Development of Flexible Piezoelectric Poly(dimethylsiloxane)–BaTiO3 Nanocomposites for Electrical Energy Harvesting

Multi-Layered Triboelectric Nanogenerators with Controllable Multiple Spikes for Low-Power Artificial Synaptic Devices

In the domains of wearable electronics, robotics, and the Internet of Things, there is a demand for devices with low power consumption and the capability of multiplex sensing, memory, and learning. Triboelectric nanogenerators (TENGs) offer remarkable versatility in this regard, particularly when integrated with synaptic transistors that mimic biological synapses. However, conventional TENGs, generating only two spikes per cycle, have limitations when used in synaptic devices requiring repetitive high-frequency gating signals to perform various synaptic plasticity functions. Herein, a multi-layered micropatterned TENG (M-TENG) consisting of a polydimethylsiloxane (PDMS) film and a composite film that includes 1H,1H,2H,2H-perfluorooctyltrichlorosilane/BaTiO3 /PDMS are proposed. The M-TENG generates multiple spikes from a single touch by utilizing separate triboelectric charges at the multiple friction layers, along with a contact/separation delay achieved by distinct spacers between layers. This configuration allows the maximum triboelectric output charge of M-TENG to reach up to 7.52 nC, compared to 3.69 nC for a single-layered TENG. Furthermore, by integrating M-TENGs with an organic electrochemical transistor, the spike number multiplication property of M-TENGs is leveraged to demonstrate an artificial synaptic device with low energy consumption. As a proof-of-concept application, a robotic hand is operated through continuous memory training under repeated stimulations, successfully emulating long-term plasticity.

Dielectric and Mechanical Properties of PDMS-La2Ba2XZn2Ti3O14 (X = Mg/Ca/Sr) Nanocomposites

Flexible polydimethylsiloxane-La2Ba2XZn2Ti3O14 (X = Mg/Ca/Sr) [PDMS-LBT] nanocomposites with high permittivity (dielectric constant, k) are prepared through a room-temperature mixing process. The LBT nanoparticles used in this study are prepared through a high-temperature solid-state reaction. It is observed that LBT (X = Mg/Ca) nanoparticles are spherical in nature, with particle size ∼20 nm, as observed from the HRTEM images, whereas LBT (X = Sr) nanoparticles are cubical in nature with particle size ≥100 nm. These LBT (X = Mg/Ca/Sr) nanoparticles are crystalline in nature, as apparent from the XRD analysis and SAED patterns. The permittivity of LBT nanoparticles is higher when "Ca" is present in place of "X". These three oxides show a temperature-dependent dielectric behavior, where LBT nanoparticles with "Sr" show a sharp change in permittivity at a temperature of ∼105 °C. These kinds of oxide materials, especially LBT (X = Sr) nanoparticles/oxides, can be used in dielectric/resistive switching devices. The effect of LBT nanoparticle concentration on the dielectric and mechanical properties of PDMS-LBT nanocomposites is widely studied and found that there is a significant increase in dielectric constant with an increase in the concentration of LBT nanoparticles. There is a decrease in the volume resistivity with the increase in the LBT nanoparticle concentration. All the PDMS-LBT nanocomposites have low dielectric loss (ε″) compared to the dielectric constant value. It is found that both permittivity (ε') and AC conductivity (σac) of PDMS-LBT composites are increased with the temperature at a frequency of 1 Hz. The % elongation at break (% EB) and tensile strength (TS) decrease with the LBT nanoparticle concentration in the matrix PDMS, which is due to the non-reinforcing behavior of LBT nanoparticles. The distribution and dispersion of LBT nanoparticles in the matrix PDMS are observed through HRTEM and AFM/SPM.

Al@SiO2 Core-Shell Fillers Enhance Dielectric Properties of Silicone Composites

Over the past decade, there has been significant interest in polysiloxane-based dielectric elastomers as promising soft electroactive materials. Nevertheless, the natural low permittivity of polydimethylsiloxane has limited its practical applications. In this study, we have developed silicone rubber/Al@SiO2 composites with a high dielectric constant, low dielectric loss, and high electrical breakdown strength by controlling the shell layer thickness and the content of the core-shell filler. We also investigated the dielectric behavior of the composites. The use of core-shell fillers has increased the Maxwell-Wagner-Sillars (MWS) relaxation process while reducing the dielectric loss of direct current conductance in silicone rubber composites. Moreover, the temperature dependence of the MWS relaxation time in the composites follows the Arrhenius equation. This strategy of increasing the permittivity of silicone composites through core-shell structural fillers can inspire the preparation of other high dielectric constant composites.

