**Controllable Electrically Guided Nano-Al**/**MoO3 Energetic-Film Formation on a Semiconductor Bridge with High Reactivity and Combustion Performance**

### **Xiaogang Guo 1,2, Qi Sun 3,\*, Taotao Liang <sup>4</sup> and A. S. Giwa <sup>5</sup>**


Received: 17 April 2020; Accepted: 14 May 2020; Published: 18 May 2020

**Abstract:** Film-forming techniques and the control of heat release in micro-energetic chips or devices create challenges and bottlenecks for the utilization of energy. In this study, promising nano-Al/MoO3 metastable intermolecular composite (MIC) chips with an uniform distribution of particles were firstly designed via a convenient and high-efficiency electrophoretic deposition (EPD) technique at room temperature and under ambient pressure conditions. The mixture of isopropanol, polyethyleneimine, and benzoic acid proved to be an optimized dispersing agent for EPD. The kinetics of EPD for oxidants (Al) and reductants (MoO3) were systematically investigated, which contributed to adjusting the equivalence ratio of targeted energetic chips after changing the EPD dynamic behaviors of Al and MoO3 in suspension. In addition, the obtained nano-Al/MoO3 MIC energetic chips showed excellent heat-release performance with a high heat release of ca. 3340 J/g, and were successfully ignited with a dazzling flame recorded by a high-speed camera. Moreover, the fabrication method here is fully compatible with a micro-electromechanical system (MEMS), which suggests promising potential in designing and developing other MIC energetic chips or devices for micro-ignition/propulsion applications.

**Keywords:** nano-Al/MoO3 MIC; stable suspension; electrophoretic deposition; kinetics; micro initiator

### **1. Introduction**

In recent decades, increasing attention has been paid to energetic fuels with high energy density (e.g., metastable intermixed composites (MICs) or nanothermites). They can generate superior combustion performance, so they are widely used in high-efficiency propellants [1], welding auxiliary devices [2], pyrotechnics [3], and specialized igniters or energetic chips [4] for a variety of military purposes and civilian applications. Generally, MICs are regarded as excellent fuels, which generally consist of metal-fuel (e.g., Al, Mg) and oxidizers that include metal oxides (e.g., MoO3 [5], Fe2O3 [6], polyvinylidene fluoride [7], NiO [8], CuO [9], and iodates [10]). Compared to traditional micro-MICs, nano-structured MICs have been gradually verified as a promising candidate for highly reactive energetic materials or composites due to their higher heat-release properties, greater contact area

between fuels and oxidizers, and faster detonation velocity [11–13]. Therefore, there is an emerging research area of interest in designing novel nano-structured MICs via facile techniques.

The Al/MoO3 MIC, as a desirable energetic system, has continuously aroused great interest, owing to its high heat of reaction (4698 J/g) and adiabatic flame temperature (3547 ◦C, higher than that of Al/Fe2O3, Al/CuO, etc.). Recently, different fabrication methods have been explored to prepare Al/MoO3 MICs or nanothermites for developing their exothermic performances, including the thermal co-evaporation method [14], magnetron sputtering [15,16], sonic wave-assisted physical mixing [17], and arrested reactive milling laser irradiation [18]. For example, M.R. Zachariah et al. designed Al/MoO3 MICs with different multilayer internal structures via the magnetron sputtering method on a semiconductor bridge as a promising micro-energy storage device, and analyzed the condensed state reaction process in the obtained nano-multilayered films [19]. E.L. Dreizin et al. reported low-temperature exothermic reactions in fully-dense Al/MoO3 nanocomposite powders fabricated by the arrested reactive milling technique [20]. In addition, Al/MoO3 MICs fabricated by the traditional mechanical mixing technique were shown to fuel a dramatic combustion exothermic process with a high burning rate of 100 ± 4 m/s and a high pressurization rate of 35 kPa/μs [21]. Nevertheless, it is relatively difficult to simultaneously meet the requirements of being low cost and easy to operate, with high-film-forming efficiency, using most reported methods. It is worth noting that using a portable electrophoresis, electrophoretic deposition (EPD) has technically demonstrated advantages in controllability of the composition and deposition efficiency for the target products from the charged micro/nanoparticles [22,23], or polymer molecules [24,25] in a stable suspension. The fabrication of Al/CuO and Al/NiO energetic films with uniform distribution was demonstrated by the K. T. Sullivan group [26] and the D. X. Zhang group [27], respectively. In our previous research work, the EPD technique was successfully used to prepare an Al/Bi2O3 MIC system [28]. Moreover, for practical application, it is essential to combine MICs with micro-electromechanical system (MEMS) technology [29] (i.e., so-called "nanoenergetics-on-a-chip" technology), constructing miniature energy-demanding devices with wide applications. However, there are few reports of the controlled design of MIC (e.g., Al/MoO3) chips via the EPD technique.

Thus, for these reasons, a novel nano-Al/MoO3 MIC chip combined with Al/MoO3 nanolaminates and a typical micro-semiconductor bridge was firstly designed via the facile EPD method using isopropanol, polyethyleneimine, and benzoic acid as an optimized dispersion system. As a type of binary energetic chip, the composition of the nano-Al/MoO3 MIC can be affected mainly by the EPD dynamic behaviors of the fuel (Al) and oxidizer (MoO3). Thus, further exploration of this aspect was analyzed and verified theoretically. Finally, the heat-release properties and ignition test of the product were investigated.

### **2. Experimental Section**

### *2.1. Reagents and Materials*

Polyethyleneimine, PEG-2000, benzoic acid, and nano-Al powders (99.9%) were purchased from Aladdin Inc. Corporation. (Shanghai, China). Isopropyl alcohol was purchased from Kelong Industrial Inc., (Chengdu, China). The other reagents (including hydrogen peroxide and ethanol) from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) were used as analytical grade purity without further purification. Deionized water (18.2 Ω) was used in all experiments.

### *2.2. Controllable Design of Nano-Al*/*MoO3 MIC*

In the fabrication of the nano-Al/MoO3 MIC, the EPD technique was exploited, and the corresponding detailed schematic diagram is displayed in Figure 1. Firstly, a classic one-step method was developed to prepare flake-like MoO3 powders. To be specific, 0.25 M Mo powders were added into 200 mL deionized water with trace PEG2000 marked as mixture *A*, then H2O2 (30 wt%) was dripped into mixture *A* slowly, until the yellow molybdenum peroxide sol appeared. After ultrasonic treatment for 0.25 h, the obtained sol was moved into a hydrothermal reactor at 110 ± 2 ◦C for 4 h, and the MoO3 powders were fabricated after repeated centrifugal cleaning at a rotation speed of 10,000 r/min and vacuum drying.

**Figure 1.** Schematic diagram of the facile fabrication of the nano-Al/MoO3 metastable intermolecular composite (MIC) chip.

Then, the nano-Al and MoO3 powders with different mass ratios were added into the optimized dispersant of isopropanol, polyethyleneimine, and benzoic acid with a volume ratio of 50:1:1 to obtain a stable suspension after ultrasonic treatment for 20 min at 25 ◦C. During EPD, a micro-ignition bridge was the working electrode, and the copper sheet with the same area was used as the counter electrode; the detailed size of electrodes is shown in Figure S1 (Supporting Information). The distance of the two electrodes ranged from 0.4 to 2.4 cm. In addition, the EPD process was conducted under different field strengths. The EPD time ranged from 0 to 16 min. After EPD, the working electrode was removed from the suspension, and dried in an oven at 80 ◦C for 1.5 h. The nano-Al/MoO3 MIC chip was finally obtained after cooling to room temperature for the subsequent ignition experiments. The deposited efficiency (deposit weight per area (mg cm<sup>−</sup>2)) of the deposits was calculated by dividing the increased weight of the working electrode after EPD by the deposition area. Each experiment was repeated five times, and the average value of five parallel experiments was used as the final valid result.

### *2.3. Material Characterization*

The morphology, element distribution, and crystalline structures of the nano-Al/MoO3 MIC were measured with a field emission scanning electron microscope (FESEM, JSM-7800F, Tokyo, Japan) equipped with energy dispersive X-ray spectroscopy (EDX), and X-ray diffractometer (XRD-6000, Shimadzu, Tokyo, Japan) with a scanning rate of 5◦/min, respectively. Atomic absorption spectroscopy (AAS, 180-80, Exter Analytical, Tokyo, Japan) was used to determine the mass or mole ratio of Al and MoO3 in deposited energetic film. The heat-release (Q) of the energetic chip was analyzed by differential scanning calorimetry (DSC, STA449F3, NETZSCH, Berlin, Germany) measured in a temperature range from 25 to 1000 ◦C at a low heating rate of 10 K/min under a 99.999% argon flow. Ignition of the product was studied using home-made capacitor charge/discharge initiating equipment, and video recordings of the deflagration were recorded by a high-speed camera (Phantom v7.3, Vision Research, Inc., Wayne, NJ, USA) at an imaging speed of 104 f/s.

