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Article

The Effects of HMX and CL-20-Based Co-Particles on the Ignition and Combustion Performances of Aluminum Powders

by
Zhihua Xue
1,
Weimeng Zhang
1,
Ruixuan Xu
1,
Sulan Yang
2 and
Qilong Yan
1,*
1
National Key Laboratory of Solid Rocket Propulsion, Northwestern Polytechnical University, Xi’an 710072, China
2
Aerospace College, University of Electronic Science and Technology of China, Chengdu 611731, China
*
Author to whom correspondence should be addressed.
Aerospace 2025, 12(4), 272; https://doi.org/10.3390/aerospace12040272
Submission received: 27 February 2025 / Revised: 19 March 2025 / Accepted: 20 March 2025 / Published: 24 March 2025
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Abstract

:
Energetic co-particles have been proven effective in balancing high-energy and safety performance, which might be used as insensitive oxidizers in solid propellants. In this work, the high temperature interactions between several co-particles and aluminum (Al) powders in the presence of ammonium perchlorate (AP) have been studied. The co-particles are based on octogen (HMX) and hexanitrohexaazaisowurtzitane (CL-20), with balanced energy content and safety performance. They are used to combine with Al and AP to form either binary or ternary systems. Their energy release rate during decomposition and combustion have been fully evaluated. Due to the intimate contact between components in co-particles, the binary/ternary systems exhibit superior reaction efficiency compared to relevant mechanical mixtures with the same formulations. These novel energetic systems have maximum two times higher pressurization rate, 10% higher heat of explosion, 53.8% higher flame propagation rate, and much shorter ignition delay than the corresponding normal mixtures. For both HMX- and CL-20-based co-particle systems, the median size of condensed combustion products (CCPs) is smaller than those of the mechanical mixtures, with higher content of Al2O3. This indicates that co-particles have advantages in improving combustion efficiency of Al particles by eliminating their agglomeration.

1. Introduction

Solid propellants, which serve as the core components of propulsion systems in modern rockets, missiles, and spacecraft, are mainly composed of fuel, oxidizer, binder, curing agent, and various other additives [1,2]. The energy performance is crucial as it directly determines vital propulsion parameters, including thrust, specific impulse (Isp), and range [3,4]. Aluminum powders (Al), a widely used metallic fuel, enhances Isp and energy release efficiency due to its high energy density and combustion heat [5]. However, Al faces practical challenges. First, its susceptibility to oxidation results in a 2–5 nm amorphous oxide shell that inhibits the ignition and combustion of the inner Al [6]. Second, Al tends to agglomerate during combustion, forming large particles. This leads to incomplete combustion, two-phase flow losses, and lower energy-release efficiency [7,8].
To address these challenges, researchers have explored various strategies in recent years to enhance the combustion performance of Al, including surface coating techniques [9,10,11], nanoscale processing [12], and the synthesis of highly reactive metastable composite thermites (MICs) [13,14]. Especially, interfacial regulation between nitramines and Al effectively enhances the reactivity and efficiency of Al [15,16,17,18,19,20]. For instance, the reaction efficiency of Al can be enhanced by incorporating inside HMX with the precise catalysis of graphene-based carbohydrazide complexes [15]. Compared with the mechanical mixture, the ignition delay of Al@HMX can be shortened by 43.7% and the particle size can be significantly reduced, indicating an improved combustion efficiency. Zhang et al. [21] further explored the effect of mass ratio on the laser ignition and combustion performance of Al/HMX composites. It was found that when the mass ratio of Al:HMX = 65:35, the lowest ignition threshold of 0.43 J and the highest combustion rate of 0.35 m s−1 were obtained. Fang et al. [22] revealed that increasing Al content enhances flame height and burning intensity while reducing burning duration by 0.21–0.91 ms, significantly improving combustion performance. Studies also indicated that the improved reaction efficiency and inhibited agglomeration can be realized by combining nitramines (e.g., HMX or CL-20) with ammonium perchlorate (AP) [23,24,25].
HMX and CL-20, known for their high energy density and excellent detonation performance, are highly valued in propellants [26]. Despite their advantages, the high sensitivity of HMX and CL-20 remains a major safety concern [27]. To address this challenge, researchers have proposed a concept of “co-particle” [28], to enhance both the energy and safety properties of nitramines. Zhou et al. [29] used a hydrothermal assembly method to create RDX/TATB co-particles, achieving strong interfacial interactions that reduced reactivity under thermal and mechanical stimuli. Patil et al. [30] synthesized HMX, RDX, and CL-20 co-particles with diaminotrinitrobenzene (DATB) via hydrothermal methods, confirming the relationship between thermochemical properties and shock wave sensitivity through Raman and FTIR analyses. Furthermore, Veerabhadragouda et al. [31] utilized a co-agglomeration approach to prepare co-agglomerates of TATB with RDX, β-HMX, 2,4,6,8-tetranitro-2,4,6,8-tetraazabicyclo[3.3.0]octane (BCHMX), and ε-CL-20. Compared to DATB, TATB-based co-agglomerates exhibited lower sensitivity. Additionally, co-agglomeration and co-crystallization methods have been explored to enhance the properties of ε-CL-20 and BCHMX, two prominent cyclic nitramines. Although some co-agglomerates exhibit lower detonation energy than anticipated, studies indicate their promising potential for application in propellants [32]. Furthermore, by introducing an interfacial layer of polydopamine (PDA), nitramine/interfacial layer/insensitive explosive co-particles architecture based on strong intermolecular interactions were prepared [33,34]. This strategy maintains the high energy of nitramines while significantly improving their safety performance.
In this study, insensitive explosives including 1,3,5-triamino 2,4,6-trinitrobenzene (TATB), 2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105), and 1,1-diamino-2,2-dinitroethylene (FOX-7) are selected. TATB is renowned for its exceptional thermal stability and insensitivity to mechanical stimuli, making it a benchmark for safety in high-energy materials [35]. LLM-105 and FOX-7 exhibit excellent detonation performance, with significantly low friction and impact sensitivities [36,37]. These properties make TATB, LLM-105, and FOX-7 ideal candidates for co-particle formulations, as they enhance safety without compromising energy performance. CL-20-based and HMX-based co-particles, containing 10% insensitive explosives, were fabricated using PDA or triaminoguanidine-glyoxal polymer (TAGP) as the interfacial layer. Detailed information about these co-particles can be found in published works [33,34]. CL-20-based and HMX-based co-particles are mixed with Al and AP to prepare co-particle/Al binary and co-particle/Al/AP ternary systems. The combustion performance, ignition characteristics, and energy levels of these systems were evaluated using a multifunctional combustion diagnostic system, a CO2 laser ignition device, and an oxygen bomb calorimeter, respectively. Mechanical mixtures with identical compositions were used as control groups to compare the energy-release advantages conferred by the co-particles. The findings provide foundation for the potential application of co-particles in solid propellants.

