Yingdi Liu

Oxidation of the two-phase Nb/Nb5Si3 composite: The role of energetics, thermodynamics, segregation, and interfaces

Oxidation behavior of the two-phase Nb/Nb(5)Si(3) composite is of significant importance for the potential applications of the composite at high-temperature conditions. We investigate the atomic-scale oxidation mechanism of the Nb/Nb(5)Si(3) composite with first-principles density-functional theory and thermodynamics analysis. In particular, the effects of energetics, thermodynamics, segregation, and interfaces are identified. The clean composite surface is found to be composed of both Nb(110) and Si-terminated Nb(5)Si(3)(001). Energetics and thermodynamics calculations show that, during the oxidation process, the Nb(110) surface is oxidized first, followed by the segregation of niobium of the Nb(5)Si(3)(001) surface and subsequent oxidation of the Nb element of Nb(5)Si(3). High coverage of oxygen results in dissolved oxygen in bulk Nb through the diffusion of oxygen in the surface and at the interface. The theoretical investigation also provides an explanation, at the atomic-scale, for the experimental observation that the oxidation layer is essentially composed of niobium oxide and almost free of silicon. Furthermore, the methodology of this work can be applied to investigations of the oxidation behavior of other two-phase and multi-phase composites.

Phase transformations of nano-sized cubic boron nitride to white graphene and white graphite

We report quantum-mechanical investigations that predict the formation of white graphene and nano-sized white graphite from the first-order phase transformations of nano-sized boron nitride thin-films. The phase transformations from the nano-sized diamond-like structure, when the thickness d > 1.4 nm, to the energetically more stable nano-sized white graphite involve low activation energies of less than 1.0 eV. On the other hand, the diamond-like structure transforms spontaneously to white graphite when d ≤ 1.4 nm. In particular, the two-dimensional structure with single-layer boron nitride, the so-called white graphene, could be formed as a result of such transformation.

Passivation effects of fluorine and hydrogen at the SiC–SiO2 interface

High concentrations of defects at the SiC–SiO2 interface significantly reduce the efficiency of the SiC-based microelectronics. Investigations of the defect passivation are thus of great importance. We report first-principles density-functional-theory calculations for the effects of fluorine and hydrogen in passivating the defects at the SiC–SiO2 interface. The calculations show that the isolated point defects involving excessive carbon atoms can be passivated by atomic fluorine and hydrogen, separately or combined. The results further suggest that molecular fluorine may be more effective for the passivation of the interface defects than molecular hydrogen and hydrogen fluoride.