Effect of synthesis conditions on the microstructure of TEOS derived silica hydrogels synthesized by the alcohol-free sol–gel route

Silica matrices synthesized from a pre-hydrolysis step in ethanol followed by alcohol removal at low pressure distillation, and condensation in water, are suitable for encapsulation of biomolecules and microorganisms and building bioactive materials with optimized optical properties. Here we analyze the microstructure of these hydrogels from the dependence of I(q) data acquired from SAXS experiments over a wide range of silica concentration and pH employed in the condensation step. From the resulting data it is shown that there is a clear correlation between the microscopic parameters—cluster fractal dimension (D), elementary particle radius (a) and cluster gyration radius (R)—with the attenuation of visible light when the condensation step proceeds at pH < 6. At higher pHs, there is a steep dependence of the cluster density (~R ^D−3) with the condensation pH, and non-monotonous changes of attenuance are less than 20%, revealing the complexity of the system. These results, which were obtained for a wide pH and silica concentration range, reinforce the idea that the behavior of gels determined in a restricted interval of synthesis variables cannot be extrapolated, and comparison of gelation times is not enough for predicting their properties.     

Bilevel optimization applied to strategic pricing in competitive electricity markets

In this paper, we present a bilevel programming formulation for the problem of strategic bidding under uncertainty in a wholesale energy market (WEM), where the economic remuneration of each generator depends on the ability of its own management to submit price and quantity bids. The leader of the bilevel problem consists of one among a group of competing generators and the follower is the electric system operator. The capability of the agent represented by the leader to affect the market price is considered by the model. We propose two solution approaches for this non-convex problem. The first one is a heuristic procedure whose efficiency is confirmed through comparisons with the optimal solutions for some instances of the problem. These optimal solutions are obtained by the second approach proposed, which consists of a mixed integer reformulation of the bilevel model. The heuristic proposed is also compared to standard solvers for nonlinearly constrained optimization problems. The application of the procedures is illustrated in case studies with configurations derived from the Brazilian power system.     

Synthesis of carbon nanotubes by CVD: Effect of acetylene pressure on nanotubes characteristics

The effect of acetylene partial pressure on the structural and morphological properties of multi-walled carbon nanotubes (MWCNTs) synthesized by CVD on iron nanoparticles dispersed in a SiO₂ matrix as catalyst was investigated. The general growing conditions were: 110 cm³/min flow rate, 690 °C synthesis temperature, 180 Torr over pressure and two gas compositions: 2.5% and 10% C₂H₂/N₂. The catalyst and nanotubes were characterized by HR-TEM, SEM and DRX. TGA and DTA were also carried out to study degradation stages of synthesized CNTs. MWCNTs synthesized with low acetylene concentration are more regular and with a lower amount of amorphous carbon than those synthesized with a high concentration. During the synthesis of CNTs, amorphous carbon nanoparticles nucleate on the external wall of the nanotubes. At high acetylene concentration carbon nanoparticles grow, covering all CNTs’ surface, forming a compact coating. The combination of CNTs with this coating of amorphous carbon nanoparticles lead to a material with high decomposition temperature.     

Enhanced visible light photoelectrochemical performance of β-Bi2O3-TiO2/ITO thin films prepared by aqueous sol-gel

Journal of Solid State Electrochemistry - Semiconductor heterojunction of β-Bi2O3-TiO2/ITO thin films was prepared by aqueous sol-gel and their photoelectrochemical and photoelectrocatalytic...     

Effect of catalyst preparation on the yield of carbon nanotube growth

Multi-wall carbon nanotubes (MWCNTs) were synthesized by catalytic chemical vapor deposition (CVD) on catalytic iron nanoparticles dispersed in a silica matrix, prepared by sol gel method. In this contribution, variation of gelation condition on catalyst structure and its influence on the yield of carbon nanotubes growth was studied. The precursor utilized were tetraethyl-orthosilicate and iron nitrate. The sols were dried at two different temperatures in air (25 or 80 °C) and then treated at 450 °C for 10 h. The xerogels were introduced into the chamber and reduced in a hydrogen/nitrogen (10%v/v) atmosphere at 600 °C. MWCNTs were formed by deposition of carbon atoms from decomposition of acetylene at 700 °C. The system gelled at RT shows a yield of 100% respect to initial catalyst mass whereas the yield of that gelled at 80 °C was lower than 10%. Different crystalline phases are observed for both catalysts in each step of the process. Moreover, TPR analysis shows that iron oxide can be efficiently reduced to metallic iron only in the system gelled at room temperature. Carbon nanotubes display a diameter of about 25–40 nm and several micron lengths. The growth mechanism of MWCNTs is base growth mode for both catalysts.     

