The Modulatory Effect of Inhibitors on the Thermal Decomposition Performance of Graded Al@AP Composites

Abstract

In this paper, a series of graded Al-based composites, including Al@AP, Al@AP/BM−52, and Al@AP/BPE−1735, have been prepared by spray drying technology. The thermal decomposition characteristics, kinetic parameters of the decomposition reaction, and Pyro-GC/MS products were comprehensively investigated. The results showed that two inhibitors, BM−52 and BPE−1735, had a significant effect on the thermal decomposition of AP. The addition of BM−52 conspicuously enhanced the thermal interaction, resulting in a more complete decomposition reaction of AP. Meanwhile, the incorporation of BPE−1735 significantly enhanced the heat releases of AP, leading to a significant enhancement in the energetic performance during the decomposition process of AP. BM−52 and BPE1735 inhibit AP decomposition as evidenced by higher activation energies for thermal decomposition and altered physical models of decomposition. Pyro-GC/MS results reveal that the fundamental pathway of Al@AP thermal decomposition remains unaltered by BM−52. However, the proportion of oxygen-containing compound products is moderately reduced. In contrast, for Al@AP/BPE−1735, in addition to the same products as those from Al@AP pyrolysis, new pyrolysis peaks emerge. It is implied that specific chemical reactions or interactions are triggered during the thermal decomposition process, thereby resulting in the formation of distinct chemical species.

1. Introduction

Solid propellants have been extensively utilized in rocket propulsion within the aerospace and military fields, serving as one of the principal energy sources [1]. Ammonium perchlorate (AP), which is among the most prevalently used inorganic oxidizers in contemporary solid propellants, exhibits thermal decomposition characteristics that directly influence the combustion performance of solid propellants [2]. Although AP-based propellants typically possess relatively high burning rate characteristics, such characteristics can be effective in providing sufficient power to the engine to ensure efficient operation in a wide range of routine missions. However, in certain specific application scenarios entailing special requirements, exemplified by the engine design of cruise missiles, an overly high burning rate can transform into an adverse circumstance. These weapons require their propellants to have a low rate of ignition in order to achieve long-term, sustained thrust to achieve long-range attack operational objectives. In such circumstances, a lower burning rate level is urgently needed for propellants [3,4].

For the purpose of flexibly and precisely modulating the burning rate of solid propellants to optimally fulfill the special requirements of diverse application scenarios, researchers customarily incorporate burning-rate modifiers into solid propellants. These modifiers are categorized into catalysts and inhibitors. Among them, the inhibitors assume an extremely crucial function in the solid propellant system. The central objectives of inhibitors are to effectively impede the thermal decomposition process of the oxidant in the propellant. During the combustion process of solid propellants, the thermal decomposition of AP represents the pivotal initiating step that instigates the entire combustion reaction [5]. The inhibitors are capable of, by virtue of their unique chemical structure and properties, intervening in the thermal decomposition reaction pathway of AP. It participates in interactions with AP or the intermediate products decomposed during its decomposition process, modifies the activation energy of the reaction, impacts the rate constant of the reaction, or interferes with the direction of the reaction, thereby leading to a significant reduction in the thermal decomposition rate of AP.

Amide-based compounds constitute common and effective inhibitors in practical applications. They encompass inorganic ammonium salts such as phosphorous salts [6] and ammonium oxalate [7,8], as well as organic ammonium salts such as urea and melamine [9,10,11]. Thermal analysis results show that the decomposition process of amide-based compounds is preceded by that of AP and is accompanied by a substantial endothermic effect. The gaseous products (such as NH₃) generated from them can inhibit the decomposition of AP [9,10,11]. Cyclic azines are acknowledged as a distinctive category of organic amines. Upon heating, in addition to the prevalent low-molecular-weight gases such as NH₃, CO₂, HCN, and N₂O, they are also capable of producing large thermally stable melamine and melon [12]. The heat and mass transfer efficiency are reduced, and NH₃ is released during the cyclization process, whereby a simultaneous reduction in the burning rate and pressure index of the propellant is achieved. A role as inhibitors for AP can be fulfilled by certain metal salts, such as LiF, SrCO₃, and CaCO₃. When 2–3% of these metals are added, the burning rates are, respectively, reduced by 14.0%, 29.1%, and 28.3% under a pressure of 7 MPa [8,13,14]. Moreover, higher working pressures of up to 14 MPa can also be accommodated. Nevertheless, a series of problems can be caused by the addition of metal salts, including increased hygroscopicity, deteriorated mechanical properties, and the production of additional solid metal oxide particles during the combustion process, which will aggravate two-phase flow losses and nozzle erosion [15], etc. The application and in-depth research of these in a broader range of fields have been limited by these problems. Quaternary ammonium pyrolysis salts are recognized as a kind of highly efficient inert inhibitors. Both halide and sulfite quaternary ammonium salts possess the capacity to reduce the burning rate of propellants. Moreover, a better deceleration effect can be achieved by adding only 0.3% of quaternary ammonium salts than adding 1.5% of oxamide [16]. Although the added amount of quaternary ammonium salts is small, the most remarkable effect is produced by it. The specific impulse of the propellant is neither reduced by it, nor is the curing reaction interfered with, and the physical properties of the propellant are neither by it. Quaternary ammonium salts can form hydrogen bonds with N and O on the surface of AP, respectively, thereby inhibiting the combustion decomposition of AP [17]. It is precisely due to the fact that quaternary ammonium salts can adsorb on the surface of AP that they are regarded as the most effective combustion inhibitors.

