RESUMEN
Achieving relatively uniform dispersion in organic-inorganic composites with overwhelming differences in surface energy is a perennial challenge. Herein, novel eliminated polyvinylidene fluoride (EPVDF)/EPVDF functionalized barium titanate nanoparticles (EPVDF@BT) flexible piezoelectric nanogenerators (PENGs) with strong interfacial adhesion are developed via thermal stretching following sequential click chemistry. Thanks to the strong interfacial adhesion, the optimal PENGs containing ultra-high ß-phase content (97.2%) exhibit not only optimized mechanical and dielectric behaviors but also excellent piezoelectric properties with high piezoelectric output (V = 10.7 V, I = 216 nA), reliable durability (8000 cycles), ultrafast response time (20 ms), and good sensitivity (2.09 nA kPa-1), far outperforming most reported PVDF-based composites. Furthermore, COMSOL finite element simulations (FEM) confirm that the elevated stress transfer efficiency induced by the strong interfacial adhesion is the main driving force for enhanced piezoelectric performances. For practical applications, self-powered PENGs can simply but stably capture mechanical energy, drive tiny electronic devices, and serve as potential multifunctional and durable sensors for detecting human physiological motions. This work opens a pioneering avenue to break the trade-offs between piezoelectric and other properties, which is of great importance for developing self-powered flexible sensors.
RESUMEN
Constructing porous structures in electromagnetic interference (EMI) shielding materials is a common strategy to decrease the secondary pollution caused by the reflection of electromagnetic waves (EMWs). However, the lack of direct analysis methods makes it difficult to fully understand the effect of porous structures on EMI, hindering EMI composites' development. Furthermore, while deep learning techniques, such as deep convolutional neural networks (DCNNs), have significantly impacted material science, their lack of interpretability limits their applications to property predictions and defect detection tasks. Until recently, advanced visualization techniques provided an approach to reveal the relevant information behind DCNNs' decisions. Inspired by it, a visual approach for porous EMI nanocomposite mechanism studies is proposed. This work combines DCNN visualization with experiments to investigate EMI porous nanocomposites. First, a rapid and straightforward salt-leaked cold-pressing powder sintering method is employed to prepare high-EMI CNTs/PVDF composites with various porosities and filler loadings. Notably, the solid sample with 30 wt % loading maintains an ultrahigh shielding effectiveness of 105 dB. The influence of porosity on the shielding mechanism is discussed macroscopically based on the prepared samples. To determine the shielding mechanism, a modified deep residual network (ResNet) is trained on a dataset of scanning electron microscopy (SEM) images of the samples. The Eigen-CAM visualization of the modified ResNet intuitively shows that the amount and depth of the pores impact the shielding mechanisms and that shallow pore structures contribute less to EMW absorption. This work is instructive for material mechanism studies. Besides, the visualization has the potential as a porous-like structure marking tool.
RESUMEN
With the rapid development of wearable self-powered devices, the piezoelectric materials having deformable and switchable characteristics are attracting extensive attention. Herein, the cross-linked polyvinylidene fluoride (cPVDF) was fabricated through an alkali-catalyzed defluorination and chemical cross-linking method by introducing trimethylhexamethylenediamine (THDA). The system filled with 1 wt % THDA (CP1) was proved to possess balanced cross-linking density and crystallinity, which would play a crucial role in achieving a switchable piezoelectric effect. In comparison to pristine PVDF, the cross-linked one exhibited repeatable shape memory characterization due to restrained plastic deformation above the melting transition. Both the shape-fixing and shape-recovery ratios were stably maintained above 90%. More significantly, the thermo-mechanical program also triggered the α-ß-α crystal transition accompanied by the variation of conformational entropy. The largest amount of ß crystals was produced in the temporary shape, whereas the original and recovery shapes were dominated by α crystals. Such structural transition occurred repeatedly in the successive shape memory cycles, which thereby induced the periodic fluctuation of the piezoelectric constant (d33). For the CP1 sample, its d33 was only about 2 pC/N in the original and recovery shapes but reached up to 9.4 pC/N in the temporary shape. When the latter one was fabricated into a piezoelectric device, alternating voltage and current were generated by performing periodic impact force and were demonstrated to be capable of monitoring some pressure-related motions in real time without an external power supply. Finally, the switchable piezoelectric effect of the CP1 at different shape memory stages was further revealed through its electroactive response to the sinusoidal voltage stimulation. This work offers a special perspective in tailoring piezoelectric performance through the structural transition in shape memory progress, which is of great significance for enriching the types and applications of piezoelectric polymers.