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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

3.3.1. Preform ceramics

3.3.1. Preform ceramics The seleced ceramic powders of Al2O3, TiO2, MgO used for preform fabrication along with their properties are listed in Table 3.3. Table 3.3 Properties of dense oxide ceramic materials. Density and coefficient of thermal expansion (CTE) and Young’s modulus (149) ,heat capacity cp (150) , thermal conductivity λ (151) , micro hardness (152) . Temperature (°C) Al 2O 3 TiO 2 MgO Density ρ 25 kg/m³ 3980 4260 3580 Thermal conductivity λ 700 W/m·K 7,5 1,6 9,1 CTE 20..1000 10 -6 1/K 7,4 9,0 13,5 Heat capacity c p 100..1000 J/g·K 1,13 0,89 1,19 Elastic modulus E 25 GPa 410 270 317 Micro hardness HV 25 N/mm² 1860 1230 700 In contrast to the fabrication of the substrates for the sessile drop tests, where fine grained powders exhibiting median grain sizes below 1 µm were used, the particles used for preforms were coarser. This was to promote intragranular pore dimensions when sintering at relatively low temperatures. The properties of the powders used for preform preparation are listed in Table 3.4. Table 3.4 Designation used in this work and product specific data provided by the suppliers of the powders, which were used for preform fabrication. Designation code AO TO MO Ceramic type Al 2O 3 corundum TiO 2 rutile MgO Powder supplier /type Alcoa CL2500 Kronos 3025 Magnifin AM B21 Purity % 99.8 >99.0 99.5 Median grain size d 50 µm 1.8- 2.4 1.4- 1.8 0.8 The particle size distribution of the powder materials were measured at the University of Karlsruhe using a Sympatec Helos laser scattering (LS) particle analyser. The specific surface 69

areas, SsBET ,of the powders were measured by the nitrogen adsorption method using a Micromeritics ASAP 2010 system (153) . One type of preform, designated as AG, was fabricated by bonding the particles of the powder AO with a glassy binder in order to get preform chemical conditions close to that of Saffil preforms, where Al2O3 fibres are bonded together with a 2-5% by volume of a silicate binder (119) . The binder type 90263, which was purchased from DMC² (Germany), consisted of SiO2 with traces of Na2O and K2O. The starting point of softening was specified to be 570°C (154) . The ratio of binder to alumina particles was in accordance with that reported for Saffil-fibre preforms (119) . The composition of the AG replicated that used by Staudenecker (155) and thus no optimisation steps toward the target porosity range were required. 3.3.2. Pore forming agents (PFA) As the target ceramic volume fraction of 0.30 to 0.40 could not be reached solely by partial sintering of powder compacts, pore formers had to be added. Two types of organic agents, as specified in Table 3.5, were considered as suitable to form irregular or elongated pores. For irregular geometries, cellulose particles (PC) were used. For tubular pores, carbon fibres (PF) were added. In the as-purchased state, the carbon fibres were agglomerated and they could not be dispersed in the slurry. Thus the fibres had to be chopped by milling in a ball mill with zirconia balls using the same milling equipment as described in section 3.2.1. Different milling times were applied in order to reach the optimum compromise between dispersion in the slurry and fibre aspect ratio. The fibre length distributions were evaluated using a Zeiss Axioplan microscope equipped with an XYZ-stage and the Zeiss KS 400 image analyser. The particle size distribution of the cellulose particles was measured at the University of Karlsruhe using a Sympatec Helos laser scattering particle analyser. 70

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