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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

the 10% uncertainty

the 10% uncertainty of the model (125) is taken into account this minimum is higher than the Vpo of 0.68 found in AODY30. Consequently no windows may be expected in AODY30 which was not the case as the predominant pore fraction in this preform type was reached by the infiltration alloy and therefore most of the pores were interconnected with windows. For the AODY30 preform the model (Equation 35) is not valid and may be improved in the future. 5.2.2 Pyrolised pore formers For the self-fabricated preforms, the target porosity range of 60 to 70% was achieved by using incomplete densification of the particles to give the intragranular fine pores, in combination with larger pores which were formed by the pyrolysis of the pore forming additive (PFA). At the relatively low sintering temperatures and times employed, the volume diffusion was kept low and sintering led to small contact points, as shown in the preform microstructure (Figure 4.22). Despite this ability to control the microstructure, the target porosity range was only reached with PFA, indicated by Φtot of 37% (AO) to 50% (MO) for preforms without PFA. Particles with d50 values between 1 and 3 µm were selected as a compromise between ease of infiltration and maximum strengthening effect. Figure 2.6 shows the relationship between particle diameter and strength. The maximum gradient was reached with particles below 0.1 µm (31) . Even though the infiltration process was optimized in order to reduce residual porosity, the fine pores of MOPC20 sintered at 800°C with pore diameters below 0.2 µm (Figure 4.30) prevented full infiltration into interparticulate pores, Figure 4.56 b). As an already established (155,163) wet powder processing route was used for the current investigations, ceramics with high hydrolysis potential like CaO were discarded. As reported by Kuang et al. (180) , waterless agents were obligatory for CaO powder processing. Even though it is well known that MgO forms hydroxides when in contact with water, the reaction 203

kinetics were reported to be rather slow, depending on parameters such as the pH-value of the aqueous solution and the surface area of the powder (181) . The latter was reported to play the predominant role. However, even at very large specific surface areas, the overall room temperature hydrolysis reaction rate was shown to be comparatively slow when nanosized MgO was exposed to different aqueous solutions (182) . The reaction kinetics were characterized by a fast initial reaction followed by gradual slowing to a low rate steady-state reaction. The main reason for the different kinetics between CaO and MgO hydrolysis lies in the solubility of the respective hydroxids in water. The solubility product of Mg(OH)2 is five decades lower than that of Ca(OH)2 (171) . On the basis of these results, it proved possible to fabricate MgO preforms using the aqueous processing route applied to Al2O3 and TiO2 preforms. Mass loss measurements of magnesia preforms, Figure 4.16, confirmed that the hydrolysis of MgO during aqueous processing led to marginal Mg(OH)2 fractions compared to the feed powder, therefore demonstrating that it is possible to process MgO in an aqueous solution. The fundamentals of PC-type pore formation in TiO2 (TOPC) and Al2O3 preforms (AGPC) were investigated by Schneele (163) and Staudenecker (155) respectively. No additions were used to aid sintering in the pure oxide systems and the intergranular porosity was controlled by the sintering temperature. In order to preserve the fibre properties in the purchased fibre reinforced system (FA), glass forming binders were added and the sintering temperatures were lowered accordingly below 1000°C (117) . The pyrolysis behaviour of carbon fibres (PF) was compared to that of cellulose particles (PC). In contrast to PC, the thermogravimetric investigations for PF showed that minor mass losses occurred in an inert atmosphere up to 800°C (Figure 4.16 a). Under oxidizing conditions (air), the main decomposition started 200°C higher (at 500°C - Figure 4.16 a), 204

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