### **3. Results and Discussion**

### *3.1. EPD Dynamic Studies*

A successful EPD generally largely depends on various dispersing agents, and a large number of experimental studies have been carried out to compare the dispersion systems for EPD of specific sorts of particles [24,30,31]. After a large number of comparison attempts, the optimal dispersing agent of mixture of isopropanol, polyethyleneimine, and benzoic acid was used to fabricate the nano-Al/MoO3 MIC by EPD at normal tempearture and pressure. For verifying the EPD controllability, dynamic behaviours of particles in optimized suspension were studied in detail. Shown in Figure 2a is the deposited efficiency (mg/cm2) as a function of deposited time under applied electric field strengths ranging from 6 to 12 V/mm during EPD of Al/MoO3 MIC films. It was obviously observed that the deposition efficiency increased with the EPD time when the field strength was fixed at 6 V/mm, and a similar trend was also seen in a higher field strength of 9 or 12 V/mm. The higher field strength provides a higher EPD efficiency at a certain deposited time (e.g., 10 min). Moreover, the deposited efficiency increased linearly with deposited time increasing from 0 to Tc (the critical time between linear and non-linear EPD dynamic in the critical region (black circle)) in Figure 2a,b, which is consistent with the research of the Zhang group [32]. In addition, *Tc* decreased with an increase of applied field strength; that is, *Tc* for 6 V/mm was larger than *Tc* for 9 and 12 V/mm, which was primarily due to the more severe precipitation and collision of particles under higher field strengths. Thus, the EPD process of Al/MoO3 MIC can be more precisely controlled in the linear control region (*t* < *Tc*) in this study, which is due to the relatively complex relationship of deposited efficiency and EPD time in the nonlinear variation region for all field strengths. Furthermore, the effect of the distance of electrodes on the deposited efficiency of the nano-Al/MoO3 MIC is analyzed in Figure 2c. When the solid loading concentration, EPD time, and applied field strength were set at 0.5 g/L, 8 min, and 12 V (blue line), respectively, the deposited efficiency increased gradually with the distance of electrodes rising to 1.2 from 0.4 cm. It then decreased slowly, as the distance of electrodes continued to increase. This result is perhaps caused by the more violent disturbance of particles under a smaller distance of electrodes, and the higher degree of the settlement of particles under a longer distance of electrodes that leads to a lower EPD efficiency. Similar change trends were observed at higher field strengths (Figure 2c), which provides a valuable reference for realizing controllable EPD of different particles.

In addition, the exothermicity of MICs is a key indicator that largely depends on the mass or mole ratio of fuel (e.g., Al) and oxidizer (e.g., MoO3). Generally, in MIC energetic reactions, the equivalence ratio (Φ) is defined as the actual ratio of fuel to oxidizer divided by the stoichiometric ratio of fuel to oxidizer in an energetic reaction, that is Φ = (F/O)actual/(F/O)stoich [26]. For the codeposition process of the Al and MoO3 particles, the equivalence ratio in the starting suspension (Φs) was adjusted accurately in weighed samples, and the equivalence ratio in the deposited product (Φd) was determined by EDX and AAS techniques. Figure 2d displays the Φ<sup>d</sup> of Al and MoO3 particles in the Al/MoO3 MIC chip as a function of Φ<sup>s</sup> of nano-Al and MoO3 particles in the starting suspension. Clearly, it can be seen that Φ<sup>d</sup> increased linearly with Φ<sup>s</sup> by EDX and AAS analysis, and the fitted equations were similar (*Y* = 1.97*X* <sup>−</sup> 1.04, *R*<sup>2</sup> = 992 for EDX analysis, and *Y* = 2.02*X* <sup>−</sup> 0.96, *R*<sup>2</sup> = 998 for AAS analysis). Thus, <sup>Φ</sup><sup>d</sup> in the nano-Al/MoO3 MIC could be adjusted by changing Φ<sup>s</sup> in suspension, which contributes to optimizing the proportion of components in product, and further developing the exothermic performance of the product.

**Figure 2.** (**a**) Deposited efficiency (mg/cm2) of nano-Al/MoO3 MIC as functions of deposition time under different applied electric field strengths; (**b**) the local amplification image of the black circle in (**a**); (**c**) the relationship of deposited efficiency and the distance of two electrodes under different loading concentrations; and (**d**) Φ<sup>d</sup> of Al and MoO3 in the Al/MoO3 MIC chip as a function of Φ<sup>s</sup> of nano-Al and MoO3 particles in the starting suspension.

### *3.2. Characteristics of Nano-Al*/*MoO3 MIC*

XRD analysis was used to investigate the crystal structures of the nano-Al/MoO3 MIC in Figure 3. It can be clearly seen that two groups of distinct diffraction peaks are marked in good agreement with that of the standard spectra for pure Al (JCPDS card No. 04-0787; Fm-3m (225)) and MoO3 (JCPDS card No. 35-0609; Pb nm (62)) on the product, demonstrating the successful co-EPD of the Al and MoO3 particles. In addition, the fact that there are no other clear peaks of Al2O3 and Mo in Figure 3 indicates the high purity of the product, and that no thermite reactions took place during the EPD process.

The as-obtained nano-Al/MoO3 MIC films via EPD are displayed in Figure 4. Regions of large-scale local sags, crests, and separations are not seen optically in the target product surface (Figure 4a), which exhibits significant coating characteristics and uniformity. Clearly, in the FESEM image of product in Figure 4b, the nano-Al/MoO3 MIC appears to be uniformly distributed, without rare unmixed zones. The higher-resolution images in Figure 4c indicate that the nano-Al particles were scattered or distributed randomly in flake-like MoO3, which significantly helps to enlarge the contact areas of Al and MoO3, and shorten the mass transportation length (MSL) during the thermite exothermic reaction of nano-Al/MoO3 MIC. Moreover, there were numerous gaps among the particles (Figure 4b,c), contributing to providing a large number of heat-release channels or multiple spatial streams, and further improving the exothermic performance of the product [27]. Moreover, the elemental compositions in the nano-Al/MoO3 MIC were analyzed by the EDX technique, as shown in Figure 4d, where the EDX spectrum indicates that all expected elements of Al, Mo, and O existed in the energetic film surface, consistent with the results of the XRD analysis. It is worth noting that the mole ratio of Al, Mo, and O

was close to 2:1:3 (0.336:0.16:0.50) (seen in Figure 4d and Table S1 in Supporting Information), and the corresponding reaction mole ratio of Al and MoO3 was close to 2:1, which contributed to realizing a sufficient aluminothermic reaction (2Al + MoO3 → Al2O3 + Mo + HHeat−release, ΔH = 4698 J/g) [17,33]. In addition, we conducted a comparative study of FESEM mapping and the corresponding results are similar in three random regions (Table S1), where the mole ratio of aluminum to nickel is close to 1:1, which indicates the uniform distribution of the product. Moreover, the percentage errors of the mole ratio of elements are approximate 5% in six random regions, according to both EDX and AAS analysis, further demonstrating the homogeneously mixed nano-Al/MoO3 MIC obtained by EPD.

**Figure 3.** Typical X-ray diffractometer (XRD) pattern of the as-obtained nano-Al/MoO3 MIC.

**Figure 4.** (**a**) Optical and (**b**,**c**) typical field emission scanning electron microscope (FESEM) images of the nano-Al/MoO3 MIC films prepared using electrophoretic deposition (EPD); (**d**) energy dispersive X-ray spectroscopy (EDX) spectrum of the product with an inserted table showing the mole content (%) of all elements.

