Regina Monteiro

With a 4f6 electronic configuration, europium ions in the trivalent charge state are known to be efficient activators in wide band gap matrices. Embedded in the aluminophosphate (Li2O-BaO-Al 2O3-La2O3-P2O 5) glasses the optically activated Eu3+ ions lead to intense room temperature orange/red luminescence with 16-23 Cd/m2 by using ultraviolet pumping. The as-prepared and heat treated europium doped glasses for temperatures below and above Tg were studied by room temperature Raman spectroscopy, absorption, photoluminescence excitation, temperature dependent and time dependent photoluminescence. When the samples are excited by 325 nm wavelength photons, an enhancement of the red luminescence intensity by ca. one order of magnitude was found to occur for temperatures between 14 K and 350 K, for all the doped glasses. On the other hand, by using resonant excitation on the 5L6 Eu3+ excited state (λexc 390 nm) the ion emission intensity was found to be nearly constant for temperatures up to 500 K. For higher temperatures a steeper decrease of the luminescence intensity occurs due to non-radiative competitive channels described by activation energies of ca. 235 meV and 450 meV by using 325 and 390 nm wavelength photons as excitation, respectively. The lifetime of the 5D0 level in these glasses is ca. 2.93 ms. A discussion of the thermal population and de-excitation mechanisms is performed. © 2013 Elsevier B.V.

Coal fly ashes have been vitrified by melting with Na2O and CaO as fluxing additives. Adequate heat treatments on the fly ash derived glass produced attractive dark green glass-ceramics. These glass-ceramics exhibited fine-grained microstructures consisting of esseneite and nepheline crystals, with average size below 200 nm, homogeneously dispersed in a residual glassy matrix. Several properties, such as density, thermal expansion coefficient, bending strength, hardness and brittleness index were determined and the correlation microstructure-properties is discussed. The results suggest that these coal ash-based glass-ceramics have potential applications as structural materials or as cladding materials.

Different glasses based on the ZnO-B2O3-SiO2 system, with a ZnO content ≥ 60 mol% and a fixed SiO2 content (20 mol%), were synthesised using a melt-quenching method. Glass samples with zinc oxide concentrations as high as 65 mol% were prepared successfully without deteriorating the glass-forming ability. The glass samples were submitted to controlled heat-treatments, and the effect of the ZnO/B2O3 molar ratio on the formation of crystalline phases within the glass matrix was investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). DTA results indicated that the glass transition and the onset crystallization temperatures of the studied glasses tended to increase with the increment on the ZnO/B2O3 ratio. XRD data showed that the zinc binary crystalline phases, willemite (Zn2SiO4), a zinc orthosilicate, and zinc borate (Zn3B2O6) could be present in the crystallized glasses, depending on the heat-treatment conditions. The formation of willemite was promoted by an increasing ZnO/B2O3 ratio. Microstructural observations performed by SEM indicated that under controlled experimental parameters (glass composition, heat-treatment temperature and time) the precipitation of nanocrystals within the glass matrix can be achieved, resulting in transparent and translucent willemite glass-ceramics. © 2016 Elsevier B.V. All rights reserved.

The glass transition and crystallization kinetics of a glass with a molar composition 60BaO-30B2O3-10SiO2were investigated by differential scanning calorimetry (DSC) under non-isothermal conditions. DSC curves exhibited an endothermic peak associated with the glass transition and two partially overlapped exothermic peaks associated with the crystallization of the glass. The dependence of the glass transition temperature (Tg) and of the maximum crystallization temperature (Tp) on the heating rate was used to determine the activation energy associated with the glass transition (Eg), the activation energy for crystallization (Ec), and the Avrami exponent (n). X-ray diffraction (XRD) revealed that barium borate (β-BaB2O4) was the first crystalline phase to be formed followed by the formation of barium silicate (Ba5Si8O21). The variations of activation energy for crystallization and of Avrami exponent with the fraction of crystallization (χ) were also examined. When the crystallization fraction (χ) increased from 0.1 to 0.9, the value of local activation energy (Ec(χ)) decreased from 554 to 458 kJ/mol for the first exothermic peak and from 1104 to 831 kJ/mol for the second exothermic peak. The value determined for the Avrami exponent was near 2 indicating a similar one-dimensional crystallization mechanism for both crystalline phases. This was confirmed by the morphological studies performed by scanning electron microscopy (SEM) on glass samples heat-treated at the first and at the second crystallization temperatures. © 2014 AIP Publishing LLC.

