Note: Descriptions are shown in the official language in which they were submitted.
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D-4263 C-3250
METHOD FOR GROWING GRAPHITE FIBERS
Background of the Invention
This invention relates to the manufacture
of graphite fibers that are suitable for use as a
filler in plastic or other composites. Said
graphite fibers are preferably 5 to 15 microns in
diameter and up to several centimeters in length.
It is an object of this invention to
provide a new method for manufacturing graphite
fibers by the pyrolysis of a methane-containing gas,
which fibers are suitable for filler in plastic
or other composites.
More particularly, it is an object of
this invention to provide a relatively inexpensive
method for producing a high yield of graphite fibers
from commercial natural gas, which method comprises
thermally decomposing methane from the gas in contact
with a thin, hydrogen-permeable iron-chromium alloy
wall, while remotely contacting the wall with wet
hydrogen gas so as to create conditions at the
natural gas-wall interface that cause the decomposing
methane to form the fibers~
Summary of the Invention
-
In accordance with a preferred embodiment,
graphite fibers are grown within a thin wall stain-
less steel tube by flowing natural gas through the
tube, while surrounding it with wet hydrogen gas,
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and hea-ting the gases and tube between 925C and
1075C. Under these conditions, the natural gas
readily decomposes within the tube and grows thin~
straight graphite fibers that protrude obliquely from
the inner tube surface in a downstream direction.
The fibers are characterized by a cross-section
resembling a rolled-up scroll. Pyrolysis is
continued for a sufficient time to form fiber
diameters between 5 to 100 microns, preferably
between 5 to 15 microns, which fibers may be up to
several centimeters long. The product fibers are
well suited for plastic filler and are relatively
inexpensive, in part because they are derived from
relatively inexpensive natural gas.
Description of the Drawings
The only Figure is a cross-sectional
schematic view of an apparatus for growing fibers
in accordance with this invention.
Detailed Description of the Invention
Referring now to the Figure, in the
preferred embodiment, graphite fibers were grown
within a cylindrical steel -tube 10. Tube 10 was
preferably composed of type 304 stainless steel
containing about 20% chromium. The outer diameter
was 1.25 centimeters and -the wall thickness was
about 0.5 millimeter. The overall length was
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about 75 centimeters, although fibers were grown
only within a portion about 20 centimeters long.
At one end of tube 10 was a gas inlet 12 that
was suitably connected to a source of natural gas
14, through a valve 16 and a flow meter 18 for
regulating the natural gas flow. A gas outlet
20 having a valve 22 was connected to the other
end of tube 10 for venting exhausted gases.
A jacket 24 comprising an alumina cylin-
der 25, coaxially surrounded a central portion of
tube 10. The inner diameter of cylinder 25 was
3.1 centimeters and the length was 42.5 centi-
meters. Metal end caps 26 hermetically sealed
tube 10 to cylinder 25 and included bellows 27
to compensate for differential thermal expansion.
Compression fittings (not shown) between tube 10
and caps 26 permit the tube to be uncoupled and
removed for conveniently collecting fibers. An
inlet 28 to jacket 24 at the end near tube inlet
12 was suitably connected to a source of hydrogen
29 through a valve 30, a bubbler 32 containing
water 34 and a flowmeter 36. An outlet 38
comprising a valve 40 was provided at the other
end of jacket 24.
Tube 10 and jacket 24 were positioned
through an insulated furnace 42 having a coiled
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resistance heating element 44. Furnace 42 heated
about 20 centimeters of tube 10 at the desired
temperature and fiber growth occurred substan-
tially in that portion of the tube.
Before heating furnace 42, tube 10 and
jacket 24 were evacuated and checked for air
leaks, and then thoroughly flushed with natural
gas and hydrogen, respectively. Tube 10 and
jacket 25 were then heated to about 970C. The
natural gas flow through tube 10 was adjusted to
about 20 cc/min, corresponding to a residence
time of about 60 seconds within the 20 centimeter
length where fiber growth occurred. The hydrogen
flow through jacket 24 was adjusted to about
15 200 cc/min. Bubbler 32 was maintained at about
room temperature and substantially saturated the
hydrogen flowing therethrough. The gas pressures
within the tube and the jacket were about atmos-
pheric.
After a few hours under these conditions,
fibers began to grow out from the inner surface
of tube 10 at an acute angle thereto pointing
generally downstream. ~ery thin fibers rapidly
grew to substantially full length and thereafter
principally grew radially. After about 24 hours,
tube 10 was uncoupled from the remaining equipment
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and the fibers were knocked out using a brush.
The product fibers were generally
straight and cylindrical and resembled very hard, very
thin pencil leads. The fibers varied in length froTn
less than a centimeter up to about 12 centimeters.
However, the fibers were remarkably uniform in
diameter, ranging, for example, between 10 and 15
microns. Electron microscopic examination re-
vealed that the fiber cross-section was spiral or
scroll-like. That is, the graphite basal planes
were helically oriented, in contrast to the
radial basal plane orientation found in commer-
cially available graphite fibers derived from
pitch pyrolysis. Young's modulus was measured for
a representative batch of fibers using an Instron
tensile testing machine and was found to range
between 0.8 and 3.8 x 10" Pascals, thinner fibers
generally having a higher modulus. Thus, the fibers
were well suited for use as a filler material.
