Tham khảo tài liệu 'volume 18 - friction, lubrication, and wear technology part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | used with the SEM because this instrument is applied primarily to the study of surface features of bulk specimens. In general as will be discussed later an accelerating voltage is selected that best suits the application at hand. Transmission electron microscopes are available in three different accelerating voltage ranges. The most commonly used instruments operate at a maximum of 120 kV but allow the selection of voltages as low as 20 kV. With so-called intermediate-voltage microscopes the maximum voltage is typically 200 to 400 kV. High-voltage instruments are capable of operating at 106 V and higher. In general a higher accelerating voltage permits penetration of thicker specimens and provides improved resolution. However the gain in going from the intermediate-voltage range to the high-voltage range is relatively small for all but the most specialized applications and is achieved at a very substantial increase in cost. Intermediate-voltage instruments allow routine observation of the atomic structure of all classes of crystalline materials. This together with increased penetration improved EELS capabilities and the fact that specially constructed laboratory facilities necessary for high-voltage instruments are not required has led to an increase in the popularity of intermediate-voltage instruments. In the SEM the specimen is normally located below the final lens in the illumination system. For improved resolution however some instruments provide a second position within the final lens. The electron beam is focussed to a small spot and scanned serially over the specimen to form a rectangular raster. Secondary electrons or one of the other sources in Fig. 2 are collected to provide a signal that is amplified and used to modulate the intensity of the electron beam in a CRT The CRT beam is scanned in synchronism with the electron beam incident on the specimen resulting in an observable image. The magnification of the image is determined by the ratio of the .