2. Experimental and Theoretical Calculation

2.1. Formulation Design and Energy Optimization

The theoretical thermodynamic parameters of the binary and ternary systems were calculated using NASA CEA software to optimize their compositions. The variation of Isp and combustion temperature with co-particle content is shown in Figure S2. For binary systems, Isp gradually increases with co-particle content when the mass fraction of co-particles is below 65%, while the combustion temperature remains relatively stable. When the co-particle content exceeds 65%, both combustion temperature and Isp initially increase and then decrease with increasing co-particle mass fraction. Notably, the (co-TATBP10%/CL-20)/Al binary system achieves maximum combustion temperature (4013.5 K) and Isp (273.9 s) when the co-TATBP10%/CL-20 mass fraction is 85%.
To investigate the synergistic effects of co-particles and AP, ternary systems (co-particles/Al/AP) with 20% Al were prepared. The theoretical Isp and combustion temperature of these ternary systems are shown in Figure S3. The Isp of all ternary systems increases with co-particle content, reaching a maximum at 75% co-particles. In contrast, the combustion temperature initially rises and then declines with increasing co-particle content. Based on these calculations, the optimal mass ratios were determined to be co-particle/Al = 85/15 for binary systems and co-particle/Al/AP = 75/20/5 for ternary systems.

2.2. Preparation Methods

2.2.1. Preparation of Co-Particles

To provide an intimate contact between nitramines and insensitive explosives, PDA was used to coat TATB, LLM-105 and FOX-7, and the detailed information can be found in our previous work [33,34]. The CL-20-based and HMX-based co-particles were prepared using a spray-drying technique, as illustrated below (taking co-TATBP10%/CL-20 as an example): (1) Precursor solution preparation: 5.4 g of CL-20 was completely dissolved in 36 mL ethyl acetate by continuous mechanical stirring; 0.6 g of nTATB@PDA was added into the solution and ultrasonicated for 10 min to ensure uniform dispersion. (2) Spray-drying process: the prepared precursor solution was fueled by a peristaltic pump with a rate of 4.5 mL min−1; the solution was atomized into small droplets by the nozzle, and ethyl acetate was evaporated immediately, followed by the recrystallization of CL-20, with nTATB@PDA as nucleation site; the evaporation temperature was set to be 60 °C; the products can be collected after the complete evaporation of solvent. The prepared co-particles were named according to their compositions. For example, ‘co-TATBP10%/CL-20’ refers to CL-20-based co-particles combined with 10 wt.% TATB and PDA as the interfacial layer.
For the preparation of HMX-based co-particles, dimethyl sulfoxide (DMSO) was used and sample mass was maintained the same. The procedure can be divided into two steps: (1) Precursor solution preparation: Accurately weigh TAGN, HMX, and LLM-105 and dissolve them in DMSO under continuous stirring until a homogeneous solution is achieved. Add a glyoxal solution to the mixture to initiate the cross-linking reaction, allowing sufficient time for the reaction to complete. (2) Spray-Drying Process: Feed the precursor solution into the spray-drying equipment using a peristaltic pump. After the DMSO evaporates, collect the resulting solid particles, yielding co-LLM-105T10%/HMX co-particles. Further information for the preparation of HMX-based co-particles can be found in the Supporting Information. For example, ‘co-LLM-105P10%/HMX’ refers to HMX-based co-particles combined with 10 wt.% LLM-105 and TAGP as the interfacial layer.

