  We observed a huge volume expansion of aligned single walled carbon nanotube
(SWNT) arrays accompanied by structural transformation during electrical
breakdown in vacuum. The SWNT arrays were assembled between prefabricated Pd
source and drain electrodes of 2 \mu m separation on Si/SiO_2 substrate via
dielectrophoresis. At high electrical field, the SWNT arrays erupt into large
mushroom-like structure. Systematic studies with controlled electrical bias
show that above a certain field SWNTs swell and transform to nanoparticles and
flower-like structures with small volume increase. Further increase in
electrical bias and repeated sweeping results into amorphous carbon as
determined from scanning and transmission electron microscopy (TEM). Cross
sectional studies using focused ion beam and TEM show the height of 2-3 nm SWNT
array increased to about 1 \mu m with a volume gain of ~ 400 times. The
electron energy loss spectroscopy reveals that graphitic sp^2 networks of SWNTs
are transformed predominantly to sp^3. The current-voltage measurements also
show an increase in the resistance of the transformed structure.
