Full Length ArticleEnhanced energy density and thermostability in polyimide nanocomposites containing core-shell structured BaTiO3@SiO2 nanofibers
Graphical abstract
Core-shell structured BT@SiO2/PI nanocomposite films with enhanced breakdown strength and energy density were synthesized via the solution casting method.
Introduction
Dielectric capacitors with a high energy density and ultrafast charging-discharging response have been widely used in advanced electronic and electrical power system [1], [2], [3], [4], [5]. Compared to ceramic counterparts, polymer capacitors are more applicable in higher electric fields because of their high electric breakdown field, low dielectric loss, flexibility and easy processing [6], [7], [8]. However, most polymer capacitors are limited to relatively low working temperatures, and fail to meet the further development on electricity under the extreme conditions. For instance, the best commercial polymer, biaxially oriented polypropylene (BOPP) (≈1.2 J cm−3 at 640 kV/mm) can operate only at temperatures below 105 °C. Therefore, polymer-based dielectrics with high operating temperature are desirable for applications [9], [10], [11]. Various high-performance engineering polymers have been studied as high-temperature resistance dielectric materials. Polyimides (PI) were one of the most studied polymers because of its high glass transition temperature (Tg) and thermal stability. In addition, the dielectric loss is 0.001–0.03, such a low dielectric loss of the PI is excellent for practical applications. Thus, PI is suitable for use in a dielectric field as a high-temperature resistant film capacitors. Our group demonstrated that the energy density of PI nanocomposite films with a sandwich structure can be significantly improved to 1.95 J cm−1 [12].
The energy density (Uc) is determined by the electric displacement (D) and applied electric field (E) of the dielectrics as:
The electric displacement (D) is related to relative dielectric constants (εr) and the polarization (P) by D = P +εrε0E, where ε0 is the vacuum permittivity. For liner dielectrics, Uc = 1/2εrε0Eb2, where Eb is the breakdown strength of the dielectrics. Thus, the effective approach to increase energy density is to enhance the breakdown strength or to increase the dielectric permittivity. Usually, introducing high dielectric constant ceramic nanoparticles into the polymer matrix, such as TiO2, Al2O3, BaTiO3 and CaCu3Ti4O12 [13], [14], [15], [16], [17], [18], is an effective method to increase the dielectric permittivity of polymer nanocomposites. However, increased dielectric constant is usually achieved by the high loading of ceramic nanoparticles and leads to a dramatically decreased breakdown strength of polymer nanocomposites. Theoretical studies and experimental practice have found that one-dimensional (1D) fillers of large aspect ratios, such as TiO2 nanofibers, BaTiO3 nanofibers [19] and (Ba1-xSrx) TiO3 nanofibers [20], [21], [22], [23], can not only increase dielectric permittivity but also enhance breakdown strength at low concentrations (1–5 vol%). The improvement could be attributed to the large aspect ratio and small specific surface areas of the 1D nanofillers, which can reduce the surface energies and prevent them from aggregation in the polymer matrix. However, the breakdown strength of nanocomposites with high loading of 1D nanofillers still decreased because of the more structure imperfections and electric field concentration exist at the interface between the ceramic nanofillers and the polymer matrix.
