Coal comes in four different types, all characterized by their composition, age, and the degree of compression applied to them over the millennia. Specifically, they range from lignite, a soft brown combustible sedimentary rock formed from naturally compressed peat, through to sub-bituminous, bituminous and lastly to anthracite, a compacted variety with a high carbon content and fewer impurities than the other types. In developing their application, the MIT researchers analyzed the chemical, electrical, and optical properties of thin films of all four different types of coal, and decided on anthracite as the most suitable for the devices constructed in their experiments.
To prepare the coal for the research, the scientists developed a process to crush the anthracite to a powder, suspend it in solution, and deposit it in thin uniform films on a substrate, similar to the way it is done in fabricating other electronic devices from graphene or silicon. However, unlike silicon that must be refined to a purity of over 99 percent, coal can simply be used in its crushed form without further refining.
Despite coal being one of the most abundant substances used by humans, the vast bulk of it has simply been used as a fuel for burning, with almost all of its electrical and optical properties rarely studied for use in electronic devices.
"The material has never been approached this way before, to find out what the properties are, what unique features there might be." said MIT doctoral student Brent Keller.
As an initial proof of concept for what the team sees as a wide range of possible uses, the researchers built an electrical heating device that could see duty in anything from being part of a part of a biomedical implant, to helping defrost car windows or aircraft wings. Along with the properties of this device, the researchers also found that by altering the temperature used to process the coal, a large range of the optical and electrical properties of the material could be exactly tailored to meet specified values.
But even this new found capability for coal is just the beginning of its potential, according to the researchers. Given that the four main varieties of coal selected have many other subsets all with differing compositions, the team believes that there could as yet be strikingly useful differences that could be exploited in other kinds of electronic devices.
"When you look at coal as a material, and not just as something to burn, the chemistry is extremely rich," said Professor Jeffrey Grossman, from the Department of Materials Science and Engineering at MIT.
According to Professor Grossman, the major advantage of the new material is its low cost to produce from an incredibly cheap base material, allied with an uncomplicated solution procedure that allows exceptionally low fabrication costs. Compared to silicon or graphene for use in electronic semiconductors, coal thin film is far and away the simplest to produce because it doesn't require such high levels of purification.
Silicon and graphene may also be relatively abundant, but the purified form of silicon, for example, is vastly more expensive than coal by comparison. It is also easily and readily tunable across conductive and resistive paths, and is very robust in nature with a naturally high thermal stability.
"[A] very extensive and informative study to facilitate the understanding of unrefined coal's potential for practical significance." said Shenqiang Ren, an associate professor of mechanical engineering at Temple University who was not involved in this research, but studied the results. "This is a significant step (probably the first) to utilize nanocarbon materials, directly from unrefined coal, with controllable electronic properties and excellent stability and scalability."
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