Study On Surface Modification And Mechanism Of Ignition And Combustion Of Boron Powder

Boron（B）powder possesses high volumetric and mass heat values,promising extensive applications in the field of energetic materials,including solid propellants and explosives.However,its difficulty in ignition and low combustion efficiency impede the effective utilization of its thermodynamic advantages.Therefore,the exploration of methods to enhance the rapid ignition and efficient combustion of B powder,as well as the elucidation of the mechanisms governing ignition,combustion,and energy release,holds paramount significance in advancing the widespread application of B powder.In this study,various performance-enhancing materials,namely nickel oxide（Ni O）,ammnium perchlorate（AP）,and polyvinylidene fluoride（PVDF）,were employed for the modification of B-based composites.These modifications were achieved through distinct techniques including ultrasonic blending,solvent evaporation,solvent-nonsolvent methods,and electrospinning,both as single-material modifications and multicomponent synergistic modifications.Thermal analysis and laser ignition experiments,complemented by physicochemical property assessments such as SEM,XRD,and particle size analysis,were conducted to investigate the ignition,combustion,and energy release characteristics of B powder modified with either single materials or multiple components.Additionally,the influence of particle size variation on the ignition and combustion performance of B powder was analyzed.Furthermore,the mechanisms by which the modification materials promote the ignition and combustion of B powder were explored,taking into consideration the characteristics of combustion products.In conjunction with experimental investigations,molecular dynamics simulations were employed to elucidate the microscale mechanisms underlying thepromotion of PVDF on B powder ignition and combustion.The main findings are as follows:（1）Investigation of ignition and combustion mechanisms in Ni O-modified B powders with varied particle sizesThe oxidation-reduction reactions between Ni O and B powder release thermal energy,contributing to the elevation of the surface temperature of B powder.This elevation improves the surface microenvironment of B powder,facilitating the volatilization of surface oxide films and increasing the contact area between B powder and oxygen.Consequently,this benefits the smooth progression of oxidation reactions.In the modified B/Ni O system,the ignition temperature can be reduced to as low as 658.76°C,and the maximum heat release reaches 15.47 k J/g.Ni O rapidly establishes oxygen pathways on the surface of B powder through oxygen vacancies,and its own content remains unaffected during combustion,thereby continuously catalyzing the combustion of B powder.Consequently,the self-sustaining combustion time of the modified B/Ni O system significantly extends,leading to increased combustion intensity and elevated combustion temperature.At the same Ni O concentration,the combustion temperature of B powder increases with decreasing particle size.When the Ni O content is 5 wt.%,the combustion time of 50 nm particle-sized B powder is twice that of the unmodified B powder with the same particle size,reaching a maximum combustion temperature of 3000°C.Furthermore,Ni O replaces some of the oxidation reactions of B powder through its oxidation-reduction reactions,reducing the formation of partially combusted products like HBO₃,thereby further improving the combustion efficiency of B powder.The combustion efficiency of the modified B/Ni O system can increase by up to 11.25%compared to the unmodified B powder.However,excessively high Ni O concentrations lead to a reduction in overall energy density,resulting in decreased combustion temperature and efficiency.Therefore,the Ni O concentration for B powder modification should not exceed 10 wt.%.（2）Investigation of ignition and combustion mechanisms in AP-modified B powders with varied particle sizesB powder can catalyze the thermal decomposition of AP,converting the two-stage weight loss process of AP into a single stage,ultimately advancing the decomposition temperature by up to 77°C.Under an argon environment,B/AP composite particles exhibit a two-stage combustion phenomenon.For B powder with the same particle size,increasing the AP concentration from 10wt.%to 30 wt.%can reduce the ignition delay time of B/AP composite particles by a maximum of 14.33 ms.With the same AP concentration,as the particle size of B powder decreases,the ignition time of B/AP composite particles can be reduced by up to 9.81 ms.B powder with a particle size of 50 nm,at an AP concentration of 30 wt.