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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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