Mechanisms of Flash Sintering in Cubic Zirconia

The recently discovered flash sintering technique has shown that the application of a sufficiently large dc electric field (E-field) to a ceramic during sintering can cause sintering at temperatures several hundred degrees below conventional temperatures with sintering rates that allow for sintering in seconds rather than hours. This technique has already been demonstrated in wide range of ceramic materials including ionic conductors, electronic conductors, semi-conductors and insulators. The application of this techniques to a large range of materials and the dramatic lowering of sintering temperatures and times means this technique has the possibility to revolutionize the ceramics field. Because the field of flash sintering is still young, the dominant mechanisms are still yet to be identified. So far, the mechanisms active during flash sintering remain only hypothetical. The key to the mechanism is that it must connect two phenomena controlled by different species: conductivity, which is determined by the fast moving species, and sintering, which is controlled by the slowest moving species. At this point the primary hypothesis is that the high E-fields used cause the nucleation and separation of Frenkel defects allowing for both high sintering rates and high conductivities. This work aims to identify the mechanisms involved in the flash sintering of cubic 8 mol% yttria stabilized zirconia (8YSZ). This materials has been selected because it was one of the first materials show flash sintering properties, and that it has been very well characterized because of its use as a fast oxygen ion conductor. This work began with the construction and initial testing of flash sintering equipment. Mechanistic investigations were performed on fully dense presintered samples of various grain sizes and single crystalline samples subject to the voltages similar to those used during flash sintering at constant temperatures between 600°C and 1000°C. Because flash sintering has a sintering component and electrochemical component, dense samples were used to remove the sintering so observations can be made only on the electrochemical behaviors during these electrical treatments. Additionally, the range of E-fields used for flash sintering of green 8YSZ samples was increased by an order of magnitude from the 150 V/cm to 2250 V/cm. Treatment of fully dense and single crystal samples show that the electrolytic reduction of ZrO2 to ZrO2-δ give I-V relations and power dissipations that are similar to those observed in flash sintering of powder samples. Flash sintering using E-field up to 2250 V/cm reveals several relationships between temperature, E-field, current density and power dissipation and their relation to the onset of sintering and densification. Most importantly the onset of sintering can be determined by the power relationship TOnset = 2440 E-1/5.85, where TOnset is the furnace temperature when flash sintering starts in K and E the applied E-field. Based on these observations, a new mechanism based on electrolytic reduction in the material is proposed to explain the onset and early stages of flash sintering and the observed electrical behaviors. Based on this, the movement of reduced zirconia in the E-field is proposed as a possible sintering mechanism.