U.S. patent application number 10/191973 was filed with the patent office on 2004-01-15 for silicon carbide fibers essentially devoid of whiskers and method for preparation thereof.
This patent application is currently assigned to Advanced Composite Materials Corporation. Invention is credited to Angier, Derek J., Rhodes, James F., Rogers, William M..
Application Number | 20040009112 10/191973 |
Document ID | / |
Family ID | 27757332 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040009112 |
Kind Code |
A1 |
Angier, Derek J. ; et
al. |
January 15, 2004 |
Silicon carbide fibers essentially devoid of whiskers and method
for preparation thereof
Abstract
Silicon carbide fibers are produced by mixing discontinuous
isotropic carbon fibers with a silica source and exposing the
mixture to a temperature of from about 1450.degree. C. to about
1800.degree. C. The silicon carbide fibers are essentially devoid
of whiskers have excellent resistance to heating and excellent
response to microwave energy, and can readily be formed into a
ceramic medium employing conventional ceramic technology. The
fibers also may be used for plastic and metal reinforcement.
Inventors: |
Angier, Derek J.;
(Simpsonville, SC) ; Rhodes, James F.; (Greer,
SC) ; Rogers, William M.; (Taylors, SC) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Assignee: |
Advanced Composite Materials
Corporation
Greer
SC
|
Family ID: |
27757332 |
Appl. No.: |
10/191973 |
Filed: |
July 10, 2002 |
Current U.S.
Class: |
423/345 |
Current CPC
Class: |
C04B 2235/5256 20130101;
D01F 9/08 20130101; C04B 35/62281 20130101; C04B 2235/77 20130101;
C04B 2235/722 20130101; Y10T 442/638 20150401; C04B 2235/5248
20130101; C04B 2235/72 20130101; C04B 2235/726 20130101; C04B
35/62675 20130101; Y10T 428/249921 20150401; C04B 2235/5264
20130101; C04B 2235/449 20130101; C04B 2235/526 20130101; C04B
2235/723 20130101; C04B 2235/3418 20130101; C04B 2235/448 20130101;
C04B 2235/3272 20130101; C04B 2235/3208 20130101; Y10T 428/2913
20150115 |
Class at
Publication: |
423/345 |
International
Class: |
C01B 031/36 |
Claims
We claim:
1. A method for making discontinuous silicon carbide fibers
essentially devoid of whiskers, comprising the steps of: (a)
admixing discontinuous isotropic carbon fiber and silica to form a
fiber/silica mixture; (b) drying the fiber/silica mixture; and (c)
reacting the dried fiber/silica mixture in an essentially inert
atmosphere in a resistance furnace for a time and at a temperature
sufficient to form the discontinuous silicon carbide fibers
essentially devoid of whiskers.
2. The method of claim 1 wherein the discontinuous isotropic carbon
fiber is isotropic pitch carbon fiber.
3. The method of claim 1 wherein the silica is present in a molar
excess relative to the discontinuous isotropic carbon fibers.
4. The method of claim 1 wherein the essentially inert atmosphere
comprises argon.
5. The method of claim 1 wherein the discontinuous isotropic carbon
fibers has a sulfur concentration greater than about 0.25 wt
percent.
6. Discontinuous silicon carbide fibers essentially devoid of
whiskers, said fibers prepared by (a) admixing discontinuous
isotropic carbon fiber and silica to form a fiber/silica mixture;
(b) drying the fiber/silica mixture; and (c) heating the dried
fiber/silica mixture in a resistance furnace in an essentially
inert atmosphere for a time and at a temperature sufficient to form
the discontinuous silicon carbide fibers.
7. The fibers of claim 6 wherein the silicon carbide fibers are
essentially pure .beta.-silicon carbide.
8. Discontinuous silicon carbide fibers essentially devoid of
whiskers made by reacting discontinuous isotropic carbon fibers and
silica, said silicon carbide fibers comprising at least about 90 wt
percent .beta.-silicon carbide, a diameter between about 3 and
about 25 microns, a length of less than about 1 mm, and essentially
the same morphology of the discontinuous isotropic carbon
fibers.
9. A method for making discontinuous silicon carbide fibers
essentially devoid of whiskers comprising the steps of: (a)
admixing discontinuous isotropic carbon fiber and silica, and a
promoter to form a fiber silica mixture; (b) drying the
fiber/silica mixture; and (c) reacting the dried fiber/silica
mixture in an essentially inert atmosphere in a resistance furnace
for a time and at a temperature sufficient to form the
discontinuous silicon carbide fibers essentially devoid of
whiskers.
10. The method of claim 9 wherein the essentially inert atmosphere
comprises argon.
11. The method of claim 9 wherein the discontinuous isotropic
carbon fiber is isotropic pitch carbon fiber.
12. The method of claim 9 wherein the silica is present in a molar
excess relative to the discontinuous isotropic carbon fibers.
13. The method of claim 9 wherein the discontinuous isotropic
carbon fiber has a sulfur concentration greater than about 0.25 wt
percent.
14. The method of claim 9 wherein the promoter is selected from the
group consisting of the salts, compounds, and complexes of iron,
cobalt, or nickel, and blends thereof, and the salts, compounds,
and complexes of alkali metals or alkaline earth metals and blends
thereof.
15. The method of claim 14 wherein the promoter is selected from
the group consisting of ferrous sulfate and calcium oxalate.
16. A method for making discontinuous silicon carbide fibers,
essentially devoid of whiskers comprising the steps of: (a)
admixing discontinuous isotropic carbon fiber and silica, and at
least two promoters to form a fiber/silica mixture; (b) drying the
fiber/silica mixture; and (c) reacting the dried fiber/silica
mixture in an essentially inert atmosphere in a resistance furnace
for a time and at a temperature sufficient to form the
discontinuous silicon carbide fibers essentially devoid of
whiskers.
17. The method of claim 16 wherein the essentially inert atmosphere
comprises argon.
18. The method of claim 16 wherein the discontinuous isotropic
carbon is isotropic pitch carbon fiber.
19. The method of claim 16 wherein the silica is present in a molar
excess relative to the carbonized fibers.
20. The method of claim 16 wherein the discontinuous isotropic
carbon fibers has a sulfur concentration greater than about 0.25 wt
percent.
21. The method of claim 20 wherein the promoters comprise (a) a
first promoter selected from the group consisting of the salts,
compounds, and complexes of iron, cobalt, or nickel, and blends
thereof; and (b) a second promoter selected from the group
consisting of the salts, compounds, and complexes of alkali metals
or alkaline earth metals, and blends thereof.
22. The method of claim 21 wherein promoter (a) comprises an amount
equivalent to about 0.5 to about 5.0 wt percent of ferrous sulfate
based on the combined weight of the carbonized fiber and silica and
promoter (b) comprises an amount equivalent to about 0.2 to about
3.0 wt percent of calcium oxalate, based on the combined weight of
the carbonized fiber and silica.
23. The method of claim 22 wherein promoter (a) is ferrous sulfate
and promoter (b) is calcium oxalate.
24. A regenerable medium for filtering volatile organic compounds
from fluids comprising silicon carbide fiber made in accordance
with the method of claim 16.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The present invention is directed to discontinuous silicon
carbide fibers and a process for producing them. In particular, the
invention is directed to discontinuous silicon carbide fibers that
retain the morphology of the carbon source, respond to microwave
energy, and are essentially devoid of whiskers.
