**Preface**

Energetic Materials (EMs) are a traditional branch of materials. They started more than 2000 years ago in China with black powder or something very close to it. In recent years, the demand for industrial and defense applications for energetic materials, including pyrotechnics, explosives, and propellants, has inspired new developments in this field. The occurrence of advanced energetic materials, in particular, offers a unique new opportunity to improve the performance of energetic formulations. DB propellants have been widely applied to solid rocket propulsion as homogeneous mixtures of NC (nitrocellulose) with NG (nitroglycerin). The composite modified double base (CMDB) propellant, often used in military missiles and space vehicles, properly combines specific features of the two previous kinds of propellant. They are smokeless and are suitable for free-standing motor grain configurations thanks to their high strength and elastic modulus. Composite propellants are a multi-phase mixture of oxidizing particles (such as AP) and metal fuel particles (such as Al) with high polymers as the matrix. For example, hydroxyl-terminated polybutadiene (HTPB) is a polymer widely used in propulsion for composite solid propellants and hybrid fuels. As a typical composite propellant, high-energy HTPB propellant has the advantages of excellent combustion performance and mechanical properties, low flame temperature, low molecular combustion products, and low infrared radiation. However, it also has a high probability of detonation and the risk of detonation. Today, an important challenge concerns solid oxidizers and insensitive compositions. On top of the energetic performance, large density, low cost, low sensitivity to multiple stimuli, low characteristic signature, slow aging, reliable safety, green features before and after burning easy disposal, and reuse technology are also of great interest to researchers and users of solid propellants. Innovative, energetic materials, such as DNTF, RDX@FOX composite, and so on, are often incorporated into propellant composition and require attention. When they were introduced to the energetic system, the performance of the system would be influenced significantly. It is critical to fully consider the properties of both the material and the composite system when selecting the cladding material to ensure that the propellant energy, ignition, density, and other characteristics are maintained while effectively improving the crystallization of DNTF and maintaining stable control of its crystallization amount. Recently, the emergence of high-energy density compounds, such as CL-20, 3,4-Bis(3-nitrofurazan-4-yl) furozan (DNTF), and Ni/Al energetic structural materials, allowed the formulation of new propellants with an increased energy density. Thus, high-energy, low vulnerability, and green solid propellants, laser-driven combustion, etc., are now hot topics worldwide. Moreover, with the development of computer science and technology, many theoretical simulation methods, such as molecular dynamics simulation, artificial neural networks (ANN), nonlinear dynamics software LS-DYNA, etc., can be used to investigate the performance of EMs. For example, the high-energy materials genome (HEMG) is based on the experimental data on the combustion and detonation characteristics of various high-energy materials (HEMs) under various conditions, being also based on the metadata on the quantum and physicochemical characteristics of HEMs components, as well as the thermodynamic characteristics of HEM as a whole.

> **Rui Liu, Yushi Wen, and Weiqiang Pang** *Editors*

### *Editorial* **Advanced Energetic Materials: Testing and Modeling**

**Rui Liu 1, Yushi Wen <sup>2</sup> and Weiqiang Pang 3,\***


Energetic Materials (EMs) are a traditional branch of materials. It started more than 2000 years ago in China with black powder, or something very close to it. In recent years, the demand for industrial and defense applications for energetic materials, including pyrotechnics, explosives, and propellants, inspired new developments in this field. The occurrence of advanced energetic materials in particular offers a unique new opportunity to improve the performance of energetic formulations. DB propellants, as homogeneous mixtures of NC (nitrocellulose) with NG (nitroglycerin), have been widely applied to solid rocket propulsion. The composite modified double base (CMDB) propellant, often used in military missiles and space vehicles, properly combines specific features of the two previous kinds of propellant. They are smokeless and are suitable for free standing motor grain configurations thanks to their high strength and elastic modulus. Composite propellants are a multi-phase mixture of oxidizing particles (such as AP) and metal fuel particles (such as Al) with high polymers as the matrix. For example, hydroxyl-terminated polybutadiene (HTPB) is a polymer widely used in propulsion both for composite solid propellants and hybrid fuels. As a typical composite propellant, high-energy HTPB propellant has the advantages of excellent combustion performance and mechanical properties, low flame temperature, low molecular combustion products, and low infrared radiation. However, it also has a high probability of detonation and the risk of detonation. Today, an important challenge concerns the solid oxidizers and insensitive compositions. On top of the energetic performance, large density, low-cost, low-sensitivity to multiple stimuli, low characteristic signature, slow aging, reliable safety, green features before and after burning, easy disposal, and reuse technology are also of great interest to researchers and users of solid propellants. Innovative energetic materials, such as DNTF, RDX@FOX composite, and so on, are often incorporated into propellant composition and require attention. When they were introduced to the energetic system, the performance of the system would be influenced significantly. It is critical to fully consider the properties of both the material and the composite system when selecting the cladding material to ensure that the propellant energy, ignition, density, and other characteristics are maintained while effectively improving the crystallization of DNTF and maintaining stable control of its crystallization amount. Recently, the emergence of high-energy density compounds, such as CL-20, 3,4-Bis(3-nitrofurazan-4-yl) furozan (DNTF), and Ni/Al energetic structural materials, allowed formulating new propellants with an increased energy density. Thus, high-energy, low vulnerability, and green solid propellants, laser-driven combustion, etc., are now hot topics worldwide. Moreover, with the development of computer science and technology, many theoretical simulation methods, such as molecular dynamics simulation, artificial neural networks (ANN), nonlinear dynamics software LS-DYNA, etc., can be used to investigate the performance of EMs. For example, the high-energy materials genome (HEMG) is based on the experimental data on the combustion and detonation characteristics of various high-energy materials (HEMs)

**Citation:** Liu, R.; Wen, Y.; Pang, W. Advanced Energetic Materials: Testing and Modeling. *Crystals* **2023**, *13*, 1100. https://doi.org/10.3390/ cryst13071100

Received: 7 July 2023 Accepted: 13 July 2023 Published: 14 July 2023

**Copyright:** © 2023 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 (https:// creativecommons.org/licenses/by/ 4.0/).

1

under various conditions, being based also on the metadata on the quantum and physicochemical characteristics of HEMs components as well as the thermodynamic characteristics of HEM as a whole.