2.2.2. Preparation of Binary and Ternary Systems

The binary and ternary systems were prepared by grinding typical co-particles with Al and AP. For example, the (co-TATBP10%/CL-20)/Al binary system was prepared as follows: (1) 1.7 g of co-TATBP10%/CL-20 and 0.3 g of Al were weighed and mixed in an agate mortar. (2) 2 mL of ethanol was added as an auxiliary solvent, and the mixture was grinded for 5 min to evaporate ethanol. (3) the above-mentioned grinding process was repeated three times, and the binary system was obtained after freeze-drying for 24 h. The ternary systems were prepared by a similar procedure, with an additional 5 wt.% AP added. The binary and ternary systems were named according to their compositions (Table 1). For example, binary and ternary systems can be named as ‘BC-X’ and ‘TC-X’, respectively, where ‘B’ and ‘T’ denotes ‘binary’ and ‘ternary’, respectively, ‘C’ represents ‘co-particle’, and ‘X’ is the sample number.
For comparison, mechanical mixtures of insensitive explosives and nitramines were prepared, as detailed in Table S1. Binary and ternary systems containing these mechanical mixtures were also fabricated. For example, the m-TATBP10%/CL-20/Al binary system was prepared as follows: (1) Weigh 1.7 g of m-TATBP10%/CL-20 and 0.3 g of Al and mix them in an agate mortar. (2) Add a small amount of ethanol as an auxiliary solvent and grind the mixture after the ethanol has evaporated. (3) Repeat the grinding process three times, collect the samples, and freeze-dry them for 24 h to obtain the binary system. The prepared samples were named according to their composition, such as “BM-X”, where “B” denotes the binary system, “M” represents the mechanical mixture, and “X” is the sample number.

2.3. Characterization Methods

To comprehensively characterize the ignition and combustion properties of the co-particle system, a series of experimental methods were employed. The pressurization rate was investigated using a bomb calorimeter equipped with a pressure sensor, where 1 g of the co-particle system was ignited under 3 MPa argon, and pressure changes during combustion were recorded in real time. The heat of the explosion was measured using a microcomputer automatic calorimeter under the same conditions to evaluate the energy level of the co-particle system. The flame propagation rate and dynamic combustion process were studied using a self-built multifunctional combustion diagnostic system and a high-speed camera, with samples ignited in a 0.5 MPa argon atmosphere. The ignition performance was assessed using a CO2 laser ignition test system, including a water-cooled CO2 laser ignition device (Shanghai Yuhong Laser Equipment Co., Shanghai, China, 100 W) and a fiber-optical spectrometer (Avantes BV Co., AvaSpec2048, Apeldoorn, The Netherlands, integral time 3 ms). Finally, the condensed combustion products (CCPs) were analyzed using scanning electron microscope (SEM, ZEISS, Jena, Germany, EVO MA 10), laser particle size testing (Malvern Instruments Ltd., Malvern, UK, Mastersizer 2000), and X-ray diffraction (XRD, Bruker AXS, Karlsruhe, Germany, D8) to characterize their morphology, particle size distribution, and phase composition. The detailed parameter settings are presented in the Supporting Information.

