Purdue Researchers Create Mini Shockwave with Custom Inkjet 3D Printer and Energetic Materials

If you’ve ever thought about the science behind how your car’s airbag deploys, you have energetic materials to thank for it. The materials in this class contain a high amount of stored chemical energy that can be released – think explosives, pyrotechnics, and propellants. Many micromechanical systems use energetic materials: case in point, automotive airbags use a small amount of solid propellant to deploy properly. Now, researchers at Purdue University are using 3D printing inkjet technology to carefully deposit tiny amounts of these materials at what the university calls “unprecedented levels of precision and safety.”

Allison Murray, who built the custom inkjet printer and is a PhD candidate in Mechanical Engineering, said, “Energetic materials is a fairly understood field, and so is additive manufacturing. What’s unique about this project is the intersection of those two fields, and being able to safely deposit energetic materials with this level of precision.”

As our devices continue to get smaller, the critical need for energetic materials on the micro-level grows, and with it the need to keep the process safe. This is why research like the kind taking place at Purdue, which was funded in part by a grant from the DOD’s Defense Threat Reduction Agency, is increasingly important.

“Our solution is to combine two components as we’re printing them. We can have a fuel and an oxidizer in two separate suspensions, which are largely inert,” explained principal project investigator and Mechanical Engineering Professor Jeff Rhoads. “Then with this custom inkjet printer, we can deposit the two in a specific overlapping pattern, combining them on a substrate to form nanothermite.”

Inkjet 3D printing technology, also referred to as binder jetting, is very versatile, and can be used to deposit a wide variety of materials, such as antibodies, ceramics, and metal. One of the difficulties Purdue faced was designing a machine that could work with very small amounts.

“We’re talking about picoliters of material. It was a challenge to get the right droplet volume and the right pattern,” Murray said.

The team also had to make sure the custom inkjet 3D printer would be able to deposit the tiny droplets accurately, and achieved this by using the machine to hold the nozzle still, while a stage below it moves to form the required shape.

Murray said, “The stage can move with a 0.1 micron precision, which is basically a thousandth the width of a human hair.”

The researchers published a paper on the results on their project, titled “Two-component additive manufacturing of nanothermite structures via reactive inkjet printing,” in the Journal of Applied Physics; co-authors include Murray, Tugba Isik, Volkan Ortalan, I. Emre Gunduz, Steven F. Son, George T.-C. Chiu, and Rhoads.

The abstract reads, “With an eye towards improving the safety of the deposition of energetic materials while broadening the scope of materials compatible with inkjet printing, this work demonstrates the use of combinatorial inkjet printing for the deposition of energetic materials. Two largely inert colloidal suspensions of nanoaluminum and nanocopper (II) oxide in dimethylformamide with polyvinylpyrrolidone were sequentially deposited on a substrate using piezoelectric inkjet printing. The materials were deposited in such a way that the aluminum and copper (II) oxide droplets were adjacent, and overlapping, to allow for in situ mixing of the components. The alternating deposition was repeated to create a sample with multiple layers of energetic materials. Energetic performance was subsequently tested on samples printed with 3, 5, and 7 layers of materials using a spark igniter. This ignition event was observed with a high speed camera and compared to representative samples printed with pre-mixed nanothermite. High speed thermal imaging supported a conclusion that the maximum reaction temperature of comparable samples printed with the dual nozzle technique was nominally 200 K less than the samples printed with a single nozzle. Scanning transmission electron microscopy images confirmed a claim that the material constituents were comparably mixed with the single and dual nozzle techniques. This work proves the feasibility of reactive inkjet printing as a means for depositing energetic materials from two largely inert suspensions. In doing so, it opens the doors for safer material handling and the development of a wide array of energetic materials that were previously deemed incompatible with inkjet printing.”

A mixture of finely powdered aluminum and iron oxide, known as thermite, is used often in incendiary bombs, because it can produce very high temperatures upon combustion. By combining nanoaluminum oxide and nanocopper oxide with the water-soluble polymer PVP, the researchers were able to use Murray’s custom 3D printer to deposit nanothermite, which reacts with as much speed and power as traditionally applied thermites do.

Murray said, “It burns at 2,500 Kelvin [over 4,000 degrees Fahrenheit]. It generates a lot of thrust, a lot of heat, and makes a nice loud shockwave!”

Many times, it takes people from different disciplines and departments coming together for a single purpose to make a project successful. Ten researchers and four faculty members from different Mechanical Engineering disciplines contributed to the 3D inkjet printing research efforts, including Rhoads, who studies micro-electromechanical systems, inkjet printing expert Chiu, and Gunduz and Son, who both study energetic materials at Purdue’s Zucrow Labs, the largest university propulsion laboratory in the US.

“It’s a defining feature of Purdue that professors from such different backgrounds can work together on a project like this. We can combine all of our experiences to collaborate on technologies that weren’t previously realizable,” said Rhoads.

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[Source/Images: Purdue University]