Nanostrands are an excellent conductive additive for polymer based systems. This is also true for fiber reinforced composites, but with an added consideration. Fiber reinforced composites are generally designed for mechanical performance, after which 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. These secondary topical treatments add time and cost to finished parts. The design rules for determining mechanical properties of composites are well understood. For instance, if one understands the modulus, strength, and strain 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 designed. This analogy can be extended to the 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 enable the ability to control the electrical conductivity of the matrix phase, while the use of nickel coated fibers of engineered coating thicknesses can be used to control the conductivity of the fiber phase of the composite. By making a composite with both of these materials, the electrical properties of the composite can be targeted to a wide variety of applications.
In the bar graph above, the
"specific conductivity" of four composites systems is
compared. Specific conductivity is defined as the actual
conductivity (in Siemens/cm) divided by the density of
the material. 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. Also keep
in mind that this reading only takes surface behaviors
into account, and advantageous volume properties are also
exhibited by these conductive composite systems.
The short bar at
the left is that of a normal carbon fiber/epoxy
composite. The 0.01 S/cm/gm/cc value 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
a nickel coated carbon fiber composite with standard
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 nature of a composite is
that there are distinct phases, specifically fiber phases
and matrix phases, where the matrix phases exhibit very
poor electrical conductivity. 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.
The best conductivity results are achieved by using both
conductive fibers and nanostrands, as shown by the bar on
the right. The conductivity of these composites is 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.
