High energy/capacitance density poly(vinylidene fluoride) based polymers for energy storage capacitor applications.

摘要

The increased energy levels and continued demands for miniaturization of many devices such as hybrid electric vehicles, pulsed power systems, and switched-mode power supplies call for advanced polymer film capacitors with a high energy density (HED) [1], which cannot be met in current low dielectric constant (3.2) polymers (energy density ∼2 J/cm3) [2]. Poly(vinylidenefluoride) (PVDF) features a high dielectric constant (12) [3], and has the potential to reach a high energy density. This dissertation introduces general considerations leading to and the results of ultra-high energy density (>25 J/cm3) in PVDF-based copolymers P(VDF-HFP) (HFP: hexafluoropropylene) 95.5/4.5 mol% and P(VDF-CTFE) (CTFE: chlorotrifluoroethylene) 91/9 mol% [4], [5], which represents an order of magnitude improvement of the energy density over currently used polymers.;In addition, this dissertation is devoted to developing a fundamental understanding of several newly observed phenomena in these HED polymers, which are not present in the currently low dielectric constant polymers. In polymer film capacitors, high fields have been used to realize high energy density. Therefore, the emphasis is paid to understand the response behaviors of these HED polymer dielectrics at high fields, particularly the losses and the breakdown mechanism. Based on these investigations and fundamental understandings, different approaches are introduced to further improve performance of these HED polymers.;This dissertation demonstrates that in these HED fluoropolymer films the losses increase rapidly with applied electric fields. Immediately beyond the weak field, the losses can be caused by the ferroelectric domain wall type motions, similar to those in magnetic materials as described by Rayleigh's law [6]. On the other hand, a complex notation has been extensively used to describe the dielectric behavior [7]. In this dissertation, we extend this complex notation to the non-linear region to include the losses [8]. As the field increases further (> 100 MV/m), the loss due to the ferroelectric switching dominates. At very high fields (> 250 MV/m), it is the conduction loss that dominates. Even for state-of-the-art capacitor films that are widely regarded as "linear" dielectrics, the conduction loss can become higher at high fields due to a non-linear increase in the conduction [9].;In PVDF-based polymers, it is well known that polymer modifications and processing conditions can significantly influence the ferroelectric loss [10]. Therefore, two approaches were investigated to reduce the ferroelectric switching loss: (1) the irradiation method [11] to destabilize the polar conformation and correspondingly reduce the ferroelectric loss and (2) the biaxial stretching method. The film processing study revealed that the orientation of polymer chains parallel to the film surface improves the breakdown strength and reduces the conduction loss in PVDF-based polymers, while a random orientation of polymer chains along the film surface is desired to reduce the ferroelectric loss.;In order to reduce the conduction loss, we take the general approach to employ a blocking layer which possesses a higher resistivity compared to the original film [12]. However, for these HED polymers, the blocking layer should also meet the requirements: (1) a dielectric constant closer to the original film (∼13) to maintain a high energy density and (2) a low temperature fabrication because of the low melting temperature (∼160°C) of PVDF-based polymers. Hence, insulating polymers of low dielectric constants (3.2) cannot meet the first requirement and will significantly reduce the energy density. On the other hand, ceramics can meet the first requirement. However, their high temperature fabrication process (>300°C) [13] is not compatible with PVDF-based polymers. In this study, we demonstrated that very high resistivity with a dielectric constant of ∼7 can be obtained with Si3N 4 deposited at 100°C and that the conduction loss of the resulting bilayered films can be much less than a single layer of PVDF-based copolymers.;In the study of the electrical breakdown in these HED capacitor films, it was observed that although the temperature dependence of the breakdown strength in the P(VDF-HFP) 95.5/4.5 mol% films is consistent with the electromechanical (EM) breakdown [14], the widely accepted EM breakdown model of Stark-Garton significantly overestimates the breakdown strength. We show that this discrepancy lies in the fact that the Stark-Garton model fails to capture the mechanical properties of the polymers that experience a plastic deformation. Furthermore, we introduce a more general power law relation to characterize the elastic-plastic deformation of polymers. This newly developed model agrees well with the experimental data [15], and should be applicable to any polymer dielectrics in their electromechanical breakdown because of the universal validity of this model to describe the mechanical behavior of polymer dielectrics.