Effect of Nanosized TiO2 on Nucleation and Growth of Cristobalite in Sintered Fused Silica Cores for Investment Casting

Sintered fused silica is often used for making sacrificial cores in investment castings of Ni superalloys. Their usage is fundamental in the manufacture of precise superalloy gas turbine components with complex internal cooling passages. In this study SiO2/ZrSiO4/TiO2 cores were prepared from fused silica powders with different grain size and zircon and TiO2 content by slip casting method. Green samples were sintered at 1230°C at various soaking time: from 0,5 to 10 hours. Thermomechanical and microstructural properties of optimized silica obtained by add of 1,5%wt of TiO2 to SiO2/ZrSiO4 composition have been investigated by three point bending tests, XRD and Hg porosimetric analysis. The influence of cristobalite content on thermal stability at high temperature was studied by an optical dilatometer. At temperature below 1200°C TiO2 appears to act as a phase transformation inhibitor reducing the transformation rate of fused silica to cristobalite at high temperatures. At higher temperature it speeds up the formation of cristobalite. A comparison with commercial silica cores made by injection moulding has been performed. A prototype core was obtained and an investment casting was performed on that

Nanocrystalline TiO2 based films onto fibers for photocatalytic degradation of organic dye in aqueous solution

Nanocrystalline titania (TiO2) synthesized via sol–gel, by using an alkoxide precursor were deposited onto commercially available silica and alumina fibers, namely E-Glass and Nextel 650, respectively. Different processing conditions and material preparation parameters, such as amount of TiO2, film composition and annealing temperature were tested in order to obtain nanocrystalline TiO2 with different morphological and structural characteristics. The materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and the Brunauer, Emmett, and Teller (BET) surface area measurements. The photocatalytic activity of the obtained coated fibers was investigated by monitoring the degradation of a model molecule, an azo dye (Methyl Red), under UV irradiation in aqueous solution. The detected photocatalytic performance of the sol–gel derived nanocrystalline TiO2 was explained on the basis of mechanism associated to the photocatalytic decomposition of organic molecules using semiconductor oxides and accounted for the structural and morphological characteristics of the TiO2 based coating. The materials with the most suited characteristics for photocatalysis were used to scale up the deposition onto a larger sample of fiber and then tested in a photocatalytic reactor. A commercially available TiO2 standard material (TiO2 P25 Degussa) was used as reference, in order to ultimately assess the viability of the coating process for real application

Is nano-TiO2 alone an effective strategy for the maintenance of stones in Cultural Heritage?

TiO2-based nanocoatings have been becoming more and more widespread during last years in Cultural Heritage: they seem to be able to keep stone surfaces self-cleaned and to prevent the formation of biofouling. However, the efficiency of these coatings is strongly dependent on the substrate (i.e.: porosity and roughness) and on the amount of TiO2. Thus, this study experimentally investigates on the self-cleaning and anti-biofouling efficiency of a nano-TiO2 dispersion (without any organic or inorganic additive) applied on six different types of natural stones (three limestones, two sandstones and one tuff) usually used in Cultural Heritage, where high porosity and roughness can be found and the TiO2 amount cannot be increased in order to avoid any chromatic variation of the substrate. Water was used as solvent so as to reduce the risk of exposition of hazardous materials and to eliminate any chemical action on stones. The self-cleaning power of the coating was evaluated by measuring its ability at discolouring organic dye Methylene Blue, while its anti-biofouling efficiency was assessed by an accelerated growth test under controlled climatic conditions of two algal microorganisms, namely Chlorella mirabilis and Chroococcidiopsis fissurarum. Results show that, even if the photocatalytic and biocide power of nano-TiO2 itself is well known in literature, its application for the maintenance of stones in Cultural Heritage does not seem to be an effective strategy, especially when stones are highly porous and rough. Roughness and porosity of stones, in fact, can limit the efficiency of TiO2, which is thus not able to powerfully keep the stone substrate cleaned or slow down algal proliferatio

Influence of glass phase on Al2O3 fiber-reinforced Al2O3 composites processed by slip casting

The present work describes the processing of alumina fiber reinforced alumina ceramic preforms consisting of chopped Al2O3 fibers (33 wt%) and Al2O3 (67 wt%) fine powders by slip casting. The preforms were pre-sintered in air at 1100 °C for 1 h. A lanthanum based glass was infiltrated into these preforms at 1250 °C for 90 min. Linear shrinkage (%) was studied before and after glass infiltration. Pre-sintered and infiltrated specimens were characterized by scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, porosimetry and flexural strength. The alumina preforms showed a narrow pore size distribution with an average pore size of ∼50 nm. It was observed that introducing Al2O3 fibers into Al2O3 particulate matrix produced warp free preforms with minor shrinkage during pre-sintering and glass infiltration. It was observed that the infiltration process fills up the pores and considerably improves the strength and reliability of alumina preform