Influence of Mg/Al Coating on the Ignition and Combustion Behavior of Boron Powder

Abstract

Amorphous boron powder, as a high-energy fuel, is widely used in the energy sector. However, its ignition and combustion difficulties have long limited its performance in propellants, explosives, and pyrotechnics. In this study, Mg/Al-coated boron powder with enhanced combustion properties was synthesized using the electrical explosion method. To investigate the effect of Mg/Al coating on the ignition and combustion behavior of boron powder, four samples with different Mg/Al coating contents (4 wt.%, 6 wt.%, 8 wt.%, and 10 wt.%) were prepared. Compared with raw B95 boron powder, the coated powders showed a significant reduction in particle size (from 2.9 μm to 0.2–0.3 μm) and a marked increase in specific surface area (from 10.37 m²/g to over 20 m²/g). The Mg/Al coating formed a uniform layer on the boron surface, which reduced the ignition delay time from 143 ms to 40–50 ms and significantly improved the combustion rate, combustion pressure, and combustion calorific value. These results demonstrate that Mg/Al coating effectively promotes rapid ignition and sustained combustion of boron particles. Furthermore, with the increasing Mg/Al content, the ignition delay time decreased progressively, while the combustion rate, combustion pressure, and heat release increased accordingly, reaching optimal values at 8 wt.% Mg/Al. An analysis of the combustion residues revealed that both Mg and Al reacted with boron oxide to form new multicomponent compounds, which reduced the barrier effect of the oxide layer on oxygen diffusion into the boron core, thereby facilitating continuous combustion and high heat release. This work innovatively employs the electrical explosion method to prepare dual-metal-coated boron powders and, for the first time, reveals the synergistic promotion effect of Mg and Al coatings on the ignition and combustion performance of boron. The results provide both experimental data and theoretical support for the high-energy release and practical application of boron-based fuels.

1. Introduction

Boron (B), as a high-energy metal fuel, possesses an exceptionally high gravimetric heat of combustion, with a theoretical value reaching approximately 58,000 kJ/kg—about 2.3 times that of magnesium and 1.9 times that of aluminum—ranking among the highest of all known metallic elements, second only to beryllium [1,2,3]. Owing to its high energy density, boron has been widely employed in high-energy propellants and explosive systems [4,5,6]. However, several challenges persist in the practical combustion of boron powders. Primarily, a native oxide layer on the surface of boron particles serves as a protective barrier, hindering the contact between oxygen and active boron, thus resulting in ignition difficulties [7]. Moreover, this oxide layer tends to thicken during combustion, further restricting oxygen diffusion and impeding the complete combustion of the boron core [8]. Additionally, boron trioxide (B₂O₃) has a relatively low melting point (718 K) and a high boiling point (2316 K), which facilitates the formation of a viscous liquid film during combustion. This film tends to adhere to the particle surface, blocking oxygen diffusion and significantly reducing the combustion efficiency [9,10,11]. Consequently, boron fuel exhibits notable ignition delays and poor sustained combustion behavior, with incomplete energy release. To address these limitations, surface modification of boron particles has been widely adopted as a synergistic strategy to regulate their ignition and combustion behavior [12,13,14,15]. By introducing suitable modifiers, interfacial reactions between the coating and the boron core can be promoted, leading to the formation of reactive structures under high-temperature conditions. These structures facilitate ignition and enhance combustion, thereby effectively lowering the ignition temperature, shortening the ignition delay time, and improving overall combustion efficiency [16,17,18,19].