Research on quaternary ammonium salts serving as inhibitors in HTPB propellants has already been conducted [6,8,18,19]. However, in those propellant systems, the Al powder typically has a single size. The monodisperse Al powder comes with numerous drawbacks. The dense Al₂O₃ layer on its surface can hinder the internal Al core from participating in the combustion reaction. As a result, some Al powder above the combustion surface of the propellant is easily subject to the influence of gravity and surface tension, liable to accumulation and aggregation phenomena, and further forming large-sized agglomerates. This not only leads to a reduction in the combustion efficiency of the Al powder but also exerts a negative impact on the overall combustion stability of the propellant [20,21,22].

In contrast, the employment of a combination of nano-sized and micro-sized Al powders can effectively improve this situation. Owing to its minute size, the nano-sized Al powder is capable of direct and rapid combustion without undergoing melting [23]. It can be promptly oxidized on the combustion surface, and the resultant oxide assumes a misty state and migrates along with the combustion gas subsequent to attaining thermal equilibrium therewith [20]. The micro-sized Al powder, on the other hand, can supplement the energy release during the overall combustion process to a certain extent. The two types of powders interact and cooperate with one another, which is conducive to enhancing the combustion stability, diminishing the occurrence of agglomeration phenomena, and thereby optimizing the combustion performance of the propellants [24].

Therefore, in this paper, the effects of two novel rate reducers, quaternary ammonium derivatives (BM−52) and adamantane derivatives (BPE−1735), on the thermal decomposition of AP in graded Al@AP composites are thoroughly investigated. In line with the principle of maximum packing density, the grading ratio of n-Al powder to μ-Al powder was determined. Subsequently, two types of Al-based composites, including Al@AP/BM−52 and Al@AP/BPE−1735, were successfully prepared using the spray drying technique. The synchronous differential scanning calorimeter and thermogravimetric (DSC/TG) apparatus were used to characterize the thermal decomposition behavior of Al@AP in the presence of BM−52 and BPE−1735, and the corresponding decomposition kinetic parameters were determined. Additionally, the pyrolysis products of the composites were comprehensively analyzed by means of pyrolysis-gas chromatography-mass spectrometry.

2. Experiment

2.1. Materials

Micron-sized aluminum powder (µ-Al, 25 µm) and nano-sized aluminum powder (n-Al, 200 nm) were purchased by Shanghai Lianghan Nano Technology Development Co., Ltd. (Shanghai, China) Dopamine hydrochloride (DOPA-HCl, 98%) and Tris (hydroxymethyl) aminomethane (Tris, 99%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Deionized water (DI) was provided by Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). Ammonium perchlorate (AP, Class III) was sourced from Xi’an Modern Chemistry Research Institute. BM−52 (a new type of perchlorate quaternary ammonium salt) and BPE−1735 (an adamantane derivative) were provided by the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences (Shanghai, China).

2.2. Preparation Methods

Owing to the highly hydrophobic and low adsorptive characteristics of the Al powder surface, it is difficult for the oxidant solution to adhere to it and precipitate a dense crystallization. Hence, it is initially essential to perform a surface wetting modification treatment on the nano-Al and micro-Al powders with polydopamine (PDA), thereby improving their surface functionality and suspension dispersion ability in the solution. Regarding the specific experimental procedures, please refer to our previous paper [25].

