Seung-Hyun Lee

Highly‐Efficient, Flexible Piezoelectric PZT Thin Film Nanogenerator on Plastic Substrates

A highly-efficient, flexible piezoelectric PZT thin film nanogenerator is demonstrated using a laser lift-off (LLO) process. The PZT thin film nanogenerator harvests the highest output performance of ∼200 V and ∼150 μA·cm(-2) from regular bending motions. Furthermore, power sources generated from a PZT thin film nanogenerator, driven by slight human finger bending motions, successfully operate over 100 LEDs.

Direct numerical simulation of the turbulent boundary layer over a rod-roughened wall

The effects of surface roughness on a spatially developing turbulent boundary layer (TBL) are investigated by performing direct numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was varied in the rangeReθ = 300 ∼ 1400. The roughness elements were periodically arranged two-dimensional spanwise rods, and the roughness height was k =1 .5θin ,w hereθin is the momentum thickness at the inlet, which corresponds to k/δ =0 .045 ∼ 0.125, δ being the boundary layer thickness. To avoid generating a rough-wall inflow, which is prohibitively difficult, a step change from smooth to rough was placed 80θin downstream from the inlet. The spatially developing characteristics of the rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value, and self-preserving forms of the turbulent Reynolds stress tensors were obtained. Introduction of the roughness elements affected the turbulent stress not only in the roughness sublayer but also in the outer layer. Despite the roughnessinduced increase of the turbulent Reynolds stress tensors in the outer layer, the roughness had only a relatively small effect on the anisotropic Reynolds stress tensor in the outer layer. Inspection of the triple products of the velocity fluctuations revealed that introducing the roughness elements onto the smooth wall had a marked effect on vertical turbulent transport across the whole TBL. By contrast, good surface similarity in the outer layer was obtained for the third-order moments of the velocity fluctuations.

Self-powered fully-flexible light-emitting system enabled by flexible energy harvester

Energy-harvesting technology utilising mechanical energy sources is a promising approach for the sustainable, independent, and permanent operation of a variety of flexible electronics. A new concept of a fully-flexible light-emitting system, self-powered by a high-performance piezoelectric thin-film energy harvester has been first established by manipulating highly-robust, flexible, vertically structured light emitting diodes (f-VLEDs). The f-VLEDs fabricated by anisotropic conductive film bonding and entire wafer etching show stable and durable performances during periodic mechanical deformations. A high-output energy harvester capable of generating up to 140 V and 10 μA can be fabricated via laser lift-off (LLO) process widely used in industries, in a safe and robust manner. In particular, this LLO process is of great benefit for the fabrication of mechanically stable, flexible piezoelectric devices, without causing any degradation of piezoelectric properties. In this process, self-powered all-flexible electronic system with light emittance can be spontaneously achieved by the electricity produced from flexible thin-film generator by applying slight biomechanical energy without any externally applied energy storage. This conceptual technology of self-powering based on the conversion of mechanical energy to electrical energy can open a facile and robust avenue for diverse, self-powered, bio-implantable applications, as well as commercial display applications.

Flexible Crossbar‐Structured Resistive Memory Arrays on Plastic Substrates via Inorganic‐Based Laser Lift‐Off

Crossbar-structured memory comprising 32 × 32 arrays with one selector-one resistor (1S-1R) components are initially fabricated on a rigid substrate. They are transferred without mechanical damage via an inorganic-based laser lift-off (ILLO) process as a result of laser-material interaction. Addressing tests of the transferred memory arrays are successfully performed to verify mitigation of cross-talk on a plastic substrate.

Mass type-specific sparse representation for mass classification in computer-aided detection on mammograms.

BackgroundBreast cancer is the leading cause of both incidence and mortality in women population. For this reason, much research effort has been devoted to develop Computer-Aided Detection (CAD) systems for early detection of the breast cancers on mammograms. In this paper, we propose a new and novel dictionary configuration underpinning sparse representation based classification (SRC). The key idea of the proposed algorithm is to improve the sparsity in terms of mass margins for the purpose of improving classification performance in CAD systems.MethodsThe aim of the proposed SRC framework is to construct separate dictionaries according to the types of mass margins. The underlying idea behind our method is that the separated dictionaries can enhance the sparsity of mass class (true-positive), leading to an improved performance for differentiating mammographic masses from normal tissues (false-positive). When a mass sample is given for classification, the sparse solutions based on corresponding dictionaries are separately solved and combined at score level. Experiments have been performed on both database (DB) named as Digital Database for Screening Mammography (DDSM) and clinical Full Field Digital Mammogram (FFDM) DBs. In our experiments, sparsity concentration in the true class (SCTC) and area under the Receiver operating characteristic (ROC) curve (AUC) were measured for the comparison between the proposed method and a conventional single dictionary based approach. In addition, a support vector machine (SVM) was used for comparing our method with state-of-the-arts classifier extensively used for mass classification.ResultsComparing with the conventional single dictionary configuration, the proposed approach is able to improve SCTC of up to 13.9% and 23.6% on DDSM and FFDM DBs, respectively. Moreover, the proposed method is able to improve AUC with 8.2% and 22.1% on DDSM and FFDM DBs, respectively. Comparing to SVM classifier, the proposed method improves AUC with 2.9% and 11.6% on DDSM and FFDM DBs, respectively.ConclusionsThe proposed dictionary configuration is found to well improve the sparsity of dictionaries, resulting in an enhanced classification performance. Moreover, the results show that the proposed method is better than conventional SVM classifier for classifying breast masses subject to various margins from normal tissues.

