Abstract
The oxidation of boron (B) is of great significance in catalysis, metallurgy, corrosion, and combustion. However, understanding the early stages of oxidation and identifying intermediate phases remain a long-standing challenge. Here we reveal an oxidation mechanism termed the W–J model; unlike classical models that rely on the diffusion of oxygen or boron through a B2O3 layer, the initial reaction in the W–J model takes place at the interface between the boron core and the B2O3 shell. This interfacial reaction produces an oxidized intermediate, B6O, which is then further oxidized to form B2O3. The formation of B6O plays a crucial role in regulating the oxidation process. Its high thermal stability and strong oxygen affinity act as barriers to continuous oxidation, thereby lowering the ignition and combustion efficiency of boron. These findings highlight a fundamentally different reaction pathway that may help explain the limited reactivity observed in practical applications.
Introduction
Elemental boron (B) exhibits exceptional mechanical, electrical, and chemical properties1,2,3 and diverse applications, including catalysis4,5, capacitors6,7, metallurgy8, and microelectromechanical systems9, as well as being the highest calorific value fuel for energy storage and release10,11,12,13. The B oxidation is critical to the deactivation of B-based catalysts and capacitors, and the energy release rate in high-energy fuel systems is closely tied to the oxidation pathway. Thus, extensive research has focused on understanding the oxidation reaction mechanism to improve the stability, longevity, and ignition and combustion efficiency of solid rocket motors. Generally, the B2O3 shell on B particles, with a low melting point (723 K) and a high boiling point (2133 K), remains intact during the oxidation process10,14,15. Additionally, Marangoni convection16,17,18 contributes to the self-healing of the B2O3 layer, complicating its removal. Therefore, a thorough understanding of the oxidation reaction mechanism of B requires investigating the interface between B and B2O3 during the ignition and combustion processes.
Currently, two classical models describe the interfacial reaction between B and B2O3: the King model14,19,20 and the L-W model21. The King model proposes that oxygen (O2) diffuses into B2O3 layer and reacts with internal B. Experimental observations by Dreizin et al.22 confirmed the diffusion of O2 within the B2O3 layer; however, the compact, glassy nature of B2O3 at elevated temperatures hinders this process. Additionally, the highly oxidized state of B2O3 impedes efficient oxygen transfer. In contrast, the L-W model suggests that B diffuses outward through B2O3 layer, reacting with external O2, generating the intermediate polymer (BO)n, which then infiltrates the B2O3 shell and reacts with O2. Theoretical work by Glassman et al.23 indicates that solubility of B in liquid oxides exceeds that of O2, while investigation by Yeh et al.24,25 shows that the diffusion rate of (BO)n is significantly higher than that of O2 at elevated temperatures. The L-W model emphasizes the formation and oxidation of the (BO)n intermediate as key to the interfacial reaction. However, there remains an insufficiency of experimental and theoretical data to fully characterize the creation process and unique structure of (BO)n. Furthermore, inconsistencies between the King and L-W models highlight unresolved issues in understanding the underlying causes of B’s low ignition and combustion efficiency.
In this study, to explore the oxidation mechanism of B and identify intermediate species, three reaction interfacial models of B-B2O3, B-B2O3-O2, and B-O2 are developed (Fig. 1). The fluorine-containing rubber F14, known for its excellent mechanical strength, chemical stability, and corrosion resistance, was used as a surface coating material. Through surface etching, it enabled the controlled construction of the B–O2 reaction interface (B@F). The formation and structure of the intermediate (BO)n were systematically investigated by advanced imaging and spectroscopic techniques. Simultaneously, molecular dynamics simulations are employed to study the interface reaction of these models between B and B2O3. The results reveal that the initial reaction occurs on the interface between B core and B2O3 shell, leading to the formation of B6O, which is subsequently oxidized by O2 to produce B2O3. The B6O intermediate acts as a barrier, hindering further oxidation of B and resulting in reduced ignition and combustion efficiency.
The oxidation mechanism of boron was elucidated through the examination of three interfacial reaction processes using three distinct models. a Model 1: The B-B2O3 model was developed by introducing boron into an argon environment, and the chemical pathway at the B/B2O3 interface was examined. b Model 2: The B-B2O3-O2 model was developed by introducing boron into the air atmosphere, and the chemical process at the B/B2O3/O2 interface was examined. c Model 3: The fluoropolymer-modified boron (B@F) was subjected to an air environment to establish a B-O2 model, and the chemical process at the B/O2 interface was investigated.