Piezoelectric Nanogenerators Based On BaTiO3/PDMS Composites for High-Frequency Applications

A series of highly flexible and environmentally friendly composites based on polydimethylsiloxane (PDMS) filled with 200 nm size ferroelectric BaTiO₃ (BTO) particles at different concentrations (from 7 to 23 vol %) have been fabricated by a simple dispersion method. The dielectric, piezoelectric, and ultrasonic properties have been studied. The ferroelectric state of BTO was confirmed by differential scanning calorimetry and ultrasonic spectroscopy. The addition of BTO into PDMS strongly affects the dielectric properties of the composites. At low temperatures close to 160 K, the PDMS matrix exhibits a dielectric anomaly related to a dynamic glass transition, which shifts to higher temperatures as the BTO content increases due to the strong interaction between polymer chains and nanoparticles. Ultrasonic measurements demonstrate the appearance of a piezoelectric voltage signal on a thin plate of the composite with the highest available filler concentration (23 vol %) under longitudinal stress applied by a 10 MHz ultrasonic wave. As a result, at room temperature, the detected signal is characterized by output voltage and specific stored energy values of 10 mV and 367.3 MeV/m², respectively, followed by a further increase with cooling to 35 mV at 150 K. The proposed BTO/PDMS composite system is thus a potential candidate for nanogenerators, namely, a simple, flexible, and lead-free device converting high-frequency (10 MHz) mechanical vibrations into electrical voltage.

Metal oxide nanocomposite based flexible nanogenerator: synergic effect of light and pressure

Here, we report the fabrication of nanocomposite comprising of CuO and poly (vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) for application in flexible piezoelectric nanogenerators (PENG). The chemically grown CuO nanostructures have been characterized through electron microscopy, x-ray diffraction, and spectroscopic techniques. It has been found that the incorporation of optimal CuO nanostructures in PVDF-HFP can increase the output voltage of the PENG by 22 times and is assigned to the increment in the effective dielectric constant of host PVDF-HFP. Further, the nanogenerator exhibits a maximum power of ∼20μW cm^-2at 3 MΩ load and can charge a capacitor under continuous bio-mechanical impart. Further, upon slight alteration of the device configuration, the output of the nanocomposite-based nanogenerator can be enhanced under illumination condition. The increment in overall piezopotential through photoexcitation in optically active CuO nanostructures can be assigned to the increment in output voltage. The wavelength dependent output variation reveal the maximum output of the PENG under blue light. Further, under white light illumination, the nanogenerator exhibits a maximum power which is 3 times higher than in dark condition and can charge a capacitor 52 times faster. The development of such superior flexible and optically active nanogenerators are quite promising for futuristic self-powered devices operated under mechanical and solar energies.

Physical and mechanical properties of hybridized elastomeric foam based on ethylene-propylene-diene-monomer, multiwall carbon nanotube, and barium titanate

The use of hybrid fillers in rubbers can provide additional benefits to rubber foams compared to individual micro- or nano-scale particles due to an optimum packaging and synergic effects. The present work reports the development of vulcanized ethylene-propylene-diene-monomer nanocomposite hybrid foams filled with barium titanate and multiwall carbon nanotube (BT/MWCNT), prepared via a scalable protocol. The developed foams presented a high shear-thinning behavior, suggesting the formation of a 3D interconnected physical network of MWCNT within the polymer matrix. This network resulted in a notable improvement of the mechanical properties under tension and compression with increasing of MWCNT content. Also, the incorporation of MWCNT and BT enhanced thermal stability and thermal conductivity. Meanwhile, BT did not show any influence on the measured physical properties, due to the lack of interaction between BT and the EPDM matrix.

