Process for preparing ceramic fibers

A process for preparing silicon carbide fiber by the carbothermal reduction of silica fiber. In the first step of the process, a specified silica fiber is contacted with a source of elemental carbon to produce a reactant mass; the silica fiber is comprised of at least about 99.5 weight percent of silica, has a density of at least about 2.15 grams per cubic centimeter, has a diameter of from about 1 to about 100 microns and an aspect ratio of at least about 30. From about 3.2 to about 5.0 moles of carbon are present in the carbon source for each mole of the silica. The reactant mass is heated at a temperature of from about 1,400 degrees centigrade to about 2,300 degrees centigrade for at least about 0.5 hours.

FIELD OF THE INVENTION 
A process for preparing a non-oxide ceramic fiber in which an oxide ceramic 
fiber is heated in the presence of a carbon-containing material and/or 
nitrogen containing material. 
BACKGROUND OF THE INVENTION 
Silicon carbide fibers, because of their excellent physical and mechanical 
properties, are widely used in industry. Thus, for example, silicon 
carbide fabric surfaces are preferred over silica fabrics as part of 
thermal protection system (TPS) for space shuttle vehicles since they 
survive higher heating loads than silica fabrics. For example, no 
observable deterioration was observed on the silicon carbide fabric 
surface at heating rate exposures of 37 W/m.sup.2 whereas a silica fabric 
became brittle after exposure to 10 W/m.sup.2. Also, the higher emissivity 
of silicon carbide over silica was found to be another advantage for 
higher temperature environments. 
Silicon carbide fibers can be used as reinforcements to toughen ceramics 
for use at high temperatures; they are inherently more resistant to 
oxidation, have a high elastic modulus, have better creep resistance (the 
ability to carry load at high temperatures without substantial 
deformation), and finally they withstand gaseous and high temperature 
liquid corrosion products that have either alkaline or acidic 
characteristics. Of these desirable properties, their superior strength at 
elevated temperatures is of great value. In ceramic matrix composites 
where matrix failure precedes failure of the fiber bundle, these fibers 
carry the load and contribute to the so-called graceful failure of the 
composite. Essentially, instead of the catastrophic failure that one 
encounters in using monolithic ceramics, ceramic matrix composites with 
silicon carbide fibers as reinforcing agents have the ability to increase 
failure strain substantially so that the user is alerted and has time to 
either repair or replace the damaged area. Thus, catastrophic system 
failures can be avoided. 
Currently, three classes of silicon carbide fibers have been produced. The 
first type of silicon carbide fiber is made via a chemical vapor 
deposition process in which silicon carbide is deposited on a carbon or a 
tungsten fiber core. This process has been taught in U.S. Pat. Nos. 
4,068,037 and 4,702,960; the disclosure of each of these U.S. patents is 
hereby incorporated by reference into this specification. This fiber is 
made and marketed by Textron Specialty Materials Company of Lowell, Mass. 
This class of fibers utilizes a chemical vapor deposition (CVD) process, 
in which silicon carbide is coated on either tungsten or carbon filaments 
to produce large diameter filaments (100 to 150 microns in diameter). For 
this CVD process, various chlorosilanes or mixtures of chlorosilanes 
(including CH.sub.3 SiC.sub.3, (CH.sub.3).sub.2 SiCl.sub.2, and CH.sub.3 
SiHCl.sub.2) are used to produce the silicon carbide coating. 
In the aforementioned CVD process, although the fiber produced has a mantle 
of silicon carbide in the form of beta-silicon carbide crystallites, this 
mantle often contains many other undesired species, including carbon, 
depending on the particular gases that are mixed with the silanes. 
Reference may be had, for example, to P. Martineau, M. Lahaye, R. Paileer, 
R. Nalsan, M. Couzi, and F. Cruege, "Silicon carbide Filament/Titanium 
Matrix Composites Regarded as Model Composites, Part 1, Filament 
Microanalysis and Strength Characterization", J. Mater. Sci. 19 8!2731-48 
(1984). Analytical studies have shown that the silicon carbide fibers 
produced by CVD are complex composite fibers which can vary considerably 
in composition and properties. See, for example, articles by P. Martineau 
et al. appearing in the Journal of Materials Science 19 8!2371-48 (1984), 
by S. R. Nutt et al. appearing in the Journal of Materials Science 
206!1953-60 (1985), and by J. A. DiCarlo appearing in the Journal of 
Materials Science 21 1!217-224 (1986). 
Another process for preparing silicon carbide fibers was developed by 
Yagima et al. who synthesized a polycarbosilane from dichloromethylsilane; 
see, e.g., U.S. Pat. No. 4,052,430. The disclosure of this United States 
patent is hereby incorporated by reference into this specification. 
In the Yagima et al. process, polydimethylsilane was synthesized from 
dichloromethylsilane by chlorination with sodium metal in xylene solvent. 
This was then melt spun at 350 degrees centigrade to form a precursor 
fiber that was then heat treated in vacuum or an inert gas at 1000 degrees 
centigrade to produce a continuous filament containing beta-silicon 
carbide crystallites. This process produced a smaller diameter fiber 
compared to the large diameter fibers produced by the CVD process 
mentioned earlier. 