### *3.3. Thermal Studies*

Exploration of exothermic performance is essential to energetic materials, including MICs [34–37], and is shown in Figure 5 in detail. Figure 5a displays the DSC data measured from the nano-Al/MoO3 MIC with the Φ<sup>d</sup> of ~1.0 and a low heating rate of 10 K/min. In addition to an unobservable exothermic peak at ~400 ◦C, probably due to the reaction between Al nano-particles with much smaller-sized MoO3 particles, there are several observable exothermic peaks in Figure 5a, where the exothermic peak (green rectangle area and yellow rectangle area) is mainly because of the reaction between Al particles with smaller-sized MoO3 particles [38], and the latter two exothermic peaks at 703.4 ◦C and

735.9 ◦C (blue rectangle area) are from the reaction of Al and bigger-sized MoO3 particles, which is consistent with the results from the Zhu group [15]. In addition, there was also an endothermic peak (Figure 5a, primrose yellow rectangle area) at ca. 660 ◦C, caused by the melt of metal-Al. After a fitting calculation, the value of the heat-release of the nano-Al/MoO3 MIC was as high as ~3340 J/g, which was >70% of the theoretical value, indicating the relatively sufficient thermite reaction. Furthermore, the effect of the deposited time on the heat-release of the product is analyzed in Figure 5b. There was a similar trend for different field strengths from 6 to 12 V/mm, that is, the output of heat was almost stable as the deposited time increased, showing the great controllability of EPD dynamic behaviors of Al or MoO3 particles in suspension. The heat-release values as functions of Φ<sup>d</sup> and Φ<sup>s</sup> of Al and MoO3 are clearly shown in the 3D histogram (Figure 5c). The heat-release values increased first and then gradually decreased with Φ<sup>d</sup> of Al and MoO3, and were highly associated with Φ<sup>s</sup> of Al and MoO3 in the starting suspension. The peak value of the heat release of the nano-Al/MoO3 MIC can be obtained when Φ<sup>d</sup> of Al and MoO3 was close to 1.0.

**Figure 5.** (**a**) Differential scanning calorimetry (DSC) curve of the obtained nano-Al/MoO3 MIC; (**b**) the relationship of heat release and deposited time under different applied electric field strengths; and (**c**) 3D histogram of heat release as a function of Φ<sup>d</sup> and Φ<sup>s</sup> of Al and MoO3 particles in the product.

The thermite reaction deflagration processes of an electric explosion for the nano-Al/MoO3 MIC chip were realized a self-regulating capacitor charge/discharge initiating device, and recorded synchronously by a high-speed camera. The detonation schematic diagram is displayed in Figure 6a. When the ignition circuit was switched on, the energetic target chip was quickly detonated with a dazzling blaze. The corresponding flame propagation images of the nano-Al/MoO3 MIC chip are observed in Figure 6b, where the interval time between adjoining images is 0.1 ms. The flame duration time of the nano-Al/MoO3 MIC chips was >1 ms, and loud sounds during the ignition test indicated that the thermite reactions were so intense that energy was released quickly [39–42]. In addition, the observed intense deflagration was in accordance with the DSC results, which provided a facile route to nano-MIC energetic chips for MEMS application. In addition, the heat-release performance of MIC chips can be optimized by building a theoretical bridge between the equivalence ratio of oxidant and reductant in starting suspension (Φs) and target energetic films or initiators (Φd).

**Figure 6.** (**a**) Schematic of the ignition system for the micro nano-Al/MoO3 MIC chip initiator, and ignition process recorded by a high-speed camera; (**b**) series of still images taken from a typical ignition deflagration study of the nano-Al/MoO3 MIC fabricated by EPD process with Φ<sup>d</sup> = 1.0; the time interval between images is 0.1 ms (*tn* − *tn*−<sup>1</sup> = 0.1 ms, *n* ≥ 2).

### **4. Conclusions**

In this study, a novel Al/MoO3 MIC chip initiator was firstly fabricated by a high-efficiency EPD technique in an optimized mixture dispersant of isopropanol, polyethyleneimine, and benzoic acid at normal temperature and pressure. The microstructures and chemical compositions of the product were demonstrated by FESEM, EDX, and XRD. The deposited energetic films exhibited even mixing between the oxidizer (Al) and reductant (MoO3), contributing to enhancing their exothermic performance. The EPD dynamic behaviors of nano-Al and MoO3 particles were studied, which can act as a theoretical bridge for connecting the Φ<sup>s</sup> in starting suspension and Φ<sup>d</sup> in energetic chips. DSC results showed the apparent exothermic peaks of nano-Al/MoO3 MIC chips, due to the thermite reaction between Al and MoO3, and the corresponding total heat-release was as high as ca. 3340 J/g when Φ<sup>d</sup> of Al and MoO3 was close to 1.0. In addition, the Al/MoO3 MIC chip initiator can be successfully ignited with a typical capacitor charge/discharge ignition device, exhibiting outstanding detonation performance with a short burst time and a dazzling flame. In short, the design of the Al/MoO3 MIC chip initiator in this study will provide a universal approach for fabricating other thermite energetic chips with wide civilian and military applications, especially in micro-initiation or micro-propulsion systems.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/5/955/s1, Figure S1: The size specification of the working and counter electrodes used for EPD dynamic research and ignition test. All yellow rectangle zones are parts of electrodes without touching the optimized suspension., Table S1: Molar content results of different elements in products by EDX and AAS analysis in three random regions.

**Author Contributions:** X.G. and T.L. conceived and designed the experiments; T.L. and Q.S. conducted the experiments; X.G. and Q.S. analyzed the data; X.G. and T.L. contributed reagents/materials/analysis tools; X.G. and T.L. wrote the paper; X.G., Q.S., A.S.G. and T.L. revised the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (21805014), the scientific and technological research program of Chongqing Municipal Education commission (KJQN201801424), Natural Science Foundation of Chongqing (No cstc2019jcyj-msxmX0675), Yangtze Normal University (No. 2018QNRC10) and the opening project of material corrosion and protection key laboratory of Sichuan province (2018CL19).

**Acknowledgments:** The authors acknowledge the financial support from National Natural Science Foundation of China (research core funding No. 21805014), Chongqing Municipal Education commission (research core funding No. KJQN201801424), Natural Science Foundation of Chongqing (research core funding No. cstc2019jcyj-msxmX0675), Yangtze Normal University (young scientist research funding No. 2018QNRC10) and the opening project of material corrosion and protection key laboratory of Sichuan province (research core funding No. 2018CL19).

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Preparation and Characterization of Al**/**HTPB Composite for High Energetic Materials**

**Alexander Vorozhtsov 1, Marat Lerner 1,2, Nikolay Rodkevich 2, Sergei Sokolov 1,\*, Elizaveta Perchatkina <sup>1</sup> and Christian Paravan <sup>3</sup>**


Received: 13 October 2020; Accepted: 6 November 2020; Published: 8 November 2020

**Abstract:** Nanosized Al (nAl) powders offer increased reactivity than the conventional micron-sized counterpart, thanks to their reduced size and increased specific surface area. While desirable from the combustion viewpoint, this high reactivity comes at the cost of difficult handling and implementation of the nanosized powders in preparations. The coating with hydroxyl-terminated polybutadiene (HTPB) is proposed to improve powder handling and ease of use of nAl and to limit its sensitivity to aging. The nAl/HTPB composite can be an intermediate product for the subsequent manufacturing of mixed high-energy materials while maintaining the qualities and advantages of nAl. In this work, experimental studies of the high-energy mixture nAl/HTPB are carried out. The investigated materials include two composites: nAl (90 wt.%) + HTPB (10 wt.%) and nAl (80 wt.%) + HTPB (20 wt.%). Thermogravimetric analysis (TGA) is performed from 30 to 1000 ◦C at slow heating rate (10 ◦C/min) in inert (Ar) and oxidizing (air) environment. The combustion characteristics of propellant formulations loaded with conventional and HTPB-coated nAl are analyzed and discussed. Results show the increased burning rate performance of nAl/HTPB-loaded propellants over the counterpart loaded with micron-sized Al.

**Keywords:** HTPB; aluminum nanopowders; solid propellants; burning rate; coated aluminum

### **1. Introduction**

The common composition of mixed high-energy material (HEM) as heterogeneous solid propellants is based on three main ingredients: an oxidizing agent, an organic binder, and a high energy density fuel. Ammonium perchlorate (AP) or ammonium nitrate (AN) are typically used as oxidizer, low-molecular-weight polybutadiene (usually cured by isocyanates) is a common binder, while metal powders are considered as fuels. Aluminum powder is the most readily available, intensively studied and efficient metallic fuel in various HEM formulations. As the aluminum particle size decreases, the ignition temperature and combustion time decrease [1,2]. In solid propellants, the replacement of micron-sized Al powders (μAl) with nanosized aluminum powders (nAl) leads to an increase in the burning rate and a decrease in the size of the condensed combustion products (aggregates/agglomerates) in the near-surface zone [3]. An analysis of the effects of Al powder size reduction on the burning rate of solid propellants and on the condensed combustion products formation is reported in [4]. In this latter study, the burning rate of solid propellants loaded with Al particles with size of 30 μm and 170 nm was measured [4]. Under the investigated conditions, the burning rate was almost doubled when μAl powders were replaced by nAl [4].