[0003] 2. Description of the Related Art
[0004] Silicon carbide is used as reinforcement for both ceramics
and plastics subjected to high temperatures. Silicon carbide
materials have many desirable qualities including high resistance
to oxidation, excellent mechanical strength, and the ability to
withstand multiple exposures to high temperatures without
deformation. The importance of such qualities has led to the
development of many methods by which various shapes of silicon
carbide materials are made. The different shapes are useful in a
plethora of industrially important products.
[0005] Silicon carbide is commonly available in particulate,
whisker, fiber, and cloth forms. Each form has distinct properties
and characteristics exploitable in divers industrial
applications.
[0006] Various methods have been developed to produce silicon
carbide having these forms. For example, Evans, GB 998,089,
describes a method for making silicon carbide cloth. First, carbon
cloth is heated in an inert atmosphere, then embedded in fine
powdered silicon (99.9 percent purity). The silicon-embedded cloth
is then heated in an inert atmosphere to 1410.degree. C., i.e.,
just below the melting point of silicon, to produce a cloth of
silicon carbide.
[0007] Methods for making silicon carbide whiskers, i.e., elongated
single crystals of silicon carbide, are well-known. Liquid- and
gas-phase reaction systems are often used to form these elongated
single crystals. Typical methods of making silicon carbide whiskers
include: (1) solidification from liquid silicon carbide at high
temperature and high pressure, (2) dissolving carbon into molten
silicon and crystallizing the silicon carbide, (3) sublimation of
silicon carbide powder and subsequent re-deposition, and (4)
deposition of silicon carbide crystals from the vapor of silicon
compounds.
[0008] For example, Wainer, U.S. Pat. No. 3,269,802 is directed to
preparation of metal carbide products by exposing a carbonized
product to an atmosphere comprising volatilizable metal-containing
material, such as a metal halide or a metal carbonyl. The product
takes the general form of the carbonized material, but also appears
in other forms, including whiskers, fibers, and coatings. Thus, the
method does not form a single product and produces environmentally
undesirable waste gas.
[0009] Another method for producing metal carbide shapes is set
forth in Hamling, U.S. Pat. No. 3,403,008. Organic material in the
desired shape is impregnated with a metal compound solution. The
impregnated form then is heated in two steps: first, to carbonize
the organic material, then to form the metal carbide.
[0010] Cutler, U.S. Pat. No. 3,754,076, is directed to a method for
producing silicon carbide whiskers from rice hulls, which comprise
about 15-20 percent silica and carbon. A metal-containing
composition, typically metal oxide, is used to catalyze the
reaction. Iron and iron oxide are suitable catalysts.
[0011] Yamada, U.S. Pat. No. 4,849,196, is directed to a process
for producing silicon carbide whiskers. In Yamada's method, Fe, Co,
or Ni are added in any combination to minimize the production of
silicon carbide powder while maximizing the yield of silicon
carbide whiskers.
[0012] Weaver, U.S. Pat. No. 4,873,069, discloses a process for
production of silicon carbide whiskers. In accordance with Weaver's
process, discontinuous fluffy carbonized fibers (having a void
volume of at least 40 percent) and ultra fine silica are heated to
1600-1900.degree. C. for about 2 hours to produce silicon carbide
whiskers. Boron oxide, alone or mixed with aluminum, serves as a
catalyst. A preferred carbon source is carbonized cotton fiber
having a diameter of 4-15 .mu.m and an average length of about 2
mm. The whiskers have a smooth surface, a diameter of 0.5 to 10
.mu.m and a length of up to 1 mm. Nixdorf, U.S. Pat. No. 5,087,272,
uses the process described in Weaver to generate silicon carbide
whiskers having a diameter of 1-3 microns which are then
incorporated into ceramic filters for removing volatile organic
compounds from gas streams.
[0013] Other methods for producing silicon carbide whiskers include
use of iron to catalyze the formation of whiskers from rice hulls
(Home, U.S. Pat. No. 4,283,375). Similarly, Home, U.S. Pat. No.
4,284,612, is directed to use of iron to catalyze production of
silicon carbide whiskers from the combination of ground carbonized
organic fibers, silica, and rice hulls.
[0014] Silicon carbide whiskers are not satisfactory for all
purposes. For example, the production of respirable pollutants from
silicon carbide whisker handling, from devices containing whiskers,
and in particular from filtering devices that are repeatedly
exposed to high temperatures, are sources of concern. As can be
seen from the methods described herein, whiskers are relatively
expensive and technically difficult to make. Proper handling of
whiskers is especially important so as to minimize the number of
inhalable fine particles. In addition, facilities which can sustain
the high temperatures required for the production of silicon
carbide whiskers are expensive to build and difficult to maintain.
Thus, commercial production of silicon carbide whiskers is not
entirely satisfactory.
[0015] Silicon carbide fiber and filament forms avoid some of the
failings of silicon carbide whiskers. Woven and composited forms of
silicon carbide materials may also avoid some of the problems
presented by whiskers. Fiber, filament, and woven forms comprise
particles larger than whiskers, and are therefore, less likely to
yield airborne respirable particles.
[0016] Wei, U.S. Pat. No. 4,481,179, is directed to a method of
producing silicon carbide bonded fiber composites, starting from a
carbon-bonded carbon fiber composite. Galasso, U.S. Pat. No.
3,640,693, is directed to forming a silicon-containing fiber by
casting silicon metal in a glass tube, drawing composite filaments,
removing the glass sheath, then exposing the silicon metal to
carbon or nitrogen to produce silicon carbide or silicon nitride,
respectively. Debolt, U.S. Pat. No. 4,127,659, is directed to
coating a refractory substance, such as carbon, with silicon
carbide by chemical vapor deposition to produce a silicon carbide
filament containing a core and a coating of carbon-rich silicon
carbide. Srinivasan, U.S. Pat. No. 5,729,033, is directed to a
method of producing silicon carbide material (fiber, fabric, or
yarn) by carbothermal reduction of silicon material. Particular
proportions of silica and carbon are preferred. DeLeeuw, U.S. Pat.
No. 5,071,600 and U.S. Pat. No. 5,268,336, are directed to methods
for producing silicon carbide fibers by the reaction of
polycarbosilane and methylpolydisilylazane resins in the presence
of boron. Tokutomi, U.S. Pat. No. 5,344,709, describes a silicon
carbide fiber produced from polycarbosilane fiber and having an
amorphous layer of carbon thereon.
[0017] Each of these methods has disadvantages. The continuous
silicon carbide filaments produced by the chemical vapor deposition
method are not homogenous and, when chopped to obtain fibers, a
carbon core is exposed. The resultant fiber product has reduced
resistance to oxidation. All of the polymer conversion methods are
disadvantageous in that they require synthesis of the starting
material which must then be spun, cured, and pyrrolized to burn off
the organic material. The submicron silicon carbide powder process
incorporated by reference in Srinivasan is expensive and difficult
to implement because the polymer carrier requires further
processing to effectuate its removal.
[0018] Other methods of producing silicon carbide fibers,
essentially without whiskers have been recently developed. Okada et
al. (Okada, et al. U.S. Pat. No. 5,618,510 and U.S. Pat. No.
5,676,918 and Nakajima et al. U.S. Pat. No. 5,922,300) developed
methods which involve activating carbon fibers by, for example,
contact with water vapor, to yield porous activated carbon fibers.
