1. Field of the Invention
The present invention is directed to a varistor or capacitor, and more particularly to a monolithic ceramic device of the type described.
2. General Discussion
A ceramic varistor comprises essentially an intergranular barrier layer capacitor including a monolithic ceramic body having a multiplicity of electrode layers separated by ceramic layers. The odd numbered electrode layers, i.e. the first, third, fifth, etc., are electrically connected as are the even numbered electrode layers. The varistor is typically employed in shunting relation of an electronic circuit to be protected. It is the function of a varistor to provide a high resistance (and a degree of capacitance) when voltages impressed on the electronic circuit are maintained below a predetermined threshold voltage, and to provide a low resistance shunt when voltages exceed the threshold.
Heretofore, it has been exceedingly difficult to manufacture varistors having a predictable breakdown voltage, and particularly varistors which are rendered conductive at low voltages, i.e. 15 volts or less. The practical solution heretofore adopted by the industry has been to manufacture the varistor in a conventional manner, i.e. in the same manner as capacitors are conventionally manufactured utilizing known formulations compounded to function as varistors. Thereafter, the varistors produced are individually tested as to break down voltage and classified. It will be readily recognized that the individual testing or batch testing of varistors constitutes a complicating and costly step in the manufacture of varistors. A further desirable characteristic of a varistor is that when the same becomes conductive as a result of exposure to voltages beyond a threshold voltage, that the current carrying capacity of the varistor be as great as is possible. This characteristic is best realized where substantially the entirety of the ceramic components become simultaneously conductive thus providing the greatest current path between the various electrodes of opposite polarity. In conventional varistors, and even those varistors which have been classified to break down at a particular voltage, the break down does not occur uniformly, especially where the impressed voltage only slightly exceeds the threshold voltage. As a result, the ability of such varistors to function as an effective shunt is greatly reduced since conduction between opposed electrodes is focused at limited areas with remaining areas of ceramic continuing to be highly resistive.
It has been experimentally determined that the breakdown voltage of a varistor-ceramic formulation is a function of the number of grain boundaries of the ceramic grains intervening between adjacent electrode layers. The greater the number of boundaries between adjacent layers, the higher the break down voltage necessary to provide a conductive path. Conversely, in the event of a grain size such that grains of ceramic directly span the distance between adjacent electrodes, the device will exhibit break down or pass current at extremely low voltages. From the foregoing experimental findings, it will be evident that a highly undesirable condition results where the number of grain boundaries between adjacent electrodes varies greatly across the expanse of the ceramic layers. In such case, the break down voltage will be a function of and will occur at that area or those areas where there are concentrations of a limited number of grain boundaries. Where the break down is concentrated in a limited number of areas, it will be readily recognized that the current carrying capacity is substantially lower than would be the case if the break down occurred more or less uniformly throughout the entire area of the ceramic.
Efforts have been made to provide a ceramic having uniform grain boundary concentrations across the thickness of the ceramic. These efforts have heretofore been relatively unsuccessful on a commercial scale. Such efforts have included close control of the ceramic particle size embodied in the "green" ceramic layers; processing the ceramic under carefully controlled heating conditions during the sintering procedure; modifying sintering times, etc. As noted, none of the above methods have proven satisfactory.
A particularly acute problem arises when it is desired to provide a varistor having a relatively low break down voltage, i.e. in the order of 15 volts or less. The manufacture of such varistors to provide for break down at low threshold voltages and yet provide high current carrying capacity when the threshold voltage is exceeded has heretofore been very difficult.