Preparation And Dielectrc Properties Of Hollow Sphere Type HS-BST/PVDF Functional Composites

BST/PVDF functional composites are expected to be desirable materials for tunable electric devices,microwave phase shifters and flexible wearable devices due to their appropriate dielectric constant,low dielectric loss and high dielectric tunability.There are still challenges like the unclear effects of BST powders with different shape and size on the dielectric properties of BST/PVDF composites,although many researchers have devoted themselves to develop this kind of composites.In this work,0-dimensional BST hollow spheres(HS-BST)were synthesized by hard template method and HS-BST/PVDF functional dielectric composites were fabricated by tape casting process.Effects of particle size and shell thickness of HS-BST on dielectric properties of HS-BST/PVDF composites were studied.The relationship between HS-BST powders and shape factor n was caculated by the dielectric tunability theoretical model of inorganic/organic composites.Firstly,the templates of monodispersed carbon spheres were prepared by hydrothermal method.A two-step coating process was employed to synthesize the C@BST hybrid spheres and the pure perovskite BST powders with evident hollow structure were obtained by calcining the C@BST hybrid spheres.The particle size and shell thickness were controlled by the concentration of metal ion and the size of carbon spheres.The effects of the coating temperature,coating time and titration speed on the morphorlogy of C@BST composite spheres,as well as the formation mechanism of HS-BST by hard template method,were investigated.Secondly,the microstructure and dielectric properties of HS-BST/PVDF composites with micro-and nano-sized HS-BST powders were investigated.It is demonstrated the uniform dispersibility of HS-BST powders in the PVDF polymer matrix.The interface compatibility between HS-BST and PVDF matrix were gradually strengthened with the particles size decreasing.In addition,with the decreasing particle size and shell thickness,the dielectric constant and dielectric loss decreased.The dielectric breakdown strength and energy storage density decreased at first and then increased.The dielectric constant and dielectric loss of HS-BST/PVDF composites showed a strong relaxation character.Finally,the shape factor of HS-BST was calculated by the dielectric tunability theoretical model of organic/inorganic composites.The shape factor n in the model is 2~2.5,corresponding to the 0-dimensional HS-BST particle size range from 0.44 μm to 2.57 μm.