3. Results and Discussions

3.1. Heat of Explosion and Pressurization Rate

To investigate the energy levels of HMX-based and CL-20-based binary and ternary systems, the heat of the explosion was measured and compared with that of the mechanical mixtures, as shown in Figure 1. For the co-TATBP10%/HMX co-particle system, BC-1 exhibited a heat of explosion of 5612.1 J g−1, which is 30.6 J g−1 (0.5%) and 109.1 J g−1 (2.0%) higher than BC-2 and BC-3, respectively. The addition of AP further enhanced the energy release, with the co-FOX-7T10%/HMX co-particle system showing the most significant improvement with an increase of 392 J g−1 (7.0%) compared to the binary system. This demonstrates that AP plays a crucial role in boosting the energy levels of these systems. Compared to mechanical mixture systems, BC-1, BC-2, and BC-3 exhibited increases in heat of explosion by 197.0 J g−1 (3.6%), 193.2 J g−1 (3.5%), and 140.5 J g−1 (2.6%), respectively. A similar trend was observed in the ternary systems, with TC-3 showing an increase of 368.9 J g−1 (6.7%) compared to TM-3 (5526.1 J g−1). These results clearly indicate that HMX-based systems with co-particles achieve higher heat of explosion values than mechanical mixtures, underscoring the advantages of the intimate structural design of co-particles in enhancing energy release.
The CL-20-based co-particle systems exhibited higher heat of explosion than the HMX-based systems due to CL-20’s higher energy content, which releases more heat during reactions and promotes the exothermic combustion of Al. For the co-TATBP10%/CL-20 co-particle system, BC-4 had a heat of explosion of 6589.5 J g−1, which is 74.5 J g−1 (1.1%) and 96.2 J g−1 (1.5%) higher than BC-5 and BC-6, respectively. Upon adding AP, the heat of explosion increased for all three systems, with the co-LLM-105P10%/CL-20 co-particle system showing the most significant improvement with an increase of 418.9 J g−1 (6.4%) compared to the binary system. This indicates that AP effectively enhances the reaction efficiency and energy release of the co-particle system. According to Figure 1b, the heat of explosion of binary systems BC-4, BC-5, and BC-6 increased by 585.3 J g−1 (9.7%), 217.2 J g−1 (3.4%), and 152.3 J g−1 (2.4%), respectively, compared to their mechanical mixtures. The ternary systems followed the same trend, with TC-6 showing an increase of 163.8 J g−1 compared to TM-6 (6730.2 J g−1). This further demonstrates that CL-20-based co-particles significantly enhance the heat of explosion of the systems.
The pressurization rate, defined as the rate of pressure change over time, reflects the rate of gas release during the reaction. Figure 1 shows the pressurization rates of HMX-based and CL-20-based co-particle systems. For binary systems containing co-TATBP10%/HMX, the pressurization rate of TC-1 (1.3 MPa s−1) is 0.5 MPa s−1 (62.5%) higher than that of BC-1 (0.8 MPa s−1). Similarly, the pressurization rates of TC-2 and TC-3 increased by 3.0 MPa s−1 (333.3%) and 5.4 MPa s−1 (300.0%), respectively, compared to BC-2 and BC-3. This indicates that the addition of AP significantly enhances the pressurization rate. Notably, CL-20-based co-particle systems exhibit significantly higher pressurization rates and heat of explosion compared to HMX-based co-particle systems. The pressurization rate of CL-20-based systems is an order of magnitude higher than that of HMX-based systems, attributed to CL-20’s higher energy density and faster reaction rate. For binary systems containing co-TATBP10%/CL-20, the pressurization rate of TC-4 (108.0 MPa s−1) is 24.1 MPa s−1 (28.7%) higher than that of BC-4. Similarly, the pressurization rates of TC-5 and TC-6 increased by 26.5 MPa s−1 (28.6%) and 57.6 MPa s−1 (58.4%), respectively, compared to BC-5 and BC-6.
The type of insensitive explosive in HMX-based co-particles also affects the pressurization rate. For binary systems, BC-3 has the highest pressurization rate (1.8 MPa s−1), which is 1.0 MPa s−1 and 0.9 MPa s−1 higher than BC-1 and BC-2, respectively. After adding AP, the ternary systems show a similar trend, with TC-3 having the highest pressurization rate of 7.2 MPa s−1. The order of effectiveness of different insensitive explosives in enhancing the pressurization rate is FOX-7 > LLM-105 > TATB. This is primarily related to their thermal decomposition properties and energy levels. Additionally, the pressurization rate of BC-3 (1.8 MPa s−1) is 0.3 MPa s−1 (20.0%) higher than that of BM-3, indicating that the co-particle structure is more effective than mechanical mixing in enhancing the reaction rate. For CL-20-based systems, the pressurization rate of BC-6 is 6.6 MPa s−1 (7.2%) higher than that of BM-6, and TC-6 is 41.8 MPa s−1 (36.5%) higher than TM-6. The same trend is observed in other systems. These findings collectively demonstrate that co-particles are more effective than mechanical mixtures in enhancing the pressurization rate of both HMX-based and CL-20-based systems.

3.2. Ignition Delay Time

The ignition delay time and spectral intensity are typical characteristic parameters for evaluating the ignition and combustion performance of co-particle systems. The spectral intensity directly characterizes the energy-release intensity and combustion intensity during the reaction. Figure 2 shows the laser ignition peak spectra of HMX-based co-particle systems and their corresponding mechanical mixtures. The co-particle components significantly influence the maximum radiation intensity. Specifically, the maximum radiation intensity of binary systems with HMX-based co-particles is notably higher than that of mechanical mixtures, indicating more intense reactions between HMX-based co-particles and Al. This enhanced reactivity can be attributed to the dense structure and high reactivity of the co-particles. The ternary HMX-based co-particle/Al/AP systems further validate this conclusion, as the presence of AP significantly enhances the spectral intensity of the ignition reaction due to its strong oxidizing properties, which promote the oxidation of Al. Similar trends are observed in CL-20-based systems, where the maximum radiation intensity of both binary and ternary systems is significantly higher than that of their mechanical mixtures, demonstrating that CL-20-based co-particles also enhance the reaction intensity of Al.
The ignition delay time serves as a key indicator of ignition reliability and reflects the efficiency of heat transfer between components. Figure 3 compares the ignition delay times of HMX-based co-particle systems and their mechanical mixtures. The co-particle systems exhibit shorter ignition delay times than the mechanical mixtures. For instance, the binary system (BC-1) containing co-TATBP10%/HMX co-particles has an ignition delay time of 9.5 ms, which is 2.5 ms (11.2%) and 13.5 ms (53.0%) shorter than that of BC-2 and BC-3, respectively. This indicates that co-TATBP10%/HMX co-particles are more effective in promoting ignition than co-LLM-105T10%/HMX and co-FOX-7T10%/HMX. Similarly, for CL-20-based systems, BC-5 containing co-LLM-105P10%/CL-20 co-particles has an ignition delay time of 19.2 ms, which is 6.1 (2.4%) and 3.5 (3.3%) shorter than that of BC-4 and BC-6, respectively.
The introduction of AP into HMX-based co-particle systems further reduces the ignition delay times of the ternary co-particle/Al/AP systems. For example, the ignition delay times of TC-1, TC-2, and TC-3 are 0.9 ms (9.5%), 1.5 ms (12.5%), and 4.7 ms (20.4%) shorter, respectively, than those of the binary systems. This reduction highlights the role of AP in improving ignition reliability by decomposing to produce oxidizing gases that promote rapid Al combustion. Similar observations are made for CL-20-based ternary systems, with TC-4, TC-5, and TC-6 showing ignition delay times of 19.3 ms, 11.8 ms, and 14.4 ms, respectively, which are 6.0 ms (23.7%), 7.4 ms (38.5%), and 7.3 ms (36.6%) shorter than those of the binary systems. As a strong oxidizer, AP decomposes to produce oxidizing gases that promote rapid Al combustion, thereby shortening ignition delay time. The system containing co-FOX-7T10%/HMX co-particles exhibits the most significant reduction in ignition delay time, indicating that AP has the most pronounced effect on this system. Overall, AP not only improves ignition reliability but also enhances the efficiency and concentration of energy release. The ternary systems consistently show significantly shorter ignition delay times compared to mechanical mixtures.

