Development of a ReaxFF Reactive Force Field for Ammonium Nitrate

Article pubs.acs.org/JPCA

Development of a ReaxFF Reactive Force Field for Ammonium Nitrate and Application to Shock Compression and Thermal Decomposition Tzu-Ray Shan,*,† Adri C. T. van Duin,‡ and Aidan P. Thompson† †

Sandia National Laboratories, Albuquerque, New Mexico 87185, United States Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States

‡

S Supporting Information *

ABSTRACT: We have developed a new ReaxFF reactive force field parametrization for ammonium nitrate. Starting with an existing nitramine/TATB ReaxFF parametrization, we optimized it to reproduce electronic structure calculations for dissociation barriers, heats of formation, and crystal structure properties of ammonium nitrate phases. We have used it to predict the isothermal pressure−volume curve and the unreacted principal Hugoniot states. The predicted isothermal pressure−volume curve for phase IV solid ammonium nitrate agreed with electronic structure calculations and experimental data within 10% error for the considered range of compression. The predicted unreacted principal Hugoniot states were approximately 17% stiffer than experimental measurements. We then simulated thermal decomposition during heating to 2500 K. Thermal decomposition pathways agreed with experimental findings.

1. INTRODUCTION Ammonium nitrate (AN, NH4NO3) is a strong oxidizer. It is commonly used as a high-nitrogen fertilizer, but it becomes an explosive when mixed with a few percent of fuel oil (hydrocarbons), which is widely used in mining, excavation, and other industrial activities.1 AN is also responsible for many large accidents (most recently the 2013 fertilizer factory explosion in West, Texas) as well as notorious terrorist bombings over the past century.2,3 AN forms monovalent polyatomic NH4+−NO3− ion pairs in crystalline solid phases and neutral acid−base pairs in the gas phase.4 AN crystallizes into five well-characterized phases at atmospheric pressure, ranging from the highest temperature phase I to the lowest temperature phase V.5−12 Crystallographic information and stable temperature ranges of these phases are summarized in Table 1. Phase IV of AN crystal is stable at room temperature and atmospheric pressure. Illustrated in Figure 1 is a ball-andstick model of phase IV of AN crystal from experimental crystallographic data.8 A Hugoniot state is a thermodynamic equilibrium state reached behind a shockwave. Given an initial equilibrium state with defined initial pressure, density, and energy, the corresponding Hugoniot is the locus of new thermodynamic states of increasing pressure, density, and energy satisfying the conditions of mass, momentum, and energy conservation.13,14 The principal Hugoniot is the locus of states associated with an initial, uncompressed state that is at ambient conditions (room temperature and atmospheric pressure). The principal © 2014 American Chemical Society

Hugoniot is often expressed in the form of an approximate linear relationship between shock velocity US and particle velocity UP. Several experimental studies have been reported pertaining to the static and dynamic loading of AN. On the basis of measured bulk sound speed in porous AN samples, Dremin et al.15 estimated the Hugoniot for AN at solid density 1.73 g/cm3 to be US = 2.20 + 1.96 × UP, where US and UP are both in units of km/s. Work by Sandstrom et al.16,17 utilized explosively generated plane shock waves, resulting in the estimate US = 3.07 + 1.89 × UP for 0.28 ≤ UP ≤ 0.94 and US = 2.23 + 2.18 × UP for 0.72 ≤ UP ≤ 2.12. Courchinoux and Lalle18 reported US = 1.80 + 1.80 × UP using the impedance mismatch method. Robbins et al.19 estimated the solid AN Hugoniot to be US = 3.10 + 1.60 × UP with pressed AN crystals in a two stage light gas gun-driven plate impact experiment. In all these experiments15−19 no indication of reaction or initiation were found. The substantial variations in these reported Hugoniot lines can be attributed to differences in density and experimental technique. In addition to experimental efforts in investigating the properties of AN, several modeling and simulation studies have been reported in this field. For example, Sorescu et al.20 have performed plane-wave ab initio calculations to investigate the structural and electronic properties of phases II, III, IV, and V Received: August 21, 2013 Revised: January 24, 2014 Published: January 30, 2014 1469

dx.doi.org/10.1021/jp408397n | J. Phys. Chem. A 2014, 118, 1469−1478

The Journal of Physical Chemistry A

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Table 1. Crystal Structures of Ammonium Nitrate Phases under Atmospheric Pressurea crystal space group temperature range (K) a

I5

II6

III7

IV8

V9

cubic Pm3m 398−442

tetragonal P421m 357−398

orthorhombic Pnma 305−357

orthorhombic Pmmn 255−305

orthorhombic Pccn 1.0 km/s.

