Energy Harvesting Technologies for Structural Health Monitoring of Airplane Components-A Review.

With the aim of increasing the efficiency of maintenance and fuel usage in airplanes, structural health monitoring (SHM) of critical composite structures is increasingly expected and required. The optimized usage of this concept is subject of intensive work in the framework of the EU COST Action CA18203 "Optimising Design for Inspection" (ODIN). In this context, a thorough review of a broad range of energy harvesting (EH) technologies to be potentially used as power sources for the acoustic emission and guided wave propagation sensors of the considered SHM systems, as well as for the respective data elaboration and wireless communication modules, is provided in this work. EH devices based on the usage of kinetic energy, thermal gradients, solar radiation, airflow, and other viable energy sources, proposed so far in the literature, are thus described with a critical review of the respective specific power levels, of their potential placement on airplanes, as well as the consequently necessary power management architectures. The guidelines provided for the selection of the most appropriate EH and power management technologies create the preconditions to develop a new class of autonomous sensor nodes for the in-process, non-destructive SHM of airplane components.

Oxide-Halide Perovskite Composites for Simultaneous Recycling of Lead Zirconate Titanate Piezoceramics and Methylammonium Lead Iodide Solar Cells.

Global concerns over energy availability and the environment impose an urgent requirement for sustainable manufacturing, usage, and disposal of electronic components. Piezoelectric and photovoltaic components are being extensively used. They contain the hazardous element, Pb (e.g., in widely used and researched Pb(Zr,Ti)O3 and halide perovskites), but they are not being properly recycled or reused. This work demonstrates the fabrication of upside-down composite sensor materials using crushed ceramic particles recycled from broken piezoceramics, polycrystalline halide perovskite powder collected from waste dye-sensitized solar cells, and crystal particles of a Cd-based perovskite composition, C6 H5 N(CH3 )3 CdBr3 x Cl3(1- x ) . The piezoceramic and halide perovskite particles are used as filler and binder, respectively, to show a proof of concept for the chemical and microstructural compatibility between the oxide and halide perovskite compounds while being recycled simultaneously. Production of the recycled and reusable materials requires only a marginal energy budget while achieving a very high material densification of >92%, as well as a 40% higher piezoelectric voltage coefficient, i.e., better sensing capability, than the pristine piezoceramics. This work thus offers an energy- and environmentally friendly approach to the recycling of hazardous elements as well as giving a second life to waste piezoelectric and photovoltaic components.

Triboelectric Response of Electrospun Stratified PVDF and PA Structures.

Utilizing the triboelectric effect of the fibrous structure, a very low cost and straightforward sensor or an energy harvester can be obtained. A device of this kind can be flexible and, moreover, it can exhibit a better output performance than a device based on the piezoelectric effect. This study is concerned with comparing the properties of triboelectric devices prepared from polyvinylidene fluoride (PVDF) fibers, polyamide 6 (PA) fibers, and fibrous structures consisting of a combination of these two materials. Four types of fibrous structures were prepared, and then their potential for use in triboelectric devices was tested. Namely, individual fibrous mats of (i) PVDF and (ii) PA fibers, and their combination-(iii) PVDF and PA fibers intertwined together. Finally, the fourth kind was (iv), a stratified three-layer structure, where the middle layer from PVDF and PA intertwined fibers was covered by PVDF fibrous layer on one side and by PA fibrous layer on the opposite side. Dielectric properties were examined and the triboelectric response was investigated in a simple triboelectric nanogenerator (TENG) of individual or combined (i-iv) fibrous structures. The highest triboelectric output voltage was observed for the stratified three-layer structure (the structure of iv type) consisting of PVDF and PA individual and intertwined fibrous layers. This TENG generated 3.5 V at peak of amplitude at 6 Hz of excitation frequency and was most sensitive at the excitation signal. The second highest triboelectric response was observed for the individual PVDF fibrous mat, generating 2.8 V at peak at the same excitation frequency. The uniqueness of this work lies in the dielectric and triboelectric evaluation of the fibrous structures, where the materials PA and PVDF were electrospun simultaneously with two needles and thus created a fibrous composite. The structures showed a more effective triboelectric response compared to the fibrous structure electrospun by one needle.

Comparative Study of PVDF Sheets and Their Sensitivity to Mechanical Vibrations: The Role of Dimensions, Molecular Weight, Stretching and Poling.

