Nanostrands are an excellent conductive additive for polymer based systems. This is also true for fiber reinforced composites, but with an added twist.
Fiber reinforced composites are generally designed for mechanical performance, after which parasitic modifications are often made to achieve desired electrical properties. Such modifications are usually a conductive surface treatment, such as a metal mesh or conductive coating.
The design rules for determining the mechanical properties of composites are well understood. For instance, a simple explanation is that if one understands the modulus and density of both the fiber and of the matrix, and understands the interface between these two phases, then the basic mechanical and physical properties of the composite can be predicted.
This analogy can be extended to the world of electrical properties of a composite. That is, if the electrical conductivity of the fiber and the electrical properties of the resin can be controlled, then the conductivity of the composite and its component phases can also be controlled.
Nanostrands open up the opportunity to control the electrical conductivity of the matrix phase, while the use of nickel coated fibers of controlled coating thickness can be used to control the conductivity of the fiber phase of the composite. By making a composite with both of these materials, then the electrical properties of the composite can be targeted to a wide variety of applications.
In the bar graph above, the "specific conductivity" of composites is compared. The specific conductivity is defined as the actual conductivity (in Siemens/cm) divided by the density of the material. The normalization for density is considered because, although nickel is an excellent conductive member of these systems, it is also very dense. Thus the normalization to specific conductivity takes into consideration the benefit of the improved conductivity as a ratio to the added weight to the composite.
The short bar at the left is that of a normal carbon fiber/epoxy composite. The .01 S/cm/gm/cc is typical of such composites. The second bar from the left is the same type of composite, but 5% nanostrands have been added to the resin. Notice that this slight modification improves the conductivity by an order of magnitude.
The third bar is the conductivity level of a nickel coated carbon fiber composite with neat resin. Notice that this is many times more conductive than either of the carbon fiber composites. One might think that this fiber would make a sufficiently conductive composite. But the real nature of a composite is that there are many regions of the composite, particularly in interlaminar regions, where there are regions poor in fiber and rich in resin. These resin rich areas are of particular concern when considering the electrical properties of an overall composite. Thus the role of the nanostrands is to increase the electrical conductivity of the resin phase of the composite, and to lower the dielectric contrast between the phases of the composite. Thus the best conductivity is achieved by using both conductive fibers and nanostrands, as shown by the bar on the right. The conductivity of these composites are in the range of 0.1 to .01 ohm-cm, depending on the amount of nanostrands in the resin and the amount of nickel on the fiber.
The micrograph below is a 5000 x SEM cross section of nickel nanostrands dispersed between nickel coated carbon fibers. The fibers are 7 micron in diameter. The coating is about 0.2 microns thick, while the nanostrands are about 0.5 microns in diameter.
