Method for determining the junction temperature of alternating current light-emitting diodes

A numerical simulation was used to simulate the temperature distribution during AC and DC operations of an alternating current light-emitting diode (AC LED). The relationship between the junction temperature and the temperature at the centre of the bottom surface of the submount of an AC LED was measured under DC operation. This relationship was confirmed by numerical simulation. The numerical results were consistent with the experimental observations in that the temperature at the centre of the bottom surface of the submount was insensitive to the current variations that occur in an AC LED, probably because of the large mass of the submount. However, it was difficult to measure the temperature oscillation at the junctions in an AC LED, although this oscillation can be clearly seen in the numerical results. Thus, the authors propose a formula for predicting the range of the oscillating junction temperature for an AC LED.     

Effect of Nafion gradient in dual catalyst layer on proton exchange membrane fuel cell performance

The role of Nafion^® binder in the electrodes was evaluated by changing its content for the membrane electrode assembly (MEA) fabrication. In the study, we prepared MEAs that have two different compositions of catalyst layers in electrodes. One layer which is close to the electrolyte membrane has the higher Nafion^® content. The other which is near the gas diffusion media (GDM) has the lower one. Also, we changed the thickness of two layers to find the ideal composition of the binder and Pt/C in the electrode. The dual catalyst layer coated MEA showed higher cell performance at high current density region than the pristine MEA.     

Covalently cross-linked sulfonated poly(ether ether ketone)/tungstophosphoric acid composite membranes for water electrolysis application

Composite polymer electrolyte membranes consisting of covalently cross-linked sulfonated polyether ether ketone (SPEEK) with tungstophosphoric acid (TPA) are prepared and their electrochemical and mechanical properties are investigated with regards to application in water electrolysis. Covalently cross-linked membranes (CL-SPEEK) comprised of sulfochlorinated SPEEK membranes and SPEEK partially lithiated by LiCl, are prepared by reaction with 1,4-diiodobutane, and blended with TPA to avoid excessive water swelling and to reinforce their mechanical properties. These ion-exchange membranes show good electrochemical properties, including proton conductivity, ion exchange capacity (IEC), thermal stability, anti-oxidative stability, and satisfactory mechanical characteristics, such as tensile strength and elongation. In particular, among the TPA-composite membranes, the CL-SPEEK/TPA30 (30 wt.% TPA) membrane displays higher proton conductivity (0.128 S cm⁻¹) and tensile strength (75.01 MPa) than Nafion^® 117 at 80 °C. The ion-exchange membranes are used to construct membrane electrode assemblies (MEAs) of use in polymer electrolyte membrane electrolysis (PEME). The MEA are prepared using a non-equilibrium impregnation–reduction (I–R) method. The electrochemical surface area (ESA) and roughness factor of the MEA prepared with CL-SPEEK/TPA30 electrolyte measured by cyclic voltammetry are 25.11 m² g⁻¹ and 321.4 cm² Pt cm⁻², respectively. The prepared MEAs are used in the water-electrolysis unit cells. The cell voltage is 1.78 V at 1 A cm⁻² and 80 °C, with a platinum loading of 1.28 mg cm⁻². The results of the present study suggest that the good conductivity and mechanical properties of covalently CL-SPEEK/TPA composite membranes make them well suited for use in PEME.     

CTAB facilitated spherical rutile TiO2 particles and their advantage in a dye-sensitized solar cell

Spherical rutile TiO₂ particles (14–20 nm) and their corresponding well-defined round clusters (500–600 nm) were obtained by using a cationic surfactant cetyltrimethylammonium bromide (CTAB). The surfactant was employed in two stages, i.e., in the hydrolysis of TiCl₄ and then in the precipitation of the corresponding Ti(IV) polymers at approximately 46 °C. On the other hand, without CTAB in the hydrolyzing solution, irregular clusters consisting of typical ellipsoidal TiO₂ particles were produced. The advantage of such spherical rutile TiO₂ particles and clusters was examined in terms of photovoltaic characteristics of a dye-sensitized solar cell (DSSC). Significantly higher overall solar energy conversion efficiency was obtained for a DSSC using the film of these spherical rutile TiO₂ particles, compared with that of a cell using a TiO₂ film of ellipsoidal particles. A mechanism for the formation of these spherical rutile particles and clusters is proposed.     

Optimization of corrosion protection potential for stress corrosion cracking and hydrogen embrittlement of 5083-H112 alloy in seawater

We report on the optimum corrosion protection potential range for stress corrosion cracking and hydrogen embrittlement of 5083-H112 Al alloy specimens using electrochemical methods and slow strain rate testing (SSRT) in seawater. In the results of the cathodic polarization curve, the concentration polarization due to dissolved oxygen reduction reaction correspondeds to a protection potential of OCP≈ −1.55 V. However, a potential of −1.2 V in the SSRT showed little effect of atomic hydrogen evolution. Potentials less than −1.6 V are affected by atomic and molecular hydrogen. We thus concluded that the effect of atomic hydrogen predominates. Overall, the optimum corrosion protection range for SCC and hydrogen embrittlement of 5083-H112 seems to be between −0.9 V and −0.7 V.     

