Zenbe-e Nakagawa

Sonochemical powder processing of iron hydroxides

Iron(II) hydroxide was oxidized in an ultrasonic-activated solution to produce α-FeOOH (goethite) or Fe3O4 (magnetite). This process involves the dissolution of Fe(OH)2, the oxidation of Fe2+ to Fe3+, and the precipitation of the products. It was found that the ultrasonic treatment greatly fasten the appearance and the growth of the products if oxygen was sufficiently supplied. When the oxygen supply was insufficient, on the other hand, an obvious phenomenon of sonochemical switching due to the enhanced oxidation was observed; that is, ultrasound promoted the α-FeOOH formation and suppressed the formation of Fe3O4 which has lower oxidation number (Fe = +83) then α-FeOOH (Fe = +3).

AGGLOMERATION OF SILICA SPHERES UNDER ULTRASONICATION

Power ultrasound of 20 kHz was applied to the synthesis of silica spheres via the controlled hydrolysis of tetraethoxysilane (TEOS). Silica spheres of about 0.3 μm were agglomerated to form tolerably uniform, dense particles of about 2 μm through 90 min sonication. This agglomeration behavior was examined by laser diffraction particle size analysis and transmission electron microscopy. It was found that the agglomeration process involves (I) an incubation period in which no agglomeration occurs, (II) rapid formation of ramified particles, and (III) their densification. It was inferred that sonication enhances collision among silica spheres.

Effect of ultrasound on synthesis of spherical silica

Spherical particles of silica were prepared in the presence of ultrasound at various frequencies (20, 1740, 4000 kHz) and intensities (< 25.4 W/cm2). It was found that the sphere size increased with increasing ultrasonic intensity and that the 20 kHz sonication induced agglomeration among the spheres. It was inferred that such agglomeration is dominated by a silanol group (-OH) at the surface of silica spheres. Size parameters, such as sphere size, agglomeration size, and ultrasonic vibration amplitude are discussed.

Oxygen Self‐diffusion in Cylindrical Single‐Crystal Mullite

A mixture of SiO 2 and Al 2 O 3 was prepared from metal alkoxides and fired at 1650°C to form mullite and an SiO 2 -rich glass phase. After leaching out the glass phase, crystalline mullite powder was obtained. Transmission electron microscopy examination revealed the powder to be cylindrical particles of single-crystal mullite. A diffusion experiment was performed on the mullite to determine the oxygen diffusion coefficient. This could be expressed as D ox = 1.32 × 10 -6 exp[-397 kJ/RT] m 2 s -1 in the range of 1100 to 1300°C. This falls along the line extended through the oxygen diffusion coefficient of forsterite, which is similar in crystal structure to mullite.

Crack propagation resistance of aluminium titanate ceramics

Abstract The crack propagation behaviour of aluminium titanate ceramics containing grain boundary microcracks has been investigated. The mean grain size of the aluminium titanate was 0·6 μm at 1300°C and 2·3 μm at 1500°C. Specimens sintered at 1450 and 1500°C contained grain boundary microcracks, while the specimen sintered at 1300°C had few cracks. Grain boundary microcracks decreased the bend strenght and Youngs modulus of the specimens. At the propagation of a crack, the microcracks deflected and caused branching of the crack, and blunted the crack tip; consequently, work of fracture of the specimen increased to more than twice the original value and crack propagation velocity decreased to 1/2000 of that in a microcrack-free aluminium titanate ceramic.

Effects of Cd doping on Bi-contained superconductors from rapidly quenched amorphous films

The effect of Cd substitution for a part of the Ca atoms in Bi 2S r 2 Ca 2 Cu 3 O q and Pb 0.2 Bi 0.8S r 0.8 CaCu 2 O q has been studued using the rapid quenched methpd. The Cd doped films in both systems yielded a highly oriented array of the 80 K phase. The addition of Cd content decreased the resistivity at room temperature and vanished the tailing of superconducting transition. Tc(endpoint) increased from 67 K (x=0.0) to 87 K (x=0.4) in Bi 2S r 2 Ca 2−x Cd x Cu 3 O q and from 67 K (y=0.0) to 83 K (y=0.2) in Pb 0.2 Bi 0.8 Sr 0.8 Ca 1−y Cd y Cu 2 O z

EFFECT OF ULTRASOUND ON CRYSTALLIZATION FROM AMORPHOUS GELS IN SOLUTION

Amorphous or poorly-crystalline gels in the system either Al-OH or TiC2O4-OH were ripened to form crystalline in the mother solution with or without ultrasonic irradiation at two different frequencies (20kHz and 1740kHz). The crystallinity of bayerite which was formed during the ripening of amorphous aluminogels was lowered by the ultrasonic treatments. While, titanium oxalate hydroxide gels transformed into different crystalline phases when ripened with high-frequency ultrasound. These effects of ultrasound on ripening are considered as (1) the acceleration of the dissolution of the gels and (2) the promotion and/or the alteration of the nucleation of the precipitates.

PREPARATION OF OXIDE MATERIALS USING ACTIVE SELF–FLUX OF NITRATES

Some oxides were prepared at lower temperatures using nitrate melts and their decomposition. Nitrate melts were kept by controlling their decomposition under high pressures. Mg-Al spinel was prepared at 500°C, corresponding to the product at 1200°C in air. Ba2Cu3O5+X that is not synthesized from oxide mixture in air could be prepared in this method. Ba2YCu3O7-d, a high Tc oxide-superconductor, was also synthesized directly in the form of orthorhombic, having a twin free microstructure.

APPLICATION OF SUPERCRITICAL FLUIDS FOR FORMATION OF FERROELECTRIC FINE POWDER

Ba-Ti acetate gels were prepared from titanium (IV) ethoxide, barium acetate, and acetic acid, and 2-propanol as a solvent. Wet gels were dried in various supercritical conditions using the drying media of CO2, MeOH, EtOH, and 2-propanol. Dry efficiency was different, depending on the supercritical conditions and the drying media. High drying efficiency, 82%, was obtained at the condition of 240 ° and 8MPa, using MeOH. The dried material was a porous body which is expected to be easy to control ceramic preparations.

Electrical Properties and Microstructure of ZnO-Nb2O5-MnO Ceramics Sintered in the Liquid Phase

Effect of several additives to ZnO-Nb205-MnO system on varistor characteristics were investigated. BaO and Si02 improved the varistor characteristics, but Co0 decreased the nonlinear exponent. The specimens with Si02 fired below the eutectic temperature also showed the nonlinear characteristics. The concentration difference of some components between the ZnO grains and the intergranular phases, and the temperature dependence of these distribution coefficients are essential for the appearance of varistor characteristics. Liquid phase sintering and high wettability are very convenient to elevate the characteristics. The addition of CeO2 markedly decreased the electrical resistivity and did not show the nonlinear characteristic. The addition of Pr6011 did not change the electrical properties but improved the stability of current.