Friday, May 05, 2006


I guess this is a good time to discuss a little about what happens after I make these alumina membranes. I can not get too specific about the research that anyone else is doing so I am just going to give information as to what has been done in the past with CVD, carbon nanotubes, and alumina membranes. As far as the synthesis of the CNT goes this is the end of the journey.

Chemical Vapor Deposition
The idea is simple - make the carbon nanotubes in the nanopores that we got with the alumina membrane.
The task is difficult. How can one get carbon into tubes and tunnels all the way through? The easiest way is to let the carbon do the work by moving through an atmosphere at a slightly elevated pressure. That sounds good but what about the carbon? We could by it in powder form and blow it through but that could clog up any tiny instruments used not too mention you can see the carbon with your naked eye and we need it in nano sized clumps.
The solution - heat a gas of hydrocarbons around the alumina membranes until it naturally breaks down and deposits onto the membrane. This is how in general, the CVD procedure works.
After inserting your alumina membrane samples into a furnace (usually in some kind of glass or quarts tube) you eliminate all of the air in the tube by blowing a heavy gas (in my case Argon) so when the carbon is released it doesn't react with the atmosphere and oxidize. While you are blowing Ar through the tube, the furnace is heating up so when you switch to the hydrocarbon gas it immediately meets the high temperature and breaks down. You also do not want a lot of carbon. The goal is to make "tubes" meaning there is some finite diameter that the carbon must leave open so we can do experiments. A mixture of hydrocarbons and a lighter gas that won't decompose or deposit onto the membranes is sufficient. The process takes around 6 hours not to mention the heating up and cooling down time for the furnace.

The Purification
We now have our glossy carbon coated membranes with carbon deposited into the nanotubes. Now to get the nanotubes out.... OH NO! I forgot my nano-sized tweezers in my other pants. Well it isn't any near that tedious. To remove all the unused membrane from around our nice CNT, we simply have to break them up in basic solution that will dissolve the membranes. This also requires some heat for about 3-4hours. Once time is up, just filter and neutralize the nanotubes, and you are set. Well until you need to anneal them that is....


At 5:58 PM, Blogger Jean-Claude Bradley said...

I think the CVD temp here has to be carefully adjusted to give enough time for the gas to equilibrate before decomposing to carbon. If the temperature is too high the pores will fill up more quickly at the edges and plug up the hole.

At 6:39 PM, Blogger James G said...

You are correct. I can set the furnace so the temp does not increase beyond my setting which has been determined experimentally by others in the group. If you let the hydrocarbon gas in the tube as the furnace heats up it will not deposit the carbon at all however until it reaches that temp where it starts to break down. Too hot and it will plug up the pores and you will get more of a mixture of single, double and even triple bond hydrocarbons that are deposited. I don't know the usual mixture percentage or how much of the 6 hours is needed to let the gas equilibrate but I imagine it depends on the hydrocarbons used.

At 7:27 AM, Blogger Jean-Claude Bradley said...

I noticed that some of the nanotubes are curved - do you observe that often? Do you think that is due to curved pores or are the nanotubes really flexible?

At 9:46 AM, Blogger James G said...

The outer surface of the nanotubes are usually rigid due to the non-uniformity of the pores. The nanotubes I think you are refering to are in one of the cross section pictures where it looks like the tube has almost like a hook. That is most likely due to the friction and strain that occured when the membrane was broken to take the SEM picture. It can also happen that the tube may be so many micrometers long that it bends under its own weight. The nanotubes are very fragile and while it can obviously happen that they bend due to stress, more often they will break into smaller tubes.

At 9:34 PM, Anonymous Anonymous said...

you are a nerd~

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