Based on the theory of maximum packing density, the mass ratio of nano-Al powder to micro-Al powder was determined (refer to Supporting Information for details). In accordance with this ratio, the graded Al powder was obtained by mixing the Al powders. Subsequently, 5 g of AP was dissolved in 50 mL of deionized water and magnetically stirred at room temperature until the complete dissolution of AP was achieved. Subsequently, the graded Al was added to the AP solution. The magnetic stirring was sustained for 0.5 h at room temperature to formulate a homogeneous suspension. Thereafter, the suspension was subjected to spray drying technology, which facilitated the rapid crystallization of AP on the surface of the graded Al powder, thereby preparing the graded Al@AP composite with a core–shell structure. For Al@AP/BM−52 and Al@AP/BPE−1735 composites, merely 0.25 g of BM−52 or BPE−1735 was required to be incorporated into the prepared suspension. Upon magnetic stirring until a state of complete and uniform dispersion was attained, the spray drying technology could be used to prepare the composites. The parameters of the spray drying technology were set as follows: the diameter of the feed inlet was 1 mm, the fluid peristaltic rate was 8 mL·min⁻¹, and the inlet air temperature was maintained at 110 °C. Finally, the composite was collected within a glass container by means of a cyclone separator. The preparation process is shown schematically in Figure 1.

2.3. Characterization Technique

The morphologies and elemental distributions of the raw materials and the prepared composites were analyzed through the utilization of scanning electron microscopy (FESEM, Hitachi 4800S, Tokyo, Japan) coupled with energy dispersive spectroscopy. The thermal decomposition characteristics of the composites were evaluated with differential scanning calorimetry and thermal gravimetric (DSC/TG, STA449-F5, Netzsch, Germany) analyses under the 50 mL·min⁻¹ Ar atmosphere. The temperature range was set from 50 to 500 °C, and the heating rates were set at 5, 10, 15, and 20 °C·min⁻¹. The kinetic parameters were determined based on the DSC data [26,27,28,29]. Specifically, the kinetic parameters were calculated using the Kissinger method; the dependence of kinetic parameters activation energy (E_a) on the conversion rate was determined by employing the Friedman method [30]. Moreover, the decomposition physical model of AP was established via the combined kinetic method [31]. Pyrolysis gas chromatography mass spectrometry (Pyro-GC/MS, GC1290, Shanghai Shunyu Hengping Scientific Instrument Co., Ltd., Shanghai, China) was employed to identify the pyrolysis products decomposed from the composites upon reaching a temperature of 500 °C (refer to Supporting Information for details) [32]. In order to reduce the potential systematic errors in the experimental process and to ensure the reproducibility of the data, three parallel experiments were used in this study, and the arithmetic mean of the experimental results was taken as the final data.

3. Results and Discussions

3.1. Morphology and Composition Characterization

The SEM images of the raw materials and the prepared composites are presented in Figure 2, Figures S1 and S2. Figure 2a and Figure S1 show the distinct surface morphology characteristics of 25 μm and 200 nm spherical Al powders, respectively. The particles display a relatively regular spherical geometry, possess a relatively smooth surface, and exhibit favorable uniformity in particle size. Figure 2b reveals that the AP particle presents a typical oval structure. The ratio of the major axis to the minor axis of the particles is appropriate, and the powder exhibits favorable dispersion with no significant agglomeration. The oval outlines of individual particles are distinctly recognizable. Figure S2a,b and Figure 2c are the surface morphological of graded Al@AP, Al@AP/BM−52, and Al@AP/BPE−1735. It is evidently observable from the figure that these three composites generally present a spherical structure. Their morphologies are not only quite uniform but also rather regular, with the particle size distribution spanning from 200 nm to 30 μm. Based on the EDS elemental mapping images of Al@AP/BPE−1735 presented in Figure 2(c₁–c₄), it can be clearly seen that the n-Al particles are tightly attached to the surface of the μ-Al particles, with the entire structure being uniformly encapsulated by AP. The genesis of this distinctive structure will remarkably influence various properties of the composites, particularly with respect to thermal reaction rate.

3.2. Thermal Decomposition Behavior of AP

The TG-DSC technique was used to conduct the thermal analysis of AP, Al@AP, Al@AP/BM–52, and Al@AP/BPE–1735 composites at a heating rate of 10 °C·min⁻¹. The corresponding curves are presented in Figure 3. The TG/DTG and DSC parameters are respectively shown in

Table 1. The exothermic peak parameters of the TG-DTG curves for the studied composite.

