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The Electrical and Thermal Conductivity of Woven Pristine and Intercalated
Graphite Fiber-Polymer Compositess
A series of woven fabric laminar composite plates and narrow strips
were fabricated from a variety of pitch-based pristine and bromine intercalated
graphite fibers in an attempt to determine the influence of the weave
on the electrical and thermal conduction. It was found generally that these
materials can be treated as if they are homogeneous plates. The rule of
mixtures describes the resistivity of the composite fairly well if it is
realized that only the component of the fibers normal to the equipotential
surface will conduct current. When the composite is narrow with respect to
the fiber weave, however, there is a marked angular dependence of the resistance
which was well modeled by assuming that the current follows only along the
fibers (and not across them in a transverse direction), and that the contact
resistance among the fibers in the composite is negligible. The thermal
conductivity of composites made from less conductive fibers more closely
followed the rule of mixtures than that of the high conductivity fibers,
though this is thought to be an artifact of the measurement technique. Electrical
and thermal anisotropy could be induced in a particular region of the structure
by weaving together high and low conductivity fibers in different directions,
though this must be done throughout all of the layers of the structure as
interlaminar conduction precludes having only the top layer carry the anisotropy.
The anisotropy in the thermal conductivity is considerably less than either
that predicted by the rule of mixtures or the electrical resistivity.
![[horizontal rule]](file:///W:/epbranch/images/rule.gif)
Electrical and Thermal Conductivity of Carbon
Fiber-Polymer Composites Plates
Carbon fiber-polymer composite plates were fabricated using 0°-90°
woven fabrics of a variety of pristine and bromine intercalated carbon
fibers. The fibers had electrical resistivities varying from 50 to 1800
µ ohm-cm, and thermal conductivities varying from 8.5 to 520 W/m-K. Anisotropic
composites were also fabricated from fabrics with low conductivity fibers
in the warp direction and high conductivity in the weft. Composite electrical
resistivity was measured using an eddy current technique and a four-point
technique, and calculated using a geometry- corrected rule of mixtures.
Composite thermal conductivity was measured using an optical heating technique
and infrared scanning of the surface as well as being calculated from
the rule of mixtures. Woven fabrics were shown to behave like homogeneous,
isotropic plates both electrically and thermally as long as the samples
are large with respect to the weave size of the fabric. The four-point resistivity
was somewhat higher than that predicted by the rule of mixtures. The resistivity
as measured by the eddy current method was in all cases higher than both
the four-point and rule of mixture resistivities. The thermal conductivities
of the composite were in fairly good agreement with the rule of mixtures
for relatively low conductivity fibers, but much lower than predicted for
high conductivity fibers. Anisotropic composites could only be made by
stacking the anisotropic fabrics in a 0°-0° geometry. Even under those conditions
the anisotropy, especially of the thermal conductivity, was considerably
less than would be expected from the rule of mixtures.
![[horizontal rule]](../images/rule.gif)
Carbon Materials Embedded With Metal Nanoparticles
as Anode in Lithium-Ion Batteries
Carbon materials containing metal nanoparticles that can form an
alloy with lithium were tested for their capacity and cycle life to store
and release lithium electrochemically. Metal nanoparticles may provide
the additional lithium storage capacity as well as additional channels
to conduct lithium in carbon. The cycle life of this carbon-metal composite
can be long because the solid-electrolyte interface (SEI) on the carbon
surface may protect both lithium and the metal particles in the carbon interior.
In addition, the voids in the carbon interior may accommodate the nanoparticle’s
volume change, and such volume change may not cause much internal stress
due to small sizes of the nanoparticles. This concept of improving carbon’s
performance to store and release lithium was demonstrated using experimental
cells of C(Pd)/0.5M LiI-50/50 (vol %) EC and DMC/Li, where C(Pd) was graphitized
carbon fibers containing palladium nanoparticles, EC was ethylene carbonate,
and DMC was dimethyl carbonate. However, such improvement was not observed
if the Pd nanoparticles are replaced by aluminum, possibly because the aluminum
nanoparticles were oxidized in air during storage, resulting in an inert
oxide of aluminum. Further studies are needed to use this concept for practical
applications.
Effects of Surface Oxygen on the Performance of
Carbon as an Anode in Lithium-Ion Batteries
Carbon materials with similar bulk structure but different surface
oxygen were tested electrochemically. Using x-ray photoelectron spectroscopy
(XPS), the chemical state of the surface oxygen was characterized according
to the binding energy of its 1s electron. Three types of surfaces were
found and examined in this research: surface with C=O type oxygen, surface
with C-OH and/or C-O-C type oxygen, and surface with low oxygen content
and high concentration of active sites. A carbon/saturated LiI-50/50 (vol
%) EC and DMC/lithium half cell was used to test each sample. All tests
involve monitoring the voltage differences between the carbon electrode
and the lithium metal reference electrodes during cycles of lithium ion
insertion and release at a constant current of 10 mA/gm of carbon. Their
capacitance and cycle lives in terms of their lithium insertion-release
cycles were then studied. The formation of solid-electrolyte interface (SEI)
and its relation to the surface oxygen were studied based on a detailed
examination of the electrochemical data for the first half cycle of every
sample. The differences among the samples in their history of SEI formation
were then used to explain their differences in their performance as the anodes
in lithium-ion battery. Results suggest that the effects of surface oxygen
on the carbon’s performance in lithium-ion battery depend on the chemical
state of the surface. The SEI resulting from the presence of adsorbed oxygen,
HO-C and/or C-O-C type oxygen, active carbon sites, and C=O type oxygen was
formed when the carbon’s voltage relative to lithium metal was >1.35V,
1 to 1.35V, 0.5 to 1V and 0.67 to 0.7 V, respectively. An optimum amount
of HO-C and/or C-O-C type oxygen and a minimum amount of C=O type oxygen
was found to increase the reversible and decrease the irreversible capacity
of carbon as the anode material. Active sites on the carbon surface, on the
other hand, result in a large irreversible capacity. These active sites also
create a second lithium insertion-release mechanism, but this new mechanism
has a short cycle life.
