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Porous coating on aluminum

1998

I am designing a rotary heat exchanger using an aluminum matrix. I would like to apply a porous coating over the aluminum so the heat exchager can adsorb water vapor and de-sorb water vapor at different vapor pressure conditions.

Some companies use desiccants such as silica gel or molecular seive glued to the aluminum, however the desiccant degrades during the thermal process. Can you please suggest a chemical compound that I can apply to the aluminum that will be permanetly bonded and have the porous feature I require? Please help.

Anthony Palucci

Anthony,

Porous coatings can easily be produced on aluminum alloys by anodizing in either sulfuric acid or phosphoric acid solutions. However, I am afraid that the pore sizes of conventional anodic coatings, usually about a few decades of nanometers, are probably too small in your application. What pore size are you expecting for your application? I believe it is possible to enlarge the pore sizes through modifying the operational parameters.

Ling

Ling Hao
- Grand Rapids, Michigan
1998

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1998

Ling,

The porous coating should have a pore diamter between 2.8 angstrom and 5 angstrom. The coating should be capable of adsorbing water vapor molecules which are about 2.8 angstrom. I was told that a Potassium compound could give me the required porous coating! However, we must make sure that this coating will be permenantly bonded. The coating should be capable of withstanding 100 psi water pressure cleaning. What do you think?

Anthony Palucci

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Anthony,

The pore sizes you are looking for are actually much smaller than those of micro pores in conventional anodic coatings. Fortunately, you can achive the pore sizes through adjusting the operational parameters of anodizing processes.

I do not know what kind of potassium compounds is suitable for making porous coatings on aluminum in your application. To my knowledge, all inorganic potassium compounds are quite soluble in water. You may produce a porous coating on aluminum with potassium compounds. I am afraid that, however, hot water and steam will wash the porous coating away easily.

Anodic coatings are of course permanently bonded to the aluminum substrates. They are able to withstand the pressure you specified as well. The problem is that the micro pores will be sealed in boiling water, which eventually leads to the complete "plug" of the micro pores. I guess this effect might be not what you expect to happen in your application.

Ling

L. Hao
1998

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1998

Ling,

What about ceramics? Is there a way to coat ceramic substrate onto aluminum? Ceramic is a porous material, and is very strong. Can this be done....

Anthony Palucci

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1998

Anthony,

It is worth trying ceramics. I have one suggestion for you to take into consideration. The adhesion between aluminum substrate and ceramic coating is probably not good enough to withstand the continuous bubbling force at a relatively high pressure. Good luck.

Ling

L. Hao

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1998

Anthony,

Thermally sprayed aluminum is used as a porous coating on aluminum and other materials such as steel to increase the adhesion of paints to the surface, etc. The porosity of the aluminum coating can be controlled by the deposition materials and condiions.

Donald M. Mattox
Society of Vacuum Coaters
Albuquerque, New Mexico

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1998

Don,

I never heard of thermally sprayed aluminum! However, I get the impression that anything that is sprayed can not cover the entire surface equally. If the coating is applied through a dipping process, I can get a better surface coverage. In my case, it is extremely important that the coating is evenly applied because it maximizes my water vapor exchange rate; the more porous surface I have, the more water vapor I can adsorb into the pores. If you have more detailed information on this process, please forward to the e-mail address below. Thank you for responding.

Regards,

Anthony Palucci

What you are describing is a cutting edge advanced technology. You seem to need a membrane and not a coating. Check with TDA Research in Denver area, CO. They might guide you. There are many companies making progress in filtering gases or toxins this way. I could also suggest conducting a search of the SBA's SBIR sites for funded topics. I have seen something like that there.

Mandar Sunthankar
- Fort Collins, Colorado
1998

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1998

Normally applied thermal spray (flame spray or plasma) will not have the quantity of pores that you are looking fore. Its porosity tends to be random in size and spacing. Pore size tends to be much larger the sizes you mentioned.

Spray parameters can be adjusted to degrade the coating by increasing porosity, but very probably would not meet your desires.

There are stainless filters for crossflow filtration that is in the range that you are looking for. Tubes that I have seen are in the 1 inch range. They might be able to be manufactured in other sizes, but will be very expensive to experiment with (by my cheap standards).

James Watts
- Navarre, Florida

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Deliquescence, is what you are trying to avoid. It is the degradation of the absorbent through repetitive cycles; matrix grain depression; loss of polarity on the inorganic absorbent molecule; the consequent loss of surface energies and therefore loss of adhesion, among other things. If all the above did occur while adhesion was maintained you might still end up with either an erratic or unacceptable isotherm. First determine what isotherm you're looking for and whether or not it can be characterized after it has been adhered to the surface of the aluminum. Additionally it must be determined whether or not the deposition of the absorbent is to be configured as a molecular mono-layer or quassi three-dimensional matrix.Once you have decided upon the above parameters you will know what kind of adhesive strategy to adopt, be it organic or inorganic.Each adhesive strategy will affect the isotherm differently.This can be predicted depending upon data available to you and the a! bility to manipulate materials t o mutually support rather than the mutually compromise their important properties. best of luck to you the applications are important and extend beyond your immediate concerns.

C. Kyricos
1999

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