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Using ZnO as a model system, a strong dependence of the onset flash sintering temperature on the atmosphere has been discovered. In the best cases, ZnO specimens have been sintered to ...>97% relative densities (with fine grain sizes of ∼1μm) in ∼30s at furnace temperatures of <120°C in Ar+5mol.% H2. The enhanced conductivities of ZnO powder specimens in reduced atmospheres are responsible for the substantial decreases of the onset flash sintering temperatures.
DC flash sintering of both pure and 0.5mol.% Bi2O3-doped ZnO at a relatively high activating field of 300V/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 electric-potential-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, Bi2O3 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.
Using ZnO as a model system, the densification mechanisms of flash sintering are investigated. Controlled experiments via limiting the maximum current or the effective ramp rate suggest that both the ...maximum specimen temperature and the high heating rate (on the order of 200 °C/s) are essential for the rapid densification during the flash sintering. Moreover, benchmarking rapid thermal annealing (RTA) experiments, which were conducted to mimic the heating profiles in the flash sintering, achieved similar densification and grain growth rates with comparable heating profiles, attesting that the ultrafast densification is mainly enabled/determined by the T(t) profile. The combination of these experiments suggest that, at least for ZnO, the rapid heating profile is a key factor for the observed rapid densification in flash sintering, while various electric field/current effects could also exist. A clear and consistent correlation between the grain sizes and relative densities is also evident for specimens made by both flash sintering and RTA with different conditions, suggesting the same conventional grain growth mechanism in both cases under the current experimental conditions.
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Seven equimolar, five-component, metal diborides were fabricated via high-energy ball milling and spark plasma sintering. Six of them, including (Hf
Zr
Ta
Nb
Ti
)B
, (Hf
Zr
Ta
Mo
Ti
)B
, (Hf
Zr
Mo
Nb
...Ti
)B
, (Hf
Mo
Ta
Nb
Ti
)B
, (Mo
Zr
Ta
Nb
Ti
)B
, and (Hf
Zr
Ta
Cr
Ti
)B
, possess virtually one solid-solution boride phase of the hexagonal AlB
structure. Revised Hume-Rothery size-difference factors are used to rationalize the formation of high-entropy solid solutions in these metal diborides. Greater than 92% of the theoretical densities have been generally achieved with largely uniform compositions from nanoscale to microscale. Aberration-corrected scanning transmission electron microscopy (AC STEM), with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping, has been conducted to confirm the formation of 2-D high-entropy metal layers, separated by rigid 2-D boron nets, without any detectable layered segregation along the c-axis. These materials represent a new type of ultra-high temperature ceramics (UHTCs) as well as a new class of high-entropy materials, which not only exemplify the first high-entropy non-oxide ceramics (borides) fabricated but also possess a unique non-cubic (hexagonal) and layered (quasi-2D) high-entropy crystal structure that markedly differs from all those reported in prior studies. Initial property assessments show that both the hardness and the oxidation resistance of these high-entropy metal diborides are generally higher/better than the average performances of five individual metal diborides made by identical fabrication processing.
Minor impurities can cause catastrophic fracture of normally ductile metals. Here, a classic example is represented by the sulfur embrittlement of nickel, whose atomic-level mechanism has puzzled ...researchers for nearly a century. In this study, coupled aberration-corrected electron microscopy and semi-grand-canonical-ensemble atomistic simulation reveal, unexpectedly, the universal formation of amorphous-like and bilayer-like facets at the same general grain boundaries. Challenging the traditional view, the orientation of the lower-Miller-index grain surface, instead of the misorientation, dictates the interfacial structure. We also find partial bipolar structural orders in both amorphous-like and bilayer-like complexions (a.k.a. thermodynamically two-dimensional interfacial phases), which cause brittle intergranular fracture. Such bipolar, yet largely disordered, complexions can exist in and affect the properties of various other materials. Beyond the embrittlement mechanism, this study provides deeper insight to better understand abnormal grain growth in sulfur-doped Ni, and generally enriches our fundamental understanding of performance-limiting and more disordered interfaces.
Water can trigger flash sintering of ZnO powder pellets at room temperature to achieve ~98% of the theoretical density in 30s without any external furnace heating. The specimen conductivity can be ...increased by >10,000 times via absorbing water vapor to enable the room-temperature flash. The initial electric field must be higher than a critical threshold to lead to densification, suggesting bifurcation in kinetic pathways. This new cost and energy saving water-assisted flash sintering (WAFS) technology can potentially be applied to consolidate other ceramic materials.
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The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic ...patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains. This discovery shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.
A grain boundary (GB) “phase” (complexion) diagram is computed via a lattice type statistical thermodynamic model for the average general GBs in Bi-doped Ni. The predictions are calibrated with ...previously-reported density functional theory calculations and further validated by experiments, including both new and old aberration-corrected scanning transmission electron microscopy characterization results as well as prior Auger electron spectroscopy measurements. This work supports a major scientific goal of developing GB complexion diagrams as an extension to bulk phase diagrams and a useful materials science tool.
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•A grain boundary complexion (phase) diagram is computed for Ni-Bi.•Predictions have been verified by experiments.•Complexion diagrams are an extension to bulk phase diagrams.
Using TiO2 as a model system, the effects of different doping (un-doped, V-doped vs. N-doped) and starting phases (anatase vs. rutile) on the flash sintering of TiO2 are investigated. The doping and ...starting phase not only alter the onset flash sintering temperatures via changing the temperature-dependent electric conductivities of the green specimens, but also significantly affect the densification and microstructural development during the flash sintering. In all six cases, the coupled thermal and electric runaway temperatures predicted from measured specimen conductivities agree well with the observed onset flash temperatures (with less than 5°C in differences), supporting a recently-developed quantitative model.
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