Differences between electron microscope and metallographic microscope
Principle of scanning electron microscope
Scanning ElectronMicroscope (SEM) is a complex system. Electronic optics technology, vacuum technology, fine mechanical structure and modern computer control technology are concentrated. Scanning electron microscope (SEM) collects electrons emitted by electron gun into fine electron beams through multistage electromagnetic lenses under the action of accelerated high voltage. Scanning the sample surface can stimulate various information, which can be received, amplified, displayed and imaged to analyze the sample surface. The incident electrons interact with the sample to generate information types as shown in Figure 1. The two-dimensional intensity distribution of these information varies with the characteristics of the sample surface (these characteristics include surface morphology, composition, crystal orientation, electromagnetic characteristics, etc.). The information collected by various detectors is converted into video signals in sequence and in proportion, and then transmitted to the synchronous scanning kinescope to modulate its brightness, so that a scanning picture reflecting the surface condition of the sample can be obtained. If the signal received by the detector is digitized and converted into a digital signal, it can be further processed and stored by the computer. Scanning electron microscope (SEM) is mainly used to observe thick samples with large height difference and roughness, so it highlights the depth of field effect in design and is generally used to analyze fractures and natural surfaces without manual treatment.
Electron microscope and metallographic microscope
First, the light source is different: the metallographic microscope uses visible light as the light source, and the scanning electron microscope uses electron beam as the light source for imaging.
Second, the principle is different: metallographic microscope uses geometric optics imaging principle to image, scanning electron microscope uses high-energy electron beam to bombard the sample surface, which excites various physical signals on the sample surface, and then uses different signal detectors to receive physical signals and convert them into image information.
Third, the resolution is different: because of the interference and diffraction of light, the resolution of metallographic microscope can only be limited to 0.2-0.5um. Because scanning electron microscope uses electron beam as light source, its resolution can reach 1-3nm, so the microstructure observation of metallographic microscope belongs to micron-scale analysis, and the microstructure observation of scanning electron microscope belongs to nano-scale analysis.
Fourth, the depth of field is different: Generally, the depth of field of metallographic microscope is between 2-3um, so the surface smoothness of the sample is extremely high, so the sample preparation process is relatively complicated. The scanning electron microscope, on the other hand, has a great depth of field, a large field of vision and a three-dimensional image, which can directly observe the fine structure of the uneven surfaces of various samples.
