Oxidation resistant carbon containing alumina-silica articles

A shaped article comprises a continuous alumina-silica first phase, an in-situ generated discontinuous carbon second phase, and optionally a discontinuous silicon carbide third phase, said article being stable to an oxidative atmosphere when heated to 1300.degree. C. for at least 30 minutes.

FIELD OF THE INVENTION 
This invention relates to an oxidation resistant ceramic shaped article 
comprising a continuous alumina-silica phase and a discontinuous carbon 
phase and, optionally, a discontinuous silicon carbide phase. In another 
aspect, it relates to a process for preparing ceramic shaped articles of 
the invention. The shaped articles are useful in high heat emissivity 
applications. 
BACKGROUND ART 
Within the last decade, an amount of literature has been published 
describing various polycrystalline, microcrystalline, or non-vitreous 
fibers and other shaped articles of refractory metal oxides. These 
articles are made by various non-melt processes, such as by drying films 
of solutions of oxygen-containing metal compounds, or drying organic 
polymeric bodies, such as cellulose or rayon, impregnated with such a 
solution, or by extruding and drawing, or spinning, viscous fluids of such 
metal compounds into fibers. The articles are then heated to remove water, 
organic material, and other volatile material to produce refractory 
articles. 
Art in the area of polycrystalline inorganic fibers includes British Pat. 
No. 1,287,288, U.S. Pat. Nos. 3,385,915, 3,632,709, 3,663,182, 3,846,527 
and the art cited in U.S. Pat. Nos. 3,709,706 and 3,795,524. Oxide fibers 
other than those identified as fiberglass are still in the relatively 
early stage of development. 
In many technologies, there is a need for a relatively inexpensive 
continuous refractory fiber product with desirable physical properties, 
such as high strength, high heat emissivity, high modulus of elasticity, 
chemical resistance, and the retention of such properties after exposure 
to high temperatures beyond the capability of presently commercially 
available fiber materials. 
U.S. Pat. No. 4,010,233 broadly discloses inorganic fibers comprising a 
metal oxide phase and a finely divided carbon dispersed phase. There are 
no examples drawn to alumina-silica fibers containing carbon as the 
dispersed phase or to the superior stability in an oxidative atmosphere 
provided by such fibers. The patentee states that mullite fibers which did 
not contain SiC disintegrated into dust when touched (col. 21, lines 
34-36). 
SUMMARY OF THE INVENTION 
Briefly, the present invention provides a shaped article comprising a 
continuous alumina-silica matrix first phase and an in-situ generated 
discontinuous carbon second phase, said article being stable to an 
oxidative atmosphere when heated at 1300.degree. C. for at least 30 
minutes. 
In another aspect, a process is disclosed for providing shaped articles 
such as fibers, flakes, beads, bubbles, granules, or small molded shapes 
which are black in color and have high heat emissivity. In addition to the 
inorganic oxide refractory components, the refractory shaped articles 
contain a dispersed carbon phase which is resistant to a high temperature 
oxidative atmosphere. The black alumina-silica articles have utility in 
high emissivity applications. 
In addition, optionally, silicon carbide can be incorporated into the 
precursor solution or suspension to provide up to 30 weight percent SiC in 
the shaped article. Silicon carbide enhances the retention of the black 
color in the article after high temperature treatment (e.g., 1300.degree. 
C. for 20 to 50 hours). 
The shaped articles of the present invention can be made by a non-melt 
process comprising forming a viscous concentrate of a precursor liquid 
into the desired shape and then dehydratively or evaporatively gelling or 
hydrolyzing the shaped articles. These articles can subsequently be dried 
to result in "green" or non-refractory shaped articles. Heating and firing 
the shaped green article removes water, decomposes and volatilizes 
undesired fugitive constituents, and converts them into the refractory 
shaped articles of the invention. Preferably, the shaped articles are 
fibers. 
The source of the silica matrix component is critical to the article of the 
present invention. The silica source is an organosilane in combination 
with either a silica sol (e.g. amorphous silica), a silicon carbide sol, 
or both. When such a mixed source is present, the resulting article will 
be black and oxidatively stable. The presence of boria can change this 
mixed silica source requirement, as is discussed below. 
In this application: 
"ceramic" means inorganic non-metallic material consolidated by the action 
of heat, such as metal and non-metal oxides; 
"sol" means a fluid solution or a colloidal suspension of metal and 
nonmetal oxides and compounds; 
"non-vitreous" means not formed from a melt of the oxide composition; 
"green" refers to the ceramic articles which are unfired, uncalcined, 
untreated or incompletely treated, that are not in their final ceramic 
form; 
"amorphous" means a material having a diffuse x-ray or electron diffraction 
pattern without definite lines to indicate the presence of a crystalline 
component; 
"crystalline" means a material having an x-ray diffraction pattern 
characteristic of the material; 
"dehydrative gelling" or "evaporative gelling" mean that sufficient water 
and volatile material are removed from the shaped green fibers (or other 
articles) so that the form or shape of the fiber is sufficiently rigid to 
permit handling or processing without significant loss or distortion of 
the desired fibrous form or shape. Therefore, all the water in the shaped 
fiber need not be removed. Thus, in a sense, this step can be called 
partial dehydrative gelling. The shaped fibers in their green form are 
generally transparent to visible light and clear under an optical 
microscope unless silicon carbide is included as an ingredient. Inclusion 
of silicon carbide renders the green fibers opaque and either dark brown 
or black. 
"continuous fiber" means a fiber (or multi-fiber article such as a strand) 
which has a length which is infinite for practical purpose as compared to 
its diameter; 
"continuous alumina-silica matrix first phase" means a homogeneous 
alumina-silica phase in which can be embedded a dispersed carbon phase, 
particles of the carbon phase being submicron in diameter; optionally, an 
additional dispersed phase can comprise silicon carbide; 
"stable" means retention of at least 40 weight percent, preferably 40 to 70 
weight percent, and more preferably 40 to 90 weight percent of the carbon 
present in the elemental form after heating for 0.5 hour at 1300.degree. 
