Advances in the development and characterization of combustible cartridge cases and propellants: Preparation, performance, and future prospects

2.1 Extraction filtration molding method

The extraction filtration molding approach, as shown in Figure 1, is a one-time molding procedure to achieve a high initial combustion activity and permeable combustion [16]. Batch pulping, suction filtration, and molding are mainly used for the fabrication of CCC [17]. The cellulose is thoroughly crushed by the pulverizer, and the obtained debris is mixed and stirred in a reactor with NC (energetic material), resin (binder), additives, and stabilizers. A homogeneous slurry is obtained after thorough mixing [18]. The binder, as a fundamental component of CCCs, plays a critical role in increasing the intensity and combustion performance of the cartridge [19]. The stabilizer also affects the thickening and bonding properties of CCCs [20]. Then, the slurry is poured on a porous filtration plate, which is connected to a suction tank. A circular wet blank of a specified size is created under specific pressure and time. The wet blank is then placed in a hot-pressing mold and under specified mold temperature and vapor pressure for a particular time. Finally, the desired CCC is obtained by vacuum drying.

Figure 1

Process of preparing CCC by molding method.

2.2 Rolling method

In the rolling method, CCCs are made of NC paper, which is already soaked in trinitrotoluene (TNT) or other highly energetic materials [21]. These cartridge cases possess a multi-layered structure, and the overall structure of the cartridge is loose and porous, resulting in a high amount of NC in corresponding components [22,23]. Under a humid atmosphere, these cartridge cases quickly absorb water and attain moisture equilibrium. Consequently, the high humidity in the surroundings leads to high water absorption. The humidity level has a significant impact on the as-rolled CCCs. The combustion characteristics alter with the increase in humidity, decreasing the combustion rate. Correspondingly, some waterproof measures need to be taken to improve the performance of as-rolled cartridge cases.

2.3 Wire winding method

The wire winding method involves winding resin-saturated metallic or non-metallic wire on a suitable axis for forming, curing, and demolding [24]. The preparation procedure can be summarized as follows. The NC paper roll is cut, brushed, and transferred to the spinning shaft. Then, each cartridge is wrapped with several layers of the wire, assisting the subsequent wire winding. The paper roll in each layer (except the first layer) must be covered with an adhesive coating on the upper surface of the paper using the feeding roller [25,26]. The CCC is obtained after multiple wire windings. These CCCs render a high tensile strength and elongation. However, they readily absorb moisture due to the porous surface and the presence of a significant amount of hydrophilic cellulose, compromising long-term storage and transit performance [27,28]. As a result, the weapon’s internal ballistic performance is altered, resulting in a considerable amount of combustion residue, reduced ammunition firing speed, and even barrel explosion mishaps. Furthermore, steps should be taken to modernize and simplify the procedure to overcome the complexity.

2.4 Semi-solvent method

Recently, the semi-solvent approach has been attracting more attention in the preparation of CCCs. Wang et al. synthesized an nitrated bacterial cellulose (NBC)/NC composite using a semi-solvent method and investigated the morphology, composition, mechanical strength, thermal behavior, and combustion performance, concluding that the space mesh material could effectively improve the impact resistance and combustion rate of CCC [29]. The nitration of bacterial cellulose results in NBC after the addition of nitrososulfuric acid. The synthesis method can be summarized as follows. The culture medium is used to obtain the bacterial cellulose gel, which is purified with sodium hydroxide and dehydrated with deionized water to neutralize. The bacterial cellulose is vacuum-dried at 80°C and then crushed and ground to the powder. For nitrification, powdered bacterial cellulose and nitrososulfuric acid are introduced into the reactor. After repeated alkali and water washing, the resulting product is dried and processed. The three-dimensional structure of NBC, which is one of nature’s hardest materials and possesses exceptional mechanical strength and crystallinity, is depicted in Figure 2 [30,31,32,33]. As a result, NBC is widely used in synthetic high-strength composites to improve mechanical performance. The CCCs in the application of firearm ammunition fields benefit from the semi-solvent approach, showing the promise and application prospects of the semi-solvent approach for firearm ammunition.

Figure 2

Nitrobacterial cellulose prepared by semi-solvent method under SEM [29].

2.5 Solvent-free method

The extraction filtration molding has been updated to produce CCCs without the use of solvents. In comparison to the extraction filtration molding, this method avoids not only the solvent extraction process but also the loss of the cartridge’s surface compactness and bulk density induced by long-term immersion in the solvent solution [34,35]. By using a solvent-free approach, Yang et al. successfully synthesized an azide nitramine propellant with diazidonitrazapentane as a plasticizer and a double base propellant with nitroglycerin (NG) as a plasticizer and investigated the internal structure and mechanical properties in detail [36]. The cost of producing CCCs has been considerably decreased, and the production cycle has been shortened, thanks to the extensive study of the solvent-free approach, which promotes the continuous progress of the preparation process of CCCs.

