U.S. patent application number 11/186941 was filed with the patent office on 2006-05-18 for silicon carbide fibers essentially devoid of whiskers and products made therefrom.
This patent application is currently assigned to Advanced Composite Materials Corporation. Invention is credited to Derek J. Angier, James F. Rhodes, William M. Rogers.
Application Number | 20060104882 11/186941 |
Document ID | / |
Family ID | 27757332 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060104882 |
Kind Code |
A1 |
Angier; Derek J. ; et
al. |
May 18, 2006 |
SILICON CARBIDE FIBERS ESSENTIALLY DEVOID OF WHISKERS AND PRODUCTS
MADE THEREFROM
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 oxidation 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.: |
11/186941 |
Filed: |
July 22, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10191973 |
Jul 10, 2002 |
|
|
|
11186941 |
Jul 22, 2005 |
|
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Current U.S.
Class: |
423/345 |
Current CPC
Class: |
Y10T 428/2913 20150115;
Y10T 442/638 20150401; C04B 2235/5248 20130101; C04B 2235/722
20130101; C04B 2235/77 20130101; C04B 2235/723 20130101; C04B
35/62675 20130101; C04B 2235/3418 20130101; C04B 2235/3208
20130101; C04B 35/62281 20130101; C04B 2235/448 20130101; C04B
2235/726 20130101; C04B 2235/72 20130101; C04B 2235/5264 20130101;
D01F 9/08 20130101; Y10T 428/249921 20150401; C04B 2235/5256
20130101; C04B 2235/449 20130101; C04B 2235/3272 20130101; C04B
2235/526 20130101 |
Class at
Publication: |
423/345 |
International
Class: |
C01B 31/36 20060101
C01B031/36 |
Claims
1. Discontinuous silicon carbide fiber essentially devoid of
whiskers, essentially devoid of boron, and having less than about
1.25 wt percent nitrogen and an apparent density of greater than
about 1.65 g/cc.
2. Silicon carbide fiber of claim 1, wherein the silicon carbide
fiber has less than about 1.1 wt percent nitrogen.
3. Silicon carbide fiber of claim 2, wherein the silicon carbide
fiber has less than about 0.9 wt percent nitrogen.
4. Silicon carbide fiber of claim 1, wherein the silicon carbide
fiber has an apparent density of at least about 1.85 g/cc.
5. Silicon carbide fiber of claim 4, wherein the silicon carbide
fiber has an apparent density of at least about 2.0 g/cc.
6. Silicon carbide fiber of claim 5, wherein the silicon carbide
fiber has an apparent density of at least about 2.2 g/cc.
7. Silicon carbide fiber of claim 2, wherein the silicon carbide
fiber has an apparent density of at least about 1.85 g/cc.
8. Silicon carbide fiber of claim 7, wherein the silicon carbide
fiber has an apparent density of at least about 2.0 g/cc.
9. Silicon carbide fiber of claim 8, wherein the silicon carbide
fiber has an apparent density of at least about 2.2 g/cc.
10. Silicon carbide fiber of claim 3, wherein the silicon carbide
fiber has an apparent density of at least about 1.85 g/cc.
11. Silicon carbide fiber of claim 10, wherein the silicon carbide
fiber has an apparent density of at least about 2.0 g/cc.
12. Silicon carbide fiber of claim 10, wherein the silicon carbide
fiber has an apparent density of at least about 2.2 g/cc.
13. Silicon carbide fiber of claim 4, wherein the silicon carbide
fiber has less than about 0.9 wt percent oxygen.
14. Silicon carbide fiber of claim 7, wherein the silicon carbide
fiber has less than about 0.9 wt percent oxygen.
15. Silicon carbide fiber of claim 10, wherein the silicon carbide
fiber has less than about 0.9 wt percent oxygen.
16. Silicon carbide fiber of claim 12, wherein the silicon carbide
fiber has less than about 0.9 wt percent oxygen.
17. Silicon carbide fiber of claim 1 wherein the silicon carbide
fiber has a true density of at least 3.0 g/cc.
18. A regenerable medium for filtering combustible carbonaceous
compounds from fluids comprising silicon carbide fiber of claim
1.
19. A fibrous web comprising silicon carbide fiber of claim 1.
20. The web of claim 19, wherein the web is non-woven.
21. The web of claim 20, wherein the web is wet laid.
Description
[0001] This application is a Continuation-in-Part of application
Ser. No. 10/191,973 filed Jul. 10, 2002, which is incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of Invention
[0003] The present invention is directed to discontinuous silicon
carbide fibers and products made therefrom. In particular, the
invention is directed to dense discontinuous silicon carbide fibers
that retain the morphology of the carbon source, are essentially
devoid of boron, respond to microwave energy, and are essentially
devoid of whiskers.
[0004] 2. Description of the Related Art
[0005] 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.
[0006] Silicon carbide is commonly available in particulate,
whisker, fiber, and cloth forms. Each form has distinct properties
and characteristics exploitable in divers industrial
applications.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] Other methods for producing silicon carbide whiskers include
use of iron to catalyze the formation of whiskers from rice hulls
(Horne, U.S. Pat. No. 4,283,375). Similarly, Horne, 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.
[0015] Silicon carbide whiskers are not satisfactory for all
purposes. Whiskers are very small. Therefore, whiskers are not
satisfactory for applications in which relatively long fibers are
preferred. For example, whiskers often are so small as to be
difficult to incorporate into a fibrous web.
[0016] Whiskers also present an environmental problem. Airborne
whiskers could present a health hazard. For example, the production
of respirable particles 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. As can be seen,
therefore, when whiskers are not the desired product, it is
important to avoid production of whiskers as a by-product.
[0017] 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. However, the prior art does
not include a suitable method to produce such products essentially
without whiskers.
[0018] 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.
[0019] 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. Yajima, U.S. Pat. No. 4,100,233, describes a method of
producing continuous 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.
[0020] SYNTHESIS OF SIC-BASED FIBERS DERIVED FROM HYBRID POLYMER OF
POLYCARBOSILANE AND POLYVINYLSILANE, Proceedings of the
International Symposium on Novel Synthesis and Processing of
Ceramics, 107-112 (1997), A. Idesaki, M. Narisawa, K. Okamura, M.
