Ultra-Wideband Electromagnetic Composite Absorber Based on Pixelated Metasurface with Optimization Algorithm.

An ultra-wideband electromagnetic (EM) absorber is proposed. The proposed absorber consists of two thin metasurfaces, four dielectric layers, a glass fiber reinforced polymer (GFRP), and a carbon fiber reinforced polymer (CFRP) which works as a conductive reflector. The thin metasurfaces are accomplished with 1-bit pixelated patterns and optimized by a genetic algorithm. Composite materials of GFRP and CFRP are incorporated to improve the durability of the proposed absorber. From the full-wave simulation, more than 90% absorption rate bandwidth is computed from 2.2 to 18 GHz such that the fractional bandwidth is about 156% for the incidence angles from 0° to 30°. Absorptivity is measured using the Naval Research Laboratory (NRL) arch method in an EM anechoic environment. It was shown that the measured results correlated with the simulated results. In addition, the proposed absorber underwent high temperature and humidity tests under military environment test conditions in order to investigate its durability.

Enhancing Diagnosis of Rotating Elements in Roll-to-Roll Manufacturing Systems through Feature Selection Approach Considering Overlapping Data Density and Distance Analysis.

Roll-to-roll manufacturing systems have been widely adopted for their cost-effectiveness, eco-friendliness, and mass-production capabilities, utilizing thin and flexible substrates. However, in these systems, defects in the rotating components such as the rollers and bearings can result in severe defects in the functional layers. Therefore, the development of an intelligent diagnostic model is crucial for effectively identifying these rotating component defects. In this study, a quantitative feature-selection method, feature partial density, to develop high-efficiency diagnostic models was proposed. The feature combinations extracted from the measured signals were evaluated based on the partial density, which is the density of the remaining data excluding the highest class in overlapping regions and the Mahalanobis distance by class to assess the classification performance of the models. The validity of the proposed algorithm was verified through the construction of ranked model groups and comparison with existing feature-selection methods. The high-ranking group selected by the algorithm outperformed the other groups in terms of training time, accuracy, and positive predictive value. Moreover, the top feature combination demonstrated superior performance across all indicators compared to existing methods.

Fabrication of Highly Sensitive Capacitive Pressure Sensors Using a Bubble-Popping PDMS.

Attempts have been made to introduce microstructures or wrinkles into the elastomer surface to increase the sensitivity of the elastomer. However, the disadvantage of this method is that when a force is applied to the pressure sensor, the contact area with the electrode is changed and the linear response characteristic of the pressure sensor is reduced. The biggest advantage of the capacitive pressure sensor using an elastomer is that it is a characteristic that changes linearly according to the change in pressure, so it is not suitable to introduce microstructures or wrinkles into the elastomer surface. A method of increasing the sensitivity of the capacitive pressure sensor while maintaining the linearity according to the pressure change is proposed. We proposed a bubble-popping PDMS by creating pores inside the elastomer. The sensitivity of the pressure sensor made of the bubble-popping PDMS was approximately 4.6 times better than that of the pressure sensor without pores, and the pressure sensor made of the bubble-popping PDMS showed a high linear response characteristic to the external pressure change. These results show that our pressure sensor can be used to detect applied pressures or contact forces of e-skins.

Multi-Geometry Parameters Optimization of Large-Area Roll-to-Roll Nanoimprint Module Using Grey Relational Analysis and Artificial Neural Network.

Micro- and nanofabrication on polymer substrate is integral to the development of flexible electronic devices, including touch screens, transparent conductive electrodes, organic photovoltaics, batteries, and wearable devices. The demand for flexible and wearable devices has spurred interest in large-area, high-throughput production methods. Roll-to-roll (R2R) nanoimprint lithography (NIL) is a promising technique for producing nano-scale patterns rapidly and continuously. However, bending in a large-scale R2R system can result in non-uniform force distribution during the imprinting process, which reduces pattern quality. This study investigates the effects of R2R imprinting module geometry parameters on force distribution via simulation, using grey relational analysis to identify optimal parameter levels and ANOVA to determine the percentage of each parameter contribution. The study also investigates the length and force ratio on a backup roller used for bending compensation. The simulation results and the artificial neural network (ANN) model enable the prediction of nip pressure and force distribution non-uniformity along the roller, allowing the selection of the optimal roller geometry and force ratio for minimal non-uniformity on a specific R2R system. An experiment was conducted to validate the simulation results and ANN model.