%,can achieve the shortest ignition delay time,as low as 28.81 ms.As the AP concentration increases,the release of oxygen from B/AP composite particles effectively elevates the oxygen concentration in the vicinity of B powder,accelerating the oxidation reaction rate of B powder and reducing its ignition delay time.On the combustion front,small agglomerates of B particles aggregate into larger agglomerates through"liquid bridging."After leaving the combustion surface,these large agglomerates rise with the thermal updraft,descend,deform upon reaching the bottom,and then rebound.Following this"rise-descend-rebound"process,they ultimately settle on the combustion surface.During this motion,as the agglomerates continuously release heat to the external environment,their outer shells solidify,forming a core-shell structure.The decomposition of AP releases a significant amount of gas,leading to an increase in internal pressure within the agglomerates.When this pressure exceeds the limits of the outer shell,micro-explosions occur.Due to the self-healing properties of the oxide film,these micro-explosions of B agglomerates exhibit dispersed characteristics rather than fragmentation-type explosions.These micro-explosions can occur multiple times in succession,dispersing unburned B particles into the oxidizing environment,increasing the contact area between B powder and surrounding oxidizing gases,enhancing flame disturbance,and thus improving the combustion efficiency of B powder while reducing the size of agglomerates.Under oxygen-deficient conditions,the combustion efficiency of B powder is generally low,around 30%.Oxygen concentration is a crucial factor influencing combustion efficiency.Therefore,increasing the AP concentration from 10 wt.%to 30 wt.%can raise the combustion efficiency of B to around 37%.The influence of AP on the ignition and combustion mechanisms of B powder primarily manifests in three aspects.Firstly,AP decomposition releases oxygen,promoting the ignition and combustion of B powder.Secondly,AP decomposition generates a substantial amount of heat,raising the surface temperature of B powder,accelerating the evaporation of the oxide film on the B powder surface,and increasing the oxidation rate of B powder.Additionally,AP decomposition produces numerous gas fragments,which aid in rupturing the oxide film on the B powder surface and facilitate micro-explosions within the agglomerates.These factors effectively enhance the combustion efficiency of B powder and reduce the size of combustion products.（3）Investigation of ignition and combustion mechanisms in PVDF-modified B powders with varied particle sizesThere exists an interaction between B powder and PVDF.B powder lowers the initial decomposition temperature,reduces the low-temperature decomposition peak temperature,and advances the decomposition peak of PVDF.When the PVDF content is 10 wt.%,the initial decomposition temperature of 50 nm B powder composite material is reduced by 152°C compared to pure PVDF,the low-temperature decomposition peak temperature is reduced by 122°C,and the decomposition peak temperature advances by139°C.PVDF decomposition generates small molecular fluorides（HF）that can react with the oxide film on the surface of B,accelerating the removal of the oxide film,increasing the contact area between B and oxygen,promoting the smooth progress of the oxidation reaction,and enhancing the oxidation heat release of B powder.In B/PVDF composite materials,both the unit mass gain and heat release of B powder are greater than those of unmodified B powder at the same particle size.However,excessively high PVDF content leads to a reduction in the overall energy density of B/PVDF composite materials,resulting in a decrease in overall heat release.At a PVDF content of 20 wt.%,50 nm particle-sized B powder exhibits the highest oxidation heat release,reaching up to 13.94 k J/g.PVDF decomposition produces a substantial amount of gas,and its decomposition products can react with B to generate gaseous combustion products,dispersing B particles in space.This exposes more internal B particles to the oxygen-rich environment,increasing the contact area between B particles and the oxidizer.Consequently,it promotes the oxidation process of B,enhancing its combustion intensity,raising its combustion temperature,and improving its combustion efficiency.The modification effect of PVDF is more pronounced on small particle-sized B powder.At a PVDF concentration of 30 wt.%,50 nm particle-sized B powder can achieve a maximum combustion temperature of 2700°C and an improved combustion efficiency of 62.91%.PVDF primarily promotes the ignition and combustion of B powder through:gas production,oxide film removal,and increased heat release.