飛秒激光燒蝕含能材料的分子動力學模擬

楊　鎮　劉　海　何遠航,*

(1北京理工大學，爆炸科學與技術國家重點實驗室，北京100081；2中國空氣動力研究與發展中心，超高速空氣動力研究所，四川綿陽621000)

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飛秒激光燒蝕含能材料的分子動力學模擬

楊鎮1劉海2何遠航1,*

(1北京理工大學，爆炸科學與技術國家重點實驗室，北京100081；
2中國空氣動力研究與發展中心，超高速空氣動力研究所，四川綿陽621000)

為了得到飛秒激光侵蝕(FLA)1,3二硝基甲苯(簡稱DNB，分子式：C6H4N2O4)，六硝基六氮雜異伍茲烷(簡稱CL20，分子式：C6H6N12O12)和CL20/DNB共晶系統的物理和化學響應過程，本文采用ReaxFF/lg反應力場對其過程進行模擬。計算結果表明，CL20/DNB系統的溫度和壓力在飛秒激光加載過程中出現階躍，激光加載過程后系統有一個冷卻過程，然后系統的溫度和壓力逐漸升高達到最大值并維持平衡。研究發現，在此過程中CL20和CL20/DNB系統觸發反應均為CL20分子中的N―NO2斷裂。CL20系統的分解速率大于CL20/DNB共晶系統，這可能是因為共晶系統在反應初期具有大量的DNB分子以及分解產物中含有比較穩定的苯環減少了CL20及其產物之間的有效碰撞。

飛秒激光；CL20/DNB共晶；ReaxFF/lg；反應機理；分子動力學

www.whxb.pku.edu.cn

1　Introduction

Femtosecond laser pulse1,2is characterized by its short duration and ultra-high peak power.By passing through focusing lens,the laser pulse can invoke nonlinear effects upon interaction with other materials and thereby achieve the capability of ultra-fine processing3.FLA has registered pivotal applications in many works including micromachining4－6,laser propulsion7,and machining energetic materials8,to name just a few.

Femtosecond laser pulse causes no heat transfer and a lowimpact pressure in fine-machining energetic materials,which not only helps dismantle explosives but satisfies sophisticated production of small arms in the arsenal as well8.

Up to date there are only a few reports on the employment of the femtosecond laser pulse in machining energetic materials, showing its feasibility9,10and non-involvements of heat transfer and chemical reactions11,12during the processes.By using ANSYS (large finite element analysis software),Chen and co-workers14have further established heat transfer models for the femtosecond laser in cutting propellant13and ablating Mg/polytetrafluoroethylene.According to detailed calculations of the internal temperature distributions and heat releases,these authors have reached the conclusions that the surface temperatures rise up to over 2500 and 3000 K for the above-mentioned processes,respectively.

To apply the femtosecond laser machining technology in energetic materials in a safe manner,it is vital to understand the fundamental response processes of the energetic materials involved in their femtosecond laser machining.Due to the ultrashort time scale of the femtosecond laser process,practical experiments encounter tremendous challenges in the attempt to unveil every subtle detail,which becomes actually impossible at the present stage of technology development.This is actually where the theory can fit in.For example,the ReaxFF reactive force field can uncover the physical and chemical response processes of the energetic materials by simultaneous simulations of millions of condensed phase atoms,which is of great complement to the experiment.

ReaxFF reactive force field has been widely applied to study the energetic materials.Numerous simulations15－23have been carried out to investigate the reaction processes of energetic materials under extreme conditions via the ReaxFF reactive force field at the atomic/molecular levels and with a temporal resolution of femtosecond for the initial reaction processes and mechanisms, providing subtle information that can be neither achieved by realistic experiments nor solved with analytical quantum mechanics. However,no studies have been conducted on the FLAof energetic materials by using the ReaxFF reactive force field.

This paper aims at simulating FLA of the hexanitrohexaazaisowurtzitane/1,3-dinitrobenzene(CL20/DNB)co-crystal with the ReaxFF/lg reactive force field from LAMMPS Molecular Simulator24program package,exploring the response processes at the atomic scale,and providing the theoretical guidance for the machining safety of energetic materials for practical applications.

