M.M.R.A. Lima

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 ...

Development of Structural Ceramics from Fly Ash and Shale

The possibility of using fly ash and shale as alternative raw materials for the production of structural ceramics was investigated. Fly ash is a by-product from coal-burning power plants, and shale is a sedimentary rock that if ground finely enough can exhibit a clay-like plasticity. Ceramic samples containing 10-50 wt% fly ash were formed from mechanical mixing of both kinds of powders that were packed and sintered in the temperature range 950-1200°C. It was verified that powders with larger fly ash content exhibited lower packing density resulting in compacts with a lower sintered density. Although an increase in fly ash content was associated to a larger presence of porosity in the sintered samples, as confirmed by microstructural analysis, all studied compositions when sintered at the highest temperatures exhibited satisfactory values for water absorption (< 10%), for flexural strength (20-64 MPa) and for hardness (20-30 GPa) indicating that they have potential to be applied in the production of structural ceramics.

Crystallization of a Borosilicate Glass during Sintering Studied by Dilatometry and XRD Analysis

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 800oC, performed at different heating rates (1, 2, 5, 8 and 10oC/min), revealed that the compacts started to shrink above ~ 600oC and that the shrinkage decreased with the increase of the heating rate for temperatures up to ~750oC. Above this temperature, and specifically when the samples were heated at heating rates < 5oC /min, the shrinkage was hindered, while samples heated at heating rates 5oC/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 700-850oC 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 725oC and 765oC). Quartz dissolution took place when the glass samples were sintered at 850oC. At this temperature and whatever the sintering time, cristobalite was the only crystalline phase present in the sintered compacts. Introduction In many glass systems devitrification occurs frequently. The rate and mechanism of devitrification are dependent upon the chemical composition of the glass, the temperature, the furnace atmosphere and the impurities in the glass network [1, 2]. Usually, devitrification takes place during the sintering of glass powders [3]. Viscous flow is the main densification mechanism during sintering of glass powders [2, 3]. The precipitation of a crystalline phase during the sintering of a glass will originate a matrix of higher viscosity than the initial single glass phase hindering the densification and thus, it is desirable that the glass remains in the amorphous state during and after firing [3]. Some borosilicate glasses in powdered form have considerable interest, either for use in the production of sintered glass with controlled porosity [4] or for application in the manufacturing of composite materials [5-9], with particular relevance as borosilicate glass matrix composites for microelectronic substrates [9]. Borosilicate glasses have a low thermal expansion coefficient (3-6 x10 -6 C -1 ) [10], a low dielectric constant and a high resistance to chemical attack [5, 9]. The precipitation of cristobalite takes place frequently during the sintering of borosilicate glass powders [3, 10]. Cristobalite is an unfavourable transformation product in view of its large thermal expansion coefficient (50x10 -6 C -1 ) [2]. The large volume change associated with cristobalite martensitic transformation from the tetragonal ( -cristobalite) to the cubic phase ( -cristobalite) above 272oC reduces dramatically both the thermal shock resistance and the mechanical strength of the glass composite [11]. Materials Science Forum Online: 2004-05-15 ISSN: 1662-9752, Vols. 455-456, pp 212-215 doi:10.4028/www.scientific.net/MSF.455-456.212

Shrinkage Behaviour of Borosilicate Glass-Al2O3 Composites during Isothermal Sintering

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 (Al2O3) 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.

Effect of Dolomite Addition on the Densification of Fly Ash Based Ceramics

Fly ash from Tapada do Outeiro, a coal power plant in the north of Portugal, has been processed by a powder technology route in order to obtain durable and mechanical resistant ceramics. Dolomite (CaCO3.MgCO3) was added in different proportions to the waste material, from zero up to 10 wt%, and the effect of this addition on the densification behaviour of the fired samples was investigated by measuring the apparent density, the open porosity and the linear shrinkage. The powder mixtures were uniaxially dry pressed in a steel die and fired at temperatures ranging from 950 to 1150°C. XRD and SEM were used to identify the phases present in the sintered materials and the degree of densification. The results revealed that added dolomite was responsible for the appearance of anorthite, an extra phase besides mullite and quartz, for an increase in the amount of liquid phase and for the swelling of closed pores at the highest firing temperatures. Significant morphological changes and phase transformations occurred during sintering and their effects on the physical-mechanical and leaching characteristics of the sintered materials were analyzed.

Study of the Crystallization of a Borosilicate Glass

The crystallization of a commercial borosilicate glass powder has been studied in the temperature range 750-900oC. 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 750oC, 9-12 min at 775oC, 5-10 min at 810oC, and 0-5 min at 840oC. 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.