DubaiAmateurRocketry
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Least Amount of Coating for Reactive Hazardous/Metallic powders ?
So, rather niche question, but say I want to coat the surface of micron level aluminum powder or any powder that's reactive (CL20, hydrides, etc) to
reduce oxidation or reaction with the atmosphere.
Of course, storing it in mineral oil seems like an obvious option, however what if one wants to use the aluminum powder (pure crystalline without
Al2O3 coating for highest reactivity) for pyrotechnic / propellant purposes? If one coated aluminum powder in a thin layer binding or any unreactive
polymer, how would one make sure not a single void is left? assuming any gaps in this coating is detrimental for the entire mixture.
[Edited on 4-7-2018 by DubaiAmateurRocketry]
[Edited on 4-7-2018 by DubaiAmateurRocketry]
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Sulaiman
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EASY !
Slowly introduce atmospheric air and each particle of aluminium will protect itself with an oxide coating 
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XeonTheMGPony
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or handle in an oxygen depleted space (Glove box), same for storage, over pressurize container by 2 psi and good to go
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DubaiAmateurRocketry
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Quote: Originally posted by Sulaiman  | EASY !
Slowly introduce atmospheric air and each particle of aluminium will protect itself with an oxide coating |
Well if you read the post, I mentioned pure crystalline without Al2O3 coating for highest reactivity.
Take powdered potassium as an example may be more suitable, it is reactive with moisture in the air and can form oxides with oxygen. Whats the optimal
binder polymer to use to coat micron size of potassium powders? Also what technique can make sure this coating has no voids, assuming any void in a
potassium mixture such as moisture can start an irreversible disastrous chain reaction.
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coppercone
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Well think about it, basically if you mill or produce a bunch of aluminum in a environment that's very devoid of oxygen (I mean like getter cleaned),
you will have pure aluminum powder with a free surface.
Then you introduce oxygen slowly, and an oxide layer will form. Read about how thick oxide layers are on aluminum, they are rather thin.
What will happen though is when the powder is moved, it will possibly abrade itself, and if it is particularly jagged, the particles will be hard and
they will tend to self polish themselves to a less reactive state with less surface area.
The aluminum specifications you use might be different if you ball mill it because the ball milled bits might have similar properties to extruded
aluminum.
The problem with using any kind of coating is that it would need to be very much oxygen free. You can do a rough calculation to see what the surface
area of your aluminum is by making a batch, sieving it, then use bin sorting, i.e. the aluminum you produce will be 10% ~ this mesh, 40~that mesh, 30%
this mesh and so on.
Then you use mass and assume spherical dimensions for your meshes to get a first order approximation of the surface area, then use a rule of thumb for
aluminum oxidzation under your conditions to get a surface thickness of oxide, estimate the crystal structure spacing/lattice properties of the oxide
layer to determine how much aluminum is used in it, then use stoichometry to determine the amount of aluminum that is consumed before a stable oxide
layer is formed.
Since you know the molecule ,you can then determine how much oxygen is required to coat your sphere.
You would need to play with geometry and surface roughness to get a good idea of the actual surface area from your sphere.
Anyway, once you have an idea how much oxygen can react to coat your aluminum (or perhaps you can determine this experimentally by using mass,
possibly by using excess oxygen then recompressing the oxygen then reacting it with something to measure mass change (probably difficult to do unless
the quantities you use are large)
you can then use that figure of oxygen requirements to coat your aluminum powder, and make sure that the volume of coating material you use has
sufficently little oxygen, so that even if it is hermatically sealed, only a thinner or even spotty oxide layer grows.
Aluminum is particularly reactive and i don't know if you can store it under degassed solvent in atmosphere without some kind of problematic diffusion
occuring.
If you want pure aluminum I would advise you use shlecnk chemistry guidelines and handling to maintain it being oxygen free.
you might wanna look at microscope or electron microscope pictures of aluminum flakes to get an idea of how they form to get some kind of estimate on
their actual surface area, unless you are using spherical aluminum, which I assume would also have some pitting and stuff,
I am bad a visual approximations like this, but I assume you might be in the area of 10-100 times the surface area of the theoretical sphere?
quite fascinating if you think about it deeply. Looking at surface area of molecular seives might give you the 'worst case' surface area. but i don't
think it will be realistic. I am sure someone can chime in.
[Edited on 5-7-2018 by coppercone]
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AJKOER
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One could place the reactive Al powder in a vessel, add a paper boundary, then pour on a more inert Al powder forming a buffer layer.
Note, the less active layer is also safer to ignite, but whether it all matters depends on how fast you can run!
[Edited on 7-7-2018 by AJKOER]
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