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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

coarsening

coarsening of the fine MgO, as indicated by a reduction in the SiHg at increased sintering temperatures, Figure 4.31. The reason for the steep decrease from 3.78 m²/g to 0.70 m²/g when the sintering temperature was increased from 1200 to 1300°C is unclear and may be investigated in future. The pore size distributions of the MOPC20 preforms sintered at 800°C and 1300°C in Figure 4.30 show a shift of the main peaks towards lower values and was attributed to the coarsening of the ceramic particles and associated neck growth at higher sintering temperatures. It is interesting to note that the large pore peak also shifts towards smaller pore sizes, as a result of a reduction in the diameter of the PFA formed channels. Thus, there had to be a migration of particles from the border toward the inside of the channel which was found in the microstructure of the MMC in Figure 4.56 b). The migration was a result of the volume increase as MgO reacted to Mg(OH)2 during the degradation of PC. The density of the oxide is 3580 kg/m³ whereas that of the hydroxide is 2410 kg/m³ (160) which led to a volume increase of 32.7%. The expansion was predominantly towards the cavity originally occupied by the pore former. The particles remained in this arrangement after calcination. This was observed in the MOPC20IS where the preform was sintered at 800°C (Figure 4.56 a). Sintering at higher temperatures (e.g. 1300°C) led to coarsening of the particles due to volume diffusion. Further migration from the PFA-formed cavity toward the intergranular regions occurred and resulted in a shift of the large pore peak toward larger values. 5.3. Saturation of porous media As shown in section 5.1, residual porosity is detrimental to the MMC properties. It is therefore important to strive for full infiltration. A prerequisite for this is an open-cell structure. In the preforms investigated in this thesis, the predominant fraction was accessible to the fluid as the preforms had between 0.2 and 2.0% closed cell porosity. 207

In order to achieve minimum porosity during infiltration processing, the influencing factors have to be known. During the constant pressure infiltration of SiC particle compacts, an uneven infiltration front was found where larger pores were filled prior to the smaller ones (103) . Even though Darcy´s law is based on the saturated flow assumption, characterised by a distinct infiltration front, Garcia-Cordovilla et al. (103) proposed it to be applicable to their infiltration problem. Consequently, their modelling accuracy was rather poor. Dopler et al. (113) also found unsaturated flow in Saffil fibre infiltration and reported that the dynamic variable saturated flow infiltration of Saffil fibre preforms could be modelled accurately using the saturation relation developed by van Genuchten (115) (Equation 29) which originated from soil sciences. Evidence of unsaturated flow was found in the preforms of the present investigation. This was indicated in the saturation profiles of the constant pressure infiltrated AOPC20 preforms, where a saturation gradient was visible (Figure 4.43). This was further supported by the shape of the infiltration curves obtained in constant flux infiltration in the DSQC mode, shown for IS infiltration in Figure 4.48. The non-linearity at higher saturations indicates Darcy´s law is not fulfilled. Table 5.1 Preform parameters of FA24 (105) , AOPC20 and TOPC10 relevant for preform infiltration modelling.. FA24 (105) 208 AOPC20 TOPC10 Preform porosity Ф tot 0.76 0.65 0.62 Specific permeability K SI 10 -14 ·m² 96 23.7 13.1 Threshold pressure P 0 10 6 ·Pa 0.25 0.72 2.02 Shape factor α 10 -6 ·Pa -1 21 2.25 1.25 In order to solve Equation 29 for the present preforms, Φtot , P0 and the shape factor α have to be known. The parameters were evaluated (Table 5.1) for AOPC20 and TOPC10 using their

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