High Temperature Stability of Bromine Intercalated
Graphite Fibers
P-55, P-75, P-100, and K-1100 pitch-based graphite fibers were intercalated
with bromine and subjected to high temperature in an inert atmosphere in
order to gauge their stability. Thermogravimetric analysis of the fibers
heated to a temperature of at least 960°C showed no mass loss features other
than a small loss of 2-6 percent above 800°C, which was also observed in
pristine fibers. This is presumed to be oxidation due to imperfect purging
of the system. X-ray diffraction patterns of most of the fibers before and
after heating showed no changes, indicating that there were no gross structural
changes after heating. The lone exception was for P-55, which did degrade
according to the diffraction pattern. The resistivity of the fibers slightly
increased on heating, with the more graphitic fibers degrading proportionally
more. This is expected from earlier stability studies in air, but casts
doubt on the diffraction results of the P-55 intercalated fibers. The temperature
dependence of the resistivity, a sensitive indicator of conduction, also
showed little change after the fibers were heated. Thus, bromine intercalated
pitch-based fibers have been shown to be essentially stable to temperatures
at least as high as 960°C in an inert atmosphere.
Brazing of Graphite Fibers to Inconel™ 718
Pitch-based graphite fibers offered a number of potentially attractive
properties, including high thermal conductivity and high solar absorptance.
In many conventional applications, these fibers are embedded in an epoxy
matrix. However, the epoxy is limited to use at temperature below 300°C
and adds little to the thermal conductivity of the end product. To make
use of the high thermal conductivity and high solar absorptance of pitch-based
graphite fibers for solar thermal applications, a research effort was initiated
to develop a technique to attach graphite fibers directly to a high temperature
alloy, Inconel™ 718, for the purpose of providing a good thermally conductive
pathway from the fibers to the Inconel™ 718. Several different vacuum brazing
materials were evaluated. Incusil™-ABA was found to be the brazing material
of choice. The technique chosen to braze pitch-based graphite fiber fabric
to the Inconel™ 718 is discussed. A discussion of future activities is
also presented.
Intercalation of Lithium in Pitch Based Graphitized
Carbon Fibers Chemically Modified by Fluorine: Softer Carbon With or Without
an Oxide Surface
The effects of carbon structure and surface oxygen on the carbon’s
performance as the anode in lithium-ion battery were studied. Two carbon
materials were used for the electrochemical tests: soft carbon made from
defluorination of graphite fluoride, and the carbon precursor from which
the graphite fluoride was made. In this research the precursor was graphitized
carbon fiber P-100. It was first fluorinated to form CF0.68, then defluorinated
slowly at 350-450°C in bromoform, and finally heated in 1000°C nitrogen
before exposed to room temperature air, producing disordered soft carbon
having basic surface oxides. This process caused very little carbon loss.
The electrochemical test involved cycles of lithium intercalation and
deintercalation using C/saturated LiI-50/50 (vol %) EC and DMC/Li half
cell. The cycling test had four major results. (1) The presence of a basic
oxide surface may prevent solvent from entering the carbon structure and
therefore prolong the carbon’s cycle life for lithium intercalation-deintercalation.
(2) The disordered soft carbon can store lithium through two different mechanisms.
One of them is lithium intercalation, which gives the disordered carbon
an electrochemical behavior similar to its more ordered graphitic precursor.
The other is unknown in its chemistry, but is responsible for the high-voltage
portion (>0.3V) of the charge-discharge curve. (3) Under certain conditions,
the disordered carbon can store more lithium than its precursor. (4) These
sample and its precursor can intercalate at 200 mA/g, and deintercalate
at a rate of 2000 mA/g without significant capacity loss.
Lightweight Highly Conductive Composites for EMI
Shielding
Triton Systems, Inc., in cooperation with NASA/Glenn Research Center,
has successfully addressed the problem of shielding electronic devices
in space from EMI (electro-magnetic interference) – with lower weight composite
shields compared to presently used aluminum or tantalum shields. Triton
has developed a new unique low density composite EMI shield using NASA developed
bromine intercalated graphite paired to Triton’s electrically conductive
epoxy matrix. Both 1- and 2-ply composites have been prepared (only 0.36
and 0.72 mm thick) that have been shown to have EMI shielding equal to
that of an aluminum control [for Q-band microwave radiation]. Because present
aluminum EMI shields must be about 2 mm in thickness for strength, and
because the new Triton/NASA shield can be as this as 0.36 mm for equal strength
and shielding, we have developed a potential weight savings of 88% compared
to aluminum. Composites of bromine intercalated graphite in epoxy were developed
by NASA for EMI and have been improved upon by addition of Triton’s unique
conductive epoxy resin for the composite matrix. Triton’s 100% polymeric
conductive epoxy increases shielding effectiveness through enhanced surface
and internal conductivity of the entire composite. Typical 2-ply composites
have provided Q-band EMI shielding greater than 85 dB. Furthermore, lightweight
composites with high strength and stiffness can be made by conventional
composite processing techniques. Data are presented on the EMI shielding
performance of the Triton/NASA composite system.
New Materials for EMI Shielding
Graphite fibers intercalated with bromine or similar mixed halogen
compounds have substantially lower resistivity than their pristine counterparts,
and thus should exhibit higher shielding effectiveness against electromagnetic
interference. The mechanical and thermal properties are nearly unaffected,
and the shielding of high energy x-rays and gamma rays is substantially
increased. Characterization of the resistivity of the composite materials
is subtle, but it is clear that the composite resistivity is substantially
lowered. Shielding effectiveness calculations utilizing a simple rule of
mixtures model yields results that are consistent with available data on
these materials.
New Materials for EMI Shielding
Gaier. James R., “New Materials for EMI Shielding”, NASA TM-209054,
April 1999
Graphite fibers intercalated with bromine or similar mixed halogen
compounds have substantially lower resistivity than their pristine counterparts,
and thus should exhibit higher shielding effectiveness against electromagnetic
interference. The mechanical and thermal properties are nearly unaffected,
and the shielding of high energy x-rays and gamma rays is substantially
increased. Characterization of the resistivity of the composite materials
is subtle, but it is clear that the composite resistivity is substantially
lowered. Shielding effectiveness calculations utilizing a simple rule of
mixtures model yields results that are consistent with available data on
these materials.
Electrical Characterization of Pristine and Intercalated
Graphite Fiber Composites
The high strength and low density of graphite fiber polymer composites
make them attractive materials for many aerospace applications. These
composites also have electrical conductivities which could be exploiting
for many applications such as EMI shielding and electrical ground returns.
Few of these applications have come to fruition, and certainly one of the
contributing problems has been the difficulty in characterizing these materials,
and modeling how current will flow through them. Much of the work that has
been done has been with isotropic filled composites, though many of the
high performance applications utilize laminar composites.
