Metallographic and electron microscopy variations
Principles of Scanning Electron Microscopy
Scanning Electron Microscope (SEM), abbreviated as SEM, is a complex system; it condenses electron optical technology, vacuum technology, fine mechanical structure and modern computer control technology. The scanning electron microscope collects the electrons emitted by the electron gun into a fine electron beam through a multi-stage electromagnetic lens under the action of accelerated high voltage. Scan the surface of the sample to stimulate various information, and analyze the surface of the sample by receiving, amplifying and displaying the information. The interaction of the incident electrons with the sample produces the types of information shown in Figure 1. The two-dimensional intensity distribution of these information changes with the characteristics of the sample surface (these characteristics include surface morphology, composition, crystal orientation, electromagnetic properties, etc.), and the information collected by various detectors is sequentially and proportionally converted A video signal is sent to a synchronously scanned picture tube and its brightness is modulated to obtain a scan image reflecting the surface condition of the sample. If the signal received by the detector is digitized and converted into a digital signal, it can be further processed and stored by a computer. The scanning electron microscope is mainly used to observe thick samples with large height difference and roughness, so the depth of field effect is highlighted in the design, and it is generally used to analyze fractures and natural surfaces that have not been artificially processed.
Electron Microscope and Metallographic Microscope
1. Different light sources: metallographic microscopes use visible light as the light source, and scanning electron microscopes use electron beams as the light source for imaging.
2. The principle is different: the metallographic microscope uses the principle of geometric optical imaging for imaging, and the scanning electron microscope uses high-energy electron beams to bombard the sample surface to stimulate various physical signals on the sample surface, and then use different signal detectors to receive physical signals and convert them into images information.
3. The resolution is different: due to the interference and diffraction of light, the resolution of the metallographic microscope can only be limited to 0.2-0.5um. Because the scanning electron microscope uses electron beams as the light source, its resolution can reach between 1-3nm. Therefore, the tissue observation of the metallographic microscope belongs to micron-scale analysis, and the tissue observation of scanning electron microscope belongs to nano-scale analysis.
4. The depth of field is different: the depth of field of a general metallographic microscope is between 2-3um, so it has extremely high requirements on the smoothness of the surface of the sample, so the sample preparation process is relatively complicated. The scanning electron microscope has a large depth of field, a large field of view, and a three-dimensional imaging, which can directly observe the fine structure of the uneven surface of various samples.