Sugimoto, T. Seguchi, and M. Itoh, discloses a fiber comprising
oxygen, silicon, and carbon. Polycarbosilane or a mixed
polycarbosilane/polymethylsilane solution was partially crosslinked
by heating, then melt-spun. The melt-spun material then is cured by
heating in air at least to 1,000.degree. C. to carbonize the
material. The resultant product is identified as silicon carbide
fiber. However, this is a misnomer because the product comprises
between about 6 and 13 wt percent oxygen. The presence of oxygen in
the backbone of this so-called silicon carbide fiber renders it
unsuitable, especially in a moist environment.
[0021] 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.
[0022] Okada, U.S. Pat. No. 5,618,510, discloses a method for
producing silicon carbide fiber, sheets, and three-dimensional
articles having a silicon nitride coating. Carbon fiber is
activated in a known manner. The porous activated carbon material
having a specific surface area of 100 to 2500 m.sup.2/g is treated
with silicon monoxide gas at a temperature of 800 to 2000.degree.
C. Pressure during silicidation must be 10 Pa or less to fully
convert the carbon and prevent formation of whiskers. The resulting
silicon carbide fiber material then is heat treated in nitrogen in
the absence of oxygen to reduce porosity of the surface and to coat
the material with silicon nitrides. The product has an oxygen
content of 1.0 wt percent and a nitrogen content of 2.0 wt
percent.
[0023] PREPARATION OF SILICON CARBIDE FIBER FROM ACTIVATED CARBON
FIBER AND GASEOUS SILICON MONOXIDE, Okada, K., H. Kato, R. Kubo,
and K. Nakajima Ceramic Engineering and Science Proceedings 16(4):
45-54, 1995, discloses manufacture of silicon carbide fiber from
activated carbon fiber having specific surface area of 500 to 2500
m.sup.2/g, and exemplified use of carbon fiber having a diameter of
10 microns and a specific surface area of 960 m.sup.2/g. The carbon
fiber is reacted with silicon monoxide at 10 Pa pressure. The
resultant silicon carbide fiber was treated in oxygen at
800.degree. C. to combust materials other than silicon carbide. The
resultant silicon carbide fiber shows a granular structure not
present in the activated carbon fiber. The specific surface area
was reduced from 960 to 50 m.sup.2/g due to growth in pore size.
Although photomicrographs indicate that granularity was reduced,
the density appears to have remained the same, as the dimensions of
the fibers appear to remain unchanged. The particles are described
as more dense after treatment with nitrogen at 1600.degree. C.
Although silicon nitride was not found by x-ray diffraction, the
nitrogen content of the particles was 2.0 wt percent. The nitrogen
content is found in the 3 micron surface layer, thus essentially
permeating the 10 micron fiber, or approximately 85 percent by
weight.
[0024] Okada, U.S. Pat. No. 5,676,918, discloses a method for
producing silicon carbide fiber having a high tensile strength and
a uniform structure without whiskers. Activated, porous carbon
fiber having a specific surface area of 100 to 3000 m.sup.2/g and
length at least 5 mm are reacted with silicon monoxide at
800-2000.degree. C. at a pressure no more than 100 Pa. The fibers
must be held in tension to produce the desired result. The low
pressure is required to reduce whisker formation. The silicon
carbide fiber has the same form as the carbon fiber, and the
dimensions of the silicon carbide fiber and the carbon fiber are
essentially unchanged. Silicon carbide fiber produced from
unactivated, non-porous carbon fiber having specific surface areas
lower than 100 m.sup.2/g remained unreacted at the core.
[0025] Nakajima, U.S. Pat. No. 5,922,300, discloses a method for
converting porous carbon filaments, yarns, and woven and non-woven
fabrics having a specific surface area of 300 to 2000 m.sup.2/g to
silicon carbide fiber. The carbon filaments are mixed with a
silicon-containing powder, such as silicon or silicon dioxide, and
heated to 1200 to 1500.degree. C. The patent discloses that
whisker-producing catalysts, such as iron, must not be present, and
the pressure is 1000 Pa or less to prevent whisker formation. The
resultant silicon carbide fiber product is separated from the
remaining particles by sieving or washing. The resultant silicon
carbide fiber product has essentially the same dimensions (length
and diameter) as the carbon fiber, and thus has a relatively low
density. Example 1 discloses that Renoves.RTM. A-10 is suitable as
activated carbon fiber precursor. This material, commercially
available from Osaka Gas K.K., has a specific surface area of 1100
m.sup.2/g and a pore volume of 0.54 cm.sup.3/g. Therefore, the
calculated apparent density, as that term is defined herein, of the
exemplified product of Example 1 is 1.17 g/cc.
[0026] Okada, U.S. Pat. No. 6,316,051, discloses a method for
manufacturing silicon carbide fiber, yarn, or fabric by reaction of
activated carbon fiber having a specific surface area of 700 to
1500 m.sup.2/g with silicon or silicon monoxide powder at
1200-1500.degree. C. under reduced pressure. The fibers then are
treated with a boron-containing substance in an amount sufficient
to provide at least 0.1 wt percent boron and heat-treated at a
temperature of 1700-2300.degree. C. Whisker production is minimized
by removing volatiles from the activated carbon fiber. Silicon
carbide fiber product has the same structure as the carbon fiber
from which it was made, as does the product heat-treated without
boron. However, the boron-containing, heat-treated product is said
to have a higher density.
[0027] Nixdorf, U.S. Pat. No. 6,767,523, discloses a method for
producing silicon carbide fiber having up to 1 wt percent whiskers.
Carbonized cotton fibers and fumed silica powder are reacted in the
presence of ferrous sulfate and calcium oxalate at about
1700.degree. C. According to the patent, carbonized cotton fiber is
the sole carbon source that will produce silicon carbide fiber
having up to 1 wt percent whiskers, and no other carbonized organic
fiber will work in this method. The resultant fibers retain the
morphology of the carbon fiber, and so remain not very dense. The
size of the silicon carbide fiber is limited by the length of the
chopped carbon fiber derived from cotton. The length is short,
about 1/8 to 1/2 inch.