Results and Discussion
The formation mechanism of intermediate B6O
The initial construction of a B-B2O3 reaction interface is introduced to investigate the interaction between B and B2O3 in the B-B2O3 model. Amorphous boron, with its native B2O3 shell, was heat-treated in argon at various temperatures to elucidate the interfacial reaction process. X-ray diffraction (XRD) patterns in Fig. 2a reveal the primary phases of B (PDF#31-0207), B2O3 (PDF#06-0297), and B6O (PDF#50-1505) prior to heat treatment. As the calcination temperature increases, the intensity of the B2O3 diffraction peaks gradually decrease, while the B6O peaks increase. Notably, no B2O3 diffraction peaks are observed at 1400 °C, suggesting that B2O3 may become amorphous and undetectable by XRD once the temperature exceeds its melting point (450 °C). Additionally, chemical reactions on the interface between B core and B2O3 shell likely contribute to the reduction of B2O3 and the formation of B6O. X-ray photoelectron spectroscopy (XPS) spectra (Fig. S1) further confirm the results found in XRD, showing a significant decrease in the proportions of B2O3 (from 7.04% to 6.41%) and B (from 72.06% to 54.04%), while the concentration of B6O increases from 25.50% to 39.55%. These results demonstrate that the B/B2O3 interface reacts to form B6O (Eq. (1)), leading to a decrease in B2O3 and an increase in B6O when B is heated in an oxygen-free environment. Equation (1) represents this process.
a the XRD patterns of boron after calcination in argon at different temperature. b representative HRTEM images and DDPs of B/B2O3 composites before calcination in argon (top), and representative HRTEM images and DDPs of B6O obtained from the B/B2O3 composites after calcination in argon (bottom). c the B 1s XPS spectra of B after calcination in air at 25 °C, 500 °C, and 900 °C temperature. d the percentage of B2O3, B6O and B at different temperature in air according to XPS. e Representative HRTEM images of B particle after calcination in air at 500 °C (top left) and detail HRTEM image of B-B6O interface (top right). DDPs of the small areas marked with (i) and (iii) refer to the B and B6O, respectively, which are shown at the bottom left and bottom right. The middle panels correspond to the sum of both DDPs. f The XRD patterns and SEM images of combustion products of B@F (top) and B (bottom). g schematic diagram of the interfacial reaction mechanisms of B and B2O3.
Given the minimal presence of the native oxide layer on the B surface, B/B2O3 composites were synthesized based on the stoichiometric ratio outlined in Eq. (1). The morphology and structure of these composites after high-temperature heat treatment were investigated to better understand the interfacial reaction between B and B2O3. XRD data (Fig. S2a) reveal that prior to calcination, the main phases are B and B(OH)3, which forms through the hydrolysis of B2O3. After calcination, the primary phase transitions to B6O, with XPS analysis (Fig. S2b) showing the disappearance of B and B2O3 peaks, replaced by distinct B6O peaks. While recent reports26,27,28,29 suggest that B6O forms during the combustion of B, attributing its generation to incomplete combustion, these studies fail to address the specific process and role of B6O in B oxidation.
High-resolution transmission electron microscopy (HRTEM) images (Fig. 2b) of the B/B2O3 composites before and after the reaction show a uniformly adherent, dense amorphous B2O3 layer on the surface of the B crystals. The diffraction pattern from the Fourier transformation of the region marked by the yellow box is shown in the inset. The differences in HRTEM images and diffraction spots suggest that a reaction occurred on the interface between B core and B2O3 shell, leading to the formation of a new compound upon calcination. The angles between the (012) and (003) planes, and between the (101) and (003) planes, were measured to be 55° and 69°, respectively. These values are in close agreement with those from the XRD standard card and the single-crystal structure of B6O, which report angles of 52.87° and 69.26°, respectively. After the reaction, both B and B2O3 are absent, confirming that B6O forms directly from the interaction of B and B2O3. Our results suggest that B6O is formed at the B/B2O3 interface, not as a result of incomplete oxidation, supporting the hypothesis that B6O could be a potential structure for the (BO)n intermediate proposed in the King and L-W models.
