Three dimensional macroscopic assemblages of randomly oriented carbon fibrils and composites containing same

It has now been found that highly advantageous three-dimensional, macroscopic assemblages of randomly oriented carbon fibrils can be prepared which have relatively uniform physical properties along one, preferably two and most desirably three-dimensional axis of the three-dimensional assemblage. Preferred compositions prepared according to the methods of the invention have uniform physical properties along at least one dimensional axis and have relatively isotropic physical properties in at least one plane of the assemblage and most desirably are isotropic throughout the entire three-dimensional structure.

FIELD OF THE INVENTION 
The invention relates generally to assemblages of carbon fibrils. More 
specifically, the invention relates to three dimensional, macroscopic, 
assemblages of randomly oriented carbon fibrils having a bulk density of 
from 0.001 to 0.50 gm/cc and to methods for preparing such assemblages. 
Even more specifically, the invention relates to such assemblages for use 
as catalyst supports, electrodes, chromatographic media, etc. and to 
composite structures comprising the assemblage and a second material 
contained within the assemblage. 
BACKGROUND OF THE INVENTION 
Carbon fibrils are vermicular carbon deposits having diameters less than 
500 nanometers. They exist in a variety of forms, and have been prepared 
through the catalytic decomposition of various carbon-containing gases at 
metal surfaces. 
Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are free of 
a continuous thermal carbon overcoat and have multiple graphitic outer 
layers that are substantially parallel to the fibril axis. As such they 
may be characterized as having their c-axes, the axes which are 
perpendicular to the tangents of the curved layers of graphite, 
substantially perpendicular to their cylindrical axes. They generally have 
diameters no greater than 0.1 micron and length to diameter ratios of at 
least 5. Desirably they are substantially free of a continuous thermal 
carbon overcoat, i.e., pyrolytically deposited carbon resulting from 
thermal cracking of the gas feed used to prepare them. 
Tubular fibrils having graphitic layers that are substantially parallel to 
the microfiber axis and diameters between 3.5 and 75 nanometers, are 
described in Tennent et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986, 
refiled as continuation application Ser. No. 593,319 filed Oct. 1, 1990, 
now U.S. Pat. No. 5,165,909, issued Nov. 24, 1992 ("Novel Carbon Fibrils, 
Method for Producing Same and Compositions Containing Same"), Tennent et 
al., U.S. Ser. No. 871,675 filed Jun. 6, 1986, refiled as continuation 
application Ser. No. 492,365 filed Mar. 9, 1990, now U.S. Pat. No. 
5,171,560, issued Dec. 15, 1992 ("Novel Carbon Fibrils, Method for 
Producing Same and Encapsulated Catalyst"), Snyder et al., U.S. Ser. No. 
149,573 filed Jan. 28, 1988, refiled as continuation application Ser. No. 
494,894, filed Mar. 13, 1990, refiled as continuation application Ser. No. 
694,244, filed May 1, 1991 ("Carbon Fibrils"), Mandeville et al., U.S. 
Ser. No. 285,817 filed Dec. 16, 1988, refiled as continuation application 
Ser. No. 746,065, filed Aug. 12, 1991, refiled as continuation application 
Ser. No. 08/284,855, filed Aug. 2, 1994 ("Fibrils"), and McCarthy et al., 
U.S. Ser. No. 351,967 filed May 15, 1989, refiled as continuation 
application Ser. No. 823,021, refiled as continuation application Ser. No. 
117,873, refiled as continuation application Ser. No. 08/329,774, filed 
Oct. 27, 1994 ("Surface Treatment of Carbon Microfibers"), all of which 
are assigned to the same assignee as the present application and are 
hereby incorporated by reference. One aspect of substantial parallelism is 
that the projection of the graphite layers on the microfiber axis extends 
for a relatively long distance in terms of the external diameter of the 
microfiber, preferably at least two microfiber diameters, more preferably 
at least five diameters, as described in Snyder et al., U.S. Ser. No. 
149,573. 
