The extraordinary performance characteristics of graphene is driving a plethora of process developments for solving its production challenges.
Graphene is an atomic-scale honeycomb lattice made of single layers of carbon atoms. First isolated and produced by Andre Geim and Konstantin Novoselov at the Univ. of Manchester, U.K., in 2003, the researchers won the Nobel Prize in Physics in 2010 “for their groundbreaking experiments regarding 2-D graphene.” Graphene had been theorized for many years before Geim-Novoselov’s production discovery. It was imaged on a transmission electron microscope (TEM) in 1961 by chemist Hanns-Peter Boehm at Ludwig-Maximilians Univ., and identified and named in an IUPAC report in 1994.
The material’s extraordinary physical properties made it an immediate target for researchers attempting to find ways to produce the material in commercial quantities. In terms of physical properties, graphene has 200 times the strength of steel by weight, with the highest tensile strength of any material ever tested. It also has nearly three times the surface area of carbon nanotubes (CNTs); it’s a zero gap semiconductor with numerous electronic properties, exceeding those of silicon; it’s resistivity is lower than silver’s; it has unique optical properties; and it has more than twice the thermal conductivity of pyrolytic graphite.
The market for graphene in 2014 was estimated by Future Markets at $15 to $20 million, with a forecast of 40% CAGR through 2020 to $140 to $160 million, according to market research firms Yole Developments, Lux Research and IDTechEx. Research use for graphene over the next five years is split equally in composite materials, conductive inks and coatings, energy storage (batteries) and basic research applications. Minor uses for these materials include high-performance electronics, transparent conductive films and water filtration applications.
With all its strong properties, the continuing challenge for graphene researchers has been to find a way to produce it in commercial quantities, while maintaining its desired properties. “There are many diverse methodologies being investigated for large-scale production of graphene,” says Felix Miranda, Chief, Advanced High Frequency Branch at NASA Glenn Research Center, Cleveland. “Some of the methods, while simple, have limitations regarding large surface area coverage, as well as control of the numbers of layers of graphene (as the number of atomic layers increase, the properties decline). Nonetheless, mechanical exfoliation is appropriate for small-scale, research-type physics, biology and chemical experiments.”
Despite the lack of significant process successes, nor the emergence of a “killer” application, current interest in graphene remains strong. In 2013, the European Union (EU) committed more than $1 billion over the next decade for research on graphene and other 2-D materials. South Korea and the U.K. governments have similarly committed $40 and $27 million, respectively, over the past two years. And market research firm IDTechEx estimates that more than $60 million in private investments have been made in graphene R&D over the past several years.
In the U.S., the National Science Foundation, the Air Force Office of Scientific Research, the Dept. of Energy, NASA and other government agencies are funding academia to optimize graphene properties and support the development of applications.
“One of the graphene production processes being examined is chemical vapor deposition (CVD), but there are other emerging ones, such as ink-jet printing,” says Miranda. “Last year, the AMBER (Advanced Materials and Bioengineeering Research at Trinity College) group in Dublin, Ireland, produced large quantities of defect-free few-layer graphene by shear exfoliation in liquids. Other original techniques used were mechanical exfoliation and unzipping of multi-walled carbon nanotubes. Graphene can also be produced by spin-coating and spraying graphene oxide or graphene flake dispersion from liquid-phase exfoliation.”
“Reduction of graphene oxide and liquid exfoliation of graphite provide small graphene flakes, while CVD processes yield large intact graphene sheets,” says Ivan Vlassiouk, research scientist in the Energy and Transportation Science Div. at Oak Ridge National Laboratory. “The graphene oxide and liquid exfoliation processes result in lower costs, and are basically ready for scale-up. However, the drawback is the large size of the flakes (1 to 10 μm or larger) and a large number of defects in graphene sheets. The current CVD approach for graphene synthesis gives the best quality/cost ratio.”
Using their CVD process, Vlassiouk’s team has fabricated polymer composites containing 2-in-by-2-in sheets of one-atom- thick hexagonally arranged carbon atoms. “Before our work, superb mechanical properties of graphene were only able to be shown at a micro scale,” says Vlassiouk. “We have extended this to a larger scale, which considerably extends the potential applications and market for graphene.” The composite structure they produced contains multiple graphene layers, each sandwiched between polymer layers. The nanocomposite laminate is electrically conductive, with graphene loading that’s 50 times less compared to other current state-of-the-art graphene samples.
Researchers at Massachusetts Institute of Technology (MIT) also recently announced a CVD-based process for fabricating graphene in large continuous sheets. This process is an adaptation of a CVD method already used at MIT, which uses a small vacuum chamber in which carbon vapor reacts on a horizontal substrate, such as a copper foil. The new MIT system uses a similar vapor chemistry, but the chamber consists of two concentric tubes, one inside the other, and the substrate is a thin ribbon of copper that slides over the inner tube. Gases flowing into the tubes are released through precisely placed holes, allowing the substrate to be exposed to two mixtures of gases sequentially. The first region is termed an annealing region, which is used to prepare the substrate surface. The second region is a growth zone, where the graphene is formed on the copper ribbon. The chamber is heated to approximately 1,000 C to perform the reaction.
