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In this paper, two new ternary layered MAX phase ceramics, Sc2GaC and Sc2InC, were successfully synthesized by spark plasma sintering. The crystal structures of two new compounds were determined by X‐ray diffraction. The lattice parameters were a = 3.3609(0) Å and c = 14.5849(9) Å for Sc2GaC, and a = 3.3453(6) Å and c = 15.2236(3) Å for Sc2InC, wherein the Sc atoms were located at (1/3, 2/3, 0.07330[Ga]/0.08268[In]), the A atoms were located at (1/3, 2/3, 3/4), and the C atoms were located at (0, 0, 0). Scanning electron microscopy images of two new MAX phase ceramics were also given in this paper, showing obvious layered characteristics. [ABSTRACT FROM AUTHOR]

In this work, two new 312 MAX phases of Zr3PbC2 and Hf3PbC2 were successfully synthesized by spark plasma sintering. It is the first discovery of lead‐containing 312 MAX phases, which together with M2PbC (M = Ti, Zr, Hf) form the lead‐containing MAX phase family. Considering the extremely low electrical conductivity of Hf2PbC, these two new compounds are of great research value. Based on the Rietveld refinement results, their lattice parameters and atomic positions were well determined, as a = 3.3771(5) Å, c = 20.0070(9) Å for Zr3PbC2 and a = 3.3357(1) Å, c = 19.7659(8) Å for Hf3PbC2, where M atoms are located at (0, 0, 0) and (1/3, 2/3, 0.1258(6)[Zr]; 0.1255(2)[Hf]), Pb atoms are located at (0, 0, 1/4), and C atoms are located at (1/3, 2/3, 0.0663(2)[Zr]; 0.0641(3)[Hf]), respectively. Additionally, the typical laminar microstructure of Zr3PbC2 and Hf3PbC2 grains was observed. [ABSTRACT FROM AUTHOR]

Dense and light B4C–(Ti0.9Cr0.1)B2 composites with excellent mechanical properties were designed and reactively densified from boron, TiC, and Cr3C2 powder mixtures by spark plasma sintering in this work. Due to solid solution effects, the as‐obtained B4C–(Ti0.9Cr0.1)B2 composite exhibited obviously enhanced hardness (43.2 ± 3.0 GPa at 9.8 N) and higher specific hardness (12.82 GPa cm3 g−1) together with improved flexural strength (663 ± 39 MPa) and fracture toughness (KIC, 5.40 ± 0.25 MPa m1/2), compared to the counterparts such as B4C–TiB2 composite and B4C. Toughening contributions in the as‐sintered ceramics were quantitatively analyzed, and higher KIC in B4C–(Ti0.9Cr0.1)B2 was mainly due to their larger initial fracture toughness and compressive stress toughening. The combination of these properties makes B4C–(Ti0.9Cr0.1)B2 composites exhibit great potentials in the application as lightweight structural materials. This work provided an inspiration to achieve lightweight materials with high performance through doping minor‐amount atoms into the matrix. [ABSTRACT FROM AUTHOR]

The electromagnetic (EM) wave absorbing properties of Cr2AlB2 powders and those after high‐temperature oxidation were investigated. Coupling of magnetic and dielectric loss enables Cr2AlB2 with good absorption properties. The minimum reflection loss (RL) value is −44.9 dB at 8.5 GHz with a thickness of 2.7 mm, and the optimized effective absorption bandwidth (EAB) is 4.4 GHz (13.0‐17.4 GHz) with a thickness of 1.6 mm. After oxidation at 750, 900, and 1000°C for 2 h, the minimum RL values, respectively, are −23.9 dB (17.5 GHz, 1.5 mm), −41.4 dB (16.5 GHz, 1.5 mm), and −39.5 dB (8.0 GHz, 3.0 mm); and the corresponding EAB values, respectively, are 3.8 GHz (13.6‐17.4 GHz, 1.7 mm), 4.1 GHz (13.5‐17.6 GHz, 1.6 mm), and 4.4 GHz (13.0‐17.4 GHz, 1.7 mm). With an absorber thickness of 1.5‐4.0 mm, the EAB with a RL value of less than −10 dB can be tuned in a broad‐frequency range 5.0‐18.0 GHz, which basically covers C (4‐8 GHz), X (8‐12 GHz), and Ku (12‐18 GHz) bands. These results demonstrate that Cr2AlB2, as a high‐efficient and oxidation‐resistant absorber, is a promising candidate for microwave absorption applications and can retain good EM wave absorbing properties after high‐temperature oxidation. [ABSTRACT FROM AUTHOR]