[0028] Silicon carbide fiber products produced by these methods are
not completely satisfactory. The resultant silicon carbide fiber is
porous and not dense, is dense only at the surface, or is
contaminated with densifying agents such as nitrogen, boron, or
silicon nitride. Some products are limited in size by the
limitations of the raw materials and include undesirable whiskers.
Whiskers are air pollutants and must be controlled to minimize
health problems, especially of those who handle them.
[0029] Thus, there remains a need for an easily implemented,
economical, and environmentally benign method of producing
homogenous, dense, discontinuous, silicon carbide fibers
essentially devoid of whiskers.
SUMMARY OF THE INVENTION
[0030] The invention is directed to discontinuous silicon carbide
fiber essentially devoid of whiskers, essentially devoid of boron,
and having less than about 1.25 wt percent nitrogen, and an
apparent density greater than about 1.65 g/cc, and to products made
with the fibers. Skilled practitioners recognize that such fibers
are not single crystals. The silicon carbide fibers of the present
invention are produced at a high yield, are essentially devoid of
whiskers, and are dense because they retain the morphology of the
carbonized fiber if promoters are used. The silicon carbide fibers
of the invention, and especially silica-coated fibers of the
invention, can be readily incorporated into other media, such as
ceramics, plastics, and metals, via conventional processing
technology.
[0031] Because silicon carbide fiber of the invention is
essentially devoid of whiskers, it is easier to handle in an
environmentally responsible manner. The fibers of the invention are
not coated with boron or silicon nitride, and have low oxygen and
nitrogen concentrations. The silicon carbide fiber of the invention
can be coated with silica. Silicon carbide fiber of the invention
also can be produced in preselected lengths or ranges of
lengths.
[0032] The silicon carbide fibers of the present invention are
economically produced, as the reaction is carried out at
atmospheric pressure. Silicon carbide fibers of the invention are
exceptionally responsive to microwave energy, and have excellent
resistance to 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 combustible
carbonaceous compounds from fluids such as diesel engine exhaust.
Silicon carbide fiber of the invention also is adapted to form
non-woven webs for various uses.
[0033] 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.
[0034] Thus, 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
whiskers, and so do not require expensive handling techniques and
do not present health hazards associated with respirable
whiskers.
[0035] These discontinuous silicon carbide fibers are particularly
useful for, but not limited to, incorporation into a filter-heater
apparatus for the removal of combustible carbonaceous 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 combustible carbonaceous
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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] 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.
[0037] FIG. 2 is an SEM photograph of silicon carbide product
prepared from Cytec ThermalGraph.RTM. DKD X mesophase pitch-based
carbon fiber at 200.times. magnification.
[0038] FIG. 3 is an SEM photograph of silicon carbide product
prepared from Fortafil.RTM. PAN M275 carbon fiber at 200.times.
magnification.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] FIG. 12 is a plot of x-ray diffraction data for silicon
carbide fiber of the invention.
[0048] FIG. 13 is an SEM photograph at 500.times. magnification of
silicon carbide fiber made under a nitrogen atmosphere.
[0049] FIG. 14a is a photomicrograph at 200.times. magnification of
silicon carbide fiber having 0.1 wt percent whiskers made in
accordance with the method disclosed in Nixdorf, U.S. Pat. No.
6,767,523. FIGS. 14b-d are photomicrographs at 200.times.
magnification of the sample of FIG. 14a to which whiskers were
added to yield 0.2, 0.6, and 1.0 wt percent whiskers,
respectively.
[0050] FIGS. 15a-d are scanning electron microscope (SEM) images at
300.times. magnification of samples depicted in FIGS. 14a-d.
DETAILED DESCRIPTION OF THE INVENTION
[0051] The invention is directed to discontinuous silicon carbide
fibers essentially devoid of whiskers, and to products comprising
the fibers. Silicon carbide fiber of the invention may be uncoated,
or may be coated with silica. The fibers are dense, are low in
oxygen and nitrogen, and are devoid of silicon nitride. In
accordance with the method of the invention, discontinuous
isotropic carbon fibers from a melt spun or melt extruded process
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.
[0052] 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
therefore are relatively dense and uniform. The fibers are
especially suited for incorporation into media such as ceramics,
plastics, and metals to, e.g., improve strength and other
characteristics thereof. Silica 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, thus indicating the relatively low nitrogen
content.
[0053] Silicon carbide fiber of the invention has relatively low
oxygen content because the reaction of silica and carbon is
essentially complete. Silicon carbide fiber of the invention has
relatively low nitrogen content and is devoid of densifying
compounds such as boron or silicon nitride. Because the product
silicon carbide fiber is dense, it is not necessary, as it is for
silicon carbide fiber made with activated, low-density carbon
sources, to undertake to increase the density of this product.
Silicon carbide fiber of the invention is not coated with silicon
nitrides resulting from a nitrogen post-formation densification
treatment.
[0054] The method of the invention is economical. The method is
carried out at atmospheric pressure in a standard graphite
resistance furnace. Therefore, furnaces capable of operating at
pressures of less than 1000 Pa, or to as little as 1 Pa, that are
required in the prior art and that are expensive and costly to
operate, are not necessary.
[0055] The method of the invention also does not require additional
densification steps after formation of the fiber, thus saving both
time and money. 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 combustible carbonaceous 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
combust carbonaceous compounds trapped by the filter. Silicon
carbide fibers of the invention are resistant to degradation even
after numerous exposures to microwave energy.
[0056] 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.
[0057] 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 an area of sample is
magnified under a light microscope at a magnification of about 200
or 250.times.. If a whisker can be seen when examining this area of
a first sample in a light microscope at a magnification of about
200.times. or 250.times., then a second sample is examined in a
Scanning Electron Microscope (SEM) at a magnification of about
200.times. or 250.times.. The approximate area encompassed in such
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
(except for FIGS. 15a-d) in this application.
