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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

fraction to be

fraction to be calculated as shown in Figure 4.14. The median fibre length was 28 µm. As shown in Figure 4.15 a), the diameter was consistent at the specified value of about 8 µm therefore the median aspect ratio of PF was 3.5. The results of the particle size distribution of PC, which was measured using a laser scattering technique, are shown for comparison in Figure 4.14. The median particle size d50 of PC was 51 µm. More than 90% of the particles were between 40 and 60 µm. Figure 4.15 b) shows a macrograph of the cellulose particles which exhibited an irregular shape. Most of the particles were flat and elongated with an estimated aspect ratio between 2 and 4. (a) (b) Figure 4.15 Microstructure of the pore formers (a) carbon fibre PF and (b) cellulose PC; both in the as-purchased state. 4.4.2 Influence of pore former chemistry on degradation properties The pyrolysis behaviour of the pure pore formers, as well as that of a green part with MgO particles, was assessed using thermogravimetric analysis over the temperature range 20°C to 800°C. The relative residue as a function of temperature, shown in Figure 4.16, was calculated as the ratio of the mass at the respective temperature to the initial mass. The pore formers were pyrolysed in inert and oxidizing atmospheres. In the inert atmosphere up to the maximum pyrolysis temperature, the initial mass of the carbon fibre PF was reduced by only 2%. In contrast, about 10% of the mass of PC was transformed into gaseous products between 105

oom temperature and 270°C, with a further 70% lost rapidly as the temperature was raised to 350°C. At 800°C, the residue reduced to 4%. Relative Relative residue residue () 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 Inert Oxidizing PC 0 100 200 300 400 500 600 700 800 Temperature (°C) /°C 106 PF PC PF MOPC20 Figure 4.16 Decomposition behaviour of the pore formers PC and PF in inert and oxidizing atmospheres and of the green MOPC20 preform in an oxidizing atmosphere. The behaviour in an oxidizing atmosphere was different. The carbon fibre PF started to degrade at 400°C which was followed by a steep decrease at about 500°C which led to less than 1% of residue above 600°C. All the carbon fibre had transformed to gas when the maximum temperature was reached. The decomposition behaviour of PC up to 270°C in an oxidizing atmosphere was similar to that in an inert atmosphere. As the temperature was increased up to 320°C, a steep decrease in the residue took place to less than 3%. The residue was reduced to zero at 800°C. In the green part of magnesia with 20 wt-% of the pore former PC (MOPC20), the first main gas formation started at about 200°C and ended at 400°C. At 620°C, another mass loss took

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