normal atmospheric pressure is sufficient

@Shiv Singh -can you check the parameters that I am using ........ diameter size of tube is 3  inch.

The pressure depends mainly on the system (catalyst and carbon containing gas) you are using.

To obtain long tubes you need to push the growth and reduce the termination process. To do this you have to find the ideal carbon concentration in your catalytic metal. If the concentration of carbon is too low, the growth is slow, if it is too high it get encapsulated so the process ends in short tubes. Please see: Angewandte Chemie International Edition 52 (36), 9372-9387 or Journal of nanoscience and nanotechnology 8 (11), 6141-6152.

What happens on the metal depends on its composition, but also from the chemical potential of the carbon containing gas, so its nature and pressure. In general, to get super long CNT (mm to cm) you really need to suppress the deactivation process, so relatively low pressures (500-700 mbar) are used and some active components (like H2O) are added in ppm to "clean" the dead catalyst particle without harming the growing CNTs.

However there are processed like HiPCO or CoMoCAT that use pressures from 5 bar up to several hundreds bar and get pretty long CNTs too.

Pressure may not be a critical parameter here, you can achieve the growth of long CNT at low P as well as atmospheric P... as Giulio said, the composition / concentration of the carbon source and the composition / thickness of the catalyst layer would play a much more significant role.

You may find some insights in this "Super-Growth Manual":

http://www.nanocarbon.jp/index_e.shtml

but keep in mind that there is no "magic recipe" since parameters related to your setup will have direct repercussion on the CNT growth kinetics (for instance your furnace geometry impacting on the residence time of the C at the catalyst surface).

Thanks all,

@ Bastein one quick question, I cannot increase pressure more than 75m torr because of some issue, does it matter then, I optimize the gases flow rate and catalyst thickness but still I am getting short CNT 

What type of catalyst do you use?

Iron/Aluminium/SiO2/ Si

Fe=2nm

Al=20nm

When you say "long CNT", what length are you actually trying to reach?

I agree with Sten, you should try thinner Fe layer. To boost the growth kinetics, you may also try to increase C2H4 flow.

Do you have catalyst poisoning issues? (is the bottom part of your array more entangled?) 

thanks Sten and Bastein,

                            I need CNT near about 1mm. I cant use aluminum di oxide as a buffer because of nonconducting nature, yeah I can reduce thickness of iron from 2 to 1nm. 

Thanks Stein, I didnt check the deactivation of catalyst but for 1 hour of processing I got 14 micrometer of height at 30 to 50 mtorr. flow rate of gases are 30sscm of C2H4 and Nh3=200sscm at 770C

no I didnt break vacuum after depositing aluminum.

without water

yeah I can use H2, OK I will try to grow CNT at 650C can you suggest me flow rate or I keep this only.

Hey Stein one quick question can I try Al rather than Al2O3, because I am gonna use this cnt for sensors.