The investigation into the role of B6O in the ignition and combustion processes of B was carried out by constructing the B/B2O3/O interface, as indicated by the results above. In this context, the native B2O3 oxide layer on the surface of amorphous B was utilized, and B was subjected to heat treatment in oxygen at varying temperatures to study the interfacial reaction between B, B2O3, and O2. The XRD patterns (Fig. S3a) reveal that, as temperature increases, the diffraction peak corresponding to B diminishes while peaks for B2O3 become more prominent. Additionally, heat-treated samples display characteristic peaks for B6O, with the intensity of these peaks initially increasing before decreasing at higher temperatures. To further characterize the chemical valence states of B, XPS spectra were analyzed, and the relative proportions of B2O3, B6O, and B were quantified by peak area analysis (Figs. 2c, d, Fig. S3b). The data show a progressive increase in B2O3 content and a corresponding decrease in B as the temperature rises, while B6O content initially increases and then decreases. This trend aligns with the XRD results, supporting that B6O forms as an intermediate product in the interface reaction between B and B2O3, facilitating the outward mass transfer of B to the particle surface, where it subsequently reacts with O2 to form B2O3.
HRTEM images of the calcined samples (Fig. 2e) reveal the coexistence of B and B6O at the nanoscale, particularly at the crystal boundaries within the same crystallites. The exposed (003) crystal plane of B and the (101) crystal plane of B6O form an angle of 75°. Detailed analysis of the digital diffraction patterns (DDPs) from various regions of the calcined composite indicates that region (i) corresponds to B, region (iii) to B6O, and region (ii) to the crystal boundary where diffraction spots of both B and B6O overlap. This boundary exhibits the (003) plane of B and the (101) plane of B6O, maintaining the 75° angle. These observations suggest that B6O forms through the reaction between B and B2O3, developing along the (003) crystal plane of B, consistent with previous analyses in B-B2O3 model.
Further insights are provided through infrared spectroscopy (Fig. S4) and electron microscopy data (Fig. S5). Upon the formation of B6O at the B/B2O3 interface, B6O gradually diffuses to the particle surface, where it reacts with O₂ to produce B2O3, thus facilitating the reaction between B and O. The chemical process involved can be represented by Eqs. (2) and (3).
The results indicate that B6O is exclusively formed as the primary reaction product in the presence of B2O3, while in its absence, B2O3 is the predominant product. Firstly, to establish the reaction interface of B-O2, B covered with fluoropolymer (B@F) was synthesized, based on the surface etching reaction between F and B2O3. The morphologies and particle size distribution of B@F, shown in Fig. S6, confirm its successful preparation. Experiment 4 (Fig. S7) and our previous work30,31 demonstrates the effective removal of the oxide layer by the fluoropolymer. Then, the above hypothesis is further substantiated by analyzing the combustion products of B and B@F, which were burned in a 1 MPa oxygen environment. XRD patterns in Fig. 2f reveal that the combustion of B predominantly results in B6O, while B@F combustion yields both B2O3 and a small amount of B6O. XPS spectra in Fig. S8 show a higher proportion of B6O in the combustion products of B and a greater proportion of B2O3 in those of B@F. Further analysis of the surface structures (Fig. 2f and Fig. S9) reveals that B exhibits rod-like structures with a B to O atomic ratio of approximately 6:1, consistent with the presence of B6O. In contrast, B@F exhibits a flaky structure with a B to O atomic ratio of 2:3, indicating that it primarily consists of B2O3.
The fluorine atoms in B@F interact initially with the B2O3 shell, exposing the interior active B, which then reacts with O2 to generate additional B2O3. Marangoni convection causes the newly formed B2O3 shell to envelop the unreacted B, thus creating a new B/B2O3 interface. This results in minimal B6O formation in B@F. The overall process can be described by Eqs. (4)–(6). These results support the hypothesis that the formation of B6O is contingent upon the presence of B2O3, and they systematically elucidate the reaction mechanism on the interface between B core and B2O3 shell (Fig. 1g).
