Microscopy and material analysis
Let’s start with the microscopy. It is quite interesting to observe how a paste spreads and when it actually tears up and down on the glass slide. You can see that the Arctic MX-6 adheres very well, but cannot be peeled off endlessly thin. You only end up with a few zinc particles and the silicone, while the rest tears off. This is probably also due to its composition. The silicone matrix used (polysiloxanes) is relatively thin and the final consistency is mainly achieved by the fillers. I would have preferred to see a firmer matrix and less “sludge” from fillers to thicken the paste.
The risk of outgassing or bleeding of the silicone base is quite real. This is also where my slight concerns about the long-term durability at high temperatures come in. If I can pull the paste apart mechanically like this, then it is certainly no different under heat and pressure. We’ll have to see how well it all performs after 6 to 9 months. However, I am already working on an ageing test.
Now let’s see what’s in it and what’s not. The paste contains a lot of fillers, mainly corundum and a lot of fine zinc oxide. The paste relies on relatively large corundum beads, which also explains why it cannot be pressed together as thinly as some other pastes.
Test equipment for material tests, accuracy and test preparation
The material testing and measurement of the pastes and pads is carried out by my Keyence VHX 7000 with EA-300, which enables both exact measurements and fairly precise mass determinations of the chemical elements. But how does it actually work? The laser-induced breakdown spectroscopy (LIBS) I used for this article is a type of atomic emission spectroscopy in which a pulsed laser is directed at a sample in order to vaporize a small part of it and thus generate a plasma.
The emitted radiation from this plasma is then analyzed to determine the elemental composition of the sample. LIBS has many advantages over other analytical techniques. Since only a tiny amount of the sample is needed for analysis, the damage to the sample is minimal. The real damage is caused in today’s article by my rather coarse cutting and separating tools. This still quite new laser technique generally requires no special preparation of the samples for material analysis. Even solids, liquids and gases can be analyzed directly.
LIBS can detect multiple elements simultaneously in a sample and can be used for a variety of samples, including biological, metallic, mineral and other materials. And you get true real-time analysis, which is a huge time saver. As LIBS generally requires no consumables or hazardous reagents, it is also a relatively safe technique that does not require a vacuum as with SEM EDX. As with any analytical technique, there are of course certain limitations and challenges with LIBS, but in many of my applications, especially where speed, versatility and minimally invasive sampling are an advantage, it offers distinct advantages.
I would first like to point out that the results of the percentages in the overviews and tables have been intentionally rounded to full percentages (wt%, i.e. weight percent), as it happens often enough that production variations can occur even within what is presumably the same material. Analyses in the parts per thousand range are nice, but not very useful today when it comes to reliable evaluation and not trace elements. However, every day in the laboratory starts with the same procedure, because when I start, I work through a checklist that I have drawn up. This takes up to 30 minutes each time, although I have to wait for the laser to warm up and the room to reach the right temperature anyway.
- Mechanical calibration of the X/Y table and the camera alignment (e.g. for stitching)
- White balance of the camera for all lighting fixtures used
- Check alignment of LIBS optics and standard lens, calibrate alignment of laser to own optics (x300)
- Test standard samples of the materials to be measured and correct the curve if necessary (see image above)
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