Jungho Hwang

Electrohydrodynamic printing of silver nanoparticles by using a focused nanocolloid jet

As a direct write technology, the electrohydrodynamic printing of silver nanoparticles by using a focused nanocolloid jet is introduced. In this letter, two categorized types of examples of two-dimensional patterning were printed by using the electrohydrodynamic printing method. A spiral-type inductor was printed to demonstrate the feasibility of the electrohydrodynamic printing as a fabrication process. The printed spiral inductor produced 9.45μH and exhibited approximately five times larger resistivity (9.5μΩcm) than that of bulk silver after the sintering process. Then, complex geometries having square- and round-shape patterns were also printed.

In situ observation of the cracking behavior of TiN coating on 304 stainless steel subjected to tensile strain

Abstract The validity of a periodic cracking model capable of evaluating the interfacial shear strength of a ceramic film on metal substrate system was tested in a highly residual-stressed TiN coating-304 stainless-steel substrate system. In situ observation of the cracking behavior of the coating was conducted while the TiN-coated tensile specimen was being pulled inside the chamber of a scanning electron microscope. Four sequentially developed morphologies of the crack and/or inter-crack-spacing of the coating, i.e. multiplication, stabilization, cross-linking and spallation, were observed. Inter-crack-spacings of the fragmented coating were analyzed and identified with their respective initiation mechanisms, thereby testing the claims made by the model. Stringent criteria for selecting the characteristic inter-crack-spacing were articulated to improve the accuracy of the calculated maximum interfacial shear stress. Lastly, a modified periodic cracking model, which incorporates the contribution from residual stress and the new rule for selecting the characteristic inter-crack-spacing, was presented. The interfacial shear strength of the subject system evaluated through the modified model is 0.65±0.06 GPa, which is of the order of magnitude of the substrates yield shear strength.

A tensile-film-cracking model for evaluating interfacial shear strength of elastic film on ductile substrate

Abstract The stress distribution associated with the film cracking model originally proposed by Agrawal and Raj is further investigated through the use of the finite element analysis (FEA) technique. The result of the FEA analysis indicates that the interfacial shear stress can be approximated by a quarter segment of an elliptical function, rather than by a sine function of a full wavelength as employed in the original model. A modified model related to the maximum interfacial shear stress is thus proposed. The validity of the new model has been tested in a TiN coating–304 stainless steel system. It is demonstrated that the interfacial shear strength evaluated through the new model is of the same order of magnitude as that of the yield shear strength of the substrate.

Effect of particle loading on the collection performance of an electret cabin air filter for submicron aerosols

Electret filters are composed of permanently charged electret fibers and are widely used in applications requiring high collection efficiency and low-pressure drop. We tested electret filter media used in manufacturing cabin air filters by applying two different charging states to the test particles. These charging states were achieved by spray electrification through the atomization process and by bipolar ionization with an aerosol neutralizer, respectively. Polydisperse solid NaCl particles with 0.1%, and 1% solutions or liquid dicotyl sebacate (DOS) particles were generated from an atomizer, and they were loaded on the filter media. The amount of charge, the mean particle size, and the particle material significantly affected the collection performance of the electret filter media for submicron particles. The collection efficiency of the electret filter media degraded as more particles were loaded, and showed minimum efficiency at steady state. The electret filter media captured the highly charged particles more efficiently during the transient state. At steady state, the filter media loaded with smaller NaCl particles showed lower collection efficiency. The filter media loaded with liquid DOS particles showed collection efficiency much lower than those loaded with solid NaCl particles.

Fabrication of silver nanowire transparent electrodes using electrohydrodynamic spray deposition for flexible organic solar cells

Fabrication of a flexible organic solar cell is demonstrated with an anode that is free of tin-doped indium oxide (ITO) by electrohydrodynamic (EHD) spraying of silver nanowires (Ag NWs). This methodology is applicable to fabricate patterned Ag NW thin-film electrodes for organic solar cells (OSCs). By optimizing the spray parameters and post-processing conditions, transparent electrodes with sheet resistances of ∼11 Ω sq−1 (on glass) and ∼20 Ω sq−1 (on polyethylene terephthalate, PET) and DC to optical conductivities of ∼70 (on PET) can be obtained at an optical transmittance of ∼80%. Bulk heterojunction OSCs are demonstrated with patterned Ag NW films serving as bottom transparent anodes on both glass substrates and flexible PET substrates. The photoactive layers are based on low band-gap polymers, poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4.5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ehtylhexanoyl)-thieno[3,4-b]thiophene)]-2,6-diyl] and phenyl-C61-butyric acid methyl ester. Under AM 1.5 illumination, fabricated cells have high power conversion efficiencies of 5.27% (on glass) and 3.76% (on PET). This study indicates that a Ag NW electrode prepared by EHD spraying can serve as an alternative to the ITO electrode, which also enables its potential application in practical and flexible OSCs.

Removal of gaseous toluene and submicron aerosol particles using a dielectric barrier discharge reactor.