[0058] Calculations illustrate that identification of a sole
whisker at 250.times. magnification means that the whisker content
is 1 in 16,600, or about 0.006 wt percent (based on having 20
silicon carbide fibers in the view). If the second view is free of
whiskers, the combined observations illustrate a whisker content of
3.6.times.10.sup.-5 wt percent. (A third whisker-free view would
reduce the content to 2.1.times.10.sup.-9 wt percent, or 2.1
ppb.)
[0059] The importance of essentially eliminating whisker production
during manufacture of silicon carbide fiber is significant. A
whisker content of 1 wt percent, as contemplated by U.S. Pat. No.
6,767,523, may generate a health hazard and certainly requires
careful handling with appropriate protective equipment during
manufacture and subsequent handling. At an average whisker length
of 105 microns and a diameter of 1.0 microns (as disclosed in U.S.
Pat. No. 4,873,069) and an average fiber length of 1550 microns
with a diameter of 7.5 microns (as disclosed in U.S. Pat. No.
6,767,523), it is calculated that there are 43 billion whiskers, or
8.3 whiskers per fiber, in 1 pound of silicon carbide fiber
contaminated with 1 wt percent whiskers.
[0060] The American Conference of Governmental Industrial Hygenists
(ACGIH) recommends a time weighted average (TWA) threshold limit of
0.1 respirable fibers per cubic centimeter per 8 hours for silicon
carbide whiskers and fibers. [2003 ACGIH.RTM. Silicon Carbide, CAS
No. 409-21-2.] Therefore, as can be calculated, processing 1 pound
of silicon carbide fiber having 1 wt percent whiskers would require
430,000 m.sup.3 of air introduced in laminar fashion to ensure a
whisker concentration of 0.1 fiber/cc. Skilled practitioners
recognize that at least this much air would have to be evacuated
from the workplace and then filtered before reintroduction to the
atmosphere. Also, workers would be required to wear personal
protective clothing, including respirators, and to exercise special
precautions when leaving the plant. Further, there exists the
possibility that whiskers may be liberated from products into which
they are incorporated. The environmental impact can be
significant.
[0061] 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
generalized formula: 3 C+SiO.sub.2.fwdarw.SiC+2 CO.
[0062] Carbon fibers suitable for use in the invention are known to
skilled practitioners melt spun or melt extruded isotropic carbon
fibers. Isotropic carbon fibers, which are melt-spun or
melt-extruded from isotropic pitch, are one type of isotropic
fiber. Such isotropic carbon fibers are not high-performance fibers
and exhibit performance, in areas such as modulus and tensile
strength, significantly inferior to that of carbon fibers made from
mesophase pitch or poly-acrylonitrile (PAN).
[0063] The structure of carbon fiber is derived from two causes.
First, the fiber takes on an orientation during spinning and
drawing. Then, during pyrolytic conversion, or carbonization, most
non-carbon atoms are removed and the structure tends to become more
graphitic. Isotropic carbon fibers are produced without
introduction of graphitic or crystalline structure during formation
and carbonization. High-performance carbon fibers derive their
properties and characteristics primarily from the orientation
introduced during formation, and this graphitic structure persists
after carbonization. Both mesophase pitch and PAN fibers are
high-performance fibers with a high degree of orientation
introduced during spinning and pyrolytic conversion of the
precursor fiber.
[0064] Carbon fiber suitable for use in the invention is known to
skilled practitioners as isotropic carbon fiber, even though such
fiber is not purely isotropic. Rather, isotropic carbon fiber has
essentially no anisotropy, or visible crystallinity. Isotropic
carbon fiber has a small degree of orientation or crystallinity
because it is a spun and drawn product. However, isotropic carbon
fiber is from an isotropic petroleum pitch or a coal tar pitch and
is spun under conditions that minimize anisotropy. In
contradistinction, mesophase fibers are processed under specific
conditions that form "liquid crystals" during initial processing.
These crystals then introduce a significant degree of anisotropy,
or of crystallinity, into the spun fibers. Mesophase fiber is such
a high-crystallinity anisotropic fiber. PAN fiber also is a
crystalline fiber that, on carbonization, yields a highly
crystalline high performance fiber.
[0065] Mesophase pitch and PAN fibers may be readily distinguished
from isotropic fibers by examination under a petrographic optical
microscope under crossed Nichols prisms, which skilled
practitioners recognize will reveal the high degree of structure in
the mesophase and PAN fibers. Mesophase pitch fibers yield superior
physical properties, but are not suitable for use in the invention.
Similarly, PAN fibers also exhibit high levels of fiber structure
and superior mechanical properties. For example, the tensile
strength of a "high performance" carbon fiber is approximately 10
times that of an isotropic carbon fiber. Highly structured fibers
are not suitable for use in the invention. Graphitic structure is
not found in isotropic fiber as it is supplied, although such
structure can be introduced by graphitization, i.e., heating the
fiber to a high temperature (at least about 1800.degree. C.; more
typically, about 2500.degree. C.) for at least about 7 hours.
However, a minor presence of such graphitic structure in an
isotropic fiber does not cause production of whiskers.
[0066] Isotropic carbon fiber suitable for use in the invention is
melt-spun or melt-extruded. This method of manufacture imbues the
resultant fiber with a uniform cross-section and a smooth, uniform
surface generally free of cracks and fissures. 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 production
is visible in an SEM photograph at 200.times. magnification.
[0067] Suitable carbon fiber is melt-spun or melt-extruded from
isotropic pitch, then is pyrolized. Suitable carbon fiber of this
invention does not have graphitic structure or crystallinity.
Typically, carbonized melt-spun or melt-extruded homogenous carbon
fiber is devoid of a graphitic structure. However, as noted above,
minor portions of graphitic structure present in an otherwise
homogeneous isotropic fiber does not cause production of whiskers
in accordance with the method of the invention. Carbon fiber
preferred in the invention forms no more than incipient graphitic
structure after treatment at 1800.degree. C. for 7 hours in an
argon atmosphere.
[0068] Another form of suitable carbon fiber is derived from a
low-orientation, predominantly amorphous, melt-spun polymeric
filament. This polymeric filament is subsequently pyrolized into a
low-orientation carbon fiber that is devoid of graphitic structure.
