Thermogravimetric Study of the M-Co-O System: I, M = Ta and Nb at 1200°C

Phase equilibria in the Ta-Co-O and Nb-Co-O systems have been studied at 1200°C at oxygen partial pressures from 10^−0.68 to 10^−13.50 atm for the former and from 10^−0.68 to 10^−13.30 atm for the latter. In both systems, M₂CoO₆ and M₂Co₄O₉ are stable ternary compounds under the experimental conditions, and a new phase, Nb₅Co₂O₁₄, has been identified. The Ta-Co-O system is simple, whereas the Nb-Co-O system is somewhat more complicated because of the extra phase. The lattice constants of the ternary compounds have been determined and compared with previous values. The standard Gibbs energies of reactions have been determined using oxygen partial pressures in equilibrium with three solid phases.     

An expert system for XPLE cable deterioration diagnosis and life forecasting

The recent increase in the demand for power in urban areas has caused changes in the way power is supplied. Underground distribution systems have been introduced for convenience in urban areas and to prevent injuries in case of a crisis. As a result, many new types of power cables are being used. One type, XPLE cables (also called CV cables) have been rapidly taking the place of conventional cables such as OF or SL cables since the 1960s because of their relatively easy installation and maintainability. However, distribution cables used under severe environmental conditions deteriorate rapidly (due mainly to water treeing), and some cables experience insulation breakdown. Therefore, many diagnostic methods have been developed, though they have not been very effective. By combining methods, a high reliability can be achieved in comparison to the use of a single method. However, processing the data is difficult for on-site personnel, so many experts are required. But there is a lack of qualified experts, hence we have turned to the development of expert systems.     

A digital automatic current control system based on an optimumassumed model

In this paper, a new ACR (automatic current regulator) scheme is introduced for brushless DC motors. It consists of a model closely resembling the motor and PWM inverter, an assumed rising pattern of the current, and a parameter estimation. Using the control scheme, the mean value of the motor current is kept at the reference current, and the transient current of the motor is kept equal to the rising pattern. For the disturbance, because of DC-line voltage change, the proportional path from the error to the duty cycle of the PWM signal stabilizes the ACR. This ACR scheme can be realized with a microcomputer     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

Ten-user truly asynchronous gigabit OCDMA transmission experiment with a 511-chip SSFBG en/decoder

A ten-user truly asynchronous gigabit coherent-optical-code-division-multiple-access (OCDMA) transmission was experimentally demonstrated without using any timing coordination. The enabling technologies are a record-length 511-chip superstructured-fiber-Bragg-grating (SSFBG) en/decoder and a supercontinuum (SC)-based optical-thresholding technique to significantly suppress the signal interference beat noise as well as the multiple-access-interference (MAI) noise.     

Preparation of heterotactic-rich poly(methyl methacrylate) with narrow molecular weight distribution bytert-butyllithium/bis(2,6-di-tert-butylphenoxy)methylaluminum

Summary Polymerization of methyl methacrylate (MMA) withtert-butyllithium (t-C₄H₉Li) in toluene in the presence of aluminum alkoxides such as ethoxide,tert-butoxide and 2,6-di-tert-butylphenoxide, were examined at various Al/Li ratios. In the cases of ethoxide andtert-butoxide, predominantly isotactic polymers with broad molecular weight distribution were obtained. Combinations oft-C₄H₉Li and bis(2,6-ditert-butylphenoxy)methylaluminum [MeAl(ODBP)₂] were found to be an efficient initiating system for heterotactic polymerization of MMA, which gives PMMA rich in heterotactic triads up to 68% with narrow molecular weight distribution (Mw/Mn=1.09–1.17). End group analysis by¹H NMR indicated thatt-C₄H₉Li initiates the polymerization and MeAl(ODBP)₂ works as a stereospecific modifier. From stereosequence analysis of the heterotactic PMMA by¹³C NMR, it was found that the calculated pentad fractions from the first-order Markovian statistics (Pm/r=0.742, Pr/m=0.627) fitted the observed ones better than those from Bernoullian statistics. The glass transition temperature of the heterotactic PMMA was 13°C lower than that of syndiotactic PMMA, and the intrinsic viscosity in tetrahydrofuran was close to that of isotactic PMMA with a similar molecular weight but higher than that of syndiotactic PMMA.     

