Functionalised graphene sheets as effective high dielectric constant fillers
© Romasanta et al; licensee Springer. 2011
Received: 31 May 2011
Accepted: 25 August 2011
Published: 25 August 2011
A new functionalised graphene sheet (FGS) filled poly(dimethyl)siloxane insulator nanocomposite has been developed with high dielectric constant, making it well suited for applications in flexible electronics. The dielectric permittivity increased tenfold at 10 Hz and 2 wt.% FGS, while preserving low dielectric losses and good mechanical properties. The presence of functional groups on the graphene sheet surface improved the compatibility nanofiller/polymer at the interface, reducing the polarisation process. This study demonstrates that functionalised graphene sheets are ideal nanofillers for the development of new polymer composites with high dielectric constant values.
PACS: 78.20.Ci, 72.80.Tm, 62.23.Kn
Keywordsdielectric properties graphene interfacial polarisation nanocomposites silicones
In recent years, elastomeric materials with high dielectric constant have been considered for different functional applications such as artificial muscles, high charge-storage capacitors and high-K gate dielectric for flexible electronics [1, 2]. Several methods have been explored in order to increase their dielectric permittivity although the most common approach involves the addition of high dielectric constant ceramics to the elastomeric matrix. This strategy usually requires high loading fractions and, hence, produces an unwanted increase of the system rigidity for the applications already mentioned [3–5]. In some other cases, dielectric constant increments have been met with relatively high loss tangent values (tg (δ)) and frequency dependence which is also undesirable for capacitor applications [6, 7]. Obtaining composites with both high dielectric permittivity and low loss tangent values at the same time is specially challenging due the interfacial polarisation or Maxwell-Wagner-Sillars (MWS) process. This mechanism occurs at the interface between materials with different permittivities and/or conductivities and involves rather high ε' and tg (δ) values at low frequencies due to the accumulation of virtual charges at the filler/polymer interface . Altering the interfacial interaction between filler and polymer matrix can regulate the dielectric contrast between matrix and filler and thus, prevent the MWS polarisation [9–11]. Therefore, chemical modification of filler particles has to be taken into account in order to achieve high permittivity composites with low dielectric losses. Nevertheless, filler surface modifications can significantly raise the production costs and thus, make them unfeasible to be produced on large scale.
Thermally expanded graphene sheets are of great interest to overcome the aforementioned problems. The thermal reduction of the graphite oxide has the advantage to produce chemically modified graphene sheets (or so-called functionalised graphene sheets FGS) without the need of further modification steps. Besides, the huge aspect ratio of these carbon-based nanoparticles (experimental value 1850 m2 g-1)  reduces considerably the percolation threshold compared to any other type of high dielectric constant filler. Accordingly, very small loading fractions can offer interesting permittivity enhancements without adversely affecting the dielectric losses and mechanical properties of a given polymer matrix.
In this work, as-produced carbon nanotubes (CNTs) and thermally expanded graphene sheets are compared for their possible enhancing effect on an elastomer dielectric response. Results show that FGS are an ideal candidate as high dielectric constant fillers in capacitor applications. The presence of remaining functional groups at their surface is able to improve the filler-matrix compatibility, enhance the nanoparticle distribution and make them suitable to develop novel, flexible and easy to process capacitors with relatively high dielectric constant and low tg (δ) values.
A commercial poly(dimethyl)siloxane (PDMS) kindly supplied by BlueStar Silicones (Rhodorsil MF620U) was used as elastomeric matrix.
Both CNTs and FGS employed in this study were synthesised in our laboratories as follows: aligned multi-wall CNTs were produced by chemical vapour deposition (CVD) injection method using toluene as the carbon source and ferrocene as the catalyst. A 3 wt.% ferrocene/toluene solution was injected into a hot quartz tube reactor (760°C) at 5 ml h-1 under inert atmosphere. FGS were produced by reduction and thermal exfoliation of graphite oxide (GO). GO was previously produced using natural graphite powder (purum powder ≤ 0.1 mm, Fluka, Sigma-Aldrich Corp. St. Louis, MO, USA) according to the Brödie method . Rapid heating (30 s at 1,000°C) of the graphite oxide under inert atmosphere produced the partial thermal decomposition of the functional groups (epoxy, hydroxyl and carboxyl groups) present in the GO, splitting the GO into FGS through the evolution of CO2 (gas). Both CNT and FGS were used without further treatments.
