Piezoresistive response of spray-printed carbon nanotube/poly(vinylidene fluoride) composites
Introduction
Surface coatings are important in science and technology with applications that span domains as diverse as biomaterials, optical coatings, photovoltaic cells or corrosion protection. Intense efforts are being directed toward the development of solution-based methods such as ink-jet printing, screen-printing, doctor blading, and spray coating that can meet the manufacturing requirements for cost-effective and large area processing [1], [2].
Many methods are also specific for a given application and only a few are applicable to a wide range of materials and surfaces. An effective and simple solution-based method for the preparation of composite nanoscale films on surfaces is spray-coating. The main advantages of this technology are being cost-effective, allowing high production speed, efficient use of materials, good reproducibility and compatibility with different substrates [3], [4], [5].
Increasingly attractive for the development of smart coatings, are functional materials with sensor and/or actuator capabilities such as piezoelectric, pyroelectric, electrostrictive, magnetostrictive or piezoresistive materials, among others, though their use is still very limited for large area applications [6], [7], [8].
In this scope, force, pressure and deformation measurements have particularly large significance for practical applications. Different piezoresistive materials whose response is based on different physical mechanisms have been developed: metals typically show gauge factors around 2.0 to 3.2 [9]. In the case of silicon, the gauge factors can reach values up to 120 and silicon based devices have been fabricated with gauge factors as high as 843 [10]. However, these materials are mechanically fragile, show very limited flexibility and are difficult to shape, requiring also complex manufacturing processes and being limited to small area applications.
These facts led to the development of piezoresistive polymer based sensors than can be overcome the aforementioned limitations. Polymer based piezoresistive sensors are typically based either in conductive polymers [11] or in polymer composites with conductive fillers [12].
Conductive fillers include carbon black [13], [14], metal powder [15], carbon nonofibers (CNF) [16] and carbon nanotubes (CNT) [17], [18], [19], In particular, composites based on CNT and CNF have been developed with gauge factors up to ∼4.6 leading to suitable polymer-based strain sensors [20], [21].
Poly(vinylidene fluoride) – PVDF – has been used as a polymer matrix for the development of composites mainly due to its piezoelectric properties, when the material is in its β-phase [22]. Sensor and actuator devices have been thus developed based on this polymer and their composites [8], [23], [24], [25], [26], [27]. On the other hand, PVDF in the nonpolar α-phase is also an interesting material for the development of polymer composites due to its large dielectric constant, chemical inertness, thermal stability and suitable mechanical properties [28], [26], [29]. Further, the development of CNT filled PVDF composites will add suitable electrical characteristics to the material that, at specific filler contents, will induce electro-mechanical responses appropriate for strain sensor applications [30], [31].
Taking all of the above into account, the present work reports on the piezoresistive response of CNT/PVDF composites prepared by spray printing in order to provide easy fabrication of large area and patterned –through suitable masks-force and deformation sensors. Further, the sprayed materials are analysed in comparison to similar composites prepared by solvent casting and hot pressing in order to study the origin of the conduction mechanism of the composites.
Section snippets
Sample preparation
Single-walled carbon nanotubes (SWCNT, AP-SWNT grade) were purchased from Carbon Solutions Inc., Riverside, California. This SWCNT powder material is synthesized by the electric arc reactor method using Ni/Y catalyst and contains ∼30 wt.% metal residue. The average diameter and length of the SWCNT is 1.89 nm and 509 nm, according to atomic force microscopy measurements [32]. Poly(vinilydene fluoride), PVDF, was purchased from Aldrich (Mw ∼ 534,000, as obtained by gas phase chromatography).
The
Morphology and CNT dispersion
The observation of the samples' morphology by SEM with different amplifications (Fig. 2) shows a fibrilar-porous microstructure with good CNT dispersion (Fig. 2a). This means that the preparation method strongly affects the morphology of the samples and the dispersion of the CNT within the polymer matrix as CNT agglomerates in a polymer compact structure is observed for the samples prepared by hot-pressing (Fig. 2b) as shown in Ref. [31].
Electrical conductivity
Representative IV curves for the PVDF composites with
Conclusion
A method for the preparation of large scale piezoresistive sensors based on spray-printing is presented. It is shown that the electrical properties and the electromechanical response of the material are independent of the preparation method: spray-printing or hot-pressing. The piezoresistive response of the CNT/PVDF composites, as quantified by the GF, is ruled by tunnelling and reaches values above 4.4, which indicates a main contribution from intrinsic piezoresistive effect and demonstrating
Acknowledgements
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 and projects PTDC/EEI-SII/5582/2014 and PTDC/CTM-ENE/5387/2014. The authors also thank FCT for financial support under project PTDC/CTM-NAN/112574/2009. AF thanks the FCT for grant SFRH/BPD/102402/2014.
The authors thank financial support from the Basque Government Industry Department under the ELKARTEK
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