How-to: high-quality fiber analysis through proper SEM sample preparation

By Marijke Scotuzzi - Jan 18, 2018

Fibers are generally imaged in a scanning electron microscope (SEM), which provides high-resolution images, elemental analysis, and the possibility of automatically measuring thousands of fibers in mere minutes. But in some cases, imaging fibers with a SEM also presents challenges as the nature of some fibers might compromise the quality of your analysis. With this in mind, this blog describes how you can obtain high-quality imaging and fiber analysis through proper SEM configuration and sample preparation.

Learn SEM sample preparation best practices for obtaining high-quality fiber analysis
Download the free sample preparation e-Guide

 

Fiber types and uses

We can distinguish between two different fibers types, natural and man-made. Natural fibers can be classified according to their origin. For example, vegetable fibers based on cellulose and used in the manufacturing of paper or cloth; animal fibers, such as wool; mineral fibers, like asbestos; and biological fibers, including muscle proteins, spider silk and - hair.

Man-made fibers can range from the synthetic fibers used in the petrochemical industry, to metallic fibers, fiberglass and optical glass; They also include polymer fibers, which are made of polyethylene, the most common plastic used for packaging.
Fibers can be woven into textiles or deposited as non-woven sheets to make filters, insulation, envelopes, or disposable wipes.

In the production process of these objects and devices, checking the quality of fibers plays a very important role, the key parameters being the fiber diameter and the size distribution of the fibers. For this step, sophisticated analysis techniques are required to ensure the fibers’ quality during manufacturing.

For example, in the filtration industry, checking the quality of the manufactured fiber textiles is of upmost importance to guarantee the filtration efficiency.

Imaging of conductive fibers with SEM: what is important?

Conductive fibers such as metal grids can be easily imaged in a SEM without any difficult sample preparation. The specimen containing the fibers is positioned on a pin stub and then placed on a holder that can be inserted into the microscope.

For high-resolution imaging, we recommend high acceleration voltage (10kV or 15kV) and low current, whereas for composition elemental analysis high current is preferred. Figure 1 shows two examples of metallic fibers in a regular grid, imaged using a 15kV (left) and a 10kV (right) beam.

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Figure 1: SEM images of two metal grids, using 15kV (left) and 10kV (right) beam.

Imaging of non-conductive fibers with SEM: what is important?

While imaging conductive samples is, in most cases, rather straightforward, in the case of non-conductive samples, the sample preparation plays a crucial role in successfully acquiring informative images. In fact, the imaging in a SEM is done by scanning the electron beam on the surface of a specimen.

If the sample is non-conductive, the negative charges build up on the surface, leading to a charging effect that compromises the quality of the analysis. A few different tricks can be applied during sample preparation to limit the effects of charging.

To limit the effect of charging, insulating samples can be imaged with a low acceleration voltage and low beam current. However, with this method the image resolution deteriorates because of the low electron energy. To overcome this limitation, non-conductive fibers can be coated with a thin conductive film that allows imaging at high acceleration voltage.

Figure 2 shows a SEM micrograph of a fabric covered with a 10 nm gold film deposited using a sputter coater. In this example, no charging artefacts are visible, and the image quality is preserved. However, because of the 3D dimensionality of fibers, in some cases the sputtered metal might not reach the underlying fibers, which will therefore charge under the electron beam.


sem-image-cotton-fabric-coated-gold.jpeg

Figure 2: SEM image of a fabric coated with 10 nm of gold using a 15kV beam.


To avoid charging, it is also possible to image non-conductive fibers in low vacuum mode. The presence of air molecules in the microscope chamber allows the electrical charges to find a conductive path and leave the specimen surface.

Figure 3 shows the SEM images of a human hair taken in high vacuum (left) and low vacuum (right), using the charged reduction sample holder. In the first image, the charging effect shows up as a brightening of the top surface of the hair, hiding the surface details. In the second image, the charging effects are eliminated by using the low vacuum mode and the surface details are now visible.

 

            sem-image-human-hair-high-vacuum.jpeg sem-image-human-hair-low-vacuum.jpeg

Figure 3: SEM images of a human hair imaged in high vacuum (left) where charging is visible and in low vacuum (right), using the charged reduction sample holder

Checking fiber quality: the tensile test

Tensile tests are also required in some production lines to check how resistant fibers are when stretched. Performing tensile tests in a SEM allows the user to check, in real time, how the fiber textile stretches under the presence of a force and how the fibers break. Figure 4 shows a strip of paper that was previously covered with 10 nm of gold, being torn using the tensile stage.

 

sem-image-strip-paper-coated.jpeg

Figure 4: SEM images of a strip of paper being stretched using the tensile stage

Sample preparation techniques for SEM

As you can see, sample preparation best practices are crucial for obtaining high-quality SEM images (of fibers). These best practices are described in more detail in our sample preparation e-Guide.

If you would like to discover more sample preparation tips and tricks, I highly recommend downloading the free guide below. I really think you will find it useful!

 desktop-sem-sample-preparation-e-guide


About the author

Marijke Scotuzzi is an Application Engineer at Thermo Fisher Scientific, the world leader in serving science. Marijke has a keen interest in microscopy and is driven by the performance and the versatility of the Phenom desktop SEM. She is dedicated to developing new applications and to improving the system capabilities, with the main focus on imaging techniques.

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