Class: Material Science EngineeringStudent : Hoang Van Tien -2012 Hanoi
DID YOU KNOW? Carbon nanotubes, composed of interlocking carbon atoms, are 1000x thinner than an average human hair – but can be 200x stronger than steel.
outline Definition Theory and properties Synthesis Potential and current applications Challenges and future
What are carbon nanotubesCNT: Rolling-up a graphene sheet to form a tube Schematic STM image of a CNT of CNT 4
Carbon nanotubes are hexagonally shaped arrangements of carbon atoms that have been rolled into tubes. These tiny straw-like cylinders of pure carbon have useful electrical propeties. They have already been used to make tiny transistor and one-dimentional copper wire
Types of CNTs Single Wall CNT (SWCNT) Multiple Wall CNT (MWCNT) Can be metallic or semiconducting depending on their geometry.
Single- walled-Most single-wallednanotubes (SWNTs)have a diameter of cloesto 1 nanometer,with atube length that can bemany millions of timelonger-The structure of aSWNTs can beconceptualized bywrapping a one-atom-thick layer of graphitecalled graphene in to aseamless cylender
If: m=0 , the nanotubes are called zigzag n=m ,the nanotubes are called armchair Otherwise ,they are called chiral
Multi-walled Multi-walled nanotubes (MWNTs) consist of multiple rolled layer( concentric tubes) of graphene Triple-walled armchair CNTs
In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, e.g., a (0,8) single- walled nanotube (SWNT) within a larger (0,17) single- walled nanotube. In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.4 Å. The Russian Doll structure is observed more commonly. Its individual shells can be described as SWNTs, which can be metallic or semiconducting.
compare Single –walled CNTs exhibit electric properties that are not shared by the multi- walled CNTs variant SWNTs is useful in the development of the first intramolecular field effect transistors (FET)
torus-Torus is theoretically describedas carbon nanotube bent into atorus (doughnut shape) .-Nanotorus are predicted tohave many unique propertiessuch as : +magnetic moments +thermal stability …-Vary widely depending onradius of the torus and theradius of the tube
In nanotechnology,carbon nanobuds form a material (discovered and synthesized in 2006) which combines two previously discovered allotropes of carbon: carbon nanotubes and spheroidal fullerences (or, in short, fullerenes) fullerenes are covalently bonded to the outer sidewalls of the underlying nanotube=>>>> Has useful properties of both fullerenes and carbon nanotubes
Extreme carbon nanotubes-The observation of the longest carbonnanotubes (18.5 cm long) was reported in2009. These nanotubes were grown on Sisubstrates using an improved chemicalvapor deposition (CVD) method andrepresent electrically uniform arrays ofsingle-walled carbon nanotubes.-The shortest carbon nanotube is theorganic compound cycloparaphenylene-The thinnest carbon nanotube isarmchair (2,2) CNT with a diameter of 3Å. This nanotube was grown inside amulti-walled carbon nanotube-The thinnest freestanding single-walledcarbon nanotube is about 4.3 Å indiameter
PROPERTIES OF CARBON NANOTUBES
Strength and elasticity CNTs are expected to be the ultimate high-strength filber Single wall carbon nanotubes are stiffer than steel and are very resistant to damage from physical forces Carbon Nanotubes (CNTs) very useful as probe tips for very high-resolution scanning probe microscopy.
elasticity -The current Young’s modulus value of single walledcarbon nanotubes is about 1 terapascal -The modulus of the multi walled carbon nanotubescorrelates to the amount of disorder in the carbon nanotubewalls -when multi walled carbon nanotubes break, theoutermost layers break first
Electrical properties For a given (m,n) nanotubes: If n=m(armchair) the CNTS is metalic If n-m is multiple of 3 ,the CNTs is semiconducting with a small band gap Otherwise ,the CNTs is moderates semiconductor
Thermal property All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction ", but good insulators laterally to the tube axis. SWNT has a room-temperature thermal conductivity along its axis of about 3500 W·m−1·K−1. in copper : 385 W·m−1·K−1 A SWNT has a room-temperature thermal conductivity across its axis (in the radial direction) of about 1.52 W·m−1·K−1. (same as oil)
defect As with any material, the existence of a crystallographic defect affects the material properties. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on its weakest segment in a similar manner to a chain, where the strength of the weakest link becomes the maximum strength of the chain. Crystallographic defects also affect the tubes electrical properties and thermal properties.
Toxicity . Under some conditions ,nanotubes can cross membrane barriers ,which suggests that if raw materials reach the organs, they can include harmful effects such as inflammatory and fibrotic reactions CNTs were capable of producing inflammation, epithelioid granulomas (microscopic nodules), fibrosis, and biochemical/toxicological changes in the lungs =>>>carbon nanotubes can pose a serious risk to human health
synthesisTechniques havebeen developedto produce carbonnanotubes insizeable quanlities,some of them are:- Arc discharge- Laser ablation- Chemical vapordeposition(CVD)
Arc discharge Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps , that was intended to produce fullerenes. the carbon contained in the negative electrode sublimates because of the high-discharge temperatures. The yield for this method is up to 30% by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects
Laser ablation a pulsed laser vaporizes a graphite target in a high- temperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes. The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature . However, it is more expensive than either arc discharge or chemical vapor deposition.
Chemical vapor deposition a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron , or a combination. The substrate is heated to approximately 700°C. two gases are bled into the reactor: a process gas (such as ammonia , nitrogen or hydrogen ) and a carbon- containing gas (such as acetylene , ethylene , ethanol or methane ).>>Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes.
Laser-assisted thermalchemical vapour deposition
the growth mechanism .
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 +ethylene multi-walled +acetylene carbon nanotubes
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.
applications Nanotubes are rolled-up graphene sheets, and graphene is one of the stiffest materials when subjected to deformations parallel to the sheet. nanotubes show exceptional mechanical properties, especially a high strength-to-weight ratio.Applications: Field emission Nanotube sensors Nanotube transistors Nanotubes as SPM tips Energy Storage Conductive Adhesives and Connectors Biomedical Applications……….
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.the VGCG provides aconvenient and robust technique for mounting the MWNT probe for use ina scanning probe instrustment
Faster computer chips
cancer treatment Use carbon nanotubes as drug delivery tools
Challenges and potentials Potentials : the new wonder material, can useful in many types of industry ,especially in electronics applications The field of research for CNTs has become so broad over the past 20 years that they are being tested for use in fields . Challenges : Too expensive (~ $ 200per gram) toxicological issues. hard to purification and the ability to manipulate structures at the atomic scale….
summary Carbon nanotubes have very different properties compared to the other carbon allotropes - these unique properties offer huge potential in product development. Nanomaterials, particularly carbon nanotubes (CNTs), hold great promise for a variety of industrial, consumer, and biomedical applications, due to their outstanding and novel properties.