One suitable polymer is phenol-formaldehyde resin. Carbon fiber
formed from such resin and devoid of graphitic structure is within
the class of isotropic carbon fiber for the purposes of this
invention. Kynol is the registered trademark for commercially
available low-orientation carbon fibers derived from low
orientation, predominantly amorphous, melt-spun phenol-formaldehyde
resin. Kynol also is within the class of isotropic fibers suitable
for use in this invention.
[0069] The skilled practitioner recognizes that many measures of
density exist. As used herein, "apparent density" is defined as the
mass of a composition divided by the total volume of the solid
matter plus the volume of both the open and the closed pores.
Apparent density is determined by mercury intrusion porosimetry.
"True density" is defined as the mass of the material divided by
its volume, excluding open and closed pores. True density is
determined by helium pycnometry. ASTM D5004 defines bulk density as
the mass of particles divided by the volume they occupy, including
the spaces between particles. Bulk density is determined by mercury
displacement. Theoretical density is the ratio of the mass of a
collection of discrete pieces of solid material having an ideal
regular arrangement at the atomic level to the sum of the volumes
of said pieces. Theoretical density is determined by x-ray
diffraction.
[0070] Suitable carbon fiber is more dense than carbonized cotton
fibers and activated carbon fiber. Indeed, activated carbon fiber
is made from dense carbon fiber by treatment that makes the fiber
porous and lowers its density. Typically, such activated carbon
fibers have an apparent density of between about 0.9 to about 1.15
g/cc, based on mercury intrusion porosimetry. One suitable porosity
determining device is the "AutoPore IV 9500" available from
Micromeritics Instrument Corporation in Norcross, Ga. The greater
apparent density of suitable carbon fiber for use in the invention
is the result of the dense, homogeneous structure of such fibers,
which has few pores. Typically, the apparent density of carbon
fiber used in the invention is greater than about 1.3 g/cc, based
on mercury intrusion porosimetry. Preferably, the apparent density
of suitable carbon fiber is greater than about 1.35 g/cc, more
preferably greater than about 1.5 g/cc, and most preferably greater
than about 1.8 g/cc. The skilled practitioner will, with the
guidance provided herein, be able to identify suitable carbon
fiber.
[0071] 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 25 mm. Fibers
that are longer than about 1 mm will yield silicon carbide product
having low bulk density (even though the particles are dense),
increasing the cost of furnace treatment, packaging,
transportation, and storage.
[0072] 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 specific surface area of less
than about 100 m.sup.2/g, more typically less than about 50
m.sup.2/g, and most typically between about 10 and about 35
m.sup.2/g. The ability to select the length and, to a lesser
extent, to select the diameter, of the carbon fiber used in the
invention provides the skilled practitioner the ability to select
and control properties and characteristics of the silicon carbide
fiber product. Thus, the skilled practitioner is afforded the
opportunity to affect, for example, filtration efficiency,
permeability, and other characteristics of products using silicon
carbide fibers of the invention. Silicon carbide fibers derived
from cotton (such as used in Nixdorf, U.S. Pat. No. 6,767,523) do
not offer this design flexibility.
[0073] Isotropic pitch carbon fibers 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. Anshan Carboflex.RTM. C-25, a 25 mm long carbon
fiber made from melt-spun isotropic pitch, also is a preferred
carbon fiber. 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. These
Anshan fibers are believed to be milled or chopped, then classified
by length.
[0074] 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, slightly reduces the
average diameter and degrades the length. The specific 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. Such pre-treatment adds dimensional stability.
[0075] Particulate silica from any source may be used in the
present invention, including, but not limited to, granular silica,
solid aerosol silica, and colloidal suspensions of silica.
Regardless of the silica source, it is preferred that the particles
be no larger than about 0.5 .mu.m (5000 .ANG.), preferably less
than about 0.3 .mu.m (3000 .ANG.), and most preferably less than
about 0.1 .mu.m (1000 .ANG.). A preferred silica source is
Cab-O-Sil.RTM. grade M5, available from Cabot Corp., Tuscola, Ill.
This product is flumed silica having a surface area of about 220
m.sup.2/g and an approximate bulk density of 0.07 g/cc.
[0076] 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 650.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.
[0077] 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.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] 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.
[0082] 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.
[0083] Silicon carbide fiber of the invention maintains the
morphology of the isotropic carbon fiber from which it is made.
Skilled practitioners recognize that some degradation in fiber
length is to be expected during manufacturing and handling.
However, silicon carbide fiber of the invention will be dense,
smooth fibers essentially devoid of whiskers, as illustrated in the
SEM photographs and photomicrographs shown in the Figures.
[0084] Skilled practitioners recognize that the theoretical density
of .beta.-, or cubic, silicon carbide is 3.21 g/cc. The true
density of silicon carbide fiber of the invention is at least about
3.0 g/cc, preferably at least about 3.05 g/cc, and more preferably
at least about 3.1 g/cc.
[0085] The high true density of silicon carbide fiber of the
invention contributes significantly to the high apparent density of
silicon carbide fiber of the invention. The high true density is
indicative of nearly total conversion to silicon carbide. This high
true density is achieved in accordance with the method herein,
without addition of densification agents such as boron or
densification steps, such as a high-temperature treatment in the
presence of nitrogen to densify the fiber. Rather, silicon carbide
fiber of the invention is homogenous, devoid of boron, and low in
nitrogen.
[0086] The apparent density of discontinuous silicon carbide fiber
of the invention is between about 0.35 to about 0.65 g/cc higher
than the apparent density of the starting carbon fiber. Therefore,
the apparent density of discontinuous silicon carbide fiber of the
invention is greater than about 1.65 g/cc, and preferably greater
than about 1.85 g/cc. More preferably, the apparent density of
silicon carbide fiber of the invention is greater than about 2.0
g/cc, and most preferably is greater than about 2.2 g/cc. Skilled
practitioners recognize that apparent densities greater than about
1.65 g/cc have not been obtained in the prior art with
discontinuous silicon carbide fibers made essentially devoid of
whiskers and without densifying agents.