The chemical reactivity of intermediate B6O
As an intermediate compound in the B-B2O3 interface reaction, B6O significantly influences the combustion and ignition characteristics of B. Notably, the complete oxidation of each mole of B requires three-quarters of a mole of oxygen (Eq. (3)), whereas the complete oxidation of each mole of B6O consumes four moles of oxygen (Eq. (5)), more than five times the amount required for B. This high oxygen consumption makes B6O less reaction efficiency. To further assess the combustion reactivity of B6O, we compare the flame propagation, pressure output, ignition delay time, and thermal processes of B, B@F, and B6O. B6O was synthesized by calcining B/B2O3 composites under a N2 atmosphere, as detailed in the preparation of B6O section. The results show that both B and B@F exhibit a larger flame area and higher light intensity (Fig. 3a, 3b), while B6O demonstrates reduced combustion reactivity. Pressure output data (Fig. 3c) reveal that B6O generates gas at a much slower rate (99.9 kPa·s-1) than B (103.9 kPa·s-1) and B@F (2451.5 kPa·s-1), indicating its lower heat release efficiency (Fig. 3d). Furthermore, B6O exhibits the longest ignition delay time (Fig. 3e), confirming its diminished reactivity and prolonged ignition delay. In conjunction with the findings of section 2.1, it is evident that the concentration of oxidized intermediate in B@F, B, and B6O samples rises during the early reaction period, which directly accounts for the significant disparity in combustion performance among B@F, B, and B6O. These findings emphasize the inhibitory effect of B6O on the ignition and combustion of B particles, in contrast to B@F’s role in effectively removing the inert B2O3 layer from B’s surface, thereby enhancing combustion performance.
a the flame propagation process in atm air ignited by Ni-Cr wire. b the flame area calculated according to flame imagens. c the time-dependent pressure evolution curves. d the combustion heat in different oxygen pressure. e the ignition delay time in different laser power. f the thermal reaction process analyzed by DSC curves. g the thermal reaction process analyzed by TG curves. h the thermal reaction process analyzed by DTG curves.
The thermal reaction processes of B, B@F and B6O were analyzed using simultaneous thermal analysis (TG-DSC). DSC curves in Fig. 3f reveal that B@F exhibits the highest heat release and the lowest peak temperature, whereas B6O shows the smallest heat release and the highest peak temperature. Additionally, TG curves (Fig. 3g) indicate that B6O has the lowest reaction efficiency. DTG curves in Fig. 3h reflect the reaction rates, indicating that B6O has the lowest reaction rate compared to B and B@F. Additionally, particle morphologies and components of B@F at various temperatures, shown in Fig. S10-S12, confirm that the reaction efficiency of B@F surpasses that of B across all temperatures. As B@F removes the B2O3 shell due to the surface etching of F, the initial reaction of B@F transitions from an interfacial reaction between B core and B2O3 shell to a direct reaction between B and O2, resulting in a reduction of B6O generated in the reaction process, thereby improving combustion efficiency and rate. These findings align with the results in Fig. 3a-d, further illustrating that B6O has a highly negative reactivity and oxygen consumption, allowing substantial amounts to form without continually reacting to create B2O3 during B combustion.
Reactive molecular dynamics simulations
To gain insights into the atomic-level reaction processes of B, reactive molecular dynamics (RMD) simulations were conducted using four models of B-B2O3, B-B2O3-O2, B-O2, and B6O-O2 according to the experiments, as illustrated in Fig. S13a. The simulations utilized a canonical ensemble (NVT) with a Nose-Hoover thermostat and employed the reactive force field (ReaxFF) for H/O/N/B developed by Weismiller et al.32. This approach has proven effective in elucidating the oxidation behavior of B particles18,33,34 (Fig. S13b, c).
The morphological snapshot of the B-B2O3 model in Fig. S14 demonstrates that oxygen atoms in the B2O3 layer diffuse inward, while boron atoms in the B layer move outward. The oxidation process of B is a critical focus of this study. A representative oxidation pathway for a boron atom at the B-B2O3 interface is illustrated in Fig. 4a. As heating occurs, the boron atom in B layer quickly migrates to the surface, resulting in a decrease in B-B bonds to four and then three at 4 ps and 4.4 ps, respectively. At 4.5 ps, the boron atom contacts an oxygen atom in the B2O3 layer, forming a B-O bond, and subsequently forms two additional B-O bonds at 5 ps. The number of B–O bonding reaches three at 200 ps and remains constant thereafter. This oxidation state persists throughout the simulation, indicating that the interactions in the B-B2O3 model involve chemical reactions rather than mere melting and physical diffusion. Coupled with the unique coordination of B6O and B2O3 shown in Fig. S13b, c, these results suggest that B6O is produced during the reaction between B and B2O3 and progressively diffuses outward, consistent with previous experimental results (Fig. 2a).
