NiO@C Nanoenergetic Materials through

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Surfaces, Interfaces, and Applications

Tuning the Reactivity of Al/NiO@C Nanoenergetic Materials through Building an Interfacial Carbon Barrier Layer Xiang Ke, Bingwang Gou, Xiaolian Liu, Ning Wang, Gazi Hao, Lei Xiao, Xiang Zhou, and Wei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09723 • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019

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ACS Applied Materials & Interfaces

Tuning the Reactivity of Al/NiO@C Nanoenergetic Materials through Building an Interfacial Carbon Barrier Layer Xiang Kea, Bingwang Goub, Xiaolian Liuc, Ning Wang a, Gazi Haoa, Lei Xiao a, Xiang Zhoua, *, and Wei Jianga, *

a National

Special Superfine Powder Engineering Research Center, Nanjing University

of Science and Technology, Nanjing 210094, China b

Xi’an Modern Chemistry Research Institute, Xi’an 710065, China

c

Safety Technology Research Institute of Ordnance Industry, Beijing 100053, China

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Graphic abstract

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Abstract: Inspired by the crucial role of interface layer in tuning the reactivity of nanoenergetic materials (nEMs), in this study, we report a new method to tune the energetic performances of Al/NiO@C nEMs by designing the interfacial barrier layer between the fuel and oxidizer. Carbon shell in special core-shell NiO@C nanorods derived from nickel-based metal-organic frameworks functions as a homogeneous interfacial diffusion-resistant layer between Al and NiO nanoparticles. Under the guidance of experimental time-resolved oxidation curves and theoretical simulation results, the carbon content can be easily controlled, thereby achieving the goal of tuning energetic performances. It is found that the chemical nature of the carbon barrier layer, rather than its content, provides the resistance against interdiffusion of Al and O atoms in the solid-state reaction, thus leading to a higher reaction onset temperature. The importance of the interfacial layer on the thermal properties of nEMs is also emphasized when compared with physically mixed ones. Combustion tests reveal that both interfacial resistance and gas generation play roles in tuning the combustion propagation, flame temperature, ignition delay time and pressurization rate. These results indicate the promising potential of pre-engineered interfacial structure for targeted reactivity of carbon-based nEMs. Keywords: interface layer; interfacial diffusion-resistance; core-shell nanorods; combustion propagation; thermal properties

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1. Introduction Nanoenergetic materials (nEMs), generally composed of nano-sized active metals and oxidizer components in one system, can rapidly convert the internal chemical energy into heat and/or light once triggered by an external threshold energy input.1-3 The improved interfacial contact and reduced diffusion distance make these nEMs superior to conventional micron-sized ones, as nEMs possess faster reaction rate, higher energy-release efficiency, and significantly enhanced combustion performance,4-6 opening opportunities for devices that require large energy output in a short time such as micro thrusters,7, 8 micro igniters9-11 and automobile safety airbag.12, 13 In order to address the increasing requirements for microdevices as well as extend their applications, high-performance nEMs with controllable energy-release characteristics have been developed through designing nanostructures, optimizing stoichiometric ratios, and adjusting the interfacial contact of reactants.14-17 However, some bottlenecks of binary nEMs, such as limited adjustable gas generation ability and detonation, are hard to overcome. One approach to add greater flexibility in combustion tenability is to add a third component which can provide either an additive or synergistic effect.18, 19 Using nEMs included high explosives as an example, on one hand, a metal or metallic oxide can catalyze the thermal decomposition of explosives and reduce the activation energy of decomposition; on the other hand, pressure loss during the combustion of binary nEMs can be compensated by gases produced by decomposition, and thus, convection dominated combustion propagation process can be significantly enhanced with a higher pressurization rate, promoting fast transition from combustion to detonation.20-22 More importantly, ternary nEMs can be used for special purposes by selecting other additives. Ternary nEMs containing biocidal agents like iodine or iodate show their long-duration effective potential use as sporicidal reagents through a synergistic function coupling both chemical and thermal effects.23, 24 Incorporating fluorocarbon into binary nEMs formulations offers nEMs with corrosion-resistant properties, long-term storage ability and ignition capability under water.25-27 All these results demonstrate that ternary nEMs 4


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with controllable energetic performance have shown their unique superiorities in contrast to regular binary ones. Carbon materials have attracted attention due to their capability in changing the ignition and combustion performance of nEMs. For example, Kim’s group studied the influence of various types of carbon additives including carbon black nanoparticle (CB),28 sea-urchin-like carbon nanotubes (SUCNTs),29 single-walled and multi-walled carbon nanotubes (SWCNTs and MWCNTs)5,

