Study On Energy Release Properties Of Boron Based Fuel Rich Solid Propellants Primar Combustion Products Systems

Boron is a metal fuel with both high gravimetric and volumetric calorific values.By taking amorphous boron as the main energy source,boron-based fuel-rich solid propellants become ideal fuels for solid ducted rockets.Primary combustion products of boron-based propellants(PCPs for short)are incomplete combustion products that are emitted from the gas generator of a solid ducted rocket,thus are the major link between theprimary and secondary combustion processes.Studying the energy release properties of boron-based fuel rich solid propellants primary combustion products systems(include PCPs,multicomponent systems containing boron,and pure boron particles)provides valuable information about the primary combustion process and helps to better organize the secondary combustion process.The qualitative analysis of the PCPs sample composition was performed by using x-ray diffraction,x-ray photoelectron spectroscopy,and thermogravimetry-differential scanning calorimetry experiments.Based on these results,an integrated quantitative analysis of the sample composition was conducted.The quantitative analysis methods include tube furnace heating,ion chromatography,infrared spectroscopy,and inductively coupled plasma emission spectroscopy.The PCPs sample mainly contain B,C,BmCn,H3BO3,B2O3,BN,Mg,MgCl2,and NH4Cl.BmCn(22-24%),H3BO3(20%),and B(16.8%)were the three major components,while BmCn,B,and C(9.8-11.8%)were the three combustible components present in the highest amounts.The oxidant NH4ClO4 was completely consumed.In general,the combustion completeness of the primary combustion products was rather low.Therefore,better understanding and controlling of the secondary combustion process is very important to improve the performance of boron-based propellants.The difference and correlation of ignition and combustion characteristics between amorphous boron and the PCPs were comparatively studied by laser ignition experiment and thermogravimetry-differential scanning calorimetry.Besides,effects of different metal additives(Mg-Al alloy.Mg.Al.and Ti)and gas generator pressures on ignition and combustion characteristics of the PCPs samples were studied using a laser ignition experimental system.The contents of boron in PCPs samples were far lower than that in the amorphous boron sample.However,the full-time spectral intensity of the former at a wavelength of 580 nm was only～2%lower than that of the latter,and the maximum spectral intensity at full wave was even～5%higher.The ignition delay time of PCPs was～150 ms shorter than that of amorphous boron,and the self-sustaining combustion time of the former was～200 ms longer than that of the latter.The thermal oxidation process of amorphous boron involves water evaporation(weight loss)and boron oxidation(weight gain).The thermal oxidation process of primary combustion products involves two additional steps:NH4Cl decomposition(weight loss)and carbon oxidation(weight loss).The presence of Mg-Al alloy effectively increased the combustion intensity of the PCPs samples.The average combustion temperature of the PCPs sample with added Mg-Al alloy reached 1440.36 ℃.The ignition delay time of the PCPs samples ranged between 61 and 146 ms,while the self-sustaining combustion time ranged between 1174 and 1254 ms.Mg-Al alloy and Ti both helped to shorten the ignition delay time and prolong the self-sustaining combustion time.Among the four metal additives.Mg-Al alloy was the most beneficial to the energy release of the primary combustion products,whereas Al had the weakest effect.Besides,the PCPs samples obtained under higher gas generator pressures exhibited inferior combustion characteristics.A two-camera two-filter flame shape test system and thermogravimetry-differential scanning calorimetry-Fourier transform infrared spectroscopy combined thermal analysis system were used to study the ignition and combustion characteristics and thermal oxidation characteristics of B/B4C binary system samples,respectively.A laser-ignition solid micropropulsion test system was set up to analyze the propulsive performance of B/NH4ClO binary system samples in the tube combustors.In addition,the thermal analysis and kinetic analysis of B/hydroxyl-terminated polybutadiene(HTPB)/oxidants ternary system samples were carried out using a thermogravimetry-differential scanning calorimetry combined thermal analysis system.The ignition and combustion processes of the B/B4C binary system sample could be divided into three stages:a developmental stage,a stable combustion stage,and an extinction stage.After the sample was ignited,a bright green flame could be observed,and the flame shape was tapered at the stable combustion stage.The samples experienced a slight mass loss during the low temperature section of the thermal oxidation process.During the high temperature section,the oxidation of B and B4C caused