Fabrication and Resistivity of IBr Intercalated
Vapor-Grown Carbon Fiber Composites
Composites using vapor-grown carbon fibers (VGCF), the most conductive
of the carbon fiber types, are attractive for applications where low density,
high strength, and at least moderate conductivity are required, such as
electromagnetic interference shielding covers for spacecraft. The conductivity
can be enhanced another order of magnitude by intercalation of the VGCF.
If a high Z intercalate is used, the protection of components from ionizing
radiation can be enhanced also. Thus, the intercalation of VGCRF with IBr
is reported. Since composite testing is required to verify properties, the
intercalation reaction optimization, stability of the intercalation compound,
scale-up of the intercalation reaction composite fabrication, and resistivity
of the resulting composites is also reported. The optimum conditions for
low resistivity and uniformity for the scaled up reaction (20-30 g of product)
were 114( C for at least 72 hr, yielding a fiber with a resistivity of
8.7 ± 2 ((-cm. The thermal stability of these fibers was poor, with degradation
occurring at temperatures as low as 40( C in air, though they were insensitive
to water vapor. Composite resistivity was 200 ± 30 ((-cm, as measured by
contactless conductivity measurements, about a factor of five higher than
would be expected from a simple rule of mixtures. The addition of 1.0 percent
Br2 intercalated microfibers increased the resistivity of the composites
by more than 20 percent.
Fabrication and Resistivity of IBr Intercalated
Vapor-Grown Carbon Fiber Composites
Gaier, James R., Smith, Jaclyn M., Gahl, Gregory
K., Stevens, Eric C., Gaier, Elizabeth M., “Fabrication and Resistivity
of IBr Intercalated Vapor-Grown Carbon Fiber Composites”, NASA-TM208493,
1998
Composites using vapor-grown carbon fibers (VGCF), the most conductive
of the carbon fiber types, are attractive for applications where low density,
high strength, and at least moderate conductivity are required, such as
electromagnetic interference shielding covers for spacecraft. The conductivity
can be enhanced another order of magnitude by intercalation of the VGCF.
If a high Z intercalate is used, the protection of components from ionizing
radiation can be enhanced also. Thus, the intercalation of VGCF with IBr
is reported. Since composite testing is required to verify properties, the
intercalation reaction optimization, stability of the intercalation compound,
scale-up of the intercalation reaction, composite fabrication, and resistivity
of the resulting composites is also reported. The optimum conditions for
low resistivity and uniformity for the scaled up reaction (20-30 g of product)
were 114 ºC for at least 72 hr, yielding a fiber with a resistivity of
8.7 ±- 2 μΏ-cm. The thermal stability of these fibers was poor, with degradation
occurring at temperatures as low as 40 C in air, though they were insensitive
to water vapor. Composite resistivity was 200 ± 30 μΏ-cm, as measured by
contactless conductivity measurements, about a factor of five higher than
would be expected from a simple rule of mixtures. The addition of 1.0 percent
Br2 intercalated microfibers increased the resistivity of the composites
by more than 20 percent.
Optimization of the Iron III Chloride Interaction
of Graphite Fibers
Intercalated graphite fibers have been proposed for several applications
where high strength, low density, and at least moderately high electrical
conductivity are required. Before these fibers could be utilized, production
methods must be scaled up from laboratory scale to production scale. Some
intercalation reactions, such as those with bromine, appear to be remarkably
insensitive to reaction conditions, but others, such as ferric chloride
(FeCl3) are not so forgiving. FeCl3 intercalated graphite
has been produced under a variety of conditions, utilizing a variety of
host graphites. In this study a response surface methodology (RSM) was utilized
in an attempt to optimize the conditions to make low electrical resistivity
P-100 graphite fibers. The strategy of RSM is to vary the process conditions
in small increments in a statistically guided manner to move the process
from an initial region of operation to a region of optimum operating condition.
A laboratory optimization will guide the scale-up of this reaction.
Effect of Intercalation in Graphite Epoxy Composites
on the Shielding of High Energy Radiation
The mass absorption coefficients of 13.0 keV x-rays, 46.5 keV g -rays
and 1.16 MeV b q particles have been measured for aluminum and for pristine
and intercalated pitch-based graphite fiber composites. Intercalation was
found to increase the mass absorption coefficient for ionizing radiation
form 40 percent of the mass absorption of aluminum to 170 percent for bromine
intercalation and 300 percent for iodine monobromide intercalation. The
mass absorption coefficient for b q particles of both the composites and
aluminum was found to be 17.8±0.9 cm²/g. Inelastic scattering processes
were significant in b q particle shielding, and similar in all of the materials.
Temperature Dependence of the Intercalation of
Bromine into Pitch-based Fibers
Bromine intercalated pitch-based graphite fiber composites have been
proposed as a substitute for aluminum in electromagnetic interference
(EMI) shielding covers for weight critical applications. Because of their
exceptionally high strength and modulus, and their low density, a simple
swap-out of covers could save in excess of 80% of the cover mass. Since
covers comprise about 20% of the power system mass in a typical spacecraft,
this reduction in the power system mass and corresponding increase in payload
mass is significant. Before a bulk use of intercalated graphite can be
initiated, the reaction must be scaled from typical laboratory experiments,
which range in scale from single filament to mg quantities, to kg quantities.
Unusual thermodynamic properties such as the unusual temperature dependence
which bromine intercalation reactions have and the fact that pitch-based
graphite fibers react only to form a single intercalation compound prompted
an investigation into the temperature dependence of the dynamics in this
reaction. The goal was twofold. An understanding of the reaction kinetics
would shed light on the nature of the intercalation process of imperfectly
ordered carbons, and perhaps on the nature of the bonding in bromine graphite
intercalation compounds. Also, the determination of the optimum intercalation
conditions would facilitate efficient mass production of this material.
Effect of Intercalation on the Ionizing Radiation
Shielding of Graphite Fiber Composites
Intercalation not only makes up the deficiency of conventional composites
in shielding components from ionizing radiation, but in the case of IBr,
actually confers an advantage over aluminum. Composites made from IBr intercalated
fibers can be made with one-third the mass of aluminum shields in those
applications where shielding of ionizing radiation is the limiting factor.
The Electrical Resistivity of Woven Graphite
Fiber Fabric Polyisocyanate Resin Composites
The use of carbon-fiber polymer matrix composites as light-weight,
high strength and high stiffness substitutes for metals is becoming increasingly
common. Graphite polymer composites however, have not been as successful
in replacing metallic structures where electrical properties are important.
