Microstructural imaging in the scanning electron microscope

A scanning electron microscope (SEM) is used instead of an optical microscope to examine microstructural regions whenever the following features are required  

These features have become routine in the examination of both fracture surfaces and polished sections of ceramic materials. 

The essential elements of a scanning electron microscope are shown in Fig. 36  

The electrons emitted by the electron gun are focused on the specimen by the Wehnelt cylinder and two to three electromagnetic lenses. As the narrowly focused electron beam strikes the specimen surface, it has a focal diameter of 2-10 nm. A sweep generator is controlled in such a way that the electron beam scans the specimen surface line-by-line and point-by-point. The electron beam also sweeps across a cathode-ray tube. These two motions of the electron beam are synchronized so that each point on the specimen surface is depicted on the screen. The image on a second screen is recorded by a camera. 

When the electron beam (consisting of primary electrons) strikes a point on the specimen, the specimen emits secondary electrons (SE), backscattered electrons (BSE), and X-rays (Fig. 37). 

---