Ti₂AlC is a typical MAX (M: early transition metal, A: main group element, and X: carbon and/or nitrogen) phase with ceramic and metallic properties due to its unique nano-layered structure. In order to investigate the interaction behavior between Ag and Ti₂AlC, a sessile drop experiment was conducted at 1080 °C for 5 min. The atomic rearrangement occurred at the Ag–Ti₂AlC interface was revealed using high-angle annular dark-field scanning transmission electron microscopy coupled with high-resolution transmission electron microscopy analysis. The results show that Ag nanoclusters generally appeared in most of the Ag–Ti₂AlC interaction regions thermally processed at 1080 °C. In addition, Ag can also substitute for Al and Ti atoms in the Ti₂AlC, promoting local structural decomposition of the Ti₂AlC and producing 4H–Ag with a hexagonal close-packed (hcp) structure. Additionally, Al atoms released from the Ti₂AlC lattices can dissolve locally into the liquid Ag, particularly at the grain boundaries. When the loss concentration of Al exceeded the critical level, the Ti₂AlC started to decompose and the residual Ti₆C octahedrons and Al atoms recombined, giving rise to the production of anti-perovskite Ti₃AlC with a cubic structure. Lastly, the discrepancy in substitution behavior of Ag in the Ti₂AlC was compared when thermally processed at different temperatures (1030 °C and 1080 °C). This work contributes to the understanding of the intrinsic stability of Ti₂AlC MAX ceramics under high-temperature treatment.

The present work introduced first-principles calculation to explore the substitution behavior of Ag atoms for Al or Ti atoms in the Ti₂AlC MAX phase ceramic. The effect of Ag substitution on supercell parameter, bonding characteristic, and stability of the Ti₂AlC was investigated. The results show that for the substitution of Ag for Al, the Al-Ti bond was replaced by a weaker Ti-Ag bond, decreasing the stability of the Ti₂AlC. However, the electrical conductivity of the Ti₂AlC was enhanced after the substitution because of the contribution of Ag 4d orbital electrons toward the density of states (DOS) at the Fermi level coupled with the filling of Ti d orbital electrons. For the substitution of Ag for Ti, new bonds, such as Ag-Al bond, Ag-C bond, Al-Al bond, Ti-Ti anti-bond, and C-C anti-bond were generated in the Ti₂AlC. The Ti-Ti anti-bond was strengthened as well as the number of C-C anti-bond was increased with increasing the substitution ratio of Ag for Ti. Similar to the substitution of Ag for Al, the stability of the Ti₂AlC also decreased because the original Al-Ti bond became weaker as well as the Ti-Ti and C-C anti-bonds were generated during the substitution of Ag for Ti. Comparing with the loss of Ti d orbital electrons, Ag 4d orbits contributed more electrons to the DOS at the Fermi level, improving the electrical conductivity of the Ti₂AlC after substitution. Based on the calculation, the substitution limit of Ag for Al or Ti was determined. At last, the substitution behavior of Ag for Al or Ti was compared to discriminate that Ag atoms would tend to preferentially substitute for Ti atoms in Ti₂AlC. The current work provides a new perspective to understand intrinsic structural characteristic and lattice stability of the Ti₂AlC MAX phase ceramic.

Reliable hydrogen leakage detection requires both adequate concentration resolution and a response compatible with the evolution of a leak. Hydrogen presents a particular challenge for absorption spectroscopy because its homonuclear structure makes electric-dipole rovibrational transitions forbidden, leaving weak quadrupole absorption features. This review examines hydrogen sensing by cavity-enhanced Raman spectroscopy, cavity-enhanced absorption and ring-down spectroscopy, photoacoustic and acoustic methods, light-induced thermoelastic spectroscopy, and tunable diode laser absorption spectroscopy. Conventional electrochemical, semiconductor, catalytic, and coated-fiber sensors provide application context. Representative studies are assessed in terms of the measured quantity, detection limit, acquisition time, optical configuration, environmental dependence, and practical deployment. Indirect measurements based on gas-dependent resonator properties are also considered. The comparison distinguishes hydrogen-specific demonstrations from techniques demonstrated for other gases, and separates gas-cell sensitivity from open-path and standoff performance. Recent advances show that lower reported detection limits can require longer averaging, optical enhancement, or more complex excitation and detection schemes. No single method is preferable for all leakage scenarios. Priorities include comparable performance reporting, interference and drift control, and validation with transient hydrogen plumes under realistic field conditions.