Molecular Simulation Studies on the Growth Process and Properties of Ammonium Dinitramide Crystal

Ammonium dinitramide (ADN) is an energetic material and an oxidizer. It is a relatively new environmentally friendly oxidizer component of propellant without halogens and carbon elements. However, ADN has high hygroscopicity properties when exposed to high humidity air, restricting its military applications. In this study, the important crystal faces and crystal habit of ADN in a vacuum were predicted. According to the prediction results of ADN crystal habit, the hygroscopicities and sensitivities of ADN crystal faces were investigated and compared to determine total ADN crystal properties. Using the grand canonical Monte Carlo method, the adsorption capacities of water molecules on ADN crystal surfaces were predicted to determine which surface of ADN crystal attracts moisture in air. The solubility parameter of ADN crystal was calculated, and interaction type was investigated to explain ADN dissolution characteristics. Furthermore, the solubility parameter of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) was also calculated and compared with experimental values to verify accuracy of the calculation method. Before the calculations of crystal properties and crystal growth were performed, density, crystal parameters, and distribution of bond lengths were calculated and analyzed to obtained precise calculation parameters. In the study of the ADN crystal growth process, calculation models were constructed to investigate the impurity effect, temperature effect, and solvent effect on crystal growth, and molecular dynamics simulations were performed. On the basis of the Burton–Cabrera–Frank spiral growth theory, the adsorption energies of ammonium dinitramide on flat and stepped surfaces were calculated to investigate crystal growth from the viewpoint of thermodynamics.