Grain-growth Phenomena in a Zno Varistor Doped With Cobalt and Praseodymium Oxides

Willot, I. Delannay, Francis [UCL] Rapaille, J.

We studied the kinetic of grain growth in a ZnO varistor doped with cobalt and praseodymium oxides (1,5 wt % CO3O4, 0,3 wt % Pr6O11) during sintering in air from 600-degrees-C to 1400-degrees-C. The results are discussed in terms of the general expression G(n) - G0n = Kt. Thermal analysis and XR diffraction indicated that cobalt oxide forms a solution in the ZnO lattice whereas praseodymium oxide inhibits grain growth up to 1300-degrees-C by segregating along the grain boundaries. Indeed, an important grain growth is restored at 1400-degrees-C but the VDR effect has disappeared. Praseodymium oxide is then not finely dispersed anymore. Instead, big spherical agglomerates form along grain boundaries and also inside the grains. A possible cause of these phenomena is the appearance of a melted phase at this temperature.