The term 'energetic materials' is used to describe the explosive components used in fireworks, demolition and defence applications. Crucial to their use is the knowledge of what stimulus is needed to activate the material. This property is known as the energetic material sensitivity. In other words, what does it take to make it explode? This is important for both designing the system and also for safety concerns as, if something is too sensitive, then it will not be safe to use or store.
Despite this being a crucial property, little is understood about what it is about an energetic material that determines the stimulus needed for activation. At the moment, researchers must prepare new energetic materials 'blindly' in the laboratory and run extensive testing to measure the sensitivity. Without knowing anything about the material, this procedure can be dangerous for the researchers involved.
To solve this problem, the teams of Dr Adam Michalchuk at the Federal Institute for Materials Research and Testing in Berlin, Germany, and Professors Carole Morrison and Colin Pulham from the University of Edinburgh have developed a computational model to predict how sensitive energetic materials are, before making them experimentally.
Their model simulates how the vibrational energy from a sudden shock flows through the structure of an energetic material and causes a chemical reaction. Using the Tosca beamline at ISIS, they were able to use inelastic neutron scattering to input real data into their model.
“Any theoretical model needs high quality experimental data to validate it," explains Dr Michalchuk. “We are delighted to have developed a strong collaboration with Tosca scientist Dr Svemir Rudic which allows us to obtain this invaluable data that makes our developments possible.'
Building on their model, the group studied one material in particular, FOX-7. This highly explosive material can form different crystal structures, or polymorphs, depending on the experimental conditions. Despite having the same chemical formulation, these different polymorphs actually have different sensitivities but, until now, this remarkable behaviour had not been explained.
In this recent study, published in Chem Comm, they used further experiments on Tosca to introduce the effects of these subtle changes in crystal structure into their model. This enabled them to predict the sensitivity of the polymorphs of the material based on their differing vibrational behaviour.
Their work also highlighted that the experimental method used to measure impact sensitivity induced a high-pressure phase transition in the material. This highlighted the previously unconsidered possibility that the very act of measurement changes the structure, and thus its property. This further justifies the need for robust impact sensitivity prediction models to be able to reliably explore the relationships between structure and properties for energetic materials.
The ability to understand individual polymorphs of a material widens the suitability of their model. Not only are they able to rank current energetic materials in terms of their impact sensitivity, but they can also use it as a tool to guide the targeted design of future energetic materials, tailoring their properties to different applications.
Their most recent study on FOX-7 can be found at DOI: 10.1039/D1CC03906G.
A previous study, developing their model can be found at DOI: 10.1039/c9ta06209b.