Actuator Performance of Dielectric Elastomers Comprising Hydrogenated Carboxylated Acrylonitrile-Butadiene Rubber/Nitrile Group-Modified Titanium Oxide Particles

We prepared a dielectric elastomer actuator composed of hydrogenated carboxylated acrylonitrile-butadiene rubber (HXNBR)/nitrile group (CN)-modified and non-modified titanium oxide (TiO₂) particles with insulation properties. The CN group-containing silane coupling agent was synthesized via a thiol-ene reaction between acrylonitrile and 3-mercaptpropyltrimethoxysilane and immobilized onto the TiO₂ particle surface. The HXNBR/CN-modified and non-modified TiO₂ particle composite elastomer showed a high relative dielectric constant and generated stress in a low electric field. The relative dielectric constant increased proportionally with the amount of CN-modified TiO₂ particles, showing a value of 22 at 100 Hz. As the dielectric constant increased, the volumetric resistivity decreased; however, the dielectric breakdown strength was maintained at 95 V/mm. The generated stress of the composite elastomer increased in proportion to the relative dielectric constant, showing a maximum of 1.9 MPa. The card-house structure of TiO₂ particles in the composite elastomer is assumed to suppress the dielectric breakdown in a low electric field. Thus, we demonstrated that an elastomer containing a high dipole group on an insulating particle surface is capable of improving the power performance of soft actuators.

Actuator Performance of a Hydrogenated Carboxylated Acrylonitrile-Butadiene Rubber/Silica-Coated BaTiO3 Dielectric Elastomer

We synthesized silica-coated barium titanate (BaTiO₃) particles with different silica shell thicknesses and evaluated the effect of silica coating on the relative dielectric properties of silica-coated BaTiO₃ particles. Furthermore, composite elastomers were prepared using hydrogenated carboxylated acrylonitrile-butadiene rubber (HXNBR) with a high relative dielectric constant (ε_r) and silica-coated BaTiO₃ particles, and their performance as an actuator was evaluated. Both ε_r and relative dielectric loss of non-coated BaTiO₃ particles increased at low frequencies (<200 Hz) associated with ionic conduction. However, ε_r and relative dielectric loss were reduced for the silica-coated BaTiO₃ particles with thick silica shells, indicating that silica coating reduced ion migration. The dielectric breakdown strength increased with the thickness of the silica shell; it increased up to 80 V/μm for HXNBR/silica-coated BaTiO₃ particles with 20 nm-thick silica shells. The maximum generated stress, strain, and output energy density of the composite elastomer with HXNBR (with a high relative constant) and silica-coated BaTiO₃ were 1.0 MPa, 7.7%, and 19.4 kJ/m³, respectively. In contrast, the values of the same parameters for a reference elastomer (acrylic/BaTiO₃; with low ε_r) were 0.4 MPa, 6.7%, and 6.8 kJ/m³ at the dielectric breakdown strength of 70 V/μm. The results indicated that the elastomers composed of HXNBR and silica-coated BaTiO₃ exhibited higher generated stress, strain, and output energy density than elastomers for conventional dielectric actuators.

Structure and Dielectric Properties of Electroactive Tetraaniline Grafted Non-Polar Elastomers

Intrinsic modification of polybutadiene and block copolymer styrene–butadiene–styrene with the electrically conducting emeraldine salt of tetraaniline (TANI) via a three-step grafting method, is reported in this work. Whilst the TANI oligomer grafted at a similar rate to both polybutadiene and styrene–butadiene–styrene under the same conditions, the resulting elastomers exhibited vastly different properties. 1 mol% TANI-PB exhibited an increased relative permittivity of 5.9, and a high strain at break of 156%, whilst 25 mol% TANI-SBS demonstrated a relative permittivity of 6.2 and a strain at break of 186%. The difference in the behaviour of the two polymers was due to the compatibilisation of TANI by styrene in SBS through π-π stacking, which prevented the formation of a conducting TANI network in SBS at. Without the styrene group, TANI-PB formed a phase separated structure with high levels of TANI grafting. Overall, it was concluded that the polymer chain structure, the morphology of the modified elastomers, and the degree of grafting of TANI, had the greatest effect on the mechanical and dielectric properties of the resultant elastomers. This work paves the way for an alternative approach to the extrinsic incorporation of conducting groups into unsaturated elastomers, and demonstrates dielectric elastomers with enhanced electrical properties for use in actuation devices and energy harvesting applications.

