Month: June 2019

Saying What You Mean and Meaning What You Say!

Shawn Wallace, Applications Engineer, EDAX

A recent conversation on a list serv discussed sloppiness in the use of words and how it can cause confusion. This made me consider that in the world of microanalysis, we are not immune. We are probably sloppiest with two particular words. They are resolution and phase.

Let us start with how we use the word phase and how phases are commonly defined in microanalysis. In Energy Dispersive Spectroscopy (EDS), we use phase for everything, for example, phase mapping, phase library. In Electron Backscatter Diffraction (EBSD), the usage is a little more straightforward.

So, what is a phase? Well to me, a geologist, a phase has both a distinct chemistry and a distinct crystal structure. Why does this matter to a geologist? Two different minerals with the same chemistry, but with different structures, can behave in very different ways and this gives me useful information about each of them.
The classic example for geologists is the Al2SIO5 system (figure 1). It has three members, Kyanite, Sillimanite, and Andalusite. They each have the same chemistry but different structures. The structure of each is controlled by the pressure and temperature at which the mineral equilibrated. Simple chemistry tells me nothing. I need the structure to tease out that information.

Figure 1. Phase Diagram of the Al2SiO5 system in geological conditions. Different minerals form at different pressures and temperatures, letting geologists know how deep and/or the temperature at which the parent rock formed.**

EDS users use the term phase much more loosely. A phase is something that is chemically distinct. Our phase maps look at a spectrum pixel by pixel and see how they compare. In the end, the software goes through the entire map and groups each pixel with like pixels. The phase library does chi squared fits to compare the spectrum to the library (figure 2).

Figure 2. Our Spectrum Library Match uses as Chi-squared fit to determine the best possible matches. This phase is based on compositional data, not compositional and structural data.

While the definition of phase is relatively straight forward, the meaning of resolution gets a little murkier. If you asked someone what the EDS resolution is, you may get different answers depending on who you ask. The main way we use the term resolution when talking about EDS is spectral resolution. This defines how tight the peaks in a spectrum are (figure 3).

Figure 3. Comparison of EDS vs. WDS spectral resolution. WDS has much higher resolution (tighter peaks) than EDS, but fewer counts and more set-up are required.

The other main use of resolution, in EDS is the spatial resolution of the EDS signal itself (figure 4). There are many factors which determine this, but the main ones are the accelerating voltage and sample characteristics. This resolution can go from nanometers to microns.

Figure 4. Distribution of the electron energy deposited in an aluminum sample (top row) and a gold sample (bottom row) at 15 kV (left column) and 5 kV (right column). Note the dramatic difference in penetration given by the right hand side scale bar.

The final use of resolution for EDS is mapping resolution. This is by far the easiest to understand. It is just the step size of the beam while you are mapping.

Luckily for us, the easiest way to find out what people mean when they use the terms resolution or phase, is just to ask. Of course, the way to avoid any confusion is to be as precise as possible with your choice of words. I resolve to do my part and communicate as clearly as I can!

** Source: Wikipedia

A New Light on Leonardo

Sue Arnell, Marcom Manager, EDAX

I recently spent 10 days’ vacation back in the UK, but my visit “home” turned into somewhat of a busman’s holiday when I visited the current exhibition at the Queen’s Gallery in London: LEONARDO DA VINCI: A LIFE IN DRAWING. While all the drawings were very interesting, one particular poster particularly caught my eye.

Figure 1: Poster showing the use of X-ray Fluorescence (XRF) analysis on one of the drawings in the exhibition.

It may be hard to see in this small image, but the drawing in the bottom left corner of the poster showed two horses’ heads, while the rest of the sheet showed very indistinct lines. When viewed under ultraviolet light, however, it is clear that there were an additional two horses depicted on the same page.

Figure 2: Drawing of horses seen under ultraviolet light

A video on the exhibit site shows a similar result with a second page:

Figure 3: Hand study seen in daylight

Figure 4: Hand study seen under ultraviolet light

According to the poster, researchers* at the Diamond Light Source at Harwell in Oxfordshire used X-ray fluorescence, which is non-destructive and would not therefore harm the priceless drawing, to explain the phenomenon in the first drawing of the horses. Scanning a small part of the drawing to analyze individual metalpoint lines, they were able to extract the spectrum in Figure 5.

Figure 5: the results of XRF analysis on the drawing showing the presence of copper (Cu) and Zinc (Zn) in the almost invisible lines and almost no silver (Ag).