Metals are common components in solid propellants, contributing to an increased decomposition heat and density of the formulations [20,21]. Existing studies have demonstrated that coating boron particles with active metals—such as Mg, Al, Ti, and Fe—can significantly enhance their ignition and combustion performance [22]. Unlike the conventional approach in propellant preparation, where boron and active metal powders are simply mixed, this method achieves interfacial modification of boron through composite coating, thereby inducing notable changes at the particle surface. Among the various metal-based materials used to modify the surface of boron particles, active metals such as magnesium (Mg) and aluminum (Al) have demonstrated particularly notable effects. This is not only because these metals have already been successfully applied in propellant systems—thereby avoiding compatibility issues—but also due to their high heats of combustion, which facilitate the ignition of boron. Magnesium powder is a commonly used additive in propellants, valued for its low oxygen consumption, excellent ignition properties, and low molecular weight of combustion products. Qin [23] investigated magnesium-coated boron powder and found that the Mg coating effectively reduced the ignition point of boron, thereby promoting its ignition and combustion. Compared with burning in air, Mg-coated boron exhibited greater flammability in an oxygen atmosphere, with its ignition temperature significantly reduced to just 195.92 °C. The addition of magnesium to the boron surface can effectively lower the ignition temperature and enhance the combustion behavior of boron. At high temperatures, Mg reacts with B₂O₃ to produce elemental boron, a reaction that shortens the ignition delay and improves the combustion efficiency [24]. Although aluminum is slightly less effective than magnesium in lowering the ignition temperature, it has a higher gravimetric heat of combustion, and its promotion effect on boron combustion remains significant. Zhang [25] employed a wet ball-milling method to combine Al nanoparticles (average diameter ~200 nm) with boron particles (2–4 μm). Under a heating rate of 20 °C/min, the reaction temperature decreased from 758 °C to 625 °C, a reduction of 133 °C, indicating the enhanced reactivity of boron. A TG-DSC analysis in an oxygen atmosphere showed that the heat release of Al-coated boron was 444% higher than that of untreated boron, demonstrating a significant improvement in the combustion efficiency.

At present, research on active metal-coated boron powders has primarily focused on single-metal coatings, particularly Mg or Al, including their coating processes, mechanisms, and the influence of the coated boron on propellant combustion performance [26]. However, studies on the simultaneous coating of boron powder with both Mg and Al, as well as the influence of the coating content on combustion performance, remain limited. In particular, there are no publicly available reports on the innovative use of the electrical explosion method to prepare boron powders coated with dual metals. In this study, Mg/Al-coated B95 boron powders were fabricated using this novel technique, with a selected Mg/Al mass ratio of 1. The effects of varying Mg/Al coating contents on ignition delay time, burning rate, combustion pressure, heat of combustion, and thermal behavior were systematically investigated. Additionally, the combustion process of Mg/Al-coated boron powder was analyzed in detail, providing both experimental data and theoretical insights for improving the ignition and combustion performance of boron-based energetic materials.

2. Experiment

2.1. Raw Materials

In this study, amorphous boron powder with a purity of 95 wt.% (denoted as B95) was selected as the reference sample and the primary raw material for composite preparation. The main impurities in the boron powder are oxygen and magnesium, which exist in the form of compounds, with an oxygen content of 2.8 wt.% and a magnesium content of 1.9 wt.%. The second raw material used was a magnesium–aluminum alloy wire, containing 49.4 wt.% Mg and 49.7 wt.% Al, with the remainder being trace impurities; the wire had a diameter of 0.25 mm. Both the boron powder and the magnesium alloy wire were supplied by BGRIMM Advanced Materials Science & Technology Co., Ltd., Beijing, China.

2.2. Mg/Al-Coated Boron Powder Preparation

2.2.1. Boron Powder Dispersion Treatment

Ball-milling dispersion was conducted using a drum-type ball mill under an inert atmosphere. A total of 0.5 kg of boron powder and 5 kg of zirconia balls were loaded into the milling jar, and mechanical ball milling was carried out at a rotation speed of 120 rpm for 2 h. After milling, the boron powder was separated from the zirconia balls to obtain the dispersed boron powder, which was designated as FSB.

