Michio Matsuoka

Nonohmic Properties of Zinc Oxide Ceramics

Nonohmic properties of ZnO ceramics with five additives of Bi2O3, CoO, MnO, Cr2O3, and Sb2O3 are studied in relation to sintering temperature, additive content, and temperature dependence. The observation of electron photomicrographs and X-ray microanalysis proves a ceramic microstructure such that ZnO and these five oxides form, at the grain boundaries, segregation layers which are responsible for the nonohmic properties. The electrical resistivity and dielectric constant of segregation layers are estimated to be 1013 ohm-cm, and 170, respectively by using a simple model. The electric field strength corresponding to the steep rise in the current is also estimated to be 104 V/cm by taking account of the concentration of applied voltage at the segregation layer. In view of these data and simple model, a possible explanation for nonohmic properties is discussed.

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.

Some Electrical Properties of γ-Fe2O3 Ceramics

We have observed some electrical properties of γ-Fe2O3 ceramics made by oxidizing Fe3O4. The change in electrical resistance during the oxidation from Fe3O4 and the transformation to α-Fe2O3 are discussed first. The effects of additives and mechanical treatment on the transformation of γ-Fe2O3 to α-Fe2O3 were also studied, by measuring the electrical resistance. A high gas sensitivity was observed in γ-Fe2O3 ceramics, and the gas sensitivity decreased as the amount of α-Fe2O3 precipitated in the γ-Fe2O3 increased. The transformation to α-Fe2O3 could thus be investigated by evaluating the gas sensitivity.

Effects of Sulfate Ion on Gas Sensitive Properties of α-Fe2O3 Ceramics

Alpha hematite (α-Fe2O3) has been considered to have only a poor sensitivity to reducing gases. However remarkable gas sensitive properties were observed in the α-Fe2O3 prepared from iron salts containing sulfate ion (SO4-) by hydrolysis and precipitation. The sulfate ion remains in the α-Fe2O3 but vaporizes at near 760°C. The vaporization causes crystallization of the α-Fe2O3 and consequently results in a decrease in gas sensitivity. The sulfate ion is an extremely important factor in controlling the microstructure and plays a significant role in the gas sensitive properties of the α-Fe2O3 ceramics.

Enhancement of Gas Sensitivity by Controlling Microstructure of α–Fe2O3 Ceramics

We have found that the gas sensitivity of α–Fe2O3 ceramics can be greatly enhanced by controlling its microstructure by adding quadrivalent Ti, Zr or Sn. These metals have the effect on suppressing the grain growth and crystallization, and consequently increasing the specific surface area of the sensing element. With Sn in particular, the sensing element consists of ultra-fine grains as small as about 100 A and has an extremely large specific surface area of 125 m2/g. However, the additives are effective only in the presence of the sulfate ion, which is considered to localize mainly on the surface of the α–Fe2O3 grains. Thus the surface of the grains, which is fairly amorphous, is thought to play an essential role in the gas sensitive mechanism.

Grain growth control in ZnO varistors using seed grains

A method for making low‐voltage ZnO varistors having extraordinarily large grain size without necessity of sintering at much higher temperatures or for much longer times is reported. The device is made by sintering a mixture of ZnO fine powder, additives, and ZnO seed grains having grain sizes of 63–105 μm obtained by washing a ZnO sintered body containing BaO in boiling water. The device has a grain size around 500 μm and low threshold voltage of 6 V/mm. Such anomalous grain growth is caused by the difference between the radii of curvature of ZnO fine powder and ZnO seed grains.