Luis Pascual

Sintering process of glasses in the system Na2O–B2O3–SiO2

Abstract In this work, the sintering process of different glasses in the system Na 2 O–B 2 O 3 –SiO 2 has been studied. The studied compositions are suitable for sealing the gas manifolds of molten carbonate fuel cells. Sealing glasses are usually applied on the surfaces to be sealed using powder glass mixed with an organic medium. The agglomerant elimination and the sintering of the glass powder take place during the thermal treatment. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and viscosity on sintering. The control of these parameters allows optimization of the sealing conditions of the glasses. Dense materials have been characterized as well as the sintering mechanism. Two processes take place during the thermal treatment: the sintering process and the quartz crystallisation. Both processes act in opposite directions on the glass densification. Crystallisation is the dominant process at long times and high temperatures. The viscous flow Scherer model has been adequately applied to the experimental data.

Nitridation of glasses in the system R2O-MO-P2O5

Abstract Nitrided glasses with the composition 25R 2 O.20MO.55P 2 O 5 (R = Li, Na, K, M = Ba, Pb) were prepared by treatment in an anhydrous ammonia atmosphere between 650 and 800 °C for periods of 3 to 40 h. The nitrogen content of the glasses increases linearly with the treatment temperature; as a function of time, the N 2 content tends toward an upper limit, which depends on the temperature. The nitrogen introduction into these glasses causes an increase in the transformation and softening temperatures, microhardness and toughness, while the expansion coefficient decreases slightly with the nitrogen content. However, the rise of the hydrolytic resistance of the glasses by near two orders of magnitude is the most important change caused by nitrogen introduction. The structural study by IR spectroscopic and the analysis of the glass weight losses during the thermal treatment reveal that nitrogen replaces the bridging oxygens in the phosphate network.

Sintering behaviour of composite materials borosilicate glass-ZrO2 fibre composite materials

The characteristics and properties of mixtures of a borosilicate glass and ZrO2 fibres obtained from compacts isothermally sintered are presented. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and the glass matrix viscosity on sintering. The fundamental developed aspects are the sintering kinetics, the dense material characterisation and the study of the sintering mechanisms. The composite materials are suitable for sealing molten carbonate fuel cells (MCFC). # 2002 Elsevier Science Ltd. All rights reserved.

Transformation of tricalcium phosphate into apatite by ammonia treatment

Tricalcium phosphate, Ca3(PO4)2, was treated in dry ammonia at temperatures between 1100°C and 1300°C for different times of up to 16 h. X-Ray diffraction and 31P-NMR studies showed a phase transformation into apatite when treated at temperatures between 1100 and 1200°C. Nitrogen was detected in the treated samples with a maximum content of 2.9 wt%. 31P-NMR-analysis indicated no direct bonding of nitrogen to phosphorous. The presence of graphite during ammonia treatment was required for nitrogen incorporation. Infrared spectra showed increasing intensities of four new bands at 3250, 2016, 1966 and 700 cm−1 attributed to the occurrence of cyanamide ions. Taking into account the results of XRD, 31P-NMR and FTIR spectrometry, it is concluded that tricalcium phosphate transforms into calcium deficient cyanamidapatite by ammonia treatments according to the reaction: 3Ca3(PO4)2 + H2CN2 → Ca9(PO4)5(HPO4)(HCN2).

Eriochrome cyanine doped sol-gel coatings. Optical behavior against pH

Sol-gel silica coatings doped with eriochrome cyanine (EC) have been prepared upon soda lime glass substrates by dip-coating. Sols were obtained starting from a silicon alkoxide hydrolysed in acid medium. EC was added to the silica sol dissolved in water and methyl alcohol to reach 1% of the total weight. Dried coatings showed pH-sensitivity (change of optical absorption and luminescence) when dipped into liquids as well as when exposed to aqueous gases at different pH. On this basis, EC doped coatings could be used as optical sensors, for instance, in analytical probes. Coatings have been demonstrated to be stable enough to UV radiation, at least at handling temperature to be used for optical sensors in common analytical instrumentation. Additional tests pointed out the behaviour of coatings to different chemical media at temperatures below 100°C. Adherence of coatings was evaluated by scratching and peeling, as well as dipping the samples into a cleaner solution submitted to ultrasounds.

The use of blast furnace slag and derived materials in the vitrification of electric arc furnace dust

The electric arc furnace (EAF) dust was added to blast furnace granulated slag or a gel produced by dissolving slag in HNO3/EtOH followed by alkaline hydrolysis and gelification, and then it was vitrified. The proportion of EAF dust used was between 5 and 30 wt pct. The physicochemical properties of the glasses produced were studied by the X-ray powder diffraction technique, differential thermal analysis, and transmission electron microscopy (TEM). Thermal properties (dilatometric softening point, glass transition temperature, and thermal expansion coefficient) were determined. Finally, analyses were performed on leachability and ecotoxicity. The results suggest that the use of either slag or slag-derived gel produces a borosilicate glass with thermal properties similar to those of conventional soda-lime-silicate glass.

Phase evolution of KLaF4 nanocrystals and their effects on the photoluminescence of Nd3+ doped transparent oxyfluoride glass-ceramics

Transparent oxyfluoride glass-ceramics (GCs) were prepared starting from the glass composition 70SiO2–7Al2O3–16K2O–7LaF3 (mol%) doped with four different Nd3+ concentrations (0.1–2 mol%). The glass-ceramics were prepared by heat treatment above the corresponding glass transition temperatures (Tg + 10–100 °C). Depending on the Nd3+ concentration, cubic (α-phase) and hexagonal (β-phase) KLaF4 nanocrystals (NCs) precipitated from the starting glass. By X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM), the crystallization of the cubic structure was observed for all compositions, while β-KLaF4 nanocrystals were mostly detected for the glass-ceramic samples doped with 0.5 mol% NdF3. This noteworthy result is confirmed by site-selective laser spectroscopy. The presence of α-KLaF4 and β-KLaF4 crystalline phases is unambiguously identified from the emission and excitation spectra and lifetime measurements of the 4F3/2 state of Nd3+ ions. The spectroscopic results confirm those obtained with XRD and HR-TEM techniques: α-KLaF4 NCs are found to be present for all NdF3 concentrations whereas β-KLaF4 NCs are predominant in the glass-ceramic samples doped with 0.5 mol%.