Sample	TG				DTG
	Ti (°C)	To (°C)	Te (°C)	Mass Loss (%)	Tp (°C)	Lmax (%·min−1)
AP-1st	289.4	295.8	303.6	9.54	295.8	15.06
AP-2nd	392.5	421.7	430.9	81.51	421.7	18.90
Al@AP	376.4	408.8	416.4	49.08	408.8	13.87
Al@AP/BM−52-1st	258.5	260.6	298.1	9.93	293.0	1.74
Al@AP/BM−52-2nd	374.7	405.7	411.6	54.89	405.7	12.53
Al@AP/BPE−1735-1st	267.4	269.0	297.2	4.54	293.5	1.99
Al@AP/BPE−1735-2nd	366.0	395.4	404.0	50.79	395.4	12.31

Notes: T_i (°C), the initial thermal decomposition temperature. T_o (°C), the onset temperature for thermal decomposition. T_e (°C), the end of the decomposition temperature. Mass loss (%), the mass loss of thermal decomposition. T_p (°C), the peak temperature of maximum mass loss. L_max (%·min⁻¹), the maximum mass loss rate. First and second mean different decomposition steps.

and

Table 2. The parameters of the exothermic peak in DSC curves for the studied composite.

Sample	Ti (°C)	Tp (°C)	Te (°C)	Width (°C)	∆H (J·g−1)
AP-1st	287.6	298.9	312.6	21.0	323.0
AP-2nd	364.4	394.0	431.3	65.8	502.0
Al@AP	364.2	389.0	426.2	47.8	365.4
Al@AP/BM−52-1st	279.7	297.7	319.1	29.0	184.4
Al@AP/BM−52-2nd	267.4	388.6	410.6	50.2	333.5
Al@AP/BPE−1735-1st	257.6	276.3	300.2	35.1	252.5
Al@AP/BPE−1735-2nd	348.3	381.1	401.1	47.9	461.5

Notes: T_i (°C), the initial temperature of thermal decomposition. T_p (°C), the peak temperature of thermal decomposition. T_e (°C), the end temperature for heat change. Width (°C), peak width. ∆H (J·g⁻¹), heat release.

.

The TG/TDG curves of the complexes are shown in Figure 3a, where the dashed line is the TG curve of the complexes and the solid line is the DTG curve of the complexes. Analysis of the curves reveals that pure AP undergoes a discontinuous two-step mass loss process. These two steps of mass loss correspond to the low-temperature decomposition (LTD) and high-temperature decomposition (HTD) of AP. The peak temperatures of those two stages are 295 °C and 422 °C, respectively, and the maximum weight loss rates are 15.1%·min⁻¹ and 18.9%·min⁻¹, with mass losses of 9.5% and 81.5%, and a total weight loss rate of 91.1%. The graded Al powder has a significant impact on the thermal decomposition of AP. The LTD of AP is principally a dissociation and sublimation process. In this process, the dissociation and sublimation are decelerated due to the adsorption and confinement effects of nano-Al, which causes the dissociation equilibrium to shift towards the left. Consequently, a retardation in the temperature of the LTD peak of AP is exhibited [33]. The HTD of AP is a redox process between NH⁴⁺ and ClO⁴⁻. The spatial confinement of nano-Al powder and its reaction with strongly oxidizing gases have a promoting effect on the high-temperature process, shifting the HTD of AP to a lower temperature range [34]. This dual effect causes the typical two-step thermal decomposition behavior of AP to transform into a single step with the peak decomposition temperature of 408 °C and a mass loss of 49.1%.

With the introduction of BM−52 and BPE−1735, the decomposition reaction of AP is significantly enhanced. Specifically, the peak temperature in the low-temperature reaction and the low-temperature reaction in the DTG curve of AP are, respectively, 260.6 °C and 405.7 °C under the effect of BM−52. The maximum weight loss rate in the high-temperature reaction process is reduced by 1.34%·min⁻¹. While, after the addition of BPE−1735, the peak temperature of the DTG during the LTD weight loss process of AP is 267.4 °C. Moreover, with the addition of BPE−1735, the peak temperature is advanced to 395.4 °C in the HTD reaction of the DTG curve with the introduction of BPE−1735, accompanied by a reduction in the maximum high-temperature weight loss rate by 1.56%·min⁻¹. Apparently, both of those two catalysts contribute to an elevation in peak temperature during the HTD weight loss process of AP. Concurrently, the decomposition of AP is rendered more moderate as a consequence of the reduction in the maximum weight loss rate. The mass losses corresponding to the two-step decomposition of Al@AP/BM−52 are 9.93% and 54.89%, respectively. While those of Al@AP/BPE-173 are 4.54% and 50.79%, respectively. In comparison with Al@AP, the increments in the mass losses are, respectively, 32.07% and 12.73%. These results suggest that BM−52 exhibits a more pronounced thermal interaction with the thermal decomposition reaction of AP.