The activated carbon fibers then are exposed to silicon monoxide
gas generated by heating a mixture of silica and silicon to a
temperature between about 1200-2000.degree. C. under reduced
pressure to minimize formation of whiskers. In accordance with the
methods of the first two patents, range of surface area of the
activated fibers is said to be from 100 to 3,000 m.sup.2/g, with
the sole exemplification at 1,500 m.sup.2/g. The Nakajima patent
teaches that the surface area of the activated carbon fibers must
be at least 300 m.sup.2/g, lest unreacted carbon remain in the
fibers because the reaction becomes difficult to carry out
uniformly at a sufficiently high reaction rate. Yajima, U.S. Pat.
No. 4,100,233, describes a method of producing silicon carbide
fibers which involves dissolving or melting an organosilicon
compound in a solvent and spinning the solution into filaments. The
spun filaments are then heated to volatilize low molecular weight
compounds, and, finally, baked to form silicon carbide fibers.
[0019] Thus, there remains a need for an easily implemented,
economical, and environmentally benign method of producing
homogenous, discontinuous, silicon carbide fibers.
SUMMARY OF THE INVENTION
[0020] The invention is directed to a method for producing
discontinuous silicon carbide fibers from the reaction of
discontinuous carbon fiber and fine silica in the presence of
promoters in a graphite resistance furnace under an inert
atmosphere, and to the fibers thus made. Skilled practitioners
recognize that such fibers are not single crystals. The silicon
carbide fibers of the present invention are essentially devoid of
whiskers, retain the morphology of the carbonized fiber if
promoters are used, may have a silica coating, and are produced at
a high yield. The silicon carbide fibers of the invention, and
especially coated fibers of the invention, can be readily
incorporated into other media, such as ceramics, plastics, and
metals, via conventional processing technology. The silicon carbide
fibers of the present invention are economically produced, are
exceptionally responsive to microwave energy, and have excellent
resistance to, e.g., oxidation during repeated exposures to
microwave radiation. Thus, a ceramic medium having the fibers of
the invention incorporated therein is especially suited for use as
a regenerable filter medium in a device for removing volatile
organic compounds from fluids.
[0021] The discontinuous silicon carbide fibers of the present
invention are less expensive to produce, easier and less costly to
process into substrate materials, such as ceramic filter media, and
are produced by a method that is environmentally benign. Further,
the fibers of the invention will not produce airborne, respirable
particles, and so do not require expensive handling techniques and
do not present health hazards associated with respirable
particles.
[0022] These discontinuous silicon carbide fibers are particularly
useful for, but not limited to, incorporation into a filter-heater
apparatus for the removal of volatile organic compounds from a gas
stream. For such a use, the fibers are formed via ceramic
processing techniques into ceramic sheets or shapes which are then
formed into filters. Microwave energy is then applied to the filter
periodically, interacting with the silicon carbide fiber providing
heat which then burns off any volatile organic compounds such as
diesel soot. Originally, the use of silicon carbide whiskers was
investigated for this purpose, but silicon carbide fibers as made
by this invention were preferred over whiskers.
[0023] In accordance with the present invention, discontinuous
silicon carbide fibers essentially devoid of whiskers are prepared
by admixing discontinuous isotropic carbon fiber, silica, and
preferably at least two promoters to form a fiber/silica mixture;
drying the fiber/silica mixture; and reacting dried fiber/silica
mixture in a resistance furnace for a time and at a temperature
sufficient to form the discontinuous silicon carbide fibers of the
invention essentially devoid of whiskers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIGS. 1a-1d are SEM photographs of purchased Carboflex.RTM.
isotropic carbon fiber grade P-200 at 200.times. (FIG. 1a) and
500.times. (FIG. 1c) magnification, and of silicon carbide fiber
product of the invention made therefrom at 200.times. (FIG. 1b) and
500.times. (FIG. 1d) magnification.
[0025] FIG. 2 is an SEM photograph of silicon carbide product
prepared from Cytec ThermalGraph.RTM. DKD X mesophase pitch at
200.times. magnification.
[0026] FIG. 3 is an SEM photograph of silicon carbide product
prepared from Fortafil.RTM. PAN M275 carbon fiber at 200.times.
magnification.
[0027] FIG. 4 is an SEM photograph at 200.times. magnification of
silicon carbide fiber of the invention made from P-200 isotropic
carbon fiber pretreated 1 hr at 1500.degree. C.
[0028] FIG. 5 is an SEM photograph at 200.times. magnification of
silicon carbide fiber of the invention made from P-200 isotropic
carbon fiber pretreated 1 hr at 1800.degree. C.
[0029] FIG. 6 is an SEM photograph at 200.times. magnification of
silicon carbide fiber of the invention made from P-200 isotropic
carbon fiber pretreated 7 hr at 1800.degree. C.
[0030] FIG. 7 is an SEM photograph at 200.times. magnification of
silicon carbide fiber of the invention made from a stoichiometric
blend of P-200 isotropic carbon fiber and fumed silica.
[0031] FIG. 8 is an SEM photograph at 200.times. magnification of
silicon carbide fiber of the invention made from Carboflex.RTM.
P-600 isotropic carbon fiber.
[0032] FIG. 9 is an SEM photograph at 200.times. magnification of
silicon carbide fiber of the invention made from P-200 isotropic
carbon fiber without calcium oxalate.
[0033] FIG. 10 is an SEM photograph at 200.times. magnification of
silicon carbide fiber of the invention made from P-200 isotropic
carbon fiber without ferrous sulfate.
[0034] FIGS. 11a and 11b are SEM photographs of silicon carbide
fiber of the invention made from P-200 isotropic carbon fiber
without any promoters at 500.times. (FIG. 11a) and 5000.times.
(FIG. 11b) magnification.
[0035] FIG. 12 is a plot of x-ray diffraction data for silicon
carbide fiber of the invention.
[0036] FIG. 13 is an SEM photograph at 200.times. magnification of
silicon carbide fiber made under a nitrogen atmosphere.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The invention is directed to a method for producing
discontinuous silicon carbide fibers essentially devoid of
whiskers, and to coated or uncoated fibers thus produced. In
accordance with the method of the invention, discontinuous
isotropic carbon fibers and fine silica are mixed, preferably with
promoters, dried, and then heated in an essentially inert
atmosphere to a temperature and for a time sufficient to form the
discontinuous coated or uncoated silicon carbide fibers of the
invention.
[0038] The method of the invention produces a high yield of
discontinuous silicon carbide fibers essentially devoid of
whiskers. Silicon carbide fibers of the invention produced using
promoters have essentially the same morphology as the carbonized
fiber starting material, i.e., smooth discontinuous strands, and
are especially suited for incorporation into media such as
ceramics, plastics, and metals to, e.g., improve strength and other
characteristics thereof. Coated fibers of the invention made with
promoters are especially easily incorporated into such media.
Silicon carbide fibers of the invention produced without both
promoters tend to degrade into smaller particles, but do not form
whiskers. The silicon carbide fibers of the invention are
gray-green in color.
[0039] In particular, silicon carbide fibers of the invention made
with promoters are especially suited for incorporation into ceramic
filter media by conventional ceramic processing technology. Such
fiber-containing ceramic filter media then is especially suitable
for use as regenerable media for filtering volatile organic
compounds from fluids, such as is disclosed in U.S. Pat. No.
