Atomic Nanotube Welders.

We demonstrate that the incorporation of boron between double walled carbon nanotubes (DWNTs) during thermal annealing results in covalent nanotube ``Y'' junctions, DWNT coalescence and the formation of flattened multi-walled carbon nanotubes (MWNTs). The processes occur via the merging of adjacent tubes which is triggered by B interstitial atoms. In order to demonstrate the unique welder properties of B in the process, we have carried out \textit{AM1} molecular dynamics simulations at high temperatures and \textit{ab-initio }calculations. We observe that B atom interstitials between DWNTs are responsible for the rapid establishment of covalent connections between neighboring tubes (polymerization). Once B is in the lattice, tube faceting (polygonization) starts to occur, and the electronic properties are expected to change dramatically.