Non-linear current–voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals

SrTiO₃ and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current–voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO₃, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current–voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current–voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current–voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent Σ1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping.     

Thermal instability of Bi-Pb-Sr-Ca-Cu-O superconductor around 650δC

Bi_0.85Pb_0.15Sr_0.8CaCu_1.4O _y superconductor was quenched to room temperature after annealing at several temperatures in air. From the initial magnetization curve measurement for the annealed samples, it was found that the volume fraction of superconducting phases of the sample annealed at 650°C greatly decreased. TEM observation revealed that a second phase often appeared in the grain-boundary region for the sample annealed at 700°C. The second phase was identified as Bi₁Pb₂Sr_2.6Ca_1.6O _y by energy dispersive spectroscopy.     

Benign partial epilepsy with affective symptoms: hyperkinetic behavior during interictal periods

We report a 4-year-old boy with benign partial epilepsy (BPE) with affective symptoms associated with hyperkinetic behavior during interictal periods. He had had hypermobility and restlessness since about age 3. At 4 years, 6 months, he developed episodes consisting of an expression of terror without complete loss of consciousness. Although we first suspected an acute psychic problem, the ictal EEG was abnormal. After carbamazepine (CBZ) therapy, the frequency of the attacks decreased dramatically and the hyperkinetic behavior gradually diminished.     

Effects of Atmospheric Composition on the Molecular Structure of Synthesized Silicon Oxycarbides

The dependence of silicon oxycarbides' chemical composition and molecular structure on their reaction conditions was tested by varying the atmosphere under which pyrolysis was performed. To obtain the silicon oxycarbides, densely cross‐linked silicone resin particles with an averaged diameter of 2 μm were pyrolyzed in various atmospheres of H₂, Ar, and CO₂, in the temperature range 700°C–1100°C. The residual mass of resin after pyrolysis was almost constant at 700°C, although their apparent colors varied distinctly. The sample obtained from the H₂ atmosphere was white, whereas that obtained from the CO₂ atmosphere was dark brown. Fourier‐transform infrared (FT‐IR) spectra of the residues suggested that the Si–O–Si network evolution was accelerated in the CO₂ atmosphere. Beyond 800°C, the chemical compositions of the compounds obtained from a H₂ atmosphere increasingly approached near‐stoichiometric SiO₂–xSiC composition with increasing the pyrolysis temperature. Compounds from a CO₂ atmosphere approached a composition of SiO₂–xC with no free SiC as the pyrolysis temperature increased. In the products from an Ar atmosphere, SiO₂–xSiC–yC compositions were typically obtained. The observed effects of the pyrolysis atmosphere on the resulting chemical compositions were analyzed in terms of thermodynamic calculations. Electron spin resonance (ESR) spectra revealed broad and intense signals from products obtained from either Ar or CO₂. Estimating from the signal intensity, the residual spin concentrations were in the range 10¹⁸–10¹⁹ g^?1. Meanwhile, the spectra from the samples obtained in H₂ showed weak and sharp signals with estimated spin concentrations ranging from 10¹⁶–10¹⁷ g^?1. This signal attenuation may have been due to the hydrogen capping of dangling bond formed during pyrolysis.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Role of Grain-Boundary Glass Phase on the Superplastic Deformation of Tetragonal Zirconia Polycrystal

Superplastic deformation of tetragonal zirconia polycrystal (TZP) was investigated by compression test in the temperature range of 1000° to 1500°C. Special attention was paid to the role of the grain-boundary glass phase on hightemperature deformation behavior. A small addition of glass phase markedly improved the high-temperature deformability of TZP. Lithium silicate glass was much superior to aluminosilicate or lithium aluminum silicate glasses for lowering the high-temperature flow stress. The deformation mechanism was discussed on the basis of mechanical testing data and microstructural examinations.