S. Lanceros-Méndez

Influence of filler size and concentration on the low and high temperature dielectric response of poly(vinylidene fluoride) /Pb(Zr0.53Ti0.47)O3 composites

Poly(vinylidene fluoride)/Pb(Zr0.53Ti0.47)O3([PVDF]1 − x/[PZT]x ) composites of volume fractions x and (0–3) type connectivity were prepared in the form of thin films. The films were prepared by solvent casting with PZT powder of 0.84, 1.86, and 2.35xa0μm average size with filler contents up to 40xa0% volume. The crystalline phase of the polymer matrix was the nonpolar α-phase and the polar β-phase. Dielectric measurements were performed in order to evaluate the influence of the filler size and content as well as the effect of the polymer matrix in the overall response of the material. No nucleation effect of any of the phases was observed for the used fillers. The spherulitic structure of the pure α-PVDF and the characteristic porosity of the β-phase material are destroyed for high PZT volume fractions. The inclusion of ceramic particles in the PVDF polymer matrix increases the complex dielectric constant of the composites independently of the PVDF polymer matrix. The dielectric properties of the composites are mainly affected by the amount of the ceramic particles. With respect to the relaxation processes of the polymer, the activation energy of the αa-relaxation increases and the glass transition temperature decreases with increasing particle size and content. The high-temperature conductivity decreases with increasing filler content and there is an important contribution of the Maxwell-Wagner-Sillars effect to the overall dielectric response.

Influence of the β−phase content and degree of crystallinity on the piezo- and ferroelectric properties of poly (vinylidene fluoride)

The ferroelectric switching behaviour and piezoelectric response of poly(vinylidene fluoride) (PVDF) prepared by drawing at stretching ratios from 1 to 5 and temperatures from 80 to 140 °C has been studied. Stretching ratio and temperature deeply influence the α (non-ferroelectric) to β (ferroelectric) phase transformation. The variations in the phase content are accompanied by changes in the degree of crystallinity and the microstructure, all of them influencing the macroscopic piezoelectric and ferroelectric response of the material. This work shows how the piezo- and ferroelectric behaviour of PVDF depends on the aforementioned parameters and, in particular, on the crystalline β-phase content. Coercive electric field, remnant polarization and saturation polarization increase with increasing ferroelectric β-phase content in the sample. In a similar way, samples with higher β-phase content show higher d33 piezoelectric coefficients.

Development of magnetoelectric CoFe2O4 /poly(vinylidene fluoride) microspheres

Magnetoelectric microspheres based on piezoelectric poly(vinylidene fluoride) (PVDF) and magnetostrictive CoFe2O4 (CFO), a novel morphology for polymer-based ME materials, have been developed by an electrospray process. The CFO nanoparticle content in the (3–7 μm diameter) microspheres reaches values up to 27 wt%, despite their concentration in the starting solution reaching values up to 70 wt%. Additionally, the inclusion of magnetostrictive nanoparticles into the polymer spheres has no relevant effect on the piezoelectric β-phase content (≈60%), crystallinity (40%) and the onset degradation temperature (460–465 °C) of the polymer matrix. The multiferroic microspheres show a maximum piezoelectric response |d33| ≈ 30 pC N−1, leading to a magnetoelectric response of Δ|d33| ≈ 5 pC N−1 obtained when a 220 mT DC magnetic field was applied. It is also shown that the interface between CFO nanoparticles and PVDF (from 0 to 55%) has a strong influence on the ME response of the microspheres. The simplicity and the scalability of the processing method suggest a large application potential of this novel magnetoelectric geometry in areas such as tissue engineering, sensors and actuators.

Novel Anisotropic Magnetoelectric Effect on δ-FeO(OH)/P(VDF-TrFE) Multiferroic Composites

The past decade has witnessed increased research effort on multiphase magnetoelectric (ME) composites. In this scope, this paper presents the application of novel materials for the development of anisotropic magnetoelectric sensors based on δ-FeO(OH)/P(VDF-TrFE) composites. The composite is able to precisely determine the amplitude and direction of the magnetic field. A new ME effect is reported in this study, as it emerges from the magnetic rotation of the δ-FeO(OH) nanosheets inside the piezoelectric P(VDF-TrFE) polymer matrix. δ-FeO(OH)/P(VDF-TrFE) composites with 1, 5, 10, and 20 δ-FeO(OH) filler weight percentage in three δ-FeO(OH) alignment states (random, transversal, and longitudinal) have been developed. Results have shown that the modulus of the piezoelectric response (10-24 pC·N(-1)) is stable at least up to three months, the shape and magnetization maximum value (3 emu·g(-1)) is dependent on δ-FeO(OH) content, and the obtained ME voltage coefficient, with a maximum of ∼0.4 mV·cm(-1)·Oe(-1), is dependent on the incident magnetic field direction and intensity. In this way, the produced materials are suitable for innovative anisotropic sensor and actuator applications.

