Structural evolution and thermal runaway of refractory W and Mo nanotips in the vacuum under high electric field from PIC-ED-MD simulations

Abstract

We performed multiscale-multiphysics simulations for W, Mo and Cu nanotips under high electric field to investigate their structural evolution and thermal runaway process. The critical electric field values for the electric prebreakdown condition are predicted to be 311 MV m−1, 570 MV m−1 and 675 MV m−1 for Cu, Mo and W nanotips respectively (R 0 = 1 nm, H 0 = 100 nm). The boiling point of the metal is found to be a good predictor of the critical electric field strength for the initiation of thermal runaway. For metal nanotips made of refractory metals such as W and Mo, the structural thermal runaway process is determined by the rapid growth of small protrusions and their subsequent sharpening and thinning under the high electric stress on the apex region. On the other hand, the more intense atomic evaporation of Cu metal nanotips is caused by the ejection of large droplets generated by recrystallization and necking of the molten region at the apex of the nanotip. The differences in the observed structural evolutions of nanotips between refractory metals and the Cu during the thermal runaway event clearly show the strong influence of melting and boiling points on the electric prebreakdown process in nanoscale.