Polyvinyl butyral composites containing halloysite nanotubes/reduced graphene oxide with high dielectric constant and low loss

https://doi.org/10.1016/j.cej.2020.124910Get rights and content

Highlights

  • HNTs@rGO hybrid microstructures were prepared via controllable electrostatic self-assembly.

  • HNTs@rGO microstructures exhibited good dispersion in the PVB composites.

  • The composites showed high dielectric constant and low dielectric loss.

  • The mechanism of the enhancement in dielectric performance was systematically discussed.

Abstract

Polymer-based composites with high dielectric constant and low loss are highly desirable due to their inherent advantages of easy processability, flexibility, and lightweight. Herein, a functional nanofillers, halloysite nanotubes (HNTs) decorated reduced graphene oxide (rGO) hybrid microstructures (HNTs@rGO) was successfully prepared via controllable electrostatic self-assembly and in-situ heat reduction method. These hybrid microstructures combine characteristics of natural 1D ceramic nanotubes with large aspect ratio and high electric conductivity of rGO micro-sheets, which provided ideal material collocation in the construction of microcapacitors. The HNTs not only effectively prevented direct contact between the rGO micro-sheets in the composites but also played an important role in forming dielectric interface within microcapacitors. Consequently, an HNTs@rGO/polyvinyl butyral (PVB) composites containing a very low content of 5 wt% rGO exhibited an ultra-high dielectric constant of 150 and an extremely low loss of 0.12 at 103 Hz. It is believed that the unique characteristics and facile fabrication process of HNTs@rGO/PVB composite make it a potentially excellent candidate for flexible polymer-based dielectric materials applied in the capacitor fields.

Introduction

Flexible materials with high dielectric constant and low dielectric loss (tanδ) play an important role in the fabrication of thin-film capacitor and wearable devices in the modern electronic field [1], [2], [3], [4], [5]. Polymer-based composites combining intrinsic advantages from both polymer and inorganic nanoparticles possess great potential in the industrial field of dielectric materials because of their excellent flexibility, easy processability, lightweight and low cost [6]. It has been widely investigated that polymers filled with conductive nanoparticles such as metal powders, carbon nanotubes, graphenes, MXenes can achieve very high dielectric constant, but these materials are inevitably accompanied by a huge increase of dielectric loss due to the formation of conductive networks between nanoparticles, which is termed as a percolative phenomenon [7], [8], [9], [10].

Some studies indicated that introducing ceramic or polymer coating layers onto the surface of conductive fillers can suppress the current leakage and reduce the dielectric loss of the materials. Shi et al. [11] fabricated reduced graphene oxide-encapsulated barium titanate (BT-RGO) hybrid filled polyvinylidene fluoride (PVDF) composites. The composites filled with 40 vol% BT-RGO showed suppressed dielectric loss of 0.25 (103Hz) compared with RGO/PVDF (tanδ > 10). However, the relatively low dielectric constant (67.5) and deterioration of mechanical properties caused by high filler loadings (40 vol%) hindered its further application. Wan et al. [12] prepared another sort of barium titanate coated graphene (BT@TGO) nanofillers using a sol-gel method combined with the thermal treatment process. The obtained BT@TGO/PVDF materials showed a low dielectric loss of 0.16 at 103Hz as the filler content reached 9.5 wt%, which implied a suppressed current leakage due to the presence of BT coating layers. However, the dielectric constant of the composites was just 35 which cannot meet the performance requirements of dielectric functional materials with high dielectric constant (>100) in the modern electronic field. Li et al. [13] synthesized polydopamine coated reduced graphene oxide nanoparticles (PDA@RGO) through self-polymerization of dopamine onto graphene oxide (GO) and subsequent chemical reduction process. The dielectric constant of PVDF with 0.70 wt% PDA@RGO increased to 176 (103Hz), about 17 times of neat PVDF, and the loss tangent was suppressed to 0.337 due to reduction of the concentration and mobility of ionizable carboxylic groups affected by PDA. Unfortunately, the reduction process in this study was complicated and not environmentally friendly so that it was not appropriate for industrial production. Besides the examples illustrated above, an increasing number of strategies have been developed more recently to prepare high-performance dielectric materials by incorporating “core-shell structural conductive nanofillers” into polymer matrix [14], [15], [16], [17], [18]. One benefit of this sort of nanofillers was that the outer insulating shell can effectively prevent the current leakage caused by the formation of conductive networks which was the main source of dielectric loss, however, the disadvantage was that the non-conductive shell completely suppressed the charge mobility, prohibiting the formation of a large number of microcapacitors. For such a system, the contribution to the dielectric constant only came from the strengthened average electric field effect based on Maxwell-Wagner theory [18], so that the dielectric constant of the material will be much lower than that filled with normal conductive fillers. Besides, some of the ceramic coating layers usually presented poor compatibility with polymer matrix because of the lacking functional groups on the surface, thus the composites will suffer a deterioration of mechanical and dielectric properties caused by defects, and the overall performance was hardly as perfect as pre-designed.

