Direct determination of electrical conductivity of oxide cathodes

The application range of oxide cathodes, e.g. in CRTs is limited by the critical dc current density (typically 1–3 A/cm2) and hence by the electrical conduction mechanism. The electrical conductivity σel of the oxide layer has now been determined directly in a new procedure in a close-spaced planar diode configuration in an UHV chamber, together with thermionic emission data. The cathode temperature measured as a function of continuous dc load is essentially the result of a superposition of electron emission cooling—proportional to the dc current—and ohmic heating proportional to the square of the dc current. At the compensation point between electron emission cooling and ohmic heating, σel can be determined directly from the emission data. After activation a result of about 10−2 (Ω cm)−1 at 780 °C true temperature is obtained for BaSr-oxide cathodes, which is consistent with literature. It is shown that electrical conductivity decreases with operating time in accordance with reduced Ba generation. σel can be increased by oxide dopants like yttria or europia.