When the ribbon is rolled through the chamber at 25 mm/min, a uniform, high-quality single layer of graphene is created. When the ribbon is rolled 20 times faster, it still produces a graphene coating, but it’s lower quality with more defects.
“Some applications, such as filtration membranes, may require high-quality graphene, but others, such as thin-film heaters, may work well enough with lower-quality graphene sheets,” says A. John Hart, MIT mechanical engineering associate professor. “The new system produces graphene that’s not quite equal to the best that can be done by batch processing, but it’s still as good as what has been produced by other continuous processes. Further work on pre-treatment of the substrate to remove unwanted defects could improve the quality of the resulting graphene sheets.”
Researchers at Northwestern Univ. have developed a novel way to print large, robust 3-D structures with a graphene-based ink. Previous similar attempts resulted in relatively low graphene loading (20%), which failed to take advantage of the graphene properties. Higher graphene volumes generally resulted in brittle, fragile structures. Led by Ramille Shah, assistant professor of materials science and engineering at Northwestern, the researchers developed a 60 to 70% graphene-loaded ink. The graphene flakes are mixed with a biocompatible elastomer and fast-evaporating solvent. “After the ink is extruded, one of the solvents evaporates causing the structure to solidify,” says Shah. “The presence of the other solvents and the interaction with a specific polymer binder contributes to the structure’s flexibility and properties. Because it holds its shape, we’re able to build large, well-defined objects.” These objects can play a role in tissue engineering and regenerative medicine, as well as in the design of electronic devices.
While U.S. research laboratories are well supported and are creating innovative production processes, “groups from South Korea are also very strong,” says Oak Ridge’s Vlassiouk. “Several Asian companies also have demonstrated the production of large sheets of graphene as well.” These foreign researchers are also working at the fundamental level with a strong orientation toward device development.
Graphene-based applications take advantage of graphene’s unique strengths, and many of these strengths are particularly applicable to electronic devices. “In early 2014, IBM’s Shu-Jen Han reported fully functional ICs made of wafer-scale graphene with 10,000 times better performance than previously reported efforts,” says NASA’s Miranda. “IBM’s interest is to improve the state of the practice wireless devices’ communication speed, and lead the way toward carbon-based electronic devices and circuit applications beyond what’s possible with widely used silicon chips. By integrating graphene RF devices, low-cost silicon technology could enable pervasive wireless communications, allowing such things as “smart” sensors and RFID tags to send data signals at significantly larger distances.”
Another electronic-based graphene application is its implementation in 3-D supercapacitors. Building on a laser-induced graphene (LIG) production process developed in the Rice Univ. laboratory of chemist James Tour (R&D’s 2013 Scientist of the Year), researchers discovered that firing a laser at an inexpensive polymer burned off other elements and left a film of porous graphene. The researchers have since created vertically aligned supercapacitors with the LIG layers on both sides of a polymer sheet. These sections are stacked with solid electrolytes in between for a multilayer sandwich with multiple graphene-based supercapacitors. “The flexible stacks have excellent energy-storage capacities and power potentials, and can be scaled for commercial applications,” says Tour. “LIG can be made in air at ambient temperatures and in industrial quantities through a commercial roll-to-roll process.”
Yet another graphene electronics application was demonstrated by researchers at Pacific Northwest National Laboratory, Richland, Wash., with their addition of graphene into the titanium oxide components of lithium battery electrodes. The addition of small quantities of Vor-X graphene into the electrodes resulted in charging and discharging performances up to three times better than with the standard titanium dioxide electrodes. Vor-X graphene is manufactured by Vorbeck Materials Corp., Jessup, Md. Vorbeck has a number of graphene patents covering applications in conductive inks, printed electronics, composite materials and energy storage.
Yet another graphene-based electronics application was demonstrated by researchers at the Univ. of Manchester, with the 3-D printing of a graphene laminate for use in fabricating a lightweight antenna. The resulting antennae products have practically acceptable return loss, gain, bandwidth and radiation patterns. This is particularly useful for RFID tags. Other acceptable high-conductivity inks used for fabricating antenna devices are available, such as those incorporating silver, but are generally cost-prohibitive. Other choices, such as copper and aluminum, aren’t durable and quickly become corroded, but are the current material of choice due to their availabilities. The Manchester researchers used a method called “rolling compression”, which substantially increased the conductivity of the graphene. They tested the concept by producing a functional dipole antenna which radiated power effectively after being successfully printed with a binder-free graphene laminate on paper process.
“While a number of companies are already claiming the ability to fabricate large quantities of defect-free graphene, there’s still work to be done regarding the optimization of the graphene materials,” says NASA’s Miranda. “The fabrication and deposition techniques also need to be optimized for large volumes, reproducibility and outside of laboratory environments. Accordingly, the time frame for achieving large-scale production values is likely in the five to 10 year range.”
As a result, the primary graphene applications over the next five years will continue to be in the research arena. This also includes the development of proof-of-concept devices.
A recent market report by Lux Research states graphene is destined to become the “next carbon nanotube (CNT),” referring to the massive hype that surrounded the initial CNT development, which then failed to be followed by actual commercial successes. Lux analysts believe graphene won’t follow in silicon’s footsteps and find wide-ranging ubiquitous applications. Lux believes graphene will become closer to CNT markets and find limited uses in somewhat niche markets.
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