Magnetoelectric CoFe2O4/polyvinylidene fluoride electrospun nanofibres

Magnetoelectric 0-1 composites comprising CoFe2O4 (CFO) nanoparticles in a polyvinylidene fluoride (PVDF) polymer-fibre matrix have been prepared by electrospinning. The average diameter of the electrospun composite fibres is ∼325 nm, independent of the nanoparticle content, and the amount of the crystalline polar β phase is strongly enhanced when compared to pure PVDF polymer fibres. The piezoelectric response of these electroactive nanofibres is modified by an applied magnetic field, thus evidencing the magnetoelectric character of the CFO/PVDF 0-1 composites.

Large linear anhysteretic magnetoelectric voltage coefficients in CoFe2O4/polyvinylidene fluoride 0-3 nanocomposites

Free-standing flexible magnetoelectric 0–3 composite films comprising CoFe2O4 non-spherical ferromagnetic nanoparticles with strong magnetic anisotropy in soft polyvinylidene fluoride matrices with significant void ratios have been prepared at low temperatures by solvent casting, melt crystallization and mechanical stretching. Magnetoelectric voltage coefficients increase linearly with applied dc magnetic field bias up to 5xa0kOe. At this field, a maximum magnetoelectric voltage coefficient of 11.2xa0mVxa0cm−1xa0Oe−1xa0was obtained for samples with 10xa0wt% ferrite at ~50-kHz resonance. The observed linear magnetoelectric response is attributed to a linear gradient of magnetostriction with respect to magnetic field for Hxa0≤xa05xa0kOe.

Mechanical Characterization and Influence of the High Temperature Shrinkage of β-PVDF Films on its Electromechanical Properties

Tensile dynamic mechanical analysis at 1 Hz was used to characterize the solid rheological properties of β-PVDF films. Both the elastic and loss moduli and the specific damping capacity were monitored against temperature, allowing the study of the effect of anisotropy upon the viscoelastic properties of the films. The temperature range covered the β- and α-relaxations. These results are compared to dielectric relaxation results in order to elucidate the electrical and mechanical contributions to the observed relaxations. Further, an important shrinking effect upon heating above 364 K has been observed, that influences the material properties. This geometrical effect has been monitored by thermal mechanical analysis. The thermal coefficients of linear expansion have been calculated, giving two different regimes for this parameter. The variations at a molecular level have been monitored by FTIR.

Fatigue prediction in fibrin poly-ε-caprolactone macroporous scaffolds

Tissue engineering applications rely on scaffolds that during its service life, either for in-vivo or in vitro applications, are under loading. The variation of the mechanical condition of the scaffold is strongly relevant for cell culture and has scarcely been addressed. The fatigue life cycle of poly-ε-caprolactone, PCL, scaffolds with and without fibrin as filler of the pore structure were characterized both dry and immersed in liquid water. It is observed that the there is a strong increase from 100 to 500 in the number of loading cycles before collapse in the samples tested in immersed conditions due to the more uniform stress distributions within the samples, the fibrin loading playing a minor role in the mechanical performance of the scaffolds.

Optimized anisotropic magnetoelectric response of Fe61.6Co16.4Si10.8B11.2/PVDF/Fe61.6Co16.4Si10.8B11.2 laminates for AC/DC magnetic field sensing

The authors thank the FCT- Fundacao para a Ciencia e Tecnologia- for financial support under project PTDC/EEI-SII/5582/2014. P.M., S.R. and M.S. acknowledges also support from FCT (SFRH/BPD/96227/2013, SFRH/BDE/406 51542/2011 and SFRH/BD/70303/2010 grants respectively). This work was also supported by Avel-electronica Lda, Trofa, Portugal. J.G., A.L. and S.L.M. thank financial support from the Basque Government Industry Department under the ELKARTEK Program. SLM also thanks the Diputacion de Bizkaia for financial support under the Bizkaia Talent program.

Chain Reorientation in β-PVDF Films Upon Transverse Mechanical Deformation Studied by SEM and Dielectric Relaxation

Chain reorientation may be induced in polyvinylidene fluoride (PVDF) in its β-phase by applying a deformation perpendicular to the pre-oriented polymeric chains. This reorientation begins right after the yielding point and seems to be completed when the stress-strain curve stabilizes. We have studied the chain reorientation mechanism by scanning electron microscopy (SEM) and dielectric relaxation methods and we have analyzed its influence in the α- and β-relaxations of the polymer. SEM images confirm the existence of a reorientation upon transversal deformation. The decrease of crystallinity with increasing reorientation and the dependence of the dielectric permittivity upon deformation indicate that a model of stress-induced melting followed by an incomplete recrystallization properly describes the reorientation mechanism.