Alumina fibers which incorporate hafnia and optionally zirconia plus a fourth oxide exhibit surprising grain refinement on sintering, and thereafter excellent retention of strength after exposure to high temperatures.

BACKGROUND OF THE INVENTION 
This invention relates to polycrystalline ceramic fibers containing alumina 
and hafnia and optionally, other metal oxides, and novel intermediates for 
their production. The process of reducing the grain size of these 
compositions is also a part of the invention. 
Claussen et al. and Kriven et al. present studies of a system of alumina, 
zirconia and hafnia. Advances in Ceramics, Vol. 3, Science and Technology 
of Zirconia, Heuer and Hobbs, ed. 1981 and Advances in Ceramics, Vol.12, 
Science and Technology of Zirconia II, Claussen Ruhle and Heuer, ed., 
1984, respectively. L.M. Lopato et al. present a phase diagram for 
alumina/hafnia in Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, 
February 1977, pp1331-1334. Int. J. High Technology Ceramics 2 (1986) 
207-219 reports a study of systems of the oxides of aluminum, chromium, 
zirconium and hafnium. U.S. Pat. No. 4,665,040 discusses alumina/zirconia 
powders possibly containing hafnia as an impurity. None of these 
references recognizes or teaches anything about grain refinement in 
alumina/hafnia systems, and none of these references suggests 
alumina/hafnia fibers. 
A number of references teach formation of ceramic fibers. Representative of 
these references are U.S. Pat. Nos. 4,125,406; 3,308,015; 3,992,498; 
3,950,478; 3,808,015 and U.K. No. 1,360,198. None of these mention hafnia 
containing alumina fibers. One patent, U.K. No. 1,264,973 lists hafnium 
oxide as a possible inclusion in alumina fibers (page 1, line 76) but 
there is no teaching of such a fiber or any recognition of superior 
properties. None of the references suggests the preferred combination of 
oxides or the fibers of this invention. 
Ceramics, in general, are formed by shaping a mixture of powders and 
binders and/or precursors into "green" forms such as fibers and other 
articles. These "green" articles are then heated carefully to remove 
volatile matter and are then sintered at high temperatures to remove 
porosity and densify their microstructures. However during this high 
temperature sintering process, the grain sizes in the ceramics increase 
with increasing time at temperature. Generally, the longer the times 
and/or higher the temperature, the larger the size of the grains. This 
invention provides ceramic fibers with the property of grain refinement or 
reduction during sintering. These fibers also exhibit excellent retention 
of strength after exposure to high temperatures.

SUMMARY OF THE INVENTION 
The present invention provides polycrystalline ceramic fibers or other 
shaped articles which have the property of undergoing grain size 
refinement as they are sintered. Fibers are preferred. The ceramic fibers 
and shaped articles comprise 50 to 99 volume percent alpha-alumina, and 1 
to 50 percent hafnia. Preferred fibers comprise 85 to 97 volume percent 
alpha-alumina and from 3 to 15 volume percent hafnia. More preferred 
fibers contain 50-98% alumina, 1-49% hafnia and also contain zirconia in 
an amount from 1 to 48 volume percent, and a fourth oxide selected from 
the group of oxides of lithium, calcium, magnesium, yttrium or a metal of 
the lanthanide series. The fourth oxide is present in an amount equal to 
0.002 to 12 volume percent based on the total volume. A preferred 
composition contains from 75 to 98 volume percent alumina, 2 to 25 volume 
percent hafnia and 0 to 23 volume percent zirconia, and a fourth oxide as 
above. A still more preferred fiber contains 75-96% alumina, 3-15% hafnia 
and 1-22% zirconia. A fiber which is still more preferred contains 80-95% 
alumina, 3-15% hafnia and 2-17% zirconia. The preferred fourth oxide is 
yttria. 
The fibers of this invention can be used to reinforce composites wherein 
the matrix is a ceramic composition, metal, or a plastic. Fibers within 
the scope of the invention are from 10 to 125 or preferrably 10 to 50 
microns in diameter. 
The shaped articles and fibers of this invention undergo grain size 
refinement, i.e., a reduction in grain size on heating. The preferred 
heating temperature is 1800 degrees Celsius or more, but temperatures in 
excess of 1500 or 1700 degrees may also be employed. The presence of 
zirconia or a fourth oxide is not required to obtain the grain size 
refinement on sintering. Other ceramic materials may be present in the 
mixture without detracting from the operability of the grain refinement 
process, or the strength of the ceramic articles produced. 
DETAILED DESCRIPTION OF THE INVENTION 
Fibers and shaped articles may be formed from dispersions of the 
ingredients of the composition, or precursors of the ingredients of the 
composition, as is well known in the art. 
The fibers of this invention are useful in the reinforcement of resin, 
polymer, glass, ceramic metal, etc. matrices to provide structures such as 
composites, laminates, prepregs and the like. The fibers may be employed 
as continuous filaments, short fibers, combinations thereof, and/or 
hybridized with other fibers for reinforcing purposes. Sheet products 
(papers etc.) can be prepared from short fibers. The fibers may be coated 
to enhance performance for specific applications. Fibrous preforms may be 
infiltrated by pressure,squeeze or vacuum casting of molten metals. 