The Yagima et al. process had been the most widely used polymer conversion 
process for making silicon carbide fibers. Other comparable processes are 
described in U.S. Pat. Nos. 4,534,948, 4,900,531, 4,117,457, 4,847,427, 
4,743,662, 4,816,497, 4,220,600, 4,283,376, 4,342,712, 4,399,232, 
5,082,872, 5,322,822, 5,283,044, 5,344,709, 5,167,881, and the like; the 
disclosure of each of these U.S. patents is hereby incorporated by 
reference into this specification. 
All of these polymer conversion processes suffer from several major 
disadvantages. In the first place, they require the synthesis of a polymer 
precursor material which often has a complicated structure. In the second 
place, they require the spinning of this polymer precursor material into a 
fiber. In the third place, the spun fiber must be "cured" to effect 
cross-linking within the polymer to a specified degree; if this curing is 
not effected in a precise and proper manner, the fibers will fuse together 
when pyrolyzed. In the fourth place, the cured fiber must then be 
pyrolyzed at elevated temperatures to burn off the organic material; if 
the pyrolysis step is not done in a precise and proper manner, the fibers 
may rupture and/or substantial impurities may remain in the pyrolyzed 
fiber. Furthermore, during these steps, undesirable amounts of oxygen are 
often introduced into the fiber. 
The "Nicalon" fibers, as they are usually referred to, do not possess the 
true stoichiometry of silicon carbide. The nature of the polymer pyrolysis 
used to make these fibers produces fibers that contain unreacted silicon, 
carbon, and oxygen at relatively high concentrations as major impurity 
constituents; see, e.g., T. J. Clark, M. Jaffe, J. Rabe and N. R. Langley, 
"Thermal Stability Characterization of Silicon carbide Ceramic Fibers: I, 
Mechanical Property and Chemical Structure Effects", pp. 901-913, Cer. 
Eng. & Sci. Proc. 7, 7-8 (1986). 
Thus, the properties of these "Nicalon" fibers are such that their 
resistance characteristics with respect to exposure to high temperature, 
oxidizing or reducing environments, and the like are poor. These fibers 
dissociate into their constituents when exposed to such environments at 
temperatures greater than 1400 degrees centigrade, resulting in fiber 
degradation, loss of strength, and fiber integrity. This chemical and 
thermo-mechanical degradation feature is a major disadvantage, both during 
composite fabrication and later during composite use. 
Presently, ceramic matrix composites are manufactured using either hot 
pressing or over-pressure sintering or pressureless sintering or a 
combination of these processes. Essentially, the manufacturing process 
involves exposure to temperatures substantially greater than 1400 degrees 
centigrade and pressures that can exceed one atmosphere depending upon 
practice. The degradation of the Nicalon fiber during manufacture of the 
ceramic matrix composite and the reaction of the released elements and 
compounds with those of the matrix constituents poses formidable barrier 
against the use of these fibers on a wider basis. 
Because of the problems with the polymer-derived silicon carbide fibers, 
and the processes used to prepare them, a third class of silicon carbide 
fibers utilizing submicron silicon carbide powder as a raw material along 
with suitable polymeric binders that might include silicon carbide 
pre-ceramic polymer have been developed. These compositions are either 
melt spun or slurry spun into green fibers that are then sintered (using 
continuous sintering) at temperatures greater than 2000 degrees centigrade 
to yield polycrystalline sintered silicon carbide fibers. Reference may be 
had, e.g., to U.S. Pat. Nos. 4,908,340, 4,942,011, and 5,354,527, the 
disclosure of each of which is hereby incorporated by reference into this 
specification. 
Although the submicron silicon carbide powder process is arguably better in 
some respects than the aforementioned polymer conversion processes, it 
suffers from its own substantial disadvantages. In the first place, a 
polymeric carrier also must be used in this process in order to produce a 
spinnable composition, and the organic polymeric material must then also 
be removed during further processing; if the polymer removal step is not 
properly and precisely conducted, the process will produce either 
unsintered fiber and/or low-density fiber. Furthermore, unless all of the 
process variables are consistently and strictly monitored and followed, 
fiber product with non-uniform shrinkage properties and density properties 
often will be produced. Because of the inordinate amount of processing and 
process control required for this product, it is relatively expensive. 
It is an object of this invention to provide a process ceramic fibers with 
excellent physical properties. 
It is another object of this invention to provide a process for producing 
ceramic fibers which is substantially less complex and expensive to run 
than prior art processes. 
It is another object of this invention to provide a ceramic fiber which is 
substantially pure. 
It is another object of this invention to provide a novel apparatus for 
producing a ceramic fiber. 
It is another object of this invention to provide a continuous process for 
producing a ceramic fiber. 
It is yet another object of this invention to provide a batch process for 
producing a ceramic fiber. 
It is yet another object of this invention to provide a process for 
preparing ceramic whiskers. 
It is yet another object of this invention to disclose a process to produce 
ceramic fibrous sheets and ribbons. 
It is a further object of this invention to provide a process to produce 
ceramic composites containing ceramic fibers in sheet, plate, tubular, and 
other geometry. 
It is another object of this invention to provide a process to produce 
monolithic non-oxide ceramics directly from silica and carbon preforms. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a silicon dioxide fiber with a purity in 
excess of 99 percent is contacted with a source of elemental carbon and 
subjected to a temperature from 1,400 to about 2,300 degrees centigrade.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of this invention allows one to produce high-quality silicon 
carbide fiber by a relatively low-cost process. The raw materials used in 
the process are relatively inexpensive. The process utilizes a simple 
furnacing technique that involves self-heating of the charge. Most 
importantly, the process does not require either fiber spinning, fiber 
drawing, polymer chemical conversion, pyrolysis, sintering, or relatively 
expensive chemical vapor deposition (CVD) methods. 
