tailieunhanh - The Coming of Materials Science Episode 7

Tham khảo tài liệu 'the coming of materials science episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 220 The Coming of Materials Science together with . Whelan and with the encouragement of Sir Nevill Mott who soon after succeeded Bragg as Cavendish professor to apply what he knew about X-ray diffraction theory to the task of making dislocations visible in electron-microscopic images. The first step was to perfect methods of thinning metal foils without damaging them w. Bollmann in Switzerland played a vital part in this. Then the hunt for dislocations began. The important thing was to control which part of the diffracted signal was used to generate the microscope image and Hirsch and Whelan decided that selected-area diffraction always had to accompany efforts to generate an image. Their group in the person of R. Horne was successful in seeing moving dislocation lines in 1956 the 3-year delay shows how difficult this was. The key here was the theory. The pioneers familiarity with both the kinematic and the dynamic theory of diffraction and with the real structure of real crystals the subject-matter of Lal s review cited in Section enabled them to work out by degrees how to get good contrast for dislocations of various kinds and later other defects such as stacking-faults. Several other physicists who have since become well known such as A. Kelly and J. Menter were also involved Hirsch goes to considerable pains in his 1986 paper to attribute credit to all those who played a major part. There is no room here to go into much further detail suffice it to say that the diffraction theory underlying image formation in an electron microscope plays a much more vital part in the intelligent use of an electron microscope in transmission mode than it does in the use of an optical microscope. In the words of one recent reviewer of a textbook on electron microscopy The world of TEM is quite different from optical microscopy . Almost no image can be intuitively understood. For instance to determine the Burgers vector of a dislocation from the disappearance of its .

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