IET Nanodielectrics最新文献

Electrets are functional dielectrics capable of quasi-permanently storing electric charges at their surface and/or in their bulk. The electret charges are either real charges (space charges) or oriented dipoles (polarisation). Traditionally, electrets are divided into space-charge (non-polar) electrets and dipole (polar) electrets. Ferroelectrets (also called piezoelectrets) are a relatively young member added to the electret family around the end of the last century. These are non-polar polymer foams or cavity-containing polymer-film systems. The air-filled cavities carry positive and negative charges on their top and bottom internal surfaces, respectively, and thus can be considered as macroscopic dipoles, the direction of which can be switched by reversing the polarity of the charging voltage. Therefore, ferroelectrets are non-polar space-charge electrets with ferroic behaviour phenomenologically the same with that of traditional ferroelectrics.

Polymer electrets and ferroelectrets may show peculiar functionalities such as electrostatic effect, piezo-, pyro- and ferroelectricity, biological effects, non-linear optical effects, and therefore attract extensive attention from academia and industry. This special issue collects some of the latest advancements in the field of polymer electrets and ferroelectrets. In total, nine papers are accepted, which cover a wide scope of topics. One paper (of Yan et al.) presents the fundamental open-circuit thermally stimulated discharge technique for electrets. Two papers (of Yang et al. and Feng et al.) study electrets employed in energy harvesters. The papers of Chen et al. and of Jiang et al. propose an electret-based electrostatic motor that can generate a power up to 5.4 mW and electrospun PVDF microfiber sensors capable of capturing weak mechanical signals, respectively. Two papers (of Sun et al. and Wang et al.) report biological effects in electrets. The paper of Ul Hag and Wang investigates the surface potential of epoxy electrets in relation to their insulation properties, while the paper of Wang et al. brings forth compound-structured ferroelectrets that can be used as wearable devices for health monitoring. In the following a brief presentation of each paper in this special issue is given.

Yan B. et al. propose a glass-assisted open-circuit thermally stimulated discharge (GA-OCTSD) technique. The newly developed technique is applied to study fluorinated ethylene-propylene copolymer (FEP) electret films. The influences of the glass thickness, glass dielectric properties, and glass metallisation on the GA-OCTSD spectra are investigated. It turns out that the GA-OCTSD can clearly distinguish contributions from surface charge and bulk/volume charge, which is not feasible with traditional air-gap OCTSD.

Yang X. et al. report a resilient electret film-based vibrational energy harvester with a V-shaped counter electrode. A negatively charged wavy-shaped FEP electret film generates si

Piezoelectrets, also called ferroelectrets, can exhibit promising piezoelectric properties and have plenty of applications in wearable health monitoring. Usually, the cellular structure of piezoelectrets is of outstanding importance for their sensing properties, and structure improvement and optimisation would be a possible way to realise high-performance piezoelectrets. The authors proposed a compound-structured piezoelectret system, where a layer of polypropylene foam was sandwiched between two layers of solid polytetrafluoroethylene, resulting in a combination of a foam-structured and a layer-structured piezoelectrets. The compound systems are thin and flexible, they can exhibit stable electrical outputs, they have relatively broader linear working range under pressure, and promising mechanical sustainability for multiple testing. The results reveal that the compound system can be considered as a simple addition of both components, and each component contributes linearly and independently to the whole system. The application potential of this proposed compound system has been demonstrated by sleep monitoring together with carotid and radial pulse recordings, where many useful physiological information including breath, heartbeat, and pulse details can be extracted from the signals acquired by the compound system. A type of flexible sensor system that is very competitive for future portable and wearable applications may be provided.

Collection and conversion of widespread mechanical energy is one promising way to alleviate environmental pollution and energy crisis. Piezoelectric materials can effectively realise this conversion between mechanical and electrical energies. Here, via near-field electrospinning, piezoelectric poly(vinylidene fluoride) microfibers were fabricated on flexible polyethylene terephthalate substrate. Bending measurement indicated that open-circuit voltage response from piezoelectric microfibers was strain dependent but insensitive to bending frequency. The microfiber sensor could detect acoustic signals with sound pressure level between 70 and 120 dB and the recorded acoustic frequency was well consistent with the nominal frequency. Light wind from a low-power hand fan was also detected by this microfiber sensor. This simply structured and highly flexible piezoelectric microfiber sensor provided a promising and low-cost fabrication measure for weak mechanical excitation sensing.

Heat dissipation has become an important challenge and technical bottleneck for the rapid development of high-frequency microelectronic devices and high-voltage electrical equipment. Thus, there is a great urgent need for high-performance intrinsically thermally conductive polymer (ITCP) to realise effective heat dissipation. In recent year, the ITCP has received extensive attention due to excellent overall performances and clear advantages over conventional heat conductive polymer composites. The thermal transport physics and its relation with the multiscale chain conformations in polymers with diverse morphologies are reviewed. Then, the current understanding of how the chemistry of polymers, multiscale chain morphologies and conformations would affect phonon transport and the resulting thermal conductivity (TC) in both amorphous and crystalline polymers to unveil the important chemistry-structure-property relationships is discussed and anaysed. The latest advances in engineering ITCP from oriented fibre to bulk amorphous states for a high TC are summarised. Lastly, the challenges, prospects and outlook of ITCP have been proposed. The authors anticipate that the present paper will spire more fundamental and applied research in the intrinsic polymer dielectrics field to advance scientific understanding and industrial applications.

