New nanostructured energetic composites with superior heat-release characteristics and long-term storage stability

Abstract

Nanoenergetic materials, which consist of nanoscale metals and oxidizers, have shown superior performance in terms of energy release rate, ignition characteristics, and combustion behaviour compared with their micron-sized counterparts. Therefore, they have found broad applications such as reactive bonding, micro ignition and rapid initiation, micro actuation and propulsion, materials synthesis and processing, and biomedical applications. Compared with randomly mixed nanoenergetic materials, nanostructured energetic composites offer further enhanced interfacial contact and improved ingredient distribution, and thus they exhibit better performance. Several types of new nanostructured energetic composites are studied in this dissertation, with the aim of superior energetic capability and long-term storage stability. Mg is a highly reactive metal which is capable of very exothermic reaction. However, the use of nano-sized Mg in nanoenergetic materials is limited probably due to the unavailable commercial Mg nanopowders and the safety concerns incurred in the handling and storage process. A facile and green method for the synthesis of Mg/CuO core/shell nanoenergetic arrays on silicon substrate is reported, with Mg nanorods as the core and CuO as the shell. Uniform mixing and intimate contact between Mg nanorods and CuO are confirmed from both microscopic images and the heat release curves. The nanoenergetic arrays exhibit low onset reaction temperature (~300°C) and high heat of reaction (~3.4 kJ/g). Most importantly, the nanoenergetic arrays possess long-term storage stability in dry air due to the stable CuO shell. CuO as the shell can give protection to the Mg core when the composites are stored in dry air. However, when the composites are exposed to moisture-laden air, the protection will not be so effective. Moreover, theoretical heat release from thermite reaction can hardly exceed 5 kJ/g, while some polymers such as polytetrafluoroethylene can react with metal to release much more heat. Consequently, Mg/fluorocarbon core/shell nanoenergetic arrays are prepared onto silicon substrate. It is found that the as-prepared fluorocarbon consists of shorter molecular chains compared to that of bulk polytetrafluoroethylene, which contributes to the low onset reaction temperature. The Mg/fluorocarbon surface is superhydrophobic just as predicted by the Cassie-Baxter mode. The Mg/fluorocarbon material exhibits a very low onset reaction temperature of about 270°C and an ultrahigh heat of reaction approaching 9 kJ/g. Preliminary combustion test reveals the rapid combustion wave propagation, and convective mechanism is adopted to explain the combustion behavior. Nanostructured energetic composites with a superhydrophobic surface are very promising to the long-term storage stability. In the light of this, sandwich-structure superhydrophobic CuO/Mg/fluorocarbon nanoenergetic composite is prepared on the silicon substrate. Accelerated aging test is performed in a temperature and humidity chamber (35°C, 95% relative humidity). It is found that Mg layer obtains the nano-texture from CuO and engenders superhydrophobicity after being coated with a fluorocarbon layer. Both the fluorocarbon coating and the CuO layer react with Mg intermediate layer but in different temperature ranges. Because of the superhydrophobicity, CuO/Mg/fluorocarbon maintains about 82% chemical energy after 240 h exposure. The nanoenergetic composite holds more than 50% of the original energy after 6 h underwater storage. Superhydrophobic MoO3/Mg/fluorocarbon and MoO3/Al/fluorocarbon are also successfully prepared to prove the generality of this new design concept. Si is chemically stable compared with many metallic elements when used as fuel for energetic formulations, and thus Si-based energetic composites can be intrinsically suitable for long-term storage. Upright nano/sub-micron Si wire arrays are fabricated by a mask-free deep reactive ion etching method. Orderly Si wires are obtained by tuning the passivation/etching ratio in a processing cycle. As the processing time increases, the number density of Si wires decreases while the length increases. Meanwhile, the effect of power density and Ar flow rate on the F/C ratio of the sputtered fluorocarbon are investigated, but neither of them show appreciable impact within the ranges studied. Si/fluorocarbon surface is extremely superhydrophobic. Si wires and fluorocarbon react exothermically with much gas products generated but the determined heat of reaction is relatively low. Si wire arrays can also be used as scaffolds to prepare other highly superhydrophobic energetic composites.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kaili ZHANG (Supervisor) & Jian LU (Co-supervisor)