a oxidation process of a B atom (shown in green for clarity) in the B-B2O3 model. b the oxidation process of marked B atoms in the B-B2O3-O2 model. c [3]O structure in the B6O crystal. d [3]O structure in different models at 500 ps (The bright red sphere represents [3]O structure). e [3]O structure quantity in different models at 500 ps.
Figure S15 presents snapshots of the B, B2O3, and O2 layers in the B-B2O3-O2 model under heating. The results reveal the melting of B2O3, the outward diffusion of boron atoms from the B layer, and the inward diffusion of oxygen atoms from the O2 layer. Three boron atoms with the high diffusion displacements at 500 ps are identified as B1, B2, and B3, with their specific oxidation processes illustrated in Fig. 4b. The B1 atom, initially on the surface with three B-B bonds and one B-O bond, experiences a decrease in B-B bonds and an increase in B-O bonds as it diffuses, ultimately forming a structure similar to B2O3. The B2 atom starts within the B layer with four B-B bonds and transitions to a B2O3-like structure by 240 ps. The B3 atom forms two B–O bonds at 45 ps and ultimately adopts a B2O3-like configuration by 92 ps. Furthermore, the radial distribution function (RDF) analyses of the B-B2O3 and B-B2O3-O2 models at 500 ps provide additional evidence supporting the formation of B6O (Fig. S16). In all cases, B initially reacts with B2O3 to form a [1]B, that is B6O as the intermediate product. As B6O diffuses outward and O2 diffuses inward, B6O reacts with O2 to generate a [3]B, that is, B2O3. We have also simulated the chemical processes in the B6O-O2 and B-O2 models, as detailed in the supplementary material (Fig. S17 and S18). These findings indicate that B6O progressively oxidizes to form B2O3, while elemental B, lacking an oxide layer, directly reacts with O2 to produce B2O3, consistent with previous experimental and simulation results (Fig. 2).
In the crystal structure of B6O, each oxygen atom forms three B-O bonds with neighboring boron atoms, denoted as [3]O35, which constitutes a key structural feature of the B6O crystal, as shown in Fig. 4c and Fig. S13c. We analyze the distribution and quantity of the [3]O structures at a 500 ps in various models, as illustrated in Figs. 4d, 4e. In the B-B2O3 model, a total of 115 [3]O structures are generated upon heating, with a uniform distribution within the B2O3 layer. In the B-B2O3-O2 model, 77 [3]O structures are predominantly located in the molten B2O3 region. Conversely, the B-O2 model shows minimal formation of the [3]O structure, with only 15 instances observed. These results indicate that B6O primarily forms at the interface between B and B2O3, with higher quantities of B2O3 correlating to increased B6O formation.
Combined with the experimental results, it can be concluded that B6O functions both as an intermediate in the oxidation of B, facilitating mass transfer between internal B and external O, and as an interface barrier that limits the chemical reactivity of B. B6O is initially formed at the B-B2O3 interface through the chemical reaction between B and B2O3. It then diffuses to the surface of the B particles, where it reacts with O2 to generate B2O3 as the oxidation process continues. This reaction mechanism is referred to as the W-J model in this study. However, the high oxygen consumption and low reactivity of B6O hinder its oxidation, resulting in prolonged ignition delays and reduced combustion efficiency.
In summary, an interfacial reaction mechanism called W-J model, is systematically investigated at the macro and atomic scales based on experiments and RMD simulations. The results propose that the compound boron suboxide (B6O) as an intermediate resulted from the interfacial reaction between the B core and B2O3 shell. The negative reactivity and high oxygen consumption of B6O pose a notable challenge in its further oxidation to B2O3, therefore greatly impeding the ignition and combustion performance of B. The oxidation mechanism of B is empirically researched and can be utilized in the fields of aerospace, microelectromechanical systems, and civic sectors.
