Signature of the scattering between dark sectors in large scale cosmic microwave background anisotropies

We study the interaction between dark sectors by considering the momentum transfer caused by the dark matter scattering elastically within the dark energy fluid. Describing the dark scattering analogy to the Thomson scattering which couples baryons and photons, we examine the impact of the dark scattering in CMB observations. Performing global fitting with the latest observational data, we find that for a dark energy equation of state w < -1, the CMB gives tight constraints on dark matter-dark energy elastic scattering. Assuming a dark matter particle of proton mass, we derive an elastic scattering cross section of sigma(D) < 3.295 x 10(-10)sigma(T) where sigma(T) is the cross section of Thomson scattering. For w > -1, however, the constraints are poor. For w = -1, sigma(D) can formally take any value.

Quantum Mechanics Insight into the Microwave Nucleation of SrTiO3 Nanospheres

An extensive investigation of strontium titanate, SrTiO3 (STO), nanospheres synthesized via a microwave-assisted hydrothermal (MAH) method has been conducted to gain a better insight into thermodynamic, kinetic, and reaction phenomena involved in STO nucleation and crystal growth processes. To this end, quantum chemical modeling based on the density functional theory and periodic super cell models were done. Several experimental techniques were employed to get a deep characterization of structural and optical features of STO nanospheres. A possible formation mechanism was proposed, based on dehydration of titanium and strontium clusters followed by mesoscale transformation and a self-assembly process along an oriented attachment mechanism resulting in spherical like shape. Raman and XANES analysis renders a noncentrosymmetric environment for the octahedral titanium, while infrared and first order Raman modes reveal OH groups which are unsystematically incorporated into uncoordinated superficial sites. These results seem to indicate that the key component is the presence of distorted TiO6 clusters to engender a luminescence property. Analysis of band structure, density Of states, and charge map shows that there is a close relationship among local broken symmetry, polarization, and energy split of the 3d orbitals of titanium. The interplay among these electronic and structural features provides necessary conditions to evaluate its luminescent properties under two energy excitation.

Microwave Hydrothermal Synthesis and Photocatalytic Performance of ZnO and M:ZnO Nanostructures (M = V, Fe, Co)

ZnO and doped M:ZnO (M = V, Fe and Co) nanostructures were synthesized by microwave hydrothermal synthesis using a low temperature route without addition of any surfactant. The transition metal ions were successfully doped in small amount (3% mol) into ZnO structure. Analysis by X-ray diffraction reveals the formation of ZnO with the hexagonal (wurtzite-type) crystal structure for all the samples. The as-obtained samples showed a similar flower-like morphology except for Fe:ZnO samples, which presented a plate-like morphology. The photocatalytic performance for Rhodamine B (RhB) degradation confirmed that the photoactivity of M:ZnO nanostructures decreased for all dopants in structure, according to their eletronegativity. Photoluminescence spectroscopy was employed to correlate M:ZnO structure with its photocatalytical properties. It was suggested that transition metal ions in ZnO lattice introduce defects that act as trapping or recombination centers for photogenerated electrons and holes, making it impossible for them reach the surface and promote the photocatalytical process.

Novel SrTi1-xFexO3 nanocubes synthesized by microwave-assisted hydrothermal method

We report herein for the first time a facile synthesis method to obtain SrTi1-xFexO3 nanocubes by means by a microwave-assisted hydrothermal (MAH) method at 140 degrees C. The effect of iron addition on the structural and morphological properties of SrTiO3 was investigated. X-ray diffraction measurements show that all STFO samples present a cubic perovskite structure. X-ray absorption spectroscopy at Fe absorption K-edge measurements revealed that iron ions are in a mixed Fe2+/Fe3+ oxidation state and preferentially occupy the Ti4+-site. UV-visible spectra reveal a reduction in the optical gap (E-gap) of STFO samples as the amount of iron is increased. An analysis of the data obtained by field emission scanning electron microscopy points out that the nanoparticles present a cubic morphology independently of iron content. According to high-resolution transmission electron microscopy results, these nanocubes are formed by a self-assembly process of small primary nanocrystals.

Structural Refinement and Photoluminescence Properties of MnWO4 Nanorods Obtained by Microwave-Hydrothermal Synthesis

Manganese tungstate (MnWO4) nanorods were prepared at room temperature by the co-precipitation method and synthesized after processing in a microwave-hydrothermal (MH) system at 140 degrees C for 6-96 min. These nanorods were structurally characterized by X-ray diffraction (XRD), Rietveld refinements and Fourier transform (FT)-Raman spectroscopy. The growth direction, shape and average size distribution of nanorods were observed by means of transmission electron microscopy (TEM) and high resolution TEM (HR-TEM). The optical properties of the nanorods were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. XRD patterns, Rietveld refinement data and FT-Raman spectroscopy indicate that the MnWO4 precipitate is not a single phase structure while the nanorods synthesized by MH processing have a wolframite-type monoclinic structure without deleterious phases. FT-Raman spectra exhibited the presence of 17 Raman-active modes from 50 to 1,000 cm(-1). TEM and HR-TEM micrographs indicated that the nanorods are aggregated due to surface energy by Van der Waals forces and grow along the [100] direction. UV-vis absorption measurements confirmed non-linear values for the optical band gap (from 3.2 to 2.72 eV), which increased as the MH processing time increased. The structural characterizations indicated that the presence of defects in the MnWO4 precipitate promotes a significant contribution to maximum PL emission, while MnWO4 nanorods obtained by MH processing decrease the PL emission due to the reduction of defects in the lattice.

