286 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
Mesh Refinement for Anisotropic Diffusion in Magnetized Plasmas
Highly accurate simulation of plasma transport is needed for the successful
design and operation of magnetically confined fusion reactors. Unfortunately,
the extreme anisotropy present in magnetized plasmas results in thin boundary
layers that are expensive to resolve. This work investigates how mesh
refinement strategies might reduce that expense to allow for more efficient
simulation. It is first verified that higher order discretization only realizes
the proper rate of convergence once the mesh resolves the thin boundary layer,
motivating the focusing of refinement on the boundary layer. Three mesh
refinement strategies are investigated: one that focuses the refinement across
the layer by using rectangular elements with a ratio equal to the boundary
layer width, one that allows for exponential growth in mesh spacing away from
the layer, and one adaptive strategy utilizing the established Zienkiewicz and
Zhu error estimator. Across 4 two-dimensional test cases with high anisotropy,
the adaptive mesh refinement strategy consistently achieves the same accuracy
as uniform refinement using orders of magnitude less degrees of freedom. In the
test case where the magnetic field is aligned with the mesh, the other
refinement strategies also show substantial improvement in efficiency. This
work also includes a discussion generalizing the results to larger magnetic
anisotropy ratios and to three-dimensional problems. It is shown that isotropic
mesh refinement requires degrees of freedom on the order of either the layer
width (2D) or the square of the layer width (3D), whereas anisotropic
refinement requires a number on the order of the log of layer width for all
dimensions. It is also shown that the number of conjugate gradient iterations
scales as a power of layer width when preconditioned with algebraic multigrid,
whereas the number is independent of layer width when preconditioned with ILU
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
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
Why, what, and how? case study on law, risk, and decision making as necessary themes in built environment teaching
The paper considers (and defends) the necessity of including legal studies as a core part of built environment undergraduate and postgraduate curricula. The writer reflects upon his own experience as a lawyer working alongside and advising built environment professionals in complex land remediation and site safety management situations in the United Kingdom and explains how themes of liability, risk, and decision making can be integrated into a practical simulation in order to underpin more traditional lecture-based law teaching. Through reflection upon the writer's experiments with simulation-based teaching, the paper suggests some innovations that may better orientate law teaching to engage these themes and, thereby, enhance the relevance of law studies to the future needs of built environment professionals in practice.</p
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