Elsevier

Combustion and Flame

Volume 215, May 2020, Pages 86-92
Combustion and Flame

Combustion of 3D printed 90 wt% loading reinforced nanothermite

https://doi.org/10.1016/j.combustflame.2020.01.021Get rights and content

Abstract

The use of Al-based nano-energetic materials has been limited in part due to difficulties in fabrication of high-density composites. In this paper, free-standing energetic composites with loading of up to 90 wt% Al-CuO were fabricated by 3D printing. A polymer hybrid of 3 wt% hydroxy propyl methyl cellulose (HPMC), 3.5 wt% nitrocellulose (NC) and 3.5 wt% polystyrene (PS), enables fabrication of mechanically strong and highly reactive composites. The energy flux can be readily tuned through the combustion speed and flame temperature by changing equivalence ratio. The highest energy flux was found to occur under fuel rich conditions (equivalence ratio = 2.4) which also corresponds to the maximum combustion speed (25 cm/s) despite the fact that the flame temperatures was lower. The Young's modulus of free-standing burn sticks was found to be as high as ~1 GPa, which is comparable to pure polypropylene. PS polymer flakes created during the high shear direct write process is believed to be critical to the enhanced mechanical properties we observed. The burning behavior using other oxidizers corresponds closely with that observed with mixed powders but with the added strength offered in a printed structure. This study offers an attractive route for safe, reliable and scalable additive manufacturing of Al-based nano-energetic materials at high energy densities.

Introduction

The combustion enthalpy of aluminum (Al) in oxygen is as high as 80 kJ/cm3, which is substantially greater than those of monomolecular CHNO compounds (10–30 kJ/cm3). [1], [2], [3] As a result, there is a growing interest in employing Al-based nano-energetic materials in propellants, explosives and pyrotechnics systems, such as additives in aerospace propellants, self-destructing microchips, etc. [1], [2], [3], [4], [5], [6], [7] This has also led to research in preparation methods, and, ignition and combustion mechanisms [8], [9], [10], [11], [12], [13]. Assembly techniques include physical mixing [14], sol-gel [15], arrested reactive milling [16], layered deposition [17,18], electrospray [19], self-assembly [20], and metal organic frameworks (MOFs), and interface control techniques to improve the reactivity and energy output of nanothermites [21], [22], [23], [24]. However, a main challenge moving forward is fabrication at high particle loading in order to obtain high energy density, without compromising the mechanical properties of the composite. Which is critical for the prevention of catastrophic failure in for example propellants [25], [26], [27].

Mechanical integrity is thus of significant importance for stable burning and potential real-world applications. Additive manufacturing techniques like 3D printing allows one to realize the fabrication from microscale to centimeter scale in a layer-by-layer manner, thus offering a customizability that is difficult to realize via conventional fabrication methods, leading to design and fabrication of multifunctional structures for a diverse range of applications [28], [29], [30], [31], [32], [33], [34], [35]. Recent efforts in traditional energetic formulations include, Xu et al. [36] DNTF/NC/Viton composite explosives with a density of 1.785 g/cm3. Li et al. [37] and Wang et al. [38] printed CL20-based composite explosive with low impact sensitivity and stable detonation properties. Chandru et al. [39] have developed a composite solid rocket propellant with customizable port geometries and controllable porosity and Driel et al. [40] prepared a TNO-base gun propellant using stereolithography 3D printing. By contrast the availability of Al-based energetic materials using 3D printing is very limited. Murray et al. [41] prepared Al-CuO nanothermites with 8 wt% solids loading by piezoelectric inkjet printing. Slocik et al. [42] created an energetic bio-thermite ink using ferritin liquid protein in water and Durban et al. [43] recently printed thermite with micron-size Al and CuO particles in an aqueous hydrogel matrix. However, in these studies [41], [42], [43], thea mechanical properties of these 3D-printed Al-CuO composites weren't reported. While these methods might be effective at designing and fabricating nano-thermite architectures, further advances might be limited due to shortcoming in scalability or poor stability from possible reactions between Al and water.

In our recent study, an Al-CuO nanothermite ink with particle loading as high as 90 wt% was developed for the first time by using a homogenous polymer mixture of polyvinylidene fluoride (PVDF, 4 wt%) and hydroxy propyl methyl cellulose (HPMC) (6 wt%) [44]. The burn rate of the printed sticks was ~2–10 cm/s, with burn temperatures of ~2800 K, and a Young's modules of ~0.3 GPa [44]. These materials were then interrogated by in-operando microscopy/thermometry to directly observe the phenomena of “reactive sintering” and the propagation of the reaction front [45], [46], [47], [48], [49].

In this study, we advance our prior work to create free-standing 90 wt% Al-CuO nanothermite sticks with Young's modulus of >1.0 GPa, and with higher reactivity with the addition of hydroxypropyl methylcellulose (HPMC), nitrocellulose (NC) and polystyrene (PS). Burn rates up to 25 cm/s were achieved, with flame temperature as high as ~2500 K. Other Al-based nanothermites with different oxidizers; Fe3O4, Co3O4 and WO3 were also employed in this formulation. These results show that the 3D printing method using a hybrid binder strategy are well suited to a variety of Al-based nano-energetic materials yielding high reactivity and mechanical integrity.

Section snippets

Materials

Aluminum nanoparticles (Al NPs, from Novacentrix Inc.) have an average diameter of 50–100 nm (TEM, Fig. S11a) with a ~2–5 nm oxide shell resulting in a ~81 wt% active Al content. CuO was purchased from US Research Nanomaterials. The particle diameter of nano-CuO are 80–200 nm (TEM, Fig. S11b). Dimethylformamide (DMF), Fe3O4, Co3O4, WO3 and nitrocellulose (NC) (Collodion solution 4–8 wt% in ethanol/diethyl ether), Polystyrene (PS, average Mw: 280,000) were purchased from Sigma-Aldrich Corp.

Preparation of colloidal inks and morphology of the printed composite sticks

For any robust printing process, stability of composite inks is crucial for high quality 3D-printing, and the inks themselves should be resistant to phase separation over long periods of time. Using a polymer hybrid of 3 wt% hydroxy propyl methyl cellulose (HPMC), 3.5 wt% nitrocellulose (NC) and 3.5 wt% polystyrene (PS), a stable ink without any phase separation could be obtained (Figs. S1–S3). The introduction of NC was used to increase the flammability of the Al-CuO nanothermite due to its

Conclusion

We have prepared a colloidal ink with 90 wt% Al-CuO and 10 wt% hybrid polymers that enables one to print mechanically strong and highly reactive materials, that operationally behave like a dense thermite powder compact. We believe that the in-situ production of PS flakes is an important component in establishing the mechanical integrity of the materials, while still maintaining combustion performance. This approach was shown to be generic as evidenced by similar behavior with three other

Declaration of Competing Interest

There is no conflict of interest.

Acknowledgments

This work was supported by the AFOSR. We acknowledge the support of the Maryland Nanocenter and its NispLab. The NispLab is supported in part by the NSF as an MRSEC Shared Experimental Facility. Supporting Information is available online from journal website or from the authors. J. Shen would like to acknowledge the China Scholarship Council (CSC) for financial support.

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  • Cited by (0)

    1

    J.P. Shen and H.Y. Wang contributed equally to this work.

    2

    Present address: School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, China.

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