Theoretical Modeling Considerations
Overview and Significance
The theoretical effort at the Hebrew University and the Ben-Gurion University is aimed at obtaining an indepth understanding of the characteristics of new explosive materials. This is in addition to assisting the experimental investigations within ALERT in deciphering reaction mechanisms. The theoretical methods are based on ab-initio quantum chemical (QC) calculation and ab-initio molecular dynamics (AIMD). For simulating larger molecular ensembles, reactive molecular dynamics (RMD) methods were employed. These computational methods have been applied successfully in the study of explosive materials from a molecular point of view. The main sub-projects investigated at present include:
1. Understanding the influence of hot spots and shear forces on the explosive sensitivity.
2. Establishing the relationship between oxidizer-fuel ratios in homemade explosives (HMEs) detonable mixtures. The system chosen is HN3 and its mixtures with water.
3. Calculating accurate equations of state for different HMEs using RMD and AIMD calculations. The outcome can be used as input in thermodynamic codes such as CHEETHA.
4. Employing QC calculations to understand the decomposition process of HMEs in different environments (pH dependences). These types of calculations assist in the design and assessment of additives to oxidizers (such as H2O2) that will prevent using theses oxidizer for the synthesis of HMEs.
5. Studying spectroscopic characteristics of explosives that could assist in developing protocols for stand-off detection schemes.
6. Studying the interaction between short, intense laser pulses and thin films of explosives and understanding the basic mechanisms that lead to parent molecule ejections. This will allow optimization of detection schemes based on the laser ablation process.
During the last two years, we have laid the foundations for sub-projects number 1, 2, 3 and 6. A new approach, based on RMD, to calculate the THz spectra of two military explosives, was developed. The results suggested that a 2D spectroscopic method using polarized THz radiation can be used to obtain high specificity in explosives detection.
The study of the detonation of pure liquid explosive HN3, has been initiated by the development of an accurate reactive force field. This stage required extensive QC calculations on both single and bi-molecular systems. The first step consisted of the development of RMD-based methods to study the role of hot spots (voids and grain boundaries) during the initial stages of detonation. This issue is associated with the sensitivity of energetic materials and can contribute to the design of new insensitive explosives. The reactive force field (ReaxFF) developed was used to obtain thermodynamic and kinetic parameters for this complex system. The results are being summarized as a research paper. The ReaxFF will be extended to include mixtures of water with HN3. We have developed a new methodology that allows simulation of energetic materials at their C-J conditions (the C-J temperature and pressure) for long periods of time (up to nano seconds). This approach was applied to studying the role of hot spots on the sensitivity of ETN, a new HME. The methodology will be extended to study the role of shear forces on explosive sensitivity and compare them with the existence of hot spots.
During the present year we had a very fruitful collaboration with the Oxley-Smith group at URI related to the decomposition of HMTD for Project R1-A.1. The experimental study at URI was accompanied by extensive QC calculations to reveal the decomposition mechanism of HMTD in different environments. This collaboration led to a thorough understanding of HMTD decomposition routes, the energy barriers involved and the main decomposition products obtained. The results of this joint study are summarized at present as a research paper.
Lastly, we carried out model calculations of the short laser pulse ablation of thin explosive layers. This approach has been demonstrated experimentally to lead to ejection of parent explosive molecules without substantial decomposition. Our simulations, using RMD, revealed two main mechanisms that explain the experimental observations. The results of this study are summarized at present as a research paper. The results obtained in the calculations allow the optimization of molecular ejection from the irradiated film. This will assist in the design of efficient detection methods based on mass spectroscopy.
The completion of the RMD methodology of simulation explosives at their C-J states needs to be extended to also include the role of shear forces on explosive sensitivity.Phase 2 Year 2 Annual Report
Students Currently Involved in Project
- David Furman
- Natan Kalson
- Paz Elia
- Sharon Yarden