Evaluating Filters with Scanning Electron Microscopy

Fiber-based filtration systems clean everything from the air we breathe to the oil that runs our engines. Many filters, like high-efficiency particulate air (HEPA) and ultra-low particulate air (ULPA) filters, are made from glass or polymer fibers. Some are a woven mesh of metal fibers, others manufactured from are nonwoven media. Some filters even use a thin layer of nanofibers to increase performance and efficiency. Filter shapes, sizes, and compositions vary widely depending upon the application, but having access to efficient filters is important in homes, businesses, and manufacturing environments.

This application note will discuss the advantages of scanning electron microscopy (SEM) over optical microscopy for analyzing fiber-based filters. SEM provides high resolution images, elemental analysis, and - thanks to the FiberMetric software on the Phenom SEM - the ability to automatically measure thousands of fibers in mere minutes.

Advantages of SEM for Evaluating Filtration Fibers & Membranes

Advanced microscopy techniques are imperative to evaluating the performance of a filter. Optical inspection has been the industry standard for decades, allowing scientists to measure various fiber properties and monitor large contaminants. Optical inspection has become insufficient for many new applications because fiber dimensions, especially nanofibers, are often below the resolution limit of an optical microscope.

Optical vs. SEM images of air filtration media


With its higher resolution and depth of field, the scanning electron microscope (SEM) is becoming the new standard tool for characterizing filtration materials. Comparing the images taken of the same area of a nanofiber membrane using an optical microscope (left) and an SEM (right) reveals the differences in resolution and contrast between these two techniques.

SEM images of a defective pulse-cleaned air filter


These images show SEM micrographs of a nonwoven nanofiber membrane attached to the surface of a filter. Nanofiber membranes are often attached to air filtration media in order to maximize filter efficiency, airflow and lifetime. This particular filter did not pass testing because the nanofiber membrane is not complete. At some point during processing or transport, the nanofiber membrane detached, leaving exposed areas. The holes in the nanofiber membrane led to a decrease in performance for this particular filter.

Elemental Analysis for Foreign Particulates in Filtration

The contaminants captured by a filter can be identified using energy dispersive X-ray spectroscopy (EDS). This technique, which is integrated in the SEM, can give the elemental composition of a sample during imaging.

SEM image (left) of HVAC filter with particulates and corresponding EDS spectrum


The Phenom SEM: Automated Fiber Characterization

The Phenom SEM was designed to bring high quality SEM imaging into environments where a traditional SEM was not manageable or practical.  The Phenom SEM:

  • Is accessible and intuitive to use
  • Produces superior images
  • Provides high productivity and workflow 

The Phenom scanning electron microscope does not require a dedicated operator nor additional infrastructure. The innovative column design allows imaging of fibers without sputter coating. 

Automatically Measure the Dimensions of Filtration Fibers by SEM

FiberMetric HEPA filter

The Phenom desktop SEM uses FiberMetric, a proprietary fiber measurement software package, which measures fiber diameter, orientation and pore sizes.  Using the integrated motorized XY stage, data acquisition and analysis can easily be automated. This combination of software and hardware allows for hundreds of images to be acquired automatically, resulting in tens of thousands of individual fiber measurements, providing statistically significant data. More information on the FiberMetric software can be found here.

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