Fabrication, structure, and property of epoxy-based composites with metal-insulator core-shell structure fillers
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iO2 core–shell structure nanoparticles prepared by chemical method were dispersed into epoxy matrix. By comparing with the epoxy-based composites filled with the mixed Ag and SiO2 nanoparticles (Ag 1 SiO2), it is found that the Ag@SiO2 core–shell structure fillers had important effects on the improved dielectric properties of the Ag@SiO2/epoxy composites. The core–shell structure fillers introduce a duplex interfacial polarization and a small number of free charge carriers, which enhance the dielectric permittivity of the composites. At the same time, the insulating SiO2 shell layer changes the interfacial interaction between the Ag filler and the epoxy matrix, not only avoiding Ag particles to connect directly and aggregate together but also providing a rough surface to contact with the epoxy host, which enhances the compatibility between the Ag@SiO2 fillers and the epoxy matrix. As the Ag@SiO2 packing ratio increases, the permittivity of the composites straightly increases and the loss tangent decreases, reaching the maximum and minimum respectively with the filler loading up to 60%.
I. INTRODUCTION
For polymer-based dielectric composites, both high permittivity and low dielectric loss are desired for practical applications.1 The conductor/polymer composites have attracted intensive interests in the last few years because of the potential applications in many electrical devices with advantages of high permittivity, lightweight, and easy processing.2–9 However, the composites possess an insulator–conductor transition as the concentration of the conducting fillers approaches to the percolation threshold, which is called the percolation effect.10–12 The conductive fillers in conductor/polymer composites not only increase the permittivity obviously but also inevitably induce plenty of free charge carriers and cause the dramatic increase in the leakage current and dielectric loss, which limits their applications in many fields.13 To avoid dramatic increase of dielectric loss when adding more conductive fillers in conductor/polymer composites, insulator/semiconductor barrier layers coated around the conductor fillers are introduced, which prevent conducting fillers from connecting with each other and forming continuous conducting paths.14–16 Composites with metal–insulator core–shell structure fillers show high permittivity and low dielectric loss. In which, the metal cores increase the permittivity owing to the interfacial polarizations, and the insulator shells are used to control the dielectric loss a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.248 2644
J. Mater. Res., Vol. 28, No. 18, Sep 28, 2013
http://journals.cambridge.org
Downloaded: 11 Mar 2015
effectively, acting as barrier layers through blocking the electron transfer between the adjacent metal cores.17 So the composites filled with different core–shell structure fillers (Co@ZnO,18 Ag@TiO2,19 Al@Al2O3,16 Ag@C,14 and Ag@SiO220) have been prepared by different methods, and their dielectric properties are significantly
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