Jeffrey Grossman thinks we've been
looking at coal all wrong. Instead of just setting it afire, thus
ignoring the molecular complexity of this highly varied material, he
says, we should be harnessing the real value of that diversity and
complex chemistry. Coal could become the basis for solar panels,
batteries, or electronic devices, he and his research team say.
As a first demonstration of what they see as a
broad range of potential high-tech uses for this traditionally low-tech
material, Grossman, doctoral student Brent Keller, and research
scientist Nicola Ferralis 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 Nano Letters.
"When you look at coal as a material, and not just as
something to burn, the chemistry is extremely rich," says Grossman, the
Morton and Claire Goulder and Family Professor in Environmental Systems
in the Department of Materials Science and Engineering (DMSE). The
question he wanted to ask is, "Could we leverage the wealth of chemistry
in things like coal to make devices that have useful functionality?"
The answer, he says, is a resounding yes.
It turns out, for example, that 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).
That means that a given variety of coal could inherently provide the
electrical properties needed for a particular component.
Designing a process
Part of the challenge was figuring out how to process the
material, Grossman says. For that, Keller 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,"
says Keller, who carried out much of the work as part of his doctoral
thesis in DMSE, "to find out what the properties are, what unique
features there might be." 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 characterization they carried out is
just the tip of a large iceberg, the team says. The four varieties
selected are just a few of the hundreds that exist, all with likely
significant differences. And preparing and testing the samples was, from
the outset, an unusual process for materials scientists. "We usually
want to make materials from scratch, carefully combining pure materials
in precise ratios," says Ferralis, also in DMSE. In this case, though,
the process involves "selecting from among this huge library of
materials," all with their own different variations.
Using nature's complexity
While coal and other fossil fuels have long been used as
feedstocks for the chemical industry, making everything from plastics to
dyes and solvents, traditionally the material has been treated like
other kinds of raw ore: something to be refined into its basic
constituents, atoms, or simple molecules, which are then recombined to
make the desired material. Using the chemistries that nature has
provided, just as they are, is an unusual new approach. And the
researchers 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 big potential advantage of the new material, Grossman
says, is its low cost stemming from the inherently cheap base material,
combined with simple solution processing that enables low fabrication
costs. Much of the expense associated with chip-grade silicon or
graphene, for example, is in the purification of the materials. Silica,
the raw material for silicon chips, is cheap and abundant, but the
highly refined form needed for electronics (typically 99.999 percent
pure or more) is not. Using powdered coal could provide a significant
advantage for many kinds of applications, thanks to the tunability of
its properties, its high conductivity, and its robustness and thermal
stability.
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