30, 31

on the ignition and combustion

properties of multiple carbon-based Al/CuO ternary nEMs. Although the types of carbon materials varied, they all offered similar characteristics in that the threshold power and delay time of ignition gradually decreased with increasing content of carbon additives for Al/CuO nEMs, while the pressurization and burn rates slightly increased at first and then were prominently suppressed by heat dissipation and thermochemical constrainsts.5 Successful flash ignition has also been demonstrated upon addition of SUCNTs and SWCNTs into Al/CuO nEMs, due to carbons’ light absorption properties and thermal conductivity.29, 30 In another work, Al and Bi2O3 nanoparticles confined in an RGO aerogel skeleton showed enhanced open combustion rates via a “self-confining reaction” in a three-dimensional scaffold.32 The above studies demonstrate the opportunities offered by carbon addition, although many of these additives are not commercially available at a large scale. Prior work has relied on a random mix of the three components, leading to regions of poor homogeneity. Here we consider a more regular arrangement to create a more homogenous structure to evaluate if there are advantages over a random mixing. Metal-organic frameworks (MOFs), composed of metal ions and organic linkers via covalent coordination linkages, have been applied in many fields due to their tunable morphologies and structures, large surface areas and well-defined pore size distributions.33 Benefiting from their chemical compositions, MOFs have been intensively investigated as ideal self-sacrificing templates for synthesizing various carbon-coated metal oxides through a pyrogenic decomposition treatment under a user defined atmosphere.34 In this study, inspired by these prominent characteristics of MOFs, especially their inexpensive preparation and self-sacrificing conversion, special 5



core-shell structural nickel oxide@carbon (NiO@C) nanorods, assembled by the spherical nanoparticles with NiO nanoparticles as the core and C layer as the shell, are obtained from the solvothermal preparation of nickel-based MOFs (Ni-MOFs), followed by a controllable pyrolysis and oxidation. The carbon source of NiO@C nanorods is generated from the carbonization of organic ligands so that no additional carbon is needed when forming the Al/NiO@C ternary nEMs. Furthermore, the carbon content can be precisely controlled by adjusting the calcination temperature and time under the premise of complete oxidation of metallic Ni. Of particular interest is that the existence of a C layer builds a natural layer, restraining the immediate contact of Al and NiO when NiO@C nanorods are mixed with Al nanoparticles, which we expect to provide a more effective approach to tune the ignition and combustion performances.

2. Experimental Section 2.1 Sample preparation The preparation procedure of core-shell structural NiO@C nanorods is presented in Figure 1 and described in detail in the supporting information. In addition, the characterization of Al nanoparticles is also shown in the supporting information. Firstly, nickel nitrate hydrate (Ni(NO3)2·6H2O) and p-benzenedicarboxylic acid (H2BDC) were selected as the raw materials to form the Ni-MOFs through a solvothermal reaction. Then, calcination in argon successfully converted Ni2+ to metallic Ni nanoparticles and carbonized the organic ligands on the nanoparticle surface at the same time to form a core-shell structure. In a subsequent calcination step in air, metallic Ni nanoparticles were totally oxidized to NiO while carbon shell was also partially consumed, finally forming the NiO@C nanorods. Lengthening the calcination time resulted in pure NiO nanorods.

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Figure 1 Illustration of the preparation process of core-shell structural NiO@C nanorods.

The Al/NiO@C nEMs were prepared by an ultrasonic mixing method. Before the mixing process, the carbon content of NiO@C was measured by thermal analysis, and then the equivalence ratio (Φ, shown in the supporting information) of Al and NiO was fixed at 1. The pre-weighed Al and NiO@C nanoparticles were ultrasonically mixed in the solvent of 10 mL ethanol for 30 min, followed by air drying at 50 ℃ for 12 h. The Al/NiO/C used as a reference sample was prepared through the same process except that the C was from the etched Ni@C nanorods by hydrochloric acid. 2.2 Characterization The morphological characterization of Ni-MOFs, Ni@C, NiO@C and Al/NiO@C nanocomposites was conducted by a field emission scanning electron microscope (FESEM, Hitachi S-4800 II) and a high-resolution transition electron microscope (HRTEM, Tecnai G2 F30). The structural and component information were characterized by X-ray powder diffraction (XRD, Bruker D8 Advanced), X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250i), Fourier transform infrared spectroscopy (FTIR, Bruker VERTEX 70), and energy dispersive spectroscopy (EDS) attached to the FESEM and HRTEM. Raman spectroscopy (Horiba Aramis) was employed characterize the degree of graphitization of the calcined Ni@C from the intensity ratio of the D and G bands (ID/IG). The energetic capability of Al/NiO@C nEMs was evaluated by differential scanning calorimetry/thermogravimetric analysis (TGA-DSC, TA Instruments SDT600), ignition and open combustion behavior tests, and confined constant-volume 7



pressure cell test (PCT). TGA-DSC tests were conducted from 50 to 1000 ℃ with a heating rate of 10 ℃/min under Ar flow rate of 100 mL/min using Al2O3 ceramic crucible. TGA analysis was also carried out to confirm the calcination temperature and investigate the content of carbon in MOF-Ni, Ni@C and NiO@C, described in detail in the supporting information. The schematic diagram of experimental equipment for combustion and PCT tests are shown in Figure S2 and S3, respectively. For the combustion test, about 8 mg of nEMs were put in a ceramic boat and ignited with a nichrome wire (250 μm in diameter and 15 mm in length) connected with a directcurrent power supply (GWINSTEK GPS-18500) operated at 5.0 A. The combustion process was recorded by a high-speed camera (REDLAKE MotionXtra HG-100K) with a sampling rate of 10 000 frames per second. Synchronously, a Si-based photodetector (Thorlabs, DET02AFC) was employed to collect the dynamic process of the optical signal. The optical signal was recorded and transferred into a voltage signal which was displayed on an oscilloscope (Wavesurfer 3054), and further used to estimate the ignition delay time and light intensity. The combustion process is also recorded by a high-speed thermal infrared camera (Telops, FAST-IR 2k) with a recording rate of 1500 frames per second to measure the flame temperature. In terms of the PCT tests, 25 mg of nEMs were placed in the confined cell with a fixed volume of 8 mL and ignited by a nichrome wire. The dynamic pressure during the combustion process was measured by a piezoelectric pressure sensor (PCB Piezotronics, Model 112A05) attached to the confined cell and the pressure signal was transformed into a voltage signal through a sensor signal conditioner (PCB Piezotronics, Model 482C), finally being recorded by the oscilloscope. Note that all the combustion tests were conducted in air.

