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Titles:
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M. Lebron-Colon, M.A. Meador, J.R. Gaier,
and
L.S. McCorkle, “Modified Single-Wall Carbon Nanotubes for Reinforced
Thermoplastic
Polyimide”, SAMPE ’06 (2006)
The use of carbon nanotubes as an additive to improve the
mechanical
properties of polymers and/or enhance their thermal and electrical
conductivity
has been a topic of intense interest. Nanotube-modified polymeric
materials
could find a variety of applications in NASA missions including
large-area
antennas, solar arrays, and solar sails; radiation shielding materials
for
vehicles, habitats, and extravehicular activity suits; and
multifunctional
materials for vehicle structures. However, many of these applications
may
not be realized unless there are reliable methods to disperse nanotubes
into
the polymer matrix. By themselves, carbon nanotubes do not dissolve in
most
solvents, and they tend to agglomerate because of electrostatic
interactions.
Recent work at the NASA Glenn Research Center has focused on the
development
of molecular complexes between carbon nanotubes and large aromatic
hydrocarbons
to enhance the solubility of carbon nanotubes without affecting their
desirable
properties. This work has led to new nanotube complexes that form
colloidal
dispersions in organic solvents. Significantly improvement in
mechanical
and electrical properties of the thermoplastic polyimide film was
obtained
by the addition of carbon nanotube complexes. In this paper the
resultant
mechanical and electrical properties of the polyimide films loaded at
vary
weight percents of nanotube complexes will be discussed.
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Ching-cheh Hung,
Randall L. Vander Wal , Gordon M. Berger , and Lee J. Hall,
"Electrochemical Characterization Of Flame Formed Carbon Nanofibers
And Nanotubes", to be presented at Carbon Conference 2004, 11-16
July 2004, Brown University, Providence, Rhode Island
The flame formed multi-wall carbon nanotubes were grown using four
different catalysts (Ni, Cu, Co, and Fe) on two different substrates
(Ni foil and Stainless steel mesh). The carbon nanotubes were examined
using SEM and TEM. The electrochemical properties of the nanotubes to
store-release lithium were also studied. The objectives of this
research were to understand how the catalysts and substrates affect the
properties of the final carbon nanotube products, and to explore the
possibility of using carbon nanotubes as the anode material in
lithium-ion batteries. Experimental results indicated that the carbon
nanotubes/nanofibers grown from a nickel foil substrate were
graphite-like, but those grown from stainless steel mesh substrate were
hard-carbon-like. Nickel catalysts produced carbon nanofibers that,
when
used as the anode material in lithium batteries, had a higher
reversible capacity and higher irreversible capacity than the nanotubes
produced by other catalysts. All nanocarbon has defects that were
responsible for the high irreversible capacity during the cycles of
lithium insertion-release. The post-treatment of the
nanotunes/nanofibers at1000ºC in nitrogen followed by CVD carbon
coating was found to be best in reducing irreversible capacity and
improving reversible capacity of the carbon nanotube as anode materials
in lithium-ion batteries.
Ching-cheh Hung "High
Temperature Oxidation Of The Graphite Fluoride-Metal Chloride Reaction
Products," to be presented at Carbon Conference 2004, 11-16
July 2004, Brown University, Providence, Rhode Island
Reactions between metal chlorides and graphite fluoride resulted in
carbon containing nanoparticles of metal halides (fluoride and
chloride) compounds. The nanoparticles of metal halides in carbon were
oxidized
when heating these carbon-metal halides composites in air at 200-400°C.
Further oxidation at 400-800°C in air removed all carbon, resulting in
metal oxide having the shape of the original graphite fluoride
precursors.
If the >400°C heating was conducted in an inert environment, the
metal
oxides were reduced by some of the carbon host, result in metal in
carbon.
If more than one kind of metal chlorides were used as the original
reactants,
the products became mixed metal oxides or alloys that had the shape of
the
carbon host. The products fabricated in this study included nickel-iron
alloy (a magnetic material) in carbon, mixed aluminum oxide-iron oxide
fibers,
and zinc oxide fabric.