To accelerate the potential applications, various works focused on the physical and chemical characteristics through theory, experiments, and simulations. The aim of this issue is to collect comprehensive knowledge on materials synthesis, characterization, combustion, mechanical, detonation, and safety. This Special Issue *Advanced Energetic Materials: Testing and Modeling* explores innovative EMs and EMs ingredients as well as formulations test and models. It collected contributions covering recent progress and models of energetic materials in chemical propulsion. Attention was focused on the design, model, properties, and state of-the-art of this class of thermochemical propulsion devices. A total of 13 papers were selected for publication after a standard peer review process, which summarize the most recent achievements of famous research groups, participation of young authors with novel/innovative concepts was especially encouraged, of course, with the assistance of their supervisors.

To investigate the crystallization of DNTF in modified double-base propellants, glycidyl azide polymer (GAP) was used as the coating material for the in situ coating of DNTF, and the performance of the coating was investigated to inhibit the crystallization. Molecular dynamics was used to construct a bilayer interface model of GAP and DNTF with different growth crystal surfaces, and molecular dynamics' calculations of the binding energy and mechanical properties of the composite system were carried out by Qin, Y. [1]. It was found that GAP can form a white gel on the surface of DNTF crystals and has a good coating effect which can significantly reduce the impact sensitivity and friction sensitivity of DNTF. GAP could effectively improve the mechanical properties of DNTF. GAP can be referred to as a better cladding layer for DNTF, which is feasible for inhibiting the DNTF crystallization problem in propellants.

In order to study the ignition process and response characteristics of cast polymerbonded explosives (PBXs) under the action of friction, HMX-based cast PBXs were used to carry out friction ignition experiments at a 90◦ swing angle and obtain the critical ignition loading pressure was 3.7 MPa. The friction temperature rise process was numerically simulated at the macro and micro scale, and the ignition characteristics were judged by Yuan, J. [2]. It was found that the maximum temperature rise was 55 ◦C, and the temperature rise of the whole tablet was not enough to ignite the explosive. HMX crystal particles can be ignited at a temperature of 619 K under 4 MPa hydraulic pressure loaded by friction sensitivity instrument. The external friction heat between cast PBX tablet and sliding column had little effect on ignition.

To study the engine safety against fragment in complex battlefield environments, the fragment impact safety simulation study of a high-energy four-component HTPB propellant solid engine was conducted. The equation of state parameters and reaction rate equation parameters of the detonation product of HTPB propellant were calibrated by using a 50 mm diameter cylinder test and Lagrange test combined with genetic algorithm. The nonlinear dynamics software LS-DYNA was used to build a finite element model of the fragment impact engine and simulate the mechanical response of the high-energy HTPB propellant under different operating conditions by Liu, Z. [3]. It was found that the critical detonation velocity decreased with the increase in the number of fragments. When the number of fragments was more than five, the influence of this factor on the critical detonation velocity was no longer obvious. Under the same loading strength conditions, the greater the metal shell strength and the greater the shell wall thickness, the more difficult it was for the HTPB propellant to be detonated by the shock. This study can provide a reference for the design and optimization analysis of solid rocket engine fragment impact safety.

For the solid propellant burning rate prediction, high-energy materials genome (HEMG) is an analytical and calculation tool that contains relationships between variables of the object, which allows researchers to calculate the values of one part of the variables through others, solve direct and inverse tasks, predict the characteristics of nonexperimental objects, predict parameters to obtain an object with desired characteristics, and execute virtual experiments for conditions which cannot be organized or have difficultly being organized. The history and current status of the emergence of HEMG are presented herein. The fundamental basis of the artificial neural networks (ANN) as a methodological HEMG base, as well as some examples of HEMG conception used to create multifactor computational models (MCM) of solid rocket propellants (SRP) combustion, was presented by Pang, W. [4].

To study the role of complex composition of 2:17R-cell boundaries in the realization of magnetization reversal processes of (Sm, Zr)(Co, Cu, Fe)*z* alloys intended for high-energy permanent magnets, the micromagnetic simulation was performed using the modified sandwich model of a (Sm, Zr)(Co, Cu, Fe)z magnet, which includes additional domain-wall pinning barriers in the form of 2:7R or 5:19R phase layers by Zheleznyi, M. [5]. It was found that there was a possibility of reaching the increased coercivity at the expense of 180◦-domain wall pinning at the additional barriers within cell boundaries. The phase and structural states of the as-cast Sm1-*x*Zr*x*(Co0.702Cu0.088Fe0.210)*<sup>z</sup>* alloy sample with *x* = 0.13 and *z* = 6.4 were studied, and the presence of the above phases in the vicinity of the 1:5H phase was demonstrated.

Research on energetic materials continuously develops energetic materials with higher detonation performance and energy density, taking it as an eternal quest. Due to the introduction of oxygen atoms, N-oxide energetic compounds have a unique oxygen balance, excellent detonation properties, and a high energy density, attracting the extensive attention of researchers all over the world. Synthetic strategies towards azine N-oxides and azole N-oxides of N-oxides were fully reviewed. Corresponding reaction mechanisms towards the aromatic N-oxide frameworks and examples that use the frameworks to create highenergy substances were discussed. Moreover, the energetic properties of N-oxide energetic compounds were compared and summarized by She, W. [6].

As we know, aluminum (Al) has been widely used in micro-electromechanical systems (MEMS), polymer-bonded explosives (PBXs), and solid propellants. Its typical core–shell structure (the inside active Al core and the external alumina (Al2O3) shell) determines its oxidation process, which is mainly influenced by oxidant diffusion, Al2O3 crystal transformation and melt-dispersion of the inside active Al. Metastable intermixed composites (MICs), flake Al, and nano Al can improve the properties of Al by increasing the diffusion efficiency of the oxidant. Fluorine, titanium carbide (TiC), and alloy can crack the Al2O3 shell to improve the properties of Al. Furthermore, those materials with good thermal conductivity can increase the heat transferred to the internal active Al, which can also improve the reactivity of Al. The integration of different modification methods was employed by Wang, D.to further improve the properties of Al [7]. With the ever-increasing demands on the performance of MEMS, PBXs, and solid propellants, Al-based composite materials with high stability during storage and transportation, and high reactivity for usage will become a new research focus in the future.