2.2.2. Preparation

Mg/Al-coated boron powder was prepared using the electrical explosion method [27]. In this technique, a high-current pulse is applied to a Mg-Al alloy wire, causing it to rapidly heat, vaporize, and explode. The resulting metal vapor condenses onto the surface of dispersed boron particles, thereby forming a Mg/Al composite coating. A schematic diagram of the electrical explosion setup is shown in Figure 1. Taking the preparation of Mg/Al-coated boron powder with 4 wt.% Mg and 4 wt.% Al as an example, the experimental procedure is as follows:

The system was first evacuated to below −0.095 MPa using a vacuum pump, and then backfilled with argon gas to 0.015 MPa to establish an inert atmosphere. A magnesium–aluminum alloy wire was mounted onto the automatic wire feeding system and fed into the reaction chamber at a constant speed of 50 mm/s. Simultaneously, 11.5 g of boron powder pre-treated using the ball-milling dispersion was introduced through the feed inlet and uniformly dispersed into the chamber as individual particles with the aid of an ultrasonic dispersion device. To prevent particle agglomeration and unidirectional deposition, which are commonly observed in gas-phase methods, a slow argon gas flow was introduced from the exhaust port located at the bottom of the chamber. This generated a gentle upward circulation, enabling the boron particles to remain suspended and evenly distributed throughout the chamber. Once the chamber conditions stabilized, a high-voltage DC power supply was activated and maintained at 15 kV to initiate the electrical explosion process. Each 100 mm segment of the magnesium–aluminum alloy wire underwent complete explosion within 5 s, producing approximately 0.01 g of fine Mg–Al alloy powder. This process was set to automatically repeat 100 times, resulting in the uniform dispersion of approximately 1 g of the Mg–Al alloy powder within the boron matrix.

After the electrical explosion process was completed, the power supply was turned off, and the system was allowed to stand for 15 min to enable sufficient condensation and crystallization of the generated Mg–Al alloy onto the surfaces of the boron particles. Subsequently, the argon flow was stopped, and the internal chamber pressure was increased to 0.02 MPa to promote the settling of the composite powder into the collection hopper. The collected composite powder was then transferred to an argon-filled glove box, where a small amount of oxygen was introduced to carry out a mild passivation treatment. Each batch yielded approximately 10 g of passivated Mg/Al-coated boron powder with a total metal content of 8 wt.%, including approximately 4 wt.% magnesium and 4 wt.% aluminum; this sample was designated as BM₄A₄. Using the same procedure, composite powders with different metal coating contents were prepared by adjusting the wire feeding amount and the number of electrical explosion cycles. To investigate the effect of different Mg/Al coating levels on the combustion performance of boron powder, four Mg/Al-coated boron samples with total metal contents of 4 wt.%, 6 wt.%, 8 wt.%, and 10 wt.% were prepared and designated as BM₂A₂, BM₃A₃, BM₄A₄, and BM₅A₅, respectively. These samples were systematically compared with the uncoated B95 boron powder.

3. Results and Discussion

3.1. Basic Performance Analysis

3.1.1. Powder Size Distribution

The particle size distribution of the powders was measured using a Malvern Mastersizer 3000 laser particle size analyzer (Malvern Panalytical Ltd., Malvern, UK). Measurements were conducted for the original B95 boron powder, the ball-milled and dispersed boron powder, and the Mg/Al-coated boron powder. The results are presented in Figure 2.

As shown in Figure 2, the B95 boron powder exhibits a relatively broad particle size distribution, primarily ranging from approximately 1 to 10 μm, with a peak value at 2.9 μm, indicating a generally large particle size. In contrast, the samples subjected to ball milling dispersion and Mg/Al coating treatments (FSB, BM₂A₂, BM₃A₃, BM₄A₄, and BM₅A₅) show a clear leftward shift in the particle size distribution, indicating a significant reduction in the average particle size. These treated samples display particle size distributions predominantly within the 0.1–1 μm range, with peak values located near 0.2–0.3 μm. This suggests that the ball-milling treatment effectively achieved particle size reduction and dispersion.

3.1.2. Specific Surface Area

The Mg/Al-coated boron powders developed in this study are intended for use as high-energy fuels. Increasing the specific surface area of the powder is beneficial for enhancing the material’s reactivity, thereby promoting rapid combustion and efficient energy release of boron. Therefore, it is essential to investigate the changes in specific surface area between Mg/Al-coated and uncoated boron powders. Based on the single-point Brunauer–Emmett–Teller (BET) adsorption theory, the specific surface area of the powders was measured using a V-Sorb 2800 fully automatic specific surface area and porosity analyzer (Gold APP Instruments Corporation, Beijing, China). The results are shown in Figure 3.