In one aspect of this invention, there is provided a method for producing 
silicon carbide fibers, whiskers, and fibrous whisker-containing 
composites. 
In one preferred process of this invention, silica fiber is used as one of 
the starting materials. This fiber is well known to those skilled in the 
art and is described, e.g., in U.S. Pat. Nos. 5,381,229 (silica fiber and 
alumina fiber), 5,293,438 (silica fiber), 5,201,072 (silica and alumina 
fiber), 5,164,999 (alumina and silica fiber), 3,454,453 (silica fiber), 
3,428,819 (silica fiber), 5,445,634 (quartz fiber), 5,370,642 (quartz), 
4,812,654 (quartz fiber), and the like; the disclosure of each of these 
U.S. patents is hereby incorporated by reference into this specification. 
As is known to those skilled in the art, silica fiber, and yarn and fabric 
products made from it, are readily commercially available. Thus, by way of 
illustration, one may use a "Silfa" silica yam which is sold by the Ametex 
company of 900 Greenbank Road, Wilmington, Del. This material is 
preferably comprised of at least about 99.5 weight percent of silica and 
has a density of at least about 2.15 grams per cubic centimeter. 
By way of further illustration, one may use silica yarn in addition to or 
instead of the silica fiber. Silica yarn is described, e.g., in U.S. Pat. 
No. 4,549,183, the disclosure of which is hereby incorporated by reference 
into this specification. 
By way of further illustration, one may use silica fabric in addition to or 
instead of the silica fiber. Silica fabric is described, e.g., in U.S. 
Pat. Nos. 4,243,715, 3,853,576, and the like; the disclosure of each of 
these United States patents is hereby incorporated by reference into this 
specification. 
Alumina fiber may be used instead of the silica fiber. Thus, e.g., one may 
use one or more of the alumina fibers disclosed in U.S. Pat. Nos. 
5,201,082, 5,164,999, 4,101,615, 5,069,854 (alumina fiber with carbon 
inclusion), 5,320,791, 5,185,299, 5,104,713, 5,051,210, and the like; the 
disclosure of each of these U.S. patents is hereby incorporated by 
reference into this specification. One may also use an alumina fiber mat; 
see, e.g., U.S. Pat. No. 5,145,613. The disclosure this U.S. patent is 
hereby incorporated by reference into this specification. 
In the preferred process of this invention, the main raw materials that 
react for making silicon carbide fibers and silicon carbide fiber mats 
consist of commercially available silica fibers, glass and quartz fiber 
mats, a single carbon source or a combination of carbon sources consisting 
of a variety of materials such as coal tar pitch, coal (anthracite, 
bituminous, etc.), petroleum coke, natural graphite, artificial graphite, 
carbon black, lamp black, acetylene black, furfural alcohol, and 
carbon-containing syrups such as corn-syrup. The carbon-containing 
substance can be in the solid, liquid, or vapor form such as 
carbon-containing gases such as hydrocarbons, and a mixture of these. 
FIG. 1 is a schematic of a preferred furnace 1 which, preferably, is an 
electric resistance furnace. Referring to FIG. 1, it will be seen that the 
electric resistance furnace depicted in FIG. 1 is comprised of a central 
core 1 of a graphite element or a mix of petroleum coke element that has 
been pressed together. A high voltage current is passed through this 
resistance element using copper or other appropriate electrode using a 
transformer and other appropriate electrical circuitry. The cross section 
of the graphite or the pressed coke core can be circular, elliptical, 
square, rectangular, or any irregular shape. For making several individual 
and/or strands of fibers it may be preferable to use plate-like geometry 
of the current-carrying core such that maximizes fiber loading. Before the 
power is turned on, the charge is set up as follows: 
The central graphite/coke core (1) is covered with a layer of carbon source 
(2) either by painting or by layering and building by packing. This layer 
is then covered with a layer of the high purity silica fibers either 
layered along the axis of the graphite core and/or wound around akin to 
filament winding of a mandrel such as used in the making of composite 
pressure vessels(3). The silica fiber-containing layer also contains, 
intermixed with the silica fibers, carbon source materials, either in 
solid or semi solid or liquid form. The silica fibers can also be coated 
with a slip containing carbon such as graphite mold wash materials that 
are used in metallurgical foundry. This assembly is then covered with a 
silica sand/coke mixture (4) that acts as insulation layer. Another layer 
of carbon black (5) that further acts as insulation between the graphite 
walls (8) and the support fire brick side walls (6) can also be a part of 
the furnace build. If needed, further fibrous ceramic insulation fiber 
blankets can also be placed between the graphite side wall and the fire 
brick side wall. Also, if required, a mixture of fine silica sand and 
petroleum coke can loosely cover the entire assembly on the top and sides. 
The whole assembly rests on a the floor on the top of fiber bricks (7). 
Then the electric power applied to this furnace on a programmed schedule 
depending upon the amount of the furnace charge. During the furnace run, 
the mixture of silica sand and petroleum coke as well as the carbon black 
will act as thermal insulator to keep the heat inside the furnace. A view 
from inside the loaded charge, section A--A is shown in FIG. 2. 
Several layers of carbon/silica fiber alternative layers can be built to 
improve the furnace yield. Estimated temperature profile of the furnace 
gives guidance about the multi-layer furnace build. As can be realized, 
this process can be either scaled-up or scaled down depending upon product 
demand. 