It has been widely accepted that interfacial polarization is the primary mechanism of polarization in polymer nanocomposites. Modulation of interfacial polarization through core-shell strategies was proved to be versatile tools to solve the well-known paradoxes, such as high dielectric constant usually suffer from the high dielectric loss and low breakdown strength [24], [25], [26], [27]. The core-shell strategies enable the disappearance of electrical percolation transition in polymer composites filled with electrically conductive fillers, resulting in high dielectric constant, low dielectric loss and high energy density. Recent studies [28], [29], [30], [31], [32] showed that the nanofibers with moderate inorganic interfacial layers have effectively increased the energy density of polymer nanocomposites. For example, Zhang et al. [33] found an energy density of 31.2 J cm−3 in the polyvinylidene fluoride (PVDF) nanocomposite with TiO2 nanofiber embedded BaTiO3 nanoparticles. Lin et al. [34] showed that a large discharged energy density of 10.94 J cm−3 was achieved for the PVDF nanocomposite film with 3% volume fraction of BaTiO3@TiO2-nf. Pan et al. [35] demonstrated that the discharged energy density of PVDF nanocomposite film cound be significantly improved to 12.18 J cm−3 by the incorporation of BaTiO3@Al2O3. The reason of the greatly enhancement in energy density is that Al2O3 layer coating on BT nanofibers surface can suppress the leakage current and reduce the interfacial polarization. However, PVDF exhibits poor stability over a wide range of temperature due to its low Tg [36] and PVDF usually suffers from high dielectric loss, which can dissipate a fraction of their stored energy in the form of heat. Therefore, PI was selected as the polymer matrix in this paper for its high Tg and low dielectric loss.
Herein, a core-shell structure of BaTiO3@SiO2 (BT@SiO2) nanofibers has been prepared via electrospinning and the stöber method. The PI nanocomposite films consisting of the core-shell structured BT@SiO2 nanofibers and PI have been successfully synthesized by the solution casting method (Scheme 1). In the BT@SiO2/PI nanocomposite films, the dielectric permittivity as well as the breakdown strength increase significantly. The thin SiO2 layers have been employed to isolate the high dielectric loss tangent from BT nanofibers, since SiO2 has an ultra-low dielectric loss (0.00002). The SiO2 shell layers forming on the surface of BT nanofibers mitigate the local field concentration induced by the large mismatch between the dielectric permittivity of BT and PI. As a result, the PI nanocomposite film filled with 3 vol% BT@SiO2 nanofibers exhibits a maximal energy density of 2.31 J cm−3 under the field of 346 kV/mm, which is 62% over the pristine PI (1.42 J cm−3 at 308 kV/mm) and about 200% greater than BOPP (≈1.2 J cm−3). The thermogravimetric analysis (TGA) results indicate that the BT@SiO2/PI nanocomposite films have good thermal stability below 500 °C.
Section snippets
Raw materials
Pyromellitic dianhydride (PMDA, AR), 4, 4′-diamino diphenyl ether (ODA, AR), 1-methyl-2-pyrrolidinone (NMP, AR), acetic anhydride (AR), ethanol (AR), acetic acid (AR) and phosphorus pentoxide (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Tetraethoxysilane (TEOS, 99.0%), polyvinylpyrrolidone (PVP, K88-96), tetrabutyl titanate (TNBT, 99.0%) and barium acetate (AR) were purchased from Shanghai Aladdin Industrial Inc. PMDA and ODA were recrystallized from acetic
Microstructure and characterization of BT@SiO2 nanofibers
The BT nanofibers were successfully prepared through the electrospinning method and coated with SiO2 via stöber method. The SEM and TEM images of the BT nanofibers and BT@SiO2 nanofibers were shown in Fig. 1. The ceramic BT nanofibers obtained from the calcined as-electrospun have a high aspect ratio, with diameters of 200 nm and lengths of a few tens of μm. The core-shell structure of the BT@SiO2 nanofibers has been obviously observed from the TEM. Fig. 1(d) exhibited the core-shell structure
Conclusions
In summary, the core-shell structured nanofibers are effective for increasing the energy density of polymer nanocomposite. The PI nanocomposite films consisting of 1D core-shell structured BT@SiO2 nanofibers have been successfully prepared by the solution casting method. The thin SiO2 layers have been employed to isolate the high dielectric loss tangent from BT nanofibers, since SiO2 has an ultra-low dielectric loss and the SiO2 shell layer could effectively mitigate the local field
Acknowledgements
This study was financially supported by National Natural Science Foundation of China (21304018 and 21374016) and Jiangsu Provincial Natural Science Foundation of China (BK20130619 and BK20130617).
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