Synthesis, characterization and performance evaluation of nano-energetic composite
Tan, Meng Lu
Date of Issue2018-12-31
School of Materials Science and Engineering
Thermites belong to a class of energetic material comprising of a metal as a fuel and a metal oxide as the oxidizer. The research on nano-thermites has significantly emerged in the last two decades and novel ways to harness their energy with improved reactivity, reduced sensitivity and high stability remains highly desirable to date. In this work, Al/NiO nano-thermite system was studied due to their relatively gasless reaction. Gasless thermite reactions could offer potential in applications requiring little flow disturbances and vibrations. A novel self-assembly technique to promote better intermixing of the fuel-oxidizer system through surface functionalization with complimentary functional groups as well as the addition of an energetic polymer binder to reduce the sensitivity were studied. The heat release characteristics and reaction mechanism of Al/NiO nano-thermites were studied. n-Al/n-NiO with different equivalence ratio (ER) were prepared and their heat of reactions measured using a bomb calorimeter. The heat of reaction increased from a fuel lean formulation to a slightly fuel rich formulation at ER 1.2, yielding an optimized heat of reaction of 3649 J/g. Their highly exothermic nature was studied using a Differential Scanning Calorimeter (DSC). The reaction products during the different stages of the decomposition were analyzed using a powder X-ray Diffraction (XRD) to understand the reaction mechanism of this alumino-thermic reaction. Surface functionalization of n-Al and n-NiO using organosilanes with complimentary end groups (epoxide/ amino) to bring about the fuel-oxidizer self-assembly were performed. The self-assembled n-Al/n-NiO showed a better intermixing of the binary composite powder from their Scanning Electron Microscopy/Energy Dispersive X-ray (SEM/EDX) photographs. The self-assembled system displayed a larger heat release before aluminum melting as well as an increased heat release rate from their DSC profiles. The preference for reaction prior to aluminum melting (or solid-state reaction) is an indication of intimate interaction between the fuel and oxidizer. The self-assembly process was shown to increase the energy release rate of organosilane-functionalized nano-thermites in their pressure studies. The overall energy release rate of the functionalized n-Al/n-NiO was, however, not better than unfunctionalized n-Al/n-NiO. Self-assembled organosilane-functionalized n-Al/n-NiO was found to have higher activation energy barrier (240 kJ/mol) as compared to physically mixed n-Al/n-NiO (203 kJ/mol). The increase is a result of increased diffusion barrier for Al and O, introduced by the additional organosilane surface graft. The surface functionalization could significantly reduce the electrostatic discharge (ESD) sensitivity of this material. The improved ESD minimum ignition energy is higher than what the human body is capable of discharging, making them safer to handle. An energetic polymer, terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV), was added to the nano-thermite to function as a binding agent as well as improve the safety of handling the highly sensitive n-Al/n-NiO. THV fluoropolymer was added to n-Al/n-NiO in varying weight percentages (0 to 40). The effect of THV addition to n-Al/n-NiO on their reactivity and sensitivity were evaluated. A 30 wt% THV addition to n-Al/n-NiO (ER 1.2) could successfully preserve the reactivity of the nano-thermite as well as improve the safety of handling by decreasing their sensitivity to ESD and friction. Recommended future work includes surface functionalization and self-assembly using highly energetic linkers as well as application-based research for potential use as primer in detonators and in gasless door breaching or metal cutting.