inc oxide varistors are novel semiconducting ceramics having highly non-ohmic current-voltage characteristics, which originate at the grain boundaries. These varistors are widely utilized in protecting electric power lines and electronic components against dangerous voltage surges. This overview of zinc oxide (ZnO) varistors includes electrical characteristics, fabrication methods, effects of additives, grainboundary properties, and specific applications.

Rutile particles doped by Cr and Sb with Sb/Cr ratios greater than unity were active for photocatalytic O 2 evolution from an aqueous silver nitrate solution under visible light irradiation. Two vibrational bands appeared in Raman scattering resonant to the electronic absorption and were assigned to symmetric breathing modes of the CrO 6 octahedron and neighboring TiO 6 octahedra, on the assumption that Cr dopant atoms exactly replaced Ti cations in rutile. Electrons excited with 355-nm and 532-nm light pulses showed absorption of mid-IR light, which was traced as a function of the time delay in a microsecond domain. By optimizing the Sb/Cr ratio, electron-hole recombination was retarded compared with that in non-doped rutile. Chromium dopants when accompanied with antimony dopants are proposed to trap the charge carriers without much enhancement of the recombination. The optimized Sb/Cr ratios were common for both the visible lightinduced O 2 production and the retarded recombination, suggesting that the charge carriers of the retarded recombination are used to produce O 2 .

The conduction mechanism of non-Ohmic ZnO ceramics is investigated. In order to explain the non-Ohmic property, a new energy-band model composed of a thin intergranular layer with traps sandwiched between Schottky barriers formed opposite each other is proposed. According to the newly proposed energy-band model, the non-Ohmic property of ZnO ceramics is mainly governed by field emission for the reverse-biased Schottky barrier in the voltage region above the threshold voltage in the V-I curve and by thermionic emission in the voltage region below the threshold voltage. The energy-band model and the conduction mechanism discussed in this paper are appropriate for the explanation of experimental results, not only those presented in previous papers, such as the effect of additives, the V-I curve and its temperature dependence, but, additionally, the dielectric properties, the asymmetrical degradation of the V-I curve, and the thermally stimulated current.

Microstructure and phase transformation in a highly non−Ohmic metal oxide varistor ceramicThe degradation phenomena caused by dc and ac biasing in non-Ohmic ZnO ceramics are studied from the viewpoints of voltage (V)-current (I) characteristics, dielectric properties, and thermally stimulated current (TSC). As a result, it is concluded that the degradation caused by dc biasing is attributed to the asymmetrical deformation of Schottky barriers, due to ion migrations in Bi z 0 3rich intergranular layers and in the depletion layers of the Schottky barriers; and that the degradation caused by ac biasing is attributed to the symmetrical deformation of the Schottky barriers, due to ion migration in the depletion layers of the Schottky barriers. Also, the relationship between the thermal runaway life of non-Ohmic ZnO ceramics and biasing conditions, such as biasing temperature and bias voltage, is obtained.

No abstract

Pentacene attracts a great deal of attention as a basic material used in organic thin-film transistors for many years. Pentacene is known to form a highly ordered structure in a thin film, in which the molecular long axis aligns perpendicularly to the substrate surface, i.e., end-on orientation. On the other hand, the face-on oriented thin film, where the molecular plane is parallel to the substrate, has never been found on an inert substrate represented by SiO2. As a result, the face-on orientation has long been believed to be generated only on specific substrates such as a metal single crystal. In the present study, the face-on orientation grown on a SiO2 surface has first been identified by means of visible and infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) together with two-dimensional grazing incidence X-ray diffraction (2D-GIXD). The combination of the multiple techniques readily reveals that the face-on phase is definitely realized as the dominant component. The face-on film is obtained when the film growth is kinetically restricted to be prevented from transforming into the thermodynamically stable structure, i.e., the end-on orientation. This concept is useful for controlling the molecular orientation in general organic semiconductor thin films.

A mechanism of degradation in leakage currents through ZnO varistorsThe destruction process of ZnO varistors caused by high currents is described. When high currents were applied to ZnO varistors, two kinds of destruction modes were found: a cracking mode and a puncture mode. The puncture mode is caused by melting of the region where current concentrates. The puncture process was simulated by a computer, solving a set of heat transfer equations. The relationships among the destruction energy, uniformity of the sintered body and nonohmic 1-V characteristics were obtained. The simulated results are consistent with the experimental results and show that the destruction energy depends strongly on the uniformity.

The performance of an organic electronic device is significantly influenced by the anisotropic molecular structure in the film, which has long been difficult to predict especially for a solution process. In the present study, a zinc tetraphenylporphyrin (ZnTPP) thin film prepared by a solution process was chosen to comprehensively explore the molecular-arrangement mechanism as a function of representative film-preparation parameters: solvent, film-preparation technique, and thermal annealing. The anisotropic structure was first analyzed by using a combination of infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) and grazing incidence X-ray diffraction (GIXD), which readily revealed the molecular orientation and crystal structure, respectively. As a result, the real dominant factor was found to be the evaporation time of the solvent that determines the initial two different molecular arrangements, types-I and -II, while the thermal annealing was found to play an additional role of improving the molecular order. The correlation between the molecular orientation and the crystal structure was also revealed through the individual orientation analysis of the porphyrin and phenyl rings.