As shown by the data in Figure 3, the specific surface areas of the ball-milled boron powder and the Mg/Al-coated boron powder are significantly higher than that of the untreated B95 powder (10.37 m²/g), all exceeding 20 m²/g. The measured specific surface areas follow a clear trend: FSB > BM₂A₂ > BM₃A3 > BM₄A₄ > BM₅A₅ > B95. This indicates a consistent relationship between the particle size and specific surface area—finer particles exhibit larger surface areas. Furthermore, as the Mg/Al content increases, the corresponding laser particle size also increases, resulting in a decrease in specific surface area. This suggests that excessive metal coating may lead to particle agglomeration or coarsening, thereby reducing the effective surface area of the composite powders.

3.1.3. Microscopic Characterization

The microstructures of the B95 boron powder and the Mg/Al-coated boron powders were analyzed using a SU5000 field emission scanning electron microscope (FESEM, Hitachi High-Technologies Corporation, Tokyo, Japan). The results are shown in Figure 4.

As shown in Figure 4, the uncoated B95 boron powder primarily exhibits irregular or elongated shapes, with significant particle agglomeration and relatively large particle sizes. After the ball-milling dispersion, the boron particles appear much smaller and better dispersed, with most exhibiting individual particle distribution. With the increasing Mg/Al coating content, a trend of increased particle agglomeration is observed, suggesting that excessive Mg/Al may lead to localized crystallization during the electrical explosion vaporization–condensation process, thereby promoting agglomeration. This phenomenon further explains the observed decrease in specific surface area. Elemental distribution on the surfaces of the B95 and Mg/Al-coated boron powders was analyzed using an XFlash 6130 energy-dispersive spectrometer (EDS, Bruker AXS GmbH, Karlsruhe, Germany), and the results are shown in Figure 5.

As shown in Figure 5, EDS mapping was performed on samples BM₂A₂, BM₃A₃, BM₄A₄, and BM₅A₅. In the elemental distribution maps, boron (B) is represented in orange, magnesium (Mg) in cyan, and aluminum (Al) in purple. The maps clearly show that Mg and Al are distributed across the regions containing B, indicating that a Mg/Al coating layer has been successfully formed on the surface of the boron particles.

3.2. Combustion Performance

3.2.1. Ignition Delay Time

The ignition delay time and combustion rate of the powders were measured using an HB-LID02 composite ignition device (SiChuan Hbst Co., Ltd., Mianyang, China). The ignition delay times (T₁) of the five powder samples are presented in

Table 1. Ignition delay time of B95 boron powder and Mg/Al-coated boron powder.

Adhesive Type	B95	BM2A2	BM3A3	BM4A4	BM5A5
Ignition delay time (ms)	143	49	43	42	39

.

A high-speed camera was used to record the flame evolution during laser ignition of the five powder samples in a pure oxygen atmosphere. As shown in Figure 6, four sequential frames were captured for each sample: the pre-ignition state (T₀), the ignition moment (T₁, corresponding to the ignition delay time), and the flame states at 50 ms (T₂) and 100 ms (T₃) after ignition.

As shown in Table 1, laser ignition experiments conducted in a pure oxygen atmosphere revealed that the ignition delay time (T₁) of boron and Mg/Al-coated boron powders significantly decreased with increasing Mg/Al-coating content. During combustion, both the flame volume and brightness at T₂ (50 ms) and T₃ (100 ms) increased as the Mg/Al content rose. This observation indicates that Mg/Al coating exerts a pronounced promoting effect on the ignition and combustion performance of boron powders.

