Transparent glasses obtained by unconventional sintering

Glasses are conventional manufactured by melting natural sand or silicon compounds at high temperatures. The traditional melting method requires high energy input to reach the melting temperature of precursors. Conventional solid-state sintering of powders is a valid alternative to reduce the processing temperature; however it requires time consuming thermal treatment to obtain densification. With the aim of reducing the environmental impact, energy, temperature and cost, the thesis project carried out in collaboration with the University of Lyon 1 aims to develop new ways of producing glass by employing unconventional sintering approaches. During the experimental work, three different strategies are explored: synthesis of nanoparticles at a low glass transition temperature, cold sintering process on commercial sub-micrometer particles borosilicate glass, the Spark Plasma sintering process on nanostructured silica nanoparticles. - Synthesis of nanoparticles with a low glass transition temperature: using a modifying Stober synthesis, including tetraethyl orthosilicate, as silicon precursor, and boron, calcium and sodium salts or alkoxides as precursors of modifier oxides, capable of lowering the glass transition temperature. Several parameters were investigated such as the type of precursor, the molar ratio of the oxide precursors to TEOS, the concentration of the solution and the calcination temperature. The results showed that particles with a diameter of about 150 nm were obtained, with a strong presence of organic residue. The glass transition temperature was about 950°C, likely due to the lack of successful incorporation of modifier oxides into the silica network. - Cold Sintering Process: a basic sodium hydroxide solution was selected as a liquid phase to promote densification at low temperatures. The main parameters investigated were the following: temperature, pressure, liquid/solid molar ratio and residence time of the powder in the mould. Density measurements by means of the Archimedes test and pycnometer revealed the following best parameter combinations: Pressure P=300MPa, temperature T=250°C, % liquid/mass =40, residence time t in the mould = 30 min; Pressure P=300MPa, temperature T=200°C, % liquid/mass =20, dwell time t in the mould = 120 min, The lack of an unit value of relative density is an indication of sintering having only partially taken place, which is confirmed by the high porosity visible through SEM characterisation analysis, and the lack of transparency. - Spark Plasma Sintering: the silicon oxide particles pre-treated at 900°C (in order to remove water and carbonate groups absorbed) were subjected to the sintering process by Spark Plasma. During the operations, the pressure and dwell time values were chosen and kept fixed (2GPa and 3 minutes respectively), allowing the effect of the key parameter of temperature to be assessed. Characterisation of the samples produced, carried out using SEM and Raman spectroscopy, revealed the following: * Crystallization occurs at 2GPa for temperature above 1000°C * Densification and transparency occur for a temperature of 800°C. In-situ measurements of the process combined with Archimedes density test show, however, that the sintering process is not complete. *An initial sintering step takes place at temperatures around 500°C; one could develop the idea of future experiments by decreasing the temperature, so as to decrease the energy consumption required to achieve densification. This thesis work shows that it is indeed possible to produce transparent glass using unconventional sintering techniques. From these preliminary investigations, it's possibile to confirm an important role of the particles dimension for the obtainment of dense and transparent glass components.