Based on the above discussion, a novel dielectric composites combining merits from both halloysite nanotubes (HNTs) and reduced graphene oxide (rGO) micro-sheets was prepared in this paper. Herein, we chose PVB as the matrix because it was a soft polar polymer with many dipole groups, which had been widely used as security glass interlayer materials but rarely studied in electronic field [19], and HNTs as nanofillers to decorate rGO because it was a low-cost ceramic nanotubes which possessed good dielectric properties [20]. In this work, the HNTs@GO micro-sheets were firstly synthesized and then reduced to HNTs@rGO in the PVB matrix under mild heat processing. The HNTs not only prevented direct contact between the rGO micro-sheets in the composites but also acted as high dielectric ultrathin barriers, facilitating the formation of a great number of microcapacitors with large capacitance. Based on these designs, excellent dielectric property of HNTs@rGO/PVB composites with HNTs@rGO fillers (containing 5 wt% rGO) was achieved (ɛ′ = 150, tanδ = 0.12, 103Hz), which was superior to that of ceramic or polymer-coated conductive nanoparticles filled composite reported in previous research. Meanwhile, the PVB matrix provided the functional composites with desired processing performance and flexibility, which was important to the application of dielectric functional materials. The mechanism of the synergistic effect of rGO and HNTs was also investigated in this paper.

Section snippets

Materials

Polyvinyl butyral (PVB) (Mw = 50,000–80,000) was purchased from Chang Li Products & Chemical Co., China. Halloysite clay nanotubes (HNTs, average outer diameter: ~40 nm, length: 200 nm–1 μm) were obtained from Yanbo Mineral Processing Co., China. Graphene oxide (GO, average sheet size is 1~2 μm) was produced by Xianfeng Nanomaterials Tech. Co., China. γ-aminopropyl triethoxysilane (APTES, 98%) was purchased from Sigma-Aldrich., USA. Ethanol, Toluene and concentrated hydrochloric acid were

The microstructure and morphology of HNTs@GO micro-sheets

It has been demonstrated that nano or microparticles with abundant hydroxy groups on the surface are easily negatively charged in water circumstances due to proton effect [23]. Besides, the hydroxy groups on the outmost surface can be modified by other chemical substances which is beneficial to further functionalization of nanoparticles. An electrostatic self-assembly method was operated to synthesize the HNTs@GO micro-sheet. The hydroxy-rich HNT nanoparticles were firstly modified by reacting

Conclusion

A flexible HNTs@rGO/PVB functional composites were prepared by using the in-situ heat reduction method. The HNTs were adsorbed on the surface of graphene oxide and the micro-sheets exhibited a good integrity. Compared with traditional polymer materials with ceramic coated conducting fillers, the 10 wt%HNTs@5wt%rGO/PVB composites possessed an ultra-high dielectric constant (ε′) of 150 (103Hz) with a low filling content of 5 wt% rGO. Furthermore, the dielectric loss of the composites

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.

Acknowledgment

The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51873011 and No. U1664251).

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