Nanosized metal powders are characterized by very high specific surface area. Nanoparticles have different chemical and physical properties when compared to micron sized powders. Due to their increased reactivity, nanopowders are very attractive ingredients for HEM formulations [5–12].

Despite the obvious advantages for inclusion nAl powders in the HEM formulations (i.e., faster reaction rates), there are some disadvantages that limit their use. Nanosized Al is more sensitive than the micron-sized counterpart to the influence of oxidizing or corrosive environment due to the large specific surface area [13,14]. This increased sensitivity to the environment may lead to significant active aluminum content losses during nAl storage [14]. As a consequence, nAl could release less energy during the combustion, thus decreasing the combustion performance of HEMs loaded with it. To suppress/limit the nAl aging during storage, the nanoparticles surface (typically passivated by Al2O3) can be coated by a protective layer [15–17]. The encapsulation of particles with hydrocarbon/fluorohydrocarbon coatings has shown the promising results in the limitation of aging process of nAl powder [18]. Hydroxyl-terminated polybutadiene (HTPB) is a suitable material for nAl capping when solid propellant applications are targeted, since it is a common binder in composite formulations for solid rocket propulsion [19,20]. The HTPB offers very low glass transition temperature, relatively low viscosity, high combustion heat and, once cured, high mechanical properties of the final product even at high powder filling ratios.

In this paper the use of HTPB as a protective coating for nAl is discussed. The nAl powder coating process includes the use of acetylacetone together with HTPB. In the proposed strategy the acetylacetone is applied on the particle first. Thus, protective organic hydrophobic layer is created on the surface of the nanoparticles. Then, HTPB is sorbed on the hydrophobic surface of the nanoparticles increasing the compatibility of the nAl with binder and propellant components, preventing a decrease in the content of active aluminum due to aging, and can also simplify propellant production and casting [18]. For powders added to the propellant, the same polymer is used as a binder as for coating the particles. However, when mixing the nanopowders and polymer, the viscosity of the mixture increases drastically, slightly decreasing with continued mixing. Since the mixture has a high viscosity, its preparation requires a lot of time and energy. To overcome this problem, an important technological stage is the preliminary preparation of pastes based on metal nanopowders and polymer binders, followed by their inclusion in the HEM formulation.

The propellant burning rate is a key parameter for the design of a solid rocket motor and, in particular for the thrust level [21]. Together with the oxidizer decomposition and sublimation, and the metal combustion mechanism, HTPB degradation is one of the significant stages of the propellant burning [22]. Thus, the study of the effect of the propellant components on the process of destruction of HTPB is an urgent task in the field of propellant improvement.

The main aim of this work is the experimental study the characteristics of different nAl-based HTPB-containing pastes (nAl/HTPB). Various pastes were prepared, the difference between them being the nAl-to-HTPB ratio. The work focuses on two nAl/HTPB (10 wt.% and 20 wt.%) that are contrasted in terms of their characteristics and effects on the burning behavior of solid propellants loaded with them.

### **2. Materials and Methods**

Aluminum nanopowder (nAl) used in present work was produced by electric explosion of wire (EEW) method [23,24].

Hydroxyl-terminated polybutadiene (HTPB R45), acetylacetone of analytical grade and mineral spirit (used as solvents) with boiling point in the range of 70–100 ◦C were used without any further purification.

Ammonium perchlorate (AP) used for the preparation of the propellant formulations was used in the form of fine and coarse powders, the fractional composition of which is shown in Table 1.


**Table 1.** Ammonium perchlorate (AP) particle size distribution.

D0.1—the diameter below which 10% of the particles lay; D0.5—the diameter below which 50% of the particles lay; D0.9—the diameter below which 90% of the particles lay; D32—the surface-based mean diameter; D43—the volume-based mean diameter.

Microstructure and morphology of the aluminum nanopowders were captured by a JEM-100 CXII transmission electron microscope (TEM, JEOL Ltd., Tokyo, Japan) for both pristine air-passivated and HTPB-coated powders. Active aluminum content (CAl) was determined by volumetric method. The content was estimated by the evolution of the hydrogen released by the powder reaction with a 5 M NaOH solution [25].

Aluminum nanopowders specific surface area was determined by nitrogen adsorption/desorption by the Brunauer-Emmett-Teller (BET) method with a SORBTOMETR-M (Katakon, Russia) surface area analyzer. A specimen mass of 50 mg was used for the measurements. Specific surface area samples had been heated at 120 ◦C for 30 min before the surface area measurement was taken. The average particle diameter based on the specific surface area was calculated assuming a particle size distribution of uniform spheres, as:

$$\text{cl} = \frac{6000}{\rho \text{S}} \tag{1}$$

where *d*—nanoparticle diameter, nm, ρ—particle density, g/cm3, *S*—BET specific surface area, m2/g [26].

The reactivity and stability of the powder at various temperatures and heating rates was studied with use of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) Seiko Exstar 6000 (Seiko Instruments Inc., Chiba, Japan). The samples of 5–10 mg weight were heated from ambient temperature to 1273 K at heating rate 10 ◦C/min in air atmosphere. The TGA/differential scanning calorimetry (DSC) experiments in Ar were performed by a Netzsch F5 Jupiter STA analyzer (NETZSCH-Gerätebau GmbH, Selb, Germany) under the same conditions of the DTA scans.

To prepare the nAl/HTPB composition, for example with HTPB content of 10 wt.% (nAl-H10), 250 mL of solvent, 45.0 g of nAl powder, and 0.225 g of acetylacetone were used. The amount of acetylacetone was 0.5% of the weight of the bare nanopowder. The resulting mixture was stirred with a high shear homogenizer HG-15D (Daihan Scientific, Seoul, Korea) at 5000 rpm for 30 min. Then a HTPB-based solution (5.0 g HTPB in 50 mL of solvent) was added to the mixture and was then stirred for another 30 min. The solvent was removed from the mixture using an IKA 10 RV (IKA®-Werke GmbH & Co. KG, Staufen im Breisgau, Germany) rotary evaporator; the resulting product was dried at a pressure of 1.33 mbar for 16 h. The content of components in the samples are listed in Table 2.


**Table 2.** The composition of the initial and composite powders.

Resonant acoustic mixing (LabRAM I apparatus, Resodyn Acoustic Mixers, Butte, MT, USA) was used to uniformly disperse the propellant ingredients [27].

The binder was prepared starting from HTPB R45 resin cured with isophorone diisocyanate (IPDI). The curing ratio of the binder ([-NCO]/[-OH]) was 1.04. Dioctyl adipate (DOA) was used as a plasticizer, while dibutyltin diacetate was added (in excess) to the formulation as curing catalyst. For a comparative assessment the propellant formulation was prepared with the initial nAl powder as presented in Table 3. The nAl mass fraction in the tested formulation is the same on a molar basis. For all the tested formulations, the coarse-to-fine AP ratio is 65:10.

**Table 3.** Base composition of the propellants tested.


The propellant burning rate (*rb*) was determined from tests carried out in a laboratory combustion chamber equipped with windows for combustion process video recording. The tested samples are cut into parallelepipeds (4 × 4 × 30 mm). The lateral surface of the samples is inhibited to provide a one-dimensional regression of the combustion surface. A simplified scheme of the experimental setup is given in Figure 1. Nitrogen is used for the combustion chamber pressurization, and to prevent combustion products smoke from inhibiting the visualization. Quasi-steady chamber pressure is granted by electrovalves controlled by a digital pressure regulator. The rb value is determined using by a proprietary software. The latter enables the regressing surface tracking during the combustion, thus providing the quasi-steady burning rate. For a given propellant, experimental results are reduced according to the standard Vielle's law [28]:

$$\mathbf{n}\_{\mathbf{b}} = \mathbf{a} \mathbf{p}^{n} \tag{2}$$

**Figure 1.** Schematic diagram of the lab-scale strand burner for *rb* determination setup.

In the Equation (2), the *rb* is typically expressed in mm/s, with *p* in bar.

Characterization of the nanopowders developed by Tomsk State University (TSU, Tomsk, Russia) is carried out in cooperation with the Space Propulsion Laboratory of the Politecnico di Milano (SPLAB-POLIMI, Milan, Italy). Investigation of the combustion of the propellants containing nanopowders is carried out by SPLAB-POLIMI.