A homogeneous black coloured glass was obtained by melting the bottom ashes produced by a municipal solid waste incinerator at 1300°C for 2 h without any chemical additives. Based on thermal analysis data glass-ceramics were produced by heat treating the glass, doped with additional TiO2 as a nucleating agent, at temperatures between 870 and 1000°C. The crystalline phases precipitated during the heat treatments were identified by powder XRD and the microstructures were examined using SEM. After a heat treatment at 900°C for 2 h, the glass was converted into a fine grained glass-ceramic with uniform microstructure. The major crystalline phases precipitated in the glass-ceramics were augite (Ca(Mg,Fe)Si2O6), gehlenite (Ca2Al2SiO7) and clinopyroxene (Ca(Ti,Mg,Al)(Si,Al)2O6). Preliminary evaluation of the mechanical and chemical properties of the bottom ash glasses and glass-ceramics suggest that they have potential to compete with existing natural and commercial outdoor cladding materials.

The kinetics of crystallization of transparent and conductive high preferential c-axis oriented Aldoped ZnO thin films on Corning 1737 glass substrate from amorphous sol-gel precursor prepared using zinc acetate and aluminum chloride as cations source, 2-methoxiethanol as solvent and monoethanolamine as sol stabilizer has been investigated. The effect of preheating temperature on the values of the kinetic parameters and crystallization mechanism is discussed. Some data concerning the microstructure, the electrical and optical properties of the thin films are presented.

This paper is mainly focused on the characterisation of a glass material (GM) obtained from the thermal treatment of a bottom ash (BA) produced at the Municipal Solid Waste (MSW) incineration plant of Valorsul. By melting the BA at 1400°C during 2 hours, and without using any chemical additives, a homogeneous black-coloured glass was obtained. The thermal and mechanical properties of this glass were characterised. The thermal expansion coefficient, measured by dilatometry, was 9-10 × 10-6 per °C and the modulus of rupture, determined by four-point bending test, was 75±6 MPa, which are similar values to those exhibited by commercial soda-lime-silica glasses used in structural applications. The chemical and the ecotoxicological leaching behaviour of the GM were also analysed. The GM was submitted to a leaching procedure composed of 15 sequential extraction cycles. A liquid/solid (L/S) ratio of 2 1/kg was applied in each cycle. The leachates were filtered through a membrane of PTFE (porosity: 0.45 μm). The filtered leachates were characterised for different chemical parameters and for an ecotoxicological indicator (bacterium Vibrio fischeri). The GM was also submitted to a microwave acidic digestion for the assessment of the total metal content. The crude BA was also submitted to the same experimental procedures. The GM showed levels of chemical emission and ecotoxicity for V. fischeri much lower than those determined for the crude BA. Similar characterisation studies will be pursued with the glass-ceramics produced by adequate thermal treatment of the glass, in order to investigate the effect of the crystallization on the final properties.