Gaier, James R., YoderVandenberg,
Yvonne,
Berkebile, Steven, Stueben, Heather and Balagadde, Frederick, “The
Electrical
and Thermal Conductivity of Woven Pristine and Intercalated Graphite
Fiber-Polymer Composites”, Carbon, Vol. 41, No. 12, pp. 2187-2193,
2003
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.
Hung, C., “Carbon Materials Embedded With
Metal Nanoparticles as Anode in Lithium-Ion Batteries,” NASA
TM-211312, 2002.
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
Gaier, J. R., Stueben, H., Berkebile, S., and
Balagadde, F., “Electrical and Thermal Conductivity of Carbon
Fiber-Polymer Composite Plates,” abstracted in Ninth International
Conference on Composite Engineering, (D. Hui Ed.), International
Community for Composites Engineering, pp 217-8, San Diego, 2002.
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.
Hung, C., and Clark, G. W., “Effects of
Surface Oxygen on the Performance of Carbon as an Anode in Lithium-Ion
Batteries,” NASA TM-210700, 2001.
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.
Gaier, J. R., Croy, C., and Studben, H., “High
Temperature Stability of Bromine Intercalated Graphite Fibers,”
Carbon ’01: An International Symposium on Carbon, American Carbon
Society, St. Marys, PA, 2001.
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.
Hung, C. and Prisko, A., “Intercalation of
Lithium in Pitch Based Graphitized Carbon Fibers Chemically Modified by
Fluorine: Soft Carbon With or Without an Oxide Surface,” NASA
TM-1999-209437, 1999.
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.
Harris, G., Lennhoff, J., Nassif, J.,
Vinciguerra, M., Rose, P., Jaworske, D., and Gaier, J., “Lightweight
Highly Conductive Composites for EMI Shielding,” prepared for the
31st International
SAMPE Technical Conference, Chicago, IL, October 1999.
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.
Gaier, J. R., “New Materials for EMI
Shielding,” prepared for the IEEE Workshop on Electromagnetic
Compatibility, Seattle, WA, NASA TM-1999-209054, 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.
Gaier, James R., Hardebeck, Wendie
C.,
Bunch, Jennifer R.Terry, Davidson, Michelle L., Beery, Dwight B., “Effect
of Intercalation in Graphite Epoxy Composites on the Shielding of High
Energy Radiation”, J. Mater. Res., Vol. 13, No. 8, August 1998
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.
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.
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.
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.
Banks, Bruce A. and Lamoreaux, Cynthia, “Performance
and Properties of Atomic Oxygen Protective Coatings for Polymeric
Materials”, Prepared for the 24th International SAMPE Technical
Conference, Toronto, Canada, October 20-22, 1992
Polymeric materials intended for use on spacecraft surfaces in low
Earth orbit need protective coatings to prevent oxidation resulting
from
reaction with environmental atomic oxygen. The effectiveness of
atomic
oxygen protective coatings relies upon the inherent atomic oxygen
durability
of the coating itself, and the number and area of scratch and pin
window
defects in the protective coating. Highly effective coatings
result in
protected polymer oxidation mass losses which are a very small fraction
of that of unprotected materials. Such coatings are required for
high
atomic oxygen fluence missions such as Space Station Freedom.
Typically,
SiOx (where 1.9 < X < 2.0) coatings have been shown to be
effective
atomic oxygen protection. This paper will present the results of
various
protective and/or electrically conductive coatings, including
germanium,
SiOx, and indium tin oxide, which have been exposed to atomic oxygen in
RF
plasma ashers and compares the results with state-of-the-art SiOx
coatings.
Resulting protected polymeric material mass loss, electrical
conductivity,
and optical properties dependence upon atomic oxygen exposure are
presented.
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.
Gaier, James R., Hambourger, Paul, and
Slaby, Melissa E., “Resistivity of Pristine and Intercalated
Graphite Fiber Epoxy Composites”, Carbon, Vol. 29, No. 3, pp.
313-320, 1991
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.
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.
