“Why do you recommend X-ray diffraction analysis and not X-ray fluorescence analysis -as you did on our last job?”
“Why should I do both? It gives me the same result, after all.”
Most of the time, these questions are related to examinations in the context of quality monitoring. The aim is to check whether the chemical composition of the delivered refractory brick corresponds to the specifications on the data sheet – or whether the quality is just not good enough.
One method used in the broad industry is chemical determination by means of X-ray fluorescence analysis (XRF). This method uses X-rays to determine the chemistry of the material on basis of re-radiated fluorescence. The X-rays knock electrons out of their shells in the vicinity of the nuclei, and the electrons that slide in after them emit specific amounts of energy (fluorescence radiation). Single elements and mixed materials can thus be analyzed. The radiation is collected by a detector and assigned to the elements by the instrument software. A major advantage of the method is the simultaneous analysis of a powder sample or a melting tablet from traces to s high percentage range.
After the wide range of XRF analysis, the question arises as to the purpose of X-ray diffraction analysis (XRD). This is because the chemical composition can also be determined here, albeit via a detour.
This method again uses X-rays. In contrast to XRF, however, the diffraction at the crystalline lattice of the material is examined here. The X-rays strike a crystal surface and are “reflected” or diffracted at a different angle. The diffraction of the radiation at the powder results in crystal-specific spectra, because the arrangement of the atoms forming a crystal face is characteristic for each crystal. Thus, information is obtained not only indirectly about the chemical composition, but also about the crystalline phase composition, which allows a mineralogical statement about the material. This phase composition has a decisive influence on the behavior in use.
For example, the melting point of different crystals with the same chemical composition differs dramatically: (beta-)tridymite (orthorhombic SiO2) melts at 1470°C, whereas (beta-)cristobalite (cubic SiO2) melts at about 1710°C. When used in the high-temperature range, this difference decides between “stable” and “failure”.
If the material fails, conclusions can be drawn about the conditions in the environment by means of the crystalline phase state. This is because different crystalline forms of a mineral – also called modification – form under different conditions ( e.g. temperature; Figure 1).
Another prominent example is CaCO3. Calcite, aragonite and vaterite form at different temperatures. Below 30°C, calcite is found, while above 30°C, aragonite is formed. Above this, vaterite is often formed as hydrothermal precipitate or in combination with biological material (e.g. kidney or gall stones).
The disadvantage of XRD: only the crystalline phases are recorded. Glass phases, which hold components of a coarse ceramics together, or trace elements such as alkalis, which can corrode components of the stones, are not recorded.
Thus, it can be summarized that both methods have their special focus with their own pros and cons – but can also complement each other well. A short overview of the key points can be found below.
- Wide detection range (low ppm to high Ms.% range)
- Fast and cost effective analysis
- Handheld devices available for mobile measurement
- Partially high accuracy
- Statements exclusively about chemical composition
- Preparation may be time-consuming (melting tablet)
- High effort for calibration and standardization of the method
- Crystalline composition of minerals
- Information on crystallite size, mineral proportions and beyond
- Fast and inexpensive measurement as powder possible
- Lower accuracy than XRF
- Evaluation of mineral phases requires knowledge and experience
- Evaluation of crystalline components only
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