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Critical thickness of crystallization of ferroelectric poly(vinylidene fluoride-trifluorethylene) copolymer thin films was observed.

In spin-cast films of poly(vinylidene fluoride-trifluoroethylene) on metalized silicon substrate, there exists a threshold thickness of crystallization d[sub th], below which the crystallinity drops precipitously. Due to the direct link between the crystallinity and functional properties in the polymer, there is a corresponding large change in the film ferroelectric properties, including the dielectric constant, the polarization level, and polarization switching speed, as the thickness is reduced to below d[sub th]. Detail microstructure studies show that this threshold thickness is controlled by the stable crystal lamellar size along the film thickness direction. By varying the film processing condition to reduce the crystal lamellar size in the thickness direction, d[sub th] can be reduced markedly. As a result, better ferroelectric responses were obtained in ultrathin films. [ABSTRACT FROM AUTHOR]

The effect of postsintering annealing on the dielectric and piezoelectric responses in lead-zincniobate (PZN)-based ferroelectric ceramics was systematically studied in this work. Sintered PZN ceramics with the same composition were annealed at temperatures from 700 to 900 °C from 30 min to 24 h in various atmospheres. As a result, significant improvements in the dielectric and piezoelectric responses were observed in this system. After annealing, there was a huge increase of up to 130% in the dielectric constants, especially near the temperature of maximum dielectric constant. While the dielectric peaks became sharper and less diffuse, there was almost no change in frequency dispersion. Similar large improvements in the piezoelectric properties such as the coupling factor and piezoelectric constant were also observed. Optimized parameters, such as d[sub 33] = 642 pC/N, d[sub 31] = - 256 pC/N, k[sub p] =48% and ∈[sub m] = 28 000, were achieved by annealing at 850 °C in an O[sub 2]-rich atmosphere for 4-8 h. The observed improvement in the electrical properties can be attributed to an extrinsic contribution induced by domain wall motion. After annealing, the pinning effects caused by oxygen vacancies and internal stress on domain wall motion were largely reduced or eliminated, therefore a significantly reduced coercive field, an increased polarization level, and a greatly increased dielectric constant were observed in the PZN-based ferroelectric ceramics. [ABSTRACT FROM AUTHOR]