Note: Descriptions are shown in the official language in which they were submitted.
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GRAPHITE COMPOSITE STRUCTURES
EXHIBITING ELECTRICAL CONDUCTIVITY
B~CKG~OUND OF T~lE INVENTION
1. Field o~ t~u Inventlon
The subjeot invention relates to electrically
conductive composite materials and, more particularly, to
graphite epoxy composite materials formed into conductive
structures.
It has long been recognized that graphite is at
least a semiconductor of DC and RF energy. However, the
only heretofore practical application of this character-
istic has been the use of chopped fibers in an epoxy
matrix for RFI shielding and as an RF reflector surface,i.e., parabolic reflectors.
2. DescriptiQn o~ ~e~ated~
A main design consideration for almost every
structure, and especially airborne and spaceborne
structures, is weight. q'he designer needs materials
having a certain strength, while at the same time havillg
as little weight as possible. Increasingly, designer~
seek to combine multiple ~unctions in a single
structure. For example, a structure which provides a
necessary antenna configuration, while being at the same
time electrically conductive, provides required mecllani-
cal and electrical functions in a single structure.
~urther requirement is that such structure be as
lightweight as possible.
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~ s an example, in the case of a dipole antenlla
to be carried aboard a spacecraft, the antenna must be as
liglltweight as possible, in one case not exceeding 1.1 I~g
(0.5 lb.). SUCh structures must, for example, be able
towithstand harsh mecllanical vibrations associated witll
satellite launch environments. Such requirements imply a
higll stiffness-to-weigllt ratio. While aluminum and steel
can probably meet strength and electrical conductivity
requirements, they, in many cases, are far too heavy to
meet mission weight limitations.
As another example, aircraft wings may, in
addition to providing the necessary airfoil for lift
purposes, house radio or radar antennas, and wing heaters
for deicing purposes. While typically these three
functions are provlded by different materials which are
interconnected in some manner, the mere use of three
different materials results in a certain weight accumu-
lation. It would be an advance in the art if all three
functions could be provided by a single material, and if
such material were more lightweight than prior tech-
niques. Once again, a high stiffness-to-weight ratio
would be required to meet the stresses placed on an
aircraft wing.
Some investigation into the use oP conducting
plastics has been perPormed. ~lowever, such presently-
lcnown materials su~fer from severe disadvantages, sucll as
poor strength when higllly conductive, or poor conduc-
tivity wllen strength is increased. Many are not stable
under extreme temperature ranges, some degrade relatlvely
rapidly in the presence of water, and most, if not all,
do not possess sufPicient electrical conductivity ~or
many applications.
It would be an advance in the art to provide a
material having a high stiffness-to-weight ratio, having
relatively high electrical conductivity, and having
relatively low weight.
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SUMMARY OF TIIE INVENTION
It is therefore an object of the invention to
improve electrical conduc~ing devices;
It is another object of the invention to provide
a electrical conducting structure of reduced weight;
It is another object of the invention to provide
a material having relatively high electrical conduc-
tivity, relatively low weight, and a relatively high
stifEness-to-weight ratio; and
It is another object of the invention to provide
a strong, lightweight antenna structure for space-based
applications.
~ ccording to the invention, a structure is
formed of a composite material comprising elongated
fibers including graphite and a binder such as epoxy. By
establishing sufficient electrical contact to the fibers,
the structure is rendered a good conductor. To establish
contact, the binder material is removed from about the
fibers in a selected area, leaving the fibers exposed.
Conductive material is then applied so as to malce
electrical contact with the exposed fibers.
In the foregoing procedure, the wrap anyle of
the fibers may be selected to achieve a desired
electrical conductivity. In a preferred embodiment,
nickel plated graphite fibers may advantayeously be
employed with silver being used to make electrical
contact to the exposed niclcel plated graphite flbers in
the selected areas in which the binder material is
removed.
The use of graphite fibers bound with an epoxy
materlal results in a structure exhibiting a high
stiffness-to-weight ratio and much less weight than
aluminum or steel. The resulting conductive structure
may be used to configure an antenna, coaxial transmission
line, or other conducting devices, as hereafter descrihed
in more detail.
