Jae Sung Song

Nanocarbon counterelectrode for dye sensitized solar cells

Nanosize carbon powders were deposited on conducting glass substrate for counterelectrode application in dye sensitized solar cell (DSSC). Electrochemical impedance measurement shows that charge transfer resistance of carbon electrode in liquid electrolyte is 0.74Ωcm−2, which is two times less than that of screen printed platinum. Using such counterelectrode and dye sensitized TiO2 working electrode, DSSC was fabricated. Under one sun illumination (AM1.5, Pin of 100mWcm−2), carbon counterelectrode DSSC shows 6.73% overall energy conversion efficiency with good stability.

Elastic Modulus and In Vitro Biocompatibility of Ti-xNb and Ti-xTa Alloys

The effect of niobium and tantalum on the elastic modulus and the in vitro biocompatibility in binary titanium alloys was studied. The Youngs modulus of titanium was effectively lowered with additions of Nb or Ta, depending significantly upon the microstructure. Martensitic microstructures such as α and α decreased the elastic modulus, while the ω phase increased it. Ti−10%Nb, Ti−30%Nb and Ti−30%Ta alloys exhibited very low elastic moduli of 74, 80, and 58 GPa, respectively. The corrosion resistance of Ti−xTa was slightly higher than that of Ti−xNb, which was comparable to that of CP Ti or Ti−6A−4V. No ion release was detected in Hanks solution, while Ti ions were released in 0.1% lactic acid ranging from 0.03 to 0.11 μg/ml for both the Ti−xNb and Ti−xTa alloys. MG63 osteoblast-like cell proliferation on Ti−30%Ta was less active compared with Ti−30%Nb, CP Ti or Ti−6Al−4V.

Redox abilities of rutile TiO2 ultrafine powder in aqueous solutions

Redox abilities of rutile TiO2 powder with an acicular primary particle, having about a 200 m2/g BET surface area obtained by a homogeneous precipitation process in aqueous TiOCl2 solution at 50°C for 4 hrs, were investigated using a photocatalytic reaction in aqueous 4-chlorophenol, Cu- and Pb-EDTA solutions. Its abilities were then compared with those of commercial P-25 powder, together with investigating those of anatase TiO2 powder obtained from the aqueous TiOCl2 solution, having a similar surface area and primary particle shape as those of the rutile powder. The powder was more effective than the anatase or P-25 powder, while the anatase powder unexpectedly showed the slowest decomposition rate and the smallest amount in the same experiments in spite of similar particle shapes and surface area. From the results, it is found that the excellent photo redox abilities of the powder is likely to be caused by the specific powder preparation method regardless of crystalline structures even when having similar particle shapes and values in the surface area. Also, many OH− attached to the surface of the TiO2 particle appeared to interfere with the adsorption of decomposing target materials to the TiO2 surface in the solution during the photocatalytic reaction, resulting in a delay in the reaction.

Effect of Heat Treatment on Structural Characteristics and Electric Resistance in TaNx Thin Film Deposited by RF Sputtering

TaNx thin films were deposited by RF magnetron sputtering and heat-treated from 300 to 700°C. The structural characterization of the deposited films was carried out by the analysis of X-ray diffraction patterns obtained using a polycrystalline diffractometer and the variation of electric resistance during heat treatment was studied in association with the measured and calculated structural characteristics. The measured X-ray diffraction patterns show that the phase of the film changes from near amorphous phase to FCC TaN of NaCl type with increasing heat-treatment temperature. The electric resistance rapidly decreased with heat treatment at 300°C and increased from 500 to 700°C. The decrease of sheet resistance can be explained by the release of misfit strain energy, as based on the decrease of interplanar spacing, and the increase of sheet resistance at elevated temperature is supposed to be caused by the formation of Ta oxide on the surface of the film.

