High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics

High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics Joshua Gild Yuanyao Zhang Tyler Harrington Sicong Jiang Tao Hu M. Quinn William M. Mellor Naixie Zhou Kenneth S. Vecchio Jian Luo

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High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics 2016 Scientific Reports 10.1038/srep37946 gild2016-high-entropy-metal-diborides-a-new gild2016-high-entropy-metal-diborides-a-new.txt, the project's staged copy 139589913b9e6d960f59ad01ff009f65a316fe7a cc-by engineering

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0 spans located by searching the stored text for the quote's exact words, allowing any whitespace between them seven equimolar five-component metal diborides, and whether each forms a single high-entropy solid-solution phase Quotation withheld because this record is not cleared for redistribution. whether high-entropy solid solutions form in a non-cubic layered ceramic, and whether the inherited size-difference criterion predicts which compositions do Quotation withheld because this record is not cleared for redistribution. the first crystalline high-entropy non-oxide ceramics, and a size-difference measurand recomputed from lattice constants rather than atomic radii Quotation withheld because this record is not cleared for redistribution. existence Quotation withheld because this record is not cleared for redistribution. entropy stabilisation of a single solid-solution phase against decomposition into separate compounds Quotation withheld because this record is not cleared for redistribution. experiment experiment Quotation withheld because this record is not cleared for redistribution. because Gibbs free energy is H minus TS, a phase with large S becomes more stable as temperature rises, so a maximally mixed solid solution can win over separate compounds at high temperature entropy stabilisation Quotation withheld because this record is not cleared for redistribution. the rigid 2D boron net deforms the metal cations toward a strain-free spacing in the basal plane, which compresses the size differences along a and leaves the c-axis free to express them boron-net constraint on metal-metal bonds Quotation withheld because this record is not cleared for redistribution. a small size difference is necessary but not essential, and a very small value is not a guarantee of a single high-entropy phase Quotation withheld because this record is not cleared for redistribution. size differences are not the only factors - mixing enthalpy and valence electron concentration may also decide whether a single phase forms Quotation withheld because this record is not cleared for redistribution. hardness and oxidation resistance are strongly affected by the porosity and oxide inclusions that this fabrication route introduces Quotation withheld because this record is not cleared for redistribution. whether these materials are good or bad for oxidation resistance has no simple answer, and systematic property investigation is outside this study Quotation withheld because this record is not cleared for redistribution. one of the seven compositions did not form a single phase, and the reason is not settled Quotation withheld because this record is not cleared for redistribution. the Hume-Rothery binary solid-solution size rule, carried from alloy metallurgy into multicomponent ceramics and then re-measured Quotation withheld because this record is not cleared for redistribution. six individual metal diborides fabricated by the identical milling and sintering route, as the comparison group eliminates: a high-entropy advantage produced by the fabrication route rather than by the composition, since the comparison specimens were milled and sintered identically · outcome: held Quotation withheld because this record is not cleared for redistribution. porosity and oxide inclusion held constant between the high-entropy and conventional specimens rather than eliminated eliminates: hardness and oxidation differences being attributed to composition when microstructure - porosity, oxide inclusions - affects both significantly · outcome: held Quotation withheld because this record is not cleared for redistribution. MoB2 excluded from the rule-of-mixtures average, with the direction of the resulting bias stated eliminates: a rule-of-mixtures baseline inflated by a phase that is not an equilibrium bulk phase at the temperature in question · outcome: held Quotation withheld because this record is not cleared for redistribution. weight-change data outside the two temperature windows where the change exceeds measurement error eliminates: measurement error masquerading as oxidation - the weights are reported only in the two temperature windows where the change is large enough to exceed it, and not outside them · outcome: absent-declared-unreported Quotation withheld because this record is not cleared for redistribution. HEA HEA Quotation withheld because this record is not cleared for redistribution. configurational entropy configurational entropy Quotation withheld because this record is not cleared for redistribution. solid-solution phase solid-solution phase Quotation withheld because this record is not cleared for redistribution. Hume-Rothery