Once the reaction has been completed, the furnace power is turned off and 
the furnace is allowed to cool by natural air convection and radiation. 
The furnace is then dismantled and the fibers of silicon carbide are 
harvested. The loose mixture of fire sand which contains low amounts of 
silicon carbide and unused silica and carbon is then stored and re-used 
for the subsequent furnacing operation. 
As described below, the conversion of silica fibers to silicon carbide 
fiber occurs by carbothermal reduction reactions involving temperatures in 
the range of 1400 to 2300 degrees centigrade. The product of this 
invention is silicon carbide fibers of discrete lengths and diameters that 
are dictated by the discrete lengths and diameters of silica fibers that 
were used as reactant materials in the process. 
In a variation of the above process, silica fiber yarns are used instead of 
individual silica fibers as a reactant material used in the process. The 
product of this variation would be silicon carbide fiber yarns. In another 
process variation, silica or quartz cloth is used as one of the reactants 
in the process, with the resulting product being silicon carbide fibrous 
cloth. 
In another embodiment of this process, braids of silica and carbon or 
graphite fibers are interwoven and such mixed silica-carbon (graphite) is 
used along with other carbon-containing material as reactants in the 
process as previously described. In this process, the resulting product is 
still silicon carbide; however, the carbon (graphite) fiber has provided 
additional carbon source for carbothermal reduction of silica fibers. 
The resulting discrete silicon carbide fibers of this process can be 
chopped or otherwise sized to obtain silicon carbide whiskers. They can be 
also ground to fine particles. Such particles, because of the nature of 
small diameter fibers that are used in the process can lead to particles 
with sharp cutting edges which would be eminently suitable for use as 
abrasives for cutting and grinding and other tooling applications. To 
obtain such micro-grit abrasives by the traditional approaches would be 
costly since those are usually obtained by grinding super hard chunky 
silicon carbide grains through several grinding steps. 
In the preferred process of this invention, a variety of silicon carbide 
fiber diameters can be obtained even in a single furnace run dictated by 
the fiber diameters available of the silica and quart fibers. These can 
range in sizes from one to hundreds of microns or greater. 
The apparatus depicted in FIG. 1 is adapted to practice a batch process 
which uses silica fibers of finite lengths that are either layered along 
the axis of the electrode or the silica fibers of finite lengths may be 
wound around the electrode-carbon source in a coil fashion. This is a 
batch process of manufacturing fibers of finite, but variable lengths. 
However, fibers can also be produced continuously where a variety of 
furnace configurations can be used, as necessary. An extension of the 
process involves continuous silicon carbide fiber production using either 
resistance heating of the charge FIG. 3, or induction as shown 
schematically in FIG. 4. 
In an embodiment shown in FIG. 3, the silica fibers (11) are fed into the 
reaction furnace from bottom (12) to top (13) although this direction can 
be reversed in different situations. The silica fibers (11) are surrounded 
by carbon source materials (14) that are in either in the solid or 
semi-solid form in a very loosely packed configuration. Carbon-containing 
gases may also be continuously purged through the fluidized bed system. 
The charge that consists of the silica fiber (11) and carbon-containing 
material (14) is continuously heated by a graphite core (15) at the center 
of the furnace by resistive heating (16). Reaction temperature is between 
1400 and 2500 degrees centigrade. The furnace is enclosed by water-cooled 
end plates (18) and furnace enclosure (17). It is realized that a variety 
of furnace configurations such as circular cylindrical configuration, or 
rectangular or cylinders of square cross section are possible. The furnace 
can be horizontal or in an inclined plane to the horizontal also. Also, 
the central graphite core (15) that carries the electrical current and 
heats the charge can have a variety of cross sections including, but not 
limited to, circular, elliptical, square, rectangular, polygon, and 
irregular cross section. The products of the reaction are continuous 
silicon carbide fibers and gaseous products consisting of sulfur dioxide 
and carbon monoxide and other volatile and non volatile gaseous products. 
The products of the reaction that are in the gaseous state are 
continuously exhausted (19). 
The output of this process is a continuous silicon carbide fiber (13). The 
process as described herein is semi-continuous because although the fibers 
go through the furnace in a continuous manner, because carbon is consumed 
in the process for effecting the reduction of silica in a carbothermal 
reduction reaction as mentioned previously, at some point in the process, 
silica reduction will not take place. The furnace is then shut down and 
replenished with a fresh charge of carbon-containing materials. On the 
other hand, if the input fibrous reactant material consists of a mixture 
of silica and carbon fibers that were previously braided or otherwise 
inter-woven, then this modification of the process can result in making 
silicon carbide fibers in a continuous fashion. 
In another embodiment shown in FIG. 4, the silica fibers (21) are fed into 
the reaction furnace from bottom (22) to top (23), although this direction 
can be reversed in different situations. The silica fibers (21) are 
surrounded by carbon source materials (24) that are in either in the solid 
or semi-solid form in a very loosely packed configuration. 
