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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

3.3.4. Sintering

3.3.4. Sintering Sintering of the green parts was performed in a Gero HTRH (Gero GmbH, Germany) alumina lined tube furnace with an inner diameter of 60 mm. A low initial heating rate was used to ensure that the organic compounds in the green parts were removed progressively. Thus a rate of 5°C/h was used up to 475°C with an air flow rate of 20 l/h inside the tube. Subsequently a rate of 300°C/h was applied under similar dynamic atmosphere. All preforms were sintered for 2 hours at the given temperatures (Table 3.6). Due to preform cracking during sintering in an oxidizing atmosphere, all preforms with PFA content of 20 wt.% or more, apart from magnesia preforms and those with PF as the pore former, had to be heat treated in an inert atmosphere prior to the oxidising sintering cycle in order to crack the cellulose. The two stage sintering process shown in Figure 3.5 was adopted. In the first stage heating took place at 5°C/h to 800°C with a flow of 5 l/h Ar (Figure 3.5 a). Figure 3.5 b) shows the heating profile in air. Temperature Temperature T / °C(°C) 1600 1400 1200 1000 800 600 400 200 0 a) argon flux 0 5 10 15 20 25 30 73 b) air flux 0 5 10 15 20 25 30 35 40 45 Process time time t /h(h) Figure 3.5 Pyrolysis and exemplary sintering cycle: a) pyrolysis in inert atmosphere. b) oxidation of organics and sintering.

The preform sintering process was optimised in accordance with the findings of earlier work (158) . The sintered preforms, which showed low sintering shrinkage, were ground to a standard size of 61 mm x 42 mm x 8 mm. 3.3.5. Porosity fractions After sintering, the specimen dimensions and weight were measured. The total porosity Φtot was calculated using the theoretical density of the ceramics. Further, the preform types with sufficient strength for handling and porosity were measured using Archimedes´ principle to determine the fraction of open-cell porosity in accordance to DIN 51918 (159) . The sample was first dried in an oven at 100°C for 1 h and then weighed (m0). It was then evacuated down to 50 mbar and immersed in distilled water. Following that, atmospheric pressure was applied for 30 min so that water fully entered the open pores of the preform and then reweighed immersed in water (m2). Ethanol was used instead of distilled water for the measurements on the magnesia preforms. Finally, the sample was taken out of the water, drops of the liquid were removed from its surface and then it was reweighed (m1). The apparent density ρr and the fraction of open porosity Φop in the preform were calculated from the following equation: ρ m ρ 0 r = f Equation 40 m1 − m2 where ρf was the density of the fluid used at test temperature. The values were taken from tables given by Lide (160) . The open cell porosity Φop is given by: m − m 1 0 Φ op = Equation 41 m1 − m2 The closed cell porosity ΦC was calculated as the difference between the total porosity Φtot and the open cell porosity Φop. 74

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