Chemical Quantification of Mo-S, W-Si and Ti-V by Energy Dispersive X-Ray Spectroscopy (original) (raw)

Elemental chemical identification of a specimen and its quantification is fundamental to obtain information in the characterization of the materials (Angeles et al., 2000; Cortes-Jacome et al., 2005). Energy dispersive X-ray spectroscopy (EDXS) is the technique that allows obtaining information concerning the elemental chemical composition using the EDX spectrometer. Generally is attached to a scanning electron microscope (SEM) (Goldstein & Newbury, 2003) and/or in a transmission electron microscope (Williams & Barry-Carter, 1996). The technique is very versatile because the spectrometer gives results in few minutes. The instrument is compact, stable, robust and easy to use and its results can be quickly interpreted. The analysis is based in the detection of the characteristic X-rays produced by the electron beam-specimen interaction. The information can be collected in very specific local points or on the whole sample. So, both electron microscopy and EDXS, give valuable information about the morphology and chemical composition of the sample. In order to give an accurate interpretation of the data collected by the instrument is important to know the fundaments of the technique. The characteristic X-rays are produced by the atoms of the sample in a process called inner-shell ionization (Jenkins & De Dries, 1967). This process is carried out when an electron of inner-shell is removed by an electron of the beam generating a vacancy in the shell. At this moment the atom remain ionized during 10-14 second and then an electron of outer-shell fills the vacancy of the inner-shell. During this transition a photon is emitted with a characteristic energy of the chemical element and its shell ionized. The emitted photons are named by the shell-ionized type as K, L, M lines.... and , , ... by the outer-shell corresponding to the electron that filled the inner-shell-ionized. For atoms with high atomic numbers, is important to note that some transitions are forbidden. Permissible transitions can be followed by the quantum selection rules and the notation can be followed by Manne Siegbahn and/or, IUPAC rules (Herglof & Birks, 1978). During the beam-sample interaction, another X-ray source is produced and it is known as Bremsstrahlung radiation or continuum X-rays which are generated for the deceleration of the electron beam in the Coulombic field of the specimen atoms. When the electrons are braked, they emit photons with any energy value giving rise to a continuous electromagnetic spectrum appearing in the EDX spectrum as *Corresponding Author www.intechopen.com X-Ray Spectroscopy 120 background. In the case of high overvoltage, more than one electron may be ejected simultaneously from an atom and then, X-rays known as satellites peaks are generated (Deutcsh, et al., 1996). This simultaneous ejection of electrons causes a change in the overall structure of the energy levels resulting in the production of X-rays with slightly lower energies than those produced during single electron ionization appearing near to the characteristic peaks. Another important source of satellite peaks is the Auger process. Finally, in the X-ray spectrum can be detected shifts of peaks produced from a pure element and that produced by the same element contained in a compound. This variation occurs because the electronic configuration of the inner-shells of an atom is strongly influenced by the outer valence electrons. These shifts are most apparent when comparing metals with their corresponding oxides and halides. Consequently, it is not advisable to use a metallic standard material for analysis of oxide compounds. In the case of energy-dispersive X-ray analysis, the shifts of the peaks are undetectable and metal s c a n b e u s e d a s r e ference standards for quantification (Liebhafsky, 1976). All X-rays produced in the sample are detected and displayed in the EDX spectrum. Their identification in the spectrum is important because help to do an accurate identification of the chemical components remaining in the specimen and subsequently a more accurate quantification can be obtained. From all X-rays displayed in an EDX spectrum the most important are characteristic X-rays. Another important parameter to consider in the EDXS is the acquisition of the data (Kenik, 2011; Scholossmacher, et al., 2010). The X-ray processing produced in the specimen is performed in three parts: detector, electronic processor and multichannel analyzer display. The overall process occurs as follow: the detector generates a charge pulse proportional to the X-ray energy. The produced pulse is first converted to a voltage and then the voltage signal is amplified through a field effect transistor, isolated from other pulses, further amplified, then identified electronically as resulting from an X-ray of specific energy. Finally, a digital signal is stored in a channel assigned to that energy in the multichannel analyzer. The speed of this process is such that the spectrum seems to be generated in parallel with the full range of X-ray energies detected simultaneously. Currently, the new software's generation delivers a spectrum ready to analyze and the previous process is not seen. However, there are many variables that must be taken into account to make a more accurate identification and subsequently their quantification. The most important variables are the time constant (Tc), acceleration voltage (AV), dead time (DT), acquisition time (AT), magnification and work distance (WD) which have direct effect on the energy resolution, peak intensity and natural width of characteristic X-ray lines. It is important to note that the calculation of the chemical composition is carried out considering the intensity and peak broadening in conjunction with the atomic number effect (Z), absorption correction (A) and characteristic fluorescence correction (F) (Newbury et al., 1986). For every group of samples w i t h s i m i l a r c h e m i c a l c o m p o n e n t s i s r e c o m m e n d a b l e t o d o a r e v i e w o f s o m e o f t h e previously mentioned operation variables to assure the truthfulness of the results. The chemical quantification has been very well studied for metals; however for powder samples of metallic and non-metallic oxides deposited on carbon tape, little work has been realized especially when the energy lines of two elements overlap. Generally, the chemical analysis in the scanning electron microscope can be obtained at different voltage, magnification, etc. depending on the information that it wants to reveal of a sample (Chung, et al., 1974). But if the overall chemical composition of the specimen analyzed is the primary requirement then a review of operation parameters of the instrument should be performed to ensure the accuracy of the results. In this study, the influence of operation conditions is presented to www.intechopen.com