Effect of different surfactants on the shape, growth and photoluminescence behavior of MnWO4 crystals synthesized by the microwave-hydrothermal method

In this communication, we report the effect of different surfactants [cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT)] on the shape, growth and photoluminescence (PL) behavior of manganese tungstate (MnWO4) crystals synthesized by the microwave-hydrothermal (MH) method at 413 K for 45 min. These crystals were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ultraviolet-visible (UV-vis) absorption spectroscopy and PL measurements. XRD patterns proved that these crystals have a monoclinic structure. FE-SEM images showed that MnWO4 crystals exhibit different shapes and growth mechanisms depending on the surfactant employed. The CTAB cationic surfactant promotes the hindrance of small nuclei that leads to the formation of flake-like nanocrystals, while SDS and AOT anionic surfactants promote a growth of crystals to plate-like and leaf-like crystals due to considerable size effect of counter-ions (RSO4- and RSO2O-) and an increase in Na+ ion remnants. UV-vis absorption spectroscopy revealed different optical band gap values due to modifications in the shape, surface and crystal size. Finally, the effect of surfactants on the crystal shapes and average crystal size distribution causing changes in the PL behavior of MnWO4 crystals was explained. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

CeO2 nanoparticles synthesized by a microwave-assisted hydrothermal method: evolution from nanospheres to nanorods

Ceria (CeO2) plays a vital role in emerging technologies for environmental and energy-related applications. The catalytic efficiency of ceria nanoparticles depends on its morphology. In this study, CeO2 nanoparticles were synthesized by a microwave-assisted hydrothermal method under different synthesis temperatures. The samples were characterized by X-ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, electron paramagnetic resonance spectroscopy and by the Brunauer-Emmett-Teller method. The X-ray diffraction and Raman scattering results indicated that all the synthesized samples had a pure cubic CeO2 structure. Rietveld analysis and Raman scattering also revealed the presence of structural defects due to an associated reduction in the valence of the Ce4+ ions to Ce3+ ions caused by an increasing molar fraction of oxygen vacancies. The morphology of the samples was controlled by varying the synthesis temperature. The TEM images show that samples synthesized at 80 degrees C consisted of spherical particles of about 5 nm, while those synthesized at 120 degrees C presented a mix of spherical and rod-like nanoparticles and the sample synthesized at 160 degrees C consisted of nanorods with 10 nm average diameter and 70 nm length. The microwave-assisted method proved to be highly efficient for the synthesis of CeO2 nanoparticles with different morphologies.

SnO2 nanocrystals synthesized by microwave-assisted hydrothermal method: towards a relationship between structural and optical properties

The exploration of novel synthetic methodologies that control both size and shape of functional nanostructure opens new avenues for the functional application of nanomaterials. Here, we report a new and versatile approach to synthesize SnO2 nanocrystals (rutile-type structure) using microwave-assisted hydrothermal method. Broad peaks in the X-ray diffraction spectra indicate the nanosized nature of the samples which were indexed as a pure cassiterite tetragonal phase. Chemically and physically adsorbed water was estimated by TGA data and FT-Raman spectra to account for a new broad peak around 560 cm(-1) which is related to defective surface modes. In addition, the spherical-like morphology and low dispersed distribution size around 3-5 nm were investigated by HR-TEM and FE-SEM microscopies. Room temperature PL emission presents two broad bands at 438 and 764 nm, indicating the existence of different recombination centers. When the size of the nanospheres decreases, the relative intensity of 513 nm emission increases and the 393 nm one decreases. UV-Visible spectra show substantial changes in the optical absorbance of crystalline SnO2 nanoparticles while the existence of a small tail points out the presence of localized levels inside the forbidden band gap and supplies the necessary condition for the PL emission.

Lipase-catalyzed kinetic resolution of (±)-mandelonitrile under conventional condition and microwave irradiation

The kinetic resolution of (±)-mandelonitrile was carried out using lipase from Candida antarctica under conventional condition (orbital shaker) and microwave irradiation in toluene, producing the (S)-mandelonitrile acetate with high selectivity (up to > 98% ee, enantiomeric excess). The unreacted (R)-mandelonitrile under microwave irradiation and conventional condition was partially converted into benzaldehyde by spontaneous chemical equilibrium. The (S)-mandelonitrile acetate under microwave irradiation was produced with 92% ee and 35% yield for 8 h of reaction. Conventional transesterification of (±)-mandelonitrile in an orbital shaker produced unreacted (R)-mandelonitrile (51% ee) and (S)-mandelonitrile acetate (98% ee) in accordance with Kazlauskas rule for 184 h of reaction.

