中村 龍哉

Lithium-nickel layered oxides were prepared by the solid-state reaction at temperature range of 700-850℃ in O_2 atmosphere. The structural refinement with the help of Rietveld analysis showed that the crystal structure of these compounds were identified as α-NaFeO_2 type layered rock-salt, where some extra Ni ions were present in Li-layers. As the Ni content in the Li-layer increased, the lattice parameter a increased and the ratio of C/a decreased. From the magnetic susceptibility measurement, it was found that both the asymptotic Curie temperature and the spontaneous magnetization at low temperature are sensitive to the presence of extra Ni ions in Li-layers. The asymptotic Curie temperature, which was always positive, shifted toward higher temperature with an increase in the non-stoichiometry degree. It was seen that the spontaneous magnetization 4.2K is also enhanced with the non-stoichiometry. Considering these magnetic variation with the non-stoichiometry enabled us to propose the following model; Ni-Ni magnetic coupling within the transition metal layer was weak and ferromagnetic, whereas the inter-layer Ni-Ni coupling is relatively strong and antiferromagnetic. The increase in the asymptotic Curie temperature may be attributed to the development of magnetic correlation from two to three dimension, and the enhancement of the spontaneous magnetization is described by ferromagnetic cluster formation.

Li-Mn spinel oxide particles were prepared by the solid state reaction using Mn3O4 particles and Li-hydroxide monohydrate, and the effects of the reaction temperature on the produced phase were studied with the powder X-ray diffraction and the magnetic susceptibility measurement. From the lattice constant variation versus the reaction temperature, it was found that the spinel oxides prepared by low temperature reaction were cation deficient type, and that the cation deficient degree decreased with an increase in the reaction temperature. The asymptotic Curie temperature ( Weiss constant), derived from the temperature dependent magnetic susceptibility, was always negative, and it implied that the antiferromagnetic coupling among Mn ions is dominant. The absolute value of the Weiss constant increased with the reaction temperature, took a maximum at the reaction temperature around 750degreesC, and then decreased with a further increase in the reaction temperature. These were attributed to the variation of the average valence of Mn ions. At the reaction temperature, where the absolute value of the Weiss constant takes a maximum, the non-stoichiometry of Li-Mn spinel oxide becomes minimum. In the specimen prepared above the temperature, it was likely that there existed oxygen vacancy, and the phase transformation between cubic and orthorhombic structures was found in the DSC curves. (C) 2003 Kluwer Academic Publishers.

Polycrystalline Z-type hexagonal ferrites were prepared by an usual ceramic sintering method, where a part of Ba ions were replaced by Sr ions and a small amount of SiO2 was added. The complex permeability spectra of each were measured. The low-frequency permeability increased until a half of Ba ions were substituted by Sr ions, and the natural resonance frequency remained almost constant and was almost insensitive to the substitution. On further substitution, the permeability remarkably decreased. This tendency indicated the enhancement of the Snoek's product. As the adding amount of SiO2 was increased, the low-frequency permeability decreased and the natural resonance frequency shifted toward higher frequency. These spectral variations were easily understood within the magnetic circuit model. From these results, it was possible to fabricate the polycrystalline hexagonal ferrite with the controlled complex permeability, which was suitable to the improvement in the antenna performance of hand-held cellular telephones. (C) 2002 Elsevier Science B.V. All rights reserved.

Li4/3Ti2/3O2 - LiFeO2 solid solution, Li(4 - x)/3Ti(2 - 2x)/3FexO2 (0.18 less than or equal to x less than or equal to 0.67), which has the cubic rock-salt structure (Fm (3) over barm, average particle size less than 100 nm), was synthesized from Fe-Ti co-precipitates by hydrothermal reaction with excess LiOH and KClO3 at 220 degreesC. Calcination of the products with lithium hydroxide in an oxidative atmosphere leads to the oxidation of trivalent iron to a 3+/4+ mixed valence state. Hydrothermally-obtained Li1.2Ti0.4Fe0.4O2 gave maximum initial charge (266 mA h g(-1)) and discharge capacities (153 mA h g(-1) around 3 V) between 2.5 and 4.8 V. Calcination enabled us not only to improve the crystallinity, but also suppress the discharge capacity fading with cycle number. Two plateaus around 3 and 4 V were observed on discharging by decreasing the amount of Li extraction (0.4 Li per chemical formula). Although the cubic rock-salt structure was retained during both charge and discharge processes, a partial 3d-cation displacement from octahedral 4a to tetrahedral 8c sites and some oxygen loss were observed after electrochemical delithiation. In-situ Fe-57 Mossbauer spectroscopy showed evidence of the Fe3+/Fe4+ redox only around the 4 V region.

Polycrystalline Ba3Co2Fe24O41 specimens were fabricated by usual solid state sintering or spark plasma sintering method. SEM observation showed that densification occurred simultaneously with grain growth in the solid state sintering. The spark plasma sintering produced dense ceramics with fine grain structure. Their complex permeability spectra were evaluated with a help of numerical calculation based on our previous model, which is related to the microstructure. Both spin rotation and domain wall motion affected the permeability in the solid state sintered specimens. The permeability of the spark plasma sintered specimens was mainly attributed to spin rotation.