From the DSC curves (Figure 3b), it can be seen that a heat absorption peak is observed in all samples at about 250 °C, at which time the decomposition of the complexes is dominated by the thermal decomposition of AP, which corresponds to the transformation of AP from rhombohedral to cubic crystalline form. Subsequently, the exothermic decomposition reaction of AP begins [35]. For pure AP, two exothermic peaks at low and high temperatures appear at 299 °C and 395 °C, respectively, with heat releases of 323 J·g⁻¹ and 502 J·g⁻¹, respectively. These correspond to the dissociation of amino groups and hypochlorite in AP molecules, followed by the further decomposition of AP, which generates gaseous products.

For the Al@AP composite, in the presence of graded Al powder, the LTD and HTD processes of AP merge into one step, presenting a decomposition peak temperature of 389 °C and a heat release of 365.4 J·g⁻¹. Additionally, due to the interfacial effect between Al and AP, the trailing peak of the AP curve vanishes. Meanwhile, with a 26% narrowing in peak width, a more intensified and concentrated heat release is achieved. This phenomenon can be attributed to the enhanced interaction and energy transfer at the interface between Al and AP [36]. The narrowed peak width indicates a more rapid and efficient decomposition process, leading to a more pronounced and focused heat release.

When BM−52 and BPE−1735 are added, the decomposition process of AP changes from a one-step to a two-step decomposition. For Al@AP/BM−52 composite, two distinct exothermic reaction stages can be observed during the decomposition of AP, with the peak temperatures of the exothermic reactions being 297.7 °C and 388.6 °C, respectively. The total heat release amounts to 517.8 J·g⁻¹, which is 41.7% higher than that of Al@AP. In the case of the Al@AP/BPE−1735 composite, the peak temperatures of the two exothermic reactions during AP decomposition are 276.3 °C and 381.1 °C, respectively. The total heat release is 714.0 J·g⁻¹, which indicates a 95.4% increase compared to the total heat release of Al@AP. In summary, the introduction of BM−52 and BPE−1735 has a remarkable effect on enhancing the reaction heat release of AP. They can not only effectively accelerate the decomposition process of AP but also play a crucial role in improving the energy performance of AP. Based on the above findings regarding the thermal decomposition characteristics, it is essential to further investigate the decomposition kinetics of AP under the influence of BM−52 and BPE−1735 to gain a more in-depth understanding of the reaction mechanisms.

3.3. Non-Isothermal Dynamics Analysis of the Thermal Decomposition of AP

The thermal decomposition behaviors of Al@AP, Al@AP/BM−52, and Al@AP/BPE−1735 were systematically investigated via DSC under various heating rates, including 5, 10, 15, and 20 °C·min⁻¹. The respective DSC curves are depicted in Figure 4 and Figure S3.

The kinetic parameters of AP thermal decomposition were determined by means of the Kissinger method, and the corresponding results are summarized in

Table 3. The parameter of Al@AP, Al@AP/BM-52 and Al@AP/BPE-1735 calculated by the Kissinger method.

Sample	Ea/kJ mol−1	ln A/s−1	r
AP-1st	116.7	11.26	0.9981
AP-2nd	152.0	14.68	0.9934
AP-3rd	165.8	14.90	0.9944
Al@AP-1st	191.4	24.42	0.9804
Al@AP-2nd	178.7	19.64	0.9902
Al@AP-3rd	146.7	12.38	0.9804
Al@AP/BM−52-1st	131.7	14.59	0.9999
Al@AP/BM−52-2nd	193.9	22.11	0.9920
Al@AP/BM−52-3rd	214.5	24.55	0.9808
Al@AP/BPE−1735-1st	144.3	17.36	0.9951
Al@AP/BPE−1735-2nd	121.8	8.88	0.9909
Al@AP/BPE−1735-3rd	166.6	16.48	0.9630

Note: E_a, activation energy, kJ mol⁻¹; A, pre-exponential factor (s⁻¹·min⁻¹); r, fit coefficients.

. Specifically, the E_a corresponding to the two-step HTD of AP in the graded Al@AP composite was measured as 178.7 kJ·mol⁻¹ and 146.7 kJ·mol⁻¹, respectively. Upon the incorporation of BM−52, for the Al@AP/BM−52 composite, the E_a values for the two-step HTD of AP were increased by 15.2 kJ·mol⁻¹ and 67.8 kJ·mol⁻¹, respectively. In the presence of BPE−1735, the E_a of the second HTD of AP in the Al@AP/BPE−1735 composite was elevated to 166.6 kJ·mol⁻¹, representing an increment of 19.9 kJ·mol⁻¹ in comparison with that of graded Al@AP. Evidently, the addition of BM−52 and BPE−1735 led to a significant enhancement in the E_a of the AP decomposition reaction, consequently increasing the complexity and energy barrier associated with the AP decomposition.

