X-Ray Computed Microtomography

Three projects will be highlighted:

1.  A study of how random shape affects particle size measures of a random powder (crushed waste glass),
2.  A study of the pore structure of a polymeric foam and how the XCT images can be used as the basis for finite-element-based computations of mechanical properties
3.  An on-going study of the 3D shape of lunar soil.

1.  At least 10 million tons of waste glass is collected across the US every year, probably with much more going into landfills. If this waste glass is crushed to a fine powder, it can be recycled as new glass or as a component of composite materials such as concrete. The particle shape is important in itself for these applications. It is also an ideal test case for comparing the results of various particle size measurements, used widely across the US every day. Almost all of these measurements analyze a powder assuming its particles are spheres of various sizes; this is of course rarely the case. Crushed powder was prepared from a waste glass source and sieved to between 25 micrometers and 38 micrometers in “size” – sieve sizes are only loosely related to true particle size for non-spherical particles. Laser diffraction particle size measurements were also made on this powder size class. The XCT was used to scan this powder, giving the true 3D particle shapes for about 20 000 particles. The XCT data was used to explain the differences between the sieve analysis and the laser diffraction results.

2.  How the elastic properties of closed-cell foams, nominally made with the same base polymeric material, depend on variables such as total porosity and cell size distribution, was elucidated using large-scale elastic simulations acting on foam XCT images. The actual cross-sectional foam images were perfectly circular, because of the particular XCT technique used called “interior” tomography (see Figure 2). The circular boundaries are mathematically cut, not physically cut. A special finite element technique was developed that operated on the entire cylindrical 3D image (many circular images stacked together make a cylinder), so that no XCT data had to be thrown away. This project was able to show that different foams from the same manufacturer that were assumed to have identical materials making up their backbone instead had solid backbones that had quite different elastic properties. The study was also able to show how foam elastic properties depended on porosity and cell size, in agreement with experiment.

3.  The particle shape analysis techniques using the XCT has now been turned towards analyzing the 3D shape of lunar soil particles, an on-going project. Under the instigation of a colleague from the NASA JPL laboratory, two 1-gram samples of lunar soil have been borrowed from the NASA CAPTEM program, one collected during the Apollo 11 mission (first man on the moon) and the Apollo 14 mission (first golf ball hit on the moon!). The idea behind this project is the following. Measuring the way sunlight scatters from the surface of airless objects in the solar system, including most especially the moon, is the best way, bar satellites in orbit or on the ground, of getting information about that surface. The scattering is studied as a function of angle and polarization and wavelength, but what has always been lacking is precise knowledge of what at the surface is doing the scattering – in the case of the moon, it is mainly lunar soil particles. This project is first to capture the 3D particle shape, then compute the light scattering that comes from those particular shapes. With this knowledge, one will be better able to interpret the sunlight scattering data in terms of a collection of random shapes, extracting more real information from the data about the lunar surface and, by analogy, other airless solar system surfaces.