TEM Testing Principles and Common Sample Preparation Issues

Figure 1 FIB-TEM Instruments

TEM Testing Principles:

Transmission Electron Microscopy (TEM) works by directing a focused electron beam onto an ultrathin sample. Under an accelerating voltage at the anode, the high-speed electrons pass through the anode aperture and converge into a beam spot of a certain diameter, which illuminates the sample. The transmitted electron beam intensity or diffracted electrons are then magnified by the objective lens, forming a transmission electron image that reflects the sample’s internal microstructure. Further magnification by the intermediate and projection lenses produces a final image for detailed analysis.

Common Sample Preparation Issues for TEM:

1. Powder Sample Preparation Using Support Films Tends to Cause Agglomeration:

   • To mitigate this issue, ultrasonic dispersion can be used before sample preparation. Generally, powder samples should be ultrasonically dispersed in a solvent before being applied to the support film.

2. Methods for Preparing Powder Samples on Support Films:

   • Suspension Method: The powder is dispersed into a suspension using an ultrasonic disperser in a solvent that does not react with it. The suspension is then dropped onto the support film and dried.

   • Dispersion Method: Powder is directly sprinkled onto the support film, followed by tapping to remove excess particles, leaving a thin, well-dispersed layer on the film.

3. Cooling is Necessary for Electrolytic Twin-Jet Polishing:

   • Common cooling methods involve using liquid nitrogen mixed with alcohol. This is especially crucial for steel materials, where cooling is mandatory, preferably using liquid nitrogen alone.

4. Biological Sample Dehydration Requires a Gradient Approach:

   • High-concentration ethanol should not be used directly; instead, the ethanol concentration should be increased step by step to ensure proper dehydration.

5. Negative Staining is Required for Certain Biological Samples:

   • Biological macromolecules, bacteria, isolated organelles, protein crystals, viruses, and similar specimens must undergo negative staining before TEM imaging.

6. Osmium Tetroxide Used in Biological Sample Fixation is Highly Toxic and Volatile:

   • It should be handled carefully inside a fume hood due to its toxicity and volatility. Additionally, osmium tetroxide is expensive, so it must be used sparingly.

7. Uranyl Acetate Has Radioactive Properties but Poses Minimal Health Risks:

   • Although uranyl acetate is radioactive, its long half-life means it poses little radiation hazard to humans.

8. Pre-Thinning is Required Before Electrolytic Twin-Jet Polishing:

   • Samples typically need to be manually or mechanically ground to a thickness of about 50 μm before electrolytic twin-jet thinning. For magnesium alloys, the pre-thinned thickness is approximately 100 μm.

9. Advantages of Electrolytic Twin-Jet Polishing:

   • This method is easy to master, fast, causes no mechanical damage, does not alter the microstructure of the sample, and is relatively low-cost.