3.2.2. Combustion Rate

The combustion rates of B95 boron powder and Mg/Al-coated boron powder were further evaluated using an HB-LID02 composite ignition device. A 2.0 g sample was pressed into a strip-shaped specimen with dimensions of 100 mm × 10 mm × 10 mm (L × W × H). The specimen was ignited via electric ignition in a pure oxygen atmosphere, and the entire combustion process was recorded using a high-speed camera operating at 1000 frames per second (FPS). The combustion rate was calculated by dividing the sample length (L) by the time (T) required for complete combustion (i.e., L/T). During the test, flame evolution was recorded at four time points: the initial state (T₀), 400 ms (T₄), 800 ms (T₅), and full combustion (T₆), as shown in Figure 7. The calculated combustion rates for B95 and Mg/Al-coated boron powders are listed in

Table 2. Combustion rate of B95 boron powder and Mg/Al-coated boron powder.

Adhesive Type	Sample Length (mm)	Spread Time (ms)	Combustion Rate (m/s)
B95	100	1477	0.068
BM2A2	100	1010	0.099
BM3A3	100	898	0.112
BM4A4	100	817	0.122
BM5A5	100	809	0.124

.

As illustrated in Figure 7, the B95 boron powder exhibits no visible flame at the initial ignition stage, remaining dark. At 400 ms, a small, concentrated, and slender flame appears, indicating a relatively slow combustion initiation; at this time, the flame intensity significantly increases, and the flame volume expands. By 1224 ms, the flame length reaches its maximum, and the flame intensity is at its highest; however, the flame width is relatively narrow and uneven, suggesting incomplete combustion of part of the powder. In contrast, boron powders coated with Mg/Al display significantly greater flame intensity and size at various time points compared to the B95 boron powder. Furthermore, with increasing Mg/Al-coating amounts, both the flame intensity and size exhibit an increasing trend. Notably, sample BM₅F₅ demonstrates the best ignition and combustion performance, with rapid initial combustion and the most prominent flame intensity and size among the five tested fuels. These observations, combined with the combustion velocity results calculated for different samples in Table 2, further confirm that Mg/Al coating promotes combustion velocity enhancement, and that increasing the coating amount accelerates the combustion speed.

3.2.3. Combustion Pressure

The powder combustion pressure was tested using an HB-Pt300 constant-volume combustion device (SiChuan Hbst. Co., Ltd., Mianyang, China). A sample of 50.0 mg was placed on the base of a sealed explosion chamber. The pressure valve was adjusted to ensure no leakage inside the chamber, maintaining an oxygen environment at 3 MPa. Ignition was initiated by applying 10 V to metal electrodes. The entire combustion process was completed within 40 s, during which the pressure variation in the sample was recorded. Combustion pressure tests were conducted on five samples, and the results are shown in Figure 8. The combustion time (T_burn) and peak pressure (P_peak) were recorded, and the pressure rise rate per unit combustion time, R = P_peak/T_burn, was calculated. The results are summarized in

Table 3. Combustion pressure of B95 boron powder and Mg/Al-coated boron powder.

Adhesive Type	Tburn (s)	Ppeak (Kpa)	R (KPa/s)
B95	8.212	3716.58	452.58
BM2A2	6.263	3983.48	636.03
BM3A3	4.554	4023.49	883.51
BM4A4	3.139	4190.29	1334.91
BM5A5	3.094	4182.44	1351.79

.

As shown in Figure 8, the combustion pressure curves of the samples exhibit an initial increase, followed by a decrease with the increasing combustion time. Boron powders coated with Mg/Al show a significantly shortened time to reach peak pressure. This is reflected in Table 3, where an increase in the Mg/Al-coating amount corresponds to a decrease in the time to reach the peak pressure (T_burn) and a gradual increase in the peak pressure (P_peak), indicating that a higher Mg/Al coating facilitates sustained combustion of boron powder and generates more gaseous products. Furthermore, the pressure rise rate (R value) increases with greater Mg/Al-coating amounts, further confirming that Mg/Al coating accelerates the combustion rate of boron powder and leads to faster energy release, consistent with the combustion velocity analysis. It is noteworthy that the T_burn, P_peak, and R values for samples BM₄A₄ and BM₅A₅ are approximately equivalent, suggesting that an excessively high Mg/Al-coating amount does not necessarily further improve the combustion performance of boron powder.