In this work, the mechanical characterization of ceramic products processed from red clay with different amounts of added coal fly ash was investigated. Coal fly ash produced by power plants is a waste material that constitutes an alternative source of minerals for the production of traditional building ceramics, as it is a complex mixture of several oxides such as SiO2, Al2O3, CaO, Fe2O3, Na2O, TiO2, which are usually present in the composition of such ceramics. A powder technology and firing route was followed for the processing of the clay and coal fly ash based ceramics. Different proportions of waste (10, 25 and 50%, by weight) were added to red clay, and then the mixed powders were pressed to form compacts that were fired at a selected temperature in the range 850-1,150°C. The effects of waste content and of heating conditions on the microstructure and mechanical characteristics of the obtained materials were investigated. The density, porosity, water absorption, flexural strength, hardness and fracture toughness of the produced materials were evaluated. A comparison was made between the properties of the produced ceramics with those of traditional ceramic materials used in construction, e.g. floor or wall tiles, and it was observed that the clay based products with coal fly ash additions may be used in similar applications. © 2007 Springer Science+Business Media B.V.

A glass was made using bottom ash produced by a Portuguese municipal solid waste (MSW) incinerator. The bottom ash was the single batch material used in the formation of the glass, which was obtained through a conventional melt-quenching method. The glass was then converted to glass-ceramic for further recycling to construction materials. After submitting the glass samples to several heat treatments, between 820 and 1050°C and during different times, it was verified that the optimum heat treatment schedule for the ceramization of the glass was at 1000°C for 10h, as confirmed by microstructural observation and by X-ray diffraction. The major crystalline phases precipitated in the glass-ceramic were wollastonite (CaSiO3) and diopside (Ca(Mg,Al)(Si,Al)2O6). Microstructural analysis of the glass-ceramic revealed that the crystalline phases were present as dendrites and fiber-like structures that were homogeneously distributed in the material. The glassceramic showed good mechanical properties with a hardness of 5.6 MPa and a bending strength of 101 MPa. This material had a density of 2.8 gcm-3 and a thermal expansion coefficient of 9.10-6°C-1. The glass and the glass-ceramic showed an excellent chemical stability against leaching in acidic solution and in alkaline solution. In summary, both the glass and the glass-ceramic have good chemical and mechanical properties and can, therefore, be applied as construction materials.

The paper presents the properties of zinc oxide thin films deposited on glass substrate via dip-coating technique. Zinc acetate dehydrate, ethanol and monoethanol amine were used as starting materials and N2 gas was used as thermal annealing atmosphere for film crystallization. The effect of withdrawal speed on the crystalline structure, morphology, zinc and nitrogen chemical states, optical, electrical and gas-sensing properties of the thin films has been investigated using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical transmittance and photoreduction-ozone reoxidation data. © 2007 Elsevier B.V. All rights reserved.

The isoconversional methods (Friedman (FR), Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS)) were applied for evaluating the dependencies of the activation energy (E) on the mass loss (Δm) corresponding to the non-isothermal decomposition of two Zn acetate-based gel precursors for ZnO thin films whose preparation differs by the drying temperature of the liquid sol-precursor (125°C for sample A, and 150°C for sample B). Although both investigated samples exhibit similar decomposition steps, strong differences between E vs. Δm curves as well as among the characteristic parameters of the decomposition steps, directly evaluated from TG, DTG and DTA curves, were put in evidence. © 2007 Springer Science+Business Media LLC.

The work deals with optical and structural properties of aluminophosphate glasses from Li2O-BaO-Al2O3-La 2O3-P2O5 system containing Sm 3 + and Eu3 + ions. X-ray fluorescence (XRF) has been used to establish the elemental composition of these materials. The influence of Sm3 + and Eu3 + ions on the optical properties of these glasses has been investigated in relation with their structural characteristics. The optical behavior of these materials has been studied by ultra-violet-visible (UV-Vis) spectroscopy, revealing absorption maxima specific to the doping ions. Structural information via vibration modes was provided by Fourier Transform Infrared (FTIR) absorption spectra evidenced as POP symmetrical and asymmetrical stretching vibration modes, POP bend, PO 2- symmetrical and asymmetrical stretching vibration modes, PO stretching vibration mode and POH water absorbance. Raman spectra acquired by 514.5 nm laser excitation disclosed peaks specific to metaphosphate network. Information about the elemental compositional homogeneity of Sm 3 + and Eu3 +-containing glasses as well as about the defects of the doped-glasses is revealed by scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). Fluorescence spectroscopy measurements put in evidence important fluorescence peaks found at 596 nm and 643 nm for Sm 3 + ions in phosphate matrix and 611 nm and 700 nm in the case of Eu3 +-doped glass. © 2013 Elsevier B.V.