Methods
Materials
Boron particle was purchased from Zhong Nuo New Materials with an average size of 5 μm. F14 was obtained from the Institute of Chemical Materials, CAEP (Mianyang, China). Ethyl acetate (AR) and Dimethyl sulfoxide (AR) were purchased from Aladdin (Shanghai, China).
Preparation of B and B@F
The twofold solvent procedure was used to produce the B coated with F14, which was referred to as B@F. On the one hand, ethyl acetate can dissolve F14, whereas DMSO cannot. On the other hand, ethyl acetate and DMSO are fully miscible in all proportions. As a result, ethyl acetate (solvent) and DMSO (anti-solvent) together form a dual-solvent system, which serves as the foundation of the twofold solvent process. At first, F14 was fully dissolved in 60 g of ethyl acetate with the use of magnetic stirring. Next, B was combined with the previously produced F14 solution using ultrasonic dispersion for 1 hour and magnetic stirring for 2 hours at room temperature. A solution with a uniform composition was produced. The mass ratio of F14 to B was selected as 1:5 based on the oxide layer content of B being around 10 wt.% (obtained by ICP-OES and XPS analysis) and the fluorine content of F14 being approximately 53 wt.% (calculated by its molecular formula: \({m}_{F}/{m}_{{C}_{5}{F}_{7}{{Cl}}_{2}H}\)). This decision was made in accordance with the equation \({{{{\rm{B}}}}}_{2}{{{{\rm{O}}}}}_{3}\left({{{\rm{l}}}}\right)+{{{\rm{CF}}}}\left({{{\rm{s}}}}\right)\to {{{{\rm{BF}}}}}_{3}\left({{{\rm{g}}}}\right)+{{{\rm{CO}}}}\left({{{\rm{g}}}}\right)+{{{\rm{C}}}}({{{\rm{s}}}})\). Subsequently, 30 g of DMSO were blended into the precursor solution and subjected to magnetic stirring for 1 hour. Ultimately, adjusting the temperature to 40 °C and opening the fume hood facilitates the evaporation of ethyl acetate. After 2 hours, B@F was acquired. To eliminate the impact of preparation processes on the properties, the B without F14 coated, referred to as B, was additionally subjected to treatment using the twofold solvent approach.
Preparation of B/B2O3 components
7.0 g of B2O3 was dissolved in 100 mL of deionized water, then 17.6 g of amorphous B powder was added and stirred for 1 h. The obtained dispersion was placed in a vacuum oven and dried for 12 h to remove water.
Heat-treated processes
Heat-treated samples are acquired by subjecting them to a controlled environment inside a muffle furnace. More precisely, a certain quantity of a sample was accumulated in a powdered form inside a corundum crucible located within the muffle furnace. The initial temperature of all heat-treated samples was set at 200 °C. The temperature was then increased to the desired temperature at a heating rate of 10 °C∙min-1 and maintained for 15 minutes. Subsequently, the samples were allowed to cool naturally until the temperature reached 200 °C before being removed. The target temperatures were determined by considering the melting temperature of boron oxide (450 °C) and the reaction temperature of B in air (~800 °C). The selected temperatures were 500 °C, 600 °C, 700 °C, 800 °C, and 900 °C, respectively.
Preparation of B6O
Elemental B and B2O3 were mixed in stoichiometric proportions according to the reaction equation: \(16{{{\rm{B}}}}({{{\rm{s}}}})+{{{{\rm{B}}}}}_{2}{{{{\rm{O}}}}}_{3}({{{\rm{s}}}},{{{\rm{l}}}})\to 3{{{{\rm{B}}}}}_{6}{{{\rm{O}}}}({{{\rm{s}}}},{{{\rm{l}}}})\). The resulting B/B2O3 mixture was then placed in a tube furnace and calcined at 1400 °C for 12 hours under an argon atmosphere with a purity of 99.99%.
Characterizations
The morphology was characterized by scanning electron microscopy (FE-SEM, ZIESS, Sigma HD, Germany) at an accelerating voltage of 5 kV. The crystal compositions of the samples were investigated by X-ray diffraction (XRD, Bruker, D8 DISCOVER, Germany) in the range of 2θ: 5° to 40°. The chemical composition and elemental state near-surface region were measured by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha, America) and Fourier Transform Infrared Spectrometer (FTIR, Bruker, VERTEX 70, Germany).
