Volume 12 (2012): Issue 1 (March 2012)

Aerogels are internally nanostructured materials characterized with a plethora of unique properties. Monoliths with high optical transparency made of silica aerogels were some of the first and still one of the most important classes of aerogels. Experiments and theory indicate that optical transparency and structural integrity of silica aerogels are negatively correlated. Other than optimal combination of processing conditions during aerogel fabrication can result in either highly transparent but cracked or in crack-free but less transparent and even opaque aerogels monoliths. Results are presented from the study of the relationship between the properties of single-step tetramethoxysilane (TMOS) base-catalyzed silica aerogels and the processing conditions, both at the alcogel preparation step and during the supercritical liquid CO₂ drying process. Crack-free aerogel monoliths with good optical transparency were obtained with TMOS:methanol (MeOH) molar ratios of 1:16 and TMOS:ammonia (NH₄OH) molar ratios of 1:0.05, CO₂-MeOH exchange rates of about 1.25 ml/min, and autoclave decompression rates of 70 KPa/min. Adding glycerol in the sol-gel stage had a positive effect on the aerogel monolithicity but, even without glycerol, crack-free silica aerogels can be obtained by reducing the depressurization rate of the autoclave. A strict control and careful selection of the aerogel's processing conditions within the set of parameters identified will enable the fabrication of structurally sound silica aerogels with good optical properties essential for a number of applications.

In the present work, the electrodeposition of iron-nickel-molybdenum (Fe-Ni-Mo) alloy on n-type silicon (Si) is investigated. A voltamperometric study has been carried out. According to the composition of the solution and of its pH, the properties of final deposit have been investigated and analyzed by X-rays diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Under our conditions an amorphous FeNiMo alloy was formed. The morphology of deposited layer was strongly influenced by the polarization and the molybdenum dominance in the final chemical composition of the deposited film. Moreover, high percentages of molybdenum can be obtained from high molybdate concentrations and grain size increase with increasing Mo concentration. We also noticed that an absence of nickel was accomplished by applying more negative potential.

The influence of Tvergaard's parameters q_i of Gurson-Tvergaard-Needleman (GTN) material model on S235JR steel response was considered in the study. The analysis concerns the strength curves simulated numerically for notched tensile elements under static tension in complex stress state defined by high initial stress triaxiality σ_m/σ_e > 1. Typical and material-dependent values of Tvergaard's parameters q_i were examined. The influence of the Tvergaard's parameters q_i on material response was noticed at the failure range for S235JR steel in the case of high stress triaxiality.