Polymer electrets and their applications

Polymer electrets have revealed great potential application in electromechanical devices because of the low weight, large quasi-piezoelectric sensitivity, and excellent flexibility. For an electret, a permanent and macroscopic electric field exists on the surface, principally led by a macroscopic electrostatic charge on the surface or a net orientation of polar groups inside the object. Here, progress in the development of polymer electrets is reviewed. After a brief retrospect of the research courses and those typical polymer electrets that are classified into fluorine polymer and nonfluorine polymer, we present a survey on the charging methods, including corona, soft X-ray, contact, thermal and monoenergetic particle beams. The latest representative applications (i.e., power harvesting, sensors, field effect transistors, and biomedicine) based on polymer electrets are also summarized. Finally, we complete this review with a discussion on perspectives and challenges in this field.     

Electrowetting-Assisted Generation of Ultrastable High Charge Densities in Composite Silicon Oxide–Fluoropolymer Electret Samples for Electric Nanogenerators

Electric nanogenerators that directly convert the energy of moving drops into electrical signals require hydrophobic substrates with a high density of static electric charge that is stable in “harsh environments” created by continued exposure to potentially saline water. The recently proposed charge-trapping electric generators (CTEGs) that rely on stacked inorganic oxide–fluoropolymer (FP) composite electrets charged by homogeneous electrowetting-assisted charge injection (h-EWCI) seem to solve both problems, yet the reasons for this success have remained elusive. Here, systematic measurements at variable oxide and FP thickness, charging voltage, and charging time and thermal annealing up to 230 °C are reported, leading to a consistent model of the charging process. It is found to be controlled by an energy barrier at the water-FP interface, followed by trapping at the FP-oxide interface. Protection by the FP layer prevents charge densities up to −1.7 mC m⁻² from degrading and the dielectric strength of SiO₂ enables charge decay times up to 48 h at 230 °C, suggesting lifetimes against thermally activated discharging of thousands of years at room temperature. Combining high dielectric strength oxides and weaker FP top coatings with electrically controlled charging provides a new paradigm for developing ultrastable electrets for applications in energy harvesting and beyond.     

高极化钛酸钡陶瓷中的赝热电电流

对铁电陶瓷在其Ｃｕｒｉｅ温度以上施加直流电场称为“高极化”．本研究提出了一个高极化陶瓷的简化模型,计算了赝热电电流与温度的关系以及恒定温度下该电流随时间的变化．计算结果与实验符合．     

Self‐Organization of Thin Polymer Films Guided by Electrostatic Charges on the Substrate

The self‐organization of thin polymer films into functional patterns is important both scientifically and technologically. Electric fields have been exploited as an efficient and powerful means to induce the destabilization and self‐organization of soft materials. Previous attention, however, has mainly focused on externally applied electric fields. It is shown herein that the internal electric field is strong enough to guide the self‐organization of thin polymer films as well. Patterns of electrostatic charges with micrometer resolution are first introduced on a dielectric substrate. A thin polymer film is then spin‐coated onto the topographically flat substrate. Upon thermal annealing, the thin polymer film destabilizes due to a lateral gradient of electrostatic stress and flows away from the electroneutral regime to the charged area, resembling the patterns of charges on the substrate. Theoretical and numerical modeling based on the electrohydrodynamic instability shows excellent agreement with experimental observations both qualitatively and quantitatively. It is also demonstrated that the interplay between charge‐driven instability with spinodal dewetting and Rayleigh instabilities can generate finer and hierarchical polymeric patterns that are completely different from the charge patterns preintroduced on the substrate. This study provides direct evidence that the internal electric field caused by charges on the substrate is strong enough to destabilize thin polymeric films and generate patterns. This study also demonstrates new strategies for bottom‐up fabrication of structured functional materials.     

