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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

shoulder between 60 and

shoulder between 60 and 70% cumulative volume. The milled AO powder was also bimodal with a shoulder in the distribution curve between 30% and 40%. Milling reduced the d50 of the powder from 8 µm to 3.2 µm. Both values were larger than the d50 of 1.8 to 2.4 µm specified by the supplier. Figure 4.11 Microstructure of AO in the as-purchased condition. The non-milled AO powder was examined by SEM to determine the source of the deviation between the nominal and measured particle sizes. As shown in Figure 4.11, relatively large spherical agglomerates with diameters exceeding 100 µm were dominant. This diameter was larger than that evaluated by the laser-scattering method. Some breakdown occurred in sample preparation where the powders were suspended in water by ultrasonic agitation, resulting in a lowering of the median particle size to 8 µm. With the introduction of a de-agglomeration step, further crushing occurred resulting in the further lowering of d50. Nevertheless the particle size suggested by the manufacturer could not be achieved even after the de- agglomeration. Therefore, it is evident that the specified median particle size range of 1.8 to 3.2 µm is that of the single particles which were agglomerated in the as-delivered condition. 101

4.3.2 Powder specific surface area The specific surface area Ss BET of TO, MO and AO in the as-purchased state was determined with the nitrogen adsorption method. That of MO was performed additionally on powder aged for 12 months in standard atmospheric conditions and that of AO after milling. A reproducibility test showed that the scatter in Ss BET was within 5% for the same sample. Specific surface area SsBET S / (m²/g) m²/g s BET Figure 4.12 Specific surface area SsBET of ceramic powders. AO in the initial and deagglomerated state. MO before and after storage for 12 months. As shown in Figure 4.12, the powder AO showed a specific surface area of 0.57 m²/g in the initial state and a marginal reduction to 0.54 m²/g after milling which was within the scatter of the method. Therefore milling did not increase the specific surface area of AO as a result of the relatively short milling time of 300 s within which no new surface could be generated. The surface area of TO was 2.84 m²/g and that of MO was 4.8 m²/g. The latter met the specified value of 4 - 5 m²/g. The surface area increased to 6.0 m²/g when the powder was stored for 12 months. At the same time, the loss of ignition (LOI) increased from 1.78% to 3.49%. This indicates that further hydration of MgO occurred during storage and led to an increase in its surface area. 8 7 6 5 4 3 2 1 0 AO AO AO milled TO MO MO MO hydr. milled stored 102

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