3.2.4. Combustion Calorific Value

The combustion calorific values were measured using an RF-C7000 automatic calorimeter (Ruifang Energy Science and Technology Co., Ltd., Changsha, China). Approximately 0.5 g of the sample was placed in the combustion cup, and oxygen was slowly filled into the oxygen bomb to a pressure of 2.0 MPa, with a continuous oxygen supply for no less than 15 s. Ignition tests were then conducted, and the combustion calorific values were calculated upon completion. The combustion calorific values of B95 boron powder, ball-milled and dispersed boron powder (FSB), and Mg/Al-coated boron powder were measured, as shown in

Table 4. Combustion calorific value of boron powder and Mg/Al-coated boron powder.

Adhesive Type	Theoretical Calorific Value (KJ/kg)	Measured Calorific Value (KJ/kg)	Combustion Efficiency (%)
B95	55,803.00	15,600	27.96
FSB	55,803.00	15,952	28.59
BM2A2	54,686.68	17,057	31.19
BM3A3	54,128.52	19,565	36.15
BM4A4	53,570.36	21,103	39.39
BM5A5	53,012.20	20,072	37.86

. Compared with the raw B95 boron powder, the FSB exhibited a slight increase in combustion heat, indicating that particle refinement is beneficial to boron combustion. In contrast, the Mg/Al-coated boron powders showed a more significant increase in combustion heat, suggesting that the Mg/Al coating plays an important role in promoting energy release during combustion. However, the combustion heat of BM₅A₅ was slightly lower than that of BM₄A₄. This may be attributed to the more severe particle agglomeration in BM₅A₅, which could hinder energy release. Further analysis combining burning rate data reveals that if the decline in energy output were solely due to poor dispersion, the larger agglomerated particles in BM₅A₅ would also result in a reduced burning rate. However, in practice, the agglomeration in BM₅A₅ had little effect on its burning rate. Therefore, it is concluded that excessive Mg/Al coating is not necessarily beneficial. Overloading may reduce the energy density of the composite boron powder and lead to an overall decrease in the combustion efficiency. This finding is consistent with the combustion pressure test results.

3.3. Thermal Performance Analysis

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out using an SDT Q600 V20.9 Build 20 simultaneous thermal analyzer (TA Instruments, New Castle, DE, USA). The measurements were conducted in an air atmosphere with a flow rate of 50.0 mL/min, and the heating rate was set to 5 °C/min up to 1300 °C. The thermal behavior (TG-DSC) of B95 boron powder and Mg/Al-coated boron powders was tested, and the results are shown in Figure 9. Thermal performance parameters of B95 boron powder and Mg/Al-coated boron powders are presented in

Table 5. Thermal performance parameters of B95 boron powder and Mg/Al-coated boron powders.

Adhesive Type	Mass (%)	* Segregation Point Temperature/Ts (°C)	Peak Temperature/Tp (°C)	Peak Heat Release Enthalpy (J/g)
B95	99.46	746.65	779.55	11,230
BM2A2	101.48	734.16	757.05	11,484
BM3A3	102.93	703.88	741.55	11,811
BM4A4	105.87	694.01	725.19	12,632
BM5A5	107.08	686.18	721.81	11,962

* Segregation point temperature T_s (°C): Onset temperature of vigorous oxidation reaction.

.