3.3. Combustion Performances

The flame-propagation rates of HMX-based and CL-20-based systems were measured using a multifunctional combustion diagnostic system, with the burning surface regression captured by a high-speed camera in a 0.5 MPa Ar atmosphere. The flame sequence images of the binary systems are shown in Figure 4 and Figure S4. Flame brightness is closely related to heat distribution and the release rate during combustion, with higher brightness indicating more intense combustion and concentrated heat release. In co-particle systems, combustion is influenced by both the efficiency of Al combustion and the decomposition characteristics of the co-particles. Upon heating, co-particles release heat and gaseous products, while Al particles melt, vaporize, and react with oxidizers to form oxides and nitrides.
It can be seen from Figure 4 that the burning surfaces of binary HMX-based co-particle systems exhibit uniform regression, indicating a stable, linear combustion process. This confirms the homogeneity of the samples and the stability of the reaction. The bright agglomerates observed above the burning surface are formed from molten Al particles. A higher number and larger size of these agglomerates typically indicate lower combustion efficiency [38]. In contrast, CL-20-based systems exhibit brighter, taller flames with a more beam-like structure and no significant agglomeration. This suggests that CL-20-based co-particles enhance combustion efficiency due to their higher energy density. Similar findings can be observed in ternary systems (Figure 5).
The flame brightness of binary co-particle systems (BC-X) is stronger than that of corresponding mechanical mixtures (BM-X), with higher flame heights and larger flame areas. This indicates that co-particles, which combine HMX or CL-20 with insensitive explosives, enhance the reactivity and energy-release rate compared to mechanical mixtures. Additionally, bright agglomerates observed in mechanical mixtures are absent in co-particle systems, further confirming higher combustion efficiency in the latter. The flame-propagation rates of CL-20-based systems, calculated from flame sequence images, are shown in Figure 6. Co-particle systems exhibit significantly higher flame-propagation rates than mechanical mixtures. For example, the flame-propagation rates of BC-1 and TC-1 are 1.4 mm s−1 (53.8%) and 1.1 mm s−1 (32.4%) higher, respectively, than those of BM-1 and TM-1. This highlights the significant improvement in propagation rate when insensitive explosives and nitramines are incorporated into co-particles, enhancing combustion performance and energy-release efficiency.
Among HMX-based co-particle systems, co-TATBP10%/HMX co-particles show the most significant improvement in flame-propagation rate. CL-20-based systems exhibit flame-propagation rates more than twice as high as those of HMX-based systems. For instance, BC-4 has a flame-propagation rate of 12.6 mm s−1, which is 3.8 times higher than that of BC-1 (2.6 mm s−1). This is attributed to CL-20’s higher energy density and faster energy-release rate. The co-LLM-105P10%/CL-20 co-particles and Al system (BC-5) achieve the highest propagation rate of 13.1 mm s−1, which is 2.5 mm s−1 higher than that of the mechanical mixture (BM-5). The addition of AP further enhances flame-propagation rates. For example, the flame-propagation rate of the binary system BC-1 (4.0 mm s−1) increases by 12.5% to 4.5 mm s−1 upon adding AP in the ternary system TC-1. Similarly, the flame-propagation rates of TC-4, TC-5, and TC-6 increase by 0.6 mm s−1 (4.7%), 0.7 mm s−1 (5.3%), and 1.6 mm s−1 (13.1%), respectively, compared to the binary systems BC-4, BC-5, and BC-6. This trend is consistent across both HMX- and CL-20-based systems.