2.6 Three-dimensional printing of energetic materials

Nowadays, stereolithography and digital light processing technology are the most prominent research directions for propellants, leading to the growth of 3D printing technology for energetic materials [37]. Yang et al. utilized 3D printing technology to break the constraints of traditional cartridge extrusion technology successfully and employed a variety of characterization methods to test the internal structure, mechanical strength, and combustion performance of the new propellant [38]. Figure 3 depicts the fundamental experimental procedure, where cyclotrimethyltritramine (RDX) particles are employed to change the energy properties and oxygen balance of the cartridge. Initially, a digital simulation is performed to optimize the design, and then photolithography 3D printing technology is employed to create a flammable cartridge with a complicated geometric structure. Li et al. developed two types of propellants based on 3D printing of energetic materials, increasing the thermodynamic properties and considerably improving the combustion rate in the experimental tests [39]. As a novel development approach, 3D printing of energetic materials accelerates the optimization and upgrading of CCCs, promoting the intelligent and precise preparation of CCCs.

Figure 3

3D printing and analysis of energetic materials [38].

3.1 Nanometal-modified CCCs

Nanometal-modified cartridge cases exploit the nanostructure of metals with unique advantages, such as the utilization of a low amount of modifying agents, high reaction area, and excellent thermal efficiency [40,41]. Moreover, the nanoparticles of different metals, such as aluminum and magnesium, could react with inert gases to improve the ignition temperature of the flame [42]. Moreover, the metallic nanoparticles generate a sufficient amount of condensed liquid during the burning reaction, leading to the metal reunion phenomenon [43]. It affects the viscosity and thermal balance, as well as reduces the superficial burning area, resulting in lower thermal efficiency. In recent years, it has been demonstrated that the addition of some macromolecule polymers into nanometal-modified cartridges could dramatically cut down the size of metal reunion [44,45]. As shown in Figure 4, the mixture of aluminum and macromolecule polymer solved the problem of metal reunion during the burning process. The nanometal-modified cartridge cases possess tremendous potential in enhancing combustion performance and further development of modern CCCs.

Figure 4

Protective oxide layer of nano alumina particles [42].

3.2 NC CCCs

NC is an energetic material (Figure 5) known as fire cotton, which is widely used in ammunition and gunpowder fields. Conventional manufacturing is carried out by placing the dry cotton in a mixture of sulfuric and nitric acids, leading to a highly exothermic esterification reaction and resulting in NC [46,47,48]. The polyacrylonitrile-modified NC CCC, as an early CCC, improves the ductility and cohesiveness of CCCs, realizing the combination of CCCs with multiple propellants [49]. This technology boosts the application of CCCs in large-caliber ammunition. The polymer, as a binder for NC materials, forms a tough and tensile gum elastic. The gum elastic not only effectively enhances the sensitivity and glass-transition temperature of NC but also promotes flexibility and mechanical properties [50,51]. It could tremendously reduce the self-combustion phenomenon of NC under high-temperature and humid environments. The TG/DTA results reveal that the thermal stability of NC is relevant to the particle size and nitrate content [52,53]. The small particle size of NC and low nitrate content result in a high decomposition temperature.

Figure 5

NC at different temperatures [46].

Moreover, the stability of synthetic NC materials is also improved by the low content of NC and nitrate. The novel NC CCC chooses flax fibers to replace wood fibers and cotton. Also, the filtration casting technology is utilized to synthesize heat-resistant nitrocellulose CCC successfully. Nonetheless, the mechanical properties are compromised due to the intrinsic properties of flax fibers [54]. Overall, the surface and structural modifications of NC are constantly being carried out to optimize the performance of CCC.

3.3 Microporous CCCs

The microporous energetic materials, e.g., RDX, offer a variety of benefits, including variable energy content, rapid burning, minimal vulnerability, and high heat resistance. The foaming agent, e.g., SC-CO₂, can be used to produce the porous structure of microporous energetic materials [55,56,57]. The microporous energetic materials contain various pore sizes by altering the foaming time. The microporous CCCs exhibit a wide array of unique properties (Figure 6) [58]. One should note that an increase in interior surface area and porosity enhances the burning area and airflow penetration.

Figure 6

Microporous CCC structure [58]. (a) Microstructure of microporous combustible cartridge. (b) Macrostructure of microporous combustible cartridge.

Furthermore, the low density of CCCs leads to a lower ignition delay, ensuring a high burning rate and consumption during launch [58,59]. Moreover, the microporous NC CCCs form hydrogen bonds due to their inner structure, enhancing the water absorption process and severely affecting the burning process [60,61]. However, as microporous CCCs need to remove the metallic shell altogether, NC is not a suitable choice, and it is necessary to explore novel materials to meet the demands of high heat resistance and low sensitivity. The fully-CCCs are being investigated to achieve the goal of zero-combustion residue.

3.4 Resin-based CCCs

The resin-based CCCs can increase mechanical characteristics, viscosity, and ductility by connecting the resin base to the combustible part. The high-temperature variant characteristic of resin has been solved by adding a stabilizer, such as organic clay or diphenylamine [62]. The mechanical properties and glass-transition temperature of resin-based CCCs [63] have been substantially enhanced by the addition of a plasticizer. As shown in Figure 7, the NC, polyvinyl alcohol, diphenylamine, and cellulose were used to investigate the resin-based CCCs, and a series of tests were conducted, including density, ductility, thermal stability, thermal sensitivity, and ash content determination. The results reveal that the mechanical strength, burning rate, and thermal stability were improved by altering the relative proportion of raw materials. To some extent, the resin-based CCCs have rendered some notable results, but further research is required for performance optimization.

