SEM as a fiber analysis method for filtration systems quality control

By Luigi Raspolini - July 4, 2019

The production of filters and membranes undergoes several quality control steps to ensure that the properties of the product are up to specification. Different methods can be used for such fiber analysis, but only one can provide the best results. Find out in this blog how Scanning Electron Microscopes (SEM) can be used to investigate imperfections in metallic filters.

Different purpose – different filter

Whether we are aware of it or not, we use many filters on a daily basis – directly or indirectly. For example, tap water is filtered several times before undergoing chemical treatment, prior to becoming drinking water. This is to remove all kind of particles, from big rocks to small sand grains. In extreme cases, the filters can become so thick that they can catch bacteria.

Air conditioners also employ filtering systems. They are mostly used to trap dust and small particles, but combined with activated carbons, they can catch odors and purify the air.

Also, some chemical reactions catalysts are typically shaped as metallic meshes with micropores to let the chemicals flow while, at the same time, maximizing the exposed surface and minimizing the pressure required for the fluid to flow in the reactors.

Whatever their application, all filtration systems typically require cleaning or replacement, to make sure that they will operate correctly over time. The cleaning operation, combined with normal usage, will eventually determine the end of the life cycle of a filter, which will require a replacement.

            fiber analysis methodsfiber analysis sem

Comparison between BSD (image 1a) and SED (image 1b) image of a metallic mesh. In BSD images the color contrast highlights differences in the atomic weight of the analyzed material, making contamination detection easier. SED signal enhances the sample topography and the surface structure of the sample.

Deeper inspections with SEM

The easiest way to investigate a filter is obviously to observe it. Optical microscopes can, in many cases, provide plenty of information on larger filters, but in other cases the resolution that they provide is not sufficient for in-depth inspection.

Scanning electron microscopes, therefore, prove to be the best analysis technique to inspect any kind of filter thanks to the higher magnification and better resolution. SEM show not just the size of the pores in-between the fibers, but also the surface of the fibers themselves, their roughness and much more. So exactly what kind of data can be obtained?

One tool for multiple analysis

An electron microscope is an imaging tool that enables the user to perform a highly detailed visual inspection of the imaged samples. However, modern image processing techniques have turned these devices into automated analysis tools.

A clear example of this is the automated system to measure the diameter of fibers displayed in the image below. Here, software can reconstruct the direction of the fiber and measure it — an operation that would require tremendous efforts and a huge amount of time from an operator.

In the case of multi-layer materials or coated fibers, the SEM can be used to determine that the size of each layer has the right size, the distribution of the material on the substrate is homogeneous and the roughness is within specifications.

Filtration efficiency and product lifetime, as well as resistance to washing processes, can largely benefit from the improvements derived from knowing this information.

Beyond imaging: chemical composition analysis with EDS

Most electron microscopes are equipped with an EDS detector. This detector collects the EM radiation emitted by the sample to identify the nature of the element it is composed of.

This analysis can be used on the fiber’s material to determine, for example, how a different composition can enhance the lifetime of the filter. Even more interesting information can be obtained when analysis is performed on the collected particles, providing precious hints on their nature.

Image processing for measurement automation

Image processing technologies have improved dramatically over the last few years and software can now do tedious operations that would steal very precious time from an operator. Applying these techniques to fiber analysis has created a powerful tool to completely characterize not just the fibers diameter and their orientation, but also the dimensions of the holes that are formed in between.

                      Fiber analysis techniquessem-automatic-detection-fiber-analysis

An example of automatic detection of fibers (image 2a) and results displayed in a diameter size distribution histogram (image 2b).

                     sem-automatic-detection-poresscanning electron microscopy fiber analysis

An example of automatic detection of pores (image 3a) and results displayed in an area size distribution histogram (image 3b).

Discover more on how these results were obtained — download the free Fiber Metric sheet here:

                                          Download the FiberMetric software specification sheet

About the author

Luigi Raspolini is an application engineer for the Thermo Scientific Phenom Desktop SEM product range at Thermo Fisher Scientific. Luigi is constantly looking for new approaches to materials characterization, surface roughness measurements, and composition analysis. He is passionate about improving user experiences and demonstrating the best way to image every kind of sample.

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