Carbon-containing gases may also be continuously purged through the 
fluidized bed system. The charge that consists of the silica fiber (21) 
and carbon-containing material (24) are continuously heated by a graphite 
outer tube core (25) which is heated by induction heating (26). Reaction 
temperature is between 1400 and 2500 degrees centigrade. The furnace is 
enclosed by water-cooled end plates (28) and furnace enclosure (27). The 
cross sectional view is shown schematically in FIG. 4. It is realized that 
a variety of furnace configurations such as circular cylindrical 
configuration, or rectangular or cylinders of square cross section are 
possible. The furnace can be horizontal or in an inclined plane to the 
horizontal also. The products of the reaction are continuous silicon 
carbide fibers and gaseous products consisting of sulfur dioxide and 
carbon monoxide and other volatile and non volatile gaseous products. The 
products of the reaction that are in the gaseous state are continuously 
exhausted (29). 
The output of this process is a continuous silicon carbide fiber (13). The 
process as described herein is semi-continuous because although the fibers 
go through the furnace in a continuous manner, because carbon is consumed 
in the process for effecting the reduction of silica in a carbothermal 
reduction reaction as mentioned previously, at some point in the process, 
silica reduction will not take place. The furnace is then shut down and 
replenished with a fresh charge of carbon-containing materials. On the 
other hand, if the input fibrous reactant material consists of a mixture 
of silica and carbon fibers that were previously braided or otherwise 
inter-woven, then this modification of the process can result in making 
silicon carbide fibers in a continuous fashion. 
It is realized that the above embodiments that utilize carbothermal 
reduction of silica fibers to silicon carbide fibers can be adapted to the 
preparation of silicon nitride fibers. In this case, a nitridation 
reaction step is added to the carbothermal reduction process. This type of 
carbothermal nitridation has been successful in converting silica 
particles into silicon nitride powders. My invention uses commercially 
available silica fibers to make silicon nitride fibers. In this process, 
any nitrogen-containing gas, such as ammonia, can also be used to 
facilitate the fiber conversion. 
In a preferred process of this invention, silica fibers are used for direct 
carbothermal nitridation to make silicon nitride fibers. Thus, in the 
embodiments described in FIGS. 3 and 4, the reactant silica fibers can be 
reacted with nitrogen-containing gaseous reactants in conjunction with 
carbon-containing gaseous phases in order to achieve carbothermal 
reduction and nitridation of silica fibers into silicon nitride fibers. 
In another embodiment, in a manner similar to the above embodiment, a 
process is disclosed here whereby aluminum nitride fibers can be made by 
using alumina fibers as a reactant for carbothermal nitridation of these 
fibers into aluminum nitride fibers. Thus in the embodiments described in 
FIGS. 3 and 4, alumina fibers can be substituted for silica fibers and 
nitrogen-containing gaseous reactants can be used in conjunction with 
carbon-containing gaseous phases in order to achieve carbothermal 
reduction and nitridation of alumina fibers into aluminum nitride fibers. 
The embodiments described in FIGS. 3 and 4 use either semi-continuous or 
continuous mode of silicon carbide fiber preparation. An alternate method 
that can be used in a continuous mode of operation involves carbothermal 
chemical reduction as the operating mechanism and uses the following 
technique. Vertical or horizontal furnaces or furnaces that are inclined 
with respect to the horizontal floor are custom-built. Carbon-containing 
gases are passed through continuously in and around silica fiber bundles 
that are continuous and move through the furnace that are then be 
converted to silicon carbide. 
In order to accelerate the reactions, such as SiO.sub.2 +2C=Si+2CO, 
SiO.sub.2 +C=SiO+CO, it may be preferable to add carbon in an amount 
slightly higher than the stoichiometric amount. Thus, the carbon or 
carbon-containing material may be utilized in up to about 10 percent of 
excess of the stoichiometric amount. 
An embodiment of the above-described process is shown in FIG. 5. Here, 
resistance heating of the silica fiber and the gaseous charge by graphite 
heating element in a tubular form is utilized. Silica fibers (31) are fed 
into the reaction furnace from the bottom (32) to top (33), although in 
different situations such direction can be reversed. These silica fibers 
(31) are reacted with carbon-containing gases (40) such as hydrocarbons 
like methane, ethylene, etc. or carbon monoxide, and may also be mixed 
with inert gases such as argon and perhaps hydrogen at temperatures in 
excess of 1400 degrees centigrade which is obtained by the resistive 
heating of the graphite tubular heating element (35) by means of 
appropriate electrical connections (36). The graphite resistance heating 
element is insulated with standard furnace insulation consisting of 
grafoil and other fibrous materials (37) and enclosed by water-cooled 
container walls (38) and end-plates (39). The gaseous products are 
exhausted from the furnace chamber at the top (41). The furnace can be 
horizontal or in an inclined plane to the horizontal also. 
The silica fibers (31) used in this invention as a reactant raw material 
for the silicon carbide fiber production can be previously covered with 
carbon either by dip coating or by gaseous deposition techniques. In this 
case, the furnace is heated by cylindrical graphite resistance heaters 
that will form the periphery of the furnace cylinder. Argon or nitrogen is 
purged continuously to preserve the graphite heating element. The fibers 
can also be passed outside the graphite heating element that can be a 
solid cylinder or a hollow one with varying geometry such as a square, 
rectangle, polygon, or circular or an ellipse or of any irregular shape. 
In instances where the presence of silicon monoxide is required for 
conversion, provision can also be made to evaporate solid SiO and carry 
this using a carrier gas such as high purity argon. Also, hydrocarbons 
such as methane, ethylene, acetylene, either by themselves, or in 
combinations thereof, in addition to hydrogen, can also facilitate the 
production of continuous fibers by accelerating the carbothermal reduction 
reaction. The pressure of the reactive gas may be less than, equal to, or 
greater than the atmospheric pressure. Also, specific gaseous or vapor 
phase atmospheres can be established that use boron, and 
aluminum-containing additives in addition to carbon containing gas in 
order to further densify, strengthen and toughen the silicon carbide 
product. 