The conclusion was that Leonardo must have used a metalpoint based on a Cu/Zn alloy and that these metals have reacted over time to produce salts and render the lines almost invisible in daylight. However, under ultraviolet light, the full impact of the original drawings is still visible.

When I shared this analysis back in the EDAX office in Mahwah, NJ, Dr. Patrick Camus (Director of Engineering) had a few additional (more scientific) observations.

  • XRF may be useful in determining the fading mechanism by looking for elements associated with environmental factors such as Cl, (from possible contact with human fingertips), or S in the atmosphere from burning coal over the centuries. It may be related to exposure to sunlight as well.
  • The use of ultraviolet light as an incoming beam has a similar reaction but slightly different with the material as the x-rays producing emissions at much smaller energy level. This process is called photoluminescence. The incoming beam excites valence electrons across an energy gap in the material to a higher energy level which during relaxation to the base energy releases a photon. The energy of these photons is typically 1-10 eV or much less than x-ray detectors can sense. Interestingly, this excitation does not occur in conductors/metals, thus proving more evidence of the picture material being a band-gap or insulating material like a salt.
  • This example shows that a single technique does not always provide a complete picture of the structure or composition of a sample, but the use of multiple techniques can provide information greater than the sum of the individual contributions.

From my point of view, I have been trying to explain, promote and market the EDAX products and analysis techniques for over eight years now, so it was very interesting to see the value of some of ‘our’ applications in a real-world situation.

* Dr. Konstantin Ignatyev, Dr. Giannantonio, Dr. Stephen Parry

From Collecting EBSD at 20 Patterns per second (pps) to Collecting at 4,500 pps

John Haritos, Regional Sales Manager Southwest USA. EDAX

I recently had the opportunity to host a demo for one of my customers at our Draper, Utah office. This was a long-time EDAX and EBSD user, who was interested in seeing our new Velocity CMOS camera, and to try it on some of their samples.

When I started in this industry back in the late 90s, the cameras were running at a “blazing” 20 points per second and we all thought that this was fast. At that time, collection speed wasn’t the primary issue. What EBSD brought to the table was automated orientation analysis of diffraction patterns. Now users could measure orientations and create beautiful orientation maps with the push of a button, which was a lot easier than manually interpreting these patterns.

Fast forward to 2019 and with the CMOS technology being adapted from other industries to EBSD we are now collecting at 4,500 pps. What took hours and even days to collect at 20 pps now takes a matter of minutes or seconds. Below is a Nickel Superalloy sample collected at 4,500 pps on our Velocity™ Super EBSD camera. This scan shows the grain and twinning structure and was collected in just a few minutes.

Figure 1: Nickel Superalloy

Of course, now that we have improved from 20 pps to 4,500 pps, it’s significantly easier to get a lot more data. So the question becomes, how do we analyze all this data? This is where OIM Analysis v8™ comes to the rescue for the analysis and post processing of these large data sets. OIM Analysis v8™ was designed to take advantage of 64 bit computing and multi-threading so the software can handle large datasets. Below is a grain size map and a grain size distribution chart from an Aluminum friction stir weld sample with over 7 Million points collected with the Velocity™ and processed using OIM Analysis v8™. This example is interesting because the grains on the left side of the image are much larger than the grains on the right side. With the fast collection speeds, a small (250nm) step size could still be used over this larger collection area. This allows for accurate characterization of grain size across this weld interface, and the bimodal grain size distribution is clearly resolved. With a slower camera, it may be impractical to analyze this area in a single scan.

Figure 2: Aluminum Friction Stir Weld

In the past, most customers would setup an overnight EBSD run. You could see the thoughts running through their mind: will my sample drift, will my filament pop, what will the data look like when I come back to work in the morning? Inevitably, the sample would drift, or the filament would pop and this would mean the dreaded “ugh” in the morning. With the Velocity™ and the fast collection speeds, you no longer need to worry about this. You can collect maps in a few minutes and avoid this issue in practice. It’s a hard thing to say in a brochure, but its easy to appreciate when seeing it firsthand.

For me, watching my customer see the analysis of many samples in a single day was impressive. These were not particularly easy samples. They were solar cell and battery materials, with a variety of phases and crystal structures. But under similar conditions to their traditional EBSD work, we could collect better quality data much faster. The future is now. Everyone is excited with what the CMOS technology can offer in the way of productivity and throughput for their EBSD work.