Papers by Author: Jae Woo Joung

Abstract: Use of silver (Ag) nanoparticle suspension for various applications such as ink-jet printing of electronic circuits has been of prime interest. We observed the microstructure evolution of the inkjet-printed Ag thin films on Si substrates under various annealing conditions using the field-emission scanning electron microscopy (FE-SEM). Abnormal grain growth characteristics were identified when annealed at about 240 oC under ambient air. Growth characteristics of pores were found to be in accordance with that of grains. Competition between grain and pore growth is attributed to small grain sizes, low packing density and high porosity, which are characteristic of inkjet-printed Ag films as dried.

Abstract: Nickel nanoparticles were prepared by reduction of nickel-hydrazine complex with sodium borohydride (NaBH4) in water-in-oil (w/o) microemulsion of PVP/ethanol/toluene/water at 50
C. It was investigated through SEM and TEM photographs that the nickel nanoparticles with a diameter 15~20nm were nearly monodispersed. By the analysis of XRD, the resulting particles were characterized to be pure crystalline nickel with a face-centered cubic structure. The results indicate that the size, shape and distribution of particles depend on the mole ratio of [water]/[PVP] and [NaBH4]/[Ni]. Also, it was found that particle size could be controlled by the kinds of surfactants.

Abstract: The synthesis and characterization of polymer-stabilized silver nanoparticles (Ag NPs) for water-based silver inks are studied. In order to synthesize Ag NPs with spherical shape, the conventional polyol processes require an excess of poly(vinyl pyrrolidone) (PVP) (10 ~ 1000 times than AgNO3) and therefore result in low productivity per reactor volume. In this study, poly(acrylic acid) (PAA) with carboxylic acid group was used instead of PVP. Even at less molar ratio of PAA to AgNO3 (< 2), the spherical Ag NPs with average size of 30 nm were successfully synthesized at 100 gram-scale with high reaction yield of 90%. Furthermore, the Ag NPs were dispersed into alcohol-based solvent and then patterned into metallic lines through inkjet printing technology.

Abstract: Cuprous oxide nanoparticles at high concentration (up to 0.3M) were successfully synthesized by modified polyol process. The redispersion stability could be controlled by optimizing various conditions like polymer concentration, temperature, reductant, and catalyst. Reductant and catalyst play a crucial rule in redispersion of cuprous nanoparticles. Under poor conditions, particles grow to larger sizes due to aggregation of nanoparticles. This modified polyol process allows monodispersed cuprous oxide to be obtained on gram scale in a single reaction and make possible a high synthetic yield of more than 80%.

Abstract: In this study, specially used in fluidic jetting apparatus, the electrical properties of piezoelectric ceramics were investigated by the resonance method and from its vibratory motion point of view. The piezoelectric ceramics were a rectangular bar type bulk and estimated by their piezoelectric constants, for example, electromechanical coupling factor, piezoelectric strain constant and so on. The measured values were compared with the displacement by the Laser Doppler Vibrometer analysis and with the droplet properties.

Abstract: A novel selective metallization process to fabricate the fine conductive line based on drop-on-demand (DoD) inkjet printing was studied. Direct inkjet printing is an alternative and costeffective technology for patterning and fabricating objects directly from design or image files without making masks and patterns. The conductive ink used in this experiment consists of 1 to 50 nm silver particles that are homogeneously suspended in an organic carrier. A piezo-electric inkjet print head driven by a bipolar voltage signal is used to dispense 20-40μm diameter droplets. Repeatability of circuitry fabrication is closely related to the formation of steady, satellite-free droplets. Therefore, the ability to form small and stable droplets with a same size, constant velocity and the correct flight angle must be taken into consideration for fine and precise conductive lines. In this study, parameters affecting the pattern formation such as drop formation, drop placement accuracy and velocity deviation between each nozzle have been investigated. As a result, direct inkjet patterning systems equipped with several functioning modules and fine metallic patterns have been developed.