Marcia Russman Gallas

Infrared and thermogravimetric study of high pressure consolidation in alkoxide silica gel powders

High density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4.5 GPa, at room temperature. The starting material was used three days after synthesis by hydrolysis of alkoxides without additional treatment. Fourier transform infrared spectroscopy (FTIR), using the KBr technique and a high vacuum cell at temperatures up to 450°C, and thermogravimetric analysis (TGA) up to 900°C was conducted. A reduction up to 60% in the adsorbed water content of the compacted silica gel was observed. Changes in the 3000 to 3800 cm−1 region indicate that the surface silanol groups became bridged, which promotes condensation reactions of dehydroxylation. Those results suggest that the mechanism for consolidation under high-pressure, is ‘cold sintering’ process, where silanol groups at the surface of the nanoparticles condense to form siloxane bonds between the particles and water, resulting in a stiff body with closed nanopores containing trapped water.

Materiais híbridos à base de sílica obtidos pelo método sol-gel

This review deals with silica based hybrid materials obtained by the sol-gel method. It involves concepts, classifications and important definitions regarding the sol-gel method that allows obtaining materials with organic and inorganic components dispersed in a molecular or nanometric level. We discuss the properties and characteristics of hybrid materials related to experimental synthesis conditions. We devote a special attention to the nanostructured materials, where the self-organization is imposed by the organic component. Finally, we present some important applications of these materials based on their specific properties.

High pressure compaction of nanosize ceramic powders

High-density ceramic materials from nanosize ceramic powders were produced by high pressure under nearly hydrostatic environment up to 5.6 GPa, on a special configuration in a toroidal-type apparatus, at room temperature. Attempts to use a common solid pressure transmitting medium, as NaCl, resulted in cracked samples. Lead and indium, which have an extremely low shear strength, proved to be the suitable choices as a pressure-transmitting medium to compact these ceramic materials, in order to obtain high-density samples. Transparent amorphous SiO 2 -gel and translucent γ−Al 2 O 3 samples, in bulk, with volumes about 40 mm 3 , hard and crack-free were obtained. Densities over 90% of full density for the γ −Al 2 O 3 samples and over 80% for the compacted SiO 2 -gel samples were obtained. In addition, from the density-pressure curve, the yield strength ( σ ) for γ −Al 2 O 3 was estimated, for the first time, as 2.6 GPa. Vickers microhardness values were in the range of 5.7 GPa for the γ −Al 2 O 3 samples, and 4.0 GPa for the SiO 2 -gel samples, under loads of 50 g. An important and practical application of these results is the possibility of producing bulk γ −Al 2 O 3 , a new alumina material, which was not possible to prepare before due to the conversion to a phase during the normal sintering process. Additionally, specially for SiO 2 -gel, a very important application of this study is the possibility of incorporation of organic substances in an inorganic matrix, using high pressure at room temperature.

Fluorescence properties of benzoxazole type dyes entrapped in a silica matrix by the sol-gel method

For the first time, the incorporation of 2,5-bis(benzoxazol-2′-yl)-4-methoxyphenol (BM) in a silica matrix by the sol–gel method has been accomplished, producing transparent and translucent monoliths with fluorescence properties. The intensity and wavelength range of the fluorescence bands F1 (primary structures) and F2 (proton-transferred forms) were extremely sensitive to the presence of solvents and to the dye concentration in the silica pores. The F2 band was observed when the silica pores contained solvent; however, after the solvent evaporation, the interactions of the dye with the silica host hindered the proton-transfer reaction, eliminating the F2 band and indicating the polar and acidic character of this host. Nevertheless, it was observed that the proton-transfer could be recovered after insertion of suitable solvents inside the matrix, like water, ethanol or DMF. Changes in the fluorescence spectra due to the use of different dye concentrations were observed when the monoliths were prepared with ethanol as solvent. During the aging process, the F2 band persists at high dye concentrations, but it disappears for low dye concentrations. The high sensitivity of the proton-transfer reaction of this dye in a particular environment, where the solvent and the host play important roles, suggests the possible application of this dye as a photoprobe for this kind of system.

