Hot spot phenomenon originated from the grain boundary of high-Tc superconducting ceramics and the application

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When a certain voltage is applied to ReBa_2Cu_3O_<7-δ> (Re : rare-earth element) ceramic rod at room temperature, a glowing hot spot appears in the rod. This project was aimed at investigating the appearance mechanism of various functions caused by the hot spot and the feasible applications. The following results were obtained from the research.(1) The mechanism of the hot spot migration toward the negative electrode is related to the oxide ion conduction in the hot spot.(2) The heat treatment using the hot spot improves the critical current density of the rod.(3) The oxygen sensor using the hot spot has an excellent sensing property and it is feasible for various uses.(4) A hot spot appears in (Ln_<1-x>Sr_x) (Co_<1-y>Fe_y) O_<3-δ> (Ln : rare-earth element) ceramic rod. By controlling the composition of the ceramics, the sensing property of the oxygen sensor can be designed for the various uses.(5) The fine constant current characteristic caused by the hot spot is unavailable when the rod is shorter than a certain value. A decrease in the thermal conductivity of the rod is effective for the miniaturization of the device.(6) When a hot spot appears under lower oxygen partial pressure than a certain value, a damped oscillatory current runs through the rod. It is possible to design the oscillation frequency and the time constant of decay by controlling the thermal conduction of surrounding gas, the cross-section area of the rod, the applied voltage and the oxygen partial pressure in ambient atmosphere. An ultra low frequency oscillator can be realized by using this phenomenon.