Flash sintering is a novel field assisted sintering technique, whereby ceramics can be sintered in just a few seconds. For example, 3 mol% yttria stabilized zirconia (3YSZ) was shown to sinter in < 5 seconds at a furnace temperature of 850°C under electric field of 120 V/cm. Several mechanisms have been proposed for flash sintering, such as Joule heating and defect generation which is far beyond thermal equilibrium. Both factors are very critical for diffusion process, not only for sintering also for solid state reaction. Therefore, flash sintering has been expanded to synthesize oxide ceramic materials in a remarkably short time and low furnace temperature.
The motivation of this work is to utilize flash sintering technique to synthesize oxide ceramic materials. For this purpose, first, the role of defects in flash sintering has been studied, using pair distribution function analysis. For careful structure analysis, experiments on single phase are preferred, to eliminate the role of dopants and other phases which can contributes to create extrinsic defects. TiO2 is an appropriate candidate to conduct the study mentioned above. TiO2 was reported to be flash sintered under 250 V/cm, at a furnace temperature of 825°C. From the structure modelling, lattice parameter and atomic displacements can be obtained. Lattice parameters were used to determine sample temperature, using unit cell volume expansion, and corresponding atomic displacement was compared with thermally expected value. The displacements are measured to be far greater for oxygen atoms than for titanium atoms. These large displacements may indicate the important role of defects in flash sintering mechanism.
Second, applying flash sintering technique to synthesize oxide ceramic materials is evaluated, which is called reactive flash sintering. Two different system were studied; i) Al2O3 + MgO → MgAl2O4 and ii) a new concept in oxide materials, rock salt structure entropy stabilized oxide Mg0.2Ni0.2Co0.2Cu0.2Zn0.2O.
We show that flash experiments with three phase mixed-powders of yttria-stabilized zirconia (8YSZ), MgO and Al2O3 not only produce polycrystals of high density, but also the transformation of magnesia and alumina into magnesium aluminate spinel. The presence of zirconia facilitates the onset of the flash. The sintering experiments in the laboratory were extended to live experiments at the synchrotron in order to measure the time-dependent evolution of magnesium aluminate spinel. In voltage-to-current control flash experiments, the densification and the phase transformation occurred in <3 seconds during Stage II. In current rate experiments, we explore the temporal relationship between phase transformation and sintering by combining measurements of sintering with in-situ measurements of phase transformation. We show that phase transformation of powders of magnesia and alumina into magnesium aluminate spinel was completed under the current density of 75 mA/mm2, whereas sintering to full density required 100 mA/mm2. The role of zirconia in catalyzing the flash in the present study is also discussed.
We also investigate the reactive flash sintering of Mg0.2Ni0.2Co0.2Cu0.2Zn0.2O precursor powders prepared by a polymeric synthesis route. The flash experiments were performed at a constant heating rate of 10 °C/min with electric fields from 15 to 60 V/cm. The single-phase rock-salt structure was obtained in just a few seconds at furnace temperature as low as 500°C, under an electric field of 60 V/cm and a current density of 150 mA/mm2. Success at processing an entropy-driven phase in a remarkably short time and low furnace temperature demonstrates the potential of reactive flash sintering for producing entropy-stabilized materials. In-situ synchrotron XRD experiments showed that the phase transformation to entropy-stabilized oxide was accompanied with an intense endothermic reaction, which can be studied by platinum standard peak shift analysis from the surface.
Our experimental approach presents new exciting insight into flash sintering mechanisms, as well as opening up further studies of new materials that can be created by this synthesis method.