Ceramic matrices may be prepared by sol-gel infiltration of suitable 
precursors or by chemical vapor deposition techniques. The end result of 
these and related operations will be a shaped article generically termed a 
"composite". The alumina in the article is derived from a dispersion of 
alumina particles and from a soluble alumina precursor. The alumina 
particle size distribution should be as follows: 99% smaller than 1 
micron, 95% smaller than 0.5 micron, as determined by standard "Sedigraph" 
measurement. Particulate materials can be classified by any of a variety 
of known techniques. In one method for preparing such particles, alpha 
alumina (Alcoa A-16 SG) is dispersed in water at 15% solids at a pH of 4.0 
and allowed to settle in a tank. Portions of the dispersion are removed 
from the top of the sedimentation tank and concentrated to the desired 
solids level for use. It should be noted that while use of fine particles 
is preferred, larger alumina particles may be employed and this alumina 
dispersion can also be use in its commercial form without sedimentation. 
Preferred soluble alumina precursors include the basic aluminum salts, 
such as aluminum chlorohydrate, basic aluminum nitrate, and basic aluminum 
acetate, which have a basicity of 0.33 to 0.83. Aluminum chlorohydrate is 
most preferred. Basicity can be adjusted by addition of HCl or other 
chemicals. Also preferred are the reagents which provide precursors for 
two or three of the oxides required, such as aluminum-zirconium 
chlorohydrate, aluminum hafnium chlorohydrate or 
aluminum-zirconium-hafnium chlorohydrate. 
The zirconia content can be derived from a variety of zirconium containing 
chemicals, including zirconium oxychloride, zirconium acetate, and 
zirconia particulate. The zirconia particles are commercially available 
with yttrium oxide and with other stabilizers already added. 
Hafnia precursors include hafnium dichloride oxide, hafnium chloride, and 
particulate hafnia all of which are commercially available. Other reagents 
can be prepared in the laboratory. These include aluminum hafnium 
chlorohydrate and aluminum hafnium zirconium chlorohydrate which can be 
prepared as follows. An appropriate amount of aluminum hydroxide aqueous 
slurry is reacted with hafnium dichloride oxide (or a mixture of hafnuim 
and zirconium dichloride oxides) aqueous solution at about 50.degree. to 
60.degree. C. for about 2 hours (until all of the aluminum hydroxide has 
reacted). Aluminum chlorohydrate is then added and the mixture heated to 
about 80.degree. C. for about 2 hours. Amino acids such as glycine may be 
reacted with aluminum hydroxide slurry to form the glycinate before 
reaction with the dichloride oxide. The fourth oxide can be incorporated 
as any of a variety of alkali, alkaline earth or rare earth compounds such 
as chlorides and oxides. These would include Li.sub.2 O, MgO, CaO, Y.sub.2 
O.sub.3, oxides of the Lanthanide metals and mixtures thereof. These may 
be present in quantities between 0.002 and 12 volume % based on the total. 
Yttria is preferred. 
Various ways of compounding materials may be employed. The general 
procedures are described in the Seufert patent, U.S. Pat. No. 3,805,015. 
Thus, an aqueous dispersion of alumina particles may be combined, in 
appropriate quantities, with a solution of a zirconium salt, hafnium salt, 
aluminum chlorohydrate and an yttrium salt. Another method involves 
combining a slurry of hafnia particles, zirconia particles containing 
yttrium oxide with an alumina slurry and aluminum chlorohydrate. The mix 
is stirred, heated, and dewatered for sufficient time to obtain 45 to 65% 
solids at a useful viscosity. Too much heating must be avoided as this can 
cause the mix to lose its extensible viscosity. The mix can be converted 
into fiber in a variety of ways, including drawing from a beaker with a 
spatula, centrifugal spinning, and extrusion through spinneret holes. For 
extrusion through a spinneret a viscosity of 400 to 1200 poises is useful. 
As fibers are formed, they are partially dried to prevent sticking as they 
are processed further. The fibers can be collected in a variety of ways 
including being wound up on a bobbin or piddled into a basket. The fibers 
are further dried and volatiles removed by heating to 400.degree. to 1000 
.degree. C. The fibers of the invention are prepared by sintering the 
dried fibers at high temperature to complete the formation of the 
microstructure and achieve full density and strength. This sintering can 
be accomplished by placing the fiber in a furnace or flame, or drawing it 
through a furnace or flame as in U.S. Pat. No. 3,808,015. 
In one method, individual fibers are held in the flame of a propane/air 
torch for a length of time between one and twelve seconds. These fibers 
can be held at sintering temperatures for a longer period of time without 
loss of desirable properties. The fibers treated by this method become 
white hot. In the sintering process, the higher the temperature, and the 
smaller the diameter of the object being sintered, the shorter the time 
required for treatment. When sintering in a flame, the type of flame in 
terms of the fuel-to-oxidant ratio is important. Also, consideration must 
be given to the number of fibers in the yarn bundle, the diameter of the 
fibers, and the composition of the fibers in choosing the proper sintering 
conditions which will provide fibers having a desired level of grain 
refinement. It will be understood that firing conditions may vary somewhat 
from those mentioned above. 