3. Results and Discussion 3.1 Morphological, structural and compositional characterization The rod-like Ni-MOFs with a large aspect ratio are observed in Figure S4a, showing a smooth surface and a diameter of 450-550 nm. The TEM image in Figure S4b further confirms the solid structure, and XRD (Figure S4c) and FTIR (Figure S3d) confirm the composition. TGA analysis is conducted in Ar atmosphere for the 8


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guidance of later thermal treatments (Figure S5), and the calcination temperature was 800 ℃. As shown in Figure 2a, the carbonization treatment does not change their rod-like shape but greatly roughens the surface. Each of Ni@C nanorods is composed of numerous spherical nanoparticles leading to a porous surface further confirmed by TEM image in Figure 2b. The enlarged image inserted in Figure 2b directly presents the core-shell structure of Ni@C nanoparticle. EDS analysis in Figure S6a illustrates only C and Ni elements, corresponding to carbon layer and metallic Ni. As shown in Figure S7, the nanorod is assembled by hollow spheres after etched by hydrochloric acid. The HRTEM image in Figure 2c verifies that graphited and amorphous carbon (red and yellow frames, respectively) co-exist and this conclusion can be also drawn from the detailed discussion of the XRD and Raman patterns (Figure S8).35,

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In brief, the obtained Ni@C nanorods are

composed of core-shell structural nanoparticles, with Ni as the core and carbon layer as the shell, and thus, in this paper, the core-shell structural nanorods are different from traditional core-shell structure. Designing this structure contributes to our primary assumption that one can effectively restrict reaction pathways through building a uniform interface-obstruction layer between the Al and NiO, upon calcination in air.

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Figure 2 (a) SEM and (b) TEM images of Ni@C nanorods; (c) HRTEM image of carbon layer after acid treatment; (d) SEM, (e and f) TEM, (g) HRTEM and (h) the elemental mapping images of a single NiO@C particle; (i) XRD patterns of Ni@C and NiO@C

The second calcination procedure is conducted to oxidize the metallic Ni to NiO in air and synchronously eliminate part of the carbon shell. Thus, the carbon content can be controlled by modulating heating temperature and time. Here, a typical NiO@C sample, calcined at 350 ℃ for 2 h, is characterized as an example. As shown in Figure 2d and e, the morphology of the NiO@C nanorods mimics the Ni@C nanocomposites after the calcination in air. Further increasing the heating time or temperature does not have a significant influence on morphology apart from reducing adhesive strength of nanoparticles, leading to some detachment from the nanorods as shown in Figure S9ac. The enlarged view in Figure 2f clearly shows that NiO nanoparticles are still coated by a continuous carbon layer after heating treatment. The thickness of carbon shell decreases from 9.7 to 4.0 nm after different heating treatments as displayed in Figure S9d-g, indicating the decrease of carbon content in NiO@C nanorods, which will be 10


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discussed later. What’s more, although some pores and cracks (Figure S9h and i) are formed, the carbon shell could still play as an interfacial barrier layer since they are not in the location of each particle. High-magnification TEM image (Figure 2g) shows clear lattice fringes with a d-spacing of 0.24 and 0.15 nm, corresponding to the (111) and (220) planes of NiO, respectively. In contrast to Ni@C nanorods, the O element is detected in the EDS analysis (Figure S6b), illustrating the oxidation of Ni, and the inserted line-scanning profile supports the result of core-shell structural NiO@C. The EDS elemental mapping in Figure 2h displays the uniform distribution of Ni, O and C in NiO@C nanorods as well as the core-shell structure. The XRD and XPS results (Figure 2i and S10) further confirm that metallic Ni is totally oxidized after being heated at 350 ℃ for 2 h. Through the above analyses, we can summarize that NiO@C composites can be prepared with a core-shell structure. 3.2 Controlling the carbon content Based on the TG results (Figure S11), we estimate that the weight percent of carbon in Ni@C nanorods is about 24.6%. On the basis of the TG tests (Figure S12) and simulation results (Figure S13, S14 and S15), Ni@C nanorods are calcined at 350 ℃ for different durations to ensure the complete oxidation of Ni and regulation of carbon content. When calcination time is more than 1h, only NiO is detected in the XRD test as shown in Figure 3a, indicating that metallic Ni is totally oxidized. The mass loss in TG curves in Figure 3b is caused by the escape of carbon dioxide which is generated from the oxidation of C shell. The carbon contents show a decreasing trend and the calculated results for the samples after being calcined for 1h, 2h and 3h are 15.9, 10.1 and 6.8%, respectively. Another way to adjust the carbon content is to choose the calcination temperature as shown in Figure 3c and d. The XRD results show that a longer time is needed to completely convert Ni to NiO when a lower temperature is adopted. Consequently, NiO@C nanorods with lower carbon contents (4.6% for 380 ℃ and 2.2% for 400 ℃, respectively) are obtained. Overall, by taking advantage of the different oxidation temperature of metallic Ni and carbon shell and different thermal stabilities of amorphous and graphitic carbon under an air atmosphere, we can obtain special core-shell structural NiO@C nanorods with multiple carbon contents (Table S1) 11



via the selection of specific set of calcination temperature and time.

