Electromechanical Performance Research Articles

Electromechanical properties and physiological sensing enhancement of an in-situ assembled 3-D nanostructure: A comprehensive effect assessment of tannic acid treated 2-D Ti3C2Tx and 1-D graphene nanoribbon (GnR)

Electrospun strain sensors allow facile integration with wearable devices, however, achieving high sensitivity and extended working range are often hindered by their non-homogenous, 2-dimensionally (2-D) responsive network and its instability under large stimuli. In this study, a novel strategy consisting of (i) nano-scale surface functionalization, materials formulations and (ii) micro-scale in-situ coating technique were employed to address these challenges. A nanohybrid sensor composed of 2-D tannic acid (TA) functionalized Ti3C2Tx MXene and 1-dimensional (1-D) graphene nanoribbons (GnRs) were developed taking advantage of the short response time from Ti3C2Tx and GnR's bridging effect and flexibility. TA functionalization helped increasing the electronegative surface terminations of Ti3C2Tx, enhancing the interfacial interaction between GnR and Ti3C2Tx and the integrity of the conductive network. The robustness and the interfacial stability of the responsive network was also promoted via a technique combining coaxial electrospinning with simultaneous electrospraying. The styrene-butadiene-styrene (SBS) nanofibers functionalized with electropositive cetryl ammonium bromide (CTAB) to provide electrostatic attraction for conformal coating of the electronegative nanohybrid onto the SBS platform. This, along with the synergism between 2-D Ti3C2Tx and 1-D GnR, led to in-situ formation of a 3-D responsive network. Also, a comprehensive assessment was conducted to study the effect of the additives' geometry on the electromechanical performance. The reported structure provided exceptionally high sensitivity (Gauge Factor (GF) of 2090 at 550%), excellent dynamic stability (enduring 5000 cyclic test under 100% strain), an extended working range (0.1%–550% strain) and fast response and recovery times (50, and 46 ms, respectively) at a low detection limit (0.1%).

Electromechanical performance of dielectric elastomer composites: Modeling and experimental characterization

Dielectric Elastomer (DE) devices have gained widespread attention in recent years for their ability to generate large, reversible strains in response to electric actuation. However, some DE materials exhibit a viscoelastic nature that detracts from their electromechanical performance in applications requiring precise time-dependent actuation. This study presents an investigation into the feasibility of using multilayered DE composites, comprising commercially available acrylic-based VHB 4905 and natural rubber membranes, to overcome the viscoelastic effects. A comparison of the electromechanical performance of bi-layer and tri-layer DE composites with that of a mono-layer VHB 4910 membrane is carried out through numerical and experimental methods. We present a theoretical model of the DE composite-based bulging actuator, using an energy-based approach and the assumption of homogeneous deformations. The model predicts the time-dependent deflection of the inflated/deflated multilayered DE composites, taking into account the effect of the intervening air column. The predictive capabilities of the model are validated through experiments under both DC and AC operating conditions, and the electromechanical performance of the DE composites is examined for various actuation responses such as resonance, creep, and hysteresis. The bi-layer DE composite is found to be superior in terms of actuation levels, reduced hysteresis, and reduced creep response. The findings of this study hold promise for the development of DE materials with improved actuation capabilities while maintaining dielectric breakdown strength.

Nanocarbon Black and Molybdenum Disulfide Hybrid Filler System for the Enhancement of Fracture Toughness and Electromechanical Sensing Properties in the Silicone Rubber-Based Energy Harvester.