Fibrils are useful in a variety of applications. For example, they can be 
used as reinforcements in fiber-reinforced composite structures or hybrid 
composite structures (i.e. composites containing reinforcements such as 
continuous fibers in addition to fibrils). The composites may further 
contain fillers such as a carbon black and silica, alone or in combination 
with each other. Examples of reinforceable matrix materials include 
inorganic and organic polymers, ceramics (e.g., lead or copper). When the 
matrix is an organic polymer, it may be a thermoset resin such as epoxy, 
bismaleimide, polyamide, or polyester resin; a thermoplastic resin; or a 
reaction injection molded resin. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a composition of matter which 
comprises carbon fibrils and more specifically an assemblage of randomly 
oriented carbon fibrils which has a low bulk density and which can be used 
as a substrate or medium for various industrial and scientific purposes. 
It is another object of the invention to provide a composition of matter 
comprising a three-dimensional, macroscopic assemblage of a multiplicity 
of randomly oriented carbon fibrils having a low bulk density to which can 
be added one or more functional second materials in the nature of active 
catalysts, electroactive species, etc. so as to form composites having 
novel industrial properties. 
It is yet another object of the invention to provide three-dimensional, 
macroscopic assemblages of a multiplicity of randomly oriented carbon 
fibrils which have isotropic physical properties so that such compositions 
can be used reliably and interchangeably for multiple industrial purposes. 
It is a further object of the invention to provide processes for the 
preparation of such three-dimensional microscopic assemblages of carbon 
fibrils which are efficient and convenient to use in the preparation of 
low-density compositions. 
It is a still further object of the invention to provide improved catalyst 
supports, filter media, chromatographic media, EMI shielding and other 
compositions of industrial value based on three-dimensional assemblages of 
carbon fibrils. 
SUMMARY OF THE INVENTION 
Definitions 
The term "assemblage" refers to any configuration of a mass of individual 
fibrils and embraces intertwined as well as discrete fibril embodiments. 
The term "macroscopic" means that the assemblages may be of any suitable 
size to achieve an industrial or scientific purpose. 
The term "physical property" means an inherent, measurable property of the 
assemblage, e.g. resistivity. 
The term "isotropic" means that all measurements of a physical property 
within a plane or volume of the assemblage, independent of the direction 
of the measurement, are of a constant value. It is understood that 
measurements of such non-solid compositions must be taken on a 
representative sample of the assemblage so that the average value of the 
void spaces is taken into account. 
The term "relatively" means that ninety-five percent of the values of the 
physical property when measured along an axis of, or within a plane of or 
within a volume of the assemblage, as the case may be, will be within plus 
or minus fifty percent of a mean value. 
The term "substantially" means that ninety-five percent of the values of 
the physical property when measured along an axis of, or within a plane of 
or within a volume of the assemblage, as the case may be, will be within 
plus or minus ten percent of a mean value. 
The terms "relatively isotropic" and "substantially isotropic" correspond 
to the ranges of variability in the values of a physical property set 
forth above. 
The Invention 
The invention is broadly in a composition of matter consisting essentially 
of a three-dimensional, macroscopic assemblage of a multiplicity of 
randomly oriented carbon fibrils, said fibrils being substantially 
cylindrical with a substantially constant diameter, having c-axes 
substantially perpendicular to their cylindrical axis, being substantially 
free of pyrolytically deposited carbon and having a diameter between about 
3.5 and 70 nanometers, said assemblage having a bulk density of from 0.001 
to 0.50 gm/cc. 
The assemblages described above can be used to great advantage as 
three-dimensional matrixes for a number of industrial purposes. For 
example, the assemblages can be used as filter media, as catalyst 
supports, as electroactive materials for use, e.g. in electrodes in fuel 
cells and batteries, and as chromatography media. It has been found that 
the assemblages are useful in the formation of composites which comprise 
the assemblage together with either a particulate solid, an electroactive 
component or a catalytically active metal or metal-containing compound, as 
well as in composites with polymers. 
