Multifunctional Materials - Biomimetics: Biologically Inspired Technologies - page 314

designs into materials, one needs a periodic material that can accommodate the arrangement of the
wire elements. Fiber-reinforced polymer composites facilitate such arrangements due to the natural
periodicity of their fiber and laminate construction. The arrangement of fibers within each layer
provides flexibility in orientation, spacing, and geometry of the conducting wire elements. Each
layer may contain elements with orientation in only one direction, as in a uni-directional laminate,
or the elements may be woven such that each layer has bi-directional elements. Variation of the
spacing of these elements in the thickness ( z ) dimension of the material is controlled by the
sequence in which laminates are stacked to form the laminate.
As an example, we have introduced arrays of thin, straight wires into various types of composite
materials. Composite panels were made by hand-layup of preimpregnated woven fabric (prepreg).
The samples varied in the type of host material, wire diameter, and number of electromagnetic
layers. Host materials included E-glass fibers impregnated with epoxy resin, Spectra 1 (Honeywell
UHMW polyethylene) fibers impregnated with vinyl ester resin, and quartz fibers impregnated with
cyanate ester resin, chosen for their mechanical attributes and favorable dielectric characteristics.
The dielectric constant of epoxy/E-glass was 4.44 at microwave frequencies with a loss tangent of
0.01, and that of vinyl ester/Spectra was 2.45 with a loss tangent of 0.002. Cyanate ester/quartz
provided the best overall electromagnetic characteristics with a dielectric constant of 3.01 and a
loss tangent of 0.001, where a low dielectric constant and loss tangent are preferable for optimal
microwave transmission. The fiber volume fraction for each material was about 60%. The fre-
quency at which the panels behave as plasma depends upon the dimensions of the embedded wire
array. Numerical simulations were performed to predict the necessary array for plasma response in
the microwave regime. In making each panel, copper wire of 75 or 50 mm diameter was strung
across a frame to form the desired pattern and was subsequently encased in layers of prepreg. Panels
were processed at elevated temperature and pressure to cure the resin and form the solid composite
as shown in Figure 12.3. Electromagnetic characterization was performed to extract the effective
material properties through measurements in an anechoic chamber that we developed in the Physics
Department of UCSD. Additional characterizations have been performed on a focused beam
electromagnetic system in the first author's laboratories, CEAM (Center of Excellence for Ad-
vanced Materials), as is discussed in connection with Figure 12.13 later on.
Representative dispersion relations of the dielectric constant in the microwave regime for each
of these panels are given in Figure 12.4, comparing analytical and numerical predictions with the
experimental results. The graphs in this figure show the characteristic trend of changing the
dielectric constant from negative to positive values as a result of the plasmon media in a composite
panel of each type. Results for the different host materials show similar behavior, though the turn-
on frequency is shifted depending on the dielectric constant of the host material and the wire
diameter and spacing. Moreover, the results show that a host material with a lower dielectric
constant provides a wider bandwidth over which the dielectric constant of the free space can be
matched (Plaisted et al., 2003b).

12.2.1.2 Coiled Wire Plasmon Media Composites

As an alternative to processing thin wires into composites, we may incorporate thicker, more
robust wires in the form of coiled arrays. By proper design of the coil geometry, various degrees
of inductance may be achieved with thicker wires as compared with the thin straight wires. Textile
braiding of reinforcing fibers with wire is an ideal method to integrate the coil geometry into
the composite. The braiding process interlaces two or more yarns to form a unified structure.
Our process uses a two-dimensional tubular braiding machine, as shown in Figure 12.5, which
operates in a maypole action, whereby half of the yarn carriers rotate in a clockwise direction,
weaving in and out of the remaining counter-rotating carriers. This action results in a two-under
two-over braid pattern. Each yarn makes a helical path around the axis of the braid to create a
uniform coil. To integrate the wire coil into such a structure, we simply replace one of the fiber