i want to synthesis super aligned CNTs (yarns) by chemical vapor deposition
but the problem is that the synthesized CNTs aren't vertically aligned...they are horizontally aligned....what is the problem?
my product is the same as what is shown in attached image (b)
There are several ways to achieve vertically aligned CNTs. However, to answer your question, we need to know what method you are using. For instance, are you using a simple thermal CVD process, or are you also using some type of excitation such as a hot filament or plasma? Then we may be able to help you, especially with respect to the method/equipment you have available.
i prepare a substrate with Al2o3 and Fe and used a simple thermal CVD process
I am not as familiar with Al2O3 and Fe as a combined catalyst, although I know that it's been reported by others. I also use a simple thermal CVD process. We've extensively studied thin Ni films as a catalyst, however, this also results in random orientations. Some degree of vertical alignment occurs if the site density is high enough, strictly for steric reasons. More recently, we've reported using anodized aluminum oxide (AOO) nanopore templates to achieve alignment. We form the template on a substrate of choice. This can be Si, sapphire, metals, anything that will survive the CNT growth temperatures. This method yields a very highly oriented array of CNTs with high crystalline quality for the individual CNTs in the array. You can download all of our CNT papers from my page on this forum. Good luck.
Alumina is not catalyst it is just catalyst support (btw, is it alumina substrate, or alumina is deposited on other substrate as a buffer?). As Michael mentioned already for thermal CVD the vertical alignment is mainly governed by steric factor. You need to provide high site density of active CNT catalyst nuclei and more or less homogeneous activity of catalyst particles. Main parameters to control this are size distribution of catalyst particles and their surface density. They depend on: 1) method which you use for Fe deposition on substrate and annealing; 2) amount of deposited Fe(too thin layer-> you have too few active sites, too thick-> you have huge particles after annealing, correspondingly growth of either fibers or graphene coated Fe particles); 3) Initial roughness of surface; 4) temperature of annealing and CVD itself. Of course gas mixture is also important also it is not directly related to catalyst. There are huge number of publications in this field with detailed description of sample preparation and CVD parameters.
One of the easiest way to produce aligned CNT is using alumina templates. When you prepare alumina template via aluminium foil, oxalic acid and electrochemistry, it is possible to obtain a substrete with a honeycomb like pattern, then you add your catalyst using spin coating technique. After that just put the template in the CVD reactor and voila!
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Atomic, Molecular and Optical Physics
- Atoms