Energy-dispersive X-ray microanalysis (EDX, for brevity, also EDS or EDAX) is a complementary technique to SEM, enabling the operator to determine the composition of the features in the SEM image.
The principle of EDX is that the electron beam generates X-rays within the specimen. Many of these X-rays have energies characteristic of the elements that emitted them – if you can measure the energy of the X-rays, you know what elements are present in the specimen. If you control the instrumental conditions carefully you can determine not only what elements are present but their concentrations.
The principle of how the EDX system works is quite simple. The X-ray detector is positioned to intercept X-rays emitted from the specimen. When an X-ray enters the detector, it generates a small current which is then converted into a voltage pulse. The size of the voltage pulse is proportional to the energy of the X-ray. The X-ray processor measures the voltage pulses and the computer displays the information as a histogram, or spectrum. A spectrum is typically collected for a period of 10 seconds to one or two minutes, but occasionally longer. The elements present in the specimen can be determined by examining the pattern of peaks on the spectrum.
For example, in the following spectrum, the strongest peaks are due to calcium and silicon.
We can tell that these peaks are due to calcium and silicon because the pattern of a strong peak at 3.7 keV and a weaker peak at 4.0 keV is consistent with that of Ca K-alpha and K-beta X-rays, which always have these energies. Similarly, the peak at about 1.7 keV is due to silicon. Actually, this is two peaks, the Si K-alpha at 1.74 keV and the Si K-beta at 1.83 keV but the detector cannot resolve them and they appear as one peak. Note that the specimen contained oxygen, but the X-ray detector used was not sensitive to light elements and so no peak due to oxygen is present in the spectrum.
Sometimes there can be ambiguity where the energies of X-rays emitted by different elements are similar. Usually, these can be resolved as most elements detectable by EDX emit X-rays at more than one characteristic energy and it is simply a matter of checking the spectrum to see whether other peaks expected from a particular element are present.
With a little practice, one rapidly becomes familiar with the patterns of peaks produced by elements typically found. For example, someone specialising in stainless steel would be used to seeing spectra indicating iron, manganese, chromium and nickel. In cementitious work, elements most commonly encountered will be calcium, silicon, aluminium, iron, sodium, magnesium, phosphorus, sulfur, chlorine, potassium, titanium, chromium and manganese, plus oxygen and carbon. If there is any doubt, the EDX software can show you at the click of a mouse what elements are present using the element auto identification function built in to all EDX software. (However, even the computer can make mistakes, especially if the peaks are weak or overlapping, so don’t believe everything your computer tries to tell you – cement does not normally contain exotic elements such as yttrium or ruthenium).
Most older EDX detectors are of the lithium-drifted silicon, or Si(Li), type requiring liquid nitrogen to cool them. A newer type of detector, the silicon drift detector, or SDD, is now becoming the standard type of detector. Compared with the Si(Li) detector, it has the two great advantages that it can operate at a higher count rate and it doesn’t need liquid nitrogen.
In summary, EDX is a powerful technique that adds a new and vital dimension to what can be achieved using the SEM. For most cementitious work, an EDX system on the SEM is virtually essential.