No abstract

Experimental evidence shows that the acceptor-state levels in Sb-doped positive-temperature-coefficient-type BaTi03 are compensated up to a critical acceptor-state density. Using the slope of the natural logarithm of the resistivity with re-' spect to 1/T instead of maximum resistivity as a measure for the acceptor-state density, it is possible to estimate this critical value. The value obtained (4.2 X 10" m-') is believed to be the first reported estimate based on experimental data. It is in good agreement with the estimate of 6 X 10" m-' (first reported by Jonker) obtained from the spontaneous polarization of BaTi03. This shows that the ferroelectric behavior of BaTi03 is indeed a feasible explanation for the low resistivity below the Curie point, as proposed by Jonker.[Key words: positive temperature coefficient of resistivity, barium titanate, acceptor, polarization, ferroelectrics.]

A novel oxygen sensor using hot spot on ceramic rod of high-T c superconductor RBa2Cu3OT_ ~ (R: rare earth element) has been developed. The hot spot appears by the self-heating of the local part on the RBa2Cu3OT_ ~ ceramic rod when a voltage above threshold is applied at room temperature. This sensor operates without any separate heater by taking advantage of the high temperature of the hot spot wherein oxide ions can diffuse easily. The oxygen concentration is determined from the value of the current flowing through the rod by utilizing the change in the resistivity of the hot spot depending on oxygen partial pressure in atmosphere. Oxygen concentration of 0 ~ 100% can be detected with high sensitivity and the response time is several seconds. The response performance of this oxygen sensor is almost the same as that of limiting-current-type zirconia sensor operating at 500°C.