Full Length ArticleEnhancing breakdown strength and energy storage performance of PVDF-based nanocomposites by adding exfoliated boron nitride
Graphical abstract
Introduction
During the past decade, polymer/ceramic high-k nanocomposites have attracted increased research interests for their great potential in application of energy storage units, such as high-power pulse, laser gun and electromagnetic ejection equipment [1], [2], [3], [4], [5]. Despite of the great success achieved in high dielectric constant, many scientific and technological challenges are still unsolved for their practical applications as high energy storage dielectrics [6], [7], [8], such as high-pulse capacitors. Among them, the rather high energy loss (Ul) and low breakdown strength (Eb) are the biggest obstacles in front of dielectric composites for high-field applications. As suggested by , where Ue is the stored energy density of the dielectrics, εr and ε0 are permittivity of dielectrics and vacuum, respectively. It is obvious that sufficient large εr and Eb are crucial to achieve high energy density in the composites. Poly(vinylidene fluoride) (PVDF) based ferroelectric polymers are mostly employed as matrix for their rather high and tunable permittivity (5–100) among the known polymers [9], [10]. In addition, a high volume content (e.g. >30 vol%) of high-k ceramic particles is inevitably requested to further increase εr based on either simple tandem and parallel models or the modified ones [11], [12], [13]. However, the addition of inorganic particles with large quantity would form serious structural heterogeneity in the resultant composite, which would seriously damage Eb and cause huge dielectric loss owing to the interfacial polarization. The strong aggregation of particles in nanometer scale and the poor compatibility between two different materials are the major reasons.
To improve the dispersing homogeneity and the compatibility of ceramics in polymer matrix, the most popular and effective way is modifying the surface of ceramics with organic moieties by means of surface grafting polymers, such as attaching silicon-based surfactants and constructing “core–shell” structures [14], [15]. More recently, ceramic particles with higher aspect ratio, such as 1D BaTiO3 fibers, barium strontium titanate (Ba0.7Sr0.3TiO3) nanowires, and 3D BaTiO3 network were fabricated by the aid of hydro-thermal and electro-spinning methods [16], [17], [18], [19]. Exciting results including high dielectric constant and improved Ue have been thus achieved for enhanced compatibility of the two phases and reduced filler loadings in the objective composites. However, the improvement on breakdown resistance and discharge efficiency could hardly be achieved, and the heterogeneous structure of composite dielectrics due to high particle loadings could not be solved as well.
As a matter of fact, high loading content of ceramic particles results into less significant improvement onto the Ue but inevitable high Ul along with reduced Eb, especially under high electric field. Therefore, in order to balance the confliction between high εr and Eb, more recently, more attentions have been turned to the composites filled with lower particle content, i.e. <10 wt%, to ensure the high Eb for its dominant role in achieving high Ue. The previous study has indicated that when the particle content is less than 10 wt%, excellent comprehensive performances, including enhanced Eb, suitable εr and good flexibility, could be realized in the composites simultaneously [20]. Besides, the well-known Stark-Garton equation () clearly indicates that Eb of the material is proportional to its mechanical strength and can also be improved by raising the modulus [21].
In our previous work, a series of PVDF/BaTiO3 binary composites with low particle content (<10 wt%) was finely prepared. Although greatly enhanced Eb and Ue values were achieved after matrix cross-linking and particle modification, the composite dielectrics resulted in a Eb of ∼250 MV/m and Ue of ∼3.0 J/cm3 owing to its high dielectric loss (∼0.20) and relatively large conductivity (∼10−6 S/cm) [20]. To further improve the energy storage capability of the composite, in present work, two strategies were adopted as follows. First, PVDF-based polymer matrix with semi-crystallized structure is chosen to realize a higher modulus for the composites. Secondly, inspired by the recently published results [22], [23], [24], boron nitride nano-sheets (BNNS) exfoliated from BN particles with high bulk electrical resistivity (1013 Ω⋅cm) are introduced to enhance Eb of above-mentioned binary composite. By comparing the structure and dielectric properties of the two sets of composites, the effects of BNNS on the reduced dielectric loss, the improved breakdown strength, and the increased energy storage properties are finely addressed.
Section snippets
Materials
Boron nitride (h-BN, 100 nm, 99.9% purity) power was purchased from Naiou Nano-technol. Co. Ltd. (Shanghai). BaTiO3 powder with particle size of 100 nm was supplied by Shandong Sinocera Functional Material Co. Ltd. (99.9%, China). Tris(hydroxymethyl)aminomethane (Tris) buffer solution (pH = 8.5) was purchased from Chengdu Best Reagent Co. Ltd. (AR grade, China). Dopamine was purchased from Aladdin Chemicals Co. Ltd. (99%, China). N, N-dimethyl formamide (DMF) and anhydrous ethanol were provided
Morphology of BN before and after exfoliation
Fig. 1 showed typical TEM images of BN particles before and after exfoliation. The pristine BN particles possess a multi-layered structure with dark-gray color for the large thickness. Exfoliation leads to significantly reduced stacking layers of BN particles exhibiting light-gray color as indicated in Fig. 1b. In an effort to assess the exfoliation process of BN particles, AFM imaging was performed at multiple locations across the sample. The absolute thickness of BN particles before and after
Conclusion
To overcome the large dielectric loss induced by interfacial polarization of PVDF/BT composites, BN nano-sheets were introduced as insulating fillers. Thanks for the 2D structure in nano size, the high bulk electrical resistivity and the fine dispersion in PVDF of BNNS, the interfacial polarization between PVDF and BT is successfully depressed. Thus, the breakdown strength and energy storage capability of ternary polymer nanocomposites are dramatically enhanced comparing to the corresponding
Acknowledgment
The authors are grateful for the support and funding from National Nature Science Foundation of China (Grant Nos. 51773164, 51573146), Aeronautical Science Foundation of China (Grant No. 2016ZF53054), Natural Science Basic Research Plan in Shaanxi Province of China (Grant Nos. 2015JZ009, 2016JQ2010) and Fundamental Research Funds for the Central Universities (XJJ2016063).
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