Spark breakdown mechanism

Lasdy, the importance of electroceramic substrates and insulators should not be overlooked. Here one strives to raise the breakdown strength by eliminating the interesting conduction mechanisms just described. Spark plugs, high voltage insulators, and electronic substrates and packages are made from ceramics like alumina, mullite [55964-99-3] and porcelain [1332-58-7]. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

There are two principal views on dielectric breakdown generation. The first favors the cavitation-bubble mechanism, and the second involves electronic phenomena occurring in the studied system. The major differences between the bubble and electronic models of electrical breakdown lie in the importance attached to the temporal development of events which precede a spark, i.e., a moment considered as the breakdown. In the former case, it is proposed that ionization and current growth begin to occur in the gaseous phase after nucleation of a bubble, whereas, in the latter case, these processes begin first in the liquid areas. Among these two schools several models based on a different approach to the source of the increased conductivity of Uquids under electrical stress have been proposed. 

---