New roles of metal–organic frameworks: Fuels for aluminum-free energetic thermites with low ignition temperatures, high peak pressures and high activity
Introduction
Because of its high reaction enthalpy and good thermal conductivity, aluminum (Al) powders at the micrometer size range are broadly employed as fuels in conventional thermites, which are regularly used in gas generators, propulsions, explosives, thermal batteries, weldings, material synthesis, and waste disposals [1], [2], [3]. Although these thermites usually have higher energy density than organic explosives such as 2,4,6-trinitrotoluene (TNT), nitrocellulose, cyclotrimethylenetrinitramine (RDX), etc., they suffer from slow rates of energy release and super-high ignition temperatures (∼2000 °C) [4], [5]. In practical applications, a large portion of the aluminum powders often remain unreacted due to low reaction rates and long ignition delays; as a result, the energetic properties are limited [4], [6]. Alternatively, nanothermites, a subset of metastable intermolecular composites (MIC), are a relatively new class of energetic materials comprising a metallic fuel and an oxidizer at nanoscale that can rapidly release heat and pressure. In most formulations, nanoaluminum is employed as fuel because of its superior properties, and ready availability of an oxidizer (e.g., CuO, Fe2O3, MoO3, etc.) [7], [8], [9], [10], [11], [12]. Unfortunately, current aluminum-based nanothermites still suffer from high electrostatic sensitivities [13], [14], and relatively high ignition temperatures (∼800 °C) [13], [15], [16]. Meanwhile, the higher surface energy of the nanoparticles also leads to greater particle aggregation, and increases composite agglomeration, which could lead to the composite incomplete combustion [17], [18]. Apart from these, however, one of the biggest problems in the aluminum-based thermite systems is excessive oxidation of the aluminum particle before combustion, resulting in a decrease of the active aluminum content. Moreover, most of the aluminum-based thermites have relatively low peak pressures (maximum peak pressure) mainly due to the lack of gas elements (e.g., CHON elements) in their components, which lead to few gaseous products during combustion. Nevertheless, gas generation plays an important role in the altitude control of orbiting microspacecraft and in microscale devices capable of producing tens of milliNewtons (mN) of thrust [19], [20]. Hence, developing new thermites with low ignition temperatures, high peak pressures, and high activity would be very fascinating while challenging at the same time.
As an emerging class of porous materials, metal–organic frameworks (MOFs) have recently attracted considerable attention due to their high surface area, uniform pore size, controllable structures, and readily tailorable functions [21], [22], [23], [24], [25], [26], [27], [28]. Energetic MOFs are an appealing subclass and can be mainly constructed from energetic nitrogen-rich ligands and metal ions [29], [30], [31]. Compared with aluminum powders, energetic MOFs possess good stabilities, abundant gas elements (e.g., high nitrogen contents) and high heats of detonation, which would make them as ideal alternative fuels to develop a new-concept aluminum-free thermite and solve above problems. Recently, we had reported MOFs as an active component for green gas generators [32]. However, there has so far been no report about the exploration of MOFs in thermite systems, although a lot of energetic MOFs with fascinating topological structures have been synthesized over the past few years [33], [34], [35].
Here, we reported a novel type of thermites based on a three-dimensional (3D) energetic MOF [Cu(atrz)3(NO3)2]n [MOF(Cu), atrz = 4,4′-azo-1,2,4-triazole]. The energetic performances of these materials in terms of ignition temperature, heat of reaction, peak pressure, reactivity and sensitivity were assessed and compared to existing and more traditional aluminum-based thermites, exhibiting impressive energetic properties. We chose to use MOF(Cu) as the fuel for the following reasons: (1) MOF(Cu) possesses a super-high heat of detonation and a high nitrogen content (53.35%) [33], which could improve the energetic performances of the target thermites; (2) in comparison with aluminum powders, MOF(Cu) exhibits higher activity since its decomposition temperature is 243 °C, which could contribute to decreasing the ignition temperatures of the target thermites. In addition, the perchlorate salts were selected as the oxidizers because of their high oxygen contents and strong oxidizing nature, which have inspired many fascinating studies [15], [36], [37].
Section snippets
Caution
Although we have not experienced any problems during the preparation of thermites, standard safety precautions (leather gloves, face shield and ear plugs) should be taken when handling these materials.
Chemical and materials
NH4ClO4 and KClO4 were all purchased from Beijing Chemical Reagent Company without further purification. 50 nm copper oxide (CuO) was purchased from Sigma-Aldrich Corporation. 50 nm aluminum (Al) nanopowders were purchased from Beijing DK Nano Technology Co. LTD, and the purity of the aluminum
Results and discussion
According to the literature procedure [33], MOF(Cu) was prepared from a hydrothermal reaction of 4,4′-azo-1,2,4-triazole (atrz) with Cu(NO3)2 at 100 °C; this material can be synthesized with high yield and purity. Its crystal structure contains 3D metal–organic framework with 1D channels incorporating energetic nitrate anions [33]. The powder X-ray diffraction (PXRD) patterns (Fig. S1) showed that the framework of MOF(Cu) was well maintained even after it has been heated at 200 °C for 24 h in
Conclusions
In summary, we have demonstrated a novel type of aluminum-free thermites based on an energetic MOF. Compared with the aluminum-based thermites, these new-concept thermites exhibit superior performances such as low electrostatic discharge sensitivities, low ignition temperatures, high heats of reaction, high peak pressures, high activity and production of very few solid combustion residues. Among them, the measured heat of reaction, ignition temperature and peak pressure for MOF(Cu)/NH4ClO4
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
The authors acknowledge financial support from NSAF (U1530262) and the National Natural Science Foundation of China (21576026) and the opening project of State Key Laboratory of Science and Technology (Beijing Institute of Technology, ZDKT12-03).
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