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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

indicating stronger

indicating stronger bonds to crack and oxidize in PF. It is interesting to note that preforms fabricated with PC with a total pore fraction, Φtot , above 0.65 could not be used for further MMC processing as they were cracked and often too weak for handling. In contrast, 81% porosity could be obtained with PF and the preforms were still suitable for further processing. The behaviour with PC was attributed to the significant expansion of up to 7.5% during sintering, as shown in Figure 4.20. This was a result of thermally-induced relaxation of the cellulose particles due to weakening of the polyvinyl alcohol (PVA) binder during heating. Mattern (114) reports that swelling of cellulose during an aqueous preform forming route led to preform cracking during drying. In the present work instead of the aqueous route a dry pressing route was applied with which preform cracking was prevented. As low cost MMC was targeted, the PF route was discarded since the cost was 10 times higher than that of PC. Apart from the magnesia preforms (MOPC20), where two distinct peaks were found in the incremental pore size distributions, relatively homogenous pore size distributions were found for the preforms produced with PC (Figures 4.27 and 4.28). The difference in pore size distributions with similar pore former concentrations resulted from the decomposition products of the PFA. PC degraded significantly in air in the temperature interval between 250°C and 310°C where a mass loss of 85% occurred. As reported by Capart et al. (183) , dehydro-cellulose and water vapour were formed and further decomposition in the same temperature range led to water vapour, CO and CO2 and charcoal (char), Figure 5.3. cellulose (C6H10O5 )n 200- 280°C 280- 340°C dehydroyo- - cellulose + H2O Figure 5.3 Cellulose decomposition in an inert atmosphere (183) . tar 205 char + H 2 O + CO + CO 2

The newly formed water vapour led to the differences in shrinkage and pore size distribution (Figure 4.27) of MOPC20 compared to the other preforms where pores were formed with PC. The proposed progress and influence of the decomposition during heat treatment of the magnesia green parts is shown schematically in Figure 5.4. At the beginning of heat treatment (25°C), the green MOPC20 consisted of MgO, cellulose particles and PVA binder. Decomposition of cellulose started at 250°C and water vapour was formed. At this temperature, magnesia is highly reactive with water and forms Mg(OH)2 according to: MgO 2 2 (s) + H O (g) ⎯⎯→ Mg(OH) ( s) Equation 49 In the present investigations it was found, that the specific surface area of the initial MgO powder when enriched with 1.71% of moisture increased from 4.8 m²/g to 6.0 m²/g indicating the formation of Mg(OH)2. This process has been reported (184) to produce high surface area catalyst support materials. A multiple hydroxide formation-calcination procedure applied to MgO powder with a surface area of less than 50 m²/g increased it to 300 m²/g. T = 25°C T= 250°C..310°C T= 600°C MgO Mg(OH) 2 (C (C6H 6H10O 10O5) 5) n Tar, char Figure 5.4 Schematic of the influence of cellulose decomposition and its solid products on the microstructure and phase composition of the MOPC20 preforms during heat treatment in an inert atmosphere. Between 310 and 600°C the organic residues, tar and charcoal, cracked and oxidized and the Mg(OH)2 calcinated. After calcination up to 600°C, further heat treatment resulted in 206

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