5,087,272, the entirety of which is incorporated herein by
reference. When regeneration is required, the filter is irradiated
with microwave energy. Energy absorbed by the silicon carbide
fibers heats the entirety of the filter medium. Heating is
continued at a temperature and for a time sufficient to volatilize
volatile organic compounds trapped by the filter. Silicon carbide
fibers of the invention are resistant to degradation even after
numerous exposures to microwave energy.
[0040] Silicon carbide fibers of the invention may be coated with
silica. Such a coating provides additional protection against
oxidation at temperatures less than about 850.degree. C.
[0041] Silicon carbide fibers of the invention are environmentally
more acceptable than silicon carbide whiskers and silicon carbide
fibers containing whiskers, as the greater particle size of a fiber
is less likely to yield respirable airborne particles. In
accordance with the invention, silicon carbide fibers of the
invention are essentially devoid of whiskers. As used herein,
"essentially devoid of whiskers" means that no whiskers can be seen
when examining two or three separate areas in a first sample in a
light microscope at a magnification of about 200.times. or
250.times., then examining a second sample in a Scanning Electron
Microscope (SEM) at a magnification of about 200.times. or
250.times.. The approximate area encompassed in an SEM image is
about 0.2 mm.sup.2, or 2.times.10.sup.5 square microns. Indeed,
this is the area of the SEM photographs used in this
application.
[0042] In accordance with the method of the invention,
discontinuous isotropic carbon fiber, silica, and preferably
promoters are admixed. The admixture is dried, then heated in an
essentially inert atmosphere in a resistance furnace to a
temperature and for a time sufficient to form silicon carbide
fibers of the invention in accordance with the following
formula:
3C+SiO.sub.2.fwdarw.SiC+2CO.
[0043] Carbon fibers suitable for use in the invention are
isotropic carbon fibers, particularly isotropic pitch carbon fibers
spun from isotropic pitch. Such isotropic carbon fibers are not
high-performance fibers and exhibit performance significantly
inferior to that of carbon fibers made from mesophase pitch or
poly-acrylonitrile (PAN). Isotropic pitch carbon fibers as
available from Anshan East Asia Carbon Fiber Co. Ltd. Anshan,
Liaoning, China under the tradename Carboflex.RTM. grades P-200 and
P-600 are preferred in the invention.
[0044] The structure of carbon fiber is derived from two causes.
First, the fiber takes on an orientation during spinning and
drawing. Then, during carbonization, most non-carbon atoms are
removed and the structure tends to become more graphitic.
High-performance carbon fibers derive their properties and
characteristics primarily from the orientation during formation,
and graphitic structure persists after carbonization. Both
mesophase pitch and PAN fibers are high-performance fibers with a
high degree of orientation introduced during spinning. In
contradistinction, the structure of isotropic pitch carbon fibers
is derived almost entirely from heating the fiber after
stabilization to produce isotropic pitch carbon fibers Graphitic
structure thus is not found in such fiber as it is supplied,
although such structure can be introduced by heating the fiber to a
high temperature (at least about 1800.degree. C.) for at least
about 7 hours. However, with isotropic fibers, such graphitic
structure in an isotropic fiber does not cause production of
whiskers.
[0045] Mesophase pitch fibers may be readily distinguished from
isotropic pitch fibers by examination under a light microscope
under crossed Nichols, which skilled practitioners recognize will
reveal the high degree of structure in the mesophase fiber.
Mesophase pitch fibers yield superior physical properties.
Similarly, PAN fibers also exhibit high levels of fiber structure
and superior mechanical properties. Such fibers are not suitable
for use in the invention.
[0046] Isotropic carbon fiber suitable for use in the invention is
essentially isotropic, i.e., its mechanical properties are
essentially the same in each direction. Such fiber is not
"high-performance" fiber. For example, the tensile strength of an
isotropic carbon fiber is approximately 10 percent of that of a
"high performance" fiber. Typically, such fibers are straight and
smooth, essentially without kinks, knots, or other surface defects,
as can be seen in FIGS. 1a and 1c. When isotropic carbon fibers are
heated with silica in accordance with the invention, the resulting
silicon carbide fiber product is essentially devoid of whiskers. In
contrast, when mesophase pitch or PAN is used as the carbon fiber,
significant whisker contents are visible in an SEM photograph at
200.times. magnification.
[0047] The length of isotropic carbon fiber suitably used in the
invention is limited only by economics and commercial practicality.
Isotropic carbonized fiber of essentially any length can be used in
the method of the invention to yield silicon carbide fiber in
accordance with the invention. However, typically, the length of
commercially available fiber does not exceed about 20 mm. Fibers
that are relatively long, i.e., longer than about 1 mm, will yield
silicon carbide product having low bulk density, increasing the
cost of furnace treatment, packaging, transportation, and
storage.
[0048] The diameter of isotropic carbon fiber typically is less
than about 25 microns. Diameters greater than this are not
preferred because it is difficult to ensure completeness of
reaction at the core of such a relatively large diameter fiber.
Typically, such carbon fiber has a surface area of less than about
100 m.sup.2/g, more typically between about 10 and about 50
m.sup.2/g, and most typically between about 20 and about 35
m.sup.2/g.
[0049] A preferred source of discontinuous carbon fibers is Anshan
East Asia Carbon Fiber Co., Ltd., particularly grades P-200 and
P-600. Carboflex.RTM. P-200 is especially preferred. The fibers
typically have an average length of 200 microns; diameters range
from about 5 to 25 microns with an average of 15 microns. Longer or
shorter lengths can be used depending on end use. If the fibers are
longer than 1000 microns (1 mm), the blend bulk densities are
significantly lowered and processing costs thereby increased, as
set forth above. These Anshan fibers are believed to be milled,
then sieved to remove fines.
[0050] The Carboflex.RTM. P-200 fiber as obtained typically has a
surface area of between about 20 and 30 m.sup.2/g. Heat treatment
of P-200, for example at a temperature of at least about
1500.degree. C. for at least about 1 hour, reduces the average
length and diameter. The surface area of the heat treated fibers
was markedly reduced, to about the same level as the mesophase and
PAN fibers investigated, or about 0.5 m.sup.2/g.
[0051] Particulate silica from any source may be used in the
present invention, including, but not limited to, granular silica
and colloidal suspensions of silica. Regardless of the silica
source, it is preferred that the particles be no larger than about
0.1 .mu.m (1000 A). A preferred silica source is Cab-O-Sil.RTM.
grade M5, available from Cabot Corp., Tuscola, Ill. This product is
fumed silica having a surface area of about 220 m.sup.2/g and an
approximate bulk density of 0.07 g/cc.
[0052] Promoters preferably are used in the method of the invention
to enhance the integrity of the silicon carbide fiber formed. Use
of promoters yields integral silicon carbide fibers having
essentially the same morphology as the isotropic carbon fiber
starting material. Promoters most preferably are used in
combination. One type of promoter is a metal containing promoter
selected from the group consisting of salts, compounds, and
complexes of iron, cobalt, or nickel, and blends thereof. These
salts, compounds, and complexes may be converted to oxides of iron,
cobalt, or nickel at a temperature less than about 1650.degree. C.
