Development of ultrahigh-entropy ceramics with tailored oxidation behavior

21-Component compositionally complex ceramics: Discovery of ultrahigh-entropy weberite and fergusonite phases and a pyrochlore-weberite transition

“…High-entropy ceramics (HECs) [1][2][3], which can be generalized to a broader class of compositionally complex spinel [42] crystal structures, as well as more complex silicates [43], phosphates [44], aluminates [45,46], and molybates [47], have been studied. The vast majority of prior HEC studies focused on five-component equimolar compositions that produce ~1.61k B per cation ideal configurational entropy on at least one sublattice, where k B is the Boltzmann constant, but some studies also include four-component [20] and six-to ninecomponent [48] equimolar compositions [1][2][3]. Recent studies further investigated the high-entropy phase formation and transition in non-equimolar 10-and 11-component oxide systems, which can form either disordered (fluorite) or ordered (pyrochlore) phase (with either one or two cation sublattices) [31,49].…”

“…While the vast majority of prior studies investigated four- [20] to nine-component [48] (mostly equimolar) compositions [1][2][3], we propose to further explore (equimolar and non-equimolar) "many-component CCCs" (i.e., ≥ 10 components), which can offer even larger (and tailorable) compositional spaces and further (extreme) complexity (Fig. 1).…”

Equiatomic 9-cation high-entropy carbide ceramics of the IVB, VB, and VIB groups and thermodynamic analysis of the sintering process

“…Aiming at the hightemperature applications, most work on the HE carbide ceramics is concentrated in the corresponding carbides of IVB, VB, and VIB groups with high melting points. Up to now, medium-entropy carbides with 3 or 4 cations [6,[24][25][26][27][28], the most studied HE carbides with 5 cations [29][30][31][32], as well as HE carbides with 6 to 9 cations [33][34][35][36], have been reported.…”

“…The second one is the preparation of the HE carbides, and different approaches such as hot pressing (HP) and spark plasma sintering (SPS) have successfully been used to densify the powder compacts [8,27,28,38,39]. If the corresponding individual carbide compounds are used as the starting powders, the necessary mixing process is generally conducted in argon atmosphere for protecting the carbide powders from oxidation [35,36]. The third one is property evaluation, including mechanical properties [6,20,24,35,39], thermal properties [19,27,29,40,41], oxidation resistance [22,[42][43][44][45], irradiation resistance [21,46], electromagnetic wave absorbing properties [47,48], etc.…”

“…Very recently this approach is also used in HE carbide ceramic systems [40,52,53], but the effectiveness is more complicated owing to the existence of several carbothermal reduction reactions with different reaction temperatures. For decreasing the oxygen contamination during the powder mixing processing, strict oxygen-free circumstance such as glove box under Ar atmosphere is used [19,35,36]. On the other hand, in the case of using Cr 3 C 2 and Mo 2 C as starting raw materials, carbon is also added to compensate for the lack of carbon, because the carbon to metal element ratio is less than 1:1 in these carbides [36].…”