considerable mass gain.The gaseous products of the thermal oxidation process mainly included CO2 and CO.In general,the B content of 60%was the most beneficial to decrease the oxidation temperature,increase the combustion intensity and improve the heat releasing ability of the B/B4C binary system samples.Various propulsive performance parameters of B/NH4ClO binary system samples,including the thrust,specific impulse,and density specific impulse first increased then decreased with the reduce of fuel-oxidant ratio.The optimal fuel-oxidant mass ratio of the B/NH4ClO binary system samples was 40%,with a density specific impulse of 0.474 kg/m2·s.During the thermal oxidation processes of the B/HTPB/oxidants ternary system samples,coating with NH4ClO4 had better performance than mechanical mixing with the same component.Coating with KNO3 effectively improved the ignition characteristic of the samples.Coating with LiClO4 was the most beneficial one for reducing the difficult degree of B oxidation.Coating with cyclotetramethylenetetranitramine(HMX)was the most beneficial coating for the heat-release of the samples.A pressurized concentrated ignition experimental system(original system)and a jet flow pressurized concentrated ignition experimental system were used for analyzing the static and dynamic ignition and combustion characterisatics of amorphous boron particles,respectively.As the oxygen pressure increased,the combustion intensity of the molded amorphous boron samples(static)steadily increased,and the ignition delay time and combustion time both decreased.Under the oxygen pressure of 9 atm.the average ignition delay time and combustion time were 2640 ms and 2596 ms,respectively,and the highest combustion temperature reached 1561.5 ℃.The flame on the sample surface was green and the brightest,which was produced by an intermediate combustion product.BO2(corresponding molecular emission spectrum wavelength,547.3 nm).Emission spectra of another intermediate product,BO(431.9 nm)was also detected.Two different types of structures were found in the condensed combustion products of the samples.The first type was the flaky B2O3 structure,and the second type was the flocculent structure of incomplete combustion products(mainly contained B6O and B2O3).The complete oxidation ratio of the samples also increased with the oxygen pressure,and reached the maximum value of 68.71%under 9 atm.During the jet flow igniton experiment(dynamic),the average ignition time of the amorphous boron samples was shorter than the average combustion time.Both surface coating and increase in oxygen pressure can improve the ignition and combustion characteristics of the samples.Specifically,they can increase the combustion intensity,decrease the average ignition and combustion times,and reduce their variances.Comparatively speaking,nitroguanidine(NQ)coating showed more beneficial effects than NH4ClO4 coating.Characteristic peaks of BO2,BO and Na(an impurity)were found in the maximum emission spectra of the samples.Amorphous boron samples in different oxidation degrees and ignition stage during combustion were obtained A dual beam focused ion beam micro/nanofabricator was used to etch and cut the samples into thin slices(-300 nm).The slices were observed under a scanning transmission electron microscope,accompanied with energy dispersive X-ray analysis.Two different reaction modes took place in sequence during the thermal oxidation of the samples.Below 650℃,the oxidation reaction occurred only on the surface of the particle(the surface reaction mode).However,when the samples were heated to 700 ℃,the particle interior was also involved in the reaction(the global reaction mode),and many pores were formed.During the jet flow ignition experiment,O2 diffusion still existed under elevated heating rate;however,its effect was weaker and almost limited to the particle surface.By combining resultes of earlier studies,we successfully verified the diffusion mechanism on the boron surface during the low-temperature oxidation stage.Bidirectional diffusions of O2 and(BO)n indeed exist on the surface of boron particle.Then,an ignition delay kinetic model of B particle was developed.By ensuring the initial ignition temperature,the ignition delay of a single B particle can be divided into two stages:(1)heat transfer stage and(2)low-temperature oxidation stage.Only heat transfer between the B particle and surroundings occurred during the heat transfer stage,whereas both heat transfer and oxidation occurred during the low-temperature oxidation stage.The oxidation involves four global reactions:(1)evaporation of B2O3,(2)diffusion of O2,(3)diffusion of(BO)n,and(4)reaction of H2O.The final ignition delay time of a boron particle is equal to the sum of the lasting times of heat transfer stage and low-temperature oxidation stage.The results of computed ignition delay time obtained by the model are consistent with the previous experimental data under O2/H2O atmosphere.