Regina Monteiro

Two glass compositions were prepared from the system SiO2-Li2O-K2O-ZrO2-P2O5 with different SiO2/Li2O ratio (2.39 and 3.39) and the crystallization behavior was investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallization kinetic parameters (activation energy of crystallization and Avrami exponent) were evaluated by different methods from the data obtained by DTA performed at different heating rates. For both glasses, two exothermic peaks were observed in the DTA curves, and the crystallization peak temperatures increased with SiO2/Li2O ratio. XRD analysis revealed that the first peak corresponds to the crystallization of lithium metasilicate (Li2SiO3) and the second to the formation of lithium disilicate (Li2Si2O5). After heating the glasses at a temperature above the second crystallization peak (900 °C), both Li2Si2O5 and Li2SiO3 were found in samples having the lowest SiO2/Li2O ratio, whereas no Li2SiO3 was detected in samples with the highest SiO2/Li2O ratio. For both glasses, the value obtained by different methods for the activation energy of crystallization was in the range of 225-275 kJ mol-1 for the first exothermic peak and in the range of 425-500 kJ mol-1 for the second peak. The estimated Avrami exponent was close to 1 for the first exothermic peak, indicating surface crystallization, and close to 3 for the second exothermic peak, suggesting volume crystallization. This was confirmed by the morphological study made by SEM that showed needle-like crystals in the microstructure of samples with lithium metasilicate and granular crystals in the microstructure of samples having lithium disilicate. © 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

A range of CaO-Al2O3-SiO2 glasses have been prepared by fusion of pure starting materials in platinum crucibles. Compositions containing large amounts on network formers, Al2O3 and SiO2, are difficult to crystallize. If the amount of network former is reduced, glasses will self-nucleate and crystallize more readily, but the products of crystallization tend to react with water. This conflict has been partly resolved by adding MgO and ZnO and tailoring compositions so as to produce a phase, variously designated "Q" or "pleochroite", ideally Ca20Al32-2 vMg vSivO68, with v close to 4. Pleochroite crystallizes with a typically fibrous morphology. Preliminary experiments on fragments and melt-cast glass rods indicate that these compositions can be heat treated without deformation to yield highly crystalline, transparent ceramics. © 1989 Chapman and Hall Ltd.

The crystallization of a borosilicate glass, when compacts of powdered glass were sintered under various conditions, was investigated by dilatometric and XRD analysis. The dilatometry results from non-isothermal sintering experiments until 800°C, performed at different heating rates (1, 2, 5, 8 and 10°C/min), revealed that the compacts started to shrink above ∼ 600°C and that the shrinkage decreased with the increase of the heating rate for temperatures up to ∼750°C. Above this temperature, and specifically when the samples were heated at heating rates < 5°C /min, the shrinkage was hindered, while samples heated at heating rates ≥ 5°C/min showed continuous shrinkage. XRD results showed that the formation of cristobalite had occurred during the sintering at the lowest heating rates and therefore, the presence of this crystalline phase was affecting the shrinkage of the compacts, inhibiting further sintering of the glass. The crystallization of the glass when sintered at a temperature in the range 700-850°C and hold at the selected temperature during various times was also analysed. XRD results showed that both cristobalite and quartz were present in glass compacts sintered under particular conditions (for example, after heating during 24h at 725°C and 765°C). Quartz dissolution took place when the glass samples were sintered at 850°C. At this temperature and whatever the sintering time, cristobalite was the only crystalline phase present in the sintered compacts.

The crystallization kinetics of a glass with a molar composition 40BaO-20ZnO-30B2O3-10SiO2 was investigated. The kinetic parameters, activation energy for crystallization (Ec) and Avrami exponent (n), were evaluated under non-isothermal conditions using the results obtained by differential thermal analysis (DTA) performed at different heating rates. DTA curves exhibited two overlapping exothermic peaks associated with the crystallization of the glass. Barium borate (BaB4O 7) was the first crystalline phase to be formed and it was followed by the formation of barium zinc silicate (BaZnSiO4), as identified by XRD. For the first exothermic peak, when the fraction of crystallization (χ) increased from 0.1 to 0.9, the local activation energy (E c(χ)) decreased from 700 to 500 kJ/mol, while for the second exothermic peak, Ec(χ) slightly increased from 490 to 570 kJ/mol. For the range of 0.1 < χ < 0.9, the local Avrami exponent (n(χ)) increased from ∼1 to 1.4 for the first exothermic peak and it decreased from ∼1.7 to 1.4 for the second exothermic peak. Observation by SEM of the microstructure of sintered glass samples revealed that crystallization started at the surface of glass particles, with growth of lamellar crystallites, that together with some quasi-spherical nano-sized crystallites progressed towards the inside of the glass at the highest sintering temperatures. The change of the local activation energy with the fraction of crystallization suggested that a multi-step kinetic reaction took place during sintering and crystallization of the glass. © 2013 Elsevier B.V. All rights reserved.

