LIAO Ning,LI Zhao-qian,LI Wen-peng,DUAN Xiao-hui,PEI Chong-hua

(State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials Key Laboratory Southwest University of Science and Technology,Mianyang 621010,China)

1 Introduction

Nanocomposite energetic materials (NCEMs) are often composed of a fuel and an oxidizer,which are homogenous at nanometer scale[1].Generally,the nanocomposites possess excellent thermodynamic characteristics of fuel-oxidizer mixtures and high energy release rate[2].Numerous energetic composites,such as cyclotrimethylenetrinitramine (RDX)/AP/SiO2,HMX/AP/resorcinol-formaldehyde (RF),RDX/bacterial cellulose (BC)[3-5],have been prepared.Compared with conventional mixtures,these nanocomposites have less sensitivity and faster burning rate.However,RF,SiO2,and BC are not energetic materials or components of rocket propellants.Cyclotetramethylene tetranitramine (HMX) and ammonium perchlorate (AP) are the most reactive components in formulations of a large number of high performance explosives and solid propellants[6].If the well-distributed composites of HMX and AP are prepared according to a certain percentage reaching or being close to zero oxygen balance (ZOB),it will be hopeful to obtain competitive energetic compounds[7].In this work,a novel energetic polymer (EP) is used as the matrix to prepare NCEMs HMX/AP/EP with different oxygen balance by co-precipitation method.

2 Experimental

HMX,AP and EP were dissolved in a good solvent(ketones) under a magnetic stirrer and ultrasonic dispersion,and then this mixed solution was added into a bad solution(haloalkane)to form the solution with co-precipitation product.The products were filtered from the solution after keeping stirring 2 h,and finally dried through freeze-drying.The oxygen balance of HMX/AP/EP composites was moderated through changing the mass ratio of HMX/AP/EP including positive oxygen balance(POB),negative oxygen balance(NOB) and zero oxygen balance.

3 Results and discussion

3.1 Microstructures of HMX/AP/EP composite

In all HMX/AP/EP composites the content of EP is controlled to be 5.6%,HMX and AP particles uniformly deposit on the nano-network structure of EP and the network size vary in the range of 50-200 nm (Fig.1).The EDS elemental mapping results can confirm that AP (Cl) and HMX (C) are homogenous dispersion,the content of element C is higher than that of Cl,because of EP contribution for element C (Fig.2).Compared with the pure EP,the specific surface area of the HMX/AP/EP nanocomposite is reduced from 34.7 m2·g-1to 10.3 m2·g-1,and the distribution of pore-size decreases from the range of 3-90 nm to 2-40 nm (Fig.3).According to the Scherer equation[8],the mean particle diameter of HMX and AP in different samples are 59,62 nm and 63 nm,respectively,indicating that the crystallite size prepared by our method is uniform.The grains of HMX,AP and nano-network are closely bonded each other with good compatibility in a microsystem and there should be molecular interactions,such as hydrogen bonding and electrostatic interactions.

Fig.1 SEM image of HMX/AP/EP composite

Fig.2 Distribution of elements Cl and C for HMX/AP/EP composite

Fig.3 The pore-size distribution of EP and HMX/AP/NBC composite

3.2 Thermal behavior and sensitivity characteristic

The thermal behavior of HMX/AP/EP nanocomposite is greatly different from that of pure HMX and AP.The ZOB NECM has a sharp exothermic peak at 232.4 ℃.The peak temperature is much lower than that of HMX.The decomposition heat of the composite reaches to 2570 J·g-1.This value is the highest among all the samples in Fig.4.The EP can boost the decomposition and enhance the interaction of AP and HMX.The unique nanostructure of the composite broadens the contact surface.The optimum mass ratio of AP/HMX/EP contributes to the excellent decomposition heat.

Fig.4 DSC curves for pure HMX,AP and nanocomposites with POB,NOB and ZOB

The characteristic drop height of impact sensitivity (H50) of the nanocomposite reaches to 50.49 cm,which is much higher than that ofH50of 27 cm of HMX,showing that EP as matrix can reduce the sensitivity of this energetic material.Meanwhile,the presence of AP and the nano-effects have also obvious desensitization function.

4 Conclusion

A novel HMX/AP/EP nanocomposite has been prepared by the co-precipitation method which enables to easily control the stoichiometry to different OBs and the homogeneity of the HMX/AP/EP nanocomposites.This material possesses the three-dimensional nano-network structure with size of 50-200 nm,HMX and AP are uniformly deposited on EP matrix with good compatibility.EP and the optimal mass ratio to ZOB are able to boost the decomposition of the composite and make its decomposition heat up to 2570 J·g-1.Its impact sensitivity is lower than that of HMX.It will be hopefully applied in formulations of high performance explosive and solid propellant.

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