Devices and objects produced from fibers are being increasingly used in everyday life. Fibers are generally imaged in a scanning electron microscope (SEM), which provides elemental analysis, high-resolution images and the possibility of automatically measuring thousands of fibers in just minutes.
However in certain cases, imaging fibers with a SEM also puts forth challenges, as the nature of a few fibers might compromise the quality of the analysis. Concentrating on this aspect, this article explains how to obtain a high analysis quality via proper SEM configuration and sample preparation.
It is possible to distinguish two different kinds of fibers, man-made and natural. Natural fibers can be categorized in vegetable fibers, which are for example based on cellulose and employed in the manufacturing of cloth or paper, mineral fibers, like asbestos, animal fibers, such as wool and biological fibers, including spider silk, muscle proteins and also hair.
Man-made fibers range from the synthetic fibers used in the petrochemical industry, to metallic fibers, from optical glass and fiberglass to polymer fibers, which comprise of polyethylene, considered to be the most common plastic used for packaging. Fibers can be woven into textiles or deposited as nonwoven sheets in order to make insulation, filters, disposable wipes or envelopes.
In the production process of these devices and objects, the quality check of fibers plays a greatly significant role, where the size distribution and fiber diameter of the fibers are the vital parameters. For this step, refined analysis techniques are needed to guarantee the quality of the fibers during manufacturing. For instance, in the filtration industry, the quality check of the manufactured fiber textiles is of upmost importance to ensure the filtration efficiency.
Imaging of Conductive Fibers: What is Important?
Conductive fibers such as metal grids can be effortlessly imaged in a SEM without any complex sample preparation. The specimen comprising of the fibers is arranged on a pin stub and then positioned on a holder capable of being inserted into the microscope. High acceleration voltage (10 kV or 15 kV) and low current are recommended for high resolution imaging, while for composition elemental analysis high current is ideal. Figure 1 displays two examples of metallic fibers in a regular grid, imaged using a 15 kV (left) and a 10 kV (right) beam.
Figure 1 & 2. SEM images of two metal grids, using 15 kV (top) and 10 kV (bottom) beam.
Imaging of Non-Conductive Fibers: What is Important?
Even though imaging conductive samples is, in several cases, relatively straightforward, in the case of non-conductive samples the sample preparation plays a vital role in successfully obtaining informative images. The imaging in a SEM is in fact performed 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, resulting in a charging effect, compromising the quality of the analysis. It is possible to apply a few different tricks during sample preparation in order to limit the effects of charging.
Insulating samples can be imaged with a low acceleration voltage and low beam current in order to limit the effect of charging. However, with this method the image resolution deteriorates due to the low electron energy. Coating of non-conductive fibers with a thin conductive film that permits imaging at high acceleration voltage will help overcome this drawback.
Figure 2 displays a SEM micrograph of a cotton cloth covered with a 10 nm gold film deposited with the help of a sputter coater. In this example, no charging artifacts are visible and the quality of the image is preserved. However, in certain cases the sputtered metal might not reach the underlying fibers because of the 3D dimensionality of fibers, and will therefore charge under the electron beam.
It is also possible to image non-conductive fibers in low vacuum mode to avoid charging. The presence of air molecules in the microscope chamber permits the electrical charges to detect a conductive path and leave the specimen surface. Figure 3 displays the SEM images of a human hair taken in high vacuum (left) and low vacuum (right), using the charged reduction sample holder.
The charging effect shows up as a brightening of the top surface of the hair, hiding the surface details, in the first image. The charging effects are eradicated by using the low vacuum mode and the surface details are now visible as displayed in the second image.
Figure 3. SEM image of a cotton fabric coated with 10 nm of gold using a 15 kV beam.
Figure 4 & 5. SEM images of a human hair imaged in high vacuum (top) where charging is visible and in low vacuum (bottom), using the charged reduction sample holder.
Checking the Quality of the Fibers: The Tensile Test
Tensile tests are also needed in some production lines in order to check how resistant fibers are when stretched. By performing tensile tests in a SEM, the user is allowed to check, in real time, how the fibers break or how the fiber textile stretches under the presence of a force. Figure 6 displays a strip of paper that was earlier covered with 10 nm of gold, being torn using the tensile stage.
Figure 6. SEM images of a strip of paper being stretched using the tensile stage.
There are specific best practices for sample preparation that users can follow in order to obtain high quality SEM images.
This information has been sourced, reviewed and adapted from materials provided by Phenom-World BV.
For more information on this source, please visit Phenom-World BV.