Thin Explosive Films Provide Snapshot of How Detonations Start

An image from a test detonation at Sandia National Laboratories of a thin explosive film, about as thick as a few pieces of notebook paper, with a 3/8th inch tall thunderbird-shaped barrier. 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, researchers from Sandia National Laboratories studied how small-scale explosions start and grow. Sandia is the only lab in the U.S. that can make such detonatable thin films. These experiments were dwsigned to advance fundamental knowledge of detonations. The data were also used to improve 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.

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.

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 to be smooth plates by scanning-electron microscopy, yet had 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.

“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,” said Eric Forrest, the lead researcher on the project. “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.”

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 and 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.

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. Peguero plotted the velocity of the shock waves above the films with and without gaps and adapted the computer program to better match their 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.

Forrest 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.