[0087] 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.
[0088] 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. The surfaces of
the silicon carbide product look smooth even at 500.times.
magnification.
[0089] 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.
[0090] 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 appear to affect
the ultimate conversion of the carbon and silica starting materials
to silicon carbide.
[0091] Although the inventors do not wish to be bound by theory, it
is believed that silicon carbide fiber of the invention is produced
essentially devoid of whiskers in accordance with the invention
because the carbon fibers are smooth, with minimal cracks and
fissures, and are free of asperities and voids that provide
whiskers an opportunity to form. FIGS. 9-11, which are free of
whiskers, were made without promoter.
[0092] Formation of whiskers is promoted by the presence of void
space in the starting materials. U.S. Pat. No. 4,873,069, directed
to production of silicon carbide whiskers, discloses that starting
materials having void volume of at least about 40 percent, and
preferably 80-90 percent, is required for whisker growth. This
patent discloses that such void volume gives sufficient space for
whisker growth. The dense, relatively low porosity carbon fiber
used in the invention does not facilitate such growth.
[0093] As described herein, use of promoters yields silicon carbide
fiber of substantially the same length as that of the carbon fiber
starting material. Although not wishing to be bound by theory, the
inventors believe that the smooth nature of carbon fiber allows the
promoters to completely cover and coat the outside surfaces of the
fibers, thus further reducing the likelihood of whisker nucleation
and growth while improving resistance to degradation, especially in
length.
[0094] The properties and characteristics of the carbon fiber
starting material are believed to make a significant contribution
to the whisker-free nature of the silicon carbide fiber produced
herein. The surface of carbonized cotton fiber is irregular,
fibrilar, and has many asperities, and the fibers are quite porous.
Therefore, even application of promoters appears not to be
effective at eliminating whisker nucleation for such fibers. This
is shown in U.S. Pat. No. 6,767,523, wherein the fiber contains
whiskers. Although the inventors do not wish to be bound by theory,
it is believed that the fibrils and asperities of carbonized cotton
fiber cause at least some of the promoters applied to the fiber
surface to form small beads or droplets that could initiate whisker
production. Further, although the inventors do not wish to be bound
by theory, it is believed that carbonized cotton fibers may contain
trace metallic compounds (from the soil in which the cotton was
grown) not vaporized on carbonation that may act as sites for
whisker nucleation and growth.
[0095] 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.
Specific surface areas of both the initial carbon fibers and the
silicon carbide fiber product were determined employing the BET
method. TABLE-US-00001 TABLE 1 Appar- Discontinuous ent Surface
Sulfur Avg. Avg. Carbon Density, Area, Content, Length, Diameter,
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
[0096] 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 carbon
fibers derived from pitch. 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. However, the carbon fibers are suitable for use in
this invention.
[0097] The mesophase pitch carbon fiber was obtained from Cytec
Carbon Fibers, 7139 Augusta Road, Piedmont, S.C. 29673. The PAN
carbon fiber was obtained from Fortafil Fibers, Inc., P.O. Box 357,
Roane County Industrial Park, Rochester, Mich. 48306.
[0098] FIG. 12 is a plot of x-ray diffraction data for a silicon
carbide fiber of the invention exemplified in Example 1. FIG. 12
illustrates that the silicon carbide fiber of the invention is
essentially .beta.-silicon carbide with a cubic crystal structure.
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. 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
.alpha.-silicon carbide with a hexagonal crystal structure. Thus,
it can be seen that product of the invention is essentially all
.beta.-silicon carbide.
[0099] 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.
[0100] FIG. 12 also indicates that silicon carbide fiber of the
invention does not have a coating, such as boron or silicon
nitrides, and does not contain a significant amount of nitrogen.
Whereas the prior art teaches that boron, nitrogen, and silicon
nitrides are necessary to densify the porous products resulting
from the porous carbon fiber starting material, silicon carbide
fiber of the invention is free of boron and silicon nitrides. Also,
the nitrogen content of the silicon carbide fiber of the invention
is low because it is not densified in nitrogen.
[0101] Silicon carbide fiber of the invention is essentially devoid
of boron because no boron-containing compounds are added to the
surface to densify it. As used herein, `essentially devoid of
boron` means that no boron is added to the surface of the silicon
carbide fiber to densify it. Because no boron is added, silicon
carbide fiber of the invention has less than 0.1 wt percent boron,
preferably less than about 0.05 wt percent, more preferably less
than about 0.03 wt percent, and most preferably less than about
0.01 wt percent boron. Both nitrogen and oxygen will be present,
but at levels significantly lower than those found in the prior
art. Silicon carbide fiber of the invention has an oxygen content
less than about 0.9 wt percent, and a nitrogen content of less than
about 1.25 wt percent, typically less than about 1.1 wt percent,
more typically less than about 0.9 wt percent, and most preferably
typically less than 0.75 wt percent.
[0102] 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.
[0103] 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.
[0104] Skilled practitioners recognize that the water-gas reaction,
2 C+2 H.sub.2O.fwdarw.CH4+CO.sub.2, 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.
[0105] 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 decompacting and mixing.
[0106] 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 disperse 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. 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.
[0107] 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, some of the water in the
reactants and in the atmosphere may react with carbon in accordance
with the water-gas reaction described above, and some carbon may be
lost.
[0108] 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.
[0109] 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.
[0110] 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. A
Noble gas, or a blend thereof, is present in the essentially inert
atmosphere used in the method of the invention.
[0111] Pure nitrogen appears to lead to formation of whiskers, and
so is not preferred an "essentially inert" atmosphere gas. The
inventors have found that use of a nitrogen atmosphere 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
[0112] 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.
[0113] Silicon carbide fiber product of the invention has
morphology essentially the same as that of the starting carbon
fiber material. The diameter and length of the fibers remains
essentially unchanged. This means that the density of the silicon
carbide fiber product is higher than the density of the carbon
fiber starting material. The silicon carbide fiber is homogenous
and the entire carbon fiber is converted to silicon carbide fiber.
Thus, there is no unreacted core.
