Letter Cite This: ACS Macro Lett. 2019, 8, 414−420
pubs.acs.org/macroletters
Soft Piezoionic/Piezoelectric Nanocomposites Based on Ionogel/ BaTiO3 Nanoparticles for Low Frequency and Directional Discriminative Pressure Sensing Sara Moon Villa,†,‡ Vittorio Massimo Mazzola,†,‡ Tommaso Santaniello,*,†,‡ Erica Locatelli,§ Mirko Maturi,§ Lorenzo Migliorini,†,∥ Ilaria Monaco,§ Cristina Lenardi,† Mauro Comes Franchini,§ and Paolo Milani*,† †
CIMaINa, Department of Physics, University of Milan, Via Celoria 16, 20133, Milan, Italy Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy ∥ Department of Chemistry, University of Milan, Via Golgi 19, 20133, Milan, Italy
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S Supporting Information *
ABSTRACT: We report on the fabrication and electro-mechanical characterization of a nanocomposite system exhibiting anisotropic electrical response under the application of tactile compressive stresses (5 kPa) at low frequencies (0.1−1 Hz). The nanocomposite is based on a chemically cross-linked gel incorporating a highly conductive ionic liquid and surface functionalized barium titanate (BaTiO3) ferroelectric nanoparticles. The system was engineered to respond to mechanical stimulations by combining piezoionic and piezoelectric activity, generating electric charge due to a redistribution of the mobile ions across the polymer matrix and to the presence of the electrically polarized ceramic nanoparticles, respectively. The nanocomposite response was characterized in a quasi-static regime using a custom-designed apparatus. The results obtained showed that the combination of both piezo-effects led to output voltages up to 8 mV and anisotropy in the response. This allows to discriminate the sample orientation with respect to the load direction by monitoring the phase and amplitude modulation of the output signal. The integration of cluster-assembled gold electrodes produced by Supersonic Cluster Beam Deposition (SCBD) was also performed, enabling to enhance the charge transduction efficiency by a factor of 10, compared to the bare nanocomposite. This smart piezoionic/piezoelectric nanocomposite represents an interesting solution for the development of soft devices for discriminative touch sensing and objects localization in physically unstructured environments. ecent developments in the field of soft robotics and wearable electronics1−4 have stimulated the efforts toward the fabrication of a new class of energy harvesting and sensing devices to be integrated into soft polymeric platforms.5 The target is a direct and efficient conversion between mechanical and electrical energy, which can be achieved by piezoelectric, triboelectric, and electromagnetic or electrostatic induction transducers.6 To efficiently couple these devices with soft robotic components, portability, flexibility and stretchability of the devices are fundamental features.7−9 Piezoelectric polymerbased nanocomposites and soft ionic conductive gels showed to be the most promising for the fabrication of deformable functional devices.10−14
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© XXXX American Chemical Society
Piezoelectric nanocomposites are electro-mechanical transducers based on polymeric materials incorporating piezoelectric nanoparticles, such as ferroelectric perovskites (e.g., barium titanate, BaTiO3), presenting a net polarization due to their noncentrosymmetric structure. The crystallites are randomly oriented, so that the net polarization is averaged out, but it is possible to reorient the ferroelectric domains by subjecting the material to intense static electric fields and, thus, obtain a nonzero polarization.11,15 This procedure is called Received: December 28, 2018 Accepted: March 20, 2019
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DOI: 10.1021/acsmacrolett.8b01011 ACS Macro Lett. 2019, 8, 414−420
Letter
ACS Macro Letters poling. The piezoelectric response of poled perovskites determines the nanocomposite electromechanical performance and has a double contribution: a lattice contribution, due to single domains deformation, and an extrinsic contribution, due to the relative displacement of the domains. Because of these features, piezoelectric nanocomposites respond to external mechanical stimuli by generating an output voltage ranging from a few millivolts to hundreds of volts, depending primarily on the nanofiller type and amount, polymer−filler interaction, and poling conditions.16−20 Their operational output voltages render them suitable candidates for a variety of sensing and energy harvesting applications.16,21−23 They usually require high frequency stimulation,6 which implies a significant power loss in the electro-mechanical conversion of low frequency movements, as the ones typical of human motion.6 Another limitation is the requirement of high nanofiller amounts (up to 40 wt %) and severe poling conditions (field intensity around 100 kV/cm and high temperatures) to reach such performances.16 The use of large volume fractions of the filler involves a drastic decrease in compliancy and flexibility of the composite with respect to the pristine polymer and often results in a nonhomogeneous dispersion of the nanostructures in the polymer matrix.24 Piezoionic gels are an emerging class of soft smart materials constituted by a polymeric backbone filled with a fluid containing mobile ions, such as room temperature ionic liquids or a solved salt in aqueous solution.25 The piezoionic effect consists in the generation of an output voltage induced by the separation of ions with different mobility, stimulated by a differential pressure applied to the material.23,25,26 The redistribution of the ionic charges inside the polymer is driven by the matrix volumetric changes at the microscopic level induced by the imposed mechanical stress, which locally alter the ions concentration. The piezoionic effect depends on the ion type, size, and mobility, as well as on the mechanical properties of the polymeric network. These gels have low output voltages (1−10 mV)23,26−28 and respond well to low frequencies (
