I understand some of these words.
Kidding aside, what was the goal of this endeavor? What is the end product?
This of course leaves the particle itself cooler, making it less likely to receive additional deposition and resulting in a more uniform thickness across the batch.
These coated particles are probably then going to be used as a catalyst in some other process.
[1] https://www.youtube.com/watch?v=My4RA5I0FKs
(alternate is that the the precursors themselves are the fluidized bed and the co2 is used to help move the coating from the donor to the recipient. If GP called them parts and not particles i would be more inclined to this option)
https://www.brianneltner.com/thin-films
The key is conformal films and aspect ratio. If you're familiar with CVD, you might have encountered "duning" where film growth can produce unstable interfaces or find that a trench will get filled in at the top and leave a void underneath.
One common option is using ALD which uses alternating precursors so you can fully coat every surface of an arbitrarily high surface area material with a monolayer of your precursor, and then a second step converts that precursor to metal so you get atomically thick layers.
The downside of ALD is that it's quite slow in terms of growth rate because you can only grow one atom thick layer at a time. Additionally, the precursor has to have a vapor pressure high enough that the often solid precursors can actually be flowed through a reactor under vacuum. But it can get super thin films that can be conformal and which have incredible aspect ratios.
What I did was use sCO2 to dissolve chemicals like those ALD or CVD precursors in an "organic solvent" (sCO2 itself) to deliver it at super high concentrations to the reactor (at like 5000PSI) instead of at super low concentrations under vacuum. It stays supercritical through the full fluidized bed. But the reactor itself is hot so the precursors spontaneously decompose and do so preferentially on surfaces. But! The key difference is that because of the high precursor concentration, instead of the film growth rate being dictated by how fast precursor can get to the particle surface (diffusion limited) it's limited by how fast the precursor reacts on the surface (kinetically limited). The prior is notorious for unstable interfaces, the latter is much better because it's just growing at max speed at all locations simultaneously.
So the real difference is that it let me deposit thin films from a high concentration precursor stream.
My actual application was trying to coat copper flake with chromium metal to increase the oxidation temperature for use as a replacement to silver conductive pastes used in solar panel manufacturing. Silver was (might still be) like 10% of the total cost of making one, so this was of keen interest and would be pretty awesome. Ultimately, I was able to make it so that copper showed no oxidation at 300C over a minute while uncoated copper did show a lot of oxidation.
However, the amount of coating I needed to keep the oxygen away from the copper was too thick (or at least I couldn't make it thinner with that tech) and the chromium was expensive enough that it'd have ended up more expensive than just using silver.
So I moved on =) A project worth doing for sure, everyone I talked to about it simply could not guess if it would work. Sadly it worked but wasn't commercially interesting in the end.
Also:
>Almost!
You’re quite generous with your knowledge and compliments haha!
Exciting read!