Recently, hybrid fillers have been found to be more advantageous in energy-harvesting composites. This study investigated the mechanical and electromechanical performances of silicone rubber-based composites made from hybrid fillers containing conductive nanocarbon black (NCB) and molybdenum disulfide (MoS2). A hybrid filler system containing only 3 phr (per hundred grams of rubber) MoS2 and 17 phr NCB provided higher fracture strain, better tensile strength, and excellent toughness values compared to the 20 phr NCB-only-filled and 5 phr MoS2-only-filled rubber composites. The chemical cross-link densities suggest that NCB promoted the formation of cross-links, whereas MoS2 slightly reduced the cross-link density. The higher mechanical properties in the hybrid filler systems suggest that the filler particles were more uniformly distributed, which was confirmed by the scanning electron microscope study. Uniformly distributed filler particles with moderate cross-link density in hybrid filler systems greatly improved the fracture strain and fracture toughness. For example, the hybrid filler with a 17:3 ratio of NCB to MoS2 showed a 184% increment in fracture toughness, and a 93% increment in fracture strain, compared to the 20 phr NCB-only-filled composite. Regarding electromechanical sensing with 2 kPa of applied cyclic pressure, the hybrid filler (17:3 CB to MoS2) performed significantly better (~100%) than the 20 phr NCB-only compound. This may have been due to the excellent distribution of conducting NCB networks and piezoelectric MoS2 that caused symmetric charging-discharging in the toughened hybrid composite. Thus, hybrid composites with excellent fatigue resistance can find dynamic applications, such as in blood pressure measurement.

Research on a high-frequency unfilled layered symmetrical piezoelectric material with a high effective electromechanical coupling coefficient

As the demand for high-performance hydrophones for marine communication systems grows, this paper develops a high-frequency unfilled layered symmetrical piezoelectric material with high effective electromechanical coupling coefficient. The vibration mode of piezoelectric material is converted into vibration mode of the piezoelectric pillar to improve the effective electromechanical coupling coefficient of the material. The material is determined by the equivalent circuit method and ANSYS finite element simulation method. Test results show that the resonant frequency is 117.8 kHz and the effective electromechanical coupling coefficient is 0.67, which shows that the developed material has a good electromechanical conversion performance.

Hierarchical Serpentine-Helix Combination for 3D Stretchable Electronics.

3D stretchable electronics attract growing interest due to their new and more complex functionalities compared to 1D or 2D counterparts. Among all 3D configuration designs, a 3D helical structure is commonly used as it can be designed to achieve outstanding stretching ratios as well as highly robust mechanical performance. However, the stretching ratio that mainly focuses on the axis direction hinders its applications. Inspired by hierarchies in a tendon, a novel structural design of hierarchical 3D serpentine-helix combination is proposed. The structural design constructed by a sequence with repeating small units winding in a helical manner around the axis can enable large mechanical forces transferred down to a smaller scale with the dissipation of potentially damaging stresses by microscale buckling, thereby endowing the electronic components made from high-performance but hard-to-stretch materials with large stretchability (≥200%) in x-, y-, or z-axis direction, high structural stability, and extraordinary electromechanical performance. Two applications including a wireless charging patch and an epidermal electronic system are demonstrated. The epidermal electronic system made of several hierarchical 3D serpentine-helix combinations allows for high-fidelity monitoring of electrophysiological signals, galvanic skin response, and finger-movement-induced electrical signals, which can achieve good tactile pattern recognition when combined with an artificial neural network.

Supersonically sprayed PVDF and ZnO flowers with built-in nanocuboids for wearable piezoelectric nanogenerators

Healthcare implant devices that can internally generate a flow of electrical charge driven by mechanical stimuli are in demand. Such self-powered electromechanical devices are known as piezoelectric nanogenerators (PENGs). In this study, a piezo-ceramic element of hydrothermally synthesized ZnO flowers comprising built-in nanocuboids was supersonically sprayed with piezo-polymer polyvinylidene fluoride (PVDF) to fabricate ZnO-embedded PVDF films for flexible PENG devices. In the presence of ZnO, the catastrophic supersonic impact and shear flow transformed the nonpolar α-phase of PVDF into the polar β-phase. The combination of ZnO flowers with built-in nanocuboids and β-phase-rich PVDF can help enhance generated charges at interfacial sites under an applied force. The supersonically sprayed PVDF/ZnO composite exhibited a 32% increase in the β-phase compared to that exhibited by pure PVDF prior to supersonic coating. The effective piezoelectric coefficient values (d33*) of the pure PVDF and PVDF/ZnO composite films were 43 and 150 pm·V‐1, respectively; the corresponding open-circuit voltages were 1.2 and 25.5 V, respectively. These significant differences confirm the pivotal role of ZnO in increasing the β-phase within PVDF during supersonic spraying. The form of the externally applied force and frequency were varied to evaluate the electromechanical performance of PVDF/ZnO. The energy harvesting capability of the composite was demonstrated using 33 light-emitting diodes.