A second type of promoter is selected from the group consisting of
the salts, compounds, and complexes of alkali metals or alkaline
earth metals, and blends thereof These salts, compounds, and
complexes may be converted to oxides of these materials at the
reaction temperature employed. When a single type of promoter is
used, it may be any of the promoters. If two types of promoters are
used, one promoter is selected from each type. A blend of promoters
of one type will be referred to as "one promoter" herein for
convenience.
[0053] Preferred metal-containing promoters include iron oxide,
ferrous sulfate, potassium ferrocyanide, cobalt oxide, cobalt
sulfate, nickel oxide, and nickel sulfate. Ferrous sulfate
(FeSO.sub.4) is an especially preferred promoter. The especially
preferred metal-containing promoter is present in an amount between
about 0.5 and about 5.0 wt percent of the fiber/silica blend;
preferably between about 0.7 and about 3.0 wt percent; more
preferably between about 1.0 and about 2.0 wt percent; and most
preferably between about 1.3 and about 1.7 wt percent.
Metal-containing promoters other than ferrous sulfate are present
in an amount sufficient to provide the mole quantity of metal
equivalent to the mole quantity of iron.
[0054] Preferred alkali metal- and alkaline earth metal-containing
promoters include calcium oxalate, barium oxalate, strontium
oxalate, and potassium oxalate. Calcium oxalate is especially
preferred. This promoter typically is present in an amount between
about 0.2 and about 3.0 wt percent of the fiber/silica blend,
preferably between about 0.25 and about 2.0 wt percent; more
preferably between about 0.4 and about 1.0 wt percent; and most
preferably between about 0.5 and about 0.7 wt percent. Alkali
metal- and alkaline earth metal-containing promoters other than
calcium oxalate are present in an amount sufficient to provide the
mole quantity of alkali metal or alkaline earth metal equivalent to
the mole quantity of calcium.
[0055] The metal-containing promoter and the alkali metal- or
alkaline earth metal-containing promoter may be provided in a
single composition. Thus, a single composition that contains both
metal promoter and alkali metal or alkaline earth metal promoter
may be used to provide at least two promoters in accordance with
the method of the invention.
[0056] With the guidance provided herein, a skilled practitioner
can select suitable salts, compounds, and complexes to serve as
promoters. For example, a skilled practitioner recognizes that, at
the reaction temperatures used in the method of the invention, most
promoters will be converted to an oxide form. However, one must
exercise care in selecting promoter compositions. For example,
ferrous nitrate (Fe(NO.sub.3).sub.2.H.sub.2O) is not a suitable
promoter composition because it degrades if the feed mixture is
heated while still wet, whereas ferrous sulfate (FeSO.sub.4) is a
preferred promoter composition.
[0057] The promoters are used in a quantity sufficient to assist
the conversion of carbon fibers to silicon carbide and to promote
fiber quality. Suggested quantities of promoter compositions are
specified herein; with this guidance, skilled practitioners will be
able to determine appropriate quantities of other suitable
promoters.
[0058] The inventors have observed that the combination of a
metal-containing promoter and an alkali metal- or an alkaline earth
metal-containing promoter is particularly effective in providing
high quality fibers essentially devoid of whiskers and having
essentially the same morphology as the carbon fiber from which it
is made. Skilled practitioners recognize that the essentially
complete absence of whiskers is a completely unexpected result, as
either calcium or iron is used individually in whisker manufacture
to promote whisker production.
[0059] Because silicon carbide fiber of the invention may maintain
the morphology of the isotropic carbon fiber from which it is made,
or may be degraded, the largest expected size range is that of the
starting carbon fiber, i.e., diameter between about 5 and about 25
microns and typical lengths up to about 1 mm. However, it is
expected that smaller diameters, between about 3 and about 15
microns, and typical lengths up to about 500 microns, also will be
realized. Thus, the expected diameter of silicon carbide fiber
product is between about 3 and 25 microns, with lengths up to about
1 mm.
[0060] SEM photographs show that silicon carbide fiber products of
the invention are essentially devoid of whiskers even when
promoters are used individually or are omitted completely. FIGS.
1b, 1d, and 4 through 11 show that no whiskers can be seen in
product of the invention at 200.times. magnification.
[0061] In particular, SEM photographs of the product of the
invention made with promoters (FIGS. 1b, 1d, and 4-8) illustrate
that the morphology of the silicon carbide fibers is essentially
the same as that of the isotropic carbon fiber starting material.
This result is completely unexpected, as prior methods used iron
and calcium salts as promoters of whisker growth.
[0062] FIG. 9 shows that no whiskers can be seen at 200.times.
magnification in an SEM of silicon carbide fiber produced without
calcium promoter, and FIG. 10 illustrates the same phenomenon for
silicon carbide fiber produced without iron promoter. FIGS. 11a and
11b show that no whiskers can be seen at 500.times. and 5000.times.
magnifications, respectively, in silicon carbide fiber of the
invention made without promoters.
[0063] FIGS. 9-11 illustrate that, whereas promoters are not needed
to inhibit whisker production, use thereof produces silicon carbide
fibers that are not degraded. FIGS. 9-11 illustrate that without
both iron and calcium salts present, the silicon fiber product
fibers of the invention are degraded and form smaller particles of
fiber, but do not form whiskers. Promoters do not affect conversion
of the carbon and silica starting materials to silicon carbide.
[0064] FIGS. 1a and 1c are SEM photographs of Carboflex.RTM. P-200
isotropic carbon fiber at 200.times. and 500.times. magnification,
respectively. The isotropic carbon fibers appear to have very
smooth surfaces. Table 1 below summarizes physical properties and
characteristics of divers discontinuous carbon fibers, including
the samples depicted in FIGS. 1a and 1c. The specific surface area
of this fiber as obtained was determined to be 28.7 m.sup.2/g,
which would indicate the fiber has significant void volume.
However, it was found to be very difficult to obtain a good
reproducible specific surface area. Preheating the fiber made it
easier to obtain reproducible specific surface area measurements
and reduced the sulfur content of the fiber. The specific surface
areas of heated fibers obtained were much closer to that expected
of a void-free, smooth-surfaced fiber, as set forth in Table 1.
1TABLE 1 Surface Sulfur Discontinuous Density, Area, Content,
Length, Diameter, Carbon Fiber g/cc m.sup.2/g wt % microns microns
Isotropic Pitch, 1.98 28.7 1.43 201 15.1 Carboflex .RTM. P-200
Isotropic Pitch, 1.96 0.59 313 14.2 Carboflex .RTM. P-600 Carboflex
.RTM. P-200 pretreated for 1 hour at 1500.degree. C. 1.53 0.5 0.85
114 11.9 1 hour at 1800.degree. C. 1.53 0.4 0.26 114 12.1 7 hours
at 1800.degree. C. 1.54 0.3 0.36 133 11.6 Mesophase Pitch 2.19 0.5
.ltoreq.0.1 118 6.9 Fiber, Cytec ThermalGraph .RTM. DKD X PAN,
Fortafil .RTM. 1.76 0.5 .ltoreq.0.1 219 9.0 M275
[0065] The Carboflex.RTM. carbon fibers described in Table 1 are
commercially available from Anshan East Asia Carbon Fiber Co. Ltd.,
Anshan, Liaoning, China. Both P-200 and P-600 are isotropic pitch
fibers. The Anshan pretreated fibers also were heated to the
indicated temperature and held for the stated time. The pretreated
fibers were somewhat degraded in length and diameter but otherwise
appeared unchanged. XRD (x-ray diffraction) showed the incipient
formation of graphitic structure in the fiber heated for 7
hours.
