Unwanted voltage surges have long been a critical problem to circuit designers of industrial and home electrical systems. Surges generated by load switching are often repetitive and range as high as 2,500 V. Lightning generated surges can range up to or over 6,000 V.
Surge protective devices have been made from SiC. It is also known that ZnO when mixed with certain oxide additives and sintered into pellets, can exhibit non-linear V-I characteristics superior to SiC. These additive modified ZnO composition are, therefore, candidate materials for non-linear lightning arrester and similar type components.
In the sintered body, the sintered ZnO grains will be coated and bound with the oxide additives. These oxide additives are effective to produce electrical non-linearity completely within the bulk of the body. The voltage limiting characteristic of these surge protective materials is believed to be due to the character of the oxide additive within the grain boundary of the body of the material, which is near-insulating at low voltage and conducting at a high voltage.
ZnO non-linear devices have been made by mixing additive oxides, as individual powders, with ZnO powder, and then pressing and sintering, as taught by Matsuoka et al, in U.S. Pat. No. 3,663,458, In that patent, ZnO powder is mixed in a wet mill for 5 hours with additive materials such as Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, CoO and MnO, as individual powders, to produce a homogeneous mixture. A binder such as water or polyvinyl alcohol can be added. The mixture was then pressed at about 340 kg./sq. cm (4,800 psi) and sintered at 1000.degree. to 1450.degree. C. for 1 to 3 hours, providing 1.3 cm diameter by 0.05 to 0.25 cm. thick discs. Matsuoka et al, in U.S. Pat. No. 3,838,878, more thoroughly mixed ZnO powder in a wet mill for 24 hours with individual additive oxide powders and CeF.sub.3 powder, to produce a mixture to which a binder could be added. The mixture was then pressed at 250 kg./sq. cm. (3,500 psi) and sintered at 1000.degree. to 1450.degree. C. for 1 to 10 hours, to provide bulk voltage non-linear bodies for lightning arresters, with dimensions as large as 3.5 to 4 cm diameter and 2 cm thickness.
We have found that mixing the materials is one of the most important operations in making non-linear lightning arrester components and non-linear resistors, because the physical homogeneity of the product, and the reproducibility of the electrical characteristics, will depend on thoroughly mixing of the component powders. By merely milling or blend-mixing the ingredients, even for 24 hours, only a marginally acceptable product is produced, resulting in a large percentage of lightning arrester components and resistors being rejected due to varying electrical properties caused by lack of homogeneity.
The grain boundary phase has been formed, in the prior art, by chemical reaction between the individual oxide additives in the sintering step of the process to form the resistor bodies. We have found that it is essential that the grain boundary phase be completely chemically homogeneous, and represent the equilibrium condition of the oxide additive reactant products. This means that the chemical reactions of the oxide additives must go to completion during the time that a single phase glass is being formed. Conventional fabrication methods of mixing component ZnO powder with 7 or 8 individual oxide additives, and then sintering in an attempt to reduce the mixture to a 2 component system, does not achieve the desired completely homogeneous grain boundary layer.
In our cross-referenced co-pending application, the aforementioned problems are disclosed to be solved by forming an additive glass composition from a mixture of oxides containing at least Bi.sub.2 O.sub.3 prior to mixing with ZnO particles. Glasses made from these oxides are disclosed to be effective to cause electrical non-linearity with a ZnO ceramic body. The additive oxides are mill-mixed and melted to form a single phase homogeneous glass melt which is quenched. Thereafter, the glass is milled and mixed with ZnO and the mixture is pressed to form a consolidated body. Thereafter, the pressed body is heated to sintering temperatures. The process provides pellets having a microstructure with a bulk phase of ZnO particles and a boundary phase containing an oxide insulating layer binding the ZnO grains together.