It is well known that aluminum doping is very effective for improving the Voltage-Current (V-I) characteristics of ZnO varistor elements comprising Bismuth oxide and Antimony oxide. Although it is established that the presence of aluminum lowers the resistance of ZnO grains by valence control, in this study, it is proposed that the homogeneity of micro structure of ZnO elements is more effective for improvement of V-I characteristics. The relationship between electrical characteristics and microstructure at different aluminum doping levels and sintering temperatures is shown with V-I characteristics, SEM observation and capacitance measurement of ZnO elements. As the effect of Al doping, it was found that moderation of ZnO grain growth causes the homogeneity of microstructure of ZnO varistors. In detail, reduction of the volume ratio of high resistive parts like thicker grain boundaries and multiple point between ZnO grains leads to excellent non-ohmic V-I characteristics. In addition, it is suggested that the tunnel effect by thinner grain boundaries is important as the mechanisms of improvement of V-I characteristics.

A variety of magnetic metal nanomaterials have been successfully prepared at extremely low temperatures by using CaH₂, which has recently received much attention in the field of solid state chemistry due to its ability to act as a very strong solid reductant. An advantage in using CaH₂ as the reductant instead of H₂, which is the conventional and most widely used reductant, is that the reaction temperatures can be lowered by several hundred degrees Celsius. This drastic decrease in the reaction temperatures enables the structures of the nanomaterials to be precisely controlled and the development of new functionalization techniques. Our recent results are briefly summarized here: SiO₂-coated α-Fe nanoparticles, biocompatible α-Fe nanoparticles and carbon allotropes-coated α-Fe and Ni nanoparticles.

The new oxidation technique by ozone was developed, which can strongly oxidize ceramic powder in a short time in comparison with the high-pressure method. Perovskite-type BaFeO₃ and its related compounds were prepared for the first time by low-temperature oxidation using ozone, showing ferromagnetism at ambient pressure.

Magneto-volume effect means the phenomena where volume of magnet changes spontaneously with temperature or as the effect of externally applied magnetic field. In this review, we show that they are treated in the same way as the thermal expansion of solids due to lattice vibrations. In this treatment of the effect on itinerant electron magnets, the same free energy is used in the derivation of magnetic entropy and specific heat. According to its volume dependence, three independent magneto-volume coupling constants are also introduced. As the results, our magneto-volume striction consists of two different components. In this way, we show quite new properties are derived and compared well with some results of experiments.

Synthesis and electronic properties of Cr(IV) oxides with CrO₆ octrahedra are overviewed focusing on the way of the connection of the octahedra. High-pressure synthesis under 6–8 GPa is effective for the preparation of them. The reaction to make some Cr(IV) oxides is found to be promoted by higher pressures even in case that it is less promoted by elevation of reaction temperatures. Magnetic and electric properties are closely related with the deep energy level of the 3d orbitals of the Cr⁴⁺ ions. For more specific description, α-Sr₂CrO₄ and NaCr₂O₄ are taken as examples of the compounds with corner- and edge-sharing octahedra, respectively. α-Sr₂CrO₄ has interesting correlation between spin and orbital degrees of freedom, which occurs due to the reverse split of the t_2g orbitals to the expectation of the conventional crystal-field theory. NaCr₂O₄ shows a new type of double exchange interaction between Cr spins owing to ligand holes. Electrically, these two compounds are insulating with small energy gaps whereas some related compounds are metallic. Thus, the Cr(IV) oxides appear to be located near the border of Mott transition.

Development of thermoelectric materials based on iron-sulfide minerals is reviewed with focusing on n-type doped CuFeS₂. It is found that the thermoelectric properties of Zn-doped CuFeS₂ strongly depend on the synthesis method, especially on the cooling rate and heat treatment conditions after spark-plasma sintering (SPS). A high power factor exceeding 1 mW/K²m is achieved around room temperature for samples annealed and slowly cooled after SPS. With aiming at reducing thermal conductivity, several methods are applied including the high-pressure torsion technique and nano-structuring by ball milling. These experiments successfully yield reduced thermal conductivity, though experimental conditions needs to be optimized to reduce electrical resistivity simultaneously.

Possible mechanism of the enhancement in magnetic interaction in itinerant electron magnets other than band controlling has been studied by means of the clusterization of magnetic ions in the η-carbide type compounds Fe₂AM₃X (A = Si, Ga, Ge; M = Mo, W; X = C, N) in which tetrahedral iron clusters are bridged by p-block metals. Powder X-ray diffraction analysis reveals the existence of new members of η-carbide type compounds; Fe₂GaMo₃N, Fe₂GeMo₃C, Fe₂GaMo₃C, Fe₂SiMo₃C, Fe₂GeW₃C, Fe₂GaW₃C, and Fe₂SiW₃C. Magnetism of the obtained materials were studied by means of macroscopic magnetization measurements and ⁵⁷Fe Mössbauer spectroscopy. Although the parent materials, Fe₃Mo₃N, Fe₃Mo₃C and Fe₃W₃C are weak itinerant electron magnets, we found antiferromagnetic orders with Néel temperatures higher than room temperature in Fe₂GaMo₃N, Fe₂GeMo₃C and a ferromagnetic order in Fe₂GeW₃C.

The formation conditions (temperature and oxygen partial pressure) of SrZn_xFe_2−x-W-type ferrites (SrZn_xFe_18−xO₂₇, 0 ≤ x ≤ 2) were investigated. W-type ferrites were formed under higher oxygen partial pressure when sintering temperature became higher. The oxygen partial pressure for forming the ferrites also increased when the Zn content x increased. The site distribution of iron and zinc ions was determined from Rietveld analysis of synchrotron radiation X-ray diffraction and the extended X-ray absorption fine structure (EXAFS) analysis at Zn-K edge. The Rietveld and EXAFS analyses indicated that zinc ions occupied mainly the 4e and 4f_IV sites, which were tetrahedral ones with down spin. Saturated magnetizations of the ferrites at 3 K increased with the increasing of the zinc content x. The magnetic moments of Fe³⁺ and Fe²⁺ are estimated to be 4.45 and 3.70 μ_B at 3 K in the ferrites, respectively.