Effect of microstructures on dielectric breakdown strength of sintered reaction-bonded silicon nitride ceramics

Silicon nitride (Si₃N₄) was prepared from silicon by a sintered reaction-bonded silicon nitride method using yttria and magnesia as sintering additives. Post-sintering (PS) of nitrided compacts was carried out at 1850°C under a nitrogen pressure of 1 MPa. Effect of PS time on microstructure and dielectric breakdown strength (DBS) of the prepared Si₃N₄ ceramics was evaluated. The DBS was measured using specimens with four different thicknesses (0.30, 0.20, 0.10, and 0.05 mm) in order to examine the thickness dependence. The porosity of the sintered Si₃N₄ decreased by prolonging the PS time, and the full density could be achieved at the PS time of over 6 h. After full densification, rod-like β-Si₃N₄ grains grew up, and their maximum grain size increased from 45.1 to 154.7 μm by prolonging the PS time from 6 to 48 h. The DBS of the thick Si₃N₄ substrates (0.30 mm) showed little variation from 35.4 to 47.0 kV/mm, regardless of the PS time. On the other hand, that of the thin ones (0.05 mm) dramatically decreased from 99.5 to 9.8 kV/mm with increased the PS time from 6 to 48 h. Because the DBS sharply decreased at the thin substrate sintered for longer time in which some large-elongated grains might span the substrate thickness-wise throughout, it was inferred that the interface between β-Si₃N₄ grains and grain boundary phase/intergranular glassy films might be a path of the dielectric breakdown.     

N-Alkylation of chitin and some characteristics of the novel derivatives

Summary Introduction of simple alkyl groups at the C-2 nitrogen of chitin and some properties of the resulting N-alkyl-chitins have been examined. Chitosan was fully deacetylated and treated with three kinds of aldehydes, formaldehyde, acetaldehyde, and pentanal. The Schiff bases of chitosan, whose extents of substitution were dependent on the amount of aldehydes, were reduced with sodium cyanoborohydride to N-alkylated chitosans. The N-alkyl-chitosans were then transformed into the corresponding N-alkyl-chitins by acetylation with acetic anhydride followed by transesterification to remove partly formed O-acetyl groups. The resulting N-methyl-, ethyl-, and pentyl-chitins were amorphous and showed improved affinity for organic solvents. Received: 13 December 2001/Revised version: 11 January 2002/Accepted: 17 January 2002     

Dietary CLA and DHA modify skin properties in mice

This study investigated the influence of PUFA on the properties of mouse skin. Mice (3 wk old) were given free access to oils high in linoleic acid, CLA, or DHA for 4 wk. At the end of the experiment, their skins were compared by both biochemical and histological methods. No significant differences in lipid and collagen contents were detected among treatments, although the FA composition in the skin was altered depending upon the FA composition of the supplemented oils. Electron microscopy revealed that the subcutaneous tissue layers in the CLA and DHA groups were significantly thinner than that in the high linoleic acid group, whereas no differences in the thickness of dermis layers were observed among the three groups. These results suggest that skin properties in mice are readily modified by dietary FA sources within 4 wk of dietary oil supplementation.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

Crystallization of polycarbonate induced by spinodal decomposition in polymer blends

We found that amorphous polycarbonate (PC) can be crystallized in several minutes by blending poly(ethylene oxide) (PEO). When the blends were annealed in the two-phase region below the upper critical solution temperature, highly interconnected two-phase structure characteristic of the spinodal decomposition was developed and then the crystallization occurred in the PC-rich phase during the spinodal decomposition. As the molecular weight of PEO decreased, the crystallization rate decreased and the crystallizable temperature became narrower in spite of the acceleration of the polymeric segmental motion. These results suggest that the crystallization of the PC is not induced by the acceleration of the polymeric segmental motion, but by the up-hill diffusion of the liquid-liquid phase separation via spinodal decomposition. Owing to the competitive progress of the crystallization and the spinodal decomposition, the melting peak of the PC crystallites shifted to lower temperature with increasing annealing temperature.     

Preparation and characterization of sulfonated block‐graft copolyimide/sulfonated polybenzimidazole blend membranes for fuel cell application

Polymer electrolyte blend membranes composed of sulfonated block‐graft polyimide (S‐bg‐PI) and sulfonated polybenzimidazole (sPBI) were prepared and characterized. The proton conductivity and oxygen permeability coefficient of the novel blend membrane S‐bg‐PI/sPBI (7 wt%) were 0.38 S cm^?1 at 90 °C and 98% relative humidity and 7.2 × 10^?13 cm³(STP) cm (cm² s cmHg)^?1 at 35 °C and 76 cmHg, respectively, while those of Nafion^® were 0.15 S cm^?1 and 1.1 × 10^?10 cm³(STP) cm (cm² s cmHg)^?1 under the same conditions. The apparent (proton/oxygen transport) selectivity calculated from the proton conductivity and the oxygen permeability coefficient in the S‐bg‐PI/sPBI (7 wt%) membrane was 300 times larger than that determined in the Nafion membrane. Besides, the excellent gas barrier properties based on an acid ? base interaction in the blend membranes are expected to suppress the generation of hydrogen peroxide and reactive oxygen species, which will degrade fuel cells during operation. The excellent proton conductivity and gas barrier properties of the novel membranes promise their application for future fuel cell membranes. © 2015 Society of Chemical Industry     

