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The printing of conductive inks is changing the additive electronics market, and is also opening up exciting new areas of scientific research that could have major implications in a number of industries. Alex Kachkine and Luis Fernando Velásquez-García of MIT recently published an award-winning paper that showed how Voltera NOVA, a materials dispensing system that utilizes direct-ink writing technology to additively print conductive materials, could be used to fabricate field emission electron sources that can help with the propulsion of small satellites in low Earth orbit (LEO).

This article looks at how Kachkine and Velásquez-García devised a novel method involving two separate rounds of sonification to create an ink densely packed with carbon nanotubes, allowing them to obtain remarkably high performance from their printed field emission devices. In the article, we look at how additive manufacturing offers major advantages over traditional semiconductor fabrication methods, as well as the potential impact of the researchers’ work in aerospace, spectrometry, and other fields.

What is Field Electron Emission?

Field electron emission is when an electrostatic field induces the emission of electrons, typically from a solid surface into a vacuum. The process is an important area of study in quantum mechanics and can be explained by the phenomenon of quantum tunneling, in which an electron passes through a barrier despite lacking the required energy (according to classical mechanics) to actually do so.

Field emission electron sources have several practical applications, from field electron microscopy to electric spacecraft propulsion. However, fabricating them can be challenging: use of semiconductor cleanrooms is expensive, slow, and comes with significant environmental costs.

Printed Carbon Nanotube Electron Sources

The additive manufacturing of field emission electron sources shows promise for overcoming major limitations in traditional fabrication processes. Direct-ink writing (DIW), a form of additive printing suitable for electronic devices, is a low-cost, low-waste process compatible with a broad range of inks and substrates. It is also highly portable, and could potentially be deployed on spacecraft for in-situ printing.

Nonetheless, DIW has its own limitations. Today, most DIW field emission sources cannot match the peak emission currents of their cleanroom-made counterparts, while they also require greater startup voltages. This limits their practical use, as performance ultimately dictates the fabrication method to a greater extent than cost, speed, or environmental concerns.

In their recent study, the MIT researchers attempted to create an ink containing the highest possible concentration of carbon nanotubes (CNTs) in order to break through the current limitations in peak emission currents for 3D printed emitters. Their strategy was to use an unusual two-pronged sonication technique, which helped to prevent unwanted evaporation of solvents during ink preparation. The first round of sonication, during which water was used as a coolant to prevent evaporation, took place after the mixing of CNTs and N,N-Dimethylformamide. The second round took place after the filtering of undispersed CNT clumps and the introduction of ethyl cellulose.

Using this innovative technique, the researchers prepared the inks and stored them in thermally stable cartridges. They then loaded their chosen ink cartridge and a standard FR4 substrate onto the NOVA printer to begin the printing process. The machine was able to print a spiralized CNT trace to act as the emitting electrode, offset from a conventional PCB copper trace functioning as the extractor electrode. The printer could accurately print in the gaps between the pre-existing copper trace via optical alignment and surface probing, completing each print in just 30 seconds.

The Results of the Study

During their research, the team made a number of ink formulations with a CNT concentration ranging from 0.1 wt% to 4.0 wt% and an ethyl cellulose concentration ranging from 0 wt% to 50 wt%, storing each different formulation in its own printable cartridge. The idea behind making a number of different inks with different ratios of their components was to test how concentrated they could make the ink (how many CNTs could be packed into it) before it became impossible to print.

While inks with a 4.0 wt% of CNTs resulted in nozzle clogging due to solvent evaporation, making printing impossible, slightly lower concentrations produced highly promising results. Analysis revealed that the devices had a field emission startup voltage of ~40 V and an aggregate peak current of up to ~100 μA/cm2 (beyond which the devices would heat up and stop working) — a higher level of performance than state-of-the-art emitters currently available, and better than previous attempts to make printed emitters of this sort.

The researchers noted that the affordability of the NOVA system used in the study could open up the number of potential users and applications for the technique while also commenting on how the printer’s integrated surface mapping and optical alignment enabled precision printing of the spiralized CNT trace.

Potential Applications of 3D Printed CNT Electron Sources

Space Propulsion

Different spacecraft have different methods of propulsion, and one form of electric propulsion — more propellant-efficient than chemical rockets — is the ion thruster. An ion thruster makes a cloud of positive ions from a neutral gas via ionization, with electricity used to accelerate the ions and create thrust.

Electric propulsion requires the use of a neutralizer to neutralize the gas and allow it to disperse into space. Without a neutralizer, the spacecraft itself could become charged, potentially damaging fragile onboard equipment. Fortunately, field emission electron sources are ideal neutralizers, particularly in low Earth orbit where there is poor vacuum due to residual oxygen.

Mass Spectrometry & Beyond

Another application of densely packed 3D printed CNT field emitters is in mass spectrometry of gasses, in which the mass-to-charge ratio of ions is measured. Electron impact ionization can provide better signal clarity than corona discharge approaches, which are destructive. The development of 3D printed CNT emitters holds plenty of promise for this field, as it requires lower pressure from the ionizer, which can make the spectrometer more portable.

Other fields that could benefit from printed electron sources made using high-concentration CNT inks include X-ray imaging and electron beam lithography (EBL), a technique for drawing custom shapes on electron-sensitive films, known as resists. In both of these fields, a significant reduction in power consumption and the ability to work in poor vacuum makes printed field emission electron sources preferable to their mainstream thermionic emission counterparts.

Conclusion

This latest study is not the first undertaking by members of MIT’s Velásquez Group — specialists in miniaturized devices — to harness the power of a Voltera printer to make a field emission electron source. In 2019, Velásquez-García and lead author Imperio Anel Perales-Martinez fabricated the first ever 3D printed CNT field emission electron sources using a Voltera V-One. That work laid the foundations for this latest study, with its development of an electron source characterized by a much higher peak current and lower startup voltage.

This latest round of exciting findings from the Velásquez Group shows the enormous potential of direct ink writing for the additive manufacturing of critical components in a range of disciplines, from aerospace to healthcare. Overcoming challenges such as unwanted solvent evaporation and nozzle clogging at very high CNT concentrations could further enhance the performance of printed emitters.