Researcher 3D printed sensor for satellite

MIT scientists have created the first fully digitally produced plasma sensor to orbit a spacecraft. These plasma sensors, also known as retarding potential analyzers (RPAs), are used by satellites to determine the chemical composition and energy distribution of ions in the atmosphere.

The 3D printing and laser cut hardware works and the advanced semiconductor plasma sensors are manufactured in a clean room, which makes them expensive and requires weeks of complex fabrication. In contrast, 3D printed sensors can be produced for tens of dollars in a matter of days.

Due to their low cost and fast production, these sensors are ideal for CubeSats. These inexpensive, low-power, and lightweight satellites are often used for communications and environmental monitoring in the Earth’s upper atmosphere.

The researchers developed RPA using a glass-ceramic material that is more durable than traditional sensor materials such as silicon and thin-film coatings. By using glass-ceramic in a fabrication process developed for 3D printing with plastics, there is the ability to fabricate sensors of complex shapes that can withstand the wide temperature changes that spacecraft will encounter in lower Earth orbit.

“Additive manufacturing can make a huge difference in the future of aerospace hardware. Some people think that when you 3D print something, you should admit less performance. But we have shown that this is not always the case. Sometimes there’s nothing to trade off,” said Luis Fernando Velásquez-García, principal scientist at MIT’s Microsystems Technology Laboratories (MTL) and senior author of the paper that presented the plasma sensor.

Joining Velásquez-García on paper are lead author and MTL postdoc Javier Izquierdo-Reyes; graduate student Zoey Bigelow; and postdoctoral Nicholas K. Lubinsky. This research was published in Additive Manufacturing.

Versatile sensor

An RPA was first used on a space mission in 1959. The sensor detects energy in the ions, or charged particles, floating in plasma, which is a superheated mixture of molecules present in Earth’s upper atmosphere. Aboard orbiting spacecraft such as the CubeSat, the versatile instrument measures energy and performs chemical analysis that can help scientists predict the weather or monitor climate change.

The sensor contains a series of electrically charged meshes decorated with tiny holes. As the plasma passes through the holes, electrons and other particles are stripped away until only ions remain. These ions create an electric current which is measured and analyzed by the sensor.

The key to the success of the RPA is the house structure that aligns the meshes. It must be electrically insulating while also being able to withstand sudden and drastic changes in temperature. The researchers used a printable glass-ceramic material that displays these properties, known as Vitrolite.

Pioneered in the early 20th century, Vitrolite is often used in colorful tiles that are a common sight in art deco buildings.

The durable material can also withstand temperatures as high as 800 degrees Celsius without breaking, while the polymers used in RPA semiconductors begin to melt at 400 degrees Celsius.

“When you build these sensors in a clean room, you don’t have the same level of freedom to define materials and structures and how they interact together. What makes this possible are the latest developments in additive manufacturing,” says Velásquez-García.

Rethinking fabrication

3D printing processes for ceramics typically involve laser-beaten ceramic powders to combine them into shapes, but this process often roughens the material and creates weak spots due to the high heat of the laser.

Instead, the MIT researchers are using vat polymerization, a process introduced decades ago for the manufacture of additives with polymers or resins. With vat polymerization, the 3D structure is constructed one layer at a time by immersing it repeatedly into a vat of liquid material, in this case Vitrolite. Ultraviolet light is used to cure the material after each layer is added, and then the platform is submerged again in the vat. Each layer is only 100 microns thick (roughly the diameter of a human hair), enabling the creation of smooth, pore-free and complex ceramic shapes.

In digital manufacturing, the objects described in design files can be very complex. This precision allowed the researchers to create laser-cut meshes with a unique shape so that the holes line up perfectly when installed in the RPA housing. This allows more ions to pass through, leading to higher resolution measurements.

Because the sensor is cheap to manufacture and can be fabricated very quickly, the team prototyped four unique designs.

While one design is highly effective at capturing and measuring a wide range of plasmas, as satellites would encounter in orbit, the other is particularly well suited for sensing extremely dense and cold plasmas, which can normally only be measured using ultraprecision semiconductor devices.

This high precision can enable 3D printed sensors for applications in fusion energy research or supersonic flight. The rapid prototyping process could spur even more innovation in the design of satellites and spacecraft, adds Velásquez-García.

“If you want to innovate, you have to be able to fail and take the risk. Additive manufacturing is a very different way of making space hardware. I can build space hardware and if it fails, it doesn’t matter because I can build new versions very quickly and cheaply, and completely redo the designs. It is an ideal sandbox for researchers,” he said.

While Velásquez-García is happy with this sensor, in the future he wants to improve the fabrication process. Reducing layer thickness or pixel size in glass-ceramic vat polymerization can create even more precise complex hardware. Additionally, manufacturing the sensor completely additive will make it compatible with manufacturing in aerospace. He also wants to explore using artificial intelligence to optimize sensor designs for specific use cases, such as greatly reducing their mass while ensuring they remain structurally sound.

This work was funded, in part, by MIT, the MIT-Tecnológico de Monterrey Nanotechnology Program, the MIT Portugal Program, and the Portuguese Foundation for Science and Technology.

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