Particulate composite materials, their microstructure and macroscopic properties

What is it about?

Properties of a composite material made of a continuous matrix and particles often depend on microscopic details, such as contacts between particles. Material microstructure is reproduced in reconstruction process that naturally calls for implementing two opposing demands on the resulting 3D image: maximum resolution and maximum volume. When FIB-SEM tomography is used for this purpose, a stack of 2D digital intensity images is set up. The stacked 2D images must be further treated for a 3D mathematical model of spatial distribution of phases (a replica) to be obtained. The treatment includes spatial alignment of the stacked 2D images, smoothing and suppressing noise by means of filtering in the spatial domain, and, finally, segmentation that uniquely assigns a phase to each point in 3D space. Each of these steps must be performed carefully for microstructural details to be reproduced reliably as much as possible.

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Why is it important?

To demonstrate effects of careful image processing on conservation of microstructural details, we reconstructed microstructures of three mixed-matrix membranes made of a polyimide matrix (a continuous phase) and silicalite-1 particles (a dispersed phase) that differed in volume fractions of silicalite-1. To achieve this goal, we combined a diffusion filter with a graph-based segmentation algorithm. The anisotropic non-linear diffusion filter was capable of well-localizing and even enhancing phase interface, which was considered essential for the reconstruction procedure. Power watershed supplemented with a seeding algorithm based on geodesic reconstruction from the markers was found to be an excellent choice for segmenting 3D images of non-uniform illumination. The tentative reconstruction with different parameters of the individual algorithms showed that a number of silicalite-1 particles could form false contacts that affected macroscopic properties of the mixed-matrix membranes, specifically the effective permeability. In spite of this fact, we suppose that a number of silicalite-1 particles create real large clusters, the existence of which is a key microstructural feature determining the effective permeability.