### **3. Results and Discussion**

In an earlier stage of the work, a number of nAl/HTPB composites were prepared. Manufactured materials featured an HTPB content in the range 10 to 50 wt.%. The composites with HTPB content of 10 wt.% and 20 wt.% are disperse solids, while for HTPB mass fractions >20%, the obtained materials are highly viscous masses of difficult handling. Thus, activities concentrated on nAl100-H10 and nAl100-H20.

### *3.1. nAl Characterization*

A TEM image of the pristine, air-passivated aluminum nanopowder is presented in Figure 2. The shape of the visible nanoparticles is predominantly spherical. The native amorphous oxide layer consisting of bayerite α-Al(OH)3 and γ-AlOOH boehmite [13] is on the surface of aluminum nanoparticles. Content of aluminum metal is usually about 90 wt.% [17,29]. Nanoparticles have a marked clustering tendency with formation of particles subject to cold cohesion reaching sizes up to hundreds of microns. The observed clusters are rather compact, yet the cold-cohesion particle-particle interactions are typically relatively weak, and clusters can be broken down (though reversibly) by mechanical stresses (e.g., ultrasound irradiation).

**Figure 2.** TEM images of the (**a**) pristine aluminum nanoparticles and (**b**) an aluminum nanoparticle cluster.

The content of active aluminum in the aluminum nanopowders used in the present work is (85.9 ± 0.8) wt.% as presented in Table 4. The BET—derived specific surface of the nanopowder is 12.6 <sup>±</sup> 0.1 m2/g, corresponding to an average diameter of the nanoparticles [Equation (1)] of ~175 nm.

**Table 4.** Active aluminum content in the tested nAl powders.


<sup>a</sup> The expected aluminum content is estimated based on the nAl100.

Table 4 shows CAl data for all the tested powders. For the nAl/HTPB, the actual CAl is compared to the expected value resulting from the pristine powder (nAl100) and the nominal coating mass fraction (10 wt.%, 20 wt.%). The difference between the actual and the expected values reported in the Table 4 is originated by the possible presence of (minor amounts of) residual solvent in the HTPB-containing pastes. From this point of view, this observation is supported by the slightly higher CAl difference for nAl100-H20, which contains a higher HTPB mass fraction.

### *3.2. Characterization of the nAl*/*HTPB Pastes*

An image of the nAl100-H10 is reported in Figure 3, where HTPB is seen to form a continuous organic layer, partially sorbed on the surface of the nanoparticles or particle clusters. Data reported in the Table 4 suggest the absence of powder-HTPB reactions modifying the powder composition: when preparing the composite, neither the formation of soluble aluminum compounds occurs when interacting with an organic substance, nor the sorption of the solvent by the particles of the composite. The HTPB deposition at the particle surface provides a protective layer shielding the nanoparticle surface from interaction with the environment, thus offering an increased aging resistance [21]. At the same time, the HTPB deposition promotes particle clustering (see Figure 3, where separated particles are encapsulated by the HTPB layer) [21], yet the impact of this effect should be evaluated considering the powder mixing and dispersion in the HEM matrix.

**Figure 3.** Aluminum nanoparticles coated with HTPB.

### *3.3. Thermogravimetric Analysis of the Pastes*

### 3.3.1. Thermogravimetric Analysis of the Pastes in Ar

The TGA plots of HTPB and nAl-HTPB samples are shown in Figure 4. Pristine, uncured HTPB can be seen to undergo two-stage degradation in an argon atmosphere. The first stage of the destruction starts at about 225 ◦C and continues up to about 325 ◦C with a weight loss of ~11% at this stage. The second stage of the thermal degradation proceeds in the temperature range from 325 to 525 ◦C with almost complete mass loss. The rate of the HTPB degradation in this second stage, based on the TGA curve slope, is higher than that at the first stage.

**Figure 4.** TGA of HTPB and nAl/HTPB pastes in Ar (10 ◦C/min).

The thermal degradation of HTPB in the nAl/HTPB pastes proceeds as a two-stage process too. However, the first stage of the destruction begins at a lower temperature (~100 ◦C) and ends at about 210 ◦C. Weight loss at this stage is 7–8%. The weight loss at this stage is independent of the paste composition. The destruction of the polymer can be assumed to occur on the nanoparticle surface and its surface oxide layer is a destruction catalyst, since the destruction rate in this case is higher than that in the lack of contact with alumina, despite the destruction proceeds at a lower temperature.

The thermal degradation of HTPB in the second stage proceeds at a lower rate compared with that at the first stage, although at a higher temperature. The thermal degradation end occurs in the range 500–550 ◦C, which suggests that polymer pyrolysis at this stage proceeds outside of its contact with alumina of the aluminum nanoparticle and the polymer degradation mechanism does not change, with a lower destruction rate due to a lower onset decomposition temperature.

### 3.3.2. Thermogravimetric Analysis of the Pastes in Air

TGA plots of HTPB and nAl/HTPB samples performed in air are shown in Figure 5.

**Figure 5.** TGA of HTPB and nAl/HTPB pastes in air (10 ◦C/min).

Initial HTPB as can be seen from the plot undergoes two-stage mass-loss, as per the tests in Ar (Figure 4). The mass loss at the first stage of decomposition is insignificantly higher as compared to decomposition in Ar. The first stage of decomposition begins at about 200 ◦C and continues up to about 380 ◦C with a weight loss of ~7% at this stage. The second stage of reaction proceeds in the temperature range from 380 to 475 ◦C with almost complete weight loss. The rate of decomposition of HTPB at this stage, based on the TG curve slope is higher than that at the first stage.

The degradation/reaction of the HTPB applied on the surface of the nAl particles proceeds as in the Ar case in two-stages, however, the first stage of the decomposition begins at a lower temperature about 100 ◦C and ends at about 200 ◦C. Weight loss at this stage depends on the composition of the paste, weight loss being larger when increasing HTPB content. Above 200 ◦C the second stage of the destruction begins, with the completion to be about 500 ◦C. Aluminum nanoparticle oxidation begins at the temperature above 500 ◦C. In the temperature range 550–650 ◦C, oxidation of aluminum nanoparticles is observed, accompanied by their partial sintering [30]. Subsequent oxidation of aluminum is observed at temperature above 650 ◦C and continues at temperatures above 1000 ◦C.

### *3.4. Burning Rate*

The propellant burning rate generally follows Vieille's law as shown in Equation (2). Data fittings and burning rate values obtained from propellant tests are shown in Figure 6 and Table 5.

**Figure 6.** Burning rate of the propellants with aluminum nanopowders.



The data show the introduction of nAl/HTPB paste to provide an increase in the propellant burning rate. Compared to nAl100, the introduction of paste into the propellant formulation results in an increase in the rate coefficient *a* and a decrease in the value of the pressure exponent *n*. The burning rate increase suggest a better dispersion of the metal particles of the nAl/HTPB pastes in the propellant matrix. The value of pressure exponent for the propellants with introduced paste is somewhat lower compared with that for propellant with initial aluminum nanopowder. However, considering the interval of confidence on the ballistic exponent, its variations are relatively small (if any). This suggest the absence of changes in the combustion mechanism of the tested formulations.

### **4. Discussion**

nAl/HTPB composites were prepared by wet mixing method. The preparation includes two stages. The first stage is the disruption of nAl particle clusters when stirring with acetylacetone and formation of the organic chemisorbed layer on the nanoparticle surface. This layer increases the hydrophobicity and compatibility with the polymer of the nanoparticle surface and facilitates the sorption of the HTPB molecules [31]. The second stage is the application of HTPB on the nanoparticle surface. TEM images show the homogeneous continuous layer of the polymer capping the nanoparticle/nanoparticles. The capping prevents direct contacts of the nanoparticles with each other (for separated particles), and hinders the penetration of oxidant molecules to the nanoparticle surface, which increases their stability during storage and processing. This effect provides the complete protection of the nanoparticles against environmental influences but retains their high reactivity which is demonstrated by nanoparticles slow heating rate oxidation and combustion tests [18], see Figure 5 and Table 5.

TGA analysis shows the lack of a chemical bond between the nanoparticle and HTPB molecules. Thermal degradation of HTPB during slow heating rate heating proceeds as intramolecular chemical process with minimal participation of oxygen. Only part of the HTPB molecules in contact with the nanoparticle surface is degraded at a lower temperature by the action of alumina layer as catalyst. So the HTPB destruction proceeds and aluminum oxidation sequential reactions occurring independently of each other. The thermal degradation of HTPB in the nAl/HTPB pastes at the second stage proceeds in the same temperature range as that of HTPB [31]. The degradation proceeding at this stage can be assumed to be noncatalytic process. The degradation process at this stage is suggested to proceed outside of the contact with alumina of the aluminum nanoparticle and the HTPB degradation mechanism is the same both for nAl/HTPB and HTPB.

