Shinya Nariki

Flux growth and ion exchange of Cd stabilized K+-βPrime;-ferrite crystals with βPrime;-alumina structure

The large single crystals of K+-β″-ferrite with ca. 3×3×0.2 mm in size were grown from K2O-KF-B2O3 flux using CdO as a stabilizing reagent. K+ ions in the K+-β″-ferrite crystals were exchanged with M+ ions (M+: Na+, Rb+, Cs+, Tl+, NH+4 or H3O+) in the corresponding molten salts to form various M+-β″-ferrites. The K+-, Rb+- and Cs+-β″-ferrites revea higher ionic conductivities than the β-phases containing these ions.

Mixed alkali effect in ionic conduction of (K+, M+)-β -ferrites (M+: Na+ and Cs+) β -alumina structure

Abstract The ionic conductivities of mixed alkali (K + , M + )-β -ferrites (M + : Na + and Cs + ) with different alkali compositions, K + /M + , were measured. In each system, the ionic conductivity isotherm below 100°C exhibited a shallow minimum at about 0.4 mole fraction of M + ion. However, this mixed alkali effect was much smaller than that in β-ferrite. The excess free energy of mixing (Δ G ex ) was calculated using the activity coefficients of K + and M + ions in the mixed alkali (K + , M + )-β -ferrite. The Δ G ex had a slight negative value in contrast to the large negative value in β phase. This indicates that the interaction between K + and M + ions is very small. Therefore, the mixed alkali effect in β -ferrite is very small.

Crystal structure and mixed alkali effect of (K+, Cs+)-β-ferrite

Abstract Crystal structure determination has been accomplished for (K + 0.3 , Cs + 0.7 )-β-ferrite with β-alumina structure to investigate the mixed alkali effect in ionic conduction. According to the structure of conduction plane, BR sites were occupied by cesium ions and mO sites were occupied by potassium ions. It was concluded that blocking effect of cesium ions in BR sites dominate the mixed alkali effect in this ferrite.

Crystal structure of Cd stabilized K+-β″-ferrite with β″-alumina structure

Abstract The crystal structure of Cd stabilized K + -β″-ferrite was determined by the single crystal X-ray diffraction method. The K + ions occupied only BR sites in the conduction plane, which was different from the alkali distribution in Na + - or K + -β″-alumina. K + content in this crystal is 1.33 per unit plane cell, which is less than the alkali content in other β″-alumina type compounds − 1.7. The K + distribution was explained on the basis of this low K + content. In addition, the conduction slab in K + -β″-ferrite was thicker than that in K + -β″-alumina. This seems to be related to the large activation energy in ionic conduction of this ferrite.

Humidity sensitivity of β-alumina type ferrite films

Abstract The thin films of K + -β- and β″-ferrites have been prepared by dipping-pyrolysis process using potassium tert-butoxide and iron naphthenate. The films exhibited a significant decrease in impedance with an increase in relative humidity. In addition, the magnitude in impedance changes at 25°C was two or three orders and increased with a rise in firing temperature from 600° to 1000°C; that is, with the progress of the crystallization of β and β″ phases. The thin films with the composition of K 2 O·6.1Fe 2 O 3 showed the reversible changes in impedance with the humidity changes. The 95% response time to the humidity change from 43% to 80% RH was 10 s for the film with 1 μm thick obtained at 1000°C. The response time was remarkably shortened in the form of thin film.

Preparation and properties of new potassium-barium ferrite with mixed β-alumina and magnetoplumbite structure

Abstract A new hexaferrite (ideal composition: KBaFe 23 O 36 ) with mixed β-alumina and magnetoplumbite structure was synthesized by the solid-state reaction of the mixture 0.45K 2 CO 3 · 0.55BaCO 3 · 6Fe 2 O 3 at 1300 to 1350°C. The alternate stacking structure of half cells of K + -β-ferrite and Ba hexaferrite was proposed based on the X-ray powder diffraction pattern of this ferrite. A similar compound was also found in the Rb 2 OBaOFe 2 O 3 system. The saturation magnetization of the KBa ferrite was very small (1.6 emu/g at 25°C) compared to that of the Ba hexaferrite. The electronic conductivity of the KBa ferrite was 0.3 S · cm −1 at 300°C, which was too high to measure the ionic conductivity based on the β-alumina structure, but the electronic conduction was reduced by MgO doping. The ionic conductivity of Mgdoped KBa ferrite (0.45K 2 O · 0.55BaO · 5.7Fe 2 O 3 · 0.3MgO) was 6 × 10 −3 S · cm −1 at 300°C, comparable to that of K + -β-ferrite.

Thermal decomposition process and electrical conductivity of single crystals of NH+4-β- and β″-ferrites

Abstract The thermal decomposition process and the ionic and electronic conductivities of NH + 4 -β-ferrite and Cd stabilized NH + 4 -β″ ferrites single crystals were investigated. TG curves of both ferrites exhibited the two steps of weight loss at 50–200°C and 300–430°C. The first weight loss was based on dehydration, while the second was due to the liberation of NH + 4 ions. The β and β″ structure decomposed with the loss of NH + 4 ions to form α-Fe 2 O 3 and Cd doped γ-Fe 2 O 3 , respectively. The ionic conductivities of NH + 4 -β- and β″-ferrite single crystals were 4×10 −6 S cm −1 and 3×10 −5 S cm −1 at 25 °C, respectively. Temperature dependence of ionic and electronic conductivities is discussed in terms of the thermal decomposition process.