C. in an air atmosphere; and 
"mullite" means an aluminum silicate crystalline compound having an 
alumina/silica mole ratio of 3/2 and requires heating the oxide precursors 
to a temperature of about 1200.degree. C. 
Shaped articles having a ceramic matrix and dispersed therein silicon 
carbide particles, the articles having high modulus of elasticity, are 
disclosed in Assignee's copending patent application, U.S. Ser. No. 
912,830, filed the same date as this application. 
DETAILED DESCRIPTION 
In a preferred embodiment, the present invention provides a shaped article 
comprising a continuous alumina-silica or alumina-boria-silica first 
phase, said first phase comprising 55 to 99 weight percent of the total 
composition of a mixture or chemical combination of 65 to 80, preferably 
72 to 77, weight percent alumina and 35 to 20, preferably 28 to 23, weight 
percent silica, and an in-situ generated discontinuous carbon-containing 
second phase, said carbon second phase comprising 1 to 20, preferably 5 to 
15, weight percent of the total composition. Optionally, an additional 
discontinuous phase can contain silicon carbide in an amount up to 30, 
preferably 1 to 30, most preferably 10 to 20, weight percent of the total 
composition, said article being stable to an oxidative atmosphere when 
heated at 1300.degree. C. for at least 0.5 hour, and when SiC is present, 
for 20 to 50 hours. 
Preferably, the shaped article of the invention is a fiber. The continuous 
alumina-silica fibers which contain a dispersed carbon phase, and 
optionally silicon carbide, are flexible, black, and strong. With 
compositions having alumina and silica in ratios of 72-77 weight percent 
Al.sub.2 O.sub.3 (preferably 72 weight percent alumina) to 28-23 weight 
percent SiO.sub.2 (preferably 28 weight percent silica) and dispersed 
carbon and silicon carbide phases, the mole ratio of alumina to silica is 
in the range of 3:2 to 2:1. Conversion to mullite at temperatures of 
1200.degree. to 1400.degree. C. resulted in continuous fibers with 
excellent resistance to fracturing and resistance loss of carbon. 
In another embodiment, the shaped article of the present invention provides 
a continuous alumina-boria-silica first phase and an in situ generated 
discontinuous carbon second phase and a mechanically added silicon carbide 
third phase, said first phase comprising 65 to 84 weight percent of a 
composition comprising a mixture of 60 to 65 weight present alumina, 25 to 
35 weight percent silica, and up to 15 weight percent boria (preferably 
0.25 to 15 weight percent boria, more preferably 0.25 to 5 weight 
percent), said second phase comprising 1 to 5 weight percent carbon, and a 
third phase comprising 5 to 30 weight percent silicon carbide, the article 
being stable to an oxidative atmosphere when heated at 1300.degree. C. for 
at least 20 hours when boria is present in small amounts. If large amounts 
(about 10 to 15 weight percent) of boria are present, the article is 
stable in an oxidative atmosphere at 1300.degree. C. for at least two 
hours. 
The ceramic fibers of the present invention are made by a non-melt process 
comprising shaping a mixture of viscous concentrates of a precursor liquid 
into a fiber form and then dehydratively or evaporatively gelling or 
hydrolyzing the drawn or spun fibers. These fibers can subsequently be 
dried to result in "green" or non-refractory fibers. Heating and firing 
the shaped green fibers removes water, decomposes and volatilizes 
undesired fugitive constituents and converts them into the refractory 
fibers of the invention. 
The starting material or fiber precursor composition from which the first 
phase of the refractory fibers of this invention can be made comprises a 
liquid mixture of silicon compounds which preferably comprises 75 to 99.5 
weight percent of the silica present in the final article from a silane 
compound and 25 to 0.5 weight percent of the silica from an amorphous 
silica source (which is preferably an aqueous dispersion of colloidal 
silica, or silica hydrosol) and a compatible aqueous solution or 
dispersion of a water-soluble or dispersible aluminum compound. The 
silicon and aluminum compounds are those compounds which can be calcined 
to the respective oxides, i.e., silica and aluminum oxide. 
More particularly, the general procedure to prepare the fibers is a 
modification of that described in U.S. Pat. No. 4,047,965. 
Aluminum formoacetate (Niacet.TM., Niacet Corp., Niagara Falls, N.Y.) is 
dissolved in boiling water to give a clear solution containing the 
equivalent of about 10 percent alumina by weight. Low molecular weight 
alkyl alcohol (C.sub.1 to C.sub.3), for example isopropyl alcohol, is 
slowly added to the hot solution which is being rapidly stirred. 
An alternative source of alumina can be made from aluminum powder, formic 
acid, acetic acid and water. For example, the aluminum powder (120 grams) 
can be dissolved in a 90.degree. C. solution of 2200 grams water, 236 
grams formic acid, and 272 grams acetic acid, over a period of eight 
hours. 
Suitable aluminum compounds which can be used as alumina precursors 
representatively include water-dispersible alumina sols and water soluble 
aluminum compounds such as aluminum formoacetate, 
Al(OH)(OOCH)(OOCCH.sub.3), or aluminum isopropylate Al(OC.sub.3 
H.sub.7).sub.3 and mixtures thereof, the organic aluminum compounds being 
preferred, particularly aluminum formoacetate (prepared as disclosed 
above). Aluminum chloride is not a preferred source because of the 
possibility of chlorine retention which is undesirable. In fact, the 
fiberizable liquid from which the fracture resistant fibers of this 
invention are made should be essentially free of chloride, i.e., the 
chloride is less than about 1 weight percent, based on the total 
equivalent oxide weight. Thus, the green fibers are likewise essentially 
free of chloride and the refractory fibers made therefrom have at most a 
trace of chloride, the presence of significant amounts of chloride in the 
refractory fibers, e.g., 0.2 weight percent, having been found to be 
coincident with fragile fibers. 
Generally, sol stabilization aids are added to the alumina precursor 
solution. Preferred aids are lactic acid and formamide or its dimethyl 
derivative. These are used in a weight ratio of about 65% and 30%, 
respectively, based on alumina content of the mixture. 