It is difficult for the reactor to achieve uniform quality of composite material, which affects its application performance. 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and 1,1 diamino-2,2-dinitroethylene (FOX-7) are famous high-energy and insensitive explosives. The preparation of RDX@FOX-7 composites can meet the requirements with high-energy and low sensitivity. Based on the principle of solvent-anti-solvent, the recrystallization process was precisely controlled by microfluidic technology. The RDX@FOX-7 composites with different mass ratios were prepared by Yu, J. [8]. It was found that at the mass ratio of 10%, the RDX@FOX-7 composites were ellipsoid of about 15 μm with uniform distribution and quality. The advantages of microscale fabrication of composite materials were verified. With the increase in FOX-7 mass ratios, the melting temperature of RDX was advanced, the thermal decomposition peak of RDX changed to double peaks, and the activation energy of RDX@FOX-7 composite decreased. These changes were more pronounced between 3 and 10%, but not between 10 and 30%. The ignition delay time of RDX@FOX-7 was shorter than

that of RDX and FOX-7. RDX@FOX-7 burned more completely than RDX indicating that FOX-7 can assist heat transfer and improve the combustion efficiency of RDX.

Ni/Al energetic structural materials attracted much attention due to their high energy release, but understanding their thermal reaction behavior and mechanism in order to guide their practical application is still a challenge. A novel understanding of the thermal reaction behavior and mechanism of Ni/Al energetic structural materials in the inert atmosphere were reported. The reaction kinetic model of Ni/Al energetic structural materials with Ni/Al molar ratios was obtained. The effect of the Ni/Al molar ratios on their thermal reactions was discussed based on the products of a Ni/Al thermal reaction by Wang, K. [9]. It was found that the liquid Al was adsorbed on the surface of Ni with high contact areas, leading in an aggravated thermal reaction of Ni/Al.

Taking the combustion tear gas mixture as the research object, the system formula was optimized by adding a different mass fraction of 5-amino-1H-tetrazole (5AT). TG-DSC, a thermocouple, and a laser smoke test system were used to characterize the combustion temperature and velocity, as well as the smoke concentration and particle size. Starink's method, the Flynn–Wall–Ozawa method, and the Coats–Redfern method were used to evaluate the pyrolysis kinetic parameters of the samples by Zhai, H. [10]. It was found that when the mass fraction of 5AT in the system was 10%, the maximum combustion temperature of the sample decreased by nearly 70 ◦C and the smoke concentration increased by 12.81%. Adding an appropriate amount of the combustible agent 5AT to the combustion tear gas mixture can improve its combustion performance and smoking performance, which provides an important, new idea for the development of a new generation of safe, efficient, and environmentally friendly tear gas mixtures.

To study the design method and pressure relief effect of the mitigation structure of a shell under the action of thermal stimulation, a systematic research method of theoretical calculation-simulation-experimental verification of the mitigation structure was established by Liang, J. [11]. The pressure relief effect of the mitigation structure was verified by the low-melting alloy plug with refined crystal structure for sealing the pressure relief hole and the cook-off test. It was found that the critical pressure relief area is when the ratio of the area of the pressure relief hole to the surface area of the charge is AV/SB = 0.0189. When the number of openings increased to 6, the required pressure relief coefficient decreased to AV/SB = 0.0110. When the length/diameter ratio was greater than 5, the opening at one end cannot satisfy the reliable pressure relief of the shell. The designed low-melting-point alloy mitigation structure can form an effective pressure relief channel.

To study the crystal mechanical properties of DNTF and hexanitrohexaazaisowurtzitane (CL-20) deeply, the crystals of DNTF and CL-20 were prepared by the solvent evaporation method. The crystal micromechanical loading procedure was characterized by the nanoindentation method. In addition, the crystal fracture behaviors were investigated with scanning probe microscopy (SPM) by Nan, H. [12]. It was found that the hardness for DNTF and CL-20 was 0.57 GPa and 0.84 GPa, and the elastic modulus was 10.34 GPa and 20.30 GPa, respectively. CL-20 obviously exhibited a higher hardness, elastic modulus, and local energy-dissipation, and a smaller elastic recovery ability of crystals than those of DNTF. CL-20 crystals are more prone to cracking and have a lower fracture toughness value than DNTF. Compared to DNTF crystals, CL-20 is a kind of brittle material with higher modulus, hardness, and sensitivity than that of DNTF, making the ignition response more likely to happen.

In order to study the reaction growth process of insensitive Ju En Ao Lv (JEOL) explosive after ignition under cook-off, a series of cook-off tests were carried out on JEOL explosive using a self-designed small cook-off bomb system. A thermocouple was used to measure the internal temperature of the explosive, and a camera recorded macro images of the cook-off process by Wang, X. [13]. It was found that the ignition time decreased as the heating rate increased, while the ignition temperature was not sensitive to the heating rate. When the heating rate was faster, the internal temperature gradient of the explosive was larger, and the ignition point appeared at the highest temperature position. As the heating

rate decreased, the internal temperature gradient of the explosive decreased, the ignition point appeared random, and multiple ignition points appeared at the same time. The growth process of the ignition point could be divided into severe thermal decomposition, slow combustion, and violent combustion stages.

**Author Contributions:** Conceptualization, W.P. and R.L.; writing—original draft preparation, W.P.; writing—review and editing, R.L. and Y.W. All authors have read and agreed to the published version of the manuscript.

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

#### **References**


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**Haolong Zhai, Xiaoping Cui \* and Yuping Gan**

Equipment Management and Guarantee Institute, Engineering University of Armed Police Force of China, Xi'an 710086, China; gcdxzhl@yeah.net (H.Z.); gcdxgyp@yeah.net (Y.G.)

**\*** Correspondence: wjgcdxcxp@yeah.net

**Abstract:** Taking the combustion tear gas mixture as the research object, the system formula was optimized by adding a different mass fraction of 5-amino-1H-tetrazole(5AT). TG-DSC, a thermocouple, and a laser smoke test system were used to characterize the characteristic combustion parameters such as combustion temperature and velocity, as well as the end-point effects such as smoke concentration and particle size. Starink's method, the Flynn–Wall–Ozawa method, and the Coats–Redfern method were used to evaluate the pyrolysis kinetic parameters of the samples. The results show that when the mass fraction of 5-amino-1H-tetrazole in the system is 10%, the maximum combustion temperature of the sample decreases by nearly 70 ◦C and the smoke concentration increases by 12.81%. The kinetic study also found that with a different mass fraction of 5-amino-1H-tetrazole in the system, the main reaction model of the mixed agent in the first, third, and fourth stages of pyrolysis changed significantly, but for the second stage of sample pyrolysis, the main reaction model (the A4 model) showed a high degree of consistency, which can be considered as the thermal diffusion stage of the tear agent capsicum oleoresin (OC) (the temperature range is 220~350 ◦C), which is highly consistent with the results of the TG-DSC analysis. It was also confirmed that OC's thermal diffusion is mainly concentrated in this stage. The results of this study show that adding an appropriate amount of the combustible agent 5-amino-1H-tetrazole to the combustion tear gas mixture can improve its combustion performance and smoking performance, which provides an important, new idea for the development of a new generation of safe, efficient, and environmentally friendly tear gas mixtures.