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.

Turbulent boundary layers over rod- and cube-roughened walls

Direct numerical simulations (DNSs) of spatially developing turbulent boundary layers (TBLs) over two-dimensional (2D) rod-roughened walls and three-dimensional (3D) cube-roughened walls were performed to investigate the effects of the streamwise spacing of roughness elements on the properties of the TBLs. The inspection of the Reynolds stresses showed that except for the 2D rough walls with px /k = 2 and 3, the effects of the roughness on the 2D and 3D rough walls extend to the outer layer and that the magnitude of the Reynolds stresses in the outer layer increases in proportional to px /k. However, such results do not account for the variations with px /k in the form drag and roughness function, which have maximum values at px /k = 8 and 4 for the 2D and 3D rough walls, respectively. Finally, we examined turbulent structure through instantaneous analysis, linear stochastic estimation, and dynamic mode decomposition to the time-evolving flow fields to scrutinize the spatial organization of the outer-laye...

The effect of shear force on ink transfer in gravure offset printing

This paper asserts that shear force plays an important role in the printing mechanism of gravure offset line printing. To that end, a theoretical printing model showing shear force dependence on the printing angle is proposed. The decrement of the internal angle between the printing direction and the pattern-line direction increases shear force, thereby enhancing the amount of transferred ink in the off stage. A printing experiment using pattern-line widths of 80 µm and 20 µm shows the angle dependence of the line width, thickness and amount of transferred ink, reflecting the effect of shear force. The effect of the internal angle on cross-sectional differences in lines with a width of 20 µm and with angle variation is greater than that in lines with a width of 80 µm, which corresponds with the theoretical prediction that shear force has greater influence on a narrower line. The strong correlation between the experimental data and the theoretical model supports the validation of the theoretical model.

Investigation on synchronization of the offset printing process for fine patterning and precision overlay

Offset printing processes are promising candidates for producing printed electronics due to their capacity for fine patterning and suitability for mass production. To print high-resolution patterns with good overlay using offset printing, the velocities of two contact surfaces, which ink is transferred between, should be synchronized perfectly. However, an exact velocity of the contact surfaces is unknown due to several imperfections, including tolerances, blanket swelling, and velocity ripple, which prevents the system from being operated in the synchronized condition. In this paper, a novel method of measurement based on the sticking model of friction force was proposed to determine the best synchronized condition, i.e., the condition in which the rate of synchronization error is minimized. It was verified by experiment that the friction force can accurately represent the rate of synchronization error. Based on the measurement results of the synchronization error, the allowable margin of synchronization...

Mechanical and environmental durability of roll-to-roll printed silver nanoparticle film using a rapid laser annealing process for flexible electronics

Abstract We investigate the mechanical durability and environmental stability of laser annealed silver (Ag) nanoparticle (NP) film. Roll-to-roll printed Ag NP film on polyethylene terephthalate substrate is annealed with a rapid laser annealing process in ambient conditions as an alternative to the conventional thermal annealing process. The laser annealed Ag NP film exhibits superior electrical and mechanical properties, with fast annealing time and no damage on the substrate. The outer/inner bending test results demonstrate that the flexibility of the laser annealed Ag film is excellent. The failure bending radii in the outer/inner bending tests are 3xa0mm. The laser annealed film can withstand 10,000 bending cycles. A nano-scratch test indicates that the adhesion strength of the laser annealed film is comparable to that of the thermal annealed film. The environmental reliability of Ag NP film is investigated under different high-temperature and high-humidity conditions, while being subjected to cyclic bending fatigue stress. The durability of printed Ag film is found to be influenced by temperature and humidity. The laser annealed film shows relatively large increase in resistance during the bending fatigue test under high temperature and humidity condition (60xa0°C/90% RH), which is attributed to the oxidation of Ag nanoparticles and initiation of cracks. Generation of cracks is accelerated owing to the combinational effects of the cyclic stress and humidity. These results suggest that, even though the laser annealed Ag film demonstrates sufficient mechanical durability, further improvement of the film properties is required for use in extreme mechanical and environmental conditions.