Browsing by Author "Pascual, Luis"

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    Alumina and zirconia as inhibitors of crystallization during sintering of borosilicate glasses
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Pascual, M. Jesus; Pascual, Luis; Durán, Alicia; Wange, Peter; Rüssel, Christian
    The glass composition (in mol%) 16 Na2O ∙ 24 B2O3 ∙ 60 SiO2 has suitable viscosity and dilatometric properties to be used as sealing material for the gas manifolds of molten carbonate fuel cells (MCFC). Nevertheless, quartz crystallization takes place during sintering of the borosilicate glass powder at temperatures between 600 and 700 °C. The quartz crystallization leads to the formation of pores and a variation in the thermal expansion coefficient; both giving rise to defects and tensions in the seal. In this work, the possibility of inhibition of quartz crystallization by substituting AI2O3 or ZrO2 for 2 or 5 mol% of SiO2 is described. The thermal properties and the viscosity-temperature curve of the new compositions, especially those containing alumina do not differ much from the original glass.
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    Chemical composition of Medieval stained glasses from the Cathedral of León (Spain)
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Valle, Francisco José; Ortega, Pilar; Pascual, Luis; Carmona, Noemi; Fernández Navarro, José Maria
    A selection of glass samples from the stained glass windows of León Cathedral (Spain) dating from Medieval and Renaissance times has been chemically characterized. The techniques used for this characterization were X-ray diffraction, inductively coupled plasma. X-ray fluorescence and flame photometry. The analyses indicate that the samples are potash-lime glass, with the Medieval glass having a low alumina concentration. This fact has given rise to its deterioration throughout history due to environmental conditions.
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    Nitridation of silicate and borosilicate glasses prepared by sol-gel process
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Villegas, Maria Angeles; Pascual, Luis; Fernández Navarro, José María
    Samples of the systems R2O-SiO2 (R = lithium, sodium, potassium), B2O3-SiO2 and Na2O-B2O3-SiO2 were prepared by the sol-gel route. After gelation and drying, heat treatments under ammonia atmosphere were carried out up to 1100 °C. The whole nitrogen content (in wt%) of nitrided gels was determined with a Leco equipment. The maximum nitrogen content (in wt%) in lithium silicate gels was 12.6; in binary borosilicate gels 5.0 and in sodium borosilicate gels 7.6. The existence of silicon oxynitride (Si2ON) in some samples treated under ammonia has been demonstrated by X-ray diffraction. The nature and amount of the alkaline oxides play an important role in nitridation. The nitrogen percentage content incorporated seems to depend on the whole content of crystalline phases formed during thermal treatment, on the devitrification and on the phase separation of the samples. In borosilicate glasses a higher nitrogen incorporation with increasing B2O3 content was observed. In these samples, devitrification and phase separation, respectively, also affected nitridation.
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    Phase separation and crystallization of glasses in the system R2O-B2O3-SiO2 with R = Li, K, Na - Influence on thermal properties
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Pascual, M. Jesus; Pascual, Luis; Durán, Alicia
    The dilatometric properties (α, ϑ, Tg) and viscosity of glasses within the systems Li2O-K2O-B2O3-SiO2 and Na2O-B2O3-SiO2 with constant ratios B2O3/R2O =1.5 and 1 (in mol%) were characterized. These compositions have suitable thermal properties to be used as sealants for molten carbonate fuel cells (MCFC). The changes in dilatometric properties and viscosity due to phase separation and crystallization processes were measured along the different stages of a thermal treatment simulating the startup and working conditions of the MCFC. Thus, the thermal and mechanical behaviour of the studied glasses can be evaluated for the whole thermal cycle of the cell. Transmission electron microscopy (TEM) was used to analyse the development of microstructure during the thermal treatment and X-ray diffraction (XRD) to determine the appearing crystalline phases.
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    Preparation and properties of nitrided phosphate glasses
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Pascual, Luis; Durán, Alicia
    Nitrided phosphate glasses with the composition (in mol%) of 25 R2O ∙ 20 BaO ∙ 3 Al2O3 ∙ 52 P2O5 (R = lithium, sodium, potassium) have been prepared by remelting at 650 to 800 °C in a flowing dry ammonia atmosphere. The chemical durability of the glasses increases strongly with the nitrogen addition, as well as the glass transition and dilatometric softening temperatures and microhardness; whereas the thermal expansion coefficient diminishes with rising nitrogen contents. The kinetics of nitridation and the structural changes produced by nitrogen incorporation are also studied in these glasses.