**Keywords:** 5-amino-1H-tetrazole; tear gas mixture; combustible agent; combustion pyrolysis characteristics; dynamics research

#### **1. Introduction**

As the main charge of combustion tear gas, the combustion tear gas mixture plays an important role in dealing with sudden mass incidents and preventing and dealing with terrorist activities [1,2]. At present, potassium chlorate (KClO3) is used as an oxidant, sucrose (C12H22O11) is used as the combustible agent, and capsicum oleoresin (OC) is used as the tear agent in the formulation of this kind of mixed agent. The redox reaction of the oxidant and the combustible agent provides energy for the sublimation of the tear agent. However, due to the relatively poor thermal stability of OC, in order to maximize the functional efficiency of the tear agent in the mixed agent and improve its effective utilization rate, the energy released during the combustion of the mixed agent must be controlled. If the energy is too great, the combustion temperature will be too high, which will lead to the thermal decomposition of the tear agent in the process of heat release. On the other hand, if the energy is too small, it will delay the heat release efficiency of the tear agents in the system and even cause the release velocity to be too slow, making it difficult to reach the combat concentration in a short time, which will greatly reduce the technical and combat effectiveness of this kind of ammunition [3–7]. At the same time, the products formed by incomplete thermal diffusion will also aggravate the burden on the

**Citation:** Zhai, H.; Cui, X.; Gan, Y. Effect of 5-Amino-1H-Tetrazole on Combustion Pyrolysis Characteristics and Kinetics of a Combustion Tear Gas Mixture. *Crystals* **2022**, *12*, 948. https://doi.org/10.3390/ cryst12070948

Academic Editors: Rui Liu, Yushi Wen, Weiqiang Pang and Qing Peng

Received: 10 June 2022 Accepted: 3 July 2022 Published: 6 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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 (https:// creativecommons.org/licenses/by/ 4.0/).

environment. Therefore, it is very important to study the combustion characteristics of its formula in order to improve the effective utilization of the lacrimal agent OC in the system and improve the action efficiency of this kind of mixed agent [8].

Through a literature review [9–11], it was found that 5-amino-1h-tetrazole (5AT), as an environmentally friendly combustible agent, has been greatly developed as a solid propellant and in other fields. Its greatest advantage is that, compared with other nitrogencontaining compounds, its nitrogen mass fraction is as high as 82.3%, and the combustion product is harmless N2, with high gas production, which is more conducive to the diffusion of the functional elements in the mixture. 5AT is considered to be an ideal fuel in gas generators with a low combustion temperature. The latest research results have shown that adding an appropriate amount of 5AT instead of a sugar compound as the combustible agent in the formula of colored smoke pyrotechnic agents can significantly improve the smoke's performance in action efficiency and durability [12–15].

However, no attempt has been made to improve the formula of combustion-type tear gas mixtures. Based on the similar principle of action between combustion-type colored smoke agents and combustion-type tear gas mixtures [16], on the basis of an unchanged oxygen mass fraction coefficient (OB), this study attempted to introduce 5AT in different proportions such as 0%, 5%, 10%, 15%, and 20% into the system as the second combustible agent to obtain five groups of different formulas. TG-DSC, a thermocouple, and a laser smoke test system were used to characterize the characteristic parameters of combustion, such as the combustion temperature and velocity, as well as the end-point effects such as smoke concentration and particle size. The apparent activation energy, pre-exponential factor, and other thermodynamic parameters in the pyrolysis process were obtained by Starink's method and the Flynn–Wall–Ozawa method. At the same time, in order to further explore the pyrolysis mechanism of combustion, the possible reaction models in the pyrolysis process of the different formulations were deduced by the Coats–Redfern modelfitting method. The study provides valuable guidance for improving the performance and combustion mechanism of this kind of mixture.

#### **2. Experiment and Method**

#### *2.1. Materials and Main Experimental Equipment*

The main raw materials were chemically pure capsicum oleoresin, OC for short (C18H27NO3) from Aldrich, St. Louis, MO, USA, and potassium chlorate (KClO3), lactose (C12H22O11), 5-amino-1h-tetrazole (CH3N5), phenolic resin ((C8H6O2)n) and basic magnesium carbonate ((MgCO3)4·Mg(OH)2·5H2O), all of which were analytically pure and purchased from Aladdin Biochemical Technology Co., Ltd., Shanghai, China.

The main test equipment was an HS-STA-002 synchronous thermal analyzer (sensitivity: 0.01 mg) produced by Hesheng Instrument Technology Co., Ltd., Shanghai, China, with a resolution of 0.06 mV, a test temperature range of room temperature to ~1000 ◦C, a temperature test accuracy of ±0.05 ◦C, and a calorimetric sensitivity of ±0.5%.

Other equipment included an analytical balance (BSA224S-CW) produced by Saidoris Instrument System Co., Ltd., Gottingen, Germany; a K-type thermocouple (FLUKE53-2B) produced by Fluke company, Everett, America; an intelligent digital display vacuum drying oven (DHG-9140) produced by Donglu Instrument and Equipment Company, Shanghai, China; a high-speed camera (X8PRO) produced by Mingce Electronic Technology Company, Shanghai, China; and a smoke concentration test system (JCY-80e) produced by Chuangyi Environmental Testing Equipment Co., Ltd., Qingdao, China.

The samples were prepared according to different formulations designed by a uniform design method, as shown in Table 1. Figure 1 shows the sample preparation flowchart, and Figure 2 shows five different prepared samples.


