Porous, Self-Polarized Ferroelectric Polymer Films Exhibiting Behavior Reminiscent of Morphotropic Phase Boundary Induced by Size-Dependent Interface Effect for Self-Powered Sensing.

Piezoelectric poly(vinylidene fluoride) (PVDF) and its copolymer, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), have attracted considerable attention due to their potential in flexible, biocompatible energy harvesting and sensing devices. However, their limited piezoelectric performance hinders their widespread application. Inspired by the concept of morphotropic phase boundary (MPB) prevalent in high-performance piezoelectric ceramics, we successfully constructed MPB in the piezoelectric polymer P(VDF-TrFE) through size-dependent interface effects. We provided direct structural evidence using atomic force microscopy-infrared spectroscopy (AFM-IR) and significantly improved the piezoelectric performance of P(VDF-TrFE). The emergence of MPB is attributed to the interface effect induced by electrostatic interactions between ZnO fillers and the -CH2, -CF2, and -CHF groups in P(VDF-TrFE). This interaction drives a concomitant competition between the all-trans ß phase (normal ferroelectric) and the 3/1 helical phase (relaxor), resulting in enhanced piezoelectric responses in the transition region. By coupling the MPB effect with a porous structure, we developed a piezoelectric nanogenerator (PENG) that surpasses the electrical output limitation of current P(VDF-TrFE)-based PENGs. The fabricated PENG exhibits superior piezoelectric outputs (6.9 µW/cm2), impressive pressure sensitivity (2.3038 V/kPa), ultrafast response time (4.3 ms), and recovery time (46.4 ms)─notably, without the need for additional poling treatment. In practical applications, the constructed PENG can efficiently generate characteristic signals in response to various human movements and harvest biomechanical energy. This work offers insight into utilizing interface-induced MPB and proposes a simple, scalable approach for developing high-performance self-polarized piezoelectric polymer films for self-powered sensing systems toward human-machine interaction.

Distributed Subgradient Method With Random Quantization and Flexible Weights: Convergence Analysis.

The distributed subgradient (DSG) method is a widely used algorithm for coping with large-scale distributed optimization problems in machine-learning applications. Most existing works on DSG focus on ideal communication between cooperative agents, where the shared information between agents is exact and perfect. This assumption, however, can lead to potential privacy concerns and is not feasible when wireless transmission links are of poor quality. To meet this challenge, a common approach is to quantize the data locally before transmission, which avoids exposure of raw data and significantly reduces the size of the data. Compared with perfect data, quantization poses fundamental challenges to maintaining data accuracy, which further impacts the convergence of the algorithms. To overcome this problem, we propose a DSG method with random quantization and flexible weights and provide comprehensive results on the convergence of the algorithm for (strongly/weakly) convex objective functions. We also derive the upper bounds on the convergence rates in terms of the quantization error, the distortion, the step sizes, and the number of network agents. Our analysis extends the existing results, for which special cases of step sizes and convex objective functions are considered, to general conclusions on weakly convex cases. Numerical simulations are conducted in convex and weakly convex settings to support our theoretical results.

Self-Powered TENG with High Humidity Sensitivity from PVA Film Modified by LiCl and MXene.

Triboelectric nanogenerators (TENGs) are promising for a variety of applications that require a reliable output performance and stability. In this work, by utilizing the synergistic effect of lithium chloride (LiCl) and MXene, poly(vinyl alcohol) (PVA) based composite films with humidity-sensitive properties were prepared and employed as a friction layer to achieve self-powered TENGs with enhanced output performance under high humidity. The composite material demonstrates exceptional and stable output performance in the humidity range of 30-95% while exhibiting a strong linear correlation with increasing relative humidity (RH). At 95% RH, its short-circuit current increases up to 31.91 µA, which is three times the output of the TENG fabricated by PVA and PTFE (P-TENG). The rich hydroxyl group in PVA, the strong hygroscopicity of LiCl, and the microcapacitor network provided by MXene nanosheets significantly improve the water absorption capacity and surface roughness of the composite material, resulting in an excellent triboelectric output of TENG. Short-circuit current of the TENG in a wide range of RH (from 50% to 98%) responds very sensitively to humidity fluctuations in the environment and superior adsorption-desorption performance as humidity decreases. Furthermore, TENG regarded as a power supply in high humidity conditions was realized and it can light up 240 LEDs instantaneously with the transfer charge density of TENG reaching 194.37 µC m-2. This technology presents an effective method for stable energy harvesting and self-powered sensing in fog, the ocean, and other high-humidity environments.

Hydrocarbon Resin-Based Composites with Low Thermal Expansion Coefficient and Dielectric Loss for High-Frequency Copper Clad Laminates.

The rapid development of the 5G communication technology requires the improvement of the thermal stability and dielectric performance of high-frequency copper clad laminates (CCL). A cyclic olefin copolymer (COC) resin was added to the original 1,2-polybutadienes (PB)/styrene ethylene butylene styrene (SEBS) binary resin system to construct a PB/SEBS/COC ternary polyolefin system with optimized dielectric properties, mechanical properties, and water absorption. Glass fiber cloths (GFCs) and SiO2 were used to fill the resin matrix so to reduce the thermal expansion coefficient (CTE) and enhance the mechanical strength of the composites. It was found that the CTE of polyolefin/GFCs/SiO2 composite laminates decreased with the increase of SiO2 loading at first, which was attributed to the strong interfacial interaction restricting the segmental motion of polymer chains between filler and matrix. It was obvious that the addition of COC and SiO2 had an effect on the porosity, as shown in the SEM graph, which influenced the dielectric loss (Df) of the composites directly. When the weight of SiO2 accounted for 40% of the total mass of the composites, the laminates exhibited the best comprehensive performance. Their CTE and Df were reduced by 63.3% and 22.0%, respectively, and their bending strength increased by 2136.1% compared with that of the substrates without COC and SiO2. These substrates have a great application prospect in the field of hydrocarbon resin-based CCL.