TEM images were collected by using a Talos F200S G2 electron microscope operated at an acceleration voltage of 300 kV. Fresh samples were dispersed ultrasonically, dropped, and dried on a copper grid with lacy support films.
The thermal reaction processes were analyzed by a simultaneous thermal analyzer (TG-DSC, NETZSCH, STA449F5, Germany). Specifically, it was carried out by placing 2 mg sample into a corundum crucible (70 mL) in an air atmosphere (flow rate of 50 mL∙min-1) and heating from 50 °C to 900 °C at a heating rate of 10 °C∙min-1.
The flame structures underwent examination through the ignition-burning test. A 25 mg sample was placed on the platform, with a Ni-Cr wire (0.4 mm in diameter and 10 cm in length) positioned beneath the samples. The wire was quickly heated to the melting point of Ni-Cr using a custom-built DC current. The entire ignition process was captured using a high-speed camera (UX50, Japan) operating at 500 frames per second. The reaction efficiencies were further characterized by the combustion heat measurements (ZDHW-8E, China). 200 mg of powder were placed into a 330 mL closed boom and ignited by DC power supply in an oxygen atmosphere at 1 MPa, 2 MPa, and 3 MPa, respectively. Each sample underwent three tests to calculate the average value for the combustion heat. The constant volume explosive container system was used to characterize the reaction rates. The pressure-time curve was derived from a closed bomb test. The closed bomb was composed of a pressure cell (330 ml), a pressure sensor (PC290-ACAEFA1A, GAILIN), an oscilloscope, and a power supply. About 200 mg of powder samples were placed into a 5 ml crucible situated within the pressure cell, and ignited using a Ni-Cr wire embedded beneath the samples (with a diameter of 0.4 mm and a length of 10 cm). Before ignition, 1 MPa of oxygen was introduced into the closed bomb. The specific experimental setup is detailed in previous work30. A laser ignition device (wavelength was 10.6μm, the focused spot was 4.5 mm) was used to characterize the ignition delay time. 50 mg samples were lit using a laser with a power output of 15/30/45/60 W and an irradiation duration of 1000 milliseconds. The ignition and combustion process were captured by a high-speed camera operating at 1000 frames per second. Each sample underwent five tests to compute the average value for the ignition delay time. The specific experimental procedure and setup are detailed in previous work31.
Statistics and reproducibility
All tests were conducted using the same experimental setup under identical conditions, with multiple measurements. Each statistical dataset was obtained from three parallel tests, while the ignition delay time was measured five times in parallel.
Data availability
The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. The raw data files generated over the course of this study are available from the corresponding authors upon request. Source data are provided with this paper.
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Acknowledgements
J.W., K.Z., X.Z., Y.M., F.N., J.C., H.G., and J.W. were supported by the National Natural Science Foundation of China (T2222027 (J.W.), T2225027 (H.G.), and 12202416 (J.C.)). Thanks to SiChuan Hbst. Co., Ltd for combustion performance testing. We thank the Analysis and Testing Center of Southwest University of Science and Technology for providing SEM, the Beijing National Center for Electron Microscopy and the Laboratory of Advanced Materials of Tsinghua University for their help in TEM observations.
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Jian W. prepared samples, carried out the heat treatment experiment and combustion reaction performance test, analyzed the experimental data, and wrote the initial manuscript. Jun W. provided the initial inspiration for the project, conceived the project, planned and carried out the experiments, wrote the initial manuscript, and revised the manuscript; K.Z. carried out a molecular dynamics simulation and analyzed the simulation results. X.Z. and Y.M. assisted in the preparation of the sample and carried out some heat treatment experiments and SEM testing. F.N. and J.C. participated in the discussion of the experiment and assisted in the analysis. H.G. planned the experiments and wrote and revised the manuscript. All authors contributed to the discussion and revision of the paper.
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Wang, J., Zhong, K., Zeng, X. et al. Interfacial oxidation of boron proceeds through a stable B₆O intermediate. Nat Commun 16, 9800 (2025). https://doi.org/10.1038/s41467-025-64797-x
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DOI: https://doi.org/10.1038/s41467-025-64797-x
