Kyung Hyun Choi

Determination of fabricating orientation and packing in SLS process

Abstract Rapid prototyping (RP) technology has been implemented in all industries needed to reduce the time for the development of new products. An orientation and packing in RP process, especially selective laser sintering (SLS), are considered as the most important factors to maximize the utilization of space in the build vat and reduce build time. However, the decision of these parameters is mainly dependent on the skill level and experience of the SLS machine operator. This paper addresses the methodology to find the optimal build layout, by considering an orientation and packing of multiple parts in SLS processing. For the optimal orientation, the fabricating height and the feeding powder quantity to reduce build time and the surface quality improvement of prototypes is considered as major objectives. In packing process, each prototype to be fabricated is modeled as a voxel structure to deal with the inefficiency of a bounding box approach. The developed orientation and packing system employs the adapted bottom-left (BL) approach with a genetic algorithm (GA) and is demonstrated in real prototypes for processing with SLS and illustrates a good enabling optimization building system to the real industries.

Design of Microstereolithography System Based on Dynamic Image Projection for Fabrication of Three-Dimensional Microstructures

As demands for complex microstructures with high aspect ratios have increased, the existing methods, MEMS and LIGA, have had difficulties coping with the number of masks and fabricable heights. A microstereolithography technology can meet these demands because it has no need of masks and is capable of fabricating high aspect ratio microstructures. In this technology, 3D part is fabricated by stacking layers, 2D sections, which are sliced from STL file, and the Dynamic Image Projection process enables the resin surface to be cured by a dynamic image generated with DMD™ (Digital Micromirror Device) and one irradiation. In this paper, we address optical design process for implementing this microstereolithography system that takes the light path based on DMD operation and image-formation on the resin surface using an optical design program into consideration. To verify the performance of this implemented microstereolithography system, complex 3D microstructures with high aspect ratios were fabricated.

Optimization of Experimental Parameters To Determine the Jetting Regimes in Electrohydrodynamic Printing

The harmony of ink and printing method is of importance in producing on-demand droplets and jets of ink. Many factors including the material properties, the processing conditions, and the nozzle geometry affect the printing quality. In electrohydrodynamic (EHD) printing where droplets or jets are generated by the electrostatic force, the physical as well as the electrical properties of the fluid should be taken into account to achieve the desired performance. In this study, a systematic approach was suggested for finding the processing windows of the EHD printing. Six dimensionless parameters were organized and applied to the printing system of ethanol/terpineol mixtures. On the basis of the correlation of the dimensionless voltage and the charge relaxation length, the jet diameter of cone-jet mode was characterized, and the semicone angle was compared with the theoretical Taylor angle. In addition, the ratio of electric normal force and electric tangential force on the charged surface of the Taylor cone was recommended as a parameter that determines the degree of cone-jet stability. The cone-jet became more stable as this ratio got smaller. This approach was a systematic and effective way of obtaining the Taylor cone of the cone-jet mode and evaluating the jetting stability. The control of the inks with optimized experimental parameters by this method will improve the jetting performance in EHD inkjet printing.

Resistive Switching in All-Printed, Flexible and Hybrid MoS2-PVA Nanocomposite based Memristive Device Fabricated by Reverse Offset.

Owing to the increasing interest in the nonvolatile memory devices, resistive switching based on hybrid nanocomposite of a 2D material, molybdenum disulphide (MoS2) and polyvinyl alcohol (PVA) is explored in this work. As a proof of concept, we have demonstrated the fabrication of a memory device with the configuration of PET/Ag/MoS2-PVA/Ag via an all printed, hybrid, and state of the art fabrication approach. Bottom Ag electrodes, active layer of hybrid MoS2-PVA nanocomposite and top Ag electrode are deposited by reverse offset, electrohydrodynamic (EHD) atomization and electrohydrodynamic (EHD) patterning respectively. The fabricated device displayed characteristic bistable, nonvolatile and rewritable resistive switching behavior at a low operating voltage. A decent off/on ratio, high retention time, and large endurance of 1.28 × 102, 105 sec and 1000 voltage sweeps were recorded respectively. Double logarithmic curve satisfy the trap controlled space charge limited current (TCSCLC) model in high resistance state (HRS) and ohmic model in low resistance state (LRS). Bendability test at various bending diameters (50-2 mm) for 1500 cycles was carried out to show the mechanical robustness of fabricated device.

Drop-on-Demand Direct Printing of Colloidal Copper Nanoparticles by Electrohydrodynamic Atomization

Electrohydrodynamic inkjet printing technology has recently become attractive in many industrial fabrication fields mainly due to its advantage of smaller drop generation as compared to the diameter of the delivery nozzle. In this article, drop-on-demand (DoD) printing of colloidal solution containing copper nanoparticles through electrohydrodynamic atomization (EHDA) is investigated. By applying a novel forward multistep waveform (FMSW) superimposed on dc biased voltage, charged fluid drops containing copper nanoparticles are deposited on glass substrate. The main focus of this study is to generate uniform droplets of conductive copper ink in a controlled fashion by varying amplitude and frequency of the applied waveform. The deposited patterns show a series of uniform-sized drops with regular spacing. Using this DoD printing technique, it is feasible to print a variety of patterns of dots and conductive continuous or discontinuous lines.

