Papers by Author: Jae Sung Song

Abstract: Dense Li2O excess (Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3 [NKLNT] ceramics were developed by conventional sintering process without any special technique. The electrical properties of NKNLT ceramics were investigated as a function of Li2O concentration. When the sample sintered at 1050 oC for 4 h with the addition of 1 mol% Li2O, electro-mechanical coupling factor (kP) and piezoelectric coefficient (d33) were found to reach the highest values of 0.42 and 260 pC/N, respectively.

Abstract: Dense 0.95(Na0.5K0.5)NbO3-0.05LiTaO3 (NKN-5LT) ceramics, as a candidate for leadfree piezoelectric materials, were developed by conventional sintering process. The effect of additions with A-site ions in perovskite structure on abnormal grain growth and piezoelectric properties in NKN-5LT ceramics was investigated. Sintering temperature was lowered by adding A-site ions as a sintering aid. Abnormal grain growth in NKN-5LT ceramics was observed with varying additions. This grain growth behavior was explained in terms of interface reactioncontrolled nucleation and growth. The electrical properties of NKN-5LT ceramics were investigated as a function of concentration of additions. The electromechanical coupling factor and piezoelectric constant of NKN-5LT ceramics were improved in the samples with A-site ions excess NKN-5LT. These results show that the piezoelectric properties of NKN-5LT samples can be improved by control of the microstructures.

Abstract: Li2O excess 0.95(Na0.5K0.5)NbO3-0.05LiTaO3 (NKN-5LT) ceramics were developed by conventional sintering method. Abnormal grain growth in NKN-5LT ceramics was observed with varying Li2O content during sintering. In the 1 mol% Li2O excess NKN-5LT samples sintered at 1000 oC for 4h in air, electromechanical coupling factor and piezoelectric constant of NKN-5LT ceramics were found to reach the highest values of 0.37 and 250 pC/N, respectively. Lead-free piezoelectric of the composition 1 mol% Li2O excess NKN-5LT were fabricated. 10×10×3 mm3 size multilayer ceramic actuators (MLCA) were fabricated by conventional tape casting method. The displacement of the MLCA was ~ 1 μm at 150 V. These results show that the NKN-5LT ceramics with reasonable good piezoelectric properties have the potential to become the next generation material for a wide range of electro-mechanical transducer applications.

Abstract: RF sputtering process was applied to produce thin hydroxyapatite(HAp, Ca10(PO4)6(OH)2) films on Ti-6Al-4V alloy substrates. The effects of different heat treatment conditions on the bonding strength between HAp thin films and Ti-6Al-4V alloy substrates were studied. Before deposition, the Ti-6Al-4V alloy substrates were heat treated for 1hr at 850°C under 3.0×10-3torr, and after deposition, the HAp thin films were heat treated for 1hr at 400°C, 600°C and 800°C under the atmosphere, and analyzed optical microscope, FESEM, FTIR, XRD, nano-indentor, micro-vickers hardness, respectively. Experimental results represented that the HAp thin films on the heat treated substrates had higher hardness than none-heat treated substrates before the deposition.

Abstract: Na2O excess 95(Na0.5K0.5)NbO3-5LiTaO3 (NKN-5LT) ceramics were developed by conventional sintering process. Sintering temperature was lowered by adding Na2O as a sintering aid. Grain growth behaviors with Na2O addition were explained in terms of the effect of Na2O on interface reaction - controlled grain growth and the critical driving force. The electrical properties of NKN-5LT ceramics were investigated as a function of Na2O concentration. In the 1 mol% Na2O excess NKN-5LT samples sintered at 1000oC for 4h in air, electromechanical coupling factor (kP) and piezoelectric coefficient (d33) of NKN-5LT ceramics were found to reach the highest values of 0.43 and 190 pC/N, respectively.

Abstract: Dye sensitized solar cells (DSSC) have great potential alternative to expensive conventional solar cells, since high efficiency and relatively simple fabrication process. However, in large size cell, there is a key factor that delayed the entry of such cells in commercial market. Performance of large size cell is lower than small size cells, since a carrier loss occurs in high resistive TCO glass substrate. Here we demonstrate a simple method to reduce resistive loss and efficient collection of photo generated carriers via strip type cells with metal grids. Using strip type cells, we constitute series and parallel type DSSC panels in order to achieve required voltage and current respectively. Stripe cells were prepared from commercial TiO2 powder by screen printing method. In addition, metal grids were established adjacent to sealant line. Using these as unit cell, portable DSSC panels were assembled and I-V performance was carried out in indoor light condition (fluorescent light, 30mW/cm2) and standard condition (Pin 100 mW/cm2, AM 1.5).

Abstract: Dye-sensitized solar cell (DSSC) is of great interest as one of renewable energy sources. So far, many investigations related to TiO2 electrode, dye, electrolyte, Pt counter electrode, etc. were carried out. Here, we focus on Pt counter electrode for providing an enhanced catalytic property and physical stability. Pt has high-conductivity and superior catalytic properties. However, in case of Pt as counter electrode for DSSC, there exists partial problem due to poor adhesion between Pt and FTO glass. In this work, we tried to enhance the properties of Pt catalyst by two methods; first, the insertion of buffer layer under Pt thin film; second, the heat treatment of Pt films. We report the effect of buffer layer using Cr and Ti and the effect of heat treatment on Pt electrode under 400oC. Electrochemical characteristics of Pt electrodes were measured using electrochemical impedance spectroscopy and cyclic-voltammetry. A degree of electrode reaction, current density, electrode potential and interface resistance, etc were discussed with measured electrochemical data. In comparison with sputtered Pt counter electrode with and without buffer layers, heat-treated Pt counter electrode showed good performance in terms of the current density and interface resistance properties.

Abstract: Recently alkali oxide materials, such as sodium - potassium niobate have drawn much attention due to their ultrasonic applicability and are also considered as promising candidates for a piezoelectric lead-free system. However, it is difficult to sinter such NKN-based materials via conventional sintering process. Therefore, in this study, dense 0.95(Na0.5K0.5)NbO3-0.05LiTaO3 (NKN-5LT) ceramics were developed by conventional sintering process. Sintering temperature was lowered by adding Li2O as a sintering aid. The electrical properties of NKN-5LT ceramics were investigated as a function of Li2O concentration. At the addition of 1 mol% Li2O, electromechanical coupling factor (kP) and piezoelectric coefficient (d33) of NKN-5LT ceramics were found to reach the highest values of 0.37 and 250 pC/N, respectively.

Abstract: An electrophoretic solution using TiO2 nano particles is the most promising candidate in electronic paper industry because it offers various advantages such as ink-on-paper appearance, good contrast ratio, wide viewing angle, image stability in the off-state and extremely low power consumption. To prepare an electrophoretic solution of TiO2 nano particles with low apparent gravity for electronic paper, TiO2 powders were mixed with polyethylene powders using high energy planetary-mill in cyclohexane with low dielectric constant and transparency. The zeta-potential value of raw TiO2 particles in cyclohexane was measured about -80mV but that of polyethylene-coated TiO2 particles by milling was measured over -110mV, with showing good dispersibility by surfactant attached to them. Thus it was possible to control surface charge on the TiO2 particle by using optimum amount of surfactant after polyethylene coating on them for enhancement of dispersion stability.