the mean stress from ReaxFF MD simulations as functions of strain. Although deviations up to 15% can be observed for individual stress components, phase IV of the AN crystal exhibits near hydrostatic response under isotropic compression. A similar response is observed for DFT calculations. Therefore, mean stress is used to represent pressure in Figure 4a. It is interesting to note that under isotropic compression, the b-axis exhibits the lowest stress. Hence, if the supercell is compressed under truly hydrostatic conditions, the b-axis will be undergo the largest strain. This agrees with the findings of Chellappa et al.12 Overall, this indicates that ReaxFF with the AN force field correctly describes the high pressure equation of state of AN phase IV. 3.4. Bulk Unreacted Hugoniot States. To investigate shock properties of AN phase IV with ReaxFF using the AN force field, points along the principal Hugoniot are found by isotropically compressing the system to a chosen volume and then equilibrating in a ReaxFF MD simulation using the canonical ensemble (NVT with Nosé−Hoover thermostat) at

simulations with the AN force field compared to three sets of experimental data.15,16,18,19 Assuming a linear relationship between the shock and particle velocities: US = C0 + s × UP, the intercepts C0 and the slopes s are compared in Table 4. Compared to average experimental values of C0 and s of 2.64 and 1.79, respectively, values for C0 and s from ReaxFF are, respectively, 15% and 17% larger. The ReaxFF Hugoniot states for particle velocities below approximately 1.0 km/s fall within experimental variation, while ReaxFF overpredicts the shock velocity at larger shock strengths. This can be attributed to the ReaxFF model having an uncompressed density (1.81 g/cm3) that is 4% to 11% higher than the densities of the material used in the shock experiments (1.63 or 1.73 g/cm3). 1473



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NH4NO3 → NH3 + HNO3

Table 4. Intercepts and Slopes of Shock Velocity-Particle Velocity (US−UP) Hugoniot Relationships (US = C0 + s × UP) and Uncompressed Densities of AN Structures exptl, Dremin15 exptl, Courchinoux18 Sandstrom17 exptl, Robbins19 ReaxFF AN

C0 (km/s)

s

density (g/cm3)

2.20 1.80 3.07 2.23 3.10 2.64

1.96 1.80 1.89 2.18 1.60 2.10

1.73 1.69 1.63

NH4NO3 →

(R1)

1 N2 + NO + 2H 2O 2

NH4NO3 → N2O + 2H 2O NH4NO3 →

1.72 1.81

Hugoniot states on the pressure−volume (specific) plane from ReaxFF MD simulations compared to available experimental data16,19 are shown in Figure 6. At pressures below 25

NH4NO3 →

(R3)

3 1 N2 + NO2 + 2H 2O 4 2

NH4NO3 → N2 + 2H 2O +

(R2)

1 O2 2

1 1 1 N2 + NO + NO2 + 2H 2O 8 2 4

(R4)

(R5)

(R6)

Reaction R1 is generally accepted as the thermal decomposition initiating reaction, which is an endothermic proton transfer.78 When the molten salt is heated from 470 to 500 K, exothermic decomposition reactions R2 or R3 occurs, depending on the heating rate.87 Reaction R3 is more common and is the basis for commercial production of nitrous oxide (N2O).80 When heated above 500 K, reaction R4 is the dominant decomposition pathway.2,81 Reaction R5 has been suggested as the decomposition pathway in detonation, while reaction R6 has been associated with subsonic explosions.78 The following detailed mechanism has been proposed for reaction R3:80 NH4NO3 → NH3 + HNO3 → NH3 + NO2 + OH (R3a) Figure 6. Bulk unreacted pressure−volume Hugoniot states of ammonium nitrate from experiments16,19 and ReaxFF-AN simulations. Hugoniot states on the P−V plane from ReaxFF MD lie within the range of experimental variation.

OH + NH3 → NH 2 + H 2O

(R3b)

NH 2 + NO2 → H 2NO + NO → N2O + H 2O

(R3c)

For reaction R4, the following mechanism has been reported:2,81,84 NH4NO3 → NH3 + HNO3

GPa, the ReaxFF Hugoniot points in the pressure−volume plane are consistent with the data from the experiments. At the highest pressure (34 GPa, 0.385 cm3/g), the ReaxFF model once again appears to be too stiff. As mentioned above, this is likely due to the uncompressed density being 4% to 11% larger. Another source of deviation is the different ways the cells are compressed: isotropically for ReaxFF MD simulations versus uniaxially for experiments. Isotropic compression significantly limits shear deformation, hence prohibiting the relief of stress via such deformation. A more thorough investigation of bulk unreacted Hugoniot states of AN from ReaxFF MD simulations via uniaxial compression techniques, such as nonequilibrium direct shock simulations and multiscale shock technique (MSST),77 is reserved for a further study. 3.5. Thermal Decomposition of Ammonium Nitrate Phase IV. The decomposition chemistry of AN has been studied and reported quite extensively due to its application as an explosive component; interested readers are referred to review articles by Oommen and Jain78 and Chaturvedi and Dave, 79 as well as several studies of more specific aspects.2,75,80−87 The decomposition chemistry of AN heavily depends upon environmental factors such as temperature, pressure, moisture content, and the presence of additives, as well as upon experimental conditions like sample size, sample purity, and heating rate. No single mechanism can explain all possible aspects of its decomposition chemistry. Reported decomposition pathways are summarized below.

