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(a) Fictitious capacitor systems “A” and “B” both develop the same ultimate saturated polarisation (top), but at different field values. While “A” develops greater energy storage capabilities at low fields (bottom), the ultimate energy storage capabilities of “B” are superior. Polarization-electric field (P-E) loops from single crystal, ceramic and thin film BiFeO3 (b). Whilst the ultimate achievable polarization is similar for all three systems, an order of magnitude increase in the field at which the saturated polarisation develops is evident for the thin film. This is one manifestation of the so-called “dead-layer,” Data have been extracted from Refs. 10–12.
X-ray diffraction (coupled θ–2θ scan) of a typical bismuth ferrite-strontium titanate (BFST) based thin film capacitor sample. Films appear to be single phase and strongly oriented.
A schematic of the thin film capacitor structure with an ultrathin alumina layer introduced at the electrode-electroceramic interface (a). A nanoprobe energy dispersive X-ray analysis (taken on a transmission electron microscope) along the line indicated by the red-dashed arrow in (a) suggests that the thickness of the alumina layer is of the order of ∼6 nm (b).
D-E hysteresis loops from two thin film capacitor structures (a, b) before (red) and after (blue) the introduction of an alumina layer at the electrode-BFST interface. The associated recoverable energy density plotted as a function of polarization (c, d) was obtained by numerical integration and shows the increase in energy density induced by the introduction of an engineered “dead-layer.”
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