Sintering of Ceramic Materials Under Electric Field

The remarkable discovery of flash sintering came across during the early work of Cologna et al. and emerged as an attractive technique in the field of ceramic processing. In this technique the applied electric field initiates the “flash” event, while the densification is controlled by the current density set. Sintering occurs in less than 5 s at a threshold temperature for a given applied field. The objective of this thesis is to analyse the phenomena of flash sintering with different ceramic oxides; such as alumina-zirconia composite, hydroxyapatite and doped-alumina. The technique involved the application of constant electric field to a dog bone shaped specimen by means of two platinum electrodes while heating. Experiments were performed either in constant heating rate or in Isothermal condition. For the two-phase 3YSZ-alumina ceramic flash sintering was studied by constant heating rate (CHR) and isothermal sintering experiments. In CHR experiment the 50 vol% 3YSZ-alumina composite was shown to flash sinter at a furnace temperature of 1060°C under an electrical field of 150 V cm−1. Conversely, undoped single-phase alumina remains immune to sintering under fields up to 1000 V cm−1, although single-phase 3YSZ flash sinters at 750°C (furnace temperature). The mechanisms of field assisted sintering are divided into two regimes. At low fields the sintering rate increases gradually (FAST), while at high fields sintering occurs abruptly (FLASH). Interestingly, alumina/zirconia composites show a hybrid behaviour such that early sintering occurs in FAST mode, which is then followed by flash-sintering. The specimens held in the flashed state, after they had sintered to nearly full density, show much higher rate of grain growth than in conventional experiments. These results are in contrast to earlier work where the rate of grain growth had been shown to be slower under weak electrical fields. In the case of isothermal field-assisted sintering of two-phase, 50 vol% 3YSZ-alumina, the composites exhibit an incubation time for the onset of the flash event. Weaker applied fields and lower temperatures lengthen the incubation period. The effect is highly non-linear. For example at 1300°C and 150 V cm–1 the flash occurs nearly instanteously (in 10 s), but extends to two hours at 1275°C and 65 V cm–1. This behaviour is reminiscent of nucleation and growth phenomena in chemically driven experiments involving phase transformations in the solid state. Here, a model for nucleation under electrical driving forces, based upon the growth of embryos of colossal permittivity is presented. The flash sintering was also studied for composites with in-creasing volume faction of alumina in zirconia (10-50 vol%). The flash onset temperature or the incubation time for the 3YSZ-alumina composites increases with increasing the alumina volume fraction. In case of CHR experiments of hydroxyapatite, flash effect was shown at 840°C for an applied field of 2000 V cm-1. All the flash sintered samples show stable hydroxyapatite phase. However the sample sintered at 500 V cm-1 requires higher sintering temperature and shows enhanced preferred orientation due to higher diffusivity along c-axis. In case of alumina, field in excess to 1000 V cm-1 are re-quired to induce flash effect, whereas doped alumina shows flash sintering at 1000 V cm-1.