Al5BO9 is a promising thermal sealing material for hypersonic vehicles due to its low density, theoretically predicted low shear modulus, and low thermal conductivity. However, experimental investigations on the mechanical and thermal properties of bulk Al5BO9 have not been carried out. Herein, we report the mechanical and thermal properties of bulk Al5BO9 prepared by spark plasma sintering of solid‐state reaction synthesized Al5BO9 powders. The bulk (B), shear (G), and Young's (E) moduli are 148 GPa, 85 GPa, and 214 GPa, respectively, which are close to the theoretical values. The Pugh's ratio G/B is 0.574, indicating its intrinsic damage tolerance, which is also revealed by Hertzian contact test. The Vickers hardness (Hv) is 10.8 GPa, being lower than mullite. The flexural strength, compressive strength, and fracture toughness are, respectively, 277 ± 35 MPa, 814 ± 75 MPa, and 2.4 ± 0.3 MPa·m1/2, which are close to those of mullite. Al5BO9 has anisotropic coefficient of thermal expansion (CTE) in three crystallographic directions, ie αa = (4.40 ± 0.21) × 10−6 K−1, αb = (7.11 ± 0.18) × 10−6 K−1, αc = (6.70 ± 0.29) × 10−6 K−1 from Debye temperature to 1473 K, which are underpinned by its structural feature, ie lower αa is resulted from the edge‐shared AlO6 octahedron chains along the [100] direction. The average CTE is (6.05 ± 0.06) × 10−6 K−1. The thermal conductivity declines with temperature as κ = 1336.39/T + 1.97, consisting with predicted trend from Slack's model. The low thermal conductivity and low density guarantee Al5BO9 a promising candidate as ceramic wafer in the seal structure for hypersonic vehicles. [ABSTRACT FROM AUTHOR]

TiB2 exhibits a unique combination of excellent properties that make it promising candidate for applications in extreme environments, where retention of strength at high temperatures is essential. Tailoring grain boundary properties by segregation is believed a prominent way to design high‐temperature performance of ceramics. In this work, segregation tendencies of solute elements, including Sc, Y, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, in TiB2 grain boundaries and the strengthening/weakening effects induced by segregations are investigated by first‐principles calculations. The results reveal that small atoms tend to segregate to grain boundary sites with local compression strains, while large atoms prefer grain boundary sites with local expansion strains. Deteriorated grain boundary strength is usually caused by additional expansion strain induced by segregation, while improved grain boundary strength results from either enhanced local bonding induced by segregation of small atoms or increased fracture strain due to segregation of large atoms. Cr and V, especially Cr, exhibit strong segregation tendency and improvement on grain boundary strength, which provides useful guidelines for the design of high performance TiB2‐based materials. [ABSTRACT FROM AUTHOR]

