What is it about?
Solid rocket propellants are heterogeneous composites made from oxidizer crystals, metal fuel, and a polymer binder. Under intense heating, their internal structure changes rapidly before combustion becomes visible. We created a controlled heating-rate gradient within a single sample and used 4D tomography together with ultrafast radiography to observe and quantify its internal evolution from initial decomposition to burnout. The measurements showed that local heating rate, rather than bulk temperature alone, governed the material’s mesoscale evolution. Different heating histories produced distinct pathways of structural reorganization and component interaction, carrying the material through different routes from decomposition to ignition and combustion.
Featured Image
Photo by Tim Mossholder on Unsplash
Why is it important?
Rapid, nonuniform heating drives heterogeneous materials far from equilibrium, yet their internal structural response has been difficult to measure. By making this evolution visible and quantitative, the study shows why thermal history must be considered when describing materials under extreme conditions. It also connects the changing mesoscale structure to competition among chemical reaction, heat transfer, and mass transport. These measurements provide an experimental basis for models that account for dynamic structure under nonequilibrium conditions. The framework may also support studies of other intensely heated composites, including ablative heat shields.
Perspectives
Our understanding of the physical world has advanced in tandem with our ability to observe it. What excited me most was not only seeing a previously hidden, rapidly evolving process, but quantifying its dynamics and connecting them to the underlying physics. A single image tells us what a material looks like, a high-speed 4D movie reveals how it changes, and comparisons of competing timescales help explain why it follows one pathway rather than another. I hope this framework helps make nonequilibrium structural evolution a testable part of how materials under extreme conditions are understood and modeled.
Zhi Jiang
Read the Original
This page is a summary of: Heating rate gradient drives mesostructural dynamics in solid propellant under nonequilibrium conditions, Proceedings of the National Academy of Sciences, November 2025, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2508143122.
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Resources
X-ray movie of propellant ignition and combustion
Ultrafast X-ray radiography captures top-to-bottom structural collapse, ignition-site formation, AP and aluminum-cluster separation, bubble formation and rupture, and aluminum-droplet formation during the transition to self-sustained combustion.
4D X-ray movie of mesostructural evolution during gradient heating
Time-resolved synchrotron tomography combines temperature, three-dimensional mesostructure, porosity, and total-volume measurements to visualize internal evolution during rapid gradient heating.
X-ray movie of aluminum agglomeration during combustion
False-color X-ray radiography follows the agglomeration of a representative aluminum particle during combustion, with corresponding volume and circularity measurements reported in Fig. S7.
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