Cobalt oxide thin films were prepared by using the dipping sol-gel process. The films were deposited onto glass slides, starting from methanolic solution of cobalt acetate Co(CH3COO)2 · 4H2O. The effects of film thickness and sol concentration on structural and optical properties were studied. Structural analyses of Co3O4 films were performed by X-ray diffraction. The film thickness was varied by using different withdrawal speeds and the number of dipping-heating cycles. It was found that the grain size increases with the number of dipping N. The results point out to some compacting effect that increases with the number of dippings: the films exhibit direct and indirect optical transition, absorption coefficients are of the order of 104 cm-1, and upon annealing the absorption coefficient increases. © 2006 Elsevier B.V. All rights reserved.

The crystallization kinetics and phase transformation of a transparent Tb3+-doped lithium-aluminum phosphate glass, prepared by melt quenching, were investigated. The energy associated to the glass transition and the crystallization parameters (activation energy for crystallization and Avrami exponent) were evaluated by different methods using the experimental data obtained by differential thermal analysis performed at different heating rates. Using an isoconversional method to determine the change of the activation energy for crystallization with the fraction of crystallization, it was verified that with the increase in the fraction of crystallization from 0.1 to 0.9, the value of the activation energy decreased slightly from  370 to  310 kJ mol -1 and that the Avrami exponent varied from 0.8 to 1, suggesting a surface crystal growth mechanism. Observation of the microstructural evolution of heat-treated glass samples confirmed a surface crystallization process revealing spherulitic crystals constituted mainly by aluminum metaphosphate. © 2014 Springer Science+Business Media New York.

The present work investigates alumino-phosphate glasses from Li 2O-BaO-Al2O3-La2O3-P 2O5 system containing Sm3+ and Eu3+ ions, prepared by two different ways: a wet raw materials mixing route followed by evaporation and melt-quenching, and by remelting of shards. The linear thermal expansion coefficient measured by dilatometry is identical for both rare-earth-doped phosphate glasses. Comparatively to undoped phosphate glass the linear thermal expansion coefficient increases with 2 × 10-7 K-1 when dopants are added. The characteristic temperatures very slowly decrease but can be considered constant with atomic weight, atomic number and f electrons number of the doping ions in the case of Tg (vitreous transition temperature) and Tsr (high annealing temperature) but slowly increase in the case of Tir (low annealing temperature-strain point) and very slowly increase, being practically constant in the case of TD (dilatometric softening temperature). Comparatively to undoped phosphate glass the characteristic temperatures of Sm and Eu-doped glasses present lower values. The higher values of electrical conductance for both doped glasses, comparatively to usual soda-lime-silicate glass, indicate a slightly reduced stability against water. The viscosity measurements, showed a quasi-linear variation with temperature the mean square deviation (R 2) being ranged between 0.872% and 0.996%. The viscosity of doped glasses comparatively to the undoped one is lower at the same temperature. Thermogravimetric analysis did not show notable mass change for any of doped samples. DSC curves for both rare-earth-doped phosphate glasses, as bulk and powdered samples, showed Tg values in the range 435-450 C. Bulk samples exhibited a very weak exothermic peak at about 685 C, while powdered samples showed two weak exothermic peaks at about 555 C and 685 C due to devitrification of the glasses. Using designed melting and annealing programs, the doped glasses were improved regarding bubbles and cords content and strain elimination. © 2013 Elsevier B.V. All rights reserved.

