A dispersant for multiwall carbon nanotubes (MWCNTs) is proposed that fulfils the requirements of creating a uniform dispersion in the matrix and obtaining a good interface between CNTs and the ...matrix, and is soluble in generic nonpolar solvents. This dispersant is based on a long chain surfactant, called in this work dabcosil stearate, containing a stearate-based 18-carbon alkyl chain as an anion, and a silsesquioxane containing a bridged, positively charged 1,4-diazoniabicyclo2.2.2octane group. It provides not only a very good dispersion medium for the MWCNTs, but also a very good interface between MWCNTs and ceramic matrices, such as alumina and zirconia, prepared by the sol–gel method.
A set of silica-based hybrid material samples was obtained, with different molar fractions of tetraethylorthosilicate and dimethyldimethoxysilane, as inorganic and organic sol−gel precursors, ...respectively. The hybrid material was used as a matrix to disperse the 2-(4′-amino-2′-hydroxyphenyl)benzothiazole, a fluorophore with large Stokes shift caused by an excited state intramolecular proton transfer phenomena (ESIPT). UV−vis and steady-state fluorescence in the solid state were applied in order to characterize the photophysical behavior of the dye. The set of samples was characterized by infrared spectroscopy, elemental analysis, and N2 isotherms. The fluorescent pyrene dye was used as a probe for local polarity environment. For the first time, a modulation in the ESIPT fluorescence emission was achieved, being correlated to the methyl organic content of the hybrid matrix. The equilibrium between the conformers in the ground state is affected by changes in the matrix hydrophobicity. The replacement of silanol groups by methyl groups on matrix surface decreases the hydrogen bond capacity with the dye stabilizing the ESIPT conformer, in spite of the normal conformers, which will present a short wavelength emission band.
We report some regular organizations of stability phases discovered among self-sustained oscillations of a biochemical oscillator. The signature of such organizations is a nested arithmetic ...progression in the number of spikes of consecutive windows of periodic oscillations. In one of them, there is a main progression of windows whose consecutive number of spikes differs by one unit. Such windows are separated by a secondary progression of smaller windows whose number of spikes differs by two units. Another more complex progression involves a fan-like nested alternation of stability phases whose number of spikes seems to grow indefinitely and to accumulate methodically in cycles. Arithmetic progressions exist abundantly in several control parameter planes and can be observed by tuning just one among several possible rate constants governing the enzyme reaction.
The deformation mechanism of the ultrahigh-temperature ceramic, tantalum carbide (TaC), consolidated at room temperature at a very high hydrostatic pressure of 7.7GPa is investigated using ...high-resolution transmission electron microscopy. The deformation behavior of TaC at room temperature is also compared with that consolidated at high temperature (1830°C) at a similar pressure. TaC could be consolidated to a bulk structure (90% theoretical density) at room temperature. The deformation mechanisms operating at room temperature and 1830°C are found to be significantly different. The room-temperature deformation is dominated by the short-range movement of dislocations in multiple orientations, along with nanotwinning, grain rotation, crystallite misorientation with low-angle grain boundary formation and lattice structure destruction at interfaces. In contrast, at high temperature, the strain is accommodated mostly by a single slip system, forming a parallel array of dislocations. The consolidation at room temperature occurs by heavy deformation with the support from short range diffusion, whereas the consolidation at high temperature is mostly diffusion dominated, indicating a classic sintering mechanism. The improved degree of consolidation with fewer defects results in significantly improved elastic modulus and hardness in the case of high-temperature consolidate.
Anisotropic self-organized hybrid silica based xerogels were obtained. The ordered structure was imposed by the double charged 1,4-diazoniabicycle2.2.2octane chloride group bonded in a bridged way. ...This was confirmed by the presence of well defined X-ray diffraction peaks corresponding to an interplanar distance with the same length estimated for the organic bridged groups. The material was characterized by elemental analysis using CHN technique and the chloride ion was analyzed by a potentiometric titration.
13C and
29Si CP MAS solid state NMR spectroscopy and thermogravimetric analysis were also performed. The material that can be obtained in the form of powders and transparent monoliths or films, is thermally stable up to 260 °C and the samples with high organic content presented birefringence properties.