Layer-by-Layer Engineered Flexible Functional Film Fabrication with Spreadability Control in Roll-to-Roll Manufacturing.

A roll-to-roll manufacturing system performs printing and coating on webs to mass-produce large-area functional films. The functional film of a multilayered structure is composed of layers with different components for performance improvement. The roll-to-roll system is capable of controlling the geometries of the coating and printing layers using process variables. However, research on geometric control using process variables is limited to single-layer structures only. This study entails the development of a method to proactively control the geometry of the upper coated layer by using the lower-layer coating process variable in the manufacture of a double-coated layer. The correlation between the lower-layer coating process variable and upper coated layer geometry was examined by analyzing the lower-layer surface roughness and spreadability of the upper-layer coating ink. The correlation analysis results demonstrate that tension was the dominant variable in the upper coated layer surface roughness. Additionally, this study found that adjusting the process variable of the lower-layer coating in a double-layered coating process could improve the surface roughness of the upper coating layer by up to 14.9%.

Web Wrinkle Defects due to Temperature Profile in Roll-to-Roll Manufacturing Systems.

The roll-to-roll manufacturing system is extensively used for mass producing products made of plastic, paper, and fabric in several traditional industries. When flexible substrates, also known as webs, are heated and transported inside the dryer, an inconsistent temperature distribution occurs on the material in the machine direction (MD) and cross-machine direction (CMD). If rollers are not aligned in parallel on the same plane in the roll-to-roll web handling process, or if roller misalignment exists, strain deviation occurs in the web, resulting in lateral displacement and web wrinkles. Therefore, this study examined a wrinkle, which is a thermal deformation that occurs when an inconsistent web temperature distribution is formed on the material inside a dryer. The changes in the elastic modulus and thermal expansion of the web were also examined. Experiments were conducted using a PET film, and its elastic modulus and thermal expansion were examined. The results showed that the presence of a web wrinkle defect can cause a thickness deviation in the functional layer manufactured on the web. Moreover, an appropriate operating speed should be set to reduce the CMD temperature deviation, thereby reducing instances of wrinkle defects.

Ultrawideband electromagnetic metamaterial absorber utilizing coherent absorptions and surface plasmon polaritons based on double layer carbon metapatterns.

An ultrawideband electromagnetic metamaterial absorber is proposed that consists of double-layer metapatterns optimally designed by the genetic algorithm and printed using carbon paste. By setting the sheet resistance of the intermediate carbon metapattern to a half of that of the top one, it is possible to find an optimal intermediate metapattern that reflects and absorbs the EM wave simultaneously. By adding an absorption resonance via a constructive interference at the top metapattern induced by the reflection from the intermediate one, an ultrawideband absorption can be achieved without increasing the number of layers. Moreover, it is found that the metapatterns support the surface plasmon polaritons which can supply an additional absorption resonance as well as boost the absorption in a broad bandwidth. Based on the simulation, the [Formula: see text] absorption bandwidth is confirmed from 6.3 to 30.1 GHz of which the fractional bandwidth is 130.77[Formula: see text] for the normal incidence. The accuracy is verified via measurements well matched with the simulations. The proposed metamaterial absorber could not only break though the conventional concept that the number of layers should be increased to extend the bandwidth but also provide a powerful solution to realize a low-profile, lightweight, and low cost electromagnetic absorber.

A photonic sintering derived Ag flake/nanoparticle-based highly sensitive stretchable strain sensor for human motion monitoring.

Recently, the demand for stretchable strain sensors used for detecting human motion is rapidly increasing. This paper proposes high-performance strain sensors based on Ag flake/Ag nanocrystal (NC) hybrid materials incorporated into a polydimethylsiloxane (PDMS) elastomer. The addition of Ag NCs into an Ag flake network enhances the electrical conductivity and sensitivity of the strain sensors. The intense localized heating of Ag flakes/NCs is induced by intense pulsed light (IPL) irradiation, to achieve efficient sintering of the Ag NCs within a second, without damaging the PDMS matrix. This leads to significant improvement in the sensor sensitivity. Our strain sensors are highly stretchable (maximum strain = 80%) and sensitive (gauge factor = 7.1) with high mechanical stability over 10 000 stretching cycles under 50% strain. For practical demonstration, the fabrication of a smart glove for detecting the motions of fingers and a sports band for measuring the applied arm strength is also presented. This study provides an effective method for fabricating elastomer-based high-performance stretchable electronics.