It has now been found that highly advantageous three-dimensional, 
macroscopic assemblages of randomly oriented carbon fibrils can be 
prepared which have relatively uniform physical properties along one, 
preferably two and most desirably three-dimensional axis of the 
three-dimensional assemblage. Preferred compositions prepared according to 
the methods of the invention have uniform physical properties along at 
least one dimensional axis and have relatively isotropic physical 
properties in at least one plane of the assemblage and most desirably are 
isotropic throughout the entire three-dimensional structure. 
These advantageous compositions can be prepared by dispersing fibrils in 
aqueous or organic solid media and then filtering the fibrils. Low density 
compositions are advantageously prepared by forming a gel or paste of 
carbon fibrils in a fluid, e.g. an organic solvent such as propane and 
then heating that gel or paste to above the critical temperature of the 
medium, removing supercritical fluid and finally removing a low-density 
porous mat or plug from the vessel in which the process has been carried 
out.

PRODUCTION OF CARBON FIBRILS 
Fibrils are prepared by contacting a carbon-containing gas with a metal 
catalyst in a reactor for an appropriate period of time, at a suitable 
pressure, and at a temperature sufficient to produce fibrils with the 
above-described morphology. Reaction temperatures are generally 
400.degree.-850.degree. C., more preferably 600.degree.-750.degree. C. 
Fibrils are advantageously prepared continuously by bringing the reactor 
to the reaction temperature, adding metal catalyst particles, and then 
continuously contacting the catalyst with a carbon-containing gas. 
Examples of suitable feed gases, catalysts and reaction conditions are 
given in the several patent applications referenced above as well as in 
Moy et al., U.S. patent applications Ser. Nos. 887,307, filed May 22, 
1992, refiled as continuation application 08/284,742, filed Aug. 2, 1994, 
refiled as continuation application 08/469,430, filed Jun. 6, 1995 and 
887,314 filed May 22, 1992, refiled as continuation application Ser. No. 
07/320,564, filed Oct. 11, 1994, which are hereby incorporated by 
reference. 
Fibrils may be prepared such that at least a portion of the fibrils are in 
the form of aggregates. As used herein, an aggregate is defined as two or 
more entangled fibrils. Fibril aggregates typically have macroscopic 
morphologies, as determined by scanning electron microscopy, in which they 
are randomly entangled with each other to form entangled balls of fibrils 
resembling a bird's nest ("BN"); or as aggregates consisting of bundles of 
straight to slightly bent or kinked carbon fibrils having substantially 
the same relative orientation, and having the appearance of combed yarn 
("CY") e.g., the longitudinal axis of each fibril, despite individual 
bends or kinks, extends in the same direction as that of the surrounding 
fibrils in the bundles; or, as aggregates consisting of straight to 
slightly bent or kinked fibrils which are loosely entangled with each 
other to form an "open net" ("ON") structure. In open net structures the 
degree of fibril entanglement is greater than observed in the combed yarn 
aggregates (in which the individual fibrils have substantially the same 
relative orientation) but less than that of bird's nest. 
In addition to fibrils such as are described in Tennent, U.S. Pat. No. 
4,663,230, fibrils may be prepared having different macromorphologies, 
such as the so-called fishbone ("FB") morphology described in published 
European Patent Application No. 198,558 to J. W. Geus (published Oct. 22, 
1986). Fibrils of the so-called fishbone morphology may be characterized 
as having their c-axes (as defined above) at some angle less than 
perpendicular to the cylindrical axes of the fibrils. The invention 
relates to such fishbone fibrils as well as to those described in Tennent, 
U.S. Pat. No. 4,663,230. 
Carbon Fibrils 
The carbon fibrils preferably comprise a combination of discrete fibrils 
and fibril aggregates. However, the fibrils may all be in the form of 
aggregates. The aggregates, when present, are generally of the bird's 
nest, combed yarn or open net morphologies. The more "entangled" the 
aggregates are, the more processing will be required to achieve a suitable 
composition. This means that the selection of combed yarn or open net 
aggregates is most preferable for the majority of applications. However, 
bird's nest aggregates will generally suffice. 
