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Open Access Research Article Issue
21-Component compositionally complex ceramics: Discovery of ultrahigh-entropy weberite and fergusonite phases and a pyrochlore-weberite transition
Journal of Advanced Ceramics 2022, 11(4): 641-655
Published: 08 March 2022
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Two new high-entropy ceramics (HECs) in the weberite and fergusonite structures, along with the unexpected formation of ordered pyrochlore phases with ultrahigh-entropy compositions and an abrupt pyrochlore-weberite transition, are discovered in a 21-component oxide system. While the Gibbs phase rule allows 21 equilibrium phases, 9 out of the 13 compositions examined possess single HEC phases (with ultrahigh ideal configurational entropies: ~2.7kB per cation or higher on one sublattice in most cases). Notably, (15RE1/15)(Nb1/2Ta1/2)O4 possess a single monoclinic fergusonite (C2/c) phase, and (15RE1/15)3(Nb1/2Ta1/2)1O7 form a single orthorhombic (C2221) weberite phase, where 15RE1/15 represents Sc1/15Y1/15La1/15Pr1/15Nd1/15Sm1/15Eu1/15Gd1/15Tb1/15Dy1/15Ho1/15Er1/15Tm1/15 Yb1/15Lu1/15. Moreover, a series of eight (15RE1/15)2+x(Ti1/4Zr1/4Ce1/4Hf1/4)2-2x(Nb1/2Ta1/2)xO7 specimens all exhibit single phases, where a pyrochlore-weberite transition occurs within 0.75 < x < 0.8125. This cubic-to-orthorhombic transition does not change the temperature-dependent thermal conductivity appreciably, as the amorphous limit may have already been achieved in the ultrahigh-entropy 21-component oxides. These discoveries expand the diversity and complexity of HECs, towards many-component compositionally complex ceramics (CCCs) and ultrahigh-entropy ceramics.

Open Access Research Article Issue
A new class of high-entropy M3B4 borides
Journal of Advanced Ceramics 2021, 10(1): 166-172
Published: 08 December 2020
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A new class of high-entropy M3B4 borides of the Ta3B4-prototyped orthorhombic structure has been synthesized in the bulk form for the first time. Specimens with compositions of (V0.2Cr0.2Nb0.2Mo0.2Ta0.2)3B4 and (V0.2Cr0.2Nb0.2Ta0.2W0.2)3B4 were fabricated via reactive spark plasma sintering of high-energy-ball-milled elemental boron and metal precursors. The sintered specimens were ~98.7% in relative densities with virtually no oxide contamination, albeit the presence of minor (4-5 vol%) secondary high-entropy M5B6 phases. Despite that Mo3B4 or W3B4 are not stable phase, 20% of Mo3B4 and W3B4 can be stabilized into the high-entropy M3B4 borides. Vickers hardness was measured to be 18.6 and 19.8 GPa at a standard load of 9.8 N. This work has further expanded the family of different structures of high-entropy ceramics reported to date.

Open Access Short Communication Issue
A high-entropy silicide: (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2
Journal of Materiomics 2019, 5(3): 337-343
Published: 22 March 2019
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A high-entropy metal disilicide, (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2, has been successfully synthesized. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) collectively show the formation of a single high-entropy silicide phase. This high-entropy (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2 possesses a hexagonal C40 crystal structure with ABC stacking sequence and a space group of P6222. This discovery expands the known families of high-entropy materials from metals, oxides, borides, carbides, and nitrides to a silicide, for the first time to our knowledge, as well as demonstrating that a new, non-cubic, crystal structure (with lower symmetry) can be made into high-entropy phase. This (Mo0.2Nb0.2Ta0.2Ti0.2W0.2)Si2 exhibits high nanohardness of 16.7 ± 1.9 GPa and Vickers hardness of 11.6 ± 0.5 GPa. Moreover, it has a low thermal conductivity of 6.9 ± 1.1 W m−1 K−1, which is approximately one order of magnitude lower than that of the widely-used tetragonal MoSi2 and ~1/3 of those reported values for the hexagonal NbSi2 and TaSi2 with the same crystal structure.

Open Access Research Article Issue
Combining cold sintering and Bi2O3-Activated liquid-phase sintering to fabricate high-conductivity Mg-doped NASICON at reduced temperatures
Journal of Materiomics 2019, 5(2): 237-246
Published: 13 February 2019
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The cold sintering process (CSP) and Bi2O3-activated liquid-phase sintering (LPS) are combined to densify Mg-doped NASICON (Na3.256Mg0.128Zr1.872Si2PO12) to achieve high densities and conductivities at reduced temperatures. As an example, a cold-sintered specimen with the addition of 1.1 wt % Bi2O3 sintering additive achieved a high conductivity of 0.91 mS/cm (with ~96% relative density) after annealing at 1000 ℃; this conductivity is > 70% higher than that of a cold-sintered specimen without adding the Bi2O3 sintering additive, and it is > 700% of the conductivity of a dry-pressed counterpart with the same amount of Bi2O3 added, all of which are subjected to the same heating profile. The highest conductivity achieved in this study via combining CSP and Bi2O3-activated LSP is > 1.5 mS/cm. This study suggests an opportunity to combine the new CSP with the traditional LPS to sinter solid electrolytes to achieve high densities and conductivities at reduced temperatures. This combined CSP-LPS approach can be extended to a broad range of other materials to fabricate the "thermally fragile" solid electrolytes or solid-state battery systems, where reducing the processing temperature is often desirable.

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