Figure 7

Resin-based CCC [64].

3.5 Nano-NC CCCs

The nano-NC CCCs, as shown in Figure 8, are developed by using a nanometal cartridge [64]. It is worth noting that composite materials, such as nano-boron/nano-NC and NC/nano-aluminum, can improve the overall performance properties of CCCs. The nano-NC is prepared using the electrostatic spinning method, mixing solid propellants and NC. It effectively resolves the high-temperature reunion problem of nanoparticles and promotes mechanical properties and burning rate [65]. In order to improve thermal stability, a wide variety of nano-compounds can be synthesized, such as metal oxides, natural zeolite, or clay [66,67].

Figure 8

Microscopic characterization of nano nitrocellulose [69]. (a) Microscopic SEM and (b) transmission electron microscopy characterization diagrams.

By reacting with decomposition products, these compounds could dramatically restrain the catalytic effect of the decomposition reaction, which effectively guarantees security and increases the service life of CCCs. To reduce the thermal sensitivity of NC, earlier CCCs adopted some thermoplastic materials to replace sectional NC [68]. However, these CCCs exhibit inherent difficulties in molding, processing, and recycling [69,70]. Focusing on these difficulties, we should explore convenient and feasible processing techniques to promote practical applications.

3.6 Composite-coated CCCs

The composite-coated CCCs are prepared by coating one or multiple layers on the surface of the porous CCC. On the premise of not affecting the burning efficiency, these CCCs not only resolve the water absorption problem but also improve the environmental adaptability and mechanical properties [71,72,73]. The composite-coating CCCs, as one of the most advanced and deadly weapon ammunition, could be traced back to the 120 mm tank ammunition developed by the American military [74]. In our previous work, we used the sol-gel method to prepare a series of organosilicon-modified coatings that protected CCCs from burning for 168.4 s at 230°C, with a water absorption rate of only 1.98 wt% [75]. Chen et al. fabricated a multifunctional coating that dramatically improved the mechanical performance and environmental adaptability of CCCs [76]. Zhang et al. prepared a new composite coating that could maximize and improve the heat resistance time of CCCs by 50% at 220°C [77]. This kind of coating significantly promotes the application scope of CCCs without affecting the combustion performance. As shown in Figure 9, the smooth surface structure of the composite-coated CCCs is quite different from the uncoated CCCs [77]. With the continuous research effort, the performance of the composite-coated CCCs will be continuously optimized.

Figure 9

A comparison of the SEM images of the uncoated and coated combustible cartridges [77]. (a) An uncoated and (b) coated combustible cartridge.

4.1 SEM/EDS imaging analysis

As seen in Figure 10, the microstructural characterization by SEM not only provides the composition and morphology of the energetic materials but also demonstrates the post-combustion structure of the residue [77]. For instance, Li et al. have investigated the influence of pore size on the combustion properties of energetic materials using SEM [78]. Nowadays, SEM and EDS, as one of the most authoritative analytical methods, have achieved highly efficient characterization of cartridge cases by SEM equipped with energy dispersive X-ray spectrometer [79,80]. As shown in Figure 10, Kara et al. utilized SEM/EDS and determined the compositional structure of different types of residues of the cartridge cases [81]. SEM/EDS also plays a vital role in analyzing the environmental hazards of CCCs. It is expected to improve the research of combustion properties by further determining the burning residue of CCCs.

Figure 10

(a)–(d) SEM/EDS imaging of combustion residues from different ammunition types [81].

4.2 Closed vessel experiments

The closed vessel experiment is widely used as a conventional analysis method of testing combustibility and accidental flammability [81]. Cheng et al. adopted the closed vessel experiment to test the explosion of energetic materials, estimating safety and preventing hidden dangers of the energetic materials [82]. Figure 11 shows the construction of the transparent closed vessel experiment, equipped with tempered glass on both sides, to observe the inner combustion phenomenon visually [83]. The improved closed pressure vessel can also be equipped with a fast response pressure sensor on the pipe wall and a high-speed camera for real-time recording of the flaming. The closed vessel experiment, at present, has become one of the most promising analytical methods for energetic materials. One should note that the simulation of closed vessels can effectively assist the research on the combustion behavior and explosion properties of energetic materials (Figure 11) [84]. The closed vessel experiment has exerted immense significance in testing the combustion behaviors. Meanwhile, this analysis method is gradually being applied to the characterization of CCCs.

Figure 11

Closed container experimental device [84].

4.3 Thermodynamical analysis

Thermodynamic analysis, as a theoretical calculation method of energetic materials, is initially established on the basis of whole-burning materials, converting combustible materials into thermal energy and transforming it into mechanical energy [85]. The objective of the research is to determine the optimal conditions of the chemical reaction and achieve the maximum yield of the target product. Nowadays, computer simulations are commonly used in thermodynamic analysis, which cannot be completed by analytical tests, revealing energy characteristics, thermodynamical behavior, and physical properties of combustion products. Aknazarov et al. combined laser heating and thermal imaging to create a thermodynamic model for the burning process, which included thermodynamic equations of laser power density, solid propellant density, combustion surface heat production, and surface heat flux [86].

