Bottom up synthesis Solgel synthesis Precipitation Physical vapor synthesis Chemical vapor condensation Spray conversion processing
Chemical vapor condensation Chemical vapor deposition (CVD) synthesis is achieved by putting a carbon source in the gas phase and using an energy source, such as a plasma or a resistively heated coil, to transfer energy to a gaseous carbon molecule. Types :plasma enhanced CVD, thermal chemical CVD, alcohol catalytic CVD, vapour phase growth, aero gel-supported CVD and laser-assisted CVD. -Gaseous carbon sources : methane, carbon monoxide and acetylene…
Case study: preparation of carbon nano tube by chemical vapor condensation method. -plasma enhanced CVD, -thermal chemical CVD, -alcohol catalytic CVD, -vapour phase growth, -aero gel-supported CVD -laser-assisted CVD.
the growth mechanism
process The energy source is used to "crack" the molecule into reactive atomic carbon. The carbon diffuses towards the substrate, which is heated and coated with a catalyst where it will bind. Carbon nanotubes will be formed if the proper parameters are maintained.
positional control onnanometre scale CVD carbon nanotube synthesis is essentially a two-step process : -catalyst preparation -actual synthesis of the nanotube. The catalyst is generally prepared by sputtering a transition metal onto a substrate and then using either chemical etching or thermal annealing to induce catalyst particle nucleation. Thermal annealing results in cluster formation on the substrate, from which the nanotubes will grow.
Thermal chemical vapor deposition a schematic diagram of thermal CVD apparatus in the synthesis of carbon nanotubes.
Catalytic growth Schematics of a CVD deposition ovenThis method is based on the decomposition of a hydrocarbon gas over atransition metal to grow nanotubes in a chemical vapor deposition (CVD)reactor
Thermal chemical vapor deposition Catalysts : Fe , Ni, Co … Substrate : Si,SiO2,glass… Gas flow : 40ml/min t = 450-1050 oC The diameter range of the carbon nanotubes depends on the thickness of the catalytic film example : - By using a thickness of 13 nm, the diameter distribution lies between 30 and 40 nm. - By using a thickness of 27 nm is used, the diameter range is between 100 and 200 nm.
product Advantages: -Typical yield: 20-100% - Long tubes with diameter ranging from 10-240 nm for MWNT (multi walled nanotubes) and 0.6-4 nm for SWNT( single walled nanotubes). - Easiest to scale up to industrial production; long length, simple process, SWNT diameter controllable, quite pure Disadvantages: - large diameter range =>>>poorly controlled. -often riddled with defects
Laser-assisted thermalchemical vapour deposition
Sources of laser:a medium power, continuous wave CO 2 laser,perpendicularonto a substrate, pyrolyses sensitised mixtures of Fe(CO) 5 vapour and acetylene in a flow reactor. Catalyst: Fe (very small iron particles) Substrate: sillica. iron pentacarbonyl vapour, single- and multi- +ethylene walled carbon +acetylene nanotubes
product The diameters of the SWNTs range from 0.7 to 2.5 nm. The diameter range of the MWNTs is 30 to 80 nm 43 prefer grow single rather than multi-walled nanotubes . Hight purity High power requirement
Purification The main impurities :graphite (wrapped up) sheets, amorphous carbon, metal catalyst and the smaller fullerenes… Rules : -separate the SWNTs from the impurities - give a more homogeneous diameter or size distribution. The techniques that will be discussed are oxidation, acid treatment, annealing, ultrasonication, micro filtration, ferromagnetic separation, cutting, functionalisation and chromatography techniques.
applicationsNanotubes are rolled-up graphene sheets, and graphene isone of the stiffest materials when subjected to deformationsparallel to the sheet.⇒nanotubes show exceptional mechanical properties,especially a high strength-to-weight ratio.Applications: Field emission Field emission Nanotube sensors Nanotube transistors Nanotubes as SPM tips….
Schematics of a nanotube transistor, with some measurements.
Use of a MWNT as AFM tip. VGCF stands for Vapour Grown Carbon Fibre.At the centre of this fibre the MWNT forms the tip