Polycrystalline Li-Zn ferrites were prepared by the ceramic sintering method, and their complex permeability spectra were measured. The low-frequency permeability decreased with an increase in the Li-ferrite fraction. However, the natural resonance frequency shifted higher when the fraction of Li-ferrite was increased. According to our previous model, the complex permeability spectra were numerically separated into spin rotation and domain wall motion contributions. For the spin rotational component, which played an important role in the high frequency region, the static spin susceptibility decreased and the spin resonance frequency shifted higher with an increase in the Li-ferrite fraction. The product values of the static spin susceptibility and the spin resonance frequency, corresponding to the Snoek's product value, took a maximum value at a certain fraction of Li-ferrite. This was reflected to the performance of the single layered electromagnetic wave absorber using the polycrystalline ferrites.

Polycrystalline Li-Zn ferrites were prepared by the usual ceramic sintering method, and their complex permeability spectra were measured. The low-frequency permeability increased with the fraction of Zn ferrite. However, the natural resonance frequency shifted lower as the fraction of Zn ferrite was increased. According to our previous model, the complex permeability spectra were also numerically separated into spin rotation and domain wall motion contributions. For the spin rotational component, which played an important role in the high-frequency region, the static spin susceptibility increased and the spin resonance frequency shifted lower with an increase in the Zn-ferrite fraction. The product values of the static spin susceptibility and the spin resonance frequency, corresponding to the Snoek's product value, took a maximum at a certain fraction of Zn ferrite. This was reflected in the performance of the single-layered electromagnetic wave absorber using the polycrystalline ferrite. © 2002 Elsevier Science B.V. All rights reserved.

LiFeO2-Li2MnO3 solid solution was synthesized using solid-state reaction and hydrothermal-postannealing methods and characterized as a positive electrode material for rechargeable lithium batteries. Although the maximum Fe content [Fe/(Fe + Mn)] was limited up to 30% by solid-state reaction, the content can extend up to 75% by the hydrothermal-postannealing method. Neutron and X-ray Rietveld analysis reveal that the basic structure of the sample is a layered rock-salt structure isostructural with LiCoO2 (R (3) over barm) in which Fe ions exist on both Li (3a) and Co (3b) sites. Elemental analysis and Fe-57 Mossbauer spectra show Fe ions exist as 3+/4+ mixed-valence state after the samples were postannealed above 650degreesC. The initial charge capacity of Li/sample cells was above 100 mAh/g when the upper voltage limit was 4.3 V. The plateau around 4 V was observed for all Li/sample cells on first discharge. The maximum of initial discharge capacity was about 100 mAh/g down to 2.5 V for the Li/(50% Fe-substituted sample) cell, when the positive electrode was obtained by postannealing at 650degreesC in air. The capacity fading of the 4 V plateau could be suppressed by adjusting the Fe content to less than 50%, postannealing temperature between 600 and 700degreesC, and by 10% Ni substitution. (C) 2002 The Electrochemical Society.

Polycrystalline hexagonal ferrites (Y, Z and W-phases) were prepared by the usual ceramic sintering. Their complex permeability spectra were natural resonance type, and numerically separated into spin rotational and domain wall motion components. For the spin rotational component, it was found that the Snoek's product values of them were far beyond the product values of spinel ferrite, It enabled us to design the thin electromagnetic wave absorbers in the quasimicrowave region.

Li-Mn spinel oxide cathode materials were prepared by annealing of Li-Mn spinel oxide containing an oxygen deficiency, which were synthesized by high-temperature calcination, under various oxygen pressures. The effects of the annealing conditions on the produced phase were studied. From the changes of the lattice constant and the chemical composition, it was found that the spinel oxides prepared by high-temperature calcination were oxidized by the low-temperature annealing. The change in the oxidation degree during the low-temperature annealing decreased with an increase in the Li/Mn molar ratio. The effect of the preparation conditions on the reversible Li-intercalation/deintercalation reaction was also examined from the view points of the oxygen deficiency. These experimental results implied that the cycle performance in the oxygen-deficient Li-Mn spinel oxide was improved by the low-temperature annealing, and that the cycle performance of Li-Mn spinel oxide cathode depended on the oxygen stoichiometry rather than the Li/Mn molar ratio. However, it was found that the complete recovery of the oxygen defect structure was impossible when the oxygen deficiency was large enough. (C) 2000 Elsevier Science B.V. All rights reserved.

Three series of polycrystalline spinel ferrite, Ni-Zn ferrite, Mg-Zn ferrite and Ni-Zn-Cu ferrite, were prepared by the usual ceramic sintering method. The complex permeability spectra of each were measured. The low-frequency permeability increases with zinc content. However, the natural resonance frequency shifted down as the zinc content increased. The spectra were also numerically separated into spin rotational and domain wall resonance components. For the spin rotational contribution, which remained in the high frequency region, the static spin susceptibility increased and the spin resonance frequency shifted down with an increase in zinc content. Additionally, it was found that the product of the static spin susceptibility and the spin resonance frequency is proportional to the magnetization, irrespective of the kind of spinel ferrite. These relationships correspond to Snoek's limitation rule. From these results, it was clarified that polycrystalline ferrites having large magnetization are suitable for high-frequency applications. (C) 2000 American Institute of Physics. [S0021-8979(00)06312-X].