[0066] The mesophase pitch fiber was obtained from Cytec Carbon
Fibers, 7139 Augusta Road, Piedmont, S.C. 29673. The PAN fiber was
obtained from Fortafil Fibers, Inc., P. O. Box 357, Roane County
Industrial Park, Rochester, Mich. 48306.
[0067] FIGS. 1b and 1d show that silicon carbide fibers produced
using promoters in accordance with this invention have higher
specific surface areas than the heat treated precursor carbon
fibers. The surfaces of the silicon carbide product look smooth
even at 500.times. magnification.
[0068] Silicon carbide fiber of the invention is a gray-green fiber
material that is essentially all .beta.-silicon carbide and has
essentially the same morphology as the carbonized fiber starting
material if promoters are used. X-ray diffraction analyses confirm
that silicon carbide fiber of the invention is essentially
.beta.-silicon carbide. FIG. 12 is a plot of x-ray diffraction data
for a silicon carbide fiber of the invention exemplified in Example
1.
[0069] FIG. 12 illustrates that the silicon carbide fiber of the
invention is essentially .beta.-silicon carbide. As can be seen on
that Figure, there is a large peak labeled "PDF 73-1708." Skilled
practitioners recognize that `PDF` stands for Powder Diffraction
File, as compiled by the National Institute of Standards and
Technology Crystal Data Center and the International Centre for
Diffraction Data. In that identification scheme, PDF 73-1708 is the
identification of .beta.-silicon carbide. There also is a small
peak at the 2.THETA. H value of 22 degrees, which represents
cristobalite, a high temperature form of silica. Another very small
peak occurs at 2.THETA. of 45 degrees, representing hexagonal
silicon carbide. Thus, it can be seen that product of the invention
is essentially all .beta.-silicon carbide.
[0070] Silicon carbide fibers of the invention also may comprise a
coating of silica. Silica-coated fibers of the invention are more
easily processed into ceramic filter media than uncoated fibers and
are better able to resist oxidation during repeated microwave
energy exposures.
[0071] The silicon carbide fiber product quality is improved by
employing a non-stoichiometric ratio of carbon fiber and silica.
Whereas the stoichiometric ratio is 3 moles of carbon per mole of
silica, silicon carbide fiber of the invention is made using a mole
ratio of carbon to silica of between about 2.4:1 to about 3.5:1,
preferably between about 2.5:1 and about 3.0:1, and most preferably
between about 2.6:1 and about 2.8:1. If an excess of silica is
employed in the initial blend, the silicon carbide fiber may be
coated with silica. If an excess of carbon is employed in the
initial blend, unreacted carbon may be found in the core region of
the silicon carbide fiber. Any silica coating due to excess silica
in the feed may be removed by washing the fiber with hydrofluoric
acid (HF) if so desired.
[0072] Skilled practitioners recognize that the water-gas
reaction,
2C+2H.sub.2O.fwdarw.CH.sub.4+CO.sub.2,
[0073] will cause loss of some carbon during the drying period if
the temperature exceeds about 250.degree. C. Thus, this reaction
must be considered when determining the relative quantities of
carbon and silica in the reactant mix. For example, skilled
practitioners recognize that the amount of water in the reactant
mixture, and in the atmosphere in the reaction boat, will affect
how much carbon may be lost to this reaction, and thus can take
steps to minimize the quantity of free water present.
[0074] In accordance with a preferred method of the invention, the
carbon fiber first is "opened," or decompacted. Such decompacting
helps ensure that the various components can be thoroughly mixed
before heating. Typically, the "opening" can be effectuated by a
laboratory single blade mixer, especially a mixer in which the feed
component admixture is to be formed. A short period (less than 5
minutes) is sufficient for thorough mixing.
[0075] The metal-containing promoter, preferably FeSO.sub.4, then
is added. While the FeSO.sub.4 fiber mixture is being blended, the
preferred calcium-containing promoter, calcium oxalate, is added
immediately after the other promoter is added. Because the quantity
of calcium oxalate to be added is small relative to the volume of
the fibers, it is preferred to add this promoter in a volatile
carrier (e.g., ethanol or water). Preferably, a suspension of
calcium oxalate is prepared, then added to the reactants during
agitation. Skilled practitioners are familiar with techniques for
adding such quantities of promoters. Typically, 3 minutes of
blending is sufficient at this step. It is especially preferred
that both promoters be added simultaneously by forming a suspension
of calcium oxalate in an aqueous solution of ferrous sulfate. This
embodiment not only shortens the mixture preparation time, but also
minimizes the quantity of water present in the reactant mixture.
The silica then is added to the admixture from the feed. Blending
for about another two minutes typically is sufficient to form a
homogenous, free-flowing blend. The quantity of silica and carbon
fiber preferably is selected to provide a molar reactant ratio of
carbon to silica of between about 2.6:1 and about 2.8:1.
[0076] For drying and subsequent reaction, the reactant blend is
loaded into a graphite "boat" which then is capped. The "boat" is
passed into a resistance furnace through a muffle furnace. While
the boat is in the muffle furnace, the temperature is increased in
steps, e.g., to 250.degree. C., then to 500.degree. C., and then to
750.degree. C. During this heating, the water in the reactants and
in the atmosphere reacts with carbon in accordance with the
water-gas reaction described above, and some carbon may be
lost.
[0077] In accordance with the method of the invention, the boat
containing the dried reactant mixture then is moved into a graphite
resistance furnace and heated in an essentially inert atmosphere at
a temperature between about 1450.degree. C.-1800.degree. C. for a
time sufficient to form the silicon carbide fibers of the
invention. If the temperature of the furnace is low, the reaction
rate is slow, especially below about 1450.degree. C. At
temperatures above 1800.degree. C., the quality of the fibers
deteriorates; fiber length is degraded and detritus is formed.
[0078] The preferred temperature for the reaction is between about
1500.degree. C.-1775.degree. C., more preferably between about
1650.degree. C.-1750.degree. C. At 1675.degree. C., more than 95
percent of the carbon is converted to silicon carbide fibers.
[0079] As used herein, an "essentially inert" atmosphere is an
atmosphere which is essentially inert to all reactants and the
environs (e.g., the furnace itself and other objects in it), and
which does not produce whiskers. Argon is a preferred gas for use
as an essentially inert atmosphere in the invention. It is likely
that the other inert gases, also known as the "Noble gases," i.e.,
Group 18 (formerly Group VIIIA) of the periodic table of the
elements, and helium also are suitably used in the invention.
[0080] Nitrogen is not suitable for use as an "essentially inert"
atmosphere gas. The inventors have found that use of nitrogen
contributes to whisker formation. With the guidance provided
herein, a skilled practitioner will be able to identify suitable
"essentially inert" atmospheres for use in the invention
[0081] The reaction is carried out in any suitable furnace.
Graphite resistance furnaces are particularly suitable. Such
furnaces are well known to skilled practitioners. One such furnace
is described in Beatty, U.S. Pat. No. 4,837,924, the entirety of
which is incorporated herein by reference.
[0082] The yield of silicon carbide fiber by the method of the
invention is high. Conversion of 100 percent of the reactants to
silicon carbide would yield 41.7 wt percent bound silicon carbide.