Furthermore, thermodynamical analysis cannot merely suit the thermodynamical calculations of the energetic materials. However, it can also be used to simulate the surface evolution of the burning process and realize the mathematical model of 3D combustion surface regression and fluid flow coupling simulations [87,88]. The experimental results also indicate the correctness and efficacy of the regression model for the combustion surface of energetic materials. The integrated framework of regression and fluid flow coupling simulations of the combustion surfaces of energetic materials provide a dependable solution to complicated physical challenges. The thermodynamic analysis has opened avenues for further development of energetic materials from the viewpoint of fundamental science.

4.4 DSC analysis

The DSC can measure the thermal behavior of materials during a programmed temperature variation and obtain critical parameters, such as glass transition temperature, reaction temperature, reaction enthalpy, oxidation stability, and specific heat of materials. DSC is also widely employed in the field of energy materials. As shown in Figure 12a, Cheng et al. synthesized a series of energetic ferrocene compounds and measured the exothermic decomposition process of each compound using a DSC curve, demonstrating the positive influence of ferrocene compound on the thermal stability of energetic materials [89,90]. Tang et al. investigated the catalytic effect of Ti₃C₂ nitride on the combustion of ammonium perchlorate (AP) and utilized DSC to evaluate AP and modified AP and Ti₃C₂ nitride materials. The exothermic crystal transformation of AP occurs during the first stage, the lowest exothermic decomposition of AP occurs during the second stage, and the higher exothermic decomposition occurs during the third stage (Figure 12b) [91,92,93]. The presence of Ti₃C₂ nitride increases the temperature of first stage exothermic transformation process of the AP. As a result, the thermal stability of AP at low temperatures is effectively improved, the maximum exothermic decomposition temperature of AP is significantly reduced, and the thermal decomposition process is accelerated. The utilization of a catalyst considerably improves the impact of the AP on thermal breakdown. Kim et al. investigated the use of carbon black (CB) nanoparticles to improve the combustion characteristics of metal-based energetic materials and examined the changes in the DSC curve of Al/CuO under varying carbon content [94]. As shown in Figure 13, the total combustion energy of the energetic material decreased with the increase in carbon content, implying that the CB nanoparticles act as a control medium in self-propagating combustion and explosion reactions. Furthermore, Liu et al. investigated the impact of magnesium-based hydrogen storage materials on AP combustion performance, concluding that hydrogen storage materials can speed up the thermal decomposition of AP by changing the DSC decomposition temperature curve [95].

Figure 12

(a) DSC curves of ferrocene compounds. (b) DSC curves of AP and Ti₃C₂/AP composites [90,93]. (a: 1–8: (ferrocenyl methyl)-propyl dimethylammonium nitrate, butyl dimethylammonium nitrate, pentyl dimethylammonium nitrate, hexyl dimethylammonium nitrate, heptyl dimethylammonium nitrate, octyl dimethylammonium nitrate, nonyl dimethylammonium nitrate, and dodecyl-dimethylammonium nitrate).

Figure 13

DSC curves of Al/CuO nanoparticle composites [94].

4.5 TGA

TGA is widely used to study the thermal behavior of energetic materials, and it is typically employed prior to combustion studies to investigate the relationship between mass loss and temperature [96,97]. It is frequently used in conjunction with DSC to investigate the combustion mechanism of energetic materials in detail. TGA was used by Rao et al. to investigate the thermal stability of ferrocene as a propellant binder. The experimental results revealed that the presence of ferrocene increased the thermal stability of propellants without affecting the microstructure of energetic components [98]. Using the TG-DTA analytical method, Chatragadda and Vargeese investigated the effect of nano copper oxide catalyst on the thermal breakdown of AP [99]. The original three-stage decomposition of AP was changed into a two-stage decomposition process after the addition of catalyst, as shown in Figure 14. Copper oxide created hydrogen bonds and Lewis acid coordination with AP decomposition products, disrupting the decomposition reaction balance and speeding up the rate of AP decomposition. Hence, TGA is widely employed in the realm of energy materials, contributing to increased innovation and progress.

Figure 14

TG and α-T kinetic curves [99].

4.6 Terahertz time-domain spectroscopy

Terahertz waves are electromagnetic radiations whose frequency ranges from microwave to infrared bands. As the vibrational and rotational energy levels of several macromolecules are in the given range, they exhibit strong absorption and resonance to terahertz waves [100,101]. Terahertz time-domain spectroscopy, at present, is a standard analytic tool to measure the far-infrared refractive index and power absorption coefficient of the materials, which is mainly used for the transmission study and analyzes minute changes in material composition and structure [102]. Thus, terahertz time-domain spectroscopy is widely used in studying the spectral characteristics of chemical and biological molecules. Based on the terahertz time-domain imaging spectroscopy, the absorbance of different materials can be analyzed according to the different colors of each part [103]. The darker color corresponds to higher absorbance and vice versa. In addition, terahertz time-domain spectroscopy can also be used to quickly and easily identify different kinds of energetic materials through spectral fingerprint regions [104]. As shown in Figure 15, the transmission mode of the terahertz time-domain spectroscopy system consists of an ultra-fast pulse laser, translation-stage delay line, controller, and terahertz generation and detection device. Terahertz time-domain imaging spectroscopy has been applied in the research of polymeric materials and played a huge role in understanding the chemistry and structure of polymers [105]. The development of terahertz time-domain spectroscopy promotes the microscopic characterization of polymeric materials. It guarantees accurate results, playing an increasingly important role in the analysis and identification of energetic materials.

