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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

is attributed to the

is attributed to the relatively large fraction of very fine porosity in this hybrid preform of Saffil fibres and fine-grained titania powder. Table 4.2 Overview of the mercury intrusion porosimetry results of the preforms. Total porosity MIP porosity Deviation Median pore dia. 125 mass volume preform volume porosity Extrusion residual porosity Φ tot Φ Hg Φ Hg - Φ tot d Hg S sHg S pHg S iHg Φ ext µm m²/g Specific surface area per unit 10 6 ·m²/m³ AO 37% 34% -3% 0.77 1.51 3.80 10.37 0.39 AOPC20 65% 58% -7% 3.48 0.86 1.20 1.84 0.18 AOPF20 60% 56% -4% 7.00 0.69 1.10 1.83 0.69 AGPC15 66% 57% -9% 2.77 1.49 2.00 3.04 0.14 TO 44% 44% 0% 0.86 1.47 3.51 7.97 0.23 TOPC10 62% 53% -9% 1.06 2.30 3.72 6.01 0.20 TOPC20 65% 57% -8% 4.98 0.76 1.13 1.74 0.55 MO 50% 50% 0% 1.84 0.75 1.34 2.69 0.06 MOPC20 62% 56% -6% 1.50 0.78 1.05 1.68 0.13 AODY30 68% 67% -1% 9.05 2.41 3.02 4.45 0.97 FATO 71% 68% -3% 8.53 5.85 6.50 9.15 0.47 The inhouse fabricated preforms with PFA exhibited SsHg values between 0.69 m²/g and 2.30 m²/g. The latter value (TOPC10 sintered at 1050°C) resulted from the relatively low sintering temperature when compared to TOPC20, the preform fabricated with similar ceramic powder but higher PFA content and sintered at a higher temperature 1100°C. When the specific surface area per unit volume (SiHg) is compared to that per unit mass (SsHg), the preforms fabricated without PFA (namely AO, TO and MO) showed a significant difference between the two values. This was a result of the lower total porosity and therefore the larger preform volume necessary to obtain an equal surface area to that in the highly porous preforms, where PFA was used to form coarse porosity. The specific surface areas per unit pore volume of AOPC20, AOPF20, TOPC20 and MOPC20 were in the range of

1.71 to 1.98·10 6 m²/m³. The similar values enabled direct comparison in the infiltration experiments. The AGPC15 preform was sintered with the aid of a glassy frit at a relatively low temperature of 1000°C. As the Al2O3 powder showed no significant sintering activity at this temperature, the initial grain size of the AO powder was presumably maintained. Therefore a comparatively large surface area of 3.05 m²/m³ could be achieved compared to that of the Al2O3 preform without frit additions (AOPC20) sintered at significantly higher temperatures. The reference preforms, AODY30 and FATO, exhibited larger SiHg values of 4.45 m²/m³ and 9.15 m²/m³ respectively, attributed to the comparatively high fraction of small pores. The relative residual non-extruded porosity Φext describes the relative volume still intruded when an ambient pressure of 0.101 MPa is reached which corresponds to a pore diameter of 11 µm using Washburn´s equation (Equation 21). It was shown that Φext ranged between 0.06 and 0.97. The lower value for the MO preform indicates that 6% of the total mercury volume was still trapped in the preform after the extrusion procedure. In contrast, the highest value was that of the AODY30 preform, which indicates that about 97% of the mercury remained in pores with an entrance diameter of more than 11 µm. This indicates that the characteristic window connections between the larger spherical cavities predominantly had diameters smaller than 11 µm. 4.6.3 Compressibility The quasi-static compression behaviour of the preforms was investigated using two testing conditions. In the first, the preform samples were loaded between two platens such that the sample could yield laterally in the x- and y-directions. The specific compression strength σc was the ratio of the maximum load to the initial loading area. In the second, samples were loaded isostatically and the pressure dependent volumetric compression, ciso , recorded. 126

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