Applications of Scanning Electron Microscopy
Scanning electron microscope is a multifunctional instrument with many superior properties, and it is the most widely used instrument. It can perform the following basic analysis:
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(1) Observation and analysis of three-dimensional shape;
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(2) While observing the morphology, the composition analysis of the micro-area is carried out.
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① Observe nanomaterials. The so-called nanomaterials refer to the solid materials obtained by pressurizing the particles or crystallites that make up the materials in the range of 0.1 to 100 nm and keeping the surface clean. Nanomaterials have many unique physical and chemical properties that are different from crystalline and amorphous states. Nanomaterials have broad development prospects and will become the key direction of future material research. An important feature of scanning electron microscopy is its high resolution, which has been widely used to observe nanomaterials.
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② Analyze the material fracture. Another important feature of the scanning electron microscope is that the depth of field is large and the image is full of three-dimensionality. The depth of focus of a scanning electron microscope is 10 times larger than that of a transmission electron microscope and hundreds of times larger than that of an optical microscope. Due to the large depth of field of the image, the scanned electronic image obtained has a three-dimensional shape and can provide much more information than other microscopes. This feature is very valuable to users. The fracture morphology displayed by the scanning electron microscope presents the essence of material fracture from the perspective of deep level and high depth of field. It plays an irreplaceable role in teaching, scientific research and production. Aspects such as the determination of rationality are a powerful tool.
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③ directly observe the original surface of the large sample. It can directly observe samples with a diameter of 100 mm, a height of 50 mm, or larger sizes, without any restrictions on the shape of the sample, and rough surfaces can also be observed, which saves the trouble of preparing samples and can truly observe the samples The contrast of the different material components of the sample itself (back reflection electron image).
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④ Observe the thick sample. When observing thick samples, it can obtain high resolution and the most realistic shape. The resolution of scanning electron microscopy is between that of light microscopy and transmission electron microscopy. However, when comparing the observation of thick samples, because the lamination method is still used in the transmission electron microscope, and the resolution of the lamination can only reach 10 nm, and the observation is not the sample itself, therefore, use Scanning electron microscopy is more beneficial to observe thick samples, and can obtain more real sample surface information.
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⑤ Observe the details of each area of the sample. The range of movement of the sample in the sample chamber is very large. The working distance of other microscopes is usually only 2 to 3 cm, so in fact, only the sample is allowed to move in a two-dimensional space. But it is different in the scanning electron microscope, due to the large working distance (can be greater than 20 mm), the large depth of focus (10 times larger than the transmission electron microscope), and the large space of the sample chamber, therefore, the sample can be placed in the three-dimensional space There are 6 degrees of freedom in the movement (that is, three-dimensional space translation, three-dimensional space rotation), and the movable range is large, which brings great convenience to the observation of the details of each area of the irregular shape sample.
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⑥Observe the sample under a large field of view and low magnification. The field of view of the sample observed by the scanning electron microscope is large. In a scanning electron microscope, the field of view F that can observe the sample at the same time is determined by the following formula: F=L/M
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In the formula, F——field of view range;
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M - the magnification when observing;
L——The screen size of the picture tube.
If the scanning electron microscope adopts a 30 cm (12 inches) picture tube, when the magnification is 15 times, its field of view can reach 20 mm. Large field of view and low magnification to observe the topography of samples are necessary for some fields, such as criminal investigation and archaeology.
⑦ Carry out continuous observation from high magnification to low magnification. The variable range of magnification is very wide, and there is no need to focus frequently. The magnification range of the scanning electron microscope is very wide (from 50,000 to 200,000 times continuously adjustable), and after focusing once, it can be continuously observed from high magnification to low magnification, and from low magnification to high magnification without refocusing. Analysis is particularly convenient.
⑧ Observation of biological samples. The degree of damage and contamination of the sample due to electron irradiation is very small. Compared with other electron microscopes, because the current of the electron probe used for observation is small (generally about 10 -10 ~ 10 -12A), the beam spot size of the electron probe is small (usually 5 nm to tens of nanometers), and the electron The energy of the probe is also relatively small (acceleration voltage can be as small as 2 kV), and the sample is not irradiated at a fixed point, but is irradiated in a raster scanning manner, so the damage and contamination of the sample occur due to electron irradiation Very small, which is especially important for observing some biological samples.
⑨ Conduct dynamic observation. In a scanning electron microscope, the imaging information is mainly electronic information. According to the technical level of the modern electronic industry, even electronic information that changes at a high speed can be received, processed and stored in time without difficulty, so some dynamic process observations can be carried out. If accessories such as heating, cooling, bending, stretching and ion etching are installed in the sample chamber, the dynamic change process such as phase transition and fracture can be observed through the TV device. 10 Obtain various information from the surface topography of the sample. In the scanning electron microscope, not only can use the incident electrons to interact with the sample to generate various information for imaging, but also can obtain a variety of special display methods for images through signal processing methods, and can also obtain information from the surface morphology of the sample. Obtain various information. Because the scanning electron image is not recorded at the same time, it is decomposed into nearly a million pieces and recorded sequentially, so that the scanning electron microscope can not only observe the surface morphology, but also analyze the composition and elements, and through the electron channel pattern. For crystallographic analysis, the selected area size can be from 10μm to 2μm.
Due to the above-mentioned characteristics and functions of the scanning electron microscope, it has been paid more and more attention by scientific researchers and has become more and more widely used. Scanning electron microscopes have been widely used in materials science (metallic materials, non-metallic materials, nanomaterials), metallurgy, biology, medicine, semiconductor materials and devices, geological exploration, pest control, disaster (fire, failure analysis) identification, criminal Reconnaissance, gem identification, product quality identification and production process control in industrial production, etc.
