B. B. Tian

Transition of the polarization switching from extrinsic to intrinsic in the ultrathin polyvinylidene fluoride homopolymer films

Polyvinylidene fluoride homopolymer thin films have been prepared by the Langmuir-Blodgett technique, and their electrical properties have comprehensively been studied. The polyvinylidene fluoride homopolymer films show better ferroelectricity with higher polarization and higher breakdown electric field than that of the poly(vinylidene fluoride-trifluoroethylene) copolymer films. Inspection on the thickness dependence of the coercive field of the polyvinylidene fluoride films suggests an extrinsic polarization switching occurs in the thickness range from 200 to 45 nm, and a non-extrinsic switching is observed in the range between 45 and 11 nm, which is ascribed to the transition range from extrinsic to intrinsic switching.

Enhanced dielectric and ferroelectric properties in the artificial polymer multilayers

Multilayers consisting of alternating ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities in thickness were prepared. A superlattice-like structure is shown in the polymer multilayer as the periodic thickness is lower than a critical value. The dielectric constant of the multilayer with a small periodic thickness is two times higher than that of the P(VDF-TrFE) copolymer over a temperature range between 300 K and 350 K. The multilayer also shows a good ferroelectricity in the same temperature range. The enhanced electrical properties of the multilayers are due to the long-range ferroelectric coupling.

The creep process of the domain switching in poly(vinylidene fluoride-trifluoroethylene) ferroelectric thin films

The polarization switching behavior in poly(vinylidene fluoride-trifluoroethylene) thin films is studied by using a pulse transient current method. The dependence of the domain switching current on the coercive electric field was investigated. The charging current around the coercive field was found to be limited by domain switching instead of the series resistor in the measurement circuit because of the slow polarization switching in the films. The domain-switching process was explained by a creep model wherein the two-dimension domain walls motion in the transverse direction dominates the polarization switching process.

β phase instability in poly(vinylidene fluoride/trifluoroethylene) thin films near β relaxation temperature

The β phase stability in poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] thin films was studied below 300 K using X-ray diffraction and polarization-electric-field (P-E) hysteresis loops measurements. On as-grown samples, an irreversible partial order-disorder transformation at Tβ ∼ 250 K, namely, the β relaxation temperature, was evidenced by the appearance of an additional X-Ray diffraction peak above Tβ as well as changes on the P-E loops on heating after the first cooling. This order-disorder-like transformation which is attributed to an all-trans order to helical disorder transition is suggested to take place in defect-rich regions like crystal-amorphous interphases and/or crystalline areas with randomly distributed TrFE defect-like units.

Enhanced piezoelectric response in the artificial ferroelectric polymer multilayers

An actuator with a high piezoelectric response, the ferroelectric polymer multilayer actuator, is described. The ferroelectric polymer multilayers consisting of alternative ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities and fixed total thickness are prepared by the Langmuir-Blodgett technique. Both X-ray diffraction and Raman spectroscopic measurements indicate that the structure of the multilayer with thin alternating layer is similar to that of the ferroelectric copolymer. Compared with that of the copolymer, it is found that the piezoelectric coefficient of the multilayer could be improved by 57%. We attributed the enhanced piezoelectric response of the multilayers to the internal electric fields that arises from the electrostatic couplings between different layers.

The intermediate temperature T* revealed in relaxor polymers

The temperature dependence of the dielectric and optical properties, crystal structure, and infrared spectra of the relaxor poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) terpolymer films obtained from Langmuir-Blodgett method have been comprehensively investigated. All the results suggest that there exists a peculiar point at ∼370 K, which is attributed to the intermediate temperature T* recently discovered in inorganic relaxors. Here, T* results from the change in the growth rate of the trans gauche T3GT3G′ chain conformations with temperature, which is similar to the transformation from dynamic to static behavior of the so-called polar nano-regions in inorganic relaxors.

High temperature coefficient of resistance for a ferroelectric tunnel junction

An infrared detector is proposed that is based on a ferroelectric tunnel junction (FTJ) working under bolometer-like principles. Electron tunneling, either direct or indirect, through the ferroelectric barrier depends on the temperature of the devices. During tunneling, infrared radiation alters the polarization of the ferroelectric film via pyroelectricity, resulting in a change in the barrier height of the tunnel junction. A high temperature coefficient of resistance of up to −3.86% was observed at room temperature. These results show that the FTJ structure has potential to be adapted for use in uncooled infrared detectors.

The Evolution of Activation Field with Temperature in Ferroelectric Copolymer of Vinylidene Fluoride and Trifluoroethylene Films

Vinylidene fluoride and trifluoroethylene [P(VDF-TrFE)] ferroelectric thin film was spin-coated on Au-coated polyimide substrate and its polarization reversal was investigated by analyzing the evolution of activation field (α) with temperature. Although α is nearly a constant between 300 and 330 K, it increases linearly when temperature rates between 200 and 230 K, and between 230 and 300 K. On the other hand, the intensity of X-ray diffraction for P(VDF-TrFE) films from 200 to 330 K indicates that glass-transition process plays a significant role in both the microstructure and the polarization reversal of P(VDF-TrFE) copolymer.