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BRIEF DESCRIPrrION OF T11~ DRAWINGS
FIGS. la and lb show perspective views of a
graphite composite materi.al. i.n accordance with the
:invention formed into coaxial conductors according to t11e
5 preferred embodiment;
FIG. 2 is a grapll illus-trating tlle var.iation o~
conductivity over a certain ere~uency band with the wraE)
angle of graphite eibers in a graphite/epoxy compos.i.te ;n
accordance with 1:he invent.ion;
FIG. 3 is a grapl1 illustrating the variation in
conductivity of coaxial cables fabricated according to
the preferred embodiment w:it11 var.i.ous grapl1ite/epoxy
materials;
IiIG. ~ is a schematic view of a s:imple dipol.e
15 employing conductors formed in accordance with a
preferred embodiment; and
FIG. 5 ls a schen)atic diagram of one-hEIlf o~ a
dipole antenna employing i~our cross-dipoles fal~ri.cated
~rom tubular conductors in accordance wit11 the preferre(l
20 embodiment.
DESCRIPIION Ol; ll~E PI~EFERRED EMBOl)IMENrS
Fl:G. la ill.ustrates a coax.i.al conduc:to1- 1..1.
~abricated of yrap1uil:e/epoxy mal:er.i.a:l. In one embod.i.111ent
25 Oe the inventlo11, long, parallel, graplnite fibers are
l~ound ln an epoxy matrix. rllle unldirec-tlol1a1.ly orienl:e ~
fibers in the mal:rix are continuous and contact eacll otller
to form a conductive rnatrix.
Carl~on (graphi.te) J~:Ll~ers are made l~y pyrolysis
Oe organi.c precursor fibers ln an :ia~ert atmosphere.
Pyrolysi.s t.emperatures generally range from lOOOC to
3000UC. Currently three precursor materials, rayon
polyacrylonitrile, and pitCIl (from coal tar products),
35 are the most widely used :raw materials i.n -the manu~actu2e
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of carbon (graphite) fibers. Physical properties such as
Young's Modulus, ultimate strength, elongation to
failure, and electrical conductivity are determined by
processing techniques, i.e., fiber tension and pyrolitic
temperatures. Bundles consisting of 1,000 to 150,000
continuous fibers may be formed of straight (tow) or
twisted (roving) fibers to suit subsequent manufacturing
processes, i.e., unidirectional tape for hand layup or
filament winding, respectively.
While graphite has been found to be a relatively
strong material, it will buckle under compressive loads.
The addition of an epoxy binder contributes strength to
the composite so that compressive loads may be handled
without compressive instability. It has been found that
a composite having 60-65% graphite by weight works well
in the application to be described below in detail.
Graphite fibers may also be nickel plated and
are commercially available in that form, as described
below. Typically, a loose roving or tow is nickel plated
with, e.g., one-halP Angstrom of nickel and then spun
tight. Thereafter, the tightened roving or tow (whether
nic]cel plated or notj may be impregnated with epoxy resin
and placed on a spool or a support backing to ~orm a
tape. The tape or spool i8 Prozen to prevent premature
curing of the epoxy. The tape or spool may be thawed
out, wrapped on a mandrel as hereafter described in more
detail, and then cured at 250 to 350-F in an oven to
establish a desired shape.
In the prePerred embodiment, graphite fibers are
used because of their relatively low weight, their
electrical conductivity properties, and their relatively
high strength. Due to the problem with compressive
loads, they are bound toyether in a nonelectrically
conductive material such as a resin. Epoxy was discussed
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above, but if in a particular application, melting
temperature were not a concern, a thermoplastic binder
may be selected rather than epoxy.
A material which may be used for the yraphite/
epoxy composite according to the preferred embodiment is
nickel plated Hercules AS4 graphite/epoxy material
available from American Cyanamid and having a part number
of 985NCGT3290. This material employs continuous
graphite fibers of 8-micron diameter whicll have been
nickel plated as described above and which are bound
together with an epoxy. The material is in the form of
tape with the graphite fibers oriented longitudinally on
the tape.
The graphite/epoxy composite may be formed into
the outer conductor 12 by wrapping the composite in its
thawed, flexible, room temperature state around a mandrel
and applying suitable pressure to squeeze out air and any
excess epoxy. It has been found that the lowest loss is
achieved when the wrap-angle ~ at which the graphite
~ibers are wound is such that the graphite fibers 14 are
aligned in the direction of radio ~requency current
propagation. The center conductor 15 is ~rom a .325-inch
(.825 cm) coaxial line, and is held in plaae at the
longitudinal centerline o~ the o~lter conduc~or 12 by
means of ~ive dielectric splines.