Electrochemical corrosion of novel beta titanium alloys

Electrochemical corrosion behaviors of novel beta titanium alloys, Ti−30Ta−10Nb−10Zr, Ti−40Ta−10Nb−10Zr and Ti−40Ta−10Nb−4Sn were characterized in naturally aerated Hanks solution at 37°C compared with currently used biomedical titanium alloys. Ti−30Ta−10Nb−10Zr and Ti−40Ta−10Nb−10Zr exhibited comparable corrosion resistance to CP Ti or Ti−6Al−4V, while the corrosion resistance of Ti−40Ta−10Nb−4Sn was greatly inferior to the other alloys. An EIS data analysis confirmed that the resistance of passive film for Ti−30Ta−10Nb−10Zr and Ti−40Ta−10Nb−10Zr was comparable to CP Ti or Ti−6Al−4V. Ti−30Ta−10Nb−10Zr and Ti−40Ta−10Nb−10Zr alloys are promising metallic biomaterials for the future, owing to their very low elastic modulus and good corrosion resistance capabilities.

Effect of process parameters on the efficiency of dye sensitized solar cells

The impact of process parameters on the efficiency of dye-sensitized solar cell (DSSC) was studied in order to improve its performance. TiO2 working electrodes were prepared on FTO glass for various TiO2 mixing time. Also these electrodes were sintered at four different temperatures (400, 450, 500, and 550°C) for one hour in ambient atmosphere. Platinum-sputtered counter electrodes were prepared at different light transmittance (70, 50, 30, and 0%). Microstructural charecterization of these electrodes were studied by X-ray diffraction method (XRD) and Scanning electron microscope (SEM). I–V characteristics of DSSCs made up of different working and counter electrodes were studied using solar simulator. Maximum efficiency (open circuit voltage (Voc)∼0.68 V and short-circuit current density (Jsc) ∼ 12.3 mA/cm2) was observed for DSSC consisting of TiO2 working electrode (12h mixing time, 500°C sintering temperature) and 0% transmittance Pt counter electrode. These results indicate that enhancement possibility of overall performance in DSSC was obtained by controlling process parameters, especially TiO2 mixing time, sintering temperature and counter electrode transmittance.

Electromechanical Properties of NKN-5LT Multilayer Actuator

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.

Dielectric properties of Ta2O5 thin films deposited onto Ti and TiO2 layer

The present study describes the dielectric properties of RF sputtered Ta2O5 thin films as a function of the buffer layer and annealing condition. The buffer layers were Ti or TiO2. And the thin film was annealed in various conditions. The X-ray pattern results showed that the phase of the RF sputtered Ta2O5 thin films was amorphous and this state was kept stable to RTA (rapid thermal annealing) even at 700°C. Measurements of the electrical and dielectric properties of the reactive sputtered Ta2O5 fabricated in two simple metal insulator semiconductor (MIS) structures, (Cu/Ta2O5/Ti/Si/Cu and Cu/Ta2O5/TiO2/Si/Cu) indicated that the amorphous Ta2O5 grown on Ti possesses a high dielectric constant (30–70) and high leakage current (10−1–10−4 A/cm2), whereas a relatively low dielectric constant (−10) and low leakage current (−10−10 A/cm2) were observed in the amorphous Ta2O5 deposited on the TiO2 buffer layer. In addition, the leakage current mechanisms of the two amorphous Ta2O5 thin films were investigated by plotting the relation of current density (J) vs. applied electric field (E). The Ta2O5/Ti film exhibited three dominant conduction mechanism regimes contributed by the Ohmic emission at low electrical field, by the Schottky emission at intermediate field and by the Poole-Frenkel emission at high field. In the case of Ta2O5/TiO2 film, the two conduction mechanisms, the Ohmic and Schottky emissions, governed the leakage current density behavior. The conduction mechanisms at various electric fields applied were related to the diffusion of Ta, Ti and O, followed by the creation of vacancies, in the rapid thermal treated capacitors.

Low temperature sintering in 95 (Na0.5K0.5)NbO3-5LiTaO3 ceramics

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.

Effect of Li2O Addition on Piezoelectric Properties of NKN-5LT Ceramics

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.