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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

As shown in Figure 4.21

As shown in Figure 4.21 a), the cavities had an irregular shape when cellulose (PC) was used. The pore openings of AOPC20 were up to 30 µm in size. In contrast, Figure 4.21 b) shows that the pore diameters and cavities were more distinct in AOPF20 where carbon fibres were pyrolysed. The cavities were aligned with a diameter of about 10 µm which was slightly larger than the nominal fibre diameter of 8 µm. Apart from the coarse pores generated by pyrolysis of the pore formers, the regions between the large cavities comprised particles which were just lightly sintered. These areas were investigated at progressively higher magnifications. The structures of 4 preforms are shown in Figure 4.22. The tabular nature of the starting powder is reflected in the AO preforms AOPC20 and AGPC15. A greater portion was observed in AGPC15 compared to AOPC20. Most of these particles had an aspect ratio larger than 3. The glassy binder in AGPC15 could be clearly seen to bridge between some particles, as marked with an arrow in Figure 4.22 b). In contrast to the pure Al2O3 preform AOPC20, the sinter bridges in TOPC10 and MOPC20 were more distinctive and the particles were finer and more spherical. They were significantly finer in the TOPC10 titania preform than in AOPC20 and MOPC20, as can be seen by comparing Figure 4.22 c) with b) and a). Some particles were less than 1 µm in diameter. The sintering in the MOPC20 was more extensive than in the other preforms, as shown by the neck plane areas in Figure 4.22 d) indicated with an arrow. The plane faces result from the removal of particles during preparation of the preform sample for the SEM investigations. 115

(a) AOPC20 (b) AGPC15 2 µm (c) TOPC10 (d) MOPC20 Figure 4.22 SEM images of the microstructures of the preforms in the intragranular region of a) AOPC20, b) AGPC15, c) TOPC10 and d) MOPC20. The microstructures of the procured preforms are presented in Figure 4.23 for AODY30 and Figure 4.24 for FATO. The AODY30 preforms produced by Dytech Corporation (162) using a foaming route showed relatively large spherical pores with a diameter of about 120 µm in cut sections as shown in Figure 4.23 a). The preform was cut prior to the SEM investigations in order to eliminate edge effects. The spherical shape was a result gas bubbles in he initial ceramic slurry. In the final preform, the bubbles were interconnected by passages (windows) indicated by arrows in Figure 4.23 a) which were about 10 µm in diameter. The struts between the bubbles exhibited high density as shown in the high magnification image, Figure 116

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