Figure 3 (a) XRD patterns of calcined samples at different calcined times and (b) corresponding TG curves; (c) XRD patterns of calcined samples at different temperature and (d) corresponding TG curves.

3.3 Thermal analysis Several Al/NiO@C nEMs with different carbon contents were prepared to study their energetic performances. In all samples, the ratio of Al and NiO is fixed at 1.0 and the detailed formulations are listed in Table S2, the samples are defined as A-0 to A-4 with a carbon content of 0, 1.75, 3.7, 5.7 and 8.7%, respectively. The morphological and compositional characterizations of A-2 and Al/NiO/C are shown in Figure 4 and S16. For A-2, the Al nanoparticles and NiO@C nanorods are randomly distributed and Al NPs agglomerate into micron-sized botryoidal particles. EDS result reveals the presence of Al, O, Ni and C elements, which can be assigned to Al nanoparticles, NiO@C nanorods and Al2O3 shell. More details in TEM images illustrate that the fuel contacts closely with the oxide. In general, the NiO@C nanorods are surrounded by the agglomerated Al NPs and some of agglomerated Al nanoparticles closely integrated with the NiO@C agglomerates separated from the broken rods. For Al/NiO/C nEMs 12


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(Figure S16), almost the same morphological feature is shown where the Al nanoparticles, NiO and C closely integrate.

Figure 4 Morphological and compositional characterizations of Al/NiO@C nEMs: (a) SEM image, (b) EDS spectrum, (c) and (d) TEM images, and (e) schematic diagram of the contact ways.

The thermal stability and reactivity of as-prepared samples are investigated as shown in Figure 5a. As expected, two exothermic peaks and one endothermic peak are observed in sample A0. The two exothermic peaks are characteristic of Al-based nanothermites, where the first can be assigned to the solid-state reaction of Al and NiO and the latter is mostly considered to be the reaction between melted Al and NiO.37, 38 Samples A1-A3 show similar trends to that of A0, despite some peak shifting. However, sample A4 appears to have a quite different behavior, including a low temperature endotherm and an overlapped exotherm. For A4, Al and NiO reacts firstly and produces metal Ni, which can take part in the further alloying reaction, resulting in the overlapped exotherm. Additional peaks of AlNi3 are detected in A4 as shown in Figure 5b, which can support our conjecture about alloying reaction. The higher reaction resistance and more consumption of Al might cause the disappearance of the second peak. Although a detailed reaction process is not available at present, from the point of reaction products, sample A0 and A3 may undergo the same reaction process, whereas the process of A4 is more complex, resulting in a greatly different thermal behavior. 13



Figure 5 (a) DSC curves of Al/NiO@C nEMs and (b) XRD patterns of residues after DSC tests.

The temperature of the first exotherm of A1-A3 increases from 565.3 ℃ to 588 ℃, while the second one moves to a lower temperature and diminishes in amplitude. The increment of first peak temperature has no much connection with the carbon content for A1-A3, while the decrement of second peak temperature increases with the increasing carbon content and the peak even disappears, indicating that the interfacial carbon layer in the Al/NiO nEMs plays an important role in controlling the solid-state reaction rather than its content. This event is also found in Kwon’s work.39 As for the second exothermic peak, due to the high thermal conductivity of carbon shell38, more carbon in the nEMs leads to a more obvious decrease of the peak temperature. In addition, the onset reaction temperature of A0 is much lower than the other ones and its heat release is also more superior based on the peak area. The total heat release shows a decreasing trend with the increasing carbon content, suggesting that the carbon layer in the nEM matrix decreases the energetic density. As a contrast, the DSC curve of physically mixed samples in Figure S17 shows an approximate temperature of the first exothermic peak, followed by an advanced peak, suggesting that carbon nanorods in random-contact ternary components take a stimulative effect on the thermite reaction. These results illuminate that carbon shell, acting as a barrier layer for the direct contact of Al and NiO, limits the interdiffusion of the reactants and thereby modulate the reactivity. Combining the above analysis and the diffusion oxidation mechanism of the solidstate reaction,40 we show the interpretation of the DSC results as shown in Figure 6. For A0 (Figure 6a), Al and O atoms diffuse toward each other, go through the only 14


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obstruction, Al2O3 shell, and react. While for Al/NiO@C nEMs (Figure 6b), the reaction interface changes a lot through the designed special core-shell NiO@C nanorods with a stable diffusion barrier between the two reactive materials. The carbon shell hinders the direct contact of the reactants, increases the diffusion distance of Al and O increases, and thus causes a delayed solid-state reaction. On the other hand, carbon shell with high thermal conductivity can facilitate heat absorption and the migration of melted Al, resulting in a prior reaction.

Figure 6 Diagram of the diffusion of Al and oxygen atoms during the solid-state reaction process: (a) Al/NiO nEMs, (b)Al/NiO@C nEMs.