[0114] Theoretical conversion of 100 percent of the reactants to
silicon carbide would yield a maximum 41.7 wt percent bound silicon
carbide; the balance would be gaseous carbon monoxide. The yield of
silicon carbide fiber by the method of the invention is high. 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 FeSO4 and 0.6 wt percent calcium
oxalate, yielded 96.1 percent of the maximum possible conversion as
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 of the maximum possible
silicon carbide conversion.
[0115] 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.
[0116] Product composition is reliably determined by a combination
of three methods. Residual silica that did not react to form
silicon carbide fiber is measured using HF leaching to remove the
silica. Unreacted carbon is calculated by "burn out" at 600.degree.
C., i.e., heating the fiber at 600.degree. C. in air to combust any
residual carbon. The weight lost is determined. Then, after
unreacted carbon is removed, total carbon present in the form of
the silicon carbide is determined using a carbon analyzer
manufactured by the Laboratory Equipment Company (LECO) of Benton
Harbor, Mich., to determine overall conversion to silicon
carbide.
[0117] 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 at a rate of about 125.degree. C. to more than about
250.degree. C. per second.
[0118] The microwave response of silicon carbide fiber of the
invention can be used to advantage by including silicon carbide
fiber in ceramic filter media to form a regenerable diesel
particulate trap. After a cycle of absorbing particulate material
from diesel motor exhaust, the trap then is regenerated by exposure
to microwave energy and air. The silicon carbide fiber absorbs the
microwave energy and becomes hot, thus combusting and vaporizing
the particulates.
[0119] Silicon carbide fiber of the invention also can be put to
other uses. It can be formed into wet-laid non-woven webs. Silicon
carbide fiber of the invention is particularly suited for such uses
because it has the form of rigid rods of essentially the same
length.
[0120] Skilled practitioners are aware of known method for making
wet-laid non-woven webs. Typically, silicon carbide fiber of the
invention is dispersed in water to make a wet-laid furnish. The
furnish then is introduced to a Fourdrinier machine or other
suitable wet-laid forming device and is dewatered to form a
non-woven web.
[0121] Non-woven webs thus made have basis weights exceeding about
15 g/m.sup.2, and can have basis weights exceeding 500 g/m.sup.2.
Suitable binders, such as self-crosslinking emulsions and high
temperature ceramic binders, also can be used. Non-woven web thus
made is suitable for many uses with additional processing including
filter media, gaskets, mats, thermal insulation,
microwave-susceptible heating devices, and engineered constructs.
The web can be bonded with resin or coated with various
compositions by chemical or physical vapor deposition processes.
With the guidance provided herein, the skilled practitioner will be
able to put silicon carbide fiber of the invention to many
uses.
[0122] 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.
[0123] 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 even in degraded silicon carbide of
the invention in SEM photographs at 200.times. magnification.
EXAMPLES
[0124] The following examples are meant to illustrate the
invention, not to limit it in any way. For example, isotropic
carbon fibers from any source may be used. Similarly, other forms
of silica can be used. The scope of the invention is limited only
by the claims.
[0125] 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
[0126] 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.
[0127] 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.
[0128] 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.
[0129] 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.
[0130] Product properties are summarized and related SEM
photographs are identified in Table 2 below: TABLE-US-00002 TABLE 2
Product Wt % Extracta- Discontinuous SiC Unreacted ble Carbon
Fiber, Conver- Carbon, Silica, Source sion Whiskers FIG. Wt % wt %
Isotropic Fiber, 96.1 None 1b, 1d None None Anshan Detected
Detected Carboflex .RTM. P-200 Mesophase Pitch 97.0 Numerous 2 0.3
0.2 Fiber, Cytec ThermalGraph .RTM. DKD X PAN Fiber, 103.6 Numerous
3 4.6 0.5 Fortafil .RTM. M275
[0131] X-ray diffraction analysis shows the fibers prepared from
P-200 to be predominantly .beta.-silicon carbide, as illustrated in
FIG. 12.
[0132] The unreacted carbon was determined by 600.degree. C.
burn-out in air, 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.
[0133] As can be seen from Table 2, the isotropic carbon fiber of
the invention yielded silicon carbide fiber product having 96.1 wt
% silicon carbide conversion 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
[0134] Quantities of Anshan 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.
[0135] The following Table 3 summarizes characteristics of the
resulting silicon carbide products of the invention: TABLE-US-00003
TABLE 3 Anshan Product Carboflex .RTM. Specific Isotropic Carbon Wt
% SiC Surface Area, Length, Diameter, Fiber, P-200 Conversion
Whiskers m.sup.2/g microns microns FIG. As received 96.1 None 11.2
188 14.5 1b, 1d Pretreated 1 hr at 96.5 None 8.6 114 11.9 4
1500.degree. C. Pretreated 1 hr at 95.8 None 6.4 105 12.3 5
1800.degree. C. Pretreated 7 hr at 97.3 None 2.8 133 11.6 6
1800.degree. C.
[0136] 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.
[0137] 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.
[0138] 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.
[0139] 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.
[0140] The graphitic structure developed in the precursor due to
preheating the isotropic carbon fiber for 7 hours did not generate
whiskers in the silicon carbide fiber product of the invention.
However, both the mesophase pitch fiber and PAN fiber, both of
which have a highly ordered structure, give rise to whiskers when
reacted with silica.
Example 3
[0141] 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.
[0142] 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. TABLE-US-00004 TABLE 4 Product Blend molar ratio Wt %
SiC Carbon Silica, Surface Length, Diameter, Carbon/Silica
Conversion wt % wt % Area, 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
[0143] 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
[0144] 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.
[0145] 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. TABLE-US-00005 TABLE 5
Silicon Carbide Product Carbon Fiber Wt % SiC True Density, Source,
Treatment Sulfur, wt % Conversion g/cc FIG. 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
[0146] 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
[0147] 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.
[0148] 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.
TABLE-US-00006 TABLE 6 Starting Materials 2.7 moles C per 1 1.5
Product mole of wt % 0.6 wt % Wt % SiC Carbon, SiO.sub.2 FeSO.sub.4
CaC.sub.2O.sub.4 Conversion Wt % FIG. 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, 11b Detected
[0149] 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.