The silica source can be important in the making of oxidation resistant 
carbon containing fibers. Silica in the resulting ceramic fiber preferably 
originates from two sources unless boria is present. One source can be a 
nonchlorine-containing organosilane, preferably a trialkoxy 
organo-functional silane or a tetraalkoxysilane. The second silica source 
is an amorphous silica which can be derived from a dry powder or from a 
liquid sol. The silane can be added to the aluminum-containing solution in 
one-fourth increments and allowed to hydrolyze completely in about 5 to 20 
minutes depending on the solution temperature (40.degree. to 80.degree. 
C.). This procedure can be repeated until the silane is completely added. 
Then the amorphous silica can be added to the cooled solution. A preferred 
ratio of silane to isopropyl alcohol is 60 parts silane to 100 parts 
alcohol by weight. In the case of alumina-silica fibers with no or very 
low boria content (0 to 2 weight present) of the total silica used, an 
organosilane first component is the silica source for 75-99.5 weight 
percent of the total silica in the refractory fiber. Of the remaining 
silica, 25-0.5 weight percent of the total is derived from either dry 
amorphous silica or liquid form silica sol. A preferred weight ratio of 
silane to silica is 90:10. A most preferred silane is tetraethoxysilane 
because it produces ethyl alcohol on hydrolysis which is compatible with 
the lower alkyl alcohol (preferably isopropyl alcohol) in the system. When 
using dry amorphous silica, an ultrasonic disperser can be used to ensure 
a good dispersion. Subsequently, the sol can be filtered through a 10 
micrometer filter to the larger remaining agglomerates. 
Silanes useful in the present invention include the hydrolyzable monomers 
of the type SiR.sub.4 or SiR.sub.x R'.sub.(4-x) where R and R' are 
independently either an organofunctional group or a hydrolyzable group, 
and x has a value of 0,1,2,3, or 4. For instance, for SiR.sub.4, if R is 
OC.sub.2 H.sub.5, a hydrolyzable group, then the hydrolyzable monomeric 
starting silane is tetraethoxysilane (TEOS). Hydrolysis in water is 
believed to take place as shown by the equation: 
EQU 4H.sub.2 O+Si(OC.sub.2 H.sub.5).sub.4 .fwdarw.Si(OH).sub.4 +4C.sub.2 
H.sub.5 OH 
to form a reactive silanol with a limited life. If the silane contains a 
mixed ligand, e.g. when R is a hydrolyzable ethoxy group and R' is the 
organofunctional group H.sub.2 N(CH.sub.2).sub.3.sup.-, the silane is 
believed to hydrolyze according to the following equation: 
##STR1## 
Again, this hydrolyzed monomer has a limited storage life. The varieties 
of R and R' available are extensive. Any R or R' group can be useful so 
long as a volatile, a water-miscible (usually an alcohol) or an insoluble 
hydrolysis byproduct is produced. Alkoxy groups (C.sub.1 to C.sub.6) such 
as the methoxy, ethoxy, propoxy and butoxy are useful R groups in the 
practice of this invention. The R' organofunctional groups can be, for 
example, aliphatic amino and diamino groups (C.sub.1 to C.sub.6), alkenyl 
groups (C.sub.2 to C.sub.6), mercapto groups (C.sub.0 to C.sub.6), epoxy 
or (meth)acryloxy groups (C.sub.2 to C.sub.6). 
Silanes particularly preferred in the practice of the invention include 
tetraethoxysilane (TEOS) made by Petrarch Systems, Inc., 
gamma-aminopropyltrimethoxysilane (A-1100.TM., Union Carbide), and 
gamma-glycidoxypropyltrimethoxysilane (A-187.TM., Union Carbide). 
For the second silica compound, a precursor silica sol can be used with 
SiO.sub.2 concentrations of 1 to 50 weight percent, and preferably 15 to 
35 weight percent, sols of the latter concentrations being commercially 
available. The silica sol is preferably used as an aqueous dispersion or 
aquasol. The preferred silica is Nalco.TM. 1034-A (Nalco Chemical Company, 
Chicago, Ill.) an aqueous acid system. A second source of amorphous silica 
is 14 nanometer amorphous silica known as Cab-O-Sil.TM., M-5 grade, 
available from Cabot Corp., Tuscola, Ill. 
The presence of a silane in the precursor sol can be important to formation 
of the dispersed carbon second phase in the shaped articles of the 
invention. However, the source of the carbon can be any one of or a 
combination of carbon-containing materials used in the precursor sol. For 
example, the second phase carbon can originate from any one or any 
combination of the carbon-containing acetate or formate groups from 
aluminum formoacetate (Niacet), lactic acid, or formamide. The amount of 
these additives used can effect the carbon content of the final ceramic 
article. For example, if the lactic acid content is increased, the amount 
of carbon in the final article also increases. 
It is known in the art that fibers made with either dry silica or silica 
sol as the only silica source become carbon-free white alumina-silica 
fibers when calcined in air. It is also known that such fibers fired in a 
reducing or inert atmosphere will retain carbon from organic components 
and have a black color. However, such carbon-containing black fibers when 
subjected to high temperature, e.g., 1200.degree. C. or higher, in an 
oxidative atmosphere will become carbon-free and white in color in a short 
time. Usually such fibers have inferior physical properties (e.g., they 
fracture readily). 
SiC in the alumina-silica fiber of the invention is an additive (up to 30 
parts by weigh, preferably 1 to 30 parts by weight, of the total 
composition, can be added) which improves the retention of carbon in a 
1300.degree. C. oxidative atmosphere. The SiC can be made either by plasma 
synthesis or by carbothermal synthesis. 
The silicon carbide particles can be produced by plasma synthesis (from Los 
Alamos National Laboratory [LANL]) or by carbothermal processing 
EQU SiO.sub.2 +3C.fwdarw.SiC+2CO.uparw. 
where carbon black is dispersed into a silica sol, dried, crushed and fired 
in a vacuum furnace at 1400.degree. C. The resultant SiC material is ball 
milled in a solvent (acetone) and filtered to provide the desired particle 
size. 
