13313 Porosity

Although mercury porosimetry has long been used to obtain porosity measurements [23], it has two limitations: (1) only small (<1 cm3) specimens can be used, and (2) it does not yield spatial distribution information. Three-dimensional data for samples with connected porosity can be obtained with NMR imaging. This method provides information on specimens of any size and shape. To use NMR imaging methods special filler fluids with NMR-sensitive nuclei must be used, and adequate penetration of filler fluid must be obtained into all the internal volumes of interest. Ellingson et al. [25] have described one example of using MRI to measure bulk porosity on a set of partially sintered Al2O3 disks. The disks were each 25 mm in diameter and had densities of 1.640, 1.703, and 1.720 g/cm3. The filler fluid used was benzene with a proton molarity of 67.3 M compared to water with 111 M. Figure 13.6 shows the relation between NMR image data for bulk porosity and density of an Al2O3 partially sintered compact. Discrimination of the porosity in the interior and at the edges of the disks was achieved. The higher porosity at the edges was expected based on knowledge of powder compaction.

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