Coal fly ashes produced by an extinguished power plant in the north of Portugal have been melted with addition of CaCO3 and Na2CO3 to obtain glasses. One of the formulated compositions was selected for further studies and it was possible to manufacture glass-ceramics by crystallizing the parent glass through adequate time-temperature schedules. The macroscopic appearance, microstructure, mechanical, thermal and chemical properties indicated that these materials are quite attractive for cladding applications, exhibiting in some cases better performances than the conventional ceramic tiles.

The effect of rigid inclusions on the densification during isothermal sintering of glass matrix composites was investigated. Mixtures containing borosilicate glass powder and 0, 5, 10 and 25 vol. % alumina (Al 2O3) particles were prepared and powder compacts isostatically pressed at 200 MPa have been used. The sintering behaviour of the samples heated at 800°C during various times was investigated through density measurement, axial and radial shrinkage measurements. The microstructure was analysed by SEM and the crystalline phases present in the sintered composites were identified by XRD. The relative density of the isothermally treated borosilicate glass-Al2O3 composites decreased significantly with the increase in Al2O3 content because the presence of rigid inclusions retarded the densification of the compacts. The borosilicate glass exhibited anisotropic shrinkage behaviour, showing a radial shrinkage higher than the axial shrinkage and isotropic shrinkage was favoured by Al2O3 additions. Sintered glass showed a dense microstructure with some spherical closed pores. The microstructure of composites with 5 vol. % Al2O3 revealed that most of the pores were filled by capillary flow of the glass. The microstructure of composites with higher Al2O3 additions showed dense areas together with interconnected pores, which appeared at the sites of large glass particles in the green compacts.

Barium zinc borosilicate glasses with a molar composition xBaO-(60-x)ZnO-30B 2O 3-10SiO 2, where x ranged from 0 to 60 mol%, were prepared using melt-quenching method. The effect of BaO substitution for ZnO on the sintering, crystallization, and dielectric characteristics has been investigated. The behavior of the studied barium zinc borosilicate glasses was mainly determined by the relative amount of the structural modifier oxides (BaO and ZnO) and the ionic size, and field strength of the modifying cations (Ba 2+, Zn 2+). Increased amounts of BaO decreased both glass transition temperature and crystallization temperature, while increasing the relative dielectric constant. Sintering occurred before crystallization for glasses where substitution of BaO for ZnO was up to 30 mol%, but for higher substitution levels, crystallization occurred during the sintering process hindering densification. © 2012 The American Ceramic Society.

Various metal oxide-silica nanocomposite films have been recently proposed as gas-sensitive materials. This paper presents results on cobalt oxide-SiO2 mesoporous nanocomposite thin films templated by a cationic surfactant. The films were deposited on glass substrate by dip-coating process, using [Co(CH3COO)2]·4H2O and tetraethoxysilane (TEOS) as starting materials. The effect of withdrawal speed, number of layers and thermal treatment on the crystalline structure, morphology, Co-doping states, optical, electrical and gas sensing properties of the thin films has been investigated using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical transmittance and room temperature photoreduction-oxidation data. © 2007 Elsevier B.V. All rights reserved.

This study deals with SiO2-P2O5 powders obtained by sol-gel process, starting from tetraethoxysilane (TEOS) as precursor for SiO2 and either triethylphosphate (TEP) or phosphoric acid (H3PO4) as precursors for P2O5. In the case of samples prepared with H3PO4, TG-DTA data showed an accentuated weight loss associated to an endothermic effect up to about 140 °C, specific for the evaporation of water and ethylic alcohol from structural pores, and also due to alkyl-amines evaporation. Sol-gel samples prepared with TEP exhibited different thermal effects, depending on the type of atmosphere used in the experiments, i.e. argon or air. XRD analysis revealed that annealed sol-gel samples prepared with H3PO4 showed specific peaks for silicophosphate compounds such as Si3(PO4)4, Si2P2O9, and SiP2O7. XRD results for annealed sol-gel samples prepared with TEP indicated mainly the presence of a vitreous (amorphous) phase, which could be correlated with SEM images. The presence of SiO2 in the sample might be expected. Thus, we have searched for any SiO2 polymorph possible to crystallize. Only potential peaks of cristobalite were identified but some of them are overlapping with peaks of other crystalline phosphates. SEM analysis indicated a decrease of the amount of crystalline phases with the increase in the annealing temperature. © 2009 Elsevier B.V. All rights reserved.

