Development of Novel Energetic Composites Based on Nano-aluminum and Energetic Coordination Polymers

基於納米鋁和含能配位聚合物的新型高能複合物的研發

Student thesis: Doctoral Thesis

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Award date9 Jun 2020

Abstract

Energetic materials (EMs) are a class of materials with high amount of stored chemical energy which can be released upon initiation. They have a wide range of applications in mining, deconstruction, automobile airbags, fireworks, ordnance, and space technology. Nowadays, EMs are being developed in the direction of high energy and low sensitivity, which requires them to possess high heat release, excellent combustion behavior and good gas generation, while being insensitive to external stimulation.

As one of MEMS compatible EMs, nanothermite has attracted a lot of attention in recent decades. It is a metastable intermolecular composite with nano-scale fuel and oxidizer, which has the advantages of high energy density and fast energy release rate. However, the gas generation of most nanothermites is poor due to the lack of gas elements (e.g. C, H, N) in composition. In addition, nanothermite is sensitive to external stimulation, such as friction, impact and static electricity, thus having potential safety hazards in storage and transportation. In this thesis, I focus on solving these two problems of poor gas generation and high sensitivity by introducing energetic coordination polymers (ECPs) into nanothermite components and using them as oxidizers, because ECPs are generally of high gas production, high energy density and low sensitivity.

Four kinds of ECPs were synthesized using 1H,1′H-[5,5′-bitetrazole]-1,1′-bis(olate) (H2BTO) as the energetic ligand and transition metal salts as metal centers, including [Mn(BTO)(H2O)2]n, [Co(BTO)(H2O)2]n, [Ni(BTO)(H2O)2]n and [Cu(BTO)(H2O)2]n. They were respectively compounded with nano-aluminum (nano-Al) by facile ultrasonic mixing method. Compared with the pure ECPs, the Al/ECPs composites have more heat release, higher peak pressure, more intensive combustion behavior and lower sensitivity. The mechanism behind these improvements is analyzed in detail, and the "father-son" effect is proposed. ECPs are found to transfer to the corresponding metal oxides after thermal decomposition because the central metal ions combine with oxygen from the ligand or the air. In Al/ECPs composites, the “father” ECPs can release a large amount of gas and heat during decomposition, and its “son,” the corresponding metal oxide, can further react with nano-Al, which results in a second heat contribution. The gas generation and thermite reaction work synergistically and greatly promote the heat release, gas production, and combustion performance, which contributes to the formation of high-temperature and high-pressure reaction environment.

The ECPs were found to have problems of incomplete and discontinuous combustion during the burning experiments. This issue was solved by in situ growing nanoscale [Co(BTO)(H2O)2]n (ECP(Co)) on graphene oxide (GO) sheets. The GO nanosheets were found to be completely covered by the bar-like nano ECP(Co) to form GO@ECP(Co) composites. The GO@ECP(Co) nanocomposites with 5 wt% - 7.5 wt% GO content can solve the quenching and discontinuous combustion issues presented by micro-size ECP(Co), with faster energy release rate and greater heat output (up to 3137.8 J/g), due to the reduced size of ECP(Co), oxygen-release ability of GO and high thermal conductivity of reduced GO. Correspondingly, their peak pressure and pressurization rate were also greatly enhanced.

To meet the requirements of miniaturization and integration of EMs in aerospace field, Cu(OH)2/ECP@nano-Al ternary and ECP@nano-Al binary energetic arrays were constructed successfully on a substrate through a MEMS compatible processes. The ECPs nanostructures were realized in situ on copper foil via the coordination reaction of H2BTO with Cu(OH)2 nanorods and subsequent polymerization at room temperature. A layer of aluminum was then deposited on the surface of ECPs nanostructures by e-beam evaporation method. The MEMS-compatible energetic arrays were found to have a core-shell structure and own better pressure output performance than powdery Al/ECPs. The experiment of torsion pendulum was used to test the preliminary propulsive performance of the Cu(OH)2/ECP@nano-Al energetic arrays. They can be successfully ignited under a continuous laser with the power at 5W, and the displacement of the swing arm proves their ability of thrust generation. The impulse produced by 1 mg sample is approximately 82.6 µN∙s. Accordingly, the energetic arrays have great potential for being used in micro-thrusters.

    Research areas

  • energetic materials, energetic coordination polymers, nano aluminium, energetic composites, graphene oxide