Noboru Yoneda

Flux growth of potassium β-ferrite (K1+xFe11O17) with β-alumina structure

Abstract Single crystals of potassium β-ferrite (K 1+ x Fe 11 O 17 ) having a layer structure were grown from a flux system of B 2 O 3 -K 2 O-KF. The crystals have a hexagonal plate shape with well-developed (0001) faces and the maximum size was 6.6 mm × 5.4 mm × 0.8 mm.

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 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.

Chemical vapor deposition of MO, MFe2O4 (M:Ni and Co) and Ni1−xFexFe2O4 single crystals

Abstract Single crystals of MO and MFe 2 O 4 (M: Ni and Co) were grown by the vapor phase reaction of MCl 2 vapor or MCl 2 and FeCl 3 vapors with oxygen at 900–1400°C for 2 h. Representative sizes of these crystals were about 0.5–2.5 mm. Ni 1− x Fe x Fe 2 O 4 (0 x 3 vapor at 1410°C. The nucleation caused by the vapor phase reaction is discussed by applying classical nucleation theory to the reaction system.

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.

Crystal growth of α-Fe2O3 by vapor phase reaction method

Abstract Single crystals of α-Fe 2 O 3 were grown by the vapor phase reaction of FeCl 3 vapor and oxygen in air at 840–1250°C. The largest crystals were grown at higher temperature and had a maximum size of 2 mm x 2 mm x 0.1 mm after the growth at 1250°C for 2 h. The representative form of the crystals was a plate with developed (0001) face.

Flux growth of α-Fe2O3 by fast cooling from low soaking temperatures

Single crystals of α-Fe2O3 were grown through use of a low soaking temperature of 900°C and a fast cooling rate of 50°Ch by the use of various fluxes. The largest crystals having 5×4×0.1 mm in size were grown from a new flux system, V2O5-KCl. The crystal shape was plate-like with (0001) faces.