Synthesis of Sn-Incorporated Folded Sheets Mesoporous Materials (Sn-FSM-16)

Sn-incorporated folded sheets mesoporous materials (Sn-FSM-16) with various contents of Sn were synthesized by using a mixture of water glass, SnCl₄ and NaOH as starting materials. Hexadecyltrimethyl-ammonium chloride (surfactant) was used to intercalate into the layered silicate. The reaction process was followed by measurements of XRD patterns of intermediates. The Sn-FSM-16 was formed via the following mechanism: (1) layered silicates such as - - and -Na₂Si₂O₅ were formed as intermediates by the calcination of the mixture of the starting materials; (2) the surfactant was intercalated into the layered silicates; (3) the surfactant-silicate complex with hexagonal structure was obtained as a precursor of Sn-FSM-16; (4) the precursor was calcined to decompose the surfactant in the interlayer and was changed to Sn-FSM-16. The structural aspect of Sn in Sn-FSM-16 was studied by XPS profiles of Sn 3d _5/2 and Si_2p, ²⁹ Si MAS NMR and FTIR. The content of Sn in Sn-FSM increased with increasing concentrations of both Sn and NaOH in the starting materials. The surface area of Sn-FSM-16 decreased with an increase of Sn content in Sn-FSM (1160–620 m²/g).     

Fracture Energies of Tape-Cast Silicon Nitride with β-Si3N4 Seed Addition

The fracture energies of the tape-cast silicon nitride with and without 3 wt% rod-like β-Si₃N₄ seed addition were investigated by a chevron-notched-beam technique. The material was doped with Lu₂O₃–SiO₂ as sintering additives for giving rigid grain boundaries and good heat resistance. The seeded and tape-cast silicon nitride has anisotropic microstructure, where the fibrous grains grown from seeds were preferentially aligned parallel to the casting direction. When a stress was applied parallel to the fibrous grain alignment direction, the strength measured at 1500°C was 738 MPa, which was almost the same as room temperature strength 739 MPa. The fracture energy of the tape-cast Si₃N₄ without seed addition was 109 and 454 J/m² at room temperature and 1500°C, respectively. On the contrary, the fracture energy of the seeded and tape-cast Si₃N₄ was 301 and 781 J/m² at room temperature and 1500°C, respectively, when a stress was applied parallel to the fibrous gain alignment. The large fracture energies were attributable primarily to the unidirectional alignment fibrous Si₃N₄ grains.     

The Wulff Shape of Alumina: IV. Ti4+-Doped Alumina

Controlled-geometry cavities, initially ≈20 μm × 20 μm × 0.5 μm, were introduced into     -plane titanium-doped (≈210 ppma;≈500 wt. ppm) sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding. The samples were annealed in air for prolonged periods at 1600° and 1800°C to convert the titanium to the 4+ state and to allow the pore shapes to adjust. Pores with an equivalent spherical radius of ≈3.6 μm reached a quasi-equilibrium shape within 160 h at 1600°C and within 48 h at 1800°C. The Wulff shape was determined using optical microscopy, scanning electron microscopy, and atomic force microscopy. The Wulff shape of Ti⁴⁺-doped alumina includes well-defined c(0001),     , and     facets and smoothly curved sections. The     and a     facets, which are components of the Wulff shape of undoped sapphire, are not discernable. In contrast to undoped alumina, for which the r-plane has the lowest energy, the c-plane is the lowest energy plane in Ti⁴⁺-doped alumina. The surface energy sequence of the stable c, r, and p surfaces differs from that in undoped alumina. The Wulff shape varies with temperature. Samples equilibrated at 1800°C were re-annealed at 1600°C. Pore shape changes were reversible, indicating that the observed pore shapes were close to the equilibrium (Wulff) shape.     

Effect of Agglomeration on Mechanical Properties of Porous Zirconia Fabricated by Partial Sintering

Porous ZrO₂ ceramics were fabricated by compacting a fine ZrO₂ powder, followed by pressureless sintering. Two unidirectional pressures of 30 and 75 MPa were used to prepare the green compacts. The strength and the fracture toughness of porous ZrO₂ specimens sintered from the compacts prepared by 75 MPa were substantially higher than those by 30 MPa, especially for the specimens with low porosity. However, the corresponding Young's moduli were identical. This caused the strain to failure of these porous bodies to increase significantly with increasing compaction pressure. Microstructural analyses showed that a number of voids and small flaws existed in the green compacts prepared by the lower pressure, due to the agglomeration of fine ZrO₂ grains. It was revealed that the ZrO₂ agglomeration resulted in a localized nonuniform shrinkage and degraded the mechanical properties of porous ZrO₂ ceramics.