Polybutadiene Dielectric Elastomers with High Dielectric Constant and Ultralow Dielectric Loss via Tuning Grafted Polar Groups.

This work describes an easy and effective process for preparing a homogeneous polybutadiene (PB) dielectric elastomer with improved actuated strain by utilizing a photochemical thiol-ene click reaction. The carboxyl groups and various ester groups are used for grafting on the PB. Since the length of the alkyl chains of the ester groups will significantly influence the polarities of the carbonyl groups and hydrogen bonding effect, the influence of polarity and hydrogen bonding effect on the dielectric properties and mechanical properties of the modified polybutadienes is carefully discussed. Very interestingly, it is found that the increment of dielectric constant of PB modified by carboxyl groups is the lowest one compared to other modified PBs with the ester groups. Meanwhile, the modified PBs with the ester groups could achieve pretty low dielectric loss factor, and finally, a high dielectric constant (3.6), low dielectric loss factor (0.0005), and large actuated strain (≈25%) of the modified polybutadienes could be achieved by the modified PBs with butyl acrylate groups. This work provides a simple and effective method for the designing and synthesizing of a homogeneous high electromechanical performance dielectric elastomer with high dielectric constant and low dielectric loss.

On the vibrational behavior of three-layered higher-order smart porous microplates with nanocomposite piezoelectric patches

A porous microplate embedded with two nanocomposite piezoelectric layers is investigated in the current work. All three layers are functionally graded and completely attached to each other, and the microstructure lies on a bi-parameter elastic foundation that withstands shear and normal forces simultaneously. Also, an external voltage is subjected to the piezoelectric patches. The mentioned patches are reinforced with carbon nanotubes to enhance their electro-mechanical performance. A higher-order shear deformation theory and von Karman’s assumptions are hired to demonstrate the kinematic relations. Next, applying Hamilton’s principle and variational technique, the governing motion equations are derived with the help of the modified couple stress theory to capture the scale influence. To solve the system of differential equations, a closed-form Navier method is employed, and by ensuring the correctness of the results, then it turns to investigate the impact of different factors including porosity percentage and also pores’ distribution patterns, nanotubes dispersion patterns, and other important factors on the natural frequencies of the model. Obtained results show that symmetric and uniform porosity distributions have the maximum and minimum amounts of natural frequencies. Moreover, the maximum difference between natural frequencies of three different porosity distribution occurs in e0 = 0.95 and is approximately 32%. The current study results may consider to design and produce smart structures and devices with improved performance.

ELECTRO-MECHANICAL RESPONSE OF STRETCHABLE PDMS COMPOSITES WITH A HYBRID FILLER SYSTEM

With the technological development of wearable devices, there are increasing demands for stretchable conductor that have stable electro-mechanical performance. In this study, a stretchable PDMS composite electrodes using ternary systems of fillers consisting of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) / carbon nanotube (CNT) / silver nanowire (AgNW) is explored in a perspective of electro-mechanical response. PDMS matrix is mixed with binary fillers of CNT and PEDOT:PSS, which is followed by AgNW peeling-off process. The PDMS composite is mechanically reliable especially under tensile deformation, which showed a high rupture strain of ~102 % and tensile strength of ~2.7 MPa. In addition, the PDMS composites shows the stable electro-mechanical response, where high electrical conductivity is sustained even under stretchable conditions, showing an electrical resistance value of ~11.7 Ω/cm under 40% of strain. As a demonstration, a supercapacitor using the PDMS composites is demonstrated that shows reliable electrochemical performance.

Composite sandwich plates with piezoelectric layers: Structural design, modal attributes and electric potential

This study investigates the electromechanical performance of piezoelectric sandwich plates both numerically and experimentally. The effect of different inner design parameters, namely the length-to-thickness, core-to-face thickness, boundary conditions and aspect ratio on both the eigenfrequency and electric potential attributes is assessed. Moreover, vibration amplitude-dependent effects are investigated. Polymer-based, sandwich plates are additively manufactured (AM) and commercially available PZT layers are surface-bonded and experimentally probed. AM smart plates are found to yield comparable eigenfrequency properties with the ones reported for fiber-reinforced smart sandwich composites. Low length-to-thickness, low core-to-face, and rectangular rather than square designs favor the electric potential performance.

