Barium titanate coated and thermally reduced graphene oxide towards high dielectric constant and low loss of polymeric composites

Abstract

Novel barium titanate (BT) layer coated and thermally reduced graphene oxide (TGO) hybrid sheets (BT@TGO) were successfully synthesized by a facile sol-gel method combining with thermal treatment process (600 °C) under nitrogen atmosphere. The BT precursors attached on graphene oxide (GO) sheets were crystallized into perovskite structure with high permittivity and GO sheets were thermally reduced heavily at such high temperature simultaneously. The hybrids were used as filler to fabricate high performance dielectric polyvinylidene fluoride (PVDF) composites by solution blending method and their dielectric performances were studied. It was found that addition of BT@TGO decreases the electrical conductivity when compared with TGO/PVDF composites and pure PVDF, and BT@TGO/PVDF composites exhibit not only high dielectric constant but also low dielectric loss. For instance, at 10³ Hz, the dielectric constant of PVDF composites containing 8.0 wt% BT@TGO is up to ∼56.3 at room temperature, which is over 5 times than that of pure PVDF polymer (∼10.3). More importantly, the dielectric loss is suppressed and only 0.058, which should be attributed to the effective encapsulation of insulating BT layer with high permittivity on the TGO surface. In addition, the improved thermal stability and crystallization behavior of BT@TGO/PVDF composites were also investigated.

Introduction

With the continuing miniaturization and increase of power density in electronic devices, higher and stricter requirements are desired for advanced electronic packaging materials [1]. High dielectric constant (high-k) polymer-based composites exhibit great potential applications in micro-electronics such as embedded capacitor, gate dielectric and electrical energy storage due to their light-weight, good mechanical properties, simple fabrication process and facile processability [2], [3]. Generally, most polymers possess low dielectric constant (typically less than 10) due to the negligible contribution of the ionic component to total dielectric constant [4]. A promising way to increase dielectric constant of polymer is using high permittivity nanoparticles, such as barium titanate (BT) [5] and calcium copper titanate [6] as fillers. However, an effective improvement of the dielectric constant is not observed until high concentration of the filler has been added into the polymer matrix, which will induce severe aggregation and deteriorate the mechanical properties. Moreover, as a consequence of the high loading of particles, the density of polymer composites increases and it suffers from decreased electric breakdown strength [7].

Over the past few years, high-k polymer composites based on electric conductors, such as metallic particle, graphene and carbon nanotube (CNT) [8] have gained significant attention because of the low filler concentration and great possibility to integrate the performance advantages of polymer [9]. Although, polymer composites based on conductor with super high-k have been achieved [10], [11], the high dielectric loss caused by the leakage current is still an enormous challenge, which limits their applications. Therefore, the key issue is to prevent direct contact between conductive fillers to improve the dielectric constant and suppress dielectric loss of the polymer-based composites. A lot of efforts have been made to fulfill the above purpose including surface modification of conductors [12], [13], construction of multi-layered structure with conductor and insulator [9], deposition of metallic particles on the surface of BT [14], [15], and combination of CNT with insulated graphene oxide sheets [16], [17]. In our previous work, a kind of polymer chains were employed to modify the graphene sheets by forming covalent bond, which improves the dispersion of graphene and the interface between sheets and epoxy matrix, thus enhanced mechanical properties and dielectric performances were obtained [13], [18]. However, when the filler concentration is up to 1.8 wt%, the dielectric loss increases to ∼1.0 at 1 k Hz [13].

Most of the previous work focused on surface functionalization and chemical reduction of GO [19], which involves introduction of toxic reduction agents such as hydrazine hydrate. Another effective and eco-friendly method to reduce GO sheets is thermal treatment process, which usually needs 600 °C or a higher temperature to complete the optimal reduction [20]. Unfortunately, graphene sheets modified by organics (polymer chains or small molecules) are unable to withstand such high temperature [12], [21].

Nevertheless, the way of thermally reduced process provides a facile route for decoration of oxides on graphene sheet. To further improve the dielectric performance of graphene polymer composites, i.e., high-k and low dielectric loss, in this study, we provide a simple sol-gel strategy integrating with thermal treatment process (600 °C) to synthesize thermally reduced graphene oxide (TGO) hybrids covered with insulating and high permittivity BT layer (BT@TGO), in which each graphene sheet was decorated with BT layer uniformly and continuously. The thermal treatment process, on one hand, reduces the insulating GO to conductive TGO. On the other hand, it makes the amorphous precursor of BT crystallize into perovskite structure with high-k under the high temperature. Thus, an insulator-conductor-insulator sandwiched 2D nano-structure can be constructed, which is crucial to improve the dielectric constant and suppress the loss of its polymer composites simultaneously. Polyvinylidene fluoride (PVDF) was selected as matrix since it is one of the most extensively studied and attractive polymer-based matrix owing to its fascinating virtues, such as relatively high dielectric constant and thermal properties. The BT@TGO/PVDF composites were fabricated by solution-blending and hot press method. The dielectric and thermal properties of resultant composites were investigated.