New high conductivity composites have recently become available which could
change that. One of the most promising composites uses intercalabration
to increase the conductivity of the fibers, and hence the composites. Such
composites have been proposed to replace aluminum EMI shielding covers with
as much as 85 percent weight savings. One of the difficulties in this work
has been the characterization of the resistivity of composites are anisotropic
and not well understood. Non-ideal, continuous filament, woven fabrics have
not been satisfactorily dealt with theoretically, and experimental measurements
made using different techniques have not given consistent results. The object
of this study was to measure the resistivity of both pristine and bromine
intercalated graphite fiber composites using two different techniques in
order to determine which, if either, would be a better indicator of suitability
for electrical applications.
The Frequency Dependance of the Resistivity of
Pristine and Intercalated Graphite Fibers from DC to 10 MHz
The frequency dependence of the resistivity of pristine and bromine
intercalated P-55, P-75, and P-100 fibers was found to be invariant from
5 Hz to 1 MHz, with minor changes in the 1 to 10 MHz range. Skin depth
effects were not expected in even the most conductive fibers until frequencies
of about 30 MHz, and none were unambiguously observed.
Ferric Chloride Intercalation Compounds Prepared
from Graphite Fluoride
The reaction between graphite fluoride and ferric chloride was observed
in the temperature range of 300ºC to 400ºC. The graphite fluorides used
for this reaction have an sp³ electronic structure and are electrical insulators.
They can be made by fluorinating either carbon fibers or powder having
various degrees of graphitization. Reaction is fast and spontaneous and
can occur in the presence of air. The ferric chloride does not have to be
pre-dried. The products have an sp² electronic structure and are electrical
conductors. They contain first stage FeCl3 intercalated graphite.
Some products contain FeCl2•2H2O, others contain FeF3,
in concentrations that depend on the intercalation condition. The graphite
intercalated compounds (GIC) deintercalated slowly in air at room temperature,
but deintercalated quickly and completely at 370ºC. Deintercalation is
accompanied by the disappearing of iron halides and the formation of rust
(hematite) distributed unevenly on the fiber surface. When heated to 400ºC
in pure N2 (99.99 vol%), this new GIC deintercalates without losing its molecular
structure. However, when the compounds are heated to 800ºC in quartz tube,
they lost most of its halogen atoms and formed iron oxides (other than hematite),
distributed evenly in or on the fiber. This iron-oxide-covered fiber may
be useful in making carbon-fiber/ceramic-matrix composites with strong bonding
at the fiber-ceramic interface.
Synthesis and Thermal Stability of Graphite Oxide-like
Graphite oxide is typically made in a process where crystalline graphite
was mixed with H2SO4, NaNO3, and KMnO4
for overnight reaction, then mixed with water for further reaction, and
finally rinsed with methanol. In this report, crystalline graphite was substituted
by submicron graphite powder, pitch-based graphitized carbon fibers, and
activated carbon as the carbon reactants in this process. The reactions
produced graphite oxide-like material. They were amorphous, but contained
oxygen in the concentration range of the traditional graphite oxide. The
weight, chemical composition, and structures of these materials were characterized
before and after they were exposed to high temperature nitrogen. The data
thus obtained were then used to calculate the carbon and oxygen loss during
heating. They began to lose both water and carbon at a temperature below
200° C. During such decomposition, the lower the degree of graphitization,
the higher the contribution of carbon loss to total mass loss. Also, slower
heating when the temperature was lower than 150° C produced nearly no carbon
loss, but 53 percent less oxygen loss. Complete oxygen removal from the
sample, however, has never been observed in this study, in which some samples
were heated to 1000° C. The same method was used to treat 0.254 mm thick
graphite sheet. Instead of graphite oxide-like material, an intercalation
compound was produced. The graphite oxide-like materials obtaining using
activated carbon, crystalline graphite and submicron graphite powder as
precursors all reacted with AlCl3. The highest Al:C atomic ratio
in the products was estimated to be 1:1.7. This implies the possibility
of applications of this process in the area of batteries, catalysts, and
sensors.
Fabrication of Iron-Containing Carbon Materials
from Graphite Fluoride
Carbon materials containing iron alloy, iron metal, iron oxide or
iron halide were fabricated. Typical samples of these metals were estimated
to contain 1 iron atom per 3.5 to 5 carbon atoms. Those carbon materials
containing iron alloy, iron metal and/or Fe3O4 were
magnetic. The kinetics of the fabrication process were studied by exposing
graphite fluoride (CF0.68) to FeCl3 over a 280 to
420° C temperature range. Between 280 and 295° C, FeCl3 quickly
entered the structure of CF0.68 , broke the carbon-fluoride bonds,
and within 10 to 30 minutes, completely converted it to carbon made up of
graphite planes between which particles of crystalline FeF3 and
noncrystalline FeCl2 were located. Longer reaction times (e.g.
28 hours) or higher reaction temperatures (e.g. 420° C) produced materials
containing graphite, a FeCl3-graphite intercalation compound,
FeCl2· 4H2O and FeCl2· 2H2O.
These products were further heat treated to produce iron-containing carbon
materials. When the heating temperature was kept in the range of 750 to 850°
C range, and the oxygen supply was kept at the optimum level, the iron halides
in the carbon structure were converted to iron oxides. Raising the heat
to temperatures higher than 900° C reduced such iron oxides to iron metal.
The kinetics of these reactions were used to suggest processes for fabricating
carbon materials containing iron alloy. Such processes were then tested
experimentally. In one of the successful trial runs, commercially purchased
CF0.7 powder was used as the reactant, and NiO was added during
the final heating to 1200° C as a source of both nickel and oxygen. The product
thus obtained was magnetic and was confirmed to be a nickel-iron alloy in
carbon.
Formation and Chemical Reactivity of Carbon Fibers
Prepared by Defluorination of Graphite Fluoride
Defluorination of graphite fluoride (CFx) by heating to
temperatures of 250 to 450° C in chemically reactive environments was
studied. This is a new and possibly inexpensive process to produce new
carbon-based materials. For example, CF0.68 fibers, made from
P-100 carbon fibers, can be defluorinated in BrH2C-CH=CH-CH2Br
(1,4-dibromo-2-butene) heated to 370° C and then heating to 660° C in
nitrogen (N2). Furthermore, defluorination of the CF0.68
fibers in bromine (Br2) produced fragile, structurally
damaged carbon fibers. Heating these fragile fibers to 1100° C in N2
caused further structural damage, whereas heating to 150° C in bromoform
(CHBr3) and then to 1100° C in N2 healed the structural
defects. The defluorination product of CFx, tentatively called
activated graphite, has the composition and molecular structure of graphite,
but is chemically more reactive. Activated graphite is a scavenger of manganese
(Mn), and can be intercalated with magnesium (Mg). Also, it can easily collect
large amounts of an alloy made from copper (Cu) and type 304 stainless steel
to form a composite. Finally, there are indications that activated graphite
can wet metals or ceramics, thereby forming stronger composites with them
than those the pristine carbon fibers can form.