3.4 Open combustion performance A series of open combustion images are shown in Figure 7. All the samples can hold on self-sustained combustion propagation processes once being ignited except A4 with the largest carbon content. As displayed in Figure 7e, A4 cannot be ignited and only some hot sparks are observed, suggesting that excess carbon would hinder the mass transfer and stop the initiation of combustion. The consumption process of the reactants can be clearly distinguished (marked in red frame) in Figure 7b. All the samples are ignited at some point, then the flame propagates along the vertical and horizontal directions at the same time, so that unreacted ones can immediately participate in the combustion. With the increased carbon content, more and more solid particles are ejected out of the flame edge because gas products from the combustion 15



of carbon layer cause the separation of the reactants. However, too many gas products may provide a negative effect, leading to a serious sintering and incomplete combustion, even generating big hot ejected aggregates.19, 41

Figure 7 Sequential open combustion images of as-prepared samples: (a) A0, (b) A1, (c) A2, (d) A3 and (e) A4, and (f) high-speed infrared images of A0-A4.

Figure 7f shows the infrared images containing the maximum temperature for each sample and the blazing flying particles can be easily found in the images of A1 to A4 as well as in Figure S18. Unfortunately, we do not obtain the exact flame temperature of A0 as the temperature of core zone (gray color) exceeds the upper limit. The reported theoretical adiabatic reaction temperature of pure Al/NiO (Φ=1) is 3187 K in consideration of phase change.42 However, the maximum flame temperature of A1-A4 is about 2450, 2350, 1900 and 1600 K, respectively, showing a decreasing trend with the increasing carbon content. The main reason can be ascribed to the decrease in energy density after introducing carbon layer which has been proved in DSC tests. In addition, adding carbon into Al/NiO brings about some gas products during the combustion process so as to make the reactants fly out of the reaction zone, thus weakening the heat accumulation and enlarging the heat dissipation area, at last causing a reduced flame temperature.41 16


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Here, a comparative combustion experiment of Al/NiO/C is also conducted as shown in Figure S19. Compared to the A4, self-sustaining combustion of physically mixed one with numerous flying particles reveals its high reactivity, which comes from the structural differences in nature since they are chemically identical. Mixing carbon nanorods with Al and NiO nanopowders generates potentially mixed reaction pathways, in contrast, special core-shell NiO@C nanorods set up a uniform interfacial barrier for the contact of Al and NiO. When the energy input reaches the threshold, the combustion may occur between direct-touch Al and NiO, and then motivates the reaction among Al, NiO and C, forming a self-sustaining combustion in the end. However, the uniform interfacial barrier in Al/NiO@C prevents the direct contact of Al and NiO, limiting the occurrence and propagation of combustion. Optical signals during the combustion process and relevant computed results are shown in Figure 8a and b as well as in Table S3. The ignition delay time is defined as the time from turning on the power to the rise of the optical signal. The burning time is determined by the time interval of beginning rise and recovering to zero of the optical signal. From the view of maximum light intensity, the combustion becomes increasingly weak with the increased carbon. Then, the total burning time increases after introducing carbon barrier layer into the nEM matrix, potentially caused by the reduction of energy density of Al/NiO@C system and lower heat accumulation during the burning process. The ignition delay time of carbon-containing samples shows a decreasing tendency depending on their carbon content, but still larger than that of pure Al/NiO (A0). This phenomenon is a little different from what Kim’s group finds, in which the ignition delay time was low than that of Al/CuO nEM matrix and decreased considerably with increasing the number of SWCNTs or carbon black powders.28, 30 Our comparative combustion experiment also shows that Al/NiO/C ternary mixture possesses a decreasing ignition delay time from 393 ms to 330 ms (Figure S19b). In physically mixed samples, carbon materials play a positive role both in promoting the energy absorption efficiency and decreasing the threshold energy, thereby realizing a rapid local ignition at the point where the heat is concentrated.5 However, in Al/NiO@C nEMs, carbon interfacial layer plays a complex role. On one hand, it can improve the 17



energy-absorption capacity, on the other hand, it gets in the way of the reaction as we proved in the DSC tests and combustion performances. The resistance to the reaction is not related to the amount of carbon in Al/NiO@C nEMs since the initial reaction temperature in the DSC tests is almost the same, while the amount of carbon can greatly affect the energy-absorption capacity, but it can't completely offset the negative effects of interface resistance. Therefore, the ignition delay time in Figure 8b is the result under the combined effects of the two.

Figure 8 (a) Optical emission and (b) evolution of ignition delay time, combustion duration and light intensity of as-prepared samples.

Dynamic pressure curves are recorded by a fast response pressure transducer as shown in Figure 9a, and then, the maximum peak pressure and pressurization rate are calculated as shown in Figure 9b and Table S3. Noted that the pressurization rate is determined by the ratio of pressure difference from 10% to 90% peak pressure to the corresponding rise time.44 The maximum pressure presents a decreasing tendency with the increasing carbon content. It is interesting to note that the maximum pressures of A1 and A2 are slightly lower than that of A0, while a sudden decrease on both pressure and pressurization rate happens for A3. Beyond playing a role as the interfacial barrier layer, carbon shell also functions as a gas generator during the combustion process. Therefore, it can offset some of the pressure loss. As we show in Figure 7d, excessive carbon generates too many gases so as to separate the reactants from each other resulting in an interrupting reaction and a rapid decrease in energy output, presenting poor performances in both pressurization rate and peak pressure.

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Figure 9 (a) Pressure traces, and (b) pressurization rates and maximum pressure of as-prepared samples as a function of carbon content.