As shown in Figure 9, compared with B95 boron powder, Mg/Al-coated boron powder samples effectively increase the oxidation weight gain, with BM₅A₅ exhibiting the highest increase. As observed from the DSC curves, all samples exhibited a distinct exothermic peak starting at approximately 700 °C. Notably, the Mg/Al-coated boron powders showed a significantly lower onset oxidation temperature and peak temperature compared to the uncoated samples, indicating that the coated boron powders can undergo rapid and intense reactions at lower temperatures. This shift suggests enhanced reactivity and improved thermal activation due to the Mg/Al coating. Moreover, with increasing Mg/Al-coating amounts, both the Segregation point temperature and peak temperature gradually decrease, suggesting that a higher Mg/Al content facilitates easier ignition and sustained rapid combustion of the boron powders. From the peak heat release enthalpy, it can be seen that the enthalpy increases with the Mg/Al-coating amount. Among the samples, BM₄A₄ exhibited the highest exothermic enthalpy at 12,632 J/g, while BM₅A₅ showed a slight decrease. This is primarily due to the higher Mg and Al contents, which have lower calorific values than boron itself, thereby diluting the boron component and reducing the overall combustion energy release. Notably, a small endothermic peak around 450 °C corresponds to the dissolution of boron oxide on the boron powder surface. Based on the above analysis, the promoting effect of Mg/Al coating on boron combustion is evident; however, excessive addition adversely affects the overall energy density and leads to a decrease in the energy output. Additionally, between 950 and 1000 °C, the DSC curve of B95 boron powder is relatively smooth, whereas the Mg/Al-coated boron powders exhibit a small endothermic peak, which may be attributed to the endothermic decomposition of newly formed substances. Nevertheless, the overall reaction process remains predominantly exothermic.

3.4. Phase Analysis of Combustion Residues

A phase analysis of the combustion residues of B95 boron powder and BM₄A₄ was conducted using a D8 ADVANCE X-ray Diffraction (XRD) instrument (Bruker AXS GmbH, Karlsruhe, Germany). The results are shown in Figure 10.

As shown in Figure 10a, the strong diffraction peaks corresponding to two different powder combustion products are indicated. The diffraction peaks of these substances were compared with standard reference cards from the Inorganic Crystal Structure Database (ICSD). For B95 boron powder, multiple sharp diffraction peaks (marked in blue) with high intensity are observed, which correspond closely to the standard card of B₆O (PDF# 97-065-6231). This indicates that crystalline B₆O was formed after combustion, representing a stable suboxide produced under incomplete oxidation conditions. Several other diffraction peaks (marked in red) match the standard card of H₃BO₃ (PDF# 97-024-0998). As H₃BO₃ is not a high-temperature product, it is likely formed during cooling via hygroscopic hydrolysis of B₂O₃ in the presence of moisture from the air.

In Figure 10b, the strong diffraction peaks corresponding to the combustion residues of BM₄A₄ are identified. Multiple sharp diffraction peaks (marked in blue) show a high degree of matching with the standard card of Mg₂(B₂O₅) (PDF# 00-050-0850), indicating that under combustion conditions, magnesium reacts with boron through solid–solid or solid–liquid reactions to form magnesium borate with good thermal stability. Several medium-intensity peaks (marked in red) correspond to H₅Al₃(B₆O₁₂)O_1.847(OH)₄ (PDF# 97-041-3233), suggesting that aluminum and boron exist as complex hydrated aluminum borates after combustion, which are typical hydrated crystalline products formed during combustion cooling. Additionally, some peaks (marked in purple) match those of B₇O (PDF# 97-002-4656), indicating that stable suboxide structures such as B₇O were generated in localized high-temperature and oxygen-deficient regions. Significant differences are observed in the combustion residues between Mg/Al-coated boron powders and uncoated boron powders.

3.5. Analysis of Combustion Mechanism

Based on the XRD analysis of the combustion residues, during the combustion process of Mg/Al-coated boron powders, synergistic reactions among the components and differences in the thermal environment lead to the formation of multiphase oxides and complex borates.

(1) High-temperature main reaction stage.

Boron is oxidized to form B₂O₃:

4B + 3O₂ →2B₂O₃

Magnesium acts as a combustion promoter during the reaction and simultaneously reacts with B₂O₃ to form Mg₂(B₂O₅):

2Mg + B₂O₃ + O₂→Mg₂(B₂O₅)

Aluminum is oxidized to form Al₂O₃, which interacts with B₂O₃ and H₂O to generate complex hydrated aluminum borates:

3Al + 3B₂O₃ + 6H₂O→H₅Al₃(B₆O₁₂)_1.847(OH)₄

(2) High-temperature oxygen-deficient or incomplete reaction stage: B₇O is formed in localized regions due to insufficient oxygen.

