Acetylene is pyrolyzed at 600, 700 and 750 degrees C over an Al substrate in an atmosphere of nitrogen by simple thermal chemical vapor deposition. The products include linear multiwall carbon nanotubes (MWCNTs) and carbon nanotube (CNT) Y-, T-, L- and bend-junctions, all of which have bamboo structures. Bamboo structures distinguish our CNT junctions from those reported in previous studies. A mechanism of the formation of CNTs with bamboo structures is proposed. CNT junctions with joints at the intersection of the CNTs are supposed to form by welding two linear CNTs. Temperature fluctuations, caused by the melting of Al substrate and the formation of aluminum nitride (AIN) precipitates are responsible for the growth instabilities of CNTs and generate defects on the walls of CNTs. An excess of carbon sources favors the formation of heptagons and provides the curvature required to form bent CNTs, the bamboo structures and CNT Y-, T-, L- and bend-junctions. The Al substrate plays a key role. More CNT junctions are synthesized at 700 degrees C than at 600 or 750 degrees C. Sufficient carbon vapor at 750 degrees C favors formation of bent CNTs over that of bamboos or CNT junctions. (c) 2006 Elsevier B.V. All rights reserved.