Some distinctions of acting mechanisms can be found between the thermal behavior and combustion performances from above analyses when we use these special core-shell NiO@C nanorods to tune the energetic performances of Al/NiO@C nEMs. In thermal properties, a uniform interfacial barrier of carbon layer prevents the direct contact of Al and NiO and increases the resistance of interdiffusion of Al and O atoms, leading to a higher initial temperature. Besides the interfacial barrier effect and high thermal conductivity, gaseous products generated from burning of carbon should be taken into account and make the combustion process more complex, such as the nonmonotonic ignition delay time, burning time and pressurization rate. When compared with the physically mixed Al/NiO/C ternary nEMs, due to the differences in structure and interface, as-prepared Al/NiO@C shows great changes both in thermal action and combustion performances such as the initial temperature of solid-state reaction and flame propagation at high carbon content. In general, all of these results demonstrate that carbon barrier layer can be considered as a candidate for tuning the energetic performances of nEMs by controlling the carbon content.

4. Conclusion In summary, we report a new method to tune the energetic performances of Al/NiO@C nEMs through controlling interfacial contact hindrance. Benefitting from the self-transformation feature of Ni-MOFs under given conditions, special core-shell structural NiO@C nanorods with adjustable carbon content, with NiO nanoparticles as the core and carbon layer as the shell, are prepared by selecting the suitable calcination 19



temperature and time. Introducing such NiO@C nanorods as oxidants into energetic system successfully sets up a homogeneous interfacial diffusion barrier between the Al and NiO nanopowders to tailor the thermal properties and combustion performances as well as to reveal the essential role of interfacial contact in the thermite reaction. Thermal analysis shows that it is the chemical nature of carbon barrier layer, rather than its content, prevents interdiffusion of Al and O atoms in the solid-state reaction, thus leading to a higher reaction temperature. It also highlights the importance of the interfacial diffusion on the properties of nanoenergetic materials when compared with physical mixed Al/NiO/C nEMs. Mutual constraint effect of the interfacial hindrance, high thermal conductivity and gas generation results in a complex process in controlling the combustion performances, especially in ignition delay time, burning time and pressurization rate. We expect this design concept here be adapted to other nEMs by changing the MOF precursors to verify its applicability and feasibility.

Supporting information The preparation process and characterization of Ni-MOFs, the TG curves and simulation of controlling the carbon content in NiO@C nanorods and the energetic performance of contrast samples are available in the supporting information.

Author information Corresponding authors *Emails: [email protected] (X. Zhou); Tel: +86−025−84315042; [email protected] (W. Jiang) ORCID Xiang Zhou: 0000-0001-7726-7131 Jiang Wei: 0000-0001-5663-9119 Xiang Ke: 0000-0001-6695-6599 Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by the National Natural Science Foundation 20


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of China (No. 51706105 and 21805139), the Natural Science Foundation of Jiangsu Province (Grant No. BK20170846), the Qing Lan Project, a Project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Basic Product Innovation Technology Research Project of Explosives.

References (1) Zhou, X.; Torabi, M.; Lu, J.; Shen, R.; Zhang, K. Nanostructured Energetic Composites: Synthesis, Ignition/Combustion Modeling, and Applications. ACS Appl. Mater. Inter. 2014, 6, 3058-3074. (2) Sundaram, D.; Yang, V.; Yetter, R. A. Metal-Based Nanoenergetic Materials: Synthesis, Properties, and Applications. Prog. Energ. Combust. 2017, 61, 293-365. (3) Wang, J.; Jiang, X.; Zhang, L.; Qiao, Z.; Gao, B.; Yang, G.; Huang, H. Design and Fabrication of Energetic Superlattice Like-PTFE/Al with Superior Performance and Application in Functional Micro-initiator. Nano Energy 2015, 12, 597-605. (4) He, W.; Liu, P. J.; Gong, F.; Tao, B.; Gu, J.; Yang, Z.; Yan, Q. L. Tuning the Reactivity of Metastable Intermixed Composite n-Al/PTFE by Polydopamine Interfacial Control. ACS Appl. Mater. Inter. 2018, 10, 32849-32858. (5) Kim, J. H.; Cho, M. H.; Kim, K. J.; Kim, S. H. Laser Ignition and Controlled Explosion of Nanoenergetic Materials: The Role of Multi-walled Carbon Nanotubes. Carbon 2017, 118, 268-277. (6) Zhou, X.; Xu, D.; Zhang, Q.; Lu, J.; Zhang, K. Facile Green in Situ Synthesis of Mg/CuO Core/Shell Nanoenergetic Arrays with a Superior Heat-Release Property and Long-Term Storage Stability. ACS Appl. Mater. Inter. 2013, 5, 7641-7646. (7) Staley, C. S.; Raymond, K. E.; Thiruvengadathan, R.; Herbst, J. J.; Swaszek, S. M.; Taylor, R. J.; Gangopadhyay, K.; Gangopadhyay, S. Effect of Nitrocellulose Gasifying Binder on Thrust Performance and High-g Launch Tolerance of Miniaturized Nanothermite Thrusters. Propellants Explos. Pyrotech. 2014, 39, 374-382. (8) Ramachandran, R.; Vuppuluri, V. S.; Fleck, T. J.; Rhoads, J. F.; Gunduz, I. E.; Son, S. F. Influence of Stoichiometry on the Thrust and Heat Deposition of On-Chip Nanothermites. Propellants Explos. Pyrotech. 2018, 43, 258-266. (9) Xu, J.; Tai, Y.; Ru, C.; Dai, J.; Ye, Y.; Shen, R.; Zhu, P. Tuning the Ignition Performance of a Microchip Initiator by Integrating Various Al/MoO3 Reactive Multilayer Films on a Semiconductor Bridge. ACS Appl. Mater. Inter. 2017, 9, 5580-5589. (10) Jang, N.-S.; Ha, S.-H.; Jung, S.-H.; Kim, S. H.; Lee, H. M.; Kim, J.-M. Fully Packaged Paper Heater Systems with Remote and Selective Ignition Capabilities for Nanoscale Energetic Materials. Sensor. Actuat. A-Phys. 2019, 287, 121-130. (11) Zhou, X.; Wang, Y.; Cheng, Z.; Ke, X.; Jiang, W. Facile Preparation and Energetic Characteristics of Core-Shell Al/CuO Metastable Intermolecular Composite Thin Film on a Silicon Substrate. Chem. Eng. J. 2017, 328, 585-590. (12) Kim, S. B.; Kim, K. J.; Cho, M. H.; Kim, J. H.; Kim, K. T.; Kim, S. H. Micro- and Nanoscale Energetic Materials as Effective Heat Energy Sources for Enhanced Gas Generators. ACS Appl. Mater. Inter. 2016, 8, 9405-9412. 21



