How micro-/nanotechnology is revolutionizing a century-old technology

Andrew Koch
Jul 28, 2017 · 3 min read

At Modern Electron, an energy startup in the Seattle area, we’re giving an old technology some very modern upgrades. I’m fortunate to have worked in this exciting field in grad school and to be continuing this work today. To introduce this topic, I’d like to share a review article my grad school colleagues and I wrote in 2014 on thermionics and thermoelectrics, and the impact that recent advancements in micro- and nanotechnology have had on them. Here’s a summary of the article:

Thermionic conversion is essentially an old technology that is learning some new tricks. Discovered in the 1880s, thermionic conversion is experiencing new breakthroughs due to advancements in the fields of micro- and nanotechnology. As a result, the limitations of thermionic converters which prevented their advancement decades ago are now tractable.

Thermionic conversion is the process of converting heat directly to electricity by heating an electrode, called the emitter, to temperatures so high that electrons “boil off” the surface, travel across some space, and condense on a cold electrode, called the collector. If the collector is held at a higher voltage relative to the emitter, the device is like a battery being charged, and it can do work across an electrical load. The temperatures involved on the hot side of thermionic converters are usually >1000˚C. Thermionic converters are indifferent to the source of the heat, as long as temperature is high enough. This means they can use heat from a wide variety of sources, including the absorption of concentrated sunlight, nuclear fuels, traditional fuels like gas, oil, or coal, or even renewable biofuels.

The steam turbine is the most common heat-to-electricity converter in the world today. Unlike the steam turbine, thermionic converters have no moving parts, which endows them with advantages in cost, reliability, and weight. This isn’t the only advantage that thermionic converters have. Not only are thermionic converters highly scalable, but they actually perform better at higher temperatures, allowing them to convert heat directly from the combustion source, rather than what is done by turbines today, which is to first drop the temperature before being able to convert heat into electricity. The latter wastes energy and hurts efficiency.

Historically, these advantages alone have not sufficed for thermionic converters to triumph over the steam turbine. However, recent advancements in micro- and nanotechnology may finally push the efficiencies of thermionic converters to unprecedented heights. In the past, thermionic converters have faced two main limitations: 1) the high work functions of the electrodes and 2) the space-charge effect. The work functions of the electrodes act as potential barriers at the surfaces of the electrodes which limit the emission and absorption of electrons. The space charge effect is caused by the build-up of negatively charged electrons between the electrodes, creating another potential barrier for new electrons trying to travel between the electrodes.

Recent advancements in materials science and nanostructured materials, such as the intercalation of alkali metals into hollow nanomaterials, have blazed a new path forward for stable, low-work function electrodes. Additionally, advancements in micro- and nanofabrication have created innovative ways to mitigate the space charge effect, such as the addition of small auxiliary electrodes. Micro- and nanotechnologies are opening new opportunities for thermionic converters, removing the technological barriers which limited their performance decades ago.

At Modern Electron, we are harnessing many of these advancements to develop micro- and nanofabricated thermionic converters with state-of-the-art efficiencies and power densities. Our interdisciplinary team of nanofabrication and mechanical engineers, physicists, and physical chemists is integrating decades of technological advancements in nanoscale physics to push thermionic technology leaps and bounds beyond what was historically possible. Visit our website to learn more about our team and what we do. (P.S. We’re hiring!)

“Thermionics, Thermoelectrics, and Nanotechnology” IEEE Nanotechnology Magazine, 2014

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