The consistency of mechanical properties is related to the quality of the 
SiC dispersion in the precursor system. Silicon carbide in powder form (20 
nm) can be dispersed into the alumina-silica or alumina-boria-silica 
precursors by sonicating a mechanical dispersion. A preferred method is to 
partially oxidize the SiC by heating at 600.degree. C. in air for about 
three hours. The oxidized SiC is mixed into the precursor sol and fully 
dispersed by sonication. 
The addition of SiC has a positive effect on the alumina-silica system 
ceramic fibers. For example, mullite (crystalline alumina-silica) fibers 
will begin to lose the carbon after 0.5 hour in air at 1300.degree. C. 
With the addition of SiC, the carbon is retained; for example, after 
exposure for 22 hours at 1300.degree. C. in air, stable black fibers 
remained. The carbon containing fibers without SiC fired at 1300.degree. 
C. for one hour were not as strong as those containing SiC (see Example 
1). In contrast, the SiC/C (unoxidized SiC) mullite fibers had properties 
as follows: tensile strength of 897 MPa (130.times.10.sup.3 psi) and 
elastic modulus of 250 GPa (36.times.10.sup.6 psi). 
A further improvement in fiber proper can be realized by using partially 
oxidized SiC. Fibers prepared with the partially oxidized SiC can retain 
carbon for more than 50 hours at 1300.degree. C. and can possess higher 
tensile strength of up to 2587 MPa (375.times.10.sup.3 psi) and elastic 
modulus of 250 GPa (36.times.10.sup.6 psi). 
Using partially-oxidized SiC it was also possible to prepare high 
emissivity alumina-boria-silica (3:1:2 mole ratio) fibers. This could not 
be achieved without the partially oxidized silicon carbide. 
The oxidized SiC may shift the composition of the final fiber because 
oxidized SiC provides a third source of silica. For example, if 20% (wt) 
oxidized SiC (itself 42% SiO.sub.2) is added to an alumina-boria-silica 
fiber (3:1:2 mol ratio) the final fiber composition will shift to 12% SiC 
(3:1:3 mol ratio alumina-boria-silica). 
In some cases, particularly when plasma synthesized SiC is used, the SiC 
may be further oxidized during firing causing further shifts in the 
composition towards higher SiO.sub.2 contents to the extent that distinct 
SiC is not detectable in the final article. When this occurs, however, a 
beneficial retention of carbon over and above fibers made without SiC is 
realized. When coarser SiC particles are used oxidation occurs to a lesser 
extent and SiC is present in the final product. 
The presence of more than about 10 weight percent boria changes the 
requirement of using an organic silane and a silica sol to produce a black 
oxidatively stable article. Surprisingly, a black article within the 
invention can be provided using a single silica source (amorphous silica) 
if boria and SiC are included as components. 
Details of the process of the invention are as follows: 
The fiber precursor material initially can be a relatively dilute liquid, 
generally containing about 10-30 weight percent equivalent oxide, which 
can be calculated from a knowledge of the equivalent solids in the 
original materials and the amount used, or determined by calcining samples 
of the components starting materials. For the preparation of fibers, it is 
necessary to concentrate or viscosify the dilute liquid in order to 
convert it to a viscous or syrupy fluid concentrate which will readily gel 
when the concentrate is fiberized and dehydrated, for example, when the 
concentrate is extruded and drawn in air to form the fibers. For example, 
the mixture can be concentrated with a Rotovapor.TM. flask 
(Buchi/Brinkmann Rotary Evaporator, Brinkmann Instruments Inc., Westbury, 
N.Y.) under vacuum. The concentration procedures are well known in the 
prior art, see U.S. Pat. No. 3,795,524. Sufficient concentration will be 
obtained when the equivalent solids content is generally in the range of 
25 to 55 weight percent (as determined by calcining a sample of the 
concentrate), and viscosities (Brookfield at room temperature) are in the 
range of 10,000 to 1,000,000 mPa sec, preferable 40,000 to 100,000 mPa 
sec, depending on the type of fiberizing or dehydrative gelling technique 
and apparatus used and the desired shape of the gelled fiber. In making 
bubbles or beads, which utilize a dehydrating liquid rather than drying 
air, or a chemical gellation technique, a low viscosity (10 to 500 mPa 
sec) is preferred. 
In making continuous fiber, the viscous concentrate can be extruded through 
a plurality of orifices (e.g., a total of 10 to 1000) from a stationary 
head and resulting green fibers allowed to fall in air by the force of 
gravity or drawn mechanically in air by means of drawing rolls or a drum 
or winding device rotating at a peripheral speed faster than the rate of 
extrusion. The concentrate can also be extruded through orifices blown by 
a parallel, oblique or tangential stream of high pressure air, such as in 
the making of blown microfibers, the resulting blown green fibers being in 
essentially staple or short form with lengths generally 25 cm or less 
(rather than the continuous filament form) and collected on a screen or 
the like in the form of a mat. Any of these forces exerted on the 
extruded, green fibers cause attenuation or stretching of the fibers, and 
can reduce their diameter by about 50 to 90 percent or more and increasing 
their length by about 300 to 1,000 percent or more and serving to hasten 
or aid the drying of the green fibers. 
The dehydrative gelling of the green fibers can be carried out in ambient 
air, or heated air if desired, for faster drying. The drying rate can 
affect the shape of the fiber. The relative humidity of the drying air 
should be controlled since excess humidity will cause the gelled green 
fibers to stick together and excessively dry air tends to result in fiber 
breakage. Generally, air with relative humidity in the range of 20 to 60 
percent at an operative temperature of 15.degree.-30.degree. C. is most 
useful, although drying air temperatures of 70.degree. C. or more can be 
used. Continuous green fibers are made and gathered together in parallel 
alignment of juxtaposition in the form of a multi-fiber strand. 