(R4a)

HNO3 + HX → NO2 + H 2O + X (HX = NH4 +, H3O+ , HNO3)

(R4b)

NH3 + 2NO2 → NH3 + NO+NO3− → HNO3 + NH 2NO

(R4c)

NH 2NO → HN2OH → N2 + H 2O

(R4d)

Thermal decomposition simulations of phase IV of AN crystal are performed with ReaxFF using the AN force field. Perfect 4 × 4 × 4 phase IV supercells are heated gradually from 0 K to target temperatures of 500, 1000, 1500, 2000, and 2500 K, in five different simulations. In each case, the heating time is 40 ps, and the volume of the cell is kept constant. Heated samples are maintained at target temperatures for another 1000 ps. Chemical species analyses are based on time-averaged bond order values over 100 fs that are sampled every 1 fs. It is found that NH4NO3 remains stable for target temperatures of 500 and 1000 K even after 1 ns, suggesting that the first step of decomposition (reaction R1), which is the endothermic dissociation to ammonia and nitric acid, does not occur at these conditions. For the target temperature of 1500 K, we observe the endothermic dissociation of NH4NO3 (reaction R1), but no subsequent reactions occur. Stable chemical species 1474



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found after 1 ns at 1500 K are coexisting NH4+, NO3−, NH3, and HNO3 at roughly equal concentrations. Subsequent reactions involving the dissociation of nitric acid (HNO3) occur for target temperatures of 2000 and 2500 K. Illustrated in Figure 7 are the evolutions of chemical species

Figure 8. Evolution of stress and temperature during the thermal decomposition NPT MD simulation at 2500 K using the ReaxFF AN force field.

4. CONCLUSIONS We present the development of a reactive force field (ReaxFF) description for ammonium nitrate (NH4NO3, AN). The force field was trained by extending the ReaxFF description for nitramines/TATB71,72 to include AN by training the hydrogen shift energy barrier between NH4+ and NO3− against our electronic structure calculations. Lattice parameters for AN phases I, IV, and V are within ±3.0% error compared to electronic structure calculations. The predicted isothermal pressure−volume curve for phase IV AN crystal agrees with our electronic structure calculations and experimental data10−12 within 10% error. Predicted shock velocity−particle velocity (US−UP) and pressure−volume (P−V) Hugoniot states for unreacted phase IV of AN crystal fall within experimental variation15,16,18,19 below particle velocity of 1.0 km/s and pressure of 15 GPa, respectively. Deviations in the higher compression range can be attributed in part to differences between the experimental and simulation starting structures, as well as the use of isotropic compression in the MD simulations. High-temperature thermal decomposition simulations of phase IV show decomposition of molten AN to NH3 and HNO3 as the first step, with subsequent dissociation of HNO 3 , in agreement with experimental findings.2,81,84

Figure 7. Thermal decomposition of ammonium nitrate phase IV from an NPT MD simulation at 2500 K using the ReaxFF AN force field. Reaction R4a: NH4NO3 → NH3 + HNO3. Reaction R4b: HNO3 + HX → NO2 + H2O + X. Reaction R4c + R4d: NH3 + NO2 → N2 + H2O + OH. The results agree with the experimentally observed hightemperature decomposition pathway.2,81,84

found in the systems at 2500 K. The dissociation of NH4NO3 to NH3 and HNO3 (reaction R4a) initiates at approximately 1200 K. The subsequent reaction is between HNO3 and HNO3/NH4+ yielding NO2, H2O, and NO3−/NH3 (reaction R4b, where HX = HNO3 or NH4+), initiating at approximately 2400 K. This explains a faster decrease in HNO3 concentration than NH3. Figure 7 also illustrates the final reaction between NH3 and NO2 forming products H2O and N2 (combining reactions R4c and R4d), initiated after reaching 2500 K. Although not shown in Figure 7, we do find trace amounts of NO+NO3− and NH2NO in the system. Since these two products are metastable, it is expected that their concentrations remain low. Major products found in the system after 1 ns are H2O, N2, NO2, and OH. The presence of NO2 is likely due to reaction R4c not fully completing. The presence of OH is due to high-temperature homolysis of H2O. Plotted in Figure 8 are the stress and temperature profiles during the 2500 K thermal decomposition simulation. Principal stresses as shown in Figure 8 are 5.5 GPa on average. In the MD simulations, the temperature required for the first dissociation step and subsequent chemical reactions are significantly larger than is seen in experiments (1200 K versus 500 K in experiments). In addition to the larger NH4NO3 decomposition barrier (39 kcal/mol versus 30−41 kcal/mol from experiments78), this is also likely due to the relatively small sample size (2 nm × 2 nm × 2 nm), complete absence of initial defects, and fast heating rate, all of which lower the nucleation rate relative to the experimental conditions, which involve large polycrystalline samples with many defects and grain boundaries and much slower heating rates. Nevertheless, the experimentally observed high-temperature decomposition pathway (reactions R4 and R4a−R4d) is reproduced by the ReaxFF AN force field.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Reactive MD force field data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*(T.-R.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.-R.S. acknowledges helpful discussions with department manager John B. Aidun. T.-R.S. and A.P.T. acknowledge funding support from Department of Energy’s Advanced Simulation and Computing and Sandia National Laboratories’ 1475



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Development of a ReaxFF Reactive Force Field for Ammonium Nitrate

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