Application of 3D energetic metal-organic frameworks containing Cu as the combustion catalyst to composite solid propellant
Introduction
Solid propellant is the most important energy source of rocket motor. Its energy properties and combustion performances are the most important factors influencing the performance of solid rocket motor [1]. An excellent solid propellant should possess an extremely stable burning rate and a low pressure exponent [2]. One of the best ways to achieve such goal is adding a burning rate catalyst into the propellant as the ballistic modifier to tune the ballistic properties of the propellant [3]. The use of combustion catalyst can increase or decrease the burning rate of propellant, reduce the pressure index and improve the combustion stability of propellant. This is important because the internal ballistics of rocket propellants are essentially related to the combustion rate. An increase in combustion rate promotes the production of thrust, which can improve the performance of rocket and missile in terms of payload and trajectory [4].
CSP constructed with hydroxyl-terminated polybutadiene (HTPB) as the binder, ester as the plasticizer, ammonium perchlorate (AP) as the oxidizer and aluminum powder as the metal fuel have been widely used for solid rocket propulsion [4,5]. It has been found that the thermal decomposition performance of AP, a most commonly used oxidizer in propellants [6], can greatly affect the combustion performance and internal ballistic performance of CSP [7]. Therefore, combustion catalysts, such as metal [8], nano oxalates [4], copper chromite [9], iron oxide (Fe2O3) [10], [11], [12], CuO [13], [14], [15] and NiO [16], are often added to improve the combustion performance of CSP. CuO, especially nano CuO, has been demonstrated as an excellent catalyst to dramatically lower the decomposition temperature of AP and increase the burning rates of CSP [17,18]. However, the application of nano catalyst can cause obvious negative effects on the ultimate mechanical properties and processing performances of CSP [6]. In addition, the blend of combustion catalysts can cause sharp energy losses of CSP due to their inert or non-energetic properties.
Metal-organic framework (MOF) have attracted considerable attentions in recent years with their unique molecular structures and potential applications in catalysis [19], [20], drug delivery [22] and gas storage [23,24]. Among various MOF, the energetic MOF with the nitrogen rich heterocyclic compounds as the organic linker have been extensively studied because of their unique molecular structures and high formation enthalpies [25], [26], [27], [28]. The 1D energetic MOF, (Ni(NH2NH2)5(ClO4)2)n (NHP) and (Co(NH2NH2)5(ClO4)2)n (CHP), have been reported with high detonation heats and extremely high sensitivity [29,30]. However, these energetic MOF are highly sensitive to impacts due to the low rigidity characteristics of their linear polymeric structures, which makes their practical use infeasible [27]. Later, 2D energetic MOF, such as ((Co2(N2H4)4(N2H3CO2)2)(ClO4)2·H2O)n (CHHP) and ((Zn2(N2H4)3(N2H3CO2)2)(ClO4)2·H2O)n (ZnHHP), were designed using a hydrazine derivative (hydrazine-carboxylate) as the ligand to reduce the sensitivity, but showing lower heats of detonation. To balance the energetic performance and sensitivity, Li et al. prepared a novel 3D energetic MOF, [Cu(atrz)3(NO3)2]n (MOF(Cu)), by a straightforward method using 4,4′-azo-1,2,4-triazole (atrz) as the ligand, and characterized its physical and detonation properties [27]. The 3D MOF(Cu) exhibited the highest heat of detonation known for metal-based energetic compounds, even higher than those of octanitrocubane (ONC), NHP and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). In addition, the MOF(Cu) was sensitive to neither friction (0%), nor electrostatic discharge (≥ 24.75 J) with the electrostatic sensitivity of HMX as low as only 0.2 J. Compared with 1D and 2D MOF, 3D MOF can maintain higher energy levels and significantly lower sensitivity, showing a good application prospect.
As the composition for containing metal elements, combining with particular porous structure and high formation enthalpies, the energetic MOF can be used as a potential functional combustion catalyst for CSP. It is promising to resolve the problems while adding the traditional metallic combustion catalysts [21].
In the present study, the halogen-free energetic MOF(Cu) was used as a representative of the energetic MOF to study its effects on the performance of CSP. The effects of MOF(Cu) on the thermal decomposition of AP, as well as on the energetic performance, thermal decomposition, combustion and mechanical sensitivity of CSP were studied by theoretical calculation and experimental study.
Section snippets
Materials
Hydroxyl-terminated polybutadiene (HTPB, Mn = 3170 g mol−1, Hydroxyl value 0.67 mmol g−1, Li Ming Research Institute of chemical Industry) was used as the binder and 2,4-toluene diisocyanate (TDI, mass fraction of NCO 48.2%, Gansu Yinguang Juyin Chemical Co., Ltd.) was used as the curing agent. Diisooctyl sebacate (DOS, Chengdu Kelong Chemical Co., Ltd.) was used as plasticizer. Micro-sized aluminum powder (Al, d50 = 5 µm, An Shan Iron and Steel Group Co.) was used as the metal fuel. Three
Characterization of MOF(Cu)
MOF(Cu) exhibits 3D and homogenous structure as shown in Fig. 1a for its SEM image. It can be seen from Fig. 1a that the grain size of MOF(Cu) crystal is about 400–600 µm. The EDS spectrum shown in Fig. 1b reveals that it is composed of C, N, O and Cu elements, consistent with the results obtained in the previous literature [27]. Figure 1c and d show the microstructures of the fractured surfaces of CSP-2 and CSP-3 prepared with CuO and MOF(Cu) as the catalyst, respectively. The internal states
Conclusions
Our work aimed to investigate the application of energetic metal-organic framework containing Cu [MOF(Cu)] as the combustion catalyst to composite solid propellant. Unlike CuO, MOF(Cu) slightly reduces propellant energy performance parameters as its content varied from 0% to 5%. The thermal analysis results reveal that both CuO and MOF(Cu) can catalyze the thermal decomposition of AP and CSP. Compared with CuO, MOF(Cu) shows better catalytic effects on the thermal decomposition and combustion
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by “the Fundamental Research Funds for the Central Universities”.
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