High speed microscopy of propellant combustion

A reflection of the impact that my publication in Nature Communications had on my research and career.
High speed microscopy of propellant combustion

About a year ago, I had just finished up the third major chapter in my dissertation as a student at the University of Maryland which was printed in Nature Communications titled “In-operando high speed microscopy and thermometry of reaction propagation and sintering in a nanocomposite.” I had also written a short blog post for the “Behind the Paper” series in the Nature Research Device and Materials Engineering Community where I described the origin story of the paper.

Our article focused on observing the reaction dynamics in an energetic nanocomposite (which we use as a rocket propellant simulant) on the particle scale using a newly designed, home-built apparatus in the Zachariah Group at UC Riverside. In this article, we directly observed a phenomenon called “reactive sintering” – a loss of nanostructure in reacting nanocomposites which has been largely blamed for inefficiencies in exothermic reactions of metal nanoparticles. We were also able to describe a reaction mechanism for the microscale combustion behavior in these types of materials using temperature measurement and simple heat transfer estimations.

Figure 1 - Image depicting the types of results obtained by the initial paper published in Nature Communications. (Top) Typical macroscale combustion test on a high loading Al/CuO/MethoCel/PVDF burn strand. (Bottom) High speed microscopy image of the same material with 1 µm/px resolution which captures sintering events and corrugated reaction front. More details can be found in https://doi.org/10.1038/s41467-019-10843-4.

At the time of writing the article, it was clear to us that there were numerous other directions to go with this newly adopted technique that were going to be too detailed to include for this first article. Little did I expect to be developing this technique into a full suite of software, taking trips to different research facilities, and learning so much more over the next 2 years.

The microscopic images we depicted in our article revealed that there was an intrinsic microstructure to our reacting films which could be readily quantified. At the time of the Nature Communications article being pressed, I had already concluded another study which demonstrated our ability to measure the length of a combusting reaction front and estimate microscopic velocity using software I developed. This article titled “Why does adding a poor thermal conductor increase propagation rate in solid propellants?” can be found in Applied Physics Letters.

Figure 2 - Image depicting outputs from an image analysis program I wrote to track the reaction front of propagating films using data collected from the high speed microscopy experiment. More details can be found in https://doi.org/10.1063/1.5113612.

However, a major problem with this microscope technique was throughput. It was simply too difficult to get consistently clear images with our microscope setup and so we looked for other options to image at the microscale. We have since adopted a long working distance microscope after a suggestion from colleagues at the US Navy and they even offered to assist in the temperature calibration of a new lens by inviting me to the Naval Air Weapons Station China Lake (shoutout to Aaron Mason for the memorable experience). 

Another major question we had tried to tackle with our (redesigned) microscope imaging technique was the role of different heat transfer mechanisms on stable and reliable burning. From our Nature Communications paper, we knew that heat transfer was effectively the rate-limiting step that prevented material from burning faster. But what if the material could not sustain a burn at all? In an article we wrote in Combustion and Flame titled “Experimental observation of the heat transfer mechanisms that drive propagation in additively manufactured energetic materials,” we used our microscopy apparatus to estimate the different rates of heat transfer in a reacting composite similar to the way we had done in our Nature Communications paper. We found that different types of heat transfer are critical to reliable combustion in these materials.

Figure 3 - High-speed microscopy images of composites depicting particle ejections into unreacted area of the propellant strands. More details can be found in https://doi.org/10.1016/j.combustflame.2020.01.020.

There are still so many unanswered questions that can be explored via this microscopic imaging technique. What is the particle-scale mechanism of combustion on the reaction time scale? Can we ever overcome reactive sintering and enhance energy release in these composites? How can these materials be made customizable, safer, or more versatile with different types of preparation techniques? 

My time as a student at the University of Maryland has since come to a close and the work that we presented in Nature Communications was one of the most memorable experiences I had in graduate school. I was consistently involved in all types of collaborations between different people, universities, and research facilities and almost all of them (at some point or another) incorporated this technique. All of this work even led to the next chapter in my career as a researcher at the Lawrence Livermore National Laboratory where I will continue to work with energetic materials and new characterization techniques. I appreciate all of the support from the many colleagues that came along for the journey and taught me something (especially my advisor, Prof. Michael Zachariah) – hopefully you all learned something, too.

Figure 4 - Fall 2019 Zachariah Group picture at UC Riverside. From left to right: Prithwish Biswas, Feiyu Xu, Haiyang Wang, Pankaj Ghildiyal, Dr. Dylan Kline (Me), Dr. Will Gibbons, Prof. Michael Zachariah, Dr. Yong (Kevin) Yang, Miles Rehwoldt, Dr. Lucas Algrim, Zaira Alibay. Not shown/new members: Yujie Wang, Erik Hagen, and Matthew Krock.

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