Conventional Carbon Materials
Carbon Gets Interesting: Mixtures, Composites, and New Materials
Carbon Nanotubes Come into their Own
Graphene: Finding its Way
Carbon materials have been an important part of electronics throughout the industry's history. But far from being a stagnant class of materials, new developments in carbon materials are poised to make dramatic performance improvements in the applications that use them and to enable completely new applications. Eventually, these new classes of materials may even revolutionize the electronics industry as we know it.
Conventional carbon inks, pastes, and coatings make up a critical--if sometimes overlooked--class of materials in the electronics industry, providing solutions that are modestly conductive as well as cheap, easy to apply, and inert. Carbon is thus an important entry in the portfolio of materials used for conductive coatings, especially when extremely low resistivity is not required. While these conventional materials and applications are certainly not the most exciting in the electronics industry, they have been a consistent source of revenues. But now new, breakthrough materials--carbon nanotubes and grapheme--are breathing new life into the carbon materials market and making carbon "sexy". Nanocarbon materials are already enabling new applications that take advantage of conductivity much higher than that of any metal. Down the road are even more possibilities that could provide carbon the status that silicon currently holds in the electronics industry.
Conventional carbon inks, pastes, and coatings are big business. Such thick-film carbon coatings are used in numerous applications including capacitors, membrane switches, keypads, printed circuit boards, EL lighting, and batteries. In addition to these, some newer and rapidly growing applications are providing growth markets for these conventional carbon inks, pastes, and coatings, including photovoltaics, energy storage--which is becoming more and more important as the smart grid is developed--and EMI/RFI shielding and antistatic coatings as electronics and components become ever more sensitive to interference and ESD. In photovoltaics conventional carbon is largely a material for CdTe PV--carbon paste for the back contacts--but even so the rapid growth of CdTe PV has made this a growth market for carbon. Conventional carbon is also a contender as a catalyst in DSC cells, to replace costly platinum.
Also approaching a rapid stage of growth is the supercapacitor market, in which carbon provides a high-surface area material for the storage of large amounts of electrical charge. These supercapacitors will see increasing use to accommodate decentralized electricity storage as electricity generation becomes more distributed--through the growth of photovoltaics and other alternative generation technologies at smaller than utility-scale--and as the smart grid is deployed. To some extent these phenomena will also boost demand for rechargeable batteries, many of which also use carbon inks and coatings.
And as low-cost, flexible electronics begins to emerge for applications such as RFID, the need for thin-film and printed batteries, often using carbon, will increase as well. Carbon is a mainstay of the primary battery industry and, although zinc-carbon batteries are generally mediocre in performance in old-style applications such as flashlights, they are certainly adequate,and low in cost,for some printed battery applications. Carbon is also used as an electrode for some batteries based on lithium chemistry, which can also be made by thin-film techniques. Also significant is the need for low-cost antennas for RFID, and carbon is a material that has been demonstrated for some types of these antennas.
Besides being an important conductor when used by itself or as the major component of a composite paste, carbon can also be especially useful in combinations with other materials. This includes mixtures (e.g., carbon-silver pastes) as well as combinations of different discrete pastes (e.g., carbon paste applied on top of silver paste). Carbon need not be the major component of the inks, pastes, and coatings it is used in; for instance, mixtures of carbon and silver or carbon and copper can be formulated to target a wide range of electrical, thermal, and chemical properties. Here lie applications such as resistive heaters for automobiles, or resistors in general. And the conductivity of a mixture of carbon and a metal such as silver behaves in a non-linear fashion. Adding carbon in small to moderate quantities to silver has only a small effect on the conductivity; thus silver--or another metal--can be mixed with carbon to reduce costs at a given level of performance.
But carbon's inertness makes it highly desirable as a thin coating over another, more conductive material. And it is not just chemical inertness that matters here; electromigration and the formation of dendrites are physical changes in metallic layers that are extremely problematic for the devices in which the conductors are used. One important function of carbon is thus to coat conductors--like silver--that are susceptible to electromigration, to provide a stable outermost surface that will not form dendrites; the printed carbon forms an inert, conductive encapsulant against dendrite formation on silver inks and other metals. Carbon is used in this way in switches and other devices that primarily use other conductive materials like silver. The thin layer of carbon produces only a small increase in series resistance.
While there are many applications for conventional carbon materials, and many of them are quite lucrative, these conventional materials are obviously not all that carbon is about. Newer carbon materials have been discovered and developed in recent years, materials that promise to enhance the properties of carbon, in some cases far beyond those of any material known before. But besides exciting researchers, these new carbon materials have also drawn the interest of investors and capitalists who see their potential in the commercial electronics world.
These new materials are the nanoscale phases of carbon--carbon nanotubes, fullerenes, and graphene sheets--and their electrical and physical properties are truly impressive. Carbon nanotubes--depending on the nanotube structure--have extremely high electrical conductivity, higher than that of any metal, while the other carbon nanotube structures are semiconducting. Carbon nanotubes also posses extremely high thermal conductivity--also much higher than that of any of the metals--and are the strongest known materials in tension. Graphene, depending on the dimensions of the sheet, can also have high thermal conductivity and either high electrical conductivity or semiconductivity. Fullerenes--the so-called "Buckyballs" or hollow carbon spheres--are good electron acceptors and have been used in OPV cells.
These carbon nanomaterials, when added to or used in place of conventional carbon materials, can lend their enhanced properties to the applications that use them. In this way thick-film carbon materials can gain new life as high-end conductive inks and pastes while still remaining low in cost. But they also open to door to completely new applications. For instance, the high conductivity of certain carbon nanotubes has hinted at their ability to form wires more conductive than copper or silver, but their tiny size makes possible the formation of films that are uniformly conductive on the macro scale and even on the micron scale, yet thin enough to be highly transparent.
This high electrical conductivity in a diffuse film is in fact the property behind some of the most lucrative new applications for carbon nanotube inks: highly conductive films, including transparent ones. But carbon nanotubes are not all the same; only some are conductive while others are semiconductive. In fact one of the key areas of research--and limits on their commercial usefulness--is with the problem of either producing only a single type or separating the conductive ones from the semiconductive ones. Mixtures containing both types of carbon nanotubes are still quite conductive and suitable for the new classes of applications, but there is a lot of room for improvement--and additional, more demanding applications--as the technological hurdles are overcome.
Beyond the applications that rely on the electrical conductivity of carbon nanotubes, their thermal conductivity is also drawing interest for applications such as heat spreaders and heat sinks, while the mechanical properties suggest uses in mechanically operating computer memories and other nanometer scale switching applications. Farther out are applications that make use of the semiconductivity of certain nanotubes, such as single-nanotube transistors and other electronic devices. Significantly, these single-nanotube applications were widely believed to be on the verge of commercialization as recently as five years ago but have since taken a back seat to the conductive applications in terms of progress toward wide commercialization.
Even newer on the scene is graphene, the single-atom-thick graphite monolayer that can also be either very conductive or semiconductive depending on sheet structure and dimensions. While continuous sheets of graphene are frequently envisioned as making up the surface of semiconductor chips in the future, as the successor to silicon, even producing a single large sheet of graphene still maxes out our current technological abilities. In fact, for all of its potential, graphene is still widely made by the very low-tech method of peeling layers off of bulk graphite!
While many graphene researchers are quite exuberant about the commercial prospects for graphene, a cautious view suggests that, as was the case for carbon nanotubes, the most dramatic, truly novel applications are still several years away. Graphene is thus likely to be a material mainly for niche applications for many years. But those niches are already beginning to emerge and products such as graphene inks are already nearing commercialization.
Source: Opportunities for Carbon Materials
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