FIG. la presents a wrap-angle 0 of
approximately 15 degrees, while FIG. lb presents a
wrap-angle e of 0 degrees. ~8 iS seen, wrap angle
in these figures has been measured from the longitudinAl
dimension of the outer condllctor 12. This wrap-angle
effeat phenomenon is illustrated in FIG. 2, which graphs
insertion loss in d~ versus Prequency in M~lz for various
wrap angles of a 28-inch (71.12 cm) length of five
spline, .325-inch (.827 cm), nickel plated AS4 graphite/
epoxy conductor coaxial cable with TNC connectors ancl
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copper inner conductor. In the graph of FIG. 2,
lines lOl, 103, 105, 107 represent wrap angles of
0 degrees, 15 degrees, 30 degrees and 45 degrees,
respectively. These wrap angles are referenced to the
insertion loss of a .325-inch (.827 cm) diameter, five
spline, standard copper coaxial cable represented by
llne los.
FIG. 3 illustrates the conductivity of various
graphite epoxy materials wrapped at an angle ~ of
15-degrees. The materials are nickel plated I~MU,
manufactured by Hercules Aerospace Company, Magna, Utall,
line 111 (estimated): IM6, as manufactured by Hercules,
line 113: T300 as manufactured by Amoco Performance
Products, Inc., Concord, California, line 115; and,
finally, the AS4 material, line 117. The conductivity of
these materials is again referenced against that of a
five spline copper coax, line 119. FIG. 3 indicates
that, the higher the values of Young's Modulus, the
greater the RF conductivity. Numerous manu~acturers
supply graphite fibers. Their desirability as an RF
conductor is therefore expected to be directly
proportional to their Young's Modulus.
While FIG. 3 shows tllat HMU has a much lower
loss than AS4, ~IMU has a much higher Young's Modulus,
thus making it more brittle and more difficult to form
into a desired shape. These tradeoffs should be ta]~en
into consideration in any particular appliaation.
An assembly configured according to the
preferred embodiment has been measured for insertion loss
and VSWR from 20 to 1500 llz and compared with .325-incl
(.~27 cm) splined cable with a copper outer conductor.
The reeults confirmed the hypothesis that graphite fibers
could be utilized as RF radiators over the frequency
spectrum of interest.
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It has been found that it is difficult to plate
silver or copper on bare graphite fibers. Such plating
resulted in a very poor interface bond, which was
unacceptable in the application considered. However, as
noted above, graphite fibers are available which are
plated with nickel (approximately .5 microns thick).
While niclcel is not considered to be the optimum
electrical conductor, it permits strong attachment of
additional plating, which facilitates electrical
connection to structures fabricated according to the
preferred embodiment, as will be discussed below. Nickel
was found to be an acceptable conductor at dc to low
frequencies because the "s]cin depth" of these frequencies
is great enough to prevent adverse results due to
nickel's relatively poor conductive characteristics.
However, care should be exercised at wavelengths on the
order of a millimeter, where "skin depth" is very
shallow. The nickel could then beaome very lossy.
It has been found that the tubular conductor 11
is highly conductive when the surface graphite fibers
conduct the RF energy. To obtain contact with the
surface fibers, the surface epoxy is bead blasted away
from them in a selected area 13 to expose undisturbed,
nickel plated graphite fibers 14. It is sufficiell~ to
expose the first layer of fibers 14.
Electrical contact to the exposed nickel plated
fibers 14 is then established by plating the area 13 with
a conductor such as silver or copper. A conductor is
then connected to the silver, copper or other plating to
electrically join the tube 11 to another conductor, such
as another tube 11 or a Peed cable.
A method for removing the epoxy to permit
plating of the niclcel-plated graphite fibers is
required. Grit blasting using 50 ~m (micrometer)
aluminum oxide grit in a microblaster has proved to be a
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workable approach to remove the epoxy from about the
first layer of niclcel plated graphite fibers. Grit
blasting is easily controlled, was found to not damage
the nickel plating or grapllite fibers 14, and removes
epoxy from between the nicJcel-plated fibers 14, exposing
a large surface area to be subsequently plated.
Other epoxy removal methods proved undesirable.
An acid etch approach wiclced up the fibers 14, resulted
in tube contamination, and greatly reduced the physical
properties of the tube 11. Hand sanding was extremely
difficult to control and resulted in surface fiber
destruction. A plasma etch process proved to be
inadequate as it could not etch enough materlal away -
especially between the surface fibers. High pressure
(70 lb/ln.2 air, 4.93 kg/cm2) bead blasting cleaned
the epoxy away, but also damaged the surface fibe~rs. Low
pressure bead blasting with a small nozzle would probably
be adequate but was not attempted.