Kinetic Studies of the Bromine Intercalation
of Pitch-Based Graphite Fibers
In order to study the kinetics of bromine intercalation into graphite
fibers. Thornel P-55, P-75, and P-100 fibers (Amoco) were intercalated
with bromine vapor at temperatures ranging from 0 to 60° C. Additional reactions
were carried out at 20° C at varying bromine partial pressures. It was found
that low temperature favors the intercalation reaction. It was further found,
at least for P-75 and P-100, that the effect is not due to lower vapor pressure,
but is solely a temperature effect. Lower vapor pressure may play a role
in P-55 intercalation. None of the fibers exhibited partial intercalation,
implying that initiation is the rate limiting step in the reaction. A model
was proposed which explains the form of the reaction by assuming that the
deintercalation reaction is independent of the intercalation reaction, and
that their temperature dependence differs.
New Materials for EMI Shielding
Graphite fibers intercalated with bromine or similar mixed halogen
compounds have substantially lower resistivity than their pristine counterparts,
and thus should exhibit higher shielding effectiveness against electromagnetic
interference. The mechanical and thermal properties are nearly unaffected,
and the shielding of high energy x-rays and gamma rays is substantially
increased. Characterization of the resistivity of the composite materials
is subtle, but it is clear that the composite resistivity is substantially
lowered. Shielding effectiveness calculations utilizing a simple rule of
mixtures model yields results that are consistent with available data on these
materials.
Atomic Oxygen Durability of Graphite Epoxy Composite
Silver Mirrors for Space Power Applications
Two light-weight graphite epoxy composite mirrors, each having a
silver reflective layer and a silicon dioxide protection layer, were exposed
to two levels of atomic oxygen fluence in a ground-based plasma asher facility
for the purpose of evaluating their atomic oxygen durability. Total reflectivity
and specular reflectivity were monitored during the course of atomic oxygen
exposure. Optical microscope photographs were also taken during the course
of exposure to document the process of atomic oxygen undercutting at pin
window defect sites. Although there was evidence of atomic oxygen undercutting
at pin window defects sites, functional performance of the mirrors remained
fair over the course of atomic oxygen exposure.
Effect Atomic of Intercalation in Graphite
Epoxy Composites on the Shielding of High Energy Radiation
The half-thickness and mass absorption coefficient of 13.0 keV x-rays,
46.5 keV γ-rays, and 1.16 MeV βө particles have been measured for pristine,
bromine intercalated , and iodine monobromide intercalated pitch-based
graphite fiber composites. Since these materials have been proposed to
replace aluminum structures in spacecraft, the results were compared to
aluminum. Pristine graphite epoxy composites were found to have about
4 times the half-thickness, and 40% of the mass absorption of aluminum
for ionizing radiation. Bromine intercalation improved performance to
90% of the half-thickness, and 1.7 times the mass absorption coefficient
of aluminum. Iodine monobromide extended the performance to 70% of the
half-thickness and 3.0 times the mass absorption of aluminum. Thus, intercalation
not only makes up the deficiency conventional composites have in shielding
components from ionizing radiation, but actually confers advantage in mass
and thickness over aluminum. The βө particle shielding of all the materials
tested was found to be very effective. The shielding of all of the materials
was found to have nearly the same mass absorption coefficient of 17.8 ±
0.9 cm2/g. Inelastic scattering processes were found to be important in
βө particle shielding; however, the extent of inelastic scattering and thus
the distribution of energies of the transmitted electrons did not vary with
material.
Monte Carlo Computational Modeling for Simulation
of Atomic Oxygen Interactions with Composites at Defect Sites in Protective
Coatings
Spacecraft orbiting the earth at altitudes below 500 kilometers are
exposed to the remnants of the earth's upper atmosphere. This low Earth
orbital (LEO) environment consists predominantly of atomic oxygen caused
by photo-dissociation of O2 by ultraviolet radiation from the
sun. Organic matrix carbon fiber composite materials exposed to this environment
are oxidized at a rate which would limit the durability of many spacecraft
components. As a result, atomic oxygen protective coatings consisting of
metals and metal oxides are being used to protect materials from oxidation
degradation in LEO. The use of Monte Carlo computational modeling to simulate
the effects of atomic oxygen undercutting oxidation of composite materials
both in the ground laboratory and in space can greatly assist in improving
the ability to project in-space durability testing. This modeling was used
to test coating materials for performance in LEO.
Leveling Coatings for Reducing Atomic Oxygen
Defect Density in Graphite Fiber-Epoxy Composites
Pinholes or other defect sites in a protective oxide coating provide
pathways for atomic oxygen in low Earth orbit to reach underlying material.
One concept for enhancing the lifetime of materials in low Earth orbit
is to apply a leveling coating to the material prior to the material prior
to applying any reflective and protective coatings. Using a surface-tension-leveling
coating concept, a low-viscosity epoxy was applied to the surface of several
composite coupons. A protective layer of 1000 Å of SiO2 was
deposited on top of the leveling coating, and the coupons were exposed
to an atomic oxygen environment in a plasma asher. Pinhole populations
per unit area were estimated by counting the number of undercut sites observed
by scanning electron microscopy. Defect density values of 180,000 defects/cm²
were reduced to about 1000 defects/cm² as a result of applied leveling coating.
These improvements occur at a mass penalty of about 2.5 mg/cm².
Durability of Intercalated Graphite in Epoxy
Composites in Low Earth Orbit
The electrical conductivity of graphite epoxy composites can be substantially
increased by intercalating (inserting guest atoms or molecules between
the graphene planes) the graphite fibers before composite formation. The
resulting high strength, low density, electrically conducting composites
have been proposed for EMI shielding in spacecraft. Questions have been raised,
however, about their durability in the space environment, especially with
respect to outgassing of the intercalates, which are corrosive species such
as bromine. To answer those concerns, six samples of bromine intercalated
graphite epoxy composites were included in the third Evaluation of Oxygen
Interaction with Materials (EOIM-3) experiment flown on the Space Shuttle
Discovery (STS-46). Changes in electrical conductivity, optical reflectance,
surface texture, and mass loss for SiO2 protected and unprotected
samples were measured after being exposed to the LEO environment for 42 hours.