The propellants with formulation comprising capped aluminum nanoparticles exhibit higher burning rate than the counterpart loaded with nAl100. Yet, the fundamentals of the burning process remain unchanged. Quantitative parameters of the burning law allow to consider the burning process to be faster and more stable when capped aluminum nanoparticles were added.

### **5. Conclusions**

Different nAl/HTPB composites were prepared by a wet method. Of the prepared materials (HTPB mass fraction in the range 10 to 50 wt.%), two were selected for detailed analyses spanning from pre-burning characterizations to combustion analyses. The selection was made based on the availability of a disperse phase system with powder-like characteristics, and limited volability. In particular, the selected composites featured the following compositions (i) nAl100 (90 wt.%) + HTPB (10 wt.%) and (ii) nAl100 (80 wt.%) + HTPB (20 wt.%). In both materials acetylacetone served as compatibilizing agent for effective wetting of the nAl surface (Al2O3 from air passivation, with possibly, hydrated compounds) with HTPB. The nAl/HTPB composite can be an intermediate product for the subsequent manufacture of mixed high-energy materials while maintaining the quality and advantages of nAl. Capping of the nanoparticle surface with HTPB protects nanoparticles against environmental influence (i.e., material corruption due to ageing) and provides easier handling and manufacturing while granting good combustion performance as tesitifed by DTA-TGA scans and burning tests performed in a lab-scale burner. The slow heating rate, non-isothermal oxidation of the powders showed the effect of the nAl on the HTPB degradation process proceeding at lower temperature and with higher reaction rate than what is observed for the uncured polymer alone. On the other hand, no evidences of HTPB-aluminum interactions affecting the powder reactivity and oxidation mechanism were noticed. Thermal degradation effects were separately investigated in Ar and air to decouple the polymer pyrolysis effects from the composite oxidation mechanism.

The burning characteristics of the propellant formulations containing tested composites were analyzed. The inclusion of the composite in the propellant formulation provides faster burning rate with increasing burning stability at low pressure.

**Author Contributions:** Writing—original draft, N.R., S.S., E.P.; supervision, A.V.; writing—review and editing, A.V., N.R., S.S., C.P.; Methodology, M.L., C.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was carried out with financial support from the Ministry of Science and Higher Education of the Russian Federation (State assignment No. 0721-2020-0028) and (Agreement with Joint Institute for High Temperatures RAS No 075-15-2020-785).

**Acknowledgments:** This work was carried out with financial support from the Ministry of Science and Higher Education of the Russian Federation (State assignment No. 0721-2020-0028) and (Agreement with Joint Institute for High Temperatures RAS No 075-15-2020-785). The synthesis and microscopic characterization of the aluminum nanoparticles and Al/HTPB composites were performed according to the Government research assignment for ISPMS SB RAS, project No. III.23.1.1. The experimental skills and the work of Alberto Verga (SPLAB-POLIMI) were highly appreciated during the research activity.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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### *Article* **Gaseous Products Evolution Analyses for Catalytic Decomposition of AP by Graphene-Based Additives**

**Shuwen Chen 1, Ting An 2, Yi Gao 3, Jie-Yao Lyu 1, De-Yun Tang 1, Xue-Xue Zhang 1, Fengqi Zhao 2,\* and Qi-Long Yan 1,\***


Received: 13 March 2019; Accepted: 17 May 2019; Published: 24 May 2019

**Abstract:** A quantitative evaluation method has been developed to study the effects of nanoadditives on thermal decomposition mechanisms of energetic compounds using the conventional thermogravimetry coupled with mass spectrometry (TG/MS) technique. The decomposition of ammonium perchlorate (AP) under the effect of several energetic catalysts has been investigated as a demonstration. In particular, these catalysts are transition metal (Cu2<sup>+</sup>, Co2<sup>+</sup> and Ni2+) complexes of triaminoguanidine (TAG), using graphene oxide (GO) as dopant. They have been well-compared in terms of their catalytic effects on the concentration of the released gaseous products of AP. These detailed quantitative analyses of the gaseous products of AP provide a proof that the proton transfer between ·O and O2 determines the catalytic decomposition pathways, which largely depend on the type of reactive centers of the catalysts. This quantitative method could be applied to evaluate the catalytic effects of any other additives on the thermal decomposition of various energetic compounds.

**Keywords:** thermolysis; energetic materials; GO-based catalysts; quantitative analyses; decomposition mechanisms

### **1. Introduction**

Energetic materials (EMs) are widely used as propellants, explosives, and pyrotechnics. The conventional EMs, such as 1,3,5-trinitro-1,3,5-triazinane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), and ammonium perchlorate (AP), are still playing dominant roles in the formulations. The nano-sized additives are usually used to improve their performances and more innovative additives have been designed and used during past decades [1–3]. The decomposition and combustion of EMs are the key parameters that have to be investigated before their applications, which are strongly connected with their compatibility, safety, and performance. The decomposition has been found to be the initial stage of combustion, and it should be well-evaluated at laboratory scale. AP is a well-known oxidizer in solid composite propellants. In order to get the burning behavior of propellants, researchers usually focus on their thermal property first [4,5]. The thermal behavior of AP has been widely studied during the past several decades [6,7]. Nanoadditives have high concentrations of dislocations and large surface areas, and therefore they normally show significant catalytic effects on the decomposition of AP [8–11]. However, the nanoadditives used today in formulations are mostly inert metal oxides or metal oxide composites. To some extent, they may reduce the energy content of the propellants.

In order to overcome the limitations mentioned above, one has to develop energetic metal complexes or metal organic frameworks (MOFs) with great thermal stability and compatibility as energetic catalysts. It has been recently shown that graphene oxide (GO) doped transition metal complexes are promising energetic catalysts [12,13]. It has been reported that the intrinsic exothermicity of GO (1600 J g<sup>−</sup>1) is comparable to several hazardous chemicals and explosives [14]. In addition, GO could be considered as a stabilizing agent [15]. In our recent study, GO-doped transition metal (nickel, cobalt and copper) complexes of triaminoguanidine (TAG) were prepared, and their effects on the thermal properties and decomposition mechanisms of RDX studied [16]. These materials could not only catalyze the decomposition of RDX, but also improve the thermal stability of RDX due to their enhanced thermal conductivity. The decomposition behavior of GO complexes modified AP have been briefly studied based on conventional thermogravimetry coupled with mass spectrometry (DSC/TG) analysis [17]. The results showed that the hybrid catalysts can enhance the initial decomposition temperature and change the thermolysis mechanism by introducing different types of metal ions. But the detailed effect of nanoadditives, especially for GO-based energetic catalysts, on thermolysis products and reaction pathways of AP are not well investigated [18]. According to the literature, two mechanisms have been proposed for the thermal decomposition of AP: (a) electron transfer mechanism [19] and (b) proton transfer mechanism [20], depending on the temperature. A large amount of data has shown that the GO-TAG based catalysts have significant effects on heat releases and thermal stability of EMs, but the inherent chemical mechanisms are still not well-known.

Therefore, this paper intends to present a comprehensive quantitative analysis to clarify how nanoadditives affect the decomposition mechanisms of EMs. The catalytic effects of GO-based energetic additives on thermal decomposition chemical pathways and gaseous products of AP have been investigated as a typical demonstration. It is a novel method to get the catalyst mechanism by measuring the amount of gaseous decomposition products. Thermal analysis techniques including thermogravimetry coupled with mass spectrometry (TG/MS) were employed to evaluate the decomposition mechanisms [21–23]. This technique was used to evaluate the bond breaking and gaseous products formation during thermal decomposition of EMs. Thus, this work aims at studying the thermal decomposition of AP, in presence of various GO-TAG based energetic catalysts, on the basis of TG/MS technique.

### **2. Experimental Procedure**

### *2.1. Sample Preparation*

G-T-M and TAG-M composites were synthesized followed by the method reported in the previously published paper ([16], the preparation was summarized in the supporting materials). In order to get AP contained complex, a saturated solution of AP containing 160 mg of AP and 20 mL of acetone was prepared first. Then 40 mg TAG-M or G-T-M was added to this solution (M means Nickel, Cobalt or Copper ion), and stirred for 3 h at room temperature. The final G-T-M/AP and TAG-M/AP complexes were obtained after freeze drying. Warning—the solvent-free TAG-Cu complex would undergo self-ignition and fast deflagration to detonation transition reaction easily in the presence of oxygen.

