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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Table 3.5 Specified

Table 3.5 Specified properties of the pore forming additives (PFA) used for preform fabrication in accordance to the data sheets of the suppliers (156,157) . 71 PC PF Type cellulose carbon fibre Main chemical constituents C 6H 12O 6 Supplier JRS SGL Carbon Type Arbocell P290 C10 M350UNS Purity % n.a. ≥99 Average particle diameter d 50 µm 150 n.a. Fibre diameter µm n.a. 8 ± 0.5 Theoretical density kg/m³ 1500 1750 ± 50 Since it was important for the pore formers to crack and oxidise during the subsequent pyrolysis of the green parts, their pyrolysis properties were investigated in the 20 -800°C temperature range using a Mettler-Toledo DSC12E thermogravimetric analysis unit. The decomposition was performed in oxidising (air) and argon atmospheres heated at 10°C/min. 3.3.3. Powder preparation The volume of PFA, the green compaction pressures and the sintering temperatures were opti- mised by studying the range of compositions shown in Table 3.6. The target was to produce preforms with ceramic volume fractions in the range of 0.30 to 0.40 with sufficient strength that they could be handled. The formulations were also fabricated without PFA to enable the fraction of PFA-formed porosity ΦPFA of the total porosity Φtot to be determined. The powder processing was undertaken following the procedure described in 3.2.1, except that the PFA was added prior to freezing, by stirring it into the slurry until visual homogeneity was reached. The powder type TOPC10 was fabricated using a large scale production unit at Alpha-Ceramics in Aachen (Germany). The ready-to-press powder was shaped using selected pressures from 10 MPa to 100 MPa by uniaxial pressing in a steel die to give green compacts C

with dimensions of 65 mm x 46 mm x 8 mm. For TOPC10, the same powder preparation was applied apart from the drying procedure, which took place in a spray drying unit which led to near spherical grains as shown in Figure 3.4. Table 3.6 Designation, composition, compaction pressures and sintering temperatures used for the fabricated preforms. PFA addition Comp. pressure Ceramics in preform Sintering temp. wt-% MPa mass fraction °C 72 Al 2O 3 TiO 2 MgO Frit AO 0 60 1 1600 AOPC 10, 20, 30, 40 20, 60, 100 1 1450, 1500, 1600 AOPF 10, 20, 30, 40, 50 100 1 1500 AG 0 100 0.94 0.06 1000 AGPC 15 100 0.94 0.06 1000 TO 0 100 1 1100 TOPC 10, 20, 30 10, 20, 40, 60, 100 1 1050, 1100, 1150 MO 0 100 1 1300 MOPC 0, 10, 20 100 1 800, 900, 1000, 1100, 1200, 1300 The designation of the preforms denotes the ceramics phase, the pore former type and its mass content. For example, MOPC20 is a MgO preform where the pores were formed using 20% of the cellulose particles. Figure 3.4 Morphology of the spray dried powder granules (white).

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