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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

deviations were 0.01

deviations were 0.01 whereas larger scatter was observed for ISQC and HPDC modes. Here the minimum scatter was 0.02 for the preforms exhibiting low compression such as AODY30, and 0.05 for the preforms with largest compression, which was that of the FATO hybrids at an infiltration velocity of 0.36 m/s in the HPDC mode. The preform geometry remained most consistent in DSQC, with compressions ranging from 0.01 to 0.04 for AODY30 and FATO preforms respectively. Taking into account the scatter, the preforms fabricated with a pore forming additive had similar compressions expressed by cpr values of 0.02 for AOPC20 and MOPC20 to 0.03 for TOPC10 and AGPC15. Table 4.8 Evaluated mean relative compression cpr of preforms as a result of infiltration in DSQC, ISQC and HPDC modes and effect of the infiltration velocity. Superficial velocity v o (m/s) DSQC ISQC HPDC 05 10 20 40 04 09 18 36 0.018 0.18 0.36 0.72 1.44 0.07 0.18 0.36 0.72 AOPC20IS 0.02 0.21 0.22 0.26 0.33 0.07 0.18 0.26 0.28 AGPC15IS 0.03 0.10 0.09 0.17 0.23 0.05 0.10 0.11 0.14 TOPC10IS 0.03 0.07 0.04 0.15 0.14 0.14 0.22 0.22 0.23 MOPC20IS 0.02 n.a. 0.04 0.11 0.14 0.17 FATOIS 0.04 0.12 0.17 0.23 0.29 0.08 0.31 0.40 0.39 AODY30IS 0.01 0.03 0.03 0.03 0.03 n.a. 0.05 0.05 0.05 173 compression c pr () In general, higher cpr were obtained in ISQC than in DSQC and HPDC infiltration. The only exception was AODY30 which exhibited the lowest cpr of 0.03, combined with no significant influence of infiltration velocity. This behaviour was attributed to the structure of the preform with strong, predominantly dense, sintered struts and good permeability. Apart from a discontinuity in TOPC10IS between 0.18 and 0.36 m/s in ISQC, all preforms showed a steady increase or constant compression with increasing infiltration velocity. At the maximum velocity of 1.44 m/s, AOPC20 and FATO showed maximum cpr of 0.33 and 0.29 respectively.

Relative preform compression c pr () Rel. preform compression c iso 0.4 0.3 0.2 0.1 AOPC20IS_DSQC AGPC15IS_DSQC AODY30IS_DSQC AOPC20IS_ISQC AGPC15IS_ISQC AODY30IS_ISQC AOPC20IS_HPDC AGPC15IS_HPDC AODY30IS_HPDC 0.01 0.1 1 Superficial velocity v (m/s) 0 o / m/s Figure 4.74 Compression of alumina preforms as a function of superficial velocity in the DSQC, ISQC and HPDC infiltration modes. Relative preform compression c pr () Rel. preform compression c iso 0.4 0.3 0.2 0.1 TOPC10IS_DSQC MOPC20IS_DSQC FATOIS_DSQC TOPC10IS_ISQC MOPC20_ISQC - n.a. FATOIS_ISQC TOPC10IS_HPDC MOPC20IS_HPDC FATOIS_HPDC 0.01 0.1 Superficial velocity v (m/s) 0 o / m/s 1 Figure 4.75 Compression of titania and magnesia-containing preforms as a function of superficial velocity. MOPC20 was not used in the ISQC mode. 174

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