Nanoscale Energetics with Carbon Nanotubes
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NANOSCALE ENERGETICS WITH CARBON NANOTUBES Yubing Wang, Sanjay Malhotra and Zafar Iqbal* Department of Chemistry and Environmental Science New Jersey Institute of Technology, Newark, New Jersey Abstract: Single wall carbon nanotubes (SWNTs) with diameters below 1 nm prepared by chemical vapor deposition (CVD), and with diameters of 1.3 nm and higher prepared by laser ablation and carbon-arc techniques, were electrochemically functionalized with hydrogen and nitro groups, and chemically derivatized with 4-nitroaniline. Hydrogen adsorption on SWNTs was carried out in the presence or absence of electrodeposited catalytic nanoparticles of magnesium. SWNTs deposited on Teflon-coated membranes by vacuum filtration and lifted off as free-standing nanopaper, were used as the electrodes for electrochemical functionalization reactions. Hydrogen uptake on the nanotubes was characterized by micro-Raman spectroscopy, thermogravimetry and thermopower measurements. Electrochemically-induced functionalization with –NO2 groups on metallic, laser-synthesized SWNTs was clearly detected by Raman spectroscopy. Chemical functionalization was achieved on CVD-produced SWNTs by acidification to form –COOH groups followed by reaction with thionyl chloride and then with 4-nitroaniline. Photoacoustic effects that are likely to be precursors of photo-induced initiation of energetic reactions, were observed to occur at varying laser intensities for these materials in experiments using a pulsed Nd-YAG laser emitting at 532 nm. Introduction: Research at nanoscale levels has revealed that chemical reactions in energetic materials nucleate in 20-50 nm regions at interfaces and surfaces [1], initiating hot spots in the 10-3 to 10-4 cm range discussed by Bowden and Yoffe [2]. The chemical reactions nucleate initially at molecular levels, followed by energy dissipation into lattice vibrations. For shock-induced reactions a mechanism for transferring the phonon energy into highly excited intramolecular vibrations leading to bond breaking and run-away reactions, has been invoked [3]. Alternative mechanisms include: (a) initiation by the sudden release of energy stored in dislocation pile ups to form hot spots, and (b) a steric hindrance model [4] where shock compression leads directly in a non-equilibrium manner to intramolecular excitation, bond breaking and reaction via shear flow triggered by the passage of edge dislocations. This process occurs without the need for the energy to move from the lattice vibrations back to the molecular level. In crystalline solids these reaction sites appear to follow the lattice structure of the solid. In liquids, these sites are nanometer scale gas bubbles that collapse to initiate the rapid reaction. The equation for the transition from a thermally stable state to a run-away reaction, is given by the Frank-Kamenetzky equation [5]: -K∇2T + ρC.δT/δt = ρQ.A exp(-E/kT) heat loss self-heating by conduction
… (1)
production of heat by chemical reaction
where ∇2 is the Laplacian operator, T is the solid temperat
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