Ptychographic computed X-ray tomography reveals structure of porous membranes on the nano scale

Article published in Communications Materials presents significant findings and discusses the possibilities offered by this technique combined with synchrotron light sources

Porous materials play key roles in a variety of contexts, from transporting water and nutrients in biological systems to storing oil and water in reservoirs of rock. And synthetic polymer membranes are essential to separation processes, as in the case of chromatography. They have well-established applications in water desalination, hemodialysis, and gas separation, and these uses are expanding into processes that filter out pollutants from contaminated water. Their benefits include energy efficiency, smaller carbon footprint, and compact design that provides a large area of membrane in a small volume.

Membranes that can fulfill technological objectives have complex porous structures that ensure the required selectivity, mechanical stability, and characteristics for rapid transport; the effectiveness and performance of these membranes is defined by characteristics such as porosity and interconnectivity, which can be particularly difficult to measure when they are brought down to the nano scale.

The limitations of electron microscopy and advantages of ptychographic X-ray computed tomography

Techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) have helped scientists better understand the transport mechanisms in these applications and develop membranes for different purposes. But even though they are powerful, these techniques also have significant limitations: the samples must be dehydrated and covered with a metallic layer, and must also remain in a vacuum during analysis, which can affect the structure of these membranes and hinder analysis under near-real conditions.

Furthermore, when the pores of the material reach the nano scale, the total sample volume must be significantly reduced in order to attain the resolution necessary for analysis. Within this context, X-ray tomography has emerged as a good alternative. This method not only offers non-destructive visualization, but samples can be analyzed in ambient conditions with significantly larger total sample volumes.

Conventional X-ray tomography, which analyzes different absorption in different parts of a sample, faces challenges related to resolution limits when analyzing less dense materials (such as membranes). But as new fourth-generation synchrotron light sources have recently come online and ptychographic X-ray tomography has been developed, images of these materials can be obtained with nanometric resolution.

Ptychographic X-ray computed tomography (PXCT) is a powerful phase-contrast imaging technique that uses a series of two-dimensional projections of the object from different angles to reconstruct its three-dimensional structure in high resolution, revealing information about porosities and interconnectivity.

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