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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Specific surface area S

Specific surface area S (m²/g) s S / m²/g 6 4 2 0 5.43 0.186 4.51 0.222 800 900 1000 1100 1200 1300 123 3.78 0.251 Sintering temperature /°C (°C) Figure 4.31 Influence of sintering temperature on specific surface area SsHg and median pore diameter of preform MOPC20. The pore size distributions of FATO and AODY30 are shown in Figure 4.32. The foam-based AODY30 exhibited two peaks, one at a pore size of about 20 µm and another more distinct at about 8 µm, which showed the largest incremental pore fraction of 0.42. The FATO hybrid preform had a wide range of pore sizes from 20 µm to 0.05 µm. There were two main peaks, d Hg one at a pore diameter of 18 µm and the other at 0.2 µm. Relative Relative pore pore volume volume () 0.42 0.40 0.1 0.0 10 -2 10 -1 S s AODY30 FATO 10 0 Pore diameter (µm) /µm Figure 4.32 Incremental pore size distribution of the purchased preforms FATO and AODY30. 10 1 1.50 0.70 2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0 Median pore diameter d (µm) Hg Hg / µm

An overview of the specific values derived from the pore size distribution curves is shown in Table 4.2. Most of the preforms exhibited differences between the total porosity Φtot calculated on the basis of the measured preform volume and the theoretical density, and the total porosity measured using the mercury intrusion method ΦHg. These differences represent the closed cell porosity of the preform. No difference was observed with the TO and MO preforms and small differences of 3% were found with AO, AODY30 and FATO preforms. Larger differences were observed when cellulose (PC) was used as a pore forming agent. Here AOPC20 showed the smallest difference of 7%, whereas TOPC10 and AGPC15 showed the maximum differences (9%) of all the preforms investigated. The median pore diameter dHg varied significantly over the range of 0.77 µm to 9.05 µm. The titania and alumina preforms AO and TO exhibited the smallest diameter which was below 1 µm. In contrast MO showed a median diameter of 1.84 µm which was larger than 1.50 µm for the MOPC20, where cellulose was used as PFA. This behaviour was unique to the magnesia system. In all other preform types where a pore former was used, the median pore diameters ranged between 1.06 µm (TOPC10) and 4.98 µm (TOPC20) and therefore were larger than that of the respective preforms without PFA additions. The alumina based preforms AGPC15 and AOPC20 showed median pore diameters of 2.77 µm and 3.48 µm respectively. Larger values were achieved with AOPF20, where the carbon fibre was used as a pore former. The largest values of the entire project were reached with the purchased preforms FATO and AODY30 with 8.53 µm and 9.05 µm, respectively. Table 4.2 shows the specific surface area per unit mass (SsHg), normalized per unit of preform volume (SpHg), and the inner surface area per porosity volume (SiHg) which were calculated using Equation 22 and Equation 42 respectively. The SsHg values of the preforms ranged between 0.69 and 5.85 m²/g for AOPF20 and FATO respectively. The high value of the latter 124

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