Strong increase of the dielectric response of carbon nanotube/poly(vinylidene fluoride) composites induced by carbon nanotube type and pre-treatment
Introduction
Carbon nanotubes (CNTs) have been increasingly used for applications due to their excellent electrical and mechanical properties [1], [2], [3], [4]. The intrinsic properties of CNT depend on their aspect ratio (AR), chirality and the presence of impurities and defects [5]. Though typically used for improving electrical conductivity and mechanical properties of polymers, CNT/polymer composites have been recently developed with the objective of increasing the dielectric response of polymers, leading to larger dielectric constant values than the ones typically obtained for ceramic/polymer composites [6], [7], [8]. Further, as lower filler volume fractions are used, the composites also show improved mechanical properties. The increase of the dielectric constant is related to variations in the percolation threshold for the high aspect ratio fillers, giving rise to the so-called high-k dielectric materials [4], [9], [10]. The overall properties of composites with carbon nanoallotropes strongly depend on polymer type and composite preparation method, CNT dispersion and orientation as well as on CNT cluster size and distribution [5]. As the dielectric constant (ɛ′) of common polymers is typically smaller than 10 [11], these high dielectric constant composites enable their use as high-k materials in electronic devices [10].
In CNT/polymer composites there is a critical concentration, the percolation threshold, at which the electrical properties suffer strong variations [4], [12]. The percolation threshold is affected by the aspect ratio and intrinsic characteristics of the CNT as well as by the composite processing method [4].
Beyond CNT or others carbon nanoallotropes, ceramic and metallic nanoparticles are also being used for the development of high dielectric constant composites [13], [14]. Large dielectric constant materials have been obtained [15], allowing applications such as high-pressure sensors and energy harvesting devices [16], actuators and transducers [17], electroluminescent devices [18], hydrophones [10] and biomedical imaging [10], [13], among others. Further, electronic devices working at high frequencies require high dielectric constant materials, which combine good dielectric and mechanical properties [19].
The dielectric properties of polymers are much lower than for ceramic or metals and several strategies have been implemented to increase them through the development of composite materials [18].
It has been shown that the dielectric response of ceramic/polymer composites depends on the ceramic concentration, preparation method and polymer matrix. The most used ceramic filler is BaTiO3, and the highest values reported in the literature range between ɛ′ ≈40 and 200, depending on the preparation conditions and filler content [13], [16], [17], [18], [19], [20], which reaches typically up to 30% volume content. On the other hand, for large ceramic loadings, the ceramic/polymer composites become fragile, preventing their use in flexible devices [21], [22], [23].
Thus, different carbon nanoallotropes, including graphite and graphene, are being used within different polymer matrices, the dielectric constant showing a strong increase with increasing filler concentration, for lower fillers concentrations that in their ceramic or metallic counterparts. Dielectric constant values up to 240 for graphite/epoxy composites [24], [25] and 5100 for graphene platelets/poly(vinylidene fluoride) composites [11] have been obtained.
The overall properties of polymer composites depend on filler type and content, polymer matrix and preparation method [26], the compatibility and interface between filler and matrix improving the performance of the composites.
Among different polymer matrices, composites with PVDF have been investigated due to the unique properties of this polymer such as strong piezo-, pyro- and ferroelectric properties, flexibility, thermal stability and chemical resistance [27], [28]. The electroactive properties of PVDF strongly depend on its phase, microstructure and degree of crystallinity, which in turn depend on the processing conditions [28].
It has been an increasing interest in CNT/PVDF composites with high dielectric constant [29], [30], [31]. Different CNT functionalization have been used, which result in strong variations of the dielectric response of the composites. Thus, pristine CNT/PVDF composites show a maximum dielectric constant around 1600 for 6% CNT volume fraction (ϕCNT), whereas ester and carboxyl functionalized CNT show a maximum dielectric constant of 2400 and 3600 for 6.5 and 8 ϕCNT, respectively, with maximum dielectric losses around 10 [31]. The oxidation state of the CNT also influences the dielectric response of the CNT/PVDF composites, with a maximum dielectric constant from 80 to 630, depending on the CNT oxidation state [32]. These materials have been developed for electrical and electronic engineering devices such as high-energy-density capacitors and electromagnetic-wave absorption [7], among others.
The dielectric constant and the electrical conductivity of composites are typically understood by the percolation theory [12], [33].
Near the percolation threshold there are specific scaling laws that govern the system:where ɛ′ is the dielectric constant of the composite, ϕ the volume fraction and ϕc the percolation threshold. The electrical conductivity can be expressed by:where σ is the electrical conductivity of the composite. The power law exponents, s and t, are the critical exponents and only depend on the dimensionality of the system. For a 3D system s is between 0.7 and 1.0 and t is between 1.6 and 2.0 [33].
The percolation theory also predicts that near the percolation threshold:where ω is the angular frequency and u is the critical exponent, which for a random resistor networks is ≈0.73 [34].
The formation of a conductive network where the conduction between the high aspect ratio fillers mainly occurs by hopping has been described [35] by:It is important to notice that above the percolation threshold, the AC composite conductivity and dielectric constant can exhibit an anomalous power law dispersions.
Due to the fact that these power laws occur in a wide class of materials they were termed universal dielectric response [36], [37]. The origin of these anomalous power law dispersions, i.e. the universal dielectric response, is the formation of a RC network where the resistors and capacitors are randomly connected in series and parallel. One of the characteristics of the universal dielectric response is the existence of a plateau for the dielectric constant at low frequencies that changes to a power law for higher frequencies.