In a different embodiment of the above principle of silica fiber conversion 
to silicon carbide fibers, induction heating is utilized for furnacing 
operation. An embodiment of the above-described process is shown in FIG. 
6. Here, resistance heating of the reactant silica fiber and the gaseous 
charge by induction heating of graphite outer tube is described. Silica 
fibers (51) are fed into the reaction furnace from the bottom (52) to top 
(53), although in different situations such direction can be reversed. 
These silica fibers (51) are reacted with carbon-containing gases (54) 
such as hydrocarbons such as methane, ethylene, etc. or carbon monoxide, 
and may also be mixed with inert gases such as argon and perhaps hydrogen. 
Reaction occurs at temperatures in excess of 1400 degrees centigrade 
obtained by radiative heating from the heated graphite tube (55), heated 
by induction (56). The furnace is enclosed by water-cooled container walls 
(57) and end-plates (58). The gaseous products are exhausted from the 
furnace chamber at the top (60). The furnace can be horizontal or in an 
inclined plane to the horizontal also. 
A variation of the above-disclosed continuous fiber making operation is 
that the reactant silica fibers can be inter-braided with carbon or 
graphite fibers before being fed into the silicon carbide ceramic 
conversion furnace. For continuous fiber making, in order to protect the 
graphite heating element use of air-lock facility is preferred. This area 
can be flushed with nitrogen using positive flow control. 
In another embodiment of the invention shown in FIG. 1, ceramic silicon 
carbide articles with or without these silicon carbide fibers can be 
fabricated directly from raw material constituents. In the first instance, 
a process to make silicon carbide hollow tubes that contain silicon 
carbide fibers, will be described. The process is described in FIG. 7. The 
central graphite/coke core (71) can be either a solid cylinder or a hollow 
cylinder and has rectangular cross section in the schematic that is shown. 
The cylinder can be of any cross section desired of the final silicon 
carbide product, such as, but not limited to, circle, rectangle, square, 
hexagon or any polygon, and ellipse. It can also have fins arising from 
its outer surfaces. This central hollow graphite/petroleum coke core is 
covered with a layer of carbon source (72) either by painting or by 
layering and building by packing. This core is then covered with a layer 
of the high purity silica fibers either layered or wound along the axis of 
the graphite core and/or wound around akin to filament winding of a 
mandrel such as used in the making of composite pressure vessels(73). The 
silica fiber-containing layer also contains carbon source materials, 
either in solid or semi solid or liquid form as well as fine silica 
powder. The silica fibers can also be coated with a slip containing carbon 
such as graphite mold wash materials that is used in metallurgical 
foundry. This assembly is then covered with a silica sand/coke mixture 
(74) that acts as insulation layer. Another layer of carbon black (75) 
that further acts as insulation between the graphite walls (78) and the 
support fire brick side walls (76). If needed, further fibrous ceramic 
insulation fiber blankets can also be placed between the graphite side 
wall and the fire brick side wall. Also, if required, a mixture of fine 
silica sand and petroleum coke can loosely cover the entire assembly on 
the top and sides. The whole assembly rests on a the floor on the top of 
fiber bricks (77). Then the electric power applied to the graphite core on 
a programmed schedule depending upon the amount of the furnace charge. 
During the furnace run, the mixture of silica sand and petroleum coke as 
well as the carbon black will act as thermal insulator to keep the heat 
inside the furnace build. A view from inside the loaded charge, section 
B--B is shown in FIG. 8. 
Once the reaction has been completed, the furnace power is turned off and 
the furnace is allowed to cool by natural air convection and radiation. 
The furnace is then dismantled and the silicon carbide tubular article 
containing silicon carbide fibers supported by silicon carbide grains is 
recovered simply by dislodging the product. The loose mixture of fire sand 
which contains low amounts of silicon carbide and unused silica and carbon 
is then stored and re-used for the subsequent furnacing operation. 
As has been previously discussed, the conversion of silica fibers to 
silicon carbide fiber product occurs by carbothermal reduction reactions 
involving temperatures in the range of 1400 to 2300 degrees centigrade. 
The tubular product of this invention can contain silicon carbide fibers 
of discrete lengths and diameters that are dictated by the discrete 
lengths and diameters of silica fibers that were the initial reactant 
materials that were used as raw materials for the process. 
In a variation of the above process, silica fiber yarns are used instead of 
individual silica fibers as a reactant raw material component. The tubular 
product of this embodiment would be silicon carbide tubes containing 
silicon carbide fiber yarns. In another process variation, silica or 
quartz cloth is used in the process, with the resulting tubular product 
containing being silicon carbide fibrous cloth in layered form that are 
bonded together by fine silicon carbide grains that result from this 
process. 
In another embodiment of this process, braids of silica and carbon or 
graphite fibers are interwoven and such mixed silica-carbon (graphite) 
fibrous reactant raw material is used along with other carbon-containing 
material in the process as previously described. In this process, the 
resulting product is still silicon carbide tube; however, the carbon 
(graphite) fiber has provided additional carbon source for carbothermal 
reduction of silica fibers. 