3.4. Condensed Combustion Products (CCPs) Analyses

3.4.1. Morphology and Size Distribution

To further evaluate the combustion efficiency of the systems, the condensed combustion products (CCPs) of HMX-based and CL-20-based systems were collected and characterized, as shown in Figure 7, Figure 8, Figure 9 and Figure 10 and Figures S6–S9. The microscopic morphology and particle size distribution of the CCPs from binary and ternary systems are shown in Figure 7 and Figure 8, respectively. The CCPs primarily consist of Al combustion products with a small amount of unreacted Al. The CCPs exhibit a regular spherical morphology, consistent with Al oxides [39]. However, the particle size distribution varies among different systems, indicating that co-particles influence the combustion agglomeration of Al. Previous studies have shown that the sintering phenomena in the combustion residues of Al/HMX composite samples are significantly improved compared to mechanical mixtures, leading to enhanced combustion efficiency [20]. This study further demonstrates that the introduction of co-particle structures can improve reaction efficiency, resulting in the formation of highly spherical Al oxides in the combustion products.
As shown in Figure S6 in the Supporting Information, the average particle size (D50) of CCPs from co-particle samples is smaller than that of corresponding mechanical mixtures, suggesting that co-particles promote complete reactions with Al and reduce agglomeration. For example, BC-2 has fewer agglomerates with an average particle size of 0.60 μm, indicating more complete combustion of Al particles in the presence of co-LLM-105P10%/HMX co-particles. In contrast, BM-2 has a larger average particle size of 3.17 μm (4.3 times larger than BC-2), indicating lower combustion efficiency of Al particles in mechanical mixtures.
Co-particles exhibit higher reactivity than mechanical mixtures. When subjected to thermal stimulation, they rapidly release heat and gaseous products. This facilitates the ignition and combustion of Al particles, enhancing overall combustion efficiency. The rapid decomposition and micro-explosion effects of nitramine systems also help inhibit Al agglomeration [40]. In contrast, mechanical mixtures have limited inter-component contact, resulting in lower reaction efficiency and larger CCPs.
The type of co-particles has a minor effect on the average particle size of combustion products. For example, the average particle sizes of CCPs from BC-4, BC-5, and BC-6 are 0.38 μm, 0.31 μm, and 0.58 μm, respectively. The average particle sizes of CCPs from ternary systems (TC-1, TC-2, and TC-3) are 0.33 μm, 0.53 μm, and 0.59 μm, respectively, all smaller than those of mechanical mixtures. This confirms the enhanced combustion efficiency of Al in co-particle systems. Additionally, the D50 values of combustion products from ternary systems are smaller than those from binary systems. For example, the D50 values of TC-1, TC-2, and TC-3 are 0.45 μm (57.7%), 0.07 μm (11.7%), and 0.03 μm (4.8%) smaller than those of BC-1, BC-2, and BC-3, respectively. The D90 values of CCPs from TC-1, TC-2, and TC-3 are also significantly reduced compared to corresponding mixtures.
This suggests that the reaction efficiency varies among HMX-based co-particles containing different insensitive explosives and AP/Al. Co-LLM-105T10%/HMX appears to synergize with AP, promoting a more uniform reaction and smaller CCPs. In contrast, ternary mixtures with insufficient component contact and uneven distribution of HMX and insensitive components exhibit localized incomplete combustion, resulting in larger CCPs. CL-20-based systems show similar trends to HMX-based systems.
The smaller particle sizes observed in co-particle systems, compared to mechanical mixtures, indicate more complete combustion and reduced agglomeration of Al particles. The underlying mechanisms can be attributed to several key factors. The close interfacial interaction between nitramines (HMX or CL-20) and insensitive explosives (TATB, LLM-105, or FOX-7) in co-particles facilitates rapid heat release and gas generation upon ignition. This accelerates the melting, vaporization, and oxidation of Al particles, resulting in smaller CCPs and a higher Al2O3 content. At the same time, the rapid decomposition of nitramines within co-particles generates localized micro-explosions, which effectively disrupt the agglomeration of Al particles and enhance their dispersion. This leads to more uniform combustion and finer CCPs.

3.4.2. Phase Composition

The phase composition of the CCPs from different systems was characterized using XRD, with results shown in Figure 9 and Figure 10. The CCPs from binary co-particle-based systems generally consist of Al, Al2O3, and AlN. The formation of AlN, which occurs at high temperatures through the reaction of carbon with Al2O3 and N2, reflects the system’s reaction intensity and energy-release characteristics. To assess the combustion efficiency of Al particles, the characteristic diffraction peaks of Al and Al2O3 in the combustion products were compared with standard reference cards (Al, PDF#04-0787; Al2O3, PDF#50-0741). The intensity of the Al2O3 peaks in the CCPs of binary systems is significantly higher than that of Al and also exceeds the Al2O3 peaks in mechanical mixtures. This indicates that most Al particles in these systems are oxidized to Al2O3, demonstrating that co-particles enhance Al combustion efficiency. Similar observations were made in CL-20-based systems, where co-particles also improve Al combustion efficiency, consistent with the findings for HMX-based systems.
In ternary HMX-based co-particle systems, the CCPs primarily consist of Al, AlN, and Al2O3. The intensity of the Al2O3 peaks in co-particle systems is significantly higher than in mechanical mixtures, further confirming that the co-particle structure enhances Al combustion efficiency. The phase composition of CCPs in both binary and ternary systems was quantitatively analyzed using XRD, with a focus on the relative contents of Al2O3, AlN, and Al, as summarized in Table 2. The results indicate that Al2O3 dominates the combustion products in both systems, but its content is generally higher in the ternary system compared to the binary system. Specifically, in the binary system, the Al2O3 content in the co-particle-based systems is significantly higher than that in the mixtures. This suggests that the unique structural characteristics of the co-particle-based systems promote more efficient and complete oxidation of Al.
In the ternary system, the Al2O3 content is further increased. This demonstrates that the addition of AP in the ternary system further optimizes the combustion reaction, particularly enhancing the oxidation efficiency of Al in the co-particle-based systems. This is attributed to two factors: (1) the co-particle structure’s faster thermal decomposition rate, which enables rapid energy release and promotes Al combustion; (2) the uniform reaction environment provided by co-particles, facilitating full contact and reaction between Al particles and oxidizers. This leads to more concentrated energy release and improved overall combustion efficiency. These findings demonstrate that co-particles significantly enhance combustion efficiency and energy release, providing a foundation for more efficient reactions.

