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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

(AGPC15). That

(AGPC15). That of MOPC20 could not be measured due to hydrolysis of magnesia in contact with water. As an alternative, ethanol was used but these measurements were not successful, as the mass of the saturated preform (m1 in Equation 40) could not be evaluated accurately due to the evaporation of ethanol. Table 4.1 Fabrication parameters and the measured resulting porosities of the preforms used for further composite processing. Compaction Sintering PFA addition Porosity pressure temperature P comp. T sint total closed cell open cell PFA formed wt-% vol-% MPa °C Φ tot Φ c Φ op Φ PFA AOPC20 20 40 60 1600 65% 0,2% 65% 46% AOPF20 20 36 100 1500 60% 0,3% 60% 32% AGPC15 15 32 100 1000 65% 2,0% 63% 27% TOPC10 10 21 100 1050 62% 1,4% 61% 22% TOPC20 20 42 100 1100 65% 1,8% 63% 40% MOPC20 20 37 100 1300 62% n.a. 62% 24% 4.5.4 Dimensional change during sintering The relative change in dimensions along the three axes of preform type was evaluated and the results are shown in Figure 4.20. It was similar along the x- and y-axes (sx and sy) and in the range 0% to 7.5% for all preforms. The lowest value was achieved with AGPC15 and the highest with TOPC20. In the z-direction (sz) the change in dimension was significantly different to sx and sy. AOPF20, TOPC10 and MOPC20 showed a reduction in the height of the preform whereas AOPC20, AGPC15 and TOPC20 showed an increase in the height. The shrinkage of AOPC20 and AGPC15 was similar (-7.5%) whereas that of TOPC20 was lower (-4.5%). 113

Relative change in dimension s x, s y , s z (%) Rel. change in dimension s x , s y, s z /% 10 5 0 -5 -10 AOPC20 AOPF20 AGPC15 TOPC10 TOPC20 MOPC20 Figure 4.20 Dimensional change due to sintering of the preforms in the three normal directions. 4.6. Preform characteristics 4.6.1 Preform microstructure Pyrolysis of the pore formers left cavities in the preform. The influence of the pore former geometry on the coarse porosity in the preform microstructure is shown in Figure 4.21. (a) AOPC20 (b) AOPF20 2 40 2 40 µm µm 114 z sx sy sz y x 2 40 2 40 µm µm Figure 4.21 SEM images of the preform microstructures: a) AOPC20 and b) AOPF20.

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