[0150] 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.
[0151] 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 carbon fiber used. Although both the
diameter and length of product fibers typically have reduced
slightly, as illustrated in these Examples, the resultant silicon
carbide fiber is a smooth-surfaced cylinder sharing dimensions
similar to the dimensions of the carbon fiber starting
material.
Example 6
[0152] Silicon carbide fiber of the invention having a length of 25
mm was made in accordance with the method of Example 1, with the
exception that preparation times were extended to ensure that the
long fibers were thoroughly covered with promoters and thoroughly
admixed with the silica. Anshan Carboflex.RTM. C-25 fibers were
substituted for the Carboflex.RTM. P-200. The "opening" of the
fibers was extended from one minute to eight minutes. Blending of
the fibers plus promoters was for 5 minutes. Final mixing with the
silica was for an additional ten minutes. Silicon carbide fibers of
the invention approximately 25 mm long were made and were devoid of
whiskers.
Example 7
[0153] Silicon carbide fibers of the invention having a length of 6
mm were dispersed in water and a wet-laid furnish created. The
furnish was cast on Whatman Filter Paper No. 4 as the forming
surface and a non-woven web was formed. The basis weight of the web
was 417 g/m.sup.2. After drying the web at 100-150.degree. C. for a
time significant to remove the water and form a bone dry substrate,
the substrate was treated with Rohm & Haas GL-618
self-crosslinking acrylic emulsion, which acts as a polymeric
binder. Binder was applied at a rate of 2 wt percent based on
solids. Final curing of the fibrous web and binder was at
200.degree. C. for 30 minutes. The resulting non-woven article is
suitable for use as filter media, gasket material, and other
products, and was devoid of whiskers.
Example 8
[0154] 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.
Example 9
[0155] To illustrate the number of whiskers present in prior art
products, samples were prepared at various whisker contents between
0.1 and 1.0 wt percent by adding commercial whiskers to a product
that had 0.1 wt percent, whiskers as produced. Silicon carbide
fiber having 0.1 wt percent whiskers was produced in accordance
with the method of U.S. Pat. No. 6,767,523. This product then was
used to form samples having 0.2, 0.6, and 1.0 wt percent whiskers
by adding the appropriate quantity of whiskers. The whiskers were
dispersed in the silicon carbide fiber product by ultrasonic
dispersion in denatured alcohol, dried, and then re-dispersed in
transparent microscopy immersion oil.
[0156] Photomicrographs and SEM photographs clearly illustrated the
number of whiskers. However, the total number of whiskers could not
be visualized clearly because of depth-of-field limitations.
Photomicrographs at 200.times. magnification and SEM photographs at
300.times. magnification were obtained, as set forth in the
following table: TABLE-US-00007 TABLE 7 Whisker content, wt percent
0.1 0.2 0.6 1.0 Photomicrograph, FIG. 14 a b c d SEM Photograph,
FIG. 15 a b c d
[0157] The photomicrographs each clearly showed more than 1
whisker, even when few fibers are in the view. At 0.6 and 1.0 wt
percent whiskers, the number of whiskers clearly exceeded the
number of fibers. The SEM photographs more clearly illustrated the
number of whiskers present, even at the 0.1 wt percent whisker
content. These photographs also illustrated that silicon carbide
fiber from carbonized cotton fibers were not smooth, but rather
were twisted, misshapen, often showed significant texture, and had
many frayed edges and asperities. Each of these factors may lead to
nucleation and growth of whiskers.
Example 10
[0158] The oxygen and nitrogen concentrations of products of the
invention were determined and compared. Silicon carbide fiber was
prepared in accordance with the method of U.S. Pat. No. 6,767,523
and was analyzed for nitrogen and oxygen by inert gas fusion on a
Horiba EMGA 620W. Silicon carbide fiber of the invention of Example
6 also was analyzed in the same way. U.S. Pat. No. 5,618,510
discloses the nitrogen and oxygen contents of silicon carbide fiber
prepared in accordance with the method disclosed therein.
[0159] The following table summarizes the data obtained:
TABLE-US-00008 TABLE 8 Wt percent Wt percent Nitrogen Oxygen
Nixdorf, U.S. Pat. No. 6,767,523 1.4 2.0 Okada, U.S. Pat. No.
5,618,510 2.0 1.0 Example 6 of the invention 0.31 0.70
[0160] The table shows that the oxygen and nitrogen contents of
silicon carbide fiber of the invention are significantly lower than
those of the prior art.
Example 11
[0161] The apparent densities of carbon fibers and silicon carbide
fiber products made therefrom in accordance with the method of the
invention (Example 1) and of U.S. Pat. No. 6,767,523, respectively,
and of the activated carbon fiber used in Nakajima, U.S. Pat. No.
5,922,300; were determined by mercury intrusion porosimetry. The
apparent density of silicon carbide fiber of Nakajima, U.S. Pat.
No. 5,922,300 was calculated.
[0162] The following table summarizes the results of the
determinations: TABLE-US-00009 TABLE 9 Apparent density, Apparent
density, Silicon carbide Carbon fiber, g/cc fiber, g/cc Nixdorf,
U.S. Pat. No. 6,767,523 1.14 1.02 Nakajima, U.S. Pat. No. 5,922,300
0.90 1.17 Example 1 of this invention 1.35 1.83
[0163] The apparent density of Nakajima silicon carbide fiber was
not determined because no sample was available; the value in Table
9 was calculated, based on the measured apparent density of the
carbon fiber. Although the inventors do not wish to be bound by
theory, it is believed that the open and frangible structure of the
porous carbon fiber of Nakajima acts in the same way as the porous
carbonized cotton fiber of U.S. Pat. No. 6,767,523. Further, even
the products of Nakajima that are densified have the same
morphology as the starting carbon fiber, and are densified only on
the surfaces. Therefore, the apparent density of Nakajima's silicon
carbide fiber product would be expected not to increase by an
amount greater than the increase in the example, or less than or
about 0.30 g/cc.
[0164] 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 .beta.-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 1b.
* * * * *