SiO2 protected samples showed no degradation, verifying conventional
protection strategies are applicable to bromine intercalated composites.
The unprotected samples showed that bromine intercalation does not alter
the degradation of graphite-epoxy composites. No bromine was detected to
have been released by the fibers allaying fears that outgassing could be
disruptive to the sensitive electronics the EMI shield is meant to protect.
Resistivity of Pristine and Intercalated Graphite Fiber Epoxy Composites
Laminar composites have been fabricated from pristine and bromine
intercalated Amoco P-55, P-75, and P-100 graphite fibers and the Hysol-Grafil
EAG101-1 film epoxy. The thickness and rf eddy current resistivity of
several samples were measured at grid points and averaged point by point
to obtain final values. Although the values obtained this way have high
precision (<3% deviation), the resistivity values appear to be 20 to
90% higher than resistivities measured on high aspect ratio samples using
multipoint techniques, and by those predicted by theory/ The temperature
dependence of the resistivity indicates that the fibers are neither damaged
nor deintercalated by the composite fabrication process. The resistivity
of the composites is a function of sample thickness (i.e., resin content).
Composite resistivity is dominated by fiber resistivity, so lowering the
resistivity of the fibers, either through increased graphitization or intercalation,
results in a lower composite resistivity. A modification of the simple
rule of mixtures model appears to predict the conductivity of high aspect
ratio samples measured along a fiber direction, but a directional dependence
appears which is not predicted by the theory. The resistivity of these
materials is clearly more complex than that of homogenous materials.
Prospects for Using Carbon-Carbon Composites
for EMI Shielding
Since pyrolyzed carbon has a higher electrical conductivity than most
polymers, carbon-carbon composites would be expected to have higher electromagnetic
interference (EMI) shielding ability than polymeric resin composites.
A rule of mixtures model of composite conductivity was used to calculate
the effect on EMI shielding of substituting a pyrolyzed carbon matrix for
a polymeric matrix. It was found that the improvements were small, no
more than about 2 percent for the lowest conductivity fibers (ex-rayon)
and less than 0.2 percent for the highest conductivity fibers (vapor grown
carbon fibers). The structure of the rule of mixtures is such that the
matrix conductivity would only be important in those cases where it is
much higher than the fiber conductivity, as in metal matrix composites.
Density of Intercalated Graphite Fibers
The densities of Amoco P-55, P-75, P-100, and P-120 pitch based graphite
fibers and their intercalation compounds with bromine, iodine monochloride,
nickel (II) chloride, and copper (II) chloride have been measured using
a density gradient column. The distribution of densities within a fiber
type is found to be a sensitive indicator of the quality of the intercalation
reaction. In all cases the density was found to increase, indicating that
the mass added to the graphite is dominant over fiber expansion. Density
increases are small (less than 10%) giving credence to a model of the intercalated
graphite fibers with regions that are intercalated and regions that are
not.
Effect of Heat-Treatment Temperature of Vapor-Grown
Graphite Fibers – I. Properties of Their Bromine Intercalation Compounds
Vapor-grown graphite fibers, which have been heat treated to 2000,
2200, 2400, 2600, 2800, and 3000 C, are treated with bromine vapor at room
temperature for two days. The fibers are characterized by X-ray diffraction
(XRD), differential scanning calorimetry (DSC), density and resistivity
measurements. Fibers heat treated at any single temperature exhibit a
wide range of properties. Bromination products of fibers that have been
heat treated to 2600 ºC and above exhibit a DSC peak new near 100 ºC which
is used as a signature of intercalation. The XRD, density and temperature
dependence of the resistivity suggest fibers with regions of pristine graphite
and regions of stage-two intercalation compounds. Fiber diameter is found
to be an important variable, with fibers having a diameter greater than
about 13 μm exhibiting low resistivities (50 μΏ cm or less) independent
of their heat-treatment temperature. The temperature dependence of the
resistivity suggests that 6 μΏ cm is the minimum resistivity of this system
unless more uniform intercalation can be achieved.
Effect of Heat-Treatment Temperature of Vapor-Grown
Graphite Fibers - II. Stability of Their Bromine Intercalations Compounds
Portions of a batch of graphite fibers grown from benzene precursor
are heat treated to 2000, 2200, 2400, 2600, 2800 and 3000 ºC. The fibers
are then subjected to about 165 Torr of bromine vapor for two days and
subsequently allowed to outgas for at least two weeks. Fiber resistivities
are monitored while they are subject to ambient conditions, high vacuum
(10-4 Pa), high humidity (100% at 60 ºC) and high temperature (up to
400 ºC in air). Vapor-grown graphite fibers, when heat treated to
high temperatures and brominated, have resistivities as low as 8 μΏ cm.
After the two-week outgassing, fiber resistivities are invariant at ambient
and vacuum conditions. At high humidities they degrade only minimally over
several weeks. When the fibers are exposed to high temperatures, degradation
occurs at higher temperatures for fibers heat treated to lower temperatures.
The onset of degradation ranges from 200 ºC for fibers heat treated to
2800 ºC and above, to 400 ºC for 2000 ºC heat treatment. A comparison
of these results with similar studies on pitch-based fibers with radial
grapheme-plane orientation reveals that the stability of bromine intercalation
compounds is more dependent upon the bromine-graphite interaction than
on the orientation of the grapheme planes. Unlike the rates of the bromination
and debromination reactions, which are strongly dependent on the grapheme-plane
orientation, the rates of degradation at high temperature in air for fibers
of similar resistivity are comparable, independent of their grapheme-plane
orientation.
The Effect of Length and Diameter on the Resistivity
of Bromine Intercalated Graphite Fibers
The resistivity of bromine intercalated graphite fibers has been shown
to vary with both the diameter and the length of the fibers. This is due
to bromine depletion from the fiber surface. Model calculations assuming
a 1.0 μm bromine depletion zone for P-100, and 3.0 μm for vapor-grown graphite
fibers fit the respective diameter dependence of their resistivities quite
well. Length dependence data imply a bromine depletion zone along the length
of P-100 fibers which is also a few microns, but that of vapor grown fibers
appears to be as large as 300 μm. Despite these values, microfilaments,
which are much smaller than the expected depletion zones, do form residual
bromine intercalation compounds with resistivities about one-half of their
pristine value.
