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Posted on March 11, 2013 by  &  with 1 Comment

Cost is the main differentiator between graphene and other nanocarbons

Graphene, an atom-thin layer of carbon, is transparent enough to see through yet can stretch up to 20 percent of its length. The pliant material also is stiffer than a diamond and 100 times stronger than steel. These and other characteristics continue to give graphene traction in the marketplace. In a 2013 post called 'Things To Watch,' New York Times' Alec Foege says Samsung and Sony are testing graphene-infused touch screens that have a flexible, paper-like feel but are 100 times stronger than steel and as conductive as copper. Recent attention has focused primarily on two-dimensional graphene.
"In many applications, graphene will likely be selected over carbon nanotubes and fibers," says David Burton, executive vice president at Angstron Materials Inc. "The material's performance will be on par with carbon nanotubes and fibers in most systems but available at a much lower price." Reports on the existence of carbon nanotubes surfaced as early as 1952 but the fabrication process that could synthesize the material didn't exist leaving the technology to lie dormant until interest in it revived in 1991. "There's been a very long learning curve associated with the use and application of carbon nanotubes and fibers," Burton says. "Fortunately, the lessons learned by the nanomaterials industry to date will greatly facilitate the commercialization of graphene." Graphene production is simpler requiring material refinement and separation versus the techniques used to manufacture carbon nanotubes. Chemical vapor deposition is the most popular method for commercial production of carbon nanotubes. Carbon nanotubes are grown from metal nanoparticles placed on a substrate and heated to temperatures greater than 1200 degrees Fahrenheit. Nanotubes form with the introduction of gases.
Dr. Bor Jang, co-founder and CEO of Dayton, Ohio-based Angstron, was the first to successfully produce nano-graphene sheets in 2001 by thermally exfoliating graphite intercalation compounds. The exfoliated graphite was subjected to mechanical shearing treatments, such as ball milling, to obtain nano-graphene platelets (or NGPs) in single to five-layer structures. The specific surface area of the prepared samples was found to be in the range of 300-1,300 m2/g, as measured by the Brunauer-Emmett-Teller method. Jang submitted the first patent application on single-layer graphene in 2002 and in 2004 filed a patent application for discovery of several thermal exfoliation processes for producing graphene from graphite particles using chemical attack (e.g. oxidation); acid/oxidizing agent intercalation, and intercalation with a foaming agent.
Angstron is an IP leader and the world's largest producer of NGPs. The only manufacturer able to produce pristine graphene, the company has since developed six different graphene products, both oxide and pristine, in varying thicknesses and dimensions. The company developed its NGPs to provide a lower-cost, high-quality material. To help support cost reductions of the advanced material, Angstron is building a new multi-million dollar plant expected to open later this year. "Historically most graphene producers have been limited to manufacturing smaller quantities of graphene which contributes to higher costs per kilogram," Burton says. "Over the last five years or so, Angstron has aggressively pursued lower cost processing techniques and worked with OEMs to optimize equipment design to increase throughput without sacrificing quality. Angstron is investing in its new production facility to enable further cost reductions through economies of scale."
Angstron's NGPs are especially suited to EMI, thermal and electrical applications. Performance characteristics include:
  • Highest thermal conductivity known today (up to ~ 5,300 W/(mK) for faster thermal dissipation
  • Exceptional in-plane electrical conductivity (up to ~104 S/cm) a material density four times lower than copper resulting in lighter weight components
  • High specific surface area (up to ~ 2,675 m2/g) for energy storage or heavy metals adsorption
  • Resistance to gas permeation
The material's attributes, coupled with the ability to tailor graphene to be compatible with the target matrix, make it a flexible option for manufacturers. NGPs can be mixed with other thermally conductive materials such as silver or copper to enhance thermal management performance and help reduce the amount of costly metals needed. NGPs also can be used to work in concert with carbon nanotubes or fibers to help improve viscosity. The long and thin-shaped carbon nanotubes and fibers tend to become tangled forming a 'bird's nest' structure. Loading these conductive nanofillers can increase the viscosity of a matrix resin to a level that is not conducive to composite processing. NGP-resin systems permit low resistance-to-shear flow, even with a relatively high NGP proportion, because the two-dimensional platelets have the capability to slide over one another. This feature also enables easier application of structural adhesives and more convenient melt processing of polymer nanocomposites containing a high NGP loading.
But the young industry still faces commercialization hurdles. "One of the biggest challenges is educating customers on how to process the material," says Burton. "To address this challenge, we prefer to partner with companies to help them select the right material or engineer variants to meet their needs. Although graphene offers nearly limitless uses in thermal management, energy, aerospace, automotive and other markets, the material can be problematic to implement due to its complex behavior and properties. We can accelerate commercialization by helping customers develop end-products or by performing the research and development work for them."
Standardization of the techniques used to characterize material properties are needed by the industry adds Burton. "Methods that are relevant to applications need to be evaluated and identified," he says. "In addition to standardized testing, manufacturers need to be cognizant of EPA requirements here in the U.S. and REACH, the European community regulation on the safe use of chemicals, and be able to explain the impact of such regulations to their customers." Angstron tests the chemical purity its graphene products in-house. BET measurements help assess and ensure the specific surface area of the manufacturer's graphene products. The x and y average size is measured by a particle analyzer which gives an equivalent x and y dimension. Angstron also uses microscopies including optical, SEM and TEM to check the quality of its samples. The shape of its graphene particles is examined by these methods as well. In addition Angstron periodically sends samples to independent laboratories to ensure its graphene products meet internal standards for chemical purity.
Along with material quality, application requirements primarily dictate graphene choices. "A manufacturer can use a company's technical data sheets to evaluate the properties of nano graphene platelets," explains Burton. For Angstron's powder forms of graphene, technical data shows oxygen content for quality graphene ranging from 0.80 to 2.10 percent. Specific surface area can, depending on the application, be as small as 13 m2/gm or as high as 800 m2/gm. Average x and y dimensions typically fall with the range of 5 µm to 44 while z dimensions (thickness) can average less than 1 nm up to 100 nm. Carbon content for powder graphene should be between 97 and 99 percent. In the case of graphene oxide (liquid form), the average thickness or z dimension for quality material ranges from less than 1 nm up to 1.2 nm with x and y dimensions at most 554 µm. Carbon content for graphene oxide in a water-based solution is approximately 46 percent.
According to Burton, Angstron is rapidly moving towards ISO certification. "As the market grows we want to be strategically positioned to meet customer requirements," he says. "We don't see graphene supplanting traditional reinforcing materials but it will certainly be a tool the polymer industry can rely on to enhance matrix properties and to work in synergy with carbon fibers and fiberglass to boost overall composite properties."
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