Bio-compatible organic humidity sensor transferred to arbitrary surfaces fabricated using single-cell-thick onion membrane as both the substrate and sensing layer.

A bio-compatible disposable organic humidity sensor has been fabricated that can be transferred to any arbitrary target surface. Single cell thick onion membrane has been used as the substrate while it also doubles as the active layer of the sensor. Two different types of sensors were fabricated. In type-1, the membrane was fixed into a plastic frame with IDT patterns on one side while the other side was also exposed to environment. In type-2, onion membrane was attached to a glass substrate with one side exposed to environment having an IDT screen-printed on top of it. The electrical output response of the sensors showed their ability to detect relative humidity between 0% RH and 80% RH with stable response and good sensitivity. The impedance of the sensors changed from 16 MΩ to 2 MΩ for type-1 and 6 MΩ to 20 KΩ for type-2. The response times of type-1 and type-2 were ~1 and 1.5 seconds respectively. The recovery times were ~10.75 seconds and ~11.25 seconds for type-1 and type-2 respectively. The device was successfully transferred to various randomly shaped surfaces without damaging the device.

Development of a precision reverse offset printing system.

In printed electronics technology, the overlay accuracy of printed patterns is a very important issue when applying printing technology to the production of electric devices. In order to achieve accurate positioning of the printed patterns, this study proposes a novel precision reverse offset printing system. Furthermore, the study evaluates the effects of synchronization and printing force on position errors of the printed patterns, and presents methods of controlling synchronization and printing force so as to eliminate positional errors caused by the above-mentioned reasons. Finally, the printing position repeatability of 0.40 µm and 0.32 µm (x and y direction, respectively) at a sigma level is obtained over the dimension of 100 mm under repeated printing tests with identical printing conditions.

Multi-layer electrode with nano-Li4Ti5O12 aggregates sandwiched between carbon nanotube and graphene networks for high power Li-ion batteries.

Self-aggregated Li4Ti5O12 particles sandwiched between graphene nanosheets (GNSs) and single-walled carbon nanotubes (SWCNTs) network are reported as new hybrid electrodes for high power Li-ion batteries. The multi-layer electrodes are fabricated by sequential process comprising air-spray coating of GNSs layer and the following electrostatic spray (E-spray) coating of well-dispersed colloidal Li4Ti5O12 nanoparticles, and subsequent air-spray coating of SWCNTs layer once again. In multi-stacked electrodes of GNSs/nanoporous Li4Ti5O12 aggregates/SWCNTs networks, GNSs and SWCNTs serve as conducting bridges, effectively interweaving the nanoporous Li4Ti5O12 aggregates, and help achieve superior rate capability as well as improved mechanical stability of the composite electrode by holding Li4Ti5O12 tightly without a binder. The multi-stacked electrodes deliver a specific capacity that maintains an impressively high capacity of 100 mA h g(-1) at a high rate of 100C even after 1000 cycles.

Long-term sustainable aluminum precursor solution for highly conductive thin films on rigid and flexible substrates.

To fabricate the highly conductive Al film via a solution process, AlH3 etherates have been a unique Al source despite their chemical instability in solvents and thus lack of long-term sustainability. Herein, we suggest an innovative solution process to overcome the aforementioned drawbacks in AlH3 etherates; AlH3 aminates powder, which can be stored in low temperature surroundings and redissolved in solvents whenever it is needed. Since refrigeration of AlH3 aminates, AlH3{N(CH3)3}, was very effective to prevent its chemical degradation, Al film with excellence and uniformity in electrical and mechanical properties was successfully fabricated even by the 180-day stored AlH3{N(CH3)3} dissolved in solvents. Moreover, the applicability of long-term stored AlH3{N(CH3)3} to electronic devices was experimentally demonstrated by the successful operation of LED lamps connected to the Al pattern films on glass, PET, and paper substrates.

Development of nanostructured ZnO thin film via electrohydrodynamic atomization technique and its photoconductivity characteristics.