Facile Fabrication of Environmentally-Friendly Hydroxyl-Functionalized Multiwalled Carbon Nanotubes/Soy Oil-Based Polyurethane Nanocomposite Bioplastics with Enhanced Mechanical, Thermal, and Electrical Conductivity Properties

It is challenging to prepare polyurethane bioplastics from renewable resources in a sustainable world. In this work, polyurethane nanocomposite bioplastics are fabricated by blending up to 80 wt % of soy-based polyol and petrochemical polyol with hydroxyl-functionalized multiwalled carbon nanotubes (MWCNTs-OH). The scanning electron microscope (SEM), transmission electron microscope (TEM), and Fourier transform infrared spectroscopy (FTIR) analyses reveal homogeneous dispersion of the MWCNTs-OH in the matrix, as well as interaction or reaction of MWCNTs-OH with the matrix or polymeric methylene diphenyl diisocyanate (pMDI) in forming the organic-inorganic hybrid bioplastic with a three-dimensional (3D) macromolecule network structure. Mechanical properties and electrical conductivity are remarkably enhanced with the increase of the multiwalled carbon nanotube (MWCNTs) loading. Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) results show that the bioplastics with MWCNTs-OH have a better thermal stability compared with the bioplastics without MWCNTs-OH. The composition of the nanocomposites, which defines the characteristics of the material and its thermal and electrical conductivity properties, can be precisely controlled by simply varying the concentration of MWCNTs-OH in the polyol mixture solution.

Intrinsic Tuning of Poly(styrene-butadiene-styrene)-Based Self-Healing Dielectric Elastomer Actuators with Enhanced Electromechanical Properties

The electromechanical properties of a thermoplastic styrene-butadiene-styrene (SBS) dielectric elastomer was intrinsically tuned by chemical grafting with polar organic groups. Methyl thioglycolate (MG) reacted with the butadiene block via a one-step thiol-ene "click" reaction under UV at 25 °C. The MG grafting ratio reached 98.5 mol % (with respect to the butadiene alkenes present) within 20 min and increased the relative permittivity to 11.4 at 10³ Hz, with a low tan δ. The actuation strain of the MG-grafted SBS dielectric elastomer actuator was 10 times larger than the SBS-based actuator, and the actuation force was 4 times greater than SBS. The MG-grafted SBS demonstrated an ability to achieve both mechanical and electrical self-healing. The electrical breakdown strength recovered to 15% of its original value, and the strength and elongation at break recovered by 25 and 21%, respectively, after 3 days. The self-healing behavior was explained by the introduction of polar MG groups that reduce viscous loss and strain relaxation. The weak CH/π bonds through the partially charged (δ+) groups adjacent to the ester of MG and the δ- center of styrene enable polymer chains to reunite and recover properties. Intrinsic tuning can therefore enhance the electromechanical properties of dielectric elastomers and provides new actuator materials with self-healing mechanical and dielectric properties.

Stretchable Electrospun PVDF-HFP/Co-ZnO Nanofibers as Piezoelectric Nanogenerators

Herein, we investigate the morphology, structure and piezoelectric performances of neat polyvinylidene fluoride hexafluoropropylene (PVDF-HFP) and PVDF-HFP/Co-ZnO nanofibers, fabricated by electrospinning. An increase in the amount of crystalline β-phase of PVDF-HFP has been observed with the increase in Co-doped ZnO nanofiller concentration in the PVDF-HFP matrix. The dielectric constants of the neat PVDF-HFP and PVDF-HFP/2 wt.% Co-ZnO nanofibers are derived as 8 and 38 respectively. The flexible nanogenerator manipulated from the polymer nanocomposite (PVDF-HFP/Co-ZnO) exhibits an output voltage as high as 2.8 V compared with the neat PVDF-HFP sample (~120 mV). These results indicate that the investigated nanocomposite is appropriate for fabricating various flexible and wearable self-powered electrical devices and systems.

Synthesis of MnO2 nanoparticles and their effective utilization as UV protectors for outdoor high voltage polymeric insulators used in power transmission lines

The mechanism of the UV protection of polymers by MnO₂ where UV radiation is absorbed by MnO₂ particles present in the PDMS/EVA–MnO₂ composite used in high voltage outdoor insulators for power transmission lines.