In this context, this work reports on the production of high dielectric constant CNT/PVDF composites. The effect of CNT aspect ratio and preparation conditions, involving solvent dispersion and corona discharge are discussed. Thus, a simple and scalable method for the production of high-k materials is provided.
Section snippets
Sample preparation
The preparation of the composites involved several steps, and in order to understand the influence of each different preparation step in the electrical response of the composites, a systematic variation of these procedures was carried out. The complete processing of the samples includes, initially, that a specific amount of CNT is placed in an Erlenmeyer, soaked in toluene and placed in an ultrasound bath for 6 h to promote a CNT disaggregation. Thereafter, toluene is evaporated in an oven at a
Results and discussion
The electric and dielectric properties of the CNT/PVDF composites strongly depend on the CNT type and characteristics, as shown in Fig. 1, for composites with all the steps of the preparation method (solvent dispersion and corona discharge). Thus, the dielectric constant of the CNT/PVDF composites with 0.0172 CNT volume fraction (ϕCNT) at 1 kHz is ɛ′ ≈ 75.5 for DWCNT, ɛ′ ≈ 95.4 for MWCNT from NanoAmor and ɛ′ ≈ 1570 for the MWCNT from Baytubes. This strong variation is fully attributed to the
Conclusions
It is shown that the electrical and dielectric properties of CNT/PVDF composites depend both on the intrinsic properties of the CNT and the preparation method. Further, a novel preparation method is presented allowing to obtain high-k composites. This method includes the use of toluene dispersion and corona treatment of the CNT. In fact, it is shown that the dielectric constant can change from 6 for pure PVDF (α-phase) to near 3000 for composites with 0.0419ϕCNT with a dielectric losses of
Acknowledgments
This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2014. PC thanks the FCT for the SFRH/BD/64267/2009 grant. SLM thanks financial support from the Basque Government Industry Department under the ELKARTEK Program and the Diputación Foral de Bizkaia for finantial support under the Bizkaia Talent program; European Union's Seventh Framework Programme; Marie Curie
References (50)
- et al.
A simple model for thermal conductivity of carbon nanotube-based composites
Chem Phys Lett
(2003) - et al.
Progress on the morphological control of conductive network in conductive polymer composites and the use as electroactive multifunctional materials
Prog Polym Sci
(2014) - et al.
Carbon nanotube–polymer composites: chemistry, processing, mechanical and electrical properties
Prog Polym Sci
(2010) - et al.
Fundamentals, processes and applications of high-permittivity polymer–matrix composites
Prog Mater Sci
(2012) - et al.
Preparation, characterization and properties of novel 0–3 ferroelectric composites of Ba0.95Ca0.05Ti0.8Zr0.2O3–poly(vinylidene fluoride-trifluoroethylene)
Mater Chem Phys
(2012) - et al.
Synthesis and giant dielectric behavior of CaCu3Ti4O12 ceramics prepared by polymerized complex method
Mater Chem Phys
(2008) - et al.
Piezoelectric and dielectric properties of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3–PVDF composites
Ceram Int
(2012) - et al.
Dielectric, mechanical and thermal properties of polymer/BaTiO3 composites for embedded capacitor
Compos Part B Eng
(2013) - et al.
Dielectric properties of 0.25(BZT–BCT)–0.75[(1−x)PVDF–xCCTO] (x=0.02, 0.04, 0.06, 0.08 and 0.1) composites for embedded capacitor applications
Compos Sci Technol
(2013) - et al.
High dielectric constant polyaniline/epoxy composites via in situ polymerization for embedded capacitor applications
Polymer
(2007)
Modifications of carbon for polymer composites and nanocomposites
Prog Polym Sci
Electroactive phases of poly(vinylidene fluoride): determination, processing and applications
Prog Polym Sci
A comprehensive picture of the electrical phenomena in carbon black–polymer composites
Carbon
Large dielectric constant of the chemically functionalized carbon nanotube/polymer composites
Compos Sci Technol
Physics of inhomogeneous inorganic materials
Prog Mater Sci
Effects of corona discharge on the surface structure, morphology and properties of multi-walled carbon nanotubes
Appl Surf Sci
Electrical properties of polyvinylidene fluoride (PVDF)/multi-walled carbon nanotube (MWCNT) semi-transparent composites: modelling of DC conductivity
Compos Part A Appl Sci
The influence of matrix mediated hopping conductivity, filler concentration, aspect ratio and orientation on the electrical response of carbon nanotube/polymer nanocomposites
Compos Sci Technol
Effect of cylindrical filler aggregation on the electrical conductivity of composites
Phys Lett A
The role of disorder on the AC and DC electrical conductivity of vapour grown carbon nanofibre/epoxy composites
Compos Sci Technol
Effect of carbon nanotube type and functionalization on the electrical, thermal, mechanical and electromechanical properties of carbon nanotube/styrene–butadiene–styrene composites for large strain sensor applications
Compos Part B Eng
The effect of fibre concentration on the alpha to beta-phase transformation, degree of crystallinity and electrical properties of vapour grown carbon nanofibre/poly(vinylidene fluoride) composites
Carbon
Effect of tensile strain on morphology and dielectric property in nanotube/polymer nanocomposites
Appl Phys Lett
Carbon nanotube networks: sensing of distributed strain and damage for life prediction and self healing
Adv Mater
Microstructure and electromechanical properties of carbon nanotube/poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) composites
Adv Mater
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