Li-Mn spinel oxide cathode materials were prepared using Mn2O3 particles, which were derived from the thermal decomposition of Mn-acetate or Mn-oxalate particles. The effects of the molar ratio of Li/Mn and the reaction temperature on the produced phase were studied. From the lattice constant variation, it was found that the spinel oxides prepared by low-temperature reaction were cation deficient type. The cation deficiency decreased with an increase in the reaction temperature, due to the lowering in the oxidation potential. The temperature, in which the cation deficient degree attained was almost zero, was dependent on the molar ratio of Li/Mn and the kind of the Mn2O3 starting material. The higher temperature reaction produced spinel oxides with oxygen deficiency. The effect of the preparation conditions on the reversible Li-intercalation/deintercalation reaction was also examined from the view points of the structural deficiency. These experimental results implied that the defect structure in the Li-Mn spinel oxide is not always in the thermal equilibrium state, but depends on the other factors. (C) 1999 Published by Elsevier Science BN. All rights reserved.

Aluminum bearing magnetite particles were synthesized by aerial oxidation of alkaline suspensions containing both ferrous and aluminum ions. It was found that the mixing procedure was critical in the preparation of ferrite particles without the formation of alpha-ferric oxyhydroxide particles: the neutralization of Al ions was required before the precipitation of Fe ions. XPS analysis of the Al(2p) electronic state indicated that aluminum ions in the obtained particles were in both hydroxide and oxide states, and that their molar fractions were approximately 20% and 80%, respectively. From the magnetization measurement at low temperature, it was found that most of the aluminum ions in the oxide state were incorporated into the octahedral site. The measured lattice constant was reproduced by the calculation using the Vegard's law in Fe3O4-gamma-Fe2O3-FeAl2O4 solid solutions and by taking into account of the existence of aluminum hydroxide. (C) 1998 Elsevier Science B.V. All rights reserved.

The low-temperature sintering of Ni-Zn-Cu ferrite was investigated using the usual ceramic technique. We found that the post-sintering density and the permeability of the sintered ferrite are strongly affected by the size of the starting oxide powders and the pre-sintering temperature. The most effective method of preparing high-permeability ferrite is to utilize fine particles of iron oxide and to calcine at about 800 degrees C. Sintered ferrite with a density greater than 4.5 g/cc and a permeability at 10 MHz greater than 200 can be obtained at a relatively low sintering temperature (about 900 degrees C). This condition is suitable for producing multilayer chip inductors. Additionally, the complex permeability of sintered ferrite was well-described as the summation of the spin rotational contribution and the domain wall motion component. The permeability in the 100 MHz region was determined mainly by the spin rotation magnetizing mechanism and then it depended only on the ferrite volume loading, i.e. the post-sintering density. However, the contribution of the domain wall motion was not negligible in the 10 MHz region and the domain wall contribution was controlled both by altering the post-sintering density and by varying the ferrite grain size.

Mn-Zn ferrite ceramics were prepared through the ceramic method starting from metal oxides. The ceramics' microstructural and electromagnetic properties were then evaluated. It was found that sintering density and ferrite grain size depend on both the sintering temperature and the kinds of the raw materials used. The changes in the microstructure were responsible for variations in magnetization, permeability, and electrical resistivity. The grain boundary resistivity, evaluated from the ac resistivity analysis, was especially affected. Increases in the electrical resistivity were counteracted by increases in the magnetization and permeability. This is also reflected in a trade-off of the hysteresis loss reduction and the lowering of the eddy current loss. This relationship between microstructural and electromagnetic properties can be explained by using the nonmagnetic and insulating grain boundary model. (C) 1996 American Institute of Physics.

The surface of acicular gamma-Fe2O3 particles was modified using a Co2+ containing aqueous suspension. Since octahedrally coordinated Co2+ ion in the spinel lattice produces strong magnetocrystalline anisotropy, the coercivity of the particles was enhanced. It was found from the XPS analysis that, in the case of the particles pre-modified with some Al3+ oxide or oxyhydroxide, a large amount of Co2+ ions did not enter the spinel phase but remained in the hydroxide phase. As the temperature of the modification reaction was raised, the transformation of Co2+ into the spinel lattice increased, resulting in the enhancement of coercivity.

Permeability spectra of Ni-Zn ferrite composite materials, prepared by mixing the ferrite particles with EVA resin, have been studied. In the sintered ferrite (volume fraction 1.0), the spin resonance is around 9 MHz and the static permeability about 1400. As the ferrite content decreases (composite materials), the static susceptibility of the spin component decreases and the spin resonance frequency shifts higher. The real part of the permeability in the ferrite composite materials becomes larger than that of the sintered ferrite in the rf frequency region. These features have been analyzed using the magnetic circuit model. The application of Snoek's limit extended to ferrite composite materials is also proposed.