Interface-Dominated Time-Dependent Behavior of Poled Poly(Vinylidene Fluoride–Trifluoroethylene)/Barium Titanate Composites

Composites in which particles of ferroelectric ceramic phase are randomly dispersed in a polymeric matrix are of interest because of flexibility, conformability, and ease of processing. However, their piezoelectric properties are rather low, unless very high volume fractions of ceramics are used. This brings agglomeration and porosity issues due to the large mismatch between the surface energies of the ceramics and of the polymer. Particle surface modification is a common approach for better dispersion; however, it may bring other effects on the properties of the composites, which are usually concealed by the huge improvement in performance due to the more homogenous microstructure. In this work, we compared poly(vinylidene fluoride–trifluoroethylene)/barium titanate composites containing 15 vol.% and 60 vol.% of pristine ceramic particles or particles modified with an aminosilane or a fluorosilane. Similar morphology, with good particle dispersion and low porosity, was achieved for all composites, owing to an efficient dispersion method. The materials were poled with two different poling procedures, and the piezoelectric coefficient d33, the relative permittivity, and the poling degree of barium titanate were followed in time. We highlighted that, although similar d33 were obtained with all types of particles, the nature of the particles surface and the poling procedure were associated with different charge trapping and influenced the evolution of d33 with time.

Effect of silane coupling agent on the morphology, structure, and properties of poly(vinylidene fluoride-trifluoroethylene)/BaTiO3 composites

Sara Dalle Vacche, Dragan Damjanovic, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

Micron- and submicron-sized barium titanate (BaTiO3) particles, untreated and surface modified with aminopropyl triethoxy silane, were incorporated in poly(vinylidene fluoride-trifluoroethylene) to fabricate composites with up to 60 vol% of ceramic phase. The morphology and structure of solvent cast and compression-molded films, and their thermal, viscoelastic, and dielectric properties were investigated. When surface-modified BaTiO3 was used, it was possible to decrease both the viscoelastic and the dielectric losses of highly filled solvent cast films, while their storage modulus and relative permittivity either increased or remained equal, owing to reduced porosity and improved matrix-filler compatibility. The effect of BaTiO3 surface modification on the morphology of compression-molded films was less marked, leading to unchanged viscoelastic properties, and lower permittivity and dielectric losses. For all composites the frequency dependency of the dielectric properties at low frequencies was suppressed with modified BaTiO3.

Springer Verlag2014

The Effect Of Processing Conditions On The Morphology, Mechanical And Electrical Properties Of Batio3-Polymer Composites

Sara Dalle Vacche, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

Ferroelectric ceramic – polymer composites have a unique mix of electrical and mechanical properties. Piezoelectric and pyroelectric activity, a wide range of dielectric constants and high breakdown strength are combined with mechanical flexibility, formability and low acoustic impedance. Furthermore their properties may be tailored by the judicious choice of the polymeric matrix and ceramic filler, of their volume fraction, and of the type of connectivity, making these materials very attractive for applications as embedded capacitors, sensors and actuators. Composites with 0-3 connectivity, i.e. a continuous polymer matrix filled with a ceramic phase discontinuous in all directions, are particularly interesting because of their relative ease of fabrication and processing. The functionality of these composite materials is mostly due to the dielectric, ferroelectric and piezoelectric properties of the ceramic phase, which therefore often needs to constitute more than 50% in volume. On the other hand the possibility of integrating these composites as embedded devices in host structures, which are often polymer based, or using them as efficient actuators depends greatly on their thermomechanical properties. Most of the studies on ferroelectric ceramic-polymer composites, however, focus on their electrical properties, and relatively few systematic studies on their thermal and mechanical behaviour have been published so far. Furthermore the ceramic volume fractions considered are often much lower than those used in most applications [1, 2], studies of highly filled composites mainly being limited to lead-based ferroelectric materials [3]. In the present study 0-3 BaTiO3 – polymer composites were prepared, containing up to 60% volume fraction of ceramic phase. BaTiO3 was chosen as the ceramic filler as it is a common piezoelectric material with high dielectric constant, having the additional benefit from an environmental point of view of being lead-free. For the matrix, a low viscosity epoxy resin and a polyvinylidene fluoride based polymer were chosen. The first is a thermoset resin commonly used for several applications, and the second is a thermoplastic semicrystalline polymer which displays piezoelectric activity. A common route for achieving very high loading of ceramic filler is the use of solvents to reduce the viscosity during processing. However, as the materials prepared via the solvent route were found to have poor mechanical properties, attributed to the presence of residual solvent, most of the effort focused on obtaining high ceramic volume fractions via a solvent free process. The thermomechanical, dielectric, ferroelectric and piezoelectric properties of the obtained composites were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. X-ray diffraction was carried out to determine the crystal structure of the BaTiO3 in the pure powder and in the composites, confirming that processing did not alter the crystallographic structure of the ceramic phase. Scanning electron microscope observation of fracture surfaces of the composites evidenced that a satisfactorily homogeneous dispersion of BaTiO3 in the matrix could be obtained, although aggregates with diameter of some micrometers and some porosity were detected. The thermal and mechanical behaviour of the composites was studied by differential scanning calorimetry and dynamic mechanical analysis. The glass transition temperature was found not to significantly depend on the amount of filler, while both the storage and loss moduli increased upon addition of BaTiO3. Furthermore, all the composites displayed a region of linear stress-strain behaviour smaller than the pure matrices. Finally, experimental results also evidenced the possibility to obtain low leakage current and high breakdown voltage.

2010

Effect of interfacial interactions on the electromechanical response of poly(vinylidene fluoride-trifluoroethylene)/BaTiO3 composites and its time dependence after poling

Sara Dalle Vacche, Dragan Damjanovic, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

Solvent cast and compression molded composite films based on poly(vinylidene fluoride-trifluoroethylene), containing pristine and silylated BaTiO3 particles, were poled, and their electromechanical response (d(33)), their dielectric properties, and the crystalline structure of the embedded BaTiO3 particles, probed by X-ray diffraction, were recorded during one week. Their d(33) was also measured after long-term aging (>3 years). During this time the d(33) decreased for all composites. The d(33) of solvent cast composites was higher when surface modified particles were used, both immediately after poling and after aging, while for compression molded composites only slight differences in the initial and aged d(33) of materials containing pristine versus modified particles were found. However the decay rate of d(33) in the short time after poling was highly affected by the surface modification of the particles for both the solvent cast and the compression molded composites. The results suggest that other phenomena besides the polarization of the ceramic particles and of the polymer matrix contribute to the measured electromechanical response of the composites. Charge accumulation and differences in the charge distribution due to the presence of the silane layer on the surface of BaTiO3 may play an important role. (C) 2015 Elsevier Ltd. All rights reserved.

Elsevier2015

The Effect Of Processing Conditions On The Morphology, Mechanical And Electrical Properties Of Batio3-Polymer Composites

Sara Dalle Vacche, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

2010