Borosilicate glass-alumina composites with (1 - x) Glass + x Al 2O 3 (x = 0, 5, 10, 25 vol.%) were prepared and the effect of Al 2O 3 addition on the structural, electrical and thermal characteristics was investigated. XRD patterns revealed the presence of cristobalite (SiO 2) in sintered borosilicate glass and that the addition of Al 2O 3 hinders cristobalite formation. This behavior is due to the diffusion of some Al 3+ ions from alumina to glass, which leads to changes in glass structure and composition as identified by SEM/EDS. Cristobalite was undetected in composites containing 10% Al 2O 3 that attained the lowest thermal expansion coefficient value (∼4.6 × 10 -6 °C -1). Conductivity (dc and ac) increased with the amount of Al 3+ ions present in the glass structure as modifiers and formers. Dielectric constant values, in the range 5.0-7.2, increased with Al 2O 3 addition and the values of loss tan δ (1.5-2.1 × 10 -2) indicate that these materials are good insulators. © 2012 Elsevier B.V. All rights reserved.

The crystallization of a commercial borosilicate glass powder has been studied in the temperature range 750-900°C. Crystal growth was investigated by high temperature XRD and cristobalite precipitation was identified. Glass devitrification exhibited a characteristic incubation period that decreased with increasing temperature: 25-30 min at 750°C, 9-12 min at 775°C, 5-10 min at 810°C, and 0-5 min at 840°C. Cristobalite is an unfavorable transformation product in terms of thermal expansion behavior. The precipitation of cristobalite in sintered glass compacts was confirmed by dilatometric analysis, where the increase in thermal expansion coefficient due to the presence of cristobalite and its transition from the tetragonal to the cubic phase were verified. Correlation between the XRD results and the dilatometric data from sintered glass compacts showed the partial dissolution of cristobalite when the glass was heated at the highest temperatures.

Hall effect measurements are one of the most powerful techniques for obtaining information about the conduction mechanism in polycrystalline semiconductor materials, which is the basis for understanding semiconductor gas sensors. In order to investigate the correlation between the microscopic characteristics and the macroscopic performances exhibited by undoped tin oxide gas sensors deposited by spray pyrolysis, Hall effect measurements were performed at different temperatures, from room temperature up to 500 K, and in the presence of two different atmospheres, air and methane. From these measurements, it was possible to infer the potential barrier and its dependence with the used atmosphere. The obtained results were analysed in terms of the oxygen mechanism at grain boundaries on the basis of the grain boundary-trapping model. In the presence of methane gas, the electrical resistivity decreases due to the lowering of the inter-grain boundary barrier height.

The surfaces of cordierite and glass particles were modified by coating them with an alumina precursor using a precipitation process in the presence of urea. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy, X-ray diffraction, electrophoresis, and rheological measurements were used to characterize the coated powders. SEM and transmission electron microscopy morphologies of the coated powders revealed that amorphous and homogeneous coatings have been formed around the particles. The morphology of the coated powders showed a coiled wormlike surface. The coating Al2O3 layer dominated the surface properties of the coated glass and cordierite powders. The influence of the coating layer on the processing ability of cordierite-based glass-ceramics substrates by tape casting was studied in aqueous media. It could be concluded that the coating of the powders facilitates the processing and yields green and sintered tapes with denser, more homogeneous microstructures compared with the uncoated powders.