**Table 1.** Formula of the mixed agents at OB = −0.18 based on a uniform design method.

**Figure 1.** Sample preparation process.

**Figure 2.** Tested samples with different formulations.

*2.2. Test of Combustion Characteristics*

#### 2.2.1. Combustion Temperature Test

In order to reduce the influence of the oxygen concentration in the external environment on the combustion environment of the sample, the test ignited each sample in an N2 environment, measured the temperature with a K-type thermocouple, and recorded the whole process with a high-speed camera. Three groups of tests were conducted for each group of samples, and the average value of the three groups of data was taken as the measurement result [17].

#### 2.2.2. Burning Rate Test

The burning rate is also one of the important indexes used to measure the combat effectiveness of combustion tear gas mixtures. The burning rate can have a direct impact on the smoke effect of the agent. Linear velocity or mass velocity is usually used for pyrotechnic agents. In general, the burning rate generally refers to the linear burning rate, which refers to the displacement of the combustion wave in front of the mixed grain along its normal direction in units of time [18], and it is expressed as:

$$v = \frac{dl}{dt}(\text{mm/s})\tag{1}$$

where *v* is the linear burning rate of the mixed grain, and *dl* is the displacement of the combustion wave of mixed grain along its normal direction in time *dt* (unit: mm/s). During the test, an electric igniter was used to ignite the grain, and the test was carried out in the smoke box in an N2 environment.

#### *2.3. Test of Combustion Smoke Characteristics*

In order to characterize the combustion smoke concentration and particle size distribution of the sample, a laser smoke concentration tester was used for testing, and the data were collected and analyzed with software. Its principle is shown in Figure 3. The samples' specifications are cylinders with a diameter of 15 mm and a height of 20 mm. The specifications of the smoke collection box are 50 cm × 50 cm × 50 cm.

**Figure 3.** Schematic diagram of the laser smoke concentration tester.

#### *2.4. Thermal Behavior Analysis*

In order to study the thermal behavior of the sample, an synchronous thermal analyzer was used. Before the test, we first calibrated the differential thermal analysis baseline and temperature of the synchronous thermal analyzer and then placed about 8–10 mg of the different samples into the ceramic crucible and heated the samples from 30 ◦C to 600 ◦C at a heating rate of 5, 10, 15, and 20 ◦C·min−1. In order to avoid environmental impact, the whole test process was carried out in an N2 atmosphere, and the ventilation rate was 40 mL·min<sup>−</sup>1.

#### *2.5. Pyrolysis Kinetics*

In order to further explore the reaction mechanism of each stage in the combustion process of the combustion tear gas mixture, Starink's method and the Flynn–Wall–Ozawa method with high accuracy were selected to calculate the corresponding thermal decomposition kinetic parameters [19–22] and the Coats–Redfern equation was used to predict the pyrolysis reaction model of each stage of the sample and to thus determine the reaction type of each stage so as to provide a certain theoretical basis for an in-depth study and improvement of its combustion environment.

The equation of Starink's method [23] is:

$$\ln \frac{\beta}{T^{1.8}} = -1.008 \cdot \frac{E\_a}{RT} + \text{C} \tag{2}$$

The equation of the Flynn–Wall–Ozawa method [24,25] is:

$$
\ln \beta + 0.4567 \frac{E\_a}{RT} = \text{C} \tag{3}
$$

The Coats–Redfern method equation [26] is:

$$\ln \frac{g(a)}{T^2} = \ln \left[ \frac{AR}{\beta E\_a} \left( 1 - \frac{2RT}{E\_a} \right) \right] - \frac{E\_a}{RT} \tag{4}$$

where *β* is the heating rate, *T* is the characteristic temperature, *Eα* is the activation energy of the reaction, *R* is the molar gas constant, and *A* is the pre-exponential factor of the reaction.

By combining 17 common *g(α)* substitutes into Equation (4), we can solve the linear correlation coefficient between ln[*g(α)/T2*] and 1/T. When the calculated linear correlation coefficient reaches the maximum, the corresponding reaction model of the selected *g(α)* is the reaction model of the sample at this stage. The 17 commonly used reaction models are shown in Table 2 [27].


**Table 2.** Thermal decomposition reaction models of 17 common solid substances.

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

*3.1. Analysis of the Combustion Characteristics*

3.1.1. Combustion Temperature Analysis

Figure 4 shows the combustion temperature-time distribution of Samples P1–P5 measured by the thermocouple method. It can be seen that the order of the maximum combustion temperature (Tmax) of the samples is Tmax (P5) > Tmax (P1) > Tmax (P4) > Tmax (P2) > Tmax (P3). At the same time, it is not difficult to see that when the mass fraction of 5AT is 10%, the combustion temperature of the sample is the lowest (588 ◦C), but when the mass fraction of 5AT is 20%, the combustion temperature of the sample is the highest (676 ◦C); the difference between them is nearly 90 ◦C. This shows that 5AT has a great influence on the combustion temperature of the system.

**Figure 4.** Combustion temperature-time distribution of Samples P1–P5 measured by a thermocouple.

Figure 5 shows the variation trend of the maximum combustion temperature of samples with different 5AT mass fractions in the system. It was found that with an increase in the 5AT mass fraction, the maximum combustion temperature first decreases and then increases. When the mass fraction of 5AT is less than 10%, the combustion temperature of the system decreases with an increase in the 5AT mass fraction, but when the mass fraction of 5AT is more than 10%, the combustion temperature of the system increases with an increase in the 5AT mass fraction. This is mainly related to the redox reaction of 5AT with the oxidant KClO3 and its own pyrolysis reaction. Among these, the former is an exothermic reaction and the latter is an endothermic reaction. When the mass fraction of 5AT in the system is less than 10%, the heat released by 5AT participating in the redox reaction in the system is less than the heat absorption required for its own pyrolysis, so the overall combustion temperature of the system decreases. When the mass fraction of 5AT in the system is higher than 10%, the heat release of 5AT participating in the reaction is greater than the heat absorption required for its own pyrolysis, so the overall combustion temperature of the system will rise.

**Figure 5.** Variation trend of the maximum combustion temperature of samples with different 5AT mass fractions in the system.