In the realm of materials science, particularly within the investigations concerning the thermal decomposition mechanism of AP, a set of well-defined procedures is adhered to. During the heating progression, a proton transfer phase is initially experienced by AP. Under low-temperature circumstances, absorbed NH₃ and HClO₄ are progressively generated on the surface of AP particles. As the temperature increases, these adsorbed molecules are successively desorbed and converted into gaseous entities. When the temperature attains a specific threshold or is subjected to a relatively elevated environmental pressure, a secondary decomposition reaction is instigated between the desorbed NH₃ and HClO₄. Simultaneously, a redox reaction is initiated among the decomposition products, thereby giving rise to the HTD of AP [5]. Taking quaternary ammonium salts as a typical example, when they are thermally decomposed, a substantial quantity of NH₃ is released, and a pronounced internal heat effect is accompanied in this process [7]. A highly significant inhibitory influence is exerted on the HTD process of AP by this phenomenon. Owing to the perturbation caused by the additional NH₃ and the internal heat effect, the original reaction system is modified, the reaction pathway is rendered more intricate, and the reaction complexity is considerably augmented. From a kinetic vantage point, a marked elevation in the activation energy of the HTD process is directly induced. Therefore, the decomposition process of AP can be effectively retarded by substances such as BM−52 and BPE−1735. Through the augmentation of the crucial factor of reaction activation energy, the precise modulation of the thermal decomposition process of AP is accomplished.

The Friedman method was employed to evaluate the relationship between the E_a and the conversion rate (a), as depicted in Figure 5. Given the sensitivity of Friedman’s method to data noise [27], the activation energy analysis in this study was restricted to the interval α = 0.3–0.7, where the kinetic parameters showed a significant linear correlation (R² > 0.95), and the relevant activation energy mean data are detailed in Table S1. During the decomposition process of graded Al@AP, the E_a of the first step decomposition of AP exhibited a relatively stable behavior, remaining around approximately 85.4 kJ·mol⁻¹. As the α increased, the E_a of the second step of decomposition increased. For instance, when α increased from 0.3 to 0.7, the E_a ascended from approximately 68.7 kJ·mol⁻¹ to about 95.3 kJ·mol⁻¹. For the third step, the E_a decreased as α increased. When α was 0.3, the E_a was approximately 137.1 kJ·mol⁻¹, and when α reached 0.7, it decreased to about 124.9 kJ·mol⁻¹.

Under the influence of BM−52, the E_a of the first step decomposition of AP remained stable at around 134.8 kJ·mol⁻¹, accompanied by a very small fluctuation amplitude of approximately ±12.8 kJ·mol⁻¹. For the second step of decomposition, when the conversion rate was in the range of 0 < α < 0.5, the E_a decreased. When α = 0.3, the E_a was approximately 218.4 kJ·mol⁻¹, and when α = 0.35, it decreased to about 218.1 kJ·mol⁻¹. Subsequently, as the conversion rate increased, it exhibited an upward trend. When α was 0.7, the E_a reached approximately 224.2 kJ·mol⁻¹. The E_a of the third step decomposition of AP continuously increased with an average value of 175.1 kJ·mol⁻¹, and throughout the process, its E_a was always higher than that of the third step of graded Al@AP, with a difference in the range of 34.9–53.2 kJ·mol⁻¹. In comparison, under the action of BPE−1735, the E_a of the first step decomposition of AP decreased as the conversion rate increased, with an average value of 144.1 kJ·mol⁻¹. When α = 0.3, the E_a was approximately 154.8 kJ·mol⁻¹, and when α was 0.7, it fell to about 137.3 kJ·mol⁻¹. The E_a of the second step of decomposition was relatively constant, approximately 212.8 kJ·mol⁻¹, with a fluctuation range not exceeding ±10.3 kJ·mol⁻¹, and it was significantly higher than that of the second step of graded Al@AP, with an average increase of approximately 130.6 kJ·mol⁻¹. The E_a of the third step decomposition of AP gradually increased as α increased, with an average value of 161.3 kJ·mol⁻¹. When α = 0.3, the E_a was approximately 154.8 kJ·mol⁻¹, and as the reaction proceeded, E_a increased to about 166.3 kJ mol⁻¹ when α was 0.7, suggesting that BPE−1735 raised the reaction energy barrier of the third decomposition stage of AP, making the reaction more difficult to proceed, and ultimately realizing a significant inhibition of the high-temperature decomposition of AP.

