A discussion on advanced methods of energy conversion- Magnetohydrodynamic power generation - New ceramics based on monoclinic zirconia

Abstract

In m.h.d. generators the mechanism of electrical conductivity of the electrodes should be electronic with a value of about 100 mho/m at 2000 °K. In addition, the material should have good thermal shock resistance. Calcia- or yttria-stabilized zirconia has sufficiently good electrical conductivity and thermal shock resistance but since their conduction mechanism is ionic they are spoilt by the passage of direct current because of electrolysis. The conductivity of pure monoclinic zirconia is mainly electronic and at 2000 °K has a value of about 50 mho/m but this declines very sharply as the temperature falls. Also the phase change at 1500 °K; from a monoclinic to tetragonal form with a concomitant volume shrinkage of 9% is mechanically destructive and precludes the use of this material for making electrodes. A new ceramic material has been designed to overcome these difficulties. A combination of zirconia, a second ceramic (which may be an insulator) and a metal is used. The metal provides electrical conductivity below 1800 °K; the second ceramic material acts as a diluent to the zirconia and reduces the effect of the phase change at 1500 °K sufficiently to prevent mechanical failure of the material, whilst the zirconia itself provides the hot electrode surface and the electrical conduction at the high temperature.