Influence Of Fractal Characteristics On The Sensitivities Of Micron And Nanometer Energetic Materials

The subject for investigating the sensitivities of energetic materials always challenges the peer researchers who want to improve their safety.In the mass,two kinds of factors influence the sensitivities of energetic materials.One is inherent properties depending on chemical structure,and the other attributes to external physical configuration,such as particle size,size distribution,morphology,crystal,charge density etc.For example,TATB is well known as"wood explosive"because of containing the low-active bases as C-NO2,C-NH2 and phenyl in its molecular structure,which may result in quite low sensitivity.However,nitroamine explosives with exploding base as N-NO₂ exhibit high sensitive,such as RDX,HMX and HNIW etc.The influence of chemical structure on sensitivity is generally difficult to alter,but the influence depending on external physical configuration may be controlled or adjusted in a large degree.Therefore,researchers inside or aboard devoted themselves to improving the safety of ammunition by adjusting the size distribution and morphology of energetic particles. They have obtained deals of experimental results,among which many discrepancy and faultiness are presented.As an appearance of influence on sensitivity of energetic materials, size distribution and morphology do act on heat conduction properties of energetic particles, and finally lead to the changes of sensitivities.This study intends to combine fractal and hot spots theories together,apply fractal dimensions to characterizing the complexity degree of particle size and morphology of energetic materials quantificationally,and establish the models heat conduction for fractal energetic particles finally.These models will be employed to investigate the influence of fractal particle size and morphology on sensitivities of micron and nanometer energetic materials by means of experimental results.Fractal dimensions(D and D_s)are also applied to assess the sensitivities of energetic materials.Firstly,fractal characteristics of particle size distribution and morphology of energetic materials are investigated,and the models heat conduction of energetic particles have been established by combining fractal and hot spot theories.Particle size fractal dimension"D"is used to characterize the size formation,span of size distribution,proportion of fine particles, and the complex degree of the asymmetric energetic particles.Another morphology fractal dimension"D_s"is employed to describe the coarse degree and proportion of surface,and complex degree of the anomalistic surface morphology of energetic particles,either.The value scope and physical signification of D and D_s are also discussed.After that,two heat conduction models had been established,which one containing D is for simple fractal energetic particles,and the other containing D and D_S is for mixed fractal energetic particles. How D and D_S influence heat conduction along with sensitivity of energetic particles are discussed in detail.The value of D could be calculated with cumulation distribution data,and D_S could be obtained from SEM images of typical particle.Both D and D_S can be a quantitative criterion to estimate heat conduction and sensitivity of energetic materials.Next,influence of fractal particle size and morphology on sensitivity of micrometer explosives is investigated in detail.Micrometer explosives as HMX,RDX,HNIW and AP with different particle size distribution and morphology were prepared by milling,solvent and non-solvent,sieving,and airflow crushing.Their mechanical sensitivities and thermal decomposition are also investigated in detail.The results indicated that sensitivity of explosive with similar morphology changes regularly along with particle size.However,these rules vary along with the morphology of energetic particles.For example,the impact sensitivity of spherical HMX samples falls,and friction sensitivity decreases as the particle size(d₅₀)decrease.Both impact and friction sensitivities of needle HMX samples fall as the particle size decrease.However,the mechanical sensitivities of polyhedral HMX samples change a little.Similarly,the sensitivities of spherical RDX and sheet RDX samples vary at different rules.After fractal analysis,we found that impact sensitivity is related to D.The heat conduction of energetic particles with high D value exhibits too excellent to form hot spots which would cause exploding.Meanwhile,friction sensitivity is related to D_s.The friction coefficient among particles with high D_s value increases,which would yield more heat to form hot spots leading to explode.In addition,thermal decomposition is related to both D and D_s.The heat conduction of energetic particles with high D and D_s become superior,and not easy to form hot spots.Thirdly,in order to investigate the sensitivity of nanoscale energetic materials,several nanometer explosives and composite explosives were prepared,and their mechanical sensitivities and thermal decomposition are also discussed.Combining sol-gel reaction and supercritical GAS method,we had prepared nanometer HNIW/Fe₂O₃,RDX/Fe₂O₃, HMX/Fe₂O₃,AP/Fe₂O₃ and ADN/Fe₂O₃ energetic composites.After etching Fe₂O₃,nanometer HNIW,RDX and HMX powders were prepared,respectively.The results indicated that the optimal technical parameters were located at Fe(NO₃)₃·9H₂O molar concentration of 0.297 mol·L_ethanol^-1,HNIW concentration within 0.438 mol·L_{ethyl acetate}^-1and 1,2-epoxypropane dosage of 8.334mL·g₍Fe(NO_/wZ,-gFe(N0₃)^-1.Nanometer HNIW powders of 50nm tolOOnm contain mainlyαcrystals,and a few e crystals.Meanwhile,nanometer RDX powders are mostly about 100nm,and nanometer HMX powders of l00nm contain mostlyβcrystals and a fewαcrystals.Experimental results indicated that both impact and friction sensitivities of nanometer composite explosives decrease as the content of explosives reduce,and exothermic peak move ahead,except that friction sensitivity of nanometer AP/Fe₂O₃ increases.In addition,the impact sensitivity of nanometer nitroamine explosives falls a bit.However,the friction sensitivity increase largely,and exothermic peak move ahead either.Finally,we tried to develop the model heat conduction of fractal energetic particles into sensitivity study of blending energetic materials.The influence of fractal characteristic of Si powders on thermal reaction and mechanical sensitivities of Si/Pb₃O₄ composites are discussed.The results indicated that for micrometer Si/Pb₃O₄ composites,as the content of superfine Si powders increase,the low temperature reaction moves ahead with added exothermic quantity.Meanwhile,the friction sensitivity increases because of superfine Si powders with high D_s.However,for nanometer Si/Pb₃O₄ composites,as the particle size of nanometer Si reduces,the low and high temperature reactions go ahead,and the activation energy(?)_a decrease either.