The Assemblages 
Broadly, the invention is in a composition of matter consisting essentially 
of a three-dimensional, macroscopic assemblage of a multiplicity of 
randomly oriented carbon fibrils, said fibrils being substantially 
cylindrical with a substantially constant diameter, having c-axes 
substantially perpendicular to their cylindrical axis, being substantially 
free of pyrolytically deposited carbon and having a diameter between about 
3.5 and 70 nanometers, said assemblage having a bulk density of from 0,001 
to 0.50 gm/cc. Preferably the assemblage has relatively or substantially 
uniform physical properties along at least one dimensional axis and 
desirably have relatively or substantially uniform physical properties in 
one or more planes within the assemblage, i.e. they have isotropic 
physical properties in that plane. In other embodiments, the entire 
assemblage is relatively or substantially isotropic with respect to one or 
more of its physical properties. 
The physical properties which can be easily measured and by which 
uniformity or isotrophy are determined include resistivity and optical 
density. 
Composites Containing the Assemblages 
Broadly, the fibril assemblages may be used for any purpose for which 
porous media are known to be useful. These include filtration, electrodes, 
catalyst supports, chromatography media, etc. In addition, the assemblages 
are a convenient bulk form of carbon fibrils and may thus be used for any 
known applications including especially EMI shielding, polymer composites, 
active electrodes, etc. 
For some applications like EMI shielding, filtration and current 
collection, unmodified fibril assemblages can be used. For other 
applications, the fibril assemblages are a component of a more complex 
material, i.e. they are part of a composite. Examples of such composites 
are polymer molding compounds, chromatography media, electrodes for fuel 
cells and batteries, fibril supported catalyst and ceramic composites, 
including bioceramics like artificial bone. 
In some of these composites, like molding compound and artificial bone, it 
is desirable that the non-fibril components fill--or substantially 
fill--the porosity of the fibril assemblage. For others, like electrodes, 
catalysts, and chromatography media, their usefulness depends on the 
composite retaining at least some of the porosity of the fibril 
assemblage. 
Methods of Preparing Fibril Assemblages 
While fibrils of any morphology may be used to prepare the assemblages of 
the invention by using the methods of the invention, it is preferred to 
use fibrils having a parallel type morphology such as CC, DD or CY. 
Methods for the preparation of fibrils having these morphologies are 
described in Moy et al., U.S. patent application Ser. Nos. 887,307 and 
887,314 filed May 22, 1992. 
Mats with a thickness between 0.02 and 0.50 millimeters have a density of 
typically 0.20 g/cc corresponding to a pore volume fraction of 0.90. Their 
electrical resistivity in the plane of the mat is typically 0.02 ohm/cm; 
resistivity perpendicular to the mat is typically 1.0 ohm/cm. 
Solid ingredients can be incorporated within the fibril mat by mixing them 
with the fibril dispersion prior to mat formation. The content of other 
solids in the dry mat may be made as high as fifty parts solids per part 
of fibrils. 
Fibrils from the synthesis reactor are dispersed at high shear in a 
high-shear mixer, e.g. a Waring Blender. The dispersion may contain 
broadly from 0.01 to 10% fibrils in water, ethanol, mineral spirits, etc.. 
This procedure adequately opens fibril bundles, i.e. tightly wound 
bundles, of fibrils and disperses fibrils to form self-supporting mats 
after filtration and drying. The application of high shear mixing may take 
up to several hours. Mats prepared by this method are not free of 
aggregates. 
If the high shear procedure is followed by ultrasonication, dispersion is 
improved. Dilution to 0.1% or less aids ultrasonication. Thus, 200 cc of 
0.1% fibrils may be sonified by a Bronson Sonifier Probe (450 watt power 
supply) for 5 minutes or more to further improve the dispersion. 
To achieve the highest degrees of dispersion, i.e. a dispersion which is 
free or virtually free of fibril aggregates, sonication must take place 
either at very low concentration in a compatible liquid, e.g. at 0.001% to 
0.01% concentration in ethanol or at higher concentration e.g. 0.1% in 
water to which a surfactant, e.g. TRITON X-100.RTM. has been added in a 
concentration of about 0.5%. The mat which is subsequently formed may be 
rinsed free or substantially free of surfactant by sequential additions of 
water followed by vacuum filtration. The three-dimensional, macroscopic 
assemblage may be a composite comprising a particulate material selected 
from aluminum oxide, silicon dioxide or silicon carbide. The composite may 
also contain and electroactive component selected from lead, lead 
compounds, manganese or a manganese compound. 
