Design,Preparation And Combustion Performance Optimization Of Polymorphic Graphene/PVDF/Al Powders

In recent years,as the demand for equipment applications in the military field has gradually increased,the demand for energetic materials with both high energy density,high reactivity,short ignition delay and high energy release rate has been further enhanced.As a metal fuel,aluminum powder is widely used in military and civilian applications such as aluminum thermite,propellant and metal-containing explosives because of its high energy density（30.46 k J/g）,wide range of sources and low price.The main advantages of micron aluminum focus on the high active aluminum content and good dispersion,but the passivation layer on its surface still hinders its energy release.Fluoropolymer/micron aluminum energetic materials have attracted the attention of researchers due to their high reactivity and high energy density,but the intrinsically low thermal conductivity of fluoropolymer hinders further energy release enhancement of the system.Come into existence as the situation requires,multicomponent energetic composites with fine structures consisting of metal-containing fuels and oxidants——metastable intermixed composites（MICs）have attracted attention.In this paper,a multilayered core-shell structured graphene/PVDF/Al composite powder with significantly optimized energy release effect and performance close to that of nano-Al powder is prepared by using graphene with high thermal conductivity additive and surface non-covalent modification of graphene to address the current problem of insufficient energy release of fluoropolymer/micron Al system.In this proposal,the polymorphic graphene/PVDF/Al composite powder and SPG/PVDF/Al composite powder samples with different SPG contents were prepared by using micron-Al powder and PVDF powder as metal fuel and oxidant,and SPG was obtained by non-covalent modification and reduction of GO using SPEEK through ultrasonic dispersion and chemical method process.By comparing the interfacial bonding between different polymorphic graphene and PVDF,the physicochemical mechanism of different interfacial bonding was elucidated,and the technical and theoretical system of structural regulation and performance improvement of sub-stable composites was established.The effect of the addition of thermal conductivity additive SPG on the combustion energy release process of SPG/PVDF/Al composite powder is investigated,and the influence of microstructure regulation on the optimization of combustion performance such as enthalpy,calorific value of combustion,energy release rate and the utilization rate of active aluminum is established.Finally,the mechanism of structure on system heat transfer and combustion performance optimization was investigated.It was found that by analyzing the morphology of polymorphic graphene encapsulation on the surface of PVDF/Al composite powder and the morphology of multilayer core-shell structure,the strong interaction between SO₃H-based dipole in non-covalent SPG and-CF₂-dipole of PVDF layer,and the optimized interfacial bonding of monolayer SPG and PVDF layer brought more desirable energy release characteristics to the material.The SPG/PVDF/Al composite powder exhibited greater exothermic enthalpy and thermal weight gain of 16710 J/g and 170.31%under slow heating conditions with both SPG and RGO addition content of 2%.The thermal reaction properties of the graphene/PVDF/Al composite powder indicated that the presence of graphene promoted the pre-ignition reaction（PIR）of the fluorine-containing system and enhanced the reactivity between PVDF and Al systems,resulting in a higher surface oxidation of the material near 600°C and a linear increase in the final main oxidation exothermic enthalpy（R²=0.9865）.The presence of graphene changes the way the internal reactive Al contacts the oxidizing atmosphere of the PVDF/Al composite powder under slow heating conditions,and the heat accumulated by the surface oxidation of the material is converted into pressure with the increase of temperature leading to the ejection of the internal reactive Al by breaking the shell,and the ejected nano-Al particles are fully utilized and the reactive Al utilization is enhanced.The degree of enhancement of peak pressure and boost rate of the material during ignition by the two different graphene is related to the graphene intrinsic content,and the higher graphene content of RGO brings greater peak pressure and higher boost rate,reaching 4773 k Pa and 13009.64 k Pa/s.However,the combination of RGO layer and PVDF layer only by physical adsorption makes the low graphene energy density is reflected,and its effect on the main oxidative exothermic lift of PVDF/Al composite powder under slow heating conditions is weaker than that of SPG.The optimized interfacial bonding of SPG/PVDF/Al composite powders provides a new strategy to enhance the energy release properties of the material.The optimized interfacial bonding crosses the disadvantage of the low energy density of graphene and achieves the enhancement of the energy release properties of the material.The increase of the combustion calorific value and the morphology of the combustion products also confirmed the shell-breaking effect of the SPG/PVDF/Al composite powder under the slow heating conditions,even the coalescence phenomenon during the combustion process,and some of the agglomerated large particles were broken through to release the active aluminum.However,with the increase of SPG content,the absolute content of polymer brought by its surface modification also increases,making the peak pressure and boosting rate of SPG on PVDF/Al composite powder exist threshold phenomenon.When the SPG content was 4%,the peak pressure and boosting rate of its ignition process reached great values of 4845.28 k Pa and 8683.58 k Pa/s.When the SPG content continued to increase,this boosting effect was weakened.By reasonably controlling the SPG content,the energy release rate of MICs can be effectively enhanced.The structural design and regulation of MICs can effectively improve the calorific value of combustion of the material,but the enhancement of the energy release rate still depends on the content of graphene and the surface modification state.