This article presents the non-vacuum technique for the preparation of nanostructured zinc oxide (ZnO) thin film on glass substrate through electrohydrodynamic atomization (EHDA) technique. The detailed process parameters for achieving homogeneous ZnO thin films are clearly discussed. The crystallinity and surface morphology of ZnO thin film are investigated by X-ray diffraction and field emission scanning electron microscopy. The result shows that the deposited ZnO thin film is oriented in the wurtzite phase with void free surface morphology. The surface roughness of deposited ZnO thin film is found to be ~17.8 nm. The optical properties of nanostructured ZnO thin films show the average transmittance is about 90% in the visible region and the energy band gap is found to be 3.17 eV. The surface chemistry and purity of deposited ZnO thin films are analyzed by fourier transform infrared and X-ray photoelectron spectroscopy, conforming the presence of Zn-O in the deposited thin films without any organic moiety. The photocurrent measurement of nanostructured ZnO thin film is examined in the presence of UV light illumination with wavelength of 365 nm. These results suggest that the deposited nanostructured ZnO thin film through EHDA technique possess promising applications in the near future.

Fabrication and characterization of flexible thin film super-capacitor with silver nano paste current collector.

Flexible thin film super-capacitors with the silver paste current collector were printed and their electrochemical characteristics were investigated to apply for a low cost solution-based printing process. The silver paste current collector was printed on a flexible Polyethylene Telephtalate (PET) substrate and the activated carbon electrode was printed in a sequence by using a mature screen printing. In experimental evaluation, three silver pastes with different solid contents were prepared and compared because sheet resistance depended on the thickness of the current collector. By using the confocal image, the thickness of the printed electrode of the activated carbon was measured to be 27.8 microm. Cyclic voltammogram, the specific capacitance and impedence together with capacitance retension were examined to determine the performance of the printed super-capacitor. The highest specific capacitance of 53.05 F/g at a scan rate of 10 mV/s was obtained. The measurement results show that the printed super-capacitors with the silver paste current collector have a great potential to apply for wearable electronics and protable electronic devices.

Fabrication of nanostructured copper indium diselenide (CIS) thin films by electrohydrodynamic atomization technique.

In this article, we report a non-vacuum electrohydrodynamic atomization (EHDA) technique for deposition of CulnSe2 (CIS) thin films. The CIS ink has been prepared with three different concentrations (7.5 wt.%, 12.5 wt.% and 15 wt.%) by using suitable solvent mixture (ethanol:terpineol as 1:1 molar ratio) with surfactant to achieve a stable dispersions. The important physical parameters for achieving homogeneous with non-agglomerated CIS layers through EHDA technique are investigated in detail. The X-ray diffraction pattern confirms the crystalline structure of CIS layers oriented in the chalcopyrite phase. The film uniformity has been investigated by field emission scanning electron microscopy. Different thickness of CIS layers has been achieved by varying the concentration of CIS particles in the precursor ink solution. The optical properties of CIS layers show the two optical band gaps in UV-visible and near infra-red (NIR) region with band gap of about 2.67-2.49 eV and 1.34-1.29 eV respectively. The energy band gap of CIS thin films have been decreased with the increase of film thickness. The X-ray photoelectron spectra confirmed presence of binding energy corresponding to CulnSe2. The electrical study observed the sheet resistivity 76-33 Omega cm with respect to film thickness.

Structural and electrical properties of Ag grid/poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) coatings for diode application through advanced printing technology.

This paper is focused on printed techniques for the fabrication of hybrid structure of silver (Ag) grid/poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) (PEDOT:PSS) on polyethylene terepthalate (PET) as a flexible substrate. Ag grid has been printed on PET substrate by using gravure offset printing process, followed by PEDOT:PSS thin film deposition on Ag grid through electrohydrodynamic atomization (EHDA) technique. The important parameters for achieving uniform hybrid structure of Ag grid/PEDOT:PSS through printed techniques have been clearly discussed. Field emission scanning electron microscope studies revealed the uniformity of printed Ag grid with homogeneous deposition of PEDOT:PSS on Ag grid. The optical properties of Ag grid/PEDOT:PSS were measured by UV-visible spectroscopy, which showed nearly 80-82% of transparency in the visible region and it was nearly same as PEDOT:PSS thin film on PET substrate. Current-voltage (I-V) analysis of fabricated hybrid device by using printed Ag grid/PEDOT:PSS as a bottom electrode showed good rectifying behavior with possible interfacial mechanisms. Capacitance-voltage (C-V) analysis was carried over different frequencies. These results suggest that fabrication of hybrid structure through printed techniques will play a significant role in mass production of printed electronic devices for commercial application by using flexible substrate.

Silver front electrode grids for ITO-free all printed polymer solar cells with embedded and raised topographies, prepared by thermal imprint, flexographic and inkjet roll-to-roll processes.