G-T-M indicates GO-doped transition metals complexes of TAG. TAG-M is transition metals modified TAG. G-T-M/AP and TAG-M/AP correspond to G-T-M and TAG-M doping AP, respectively. The components of G-T-M/AP and TAG-M/AP have been summarized in Table 1.

**Table 1.** The compositions of ammonium perchlorate (AP)-based mixtures (in weight percent).


### *2.2. Experimental Techniques*

The gaseous products from thermal decomposition of AP and AP-based mixtures using TAG-M and G-T-M as energetic nanoadditives were detected using the TG-DSC/MS technique. This experiment was realized on a simultaneous thermoanalyzer STA 449 F3 coupled with a quadrupole mass spectrometer QMS 403 C Aëolos (Netzsch Group, Selb, Germany). An alumina pan with a pin-hole cover was used as a sample pan). The sample masses for these measurements were about 3 mg, with a heating rate of 10 ◦C/min in a temperature range of 40–500 ◦C, using an argon atmosphere (gas flow: 50 mL/min).

### **3. Results and Discussion**

### *3.1. Possible Catalytic Decomposition Pathways of AP*

According to the literature [24–26], there are two possible decomposition processes for AP; the low and high temperature stages. Since AP contains plenty of nuclei with tiny pores [27], they are likely to provide highly active sites on their surface at the low temperature decomposition stage, while the high-temperature decomposition stage takes place at the surfaces of the nanocrystals, and involves adsorption and desorption of ammonia as well as perchloric acid. The decomposition of AP generally follows two steps [28,29]; a solid-gas multiphase reaction of the first decomposition step at 300–330 ◦C, and a gas phase reaction of the second decomposition step at 450–480 ◦C. The possible decomposition reactions and transformations are as follows [27]:

$$\mathrm{NH\_4ClO\_4} \rightarrow \mathrm{NH\_3} + \mathrm{HClO\_4}$$

$$\mathrm{HClO\_4} \rightarrow 2\mathrm{O\_2} + \mathrm{HCl}$$

$$\mathrm{O\_2} \rightarrow \mathrm{O\_2}^-$$

$$4\mathrm{NH\_4}^+ + \mathrm{O\_2}^- \rightarrow 2\mathrm{H\_2O} + 4\mathrm{NH\_3}$$

$$7\mathrm{ClO\_4}^- \rightarrow 2\mathrm{ClO\_3} + \mathrm{ClO\_2} + 2\mathrm{ClO} + 9\mathrm{O\_2} + \mathrm{Cl\_2}$$

$$4\mathrm{NH\_3} + 5\mathrm{O\_2} \rightarrow \mathrm{N\_2O} + \mathrm{NO} + \mathrm{NO\_2} + 6\mathrm{H\_2O}$$

As mentioned above, the TG/MS technique has been used to detect the products of thermal decomposition of AP and TAG-based catalysts coated AP. The major gas products have been found to be O2, NO, NH3, H2O, ·O, which would be used to clarify the possible catalytic decomposition mechanisms.

As shown in Figure 1a, there are two obvious gas evolution peaks at around 308–323 ◦C and 438–445 ◦C for pure AP, which are attributed to the low-temperature decomposition and high-temperature decomposition, respectively. Compared with pure AP, G-T-Cu2/AP shows only one gas release peak between 328 ◦C and 342 ◦C. The G-T-Cu2/AP decomposes with a single but wide releasing peak, so G-T-Cu2 has a catalyst effect on AP decomposition. In the presence of TAG-Ni, the decomposition peak of the first step has the same temperature range as that of pure AP, and the second peak temperature has decreased by about 40 ◦C. The addition of TAG-Ni has a larger effect on the releasing amount of NO and ·O. For the mixture of G-T-Ni/AP, as shown in Figure 1d, there is only one decomposition peak similar to that of G-T-Cu2/AP, but with a higher peak temperature in the range of 364–383 ◦C. The results suggest that G-T-Ni can improve the thermal stability of AP compared with G-T-Cu2.

A sharp decomposition peak can be observed for TAG-Co/AP with the range of 297–301 ◦C, so the presence of TAG-Co has a large catalytic effect on AP decomposition by decreasing the amount of released gases and decomposition temperature. All the gas products are releasing at almost the same temperature. Regarding G-T-Co/AP, the decomposition peak is smoother and the peak temperature increases by 10–20 ◦C with the addition of GO. It has been reported that G-T-Co can slightly increase the thermal stability of AP [17]. The results indicate that TAG-M complexes in the presence of GO can enhance thermal stability for AP, resulting in slower reaction rates at a higher temperature range.

**Figure 1.** Temperature dependency of the ion flows from non-isothermal decomposition for pure AP and coated AP under a heating rate of 10 ◦C min−<sup>1</sup> using thermogravimetry coupled with mass spectrometry (TG/MS) technique: (**a**) pristine AP; (**b**) G-T-Cu2/AP; (**c**) TAG-Ni/AP; (**d**) G-T-Ni/AP; (**e**) TAG-Co/AP; (**f**) G-T-Co/AP; TAG—triaminoguanidine.

### *3.2. The Dependence of Gases' Evolution Processes on Temperature for Catalytic Decomposition of AP*

The curves in Figure 1 were derived and summarized as a function of gaseous product types, as shown in Figure 2.

For the decomposition of pure AP (Figure 2f), the sequence of its gas releasing process is as follows: NO, ·O, O2, NH3 and H2O. The releasing rate of NO and ·O increases rapidly at around 300 ◦C, and then slows down when the temperature reaches 350 ◦C. In the meantime, the other three kinds of gases start to release. The releasing rates of all the gaseous products reach their maximum values at the temperature of 420 ◦C, and start to decrease when the temperature is over 450 ◦C. The two releasing stages correspond to two decomposition peaks in Figure 1a. Compared with pure AP, the addition of TAG-M complexes could affect the gaseous releasing processes due to the change of chemical reaction pathways, which will be discussed in more detail in a later section.

There is only a trace amount of ·O (*m*/*z* = 16) radical being formed for all involved samples. The release of ·O is even earlier than that of pure AP in the presence of G-T-Co, G-T-Cu2 and TAG-Co complexes, but TAG-Ni and G-T-Ni complexes postponed the reactions that would release ·O. The mass loss of AP in these cases should be largely caused by the transformation of O2→O2 − that forms ·O radical, which has been demonstrated by the curve of *m*/*z* = 16 in Figure 1. Five kinds of TAG-based complexes have similar catalytic effects on reaction temperature, which lead to the formation of NH3 (*m*/*z* = 17) and H2O (*m*/*z* = 18), where the releasing rate of gas is increased. In comparison, G-T-Ni postpones the temperature at which NO (*m*/*z* = 30) starts to release. It also improves the thermal stability of AP, whereas the other four types of G-T-M complexes decrease the initial gas releasing temperature of AP. The initial decomposition temperature of AP is decreased, and the reaction rate is increased in formation of O2 (*m*/*z* = 32), by using TAG-M complexes as additives.

**Figure 2.** The dependence of accumulation (α is conversion rate) of the typical gaseous products on the temperature for AP-based mixtures: (**a**) *m*/*z* = 16, ·O; (**b**) *m*/*z* = 17, NH3; (**c**) *m*/*z* = 18, H2O; (**d**) *m*/*z* = O2; (**e**) *m*/*z* = 30, NO; (**f**) summary of products evolution processes of pristine AP.

In the presence of TAG-Ni, all the gas release processes occur in two steps, which is similar to the case of pure AP. G-T-Ni/AP has a lower initial temperature for releasing O2 and ·O than that of TAG-Ni/AP. However, the initial releasing temperature for NO, NH3 and H2O of G-T-Ni/AP is higher than that of TAG-Ni/AP. If comparing G-T-Co/AP with TAG-Co/AP, the initial temperatures for the production of NO, O2, ·O and H2O are much lower, where extra NH3 was generated. The gas releasing rate of TAG-Co/AP is also higher than that of G-T-Co/AP throughout their decomposition. The G-T-Cu2 has a significant catalytic effect on AP decomposition so that the stabilization of GO is excluded. It is clear that the initial temperature of G-T-Cu2 has been decreased in comparison to pure AP. In summary, the addition of TAG-M complexes to AP may lead to significant changes in the chemical decomposition pathways.

### *3.3. Quantitative Analyses of Gaseous Products' Changes of AP in Presence of These Nanocatalysts*

To study the catalytic effect of TAG-based complexes on the decomposition mechanisms of AP, the ion flow curves (Figure 1) have been integrated and analyzed. In order to make a quantitative comparison, the characteristic parameters of these curves are calculated and summarized in Table 2.