Abstract: Ultra‐high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Various UHTCs are needed due to diverse harsh conditions that UHTCs are faced with in different applications. Due to structural similarity to ZrB2, possible high melting point and possible protective oxide scale formed in oxygen rich and water vapor environments, REAlB4 (RE: rare‐earth) is suggested a good candidate for UHTCs. In the present work, temperature‐dependent mechanical and thermal properties of both α‐YAlB4 (YCrB4 type, space group Pbam) and β‐YAlB4 (ThMoB4 type, space group Cmmm) were investigated by first principles calculations in combination with quasi‐harmonic approach. Due to the structural similarity between α‐YAlB4 and β‐YAlB4, their properties are very similar to each other, which are approximately transverse isotropic with properties in (001) plane being almost the same and differing from properties out of (001) plane. The results reveal that resistance to normal strain in (001) plane (~460 GPa) is higher than that along [001] direction (~320 GPa) and thermal expansion in (001) plane (~10 × 10−6 K−1) is lower than that along [001] direction (~17 × 10−6 K−1), which is because the stiff boron networks are parallel to (001) plane. The average thermal expansion coefficient is around 12 × 10−6 K−1, which is fairly high among UHTCs and compatible with metallic frameworks. The combination of high thermal expansion coefficient and protective oxidation scale forming ability suggest that REAlB4 is promising for practical applications not only as high‐temperature structural ceramic but also as oxidation resistant coating for alloys. [ABSTRACT FROM AUTHOR]

Abstract: As promising TBC (thermal barrier coating) candidates, perovskite oxides own designable properties for their various options of cations and structural diversity, but limited comprehensions of structure‐property relationship delay their engineering applications. In this work, mechanical/thermal properties of ABO3 (A=Sr, Ba; B=Ti, Zr, Hf) perovskites and their anisotropic nature are predicted employing density functional theory. Their theoretical minimum thermal conductivities range from 1.09 to 1.74 W·m−1·K−1, being lower than Y2O3 partially stabilized ZrO2. Reduced thermal conductivities up to 16% along particular directions are reached after considering thermal conductivity anisotropy. All compounds own high hardness while SrZrO3, SrHfO3, and BaHfO3 possess well damage tolerance. We found that small electronegativity discrepancy leads to big anisotropy of chemical bond, Young's/shear moduli and thermal conductivities, together with good damage tolerance. These results suggest that the next generation TBCs with extra low thermal conductivity should be achieved through combining material design and orientation‐growth tailoring. [ABSTRACT FROM AUTHOR]

Abstract: Layer structured MAB phases (M=transition metal, A=IIIA or IVA group element, B is boron) are promising ternary borides for high and ultrahigh‐temperature applications. Herein, a new MAB phase Y5Si2B8 consisting of alternative stacking of YB4 and Y3Si2 slabs along the [001] direction is investigated. Density functional theory (DFT) calculations on the electronic structure and chemical bonding reveal that this new MAB phase has diverse chemical bonding and properties similar to MAX phases. The strong covalent bonds in the two‐dimensional B network on (001) plane in the YB4 slab and between Si atoms on (002) plane in the Y3Si2‐slab warrant the high stiffness (Ex=288 GPa) of Y5Si2B8 in the ab plane and the weak Y2‐Si and Y1‐B2 bonds that connecting the YB4 and Y3Si2 slabs underpin the low Young's modulus in [001] direction (Ez=200 GPa). The low shear deformation resistance is due to the presence of the metallic bond and the weak bond within the B6 octahedral. The possible slip systems are {001}< 100> and {110}< 111> . Based on the low shear modulus (G = 104 GPa) and Pugh's ratio G/B, Y5Si2B8 is predicted as a damage tolerant MAB phase. Y5Si2B8 is also predicted electrically conductive and the conductivity is higher in directions parallel to ab plane. In addition, temperature‐dependent phonon and electron heat capacity are predicted based on the electron and phonon density of states analysis. [ABSTRACT FROM AUTHOR]

Searching for layered MAX phase-like materials with properties of both ceramics and metals is a topic in its infancy. Herein, through a combination of crystal structure, electronic structure, chemical bonding, and elastic property investigations, we report two MAX phase-like layered materials Rh2YSi and Ir2YSi. Rh2YSi and Ir2YSi have bulk modulus B of 150 and 185 GPa, respectively, which are comparable to the typical MAX phases like Ti2AlC, Ti3AlC2, and Ti3SiC2, but much lower shear modulus G (82 and 97 GPa for Rh2YSi and Ir2YSi, respectively) than MAX phases. The high stiffness is due to the presence of rigid Si2-M-Si3-M (M = Ir, Rh) units, while the low shear deformation resistance is due to the presence of metallic bonds and the weak bonds that link the rigid Si2-M-Si3-M (M = Ir, Rh) units. Based on the low shear deformation resistance and low Pugh's ratio, Rh2YSi and Ir2YSi are predicted as damage-tolerant silicides and promising water vapor-resistant interphase materials for SiCf/SiC composites if yttria or yttrium silicates are formed to protect the SiC fibers in oxygen containing environments. The possible slip systems are {0001} < [ABSTRACT FROM AUTHOR]