Hume-Rothery Quotation withheld because this record is not cleared for redistribution. ultra-high temperature ceramics ultra-high temperature ceramics Quotation withheld because this record is not cleared for redistribution. entropy-stabilized oxides entropy-stabilized oxides Quotation withheld because this record is not cleared for redistribution. high-entropy metal diborides high-entropy metal diborides Quotation withheld because this record is not cleared for redistribution. boron nets boron nets Quotation withheld because this record is not cleared for redistribution. spark plasma sintering spark plasma sintering Quotation withheld because this record is not cleared for redistribution. HEA = typically five or more elements in equimolar concentration, giving maximum molar configurational entropy RlnN Quotation withheld because this record is not cleared for redistribution. 'high-entropy material' HAS A CONTESTED SCOPE - glasses count only 'in a broad definition', said in parentheses Quotation withheld because this record is not cleared for redistribution. delta <= 4% or 4.3% = a NECESSARY BUT NOT SUFFICIENT criterion, and it is empirical, not derived Quotation withheld because this record is not cleared for redistribution. the same delta, recomputed from LATTICE CONSTANTS instead of atomic radii, because no radius represents the real bond lengths Quotation withheld because this record is not cleared for redistribution. 'only one necessary, but not essential, condition' - the authors' own hedge on their replacement Quotation withheld because this record is not cleared for redistribution. delta-c is the discriminating one and delta-a is not, because the boron net constrains the a-axis Quotation withheld because this record is not cleared for redistribution. contradicts → the delta <= 4% size-difference criterion for forming a single-phase high-entropy solid solution note: HEB #7 has delta about 8%, double the stated maximum, and forms a single high-entropy phase anyway - so the criterion fails as a NECESSARY condition, which is the only thing it was ever claimed to be Quotation withheld because this record is not cleared for redistribution. extends → Rost et al.'s entropy-stabilized oxide, the only previously reported crystalline high-entropy ceramic note: extends the class from oxides to borides and from cubic to layered hexagonal Quotation withheld because this record is not cleared for redistribution. configurational entropy —causes→ solid-solution phase configurational entropy solid-solution phase direction on the measure: increases · strength of the relation: 0.95 Quotation withheld because this record is not cleared for redistribution. the inherited size-difference criterion fails: the specimen with the largest atomic-radius delta still forms a single high-entropy phase Quotation withheld because this record is not cleared for redistribution. delta computed from the c lattice constant separates the six single-phase specimens from the one that failed, where the atomic-radius delta does not Quotation withheld because this record is not cleared for redistribution. a smaller delta-c appears to matter more than a smaller delta-a Quotation withheld because this record is not cleared for redistribution. STEM imaging confirms 2D high-entropy metal layers with no layered segregation along the c-axis Quotation withheld because this record is not cleared for redistribution. hardness and oxidation resistance exceed the average of the individual diborides made the same way, with porosity working against them Quotation withheld because this record is not cleared for redistribution. HEA HEA Quotation withheld because this record is not cleared for redistribution. configurational entropy configurational entropy Quotation withheld because this record is not cleared for redistribution. ultra-high temperature ceramics ultra-high temperature ceramics Quotation withheld because this record is not cleared for redistribution. five or more elements, equimolar, for maximum configurational entropy — None Quotation withheld because this record is not cleared for redistribution. the inherited threshold: delta at most about 4% or 4.3% — None Quotation withheld because this record is not cleared for redistribution. computed delta values of 3.5% to about 8%, with the 8% specimen still forming a single phase — None Quotation withheld because this record is not cleared for redistribution. delta-c 3.9% to 5.2% for the six that worked, 6.2% for the one that did not — None Quotation withheld because this record is not cleared for redistribution. sintered at 2000 degrees C for 5 minutes under 30 MPa — None Quotation withheld because this record is not cleared for redistribution. lattice-spacing standard deviation of about 0.6% of the c parameter — None Quotation withheld because this record is not cleared for redistribution. standard deviations of lattice spacings between basal planes standard deviations of lattice spacings between basal planes Quotation withheld because this record is not cleared for redistribution. hardness hardness Quotation withheld because this record is not cleared for redistribution. oxidation resistance oxidation resistance Quotation withheld because this record is not cleared for redistribution.

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