Polymeric Capacitors Research Articles

Boosted energy-storage efficiency by controlling conduction loss of multilayered polymeric capacitors

Among the organic dielectrics, polyvinylidene fluoride PVDF-based polymers present the highest level of polarizability with permittivity >35. Nonetheless, their applications in advanced electronics and industrial uses are limited by significant leakage current under high voltage, which is considered the principal cause of device energy consumption and short lifetime. Therefore, the main objective of this paper was to focus on alternative capacitor structure-based polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene (PVDF-TrFE-CTFE) terpolymers to efficiently limit the leakage current and lead to enhanced electrical breakdown with negligible degradation of polarization level. The novel bilayer capacitor was constructed by depositing a thin barrier layer of high polar PVDF-HFP copolymer on the terpolymer via a rapid solution-casting technique. The relationship between leakage current, dielectric strength and conduction mechanisms was investigated. Implementation of such a barrier layer led to a significant reduction (70%) in leakage current and ferroelectric losses (approximately 90%), thus boosting the performance of multilayered material up to 50% of enhanced energy-storage efficiency. Experimental results are very promising, allowing to confirm that combining both terpolymer and copolymer in a hybrid multilayer design makes a possibility to achieve the best compromise in terms of energy efficiency, dielectric properties, and breakdown strength.

Fluorinated poly(arylene ether ketone)s for high temperature dielectrics

There is a need for polymeric capacitors with improved energy storage density and thermal stability. In this work, the effect of polymer molecular structure and symmetry on Tg, breakdown strength, and relative permittivity were investigated. A systematic series of four amorphous poly(arylene ether ketone)s were compared. Two of the polymers had symmetric bisphenols while the remaining two had asymmetric bisphenols. Two contained trifluoromethyl groups while the other two had methyl groups. The symmetric polymers had Tg's of approximately 160 °C while the asymmetric polymers showed higher Tg's near 180 °C. The symmetric polymers had breakdown strengths near 380 kV/mm at 150 °C. The asymmetric counterparts had breakdown strengths near 520 kV/mm even at 175 °C, with the fluorinated polymers performing slightly better in both cases. The non-fluorinated polymers had higher relative permittivities than the fluorinated materials, with the asymmetric polymers being better in both cases.

Simulation of charge packet formation in layered polymer film

Purpose – The purpose of this paper is to describe a rapid and robust axisymmetric hybrid algorithm to create dynamic temporal and spatial charge distributions, or charge map, in the simulation of bipolar charge injection using Schottky emission and Fowler-Nordheim tunneling, field-dependent transport, recombination, and bulk and interfacial trapping/de-trapping for layered polymer films spanning the range from initial injection to near breakdown. Design/methodology/approach – This hybrid algorithm uses a source distribution technique based on an axisymmetric boundary integral equation method (BIEM) to solve the Poisson equation and a fourth-order Runge-Kutta (RK4) method with an upwind scheme for time integration. Iterative stability is assured by satisfying the Courant-Friedrichs-Levy (CFL) stability criterion. Dynamic charge mapping is achieved by allowing conducting and insulating boundaries and material interfaces to be intuitively represented by equivalent free and bound charge distributions that collectively satisfy all local and far-field conditions. Findings – Charge packets cause substantial increase of electric stress and could accelerate the breakdown of polymeric capacitors. Conditions for the creation of charge packets are identified and numerically demonstrated for a combination of impulsive step excitation, high charge injection, and discontinuous interface. Originality/value – Metallized bi-axially oriented polypropylene (BOPP) dielectric thin film capacitor with self-clearing and enhanced current carrying capability offer an inexpensive and lightweight alternative for efficient power conditioning, energy storage, energy conversion, and pulsed power. The originality is the comprehensive physics and multi-dimensional modeling which span the dynamic range from initial injection to near breakdown. This model has been validated against some empirical data and may be used to identify failure mechanisms such as charge packets, gaseous voids, and electroluminescence. The value lies in the use of this model to develop mitigation strategies, including re-designs and materials matching, to avoid these failure mechanisms.

