Shingo Ishida

Investigations of the electrical property, diffuse reflectance and ESR spectra of the La-(Fe, Mn)-codoped PTCR BaTiO3 annealed in reducing atmosphere

Electrical properties of La-(Fe,Mn)-codoped positive temperature coefficient of resistivity (PTCR) BaTiO3 ceramics were studied by combining their diffuse reflectance measurements and electron spin resonance (ESR) spectroscopy. La-(Fe,Mn)-codoped samples showed high durability to reducing atmosphere. It is assumed that Fe and Mn ions segregated in the grain boundary contribute to the density of surface acceptor states, meanwhile localizing electrons in a form of Ti3+ and stabilizing the chemisorbed oxygens through La3+-Mn3+,4+ or La3+-Fe3+ pairs. In addition, ESR signals of Fe3+ in annealed samples was intensified above Curie temperature (Tc), indicating that Fe ions still maintained its high valence states (Fe3+) in the grain boundary even after annealing in reducing atmosphere.

Mineralizing action of iron in amorphous silica

Abstract The mineralizing action of iron in amorphous silica was investigated by firing mixtures of silica gel and iron oxides at 1100 and 1150°C. Silica gel did not crystallize in the absence of ferrous ion below 1200°C. When silica gel mixed with ferrous oxalate was heated at 1100°C in a mildly reducing atmosphere(N2, CO2, CO), quartz, cristobalite and ferrous silicate (Fe2SiO4) were formed. Application of the reducing atmosphere from room temperature promoted the crystallization to quartz, while that from the soaking temperature after heating in N2 accelerated the formation of cristobalite. Chemical analysis revealed that the N2 atmosphere acted as a mildly oxidizing atmosphere to FeO possibly due to the coexistence of water vapor released from silica gel and impurity O2. Furthermore, tentative crystallization mechanism is proposed where mildly reduced and mildly oxidized Fe2SiO4 produced nuclei for quartz and cristobalite, respectively.

Conversion of silica gel and silica glass mixed with various metal oxides into quartz

Mixtures of silica gel and various metal oxides were heated at temperatures within the range of 500 to 1200°C for 1 h. When mixtures of silica gel and intentional additives such as Li 2 CO 3 , MgO, CaCO 3 , SrO, Ba(CH 3 COO) 2 , and ZnO were heated at temperatures from 700 to 1050°C, quartz phases formed. Similarly, heating a mixture of powdered silica glass and Li 2 CO 3 yielded quartz. In all cases the formation of quartz phases always accompanied the formation of silicate minerals. When a mixture of silica gel and 1 wt% Li 2 CO 3 was heated at 900°C, quartz phases and a very small amount of Li 2 Si 2 O 5 (hereafter denoted as Q-S mixture) were formed. Silica gel mixed with 10 wt% of Q-S mixture was transformed almost completely into quartz phases at 1000°C. The Q-S mixture consisted of only quartz phases after washing with hot H 2 SO 4 . Silica gel mixed with 10 wt% of the washed Q-S mixture was not transformed into quartz, but rather into cristobalite above 1200°C. This fact also indicates that the formation of the silicate mineral is a necessary precursor to the conversion of silica gel and silica glass into quartz.

Effect of furnace atmosphere on color of iron glaze

The glaze containing 1.8wt% Fe 2 O 3 were invariably yellow when fired under oxidizing and neutral atmospheres, but showed a wide range of color when fired under reducing atmosphere depending on the concentration of reducing gases and cooling conditions. From chemical analysis, it was found that the glazes fired in air and nitrogen contained iron mostly in a state of Fe 3+ and that those fired under reducing atmosphere contain major part of Fe 2+ and a small amount of Fe 3+ . The ESR signals of Fe 3+ and metallic iron were not observed in the greenish blue glaze. This result suggested that the greenish blue color was produced by Fe 2+ associated with a small amount of Fe 3+ . The X-ray diffraction analysis showed the presence of metallic iron in the black glaze fired under strong reducing atmosphere. From the EPMA results that iron was highly enriched in the surface layer of this black glaze it was assumed that iron aggregated into metal during firing.