Particulate solids such as MnO.sub.2 (for batteries) and Al.sub.2 O.sub.3 
(for high temperature gaskets) may be added to the fibril dispersion prior 
to mat formation at up to 50 parts added solids per part of fibrils. 
Reinforcing webs and scrims may be incorporated on or in the mats during 
formation. Examples are polypropylene mesh and expanded nickel screen. 
Methods of Improving the Stability of Assemblages 
In order to increase the stability of the fibril assemblages, it is 
possible to deposit polymer at the intersections of the assemblage. This 
may be infiltrating the assemblage with a dilute solution of polymer 
cement and allowing the solvent to evaporate. Capillary forces will 
concentrate the polymer at fibril intersections. It is understood that in 
order to substantially improve the stiffness and integrity of the 
assemblage, only a small fraction of the fibril intersections need be 
cemented. 
EXAMPLES 
The invention is further described in the following examples. 
Example I 
Preparation of a Porous Fibril Mat 
A dilute dispersion of fibrils is used to prepare porous mats or sheets. A 
suspension of fibrils is prepared containing 0.5% fibrils in water using a 
Waring Blender. After subsequent dilution to 0.1%, the fibrils are further 
dispersed with a probe type sonifier. The dispersion is then vacuum 
filtered to form a mat, which is then oven dried. 
The mat has a thickness of about 0.20 mm and a density of about 0.20 gm/cc 
corresponding to a pore volume of 0.90. The electrical resistivity in the 
plane of the mat is about 0.02 ohm/cm. The resistivity in the direction 
perpendicular to the mat is about 1.0 ohm/cm. 
Example II 
Preparation of a Porous Fibril Mat 
A suspension of fibrils is prepared containing 0.5% fibrils in ethanol 
using a Waring Blendor. After subsequent dilution to 0.1%, the fibrils are 
further dispersed with a probe type sonifier. The ethanol is then allowed 
to evaporate and a mat is formed. The mat has the same physical properties 
and characteristics as the mat prepared in Example I. 
Example III 
Preparation of a Low-Density Porous Fibril Plug 
Supercritical fluid removal from a well dispersed-fibril paste is used to 
prepare low density shapes. 50 cc of a 0.5% dispersion in n-pentane is 
charged to a pressure vessel of slightly larger capacity which is equipped 
with a needle valve to enable slow release of pressure. After the vessel 
is heated above the critical temperature of pentane (Tc=196.6.degree.), 
the needle valve is cracked open slightly to bleed the supercritical 
pentane over a period of about an hour. 
The resultant solid plug of Fibrils, which has the shape of the vessel 
interior, has a density of 0.005 g/cc, corresponding to a pore volume 
fraction of 0.997%. The resistivity is isotropic and about 20 ohm/cm. 
Example IV 
Preparation of EMI Shielding 
A fibril paper is prepared according to the procedures of Example I. Table 
I below sets forth the attenuation achieved at several paper thickness. 
TABLE I 
______________________________________ 
FIBRIL PAPER 
EMI SHIELDING 
ATTENUATION 30 MHZ TO 1 GHz 
THICKNESS, INCHES (MM) 
WEIGHT ATTENUATION 
______________________________________ 
0.002 (0.5) 12 G/M.sup.2 
27 Db 
0.005 (.125) 30 37 Db 
0.017 (.425) 120 48 Db 
______________________________________ 
Example V 
A fibril mat prepared by the method of Example I is used as an electrode in 
an electrochemiluminescence cell such as is described in PCT U.S. 85/02153 
(WO 86/02734) and U.S. Pat. Nos. 5,147,806 and 5,068,088. When the voltage 
is pulsed in the presence of ruthenium trisbipyridyl, 
electrochemiluminescence is observed.