The mixing state of fine powders measured by magnetic resonance imaging

The usefulness of magnetic resonance imaging (MRI) for the spatially resolved, quantitative characterization of mixtures of fine powders in the size range of about 10 μm is investigated. The sources of errors inherent in the signal translation process between raw MRI data and local concentrations are analyzed in detail. Possibilities for their correction are pointed out in deriving a global characteristic such as the standard deviation.The analysis of the mixing state of powders by MRI and the benefits of error correction are then illustrated by simple examples covering a wide range of compositions and mixing states. These experiments were made by mixing spherical, oil-filled (hence “visible” to MRI) melamine microcapsules in the size range of about 1-20 μm with solid melamine spheres of similar size and density. The mixture volume of typically 1 cm³ was imaged with an isotropic resolution of 235 μm, providing over 60,000 contiguous data points per measurement. The benefit of error correction is shown to be significant, provided the mixtures are homogeneous to a large extent.