Silver plating over the nickel-plated exposed
fibers 14 has resulted in maximum bond strength. Pull
testing at 90 degrees has resulted in peel strengths from
40 to 60 lb/in. (10.72 kg/cm) (three to four times that
required for a printed aircuit board). When failure
occurs, the first layer oE graphite fibers is delaminated
away from the ad~aaent underlying fibers. Excellent
solder characteristics are also obtained. Typical solder
joints have been success~ully temperature cycled ~000
times over the range of ~200-F (93.3'C) without
failure of the solder or the plating bond.
FIG. 4 illustrates a simple dipole antenna
stru¢ture configured from first and second tubes 11 and a
coaxial feed 15. Leads 17, 18 are soldered to silver
plated areas 19, 20 of the tubes 11. ~ coaxial cable may
also be made out of the graphite epoxy material with
losses similar to those shown in FIGS. 2 and 3.
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The tubular conductors 11 of the preferred
embodiment have also been used to design an ultra-
lightweight dipole assembly in the 15- to 75-M~Iz range.
One-half of such a dipole is shown in FIG. 5, the other
half being the mlrror image of the hal~ shown. The
structure of FIG. 5 was designed and constructed as an
electrically flat panel to obtain a bandwidth of at least
one octave (30 to 75 M~lz). The entire antenna (half
dipole shown and its mirror image) is consequently about
18 x 90 inches (45.72 x 228 cm).
The graphite/epoxy tubes 25, 27, 29, 31, 37, 39,
40 of FIG. 5 are held in place by a truss structure 50.
This structure 50 includes truss tubes 41, 43, 47, 49
extending from a central truss fitting 53. The central
truss fitting 53 is mounted on a central truss tube 51
through which each of the dipole elements 27, 29, 37, 39
pass. The outer truss tubes 41, 43 fit together with a
tip tube 45, while the inner truss tubes 47 are ~oined
with the graphite/epoxy tubes 29, 27, 37, 39 at outer
fittings 57. Each upper outer graphite/epoxy tube 25, 31
i8 shown joined to a respective lower tube 37, 39 by
respective elbows 35. The elbows 35 are graphite epoxy
tubes of a slightly wider diameter than the tubes 31, 37:
25, 39 to Paailitate joinder. 'rhe ~oints between the
elbows 35 and tubes 31, 37, 25, 39 employ copper
conductors to electrically join the graphite/epoxy tubes
according to the attachmellt method described ln
connection with FIG. 1. 'l'he tubes 27, 29, 37, 39 are
electrically ~oined to the lower cross tube 40 in the
same manner. The mirror image half dipole (not shown)
may be made pivotable about the tube 40 if desired.
With the antenna structure of FIG. 5, one may
achieve a natural and resonant frequency exceeding 50 llz,
a high Young's Modulus, on the order of 16 x 1016
pounds per square inch (1.11 kg/cm2), and an allowable
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weight budget of .5 pounds (.227 kg) per dipole. This
provides a much lighter but stronger structure than
heretofore available.
Ion vapor deposited aluminum on all radiating
surfaces over unplated fiber graphite tubes was
considered for achieving ~F conductivity for the
structure of FIG. 5. However, the weight added
necessitated tlle use of considerably larger tubes which
resulted in approximate doubling of the allowable weight.
An additional application of the preferred
embodiment is in the fabrication of the leading edge of
an aircraft wing. In such an application a graphite
epoxy dipole is disposed along the leading edge of the
wing within another material such as FiberglassTM or
Ke~vlarTM. In addition to functioning as an antenna,
the graphite epoxy has sufficient resistance to serve as
a deicing element for the wing, and the strength to
withstand the lift forces to which the leading edge of
the wing is subjected. The invention may also be used
for lightning protection of aomposite aircraft.
In conclusion, graphite fibers in an epoxy
matrix perform well as an RF radiator. RF components
such as spacecraft antennas, horns, phased arrays, and
transmission lines are potential applications in addition
to those disaussed herein.
From the foregoing disclosure o~ the preferred
embodiments, various modifications, configurations and
adaptations of the disclosed graphite/epoxy structures
will be apparent to one skilled in the art. Therefore,
it is to be understood tllat, within the scope of the
appended alaim~, the invention may be practiced other
than as specifically described herein.
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