Hi!
We want to investigate the tracer diffusion of Al in an NiAl alloy using
the isotope 26Al, from which we do have 0.1 Micro-Curie in 10ml of a
0.1mol HCl solution.
What methods can be used to efficiently prepare a homogeneous, thin
layer (typically 30nm) of metallic 26Al on top of the NiAl without
destroying the surface-near region of the alloy and also to avoid oxide
formation at the interface between the 26Al and the alloy?
Thanks for all suggestions
Martin
Wow. The only way Uncle Al knows of depositing 30 nm of metallic Al
and have it stick around is to vaporize or sputter the pure metal in
hard vacuum with a huge area of active getter nearby. Thin clean
aluminum is astoundingly reactive toward oxygen and water vapor. Deep
UV Al mirrors must be overcoated with MgF2 or LiF even before the
aluminum deposition filaments can cool. Immediately!
Getting hydrated aluminum chloride to the metal is a labor of
Hercules. Merely getting it to anhydrous AlCl3 is a right proper
pisser. You've also got to contain the radioisotope. Good luck.
Does anybod have some clever ideas? (clever idea = literature
citation)
and have it stick around is to vaporize or sputter the pure metal in
hard vacuum with a huge area of active getter nearby. Thin clean
aluminum is astoundingly reactive toward oxygen and water vapor. Deep
UV Al mirrors must be overcoated with MgF2 or LiF even before the
aluminum deposition filaments can cool. Immediately!
Getting hydrated aluminum chloride to the metal is a labor of
Hercules. Merely getting it to anhydrous AlCl3 is a right proper
pisser. You've also got to contain the radioisotope. Good luck.
Does anybod have some clever ideas? (clever idea = literature
citation)
Our research group is looking for some 26Al, does anybody know where to get or to buy it ? It seems to be difficult... 
If you expect your 26Al to diffuse deeper than 30nm, then implant (= ion implanter) it under the oxide layer of NiAl.
Also, you probably don't need a thin layer of pure 26Al. Use a thicker one, of less concentrated Al, if you can.
The oxide layer pre-exists on NiAl before you deposit anything, so you'll have to remove this layer in the same vacuum machine that sputters/evaporates/etc the layer. Rather common in semiconductor technology.
In case you really need such a thin pure layer, then put a protective coating on it immediately (in the same vacuum machine). LiF or MgF2 are used for their optical properties which you probably don't need; Si3N4 and SiO2 are better protective coatings against oxidation. Probably simpler, put a thick (I mean, 1µm) layer of pure ordinary aluminium on top of your 26Al. Insert a diffusion barrier between them if needed for your measurement; W, Pt are commonly used for that purpose.
What I describe is common semiconductor technology, so my suggestion would be to find someone from that field to help you. It doesn't look very complicated with that background.
Bye!
Also, you probably don't need a thin layer of pure 26Al. Use a thicker one, of less concentrated Al, if you can.
The oxide layer pre-exists on NiAl before you deposit anything, so you'll have to remove this layer in the same vacuum machine that sputters/evaporates/etc the layer. Rather common in semiconductor technology.
In case you really need such a thin pure layer, then put a protective coating on it immediately (in the same vacuum machine). LiF or MgF2 are used for their optical properties which you probably don't need; Si3N4 and SiO2 are better protective coatings against oxidation. Probably simpler, put a thick (I mean, 1µm) layer of pure ordinary aluminium on top of your 26Al. Insert a diffusion barrier between them if needed for your measurement; W, Pt are commonly used for that purpose.
What I describe is common semiconductor technology, so my suggestion would be to find someone from that field to help you. It doesn't look very complicated with that background.
Bye!
This is one of the most important experiments in materials, since the tracer diffusion of Al in NiAl is not established due to the limited availability of the radioactive isotope of Al that is very expensive. For many other elements, e.g., Ni, stable isotopes are available in the form of thin foils and one can use modern sputtering or ion beam deposition tools that can clean the native oxide before depositing the isotope. A good gettering and vacuum in your deposition chamber helps. You can perform an experiment where the native oxide is either removed or left in place and determine the effect. For our experiments, we plan on purchasing a new ion beam sputter tool that can remove the surface oxide prior to deposition.
In case of Al, where the isotope has to be deposited from solution (electroplated), the relevant question is whether the thin film error function solution can still be fitted assuming the Al tracer can penetrate the native oxide. Since the presence of the native oxide only affects the initial (or head) concentration profile and not the tail, perhaps it is not that important since dilute conditions to fit the error function solution are only met in the tail end of the concentration profile. You can also use SIMS to determine the concentration depth profile rather than the activity. I would suggest removing the native oxide with an appropriate etch solution and then immediately perform the electroplating.
In case of Al, where the isotope has to be deposited from solution (electroplated), the relevant question is whether the thin film error function solution can still be fitted assuming the Al tracer can penetrate the native oxide. Since the presence of the native oxide only affects the initial (or head) concentration profile and not the tail, perhaps it is not that important since dilute conditions to fit the error function solution are only met in the tail end of the concentration profile. You can also use SIMS to determine the concentration depth profile rather than the activity. I would suggest removing the native oxide with an appropriate etch solution and then immediately perform the electroplating.