Channels of possible synthesis of nanotubes

Channels of possible synthesis of nanotubes

  4.1 Method of electric arc:
It consists of an arc discharge between two graphite electrodes in a chamber filled with helium. One of the two graphite electrodes, the anode, is associated with a few percent of a metal such as Co, Fe and Ni. The anode will be consumed to form a plasma whose temperature can reach 6000 ° C
(Note: A plasma is an ionized gas where the atoms are broken down into positively and negatively charged ions. For example, our universe is composed of 99% plasma.).
The plasma condenses on the other electrode, the cathode, a rubbery and stringy deposit containing nanotubes. This method is very simple realization was quickly implemented throughout the world, only here, the processes occurring during the synthesis are very complex and that makes it virtually uncontrollable.
A variant of the technique known as arc discharge is the synthesis from solar energy. The graphite-catalyst mixture is sprayed in this case using a concentrated radiation solar furnace


Method of electric arc

4.2 Laser ablation
It is to destroy a composite graphite electrode-metal transition with laser radiation of high energy pulsed or continuous. The graphite is then vaporized in an argon atmosphere and give the nanotubes. The main advantage of this method is the limited number of control parameters, which makes possible the study of synthesis conditions, although this process is very expensive
Note
We have synthesized different behaviors when the laser is pulsed (it emits a certain amount of photon has a well-defined frequency) or when it is continuous.
Here we have shown what we call synthetic routes to high temperature. There are others to develop carbon nanotubes at lower temperatures.

• The catalytic decomposition of a gas

  The principle of these methods is to decompose a carbon containing gas to the surface of particles of metal catalyst in a furnace heated to a temperature between 500 ° C and 1100 ° C.


The carbon atoms released by thermal decomposition of gas will then precipitate on the catalyst surface leads to the condensation growth of tubular structures graphitized. This technique has already led to other types of nano structures (filaments, nano fiber, ...).
The first gas used was acetylene, using as a catalyst of fine particles of iron, but also cobalt and nickel. Benzene has been in the same way as methane as a carbon source. An alternative to acetylene is the carbon monoxide that Smalley and her team used by said mutation on molybdenum at 1200 ° C.
Note: The pro mutation is a redox reaction between two molecules of a compound, an oxidant and the other being reduced by it.
The oxidation number of the item, identical in the two molecules of departure is increased in one and reduced in another.
Example: Cannizzaro reaction

However there is a major drawback to this method of manufacture, the difficulty lies in the construction and control the size of catalyst particles. Their sizes to be of the order of few nm for the synthesis of nanotubes. The particles are obtained by reduction of an organometallic compound (such as ferrocene) and are either deposited on a ceramic support material (silica, alumina) is broken down into the reaction chamber where the reaction takes place with the gas carbon.
The nanotubes obtained by these methods are less good qualities they exhibit geometric characteristics (length, diameter) much more uniform, which is an advantage. It is possible to orient tube growth in a synthetic catalysts on blocks arranged on a support according to a defined geometry.
This possibility opens up exciting prospects for certain uses. In addition, the average temperature processes can be designed to obtain the means of large-scale production like carbon fiber, which is much more difficult to envisage ways to high temperature.

• The different modes of assembly of carbon nanotubes
• The single-wall nanotubes (SWNT Single Wall Nanotubes)

The SWNT are found mostly in the form of bundles (bundles), also called clusters. In each bundle, the tubes stacked in a compact and form a periodic arrangement of triangular symmetry. Their number may reach several tens in a bundle and their diameter varies between synthesis conditions of 0.6 to 1.5 nm.
S ructure of a SWNT

Cross section of a SWNT

• Multiwalled nanotubes (MWNTs Multi Wall Nanotubes) 6199

Two configurations are possible for this type of nanotube or tubes nested one inside the other like Russian dolls (a) or when the structure is constructed by wrapping a single sheet of grapheme (b).



Structure of multiwalled nanotubes

In both types of assembly, the distance between two adjacent tubes is about equal distance between two planes of grapheme, meaning that the assembly of the tubes do not change the nature of chemical bonds that are identical to what 'they are in graphite.
The two assembly modes are mutually exclusive and are obtained for the other synthesis conditions radically different. On tracks high temperature, obtaining beams monotube requires the use of a metal catalyst that is mixed up a few percent to the graphite powder.
The catalyst is a transition metal, Ni, Co, Pd, Pt nanotubes multi layer formed directly in the vapor phase at a temperature of at least 3000 ° C. Conversely, the single nanotube layer formed in a colder zone between 800 and 1400 ° C.
On tracks average temperature, the nature of the assembly is controlled by temperature and size of catalyst particles. If these conditions of synthesis are now well established, it remains that the mechanisms that control the formation and growth of the tubes are still very poorly understood and much remains to be done before we know drive the synthesis of a tube a given configuration.