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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

4.23 b). The immediate

4.23 b). The immediate border of the window was about 2 µm wide and had a thickness equal to the diameter of one to three particles. Because grain growth has occurred during the sintering process is was probable that the starting powder was smaller than one micron. (a) 2 20 2 20 µm µm 2 µm Figure 4.23 Microstructure of the purchased AODY30 preforms: (a) overview with the single windows marked and (b) detail of a window connecting the cavities. In the FATO preforms, built up of Saffil fibres and TiO2 particles, the two phases were clearly visible, Figure 4.24 a). The fibres were embedded into the fine particles. Between the clusters of particles and fibres, there were large pores 15-20 µm wide and more than 200 µm long. (a) 2 20 2 20 µm µm 117 (b) (b) 2 µm Figure 4.24 Microstructure of the FATO preform. Arrows indicate an extended pore.

At higher magnification, Figure 4.24 b) shows the alumina fibre diameter to be about 4 µm and the titania particles to be sub-micron. The loose particles on the fibre sections cut during specimen preparation suggests a weak interparticle bonding. 4.6.2 Pore size distribution The incremental and cumulative graphs of the relative pore volume of two alumina preforms, AO and AOPC20, as a function of pore diameter are shown in Figure 4.25 a) and b) respectively. The incremental plot is often easier to interpret in terms of fraction per pore size diameter class and was therefore used for further presentations of the pore size distributions. Intrusions below 0.25 µm pore diameter were not observed in any of the preforms made with pore forming additives. Therefore the pore size distribution plots of all preforms, apart from AODY30 and FATO, were limited to a minimum of 0.1 µm. Relative Relative pore pore volume () 0.3 0.2 0.1 0.0 10 -2 (a) incremental (b) AO AOPC20 10 -1 10 0 10 1 118 10 -2 Pore diameter /µm (µm) 10 -1 cumulative 10 0 10 1 10 2 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 10 Figure 4.25 Pore size distribution curves of AO and AOPC20. a) incremental and b) cumulative outline. Cumulated Cumulative rel. pore volume ()

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