Thin explosive films provide snapshot of how detonations start

By Mollie Rappe

Friday, February 26, 2021

An image from a test detonation of a thin explosive film
A LITTLE BANG — An image from a test detonation of a thin explosive film, about as thick as a few pieces of notebook paper, with a thunderbird-shaped barrier the size of the thunderbird on Sandia business cards. The “shimmering” lines to the right of the thunderbird are the shock waves from the explosion, detected by schlieren imaging, a technique that can detect differences in air density. (Photo courtesy of Eric Forrest)

Using thin films — no more than a few pieces of notebook paper thick — of a common explosive chemical, Sandia researchers are studying how small-scale explosions start and grow.

Sandia is the only lab in the U.S. that can make detonatable thin films like these for experiments that advance the fundamental knowledge of detonations. And the data improved a Sandia-developed computer-modeling program used by universities, private companies and the Department of Defense to simulate how large-scale detonations initiate and propagate.

“It’s neat, we’re really pushing the limits on the scale at which you can detonate and what you can do with explosives in terms of changing various properties,” said Eric Forrest, lead researcher on the project. “Traditional explosives theory says that you shouldn’t be able to detonate at these length scales, but we’ve been able to demonstrate that, in fact, you can.”

Eric and the rest of the research team shared their work studying the characteristics of the thin films and the explosions they produce in two recently published papers in ACS Applied Materials and Interfaces and Propellants, Explosives, Pyrotechnics.

For their studies, the team used PETN, also known as pentaerythritol tetranitrate, which is a bit more powerful than TNT, pound for pound. It is commonly used by the mining industry and by the military.

Typically, PETN is pressed into cylinders or pellets for use. The research team instead used a method called physical vapor deposition — also used to make second-generation solar panels and to coat some jewelry — to “grow” thin films of PETN.

Sandia is the only lab in the U.S. that has the skills and equipment to use this technique to make thin explosive films that can detonate, said Rob Knepper, a Sandia explosives expert involved in the project.

Growing and studying thin explosive films

Starting in late 2015, the team grew thin films of PETN on different types of surfaces to determine how that would affect the films’ characteristics. They started with pieces of silicon about the size of a pinkie nail and grew films that were about one-tenth the thickness of a piece of paper, too thin to explode. Some of the silicon pieces were very clean, some were moderately clean and some were straight-out-of-the-box and thus had a very thin layer of dirt — 50,000 times thinner than a sheet of paper.

On the very clean silicon surfaces, the PETN films formed what appeared under scanning-electron microscopy to be smooth plates, yet with tiny cracks in between plates, somewhat like dried mud on a dried lakebed. On the dirty silicon surfaces, the surface of the PETN films appeared more like even hills.

Using an X-ray-based technique, the researchers determined this is because the PETN molecules orient themselves differently on dirty surfaces compared to very clean surfaces, and thus the film grows differently, Eric said.

“This study in particular has shown that we can get not just novel, but very useful forms of traditional explosives that you would never be able to achieve via traditional means,” Eric said. “Finely controlling the film properties enables us to investigate theories to better understand explosive initiation, which will allow us to better predict reliability, performance and safety of explosive systems through improved models.”

Rob, who served as Eric’s mentor on the project, agreed. “Developing a way that we can reproducibly control the microstructure of the films, just through the surface manipulation, is important. Right now, our focus is on using these films to further our understanding of explosive properties at small scales, such as the initiation and failure of explosives.”

Once the characteristics and properties of the thin films were better understood, the research team grew thicker films — this time about the thickness of two sheets of notebook paper — on very clean pieces of plastic about the size of a pinkie finger.

Then, with a bang, they detonated the explosive films inside a specially designed safety enclosure called a “boombox,” which was engineered to prevent a detonation from starting while the enclosure was open as well as contain any debris from the detonation. Using an ultra-high-speed camera that can take up to a billion frames a second, they watched the shock wave rise up as the explosion raced across the thin film.

In collaboration with New Mexico Institute of Mining and Technology in Socorro, the research team developed a specialized setup to see the shock wave despite the smoke and debris from the test explosions using schlieren imaging, a technique that can detect differences in air density similar to the shimmering over a hot highway.

Student refines explosives

A mechanical engineering master’s student from New Mexico Tech, Julio Peguero, used the data from these experiments to refine Sandia’s explosives computer-modeling program. The program, called CTH, can be used for applications, such as to determine how to best shape explosive charges while drilling for oil, Rob said.

Julio plotted the velocity of the shock waves above the films with and without gaps and adapted the computer program to better match the experimental results on very thin films. The team engineered thin films with cracks in the middle of various sizes — ranging from one-third the width of a human hair to 1 1/3 the width of a hair — to better understand the reliability of thin films and how detonations can fail. The team found that gaps around the size of a hair could stop a detonation from continuing.

Eric was particularly interested in the gap studies because the first study found thin cracks between the very smooth plates of some of the films. Although these cracks were far smaller than even one-tenth a hair’s width, the data from the gap study provided insights into how these films would perform.

Julio, now a Sandia employee, started working on the project in January 2018, first as a student and then as a Sandia intern.

“In addition to the excitement of doing explosives research, I gained an appreciation for measurement uncertainty and risks,” Julio said. “That is especially important for national security work to ensure that our confidence in our measurements is well-understood.”

Rob agreed about the importance of the project. He said, “When you have experimental data at small scales, especially those that are relevant for the border between what can detonate and what can’t, those data can be really helpful in calibrating computer models. Also, being able to have good characterization of the explosive microstructure to go into the models helps with having parameters that can successfully predict performance over a wider range of explosive behaviors.”

The research was funded by Sandia’s Laboratory Directed Research and Development program. Michael Hargather, a professor at New Mexico Tech and a scientist at the Energetic Materials Research and Testing Center, worked on the project and mentored Julio. Numerous Sandia employees with expertise across a variety of fields also worked on the project.

Fast Shocks Video

image from explosives video

A team of Sandia researchers led by Eric Forrest made thin explosive films to study how small-scale explosions start and grow. Using an ultra-fast camera that can take up to a billion frames a second, they watched the shock waves from the thin films’ denotations. By comparing the denotation of films with and without small gaps, they advanced basic understanding of denotations, which will improve the safety and reliability of explosives. For scale, the thunderbird in the last video is 3/8th of an inch tall. View the video.