Coal redefined for thin film electronics
Rather than merely considering the material as a fossil fuel or as a raw source of carbon for the petrochemical industry, Keller and his colleagues studied the raw material properties, once processed into thin films.
The student developed a series of steps to crush the material to a powder, put it in solution, then deposit it in thin uniform films on a substrate, in order to fabricate electronic devices and characterize them.
As a first demonstration of what they see as a broad range of potential high-tech uses for this traditionally low-tech material, the MIT researchers have succeeded in making a simple electrical heating device that could be used for defrosting car windows or airplane wings, or as part of a biomedical implant.
In developing this initial application, they have also for the first time characterized in detail the chemical, electrical, and optical properties of thin films of four different kinds of coal: anthracite, lignite, and two bituminous types. Their findings have just been reported in the journal NanoLetters.
This was done with naturally occurring coal varieties, without the purifying or refining that is needed to make electronic devices out of silicon. Different coals can have a range of electrical conductivities that spans seven orders of magnitude, the researchers report, meaning that a given variety of coal could inherently provide the electrical properties needed for a particular component. The researchers also found that by simply adjusting the temperature at which the coal is processed, they could tune many of the material’s optical and electrical properties to exactly the desired values.
The simple heating device the team made as a proof of principle provides an end-to-end demonstration of how to use the material, from grinding the coal, to depositing it as a thin film and making it into a functional electronic device. Now, they say, the doors are opened for a wide variety of potential applications through further research.
The work was made possible thanks to a grant from the Bose Fellows Program at MIT, which encourages the kind of high-risk research embodied in this project. The work was also supported by ExxonMobil through the MIT Energy Initiative and the ExxonMobil Energy Fellow Program.
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