Oxidation mechanisms of SiC-fiber–reinforced Si eutectic alloy matrix composites at elevated temperatures

SiC-fiber–reinforced binary Si eutectic alloy composites have been developed for aerospace applications using the melt infiltration method. In this study, the oxidation mechanisms of various binary Si eutectic alloys were evaluated at elevated temperatures. We suggest that the oxidation resistance of eutectic alloys could be predicted using the Gibbs energy change for the oxidation reaction. Based on these calculations, eutectic alloys of Si-16at%Ti, Si-17at%Cr, Si-22at%Co, Si-38at%Co, and Si-27at%Fe were prepared. These alloys produced uniform SiO₂ layers and showed the same oxidation resistance as Si at 1000°C under humid conditions. Therefore, SiC composites using Si alloys with excellent oxidation resistance can be predicted using thermodynamic calculations.     

Genetic variation of Listeria monocytogenes isolates from domestic and imported foods in Japan

Phylogenetic analyses were carried out on a total of 118 Listeria monocytogenes isolates from foods or food processing environments, and 7 isolates from listeriosis patients in Japan to evaluate the genetic variation in the pathogen in this country. Isolates of serotypes 1/2a, 1/2b and 4b were mainly examined to assess the risk of exposure of humans to L. monocytogenes from foods in Japan. The nucleotide sequences of the part of the iap gene that contains the region encoding the threonine-asparagine repeat units were determined in order to construct phylogenetic trees of the isolates investigated. A phylogram showed high genetic diversity among lineage 2 isolates, while the lineage 1 isolates showed clonal characteristics. The results of the genetic analyses suggested the presence of rare putative lineage 3 isolates and epidemic clone I (ECI) isolates in foods in Japan. The results showed that ECI was also isolated from listeriosis patients. The genetic variation in L. monocytogenes in Japan reported here suggests the necessity of monitoring the pathogen in foods and environments in addition to surveillance of listeriosis patients.     

Preparation and properties of poly(methylmethacrylate)-silica hybrid materials incorporating reactive silica nanoparticles

In this study, poly(methylmethacrylate) (PMMA)-based hybrid materials were prepared from reactive silica nanoparticles. These nanoparticles were obtained by the reaction of 2-(methacryloyloxy)ethyl isocyanate with colloidal silica dispersed in ethyl acetate, and they were copolymerized in various ratios with methyl methacrylate. Dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA) and visible spectrometry were performed to evaluate the physical properties of the resulting hybrid materials. The PMMA-silica hybrid copolymers maintained high transparency, and their storage elastic modulus and surface hardness increased with increasing silica content. Moreover, in comparison with PMMA, the hybrid copolymers had greater heat resistance and lower volume contraction.     

Method for grinding and delaminating muscovite

A method of dry grinding muscovite based on the use of knife-mills is reported. It was possible to produce mica with a particle size below 100 and 45 μm. After grinding, the samples were submitted to sonication treatment, which promoted delamination of the material. The particle size distribution shows that sonicated mica has more particles in the size range 10-50 μm than does non-sonicated mica have.This also indicates a decrease in the average particle size. Characterization of the treated muscovite by scanning electron microscopy revealed a highly delaminated material with a plate-like structure. The products were characterized by chemical analysis and X-ray diffraction, too. This mica has already been used for the synthesis of pearlescent pigments.     

Tunnelling through an embankment using all ground fasten method

The Tohoku Shinkansen Railway between Morioka and Hachinoe was opened on December 1, 2002. In this section, there are 20 tunnels. Second Itsukaichi tunnel is one of them, and it is a relatively short tunnel of 1175 m. At the Morioka portal, this tunnel crosses under National Highway No. 4 with a small depth of overburden 2–5 m where approximately 15,000 vehicles pass a day. The tunnel was excavated by NATM method. All Ground Fasten method was used as an auxiliary method. As a result, the face was prevented from collapsing and the tunnel was excavated while keeping the ground surface settlement to a minimum without interfering with road traffic.     

The effects of surfactant on the multiscale structure of bubbly flows

It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubble's rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1mm size can be much smaller than that on a clean bubble.     

A three‐phase hybrid stepping motor drive system combined with position‐sensorless closed‐loop control

This paper proposes a drive system for a three‐phase hybrid stepping motor, combining sensorless closed‐loop control with conventional open‐loop control. It is characterized by sophisticated control providing both prevention of pulling out from synchronism and suppression of natural rotor oscillation, without any position sensor attached to the motor shaft. A switching technique in chopper control which can enlarge the speed range controllable in the sensorless closed‐loop control is described. Starting and stopping sequences are developed to reduce mechanical natural oscillation produced in the transient state. Finally, the proposed drive system is compared experimentally with a conventional constant‐current open‐loop drive system. It is shown that the proposed drive system can perform the switchover from starting to sensorless closed‐loop operation within 20 ms, and can reduce the natural oscillation caused just after positioning. © 2000 Scripta Technica, Electr Eng Jpn, 131(3): 80–90, 2000     

Control and performance of a zero‐current‐switched,three‐phase PWM inverter equipped with resonant circuits on the ac side

This paper presents a zero‐current‐switched voltage‐fed inverter equipped with resonant circuits on the ac side. The current flowing through a switching device, that is, IGBT, is the sum of the load current and the resonant current. When the amplitude of the resonant current is larger than that of the load current, the current in the switching device becomes zero at an instant in each resonant cycle. This allows the switching device to be turned on or off at the zero current. The zero‐current switching makes a significant contribution to reduction of switching losses and electromagnetic noises. In this paper, the principle of zero‐current‐switching operation, along with a novel control scheme, is described from a theoretical and practical point of view. Experimental results obtained from a laboratory system of 5 kVA verify the practicability. Moreover, the switching and conduction losses of the proposed soft‐switched inverter are compared with those of a conventional hard‐switched inverter. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 85–95, 2000     

Connecting dopant bond type with electronic structure in N-doped graphene

Robust methods to tune the unique electronic properties of graphene by chemical modification are in great demand due to the potential of the two dimensional material to impact a range of device applications. Here we show that carbon and nitrogen core-level resonant X-ray spectroscopy is a sensitive probe of chemical bonding and electronic structure of chemical dopants introduced in single-sheet graphene films. In conjunction with density functional theory based calculations, we are able to obtain a detailed picture of bond types and electronic structure in graphene doped with nitrogen at the sub-percent level. We show that different N-bond types, including graphitic, pyridinic, and nitrilic, can exist in a single, dilutely N-doped graphene sheet. We show that these various bond types have profoundly different effects on the carrier concentration, indicating that control over the dopant bond type is a crucial requirement in advancing graphene electronics.