As promising candidates for ultrahigh temperature applications, high-temperature properties, which are quite rare and fragmentary, have great significance to ZrB2 and HfB2. In this work, thermodynamic and mechanical properties of ZrB2 and HfB2 from 0 K to 2000 K were investigated by a combination of first principles calculations and quasi-harmonic approximations. The ground-state properties, including lattice parameters, elastic constants, phonon dispersion, and mode-Grüneisen parameters are calculated. The theoretical thermal expansion, elastic and thermodynamic properties at elevated temperatures show good agreement with experiments. By discussing Grüneisen parameters anisotropy, the mechanism for the thermal expansion anisotropy of ZrB2 and HfB2 is uncovered. The influence of direction-dependent sound velocities on the anisotropy of thermal conductivity is also discussed. [ABSTRACT FROM AUTHOR]

Carbon-vacancy-bearing Nb4AlC3− x has the best high-temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb4AlC3− x have been identified to be ordered, establishing an ordered compound Nb12Al3C8. Here, the spatial distribution of the ordered vacancies and their influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb12Al3C8 and vacancy-free Nb4AlC3. In Nb12Al3C8, the carbon vacancies break only relatively weak Nb-C bonds and form ordered equilateral triangular carbon-vacancy networks ( OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of 'de-metallization'. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon-vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering. [ABSTRACT FROM AUTHOR]

Surfaces play pivotal roles during the oxidation and interfacial bonding of ZrB2. To understand the surface properties, the anisotropic stability and oxygen adsorption behavior of ZrB2 surfaces, including ( [ABSTRACT FROM AUTHOR]

The electronic structure, chemical bonding, mechanical, and thermal properties of a B-containing mullite Al5BO9 were investigated using a combination of first-principles calculations, Debye model, Clack model, and Slack's equation. The results show that Al5BO9 has a wide band gap of 6.4 eV. The bonding in BO3 triangle and AlO4 tetrahedron is covalent-ionic, whereas that in AlO6 octahedra and AlO5 bipyramids is ionic-covalent. The maximum (245 GPa) and minimum Young's modulus (161 GPa) are along [100] and [010] direction, respectively. Based on the low shear modulus (78 GPa) and low Pugh's ratio ( G/B = 0.497), Al5BO9 is predicted as a damage-tolerant ceramic. The most likely slip systems are [010](100) and [100](010). The high Debye temperature (956 K) indicates good high-temperature stability. The minimum thermal conductivity is estimated to be 1.59 W·(m·K)−1, and the thermal conductivity decreases with temperature as 2071.9/T. [ABSTRACT FROM AUTHOR]

Weak interphases play a pivotal role by acting as mechanical fuse to deflect matrix cracks in future ultrahigh-temperature ceramic fiber reinforced ceramic matrix ( UHTCf/ UHTC) composites. However, the interphase materials are not available yet. In this work, the electronic structure, chemical bonding, and mechanical properties of NiB, which is a promising interphase material for UHTCf/ UHTC composites, were investigated. NiB has relatively low shear modulus of 116 GPa, moderate Young's modulus of 307 GPa, but high bulk modulus of 287 GPa. The Pugh's ratio G/B is only 0.404 and the micro hardness is predicted to be 8.2 GPa, indicating that NiB belongs to 'soft' and 'ductile' UHTCs. The possible slip systems are [100](001), [010](001), and [001](010) due to the presence of metallic Ni-Ni and weak Ni-B bonding. Details on the electronic structure and directional dependence of shear and Young's moduli are disclosed to highlight the mechanisms that underpin the properties. [ABSTRACT FROM AUTHOR]