ABSTRACT
A 2-D semianalytical model using the Legendre polynomial approach was applied for modeling the piezoelectric transformer (PT). In the model, the mechanical displacements and the electric potential were expanded in a double series of orthonormal polynomials with an appropriate analytical form. Series and parallel resonance frequencies, input electrical admittance, and voltage gain were obtained for the PT. The results are in excellent agreement with both the numerical results calculated with a 3-D finite-element analysis and experimental ones. The approach is able to predict the behavior of a wide and a long PT.
ABSTRACT
Poly(vinylidene difluoride) (PVDF) displays attractive ferroelectric/piezoelectric properties and its polar ß-crystals are specifically targeted for achieving electroactive applications. However, their direct crystallization from the melt state represents a challenging task that has never been addressed using melt-state processes. The use of poly(methyl methacrylate) (PMMA) is herein investigated to promote the PVDF polar ß-phase using melt-blending and extrusion-calendering technologies. The presence of the ß-phase is here confirmed by ATR-FTIR and WAXS experiments with blends at a PMMA content as low as 5 wt%. The key role of PMMA for the ß-phase crystallization from the melt state was unambiguously highlighted with the help of Flash DSC experiments in non-isothermal cooling mode from the melt state. PMMA is able to efficiently shift the α-to-ß crystal transition to lower cooling rates (>100-200 °C s-1), making the achievement of the PVDF polar ß-phase for these blends compatible with conventional processing tools. A crystal phase diagram is proposed for the PVDF/PMMA blends to highlight the dual effects of both PMMA and cooling rate on the PVDF crystallization during melt-processing. Ferroelectric properties were even observed for the blends containing PMMA up to 10 wt% with the highest remanent polarization obtained at 5 wt% PMMA. After 10 wt% PMMA, a progressive transition from ferroelectric to pseudo-linear dielectric behavior is observed more likely due to the presence of PMMA in the interlamellar amorphous phase of the polar PVDF spherulites as shown by SAXS experiments. In this work, we successfully demonstrated that PMMA plays a key role in the crystallization of PVDF polar crystals from the melt state, enabling large-scale and continuous extrusion processing of PVDF-based materials with attractive dielectric properties for sensing and harvesting applications.
ABSTRACT
Surface acoustic waves (SAW) are particularly suited to effectively monitoring and characterizing structural surfaces (condition of the surface, coating, thin layer, micro-cracks ) as their energy is localized on the surface, within approximately one wavelength. Conventionally, in non-destructive testing, wedge sensors are used to the generation guided waves but they are especially suited to flat surfaces and sized for a given type material (angle of refraction). Additionally, these sensors are quite expensive so it is quite difficult to leave the sensors permanently on the structure for its health monitoring. Therefore we are considering in this study, another type of ultrasonic sensors, able to generate SAW. These sensors are interdigital sensors or IDT sensors for InterDigital Transducer. This paper focuses on optimization of IDT sensors for non-destructive structural testing by using PZT ceramics. The challenge was to optimize the dimensional parameters of the IDT sensors in order to efficiently generate surface waves. Acoustic tests then confirmed these parameters.
