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The electronic, optical and structural properties of Cs2AgXY6 (X = Bi, In; Y = Cl, Br), were investigated under external pressure (0–50 GPa) using the full potential density functional theory. In this study, the Tran–Blaha modified Becke–Johnson (TB-mBJ) functional was used to evaluate the electronic and optical properties of the double perovskites, and the GGA-PBEsol functional was used to examine the structural properties. The formation energy and tolerance factor predict the stability of these compounds. With increasing pressure, the lattice parameters and unit cell volume of these compounds decreased, resulting in a reduction in the bond length between the cations and anions. Comparatively Cs2AgInCl6 has shown a low compressibility. At zero pressure, Cs2AgInBr6 and Cs2AgInCl6 exhibit direct band gap while Cs2AgBiBr6 and Cs2AgBiCl6 has indirect band gap. Band gaps decrease with pressure. High pressure decreases peak height, shifts energy lower, and widens bandwidth in density of states. The density of states plots at zero pressure showed mixed ionic and covalent bonds, and with increasing pressure, a strengthening of covalent bonds is observed. Reflectivity/optical conductivity spectra show redshifts (Cs2AgBiBr6, Cs2AgBiCl6) and blueshifts (Cs2AgInBr6, Cs2AgInCl6). This study provides insights into double perovskite behavior under pressure, aiding material design. Additionally, our work highlights the variations in energy band gap and band structure is dependent on the choice of functional utilized. [ABSTRACT FROM AUTHOR]

The Al2O3 forming MAX-phase Ti2AlC has potential to be used in the high-temperature combustion chambers of aircraft engines. In this work, Ti2AlC powder was compressed and synthesized by spark plasma sintering (SPS) at 1200°C and 30 MPa for different periods of time. The surface morphology, cross-section microstructure, mapping elemental distribution analysis, phase transition analysis, and mechanical properties were analyzed using a scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and nanoindentation techniques. The results revealed that between 24 h and 100 h, Ti2AlC structure was primarily composed of rutile TiO2, with a small amount of α-Al2O3 and numerous planar defects. When the oxidation time was increased, some additional oxides such as rutile-TiO2, α-Al2O3, TiO2-Al2TiO5, and TiO2-α-Al2O3 were also formed. Although the planar defects were reduced at 300 h due to the inward diffusion of O2− and outward diffusion of Al3+ and Ti4+, they spread throughout the entire structure rather than forming layers, leading to a significant loss in oxidation resistance. [ABSTRACT FROM AUTHOR]

A first principle study was performed to investigate the electronic and thermostructural properties of the Ti2AlC MAX-phase using quasi-harmonic Debye approximation. The thermodynamical properties of Ti2AlC MAX-phase at various temperatures and pressure were calculated via the quasi-harmonic Debye approximation and explored the role of temperature and pressure on heat capacity, bulk modulus, thermal expansion coefficient, Debye temperature, enthalpy, entropy, and Gibbs free energy. Surprisingly, both the bulk modulus and Debye temperature was observed to drop with increase in temperature. However, a rise in both occurred as the pressure gradually builds up. This suggests that the heat capacity is influenced by pressure and temperature in opposing ways. The observation of increase in both heat capacities (Cp and Cv) due to increase in temperature infers an increase in the thermal velocity of the atoms. Consequently, the thermal velocity of the atoms decreases with a decrease in pressure which affects Cp and Cv, respectively. In addition, the Gibbs free energy slope increased at a little rate at constant pressure. These novel results possessing improved thermostructural properties could be useful for high-temperature fatigue-resistant applications specially in a gas turbine engine. [ABSTRACT FROM AUTHOR]

The effect of pre-oxidation during hot corrosion test of cobalt-based alloy CoNiCrAlHf were corroded at 1000 ℃ with molten Na2SO4 salt and investigated the corrosion resistance. Pre-oxidized specimens showed good oxidation resistance and better adhesion to the oxide layer with a dense and unvarying layered structure. In the case of non-pre-oxidized specimens by increasing corrosion time, more aluminum was consumed due to the instability of aluminum content in salt, and the oxide scale decreased slightly which is not enough for corrosion resistance. Nevertheless, direct corroded specimens generated a non-protective and imperfect α-Al2O3 scale, which had thick but not uniform and compact. Therefore, non-pre-oxidized specimens permitted detrimental penetration of sulfur into the substrate. This sulfur diffused into the substrate by grain boundaries which increased the degree of thermal expansion and produced internal stress, cause spallation. The pre-oxidation treatment presented two advantages, one is the uniform and dense α-Al2O3 formation and the second one is impeded sulfur penetration. [ABSTRACT FROM AUTHOR]

The oxidation resistance of the MCrAlY binding coat is due to the formation of protective Al2O3 oxide scale at high temperature. The oxidation behavior of NiCrAlYHf alloy in 1100 °C air and air-water vapor atmosphere was studied. The effect of water vapor on the microstructure and distribution of reactive elements was discussed. The results showed that the oxide scale in air has a double layer structure composed of columnar and equiaxed crystals, while the oxide scale in water vapor contains fine alumina grains, which provides more channels for the diffusion of reactive elements. In addition, The Cr element in the oxide scale is mainly concentrated in the outer equiaxed crystal zone, and the Hf oxide is mainly concentrated in the columnar crystal boundary. In air-water vapor atmosphere, the Cr element is uniformly distributed in the oxide scale. [ABSTRACT FROM AUTHOR]

Sodium‐based halide double perovskites (HDPs) present a promising alternative to Pb‐based perovskites for safe solar and thermal energy conversion devices due to their high durability and non‐toxic elements. This study examines the electronic, thermoelectric, elastic, optoelectronic, and structural properties of Rb2NaSbZ6 (Z = I, Br, Cl) double perovskite compounds using density functional theory (DFT). These compounds, characterized by a cubic structure, show increasing structural parameters as halogens are substituted from chlorine to iodine. Structural stability is confirmed by evaluating the enthalpy of formation, Gibbs free energy, Born and Huang criteria, and tolerance factor. Pugh's ratio indicates the ductile nature of the compounds. All investigated halide compounds exhibit an indirect band gap ranging from 3.9 to 2.34 eV, with valence and conduction band extrema located at different symmetry points, and a higher effective mass of holes is noted. This study also analyzes the refractive index, optical loss, light absorption, and polarization across the energy range of 0–10 eV. The spectral characteristics indicate that these HDPs are suitable for optoelectronic and photovoltaic devices due to their absorption in the visible and near‐ultraviolet spectra. High figures of merit (0.45–0.6) derived from power factor and thermal conductivity calculations suggest the potential of these compositions as thermoelectric devices. These findings provide a comprehensive understanding of these materials, facilitating their further deployment. [ABSTRACT FROM AUTHOR]

In this study, we focus on the oxidation behavior of the oxide scale with Platinum (Pt) on free standing NiCoCrAlY coating in air with water vapor at high temperature. The Pt layer is deposited on the NiCoCrAlY coating surface by electroplating, and then heat treatment at 1000 ∘ C with Argon (Ar) protection. Cycle oxidation of the oxide scale with Pt is conducted in air and air with water vapor at 1050 ∘ C. The results indicated that the spinels (NiCr2O4) are formed on the surface of the alumina scale in air and air with water vapor. However, this kind of spinel is inhibited in the air with water vapor and the scale on the coatings with Pt presents a good adherence during oxidation process. The thermal stress and crack initiation and propagation will be discussed. [ABSTRACT FROM AUTHOR]