Hot Spot in Ceramics and Its Various Applications

The present authors observed the phenomenon that a local area of a LnBa₂Cu₃O_7-δ (Ln: rare-earth element) ceramic rod glowed orange once a voltage exceeding a certain value was applied at room temperature. The visible glowing area was named a hot spot. The hot spot moved to the negative electrode with the velocity of a few mm/min. The direction of movement was reversed by switching the polarity.

The rod with the hot spot showed various functions. The current through the rod decreased abruptly when the hot spot appeared, and remained constant with increasing voltage. The rod with the hot spot could be used as a constant-current generator without any active component. The current after the appearance of the hot spot depended on the oxygen partial pressure in ambient atmosphere, which acted as an oxygen sensor without the need for any heating system. The current was also sensitive to the gas flow around the rod, and the rod could be used as a high sensitive gas flow sensor. Under low oxygen partial pressure, the current vibrated in the form of damped sinusoidal oscillation, and the rod could be used as a new type of sine wave oscillator.

The mechanisms of appearance and migration of the hot spot, and various applications created by the hot spot are discussed.