R.C.C. Monteiro

Production of glass-ceramics from coal ashes

Abstract Coal fly ashes produced by an extinguished power plant in the north of Portugal have been melted with addition of CaCO 3 and Na 2 CO 3 to obtain glasses. One of the formulated compositions was selected for further studies and it was possible to manufacture glass-ceramics by crystallising 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.

Ash from a pulp mill boiler—Characterisation and vitrification

The physical, chemical and mineralogical characterisation of the ash resulting from a pulp mill boiler was performed in order to investigate the valorisation of this waste material through the production of added-value glassy materials. The ash had a particle size distribution in the range 0.06-53 microm, and a high amount of SiO(2) (approximately 82 wt%), which was present as quartz. To favour the vitrification of the ash and to obtain a melt with an adequate viscosity to cast into a mould, different amounts of Na(2)O were added to act as fluxing agent. A batch with 80 wt% waste load melted at 1350 degrees C resulting in a homogeneous transparent green-coloured glass with good workability. The characterisation of the produced glass by differential thermal analysis and dilatometry showed that this glass presents a stable thermal behaviour. Standard leaching tests revealed that the concentration of heavy metals in the leaching solution was lower than those allowed by the Normative. As a conclusion, by vitrification of batch compositions with adequate waste load and additive content it is possible to produce an ash-based glass that may be used in similar applications as a conventional silicate glass inclusively as a building ecomaterial.

Crystallization of CaO−Al2O3−SiO2 and CaO−MO−Al2O3−SiO2 (M=Mg, Zn) glasses

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 Ca20Al32-2vMgvSivO68, withv 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.

NON-ISOTHERMAL KINETIC STUDY ON THE DECOMPOSITION OF Zn ACETATE-BASED SOL-GEL PRECURSOR Part 1. Application of the isoconversional methods

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.

Leaching Behaviour of a Glass Produced from a MSWI Bottom Ash

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 x 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 l/kg was applied in each cycle. The leachates were filtered through a membrane of PTFE (porosity: 0.45 8m). 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.

Fine-Grained Glass-Ceramics Obtained by Crystallisation of Vitrified Coal Ashes

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.

Glass transition and crystallization kinetics of a barium borosilicate glass by a non- isothermal method

The glass transition and crystallization kinetics of a glass with a molar composition 60BaO-30B2O3-10SiO2 were 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 ...

Investigations on optical, structural and thermal properties of phosphate glasses containing terbium ions

Aluminophosphate glasses belonging to the Li2O–BaO–Al2O3– La2O3–P2O5 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, P=O stretching vibration mode as well as P-O-H water absorbance. Raman spectra acquired in the 100–4000 cm−1 range by 488, 514 and 633 nm laser excitation sources disclosed peaks also specific for the phosphate matrix, proving the role of phosphorous oxide as a vitreous network former. Differential Scanning Calorimetry and Thermogravimetric Analysis (DSC-TGA) provided information regarding the phase transformations that took place during the heating process and the associated thermal effects.

Thermoreactivity of Sol-Gel Precursor for ZnO-Based Thin Films

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 (sclarite 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 nonisothermal decomposition of solid precursors in the temperature range 170-250oC have been found.

Study of the Sensing Mechanism of SnO2 Thin-Film Gas Sensors Using Hall Effect Measurements

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. Introduction Tin oxide (SnO2) is among all the semiconducting metal oxides the most widely used material in gas sensors fabrication. Until now, the majority of the known gas sensors are based on sintered tin oxide due to its low cost (both for the raw material and for the technological process), long-life and requirement of very simple electronics, so that little maintenance is involved [1]. These sensors exhibit a good response to several gases, both reductors and oxidants, presenting the inconvenient of a lack of selectivity in the presence of a gas mixture and a gradual performance degradation with time [2]. As these properties vary with the grain size, surface morphology and internal porosity [3], the use of this material in the form of thin film starts gaining attention. Nanostructured tin oxide presents greater densities of grain boundaries and interfaces, and its strong reactivity with gaseous species is a consequence of being small-grained. The study of the transport mechanism in polycrystalline semiconductor materials is the basis for the understanding of semiconductor gas sensors [4], and through Hall effect measurements it is possible to obtain this type of information. With the purpose of improving their operation characteristics and obtaining optimal sensors with respect to electrical conduction and microstructure characteristics, a study was performed in order to understand the correlation between the microscopic characteristics and the macroscopic performances of undoped tin oxide gas sensors [5]. Experimental details Undoped SnO2 thin films were deposited by spray pyrolysis technique. A solution of SnCl4 5H2O (Riedel-de-Haën, ref. 14550 with 98% purity) dissolved in isopropilic alcohol (Pronalab with 99.7% purity) with concentrations varying from 0.1 to 0.4 M was sprayed onto glass substrates pre-heated at temperatures between 573 and 673 K using nitrogen as carrier gas at a constant flow rate of 8l/min. The substrate-nozzle distance was kept constant at 25 cm. The film thickness (d) was Key Engineering Materials Online: 2002-10-25 ISSN: 1662-9795, Vols. 230-232, pp 357-360 doi:10.4028/www.scientific.net/KEM.230-232.357