Applied Materials Today
Elaborately fabricated polytetrafluoroethylene film exhibiting superior high-temperature energy storage performance
Graphical abstract
Pure polymer dielectric films with excellent energy storage performance at high temperature are highly desired in electric and electronic industries. The elaborately fabricated PTFE films with controlled microstructure exhibit a high Eb (~350 kV/mm), high η (~94%), large Ud (~1.08 J/cm3), short t0.9 (2.95 μs), high Pd0.9 (~0.72 MW/cm3) and excellent self-healing ability at 200 °C.
Introduction
Energy storage technology plays an important role during the advancement of modern civilization in replacing the continuous huge consumption of fossil fuel with renewable source. Among various energy storage devices, the electrostatic dielectric capacitors possess excellent superiority in power density (up to 108 W/kg) which thus have been widely used in power electronic and electrical systems, such as portable electronics, hybrid electric vehicles, and medical defibrillators etc [1], [2], [3], [4], [5]. However, with the increase of the integration degree and the expansion of application fields of the power devices, the working temperature of the capacitors is upto 200 °C [6,7]. Designing and fabricating dielectric materials with high discharged energy density (Ud), high charge-discharge efficiency (η), as well as self-healing ability at high temperature is a long-standing challenge [8], [9], [10], [11], [12].
The Ud is proportional to the stored energy density (Ue) in dielectrics and the η. The Ue is determined by the displacement (D) under the external applied electric field (E) as described by Ue=∫EdD [13]. For ideal linear dielectrics, the relationship can be simplified to Ue=1⁄2 ε0 εr E2, where ε0 is the vacuum permittivity, and εr is the relative permittivity of the dielectric material [14,15]. The maximum electric field that can be applied to the dielectrics is the electric breakdown strength (Eb). Hence, an ideal dielectric energy storage material should simultaneously have a high Eb, εr and η.
Compared with the ceramic materials, polymer based dielectrics with the advantages of high Eb, large scale processability, flexibility, light weight, low cost and self-healing capability are highly desired in industry [16], [17], [18], [19], [20], [21]. However, the low polarization nature and the lack of heat resistance of polymer dielectrics restrict their application from high temperature circumstances. Recently, strenuous attempts to improve the εr and high-temperature Eb have been made by fabricating polymers, e.g. polyimide [22] and polyesterimide [23] with high glass transition temperature (Tg) [7,[24], [25], [26], [27]] or introducing nanoparticles (eg. BaTiO3 [28], BNNS [9,29], Al2O3 [30]) into them. .However, the efforts have produced limited success. For example, the Ud of pure polyimide (PI) and flourene polyester (FPE) with Tg higher than 300 °C are both only 0.05 J/cm3 at 200°C with the intended η>90% because the applied electric field has to be significantly lowered to satisfy the presupposed η value [31]. The Ud of the divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) composites filled with BNNS and Al2O3 are only 0.5 and 1 J/cm3 at 200°C, respectively, which are much lower than that of the commercial biaxially oriented polypropylene (BOPP) films (Ud~2 J/cm3) at room temperature [30,31]. The low Ud of the polymer dielectrics is due to easy occurrence of electric breakdown at high temperature. Hence, to maintain a high Eb at high temperature is the key to obtaining high Ud for the polymer dielectrics.
Polytetrafluoroethylene (PTFE), a non-polar linear polymer with excellent thermal stability upto 260°C, has found wide applications from aerospace to cookware since it was discovered in 1938. As a dielectric energy storage film produced by traditional methods of melt-extrusion or solvent-casting, the thickness of PTFE is hard to be reduced to lower than 6 μm and it usually presents poor quality with pin holes which significantly influences its insulation [24]. A thinner and better quality PTFE dielectric film with high performance of dielectric energy storage is highly desired to realize lightweight of electric power system.
In this study, PTFE films with the thickness below 5 μm were fabricated through a casting process of the suspension containing PTFE nanoparticles. The heat-treatment of the films was strictly optimized to achieve a compact microstructure and a smooth surface. The resulting films demonstrated an ultrahigh Ud (~1.08 J/cm3) and a high power density (Pd0.9 ~0.72 MW/cm3) at the temperature upto 200°C, attributing to a high Eb (~350 kV/mm), high η (~94%) and a fast discharge time (~2.95 μs).
Section snippets
Preparation of the PTFE Films
The HTE (high temperature and high elongation) copper foil with the thickness of 35μm was purchased from Chang Chun Petrochemical Co. Ltd. The PTFE water suspension with the solid content of 60% (D-210) was purchased from Daikin Industries Co. Ltd. To prepare a PTFE film, about 3 ml PTFE suspension was spin-coated (KW-4A from Institute of Microelectronics of the Chinese Academy) on the smooth surface of the copper foil with the size of 50 mm × 50 mm. The spinning lasted 18 s under the rotation
Microstructure and room temperature electric breakdown strength
The films were fabricated by coating the water emulsion of PTFE onto copper foils with optimized heat-treatment process. The particle size is about 200 nm (Fig. 1a). The PTFE particles were softened and started to coalesce with their neighbors (Fig. 1b) as the heating temperature reached 330°C, which is close to its melting point of 336°C (Fig. S2). After heating for 40 min at 330°C, many adjacent PTFE particles were coalesced with their boundaries barely discerned. As the heating temperature
Conclusion
In summary, PTFE films with a thickness of 4~5 μm were fabricated through a PTFE nanoparticles suspension, followed by a controlled heat-treatment. The film treated at 380°C for 40 min possesses a low Sd, which demonstrates a highest Eb (~410 kV/mm) at room temperature. This high Eb should be attributed to the reduced space charge density at the interface between PTFE and electrodes based on the simulation, as a result of the compact microstructure and smooth surface of the thin films. The
CRediT authorship contribution statement
Suibin Luo: Conceptualization, Methodology, Investigation, Writing - original draft. Junyi Yu: Formal analysis, Data curation. Talha Qasim Ansari: Software. Shuhui Yu: Conceptualization, Methodology, Writing - review & editing. Pengpeng Xu: Data curation. Liqiang Cao: Supervision. Haitao Huang: Funding acquisition, Writing - review & editing. Rong Sun: Funding acquisition. Ching-Ping Wong: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Author contributions
Suibin Luo and Junyi Yu contributed equally to this work. Suibin Luo and Shuhui Yu conceived the idea and designed the experiments. Suibin Luo and Junyi Yu carried out the experiments and characterizations. Suibin Luo and Shuhui Yu wrote the manuscript. All the authors discussed the results and commented on the manuscript.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (51907194 and 51777209), National key R&D Project from Minister of Science and Technology of China (2017YFB0406300), Shenzhen Peacock Innovative Research Program (KQJSCX20170731163718639), Shenzhen Science, Technology and Innovation Commission (ZG8Y) and open project of Shanghai Key Laboratory of Electrical Insulation and Thermal Aging.
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