The thermoelectric power of oxide cathodes on platinum cores

Abstract

The thermoelectric power of barium oxide, strontium oxide and mixed barium and strontium oxides on platinum cores has been investigated over the range of temperature from 70 to 800? C. Curves of thermoelectric power against temperature showed the following regions: (i) less than 1 mv degC-1 at low temperatures, (ii) thermoelectric power increasing rapidly from 180 to 300? C, (iii) constant at more than 2 mv degC-1, (iv) thermoelectric power remaining constant at temperatures up to 800? C for single oxides, but for mixed oxides falling sharply to a slightly lower value at the highest temperatures. After the oxides had been aged at 750-800? C for a year, with no current load, region (i) had disappeared and the thermoelectric power increased rapidly from 70 to 300? C. With barium oxide the disappearance of region (i) of the thermoelectric power characteristic was accompanied by a similar disappearance of the region of crystal conduction from the electrical conductivity plot (logarithm of conductivity against reciprocal temperature), but there was no change in thermal conductivity. With strontium oxide and mixed oxides there was no such simple correlation between the change in thermoelectric power and the change in electrical conductivity, and the thermal conductivity had decreased. These results are interpreted in terms of Loosjes and Vink's pore conduction theory of an oxide cathode and Metson's work on the thermal stability of the alkaline earth oxides.