Smart sensors with excellent performance are accelerating the development of biomedicine and the Internet of Energy. Nanodielectrics exhibit unique electrical and mechanical properties. As the predominant materials in optical magnetic field sensor (MFS), they can not only exert the anti-interference of optical sensing, but improve the measuring characteristics of optical sensors. For instance, the optical fibre quantum probe for the magnetic field can obtain a higher sensitivity of 0.57 nT/Hz^1/2, while the measurement range of the sensor that uses Co-doped ZnO nanorods as cladding is 17–180 mT. Here, these exciting recent achievements in the realm of optical sensing methods for magnetic field detection are reviewed, with a focus on nanodielectrics, which provide an emerging opportunity to achieve higher sensitivity and a wider measurement range of MFS.

Semiconductor packaging materials play a critical role in the development of semiconductor devices. They not only provide reliable protection and support, but also contribute to the electrical connection between the chip and the external circuit. Among many choices of packaging materials, polymer-based nanocomposites have become the mainstream candidate due to their low cost, easy processability, and tunable properties. Materials with low dielectric constant and dielectric loss, high glass transition temperature, fast thermal conductivity, suitable coefficient of thermal expansion, low viscosity, and good processability are commonly required in semiconductor packaging, yet most polymers do not meet these criteria. Therefore, modulation of the polymer matrix, introduction of suitable fillers, and modification of the filler surface are often effective approaches to enhance the performance of the composites. Here, the authors first review current research progresses of polymer-based nanocomposites for five different types of packaging applications, namely moulding compounds, thermal interface materials, underfills, die attach materials, and substrates. The authors then present prospects of developing next-generation polymer-based nanocomposites for advanced semiconductor packaging and propose some suggestions to solve the existing challenges.

Bacterial biofilm formation is an important factor in bacterial resistance. The commonly used methods to inhibit bacterial biofilms are synthetic drugs such as antimicrobial peptides, but physical methods are often safe, non-toxic and simple to prepare. This work proposes an environmentally friendly method to use electret films to provide a stable electric field during the formation of bacterial biofilms, inhibit the formation of bacterial biofilms through the action of the electric field and weaken the adhesion of bacterial biofilms. The total amount of Staphylococcus aureus biofilm decreased by 20% compared to the control group after the treatment of positive electret. The distribution of exopolysaccharides showed that the activity of biofilm also decreased. In addition, the negative electret can also inhibit the formation of bacterial biofilm. The result can be generalised to other Gram-positive bacteria and could contribute to reduce the resistance of bacteria, improve the effect of related antibiotics, reduce the dosage of antibiotics and reduce the side effects of drugs.

Environmental protection is the future trend of power equipment development, and is also a research hotspot in the field of power cable insulation in recent years. Due to the excellent electrical properties and recyclability, polypropylene (PP) based composites are regarded as promising insulating materials for eco-friendly next-generation power cables. However, the high modulus and hardness of pure PP make it difficult to be directly employed as cable insulations, which needs to be further optimised. General methods of mechanical performance regulation often result in the deterioration of electrical performance, such as breakdown strength, space charge and so on. Therefore, it is recognised that the major challenge impeding practical applications of PP power cable insulation arises from the synergetic regulation of multi-performances. The multi-level structures influencing the multi-performances of PP are introduced by the authors and the researches on the performance enhancement of PP through nanoscale structure regulation in recent years are reviewed in detail. Seven kinds of modification methods including nano-doping, chemical grafting, in-suit copolymerisation, heat treatment, nucleating agent, voltage stabiliser and elastomer blending are paid special attention. Based on the full understanding of the research status, the challenges and issues of future research are put forward for eco-friendly PP power cable insulation.

Hybrid vibration energy harvesting technology converts vibration energy into electricity using multiple transduction mechanisms to improve output power. A frequency-tunable resonant hybrid vibration energy harvester using a piezoelectric cantilever with electret-based electrostatic coupling is proposed in this article. The electrostatic coupling including electrostatic force coupling and electrical damping coupling is introduced by an electret film placed below the cantilever, where the electrostatic force acting on the cantilever realises a tunable resonant frequency and additional electrical damping boosts power output. A coupling electromechanical model is derived using Euler–Bernoulli beam theory and Kirchhoff's law. By investigating the static and dynamic stability of cantilever, the maximum electret surface potential is defined to prevent the pull-in phenomenon. The damping of the device is evaluated, and the optimal electret surface potential is determined to obtain the matching of the electrical and mechanical damping for maximum power output. The resonant frequency of hybrid vibration energy harvester can be adjusted in range of 176.1 rad/s by changing the electret surface potential and resistive load. The experimental output power of hybrid vibration energy harvester was 5.2 μW, 27.4 times higher than that of the individual piezoelectric generator. The proposed hybrid vibration energy harvester exhibits a promising potential to power microelectronic devices and wireless sensor network node.

Flexible solar wings with high energy density, lightweight, small size and large deployment area are one of the first choices for next-generation spacecraft. However, the flexible solar wings are subjected to irradiation in space and tensile mechanical stress, which produce the charge accumulation effect and result in electrostatic discharge. It is necessary to establish a test method for the conductivity and space charge behaviour of polyimide under tensile stress. The stress–strain characteristics of polyimide under different tensile stresses are studied by the authors. The longitudinal length-strain characteristics and transverse thickness evolution characteristics under different stresses are also obtained. The results show that the variation of film thickness with tensile force is only about 1% before the yield point. The polyimide films from 50 to 200 μm thick have similar yield and tensile strengths. The ultimate stress of the specimen decreases from approximately 126 to 103 MPa with increasing thickness. The thickness model of polyimide under tensile stress were obtained, which could accurately calculate the voltage amplitude applied on the specimens for measuring the conductivity under different tensile stresses. A basis for investigating the stress–strain characteristics of polyimide films under different tensile stresses are provided, which will facilitate the formulation selection and performance improvement of polyimide for flexible solar wings of spacecraft.