tailieunhanh - ENCYCLOPEDIA OF MATERIALS CHARACTERIZATIONC phần 3

chỉ có một kính hiển vi ánh sáng có thể làm điều này trực tiếp. Ánh sáng phản xạ hoặc truyền từ mẫu đi vào mắt sau khi đi qua một cột phóng đại. Tất cả các kỹ thuật kính hiển vi hình ảnh khác sử dụng một số thăm dò tương tác khác và tín hiệu phản ứng (thường là các điện tử) để cung cấp sự tương phản tạo ra một hình ảnh. | derive the local elemental concentration of each atomic species present. Additionally by studying the detailed shape of the spectral profiles measured in EELS the analyst may derive information about the electronic structure chemical bonding and average nearest neighbor distances for each atomic species detected. A related variation of EELS is Reflection Electron Energy-Loss Spectroscopy REELS . In REELS the energy distribution of electrons scattered from the surface of a specimen is studied. Generally REELS deals with low-energy elecưons 10 keV while TEM STEM-based EELS deals with incident electrons having energies of 100 400 keV. In this article we shall consider only the transmission case. REELS is discussed in Chapter 5. In principle EELS can be used to study all the elements in the periodic table however the study of hydrogen and helium is successful only in special cases where their signals are not masked by other features in the spectrum. As a matter of experimental practicality the inner shell excitations studied are those having binding energies less than about 3 keV. Quantitative concentration determinations can be obtained for the elements 3 ắ z 35 using a standardless data analysis procedure. In this range of elements the accuracy varies but can be expected to be 10-20 at. By using standards the accuracy can be improved to 1-2 at. Detection limit capabilities have improved over the last decade from IO-18 g to IO-21 g. These advances have arisen through improved instrumentation and a more complete understanding of the specimen requirements and limitations. The energy resolution of the technique is limited today by the inherent energy spread of the electron source used in the microscope. Conventional thermionic guns typically exhibit an energy spread of 2-3 eV and LaBg a spread of about 1-2 eV field emission sources operate routinely in the 1 eV range. In all cases the sample examined must be extremely thin typically 2000 Â to minimize the adverse .