(13) Jian, G.; Liu, L.; Zachariah, M. R. Facile Aerosol Route to Hollow CuO Spheres and its Superior Performance as an Oxidizer in Nanoenergetic Gas Generators. Adv. Funct. Mater. 2013, 23, 1341-1346. (14) Wang, H.; Rehwoldt, M.; Kline, D. J.; Wu, T.; Wang, P.; Zachariah, M. R. Comparison Study of the Ignition and Combustion Characteristics of Firectly-Written Al/PVDF, Al/Viton and Al/THV Composites. Combust. Flame 2019, 201, 181-186. (15) Wang, H.; DeLisio, J. B.; Holdren, S.; Wu, T.; Yang, Y.; Hu, J.; Zachariah, M. R. Mesoporous Silica Spheres Incorporated Aluminum/Poly (Vinylidene Fluoride) for Enhanced Burning Propellants. Adv. Eng. Mater. 2018, 20, 1700547-1700553. (16) Zhou, X.; Xu, D.; Lu, J.; Zhang, K. CuO/Mg/fluorocarbon Sandwich-Structure Superhydrophobic Nanoenergetic Composite with Anti-Humidity Property. Chem. Eng. J. 2015, 266, 163-170. (17) Séverac, F.; Alphonse, P.; Estève, A.; Bancaud, A.; Rossi, C. High-Energy Al/CuO Nanocomposites Obtained by DNA-Directed Assembly. Adv. Funct. Mater. 2012, 22, 323-329. (18) He, W.; Liu, P. J.; He, G. Q.; Gozin, M.; Yan, Q. L. Highly Reactive Metastable Intermixed Composites (MICs): Preparation and Characterization. Adv. Mate. 2018, 30, 1706293-1706312. (19) Zhu, Y.; Zhou, X.; Xu, J.; Ma, X.; Ye, Y.; Yang, G.; Zhang, K. In Situ Preparation of Explosive Embedded CuO/Al/CL20 Nanoenergetic Composite with Enhanced Reactivity. Chem. Eng. J. 2018, 354, 885-895. (20) Thiruvengadathan, R.; Bezmelnitsyn, A.; Apperson, S.; Staley, C.; Redner, P.; Balas, W.; Nicolich, S.; Kapoor, D.; Gangopadhyay, K.; Gangopadhyay, S. Combustion Characteristics of Novel Hybrid Nanoenergetic Formulations. Combust. Flame 2011, 158, 964-978. (21) Shen, J.; Qiao, Z.; Wang, J.; Zhang, K.; Li, R.; Nie, F.; Yang, G. Pressure Loss and Compensation in the Combustion Process of Al-CuO Nanoenergetics on a Microheater Chip. Combust. Flame 2014, 161, 2975-2981. (22) Qiao, Z.; Shen, J.; Wang, J.; Huang, B.; Yang, Z.; Yang, G.; Zhang, K. Fast Deflagration to Detonation Transition of Energetic Material Based on a Quasi-Core/Shell Structured Nanothermite Composite. Compos. Sci. Technol. 2015, 107, 113-119. (23) Wang, H.; Holdren, S.; Zachariah, M. R. Preparation and Combustion of Laminated Iodine Containing Aluminum/Polyvinylidene Fluoride Composites. Combust. Flame 2018, 197, 120-126. (24) Wang, H.; Jian, G.; Zhou, W.; DeLisio, J. B.; Lee, V. T.; Zachariah, M. R. Metal Iodate-Based Energetic Composites and Their Combustion and Biocidal Performance. ACS Appl. Mater. Inter. 2015, 7, 17363-17370. (25) Zhou, X.; Xu, D.; Yang, G.; Zhang, Q.; Shen, J.; Lu, J.; Zhang, K. Highly Exothermic and Superhydrophobic Mg/Fluorocarbon Core/Shell Nanoenergetic Arrays. ACS Appl. Mater. Inter. 2014, 6, 10497-10505. (26) Ke, X.; Guo, S.; Zhang, G.; Zhou, X.; Xiao, L.; Hao, G.; Wang, N.; Jiang, W. Safe Preparation, Energetic Performance and Reaction Mechanism of Corrosion-Resistant Al/PVDF Nanocomposite Films. J. Mater. Chem. A 2018, 6, 17713-17723. (27) Yu, C.; Zhang, W.; Gao, Y.; Ni, D.; Ye, J.; Zhu, C.; Ma, K. The Super-Hydrophobic Thermite Film of the Co3O4/Al Core/Shell Nanowires for an Underwater Ignition with a Favorable AgingResistance. Chem. Eng. J. 2018, 338, 99-106. (28) Kim, H. S.; Kim, J. H.; Kim, K. J.; Kim, S. H. Tuning the Ignition and Combustion Properties of Nanoenergetic Materials by Incorporating with Carbon Black Nanoparticles. Combust. Flame 22