High pressure loading of organic dyes into a silica matrix

Abstract High density, transparent and crack-free compacts of silica gel doped with naphthazarin, quinizarin and rhodamine 6G, were produced by sol–gel synthesis with high pressure at room temperature. Compacts are very stable, being resistent to polishing and leaching by ethanol and acetone. They show optical properties, as absorption and fluorescence, similar to those at dilute solutions, indicating that the dyes are dispersed in the silica matrix as individual molecules. Heat treatment to 150°C for 1 h led to almost no degradation.

Surfactant-based dispersant for multiwall carbon nanotubes to prepare ceramic composites by a sol-gel method.

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-diazoniabicyclo[2.2.2]octane 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.

Surface morphology of spray-dried nanoparticle-coated microparticles designed as an oral drug delivery system

This paper was devoted to studying the influence of coating material (nanocapsules or nanospheres), drug model (diclofenac, acid or salt) and method of preparation on the morphological characteristics of nanoparticle-coated microparticles. The cores of microparticles were obtained by spray drying or evaporation and the coating was applied by spray drying. SEM analyses showed nanostructures coating the surface of nanocapsule-coated microparticles and a rugged surface for nanosphere-coated microparticles. The decrease in their surface areas was controlled by the nanoparticulated system, which was not dependent on microparticle size. Optical microscopy and X-ray analyses indicated that acid diclofenac crystals were present in formulations prepared with the acid as well as in the nanocapsule-coated microparticles prepared with the salt. The control of coating is dependent on the use of nanocapsules or nanospheres and independent of either the characteristics of the drug or the method of preparing the core.

Phase separation in ion bombarded FeNi Invar alloys

Studies of the effects of ion bombardment on the phase equilibria and ordering of FeNi alloys with 30.6 and 39.5 at. % Ni were performed. The FeNi samples were bombarded with Ne, Ar, and Kr at different doses and temperatures, and analyzed using the conversion electron Mossbauer spectroscopy (CEMS) technique with 57Fe. The changes in the CEMS spectra of Fe69.4Ni30.6 and Fe60.5Ni39.5 alloys, bombarded with Ne and Ar, indicate a phase separation, but no evidence of ordering. The Kr bombardment does not produce any change, however samples of 39.5 at. % Ni already showing phase separation returned to the single phase state after Kr irradiation. Our results give strong evidences of a prevalence of displacement mixing for bombardment with heavier ions, and of radiation enhanced diffusion for bombardment with lighter ions.

Effects of annealing processes on the Curie temperature of Fe-Ni Invar alloys

The Curie temperature (Tc) of the iron-nickel Invar Alloy with composition 30.6 at.% Ni was studied as a function of the annealing time at different temperatures. An increase in Tc of more than 20 degrees C was observed with annealing at temperatures as low as 340 degrees C. The change in Tc with time was similar to that observed in metallic glasses, and was interpreted as an atomic rearrangement, with a change in the chemical short-range order. The activation energy for the process was estimated to be about 1.5 eV, indicating movement of quenched-in vacancies as the main relaxation mechanism.

High-Pressure Entrapment of Rhodamine 6G into a Silica Matrix

The entrapment of organic dyes with interesting optical properties in silica produced by sol-gel method, has been studied over the last years, aiming to obtain a new class of optical materials. The monoliths produced by this method present cracks, large shrinkage during the silica gelation and drying, and a great porosity, which decreases their optical performance. In a previous work, to overcome these difficulties, a high pressure (4.5 GPa) technique at room temperature was used to produce compacts of silica gel doped with rhodamine 6G (R6G). In this work, we extended this study to pressures up to 7.7 GPa to verify the effect of high pressure in the fluorescence spectra of the R6G and in the density and hardness of the compacts. High density (98% of the silica density), transparent and crack-free compacts were produced, being resistent to polishing. The fluorescence spectra were similar to the ones observed in ethanol solutions. These new class of materials are potentially useful for optical applications.