Adhesion
interaction of epoxy resin with the basal surfaces of
h-BN and graphite is investigated with the first-principles density
functional theory calculations in conjunction with the dispersion
correction. The h-BN/epoxy and graphite/epoxy interfaces play an important
role in producing nanocomposite materials with excellent thermal dissipation
properties. The epoxy resin structure is simulated by using four kinds
of fragmentary models. Their structures are optimized on the h-BN
and graphite surfaces after an annealing simulation. The distance
between the epoxy fragment and the surface is about 3 Å. At the
interface between h-BN and epoxy resin, no H-bonding formation is
observed, though one could expect that the active functional groups
of epoxy resin, such as hydroxyl (−OH) group, would be involved
in a hydrogen-bonding interaction with nitrogen atoms of the h-BN
surface. The adhesion energies for the two interfaces are calculated,
showing that these two interfaces are characterized by almost the
same strength of adhesion interaction. To obtain the adhesion force–separation
curve for the two interfaces, the potential energy surface associated
with the detachment of the epoxy fragment from the surface is calculated
with the help of the nudged elastic band method and then the adhesion
force is obtained by using either the Morse-potential approximation
or the Hellmann–Feynman force calculation. The results from
both methods agree with each other. The maximum adhesion force for
the h-BN/epoxy interface is as high as that for the graphite/epoxy
interface. To better understand this result, a force-decomposition
analysis is carried out, and it has been disclosed that the adhesion
forces working at both interfaces mainly come from the dispersion
force. The trend of increase in the
C
6
parameters used for the dispersion correction for the atoms included
in the h-BN or graphite surface is in the order: N < C < B,
which reasonably explains why the strengths of the dispersion forces
operating at the two interfaces are similar. Also, the electron localization
function analysis can explain why the h-BN surface cannot form an
H bond with the hydroxyl group in epoxy resin.
The morphological instability appeared at step-free 4H-SiC (0001) surfaces was investigated. The step-free surfaces were fabricated at the bottom of inverted-mesa structure by the method combining a laser digging and Si-vapor etching. By repeated Si-vapor etching treatments, randomly created crater and maze structures were cyclically appeared at the step-free surfaces. These structures were distinctly classifiable by their depths from the step-free surfaces. Crater structures have 0.2 - 0.3 nm depth and maze structures have 0.5 nm depth. The morphological evolutions indicate the process of destruction of the step-free (0001) basal plane and generation of steps from step-free surfaces in the Si-vapor etching process.
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