Making electronics out of coal

April 19, 2016 by David Chandler
“When you look at coal as a material, and not just as something to burn, the chemistry is extremely rich,” says Jeffrey Grossman. In this photo, a sample of pulverized coal (right) is shown with several test devices made from coal by the MIT researchers. Credit: Photo courtesy of the researchers

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.

Explore further: Research sheds light on sensor material behavior in harsh environments

More information: Brent D. Keller et al. Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices, Nano Letters (2016). DOI: 10.1021/acs.nanolett.5b04735

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11 comments

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Whydening Gyre
5 / 5 (5) Apr 19, 2016
Nicely done, guys...
ab3a
3 / 5 (2) Apr 19, 2016
Coal is interesting, but using it in this manner is sort of like throwing spaghetti at the wall to see what sticks.

Studying coal and its chemical constituents is useful in much the same way as the discovery of buckyballs and buckytubes were discovered in carbon soot.

But once an interesting compound is found, it will probably be easier and more practical to synthesize it than to attempt refining it from raw coal. And of course, if we heat it, the chemical compositions will change...
obama_socks
not rated yet Apr 19, 2016
That would be fine, providing that the end product isn't too brittle to work with and reproduce commercially.
axemaster
5 / 5 (5) Apr 20, 2016
We already have carbon resistors. They suck (very noisy).

As for replacing silicon for electronic devices (ICs)... not likely. We use pure materials for a reason.

That said, it's interesting work. I just wish they would avoid fishing for citations...
Eikka
5 / 5 (5) Apr 20, 2016
We already have carbon resistors. They suck (very noisy).


Carbon has a slightly negative coefficient of resistance over temperature, so it's not very suitable for use in heaters either. It tends to "run away" - ie. it's not self regulating. The hotter it gets the more it conducts, and the hotter it gets. That was the problem of the original Edison lightbulb.

Most metals have a positive coefficient, so they stabilize to some specific heating power given a stable input voltage.
antialias_physorg
5 / 5 (3) Apr 20, 2016
Jeffrey Grossman thinks we've been looking at coal all wrong.
...
making a simple electrical heating device that could be used for defrosting car windows or airplane wings


OK, the propcessing and tweaking of properties may be a novel approach, but using carbon-based resistors is as old as electronics itself.

Coal is interesting, but using it in this manner is sort of like throwing spaghetti at the wall to see what sticks.

..which is, arguably, the method by which most progress is made.
Eikka
5 / 5 (5) Apr 20, 2016
but using carbon-based resistors is as old as electronics itself.


I make long touch sensors strips by mixing powdered activated carbon with acrylic or latex paint and squeezing the resulting paste into a sheet or a strip. Just to play with the stuff, literally, as it's quite a neat way to make a playable musical instrument out of a 555 chip.

Steelwolf
not rated yet Apr 23, 2016
I would imagine that many of the types in their Library are almost entirely gone except for such things as the sample held by the Library itself since so many of the older sites were mined out completely, and then mined again for the lower grade ores later.

One MIGHT be able to find some of the compounds and even carbon formations in the slag heaps and flue ash piles from the older blast furnaces and the local environs of such may contain further, although likely degraded, samples of such coals.

Another unlikely location would be the larger iron or steel elements still existing, and possibly even the refractory brick (which went into the ground with the slag) for being able to at least trace the path of some of these 'extinct' coals.

Hey, a whole new science to put money into. Sad Sigh.
Edit: Yes, I know how heat would changes compounds, was welder but also know can be pockets, esp. w/ older iron and steel products that retain some of the original carbon forms w/ impurities
compose
Apr 23, 2016
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Da Schneib
5 / 5 (1) Apr 24, 2016
For a first cut it's not bad.

Long way to go before this is high enough quality to do something more than make a heating element though.

However, @Eikka, you should have read the abstract of the paper:
Finally, Joule heating devices were fabricated from coal-based films, and temperatures as high as 285 °C with excellent stability were achieved.
Note that "excellent stability." This indicates that it doesn't run away at up to 285°C. Edison was working at much higher temperatures; quite frankly if you've got something that's 285°C in your computer or cell phone you already have a problem.
compose
Apr 24, 2016
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