A new class of singlefluoforic submicro-sized spherical silica particles based on an ESIPT-exhibiting dye is presented in this work. The particles are able to generate a modulated response in the ...blue–green region varying the incident excitation wavelength. The submicro silica particles present monodisperse size distribution with a size range of 0.9–0.95 μm. The morphology and homogeneity combined with the observed fluorescence emission modulation and the absence of energy transfer between the fluorescent dyes open up new perspectives in the application of these fluorescent particles in the field of optical sensors and devices.
Using a high-pressure (HP) technique, samples of γ-Al2O3 were obtained by compaction at 4.5 GPa, in a toroidal-type apparatus, at room temperature (RT) and at higher temperatures. Compaction at RT ...produced crack-free, translucent, and dense samples. An improvement of these properties was observed for samples compacted at higher temperatures up to 565 °C. The nanocrystalline structure of γ-Al2O3 is retained, and the samples became transparent, showing high hardness (HV = 17 ± 1 GPa) and high density (95% of theoretical density). To understand the mechanisms of consolidation, a comparative analytical study by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) was conducted on the compacted γ-Al2O3 samples and the original powder. An FTIR study was done using the KBr technique and a high-vacuum cell, where the samples were submitted to thermal treatments up to 450 °C. For samples compacted at RT, a reduction in the content of adsorbed water was observed, compared to the original powder. Also, the surface hydroxyl groups became bridged, promoting dehydroxylation reactions, which were confirmed by TGA technique. In the dehydroxylation region, a weight loss was observed, and the water was released only at temperatures above 300 °C. For samples compacted simultaneously with temperature, the FTIR and TGA results did not show water release up to 500 °C. The compaction at temperatures higher than 565 °C yielded the formation of an aluminum hydroxide (diaspore) and the phase transformation from γ- to α-Al2O3. All these results support strongly the idea that the compaction at HP has caused the formation of a strong structure, with closed pores containing trapped water and hydroxylated internal surfaces, which confirms a proposed model for “cold-sintering”. At temperatures higher than 565 °C, this kind of structure is responsible for the formation of diaspore plus α-Al2O3.
We investigate the structural evolution of synthetic layered double hydroxides (LDH) samples, processed at room temperature and high-pressure (up to 7.7 GPa) in a toroidal chamber with two ...pressure-transmitting media, lead and graphite, using X-ray diffraction, thermogravimetry and N
2
-adsorption isotherms techniques. The X-ray patterns of compacted samples show a decrease in the peak intensities. For both pressure-transmitting media, our samples revealed a reduction of the basal d-spacing for the (003) plane when processed at 7.7 GPa. The expected high-pressure-induced amorphization was not observed. Surprisingly, we find high-pressure processing to have a strong influence on the memory effect of the LDH, due essentially to the reduction of the surface area and pore closing. Even when immersed in water, our samples did not recover the LDH structure, when either calcined at 450°C and compacted at 7.7 GPa, or calcined at 700°C and immediately compacted at 2.5 and 7.7 GPa.
Zirconium tungstate (α‐ZrW2O8) exhibits isotropic negative thermal expansion over a wide range of temperatures. Under high pressure it transforms to a metastable amorphous phase with positive thermal ...expansion coefficient. α‐ZrW2O8 can be fully recovered by annealing the amorphous phase at temperatures slightly above 600°C, at room pressure. In this work, we describe the dependency of the linear thermal expansion coefficient of amorphous ZrW2O8 with the annealing temperature, showing that it can be continuously tuned, in two stages, from about 7.5° to −9°C−1. These two stages are related to the relaxation of the amorphous phase and the onset of recrystallization, respectively.
We have produced silica-gel compacts doped with 2,5-Bis(benzoxazol-2
′-yl)-4-methoxyphenol dye using high-pressure processing of powders synthesized by the sol–gel technique. The high-pressure ...compaction of powders with three different dye concentrations was done at 4.5 GPa and room temperature. We have measured optical and mechanical properties of the obtained compacts. They were very stable, transparent, crack free, hard (3.56
±
0.07 GPa) and dense (1.95
±
0.03 g/cm
3), being resistant to polishing and leaching, which enables its use in optical applications. The Stokes shift observed was higher than 100 nm indicating that the intramolecular proton-transfer in the electronically excited state (ESIPT) of this dye is maintained, even in an OH rich environment like silica. A shift to higher wavelength in the fluorescence spectra of the compacts, attributed to the increasing in the conjugation of the π system, was observed.
