Key Engineering Materials Vol. 388

Abstract: (Mg,Zn)O films were grown on Zn- and O-face ZnO single crystal substrates by pulsed laser deposition. The surface morphologies of the films grown on the Zn- and O-face substrates were quite different, indicating that no domain inversion occurred in both films. The films showed markedly different features for valence band spectra obtained by hard X-ray photoemission spectroscopy. This suggests that the effect of film polarity should be considered in X-ray photoemission spectroscopy.

Abstract: The effects of the thermally annealing of Bi-Mn-Co-Sb2O3-added ZnO varistors on their electrical degradation were investigated. For the samples with 0.01mol% Sb2O3added and without Sb2O3, no marked difference in the non linearity index of the voltage-current (V-I) characteristics was observed upon electrical degradation for the annealed and nonannealed samples. Upon increasing the amount of Sb2O3 added, the values of  increased after electrical degradation for the annealed samples. Moreover, the value of  after electrical degradation was proportional to the full width at half maximum (FWHM) of the X-ray diffraction peak for Zn2.33Sb0.67O4-type spinel particles under various annealing conditions. The added Sb2O3 did not dissolve in the ZnO grains but became segregated at grain boundaries. Therefore, it is speculated that the increase in the FWHM for the spinel particles is due to the increase in the numbers of fine spinel particles at grain boundaries and triple points.

Abstract: The mechanism of the recovery of the nonlinearity voltage-current (V-I) characteristics after electrical degradation was investigated from various viewpoints. The recovery processes of ZnO varistors in various gas atmospheres, and at various pressures, temperatures, and absolute humidity levels were evaluated. The evaluation involved the analysis of V-I characteristics, capacitance-voltage (C-V) characteristics, and X-ray diffraction (XRD). The height of the Schottky barrier and the thickness of the depletion layer were almost constant for sintering times between 3 and 5 h. On the other hand, the rate of recovery exhibited a change with sintering time. The recovery speed of the nonlinearity was also temperature dependent. It is suggested that the recovery process is caused by the transport of oxide ions through the grain boundaries between ZnO grains. It was found that the recovery process is affected by the crystal structure of Bi2O3 particles, which exist at the boundaries between ZnO grains.

Abstract: The degradation of ZnO-based multilayer ceramic varistors (MLCV) caused by electro-static discharge (ESD) and its mechanism on Schottky barriers formed at grain boundaries were examined. ESD is an extremely fast pulse which rise time is less than 1 n sec, and the typical voltage is around 8kV. Two degradations of current-voltage (I-V) characteristics occurred depending on ESD-voltage. The minor degradation at the early stage was caused only in the low-current region by a slight ESD pulse (=0.4kV). In contrast, the major degradation occurred over the wide current range of 1μ to 1mA by a highly ESD-voltage (= 8kV). The failure of Schottky barriers by ESD was produced partially in the microstructure. The large degradation was probably caused by the extension of region of broken barriers. The properties of barriers among boundaries and the microstructure play a crucial role in the degradation. In addition, using C-V analysis was found to be extremely valuable for the detection of degradation in MLCV than I-V property.

Abstract: Photoluminescence from epitaxial ZnO thin films deposited on R-plane sapphire substrates by RF magnetron sputtering was investigated. The intensity of the near band emission (NBE) of the ZnO thin film on R-plane sapphire was stronger than that of the film formed on C-plane sapphire at a low temperature. Some experimental results suggest that NBE depends on the polarization of the excitation light, which are considered to be related to the ZnO crystal orientation on the sapphire substrate.

Abstract: The effect of ion implantation leading to contamination and diffusion of lithium impurity in ZnO ceramics substrates was investigated. The diffusion coefficients of Li in the implanted ZnO annealed at 1000 and 850°C were in good agreement with those in the non-implanted ZnO. At 700°C, Li diffusion in the implanted ZnO was strongly enhanced. Our results show that the defects introduced by the implantation enhance the impurity diffusion at low temperature annealing.

Abstract: Highly conductive and well-aligned ZnO nanowhisker films which can be used to optoelectronic devices have been fabricated on conductive F-doped SnO2 coated glass substrates using aqueous solutions with the addition of polyethylenimine at a lower temperature. ZnO nanowhiskers with well crystalline and hexagonal morphology were grown with high orientation along the c-axis. The electrical properties were investigated by a low intensity red laser irradiation (6 mW), after DNA molecules labeled with dye molecules were absorbed on the nanowhiskers. Enhanced photocurrent was detected.

Abstract: Composite electrode materials of amorphous FeOOH-based particles and carbon powder were prepared by heat treatment of composite powder obtained by hydrolyzing of mixed aqueous solutions of FeCl3 and Ti(SO4)2 into which electron conducting carbon powder was dispersed. They exhibited high capacities over 150 mAh g-1 and good cycle performance at large charge-discharge current density of 5 mA cm-2 (ca. 1 A g-1). In this case, the heat treatment was effective process to improve the cycle performance.

Abstract: Thin films of titanate were prepared by electrophoretic deposition (EPD) of a colloidal suspension of nanosheets, and their lithium intercalation properties were examined. Thickness of the obtained film increased approximately in proportion to the increase in deposition time and concentration of the colloidal suspension used for EPD bath. EPD method was revealed to be a convenient method for layer lamination of nanosheets. The reversible capacity for the obtained film was approximately 170 mA h g-1, and it was in common with anatase-type TiO2 or conventional titanate. Lithium diffusion coefficient along the thickness direction was estimated to be 6 × 10-14 cm2 sec-1.