energy density demanding applications. In this dissertation, high permittivity (8 < εr < 12) polar polymers poly(vinylidene fluoride) (PVDF) and its copolymer poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) are used as model materials to investigate the role of interfaces on low frequency, high temperature and high electric field charge transport. This work demonstrates 1) the importance of electrode/dielectric interface chemistry in controlling charge injection and conduction, and 2) how interfaces in layered dielectrics block transport of impurity ions ultimately influence dielectric performance. The high field performance of hot-pressed layered dielectrics in pure PVDF laminates was explored first. The solution processing and hot-press lamination procedure produced films containing both alpha and beta crystal phases throughout the bulk. Impedance spectroscopy at 70oC combined with equivalent circuit (EC) modeling demonstrated a blocking effect in PVDF films containing 4 layers relative to the 1-layer control. Finally, dielectric breakdown experiments, analyzed via Weibull statistics, reveal a statistically significant 16% increase in the breakdown strength of 3-layer films (490 MV/m) relative to 1-layer (415 MV/m) analyzed using a 90% confidence interval. These initial results imply that multilayer lamination low frequency charge migration which leads to higher dielectric breakdown strength, however hot-pressing proved to be a disadvantageous processing procedure: layer counts beyond 4 were not possible due poor repeatability and individual layer thickness is ~10μm. A spin cast process was developed as an alternative to hot pressing for creating reproducible P(VDF-TrFE) thin films. (~1μm thick). Electrode/dielectric interfacial chemistry on high-field conduction was studied by controlling P(VDF-TrFE) surface chemistry using a CF4/O2 reactive ion plasma treatment. It was found that oxygen based chemical moieties detected using X-ray photoelectron spectroscopy (XPS) grafted to the film surface cause Schottky barrier height lowering by approximately 0.05 eV. This reduction accounted for an order of magnitude increase in leakage current at high fields. The effect of a pure oxygen plasma treatment was then assessed in 10μm thick polyimide (PI) films. PI exhibits a non-polar chemical structure and surface chemistry modification after plasma treatment had a different effect on high-field conduction relative to P(VDF-TrFE). A combination of hopping, Poole-Frenkle, and Schottky theoretical frameworks were used to analyze charging current as a function of voltage and temperature in untreated and oxygen plasma treated PI. It was found that oxygen moieties introduced via plasma treatment caused both decrease in the leakage current of PI films at high temperature and delayed transition from bulk dominated conduction (hopping) to interface dominated conduction (Schottky) by 50oC relative to untreated PI. It is posited that the presence of electronic trapping centers introduced by chemical modification at the electrode/dielectric interface are responsible for electronic charge scattering and trapping at high temperatures. The influence of interfaces on ionic transport in P(VDF-TrFE) multilayer films was explored. Lithium perchlorate (LiClO4) is first doped into 1-layer P(VDF-TrFE), creating dielectrics in which the impurity ion species is controlled and well known. Differential scanning calorimetry (DSC) revealed a correlation between curie transition temperature of copolymer’s beta phase and LiClO4 concentration added into the material. EC modeling of impedance spectra as a function of temperature captured Li+ ion interaction with crystalline phases distributed throughout the bulk as well as the electrode dielectric interface at low frequency and high temperature. It was found that the impedance of crystalline interfaces and the electrode/dielectric interface is a major contributor to the overall electrical response of the film, indicating that ions are blocked at interfaces. Finally, multilayered composites were created with alternating doped P(VDF-TrFE) and thin (500nm) poly(vinyl alcohol) (PVA) layers used to develop an ion depleted interface. The EC model used to describe 1-layer films of P(VDF-TrFE) was used to develop a model that predicts the impedance behavior of doped layered composites. Impedance spectroscopy, EC modeling, and TSDC are used to prove the occurrence of impurity ion blocking at the interface leading to substantial space charge distribution through the bulk of the composites at low frequencies and high fields. Finally, high voltage dielectric breakdown experiments were performed, and the defect dominated breakdown mechanism showed most significant effect due to layering and is described well using Weibull statistics.