The rational design of polyurea & polyurethane dielectric materials

When designing polymeric capacitors, it is important to understand the structure–property relationship between chemical functionalities and dielectric properties to tailor materials for specific applications in terms of dielectric constant, dielectric loss, band gap, breakdown strength, etc. Herein, we report a clear structure–property relationship with dielectric constant and dielectric loss in a series of polyurea and polyurethane thin films. We demonstrate that the dielectric constant systematically increases and the dielectric loss decreases as the number of carbons between polarizable functional groups decreases. Our syntheses are guided with data obtained from high-throughput density functional theory calculations. By modeling 382 polymer systems, we have determined that a dielectric constant >4 is achieved with at least one aromatic group and at least one of the following moieties: –NH–, –C(O)–, or –O–.

Novel self-directed dual surface metallisation via UV-curing technique for flexible polymeric capacitors

Polymer composites with metal nanoparticles have attracted considerable attention over the past years due to numerous solid state potential applications in organic electronics and microelectronic devices. Here we report the first example of a novel, rapid and convenient self-directed dual surface nano-silvering of a polymer film by a simple single-stage process of free-radical photopolymerisation (UV-curing). The major component of the composite is bisphenol A ethoxylate dimethacrylate (BEMA) which can be photopolymerised by a photo-reactive initiator (Darocur 1173) under UV light. Under the same conditions of UV irradiation, silver nitrate decomposes and the silver ion is deposited as metal while the pyrrole is oxidised to polypyrrole. It is demonstrated that this versatile method can open up promising prospective in obtaining innovative composite polymeric capacitors for highly flexible and/or non-planar electronics application. The obtained films are characterised by Scanning Electron Microscopy (SEM) and electrical measurements.

Electrochemical and morphological characterization of poly(12-pyrrol-1′-yl dodecanoic acid)

12-Pyrrol-1′-yl dodecanoic acid was prepared and electrochemically polymerized on ITO electrodes by three different methods. The electrochemical and morphological properties of poly(12-pyrrol-1′-yl dodecanoic acid) films were investigated by cyclic voltammetry, galvanostatic charge–discharge curves and AFM, which characteristics varied as function of deposition method. Films deposited by potentiodynamic and galvanostatic methods showed capacitive properties and are promising for application in polymeric capacitors.

Enhanced breakdown strength of multilayered films fabricated by forced assembly microlayer coextrusion

There is a need in electronic systems and pulsed power applications for capacitors with high energy density. From a material standpoint, capacitive energy density improves with increasing dielectric constant and/or breakdown strength. Current state-of-the-art polymeric capacitors are, however, limited in that their dielectric constant is low (2–4). Our approach to improve polymer film capacitors is to combine, through microlayer coextrusion, two polymers with complementary properties: one with a high breakdown strength (polycarbonate) and one with a high dielectric constant (polyvinylidene fluoride-hexafluoropropylene). As opposed to the monolith controls, multilayered films with various numbers of layers and compositions subjected to a pulsed voltage exhibit treeing patterns that hinder the breakdown process. Consequently, substantially enhanced breakdown strengths are measured in the mutilayered films. It is further shown, by varying the overall film thickness, that the charge at the tip of the needle electrode is a key parameter that controls treeing. Based on the acquired data, a breakdown mechanism is formulated to explain the increased dielectric strengths. Using the understanding gained from these systems, selection and optimization of future layered structures can be carried out to obtain additional property enhancements.

Injection charging of polymeric capacitors

The charging of non-polymer capacitors due to a step voltage is studied as a function of time. Apart from the classical charging, which is completed in nanoseconds, it is found that the current flowing into the capacitor can be described as a sum of two exponential functions of time, the time constants differing by a factor of ten or more. In the case of polyethylene, at least, it is clear that the shorter time constant is connected with carrier injection into the polymer from the metal surface. The longer time constant is presumably due to carrier migration within the volume of the dielectric. These findings may be correlated with our previous results on the field dependence of electrostatic charging and support the model of carrier injection to a finite depth.