This paper is focused on the comparative study of the vibration sensing capabilities of poly(vinylidene fluoride) (PVDF) sheets. The main parameters such as molecular weight, initial sample thickness, stretching and poling were systematically applied, and their impact on sensing behavior was examined. The mechanical properties of prepared sheets were investigated via tensile testing on the samples with various initial thicknesses. The transformation of the α-phase to the electro-active ß-phase was analyzed using FTIR after applying stretching and poling procedures as crucial post-processing techniques. As a complementary method, the XRD was applied, and it confirmed the crystallinity data resulting from the FTIR analysis. The highest degree of phase transformation was found in the PVDF sheet with a moderate molecular weight (Mw of 275 kDa) after being subjected to the highest axial elongation (500%); in this case, the ß-phase content reached approximately 90%. Finally, the vibration sensing capability was systematically determined, and all the mentioned processing/molecular parameters were taken into consideration. The whole range of the elongations (from 50 to 500%) applied on the PVDF sheets with an Mw of 180 and 275 kDa and an initial thickness of 0.5 mm appeared to be sufficient for vibration sensing purposes, showing a d33 piezoelectric charge coefficient from 7 pC N-1 to 9.9 pC N-1. In terms of the d33, the PVDF sheets were suitable regardless of their Mw only after applying the elongation of 500%. Among all the investigated samples, those with an initial thickness of 1.0 mm did not seem to be suitable for vibration sensing purposes.

Scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis of electrochemically etched graphite tips created from pencil lead.

Modern day pencil lead is a material of many possibilities. Manufacture process is fast, easy, and well established, yet the full potential of its use still remains to be uncovered. Graphite content ratio to binding clays determines basic properties of the lead like its toughness and color, but more interesting qualities like conductivity and reactivity as well. Properly employed electrochemical etching with a bubble membrane creates sharp and smooth graphite tips, which can be, given enough graphite content, used as probes in several measurement techniques. Observing and adjusting the tip creation process and the results for use in further research are the objectives of this paper.

Structure-Properties Relationship of Electrospun PVDF Fibers.

Electrospinning as a versatile technique producing nanofibers was employed to study the influence of the processing parameters and chemical and physical parameters of solutions on poly(vinylidene fluoride) (PVDF) fibers' morphology, crystallinity, phase composition and dielectric and piezoelectric characteristics. PVDF fibrous layers with nano- and micro-sized fiber diameters were prepared by a controlled and reliable electrospinning process. The fibers with diameters from 276 nm to 1392 nm were spun at a voltage of 25 kV-50 kV from the pure PVDF solutions or in the presence of a surfactant-Hexadecyltrimethylammonium bromide (CTAB). Although the presence of the CTAB decreased the fibers' diameter and increased the electroactive phase content, the piezoelectric performance of the PVDF material was evidently deteriorated. The maximum piezoelectric activity was achieved in the fibrous PVDF material without the use of the surfactant, when a piezoelectric charge of 33 pC N-1 was measured in the transversal direction on a mean fiber diameter of 649 nm. In this direction, the material showed a higher piezoelectric activity than in the longitudinal direction.

Characterization of Polyvinylidene Fluoride (PVDF) Electrospun Fibers Doped by Carbon Flakes.

Polyvinylidene fluoride (PVDF) is a modern polymer material used in a wide variety of ways. Thanks to its excellent resistance to chemical or thermal degradation and low reactivity, it finds use in biology, chemistry, and electronics as well. By enriching the polymer with an easily accessible and cheap variant of graphite, it is possible to affect the ratio of crystalline phases. A correlation between the ratios of crystalline phases and different properties, like dielectric constant as well as piezo- and triboelectric properties, has been found, but the relationship between them is highly complex. These changes have been observed by a number of methods from structural, chemical and electrical points of view. Results of these methods have been documented to create a basis for further research and experimentation on the usability of this combined material in more complex structures and devices.

A Game Changer: A Multifunctional Perovskite Exhibiting Giant Ferroelectricity and Narrow Bandgap with Potential Application in a Truly Monolithic Multienergy Harvester or Sensor.

An ABO3 -type perovskite solid-solution, (K0.5 Na0.5 )NbO3 (KNN) doped with 2 mol% Ba(Ni0.5 Nb0.5 )O3-δ (BNNO) is reported. Such a composition yields a much narrower bandgap (≈1.6 eV) compared to the parental composition-pure KNN-and other widely used piezoelectric and pyroelectric materials (e.g., Pb(Zr,Ti)O3 , BaTiO3 ). Meanwhile, it exhibits the same large piezoelectric coefficient as that of KNN (≈100 pC N-1 ) and a much larger pyroelectric coefficient (≈130 µC m-2 K-1 ) compared to the previously reported narrow-bandgap material (KNbO3 )1-x -BNNOx . The unique combination of these excellent ferroelectric and optical properties opens the door to the development of multisource energy harvesting or multifunctional sensing devices for the simultaneous and efficient conversion of solar, thermal, and kinetic energies into electricity in a single material. Individual and comprehensive characterizations of the optical, ferroelectric, piezoelectric, pyroelectric, and photovoltaic properties are investigated with single and coexisting energy sources. No degrading interaction between ferroelectric and photovoltaic behaviors is observed. This composition may fundamentally change the working principles of state-of-the-art hybrid energy harvesters and sensors, and thus significantly increases the unit-volume energy conversion efficiency and reliability of energy harvesters in ambient environments.