Electron microscope structure and imaging principle
The electron microscope consists of three parts: the mirror barrel, the vacuum system and the power cabinet. The barrel is mainly composed of electron gun, electron lens, sample holder, fluorescent screen and photographic mechanism, which are usually assembled into a column from top to bottom; the vacuum system is composed of mechanical vacuum pump, diffusion pump and vacuum valve, etc., and connected with the barrel through pumping pipeline; the power supply cabinet is composed of high-voltage generator, excitation current stabilizer and various regulation control units.
◆Electron Lens
Electron lens is the most important part of the electron microscope tube, which uses a space electric field or magnetic field symmetrical to the axis of the tube to make the electron track to the axis of the formation of the focusing of the role of the glass convex lens to make the beam of light focusing the role of the similar, so it is called an electron lens. Most of the modern electron microscopes use electromagnetic lenses, in which the electrons are focused by a strong magnetic field generated by a very stable DC excitation current through a coil with a pole shoe.
◆Electron gun
An electron gun is a component consisting of a tungsten hot cathode, a gate and a cathode. It emits and forms an electron beam with uniform velocity, so the stability of the accelerating voltage is required to be not less than one part in ten thousand.
[Imaging Principle
■Optical microscope imaging principle
When the object to be observed on the outside of the focus of the objective lens near the focal point, after the objective lens of the real image formed just inside the eyepiece focus near the focal point of the eyepiece, the eyepiece is again magnified into a virtual image. What is observed is an inverted virtual image after two magnifications.
■ electron microscope imaging principle
Electron microscope is based on the principle of electron optics, the electron beam and electron lenses instead of light beams and optical lenses, so that the fine structure of the material in a very high magnification of the imaging instrument.
The resolving power of an electron microscope is expressed by the minimum distance between two neighbouring points that it can resolve.In the 1970s, the resolution of a transmission electron microscope was about 0.3 nanometres (the resolving power of the human eye is about 0.1 millimetres). Nowadays, the maximum magnification of an electron microscope is more than 3 million times, while the maximum magnification of an optical microscope is about 2,000 times, so that the atoms and crystals of certain heavy metals can be directly observed through an electron microscope with a neatly arranged array of atomic dots.