Effects of CuO Addition on Electrical Properties of 0.6BiFeO3–0.4(Bi0.5K0.5)TiO3 Lead‐Free Piezoelectric Ceramics

0.6BiFeO₃–0.4(Bi_0.5K_0.5)TiO₃ (0.6BF–0.4BKT) ceramic samples with 0.0–4.0 mol% CuO were prepared by the solid‐state reaction. The CuO addition aided the densification of the samples and slightly increased the lattice constant. The relaxor‐like defuse dielectric peak of 0.6BF–0.4BKT became sharper with increasing the CuO content. Polarization–electric field curve of the undoped 0.6BF–0.4BKT was a pinched loop in the as‐sintered state, while that was a square hysteresis with a large remanent polarization of 48 μC/cm² after the thermal quenching, demonstrating a strong domain wall pinning due to defect dipoles. We found that the CuO addition up to 2.0 mol% facilitates the polarization switching in the as‐sintered samples to increase the remanent polarization and the piezoelectric d₃₃ coefficient. The results of the structural and electrical investigations suggested that the copper ion acts as a donor in 0.6BF–0.4BKT by compensating the potassium vacancy created by the evaporation of K₂O during the calcination and sintering processes.     

Relationship between degree of polymerization and optical and thermal properties of fluorene in polycarbonate polymers

The influences of average degree of polymerization (D_p) and terminal group on thermal and optical properties of high refractive indexed transparent polymers were investigated. In this study, 9,9‐bis[4–(2‐hydroxyethoxy) phenyl] fluorene (BPEF) homo polymer was selected because it has been used as a representative monomer in high refractive index polymers as well as its unique property. BPEF has stable amorphous phase and reacts like a polymer. Its unique reaction allows continuous investigation from monomer to polymer. For hydroxyl‐terminated polymer, the refractive index (n_d) decreased with increasing D_p. On the other hand, for a phenolic‐terminated group, n_d increased with increasing D_p, and both converged to same value in high D_p region. As for glass transition temperatures (T_g), both terminal group series were increased as D_p increased. Though T_g of hydroxyl‐terminated polymer was higher than that of phenolic‐terminated polymer in the low D_p region, both converged to the same value and the inverse number of T_g had linear correlation against the weight percentage of carbonyl groups (CO), which was calculated by D_p. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45042.     

Incorporation of Dust Particles into a Growing Film During Silicon Dioxide Deposition from a TEOS/O2 Plasma

The effects of gas flow rate on particle formation and film deposition during the preparation of silica thin film using a TEOS/O₂ plasma were investigated. Particle formation and growth are suppressed with increasing gas flow rates. The film deposition rate increases with increasing gas flow rate, reaches a maximum value, and eventually decreases again. However, the uniformity of the film tends to degrade at high gas flow rates. At a high gas flow rate, some particles trapped in the sheath near the grounded electrode pass through the sheath to reach the substrate and are then embedded in the growing film. A self-consistent sheath model combined with particle force balance based on charge fluctuation was developed to explain these experimental findings qualitatively. The model reveals that charge fluctuation is a key factor for the particle to overcome the potential barrier of the negatively charged particles to pass through the sheath, eventually reaching the substrate. The model further shows that the probability of a particle being deposited on the substrate is higher for increased gas flow rates, which correctly predicts the experimentally observed trend.     

Unipolar Charging of Nanosized Aerosol Particles Using Soft X-ray Photoionization

A unipolar charging device based on a soft X-ray (<9.5 keV) photoionization was developed to investigate the charging efficiency of aerosol nanoparticles. Unipolar charging using a 241 Am charger was also evaluated as a comparison with the characteristics obtained by X-ray charging. The production rate and the concentration of ions generated by the X-ray and 241 Am unipolar chargers were estimated from ion current measurements. Theoretical calculations by the unipolar diffusion charging theory were also carried out and the calculated data were compared with the experimental results. For acquiring a high number of standard nanoparticles, the classification of monodisperse nanoparticles from polydisperse aerosol particles using the X-ray unipolar charger and a differential mobility analyzer (DMA) was also evaluated. The ion production rate of the X-ray unipolar charger was at least 5.5 times higher than that of the 241 Am unipolar charger and the ion concentration was about three times higher. Therefore, the X-ray unipolar charger showed a higher capability for charging aerosol particles of 10-40 nm size in diameter than the 241 Am charger. The charging state of particles produced by the X-ray unipolar charger was in good agreement with theoretical calculations. The X-ray unipolar charger developed herein has potential for use in charging a high number concentration of nanoparticles for use in nanotechnology investigations.