High temperature evaporation characteristics of amorphous Si3N4-C composite prepared by chemical vapour deposition

Evaporation characteristics of amorphous Si₃N₄ and amorphous Si₃N₄-C composite (6 wt % C) prepared by the chemical vapour deposition (CVD) were investigated in the temperature range of 1400 to 1650°C in a vacuum of about 10^?6 torr. The weight loss due to the evaporation was linear with time for all the samples tested. Evaporation rate of the amorphous CVD-(Si₃N₄-C) composite was 50 to 70% of that for the amorphous CVD-Si₃ N4. The activation energy for evaporation, calculated from the temperature dependence of the evaporation rates, was about 160 kcal mol^?1 for both samples. The carbon dispersed in the amorphous CVD-(Si₃N₄-C) composite reacted at the time of heat-treatment with the amorphous Si₃N₄ matrix and formedβ-SiC particles. Theβ-SiC particles were found to be about 100 nm in diameter and connected each other to form a three-dimensional network structure.     

Functionalization of unactivated carbons in 3α,6- and 3α,24-dihydroxy-5β-cholane derivatives by dimethyldioxirane

Direct remote functionalization of unactivated carbons by dimethyldioxirane (DMDO) was examined for 3α,6- and 3α,24-dihydroxy-5β-cholane derivatives. DMDO oxidation of stereoisomeric methyl 3α,6-diacetoxy-5β-cholanoates caused the direct, unexpected 14α- and 17α-hydroxylations, in analogy with that of the 5α-H analogs, regardless of the differences in stereochemical configuration of the A/B-ring junction and of the acetoxyl groups at C-3 and C-6. On the other hand, the ester derivatives of 3α,24-dihydroxy-5β-cholane with DMDO were transformed into the corresponding 5β-, 14α-, and 17α-hydroxy compounds, whereas the ether derivatives yielded the 5β-hydroxy, 3-oxo, and C-24 oxidized products, accompanied by their dehydrated ones.     

Recent progress in high-power blue-violet lasers

The property of GaInN-AlGaN heterostructures and GaInN multiple quantum well (MQW) gain GaInN laser diodes with low internal loss are described. GaInN blue-violet laser diodes have been developed as a light source for optical disk recording. However, the threshold current density of these diodes has been difficult to reduce and remains high at around 3-4 kA/cm/sup 2/. This is thought to be due to the large transparency current density Jt and the large optical internal loss /spl alpha//sub i/. Recently, the internal loss was successfully reduced to 13.6 cm/sup -1/ by optimizing the design of the near active region and achieved stable continuous operation under 50-mW continuous wave at 70/spl deg/C. Other laser characteristics such as far-field patterns and laser noise have also been improved for optical disk use.     

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CW iodine-ion laser in a positive-column discharge

In a positive-column He-I2discharge of a cataphoresis-type, CW laser oscillation of an output power around 10 mW for five visible lines has been first observed and output characteristics of three lines have been studied.     

Microcollimated laser diode with low wavefront aberration

A microcollimated laser diode (MCLD) utilizing a 1-mm short focal length, φ0.5-mm small diameter micro Fresnel lens (MFL) as the collimating lens is discussed. The MCLD is assembled with a 780-nm quantum-well laser diode dice and an MFL in the smallest commercially available laser package. The radiated laser beam from the MCLD has a power of higher than 2 mW at 50 mA driving current, a φ2 mm beam diameter with a nearly Gaussian intensity profile, and a low wavefront aberration of less than λ/14 (RMS value) measured at a 1-m distance     

Polymer composite of rigid and flexible molecules: Blend systems of poly(p-phenylene terephthalamide) and ABS resin

Homogeneous coagulant of poly(p-phenylene terephthalamide) (PPTA) and ABS resin was obtained by pouring the dimethylsulfoxide solution of N-sodium PPTA and ABS into acidic water. Transmission electron microscopic observation proved that PPTA was dispersed in the matrix in a form of microfibril with a diameter of 10–30 nm. The T_g of the resin component in ABS shifted to higher temperatures with increasing fraction of PPTA. Stress-strain behavior of the polymer composite showed increased tensile modulus and strength with addition of PPTA. The transition temperature from brittle to ductile fracture, however, shifted to higher temperature resulting in lower extensibility. Incorporation of the block copolymer of PPTA and polybutadiene into ABS improved the ultimate extensibility, i.e., increased toughness was provided compared with the simple composite systems of ABS and PPTA microfibrils. Scanning electron microscopic observation showed that the polymer composite made with the block copolymer generated many crazes upon deformation, while the composite with PPTA homopolymer fractured without remarkable craze formation. Thus, a new type of thermoplastic with improved mechanical properties was obtained by use of PPTA block copolymer as compatibilizer.