The fibers in the green or unfired gel form are dry in the sense that they 
do not adhere or stick to one another or other substrates and feel dry to 
the touch. However, they still may contain water and organics, and it is 
necessary to heat and fire the green fibers in order to remove the 
remaining fugitive materials and convert the green fibers into refractory 
fibers. The green fibers in their continuous form are preferably gathered 
or collected in the form of a strand, the strand then accumulated in a 
relaxed, loose, unrestrained configuration of offset or superimposed loops 
as in a "Figure 8" in preparation for firing. These green fibers can be 
heat treated by placing them in a box furnace or a belt furnace with an 
air atmosphere wherein the temperature is raised from room temperature to 
900.degree. to 1200.degree. C. in a period of about 1 hour. Generally, at 
about 900.degree. C. or above the fibers begin to assume crystalline form. 
The black fibers can be further heat treated to about 1300.degree. C. in 
air and still retain the dispersed carbon phase. These black, high 
temperature fibers have utility where thermal emissivity is needed, e.g., 
space shuttle tile for re-entry protection. 
In firing the green fibers, care should be exercised to avoid ignition of 
combustible organics in the fiber, volatile byproducts and fiber size. 
Such ignition may cause excessive crystallization and defects leading to 
embrittlement and poor quality fiber or even dust, controlled rates of 
heating can be used to volatilize the combustibles so as to avoid 
ignition. 
The shaped articles of the invention can be useful in composites. For 
example, fibers can be used as one component in a mixed fiber composite 
structure and impart a retainable black color in the composite. Such a 
composite can provide heat emissivity properties to lightweight 
structures. 
The procedure for testing tensile strength used a metal chain attached to a 
single fiber. The load applied to the fiber was measured by increasing the 
chain length electromechanically until a break occurred and then weighing 
the minimum length of chain necessary for break. The tensile strength (TS) 
is calculated as 
##EQU1## 
W=weight of chain length at break A=cross-section area of the fiber. 
The modulus of elasticity was determined from flexural vibration as 
described by E. Schreiber and others in Elastic Constants and Their 
Measurement (New York; McGraw-Hill, 1973, Chpt. 4.4) The general equation 
which relates modulus of elasticity (Young's modulus) and the flexural 
resonant frequency (f.sub.E) is: 
##EQU2## 
where K=radius of gyration of the cross-section about the axis 
perpendicular of the plane of vibration. 
m=constant depending on the mode of vibration. 
T=shape factor, which depends upon the shape, size, and Poisson's ratio of 
the specimen and the mode of vibration. 
l=length of the specimen. 
.rho.=density. 
Objects and advantages of this invention are further illustrated by the 
following examples, but the particular materials and amounts thereof 
recited in these examples, as well as other conditions and details, should 
not be construed to unduly limit this invention. All percents are by 
weight unless otherwise stated. In all cases, unless otherwise stated the 
fibers of the invention were stable to an oxidative atmosphere when heated 
to 1300.degree. C. for at least 30 minutes.

EXAMPLE 1 
An alumina precursor solution was made by dissolving 240 grams aluminum 
powder in a hot solution of 4400 grams water, 472 grams formic acid and 
544 grams acetic acid. The mixture was heated at a temperature of more 
than 90.degree. C. for up to eight (8) hours to dissolve most of the metal 
powder. Hydrogen gas was evolved during this reaction. The resulting 
solution was cooled to room temperature. 
Lactic acid (37 grams), formamide (17 grams) and isopropanol (100 grams) 
were added to a 700 gram portion of the alumina precursor solution. 
Methanol may be used in place of isopropanol. Then 57 grams 
tetraethoxysilane (TEOS) (Petrarch Systems, Inc., Bristol, Pa.) were added 
in four equal increments allowing sufficient time between additions for 
completion of hydrolysis (up to 30 minutes), then 14 grams silica sol 
(Nalco.TM. 1034A, Nalco Chemical Co., Oakbrook, Ill.) were added. The 
resulting mixture was filtered through a 14-micrometer filter and 
concentrated in a rotating flask, (Rotovapor.TM. flask, Buchi/Brinkmann 
Rotary Evaporators, Brinkmann Instruments, Inc., Westbury, N.Y.) to a 
viscosity of about 100,000 mPa sec at 25.degree. C. as measured with a 
Brookfield viscometer (Brookfield Engineering Laboratories, Stoughton, 
Mass.). The concentrated fluid was extruded through a 20 hole spinnerette 
under 1.38 MPa (200 psi) pressure. Each hole was 76 micrometers (3 mil) in 
diameter. The extruded filaments were extruded and simultaneously 
stretched with a mechanical wheel to draw the filament diameter to a 
smaller dimension. This process is commonly referred to as a dry spinning 
process. The collected filaments were removed from the wheel and fired in 
an air atmosphere to about 1150.degree. C. over about a 1 hour period to 
produce black, strong and shiny fibers that contained 4.0% (wt) carbon in 
the 70% alumina 30% silica matrix. After heating at 1300.degree. C. in air 
for 30 minutes the black fibers contained 3.4% carbon. These heat-treated 
black fibers were found to be have a tensile strength of 614 MPa 
(89.times.10.sup.3 psi) and on elastic modulus of 165 GPa 
(24.times.10.sup.6 psi) The individual filaments had an oval cross-section 
with major and minor axes of 20 and 10 micrometers, respectively. 
EXAMPLE 2 
Water (2600 grams) was heated to about 80.degree. C., and 657 grams of 
aluminum formoacetate (Niacet.TM., Niacet Corp., Niagara Falls, N.Y.) was 
slowly added with stirring until dissolved and the solution was then 
cooled to room temperature. To this alumina source, 150 grams of lactic 
acid, then 67 grams of formamide, were added with stirring to produce an 
alumina precursor having an alumina content of about 6 weight percent. To 
one-third of this solution was added 100 mL isopropyl alcohol. The 
solution was heated to about 80.degree. C. and 92 grams tetraethoxysilane 
(TEOS from Petrarch Systems, Inc., Bristol, Pa.) was added in four equal 
increments at 10 minute intervals, time enough for the hydrolysis to be 
completed. After the TEOS was completely added and hydrolyzed, the 
solution was clear with a yellow color. To this solution was added 3 grams 
powdered silica (Cab-O-Sil.TM., grade M-5, Cabot Corp., Tuscolo, Ill.). 