Figure 15

Terahertz time domain spectral system [100].

4.7 MS

To date, MS and ion migration spectrum are often combined for the detection and analysis of explosives, such as RDX, TNT, pentaerythritol tetrachitroester (PETN), and 2, 4-dinitrotoluene [106,107]. Traditional isotope-ratio mass spectrometry has also been applied in the field of explosive characterization for several decades [108]. Based on the isotope-ratio mass spectrometry, multiple components with independent isotope characteristics were analyzed to generate multivariable data for identification, and then, the characteristic information of explosives was collected. Moreover, corresponding data were created to facilitate the sample comparison and establish a relationship between variables. On the other hand, rapid and real-time mass spectrometry can be used to analyze potential by-products and decomposition products of explosives [109]. However, when analyzing explosives, the matrix effect and signal suppression of explosives were enhanced with the increase in mass of the by-products, hindering the further development of this technology.

Moreover, single-particle inductively-coupled plasma mass spectrometry (SPICP-MS) plays a vital role in the analysis of shooting residue, analyzing the ion pulses generated by each material to characterize the underlying substance of the residue. Hence, SPICP-MS can accurately detect and count nanoparticles from the gunshot residue using either single or double-element modes [110]. As shown in Figure 16, inorganic explosives and components are studied using induced dissociation mass spectrometry, which combines the collision-induced dissociation of explosives within an ion source with laser desorption/ionization mass spectrometry (LDI-MS), trace detection, volume quantification, and chemical imaging [111]. In addition to the lower organic noise, the detection of inorganic elements, molecules, and ions in explosives has been improved, and nano-level detection of inorganic elements and molecules in compounds has been achieved. Overall, MS is being used in different modes and plays a vital role in characterizing energetic materials and their residue.

Figure 16

LDI-MS imaging device [111].

4.8 Liquid-mass characterization

Liquid-mass characterization is widely employed to overcome the test limitations of lead-free and metal-free ammunition, in general, and in the field of ammunition residue, in particular. As shown in Figure 17, it is mainly utilized in forensic identification and monitoring of munitions residue pollution 32 min [112,113]. Liquid-mass analysis provides a fast and sensitive method for the analysis and identification of inorganic compounds and organic compounds in the gun-machine ammunition residue [114]. Ochsenbein et al. carried out the liquid chromatography (LC)-electrospray tandem mass spectrometry (MS) of explosive analysis of lake and tributary water sources [115]. The contents of cyclotetramethylenitramine (HMX), RDX, NG, PETN, and other trace explosives in lake water were determined by off-line solid-phase extraction (SPE) and direct injection analysis method, resulting in effective detection and identification of gun and ammunition residue. Sun et al. have developed an automatic online SPE, LC, and MS for the analysis of explosive residue in water [116], as detailed in Figure 17. This approach does not require sample pretreatment. It can automatically perform the conditioning, pre-concentration, elution, separation, and detection processes within 32 min. This approach also offers a high sample throughput, which lowers the detection limit. By examining ammunition powder, Laza et al. have broadened the application of liquid-mass analysis technology in the realm of chemical ballistics [117,118]. The liquid-mass analysis technology not only increases the accuracy of ammunition forensic identification technology but also provides accurate reference data for ammunition synthesis and preparation, as well as promotes the research and development of ammunition technology.

Figure 17

(a) and (b) Liquid-mass analysis process [116].

4.9 Near-infrared analysis

The presence of highly disharmonic bands and overtones between X–H bonds can be determined using near-infrared analysis technology, which is an early explosive analysis approach [119,120,121]. Using the near-infrared analysis technique, Zapata et al. investigated 18 different energetic materials [122]. However, these near-infrared approaches possess certain limitations when measuring NC single- or double-base propellants with a nitrogen content of >70%. Though the near-infrared analysis technology is suitable for analyzing C–H and N–H bonds in propellants, its limitation to NC analysis remains a major roadblock in its development.

4.10 Gas-qualitative analysis

As a common energetic material, NC is widely used in propellants, explosives, cartridges, and other fields. However, the analysis of NC is difficult in terms of research owing to its non-volatility and difficult detection. Chajistamatiou et al. have established a rapid identification method for NC by gas chromatography (GC)-MS [123], which can realize the classification of smokeless powder and distinguish between ordinary and colloidal explosives credibly. Moreover, the gas chromatography-solid phase extraction technique of Li et al. can improve the precision of the tested samples and achieve comprehensive characterization of the samples [124]. For the first time, Joshi et al. utilized solid-phase microextraction as sampling and pre-concentration technology to detect volatile and semi-volatile additives in smokeless powder comprehensively and combined it with gas-mass analysis technology to study 65 different smokeless powders [125]. Thus, a database of smokeless powder has been completed, which will be helpful for the further development of energetic materials. The database of gas-mass analysis technology will continue to improve and promote the characterization and analysis of energetic materials.

