Research On Composition/Performance Of Modified PVA-Based Electrically Controlled Solid Propellant

The electrically controlled solid propellant（ECSP）is a newly developed type of solid propellant that can be ignited and combusted by applying electrical voltage or current.It has unique electrically controlled combustion characteristics,such as on-line combustion,power-off extinction,and real-time adjustable burning rate,overcoming the shortcomings of traditional solid propellants that cannot be extinguished after ignition and the burning rate cannot be throttled by real-time adjustment.A solid rocket motor based on ECSP has a simple structure without an igniter and moving parts.It can work continuously for a long time or work in short-term pulses.It shows higher mobility and flexibility than traditional solid rocket motors and is an ideal power device for the main power and attitude control engines of missile weapons.In this study,the formulation and performance of a perchlorate-based electrically controlled solid propellant（P-ECSP）was investigated using a combination of theoretical analysis and experiments,and the thermal properties and thermal decomposition mechanism of a polyvinyl alcohol（PVA）binder were studied.PVA modification was explored on the basis of the low thermal decomposition temperature of PVA.To improve understanding of the effect of modified PVA on P-ECSP,this study explored the thermal decomposition and kinetics of an oxidizer/binder bicomponent matrix in the application of modified PVA as a binder of P-ECSP.On the basis of research methods for electrically controlled solid and traditional solid propellants,main components,such as oxidizer,binder,and metal powder were identified and screened.In addition,the physical and chemical properties of raw materials were comprehensively explored,and the content range of each main component was determined.Based on theoretical analysis,a single-factor method was used in investigating the curing molding performance and electrically controlled combustion characteristics of different formulations,and the basic formulation of the electrically controlled solid propellant with Li Cl O₄（Li Cl O₄-ECSP）as an oxidizer was determined.The sensitivity characteristics and safety of the propellant were analyzed and evaluated using the National Military Standard Propellant and Explosive Test Method.The results showed that the prepared Li Cl O₄-ECSP has good insensitive characteristics and meets the safety level（1.3）.Repeated ignition test was carried out at0.1–1.8 MPa,and the propellant was repeatedly ignited several times.Then,the linear and mass burning rates of the propellant were obtained at varying voltages and pressure.An electrically controlled solid rocket engine test based on Li Cl O₄-ECSP was carried out,the engine was successfully restarted several times,and the pressure and thrust curves were obtained.The thermal properties of PVA was analyzed through oxygen bomb combustion,elemental analysis and TG-DSC.The combustion heat,elemental composition,and molar heat capacity of PVA were obtained,and standard molar formation enthalpy and thermodynamic parameters were calculated,which enriched the thermal property parameters of PVA.The thermal decomposition products of PVA were analyzed and characterized through TG-FTIR,and the types of PVA thermal decomposition products were obtained.The thermal decomposition reaction mechanism of PVA and the effects of polymerization degree and alcoholysis degree on the properties of PVA were analyzed.The mechanical properties of Li Cl O₄-ECSP were studied.An electronic universal testing machine was used in studying the static tensile properties of the propellant,and the influences of solid filler,temperature,and tensile rate on the tensile properties of the propellant were examined.The results showed that Li Cl O₄-ECSP exhibits good viscoelastic properties and has a low tensile strength and high elongation at break.The temperature spectrum of dynamic mechanical parameters,such as storage modulus,loss modulus,and loss factor,of Li Cl O₄-ECSP with different solid contents at different vibration frequencies were obtained using a dynamic mechanical analyzer（DMA）.The apparent activation energy of propellant deformation was calculated using the Arrhenius equation.The results of the DMA study showed that the dynamic mechanical change process of Li Cl O₄-ECSP has only one glass transition peak and the glass transition temperature（T_g）moves to the high temperature direction as frequency increases.The mechanical strength of the propellant can be effectively improved through reasonable particle gradation.When the mass ratio of Al（5μm）/Al（65μm）was 3/7,the maximum tensile strength and storage modulus were obtained.The apparent activation energy required for glass transition is 270.19 k J/mol.The thermal decomposition temperature of PVA was increased by modifying PVA with hydroxyl as an active site.PVA-g-EPBE and PVA-g-VTMOS were synthesized with the chemical grafting method.PVA/PVAN was obtained using the solution blending method,whereas PVAN was obtained through PVAm nitration treatment as a modifier.The structure and element composition of the modified PVA（PVA-g-EPBE,PVA-g-VTMOS,and PVA/PVAN）were analyzed through Fourier transform infrared spectroscopy,nuclear magnetic resonance spectroscopy,X-ray diffraction,and elemental analysis.The results showed that PVA was successfully modified,and the TG-DTG results showed that the modified PVA exhibits higher thermal stability than PVA.According to the results of combustion heat and elemental analysis,the standard molar formation enthalpy of the modified PVA were calculated,and the effects of modified PVA on the energy characteristics of Li Cl O₄-ECSP and HAN-ECSP were studied.PVA-g-EPBE,which is composed of C,H,and O,can improve the specific impulse characteristics of the propellant,and increase in PVA-g-VTMOS and PVA/PVAN contents will reduce the specific impulse characteristics of the propellants.Thus,the amount of PVA-g-VTMOS or PVA/PVAN used to replace PVA should be strictly controlled.The effects of the modified PVA on the thermal decomposition process and kinetic characteristics of the oxidizer/binder bicomponent matrix of Li Cl O₄-ECSP under different heating rates were studied through TG-DTG analysis.The thermal decomposition temperature of Li Cl O₄/PVA-g-EPBE decreases relative to that of Li Cl O₄/PVA,and the activation energy of the three stages of thermal decomposition are reduced.Li Cl O₄/PVA-g-VTMOS has a slightly lower thermal decomposition temperature than Li Cl O₄/PVA,but the second stage of thermal decomposition and activation energy are greatly increased.The activation energy increases by 25%.The thermal decomposition temperature of Li Cl O₄/PVA/PVAN is 35℃higher than that of Li Cl O₄/PVA,but the reaction activation energy is reduced.Reaction activation energy corresponding to the different conversion rates of the oxidant/binder at different heating rates was obtained through single heating rate and multiple heating rate methods.The possible mechanism functions were preliminarily screened using the Coats-Redfern method and then combined with the average and double extrapolated methods.The most probable mechanism functions of the oxidizer/binder bicomponent matrix at different thermal decomposition stages were determined.The results showed that the thermal decomposition mechanism of the oxidizer/binder bicomponent matrix can be described by the random nucleation and subsequent growth model and the three-dimensional diffusion model.