2　Model-building for the simulations

The unit cell parameters of the crystal structures were adapted from the experimental results of the 1:1 molar ratio CL20/DNB co-crystal25,pure CL2025,and DNB26crystals.The supercells used in the simulations are:(2×2×1)for the CL20/DNB co-crystal with 32 CL20 and 32 DNB molecules,(2×2×2)for the CL20 crystal with 32 CL20 molecules and(2×2×1)for the DNB crystal with 32 DNB molecules,as shown in Fig.1.In Fig.1,the C,H,O,N are represented by grey,white,red,and blue balls, respectively.

Based on uniform velocity distribution,the initial speed of all atoms at 300 K and optimized atom locations to minimize the potential energy are determined.The whole system was relaxed for 1 ps by using the isothermal-isobaric(commonly called NPT) system to set the pressure and temperature to be 0 Pa and 300 K, respectively.The yielded unit cell parameters of all three crystals are summarized in Table 1 where the experimental results are also listed for comparisons.Then the systems were thermally elevated up to 4000 K within 24 fs by using the micro-canonical ensemble (commonly called NVE)method,and elapsed for 40 ps.Time step was set to be 0.02 fs,and the bond cutoff,0.3.The bond value determines whether or not a new bond forms between atoms. Whenever a bond order of larger than 0.3 establishes，it′s taken that new molecules are formed.

3　Results and discussion

3.1Temperature and pressure evolutions

Fig.2 shows the temperature evolutions of different systems during and after their FLA process.When the energetic materials are heated by femtosecond laser to around 4000 K,the system undergoes a rapid cooling process which lasts about 0.2 ps,and the temperature decreases to about 2800 K and dwells for about 0.3 ps,which is caused by endothermic reaction during the initial stage of the reaction.Subsequently,the temperatures for the CL20 and CL20/DNB systems gradually rose up to 5500 and 5000 K, respectively.During this process,both CL20 and CL20/DNB systems release a large amount of heat due to instant decomposition and then reach chemical equilibrium.In addition,while the temperature of the system increases,the CL20 system heats up much faster than the CL20/DNB co-crystal system.However,the temperature of the DNB system dwells at 2800 K for a period of time up to 20 ps before it increases.During the cooling process (0－0.5 ps),all three systems follow a similar temperature evolution pattern before their trigger reactions come into play.

Fig.3 shows the pressure evolutions of the three systems during and after their FLA process.During the FLA process,pressure jumps to 8 GPa for the DNB system and reaches 10 GPa for both CL20 and CL20/DNB co-crystal systems.As a matter of fact,the CL20 and CL20/DNB co-crystal systems gradually increase in pressure.It′s noticed that the pressure of the CL20 system increases at a greater rate than that of the CL20/DNB co-crystal system.Moreover,the DNB system pressure shows a remarkable declining trend,and eventually fluctuates at 5 GPa.The ultimate pressure of the system is equal to about half of Chapman-Jouguet (CJ)detonation pressure,which is consistent with the result calculated using transient detonation theory.

On the whole,in CL20 and CL20/DNB systems,endothermic reaction starts first and leads to the remarkable decrease of the temperature and pressure,and then comes a short period of equilibrium.During this initial reaction process,N―NO2of CL20 molecule breaks and the number of NO2increases sharply.Sub-sequently,dramatic exothermic reaction occurs,which results in gradual increase of the temperature and pressure until a final plateau of the system.

Fig.1　Crystal structures of DNB,CL20,and co-existent CL20/DNB

3.2Potential energy evolution

Fig.4 shows the potential energy evolution of the three systems during and after the FLA process.During the FLA process,all three systems display a jump in potential energy.After the ablation process,the potential energy for both CL20 and CL20/DNB systems rapidly decays.However,the potential energy of the DNB system initially maintains and then starts to decline.Moreover,the potential energy of the CL20 system decays the fastest(0－8 ps) in three systems,and its decay rate gradually increases.This is dueto the fact that the CL20 system is autocatalytic in its thermal decomposition27.For the CL20 and CL20/DNB systems,the potential energy depicts a slow growth after the decay,which is consistent with the slow increase in temperature.Therefore,the potential energy increase is likely caused by the temperature rise that leads to the decompositions of H2O,N2,and CO2.H2O has been previously reported to decompose at high temperatures28.