Texture Engineering Modulating Electromechanical Breakdown in Multilayer Ceramic Capacitors

Understanding the electromechanical breakdown mechanisms of polycrystalline ceramics is critical to texture engineering for high-energy-density dielectric ceramics. Here, an electromechanical breakdown model is developed to fundamentally understand the electrostrictive effect on the breakdown behavior of textured ceramics. Taking the Na0.5 Bi0.5 TiO3 -Sr0.7 Bi0.2 TiO3 ceramic as an example, it is found that the breakdown process significantly depends on the local electric/strain energy distributions in polycrystalline ceramics, and reasonable texture design could greatly alleviate electromechanical breakdown. Then, high-throughput simulations are performed to establish the mapping relationship between the breakdown strength and different intrinsic/extrinsic variables. Finally, machine learning is conducted on the database from the high-throughput simulations to obtain the mathematical expression for semi-quantitatively predicting the breakdown strength, based on which some basic principles of texture design are proposed. The present work provides a computational understanding of the electromechanical breakdown behavior in textured ceramics and is expected to stimulate more theoretical and experimental efforts in designing textured ceramics with reliable electromechanical performances.

Transparent and Stretchable Au─Ag Nanowire Recording Microelectrode Arrays.

Transparent microelectrodes have received much attention from the biomedical community due to their unique advantages in concurrent crosstalk-free electrical and optical interrogation of cell/tissue activity. Despite recent progress in constructing transparent microelectrodes, a major challenge is to simultaneously achieve desirable mechanical stretchability, optical transparency, electrochemical performance, and chemical stability for high-fidelity, conformal, and stable interfacing with soft tissue/organ systems. To address this challenge, we have designed microelectrode arrays (MEAs) with gold-coated silver nanowires (Au─Ag NWs) by combining technical advances in materials, fabrication, and mechanics. The Au coating improves both the chemical stability and electrochemical impedance of the Au─Ag NW microelectrodes with only slight changes in optical properties. The MEAs exhibit a high optical transparency >80% at 550 nm, a low normalized 1 kHz electrochemical impedance of 1.2-7.5 Ω cm2, stable chemical and electromechanical performance after exposure to oxygen plasma for 5 min, and cyclic stretching for 600 cycles at 20% strain, superior to other transparent microelectrode alternatives. The MEAs easily conform to curvilinear heart surfaces for colocalized electrophysiological and optical mapping of cardiac function. This work demonstrates that stretchable transparent metal nanowire MEAs are promising candidates for diverse biomedical science and engineering applications, particularly under mechanically dynamic conditions.

Inkjet-printed P(VDF-TrFE) film for high-frequency annular array.

An innovative processing to deposit P(VDF-TrFE) film on silicon wafers by an inkjet printing method was used to fabricate high-frequency annular array prototype. This prototype has a total aperture of 7.3 mm and 8 active elements. A polymer-based lens with low acoustic attenuation was added to the flat deposition on the wafer, setting the geometric focus to 13.8 mm. With a thickness of around 11 μm, the electromechanical performance of P(VDF-TrFE) films was evaluated with an effective thickness coupling factor of 22%. Electronics allowing all elements to simultaneously emit as a single element transducer was developed. In reception, a dynamic focusing, based on eight independent amplifying channels, was preferred. The center frequency of the prototype was 21.3 MHz, the insertion loss was 48.5 dB and the -6 dB fractional bandwidth was 143%. The trade-off sensitivity/bandwidth has rather favored the large bandwidth. Dynamic focusing on reception was applied and allowed to improvements in the lateral-full width at half maximum as shown on images obtained with a wire phantom at several depths. The next step, for a fully operational multi-element transducer, will be to achieve a significant increase of the acoustic attenuation in the silicon wafer.

