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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

3.3.6. Pore structure

3.3.6. Pore structure The pore size distribution measurements were carried out using the mercury intrusion method on an Autopore III-9400 system (Micromeritics, USA). Specimens of size 6 mm x 6 mm x 6 mm were first dried for at least 24 h at 95°C. Subsequently, they were placed into a quartz glass penetrometer, which is shown schematically in Figure 3.6 b). The penetrometer was then evacuated to a pressure of 6.7 Pa. The mercury was added and the low pressure measurement programme performed to a maximum pressure of 0.26 MPa. Subsequently the high pressure analysis was performed in the high pressure chamber in the range of 0.26 MPa to 227 MPa. The equilibration time for each pressure increment, which is shown as a single data point in the pressure-time schedule in Figure 3.6 a), was 10 s after which the volume of intruded mercury was measured. a) a) b) Pressure (MPa) 100 10 1 0.1 0.01 0 500 1000 1500 2000 2500 3000 Time (s) Figure 3.6 a) time-pressure schedule of pressurization in the porosimeter. b) schematic of the penetrometer set-up fitted in the pressurization chamber fitted with a preform sample. To determine the fraction of bottle-neck shaped pores, the pressure was decreased incrementally after the highest pressure was reached to give the extrusion curve. The lower limit in the high pressure chamber was ambient pressure. For safety reasons, low pressure extrusion analysis could not be used after the high pressure analysis due to the drastic increase 75 oil inlet penetrometer filled with Hg preform ΔΔV ΔΔV Δ Δ Δp

in the evaporation of mercury at low pressure. The pore fraction still intruded after the extrusion programme was assessed as the residual porosity after extrusion Φext. The difference between the geometrically measured Φtot and the porosity intruded by mercury, ΦHg , at the maximum pressure was calculated in order to determine the sum of the closed cell porosity and the compression of the sample. The median pore diameter dHg was determined on the basis of the intrusion curve and represents the diameter at which 50% of the total porosity was intruded. S iHg = S sHg ( 1 φ ) ρ − φ tot tot 76 Equation 42 The specific surface area per unit mass SsHg was calculated in accordance to Equation 22. The specific surface area per unit volume of porosity SiHg was calculated using Equation 42: 3.3.7. Permeability The permeability of the preforms was measured along and perpendicular to the uniaxial pressing direction of the initial green compacts. This was carried out as anisotropic permeability was expected in accordance to the work on uniaxial formed fibre preforms by Mortensen et al. (112) . For measurements along the pressing direction, disk-shaped samples 30 mm in diameter and 7 mm high were machined from the standard preform geometries. The perpendicular direction was measured applying a cross-sectional cut as shown in Figure 3.7 a). The samples were bonded into aluminium rings using a high viscosity hot-melt adhesive to prevent infiltration. Prior to testing, the samples were evacuated and immersed in water for 12 h to completely fill the open porosity. When measuring magnesia preforms, ethanol was used instead of water to prevent hydroxide formation. For measurement, the ring was mounted to the end of a vertical tube. The tube was filled with the fluid and a controlled gas pressure was applied in the range

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