Table 1　Crystal lattice parameters25for CL20,DNB,and CL20/DNB co-crystal

3.3Product analysis

Fig.2　Time evolutions of temperature for the CL20,DNB,and CL20/DNB systems

Fig.3　Time-dependent pressure evolutions of the CL20,DNB, and CL20/DNB system

Fig.4　Time-dependent evolution of the normalized potential energy for the CL20,DNB,and CL20/DNB system

Fig.5 shows main product distributions of the CL20,DNB,and CL20/DNB systems.For both CL20 and CL20/DNB systems,the CL20 molecule is consumed up in a very short period of time.The DNB consumption rate is,however,smaller in the DNB system than in the CL20/DNB system.The main products in the system are NO2,NO,N2,CO2,HNO,HONO,and HNO3.In all three systems,NO2first evolves,because N―NO2of CL20 molecule and C―NO2of DNB molecule are the weakest,and then becomes dominant as a main intermediate,which is consistent with the experimental observation that NO2is first produced in the thermal decompositions of the CL20 and DNB molecules29,30.For both CL20 and CL20/DNB systems,NO2follows a similar consumption pattern:it sharply increases at the very beginning of the reaction,reaches the maximum within about 2 ps,and then vigorously decreases due to its involvement in subsequent reaction, such as NO2→ONO→NO+oxygen radical(which in contrast proceeds less rapidly for the CL20/DNB system).The simulation results are in excellent agreement with the fact that the thermal sensitivity of the CL20/DNB system is lower than the CL20 system25.In the DNB system,the evolution of NO2undergoes an up-down pattern,but the changing rate is significantly smaller than those for both CL20 and CL20/DNB systems,and hence the time for the CL20 system to achieve its maximum(in approximately 8 ps)is much longer.It is because bond dissociation energy (BDE)of C―NO2for DNB molecule(about 305 kJ·mol－1)is larger than that of N―NO2for CL20(about 100 kJ·mol－1).

Fig.5　Product evolutions for the CL20,DNB,and CL20/DNB systems

In all three systems,N2,H2O,and CO2are the most important final products,but their evolution patterns are quite different.In the DNB system,CO2first appears at about 2 ps,and starts to gradually increase at 14 ps;H2O comes into appearance at about 5 ps,gradually increases from 10 ps,reaches its maximum at 35 ps and then remains stable;N2occurs at about 16 ps and then gradually increases.The sequence of the production rate is r(H2O)>r(N2)>r(CO2).The production rate of CO2is the smallest,because benzene is relatively stable,for BDE of C―C of benzene in DNB molecule is about 627 kJ·mol－1,which is a lot larger than that of C―N(305 kJ·mol－1）and of C―H(418 kJ· mol－1）.Compared with the results depicted in Fig.4,the potential energy for all three systems starts to decrease upon the increase ofthe N2and CO2products.For the CL20 and CL20/DNB systems, N2,H2O,and CO2follow a similar evolution pattern,augment up to a plateau before a slow decrease.During the product evolutions, the reaction rates of N2,H2O,and CO2in different systems change by r(CL20)>r(CL20/DNB);the reaction rates of the products change by r(N2)>r(H2O)>r(CO2)in the same system.

The CL20 system decomposes faster than its CL20/DNB counterpart.Fig.5 shows that all important intermediates follow a similar evolutionary pattern for both CL20 and CL20/DNB systems,and the production and consumption rates of the intermediates are higher in the CL20 system than in the CL20/DNB counterpart.The important intermediates are consumed up at 10 ps in the CL20 system and at about 15 ps in the CL20/DNB system.For instance,HNO gradually appears in 0－5 ps and disappears in 5－10 ps in the CL20 system while the time slots for the corresponding appearance and disappearance of the HNO species in the CL20/DNB system are 0－8 ps and 10－15 ps,respectively.