Effect of Length of Chopped Pristine and Intercalated
Graphite Fibers on the Resistivity of Fiber Networks
Samples of Amoco P-100 fibers were chopped to lengths of 3.14, 2.53,
1.90, 1.27, 0.66 mm, or milled for 2 hours. The two-point resistivity
of compacts of these fibers, were measured as a function of pressure from
34 kPa to 143 MPa. Samples of each fiber length were intercalated with
bromine at room temperature and similarly measured. The low pressure resistivity
of the compacts decreased with increasing fiber length. Intercalation lowered
the resistivity of each of the chopped length compacts, but raised the
resistivity of the milled fiber compacts. Bulk resistivity of all samples
decreased with increasing pressure at similar rates. Even though fiber
volumes were as low as 5 percent, all measurements exhibited measurable
resistivity. A greater change with pressure in the resistance was observed
for shorter fibers than for longer, probably an indication of tighter fiber
packing. Intercalation appeared to have no effect on the fiber to fiber
contact resistance.
Synthesis Pristine and Stability of Br2,
IC1 and IBr Intercalated Pitch-Based Graphite Fibers
This work presents a further study of the intercalation of halogens
in pitch-based fiber and the stability of the resultant intercalation
compounds. P-100 fibers were intercalated with purified IBr at 50 ºC to
produce high electrical conductivity graphite intercalation compounds (GIC’s).
After intercalation and subsequent equilibration in ambient atmosphere,
the fibers average a five-fold conductivity enhancement over the pristine
fiber, and after nine weeks in air, the conductivity, ơ, degrades by only
1%. The intercalation of IC1 in P-100 was performed in vacuum-sealed vessels
at 50 ºC, 20º and 0
ºC. Both the lowest equilibrium resistivity and its smallest ambient
gain were observed for fibers reacted at 20 ºC. P-100 brominated at 68
ºC in vacuum-sealed vessels showed no loss in electrical and stability properties
over those reacted at 20 ºC in the presence of air. Energy dispersive spectroscopy
(EDS) results confirm the existence of excess bromine and chlorine in the
iodine interhalide GIC’s, which is predicted by the oxidation mechanism
proposed for this class of intercalation reactions.
Stability of the Electrical Resistivity
of Bromine, Iodine Monochloride, Copper(II) Chloride, and Nickel(II) Chloride
Intercalated Pitch-Based Graphite Fibers
Four different grades of pitch-based graphite fibers (Amoco P-55,
P-75, P-100, and P-120) were intercalated with each of four different
intercalates: bromide (Br2), iodine monochloride (IC1), copper (II) chloride
(CuC12), and nickel (II) chloride (NiCl2). The P-55 fibers did not react
with Br2 or NiCl2, and the P-75 did not react with NiCl2. The stability
of the electrical resistance of the intercalated fibers was monitored over
long periods of time in ambient, high humidity (100% at 60ºC), vacuum (10-6Torr),
and high temperature (up to 400ºC) conditions. It was found that fibers
with lower graphitization form graphite intercalation compounds (GIC’s)
that are more stable than those with higher graphitization (i.e., P-55 (most
stable) > P-75 > P-100 > P-120 (least stable)). Br2 formed the
most stable GIC’s followed in order of decreasing stability by ICI, CuCl2,
and NiC12. Although Br2 GIC’s had the best stability, ICl had the advantages
of forming GIC’s with slightly greater reduction in resistance (by about
10%) than Br2, and the ability to intercalate P-55 fiber. The transition
metal chlorides appear to be seriously susceptible to water vapor and high
temperature.
Technological Hurdles to the Application
of Intercalated Graphite Fibers
Before intercalated graphite fibers can be developed as an effective
power material, there are several technological hurdles which must be
overcome. These include the environmental stability, homogeneity and bulk
properties, connection procedures, and costs. Strides were made within
the last several years in stability and homogeneity of intercalated graphite
fibers. Bulk properties and connection procedures are areas of active research
now. Costs are still prohibitive for all but the most demanding applications.
None of these problems, however, appear to be unsolvable, and their solution
may result in wide spread GOC application. The development of a relatively
simple technology application, such as EMI shielding, would stimulate the
solution of scale-up problems. Once this technology is developed, then more
demanding applications, such as power bus bars, may be possible.
Intercalated Graphite Fiber Composites
as EMI Shields in Aerospace Structures
Gaier, James R., "Intercalated Graphite Fiber Composites as
EMI Shields in Aerospace Structures", IEEE Transactions on Electromagnetic
Compatibility, Vol. 34, No. 3, pp. 351-356, August 1992
The requirements for electromagnetic interference (EMI) shielding in
aerospace structures are more complicated than those for ground structures
because of their weight limitations. As a result, the best EMI shielding
materials must combine low density, high strength, and high elastic modulus
with high shielding ability. EMI shielding characteristics were calculated
for shields formed from pristine and intercalated graphite fiber/epoxy
composites and compared to preliminary experimental results for these materials
and to the characteristics of shields made from aluminum. Calculations
indicate that effective EMI shields could be fabricated from intercalated
graphite composites which would have less than 12% of the mass of conventional
aluminum shields, based on mechanical properties and shielding characteristics
alone.
Properties of Hybrid CVD / Pan Graphite
Fibers and their Bromine Intercalation Compounds
Gaier, James R., Lake, Max L., Moinuddin, Alia and Marabito, Mark,
“Properties of Hybrid CVD / Pan Graphite Fibers and their Bromine Intercalation
Compounds”, Carbon, Vol. 30, No. 3, pp. 345-349, 1992
A hybrid fiber with a PAN core surrounded by a vapor-grown carbon-fiber
(VGCF) sheath has been fabricated using a proprietary process. The density,
ultimate tensile-strength, Young’s modulus, and resistivity of pristine
and bromine-intercalated fibers made by this technique, having diameters
varying from 5 to 50 μm, were compared with the values predicted from the
rule-of-mixtures model. For both the pristine and intercalated fibers,
the density, ultimate tensile strength, and Young’s modulus of the fibers
were lower than predicted, but resistivity was measured to be consistent
with predictions. Intercalation had little, if any, effect on ultimate
tensile strength and Young’s modulus, but raised the density by about 11
percent, and lowered resistivity by an order of magnitude. The diameter
dependence of the resistivity showed evidence of a depletion layer of the
type found in VGCF.