The incorporation of BM−52 and BPE−1735 has the capacity to modify the relationship between the E_a of the third step thermal decomposition reaction of AP. The conversion rate undergoes a transition from a decreasing trend to an increasing one, which correspondingly increases the difficulty of AP decomposition.

To investigate the thermal decomposition mechanism of AP, the physical models pertaining to the AP thermal decomposition process were computed via the combined kinetic method, and the corresponding results are presented in Table S1 and Figure 6. The physical model of AP decomposition is plotted as scatter points, whereas the ideal models are delineated as solid lines for comparison, as depicted in Figure 6.

The three-step decomposition physical models of pure AP follow the chain-breaking model (L2), the L2, and the phase boundary-controlled reaction model (R2) in sequence. The LTD process of AP is initiated by the proton transfer from the NH⁴⁺ cation to the ClO⁴⁻ anion and occurs within the internal pore core structure of the crystal. In contrast, the HTD involves the adsorption and desorption of NH₃ and HClO₄, which transpires on the crystal surface [35]. The interfacial modification effect between the graded Al powder and AP induces a transformation of the three-step reaction physical model of AP into a three-dimensional nucleation and nuclei growth model (A3), an autocatalytic model (AC), and AC. The A3 model refers to a reaction comprising two main consecutive steps. The initial step is a slow nucleation process taking place on the surface of the graded Al powder, which is attributed to the adsorption and confinement effect of the nano-Al powder on the AP crystal surface. The second step involves the growth of the condensed products decomposed from AP decomposition towards the core of the particle, and this is also an interactive process [37]. The nano-Al powder exerts a substantial promoting effect on the HTD of AP. Therefore, under the influence of the nano-Al powder, the HTD of AP adopts an autocatalytic model. After the addition of BM−52 and BPE−1735, the LTD of AP is minimally affected, and its decomposition model persists in conforming to L2. However, BM−52 and BPE−1735 have a more pronounced impact on the HTD of AP. Subsequent to the addition of BM−52, the physical model of the HTD reaction of AP follows a model intermediate between F1 and D2, as well as a two-dimensional nucleation and nuclei growth model (A2). In contrast, after adding BPE−1735, the physical model of the HTD of AP follows an A3 and a phase boundary-controlled reaction (R3). The changes in the physical reaction models imply that the thermal decomposition mechanism of AP has been modified.

This variation in the decomposition mechanism can be attributed to the specific interactions between the added substances and AP. BM−52 and BPE−1735 presumably modify the energy barriers and reaction pathways involved in the decomposition process. Their influence on the HTD of AP may be associated with their capacity to interact with the reactive intermediates or to alter the surface characteristics of AP.

3.4. Thermal Decomposition Mechanism

The Pyro-GC/MS technique presents remarkable advantages in terms of high sensitivity and selectivity. It empowers the accurate and precise identification of a diverse array of volatile and semi-volatile compounds. In order to explore the influence exerted by BM−52 and BPE−1735 on the product distribution during the thermal decomposition process of AP, a comprehensive analysis was carried out by means of the Pyro-GC/MS instrument [38]. The total atomic chromatogram of the composites is exhibited in Figure 7. The total ion current (TIN) of the composites is vividly illustrated in Figure 8. Each measured retention time is intrinsically linked to the thermal decomposition products of the composites. Consequently, the final composition of the decomposition products of the composite can be definitively determined by the MS spectra corresponding to the shortest retention time.

As shown in Figure 7, all composites generate mass spectral fragments at around 1.54 min, thereby facilitating the acquisition of the corresponding mass spectra. Additionally, the Al@AP/BPE−1735 composite generates mass spectral fragments at 5.2 min and 5.5 min. The MS corresponding to the shortest retention time is shown in Figure 8. To achieve a more precise and detailed description, the concentration of each product is normalized with respect to the percentage of N₂O. This normalization process allows for a more direct comparison of the relative abundances of the major decomposition products. It is determined that products possessing a concentration less than 1% will be excluded from subsequent considerations. Through concentrating on the products with substantial concentrations, a more lucid comprehension can be attained regarding the crucial chemical species that are generated during the Pyro-GC/MS process of the composites.