3.4Initial reaction pathways for CL20 and CL20/DNB

According to the product analyses,the initial reaction pathway for the CL20 crystal could be the follows.

Path I:

In these initial reaction pathways,Path I turns out to be dominating according to the product distributions in Fig.5.The simulation results point out that in the CL20/DNB co-crystal system, the CL20 molecule first decomposes in a way similar to the initial reaction pathway in pure CL20 crystal.However,the consumed DNB molecule in the initial processes does not decompose,but correspondingly reacts with the CL20 molecule and its decomposition products to convert into C10H10N14O16,H10C12N11O10and H10C12N14O15via the following reaction equations:

The involved DNB molecule in the reaction also turns into C6H4N3O6,C6H4N4O8,and C10H10N14O16.These benzene-containing molecules are rather stable and can substantially reduce the effective collision probabilities between CL20 and its intermediates, and thus lower the CL20 thermal decomposition rate.This may well explain why the CL20/DNB co-crystal decomposes at a slower rate than the CL20 crystal.

4　Conclusions

In summary,the FLA of the DNB,CL20,and CL20/DNB systems have been computationally simulated by using the ReaxFF/lg reactive molecular dynamics.The physical and chemical response processes of the energetic materials in and after the FLA process were scrutinized.Both temperature and pressure in the CL20/DNB system generally jump in the FLAprocess,and gradually reach their maxima after having experienced a cooling process.In the process,the trigger reaction in the CL20 crystal and the CL20/DNB co-crystal stems from the N―NO2bond-breaking of the involved CL20 molecule.The decomposition rate in the CL20 system is greater than that in the CL20/DNB co-crystal counterpart because the co-existent DNB molecule in the cocrystal system and rather stable benzene-containing decomposition products can significantly reduce the effective collision probabilities between the CL20 molecule and its products.Our study actually provides atom-scale information on the initial reaction processes involved in the CL20,DNB,and CL20/DNB systems upon the FLA,which may help to theoretically understand for the femtosecond laser machining of energetic materials with the effective ReaxFF/lg simulation approach.

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Molecular Dynamics Simulations of Femtosecond Laser Ablation of Energetic Materials

YANG Zhen1LIU Hai2HE Yuan-Hang1,*
(1State Key Laboratory of Explosion Science and Technology,Beijing Institute of Technology,Beijing 100081,P.R.China;2Hypervelocity Aerodynamics Institute,China Aerodynamics Research and Development Center, Mianyang 621000,Sichuan Province,P.R.China)

To understand the physical and chemical responses of energetic materials,such as 1,3-dinitrobenzene(DNB,C6H4N2O4),hexanitrohexaazaisowurtzitane(CL20,C6H6N12O12),and CL20/DNB co-crystal, tofemtosecondlaser ablation(FLA),their molecular reactiondynamicshavebeeninvestigatedusingtheReaxFF/ lg force field.The computational results indicate that the temperature and pressure of the CL20/DNB system jump during FLA.In particular,the temperature and pressure gradually reach their maxima following an initial cooling process.N―NO2bond breaking of the CL20 molecule triggers the reactions for both the CL20 and CL20/ DNB systems.However,the CL20 system prevails the CL20/DNB co-crystal in the decomposition rate simply because coexistence of DNB molecules in the mixture and generated decomposition products containing benzene rings greatly reduce the effective collision probability between CL20 and the products.

Femtosecond laser;CL20/DNB co-crystal;ReaxFF/lg;Reaction mechanism; Molecular dynamics

April 19,2016;Revised:April 28,2016;Published on Web:April 29,2016.*Corresponding author.Email:heyuanhang@bit.edu.cn;Tel:+86-10-68918878

O643；O642

10.3866/PKU.WHXB201604293

?Editorial office ofActa Physico-Chimica Sinica

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