Novel Authentic and Ultrafast Organic Photorecorders Enhanced by AIE-Active Polymer Electrets via Interlayer Charge Recombination

Organic photonic memory, featuring a variety of glamorously light-driven characteristics, is rapidly growing into an indispensable building block for next-generation optical communication systems. However, the ambiguity of their operating mechanism associated with the limitation of photoadaptive materials as an electronics promoter results in the slow development of photonic transistor-based devices. In this study, the conjugated polymers composed of donor–acceptor motifs with typical aggregation-induced emission (AIE) behaviors are designed and successfully discover high-performance photoprogrammable memory. Moreover, the mechanism of photoboosted recording behavior, attributed to the recombination of the formed interlayer excitons right after simultaneous excitation without applying vertical and parallel electric-field at the interface in-between active semiconductor and AIE polymers, is cautiously corroborated by steady-state PL and pulse PL measurements. The AIE-polymer memory devices perform ultrafast photoresponse time of 0.1 ms, an outstanding current switch ratio up to 10⁶, and retention stability over 40 000 s without significant dissipation. Furthermore, photoresponsive AIE-polymer electrets not only modulate the memory performance through the emission wavelength but easily switch storage behavior of nonvolatile memory from flash to WORM by adjusting the torsion-angle through the motif of the donor and acceptor moieties. These findings open an avenue for designing conjugated polymer electret for ultrafast optical storage devices.     

Charge storage performance of polyetherimide Ultem®1000—Influence of secondary antioxidants and purification

The performance of a polymeric electret material depends on many parameters, and besides chemical structure, charging conditions, and application temperature, other factors, such as grade, manufacturer, processing history, and additive package, are critical. Commercial polyetherimide (PEI) Ultem®1000 films exhibited satisfying electret properties as revealed by an isothermal potential decay (ITPD) to 75% of the initial surface charge after 24 h at 90°C. It was found that after purification by reprecipitation this value drops to 34% and that the same PEI synthesized by two different methods revealed to be a very poor electret with charge retention of almost zero. Assuming that an additive in the commercial material might be responsible for this behavior, we identified an organophosphonite which is commonly used as antioxidant in high‐temperature polymers. We incorporated this additive by melt compounding into purified PEI and found a dramatic increase in charge retention to 79% of the initial charge at an additive load level of 0.5 wt %. By immersing Ultem®1000 films in water, the electret behavior was further improved and almost 100% charge retention was achieved. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Porous polytetrafluoroethylene (PTFE) electret films: porosity and time dependent charging behavior of the free surface

Electrically charged porous polytetrafluoroethylene (PTFE) films are often discussed as active layers for electromechanical transducers. Here, the electric charging behavior of open-porous PTFE films with different porosities is investigated. Optimized electric charging of porous PTFE films is determined by variation of charging parameters such as electric fields and charging times. Maximum surface potentials are depending on the porosity of the PTFE films. Suitable charging leads to high surface potentials observed on non-stretched or slightly stretched porous PTFE films. Further increase of charging fields yields decreasing values of the surface potential accompanied with an increase of conductivity.     

驻极体电动机的原理研究

利用驻极体的开缝效应可以制作驻极体电动机。阐明了这种电动机的基本原理，并简要介绍了以此理论为基础而设计的实验样机的基本构思。     

2D Electrets of Ultrathin MoO2 with Apparent Piezoelectricity

Since graphene, a variety of 2D materials have been fabricated in a quest for a tantalizing combination of properties and desired physiochemical behavior. 2D materials that are piezoelectric, i.e., that allow for a facile conversion of electrical energy into mechanical and vice versa, offer applications for sensors, actuators, energy harvesting, stretchable and flexible electronics, and energy storage, among others. Unfortunately, materials must satisfy stringent symmetry requirements to be classified as piezoelectric. Here, 2D ultrathin single-crystal molybdenum oxide (MoO₂) flakes that exhibit unexpected piezoelectric-like response are fabricated, as MoO₂ is centrosymmetric and should not exhibit intrinsic piezoelectricity. However, it is demonstrated that the apparent piezoelectricity in 2D MoO₂ emerges from an electret-like behavior induced by the trapping and stabilization of charges around defects in the material. Arguably, the material represents the first 2D electret material and suggests a route to artificially engineer piezoelectricity in 2D crystals. Specifically, it is found that the maximum out-of-plane piezoresponse is 0.56 pm V⁻¹, which is as strong as that observed in conventional 2D piezoelectric materials. The charges are found to be highly stable at room temperature with a trapping energy barrier of ≈2 eV.