MIT scientists have developed an electrical heating device using thin films of coal for the first time, demonstrating that the material could have a broad range of uses in electronics, solar panels and batteries.
"We've been looking at coal all wrong. Instead of just setting it afire, thus ignoring the molecular complexity of this highly varied material, we should be harnessing the real value of that diversity and complex chemistry," said Jeffrey Grossman, from Massachusetts Institute of Technology (MIT).
As a first demonstration of what they see as a broad range of potential high-tech uses for this traditionally low-tech material, the researchers made an 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 characterised the chemical, electrical, and optical properties of thin films of four different kinds of coal: anthracite, lignite, and two bituminous types.
Naturally occurring coal varieties, without the purifying or refining that is needed to make electronic devices out of silicon, have a range of electrical conductivities that spans seven orders of magnitude (ten million times).
A given variety of coal could inherently provide the electrical properties needed for a particular component.
Part of the challenge was figuring out how to process the material, Grossman said.
For that, researchers 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 - a necessary step in fabricating many electronic devices, from transistors to photovoltaics.
Even though coal has been one of the most widely used substances by human beings for centuries, its bulk electronic and optical properties had never really been studied for the purpose of advanced devices.
"The material has never been approached this way before to find out what the properties are, what unique features there might be," said Brent Keller, doctoral student at MIT.
To do so, he developed a method for making thin films, which could then be tested in detail and used for device fabrication.
Even this new, detailed characterisation is just the tip of a large iceberg, the team said. The four varieties selected are just a few of the hundreds that exist, all with likely significant differences.
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.
The big potential advantage of the new material is its low cost stemming from the inherently cheap base material, combined with simple solution processing that enables low fabrication costs, Grossman said.
The research was published in the journal Nano Letters.