#### 3.1.2. Analysis of Burning Rate

Table 3 shows the burning rate of Samples P1–P5 in the same nitrogen atmosphere, with an air pressure of 0.1 MPa, room temperature T = 20 ◦C, and relative humidity RH = 30%. The results show that under the same atmospheric conditions, the burning rates of samples with different formulas show little difference. The maximum is 1.08 mm·s−1, the minimum is 1.03 mm·s−1, and the difference is only 0.05 mm·s−1, which is basically the same level of burning rate. This shows that when all the other conditions are the same, the addition of 5AT to the system does not affect the overall combustion rate of this kind of mixture.

**Table 3.** Test results of the burning rate of samples.


#### *3.2. Smoke Characteristic Analysis*

In order to evaluate the effect of 5AT on the thermal diffusion effect of the tear agent in combustion tear gas mixtures, the smoke concentration and particle size distribution of the samples were characterized; the results are shown in Figures 6 and 7. Figure 6 shows the particle size distribution of the combustion smoke of Samples P1–P5. It can be seen from the figure that the average particle size of the combustion smoke of Samples P1–P5 is mainly distributed between 833.4–839.8 μm. The relationship between the smoke concentration and average particle size of different samples and the mass fraction of 5AT in the system is shown in Figure 7. It is not difficult to see that when the mass fraction of 5AT in the system is less than 10%, the smoke concentration (C) and average particle size (AP) show an increasing trend. When the amount of 5AT in the system is 10%, the C and AP values of smoke reach the maximum, which are 68.59% and 839.8 μm, respectively. When the mass fraction of 5AT in the system is greater than 10%, the C and AP values of the sample smoke show a decreasing trend. When the mass fraction of 5AT in the system is 20%, the C and AP values of the smoke are the smallest: 53.58% and 833.4 μm, respectively.

**Figure 6.** Particle size distribution of the combustion smoke of Samples P1–P5.

**Figure 7.** Variation trend of the smoke concentration and average particle size of samples with different 5AT mass fractions in the system.

Compared with the ranking of the maximum combustion temperature (*Tmax*) of the different formulations measured above, the rankings for smoke concentration and the average particle size of different samples were just the opposite; that is, the higher the *Tmax*, the smaller the corresponding C and AP values. On the contrary, the lower *Tmax*, the greater the corresponding C and AP values. This may be related to the thermal decomposition of the tear agent OC during the combustion process of the system; that is, when the combustion

temperature is higher, the amount of OC will increase, and the corresponding C value will decrease. With the thermal decomposition of the tear agent OC, the corresponding smoke AP value will decrease.

#### *3.3. Pyrolysis Behavior Analysis and Related Kinetic Analysis*

#### 3.3.1. Thermal Behavior Analysis of Individual Components

Figure 8 shows the distribution of each individual component in the mixed reagent system as the TG-DSC-DTG curve at *<sup>β</sup>* = 10 ◦C·min<sup>−</sup>1. According to the TG curve, compared with other components in the system, the temperature at which the oxidant KClO3 begins thermal decomposition is higher. Near 400 ◦C, the decomposition process is mainly one stage, and the weight loss ratio is about 30%.

**Figure 8.** TG-DSC-DTG curve of each individual component in mixed reagent sample at <sup>β</sup> = 10 ◦C·min<sup>−</sup>1.

The temperature of the thermal decomposition of 5AT is the lowest, which starts near 200 ◦C. There is an obvious endothermic peak in the weight loss process of thermal decomposition, indicating that its thermal decomposition is mainly an endothermic process [28]. According to the weight loss trend of the TG curve, the weight loss process is mainly divided into three stages, for which the weight loss ratio is about 40%, 10%, and 30%.

Compared with sucrose, which is also a combustible agent, the temperature when sucrose starts thermal decomposition is slightly higher than that of 5AT; the weight loss begins near 210 ◦C, and there is an exothermic peak in the thermal decomposition process [29], indicating that the thermal decomposition of sucrose is mainly an exothermic process. According to the weight loss trend of the TG curve, the weight loss process is mainly divided into two stages: the weight loss ratio of the first stage is about 70%, and the weight loss ratio of the second stage is about 30%.

According to the TG-DSC-DTG curve of the lacrimal agent OC and previous research [8,30], the endothermic peak near 58 ◦C corresponds to its melting point. The weight loss phenomenon begins at around 230 ◦C, and the weak exothermic phenomenon does not appear until near 340 ◦C. In this temperature range, the DTG curve corresponds to an obvious pyrolysis weight loss peak, which is mainly considered to be the thermal diffusion process of OC. The second exothermic peak near 500 ◦C corresponds to the thermal decomposition of OC, and the weight loss ratio in this stage is about 10%.

According to the TG-DSC-DTG curve of the basic coolant magnesium carbonate and previous studies [31], the pyrolysis process is mainly divided into two stages. The temperature range of the first stage is 220–360 ◦C, and the weight loss ratio is about 16%. This is considered to mainly be the loss process of crystal water. The temperature range of the second stage is 360–500 ◦C, and the weight loss ratio is about 55%, which is basically consistent with the theory of complete pyrolysis to produce carbon dioxide, magnesium oxide, and water.

#### 3.3.2. Thermal Behavior Analysis of the Samples

#### DSC Analysis of the Samples

Figure <sup>9</sup> shows the TG-DSC curve of Samples P1–P5 at a heating rate of <sup>β</sup> = 10 ◦C·min<sup>−</sup>1. According to the DSC curve, in the temperature range of 30–600 ◦C, the formulae of Samples P1–P4 mainly correspond to four thermal behaviors, which are the primary endothermic phenomenon and the tertiary exothermic phenomenon successively (the corresponding peak temperatures are *T1, T2, T3*, and *T*4).

**Figure 9.** The TG-DSC curve of Samples P1–P5 at <sup>β</sup> = 10 ◦C·min<sup>−</sup>1.

However, P5 corresponds to five thermal behaviors, namely the primary endothermic phenomenon and four exothermic phenomena (the corresponding peak temperatures are *T5-1, T5-2, T5-3, T5-4,* and *T5-5*), as shown in Table 4.

**Table 4.** Characteristic peak temperatures of samples P1–P5 corresponding to the DSC curve at <sup>β</sup> = 10 ◦C·min<sup>−</sup>1.