The mixture was exposed to a sonicator (Bronson Cell Disrupter, 
Smith-Kline Co., Shelton, Conn.) until the dry powder was fully suspended. 
The total mixture was filtered through a 10 micrometer filter and 
concentrated in a Rotovapor.TM. flask (Buchi/Brinkmann Rotary Evaporators, 
Brinkmann Instruments, Inc., Westbury, N.Y.) to 100,000 cps measured at 
25.degree. C. The concentrated sol was spun using a 76 micrometer (3 mil) 
spinnerette (20 holes) and fired in an air atmosphere furnace up to 
1150.degree. C. in about 1 hour. The 90:10 weight ratio silane-amorphous 
silica sources produced fibers that contained alumina and silica in a 
70:30 weight ratio plus carbon. The black fibers were shiny and strong 
after heat treatment at 1350.degree. C. for 20 min. in air. 
EXAMPLE 3 
Example 1 was repeated except the alumina:silica molar ratio was changed to 
2:1. To a second 700 gram portion of the alumina source was added 44 grams 
TEOS and 12 grams Nalco 1034-A silica sol. The mixture was filtered, 
concentrated and spun into fibers. The fibers were fired to about 
1150.degree. C. in air producing black, strong and shiny fibers that 
contained 2.9% carbon in the 76% alumina: 24% silica (by weight) matrix. 
EXAMPLE 4 
Example 1 was repeated except that differing quantities of lactic acid to 
show the relationship between sol composition and residual carbon in the 
fiber. To separate 700 gram portions of alumina precursor 42 grams and 33 
grams lactic acid additions were used, respectively. The resultant black 
fibers contained 8% and 6% carbon, respectively. 
To two additional 700 gram portions of the alumina precursor 19 grams and 
15 grams formamide respectively were used instead of the 17 grams in 
Example 1. All other ingredients and process procedures were held constant 
to those of Example 1. The resultant black fibers contained 7.3 and 5.5% 
carbon, respectively. 
EXAMPLE 5 
An alumina precursor solution was made by dissolving 112 g aluminum 
formacetate (Niacet) in 500 ml of hot water. The solution was cooled to 
room temperature and 34 grams of lactic acid and 15 grams of formamide 
were added to the solution. In a separate round-bottom reaction flask, 50 
grams gamma-aminopropyltriethoxysilane (A-1100.TM., Union Carbide, New 
York, N.Y.) was added to 100 grams of warm 5% acetic acid solution. When 
hydrolysis was complete, the silane solution was added to the alumina 
precursor solution Finally, 0.3 grams of colloidal silica (Nalco 1034-A) 
was added. The mixture was concentrated and spun into fibers as in Example 
1. The fibers were fired in air to 1175.degree. C. These fibers were black 
but after 1400.degree. C. treatment for 30 min., the color was lighter 
where the fibers were in contact with the furnace. However, as made 
(1175.degree. C.) the fibers contained 15.5% carbon and after 1400.degree. 
C. for 30 min. the carbon analysis was 11.7%. 
EXAMPLE 6 
438 grams of alumina precursor as described in Example 2 was dissolved in 
1800 grams of water to which was added 100 grams lactic acid and 50 grams 
of formamide. To this mixture was added 21 grams of colloidal silica 
(Nalco.TM. ISJ-613.TM., Nalco Chemical Company). Then 188 grams of 
hydrolyzed A-1100 (Example 5) was added to the mixture. The fiber 
precursor was concentrated, spun, and fired (Example 1 procedure) to yield 
black ceramic fibers. 
EXAMPLE 7 
Another silane also produced black, carbon containing fibers. To 500 grams 
of an aluminum formacetate solution (Example 2) was added 25 grams lactic 
acid, 12 grams formamide and 62 grams 
gamma-glycidoxypropyltrimethoxysilane (A-187.TM., Union Carbide 
Corporation). The mixture was stirred and heated (80.degree.-90.degree. 
C.) to hydrolyze the silane and after hydrolysis completion, the mixture 
was cooled and 0.7 g silica sol (Nalco 1034-A) was added. The resulting 
mixture was filtered, concentrated in a rotary evaporator, spun, and 
resulting fibers were heated to about 1150.degree. C. in air for a 2 hour 
period to yield black fibers that contained 16.1% carbon. 
EXAMPLE 8 
Another sample was prepared to produce an alternative product form 
(bubbles). Two hundred nineteen grams aluminum formoacetate (Niacet) was 
dissolved in 900 grams hot water. After cooling 50 grams lactic acid and 
25 grams formamide were added. Then 91 grams TEOS was added as in Example 
1. Then 13.8 grams Nalco.TM. ISJ-613 silica sol (Nalco Chemical Company) 
was added to the solution. About 20 ml of the resulting mixture was 
sprayed though a 20-hole spinnerette with 50 micrometer diameter holes 
into about a pint of well stirred butanol. The resulting suspension of 
alumina-silica precursor in butanol was immediately vacuum filtered 
through #4 Whatman.TM. filter paper (Whatman Ltd., England) to isolate the 
green bubbles. The bubbles were then heated in an air atmosphere to about 
1150.degree. C. for 2 hours to convert them to black, refractory bubbles. 
Photomicrographs of a crushed sample showed that hollow spheres had been 
formed in this process. The outside diameter of the bubbles was about 20 
micrometers and the wall thickness was about 2-3 micrometers. The bubbles 
were used to prepare a light weight composite foam. 
EXAMPLE 9 
Carbon containing ceramic flakes can be prepared by using the spinning 
solution of Example 2. The solution is coated onto a glass surface and 
allowed to air dry. The flake thickness is governed by the thickness of 
the coating. The dry flake particles are then fired in an air atmosphere 
to 1150.degree. C. The particles were shiny and black. 