A transparent Eu3 +-doped lithium aluminophosphate glass was prepared by melt-quenching technique. The thermal behavior of the glass was investigated by differential thermal analysis (DTA), the structure was studied by X-ray diffraction (XRD) and the morphology was observed by optical polarization microscopy and scanning electron microscopy (SEM). The activation energy of glass transition and the activation energy of crystallization and Avrami exponent have been evaluated under non-isothermal conditions from the data obtained by DTA at different heating rates. DTA curves exhibited an endothermic peak associated with the glass transition and two exothermic peaks. The mean value calculated for the activation energy of glass transition was 545 kJ mol- 1. The activation energy of crystallization was   400 kJ mol- 1 for the first exothermic peak and   170 kJ mol- 1 for the second peak. The Avrami exponent was   1 for both peaks indicating surface crystallization. XRD results showed that the main crystalline phase, aluminum metaphosphate, Al(PO3)3, and aluminum phosphate, AlPO4, were formed together with lithium barium phosphate, Li 3Ba(PO3)7, during the first exothermic peak and together with barium pyrophosphate, Ba2P2O7, during the second peak. Morphological study of heat-treated glass samples revealed microstructural features that confirmed a surface crystallization process. © 2014 Elsevier B.V.

Glass samples with a molar composition (64+x)ZnO–(16−x)B2O3–20SiO2, where x=0 or 1, were successfully synthesized using a melt-quenching technique. Based on differential thermal analysis data, the produced glass samples were submitted to controlled heat-treatments at selected temperatures (610, 615 and 620 °C) during various times ranging from 8 to 30 h. The crystallization of willemite (Zn2SiO4) within the glass matrix was confirmed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Under specific heat-treatment conditions, transparent nanocomposite glass-ceramics were obtained, as confirmed by UV–vis spectroscopy. The influence of temperature, holding time and glass composition on crystal growth was investigated. The mean crystallite size was determined by image analysis on SEM micrographs. The results indicated an increase on the crystallite size and density with time and temperature. The change of crystallite size with time for the heat-treatments at 615 and 620 °C depended on the glass composition. Under fixed heat-treatment conditions, the crystallite density was comparatively higher for the glass composition with higher ZnO content. © 2016 Elsevier B.V.

The characterization of the bottom ashes produced by two Portuguese municipal solid waste incinerators (MSWI) was performed with the aim of assessing the feasibility of using this waste as raw material in the production of glass that can be further processed as glass-ceramics for application in construction. Density and particle size distribution measurements were carried out for physical characterization. Chemical characterization revealed that SiO2, a network glass former oxide, was present in a relatively high content (52-58 wt%), indicating the suitability for this waste to be employed in the development of vitreous materials. CaO, Na2O and K2O, which act as fluxing agents, were present in various amounts (2-17 wt%) together with several other oxides normally present in ceramic and glass raw materials. Mineralogical characterization revealed that the main crystalline phases were quartz (SiO2) and calcite (CaCO3) and that minor amounts of different alkaline and alkaline-earth aluminosilicate phases were also present. Thermal characterization showed that the decomposition of the different compounds occurred up to 1100 °C and that total weight loss was <10 wt%. Heating both bottom ashes at 1400 °C for 2 h resulted in a melt with suitable viscosity to be poured into a mould, and homogeneous black-coloured glasses with a smooth shiny surface were obtained after cooling. The vitrified bottom ashes were totally amorphous as confirmed by X-ray diffraction. The results from the present experimental work indicate that the examined bottom ashes can be a potential material to melt and to obtain a glass that can be further processed as glass-ceramics to be applied in construction. © 2007 Elsevier Ltd. All rights reserved.

The changes occurring during the heating of a borosilicate glass have been investigated by differential thermal analysis, dilatometric analysis and thermomechanical analysis. The thermal properties of this glass, such as glass transition temperature, dilatometric softening temperature and linear thermal expansion coefficient, have been determined. Viscosity measurements in the temperature range 898-1048 K were performed in a thermomechanical analyser equipped with a penetration attachment for isothermal measurement, and from the temperature dependence of viscosity a value of 290 kJ mol-1 was obtained for the activation energy for viscous flow. Devitrification of the glass was observed, specifically in finely powdered glass samples, where the precipitation of cristobalite was identified by X-ray diffraction. Glass powder compacts sintered by viscous flow and cristobalite precipitation reduced strongly the shrinkage rate. © 2001 Elsevier Science B.V.