Effect of Electric Field on the Processing and Properties of Yttria-stabilized Zirconia (YSZ)

It is well-known that the processing and properties of polycrystalline ceramics are grain size dependent; the grain size is usually controlled by thermo-mechanical treatments and the addition of solutes. More recently, application of an electric field has been found to retard grain growth and in turn influence the annealing, sintering and plastic deformation rate of ceramic materials. Moreover, it is well-established that the bulk conductivity of polycrystalline ceramics decreases with decrease in grain size.;This research deals with the effect of an applied electric field on the processing and properties of 3 mole percent yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP), whereby DC and AC fields were applied during sintering and isothermal annealing of 3Y-TZP. Regarding sintering, it was found that grain growth during sintering was retarded with the application of electric field compared to without. The ratio of the grain size with field to that without d¯E/d¯ decreased with increase in field strength up to ∼ 26 V/cm and then remained constant with further increase in field strength to ∼ 60 V/cm, the transition occurred at the ratio of d¯E/d¯ = 0.4. It was further found that an AC field had a greater effect on grain growth retardation compared to DC. In the isothermal annealing study a modest DC field with an initial strength of 14 V/cm was applied during the isothermal annealing at 1300 °C and 1400 °C. It was again found that the grain growth rate was retarded by the field.;SEM micrographs showed that the grains were tetrakaidecahedral in shape and essentially isotropic both with electric field and without. The grain size distribution was consistent with the Bitti-Di Nunzio model for both fully sintered and isothermally annealed conditions, which gives that the entire size distribution shifted to a smaller grain size. The retardation of grain growth rate was attributed to a reduction in grain boundary energy by the interaction of the applied electric field with the space charge at the grain boundary.;Physical models are proposed for the magnitudes of the GB energy components, namely electrostatic (space charge) gammabe, crystallographic mismatch (bSigma) and the ionic size misfit (bs) components. It was determined that the former two components combined comprised 40% of the total grain boundary energy in 3YSZ and the electrostatic component 60%. The reduction in be by electric field is considered to result from the bias exerted by the applied field on the space charge potential that occurs with the segregation of the yttrium ions at the grain boundaries, and is proposed to be the major factor responsible for the retardation of grain growth in 3YSZ by an electric field.;The electrical conductivity of 3 Y-TZP was investigated during the sintering and annealing processes. It was found that the bulk resistivity rho O increased with applied field E to ∼ 20-25 V/cm and then becomes relatively independent of E with further increase to ∼ 65-70 V/cm. The bulk resistivity with ac field rhoO(AC) was greater than that with dc rhoO(DC) at all but the highest sintering temperatures. Employing with brick layer model, the grain boundary resistivity rho b was 131 O-cm and 114 O-cm at 1300 °C and 1400 °C, respectively, and in turn gave the ratio of rho b/rhog(resistivity of the grain interior) =257 and 124, which are in good accord with the results obtained by impedance spectroscopy (IS) and 4-probe method. The value of rho b obtained was 173O-cm and 128 O-cm at 1300 °C and 1400 °C respectively, taking that the observed increase in bulk resistivty rho O with applied field strength E results from the corresponding decrease in grain size.