The mass spectra of pure AP are composed of six peaks, which, respectively, correspond to NH₂⁻ (m/z = 16), NH₄⁺ or H₂O (m/z = 18), N₂ (m/z = 28), NO (m/z = 30), O₂ (m/z = 32), and N₂O (m/z = 44). Among them, NH₂⁻ is a minute fragment that emerges as a consequence of the rupture of the N-H bond during the thermal decomposition of AP, while the formation of NH₄⁺ is attributed to the breakage of the H-O bond in AP under thermal stress. The composition of the thermal decomposition products of AP is contingent upon the temperature [35]. At temperatures below 300 °C, AP decomposes in accordance with the equation 4NH₄ClO₄ = 2Cl₂ + 2N₂O + 3O₂ + 8H₂O, accompanied by the formation of ClO₂, HCl, and N₂. When the temperature exceeds 380 °C, the decomposition pathway of perchlorate is 2NH₄ClO₄ = Cl₂ + 2NO + O₂ + 4H₂O, and in addition, trace amounts of HCl, NOCl, and NO₂ are also generated.

Subsequent to the introduction of the interfacial modification effect, the contact area is augmented, thereby enhancing the reaction efficiency of the gaseous products of Al and the oxidant and diminishing the oxygen content within AP. Consequently, the mass spectra of the thermal pyrolysis products of Al@AP comprise nine peaks. In comparison to pure AP, HCl (m/z = 36.5), NO₂ (m/z = 46), and Cl₂ (m/z = 71) are newly present, and the proportion of oxygen-containing complex products is reduced.

The Pyro-GC/MS products remain unaltered under the influence of BM−52. This implies that BM−52 does not substantially modify the fundamental decomposition route of Al@AP. Instead, it might interact in a more nuanced manner that does not lead to the creation of novel and distinct products. The proportion of oxygenated complex products in the AP thermal decomposition products is further reduced, which in turn affects the reaction kinetics to some extent. In the presence of BPE−1735, in addition to generating substances identical to the pyrolysis products of Al@AP, new pyrolysis peaks at 95 and 135.5 are present. These newly emerged peaks suggest that BPE−1735 induces specific chemical reactions or interactions during the thermal decomposition process, leading to the formation of unique chemical species that are not observed in the absence of BPE−1735.

4. Conclusions

In this paper, three types of core–shell graded Al@AP, Al@AP/BM−52, and Al@AP/BPE-735 composites were prepared by spray drying technology. The influence of BM−52 and BPE−1735 on the thermal decomposition performance of AP in graded Al@AP was comprehensively studied using thermal analysis techniques. Pyrolysis technology was employed to analyze the thermal decomposition products of composites. The following conclusions were reached:

(1)

The reactivity of AP is capable of being effectively enhanced by both BM−52 and BPE−1735 inhibitors. The incorporation of BM−52 facilitates a more comprehensive AP decomposition reaction. The total heat release is elevated from 365.4 J·g⁻¹ to 714.0 J·g⁻¹ with the introduction of BPE−1735.

(2)

The E_a of AP thermal decomposition can be remarkably enhanced by both BM−52 and BPE-735, thereby increasing the difficulty of the HTD of AP. Under the influence of BM−52, the physical model of HTD of AP is transformed from the chain-breaking model (L2) and the phase boundary-controlled reaction model (R2) into a model that lies between F1 and D2, as well as the two-dimensional nucleation and nuclei growth model (A2). Under the effect of BPE−1735, the physical model of HTD of AP is converted into the three-dimensional nucleation and nuclei growth model (A3) and the phase boundary-controlled reaction model (R3).

(3)

After the addition of BM−52 and BPE−1735, the proportion of oxygen-containing compound products in the Pyro-GC/MS products of AP is reduced. Evidently, a more complete decomposition of AP can be achieved with the aid of these two inhibitors. Additionally, specific chemical reactions are induced by BPE−1735 during the thermal decomposition process, leading to the generation of unique chemical species.

Overall, this research furnishes additional insights into the thermal decomposition mechanism of core–shell structured graded Al@AP composites with BM−52 and BPE-735 serving as inhibitors. The finding of this study demonstrates that after the incorporation of BM−52 and BPE−1735, the thermal decomposition of AP is substantially augmented, concurrent with an increase in the decomposition complexity. These discoveries accentuate the latent capacity of Al@AP/BM−52 and Al@AP/BPE-735 composites as fuel components for next-generation low-burning-rate propellants, which is conducive to the evolution of more efficient and dependable propulsion systems. It is anticipated that this research will function as a significant milestone and offer a robust foundation for subsequent studies and refinements in the realm of propellant technology, spurring the innovation of unprecedented formulations and manufacturing methodologies that might potentially reshape the performance and safety benchmarks of propulsion applications.