Yet another embodiment of the above process involves making open-ended 
silicon carbide tubes containing no silicon carbide fibers but grains of 
silicon carbide that are bonded together. Such a product is made of single 
crystals that have very high thermal conductivity and are fused together. 
In this invention, I disclose a silicon carbide tube making process 
directly from silica and carbon using carbothermal reduction chemical 
reaction process. 
An embodiment of making tubes that consist of silicon carbide grains that 
are fused together is shown in FIG. 9. The central graphite/coke core (81) 
can be either a solid cylinder or a hollow cylinder and has rectangular 
cross section in the schematic that is shown; it can be of any cross 
section desired of the final silicon carbide product, such as, but not 
limited to, circle, ellipse, rectangle, square, hexagon or any polygon, 
and any irregular cross section. It can also have fins arising from its 
outer surfaces. This central hollow graphite/petroleum coke core is 
covered with a layer of a mixture of fine silica sand intermixed with 
carbon source (82) and built by packing. This is then covered with a layer 
of carbon black (83) that further acts as insulation between the graphite 
walls (84) and the support fire brick side walls (85). If needed, further 
fibrous ceramic insulation fiber blankets can also be placed between the 
graphite side wall and the fire brick side wall. Also, if required, a 
mixture of fine silica sand and petroleum coke can loosely cover the 
entire assembly on the top and sides. The whole assembly rests on a the 
floor on the top of fiber bricks (86). Once the furnace has been built, 
electric power is applied to the graphite core on a programmed schedule 
depending upon the amount of the furnace charge. During the furnace run, 
the mixture of silica sand and petroleum coke as well as the carbon black 
will act as thermal insulator to keep the heat inside the furnace build. A 
view from inside the loaded charge, section C--C is shown in FIG. 10. 
Once the reaction has been completed, the furnace power is turned off and 
the furnace is allowed to cool by natural air convection and radiation. 
The furnace is then dismantled and the silicon carbide tubular article 
containing silicon carbide grains that are fused together is recovered 
simply by dislodging the product. The loose mixture of fire sand which 
contains low amounts of silicon carbide and unused silica and carbon is 
then stored and re-used for the subsequent furnacing operation. 
As described previously, the conversion of silica to silicon carbide 
tubular product occurs by carbothermal reduction reactions involving 
temperatures in the range of 1400 to 2300 degrees centigrade. The tubular 
product of this disclosure can be of a variety of discrete lengths and 
thickness as dictated by commercial demand. 
In addition to the open-ended tube making process described above, the 
process can also be modified to produce closed-end tubes such as U-tubes, 
tubes with elbows and bends, etc. The graphite core configuration and the 
power input engineering will be dependent upon the size and shape of such 
tubular configurations. 
An embodiment for making a U-tube made of silicon carbide ceramic fibers 
with silicon carbide particles or grains is shown in FIG. 11. In this 
figure which shows a cross sectional top view of the furnace, only half of 
the furnace is reproduced for clarity with the central line shown in the 
figure. The graphite core (91) is of U-configuration and is of circular 
cross section. In reality, any cross sectional shape can be utilized 
depending upon the need. This graphite core is heated by resistive heating 
(92), by utilizing appropriate electrical circuitry that can generate 
temperatures up to 2600 degrees centigrade. The graphite core (91) is then 
covered with a thin layer of carbon source material (93) which is then 
covered with a layer of silica fibers either by themselves or inter-mixed 
with carbon-graphite fibers or other carbon-containing solid or semi-solid 
or liquid material (94) as well as fine silica sand. Surrounding this all 
around is a mixture of carbon-containing material and silica sand (95). 
This layer is further covered with carbon black and a mixture of 
electrical insulation (96) that will prevent electrical path across the 
U-shaped core. The furnace is protected by graphite walls (97) and then 
with fire brick wall (98). In one-end, the distance between the fire brick 
wall and the graphite wall can be covered with thermally insulating 
fibrous materials. (99). A view from inside the loaded charge, section 
D--D is shown in FIG. 12. Here, the furnace bottom (100) made of fire 
brick is also shown. 
Once the reaction has been completed, the furnace power is turned off and 
the furnace is allowed to cool by natural air convection and radiation. 
The furnace is then dismantled and the silicon carbide tubular article 
containing silicon carbide grains that are fused together is recovered 
simply by dislodging the product. The loose mixture of fire sand which 
contains low amounts of silicon carbide and unused silica and carbon is 
then stored and re-used for the subsequent furnacing operation. 
As described previously, the conversion of silica to silicon carbide 
tubular product occurs by carbothermal reduction reactions involving 
temperatures in the range of 1400 to 2300 degrees centigrade. The tubular 
product of this disclosure can be of a variety of discrete lengths and 
thickness as dictated by commercial demand. 
These tubes can be used as thermal wells, thermocouple protection tubes, 
and depending upon the porosity achieved in the process as high 
temperature particular filters in coal gasification power generation 
systems, diesel particular filters and other similar applications. 
One other embodiment will be disclosed hem which is relevant to making 
finned ceramic radiant tubes with very high surface area that can radiate 
heat in heating molten metal and molten glass baths. Because the overall 
process technology disclosed by my invention converts silica and quartz 
cloths into silicon carbide cloths, these cloths can be attached to the 
outer surface of a silicon carbide fiber ceramic tube, all made in one 
process. In the present invention, silicon carbide sheet heat conductors 
protrude out of silicon carbide fibrous tubes. Because of their higher 
temperature capability, the present invention offers a variety of special 
industrial and scientific applications that involve dissipation of heat in 
still air or other hostile industrial processing atmospheres. The cross 
section of the final product is shown in FIG. 13. The source of heat such 
as burner (113) is inside the silicon carbide fiber/silicon carbide 
particle tube (111) which has silicon carbide sheets or fins (112) 
protruding from the surface of tube (111) which have been made together in 
the process disclosed here. 
