Carbon nanotubes have been incorporated into copper to form ultraconductive copper composites in an effort to reduce the weight of wiring present under the hood of an electric vehicle.
A new approach has produced ultraconductive copper composites that exhibit greater current capacity and better mechanical properties.
The ultraconductive copper composites also are longer in length than what has previously been synthesized.
The commitment made by the global automotive industry to develop and commercialize electric vehicles is leading to new challenges for materials that can increase efficiency without sacrificing performance. One of the challenges faced by having a growing number of electric vehicles on the road is how to handle the added stress placed on the power grid when they all need to be recharged.
In a previous TLT article,
1 research described a new approach for improving the stability of the power grid. The key is to enable localized microgrids operating in islanded mode to act autonomously from the main power grid. This is accomplished through the use of conservation frequency reduction, which permits the microgrid to adaptively adjust the microgrid frequency controller parameters based on the availability of local energy resources and expected reconnection time. The result is that any shortfall in power can be better handled by the microgrid.
Another way to look at this issue is to determine what can be done to improve the ability of an electric vehicle to efficiently utilize power generated by the battery. Copper is the most widely used metal for wiring in electric vehicles because of its low resistivity. But electrical power loss is still encountered with copper wiring.
Dr. Tolga Aytug, senior research staff member at Oak Ridge National Laboratory in Oak Ridge, Tenn., says, “Copper displays the best conductivity of any metal, but some resistivity is still encountered, which leads to a power loss in some applications. The power loss is due to the evolution of heat. From a conductivity standpoint, the next best metal is aluminum, which, while preferred over copper due to its lower weight, displays significantly inferior conductivity.”
A large amount of copper wiring is present under the hood of an electric vehicle, according to Aytug, which adds to the weight of the automobile, reducing efficiency. This problem also exists in modern airplanes where the high weight of copper wiring reduces efficiency.
One strategy for reducing weight is to incorporate carbon nanotubes (CNTs) into the copper matrix. Aytug says, “CNTs are extremely light and exhibit excellent electrical, thermal and mechanical strength properties. Over a decade, there has been work done to incorporate them into copper to form composites. Results have been promising with the development of ultraconductive copper composites (UCCs) that increased conductivity by greater than 30%. Unfortunately, this result could only be achieved with very short sample sizes (micrometer to millimeter).
Aytug believes the problems in preparing UCCs with CNTs are due to low quality CNT, lack of controlled alignment and inhomogeneous distribution. A new approach has now been developed to produce UCCs that display better performance.
Electrospinning
Aytug and his colleagues have developed UCCs that exhibit 14% greater current capacity and up to 20% better mechanical properties compared to copper. The UCCs samples also are longer in length as the researchers produced ones that are 10 centimeters long and 4 centimeters in width.
Aytug says, “We tried to reduce the resistivity as well as increase the current carrying capacity of copper.”
The key step in the process involves deposition of CNTs onto copper tape substrates using a process known as electrospinning. Aytug says, “Electrospinning has generally been used in the production of fibers. In our method, a CNT dispersion was discharged through a metallic needle attached to a syringe pump at a rate of 0.5 milliliters per hour. The dispersion was deposited in the form of fibers to a collector containing the copper tape that is rotating on a drum at a speed of 3,000 rpm under the application of a high potential (15 kilovolts) between the needle and the collector. The purpose of electrospinning is to align the resulting nanofibers into a straight line.”
After electrospinning, a thin copper layer (approximately 200 nanometers thick) is placed on top of the CNT-coated copper tape through a process known as magnetron-sputtering.
Figure 1 shows an image of an UCC.
Figure 1. The composition of an ultraconductive copper composite is shown. Use of this material has the potential to improve the efficiency of electric vehicles. Figure courtesy of Oak Ridge National Laboratory.
Several parameters needed to be adjusted to optimize the formation of the UCCs. Aytug says, “Initially, we decided to use an environmentally friendly strategy in preparing the dispersion in a water/ethanol solution. Unfortunately, the dispersions were not stable, so we moved to a polar organic solvent. Dimethylformamide works well and was less expensive than our second choice, n-methyl pyrrolidone.”
Polyvinylpyrrolidone (PVP) was added to the dispersion to impart viscosity. The researchers used PVP at various concentrations ranging from 2% to 10% but decided on the highest concentration because of the impact on viscosity. Aytug says, “We tried to reduce the concentration of PVP in the CNT dispersion but found that a loss of viscosity led to a spraying action of the CNT dispersion that caused non-uniform and random formation of fibers containing CNTs.”
After electrospinning, the organic chemicals present in the coating were removed by heat treatment.
Aytug says, “In the future, we will be working to increase the length of the UCCs to a meter scale in order to show the commercial viability of the technology. Multilayer UCCs also will be prepared to increase the conductivity, current density and, ultimately, reduce the weight of wiring and improve the efficiency of the applied electrical system (device or component).” All of these steps will make it potentially more attractive to utilize UCCs in applications such as electric vehicles.
Additional information can be found in a recent article
2 or by contacting Aytug at
aytugt@ornl.gov.
REFERENCES
1.
Canter, N. (2020), “Maintaining power grids,” TLT,
76 (7), pp. 12-13.
2.
Li, K., McGuire, M., Lupini, A., Skolrood L., List, F., Ozpineci, B., Ozcan, S. and Aytug, T. (2020), “Copper-Carbon Nanotube Composites Enabled by Electrospinning for Advanced Conductors,”
Applied Nano Materials, 3 (7), pp. 6863-6875