Achieving excellent dielectric performance in polymer composites with ultralow filler loadings via constructing hollow-structured filler frameworks
Introduction
With the fast-growing development of electrical power systems, ever-increasing attention has been devoted to high-performance energy storage technologies. Among various energy storage devices (e.g., batteries, electrochemical supercapacitors, fuel cells, etc), electrostatic capacitors aroused considerable interest due to their fast charge-discharge capabilities, excellent cycling stabilities, and huge potential for pulsed power applications, such as hybrid electric vehicles, medical defibrillator, electrocaloric cooling systems, and electrical weapon systems [1], [2], [3], [4]. However, the low energy densities of current electrostatic capacitors are far too small to satisfy the miniaturization and high integration of electronic devices. Generally, the energy density (Ue) of dielectric materials are determined by equations: Ue = 1/2ε0εrE2 for linear dielectrics and Ue = ∫EdD for nonlinear dielectrics, where ε0 and εr represent the dielectric permittivities of the vacuum and dielectrics, E is applied electric field and D = ε0εrE is electric displacement [5], [6], [7]. Accordingly, the enhancement of εr is strongly desired for the improved Ue and various strategies have been proposed to achieve high dielectric (high-k) materials. Currently, constructing polymer composites consisting of ceramic fillers (such as BaTiO3 [8], [9], SrTiO3 [10], [11], (Ba, Sr)TiO3 [12], [13], NaNbO3 [14], [15], CaCu3Ti4O12 [16], [17], TiO2 [18], [19], Na2Ti6O13 [20], [21],BN [22], [23] etc.) or conductive fillers [24], [25], [26], [27] dispersed in insulating polymer matrix is believed to be promising strategies. In these composites, the high dielectric permittivities are usually originated from two aspects: (1) the elevated effective dipole strength of the composites because of the high inherent dipole strength of the ceramic fillers according to the Effective Medium Theory [28], [29]; the interfacial polarization and equivalent micro-capacitor effects at the filler/matrix interfaces as a result of the distinct dielectric permittivity (or conductivity) contrast between the matrix and fillers [30], [31], [32].
In recent years, extensive researches have been carried out to construct ceramic/polymer high-k composites. Hao et al. [33] introduced 6.9 nm BaTiO3 nanocrystals into the poly(vinylidene fluoride-co-hexafluoro propylene)(PVDF-HFP) polymer matrix. It is demonstrated that the composite with 70 vol% BaTiO3 exhibits a significantly enhanced dielectric permittivity of ~33@10 kHz, which is almost 4 times that of the polymer matrix. Wei et al. [34] reported a design of BaTiO3/epoxy composites consisting of p-aminothiophenol (PATP) modified BaTiO3 nanoparticles hosted in epoxy matrix. An obvious enhanced dielectric constant of 18 @10 kHz, which is 6 times that of the epoxy matrix, was achieved when the BaTiO3 loading fraction reached 30 vol%.
