Thermal runaway, flash sintering and asymmetrical microstructural development of ZnO and ZnO–Bi2O3 under direct currents

Abstract

DC flash sintering of both pure and 0.5 mol. % Bi 2 O 3 -doped ZnO at a relatively high activating field of 300 V/cm has been investigated. It is demonstrated that even high-purity ZnO single crystals can "flash" at ~870 C. In comparison, flash sintering occurs at a substantially lower onset temperature of ~550 C in ZnO powder specimens, indicating the important roles of surfaces and/or grain boundaries. A model has been developed to forecast the thermal runaway conditions and the predictions are in excellent agreements with the observed onset flash temperatures, attesting that the flash starts as a thermal runaway in at least these ZnO based systems. Interestingly, enhanced grain growth is observed at the anode side of the pure ZnO specimens with an abrupt change in the grain sizes, indicating the occurrence of electricpotential-induced abnormal grain growth. With a large current density, the growth of aligned hexagonal single-crystalline rods toward the anode direction is evident in the ZnO powder specimen. Moreover, Bi 2 O 3 doping defers the onset of flash sintering, which can be explained from the formation of space charges at grain boundaries, and it homogenizes the microstructure due to a liquid-phase sintering effect. The key scientific contributions of this study include the development of a model to predict the thermal runaway conditions that are coincident with the observed onset flash sintering temperatures, the clarification of how flash starts in ZnO based specimens, and the observation and explanation of diversifying phenomena of sintering and microstructural development under applied electric currents.

Review of flash sintering: materials, mechanisms and modelling

“…This has been studied for several diffusion couples, e.g. MgO [56,96,116,117]. The grain size of FSed ZnO doubled at the cathode side, while no grain growth occurred at the anode side ( Figure 25).…”

“…Power supply (DC and AC) and frequency effects Most FS research has employed DC power supplies (V and I in the range of 10-5000 V and 0.5-15 A, respectively) [5,38,56]. This might be due to their lower cost compared to AC supplies.…”

“…The significance of atmosphere on FS has been seen as a reduction in the T Onset using ZnO as a model material system [96]. Zhang et al has specifically investigated the effect of atmosphere (air or flowing gas of Ar, Ar + 5 mol-% H 2 and O 2 ) on FS [56,99]. A more reducing hydrogen atmosphere (Ar + 5 mol-% H 2 ) led to the reduction in the T Onset down to 186°C, while pure O 2 atmosphere increased T Onset = 631°C only slightly compared to air T Onset = 599°C (shown in Figure 21(a)).…”

“…Some other effects have also been proposed. Luo [121] suggested that a complex ion transition (phaselike) might have occurred [56]: 'electrons accumulate at the anode due to the positive electric potential and interact with surfaces and/or grain boundaries to enhance the interfacial transport rates via an oxidation reaction that increases the local cation vacancy concentration, which subsequently accelerates the coarsening of particles and/or grain growth during FS'. It is clear that several concurring mechanisms could affect the asymmetric grain size distribution near electrodes Figure 24.…”

“…Such techniques have identified the formation of an apparent 'new' phase during FS of 3YSZ samples [39] and an apparent 'reversible' texturing formation only maintained while the field was applied to TiO 2 [40] samples. Even if these techniques are very powerful for in situ investigations, the interpretation of the results in terms of precise quantification (lattice parameters and crystal structure) of 'new' phase is difficult [39] mainly because of the narrow 2θ range, which is within 1°a nd the relatively slow acquisition time of 1.5 s. [56], silicon carbide (SiC) [43,61,80] and boron carbide (B 4 C) [32]; as well as several metalliclike non-oxide ceramics (ZrB 2 and MoSi 2 ) [35,80,81]. As illustrated in Figure 3, FSed materials can be grouped with respect to their type of electrical conductivity at T Onset ( Figure 9).…”