Effect of Lightening Strike on Bromine
Intercalated Graphite Fiber/Epoxy Composites
Gaier, James R., Slabe, Melissa E., Brink, Norman O., “Effect
of Lightening Strike on Bromine Intercalated Graphite Fiber/Epoxy Composites”,
NASA-TM-104507, August 1991
Laminar composites were fabricated from pristine and bromine intercalated
pitch based graphite fibers. It was found that laminar composites could
be fabricated using either pristine or intercalated graphite fibers using
standard fabrication techniques. The intercalated graphite fiber composites
had electrical properties which were markedly improved over both the corresponding
pitch based and polyacrylonitrile (PAN) based composites. Despite composites
resistivities more than an order magnitude lower for pitch based fiber composites,
the lightning strike resistance was poorer than that of the Pan based fiber
composites. This leads to the conclusion that the mechanical properties
of the pitch fibers are more important than electrical or thermal properties
in determining the lightning resistance. Based on indicated lightning strike
tolerance for high elongation to failure materials, the use of vapor grown,
rather than pitch based graphite fibers appears promising.
Durability of Intercalated Graphite Epoxy
Composites in Low Earth Orbit
Gaier, James R., Davidson, Michelle L., Shively, Rhonda, “Durability
of Intercalated Graphite Epoxy Composites in Low Earth Orbit”, NASA-TM-107157,
February 1996
The electrical conductivity of graphite epoxy composites can be substantially
increased by intercalating (inserting guest atoms or molecules between
the graphene planes) the graphite fibers before composite formation. The
resulting high strength, low density, electrically conducting composites
have been proposed for EMI shielding in spacecraft. Questions have been raised,
however, about their durability in the space environment, especially with
respect to outgassing of the intercalates, which are corrosive species such
as bromine. To answer those concerns, six samples of bromine intercalated
graphite epoxy composites were included in the third Evaluation of Oxygen
Interaction with Materials (EOIM-3) experiment flown on the Space Shuttle
Discovery (STS-46). Changes in electrical conductivity, optical reflectance,
surface texture, and mass loss for SiO2 protected and unprotected samples
were measured after being exposed to the LEO environment for 42 hours. SiO2
protected samples showed no degradation, verifying conventional protection
strategies are applicable to bromine intercalated composites. The unprotected
samples showed that bromine intercalation does not alter the degradation
of graphite-epoxy composites. No bromine was detected to have been released
by the fibers allaying fears that outgassing could be disruptive to the
sensitive electronics the EMI shield is meant to protect.
A Comparison of the Bromination Dynamics
of Various Carbon and Graphite Fibers
Gaier, James R. “A Comparison of the Bromination Dynamics of
Various Carbon and Graphite Fibers”, Synthetic Metals, 22, pp. 15-22, 1987
The electrical resistance of four grades of pitch-based graphite fibers
(Amoco P-55, P-75, P-100 and P-120), and three experimental organic vapor-derived
fibers (General Motors, GA Technologies and University of Nebraska) was
recorded in situ during bromination and subsequent exposure to ambient laboratory
air. The results of this study indicate that the least graphitic pitch-based
fiber (P-55) does not brominate to any significant extent, and that bromination
and debrominaton reactions proceed much slower for vapor-derived fibers
than for pitch-based ones. While this decreased reaction rate may be due
in part to the large diameter of the favor—derived fibers, the majority
of the effect can probably be attributed to the differences in graphene
plane orientation between the fiber types. Although the reactions are slower
in the vapor-derived than in the pitch-based fibers, the extent of reaction
as measured by the change in electrical resistance is essentially the same,
with comparable (or larger) decreases in resistivity. In both the vapor-derived
and pitch-based fibers, bromination reaction proceeds with one or more
plateaux in the resistant versus time curves, which suggests staging and
strengthens the agument that these fibers produce true intercalation compounds.
Effects of Milling Brominated P-100 Graphite
Fibers
Gaier, James R., Dillehay, Michael E., Hambourger, Paul D.,
“Effects of Milling Brominated P-100 Graphite Fibers”, Journal of Materials
Research, Vol. 2, No. 2, pp. 195-200, March/Apr 1987
Preliminary procedures have been developed for the ball milling of
pristine and brominated P-100 graphite fibers. Because of the lubricative
properties of graphite, large ball loads (50 percent by volume) are required.
Use of 2-propanol as a milling medium enhances the efficiency of the process.
The fibers, when allowed to settle from the milling medium, tend to be preferentially
aligned with rather few fibers standing up. Milled, brominated P-100 fibers
have resistivities that are indistinguishable from their pristine counterparts,
apparently because of loss of bromine. This suggests that bromine would
not be the intercalate of choice in applications where milled fibers of
this type are required. It was found that brominated graphite fibers are
stable in a wide variety of organic solvents.
The Milling of Pristine and Brominated P-100
Graphite Fibers
Dillehay, Michael E. and Gaier, James R., “The Milling of Pristine
and Brominated P-100 Graphite Fibers”, NASA-TM-88828, September 1, 1986
Techniques were developed for the ball milling of pristine and brominated
P-100 graphite fibers. Because of the lubricative properties of graphite,
large ball loads (50 percent by volume) were required. Use of 2-propanol
as a milling medium enhanced the efficiency of the process. Milled brominated
P-100 fibers had resistivities which were indistinguishable from milled
pristine P-100 fibers. Apparent loss of bromine from the brominated fibers
suggests that bromine would not be the intercalate of choice in applications
where milled fibers of this type are required. Other intercalates which
do not degas may be more appropriate for a milled fiber application. These
same results, however, do provide evidence that bromine molecules leave the
fiber surface when removed from overpressure of bromine. While exploring possible
solvent media for milling purposes, it was found that brominated fibers are
stable in a wide variety of organic solvents.
Homogeneity of Pristine and Bromine Intercalated
Graphite Fibers
Gaier, J. R. and Marino, D., “Homogeneity of Pristine and Bromine Intercalated
Graphite Fibers”, NASA TM-87016, Prepared for the 17th Biennial Conference
on Carbon, sponsored by the American Carbon Society, Lexington, KY, June
16-21, 1985
Wide variations in the resistivity of intercalated graphite fibers
and to use these materials for electrical applications, their bulk properties
must be established. The homogeneity of the diameter, the resistivity,
and the mass density of 50 graphite fibers, before and after bromine intercalation
was measured. Upon intercalation the diameter was found to expand by about
5%, the resistivity to decrease by a factor of five, and the density to
increase by about 6%. Each individual fiber was found to have uniform diameter
and resistivity over macroscopic regions for lengths as long as 7 cm. The
ratio of pristine to intercalated resistivity increases as the pristine
fiber diameter increases at a rate of 0.16 micron, but decreases with the
increasing ratio of intercalated diameter to pristine diameter at a rate
of 0.08.
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