[Fracture surface of AA6061 covetic tensile specimen; dark spots show regions of nanocarbon particles. Image courtesy Prof. Lourdes Salamanca-Riba, U. Maryland]

Covetic Nanomaterials

Covetic nanomaterials are metals that have been infused with very small particles of carbon using a unique electrical process. In general, these materials can conduct heat and electricity more efficiently than conventional metal alloys. Covetic nanomaterials are commercially important because the process is scalable to tonnage quantities with widespread implications for energy savings in thousands of potential applications such as high voltage electrical wires, high performance heat exchangers, and lightweight motors. The process can be performed on different kinds of metals, such as aluminum, copper, silver, and gold.

Derivation of the term "covetic"

The term covetic is derived from the words "covalent" and "metallic"--two different kinds of chemical bonds, with the thought that the bonding between the nanocarbon and the metal might be some type of hybrid carbon-carbon bond (covalent) and metal-metal bond (metallic). Although covetics display unusually strong bonding between the carbon particles and their metallic matrix, the exact nature of the attraction is still unknown.

Unusual characteristics

With covetic processing it is possible to add unusually high amounts of stable nanocarbon to metals--above 6 wt.%, which is well beyond the limit for thermodynamic stability in conventional phase diagrams. The crystal structure of the carbon is similar to that of single wall carbon nanotubes and in some cases amorphous carbon (from the EELS and Raman spectra in Figure 6, below) but the particles are globular or ribbon-like in shape (see SEM image of 3% nanocarbon AA6061 tensile fracture surface, above, and Figure 3 below). The particles are a second phase in these metal matrix nanocomposites--not precipitates. They are highly stable in the melt and do not float out or agglomerate, although there may be regions in which they are more concentrated. The nanocarbon is highly resistant to oxidation in the melt in the presence of air--it cannot be measured using Combustion Infrared Detection (ASTM E1019). Covetics can be remelted without significant degradation in nanocarbon content, or agglomeration of the nanocarbon. Depending on processing--and particularly in the case of alloys (as opposed to pure metals)--covetic nanomaterials can exhibit increased electrical conductivity, increased thermal conductivity, and improved resistance to softening and recrystallization at elevated temperatures. The nanocarbon does not significantly reduce the density of the composite compared to that of the base metal.

Background

In 1998, inventor Roger Scherer was experimenting with methods to add polymers into metals, and discovered that a reaction occurred when a direct current was applied to an aluminum melt into which polymers were mixed. The resulting materials displayed unusual behavior such as a large temperature drop during the reaction process, unusual solidification patterns such as significantly increased porosity, and increased thermal conductivity. Some years later, Third Millennium Metals, LLC (Dayton, Ohio) was formed to commercialize the process. They learned that it was possible to create a dispersion of significant quantities (greater than 6 wt.%) of nanoscale carbon particles in a variety of metallic systems: Al, Cu, Au, Ag, Zn, Sn, Pb and Fe. In the current process, micron-scale activated carbon powder is mixed into the melt under forced convection (via an impeller). Electrodes are inserted into the melt, with direct current applied between electrodes, and the carbon particles are converted to nanoscale particles through a mechanism that is still not understood. The nanocarbon particles are tenaciously bound to the metal, increasing the elevated temperature strength, electrical conductivity, and thermal conductivity. This represents a significant advance in both nanomanufacturing processes and in nanomaterials because it provides a pathway to bulk production of nanomaterials with tonnage scale quantities and competitive production costs. The process is able to produce structures that seem to be thermodynamically unavailable via conventional processing methods but remain stable once established. With their equipment located in Dayton, OH, Third Millennium now has the capability to produce 100 pound heats of aluminum (300 lb. heats of Cu), providing an estimated annual production capacity of about 30,000 pounds. US Patent application # 20100327233 was published in December 2010, describing a method to produce covetic copper using electrical current to convert carbon particles to their nanoscale form in the melt. A patent application for gold, silver, tin, lead, and zinc was published in January 2012 (US20120009110), and for aluminum in September 2012 (US20120244033).

Fine structure

The nanocarbon form, distribution, and carbon/metal interface is still under investigation to resolve the sometimes contradictory results that researchers have seen in their characterization work. Specifically, on a fracture surface or metallographically polished sample the carbon appears as nanoscale particles, 5-200 nm diameter (Figure 2). But in the TEM (Figure 3) we see ribbons of nanocarbon and no carbon nanoparticles like those in Figure 2. There is evidence for planes or rows of carbon atoms in between planes of metal atoms (Figure 4). This would explain why density is not significantly reduced, but does not explain why the carbon atoms do not push the metal atoms apart: XAS spectrum (Figure 5) shows no evidence of carbon changing the first shell distance. Finally, there are no structures that look like carbon nanotubes, yet both EELS and Raman spectra indicate that the structure of the carbon most resembles that of single walled carbon nanotubes (Figure 6). [2] Two Raman spectra at different wavelengths show a signal characteristic of the G-band at 1,600 cm-1 and a D-band at 1,300 cm-1. The G-band is characteristic of sp2 bonding and is, therefore, observed in graphite and carbon nanotubes. The D-band corresponds to a breathing mode of A1g symmetry which is forbidden in perfect graphite. The D-band is associated with defects in graphite and is frequently observed in carbon nanotubes.

References

  1. Salamanca-Riba, et al., "A New Type of Carbon Nanostructure Formed Within a Metal-Matrix," Nanotech 2012, Santa Clara, CA, 18 June 2012, CRC Press.
  2. Forrest, et al., "Novel Metal-Matrix Composites with Integrally-Bound Nanoscale Carbon," Nanotech 2012, Santa Clara, CA, 18 June 2012, CRC Press.

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Last Updated 28 August 2013
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