5.1 Geometric structure

Managing the combustion rate of a propellant, which is typically accomplished by controlling the combustion area, is at the heart of propellant interior ballistics theory. The successful development of existing multi-layer and porous propellants demonstrates that the combustion area of propellant can be efficiently modified by designing the geometric structure of the propellant, allowing the adjustment of combustion rate.

5.1.1 Multi-layer propellants

The basic structure of multi-layer propellants consists of an interior rapid-burning layer and an external slow-burning layer with high loading density, controlled combustion, and the ability to load a range of propellant formulas. This form of propellant may efficiently reduce the initial chamber pressure and improve combustion, thereby improving the propellant’s launch power and ballistic efficiency [129,130]. Starting with the combustion characteristics and controllability of the propellant, Wang et al. combined the high-energy propellant formula, high loading density geometry, high combustion increasing surface structure, and other technologies to investigate the programable combustion propellant charge [131]. Fu et al. [132] used the multi-layer tubular propellant as an example to compute the ballistic performance of multi-layer propellant using numerical simulations and concluded the related shape function, as shown in Figure 18. Zhang et al. selected the existing propellant raw ingredients and used the physical composite approach to create a series of multi-layered sheet propellants. It has been demonstrated that multi-layered sheet propellants possess excellent combustion properties and considerably boost initial velocity while maintaining the chamber pressure [133,134]. Liu et al. investigated the effect of multi-layers on propellant combustion performance and established the matching shape function for multi-layered tube propellant, square multi-layered flake propellant, and circular multi-layered flake propellant [135]. The enhancement in combustion performance of multi-layered tube propellant was found to be better than the multi-layered flake propellant. The multi-layered propellant can successfully improve combustion performance, expanding the application prospects of propellants.

Figure 18

Multi-layered tubular propellant [133]. l: Length of multilayer propellant, d₀: Inner diameter of inner fast burning layer, d_h: Outer diameter of inner quick burning layer, D_h: Inner diameter of outer flame-retardant layer, D₀: Outer diameter of outer flame-retardant layer.

5.1.2 Porous propellants

The gas generation law of propellant can be efficiently regulated by developing a microporous structure on its surface while maintaining the overall structure of the original propellant and not introducing other components. This propellant, which is separated into porous granular propellant and porous rod propellant, is widely used in the field of weapons. Porous rod propellants can significantly improve the loading density as compared with porous granular propellants [136,137,138,139,140]. Xu et al. studied and designed the porous rod propellant based on its constant volume combustion performance and discovered that the combustion effect is related to the same direction notch spacing. The smaller notch spacing guarantees the mechanical properties of porous rod propellant and renders a high combustion effect [141]. This study established a theoretical foundation for the development and utilization of porous rod propellants. Yang et al. used numerical simulations to investigate the ultra-porous disc propellant and developed a constant volume combustion model, as illustrated in Figure 19. The geometric configuration and quantitative relationship rendered a significant impact on the combustion performance of porous disc propellant. The physical structure of porous disc propellant can be tuned to lower the aperture and increase the thickness, improving the combustion performance [142]. The porous propellant must have a flame-retardant coating due to the porous surface layer. Yang et al. utilized an epoxy-based composite with high surface tension to cover the porous surface of the propellant. Owing to the action of surface tension during the curing process, the inner hole of the protective layer is exposed, forming a non-plugging propellant at the end face and successfully resolving the igniting issue caused by the blockage of the inner hole [143]. To a certain extent, CCCs can act as a technological extension of porous propellants.

Figure 19

The partially cut multi-perforated stick propellant [142]. (a) Incongruous incision intervals and (b) synclastic incision intervals.

5.2 Burning rate

The precise measurements of propellant combustion rate are required for propellant optimization. Currently, the burning rate of propellant can be classified into two categories: linear burning rate and mass burning rate, as discussed below.

5.2.1 Linear burning rate

Numerous experimental studies and numerical simulations have been carried out to determine the linear burning rate of a propellant. The optimal propellant formula and geometric size are optimized by analyzing the pressure–time curve, which is obtained from the corresponding simulated equation and thermochemical parameters of the propellant [144]. The electronically controlled propellant test instrument, as shown in Figure 20, can test the burning rate of a propellant under various pressures [145,146]. Zhang et al. have used laser ignition technology, highspeed video, and image processing to systematically study the linear burning rate of an AP-based composite propellant and established a corresponding numerical combustion rate calculation model, greatly simplifying the analysis of composite propellants [147]. Through numerical simulations of the composite propellant, Sangtyani et al. have deduced the matching burning rate formula. A new composite propellant is produced using ultra-fine AP or nanocombustion supporting agent, which effectively boosts the burning rate and paves the path for the development of high-performance propellants [148]. Rasmont et al. measured three distinct propellant types and numerically simulated the corresponding burning rate equations. Also, the burning rate of green ionic liquid propellant has been successfully analyzed [149]. The linear burning rate is widely employed in the research of propellants to understand and design their reaction mechanism.

Figure 20

Electrically controlled experiment of propellants [146].