Effect of pH-induced nanopowder deagglomeration on sintering, microstructure and dielectric properties of Ba0.77Ca0.23TiO3 ceramics

Ba0.77Ca0.23TiO3 ceramics were produced in this work starting from nanopowders synthesized via a polymeric precursor method. By adjusting the pH values of the precursor solutions above 7, it was possible to prepare powders weakly aggregated and with a smaller particle size, both facts which traduced into an enhanced nanopowders' sintering process at comparatively lower temperatures. Irrespective of the initial pH value, highly-dense and second phase-free ceramics were obtained following optimal sintering parameters (temperature and time) extracted from dilatometric and density measurements. By considering these and other sintering conditions, moreover, polycrystalline materials with an average grain size varying from 0.35 to 8 mm were produced, the grain growth process involving liquid phase-assisted sintering for heat treatments achieved at 1320 °C. The study of grain size effects on the ferroelectric properties of these materials was conducted, the results being discussed in the light of previous debates, including grain size-dependent degree of tetragonal distortion in such materials, as verified in this work.

On Ti-18Si-6B and Ti-7.5Si-22.5B powder alloys prepared by high-energy ball milling and sintering

Recently, a new ternary phase was discovered in the Ti-Si-B system, located near the Ti6Si2B composition. The present study concerns the preparation of titanium alloys that contain such phase mixed with α-titanium and other intermetallic phases. High-purity powders were initially processed in a planetary ball-mill under argon atmosphere with Ti-18Si-6B and Ti-7.5Si-22.5B at. (%) initial compositions. Variation of parameters such as rotary speed, time, and ball diameters were adopted. The as-milled powders were pressureless sintered and hot pressed. Both the as-milled and sintered materials were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive spectrometry. Sintered samples have presented equilibrium structures formed mainly by the α-Ti+Ti6Si2B+Ti5Si3+TiB phases. Silicon and boron peaks disappear throughout the milling processes, as observed in the powder diffraction data. Furthermore, an iron contamination of up to 10 at. (%) is measured by X-ray spectroscopy analysis on some regions of the sintered samples. Density, hardness and tribological results for these two compositions are also presented here.

Sintering and model of thermal residual stress for getting cutting tools from functionally gradient materials

Cutting tools with higher wear resistance are those manufactured by powder metallurgy process, which combines the development of materials and design properties, features of shape-making technology and sintering. The annual global market of cutting tools consumes about US$ 12 billion; therefore, any research to improve tool designs and machining process techniques adds value or reduces costs. The aim is to describe the Spark Plasma Sintering (SPS) of cutting tools in functionally gradient materials, to show this structure design suitability through thermal residual stress model and, lastly, to present two kinds of inserts. For this, three cutting tool materials were used (Al2O3-ZrO2, Al2O3-TiC and WC-Co). The samples were sintered by SPS at 1300 °C and 70 MPa. The results showed that mechanical and thermal displacements may be separated during thermal treatment for analysis. Besides, the absence of cracks indicated coherence between experimental results and the residual stresses predicted.

Spark Plasma Sintering of Si3N4-based Ceramics : Sintering mechanism-Tailoring microstructure-Evaluationg properties

Spark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain Si3N4-based ceramics with tailored microstructures, consisting of grains with either equiaxed or elongated morphology. The presence of an extra liquid phase is necessary for forming tough interlocking microstructures in Yb/Y-stabilised α-sialon by HP. The liquid is introduced by a new method, namely by increasing the O/N ratio in the general formula RExSi12-(3x+n)Al3x+nOnN16-n while keeping the cation ratios of RE, Si and Al constant. Monophasic α-sialon ceramics with tailored microstructures, consisting of either fine equiaxed or elongated grains, have been obtained by using SPS, whether or not such an extra liquid phase is involved. The three processes, namely densification, phase transformation and grain growth, which usually occur simultaneously during conventional HP consolidation of Si3N4-based ceramics, have been precisely followed and separately investigated in the SPS process. The enhanced densification is attributed to the non-equilibrium nature of the liquid phase formed during heating. The dominating mechanism during densification is the enhanced grain boundary sliding accompanied by diffusion- and/or reaction-controlled processes. The rapid grain growth is ascribed to a dynamic ripening mechanism based on the formation of a liquid phase that is grossly out of equilibrium, which in turn generates an extra chemical driving force for mass transfer. Monophasic α-sialon ceramics with interlocking microstructures exhibit improved damage tolerance. Y/Yb- stabilised monophasic α-sialon ceramics containing approximately 3 vol% liquid with refined interlocking microstructures have excellent thermal-shock resistance, comparable to the best β-sialon ceramics with 20 vol% additional liquid phase prepared by HP. The obtained sialon ceramics with fine-grained microstructure show formidably improved superplasticity in the presence of an electric field. The compressive strain rate reaches the order of 10-2 s-1 at temperatures above 1500oC, that is, two orders of magnitude higher than that has been realised so far by any other conventional approaches. The high deformation rate recorded in this work opens up possibilities for making ceramic components with complex shapes through super-plastic forming.