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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]

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]

The exaggerated grain growth, anisotropic crystallite morphology, and thermal expansion are the main reasons for the microcracking of sintered TiB2, wherein grain coarsening and anisotropic crystallite morphology are believed to be controlled by the surface stabilities of TiB2. To deeply understand the grain growth mechanism, the anisotropic stability and bonding features of TiB2 surfaces, including $\left( {11\bar 20} \right)$, two types of (0001), and three types of $\left( {10\bar 10} \right)$, are investigated by first-principles calculations. By employing the two-region modeling method, surface energies are calculated and the $\left( {11\bar 20} \right)$ surface is found to be more stable than (0001) and $\left( {10\bar 10} \right)$ surfaces. Hexagonal plate-like grain morphology is predicted. The different bonding conditions of surface Ti and B atoms contribute to the difference of surface structure relaxation between surfaces with Ti- and B-termination, which lead the B-terminated ones to be more stable. It is also found that the surface energies of TiB2 are much higher than those of ZrB2 with a similar structure, which may be responsible for the easy coarsening of TiB2. [ABSTRACT FROM AUTHOR]

In this article, a comprehensive investigation on the thermal properties of Yb3Al5O12 is conducted, including Debye temperature, thermal expansion coefficient (TEC), thermal diffusivity, heat capacity, and thermal conductivity. The calculated Debye temperature of Yb3Al5O12 from the measured elastic properties is 625 K. The linear and volumetric thermal expansions of Yb3Al5O12 from 298 to 1273 K are (7.83 ± 0.14) × 10−6 and (23.74 ± 0.42) × 10−6 K−1, respectively. The linear TEC of the polycrystalline bulk Yb3Al5O12 determined by dilatometer is (8.22 ± 0.3) × 10−6 K−1. The measured thermal conductivities of Yb3Al5O12 are 4.67 and 2.05 W (m K)−1, respectively, at 300 and 1400 K. The estimated minimum thermal conductivity, κmin, is 1.22 W (m K)−1. The high temperature thermal conductivity is close to the evaluated κmin, which is lower than most commonly used thermal barrier coating (TBC) material such as Y2O3-stabilized-ZrO2 (YSZ). The unique combination of these properties renders Yb3Al5O12 being a very promising candidate material for TBC. [ABSTRACT FROM AUTHOR]

Temperature dependences of thermal and elastic properties, such as the Grüneisen parameters, thermal expansion, bulk modulus, and heat capacity of Ti2 AlC and Cr2 AlC, are studied by combining first-principles method and lattice dynamic calculation based on the quasi-harmonic model. Experimental thermal expansion coefficient is also measured for comparison. Thermal expansion coefficients of Ti2 AlC and Cr2 AlC show different trends: Ti2 AlC exhibits anisotropic thermal expansion while Cr2 AlC shows generally isotropic character. The mechanism is explored by investigating the isotropy or anisotropy of Grüneisen parameters (phonon anharmonicity and thermal pressure) and elastic stiffness (response to thermal pressure) of Ti2 AlC and Cr2 AlC. In addition, the calculated bulk modulus of Cr2 AlC is higher at ambient temperature but decreases faster than the value of Ti2 AlC as temperature increasing. [ABSTRACT FROM AUTHOR]

In this paper, in situ Raman spectra of Ta2AlC are measured in the temperature range of 80-500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line-width of E2g ( ω3) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon-phonon scatterings. On the other hand, the line-widths of E2g ( ω1) and A1g ( ω4) modes decrease continuously with increasing temperature, which is explained by the electron-phonon couplings of these two phonon modes with the Ta 5d electrons. The electron-phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta2AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Y2SiO5 is a promising candidate for oxidation-resistant or environmental/thermal barrier coatings (ETBC) due to its excellent high-temperature stability, low elastic modulus and low oxygen permeability. In this paper, we investigated the thermal properties of Y2SiO5 comprehensively, including thermal expansion, thermal diffusivity, heat capacity and thermal conductivity. It is interesting that Y2SiO5 has a very low thermal conductivity (∼1.40W/mK) but a relatively high linear thermal expansion coefficient ((8.36±0.5)×10−6 K−1), suggesting compatible thermal and mechanical properties to some non-oxide ceramics and nickel superalloys as ETBC layer. Y2SiO5 is also an ideal EBC on YSZ TBC layer due to their close thermal expansion coefficients. As a continuous source of Y3+, it is predicted that Y2SiO5 EBC may prolong the lifetime of zirconia-based TBC by stopping the degradation aroused by the loss of Y stabilizer. [Copyright &y& Elsevier]

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]

The key requirements for a successful thermal and environmental barrier coating (T/EBC) material include stability in high temperature water vapor, low Young's modulus, close thermal expansion coefficient (TEC) with mullite, low thermal conductivity and weak mechanical anisotropy. The current prime candidates for top coat are ytterbium silicates (Yb 2 SiO 5 and Yb 2 Si 2 O 7 ). A major weakness of these two silicates is the severe anisotropy in mechanical properties and thermal expansion that would lead to cracking of the coating. Thus, searching for new materials with weak mechanical and thermal anisotropy is of signification. In this work, the crystal structure, mechanical and thermal properties of a promising T/EBC candidate, Yb 4 Al 2 O 9 , are investigated theoretically and experimentally. Good ductility, low shear deformation resistance, low Young's modulus (151 GPa) and low thermal conductivity (0.78 W m −1 K −1 ) is underpinned by heterogeneous bonding characteristic and distortion of the structure. Close TEC (6.27 × 10 −6 K −1 ) with mullite and weak mechanical anisotropy highlight the suitability of Yb 4 Al 2 O 9 as a prospective T/EBC. [ABSTRACT FROM AUTHOR]