EDX

A scanning electron microscope can also be equipped with additional instmmentation for electron-excited x-ray analysis (9). In many systems, this is performed in the mode known as energy dispersive x-ray analysis (edx). Other common acronyms for this method are eds for energy dispersive spectroscopy or edax for energy dispersive analysis of x-rays. 

Edx is based on the emission of x-rays with energies characteristic of the atom from which they originate in Heu of secondary electron emission. Thus, this technique can be used to provide elemental information about the sample. In the sem, this process is stimulated by the incident primary beam of electrons. As will be discussed below, this process is also the basis of essentially the same technique but performed in an electron spectrometer. When carried out this way, the technique is known as electron microprobe analysis (ema). 

X-rays are collected and analy2ed in ema in one of two ways. In wds, x-rays are dispersed by Bragg diffraction at a crystal and refocused onto a detector sitting on a Rowland circle. This arrangement is similar to the production of monochromati2ed x-rays for xps described above. In the other approach, edx, x-rays are all collected at the same time in a detector whose output scales with the energy of the x-ray (and hence, Z of the material which produces the x-ray.) Detectors used for ema today are almost exclusively Li-drifted Si soHd-state detectors. 

W. Auyeung and co-workers. Pin Evaluation of Technologies to Produce Fine Particle Si e EDX, ARLCD-TR-85024, ARDEC, Dover, N.J., Sept. 1985. 

The Ni3S2 constituent formed on the surface and scale formation was observed in all areas of the blade roots. The mechanism seemed to be more prevalent above the root pressure boundary than other areas of the blade root. Characterization of the scale was performed using a Scanning Electron Microscope equipped with an Energy Dispersion X-ray analyzer (EDX). 

The energy-dispersive (EDX) solid state detector (SSD, Figs 4.6, 4.7) is made of lithium-drifted Si crystal (Si(Li)). Between a thin p-type and an n-type layer lies a high-resistivity Si crystal of centimeter dimensions. The front and end planes of the crystal are coated with Au and serve as electrodes. The crystal, cooled to 77 K by liquid nitrogen, represents a p-i-n diode (Fig. 4.7). An incident X-ray photon with... 

Fig. 4.21. Schematic diagram of spectrometer arrangements for wavelength-dispersive and energy-dispersive X-ray spectroscopy (WDXS/EDXS) in electron microscopy.

Fig. 4.23. EDXS instrumentation for TEM and SEM (a) conventional EDX detector attached to a TEM/STEM (b) EDX system with Si drift-...

Fig. 4.24. Fraction of transmitted X-rays calculated for EDXS detectors with windowless Si(Li) and Gecrystals, and for different windows.

Because of the limited energy resolution in EDX spectra an overlap of peaks can often occur, depending on the composition of the material to be analyzed. The situation is much improved in WDXS, for which the energy resolution is approximately 10 eV and better. This is demonstrated in Fig. 4.25, in which the WDX and EDX spectra recorded from BaTi03 are compared. Here, WDXS enables easy resolution of the Ba-La and Ti-Ka lines this is impossible by EDXS. In addition, for WDXS the... 

Fig. 4.25. A WDX spectrum of BaTi03 plotted against energy and compared with the corresponding EDX spectrum [4.91].

An EDX spectrum typical of thin-film analysis in TEM/(S)TEM is shown in Eig. 4.26. It was obtained from a polycrystalline TiC/Zr02 ceramic by use of an Si(Li) detector at 100 keV primary electron energy. Eor spectrum recording the electron probe of approximately 1 nm in diameter was focused on the triple junction between the grains in the STEM mode (Eig. 4.26a). Besides the elements expected for the material under investigation, viz. Ti and Zr, Si, Ee, and Co were also detected, hinting at the presence of a (Ee, Co) silicide as an impurity. Eor ceramic materials it is known that... 

Fig. 4.26. Typical X-ray spectra (a) STEM bright-field image of a polycrystalline Zr02/TiC ceramic with a triple junction (b) corresponding EDX spectrum.

The procedure commonly used to quantify EDX spectra was originally outlined by Castaing [4.109], although for the general situation of investigating bulk materials. To a good approximation it can be assumed that the concentration Csp of an element present in an unknown sample is related to the concentration Cst of the same element in a standard specimen by... 

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