Hydrangea-like zinc oxide superstructures for ferroelectric polymer composites with high thermal conductivity and high dielectric constant

Abstract

Polymer composites with high thermal conductivity and high dielectric constant are highly desirable in electronic and electric industry, and particularly, for power apparatus at high voltages. In this work, a novel hydrangea-like ZnO superstructure was prepared by a template-free solvothermal method. Polyvinylidene fluoride (PVDF) composites filled with the ZnO superstructure were prepared via a solution mixing method. The microstructure, thermal conductivity, thermal stability and dielectric properties of the composites were investigated. It was found that the hydrangea-like ZnO shows marginal influence on microstructure of the PVDF matrix, but has significant enhancement effects on thermal conductivity, thermal stability and dielectric constant of the composites. Compared with the commercial ZnO nanoparticles, the hydrangea-like ZnO superstructures result in much higher enhancement of thermal conductivity and dielectric constant and slightly lower breakdown strength of the composites. This has been ascribed to the formation of percolation-like structure in the hydrangea-like ZnO composites.

Introduction

In recent years, high dielectric constant (high-k) polymer composites have drawn much interest because of their ease of processing and the huge potential applications to energy storage and dielectric materials [1]. On the other hand, the continuing minimization and ever-increasing output of consumer products inevitably lead to the large increase of power dissipation as well as significantly enhanced thermal fluxes [2], [3]. Therefore, in order to guarantee high performance and high reliability of the electronic devices and power apparatus, an ideal high-k material should simultaneously has a high heat transport capability for thermal management requirement. From this point of view, it is therefore highly desirable to develop the polymer composites with high dielectric constant and high thermal conductivity [4], [5], [6], [7].

So far, two methods have been usually applied to fabricate high-k polymer composites. One is adding electrically conductive fillers (e.g. graphene and carbon nanotube) into a polymer to achieve percolative systems [8], [9], [10], [11], [12], [13]. Although significantly enhanced dielectric constant can be observed when the filler content is close to the percolation threshold, the increase of thermal conductivity is usually marginal and the dielectric loss tangent of the resulting composites is usually extremely high [2], [14], [15], [16]. An alternative approach is introducing electrically insulating ceramic particles with high intrinsic dielectric constant (e.g. BaTiO₃ [17], [18]). However, the resulting composites usually do not exhibit large enhancement in thermal conductivity because of their low intrinsic thermal conductivity [19], [20]. Moreover, high loading levels (>50 vol%) are often required in order to achieve a high dielectric constant, which in return deteriorates the mechanical and processing properties of the composites [21]. Therefore, there are still some challenges in the design and preparation of high-k polymer composites with high thermal conductivity.

Zinc oxide (ZnO) is a unique semiconductor with photoelectric, pyro- and piezoelectric multiple properties, and is frequently used in photonic and electronic fields [22], [23], [24]. Recently, ZnO has also evoked considerable interest for polymer composites because of its semiconductive feature [25], [26], [27]. Despite the high potential of ZnO in increasing the dielectric constant of polymer composites, the dielectric properties of ZnO based polymer composites are closely associated with the filler shape. It has been reported that, at relatively low loading levels, the polymer composites filled with the sphere-like ZnO filler only show moderately enhanced dielectric constant [28], whereas the ZnO filler with a high aspect ratio or superstructure can result a significant increase of dielectric constant [29], [30]. On the other hand, the thermal conductivity enhancement of polymers by sphere-like ZnO has been well known since ZnO has an intrinsic high thermal conductivity of about 60 W m⁻¹ K⁻¹. Recently, ZnO nano-pyramidal particles were also used to enhance the thermal conductivity of polyimide by forming a so-called vertical double percolation structure [26]. In spite of this, there has not been a report on the preparation of ZnO superstructure based polymers composites with high dielectric constant and high thermal conductivity.

In this study, by using a novel hydrangea-like ZnO superstructure as filler, polyvinylidene fluoride (PVDF) composites with high dielectric constant and high thermal conductivity have been successfully prepared. Compared with the composites containing the sphere-like ZnO nanoparticles, the ZnO superstructure based composites show much more enhanced dielectric constant and thermal conductivity.