**Table 2.** A summary of TG/MS parameters of AP coated by different GO (graphene-oxide)-based catalysts.

Notes: *T*o, onset temperature of decomposition, in ◦C; *T*e, the end temperature of decomposition, in ◦C; Δ*T*, *T*<sup>e</sup> − *T*o, in ◦C; n, the amount of each released gas; nm, proportion of each product in total gaseous products.

Many investigations have shown that AP decomposition proceeds via the electron transfer from the cation NH4 <sup>+</sup> to anion ClO4 −, and the catalytic processes with nanocrystalline additives involve the electron transfer between AP and nanoadditives. Boldyrev [28] concluded that the AP decomposition process is followed by a proton transfer from the cation NH4 <sup>+</sup> to anion ClO4 −. We may assume that the addition of TAG-M complex has catalytic activity in AP decomposition. The amount of gaseous product is different depending on the type of catalysts. For TAG-M doped by GO, the nanocrystalline of TAG-M grows uniformly on the graphene nanosheets, which can increase the contact surface area between AP with the active electron transfer centers, thus resulting in accelerated thermolysis reaction processes. The GO-doped TAG-M complexes with different metal centers could either postpone or accelerate the initial decomposition of AP. According to the quantitative change of gases by TG-DSC/MS technique, the reaction pathways have been greatly changed by these catalysts, depending on the type of metal ions.

For releasing ·O, TAG-Ni and G-T-Ni have little influence on the initial temperature of decomposition, but G-T-Ni/AP can help increase the reaction rate compared with TAG-Ni/AP and AP. The addition of TAG-Co, G-T-Co or G-T-Cu2 increases the initialization temperature and shortens the reaction time. For the reaction to produce NH3, TAG-M complex decreased the initial reaction temperature of NH3 by 57.6–88.4 ◦C while increasing the reaction time. For the reaction to produce NO, G-T-Ni/AP postpones the initial reaction temperature, while the other nano- complexes decrease the initialization temperature. All of these TAG-M complexes have increased the reaction rate.

TAG-Co has shortened the reaction that produces H2O, whereas the other four types of complexes have prolonged the reaction time by 53.8% to 100%. The initial decomposition temperature of AP under the effects of all TAG-M complexes have been decreased by 61 ◦C to 138 ◦C. For the releasing of O2, TAG-Co based complex has shortened the reaction time by 35–40%, but Ni-based and Cu-based composites prolong the reaction time by 29–57%. Cu-based component reduces the initial decomposition time and slightly decreases the reaction time by 5.8%.

The amount of each gas product is quantified by integrating the peak area of ion intensity curves in Figure 1. The percentage of major gaseous products for each TAG-M complex was calculated and summarized in Figure 3. The amount of H2O and NH3 was increased with the addition of G-T-Cu2, while the amount of the other three kinds of gases was reduced. This suggests that G-T-Cu benefits the reaction of NH4 <sup>+</sup> and ClO4 −, but suppresses the reaction of oxygen conversion. For TAG-Ni/AP and G-T-Ni/AP, the reactions of producing NO and ·O were suppressed, but the reactions producing H2O, NH3 and O2 were promoted. The amount of change of gas released for G-T-Ni/AP is more significant than that of TAG-Ni/AP. For TAG-Ni/AP, the molar ratio of ·O and H2O changed by −1.8% and 7.5%, respectively. Additionally, TAG-Ni/AP shows two decomposition peaks the same as that of pure AP. The TAG-Co based additives show a similar catalyst effect to that of TAG-Cu2 complex. The reaction of ·O is very sensitive to TAG-Co, which reduces the gas amount by 40%. The decomposition curve of TAG-Co/AP is the sharpest and its reaction time is the shortest. G-T-Co/AP shows the greatest influence on the reaction of producing O2, with a decrease in amount of gas by 52%.

The addition of TAG-M complex has an obvious catalytic effect on the whole decomposition efficiency of AP, where an increase of gas production of H2O and NH3 could be found. For the molar ratio of ·O, TAG-Ni/AP presents the lowest reduction rate of gas production at 1.8%, compared to 41.1% for that of TAG-Co/AP. The amount of O2 generation was reduced for TAG-Co/AP, G-T-Co/AP, and G-T-Cu2/AP, while it increased in TAG-Ni/AP and G-T-Ni/AP. Co-based complex shows an obvious reduction in O2 product, among which G-T-Co/AP presents a reduction rate as high as 52.5%. The producing of NO is the most sensitive to G-T-Ni, with a reduction rate of 46.9%.

**Figure 3.** Comparison of molar ratio of gas products decomposed by AP coated with different TAG-M based catalysts.

Figure 4 shows that there is a pair of ions (NH4 <sup>+</sup> and ClO4 <sup>−</sup>) in NH4ClO4 crystal lattice. The proton transfers from NH4 <sup>+</sup> to ClO4 <sup>−</sup>, which leads to reaction *b* and the formation of NH3 and HClO4. Moreover, the graphene could be combined with metal oxides to improve the catalytic activity. For instance, the molar ratio change of G-T-Cu2, TAG-Co, and G-T-Co show the same trend; the amount of H2O and NH3 is increasing, while the amount of ·O, NO and O2 is decreasing. According to the amount of change, we can postulate that reactions *a*, *f*, and *e* were promoted (Figure 4).

**Figure 4.** Thermal decomposition mechanisms of AP under the catalytic effects of TAG-M complexes (arrow up means increase and arrow down means decrease).

For TAG-Co catalyst dispersed in AP, the amount of ·O has the minimum value, where one sharp decomposition peak was observed from the ion intensity curve. In the case of G-T-Co/AP, the amount of O2 shows the maximum reduction, while graphene shows additional promotion to reaction *f*. In contrast, TAG-Ni catalyst promotes reaction *d*, but suppresses reaction *g*, while the amount of ·O slightly decreases compared with that of pure AP. G-T-Ni shows catalytic effect on reaction *d*, but suppresses reactions *f* and *g*.

All TAG-M catalysts could promote the reactions of AP decomposition by increasing the amount of gas products. In addition G-T-Co, TAG-Co, and G-T-Cu2 have stronger catalytic effects than nickel-based compounds, as they could make two-step gas releasing into one step. This conclusion is consistent with the thermal results from our previous research [17], which claimed that all TAG-M based composites could catalyze the first decomposition process of AP, and Co-based and Cu-based materials have stronger catalytic effects.

### **4. Conclusions**

TG/MS was used to quantitatively study the effects of additives on decomposition mechanisms of energetic compounds, where catalytic decomposition of AP by GO-based catalysts has been selected as a typical example. It has been demonstrated that the detailed quantitative analyses of the gaseous products of AP would show the inherent mechanism changes under the effects of various additives. This method could be applied to analyze the decomposition mechanisms of any other energetic compounds.

The findings further support the literature on the catalytic decomposition kinetics of AP. The GO-based catalysts show improved catalytic efficiency due to their capability in increasing the conversion rates of NH3 and H2O. This can be explained by more O elements being transferred to react with NH4 <sup>+</sup>, which enhances the initial decomposition heat, resulting in the combination of two decomposition peaks. The amount of O2 decreases under the effect of Ni-based complexes, which promote the reaction in producing ·O. In particular, new findings suggest that TAG-Ni and G-T-Ni materials have higher stabilization effects on AP than the others. The Co-based and Cu-based composites could increase the release of O2, so they have better catalytic effect on AP decomposition. The improved reactions between ·O and the other radicals mean a better catalytic effect on the decomposition of AP.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/5/801/s1, Figure S1: The G-T-M (M=Cu2<sup>+</sup>, Ni2<sup>+</sup> and Co2<sup>+</sup>) coordination nanomaterials prepared by reaction of ammonized GO with corresponding metal nitrates: mononuclear coordination complexes could be formed based on triaminoguanidine ligand, Figure S2: The SEM photos of involved materials, where G-T-Cu, G-T-Co, G-T-Ni, TAG-Co and TAG-Ni are presented.

**Author Contributions:** Conceptualization, Q.-L.Y. and F.Z.; methodology, T.A. and X.-X.Z.; formal analysis, T.A. and Y.G.; investigation, Q.-L.Y. and S.C.; resources, J.-Y.L.; data curation, D.-Y.T.; writing—original draft preparation, S.C.; writing-review and editing, Q.-L.Y.; project administration, F.Z. and Q.-L.Y.; funding acquisition, Q.-L.Y.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number 51776176 and the Scientific Research Ordering Bureau of EDDM China with project number 61407200204.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

### **References**


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