Atsushi Iga

Degradation mechanism of non‐Ohmic zinc oxide ceramics

The degradation phenomena caused by dc and ac biasing in non‐Ohmic ZnO ceramics are studied from the viewpoints of voltage (V)‐current (I) characteristics, dielectric properties, and thermally stimulated current (TSC). As a result, it is concluded that the degradation caused by dc biasing is attributed to the asymmetrical deformation of Schottky barriers, due to ion migrations in Bi2O3‐rich intergranular layers and in the depletion layers of the Schottky barriers; and that the degradation caused by ac biasing is attributed to the symmetrical deformation of the Schottky barriers, due to ion migration in the depletion layers of the Schottky barriers. Also, the relationship between the thermal runaway life of non‐Ohmic ZnO ceramics and biasing conditions, such as biasing temperature and bias voltage, is obtained.

The Sb2O3 Addition Effect on Sintering ZnO and ZnO+Bi2O3

The effects of adding minute quantities of Sb2O3 when sintering ZnO and ZnO+Bi2O3 (0.5 mol%) were studied. To prevent non-uniform distribution of the additive, dilute solutions of Sb-salts were used instead of solid Sb2O3. Very small quantity additions of Sb2O3 retarded sintering of both ZnO and ZnO+Bi2O3 (0.5 mol%). The sintering retarding effect, however, is lost when a powder mixture composed of Bi2O3+Sb2O3 is heat-treated to have these additives reacted with each other before they are added to the ZnO matrix.

Effect of Al-doping on the Grain Growth of ZnO

Grain growth of ZnO in liquid phase sintering of ZnO–0.5 mol%Bi2O3 ceramics has been studied by increasing the doping amount of Al2O3 from 25 ppm to 200 ppm. Al2O3 was doped using an aluminum nitrate aqueous solution. When 50 ppm of Al2O3 was doped to ZnO ceramics, abnormal grain growth was observed, while 75 ppm Al2O3 retarded grain growth. Doping of Al2O3 to pure ZnO ceramics did not cause abnormal grain growth,but simply retarded grain growth . The ratio of diameters of these grain sizes sintered at 900° C: d (abnormal grain growth)/d(retarded grain growth) is 20–100. Mechanisms of abnormal grain growth and retardation are proposed as follows. When the doped amount of Al2O3 was 50 ppm, aluminum compound thin films which were formed around ZnO grains at a low temperature and retarded grain growth disappeared suddenly by diffusion into the ZnO grain and ZnO grains grew suddenly. When the doped amount of Al2O3 was 75 ppm and the aluminum compound film was thick enough, part of the film remained unchanged even when a considerable proportion of the aluminum diffused mainly into the ZnO to reach solubility, retarding grain growth.

Magnetic Properties of Molybdenum- and Wolfram-Modified Mn3B4

Magnetic properties of (Mn 1- x Mo x ) 3 B 4 and (Mn 1- x W x ) 3 B 4 , where x ranges from 0.01 to 0.667, were studied by the measurements of magnetization, unit cell dimension, rotational hysteresis and internal field of Mn 55 . Unit cell dimensions for both Mo- and W-modified Mn 3 B 4 were found to increase continuously with increasing x indicating that solid solutions are formed in this composition range. Magnetization measurements showed that Mo or W addition results in ferromagnetic part in antiferromagnetic host of Mn 3 B 4 . The expected value for the magnetic moment of a Mn atom on 4( g )-site is about 1.5µ B for all the Mn 3 B 4 -type compounds studied.

Magnetic Properties of Fe-Modified Mn3B4

Magnetic susceptibilities were studied in antiferromagnetic Fe-modified Mn 3 B 4 over the temperature range from 77°K to 600°K. It was found that the Neel temperature of (Mn 1- x Fe x ) 3 B 4 decreases and the magnetic susceptibility at the Neel temperature increases with x , for the x values up to 0.15. The results show that iron atoms strengthen the inter-layer antiferromagnetic coupling in these materials. Fe-substitution also changed the character of the magnetic susceptibility in the low temperature transition of the materials.

Ferro- and Antiferromagnetism in (Mn1-xCrx)3B4

Magnetic studies were made on the compound (Mn 1-x Cr x ) 3 B 4 , x ranging from 0 to 0.5. In the composition range 0≤x≤1/3, antiferromagnetic phase of Mn 3 B 4 and ferromagnetic phase of Mn 2 CrB 4 were found to be stable, and these two phases seemed to coexist for the intermediate compositions. A rotational hysteresis was observed with the specimen for which x=0.05. This was attributed to the exchange interaction at the boundaries of these phases.

Electrical Characteristics of Boundary Layer-Type Semiconducting SrTiO3 Ceramics

Porous SrTiO3 ceramics containing an MgO-TiO2-SiO2 compound, Nb2O5, ZrO2, and a perovskite compound of Mn, Cu or Co oxide, were prepared. After fired in air, the ceramics were reduced in an N2-H2 atmosphere and then oxidized in air at a temperature between 800° and 1200°C. Grain boundaries were mainly oxidized by the heat treatment and the ceramics exhibited the characteristics of both capacitor and varistor. The apparent value of effective dielectric constant e decreased with the rise of temperature of heat treatment, and the maximum value 14 of nonlinear exponent α was obtained by the heat treatment at 1000°C. It is concluded that oxygen diffused through pores formed at tripple grain junctions, and oxidized the grain boundaries, thus furnishing the ceramics with the characteristics of capacitor and varistor.