4. Conclusions

In this study, CL-20- and HMX-based co-particles containing 10% insensitive explosives were prepared and mixed with AP and Al to prepare co-particle/Al binary and co-particle/Al/AP ternary systems. The effects of co-particles on the pressurization rate, heat of explosion, ignition delay, flame structure, flame propagation rate, and condensed combustion products (CCPs) of these systems were investigated. The results are summarized as follows:
(1) The uniform and intimate structure of co-particles enhances the energy release in binary systems, resulting in pressurization rates one to two times higher and heat of explosion 2–10% higher than those of mechanical mixtures. The addition of AP further improves energy release in ternary systems.
(2) Co-particles improve ignition performance in both binary and ternary systems, characterized by shorter ignition delays and higher radiation intensity compared to mechanical mixtures. The introduction of AP reduces ignition delay by 10–20% in ternary systems relative to binary systems, highlighting its role in accelerating the reaction rate.
(3) Systems containing co-particles exhibit significantly higher flame-propagation rates than mechanical mixtures. CL-20-based co-particles are more effective than HMX-based co-particles in enhancing combustion, resulting in taller and brighter flames in both binary and ternary systems.
(4) The high reactivity of co-particles improves the combustion efficiency of Al, as evidenced by reduced CCP particle sizes and increased Al2O3 content. This indicates more complete oxidation of Al particles in co-particle systems compared to mechanical mixtures.
While the co-particle systems demonstrated in this study exhibit significant improvements in combustion performance and energy release, several challenges must be addressed to enable their practical application in solid propellants. These include ensuring large-scale production consistency, cost considerations, and achieving compatibility with existing propellant formulations. The current synthesis methods for co-particles are primarily laboratory-scale. Transitioning to industrial-scale production will require optimization of the spray-drying and interfacial coating processes to ensure consistent quality and performance. At the same time, further verification is needed to evaluate the long-term stability and reliability of co-particles in real-world applications, particularly regarding potential changes in their physical and chemical properties during storage.
Nevertheless, the enhanced combustion efficiency and energy release achieved by co-particle systems make them highly promising for use in solid rocket propellants, especially in next-generation aerospace propulsion systems where high performance and reliability are paramount. Furthermore, the co-particle approach holds potential for extension to other energetic material systems, such as MICs, broadening its applicability. Addressing these challenges through future research will be crucial for unlocking the full potential of co-particle systems in practical applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/aerospace12040272/s1, Section S1. Preparation and Characterization Methods. Section S2. Theoretical Calculation and Formulation Design. Section S3. Flame structure. Section S4. Condensed combustion products analysis.