It can be seen from the characteristic peak temperatures in Table 4 that the thermal behavior of the formulae of Samples P1–P4 is basically the same. From P1 to P4, with the increase in 5AT mass fraction in the formula, the first exothermic peak *T*<sup>2</sup> gradually decreases and the third exothermic peak *T*<sup>4</sup> gradually increases, while the endothermic peak *T*<sup>1</sup> and the second exothermic peak *T*<sup>3</sup> have no obvious change. In combination with the TG-DSC-DTG curve of individual components in the previous section, it can be seen that the endothermic peak *T*<sup>1</sup> and the second exothermic peak *T*<sup>3</sup> of P1–P4 correspond to the melting point of OC in the system and the temperature at which pyrolysis begins. The first exothermic peak, *T*2, is mainly caused by the exothermic oxidation–reduction reaction of the oxidant KClO3, combustible C12H22O11, and 5AT. With an increase in 5AT, the initial temperature of the reaction at this stage moves to the left, and the peak's shape gradually becomes gentle, which indicates that the addition of 5AT can slow down the intensity of the reaction, which may be related to the need to absorb some heat for the decomposition of 5AT [32,33]. Compared with P1–P4, P5 also has a weak exothermic peak in the temperature range of 200–300 ◦C. In combination with the changes in the components in the formula and the pyrolysis curve of each individual component, it is considered that the exothermic phenomenon is related to the fact that the heat released by 5AT participating in the reaction in the system begins to be greater than the heat absorbed by its own pyrolysis.

The exothermic peak of Samples P1–P5 near 330 ◦C mainly corresponds to the initial thermal decomposition of OC in the system. The exothermic enthalpy corresponding to each exothermic peak is shown in Table 5. It can be seen that with an increase in the 5AT mass fraction in the system, the exothermic enthalpy at the corresponding position first decreases and then increases. The corresponding exothermic enthalpy of P3 is the smallest, which indicates that the amount of thermal decomposition of OC in P3 is the smallest, which is the same as the smoke concentration of P3 measured above. The results are basically consistent with those of the largest average particle size.

**Table 5.** Exothermic enthalpy corresponding to the exothermic peak of Samples P1–P5 near 340 ◦C.


TG-DTG Analysis of the Samples

Figure <sup>10</sup> shows the TG-DTG curve of Samples P1–P5 at <sup>β</sup> = 10 ◦C·min<sup>−</sup>1. In the DTG curve of the samples, the thermogravimetric process of the samples can be divided into several different stages according to the peak value corresponding to the mass loss rate of the samples.

**Figure 10.** The TG-DTG curve of Samples P1–P5 at <sup>β</sup> = 10 ◦C·min<sup>−</sup>1.

Table 6 shows the characteristic values such as the initial temperature, the temperature corresponding to the DTG peak value, and the thermal weight loss ratio at each stage. From the TG-DTG curve, it can be seen that the pyrolysis weight loss process of P1–P5 is mainly divided into four stages in the temperature range of 30–600 ◦C. Combined with the curve in Figure 10, the first weight-loss stage is 150–220 ◦C, and an obvious DTG peak can be observed at this stage, in which the peak shape of P1 is the sharpest, indicating that the reaction is violent, and the peak value of the corresponding curve is 0.71% ◦C−1. With an increase in the 5AT mass fraction in the system, the exothermic peak tends to be gentle. The peak values of the corresponding curves of P2–P5 are 0.4% ◦C<sup>−</sup>1, 0.14% ◦C<sup>−</sup>1, 0.19% ◦C<sup>−</sup>1, and 0.31% ◦C−1, respectively. Compared with P1, the DTG peak in the corresponding stages decreases significantly. According to Table 6, the thermal weight loss ratio ML1 corresponding to P1 at this stage is the largest. In combination with the previous research results of this kind of mixed agent [34], it can be considered that the redox reaction between the combustible agent and the oxidant has occurred in this stage. The peak value of P1 s curve is the largest, and the thermal weight loss ratio is the largest, which is caused by the violent reaction between the combustible C12H22O11 and the oxidant KClO3 at this stage.

The temperature range of the second stage is 190–320 ◦C. At this stage, except for P5, which corresponds to a weak exothermic peak, the other formulae have no obvious heat absorption and exothermic phenomena. The DTG curve of this stage corresponds to an obvious peak, indicating that the thermal weight loss at this stage is obvious. In combination with the properties of each component in the mixed agent and relevant research results, it can be determined that this mainly corresponds to the thermal diffusion process of OC in the mixed agent; that is, the greater the weight loss ratio at this stage, the greater the amount of OC for effective thermal diffusion. When the heating rate of Samples P1–P5 is 10 ◦C min−1, the thermal weight loss ratio of this stage is 26.7%, 27.5%, 28.9%, 27.1%, and 24.8%, respectively. It can be seen that the weight loss ratio of P3 is the largest, while the weight loss ratio of P5 is the smallest. The corresponding order is consistent with the concentration of each sample measured above. Therefore, appropriately increasing the mass fraction of 5AT in the system can improve the effective utilization rate of the lacrimal agent OC in the system.


**Table 6.** Starting and ending temperatures and corresponding characteristic values of Samples P1–P5 at each stage, based on the DTG curve.

Note: *To* is the initial temperature; *TP* is the peak temperature; *Tf* is the cut-off temperature; ML refers to the mass ratio of thermal weight loss in this stage; *To, TP, Tf* unit: ◦C; ML unit: %.

The temperature range of the third stage is 270–440 ◦C. A weak exothermic peak can be observed at this stage, and the temperature of this exothermic peak is consistent with the corresponding exothermic peak in the TG-DTG curve of the individual component OC. Therefore, it can be determined that this exothermic peak is related to the exothermic pyrolysis of OC.

#### 3.3.3. Analysis of the Pyrolysis Kinetics of the Samples

In order to further explore the thermal decomposition mechanism of Samples P1–P5 and to calculate the kinetic parameters of each stage of the reaction, Starink's method and the Flynn–Wall–Ozawa method were used [35–37]. The TG-DTG curve of Samples P1–P5 at different heating rates is shown in Figure 11. The corresponding characteristic peak temperature of each stage at different heating rates for each sample is shown in Table 6.

#### Starink's Method

Based on the measured TG-DTG curves of P1–P5 at different heating rates, combined with the characteristic peak temperatures of the four stages in Table 6, the value of *ln(β/TP* 1.8*)* and 1*/TP* can be obtained for the sample across the four pyrolysis stages [23]. Taking 1*/TP* as the independent variable and *ln(β/TP* 1.8*)* as the dependent variable, we then carried out linear fitting to obtain the slope of the fitting line and substituted it into Equation (2) to obtain the activation energy Ea at this stage.

