Would Diamond Nanorods Be Stronger than Fullerene Nanotubes?

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 macroand 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 TPa3) 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 reported. * Corresponding author. † International Technology Center. ‡ North Carolina State University. § Northwestern University. Table 1: Selected Measured and Predicted Mechanical Properties of Bulk Diamond along Different Orientations, Graphite, and Graphene (related strains in parentheses) structure orientation Young’s modulus (GPa) ab initio tensile