The development of low-cost, high quality silicon carbide fibers produced 
by this invention that uses relatively straight-forward technique forms 
the foundation for the manufacture of a variety of components in many 
industries. Such fibers will find commercial use in heat exchangers and 
chemical reactors subject to high-temperature, high-pressure, and highly 
corrosive environments. Other applications include: waste heat recovery 
air preheaters in aluminum re-melt facilities, ceramic radiant tubes in 
aluminum melting and holding furnaces, ferrous heat treatment furnaces, 
and applications in high-temperature incineration of municipal and 
industrial waste systems. These fibers can also be potentially useful in 
the manufacture of hazardous and toxic waste containment vessels. These 
silicon carbide ceramic fibers can also be made into chain curtains either 
during the carbothermal reduction process as practiced by this invention 
or by subsequent braiding operation of the fibers resulting from this 
invention. These special curtains can be hung before the uncovered opening 
of a furnace. This will then effectively cut down on heat, glare, gases 
and sparks which escape (thus contributing to safety) during a furnace 
operation. It also keeps the outside cold air from entering the furnace. 
Thus savings can be realized in fuel consumption because heat loss from 
radiant heat from the furnace is reduced. Because the curtains are made of 
silicon carbide chains or cloths, that withstand serve corrosive 
atmospheres encountered in furnace operations, these can be selectively 
installed inside furnaces to separate temperature zones. 
Advantage of porous silicon carbide fiber-containing composite is that it 
is inherently more resistant to cracking induced by thermal loads and 
thereby can exhibit improved resistance to thermal spalling. Therefore, 
the articles made of this invention are amenable to use as furnace walls, 
mufflers, abrasives, and rocket nozzles. Also, the process is eminently 
suitable to make tubes that can be used as high-temperature filters in 
coal-gasification power generation systems and the like. Tubes with 
U-configuration and other specialized configuration can be constructed 
using this process. 
The silicon carbide fibers and fibrous products of this invention can be 
used for crucibles, boats and processing tubes in the semiconductor 
industry where higher temperatures are used for heat treatment. Other 
applications for products derived from this invention include rocket 
nozzles, solid, fluid gas, and compounds containing a mixture of these 
transfer nozzles that have to accommodate constant and variable pressure 
in any application. 
The silicon carbide cloths produced from this invention can be used in 
thermal protection systems instead of silica cloths due to its superior 
resistance to increased thermal exposure rates and superior emissivity 
characteristics when compared with such properties for the currently used 
silica cloths. 
The fibrous products of this invention may be used as thermocouple 
protection tubes in high temperature environments, protective tubes that 
can house metallic and ceramic heating elements and igniters used in 
appliances such as dryers as well as high temperature kilns and furnaces. 
The process lends itself to the making of products of a variety of size and 
shape capabilities. Plates of silicon carbide fiber containing ceramic 
composites made of this process can be used as furnace furniture with 
superior higher temperature bending strength and creep resistance. Thus, 
furnacing capacity can be increased allowing remarkable improvements in 
furnace throughput Also, silicon carbide fiber containing silicon carbide 
composite plates, in relatives thin structures can be used as liners of 
refractory materials. Such use will help contain the heat within the 
furnace more because of the higher thermal emissivity of silicon carbide. 
In addition, the improved corrosion resistance will increase the useful 
life of refractory walls, roofs and furnace bottoms. 
The advantages of this invention over existing silicon carbide fiber making 
technology include: 1) low-cost production relative to existing technology 
due to the use of low-cost readily available raw materials and novel 
furnacing technique involving relatively low technology that can be 
practiced world-wide, 2) crystalline silicon carbide fiber that will 
withstand high temperature (&gt;1400 degrees centigrade) use, 3) relatively 
high flexibility in scale-up or scale-down to large or small volume 
production dictated by market demand, and 4) the process imposes no 
processing-related restriction on the fiber diameter. Rather, it is 
dictated by the raw material that is used and the process is applicable to 
all available silica containing fibers and fiber and whisker mats that can 
be used as reactant materials. 
In applicant's preferred process, in which silica fiber is used, the silica 
fiber preferably has the purity and density properties described elsewhere 
in this specification. It also preferably has an aspect ratio (the ratio 
of its length to its diameter) of at least about 30; its diameter is 
preferably from about 1 to about 100 microns and, more preferably, from 
about 10 to about 20 microns. During the preferred process, the silica 
fiber and the carbon source are subjected to a temperature of from about 
1,400 to about 2,300 degrees centigrade (and preferably from about 1,800 
to about 2,200 degrees centigrade) for at least about 0.5 hours. 
In applicant's preferred process, from about 3.2 to about 5 moles of carbon 
from the carbon source are used for each mole of silica in the silica 
fiber. 
It is to be understood that the aforementioned description is illustrative 
only and that changes can be made in the apparatus, in the ingredients and 
their proportions, and in the sequence of combinations and process steps, 
as well as in other aspects of the invention discussed herein, without 
departing from the scope of the invention as defined in the following 
claims.