Editing Carbon nanotube (section)

=== Mechanical ===
{{Main|Mechanical properties of carbon nanotubes}}
[[File:CNTSEM.JPG|thumb|A [[scanning electron microscopy]] image of carbon nanotube bundles]]
Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of [[tensile strength]] and [[elastic modulus]]. This strength results from the covalent sp<sup>2</sup> bonds formed between the individual carbon atoms. In 2000, a multiwalled carbon nanotube was tested to have a tensile strength of {{convert|63|GPa|abbr=on}}.<ref name="Strength and Breaking">{{cite journal | vauthors = Yu MF, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS | title = Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load | journal = Science | volume = 287 | issue = 5453 | pages = 637–640 | date = January 2000 | pmid = 10649994 | doi = 10.1126/science.287.5453.637 | bibcode = 2000Sci...287..637Y | s2cid = 10758240 }}</ref> (For illustration, this translates into the ability to endure tension of a weight equivalent to {{convert|6422|kgf}} on a cable with cross-section of {{convert|1|mm2|abbr=on}}). Further studies, such as one conducted in 2008, revealed that individual CNT shells have strengths of up to ≈{{convert|100|GPa|abbr=on}}, which is in agreement with quantum/atomistic models.<ref name="B. Peng">{{cite journal | vauthors = Peng B, Locascio M, Zapol P, Li S, Mielke SL, Schatz GC, Espinosa HD | title = Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation-induced crosslinking improvements | journal = Nature Nanotechnology | volume = 3 | issue = 10 | pages = 626–631 | date = October 2008 | pmid = 18839003 | doi = 10.1038/nnano.2008.211 }}</ref> Because carbon nanotubes have a low density for a solid of 1.3 to 1.4&nbsp;g/cm<sup>3</sup>,<ref name="nanotubes for electronics">{{cite journal | vauthors = Collins PG, Avouris P | title = Nanotubes for electronics | journal = Scientific American | volume = 283 | issue = 6 | pages = 62–69 | date = December 2000 | pmid = 11103460 | doi = 10.1038/scientificamerican1200-62 | bibcode = 2000SciAm.283f..62C }}</ref> its [[specific strength]] of up to 48,000&nbsp;kN·m/kg is the best of known materials, compared to high-carbon steel's 154&nbsp;kN·m/kg.

Although the strength of individual CNT shells is extremely high, weak shear interactions between adjacent shells and tubes lead to significant reduction in the effective strength of multiwalled carbon nanotubes and carbon nanotube bundles down to only a few GPa.<ref name="T. Filleter, R. A 2011">{{cite journal | vauthors = Filleter T, Bernal R, Li S, Espinosa HD | title = Ultrahigh strength and stiffness in cross-linked hierarchical carbon nanotube bundles | journal = Advanced Materials | volume = 23 | issue = 25 | pages = 2855–2860 | date = July 2011 | pmid = 21538593 | doi = 10.1002/adma.201100547 | bibcode = 2011AdM....23.2855F | s2cid = 6363504 }}</ref> This limitation has been recently addressed by applying high-energy electron irradiation, which crosslinks inner shells and tubes, and effectively increases the strength of these materials to ≈60 GPa for multiwalled carbon nanotubes<ref name="B. Peng" /> and ≈17 GPa for double-walled carbon nanotube bundles.<ref name="T. Filleter, R. A 2011" /> CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo [[buckling]] when placed under compressive, torsional, or bending stress.<ref>{{cite journal | vauthors = Jensen K, Mickelson W, Kis A, Zettl A |title=Buckling and kinking force measurements on individual multiwalled carbon nanotubes |journal=Physical Review B |date=26 November 2007 |volume=76 |issue=19 |page=195436 |doi=10.1103/PhysRevB.76.195436 |bibcode=2007PhRvB..76s5436J }}</ref>

On the other hand, there is evidence that in the radial direction they are rather soft. The first [[Transmission electron microscopy|transmission electron microscope]] observation of radial elasticity suggested that even [[van der Waals force]]s can deform two adjacent nanotubes. Later, [[nanoindentation]]s with an [[atomic force microscope]] were performed by several groups to quantitatively measure the radial elasticity of multiwalled carbon nanotubes and tapping/contact mode [[atomic force microscopy]] was also performed on single-walled carbon nanotubes. Their high [[Young's modulus]] in the linear direction, of on the order of several GPa (and even up to an experimentally-measured 1.8 TPa, for nanotubes near 2.4 μm in length<ref>{{cite journal |last1=Treacy |first1=M. M. J. |last2=Ebbesen |first2=T. W. |last3=Gibson |first3=J. M. |title=Exceptionally high Young's modulus observed for individual carbon nanotubes |journal=Nature |date=June 1996 |volume=381 |issue=6584 |pages=678–680 |doi=10.1038/381678a0|bibcode=1996Natur.381..678T }}</ref>), further suggests they may be soft in the radial direction.