**Figure 11.** TG-DTG curve of Samples P1–P5 at different heating rates.

The linear fitting results of *ln(β/TP* 1.8*)* and 1*/TP* are shown in Figure 12, and the results of calculating the activation energy Ea for each stage are shown in Table 7. The correlation coefficient R<sup>2</sup> represents the accuracy of the fitting results, and the closer it is to 1, the higher the reliability.

**Figure 12.** Activation energy curve of Samples P1–P5 at each stage of pyrolysis calculated via Starink's method.


**Table 7.** Reaction kinetic parameters of Samples P1–P5 at each stage of pyrolysis calculated via Starink's method.

Flynn–Wall–Ozawa method.

It can be seen from the results that with the same OB, with the addition of 5AT to the mass fraction, the activation energy of the first and second stages of P2–P5 shows a decreasing trend compared with P1, which confirms that the starting temperature of the component reaction after the addition of 5AT mentioned in the pyrolysis behavior analysis is significantly lower, which plays a certain role in promoting the reaction at this stage. In addition, if we compare the activation energies of the first and second stages of each formula, it can also be seen that only the activation energy of P3 in the second stage is significantly lower than that of the first stage, indicating that P3 can spontaneously carry out the second stage reaction after the first-stage reaction, which again confirms the reason why the weight loss ratio of P3 in the second stage is significantly higher than that of other samples. The activation energy of the third stage of P2–P5 increases significantly compared with that of P1, which may be related to a large amount of heat absorbed by the pyrolysis of 5AT. With a continual increase in the mass fraction of 5AT, the activation energy corresponding to this stage also increases accordingly.

Based on the measured TG-DTG curve data (Table 6), the corresponding temperature value 1*/T* at the same value of conversion α is an independent variable and *lnβ* is a dependent variable. The obtained data points were linearly fitted, then the slope of the straight line was obtained. By substitution in Formula (3), the activation energy Eα corresponding to the reaction conversion α can be obtained at this stage [24,25]. Figure 13 shows the activation energy curve of Samples P1–P5 at each stage obtained via the Flynn– Wall–Ozawa method.

The activation energy of each stage for P1–P5 obtained by the Flynn–Wall–Ozawa method (Figure 13) is basically consistent with the activation energy of each stage obtained by Starink's method (Table 7), which further verifies the reliability of the kinetic parameters obtained by this method.

In addition, according to the activation energy curve of each stage of the mixed reagent obtained by the Flynn–Wall–Ozawa method, the reaction activation energy at the first, third, and fourth stages of thermal decomposition of Samples P1–P5 varies with α. This shows that the three-stage reaction process is a multi-step reaction, which is basically consistent with the results of the pyrolysis analysis. In the second stage of thermal decomposition, when α > 0.3, there is an independent linear relationship between the corresponding reaction activation energy and the conversion α. This shows that the thermal decomposition process of the sample at this stage is mainly a one-step reaction, which confirms that this stage is mainly the thermal diffusion process of the tear agent OC in the analysis of the pyrolytic behavior.

**Figure 13.** Activation energy curve of Samples P1–P5 at each stage obtained via the Flynn–Wall– Ozawa method.

Prediction of the Reaction Model of Samples P1–P5

In order to further explore the reaction mechanism of the thermal decomposition process of the main charge mixture, the Coats–Redfern method [26,38] was used to predict the most likely reaction model at each stage. According to 17 common reaction mechanism functions (Table 3), the linear fitting results of *ln[g(α)/T*2*]* and 1*/T* corresponding to the pyrolysis reaction at different stages are shown in Figure 14 (the maximum correlation coefficient has been marked in red in the figure). Table 8 shows the reaction models of four stages in the pyrolysis process of Samples P1–P5. In Figure 14 and Table 8, we can see the correlation coefficient obtained by fitting the data based on each reaction mechanism function and the most likely model of each stage of Samples P1–P5.


**Table 8.** Reaction model of four stages during the pyrolysis of Samples P1–P5.

From these results, it can be seen that the reaction models of the first, third, and fourth stages of the sample change significantly when different amounts of 5AT are added to the sample, which shows that the reaction models of each stage can be effectively changed by adding 5AT. At the same time, it also further explains the relevant mechanism of Samples P1–P5 corresponding to their different combustion characteristics. In addition, for the second stage, which is most suitable for OC's thermal diffusion temperature range, the reaction model maintains a high degree of consistency, which further verifies the correctness of the physical thermal diffusion weight loss theory of OC in the second stage of pyrolysis

weight loss. The discovery of this theory is of great significance for studying and improving the smoke characteristics of combustion tear gas mixtures.

**Figure 14.** Fitting curve of the most likely reaction mechanism model of Samples P1–P5 formula obtained via the Coats–Redfern method. (**a**) Fitting curve of P1 s reaction mechanism model. (**b**) Fitting curve of P2 s reaction mechanism model. (**c**) Fitting curve of P3 s reaction mechanism model. (**d**) Fitting curve of P4 s reaction mechanism model. (**e**) Fitting curve of P5 s reaction mechanism model.

#### **4. Conclusions**

The conclusions regarding the combustion pyrolysis characteristics and kinetic analysis of a combustion-type tear gas mixture based on 5AT are as follows:

1. Through a comparison of the maximum combustion temperature and the linear combustion rate of Samples P1–P5 with different amounts of 5AT, it was found that when the amount of 5AT is 10%, the maximum combustion temperature of the sample can be reduced by nearly 70 ◦C under the condition that the linear combustion rate is basically unchanged, thus improving the combustion environment of the mixture.


**Author Contributions:** H.Z. and X.C. contributed to the conception of the study; H.Z. and Y.G. performed the experiments; H.Z. and X.C. contributed significantly to analysis and manuscript preparation; H.Z. performed the data analysis and wrote the manuscript; X.C. helped perform the analysis with constructive discussions. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the equipment comprehensive research project (WJ2021 1A030013), (WJ20182A020036); basic research of university technology (WJY202146); basic frontier innovation research (WJY202236) and research on military theory (JLY2022089).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available on request from authors.

**Acknowledgments:** The authors gratefully acknowledge the research infrastructure provided by the equipment management and support of the College of Engineering University of the Chinese People's Armed Police Force.

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

#### **References**

