Thermal evolution and mechanical properties of vitreous fibres in the SiO2-Li2O-TiO2-Al2O3 system

The thermal evolution of glass fibres of the SiO₂-Li₂O-TiO₂-Al₂O₃ system has been investigated by X-ray diffraction, DTA and SEM. Within the range of the compositions investigated (2 to 10 mol % TiO₂) SEM and SAXS measurements on bulk materials have shown that a small addition of Al₂O₃ (3 mol %) does not modify the limits of the immiscibility region present in the ternary system SiO₂-LiO₂-TiO₂. Within this region TiO₂-rich droplets are more than likely to be formed. As to tensile strength, the asquenched fibres of the composition placed within the immiscibility dome give higher values than that obtained outside the liquid-liquid region and these values are higher than those measured on E-glass fibres. A slight crystallization in all the fibres leads to a continuous decrease of the initial tensile strength. With regards to the crystal phases presented by the crystallized fibres, the first exothermic peak is due to the simultaneous crystallization of-quartz (ss) and lithium disilicate whereas the second peak is due, most probably, to a-quartz (ss) -spodumene (ss) phase transformation as well as to the formation of Li₂TiSiO₅. The path of crystallization of the other detected Ti-compounds is similar to that proposed by Furukawa and White.     

On the onset of fracture as a silicon-based polymer converts into the ceramic phase

Silicon-based polymers evolve into refractory ceramics when heated gradually up to ~1000°C. The conversion is accompanied by the loss of gaseous species, and by a two-fold increase in density. The shrinkage can produce microcracks if the heating rate is too high, or if the specimen is too thick. This communication builds on earlier work whereby the measurement of gas evolution, and its relationship with viscous flow, are related to the onset of fracture in disk-shaped green (polymer) samples. The onset is determined as a function of the thickness of the disks, and of the heating rate. The results are presented in the form of a processing map. The overlay with gas evolution, and strain-rate measurements, suggest that fracture initiates with the release of hydrogen and methane, starting at temperatures near 750°C.     

High surface area methyltriethoxysilane-derived aerogels by ambient pressure drying

In this paper we report the synthesis of methyltriethoxysilane (MTES) based aerogels by non-supercritical/ambient pressure drying. The alcogels have been aged in different concentrations of silane precursor solutions before drying and aerogels with low density and high porosity were obtained. The 60% vol silane aged aerogel shows a surface area of 416 m²/g with a pore volume of 0.99 cm³/g and a maximum surface area of 727 m²/g was obtained for 80% vol silane aged aerogel. The non-silane aged sample possess a surface area of 471 m²/g with a total pore volume of 0.83 cm³/g. The aerogels show broad pore-size distribution. The FT-IR studies reveal the retention of Si–C bond in the network and the formation of a hydrophobic gel. The ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS-NMR) studies were also employed to characterize the local environment around the silicon atoms and to obtain information on the condensation degree of the gel network. By varying the hydrolysis pH, highly flexible aerogels have also been successfully prepared. The porosity studies on the flexible aerogels are also presented here.     

Gas Sensing Behavior of Mesoporous SiOC Glasses

In this study, for the first time, we report the gas sensing behavior of aerogel‐derived silicon oxycarbide (SiOC) glasses. The SiOC glass pyrolyzed at 1400°C has specific surface area of 150 m²/g with pore size in the 2–20 nm range. SiOC sensor shows good response to 5 ppm NO₂ at 300°C. NO₂ response completely disappears at 400°C, and from this temperature SiOC sensor starts respond to H₂. The optimum sensitivity for H₂ is obtained at 500°C. SiOC sensor is very selective; it is not sensitive to other gases such as acetone vapor or CO, even at high concentrations.     

