Sprayed parallelepiped samples of the following FRMs were obtained from their manufacturers: one FRM with a gypsum binder and two FRMs containing mineral fibers with a binder based on portland cement. Samples of each material were sent to a private testing laboratory (Anter Laboratories, 2004) for experimental measurement of their thermal conductivities as a function of temperature using a standard hot wire technique (ASTM, 2004). The testing laboratory reported their results for thermal conductivity to be normally within ± 3 %. For each material, nominal 25 mm (1") diameter cylindrical cores were carefully extracted using a utility knife. The cores were then placed in 27 mm inner diameter (ID) polypropylene tubes and mounted for viewing by the x-ray microtomography system at the Center for Quantitative Imaging at the Pennsylvania State University *.
Volumetric x-ray CT data were collected using the facility's microfocus x-ray source at a voltage setting of 185 kV and a low tube current of 50 µA to minimize focal spot size and thus optimize spatial resolution. We positioned the sample in the gantry to magnify the sample, and calibrated the resulting voxel dimensions against a dimensional standard for precision and to allow combining adjacent data sets without overlapping or skipping portions of the sample volume.
Because of the different microstructures present in the two types of FRMs examined in this study (e.g., characteristic feature dimensions), the microtomography data sets were acquired with different voxel dimensions. The microtomography data sets for the gypsum-based FRM were acquired with voxel dimensions of dx =dy =0.0273 mm and dz =0.0361 mm. Each data set consisted of a 1024 x 1024 x ≈750 array of 16-bit x-ray absorption values on an arbitrary scale. For this sample we collected approximately 750 slices covering a volume about 28 mm long by 28 mm in diameter. For the fiber/cement-based FRMs, in order to cover a larger representative sample volume, the magnification was reduced by repositioning the sample between the source and the detector, resulting in voxel dimensions of dx =dy =0.0586 mm and dz =0.0740 mm, for data arrays 512 pixels by 512 pixels by 492. Thus, the data cover sample volumes about 36 mm long by 30 mm in diameter. Each individual two-dimensional slice was available as a 16-bit tiff-format image for further processing as described below.