Electron microprobe phases

With a special optical system at the sample chamber, combined with an imagir system at the detector end, it is possible to construct two-dimensional images of the sample displayed in the emission of a selected Raman line. By imaging from their characteristic Raman lines, it is possible to map individual phases in the multiphase sample however, Raman images, unlike SEM and electron microprobe images, have not proved sufficiently useful to justify the substantial cost of imaging optical systems. 

The characteristic feature of solid—solid reactions which controls, to some extent, the methods which can be applied to the investigation of their kinetics, is that the continuation of product formation requires the transportation of one or both reactants to a zone of interaction, perhaps through a coherent barrier layer of the product phase or as a monomolec-ular layer across surfaces. Since diffusion at phase boundaries may occur at temperatures appreciably below those required for bulk diffusion, the initial step in product formation may be rapidly completed on the attainment of reaction temperature. In such systems, there is no initial delay during nucleation and the initial processes, perhaps involving monomolec-ular films, are not readily identified. The subsequent growth of the product phase, the main reaction, is thereafter controlled by the diffusion of one or more species through the barrier layer. Microscopic observation is of little value where the phases present cannot be unambiguously identified and X-ray diffraction techniques are more fruitful. More recently, the considerable potential of electron microprobe analyses has been developed and exploited. 

Aircraft turbines in jet engines are usually fabricated from nickel-based alloys, and these are subject to combustion products containing compounds of sulphur, such as S02, and oxides of vanadium. Early studies of the corrosion of pure nickel by a 1 1 mixture of S02 and 02 showed that the rate of attack increased substantially between 922 K and 961 K. The nickel-sulphur phase diagram shows that a eutectic is formed at 910 K, and hence a liquid phase could play a significant role in the process. Microscopic observation of corroded samples showed islands of a separate phase in the nickel oxide formed by oxidation, which were concentrated near the nickel/oxide interface. The islands were shown by electron microprobe analysis to contain between 30 and 40 atom per cent of sulphur, hence suggesting the composition Ni3S2 when the composition of the corroding gas was varied between S02 02 equal to 12 1 to 1 9. The rate of corrosion decreased at temperatures above 922 K. 

Pressure oxidation residues from the autoclaves display similarities (i.e., XAFS and electron microprobe) to Type-2 and As-rich Phase-3 compounds (Fig.2). 

Polished thin sections were made in the absence of water to prevent dissolution of soluble phases. Mineral composition was determined using a combination of X-ray diffraction, electron microprobe analysis, and scanning electron microscopy with... 

The accuracy of some isothermal techniques, particularly those that rely on observation of phases, is limited by the number of different compositions that are prepared. For example, if two samples are separated by a composition of 2at%, and one is single-phase while the other two-phase, dien formally the phase boundary can only be defined to within an accuracy of 2at%. This makes isothermal techniques more labour intensive than some of the non-isothermal methods. However, because it is now possible to directly determine compositions of phases by techniques such as electron microprobe analysis (EPMA), a substantially more quantitative exposition of the phase equilibria is possible. 

The mechanism by which these deleterious elements are controlled is still a matter of speculation. Some researchers have suggested that the rare earth elements combine with the deleterious elements to form innocuous insoluble intermetallic compounds (31). However, such particles have been observed, using the electron microprobe, only when concentrations of both the rare earths and deleterious elements were well above those levels usually found in commercial practice. Even then, the composition of the particular phases was not determined. Further, the effective level of cerium at which the beneficial effects are observed suggests that the mechanism may not be simply compound formation. 

Zhao Ewing (2000) examined altered uraninite from the Colorado Plateau with quantitative electron microprobe analysis in order to determine the fate of trace elements, including Pb, Ca, Si, Th, Zr, and REE, during corrosion under oxidizing conditions. The alteration phases identified included schoepite, calciouranoite, uranophane, fourmarierite, a Fe-rich U phase, and coffinite. The primary uraninites and alteration phases generally had low trace element contents. The electron microprobe analyses indicated that the trace elements preferentially entered the secondary U phases. Alteration caused the loss of U, Pb, and Zr, and incorporation of Si, Ti, Ca, and P into U phases. 

Although a number of secondary minerals have been predicted to form in weathered CCB materials, few have been positively identified by physical characterization methods. Secondary phases in CCB materials may be difficult or impossible to characterize due to their low abundance and small particle size. Conventional mineral identification methods such as X-ray diffraction (XRD) analysis fail to identify secondary phases that are less than 1-5% by weight of the CCB or are X-ray amorphous. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with energy dispersive spectroscopy (EDS), can often identify phases not seen by XRD. Additional analytical methods used to characterize trace secondary phases include infrared (IR) spectroscopy, electron microprobe (EMP) analysis, differential thermal analysis (DTA), and various synchrotron radiation techniques (e.g., micro-XRD, X-ray absorption near-eidge spectroscopy [XANES], X-ray absorption fine-structure [XAFSJ). 

The final sintered specimen was mounted and polished in epoxy (ME14730 epoxy resin and epoxy hardener) for electron microprobe analysis (EMPA, JEOL JXA-8600). The backscattered electron image shown in Fig. 2a confirmed a homogeneous single phase of LSFTO. Elemental compositions were determined with the probe placed at several different spots on the sample. The composition ratios were determined from the average data to be La Sr... 

As illustrated by Fig. 10.4, an electron microscope offers additional possibilities for analyzing the sample. Diffraction patterns (spots from a single-crystal particle and rings from a collection of randomly oriented particles) enable one to identify crystallographic phases as in XRD. Emitted X-rays are characteristic for an element and allow for a determination of the chemical composition of a selected part of the sample (typical dimension 10 nm). This technique is called electron microprobe analysis (EMA, EPMA) or, referring to the usual mode of detection, energy dispersive analysis of X-rays (EDAX or EDX). Also the Auger electrons carry information on sample composition, as do the loss electrons. The latter are potentially informative on the low Z elements, which have a low efficiency for X-ray fluorescence. 

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