The Study of Sublimation Rates and Nucleation and Growth of TNT and Petn on Slica and Graphite Surfaces by Optical and Atomic Force Microscopy and Ellipsometry

Abstract

Optical and atomic force microscopy (AFM) were used to study the structure and crystallization kinetics of 2,4,6-trinitrotoluene (TNT) and pentaerythritol tetranitrate (PETN) vapor deposited on silica, Muscovite mica and highly oriented pyrolytic graphite (HOPG) surfaces. Under ambient conditions, the deposited TNT and PETN on these surfaces are in supercooled liquid droplet forms. Dendritic-like crystal nucleation and growth are found for both TNT and PETN on a silica surface after the exposure to the air for 16 and 48 h, respectively. A plate-like solid TNT is also observed in the depletion zone between the grown crystals and the supercooled liquid droplet region. Both AFM and DSC measurements suggest that these plate-like TNT films are amorphous. Tapping mode AFM and ellipsometry were also employed to study the sublimation rates of TNT deposited on silica, mica and graphite surfaces. A nonlinear sublimation rate has been observed for solid TNT on both silica and mica surfaces. A theoretical model, which assumes that the potential is an exponential function with no arbitrary constants, is proposed. From the model fitting, three fundamental parameters, bulk TNT sublimation rate, surface interaction potential at TNT-silica interface and a critical decay length have been obtained. TNT on * Correspondence addressee. Atomic Force Microscopy/Scanning Tunneling Microscopy 3, edited by S H Cohen and M L. Lightbody, KluwerAcademic/Plenum Publishers, 1999 135 graphite, on the other hand, exhibited a constant sublimation rate, which can be attributed to a weak TNT-substrate interaction. Ellipsometry can provide information on the explosives' effective sublimation rates on various surfaces, over an area of˜0of˜of˜0.3 cm 2 .