18 research outputs found
Phase stability, elastic, electronic, thermal and optical properties of Ti3Al1-xSixC2 (0 LE x LE 1): First-principles study
The structural parameters with stability upon Si incorporation and elastic,
electronic, thermodynamic and optical properties of Ti3Al1-xSixC2 (0 \leq x
\leq 1) are investigated systematically by the plane wave psedudopotential
method based on the density functional theory (DFT). The increase of some
elastic parameters with increasing Si-content renders the alloys to possess
higher compressive and tensile strength. The Vickers hardness value obtained
with the help of Mulliken population analysis increases as x is increased from
0 to 1. The solid solutions considered are all metallic with valence and
conduction bands, which have a mainly Ti 3d character, crossing the Fermi
level. The temperature and pressure dependences of bulk modulus, normalized
volume, specific heats, thermal expansion coefficient, and Debye temperature
are all obtained through the quasi-harmonic Debye model with phononic effects
for T = 0-1000K and P = 0-50GPa. The obtained results are compared with other
results where available. Further an analysis of optical functions for two
polarization vectors reveals that the reflectivity is high in the
visible-ultraviolet region up to ~ 10.5 eV region showing promise as good
coating material.
Keywords: Ti3Al1-xSixC2; First-principles; Quasi-harmonic Debye model;
Mechanical properties; Band structure; Optical propertiesComment: 12 pages, 8 figure
Superconductivity in the Nb2SnC compound
Nb2SnC is a member of the large family of lamellar materials that crystallize
in the hexagonal structure with space group P63/mmc which are isomorphs with
Cr2AlC, also named H-phase. In spite of the great number of compounds which
belong to this family, the superconductivity has been reported only for two
cases: Mo2GaC and Nb2SC. In this work we show that superconductivity can be
observed in Nb2SnC depending on the synthesis method used. The quality of the
superconductor is strongly dependent of the synthesis method and the optimal
results were reached for samples synthesized at 2.5 GPa and 523 +/- 50oC. This
sample showed a critical temperature close to 7.8K, revealed from magnetization
and transport measurement, the highest critical temperature reported up to now
for an H-phase.Comment: paper with 12 pages and 4 figure
Structural, Elastic, Electronic and Optical Properties of a New Layered-Ternary Ta4SiC3 Compound
We propose a new layered-ternary Ta4SiC3 with two different stacking
sequences ({\alpha}- and {\beta}-phases) of the metal atoms along c axis and
study their structural stability. The mechanical, electronic and optical
properties are then calculated and compared with those of other compounds M4AX3
(M = V, Nb, Ta; A = Al, Si and X = C). The predicted compound in the
{\alpha}-phase is found to possess higher hardness than any of these compounds.
The independent elastic constants of the two phases are also evaluated and the
results discussed. The electronic band structures for {\alpha}- and
{\beta}-Ta4SiC3 show metallic conductivity. Ta 5d electrons are mainly
contributing to the total density of states (DOS). We see that the
hybridization peak of Ta 5d and C 2p lies lower in energy and the Ta 5d-C 2p
bond is stronger than Ta 5d-Si 3p bond. Further an analysis of the different
optical properties shows the compound to possess improved behavior compared to
similar types of compounds.Comment: 9 pages, 5 figures; PACS: 60.20.Dc; 62.20.-x; 71.15.Mb; 78.20.Ci;
Keywords: Ta4SiC3, First-principles; Elastic properties; Electronic
properties; Optical propertie
Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices
International audienceWoody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities' features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur
Rare earth rhodium borides RRh3B (R = Y, Zr, and Nb): mechanical, thermal and optical properties
We report here ab initio density functional theory (DFT) calculations of structural,
elastic, Peierls stress, thermodynamic and optical properties of
RRh3B (RÂ =Â Y, Zr and Nb) using the plane
wave pseudopotential method. The materials possess better ductile behavior in comparison
with a selection of layered MAX phases but the anisotropy is strong, particularly in
NbRh3B. The Peierls stress, approximately 3-4 times larger than in MAX
phases, show that dislocation movement may follow but with much reduced occurrences
compared to MAX phases. The temperature and pressure dependence of bulk modulus, specific
heats, thermal expansion coefficient, and Debye temperature are calculated for the first
time for two of the three compounds using the quasi-harmonic Debye model with phononic
effects for elevated temperature and pressure. The obtained results are discussed in
comparison to the behavior of other related compounds. Further the features of optical
functions obtained for the first time are discussed. The study reveals that the
reflectivity is high in the IR-UV regions up to  ~17.5 eV (YRh3B,
ZrRh3B) and 20Â eV (NbRh3B), thus showing promise as good coating
materials