（4）Investigation of ignition and combustion mechanisms enhanced by electrospinning-mediated interface control of PVDF in B powdersElectrospinning improves the dispersion of B powder in composite materials,reduces the agglomeration of B powder particles,increases the contact area between B and PVDF,shortens the interaction distance between B and PVDF,and enhances the heat and mass transfer between interface.This facilitates the rapid decomposition of PVDF,accelerates the removal of the oxide film on the surface of B powder,thereby enhancing the oxidation heat release of B powder.When compared to composites prepared using the solvent-nonsolvent method within the same composition,the composite prepared through electrospinning exhibits an advancement of 5°C in the PVDF decomposition peak temperature and an increase of 1.44 k J/g in heat release.The rapid decomposition of PVDF increases the gas production rate,aiding in the enhancement of the flame spread rate of B powder.It also increases the dispersion of B particles,enlarges the contact area between B powder and the surrounding oxidizing gases,boosts the oxidation rate of B powder,shortens its combustion duration,raises the combustion temperature,and improves combustion efficiency.The maximum flame expansion rate of composites prepared via electrospinning is 1.6 times that of composites prepared via the solvent-nonsolvent method,resulting in a 3.59%increase in combustion efficiency.（5）Mechanisms of PVDF promotion in ignition and combustion of B powdersAn adsorption configuration of PVDF on the surface of B has been established,revealing that the adsorption of PVDF on the B surface is characterized by unstable physical adsorption.Within the PVDF chain,a small portion of charges transfers from the surroundings of the H atoms,facilitating its dehydrofluorination.Kinetic studies of the reaction between B₂O₃ and HF indicate that individual HF molecules readily adsorb onto the B₂O₃ surface.However,as the quantity of HF increases,due to saturation of the active sites on the B₂O₃ surface,some free gaseous HF molecules do not react with B₂O₃.The potential energy surface and kinetics of the BO₂ and HF reaction were calculated using the B3LYP/6-311+G（3df,2p）method.By analyzing the potential energy surface of the BO₂ and HF reaction system,four transition state structures and two reaction pathways were identified:reaction pathway R1（BO₂+HF→IM1→TS1→IM2→TS2→HBO₂F）and reaction pathway R2（BO₂+HF→IM1→TS3→IM3→TS4→HBO₂F）.BO₂ and HF are more prone to react via pathway R2,thereby inhibiting the formation of the oxide film.（6）Mechanisms of ignition and combustion enhancement in B powders through synergistic multicomponent modification with PVDF/AP/Ni OA successful multicomponent modification of B powders was achieved using a layer-by-layer encapsulation method to create B composite energetic materials.Compared to unmodified B powder,the modified B EMS composite exhibited a lowered ignition temperature by 42°C,an advancement in oxidation peak temperature of approximately 69°C,and a peak heat release that increased to 1.4 times that of unmodified B.Unmodified B powder did not ignite at a 60 W ignition power,but ignition was successful at 120 W,with a visible flame at 180 W.In contrast,the synergistically modified B powder ignited at 60 W and exhibited a noticeable flame.Moreover,the synergistic interaction among the components improved the dispersion of B particles,facilitating rapid energy release,increasing the combustion intensity,elevating the combustion temperature by approximately 700°C compared to unmodified B powder,and enhancing combustion efficiency by 30.68%.These results demonstrate that the multicomponent modification materials,through synergistic interactions,effectively harness their respective advantages,promoting rapid ignition and combustion of B powders.During the ignition stage,they lower the ignition temperature of B through a combination of"temperature rise oxygen supply+film removal+catalysis,"achieving successful ignition at low energy densities.In the combustion stage,they enhance the combustion intensity,efficiency,and injection efficiency of B powders by leveraging the mechanisms of"gas production+film removal+heat release.In summary,the modification of B powders using Ni O,AP,and PVDF has yielded favorable results.Particularly,the synergistic modification of these three components has shown significant potential in promoting rapid ignition,efficient combustion,and improved injection efficiency of B powders.This approach offers new perspectives and methods for B powder modification and establishes a theoretical foundation for the study of ignition and combustion mechanisms in B powders.