An Investigation on Substitution of Ag Atoms for Al or Ti Atoms in the TiAlC MAX Phase Ceramic Based on First-Principles Calculations.

The present work introduced first-principles calculation to explore the substitution behavior of Ag atoms for Al or Ti atoms in the TiAlC MAX phase ceramic. The effect of Ag substitution on supercell parameter, bonding characteristic, and stability of the TiAlC 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 TiAlC. However, the electrical conductivity of the TiAlC was enhanced after the substitution because of the contribution of Ag 4 orbital electrons toward the density of states (DOS) at the Fermi level coupled with the filling of Ti 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 TiAlC. 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 TiAlC 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 orbital electrons, Ag 4 orbits contributed more electrons to the DOS at the Fermi level, improving the electrical conductivity of the TiAlC 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 TiAlC. The current work provides a new perspective to understand intrinsic structural characteristic and lattice stability of the TiAlC MAX phase ceramic.