Semitransparent front electrodes for polymer solar cells, that are printable and roll-to-roll processable under ambient conditions using different approaches, are explored in this report. The excellent smoothness of indium-tin-oxide (ITO) electrodes has traditionally been believed to be difficult to achieve using printed front grids, as surface topographies accumulate when processing subsequent layers, leading to shunts between the top and bottom printed metallic electrodes. Here we demonstrate how aqueous nanoparticle based silver inks can be employed as printed front electrodes using several different roll-to-roll techniques. We thus compare hexagonal silver grids prepared using either roll-to-roll inkjet or roll-to-roll flexographic printing. Both inkjet and flexo grids present a raised topography and were found to perform differently due to only the conductivity of the obtained silver grid. The raised topographies were compared with a roll-to-roll thermally imprinted grid that was filled with silver in a roll-to-roll process, thus presenting an embedded topography. The embedded grid and the flexo grid were found to perform equally well, with the flexographic technique currently presenting the fastest processing and the lowest silver use, whereas the embedded grid presents the maximally achievable optical transparency and conductivity. Polymer solar cells were prepared in the same step, using roll-to-roll slot-die coating of zinc oxide as the electron transport layer, poly-3-hexylthiophene:phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) as the active layer and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the top electrode, along with a flat bed screen printed silver grid. The power conversion efficiency (PCE) obtained for large area devices (6 cm(2)) was 1.84%, 0.79% and 1.72%, respectively, for thermally imprinted, inkjet and flexographic silver grids, tested outside under the real sun. Central to all three approaches was that they employed environmentally friendly solvents, i.e. water based nanoparticle silver inks.

Design of roll-to-roll printing equipment with multiple printing methods for multi-layer printing.

In this paper, a novel design concept for roll-to-roll printing equipment used for manufacturing printed electronic devices by multi-layer printing is presented. The roll-to-roll printing system mainly consists of printing units for patterning the circuits, tension control components such as feeders, dancers, load cells, register measurement and control units, and the drying units. It has three printing units which allow switching among the gravure, gravure-offset, and flexo printing methods by changing the web path and the placements of the cylinders. Therefore, depending on the application devices and the corresponding inks used, each printing unit can be easily adjusted to the required printing method. The appropriate printing method can be chosen depending on the desired printing properties such as thickness, roughness, and printing quality. To provide an example of the application of the designed printing equipment, we present the results of printing tests showing the variations in the printing properties of the ink for different printing methods.

Fabrication of transparent conductive electrode film using thermal roll-imprinted Ag metal grid and coated conductive polymer.

In this study, to fabricate a low-resistance and high optical transparent conductive electrode (TCE) film, the following steps were performed: the design and manufacture of an electroforming stamp mold, the fabrication of thermal-roll imprinted (TRI) poly-carbonate (PC) patterned films, the manufacture of high-conductivity and low-resistance Ag paste which was filled into patterned PC film using a doctor blade process and then coated with a thin film layer of conductive polymer by a spin coating process. As a result of these imprinting processes the PC films obtained a line width of 10 +/- 0.5 Mm, a channel length of 500 +/- 2 microm, and a pattern depth of 7.34 +/- 0.5 microm. After the Ag paste was used to fill part of the patterned film with conductive polymer coating, the following parameters were obtained: a sheet resistance of 9.65 Omega/sq, optical transparency values were 83.69% at a wavelength of 550 nm.

Roll-printed organic thin-film transistor using patterned poly(dimethylsiloxane) (PDMS) stamp.

The roll-printed gate, source, and drain electrodes of organic thin-film transistors (OTFTs) were fabricated by gravure printing or gravure-offset printing using patterned poly(dimethylsiloxane) (PDMS) stamp with various channel lengths and low-resistance silver (Ag) pastes on flexible 150 x 150 mm2 plastic substrates. Bottom-contact roll-printed OTFTs used polyvinylphenol (PVP) as polymeric dielectric and bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) as organic semiconductor; they were formed by spin coating or ink-jetting. Depending on the choice of roll-printing method, the printed OTFTs obtained had a field-effect mobility of between 0.08 and 0.1 cm2/Vs, an on/off current ratio of between 10(4) and 10(5), and a subthreshold slope of between 1.96 and 2.32 V/decade. The roll-printing using patterned PDMS stamp and soluble processes made it possible to fabricate a printed OTFT with a channel length of between 12 to 74 microm on a plastic substrate; this was not previously possible using traditional printing techniques. The proposed fabrication process was 20 steps shorted than conventional fabrication techniques.