Dielectric materials with high electric energy densities and low dielectric losses are of critical importance in a number of applications in modern electronic and electrical power systems. An organic-inorganic 0-3 nanocomposite, in which nanoparticles (0-dimensional) are embedded in a 3-dimensionally connected polymer matrix, has the potential to combine the high breakdown strength and low dielectric loss of the polymer with the high dielectric constant of the ceramic fi llers, representing a promising approach to realize high energy densities. However, one signifi cant drawback of the composites explored up to now is that the increased dielectric constant of the composites is at the expense of the breakdown strength, limiting the energy density and dielectric reliability. In this study, by expanding the traditional 0-3 nanocomposite approach to a multilayered structure which combines the complementary properties of the constituent layers, one can realize both greater dielectric displacement and a higher breakdown fi eld than that of the polymer matrix. In a typical 3-layer structure, for example, a central nanocomposite layer of higher breakdown strength is introduced to substantially improve the overall breakdown strength of the multilayer-structured composite fi lm, and the outer composite layers fi lled with large amount of high dielectric constant nanofi llers can then be polarized up to higher electric fi elds, hence enhancing the electric displacement. As a result, the topological-structure modulated nanocomposites, with an optimally tailored nanomorphology and composite structure, yield a discharged energy density of 10 J/cm 3 with a dielectric breakdown strength of 450 kV mm -1 , much higher than those reported from all earlier studies of nanocomposites. Figure 5. Electric fi eld-dependent discharged energy densities of the multilayer fi lms. For comparison, the energy densities of the BOPP is also included (data extrapolated from Ref. [ 25 ] ).
Graphene-BiFeO 3 composites are synthesized through a one-pot hydrothermal method and their photocatalytic performances are investigated under visible light irradiation. Bandgap engineering of BiFeO 3 -graphene composites is achieved by simply adjusting the concentration of OH groups during the hydrothermal treatment. XPS and Raman analysis indicate that enhanced coupling between BiFeO 3 nanoparticles and graphene is achieved by the formation of Fe-O-C bonds, which is mediated by OH groups adsorbed on the surface of graphene. The band gaps of the composites could be successfully tuned from 1.78 eV-2.24 eV, giving rise to high photocatalytic performance under visible light irradiation.This may be of significance in understanding the mechanism of the coupling between graphene and semiconductor oxides which are currently being intensively investigated as photocatalysts.
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