Fine-resolution patterning of copper nanoparticles through electrohydrodynamic jet printing

This paper presents the low-cost, fine-resolution printing of conductive copper patterns on silicon substrate. The colloidal solution containing copper nanoparticles is deposited through electrohydrodynamic printing technology. Conductive copper tracks of different width are printed by varying the operating conditions (applied voltage and flow rate) and controlling the jet diameter. The minimum pattern width achieved was approximately 12 ?m with the average thickness of 82 nm across the width after the sintering process. The achieved pattern width is five times smaller than the capillary used for patterning. The morphology and purity of the printed copper tracks were analyzed through scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray diffraction (XRD). The current?voltage (I?V) characteristic of the printed copper tracks showed linear Ohmic behavior and exhibited resistivity ranging from 5.98???10?8?? m?1?to 2.42???10?7?? m?1.

Direct synthesis of graphene quantum dots from multilayer graphene flakes through grinding assisted co-solvent ultrasonication for all-printed resistive switching arrays

Graphene quantum dots (GQD) with diameters as small as ∼2 nm were synthesized by an efficient chemo-mechanical technique. This involved mortar grinding and ultra-sonication as a means of mechanical energy transfer, while N-methyl-pyrrolidone and 1,2-dichlorobenzene were used for exfoliation and breakdown of graphene nanoplatelets. High resolution transmission electron microscopy images showed that the solution-based GQDs were about 2–4 nm in size, and had a crystalline lattice parameter of 0.24 nm. The technique proved useful for extracting GQDs of the desired size. XRD, Raman and FTIR spectroscopy were used to analyze the quality of the graphene structure within the GQDs. The UV responsive GQDs had a band-gap of 2.6 eV and stronger photoluminescence at 350 nm compared to lower wavelengths of laser excitation. An all-printed 2 × 2 array of memristors based on a GQD embedded polymer matrix fabricated on a flexible PET substrate showed an OFF/ON ratio of just over 7 when read at 100 mV, stable retention despite a high compliance current for ∼100 switching cycles, and a robustness of 200 bending cycles up to 1.5 cm bending diameter without compromise on resistive switching states.

A two-dimensional hexagonal boron nitride/polymer nanocomposite for flexible resistive switching devices

Organic–inorganic hybrid nanocomposites are an attractive choice for various electronic device applications. Owing to the unique characteristics of hybrid nanocomposites, we have explored the memory effect in a device, based on a 2D material: boron nitride (BN) and a polymer, polyvinyl alcohol (PVOH). This memory device has been fabricated on a flexible ITO coated PET substrate by using an all printed approach including electrohydrodynamic atomization (EHDA) and reciprocating head (RPC). The fabricated device displayed nonvolatile, rewritable and characteristic bipolar resistive switching at a low current compliance and small operating voltage. The conduction mechanism was deduced to be conductive filamentary and verified by the effect of temperature and device size on switching characteristics. Raman, FTIR and UV/Vis spectroscopies were employed in studying the optical properties of as-deposited hBN/PVOH thin films. The morphological characteristics were analyzed by FESEM and AFM techniques. A bendability test at various bending diameters (50–4 mm) for 1500 cycles was carried out to show the mechanical robustness of the fabricated device. The remarkably stable and repeatable results of electrical and mechanical characterization make this hybrid nanocomposite a potential candidate for future flexible, robust and low power nonvolatile memory devices.

Optical security system for the protection of personal identification information

A new optical security system for the protection of personal identification information is proposed. First, authentication of the encrypted personal information is carried out by primary recognition of a personal identification number (PIN) with the proposed multiplexed minimum average correlation energy phase-encrypted (MMACE_p) filter. The MMACE_p filter, synthesized with phase-encrypted training images, can increase the discrimination capability and prevent the leak of personal identification information. After the PIN is recognized, speedy authentication of personal information can be achieved through one-to-one optical correlation by means of the optical wavelet filter. The possibility of information counterfeiting can be significantly decreased with the double-identification process. Simulation results demonstrate the effectiveness of the proposed technique.

Hybrid Surface Acoustic Wave- Electrohydrodynamic Atomization (SAW-EHDA) For the Development of Functional Thin Films

Conventional surface acoustic wave - electrostatic deposition (SAW-ED) technology is struggling to compete with other thin film fabrication technologies because of its limitation in atomizing high density solutions or solutions with strong inter-particle bonding that requires very high frequency (100 MHz) and power. In this study, a hybrid surface acoustic wave - electrohydrodynamic atomization (SAW-EHDA) system has been introduced to overcome this problem by integrating EHDA with SAW to achieve the deposition of different types of conductive inks at lower frequency (19.8 MHZ) and power. Three materials, Poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV), Zinc Oxide (ZnO), and Poly(3, 4-ethylenedioxythiophene):Polystyrene Sulfonate (PEDOT:PSS) have been successfully deposited as thin films through the hybrid SAW-EHDA. The films showed good morphological, chemical, electrical, and optical characteristics. To further evaluate the characteristics of deposited films, a humidity sensor was fabricated with active layer of PEDOT:PSS deposited using the SAW-EHDA system. The response of sensor was outstanding and much better when compared to similar sensors fabricated using other manufacturing techniques. The results of the device and the films’ characteristics suggest that the hybrid SAW-EHDA technology has high potential to efficiently produce wide variety of thin films and thus predict its promising future in certain areas of printed electronics.