EXAMPLE 10 
High emittance fibers of 20 wt % SiC in an alumina silica matrix of mole 
ratio 3:2 (mullite) were prepared by the following process. Lactic acid 
(85 wt % aqueous solution), 6 g, and 2.75 g formamide were stirred into 
119.2 g of the aluminum formoacetate solution as described in Example 1. 
The resulting mixture was concentrated in a Rotovapor.TM. flask to about 
half of its volume. 
A dispersion was made by gradually adding 3.46 grams of ultrafine SiC 
(LANL) to 13.5 grams well-stirred TEOS and then diluting with 6.8 grams 
isopropanol. The diluted dispersion was sonicated in a Branson 
Sonifier.TM. for 10 minutes. 
The sonicated dispersion was stirred into the concentrated aluminum 
formoacetate above, allowed to hydrolyze at 99.degree. C., then 
resonicated for another ten minutes. The resulting sol was concentrated 
under vacuum in a Rotovapor.TM. flask partly submersed in a water bath at 
a temperature of 35.degree. to 45.degree. C. until it was viscous enough 
to enable the pulling of fibers with a glass rod. Fibers were formed by 
extruding this concentrate through a spinnerette having 40 holes, 102 
micrometer in diameter, and collecting the fibers on a rotating 28.0 cm 
diameter drum placed 17.8 cm below the spinnerette. The black and 
continuous fibers were cut and removed from the drum in 10 cm long 
bundles. 
A portion of these fibers was fired in air in an electric furnace 
(Lemont.TM. KHT 250, Lemont Scientific Co., State College, Pa.) at a rate 
of temperature increase of 7.5.degree. C. per min. with a 30 min. soak at 
900.degree. C. and another 30 min. soak at 1150.degree. C. The fibers were 
refired in air at 1300.degree. C. for one hour. These 1300.degree. C. 
fired fibers were black, shiny and oval in cross section. The only 
crystalline material discernible by X-ray powder diffraction analysis was 
mullite. The presence of SiC was confirmed by x-ray photoelectron 
spectroscopy. These fibers had a tensile strength of 896 MPa 
(130.times.10.sup.3 psi) and an elastic modulus of 248 GPa 
(36.times.10.sup.6 psi) 
Another portion of green fibers was fired in a belt furnace 10 meters long 
from room temp. to 1150.degree. C. over about a one hour period. These 
fibers contained 2.5% carbon. After heat treatment at 1300.degree. C. for 
16 hours in air they contained 1.6% carbon. The fibers retained a black 
color under the following conditions: 2 hours at 1400.degree. C.; 22 hours 
at 1300.degree. C.; and, more than 55 hours at 1200.degree. C. 
EXAMPLE 11 
Materials and procedure of Example 10 were followed, except different 
concentrations of the SiC were used. 
______________________________________ 
alumina source 
SiC TEOS (18% conc)* Isopropyl alc. 
______________________________________ 
1st Run 
0.15 g 1.35 g 5.5 g 1.0 g 
2nd Run 
3.4 9.2 37.0 4.6 
______________________________________ 
*Example 10 
The resulting fibers were fired in air in an electric furnace (Lemont KHT 
250) at 1300.degree. C. for one hour. The rate of temperature increase was 
7.5.degree. C. per min. with a 30 min. soak at 900.degree. C. and another 
30 min. soak at 1150.degree. C. The fired fibers were all black and shiny. 
The fibers from the first run had 9.8 wt percent SiC. A bundle of these 
felt soft and remained black in color at 1300.degree. C. for 4 hours. 
Fibers from the second run had 26.8 wt percent SiC, were relatively stiff 
and remained black in color at 1300.degree. C. up to 22 hours. 
EXAMPLE 12 
To illustrate the effect of using partially oxidized SiC (LANL) the 
following procedure was used. SiC powder (LANL) (1.7 grams) was partially 
oxidized at 600.degree. C. for 3 hours in a Lindberg.TM. furnace (Lindberg 
Furnace Co., Watertown, Wis.). The partially oxidized SiC was dispersed 
into 85 grams of alumina-silica precursor from example 1 excluding, 
however, the silica sol. The SiC was further dispersed with a Branson 
Sonifier for 10 minutes. The resulting mixture was filtered, concentrated 
and extruded in the typical process to give dark brown unfired fibers. A 
portion of these green fibers were fired on a belt furnace 10 meters long 
using a temperature cycle as in Example 10, from room temperature to 
1000.degree. C. in about 50 minutes, held for 15 minutes then increased to 
1150.degree. C. in about 10 minutes. 
The 1150.degree. C. fired fibers were black and shiny and felt very strong 
when gripped between the index finger and thumb of each hand and were 
pulled to break. 
Another portion of green fibers was fired in air in an electric tube 
furnace (Lemont KHT 250) at a rate of 7.degree. C. increase per minute 
with a 30 min. soak at 900.degree. C. and another 30 min. soak at 
1150.degree. C. They were then heat-treated at 1300.degree. C. in air for 
one hour. 
The fibers which were fired at 1300.degree. C. remained black for 50 hours 
at 1300.degree. C. A calculation based on the amount of silica in the 
oxidized SiC powder indicated that the fiber contained 12 weigh percent of 
SiC in a matrix of 3 alumina:2.6 silica (mole ratio). The only crystalline 
material discernible by X-ray diffraction analysis was mullite. 
EXAMPLE 13 
Alumina-silica precursor (Example 12) 200 grams was heated to boiling, 
boric acid, 0.76 g, was stirred into the alumina-silica sol and heated for 
about 1 minute. The precursor sol was cooled rapidly and filtered through 
a No. 54 Whatman filter paper. 1.5 g of partially oxidized SiC (LANL.TM.) 
(see Example 12 for method) was heated in air for three hours at 
600.degree. C. to partially oxidize the SiC. The SiC was stirred into 
64.12 g of the alumina-boria-silica sol. The dispersion was sonicated for 
10 min. then filtered through a No. 54 Whatman filter again. The resulting 
sol was concentrated at 42.degree. C. under vacuum to a fiberizable 
condition, e.g. a fiber formed with a glass rod. The concentrate was 
extruded using a spinnerette with 40-76 micrometer diameter holes and 
using an extrusion pressure of 1.38 MPa (200 psi). The continuous dark 
brown fibers were fired on a belt furnace (see Example 10). The 
temperature was raised from room temperature to 1125 .degree. C. in about 
50 minutes, held for 15 minutes, then increased to 1175.degree. C. and 
held for 15 minutes. 