Vickers Crack Nucleation of Glass Sheets Coated by Thin Silica Gel Layers

SiO₂ gel layers of different thickness have been deposited on glass sheets by dipping from gelling solutions with different concentrations of silicon alkoxide. Some mechanical properties of the coated samples such as hardness (H), elastic modulus (E), crack formation tendency, and tensile stress in the film (σ) have been measured by microindentation techniques and slide deflection. SiO₂ films deposited from more dilute solutions had lower H, E, and σ values, accounting for the observed higher resistance to crack nucleation in comparison with that found for samples prepared from more concentrated solutions.     

Polymer-derived ceramic molten metal filters

Journal of Materials Science - This paper reports the synthesis and the performance of polymer-derived ceramic filters for molten metal filtration. Two different filter types were studied: foam...     

Structural evolutions from polycarbosilane to SiC ceramic

The pyrolysis process of a polycarbosilane into a microcrystalline silicon carbide ceramic has been followed up to 1700 ° C mainly by means of solid state²⁹Si and¹³C nuclear magnetic resonance, transmission electron microscopy and X-ray diffraction analysis. A structural model has been proposed for the amorphous silicon carbide phase that is formed during the pyrolysis process. The ceramic obtained at high temperature is formed by a mixture of -SiC and -SiC; however, some difficulties in the identification of the crystalline phases have been pointed out.     

Crystallization Behavior of Novel Silicon Boron Oxycarbide Glasses

Homogeneous silicon boron oxycarbide (Si-B-O-C) glasses based on SiO _x C_{4– x} and BO _y C_{3– y} mixed environments were obtained by pyrolysis under inert atmosphere of sol–gel-derived precursors. Their high-temperature structural evolution from 1000° to 1500°C was followed using XRD, ²⁹Si and ¹¹B MAS NMR, and chemical analysis and compared with the behavior of the parent boron-free Si-O-C glasses. The XRD study revealed that, for the Si-O-C and the Si-B-O-C systems, high-temperature annealing led to the crystallization of nanosized β-SiC into an amorphous SiO₂-based matrix. NMR analysis suggested that the β-SiC crystallization occurred with a consumption of the mixed silicon and boron oxycarbide units. Finally, by comparing the behavior of the Si-O-C and Si-B-O-C glasses, it was shown that the presence of boron increased the crystallization kinetics of β-SiC.     

Closed porosity ceramics and glasses

In the last three decades, considerable effort has been devoted to obtain both open and closed porosity ceramics & glasses in order to benefit from unique combination of properties such as mechanical strength, thermal and chemical stability at low-relative density. Most of these investigations were directed to the production and the analysis of the properties for open porosity materials, and regrettably quite a few compositions and manufacturing methods were documented for closed porosity ceramics & glasses in the scientific literature so far. This review focuses on the processing strategies, the properties and the applications of closed porosity ceramics & glasses with total porosity higher than 25%. The ones below such level are intentionally left out and the paper is set out to demonstrate the porous components with deliberately generated closed pores/cells. The processing strategies are categorized into five different groups, namely sacrificial templating, high-temperature bonding of hollow structures, casting, direct foaming, and emulsions. The principles underlying these methods are given, with particular emphasis on the critical issues that affect the pore characteristics, mechanical, thermal and electrical properties of the produced components.     

Surface Damage Resistance of Gel-Derived Oxycarbide Glasses: Hardness, Toughness, and Scratchability

Gel-derived oxycarbide glasses have atomic network structures similar to that of vitreous silica glass but with carbon-rich regions consisting of CSi₄ tetrahedra and C–Si–O bonds finely dispersed in the glass. Therefore, oxycarbide glasses exhibit the so-called "anomalous" hardness behavior, similar to silica-rich glasses, with a substantial densification–strain component beneath the indenter. However, the role of carbon is twofold: on the one hand, the covalently bonded carbon atoms slightly affect the behavior, similar to the way network modifiers affect the behavior of silicate glasses, and favor a "normal" indentation behavior; and on the other hand, the free carbon, forming turbostratic graphite domains, provides easy crack initiation sites and low-energy fracture paths. Almost concentric shear steps and microcracks, which follow the turbostratic graphite domains, are observed after indentation. The ultimate coalescence of the microcracks produces Hertzian-type cone cracks.