Nanocomposites containing 0.5, 1.0, and 2.0 wt.% of CNT and FGS were prepared at room temperature in an open two-roll laboratory mill (speed ratio of 1:1.4) using standard mixing procedures. After that, samples were vulcanised at 170°C in an electrically heated hydraulic press using the optimum cure time (t 90), deduced from the curing curves previously determined by means of a rubber process analyser (RPA2000 Alpha Technologies, Akron, OH, USA).
Broadband dielectric spectroscopy was performed on an ALPHA high-resolution dielectric analyser (Novocontrol Technologies GmbH, Hundsangen, Germany). Cross-linked film disc-shaped samples were held in the dielectric cell between two parallel gold-plated electrodes. The thickness of the films (around 100 μm) was taken as the distance between the electrodes and determined using a micrometre gauge. The dielectric response of each sample was assessed by measuring the complex permittivity ε*(ω) = ε'(ω) - jε"(ω) over a frequency range window of 101 to 107 Hz at 23°C. The amplitude of the alternating current (ac) electric signal applied to the samples was 1 V. In this work, the real part of the complex permittivity constant will be referred simply as the dielectric permittivity constant.
Stress-strain measurements were performed on a tensile test machine (Instron 3366 dynamometer, Norwood, MA, USA) at 23°C. Dog bone-shaped specimens with thickness around 0.5 mm were mechanically cut out from the vulcanised samples. The tests were carried out at a crosshead speed of 200 mm min-1 with a distance between clamps of 2.0 mm. The elongation during each test was determined by optical measurement (video extensometer) of the displacement of two marker points placed along the waist of the tensile test sample. An average of five measurements for each sample was recorded.
Nitrogen-fractured cross-sections of the composites were examined by scanning electron microscopy (SEM), (ESEM XL30 Model, Philips, Amsterdam, Netherlands). Samples were sputter-coated with a thin layer of 3 to 4 nm of gold/palladium lead prior to imaging.
Results and discussion
FoM calculated for several types of high dielectric constant filler/silicone composites
Stress at several strains and elongation at break for silicone and its composites
Filler content (wt.%)
Stress at 100% strain (MPa)
Stress at 300% strain (MPa)
Stress at 500% strain (MPa)
0.33 ± 0.05
0.71 ± 0.09
1.49 ± 0.18
842 ± 23
0.43 ± 0.05
0.83 ± 0.10
1.69 ± 0.22
754 ± 45
0.74 ± 0.18
1.38 ± 0.30
2.76 ± 0.59
732 ± 38
0.69 ± 0.06
1.35 ± 0.15
2.67 ± 0.39
583 ± 14
0.57 ± 0.03
1.33 ± 0.07
2.75 ± 0.19
651 ± 18
0.54 ± 0.08
1.29 ± 0.21
2.46 ± 0.43
644 ± 39
0.99 ± 0.03
2.15 ± 0.09
3.38 ± 0.17
528 ± 32
The electrical properties of CNT and FGS fillers on a silicone elastomeric matrix were studied for their possible enhancing effect on the material dielectric response. The increase on the dielectric permittivity depended on the filler content and frequency; although, FGS had a larger effect on the dielectric permittivity without significantly altering the tg (δ) value. An increase in the permittivity value, about 10 times higher than that of PDMS, was obtained at low frequency for composites with 2.0 wt.% of FGS. The presence of functional groups on the graphenes' surface and their homogenous dispersion throughout the polymer matrix was effective enough to modify the dielectric characteristics of the interface, increasing the dielectric permittivity value without the introduction of loss mechanisms. The addition of both filler nanoparticles caused a slight increment in the elastic modulus at different strains, being this fact more evident for composites containing FGS. The wrinkled morphology and the high specific surface area of the FGS employed resulted in improved adhesion with the polymeric chains. A slight decrease of elongation at break values was observed for both types of composites although good stretchability was retained.
The homogeneous FGS/silicone nanocomposites prepared in this study display desirable mechanical and dielectric properties, indicating potential applications in the electronic industry.
chemical vapour deposition
functionalised graphene sheets
Figures of Merit
scanning electron microscopy.
The authors gratefully acknowledge the financial support of the Spanish Ministry of Science and Innovation (MICINN) through project MAT 2010-18749 and the 7th Framework Program of E.U. through HARCANA (NMP3-LA-2008-213277). M. Hernández acknowledges the Venezuelan Science and Technology Ministry for a Mision Ciencia fellowship.
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