Regina Monteiro

The thermoreactivity of a zinc acetate non-alkoxide solution used for the preparation of ZnO-based thin films was investigated in the temperature range 20-600°C by TG-DTA, XRD and SEM data. We found that the formation in air of ZnO crystallites from the sol-gel precursor occurs above 150°C simultaneously with the decomposition of an intermediary compound, most probably carbonate hydroxide (sciante and/or hydrozincite). At 200°C, the crystalline structure is well defined in terms of ZnO hexagonal lattice parameters, although residual organic compounds and water were not yet fully removed and an amorphous phase coexists. A kinetic investigation on the thermal decomposition of sol-gel precursor from DTA data using Kissinger differential equation is also presented. Apparent activation energy values of about. 100 kJ mol-1 corresponding to the non-isothermal decomposition of solid precursors in the temperature range 170-250°C have been found.

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.

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.

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.

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.

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.

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.

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.

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 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.

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 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.

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.

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.

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.

{Aluminophosphate glasses belonging to the Li2O-BaO-Al 2O3- La2O3-P2O 5 system doped with Tb3+ were prepared and investigated. Methods as Induced Coupled Plasma-Mass Spectrometry (ICP-MS), Induced Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and X-ray diffraction (XRD) have been used to establish the elemental composition of these vitreous materials. The influence of the Tb3+ ions on the optical properties of the phosphate glasses has been investigated in relation with the structural characteristics of the vitreous matrix. The optical behavior has been studied by ultra-violet-visible (UV-Vis) spectroscopy, revealing electronic transitions specific for terbium ions. Fluorescence spectroscopy measurements have been performed by excitation in the UV and visible domains (377 nm and 488 nm) which resulted in the most significant fluorescence peaks in the Vis domain (540 and 547 nm). Structural information via vibration modes were provided by Fourier Transform Infrared (FTIR) absorption spectra in the 400-4000 cm-1 range. Absorption peaks specific for the vitreous phosphate matrix were put in evidence as P-O-P symmetrical and asymmetrical stretching vibration modes, P-O-P bend, PO2- symmetrical and asymmetrical stretching vibration modes

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.

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.

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.

A batch of coal fly-ash, soda and lime was melted, quenched to a glass and then devitrified, by one-step heating cycles, forming coarse fibrous microstructures with pores and cracks, resulting in low strength materials. The crystallization behaviour of the based glass was further studied by adding a nucleating agent, TiC2. The resulting structural and microstrutural changes were investigated by differential thermal analysis, scanning electron microscopy, x-ray diffraction, dilatometry and density measurements. The results indicated that the addition of TiO2 could provide a finer grained microstructure, suitable for the production of structural materials.

This paper presents preliminary results on Al doped ZnO thin films prepared by the solgel method. The thin films were produced by a dip-coater technique on glass substrate, using zinc acetate dihydrate, aluminium chloride hexahydrate, 2-methoxyethanol and monoethanolamine as raw materials. The ZnO thin films were analysed by XRD, Hall effect and SEM measurements. In order to determine the influence of the thermal treatments on the film properties, a set of four different heat treatments (atmosphere and temperature) were studied. All the films are polycrystalline presenting a crystallographic c-axis orientation (002) perpendicular to the substrate. The best results were obtained for films pre-heated at 400°C and post-heated for 1 hour in air at 600°C, after annealing under a reduced atmosphere of forming gas, where a resistivity of 3.9×10-3 Ωcm, a Hall mobility of 34.1 cm2/Vs, a carrier concentration of 4.7×1019 cm-3 and an optical transmittance of 90% were achieved.

This paper presents the effect of post-heating temperature and atmosphere on the electrical and optical properties of ZnO:Al thin films prepared by the sol-gel method. The electrical properties of the n-type semiconductor thin films showed that for the final films, the values of carrier concentration ranged between 2.76 and 9.96 × 1019 cm- 3, the Hall mobility values between 7 and 34.1 cm2/V s and the resistivity values between 2.9 × 10- 3 and 5.0 × 10- 2 Ω cm, depending on the processing conditions. For the thin film doped with 2 wt.% Al, preheated at 400 °C and post-heated for 1 h in air at 600 °C, a resistivity of 2.9 × 10- 3 Ω cm has been reached after annealing under a reducing atmosphere of forming gas. The optical transmittance spectra of the only post-heated films and of the post-heated and annealed films showed a good transmittance (75-90%) within the visible wavelength region and some small effects of Al-doping concentration and annealing treatment in forming gas. © 2005 Elsevier B.V. All rights reserved.

Mixtures of fly ash, calcite and soda ash were prepared by keeping the amount of fly ash at 80 wt% and adding the other raw materials in order to obtain Na2O/CaO ratios of 1/3, one and three. On melting all mixtures led to black glassy materials. Several heat treatments were performed on these glasses at temperatures suggested by DTA. The crystalline phases precipitated during the heat treatments were identified by XRD on powdered samples and the microstructures of the obtained glass ceramics were observed by SEM. The formulations tended to precipitate pyroxene solid solutions and aluminium silicates of calcium and/or sodium, during the heat treatments. The glass ceramics mainly contained fine crystals. However some of the materials obtained from glasses with Na2O/CaO ratios of 1/3 and three had non-uniform microstructures. It is generally concluded that the microstructure of these crystallised glasses is critically dependent on both composition and heat treatment.

A uniform shiny black-coloured glass was obtained using bottom ash produced by a Portuguese municipal solid waste incinerator (MSWI). The bottom ash was the single batch material used in the formation of the glass, which was obtained by vitrification of the solid waste at 1400 °C for 2 h. Under these conditions, a homogeneous melt with an appropriate viscosity to be shaped was obtained, indicating the suitability of this waste material to be employed in the development of vitreous products. The characterization of the resulting glass was performed in order to assess its structural, physical, mechanical, thermal and chemical features. The glass had a density of 2.69 g cm -3, a hardness of 5.5 GPa, a fracture strength of 75 MPa, a thermal expansion coefficient of 9.5 × 10-6 °C-1 and it exhibited a very good chemical stability. In summary, the MSWI bottom ash glass has good mechanical and chemical properties and may, therefore, be used in several applications, particularly as a construction material. © 2005 Elsevier B.V. All rights reserved.