Dielectric property improvement of ATO particles with narrow size distribution in ATO/PI composite films
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High permittivity antimony-doped tin oxide (ATO)/polyimide (PI) composite films consisting of narrow size distribution ATO fillers prepared by inverse microemulsion method and PI host are synthesized by in situ polymerization. The microstructure and thermal stability of composite films are characterized by scanning electron microscopy and thermal gravimetric analyses, respectively. Dielectric properties of composite films with different concentrations of ATO particles of variable size are investigated in the frequency range of 102 to 2.5 106 Hz. The hydrophilic surface of ATO is not helpful of tight connection between the filler and host. The addition of ATO contributes slight increase of the thermal stability. However, the permittivity of composite films can be remarkably increased due to Maxwell–Wagner–Sillars polarization as well as a large number of tiny capacitors formed by ATO particles with narrow distribution and small size. The dielectric constant behavior of composite films fits well to the usual percolation theory. I. INTRODUCTION
High-performance dielectric polymeric films and coating materials have been widely used as circuit boards, high-performance capacitors, gate dielectrics, and power storage devices in electronic applications.1–3 Aromatic polyimide (PI) is a kind of classic polymer matrix for its high thermal stability, excellent mechanical properties, and good chemical resistance. Based on the percolation theory, a great deal of work has been carried out to get high permittivity composites by embedding high dielectric permittivity ceramic particles,4–7 metal oxides,8–11 or different conductive fillers12–16 into polymer. When the volume fraction of the filler is very close to, but does not exceed, the percolation threshold, a great enhancement in electrical conductivity and dielectric permittivity will be observed near the insulator–conductor transition. In general, the sharp increase in conductivity is either caused by the formation of conducting path network or by the tunneling conduction.17,18 Among the efforts to prepare high dielectric constant composites, much work has been focused on the preparation of ceramic–polymer composites. But the dielectric constant of ceramic–polymer composites can only be enhanced obviously by loading with high concentration of fillers, such as BaTiO3,7 LaTiO3,4 NiO,5 and CaTiO3,6 which can lead to a sharp decrease of their mechanical properties. However, the dielectric constant is just enhanced a little when metal oxides, e.g., SiO2,9 TiO2,10 and Al2O3,11 are embedded.
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2012.174 J. Mater. Res., Vol. 27, No. 19, Oct 14, 2012
Recently, more and more studies focus on conductive fillers, especially on carbon nanotube,14 graphite,13 and metal fillers, such as Ag12 and Ni.16 Generally, obtaining inorganic fillers with super high dielectric constant is the key to give high dielectric constant composites. Therefore, preparation of high dielectric constant inorganic
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