5.3 Ignition delay time

One of the most critical indicators of propellant combustion performance is ignition delay. Theoretically, a shorter ignition delay is desired for optimal performance. The electronically controlled solid propellant (ECSP) combustion diagnosis system, as depicted in Figure 21, is an analysis tool for determining propellant ignition delay. The combustion chamber, DC power supply, oscilloscope, image acquisition system, and pressure control system are the key components of this system. Using the ECSP combustion diagnosis system, Bao et al. investigated the ignition delay of propellant at various temperatures and discovered that the increase in initial temperature of the propellant improves the surface ignition energy, reduces the temperature difference with the ignition temperature, facilitates the ignition and decreases the ignition delay time [153]. Chen et al. suggested a single boron particle combustion ignition model based on the boron particle ignition and combustion experimental model and demonstrated that the ignition delay is proportional to the initial temperature of the propellant. It is reasonable to assume that a high initial temperature shortens the igniting time. However, several limitations remain, such as the inability to forecast large changes in ignition time under high pressure and high-temperature conditions [154]. Aly et al. investigated the effect of particle size on the ignition and combustion performance of magnesium and aluminum bimetallic powder and proposed a practical two-step grinding process to create magnesium and aluminum alloy powder with a desired particle size [155]. Through a single particle laser experiment, the powder cloud combustion experiment revealed that the ignition delay reduces, and the burning rate becomes similar to the pure aluminum particles. The ignition performance and burning rate of propellant can be considerably increased by adjusting the size of energetic components. Wang et al. looked into the influence of iodine-containing active compounds and oxidants on the propellant ignition delay. The nanocomposite aluminothermic reagent not only increased the propellant stability but also significantly lowered the ignition temperature and shortened the ignition delay [156]. As a result, the interior aluminothermic reagent considerably improved the ballistic performance of propellants. The surface coating is used to adjust the kinetics of this reaction, which significantly improves the energy output and combustion rate, shortens the ignition delay, and enhances the combustion performance. Liu et al. explored the optimization of the combustion performance of nano-aluminum, the surface coating of which is used to adjust the kinetics of this reaction, significantly improving the energy output and combustion rate, shortening the ignition delay and enhancing the combustion performance [157]. By integrating theoretical and experimental data, a novel approach for adjusting dynamics and enhancing propellant combustion performance is proposed. In terms of propellant ignition delay time, the research shall continue to improve the combustion performance and promote its utilization in weapons.

Figure 21

Diagram of ECSP combustion diagnosis system [154].

5.4 Internal components

The key technology of propellant design is internal components. Hence, the energy density and combustion rate can be increased by the design and optimization of internal composition. By adding appropriate energetic components, the weapon power of propellant can be efficiently increased. Dibutyl phthalate, for example, is commonly utilized in propellant composition. It can be employed not only as a desensitizer to increase propellant combustion performance but also as a plasticizer to improve the physical and mechanical properties of the propellants [158]. Kumar et al. aimed to tune the combustion performance of propellants by mixing combustible boron, magnesium, and aluminum powders into a double-base propellant, revealing that the presence of metallic powder improves the combustion heat, burning rate, and flame temperature of the propellants [159]. Also, the change in flame temperature is attained without affecting the thermal decomposition effect of the propellant. Yao et al. evaluated the effect of NC content on the capacity of single-base propellants to increase combustion. The experimental results revealed that when the amount of NC is increased, the burning rate of propellant is gradually increased, and the ability to increase the combustion of propellant is also improved [160]. The combustion performance of RDX single base propellant has been thoroughly investigated by Li et al., where the experimental results revealed that the burning rate of propellant initially decreased with the increase in RDX percentage, followed by a gradual increase [161]. Owing to the endothermic melting heat of RDX, the burning rate of propellant is reduced when the RDX level is low. When the RDX concentration is high, the overall heat of the whole system is raised, which improves the burning rate. Wang et al. investigated the combustion performance of CL20-HTPB, which improved the burning rate of propellants [162]. The inclusion of additives successfully adjusts the burning rate of propellants. Ma et al. investigated the influence of combustion catalysts on the burning rate and demonstrated that the addition of catalysts could effectively adjust the burning rate of the low-pressure section during the combustion process, optimizing the internal ballistic performance and improving the initial combustion rate of propellant. Hence, the introduction of a catalyst can significantly improve the propellant combustion rate. From the viewpoint of weaponry, the design of internal propellant components is still a research focus. The internal ballistic performance of propellant can be quickly and effectively modified due to the design of components.

7.1 Nitrogen-rich energetic materials

Nitrogen-rich energetic materials, up to now, have gained much attention due to their remarkable properties, including large nitrogen content, high energy density, good thermal stability, low sensitivity, good energetic performance, and environmental friendliness [176]. Tetrazole, among them, has the highest nitrogen and highest energy contents among the stable azoles. Zhang et al. demonstrated that CN10 has high detonation energy and heat formation, making it an ideal candidate for high energy density material [177]. Tang et al. used a one-step method to synthesize tetrazole compounds bearing the N₅ group that showed high energy performance and low sensitivities [178]. Yao et al. concentrated on exploring the compatibility of NaN₅ with traditional energetic materials. They found that NaN₅/AP, NaN₅/HMX, NaN₅/RDX, and NaN₅/CL-20 have good compatibility, whereas NaN₅/HTPB possesses moderate compatibility [179]. The nitrogen-rich energetic materials based on mono-, di-, tri-, and tetra-tetrazole skeletons were prepared and characterized successfully and have attracted increasing popularity. In designing and manufacturing practically useful high-nitrogen tetrazole-based energetic materials, a major challenge remains in achieving the delicate balance between excellent detonation performance and good insensitivity.

