255 research outputs found
Macroscopic Elastic Properties of Textured ZrN--AlN Polycrystalline Aggregates: From Ab initio Calculations to Grain-Scale Interactions
Despite the fast development of computational materials modelling,
theoretical description of macroscopic elastic properties of textured
polycrystalline aggregates starting from basic principles remains a challenging
task. In this communication we use a supercell-based approach to obtain the
elastic properties of random solid solution cubic ZrAlN system as a function of
the metallic sublattice composition and texture descriptors. The employed
special quasi-random structures are optimised not only with respect to short
range order parameters, but also to make the three cubic directions
, , and as similar as possible. In this way,
only a small spread of elastic constants tensor components is achieved and an
optimum trade-off between modelling of chemical disorder and computational
limits regarding the supercell size is achieved. The single crystal elastic
constants are shown to vary smoothly with composition, yielding
-0.5 an alloy constitution with an almost isotropic response.
Consequently, polycrystals with this composition are suggested to have Young's
modulus independent on the actual microstructure. This is indeed confirmed by
explicit calculations of polycrystal elastic properties, both within the
isotropic aggregate limit, as well as with fibre textures with various
orientations and sharpness. It turns out, that for low AlN mole fractions, the
spread of the possible Young's moduli data caused by the texture variation can
be larger than 100 GPa. Consequently, our discussion of Young's modulus data of
cubic ZrAlN contains also the evaluation of the texture typical for thin films.Comment: 10 pages, 6 figures, 3 table
Graphite under uniaxial compression along the c axis: A parameter to relate out-of-plane strain to in-plane phonon frequency
Stacking graphene sheets forms graphite. Two in-plane vibrational modes of
graphite, E1u and E2g(2), are derived from graphene E2g mode, the shifts of
which under compression are all considered as results of in-plane bond
shortening. Values of Gruneisen parameter have been reported to quantify such
relation. However, the reason why the shift rates of these three modes with
pressure differ is unclear. In this work, we introduce a new parameter to
quantify the contribution of out-of-plane strain to the in-plane vibrational
frequencies, suggesting that the compression of \pi-electrons plays a
non-negligible part in both graphite and graphene under high pressure.Comment: 8 pages, 5 figures, 1 tabl
Unexpected softness of bilayer graphene and softening of A-A stacked graphene layers
Density functional theory has been used to investigate the behavior of the π electrons in bilayer graphene and graphite under compression along the c axis. We have studied both conventional Bernal (A-B) and A-A stackings of the graphene layers. In bilayer graphene, only about 0.5% of the π-electron density is squeezed through the sp2 network for a compression of 20%, regardless of the stacking order. However, this has a major effect, resulting in bilayer graphene being about six times softer than graphite along the c axis. Under compression along the c axis, the heavily deformed electron orbitals (mainly those of the π electrons) increase the interlayer interaction between the graphene layers as expected, but, surprisingly, to a similar extent for A-A and Bernal stackings. On the other hand, this compression shifts the in-plane phonon frequencies of A-A stacked graphene layers significantly and very differently from the Bernal stacked layers. We attribute these results to some sp2 electrons in A-A stacking escaping the graphene plane and filling lower charge-density regions when under compression, hence, resulting in a nonmonotonic change in the sp2-bond stiffness
Towards predictive modelling of near-edge structures in electron energy loss spectra of AlN based ternary alloys
Although electron energy loss near edge structure analysis provides a tool
for experimentally probing unoccupied density of states, a detailed comparison
with simulations is necessary in order to understand the origin of individual
peaks. This paper presents a density functional theory based technique for
predicting the N K-edge for ternary (quasi-binary) nitrogen alloys by adopting
a core hole approach, a methodology that has been successful for binary nitride
compounds. It is demonstrated that using the spectra of binary compounds for
optimising the core hole charge ( for cubic TiAlN
and for wurtzite AlGaN), the predicted spectra
evolutions of the ternary alloys agree well with the experiments. The spectral
features are subsequently discussed in terms of the electronic structure and
bonding of the alloys.Comment: 11 pages, 9 figures, 1 tabl
- …