A lab-scale dielectric barrier discharge (DBD) reactor was fabricated, and gaseous and particulate contaminant removal tests were carried out under a range of DBD reactor operating conditions: applied voltage (5.0-8.5 kV), frequency (60-1000 Hz), upstream toluene concentration (50-200 ppm) and gas flow rate (1-5 L min(-1) or 0.48-0.096 s of gas residence time). The results suggested that the toluene removal efficiency (at 1 L min(-1), 100 ppm) increased (up to approximately 46%) either with increasing voltage (at 1000 Hz) or frequency (at 8.5 kV). The overall particle collection efficiency (at 1 L min(-1)) improved (up to approximately 60%) with increasing voltage (at 1000 Hz) whereas the penetration of the particles increased (up to approximately 40%) with increasing frequency (at 8.5 kV). The toluene removal efficiency (at 8.5 kV, 1000 Hz, 100 ppm) decreased (down to approximately 29%) with increasing gas flow rate while the particle collection efficiency decreased slightly (maintaining approximately 60%) regardless of the flow rate. In addition, the toluene removal efficiency (down to approximately 41%) and carbon dioxide selectivity (down to approximately 43%) decreased with increasing upstream toluene concentration (at 5 kV, 1000 Hz, 1 L min(-1)).

Fabrication of a multi-walled carbon nanotube-deposited glass fiber air filter for the enhancement of nano and submicron aerosol particle filtration and additional antibacterial efficacy.

We grew multi-walled carbon nanotubes (MWCNTs) on a glass fiber air filter using thermal chemical vapor deposition (CVD) after the filter was catalytically activated with a spark discharge. After the CNT deposition, filtration and antibacterial tests were performed with the filters. Potassium chloride (KCl) particles (<1 μm) were used as the test aerosol particles, and their number concentration was measured using a scanning mobility particle sizer. Antibacterial tests were performed using the colony counting method, and Escherichia coli (E. coli) was used as the test bacteria. The results showed that the CNT deposition increased the filtration efficiency of nano and submicron-sized particles, but did not increase the pressure drop across the filter. When a pristine glass fiber filter that had no CNTs was used, the particle filtration efficiencies at particle sizes under 30 nm and near 500 nm were 48.5% and 46.8%, respectively. However, the efficiencies increased to 64.3% and 60.2%, respectively, when the CNT-deposited filter was used. The reduction in the number of viable cells was determined by counting the colony forming units (CFU) of each test filter after contact with the cells. The pristine glass fiber filter was used as a control, and 83.7% of the E. coli were inactivated on the CNT-deposited filter.

The Effect of Ventilation Rate and Filter Performance on Indoor Particle Concentration and Fan Power Consumption in a Residential Housing Unit

This paper reports a study of the effect of ventilation rate and filters performance on indoor particle concentration and fan power consumption in a residential housing unit with a mechanical ventilation system. Through an adapted mass-balance model, indoor particle concentrations were calculated for various ventilation rates, filter performances and room sizes. Additionally, the influence of air-exchange effectiveness and cross-contamination around the exterior air vent on the indoor particle concentration was considered. Recirculation of indoor air was not considered. From the results, filters for which the performance was lower than MERV07 were found to be insufficient for reducing indoor particle concentrations below the levels obtained under no ventilation. A higher ventilation rate was needed for the given amount of indoor particle sources for a smaller size residential housing unit in comparison to the larger units. The minimum ventilation rate was less sensitive to variations in the air-exchange effectiveness inside the residential housing unit and the cross-contamination index around the exterior air vents. To satisfy the ventilation requirement for gaseous pollutants and keep the particle concentrations below those under no ventilation, a filter with a performance that would exceed MERV11 should be used when the size of the residential housing unit is in the range of 150—300 m3.

Design and evaluation of a silicon based multi-nozzle for addressable jetting using a controlled flow rate in electrohydrodynamic jet printing

This letter reports on the development and evaluation of a electrohydrodynamic jet printing that uses an addressable multinozzle. To reduce the interference and distortion in the electric field, a multinozzle was fabricated from a silicon wafer. The experimental conditions were optimized to prevent the jet from bending at the end of the multinozzle and to allow for independent control of each nozzle. To better evaluate this technique, simulations were performed and compared with the experimental results. We observed a strong correlation between the simulated and experimental results. In addition, each nozzle in this multinozzle could be individually controlled.

Fabrication of ordered bulk heterojunction organic photovoltaic cells using nanopatterning and electrohydrodynamic spray deposition methods

Organic photovoltaic cells with an ordered heterojunction (OHJ) active layer are expected to show increased performance. In the study described here, OHJ cells were fabricated using a combination of nanoimprinting and electrohydrodynamic (EHD) spray deposition methods. After an electron donor material was nanoimprinted with a PDMS stamp (valley width: 230 nm, period: 590 nm) duplicated from a Si nanomold, an electron acceptor material was deposited onto the nanoimprinted donor layer using an EHD spray deposition method. The donor-acceptor interface layer was observed by obtaining cross-sectional images with a focused ion beam (FIB) microscope. The photocurrent generation performance of the OHJ cells was evaluated with the current density-voltage curve under air mass (AM) 1.5 conditions. It was found that the surface morphology of the electron acceptor layer affected the current and voltage outputs of the photovoltaic cells. When an electron acceptor layer with a smooth thin (250 nm above the valley of the electron donor layer) surface morphology was obtained, power conversion efficiency was as high as 0.55%. The electrohydrodynamic spray deposition method used to produce OHJ photovoltaic cells provides a means for the adoption of large area, high throughput processes.