For example, after 1 hour in argon at 1675.degree. C., a blend of
silica and carbon in a molar ratio of 2.7 carbon per 1 silica,
together with 1.5 wt percent FeSO.sub.4 and 0.6 wt percent calcium
oxalate, yielded 96.1 percent conversion to silicon carbide. In
accordance with the method of the invention, conversion generally
is at least 80 percent, preferably is about 85 percent, more
preferably is at least about 90 percent, and most preferably is at
least above 95 percent.
[0083] It has been discovered that the sulfur content of isotropic
pitch carbon fiber, which exceeded about 0.25 wt percent even after
pre-treatment, surprisingly did not have an adverse effect on
conversion of carbon to silicon carbide. At a sulfur concentration
greater than about 0.25 wt percent, skilled practitioners would
have been expected an adverse effect on quality and conversion.
[0084] Product composition may be reliably determined by means of a
hot HF extraction technique, which removes any silica present. The
equivalent silica that has chemically reacted may then be
calculated by difference and translated into the quantity of
silicon carbide present. The unreacted carbon also may be computed
by difference. The total carbon present in the product, both in the
silicon carbide and in the unreacted carbon, was determined using
equipment manufactured by the Laboratory Equipment Company of
Benton Harbor, Mich. (LECO). This provides an independent
cross-check on the unreacted carbon computed from HF
extraction.
[0085] The absorption of microwave energy is easily and quickly
confirmed. A cavity 1.0 inches in diameter by 0.25 inches deep in a
3.times.3.times.2 inch rigid Kaowool.RTM. insulation block
(microwave transparent) is filled with silicon carbide fibers of
the invention. The fiber-filled block is placed at a specific spot
in a 1 kilowatt, 2.45 GHz microwave oven and heated until the
fibers achieved red heat, i.e., about 750-800.degree. C. Each of
the preferred fiber products described in the Examples achieved red
heat in between about 3 and about 6 seconds. Silicon carbide
whiskers require about 5 seconds to achieve red heat under the same
conditions.
[0086] As set forth above, the silicon carbide fibers of the
present invention are .beta.-silicon carbide. This determination is
made by x-ray diffraction techniques in a manner known to skilled
artisans.
[0087] The absence of whiskers in silicon carbide fiber product of
the invention is illustrated by SEM photographs of well-dispersed
samples. Whiskers are not found in degraded silicon carbide of the
invention in SEM photographs at 200.times. magnification.
EXAMPLES
[0088] The following examples are meant to illustrate the
invention, not to limit it in any way. For example, isotropic pitch
carbon fibers from any source maybe used. Similarly, other forms of
silica can be used. The scope of the invention is limited only by
the claims.
[0089] Throughout the Examples, "wt percent" means, "weight percent
based on the combined weight of the carbon fiber and silica" when
referring to starting materials.
Example 1
[0090] Quantities of discontinuous divers carbon fibers were used
to form silicon carbide fiber. Carbon fiber was placed in a plow
mixer equipped with a high-speed chopper and "opened" for 1 minute.
With the mixer still running, an aqueous dispersion of ferrous
sulfate and calcium oxalate was added. The quantity of FeSO.sub.4
was sufficient to provide 1.5 wt percent FeSO.sub.4 based on the
combined weight of the carbon fiber and silica to be added
immediately thereafter. The quantity of calcium oxalate was
sufficient to provide 0.6 wt percent calcium oxalate based on the
combined weight of carbonized fiber and silica to be added.
[0091] After 3 minutes of blending, a quantity of Cab-O-Sil.RTM.
grade M5, a fumed silica, sufficient to provide 2.7 moles of carbon
per mole of silica was added. Two additional minutes of blending
followed.
[0092] The blend product was loaded into capped graphite crucibles,
placed in a laboratory graphite furnace and slowly dried at less
than 250.degree. C. under flowing argon. Then, the temperature was
raised to 1675.degree. C. and held for one hour under flowing
argon. The sample was allowed to cool and was examined.
[0093] The carbon fibers used in this Example are the same as those
described in Table 1, and were used in equal mass quantities. Only
the silicon carbide product made from Carboflex.RTM. P-200 is an
example of the invention; silicon carbide fibers from the other two
fibers are comparative examples.
[0094] Product properties are summarized and related SEM
photographs are identified in Table 2 below:
2 TABLE 2 Product Discontinuous Unreacted Extractable Carbon Fiber,
Wt % Whiskers, Carbon, Silica, Source SiC Wt % Figure Wt % wt %
Isotropic Fiber, 96.1 None 1b, 1d None None Carboflex .RTM. P-
Detected Detected 200 Mesophase 97.0 Numerous 2 0.3 0.2 Pitch
Fiber, Cytec ThermalGraph .RTM. DKD X PAN Fiber, 103.6 Numerous 3
4.6 0.5 Fortafil .RTM. M275
[0095] X-ray diffraction analysis shows the fibers prepared from
P-200 to be predominantly P-silicon carbide, as illustrated in FIG.
12.
[0096] The unreacted carbon composition was determined by LECO
analysis, and the extractable silica by HF extraction. The wt
percent silicon carbide conversion was determined by calculating
what fraction of the carbon fiber (less unreacted carbon) formed
silicon carbide fiber. The failure of the fractional compositions
of the various samples to sum to 100 percent is not surprising, but
rather falls within the range of experimental error. The analyses
indicate the absence of unreacted carbon and of extractable silica
in silicon carbide fiber of the invention. The data also show that
product of the invention is essentially devoid of whiskers, as none
were found in the product. However, use of other discontinuous
carbon fiber types resulted in products having numerous
whiskers.
[0097] As can be seen from Table 2, the isotropic pitch fiber of
the invention yielded silicon carbide fiber product having 96.1 wt
% silicon carbide (i.e., essentially all silicon carbide, within
the range of experimental error), with neither unreacted carbon nor
extractable silica. Also, no whiskers are detectable, even at
500.times. magnification, as can be seen in FIGS. 1b and 1d. In
contradistinction, the comparative examples resulted in significant
quantities of whiskers, as can be seen in FIGS. 2 and 3. Also,
FIGS. 1a-1d highlight the fact that the smoothness of the isotropic
carbon fibers is maintained in the resultant silicon carbide fiber
product.
Example 2
[0098] Quantities of Carboflex.RTM. P-200 isotropic carbon fibers
were heated under argon for times and at temperatures as set forth
in Table 3 below. The treated carbon fibers then were reacted with
silica in the laboratory furnace in accordance with the method of
Example 1. FIGS. 4, 5 and 6 of samples of the resultant silicon
carbide fiber thus produced, show that no whiskers were produced.
Indeed, no whiskers were found after thorough examination of each
of the product samples.
[0099] The following Table 3 summarizes characteristics of the
resulting silicon carbide products of the invention:
3 TABLE 3 Product Carboflex .RTM. Wt % Specific Isotropic SiC
Surface Diam- Carbon Fiber, Conver- Whiskers, Area, Length, eter,
Fig- P-200 sion wt % m.sup.2/g microns microns ure As received 96.1
None 11.2 188 14.5 1b, 1d Pretreated 1 hr 96.5 None 8.6 114 11.9 4
at 1500.degree. C. Pretreated 1 hr 95.8 None 6.4 105 12.3 5 at
1800.degree. C. Pretreated 7 hr 97.3 None 2.8 133 11.6 6 at
1800.degree. C.