So far, enhanced dielectric permittivities of the ceramic/polymer composites are usually achieved at the expense of high loading fractions of ceramic fillers, which may result in deteriorated breakdown strength, elevated loss, and poor mechanical properties [35], [36], [37], [38]. As an alternative, Ávila et al. [39] designed a class of composites composed of electrospun BaTiO3 nanofibers instead of nanoparticles dispersed in epoxy resin. It was demonstrated that the 1-dimensional morphology of the BaTiO3 nanofibers can facilitate their mutual interconnection, hence the formation of BaTiO3 networks. Consequently, a dielectric constant of 12@10 kHz, which is three times that of the epoxy matrix, was obtained in the composite with merely 2 vol% BaTiO3 nanofibers. Although using one (or two) dimentional nanofillers could be an effective way to achieve high permittivity at low filler loadings, the enhancement of permittivity is still limited. In this regard, Luo et al. [40] constructed composites composed of three dimensional BaTiO3 networks hosted in epoxy, and ultra-high permittivities of 200 and 34.5 at 1 kHz were successfully achieved in the composites with 30 vol% and 16 vol% BaTiO3, respectively. In addition, the breakdown strengths of the composites were further improved when the epoxy matrix was replaced by the mixture of epoxy and PVDF [41]. Recently, Feng et al. [42] theoretically demonstrated, through coarse-grained molecular dynamics simulations, that constructing ceramic frameworks in polymer matrix is an effective way to achieve high dielectric permittivity at very low particle loadings. Herein, a unique design of hollow-structured BaTiO3 frameworks was fabricated by electrophoretic deposition combined with high temperature sintering and epoxy resin infiltration, as shown in Fig. 1. The BaTiO3 framework possesses a hierarchical porous structure, including the macro-pores formed by interconnected BaTiO3 tubes and the micro-pores between the BaTiO3 particles. The hierarchical pores provides huge BaTiO3/epoxy interfaces, hence the intensified interfacial polarization and improved dielectric permittivities. It is demonstrated that the epoxy composite with 18 vol% 3D-BaTiO3 exhibits a high dielectric permittivity of 126 @10 kHz, which is 31 times larger than that of the epoxy resin (εr = 4 @10 kHz). Meanwhile, a low loss tangent of 0.045 @10 kHz is maintained. Furthermore, the composite with merely 5 vol% BaTiO3 achieves an obviously improved energy density (Ue = 0.15 J/cm3 at 350 kV/cm) and well retained high charge–discharge efficiency (η = 85.8%) in comparison with the epoxy matrix (Ue = 0.06 J/cm3 at 350 kV/cm, η = 89.7%). This work offers an effective way to achieve simultaneous high permittivity, low loss and high energy density in polymer composites with ultralow filler loadings.
Section snippets
Materials
Nickel foam (350 g/m2, Hefei Kejing Material Technology Co., Ltd.), barium titanate (BaTiO3, 300 nm, >99.5%, Shandong Sinocera Functional Material CO., Ltd.), poly(ethylenimine) (PEI, M.W. 10000, 99%, Aladdin Industrial Corporation), isopropyl alcohol (≥99.7%, Sinopharm Chemical Reagent Co., Ltd.), anhydrous ethanol (≥99.7%, Sinopharm Chemical Reagent Co., Ltd.), acetone (≥99.7%, Sinopharm Chemical Reagent Co., Ltd.), epoxy resin Epon828 (Hexion Specialty Chemicals. Curing agent),
Morphology and composition characterization
Fig. 2 shows the SEM morphologies and elemental mapping images of the samples at different preparation steps, i.e., EPD & sintering, etching, and epoxy infiltration. As can be seen from Fig. 2a and d, the surface of the Ni foam was uniformly coated with a shell layer of BaTiO3 after the EPD & sintering treatment. Then, after removing the nickel foam, a three dimensional hollow-structured BaTiO3 framework with compact inner surfaces and porous outer surfaces was successfully obtained as
Conclusion
In summary, a unique design of epoxy composites based on hollow-structured BaTiO3 frameworks is developed and the dielectric performance is explored. It is demonstrated that the epoxy composite with 18 vol% BaTiO3 exhibits a high dielectric permittivity of 126 @10 kHz, which is 31 times larger than that of the epoxy resin. Meanwhile, a low loss tangent of 0.045 @10 kHz is maintained. Moreover, the composite with merely 5 vol% BaTiO3 displays a greatly improved energy density of 0.15 J/cm3 (at
CRediT authorship contribution statement
Jie Yang: Conceptualization, Methodology, Validation, Investigation, Data curation, Writing - original draft. Xiaotong Zhu: Validation, Investigation. Huanlei Wang: Resources. Xin Wang: Supervision, Resources. Chuncheng Hao: Supervision, Resources. Runhua Fan: Writing - review & editing, Supervision. Davoud Dastan: Resources. Zhicheng Shi: Writing - review & editing, Methodology, Resources, Supervision.
Declaration of Competing Interest
All 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. The authors have read the paper and confirm that this manuscript is the authors’ original work and has not been published nor has it been submitted simultaneously elsewhere.
Acknowledgements
The authors acknowledge the financial support of this work by National Natural Science Foundation of China (51773187, 51402271), Foundation for Outstanding Young Scientist in Shandong Province (BS2014CL003).
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