Gaier, James R., Gooden, Clarence E.,
Yashan, Doreen, and Naud, Steve, “Feasibility of Intercalated
Graphite Railgun Armatures”, NASA-TM 102546, Prepared for the 5th
Symposium
on Electromagnetic Launch Technology, sponsored by the Institute of
Electrical and Electronics Engineers, Fort Walton Beach, Florida, April
2-5, 1990
Graphite intercalation compounds may provide an excellent material for
the fabrication of electromagnetic railgun armatures. As a pulse of
power is fed into the armature the intercalate could be excited into
the plasma state around the edges of the armature, while the bulk of
the current would be carried through the graphite block. Such an
armature would have desirable characteristics of both diffuse plasma
armatures and bulk conduction armatures. In addition, the highly
anisotropic nature of these materials could enable the electrical and
thermal conductivity to be tailored to meet the specific requirements
of electromagnetic railgun armatures. Preliminary investigations have
been performed in an attempt to determine the feasibility of using
graphite intercalation compounds as railgun armatures. Issues of
fabrication, resistivity, stability, and electrical current spreading
have been addressed for the
case of highly oriented pyrolytic graphite.
Gaier, James R., and Slabe, Melissa E.,
“Density of Intercalated Graphite Fibers”, Carbon, Vol. 28, No.
5, pp. 669-674, 1990
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.
Gaier, James R., Hambourger, Paul D.,
and Slabe, Melissa E., “Effect of Heat-Treatment Temperature of
Vapor-Grown Graphite Fibers – I. Properties of Their Bromine
Intercalation Compounds”, Synthetic Metals, Vol. 31, pp. 229-240,
1989
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.
Gaier, James R., Slabe, Melissa A., and
Stahl, Mark, “Effect of Heat-Treatment Temperature of Vapor-Grown
Graphite Fibers - II. Stability of Their Bromine Intercalations
Compounds”, Synthetic Metals, Vol. 31, pp. 241-249, 1989
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.
Gaier, James R., “The Effect of Length
and
Diameter on the Resistivity of Bromine Intercalated Graphite Fibers”,
1989
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.
Wessebecher, Dorothy E., Forsman, William
C., and Gaier, James R., “Synthetic and Stability of Br2, IC1 and
IBr Intercalated Pitch-Based Graphite Fibers”, Synthetical Metals,
No. 26, pp. 185-194, 1988
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.
Gaier, James R., Slabe, Melissa E., and
Shaffer, Nanette, “Stability of the Electrical Resistivity of
Bromine, Iodine Monochloride, Copper (II) Chloride, and Nickel (II)
Chloride Intercalated Pitch-Based Graphite Fibers”, Carbon, Vol.
26, No. 3, pp. 381-387, 1988
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.
Gaier, James R., “Technological
Hurdles to the Application of Intercalated Graphite Fibers”,
NASA-TM 101394, Prepared for the Fall Meeting of the Materials Research
Society, Boston, MA, 15 pgs., November 28 – December 2, 1988
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.
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.
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.
Gaier, J. R., Slabe, M. E., “Effects
of Graphitization on the Environmental Stability of Brominated
Pitch-Based Fibers“, NASA-TM-88899, Prepared for the Fall Meeting
of the Materials Research Society, Boston, MA, December 1-5, 1986
The residual bromine graphite intercalation compounds of high modulus
pitch-based fibers (Amoco P-55, P-75, P-100, and P-120) were formed and
their resistances were monitored under a variety of environmental
conditions.
A threshold graphitization was observed below which the bromination
reaction does not occur to an appreciable extent. The graphitization of
the P-55
fibers falls below that threshold, precluding an extensive reaction.
The
P-75, P-100, and P-120 fibers all form bromination compounds which are
stable
at ambient conditions, under vacuum, and under high humidity (100
percent
humidity at 60 C). The thermal stability of the resistivity increased
with
decreasing graphitization, with the stable temperature for P-120 being
100
C; for P-100, 200 C; and for P-75, 250 C. When cost is a consideration,
bromination of pitch-based fibers is an economical way to achieve low
resistivities.
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.
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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