[0100] Neither unreacted carbon nor extractable silica was found in
any product. The quality of the product fibers was very good, as
can be seen from the Figures. The fiber length and diameter of the
products of pretreated fibers were somewhat lower than those of the
product of the untreated fibers, reflecting the same reductions in
the carbon fibers, as set forth in Table 1.
[0101] The preheat treatment of the isotropic fibers (Table 1)
shows that the carbon fibers are reduced in length and diameter.
These analyses, together with the data in Table 3 for the resultant
silicon carbide fibers of the invention produced, show that the
effect of preheat treating of Carboflex.RTM. P-200 carbon fiber was
to reduce both the average lengths and diameters, reduce surface
areas markedly, and lower the sulfur content of the carbon fiber.
During the heat treatment of the fiber, sulfur and sulfurous
products were detected in the exhaust gases.
[0102] FIGS. 4, 5 and 6 illustrate that the fibers of the invention
of Example 2 are essentially devoid of whiskers. The reductions of
length and fiber diameter occur mainly during the preheat
treatment, rather than during conversion to silicon carbide
fiber.
[0103] The data also show that surprisingly, the sulfur present in
the fiber in concentrations between 0.26 and 1.43 wt % did not
inhibit conversion to silicon carbide fiber.
[0104] The graphitic structure developed in the precursor due to
preheating the isotropic carbon fiber for 7 hours does not result
in the presence of whiskers in the silicon carbide fiber product of
the invention. However, both the mesophase pitch fiber and PAN
fiber have very low sulfur content less than (0.1 wt %) and give
rise to whiskers when reacted with silica.
Example 3
[0105] Silicon carbide fiber of the invention was prepared in
accordance with Example 1. However, a stoichiometric ratio of 3.0
moles of carbon to 1.0 mole of silica was used. Carboflex.RTM.
P-200 isotropic pitch carbon fiber was used.
[0106] Data for the silicon carbide product is set forth in Table
4, which also contains data relating to Carboflex.RTM. P-200 from
Example 1.
4TABLE 4 Blend Product molar Wt % ratio SiC Surface Diam- Carbon/
Conver- Carbon Silica, Area, Length, eter, Silicon sion wt % wt %
m.sup.2/g microns microns Fig. 2.7:1.0 96.1 None None 11.2 188 14.5
1b, 1d 3.0:1.0 98.4 0.2 None 12 122 12.0 7
[0107] The fiber quality of the fibers produced from the 2.7:1
blend of Example 1 was superior to that of those produced from the
stoichiometric blend (3:1) of this Example, but both were
acceptable. No whiskers were produced in either case. The quality
differences are difficult to see by comparing FIGS. 1b and 1d with
FIG. 7, but the greater reduction in length and diameter of the
product fiber of this Example lends support to this
observation.
Example 4
[0108] Silicon carbide fiber of the invention was prepared in
accordance with Example 1, except that the Carboflex.RTM. P-200 of
Example 1 was replaced by an equal weight of Carboflex.RTM. P-600,
a lower sulfur grade of isotropic carbon fiber. The molar ratio of
carbon to silica remained 2.7:1.
[0109] No whiskers were produced. Table 5 sets forth product
characteristics for silicon carbide fiber of this Example, together
with comparable data from Examples 1 and 2.
5TABLE 5 Carbon Fiber Silicon Carbide Product Source, Sulfur, Wt %
SiC Fiber Density, Treatment wt % Conversion g/cc Figure P-200, as
received 1.43 96.1 3.19 1b, 1d Pretreated 1 hr at 0.85 96.5 3.18 4
1500.degree. C. Pretreated 1 hr at 0.26 95.8 3.20 5 1800.degree. C.
Pretreated 7 hr at 0.36 97.3 3.20 6 1800.degree. C. P-600, as
received 0.59 99.3 3.20 8
[0110] Table 5 suggests that the effect of sulfur content on the
production of silicon carbide fiber produced was minimal. It is
surprising that the sulfur is not detrimental to the conversion to
silicon carbide. Further, the presence of sulfur in the carbon
fiber did not reduce the quality of the fibers.
Example 5
[0111] Silicon carbide fiber of the invention was prepared with
each type of promoter individually, and without any promoters. The
fibers were prepared in accordance with the method of Example
1.
[0112] Data on the composition and unreacted carbon for each of the
fibers made using only one or no promoter is set forth in Table 6
below. For comparison, the same information of product of Example 2
made using both promoters also is set forth in Table 6.
6TABLE 6 Starting Materials 2.7 moles C Product per 1 mole 1.5 wt %
0.6 wt % Wt % SiC Carbon, Fig- of Si FeSO.sub.4 Ca(OOC).sub.2
Conversion Wt % ure X X X 96.1 None 4 Detected X X 99.2 0.8 9 X X
92.0 0.5 10 X 97.3 None 11a, Detected 11b
[0113] No whiskers were observed in any of the product samples, as
can be seen in the Figures. This result is surprising, as the
skilled practitioner would have expected the presence of promoters
to have produced whiskers. Both iron and calcium salts are reported
to be whisker promoters.
[0114] The Figures illustrate the degraded quality of the fibers.
Crystalline structure and granularity of the silicon carbide fiber
of the invention are seen most clearly in FIGS. 11a and 11b. Such
degraded fiber material may have different commercial uses from the
uses of non-degraded fiber.
[0115] Thus it can be seen that the presence of both types of
promoters greatly increases fiber quality. Silicon carbide fiber of
the invention produced in accordance with a preferred embodiment,
in which two promoters are present, essentially retains the
morphology of the isotropic pitch carbon fiber used. Thus, the
resultant silicon carbide fiber is formed in smooth-surfaced
cylinder sharing dimensions similar to the dimensions of the carbon
fiber starting material. Although both the diameter and length of
product fibers typically have reduced diameter and length, as
illustrated in these Examples.
Example 6
[0116] Silicon carbide fiber was prepared in accordance with the
method of Example 1, except that the atmosphere was nitrogen rather
than argon. FIG. 13 is an SEM photograph at 200.times.
magnification of the silicon carbide fiber thus produced. A
significant quantity of whiskers can be seen in the photograph.
Comparison of FIG. 13 with FIG. 1b illustrates the whisker-laden
nature of fiber produced under nitrogen with the whisker-fee nature
of silicon carbide fiber of the invention prepared under argon.
[0117] Specific surface areas of both the initial carbon fibers and
the silicon carbide fiber product were determined employing the BET
method. Densities were determined using helium pycnometry. The
specific surface area of the carbon fibers was about 30 m.sup.2/g.
The silicon carbide fibers made in accordance with the method of
the Examples had specific surface areas of between about 2.5 and
12.0 m.sup.2/g and densities of about 3.2 g/cc.
[0118] Any source of fine silica can be used in the claimed
invention. Furthermore, the method of the claimed invention results
in essentially no whiskers and P-silicon carbide fiber of high
quality and gray-green color when using promoters. Fiber quality is
degraded if only one or no promoters are used, but the resultant
silicon carbide fiber of the invention is essentially devoid of
whiskers. The conversion of carbon fiber to silicon carbide fiber
does not significantly change the morphology when using two
promoters. The degradation in fiber quality experienced in the
absence of both promoters changes the morphology, as shown in FIGS.
9, 10, 11a, and 11b.
* * * * *