Based on the above analysis, the Mg/Al-coated boron powder can rapidly ignite and oxidize during the initial ignition stage, thereby increasing the local combustion temperature and promoting the early ignition of boron, effectively reducing the ignition delay time. At the early combustion stage, magnesium reacts with B₂O₃ to form the ternary solid oxide Mg₂(B₂O₅), which decreases the formation of liquid B₂O₃ and promotes oxygen diffusion into the interior of boron particles. Although the magnesium coating itself also forms a thin oxide layer, this layer is relatively thin, effectively reducing the thickness of the boron oxide layer [24] and mitigating the barrier effect of liquid boron oxide on oxygen diffusion. Additionally, Mg₂(B₂O₅) readily decomposes above 900 °C, which explains the endothermic peak observed in the DSC curves around 900–1000 °C. Similarly to magnesium, aluminum coating also significantly enhances the rapid combustion of boron [28,29]. The analysis of combustion products shows that aluminum oxidizes to form Al₂O₃, which can interact with B₂O₃ and H₂O to generate H₅Al₃(B₆O₁₂)_1.847(OH)₄. This interaction results in discontinuities in the boron oxide layer surface, providing channels for oxygen diffusion into the boron core and promoting sustained combustion and high energy release.

4. Summary and Conclusions

Mg/Al-coated boron powders with varying coating amounts were prepared via the electrical explosion method. Characterization techniques, including laser particle size analysis, specific surface area measurement, microscopic morphology observation, and surface elemental distribution analysis, were employed. The results indicate that, compared with conventional B95 boron powder, Mg/Al-coated boron powders exhibit reduced particle size and increased specific surface area. However, as the Mg/Al-coating amount increases, the particle size tends to increase, while the specific surface area decreases, possibly due to particle agglomeration. From the microscopic morphology, the boron powders show smaller and more uniformly dispersed particles after ball milling and dispersion. With an increasing Mg/Al-coating content, an increasing tendency towards particle agglomeration is observed, suggesting that excessive Mg/Al addition may promote localized crystallization during the vapor condensation stage of the electrical explosion process, leading to partial agglomeration. Nonetheless, the overall particle dispersion remains relatively good.

The results of the ignition delay time, combustion rate, and combustion pressure tests indicate that Mg/Al-coated boron powders exhibit significantly shorter ignition delay times, higher combustion rates, increased peak combustion pressures, and faster pressure rise rates compared to conventional B95 boron powder. As the Mg/Al-coating amount increases, the ignition delay time decreases progressively, while the combustion rate, peak combustion pressure, and pressure rise rate all increase. These findings demonstrate that Mg/Al coating effectively promotes both rapid ignition and sustained combustion of boron powder, with more pronounced improvements at higher coating levels. This suggests that the coating plays a critical role in enhancing combustion performance. Furthermore, the combustion calorific value results show a significant increase in heat output for Mg/Al-coated boron powders compared to B95 boron powder. However, when the Mg/Al coating amount reaches 8%, the increase in the combustion heat value becomes marginal, indicating that the promoting effect has reached saturation. Therefore, excessive coating is not recommended.

The phase analysis of the combustion residues indicates that H₅Al₃(B₆O₁₂)O₁.₈₄₇(OH)₄ and Mg₂(B₂O₅) are formed in the combustion products of Mg/Al-coated boron powders. These compounds are the result of reactions between Mg, Al, and the boron oxide layer on the surface of the boron particles. This suggests that during combustion, Mg and Al react with the surface B₂O₃ layer, either reducing its thickness or disrupting its continuity. As a result, the barrier effect of the boron oxide layer on oxygen diffusion into the boron core is diminished, which facilitates the ignition and sustained combustion of boron, thereby enabling higher energy release. Based on the optimal Mg/Al coating composition identified in this study, subsequent work can focus on scaling up the preparation process and conducting application tests in propellants, explosives, and pyrotechnic fuels, thereby enhancing the energy release performance of these related energetic materials.