Would Diamond Nanorods Be Stronger than Fullerene Nanotubes?

Abstract

On the basis of literature ab initio data, we show that diamond nanorods would have a brittle fracture force and a zero strain stiffness that exceeds carbon nanotubes for radii greater than about 1−3 nm, depending on the orientation of the diamond nanorod. The energetic stability of diamond nanorods is predicted by molecular modeling to be comparable to single-walled carbon nanotubes. It is concluded that diamond nanorods represent an important and viable target structure for synthesis.With its exceedingly high bulk modulus and hardness, diamond has historically been considered the strongest material. Recently, however, it has been claimed based on both theory and experiment that carbon nanotubes are both stiffer and stronger along their axis than diamond. A problem with this claim is that it is difficult to make a fair comparison between these two representatives from the macro-and nanoscales unless some additional assumptions about their structure are made, for example an effective "thickness" of a sheet of carbon atoms comprising a nanotube.In this paper the mechanical properties of single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs) are compared to the predicted properties of an equivalent nanoscopic-scale diamond structure, namely a diamond nanorod (DNR). Our general analysis suggests that while a SWNT will have a higher strength-to-weight ratio, above a critical radius between about 1 and 3 nm (depending on the DNR structure) the force needed for brittle fracture of a DNR exceeds that of a SWNT. This higher fracture force, which at the nanoscopic scale is a less ambiguous property than fracture stress, results from the larger load-bearing crosssectional area of DNRs compared to SWNTs at the same diameter. Similarly, the calculations show that the zero strain stiffness of DNRs will exceed that of SWNTs for radii greater than about 1 nm.Experimental loading of SWNTs in ropes has yielded estimates for the tensile Young's modulus that range from 320 GPa to 1.47 TPa, 1 and breaking strengths that range from 13 to 52 GPa (a strain of up to almost 6%), 1 values that agree well with theoretical estimates of yield strain. 2 Properties of SWNTs predicted from first principles calculations include a Young's modulus approaching that of a graphene sheet (∼1.03 TPa 3 ) and a fracture strength of ∼200 GPa (Tables 1 and 2), 4 assuming a 0.34 nm nanotube wall thickness. The failure strength estimated from theory and experiment for a SWNT is over a factor of 3 larger than established yield strengths for the strongest fibers typically used in fiber-reinforced composites (e.g., graphite, silicon nitride, silicon carbide, or aluminum oxide), which has made nanotubes attractive structures for reinforcing nanocomposites. More recently, the first observation using transmission electron microscopy of a MWNT breaking in tension was reported.5 Based on the measured force required to break the nanotube, a tensile strength of 150 GPa (experimental uncertainty ∼30%) at a deformation estimated to be ∼5% was reporte...

Analysis of diamond nanocrystal formation from multiwalled carbon nanotubes

“…32 The stability of the one-dimensional equivalent nanostructures, namely, diamond nanowires, both monolithic and in the form of hollow cylinders, also has been investigated thoroughly. [33][34][35][36][37] It has been demonstrated that the stability of these nanowires is a function of their diameter, their surface morphology, as well as the crystallographic direction of their principal axes. The above-mentioned studies refer to isolated nanocrystals [29][30][31][32] or practically infinitely long diamond nanowires.…”

Aggregated diamond nanorods, the densest and least compressible form of carbon

“…So far nobody has reported experimental observation of diamond nanorods, while several theoretical groups have predicted their possible existence and tried to calculate their properties. [4][5][6][7] For centuries diamond has been considered the hardest known material. [8][9][10] However, both theory and experiment suggest that single-walled carbon nonotubes ͑SWNT͒ along their axis are stiffer and stronger than diamond.…”

“…[8][9][10] However, both theory and experiment suggest that single-walled carbon nonotubes ͑SWNT͒ along their axis are stiffer and stronger than diamond. 11,12 Moreover, theory predicts 4 that diamond nanorod would have a brittle fracture force and a zero strain stiffness that exceeds that of carbon nonotubes for radii greater than about 1-3 nm. The results of the theoretical study 7 indicate that diamond nanowires with the diameter from 2.7 to 9 nm could be stable.…”

“…The first synthesis of bulk sample of nanocrystalline diamond from C 60 was recently reported, 8 and complex investigation has suggested that the new material was at least as hard as single crystal diamond and, at high temperature and ambient pressure, kinetically more stable with respect to graphitization than usual diamonds. The theoretical evaluations of properties of diamond nanowires and nanorods, [4][5][6][7] which predicted superior properties of the latter and made them an important and viable target for synthesis, encouraged our search for the methods of their production.…”

Nanodiamonds for field emission: state of the art

“…This could be hardly considered an unexpected result, because among the other WBGMs, diamond is characterized by very favorable properties, such as a high dielectric constant, a negative electron affinity (NEA), high thermal conductivity, mechanical strength, chemical inertness and radiation tolerance. [21][22][23][24][25] Moreover, the fact that the diamond surfaces do not adsorb chemical species has made it possible to use the emitters without the need for "conditioning" pre-treatments, consisting of complex out-gassing steps, as required in the case of uncoated CNTs. 26 Let us consider in some detail the electronic structure of diamond.…”