Dielectric and magnetic properties of ferrite/poly(vinylidene fluoride) nanocomposites

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Abstract

Particulate composite films of poly(vinylidene fluoride) and CoFe2O4 and NiFe2O4 were prepared by solvent casting and melt processing. The well-dispersed ferrite nanoparticles nucleate the piezoelectric β-phase of the polymer, but the different ferrites nucleate the whole polymer crystalline phase at different filler concentrations. The macroscopic magnetic and dielectric response of the composites demonstrates a strong dependence on the volume fraction of ferrite nanoparticles, with both magnetization and dielectric constant increasing for increasing filler content. The β-relaxation in the composite samples is similar to the one observed for β-PVDF obtained by stretching. A superparamagnetic behavior was observed for NiFe2O4/PVDF composites, whereas CoFe2O4/PVDF samples developed a hysteresis cycle with coercivity of 0.3 T.

Highlights

► We have demonstrated that ferrite nanoparticles nucleate the electroactive β-phase of the polymer. ► We have produced polymer composites with two different ferrites suitable for magnetoelectric applications. ► We fully report and interpret the variations in the structural, dielectric and magnetic properties. ► It is demonstrated that increasing nanoparticles content improves the dielectric and magnetic properties of composites.

Introduction

Poly(vinylidene fluoride) (PVDF), is a semicrystalline polymer with one of the largest pyro- and piezoelectric properties among polymers [1]. These properties combined with its high elasticity, transparency and easy processing make this material suitable for numerous technological applications [2].

PVDF is also well known for its polymorphism, showing at least four crystalline phases called α, β, δ and γ [3]. The α and β polymorphs are most common: melt processing of the material typically results in the nonpolar α-phase [4], whereas the polar β-phase is technologically the most interesting one for sensor and actuator applications as it shows the largest piezoelectric, pyroelectric and ferroelectric coefficients, as well as a high dielectric constant [1]. The β-phase of PVDF is commonly obtained by mechanical stretching of films originally in the non-polar α-phase, resulting in films mostly in the β-phase, but with some percentage of α-phase [5]. Further, this method is not appropriate for the preparation of polymer composites, as the stretching process is either hindered for high filler loadings and/or leads to non-controlled reconfigurations and eventual agglomeration of the fillers [1].

Films of β-PVDF can be also obtained directly by solution casting but the material shows high porosity leading to a decrease of the electrical and mechanical properties [4], [6]. The development of polymer nanocomposites is a subject of intensive research [7]. In the simplest case, adding nanoparticles to a polymer matrix such as PVDF can enhance its performance or provide new responses, by capitalizing on the nature and properties of the nanoscale filler [8]. This is the case of composite materials consisting of magnetic nanoparticles dispersed in a polymer matrix. In addition, the processability and mechanical quality of the matrix is an advantage compared to ferrites. On the other hand, despite a restricted particle concentration, a sufficiently high magnetic permeability can be achieved within the polymer composite [9], and, finally, the magnetoelectric (ME) effect can be also observed in such composites [10], [11], [12].

Van Suchtelen introduced the idea of the two-phase particulate composites [13], which was supported by the van den Boomgaard's synthesis conditions [14]. The composites with a ferrite and a ferroelectric phases have the ability to show product and sum properties [15]. In such composites, electromechanical coupling occurs: magnetostriction in the ferrite phase gives rise to a mechanical stress within the ferroelectric phase, resulting in variations of the electrical polarization and, therefore, in a magnetoelectric effect [16]. Multiferroic materials are, in this way, excellent candidates as memory elements, smart sensors, etc. [17].

Due to the magnetic and dielectric properties of ferrites, much interest has been focused on polymer-based composites filled with ferrite nanoparticles, such as cobalt-ferrite [18] and nickel-ferrite [19], for their applications in various areas such as information storage, electromagnetic wave absorption, bio-separation, and diagnostics. Their magnetostrictive properties also make them good candidates for magnetoelectric composites [20].