Structures and stability of lipid emulsions formulated with sodium caseinate

The physicochemical properties of emulsions play an important role in food systems as they directly contribute to texture, sensory and nutritional properties of foods. Sodium caseinate (NaCas) is a well-used ingredient because of its good solubility and emulsifying properties and its stability during heating. One of most significant aspects of any food emulsion is its stability. Among the methods used to study emulsion stability it may be mentioned visual observation, ultrasound profiling, microscopy, droplet size distribution, small deformation rheometry, measurement of surface concentration to characterize adsorbed protein at the interface, nuclear magnetic resonance, confocal microscopy, diffusing wave spectroscopy, and turbiscan. They have advantages and disadvantages and provide different insights into the destabilization mechanisms. Related to stability, the aspects more deeply investigated were the amount of NaCas used to prepare the emulsion, and specially the oil-to-protein ratio, the mobility of oil droplets and the interactions among emulsion components at the interface. It is known that the amount of protein required to stabilize oil-in-water emulsions depends, not only on the structure of protein at the interface, and the average diameters of the emulsion droplets, but also on the type of oils and the composition of the aqueous phase. Several authors have investigated the effect of a thickening agent or of a surface active molecule. Factors such as pH, temperature, and processing conditions during emulsion preparation are also very relevant to stability. There is a general agreement among authors that the most stable systems are obtained for conditions that produce size reduction of the droplets, an increase in viscosity of the continuous phase and structural changes in emulsions such as gelation. All these conditions decrease the molecular mobility and slow down phase separation.     

SYNTHESIS OF METAL OXIDE PARTICLES FROM AQUEOUS MEDIA: THE HOMOGENEOUS ALKALIN1ZAT1ON METHOD

An analysis of the urea method as a way to synthesize uniform colloidal particles is presented. Various selected cases of precipitation and coprecipitation are reviewed to illustrate the inherent complexities of the method. The importance of the kinetics of urea hydrolysis is recognized. The concept of trajectory is used to rationalized the evolution of the systems upon aging. The influence of thermodynamic and kinetic factors in the definition of the nature of the precipitating solid phases is analyzed; the overriding role of the latter is stressed. The factors that influence particle morphology are also addressed.     

Mixed oxide semiconductors based on bismuth for photoelectrochemical applications

The structural and photoelectrochemical properties of mixed oxide semiconductor films of Bi-Nb-M-O (M = Al, Fe, Ga, In) were studied in order to explore their use as photoanodes in photoelectrochemical cells. These films were prepared on AISI/SAE 304 stainless steel plates by sol–gel dip-coating. The films were characterized by scanning electron microscopy—energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), and their photoelectrochemical properties were studied by open circuit potential (OCP) measurements, linear sweep voltammetry (LSV), and cyclic voltammetry (CV). SEM micrographs show homogeneous and rough films with agglomerates on the surface. EDS analyses show that the films are composed of Bi, Nb, and M, and the agglomerates are mainly composed of Bi. XRD analyses show a predominant crystalline phase of bismuth(III) oxide (Bi₂O₃) and a secondary phase composed of Bi-M mixed oxides. It is noteworthy that there was no identified niobium-based crystalline phase. XPS results reveal that the films are composed by Bi(III), Nb(V), and M(III). CV results show that the electrochemical behavior is attributed only to the semiconductor films which indicate a good coating of the stainless steel support. OCP measurements show that all the films have n-type semiconductor properties and exhibited photoresponse to the visible light irradiation. LSV results show that the application of a potential higher than +0.1 V enhances the photocurrent which can be attributed to an improved charge carrier separation. The results indicate that these materials can be used in photoelectrochemical cells.     

Influence of filler alignment in the mechanical and electrical properties of carbon nanotubes/epoxy nanocomposites

In this work, we report the mechanical and electrical properties of carbon nanotubes/epoxy composites prepared with aligned and randomly oriented nanotubes as filler. The samples are disks of 30 mm in diameter and 3 mm in thickness. To obtain the carbon nanotubes alignment, an external electric field (250 VAC; 50 Hz) was applied through the thickness of the sample during all the cure process. The AC electrical current was measured, during the cure, as a strategy to determine the optimum time in which the alignment reaches the maximum value. DC conductivity measured after the cure shows a percolation threshold in the filler content one order of magnitude smaller for composites with aligned nanotubes than for composites with randomly oriented filler (from 0.06 to 0.5 wt%). In the percolation threshold, the achieved conductivity was 1.4×10⁻⁵ Sm⁻¹. In both cases, aligned and randomly distributed carbon nanotube composites, the wear resistance increases with the addition of the filler while the Rockwell hardness decreases independently of the nanotubes alignment.     

Magnetic binary nanofillers

Magnetic binary nanofillers containing multiwall carbon nanotubes (MWCNT) and hercynite were synthesized by Chemical Vapor Deposition (CVD) on Fe/AlOOH prepared by the sol–gel method. The catalyst precursor was fired at 450 °C, ground and sifted through different meshes. Two powders were obtained with different particle sizes: sample A (50–75 μm) and sample B (smaller than 50 μm). These powders are composed of iron oxide particles widely dispersed in the non-crystalline matrix of aluminum oxide and they are not ferromagnetic. After reduction process the powders are composed of α-Fe nanoparticles inside hercynite matrix. These nanofillers are composed of hercynite containing α-Fe nanoparticles and MWCNT. The binary magnetic nanofillers were slightly ferromagnetic. The saturation magnetization of the nanofillers depended on the powder particle size. The nanofiller obtained from powder particles in the range 50–75 μm showed a saturation magnetization 36% higher than the one formed from powder particles smaller than 50 μm. The phenomenon is explained in terms of changes in the magnetic environment of the particles as consequence of the presence of MWCNT.