Capacitive Extensometry for the Estimation of Meaningful Stretch-Dependent Dielectric Strength of Dielectric Elastomer

The electromechanical performance of a dielectric elastomer (DE) is greatly influenced by the dielectric strength of the elastomeric materials. The dielectric strength of an elastomer profoundly depends on the state of mechanical stretch. In this work, the stretch-dependent dielectric strength for DE is measured for all possible modes of stretches using compliant electrodes. Dielectric breakdown measurement using compliant electrodes considers voltage-induced deformation, which reflects the actual working condition energy transduction. Hence, the measured strength is for a meaningful value of a working DE. Capacitive extensometry and optical method are the two leading methods for the same. We compare the extensometry and optical methods in estimating the true value of dielectric breakdown strength of DEs at high prestretches. Furthermore, we propose a modified thickness stretch-based power law for the dielectric breakdown strength. Using the capacitive extensometry method, we verify the model on comprehensive experimental data on three types of elastomers, i.e., acrylic, silicone, and natural rubber. This work proposes a general framework for estimating the meaningful stretch-dependent dielectric strength of DEs.

Outstanding electromechanical performance in piezoelectric ceramics via synergistic effects of defect engineering and domain miniaturization

Piezoelectric ceramics with high mechanical quality factor Qm and large piezoelectric coefficient d33 are urgently required for advanced piezoelectric applications. However, obtaining both of these properties simultaneously remains a difficult challenge due to their mutually restrictive relationship. Here 0.5Pb(Ni1/3Nb2/3)O3–0.5Pb(Zr0.3Ti0.7)O3 piezoceramic with tetragonal (T)-rich MPB is designed as a matrix to construct the defect engineering by doping low-valent Mn ions. The strong coupling of defect dipole and T-rich phase can effectively hinder the rotation of Ps, restrict domain wall motion and improve Qm. At the same time, the substituted Mn ions will introduce local random field, destroying the long-range ordering of ferroelectric domain and reducing domain size. The miniaturized domain structure can increase poling efficiency and inhibit the reduction of d33. Guided by this strategy, Qm has increased by more than 10 times and d33 has only decreased by about 25%. The optimized electromechanical performance with Qm = 822, d33 = 502 pC/N, kp = 0.55 and tanδ = 0.0069 can be obtained in the present study.

Link between cracking mechanisms of trilayer films on flexible substrates and electro-mechanical reliability under biaxial loading

The propagation of cracks from a top layer in trilayer systems (Cr/Cu/Mo) on a polyimide substrate is studied experimentally by in situ synchrotron x-ray diffraction under equi-biaxial loading. The results show that depending on the thickness of the ductile Cu middle layer (100 or 500 nm), the propagation can be a direct vertical path through all layers or a more complex path. These effects are analyzed by monitoring the individual stresses of each layer along with electrical resistance and resulting crack patterns. Cracks starting from the upper Cr layer propagate instantaneously through the whole system for a 100 nm Cu layer but are strongly deflected in a 500 nm Cu layer, thus delaying the global fracture of the system measured by the increase of electrical resistance. Mechanisms are proposed and allowed to anticipate the electro-mechanical performances of stretchable systems constructed of several layers.

Ferroelectric, Dielectric and Electromechanical Performance of Ba0.92Ca0.08Ti0.95Zr0.05O3 Ceramics with an Enhanced Curie Temperature.

Ba0.92Ca0.08Ti0.95Zr0.05O3 (BCZT8-5) ceramic materials have been scarcely studied as lead-free piezo/ferroelectrics despite their enhanced Curie temperature (>100 °C) with respect to most studied BCZT compositions. In this work, homogeneous dense BCZT8-5 ceramics with grain size in the range of 20 μm, and optimum ferroelectric, dielectric, and electromechanical performance, were prepared by the mixed oxides route using moderate synthesis (1250 °C-2 h) and sintering (1400 °C-2 h) conditions. Thickness-poled thin disks and monomodal shear plate resonators were used for the determination of piezoelectric coefficients, coupling factors, elastic, and dielectric permittivity coefficients, including all losses, by iterative analysis of impedance curves at resonance. Furthermore, the thermal evolution of the piezoelectric characteristics at resonance was determined to assess the enhanced working range of the ceramics (≈100 °C). Ferroelectric hysteresis loops and strains vs. electric-field butterfly loops were also measured and showed soft behavior with Ec = 2 kV/cm, Pr = 12 μC/cm2 after a maximum applied field of 3 kV was used. The ceramics showed a high endurance of P-E cycles to electrical fatigue up to 107 cycles. Moreover, dielectric properties as a function of temperature were also accomplished and showed nearly normal ferroelectric behavior, characteristic of samples with low crystallographic disorder. Overall, these ceramics showed high sensitivity and higher stability than other currently studied BCZT compositions.