A nanoindentation study on the micromechanical characteristics of API X100 pipeline steel

The hardness characteristics of constituent micro-phases (ferrite and bainite) in a dual-phase API X100 pipeline steel were analyzed by nanoindentation experiments. The measured nano-hardness of the bainite phase is from 3.8 GPa to 4.9 GPa, which is much higher than that of the ferrite phase, which ranged from 1.75 GPa to 2.3 GPa. With the hardness and volume fraction of each micro-phase, attempts were made to predict the overall hardness by applying a simple rule-of-mixture. A comparison between the predicted overall hardness value and the experimentally measured value revealed that the rule-of-mixture can be successfully applied for prediction purposes. The results are discussed in terms of the grain boundary strengthening effect and the indentation size effect.     

Effects of La content on the properties of Ba1−xLaxTiO3 powders prepared by spray pyrolysis

Nano-sized Ba_1?xLa _x TiO₃ (0 ≤ x ≤ 0.03) powders were prepared by ultrasonic spray pyrolysis from the spray solution with citric acid. The mean sizes of the primary particles decreased from 110 nm to 59 nm when the x of the Ba_1?xLa_xTiO₃ powders was changed from 0 to 0.03. The BET surface areas of the powders changed from 9.1 m²/g to 16.9 m²/g. The pure BaTiO₃ and La-doped BaTiO₃ powders had both tetragonal and cubic crystal structures. The average grain size of the sintered BaTiO₃ specimen was 2.8 µm, while those of the La-doped BaTiO₃ pellets ranged from 1.04 µm to 0.68 µm according to the doping concentrations of La. The dielectric constant of Ba_1?xLa_xTiO₃ (0.01 ≤ x ≤ 0.03) pellets ranged from 2500 to 3855 when sintered at 1280 °C.     

In-depth numerical analysis on the determination of amount of CO2 recirculation in LNG/O2/CO2 combustion

The determination of proper amount of CO₂ recirculation is one of the critical issues in oxy-fuel combustion technology for the reduction of CO₂ emissions by the capture and sequestration of CO₂ species in flue gas. The objective of this study is to determine the optimum value of O₂ fraction in O₂/CO₂ mixture to obtain similar flame characteristics with LNG–air combustion. To this end, a systematic numerical investigation has been made in order to resolve the physical feature of LNG/O₂/CO₂ combustion. For this, SIMPLEC algorithm is used for the resolution of pressure velocity coupling. And for the Reynolds stresses and turbulent reaction the popular two-equation (k–ε) model by Launder and Spalding and eddy breakup model by Magnussen and Hjertager were incorporated, respectively. The radiative heat transfer is calculated from the volumetric energy loss rate from flame, considering absorption coefficient of H₂O, CO₂ and CO gases. A series of parametric investigation has been made as function of oxidizer type, O₂ fraction and fuel type for the resolution of combustion characteristics such as flame temperature, turbulent mixing and species concentration. Further the increased effect of CO₂ species on the flame temperature is carefully examined by the consideration of change of specific heat and radiation effect. Based on this study, it was observed that the same mass flow rate of CO₂ with N₂ appears as the most adequate value for the amount of CO₂ recirculation for LNG fuel since the lower C_p value for the CO₂ relative to N₂ species at lower temperatures cancels the effect of the higher C_p value at higher temperatures over the range of flame temperatures present in this study. However, for the fuel with high C/H ratio, for example of coal, the reduced amount of CO₂ recirculation is recommended in order to compensate the increased radiation heat loss. In general, the calculation results were physically acceptable and consistent with reported data in literature. Further work is strongly recommended for a large-scale combustor such as coal-fired power plant to figure out important parameters caused by the effect of increased combustor size and the presence of particle phase, etc.     

Characteristics of laser-annealed ZnO thin film transistors

We investigated the effects of laser annealing on ZnO thin film transistors (TFTs). ZnO layers were deposited on a bottom-gate patterned Si substrate by radio-frequency sputtering at room temperature. Laser annealing of the ZnO films reduced the full width at half maximum of the ZnO (002) diffraction peak from 0.49° to 0.1°. It reveals that the crystalline quality is improved by annealing effect. A SiO₂ formed in low temperature was used as the gate dielectric. Unannealed ZnO-TFTs were operated in enhancement mode with a threshold voltage of 21.6 V. They had a field-effect mobility of 0.004 cm²/Vs and an on/off current ratio of 134. Laser annealing of the ZnO-TFTs by 200 laser pulses reduced their threshold voltage to 0.6 V and increased their field-effect mobility to 5.08 cm²/Vs. The increase of mobility is originated from the crystallization enhancement of ZnO films after laser annealing.