Page 22 of 23

Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2018, 194, 264-270. (29) Kim, J. H.; Kim, S. B.; Choi, M. G.; Kim, D. H.; Kim, K. T.; Lee, H. M.; Lee, H. W.; Kim, J. M.; Kim, S. H. Flash-Ignitable Nanoenergetic Materials with Tunable Underwater Explosion Reactivity: The Role of Sea Urchin-like Carbon Nanotubes. Combust. Flame 2015, 162, 1448-1454. (30) Kim, J. H.; Ahn, J. Y.; Park, H. S.; Kim, S. H. Optical Ignition of Nanoenergetic Materials: The Role of Single-Walled Carbon Nanotubes as Potential Optical Igniters. Combust. Flame 2013, 160, 830-834. (31) Kim, K. J.; Jung, H.; Kim, J. H.; Jang, N. S.; Kim, J. M.; Kim, S. H. Nanoenergetic MaterialOn-Multiwalled Carbon Nanotubes Paper Chip as Compact and Flexible Igniter. Carbon 2017, 114, 217-223. (32) Wang, A.; Bok, S.; Thiruvengadathan, R.; Gangopadhyay, K.; McFarland, J. A.; Maschmann, M. R.; Gangopadhyay, S. Reactive Nanoenergetic Graphene Aerogel Synthesized by One-Step Chemical Reduction. Combust. Flame 2018, 196, 400-406. (33) Xu, X.; Liu, J.; Liu, J.; Ouyang, L.; Hu, R.; Wang, H.; Yang, L.; Zhu, M. A General MetalOrganic Framework (MOF)-Derived Selenidation Strategy for In Situ Carbon-Encapsulated Metal Selenides as High-Rate Anodes for Na-Ion Batteries. Adv. Funct. Mater. 2018, 28, 17075731707584. (34) Liu, X.; Liu, W.; Ko, M.; Park, M.; Kim, M. G.; Oh, P.; Chae, S.; Park, S.; Casimir, A.; Wu, G.; Cho, J. Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts. Adv. Funct. Mater. 2015, 25, 5799-5808. (35) Cho, J. S.; Won, J. M.; Lee, J. H.; Kang, Y. C. Synthesis and Electrochemical Properties of Spherical and Hollow-Structured NiO Aggregates Created by Combining the Kirkendall Effect and Ostwald Ripening. Nanoscale 2015, 7, 19620-19626. (36) Zou, F.; Chen, Y. M.; Liu, K.; Yu, Z.; Liang, W.; Bhaway, S. M.; Gao, M.; Zhu, Y. Metal Organic Frameworks Derived Hierarchical Hollow NiO/Ni/Graphene Composites for Lithium and Sodium Storage. ACS nano 2016, 10, 377-386. (37) Zhou, X.; Zhu, Y.; Ke, X.; Zhang, K. Exploring the Solid-State Interfacial Reaction of Al/Fe2O3 Nanothermites by Thermal Analysis. J Mater. Sci. 2018, 54, 4115-4123. (38) Shen, J.; Qiao, Z.; Wang, J.; Yang, G.; Chen, J.; Li, Z.; Liao, X.; Wang, H.; Zachariah, M. R. Reaction Mechanism of Al-CuO Nanothermites with Addition of Multilayer Graphene. Thermochim. Acta 2018, 666, 60-65. (39) Kwon, J.; Ducere, J. M.; Alphonse, P.; Bahrami, M.; Petrantoni, M.; Veyan, J. F.; Tenailleau, C.; Esteve, A.; Rossi, C.; Chabal, Y. J. Interfacial Chemistry in Al/CuO Reactive Nanomaterial and its Role in Exothermic Reaction. ACS Appl. Mater. Inter. 2013, 5, 605-613. (40) Schoenitz, M.; Patel, B.; Agboh, O.; Dreizin, E. L. Oxidation of Aluminum Powders at High Heating Rates. Thermochim. Acta 2010, 507-508, 115-122. (41) Wang, H.; Jian, G.; Egan, G. C.; Zachariah, M. R. Assembly and Reactive Properties of Al/CuO Based Nanothermite Microparticles. Combust. Flame 2014, 161, 2203-2208. (42) Rossi, C.; Zhang, K.; Esteve, D.; Alphonse, P.; Tailhades, P.; Vahlas, C. Nanoenergetic Materials for MEMS: A Review. J Microelectromech. S. 2007, 16, 919-931. (43) Dai, J.; Xu, J.; Wang, F.; Tai, Y.; Shen, Y.; Shen, R.; Ye, Y. Facile Formation of NitrocelluloseCoated Al/Bi2O3 Nanothermites with Excellent Energy Output and Improved Electrostatic Discharge Safety. Mater. Design 2018, 143, 93-103.

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NiO@C Nanoenergetic Materials through

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