Hierarchically interlocked helical conductive yarn enables ultra-stretchable electronics and smart fabrics

Simultaneously achieving high mechanical elasticity and electrical conductivity is essentially required for wearable electronics and smart fabrics. However, great challenges still remain to achieve high electrical conductance under large mechanical deformations due to the “trade-off” effect between them. Herein, inspired by curling structure of climbing plants, hierarchically interlocked helical conductive yarn for ultra-stretchable electronics and smart fabrics was developed, via over-twisting silver nanowires (AgNWs)/MXene multi-dimensional synergistic conductive networks that hierarchically entangled with elastic thermoplastic polyurethanes (TPU) nanofiber networks. High-aspect-ratio 1D AgNWs bridged 2D MXene nanosheets into synergistic interconnected conductive network while interlocked with elastic 3D TPU nanofiber skeleton. Unique hierarchically interlocking effect between highly conductive networks and highly elastic nanofiber networks during the twisting process induced helical conductive yarn simultaneously exhibits excellent mechanical stretchability and electrical conductivity. Excellent intrinsic elasticity of TPU macromolecular chains coupled with its hierarchically helical structure enables the conductive yarn exhibits excellent mechanical stretchability. While the hierarchically interlocked conductive structure ensures the stretchable yarn possess high electrical conductivity which could resist large mechanical deformations during stretching process. The hierarchically interlocked helical conductive yarn could still exhibit high electrical conductivity (1.12 × 105 S/m) even under large mechanical deformations (300%), which effectively avoid the “trade-off” effect. Moreover, the ultra-stretchable conductive yarn exhibits smart responsiveness to multi stimuli (mechanical/electron/light). The helical strain sensor has excellent electromechanical performance with stable GF (1.7) in the 600%–1000% linear range. The hierarchically interlocked helical conductive yarn holds great promise for ultra-stretchable electronics and smart fabrics.

Dramatical improvement in temperature stability of ZnO modified PNN-PZT ceramics via synergistic effect of doping and composite

In this work, doping and composite are simultaneously utilized to improve the Tm of 0.55 Pb(Ni1/3Nb2/3)O3-0.135PbZrO3-0.315PbTiO3 ceramic by adding ZnO (PNN-PZT:xZnO). It is found that Zn ions prefer to substitute for Ni ions, making Ni ions spontaneously crystallize at grain boundaries in the form of an oxide. Moreover, limited by the solid solubility, superfluous ZnO particles also stay at grain boundaries with the increasing ZnO content. Thus, triple contribution of ions substitution, local stress fields and electric fields coexist in the PNN-PZT:xZnO ceramics with high ZnO content, dramatically improving the Tm from 102 °C to 171 °C. The excellent comprehensive electromechanical performance is obtained in the sample with x = 0.05: d33 ∼ 824 pC/N, d33* ∼1059 p.m./V, Tm ∼140 °C, Ec ∼5.2 kV/cm, Qm ∼87. As the ceramic is heated to 125 °C, its k33 and d33 values still remain ∼67% and ∼80% of the corresponding room temperature values as evidenced by in-situ temperature stability measurement. Our results indicate that PNN-PZT:xZnO ceramics are the most promising candidates to develop high-performance medical ultrasound transducers for imaging, meanwhile, provide a fresh design strategy to optimize the performance of the piezoelectric ceramics.

Stretchable Polyaniline@Epoxidized Natural Rubber Composites with Strong 3D Conductive Networks for High Performance Strain Sensors

Front Cover: In article 2200365 by Jinyu Sun and co-workers, polyaniline (PANI) is successfully coated onto the surface of epoxidized natural rubber (ENR) nanospheres to form PANI@ENR-based flexible strain sensors. The strong 3D conductive network of PANI is firmly bonded with the substrate, endowing the strain sensors with excellent electrical and electromechanical performances. This sensor exhibits excellent strain sensing capabilities to monitor human joints movements and is suitable for smart wearable devices.