High energy density dielectric materials are desirable for capacitors and other energy storage systems. Two approaches were developed to achieve high electric energy density: explore high dielectric constant (K) materials and improve high operation electric field. Relaxor ferroelectric polyvinylidene fluoride (PVDF) based copolymers P(VDF-HFP), P(VDF-CTFE) and terpolymer P(VDF-TrFE-CFE) have been proven to possess high electric energy density. An energy density of over 25 J/cm3 has been achieved in PVDF-based polymers, which represents the state of art in high energy density polymers. Aromatic polyurea thin films were developed through vapor phase deposition, exhibiting relatively high dielectric constant, low loss, high breakdown field (>800 MV/m) and consequently high energy density (>12 J/cm3). Its thermal stability up to 200°C and high charge-discharge efficiency (>90%) make it attractive for high temperature capacitors. Investigation through SEM, AFM and other experiments indicated unbalanced aromatic polyurea could exhibit apparent high-K (~15) due to the non-uniformity of film thickness and surface morphology. This article reviews the recent development of these high performance polymers.

Recent development of high energy density polymers for dielectric capacitors

Progress, Outlook, and Challenges in Lead‐Free Energy‐Storage Ferroelectrics

Using polyimide as host in a guest‐host thin film we demonstrate the first poled‐polymer electro‐optic response stable at temperatures up to 150 °C (samples poled and cured at 250 °C). A coplanar‐electrode poling geometry is used so that the molecular alignment of the guest dye between the electrodes is coincident with the free volume of the host. We hypothesize that ‘‘high’’ temperature poling during the imidization process, when the polyimide forms rings and densifies, accounts for the excellent poled response stability.

Thermal stability of electro‐optic response in poled polyimide systems

TSDC spectroscopy of relaxational and interfacial phenomena.

In several low loss dielectric materials, it was observed that the energy loss remains very small under low and medium electric fields but dramatically increases at high field which is believed to be due to tunneling current. The increase of tunneling current at high field is due to the decrease of barrier width and height and is a universal phenomenon in all dielectric materials. Due to the requirement of high energy efficiency, high field conduction places a limit for the maximum operation field, which could be lower than the breakdown field and act as the limiting factor of energy density.

High field tunneling as a limiting factor of maximum energy density in dielectric energy storage capacitors

Electrical conduction phenomena in polyimide (Kapton) films were studied with particular attention devoted to the separation of interface and bulk phenomena. The measurements were carried out with a variety of methods in the temperature range of 50 to 270 °C at electrical fields of 104 to 6×105 V/cm and at time intervals of up to 2×104 s after voltage application. Biased, two‐side metallized samples yield, after sufficiently long voltage application, interface‐controlled steady‐state currents described by Schottky injection, modified by space‐charge layers in the vicinity of the electrodes. The effective work function for aluminum‐polyimide is estimated to be 1.7 eV in the temperature range between 100 and 270 °C. A distinct dependence of these currents on electrode material is observed. Bulk phenomena were studied on one‐side metallized samples subject to positive‐corona charge injection. At temperatures below 200 °C, significantly larger currents than those for biased, two‐side metallized samples were observed. The current‐voltage characteristics are ohmic at low fields and space‐charge limited at high fields. From these data, trap modulated mobilities for positive carriers of 4×10−12 cm2/V s at 50 °C and 10−9 cm2/V s at 200 °C, ohmic bulk conductivities of 10−16 (Ω cm)−1 at 100 °C and 10−14 (Ω cm)−1 at 200 °C and an intrinsic carrier density of 5×1013 cm−3 independent of temperature are obtained. Activation energies for the mobility are between 0.2 and 0.8 eV for the temperature range between 50 and 200 °C.

B. Hahn

Papers

Electrical conduction in polyimide films