Three kinds of bulk magnetoelectric composites have been reported: magnetic metals/alloys, laminated Terfenol-D and piezoelectric ceramics or polymers and, most recently, particulate composites of ferrite and piezoelectric ceramics, e.g., lead zirconate titanate (PZT) [21]. The ME coefficients obtained in ceramic particulate or laminated composites are typically three orders of magnitude higher than those of single phase materials [22], [23]. On the other hand, the composites become fragile and are limited by deleterious reactions at the interface regions making such ceramic composites not suitable for device applications [24]. To overcome some of the problems, polymer based magnetoelectric materials are developed such as particulate composites of magnetostrictive Terfenol-D, piezoelectric PZT, and a binding polymer matrix has been developed [25]. In these materials, the incorporation of PZT into the polymeric matrix makes the composite more brittle [25], [26] and, although Terfenol-D has the highest magnetostriction among all known materials, this rare-earth iron alloy is quite costly and also very brittle.

One approach to obtain highly flexible and non-brittle magnetoelectric composites is to use two-phase polymer composites without any additional piezoelectric ceramic filler, in which the polymer itself is piezoelectric, such as PVDF in its β-phase.

In this paper, PVDF-based nanocomposites with either Co or Ni-ferrites fillers are investigated. The effect of the filler concentration on the dielectric and magnetic properties are discussed, as they are at the base of the different potential applications of these materials. It is particularly important to notice that the electroactive phase of the polymer is nucleated by the ferrites, leading in this way to a simplified processing method for the preparation of magnetoelectric composites.

Section snippets

Preparation of the nanocomposites

Ferrite powders, purchased from Nanoamor, were characterized by XRD and single phase samples were observed of CoFe2O4 (Fd3m (227); JCPDS: 22-1086) with particle size 35–55 nm and NiFe2O4 (Fd3m (227); JCPDS: 10-325) from 20 to 30 nm and densities of 5.30 and 5.37 g cm−3, respectively [27]. N,N-Dimethylformamide (DMF, pure grade) was supplied by Fluka and poly(vinylidene fluoride) (PVDF, Solef 1010) with a density of 1.78 g cm−3 was supplied by Solvay. All the chemicals and nanoparticles were used as

Results and discussion

Fig. 1 shows typical SEM images of nanocomposite films of CoFe2O4/PVDF with volume fractions of 0.02 (a and b) and 0.25 (c and d). These images are also representative for the NiFe2O4/PVDF composites. For low Co or Ni ferrite filler concentrations, the microstructure of PVDF is spherulitic, just like pure PVDF in the α-phase [5], [28] as it can be observed in Fig. 1a and b. For higher volume fractions (ϕ = 0.08 or higher) of ferrite particles, the spherulitic structure is destroyed and the

Conclusions

Composites consisting in CoFe2O4 and NiFe2O4 nanopowders as ferrite phase and PVDF as ferroelectric phase were prepared by solution method. XRD of the composites reveals the formation of the ferroelectric phase of the polymer with increasing ferrite content. The nucleation of the β-phase of the polymer is more effectively nucleated for the Co-ferrite nanoparticles, as the whole crystalline phase of the polymer is within the ferroelectric phase for ferrite concentrations as low as ϕ = 0.02. On the

Acknowledgments

This work is funded by FEDER funds through the “Programa Operacional Factores de Competitividade – COMPETE” and by national funds by FCT – Fundação para a Ciência e a Tecnologia, project references NANO/NMed-SD/0156/2007 and PTDC/CTM/69316/2006. P. Martins thanks the support of the FCT under grant SFRH/BD/45265/2008. Technical and human support for magnetic measurements provided by SGIker (UPV/EHU, MICINN, GV/EJ, ESF) is gratefully acknowledged. J. M. Barandiaran and J. Gutierrez want to thank

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