1,189 research outputs found
Si3N4 single-crystal nanowires grown from silicon micro and nanoparticles near the threshold of passive oxidation
A simple and most promising oxide-assisted catalyst-free method is used to
prepare silicon nitride nanowires that give rise to high yield in a short time.
After a brief analysis of the state of the art, we reveal the crucial role
played by the oxygen partial pressure: when oxygen partial pressure is slightly
below the threshold of passive oxidation, a high yield inhibiting the formation
of any silica layer covering the nanowires occurs and thanks to the synthesis
temperature one can control nanowire dimensions
Synthesis of CaSiO3 whiskers in alkaline salt flux for biomaterials reinforcement
Materials reinforcement by ceramic whiskers has been employed for a long time in a variety of industrial applications. Nevertheless, the materials by which these whiskers are commonly made of (carbide and silicon nitride) do not allow their use in biomaterials field due to their high toxicity. Then, it is of interest to synthesize ceramic whiskers which could reinforce biocompatible ceramic and polymeric biomaterials without harming the patients' health. In this manner, the aim of this work is to propose and analyze the limiting process variables of a new synthetic route to produce whiskers of CaSiO3 (wollastonite): a biocompatible, bioactive and readsorbable biomaterial. It was employed the molten salt synthesis at 900 ºC to grow wollastonite crystals which were characterized by X-ray diffraction and scanning electron microscopy. The proposed method was efficient in growing whiskers; however, the dwell time was not sufficient to guarantee a 100% reaction yield, leading to the formation of cristobalite.Há tempos empregam-se whiskers cerâmicos como reforço de materiais nas mais diversas aplicações, porém os materiais com os quais estes são comumente fabricados (carbeto e nitreto de silÃcio) não permitem a sua utilização no campo dos biomateriais devido sua elevada toxicidade. Assim, torna-se interessante sintetizar whiskers biocompatÃveis capazes de reforçar biomateriais cerâmicos e poliméricos sem prejudicar a saúde dos pacientes. Dessa forma, este trabalho tem como objetivo desenvolver e determinar os parâmetros limitantes de uma nova rota de sÃntese por fusão de sais de whiskers de CaSiO3, uma biocerâmica biocompatÃvel, bioativa e reabsorvÃvel. Este método é simples, barato e permite a produção em larga escala. Utilizou-se um fluxo de NaCl/KCl a 900 ºC para sintetizar whiskers de wollastonita, que foram caracterizados por difração de raios X e microscopia eletrônica de varredura. O método proposto mostrou-se eficiente, entretanto os tempos de patamar empregados não foram suficientes para garantir 100% de rendimento da reação de formação de CaSiO3, ocorrendo a formação de cristobalita.UNIFESP Instituto de Ciência e TecnologiaUNIFESP, Instituto de Ciência e TecnologiaSciEL
Angle-resolved photoemission study of the role of nesting and orbital orderings in the antiferromagnetic phase of BaFe2As2
We present a detailed comparison of the electronic structure of BaFe2As2 in
its paramagnetic and antiferromagnetic (AFM) phases, through angle-resolved
photoemission studies. Using different experimental geometries, we resolve the
full elliptic shape of the electron pockets, including parts of dxy symmetry
along its major axis that are usually missing. This allows us to define
precisely how the hole and electron pockets are nested and how the different
orbitals evolve at the transition. We conclude that the imperfect nesting
between hole and electron pockets explains rather well the formation of gaps
and residual metallic droplets in the AFM phase, provided the relative parity
of the different bands is taken into account. Beyond this nesting picture, we
observe shifts and splittings of numerous bands at the transition. We show that
the splittings are surface sensitive and probably not a reliable signature of
the magnetic order. On the other hand, the shifts indicate a significant
redistribution of the orbital occupations at the transition, especially within
the dxz/dyz system, which we discuss
Coherent and incoherent bands in La and Rh doped Sr3Ir2O7
In Sr2IrO4 and Sr3Ir2O7, correlations, magnetism and spin-orbit coupling
compete on similar energy scales, creating a new context to study
metal-insulator transitions (MIT). We use here Angle-Resolved photoemission to
investigate the MIT as a function of hole and electron doping in Sr3Ir2O7,
obtained respectively by Ir/Rh and Sr/La substitutions. We show that there is a
clear reduction as a function of doping of the gap between a lower and upper
band on both sides of the Fermi level, from 0.2eV to 0.05eV. Although these two
bands have a counterpart in band structure calculations, they are characterized
by a very different degree of coherence. The upper band exhibits clear
quasiparticle peaks, while the lower band is very broad and loses weight as a
function of doping. Moreover, their ARPES spectral weights obey different
periodicities, reinforcing the idea of their different nature. We argue that a
very similar situation occurs in Sr2IrO4 and conclude that the physics of the
two families is essentially the same
Giant Anisotropy of Spin-Orbit Splitting at the Bismuth Surface
We investigate the bismuth (111) surface by means of time and angle resolved
photoelectron spectroscopy. The parallel detection of the surface states below
and above the Fermi level reveals a giant anisotropy of the Spin-Orbit (SO)
spitting. These strong deviations from the Rashba-like coupling cannot be
treated in perturbation theory. Instead, first
principle calculations could accurately reproduce the experimental dispersion
of the electronic states. Our analysis shows that the giant anisotropy of the
SO splitting is due to a large out-of plane buckling of the spin and orbital
texture.Comment: 5 pages, 4 figure
Symmetry breaking in commensurate graphene rotational stacking; a comparison of theory and experiment
Graphene stacked in a Bernal configuration (60 degrees relative rotations
between sheets) differs electronically from isolated graphene due to the broken
symmetry introduced by interlayer bonds forming between only one of the two
graphene unit cell atoms. A variety of experiments have shown that non-Bernal
rotations restore this broken symmetry; consequently, these stacking varieties
have been the subject of intensive theoretical interest. Most theories predict
substantial changes in the band structure ranging from the development of a Van
Hove singularity and an angle dependent electron localization that causes the
Fermi velocity to go to zero as the relative rotation angle between sheets goes
to zero. In this work we show by direct measurement that non-Bernal rotations
preserve the graphene symmetry with only a small perturbation due to weak
effective interlayer coupling. We detect neither a Van Hove singularity nor any
significant change in the Fermi velocity. These results suggest significant
problems in our current theoretical understanding of the origins of the band
structure of this material.Comment: 7 pages, 6 figures, submitted to PR
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