The 1175.degree. C. fired fibers were black, shiny, and very strong. The 
fibers were further fired at 1300.degree. C. in air for 16 hours and 
retained the black color. 
EXAMPLE 14 
The material and procedure of Example 13 were utilized except that the 
concentrated sol was extruded through the orifices into a high pressure 
air stream. The resulting blown green fibers were collected on a screen in 
the form of a mat 1.25 cm (0.5 inches) thick. The fibers were brown and 
became dark brown when fired to 1175.degree. C. in a belt furnace, as 
described in Example 13. The microfibers turned to a grey color when 
heated to 1300.degree. C. and remained grey for a 16 hour heating period. 
At 1350.degree. C., the fibers lost some of the grey color after two hours 
of heating and became a mixture of grey and white. 
EXAMPLE 15 
The material and procedure of Example 13 were utilized except 5. 3 g of 
boric acid was used to make the sol. This gave a precursor having a 3:1:2 
mole ratio of alumina:boria:silica. Partially oxidized SiC (1.5 g) as 
described in Example 12, was dispersed in 57.9 g of the 
alumina:boria:silica sol. The fibers obtained from this sol were dark 
brown in color, and were fired in the Lemont.TM. furnace in air to 
1300.degree. C. and held for 1 hour. The furnace heating rate was 
7.5.degree. C./minute. The fired fibers were black and shiny. The fibers 
maintained their black color after 3 hours at 1300.degree. C. 
The calculation based on the amount of silica in the oxidized SiC powder 
indicated that the fiber contained 12% SiC in a matrix of 3.0 alumina:1.0 
boria:2.6 silica (mole ratio). 
EXAMPLE 16 
Alternative ingredients were used to make black ceramic fibers. Aluminum 
acetate stabilized by boric acid, 11.6 grams, (Niaproof.TM., Niacet 
Corporation, Niagara Falls, N.Y.) was stirred into 30 ml water to provide 
an alumina-boria source for the desired fibers. After the Niaproof 
dissolved, 1.5 grams lactic acid (85% solution), 1.2 grams formamide and 
2.6 grams silica sol (Nalco 1034A) were added successively. Then 1.7 grams 
partially oxidized SiC (Example 12) were dispersed and sonicated for 10 
minutes. The resulting suspension was filtered through a Whatman No. 4 
filter paper, concentrated, extruded, and collected on a wheel (Example 
10). 
Portions of the unfired fibers were fired to three different temperatures - 
950.degree., 1150.degree. and 1300.degree. C. and held for 15 minutes. 
Black fibers were obtained in all cases. In addition, when samples of each 
portion were heat treated at 1300.degree. C. for 2 hours in air all the 
samples remained black. 
EXAMPLE 17 
The materials and procedures of example 10 were used to make black fibers 
except SiC powder made by carbothermal synthesis was used in place of the 
plasma produced SiC. The raw materials for the SiC were a 3:1 molar ratio 
of carbon black (Monarch.TM. 1100, Cabot Corp., Glen Ellyn, Ill.) and 
silica sol (Nalco.TM. 2327, Nalco Chemical Company, Oakbrook, Ill.). The 
following equation describes the synthesis 
EQU SiO.sub.2 +3C .fwdarw.SiC+2CO.uparw.. 
The carbon black was dispersed into the silica sol, dried, crushed and 
vacuum fired in an Astro.TM. furnace (Astro Industries, Inc., Santa 
Barbara, Calif.) at 1400.degree. C. for five hours. 
This SiC powder had a particle size range of 6.times.10.sup.-5 to 
9.times.10.sup.-5 mm (600 to 900 Angstroms). Coarse particles had been 
separated from the powder by ball milling in acetone solvent for 20 hours. 
The dispersion was filtered through a No. 4 Whatman filter and refiltered 
through a Balston filter tube grade CQ. The acetone was evaporated. SiC 
powder (2.42 g) were dispersed in 9.45 g TEOS, 5 g isopropyl alcohol and 
38.5 g of aluminum formoacetate (concentrated to 18 percent) and spun into 
fibers as in Ex. 10. 
The fibers were fired to 1300.degree. C. and held at that temperature for 1 
hour. The furnace cycle used a rate of 7.degree. C. rise per minute with a 
30 min. soak at 900.degree. C. and another 30 min. soak at 1150.degree. C. 
The fibers were black and had an average tensile strength of 782 MPa 
(113,000 psi) and an average modulus of elasticity of 283 GPa 
(41.times.10.sup.6 psi). X-ray diffraction analysis revealed mullite with 
SiC. The 1300.degree. C. fired fibers turned greyish black after 18 hours 
at 1300.degree. C. The coaser SiC particles resulting from the 
carbothermal process were identified in the final fiber by x-ray 
diffraction. 
EXAMPLE 18 
SiC powder (3.675 g) prepared as in Example 17 was ball milled in 150 g 
alumina-silica sol (at a 9.8 weight percent oxide solids) for 72 hours. 
The dispersion was filtered first through a Whatman No. 4 then a Whatman 
No. 54 filter. The precursor sol was concentrated as in Example 10. Fibers 
were spun using a 40 hole spinnerette with 76 micrometer (3 mil) diameter 
orifices and using a pressure of 1.21 MPa (175 psi) for the extrusion. The 
fibers were black when fired using the firing schedule as described in 
Example 17. The fibers were fired in air at 1300.degree. C. and remained 
black for 24 hours. After 30 hours at 1300.degree. C. the fibers had begun 
to turn to a grey color. 
Various modifications and alterations of this invention will become 
apparent to those skilled in the art without departing from the scope and 
spirit of this invention, and it should be understood that this invention 
is not to be unduly limited to the illustrative embodiments set forth 
herein.