Although not limited to any particular
theory, it is believed that thermally decomposing
methane in the natural gas adjacent one surface of
the tube wall, i.e., the inner surface in the
described embodiment, while concurrently diffusing
hydrogen through the metal to the surface, carburiæes
the surface in a manner that results in fiber growth.
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The surface initially bears a very thin, air-
derived, integral oxide film composed of iron and
chromium. Carburization transforms the oxide to
carbide comprising a mixture of iron and chromium
that is believed to catalytically decompose the
methane to grow the fibers.
In addition, in the described embodiment,
a portion of the pyrolytic carbon derived from the
natural gas diffuses through the stainless steel
wall to the outer ~ube surface and is extracted into
the wet hydrogen, presumably as a result of oxidation
by the water. It is estimated that up to one-eighth
of the carbon in the natural gas introduced into the
tube is exhausted with the hydrogen in the jacket.
While decarburization by the jacket gas is not
believed to be essential, it does significantly
improve the density of product fibers. Thus, dry
hydrogen, which is not considered decarburizing,
produces fibers, although significantly fewer in
density, provided that soot buildup within the
tube does not inhibit fiber growth.
In the apparatus of the preferred
embodiment, the density of fiber growth is relatively
insensitive to the flow rate of wet hydrogen gas
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through the jacket. Rates up to about 400 cc/min are
suitable. Also, brief interruptions do not apparently
affect fiber growth. However, fiber size is related
to the water content of the hydrogen gas. Drier
hydrogen produces generally longer fibers that are
more uniform in diameter, whereas wetter hydrogen
produces a greater density of shorter fibers. Other
hydrogen-containing decarburizing atmospheres, for
example, hot mixtures of water and inert gas or carbon
dioxide and hydrogen, are also believed suitable.
As discussed herein, fiber growth is
believed to be related to chromium oxide initially
on the inner tube surface, which in turn is related
to the chromium content of the preferred steel.
It has been found that the logarithm of the fiber
population density is directly related to the
logarithm of the chromium concentration. Type 304
stainless steel containing 20 weight percent
chromium produces a high density of fibers and is
preferred. Type 4130 steel containing about 1%
chromium also produces dense fiber growth. Steel
selected from the 1010 series and containing 0.1~
chromium produces more widely scattered fibers and
may be preferred for growing thicker fibers.
~owever, 1010-series steel containing about 0.01
chromium did not appreciably grow fibers.
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In another embodiment, the apparatus in
the Figure was modified to flow wet hydrogen
through tube 10 and natural gas through jacket 24.
Fiber growth occurred on the outer surface of the
tube. The method of this invention is also suit~
able for fiber growth on nontubular walls having
opposite surfaces in contact with natural gas and
wet hydrogen. Wall thicknesses are suitably less
than about 3.0 mm and preferably between about
0.5 to 1.0 mm. In general, thinner walls enhance
fiber growth. This is at least partially
attributed to the higher diffusion rate of carbon
or hydrogen through a thinner wall.
In the preferred embodiment, fibers were
produced from natural gas containing, by weight,
0.5% nitrogen, 0.6% carbon oxides, 4.0% ethane,
1.1% higher hydrocarbons, and the balance methane.
This gas was commercially obtained, in bottled form,
from Airco, Inc., and designated methane grade 1.3.
City natural gas containiny 1.2% nitrogen, 0.7%
total carbon oxides, 1.9% ethane, 0.6% hydrogen,
0.5% heavy hydrocarbons and the balance methane also
produces good fibers. Natural gas produces eight or
more times the fiber density as methane, indicating
that minor constituents in natural gas, such as carbon
oxides or ethane, significantly promote fiber growth.
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Although the method of this invention has
been adapted to produce fibers having diameters of
over 600 microns, the preferred fiber diameter for
filler use is between 5 and 15 microns. The length
of the longest fibers is believed to be mainly limited
by the length of the heated section of the tube. In
general, increasing the natural gas flow rate, tem-
perature or fiber-growing time increases fiber size,
particularly diameter. For the described apparatus,
flow rates suitably range between 10 to 70 cc/min,
corresponding to residence time in the fiber
growth section of between about 120 seconds -to
about 17 seconds. Diluting the natural gas before
pyrolysis, for example, with hydrogen gas, affects
fiber growth similar to reducing the undiluted
flow rate. Suitable temperatures range from about
925C to 1075C or higher, and about 970C to
1000C is preferred. Also, in the preferred
embodiment, tube and gases were heated at a con-
stant temperature for about twenty-four hours.
During this time, it is believed that the metal carbide
forms first and thereafter catalyzes fiber growth.
The growing time may be reduced by adjusting selected
reaction conditions to more rapidly form the carbide.
For example, at a flow rate of 20 cc/min, the onset
of fiber growth occurred after about twelve hours.
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Increasing to 60 cc/min reduces the time to about
three hours. Similarly, increasing the temperature
reduces the time before fibers begin to grow. After
fiber growth begins, the reaction conditions may be
adjusted for optimum fiber growth, which typically
requires only a few hours, depending on the desired
diameter. Careful removal of the fibers may permit
the carburized tube to be reused and may reduce
the time required to initiate fiber growth.
Although this invention has been described
in terms of certain embodiments thereof, it is not
intended that it be limited to the above description
but rather only to the extent set forth in the claims
that follow.