7.2 Insensitive energetic materials

Insensitive energetic materials could provide an achievable way for synthesizing environmentally compatible energetic materials to balance between energy and stability. Bhatia et al. were able to convert 3-nitrotriazoles into various explosophores, which exhibited high thermal stability [180]. Song et al. adopted polydopamine for incorporation in energetic nanocomposites, which offered a new research approach for the biological synthesis of explosive materials and energetic nanomaterials [181]. Song et al. introduced a new method of constructing host−guest explosives using solvent vapor/gas induction that significantly improved the structural stability, contributing to the high performance of the explosive skeleton and the detonation velocity [182]. Therefore, insensitive energetic materials may offer a promising approach to synthesizing novel energetic materials with outstanding detonation capability, enhanced combustion performance, and safety performance.

7.3 Silicon nano-energetic materials

Nowadays, silicon nano-energetic materials, compared to traditional energetic materials, possess excellent hygroscopicity performance that has drawn more and more attention to its preparation, mechanism, combustion performance, and surface modification [183]. Clément et al. prepared numerous pSi energetic arrays on 4-inch mono-crystalline silicon wafers, which demonstrated more effective combustion performance than analogous nitrates [184]. Yang et al. adopted a two-step metal-assisted chemical etching method to prepare silicon nanowire-based energetic materials, providing a novel approach to improve the hydrophobic properties of silicon-based energetic materials [185].

7.4 Glycidyl azide polymer (GAP)-based adhesive materials

GAP, an energetic adhesive, has gained increasing popularity due to its advantages of good burnout, high energy, high burning rate, low sensitivity, and environmentally friendly, as it only releases clean gases such as N₂ and CO₂ [186,187]. Wu et al. prepared a GAP polymer film with a tensile strength of 4.65 MPa, a 78.49% elongation before breaking, and a low residue carbon rate of only 2.47% [188]. GAP is a clean adhesive that provides excellent mechanical and combustion performance and eco-friendly development of GAP-based films.

9.1 Charging process of CCCs

The charging process of CCCs is mainly divided into preparation, charging, and full bomb assembly [12,13,14].

9.2 Difference between CCCs and metal cartridge cases

As can be seen in Table 1, we have listed the six differences between CCCs and metal cartridge cases. Compared with metal cartridge cases, CCCs not only save the metal resource and preparation costs but also reduce the weight of ammunition. Moreover, CCCs could also provide energy using combustion itself to accelerate the original firing rate. Additionally, CCCs generate small amounts of exhaust that contain fewer toxic and harmful gases. However, CCCs were mainly used in tanks with a higher rate of fire and limited volume, which was not suitable for a very high rate of fire and high-sealing tank guns.

10.1 Conclusion

In conclusion, this study provides a systematic review of advancements in CCCs, highlighting various types such as NC-based, microporous, resin-based, nano-NC, and composite-coated cartridges, each offering distinct mechanical, thermal, and environmental benefits. CCCs have shown significant potential to enhance ammunition by reducing weight, lowering costs, and minimizing environmental impact compared to traditional metal cartridges. This comprehensive analysis covers advancements in CCC preparation methods, including extraction, filtration, molding, solvent-free techniques, and three-dimensional printing, as well as innovations in composite coatings that improve storage stability and environmental resilience. This study examined the internal ballistic properties of propellants and CCCs to optimize ballistic performance, with a focus on combustion rate, ignition delay, and geometric structure. Advanced characterization techniques, such as SEM/EDS, DSC, TG, and terahertz time-domain spectroscopy, were considered to provide detailed insights into the structural and thermal properties of CCCs and propellants, guiding the development of safer and more efficient designs.

10.2 Perspectives

This study identifies several areas for further research to improve the performance and sustainability of CCCs. First, future studies should focus on developing CCCs with enhanced water resistance and salt corrosion protection, as well as on creating low-toxicity or non-toxic energetic materials that reduce environmental pollution and residue. Second, additional research is needed to optimize the combustion rate and ignition compatibility of CCCs with various propellants. Improving the burn rate compatibility between CCCs and propellants could greatly enhance internal ballistic performance and boost weapon effectiveness. Third, efforts should be made to develop CCCs suitable for modular charge systems to accommodate various ammunition calibers and configurations. Additionally, enhancing the temperature resilience of CCCs would ensure functionality in a broader range of operational environments. Fourth, expanding the use of CCCs to include large-caliber artillery, long-range tactical rockets, and other high-energy weaponry could increase their utility in military applications. Finally, focusing on the development of environmentally sustainable materials, such as eco-friendly propellants and GAP-based adhesives, is essential for reducing the ecological footprint of CCCs, providing eco-friendly alternatives without compromising performance.