Author Contributions

Conceptualization, Z.X. and Q.Y.; methodology, Z.X.; software, W.Z.; formal analysis, Z.X.; investigation, R.X.; resources, Q.Y.; data curation, Z.X. and R.X.; writing—original draft preparation, Z.X.; writing—review and editing, S.Y. and Q.Y.; funding acquisition, Z.X. and Q.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This paper was funded by NSAF project (U2030202) and Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (CX2023033).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. The pressurization rate and heat of explosion for the binary and ternary systems: (a) HMX-based co-particles/Al, (b) HMX-based co-particles/Al/AP, (c) CL-20-based co-particles/Al, (d) CL-20-based co-particles/Al/AP.
Figure 1. The pressurization rate and heat of explosion for the binary and ternary systems: (a) HMX-based co-particles/Al, (b) HMX-based co-particles/Al/AP, (c) CL-20-based co-particles/Al, (d) CL-20-based co-particles/Al/AP.
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Figure 2. The flame spectra with maximum intensity of binary and ternary systems: (a) HMX-based co-particles/Al, (b) HMX-based co-particles/Al/AP, (c) CL-20-based co-particles/Al, (d) CL-20-based co-particles/Al/AP.
Figure 2. The flame spectra with maximum intensity of binary and ternary systems: (a) HMX-based co-particles/Al, (b) HMX-based co-particles/Al/AP, (c) CL-20-based co-particles/Al, (d) CL-20-based co-particles/Al/AP.
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Figure 3. The ignition delay time for the binary and ternary systems: (a) HMX-based co-particles/Al, (b) HMX-based co-particles/Al/AP, (c) CL-20-based co-particles/Al, (d) CL-20-based co-particles/Al/AP.
Figure 3. The ignition delay time for the binary and ternary systems: (a) HMX-based co-particles/Al, (b) HMX-based co-particles/Al/AP, (c) CL-20-based co-particles/Al, (d) CL-20-based co-particles/Al/AP.
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Figure 4. The flame structure of the binary systems (co-particle/Al) in 0.5 MPa Ar atmosphere.
Figure 4. The flame structure of the binary systems (co-particle/Al) in 0.5 MPa Ar atmosphere.
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Figure 5. The flame structure of the ternary systems (co-particle/Al/AP) in 0.5 MPa Ar atmosphere.
Figure 5. The flame structure of the ternary systems (co-particle/Al/AP) in 0.5 MPa Ar atmosphere.
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Figure 6. The flame-propagation rate of (a1,a2) the binary (co-particle/Al) and (b1,b2) ternary systems (co-particle/Al/AP) in 0.5 MPa Ar atmosphere.
Figure 6. The flame-propagation rate of (a1,a2) the binary (co-particle/Al) and (b1,b2) ternary systems (co-particle/Al/AP) in 0.5 MPa Ar atmosphere.
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Figure 7. The morphology and particle size distribution of the binary systems (co-particle/Al): (a1,a2) BC-1, (b1,b2) BC-2, (c1,c2) BC-3, (d1,d2) BC-4, (e1,e2) BC-5, (f1,f2) BC-6.
Figure 7. The morphology and particle size distribution of the binary systems (co-particle/Al): (a1,a2) BC-1, (b1,b2) BC-2, (c1,c2) BC-3, (d1,d2) BC-4, (e1,e2) BC-5, (f1,f2) BC-6.
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Figure 8. The morphology and particle size distribution of the ternary systems (co-particle/Al/AP): (a1,a2) TC-1, (b1,b2) TC-2, (c1,c2) TC-3, (d1,d2) TC-4, (e1,e2) TC-5, (f1,f2) TC-6.
Figure 8. The morphology and particle size distribution of the ternary systems (co-particle/Al/AP): (a1,a2) TC-1, (b1,b2) TC-2, (c1,c2) TC-3, (d1,d2) TC-4, (e1,e2) TC-5, (f1,f2) TC-6.
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Figure 9. The X-ray-diffraction patterns of the CCPs for binary systems (co-particle/Al).
Figure 9. The X-ray-diffraction patterns of the CCPs for binary systems (co-particle/Al).
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Figure 10. The X-ray diffraction patterns of the CCPs for ternary system (co-particle/Al/AP).
Figure 10. The X-ray diffraction patterns of the CCPs for ternary system (co-particle/Al/AP).
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Table 1. Components and names of binary and ternary systems.
Table 1. Components and names of binary and ternary systems.
SystemsNamesCo-Particles/wt.%Al/wt.%AP/wt.%Nomenclatures
binaryBC-18515-co-TATBP10%/CL-20/Al
BC-28515-co-LLM-105P10%/CL-20
BC-38515-co-FOX-7P10%/CL-20
BC-48515-co-TATBP10%/HMX
BC-58515-co-LLM-105T10%/HMX
BC-68515-co-FOX-7T10%/HMX
ternaryTC-175205co-TATBP10%/CL-20
TC-275205co-LLM-105P10%/CL-20
TC-375205co-FOX-7P10%/CL-20
TC-475205co-TATBP10%/HMX
TC-575205co-LLM-105T10%/HMX
TC-675205co-FOX-7T10%/HMX
Table 2. The relative contents of Al2O3, AlN, and Al in combustion products of ternary system.
Table 2. The relative contents of Al2O3, AlN, and Al in combustion products of ternary system.
Binary SystemsTernary System
SamplesAl2O3AlNAlSamplesAl2O3AlNAl
BC-186.77%5.82%7.41%TC-188.53%5.84%5.63%
BM-181.14%9.75%9.11%TM-182.24%5.10%12.65%
BC-288.64%5.19%6.17%TC-290.73%2.02%7.26%
BM-284.14%5.29%10.57%TM-284.29%4.39%11.32%
BC-389.28%6.09%4.64%TC-392.49%3.22%4.29%
BM-384.73%5.53%9.73%TM-387.43%3.84%8.73%
BC-487.36%2.87%9.77%TC-490.02%3.84%6.14%
BM-486.00%6.71%7.29%TM-484.00%7.64%8.36%
BC-589.58%3.19%7.22%TC-590.78%4.47%4.75%
BM-586.56%6.46%6.98%TM-586.38%7.17%6.44%
BC-690.39%0.39%9.21%TC-692.53%3.03%4.44%
BM-687.29%4.24%8.47%TM-688.76%3.61%7.63%
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Xue, Z.; Zhang, W.; Xu, R.; Yang, S.; Yan, Q. The Effects of HMX and CL-20-Based Co-Particles on the Ignition and Combustion Performances of Aluminum Powders. Aerospace 2025, 12, 272. https://doi.org/10.3390/aerospace12040272

AMA Style

Xue Z, Zhang W, Xu R, Yang S, Yan Q. The Effects of HMX and CL-20-Based Co-Particles on the Ignition and Combustion Performances of Aluminum Powders. Aerospace. 2025; 12(4):272. https://doi.org/10.3390/aerospace12040272

Chicago/Turabian Style

Xue, Zhihua, Weimeng Zhang, Ruixuan Xu, Sulan Yang, and Qilong Yan. 2025. "The Effects of HMX and CL-20-Based Co-Particles on the Ignition and Combustion Performances of Aluminum Powders" Aerospace 12, no. 4: 272. https://doi.org/10.3390/aerospace12040272

APA Style

Xue, Z., Zhang, W., Xu, R., Yang, S., & Yan, Q. (2025). The Effects of HMX and CL-20-Based Co-Particles on the Ignition and Combustion Performances of Aluminum Powders. Aerospace, 12(4), 272. https://doi.org/10.3390/aerospace12040272

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