Lapa, N. a, R. a Barbosa, S. a Camacho, R. C. C. b Monteiro, M. H. V. c Fernandes, and J. S. a Oliveira. "
Leaching behaviour of a glass produced from a MSWI bottom ash."
Materials Science Forum. 514-516 (2006): 1736-1741.
AbstractThis 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.
Lima, M. M. R. A., and R. C. C. Monteiro. "
Crystallization of a borosilicate glass during sintering studied by dilatometry and XRD analysis."
Materials Science Forum. 455-456 (2004): 212-215.
AbstractThe 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.
Lima, M. M. R. A., and R. C. C. Monteiro. "
Viscous sintering in a glass-alumina system."
Materials Science Forum. 587-588 (2008): 143-147.
AbstractThe densification during sintering of borosilicate glass matrix composites with 25 vol. % alumina (Al2O3) particles was investigated. The powder compacts, isostatically pressed at 200 MPa, were sintered at a temperature within the range 800-1000° C and maintained at this temperature during various times. The sintering behaviour of the composites was investigated by density measurement and by axial and radial shrinkage measurements. The crystalline phases present in the sintered composites were identified by XRD and the microstructure was analyzed by SEM. For temperatures up to 900°C, the relative density of the composites increased continuously with sintering temperature and sintering time, while for higher temperatures, the density increased rapidly and then slowed down to achieve a nearly constant value after sintering the composites for 30 minutes. The composites exhibited isotropic shrinkage behaviour when sintered at 800°C and 850°C, but at higher temperatures slightly higher axial shrinkage than radial shrinkage was observed. The shrinkage behaviour and microstructural characteristics of the composites indicate that densification during sintering can be attributed to the viscous flow of the borosilicate glass.
Lima, M. M. R. A., F. M. Braz Fernandes, and R. C. C. Monteiro. "
Study of the crystallization of a borosilicate glass."
Key Engineering Materials. 230-232 (2002): 157-160.
AbstractThe 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.
Lima, M. M. R. A., and R. C. C. Monteiro. "
Shrinkage behaviour of borosilicate glass-Al2O3 composites during isothermal sintering."
Materials Science Forum. 514-516 (2006): 648-652.
AbstractThe 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.
Lopes, A., P. Nunes, P. Vilarinho, R. Monteiro, E. Fortunato, and R. Martins. "
Study of the sensing mechanism of SnO2 thin-film gas sensors using hall effect measurements."
Key Engineering Materials. 230-232 (2002): 357-360.
AbstractHall 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.