Microscopy and material analysis
Let’s start with microscopy. It is quite interesting to observe how a paste spreads and when it de facto tears up and down on the glass slide. You can see that the KPx adheres very well, which is probably also due to its composition. The polysiloxane matrix used, i.e. silicone, is relatively homogeneous but rather flowing, which is of course also due to its consistency. It is anything but viscous, but relatively fluid like an Arctic MX-6, for example. I think this compromise is very suitable for LN2 projects, but it is probably more of a “short-term paste” for quick cooling success.
With only 20x magnification, you can already see various particles of very different colors. Reason enough to zoom in a little further. In addition to the colored particles, we can also see other globules, which could indicate an accumulation of zinc flakes.
Of course, this triggers me to go one size larger. And we also understand why the KINGPIN KPx could not be compressed quite as thinly as the Apex or even the DOWSIL 340, for example.
Now let’s take a look at what’s inside and what’s not. The paste contains a very complex matrix, while the proportion of thermally conductive particles is lower than in the Apex reference paste. In the end, there are two completely different solution sets, whereby the proportion of zinc oxide in the KPx even outweighs that of corundum. That is also a finding. The paste is always better than the Apex when the silicone is pressed away, which also explains the measurement curves. Due to the somewhat coarser grain structure (up to 5 µm), there are also fewer phase differences in the layer. This is good for cryo-sleep, but also not for the PC at home over a longer period of time. If the matrix outgasses or leaks, the coating will suffer significantly at some point.
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 accurate 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 the presumably same material. Analyses in the parts-per-thousand range are nice, but today they are not useful 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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