Effect of carbon vacancies on carbide work functions

Tool for Designing Breakthrough Discovery in Materials Science

A database of material property relationships, which serves as a scientific principles database, and a database search system are proposed and developed. The use of this database can support a broader research perspective, which is increasingly important in the era of automated computer-aided experimentation and machine learning of experimental and calculated data. Examples of the wider use of scientific principles in materials research are presented. The database and its advantages are described. An implementation of the proposed database and search system as a prototype software is reported. The usefulness of the database and search system is demonstrated by an example of a surprising but reasonable discovery.

Reduction in Work Functions of Transition-Metal Carbides and Oxycarbides upon Oxidation

Herein, the work functions of group 4 and group 5 transition-metal (Ti, Zr, Hf, V, Nb, and Ta) carbides and transition-metal oxycarbides (TMCOs) were investigated by first-principles calculations for their potential application as electron emitters. The work functions of both groups decreased as the substitution of carbon atoms with oxygen proceeded, and the reduction in group 4 was more than that of group 5. In particular, ZrC_1-x O _x and HfC_1-x O _x (x ≥ 0.25) exhibited work functions of less than 3 eV, which were comparable with those of LaB₆- and ZrO-coated tungsten. The reduction in the work functions could be explained by the rigid-band model of the electronic density of states. The increase in valence electrons increased the Fermi energy, while it demonstrated a less significant influence on the vacuum potential, resulting in a reduction in the work functions. The phonon dispersion curves indicated that the NaCl-type group 5 TMCOs were less stable than the group 4 TMCOs. This agrees with the experimental findings that TaC_1-x O _x was not synthesized and NbC_1-x O _x was synthesized only for smaller values of x (i.e., x < 0.28). From the viewpoints of the work functions and structural stabilities, group 4 (Ti, Zr, and Hf) TMCOs exhibit better potential for application as electron emitters than group 5 (V, Nb, and Ta) TMCOs.

Bulk and surface properties of metal carbides: implications for catalysis

We present a comprehensive study of the bulk and surface properties of transition metal carbides with rock salt structures and discuss their formation energies, electronic structure and potential catalytic activity.