Imaging fibers with a SEM: how to obtain a flawless quality analysis

By Marijke Scotuzzi - Dec 6, 2018

In our daily life, we make use of a large amount of objects and devices that are produced from fibers. Fibers are usually 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 a high analysis quality through proper SEM configuration and sample preparation. 

We can distinguish two different kinds of fibers, natural and man-made. Natural fibers can be classified in vegetable fibers, that are for instance 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 also hair. Man-made fibers range from the synthetic fibers used in the petrochemical industry, to metallic fibers, from fiberglass and optical glass to polymer fibers, which comprises polyethylene, that is the most common plastic used for packaging. Fibers can be woven into textiles or deposited as nonwoven sheets to make filters, insulation, envelopes, or disposable wipes.

In the production process of these objects and devices, the quality check of fibers plays a role of great importance, where the fiber diameter and size distribution of the fibers are the key parameters. For this step, sophisticated analysis techniques are required to ensure the fibers quality during manufacturing. For example, in the filtration industry, the quality check of the manufactured fiber textiles is of upmost importance to guarantee the filtration efficiency.

Imaging of conductive fibers: 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, while 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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Fig. 1 & 2: SEM images of two metal grids, using 15kV (left) and 10kV (right) beam.

Imaging of non-conductive fibers: 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, compromising 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 cotton cloth covered with a 10 nm gold film deposited using a sputter coater. In this example, no charging artifacts 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.

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.

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Fig. 3: SEM image of a cotton fabric coated with 10 nm of gold using a 15kV beam.

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Fig. 4 & 5: 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 the fibers 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 6 shows a strip of paper that was previously covered with 10 nm of gold, being torn using the tensile stage.

 

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Fig. 6: SEM images of a strip of paper being stretched using the tensile stage

As you can see, there are specific best practices for sample preparation that you can follow to obtain a high quality SEM images. If you would like to discover more sample preparation tips and tricks, you can download our free sample preparation e-guide here

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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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