Scanning electron microscopy (SEM) can be employed to analyze the 
microstructure of the fibers. Two modes of operation of the SEM can be 
used in the analyses. An energy dispersive X-ray (SEM/EDX) technique can 
be used to identify the elemental composition of the grains within the 
fiber and a secondary electron imaging technique was used to determine 
grain sizes in a cross-section and the surface of a fiber. 
The technique used to measure grain sizes relies on the images obtained 
using a secondary electron detector of a scanning electron microscope 
(SEM). The samples are prepared by fracturing the fiber and sputter 
coating the fiber with gold and placing it in the instrument. Once placed 
in the instrument, a signal is generated from the sample and is enhanced 
by adjusting the SEM instrument's secondary scattering detector. In this 
manner, one can easily obtain a micrograph that clearly shows the 
individual grains and their sizes. 
If desired, the fibers of the invention may be coated with silica which has 
been shown in Tietz, U.S. Pat. No. 3,837,891, to have a beneficial effect 
on alumina fiber strength. 
TEST PROCEDURES AND MEASUREMENTS TENSILE TESTING 
In this method, single fibers are selected at random and their diameters 
are measured using a calibrated optical microscope. The gauge length used 
is one-fourth of one inch. The clamps of the Instron tensile tester are 
covered with "Neoprene". The clamps of the Instron tester have to be well 
aligned and the clamp pressure selected carefully so that the filaments 
are not damaged during testing. The head speed is 0.02 in/min. 
Grain Size Determination 
Images of the cross-sections and the surface of the fibers are used to 
measure the overall grain size. The individual filaments are sputter 
coated with gold to provide a conductive surface and placed in a JEOL JXA 
840 Scanning Electron Microscope. Optimization of the secondary electron 
signal is obtained on each sample and the images recorded of Polaroid type 
52 or 53 film. The instrument settings include an acceleration voltage of 
10 kV, current of 1.times.10.sup.-10 Amp, final aperture of 70 microns, 
filament consisting of tungsten hairpin, working distance of 20-30 
millimeters, and a magnification of about 10,000 X. From the images made 
by this technique, the sizes of the grains can be inferred. 
EXAMPLE 1 
Spin-Mix Preparation 
20 gms of alumina-water slurry with 56.93% alumina (determined by 
Thermogravimetric analysis) was weighed into a three-necked flask and 
stirred slowly for about 21/2 days in a water bath at 40.degree. C. The pH 
was measured to be 3.31, measured using a Fisher Accumet Model 815MP. 7.36 
gms of zirconyl acetate solution (TGA residue=28.59%) obtained from 
Harshaw Chemicals were added to the above slurry and stirred well for 
about 5 minutes. 11.68 gms of "Chlorhydrol" (Reheis Chemicals, Ctrl no. 
7536, TGA residue =48.88%) was added and stirred well for about 5 minutes. 
The pH of the solution was measured and found to be 2.90. 8.30 gms of 
Hafnium dichloride oxide (HfOCl.sub.2) obtained from Alfa (TGA residue 
=43.35%) were added and the pH was measured to be 1.21. 0.21 gms of 
YCl.sub.3.6H.sub.2 O from Aldrich Chemicals was added and stirred well. 
The mixture was of very low viscosity. Vacuum was applied gradually to the 
mixture to remove water till the mix became thick enough to pull fibers. 
The solids content of the slurry as measured by TGA was 61.18% at 
800.degree. C. at a heating rate of 20.degree. C. per minute. Several long 
fibers were pulled from the spin mix by hand and then set in 10-inch 
alumina boats and low-fired. 
Low-firing 
The filaments were low-fired using a staged cycle as shown below: 
______________________________________ 
Time 
0.degree. C. 
Temperature, .degree.C. 
Set Point, 
______________________________________ 
1045 40 150 
1100 148 145 
1129 150 150 
1310 150 350 
1318 348 350 
1343 350 350 
1407 350 350 
1408 -- 700 
1414 536 700 
1437 700 700 
1440 700 700 
1500 700 700 
1501 -- 1000 
1509 880 1000 
1520 996 1000 
1526 999 1000 
1530 1000 1000 
1535 1000 25 
______________________________________ 
Sintering 
The fibers were allowed to come to room temperature and then sintered in a 
Bernzomatic Propane-air torch. The individual filaments were held in the 
flame with a pair of tweezers for times varying from 2 seconds to 6 
seconds. The fibers reached a temperature of about 1830.degree. C. as 
measured by an optical pyrometer. Quarter-inch lengths of the sintered 
filaments were then tested in an Instron Tensile Tester. The diameters of 
the individual filaments were determined using a microscope with a 45X 
objective and a 15X eyepiece. The filaments had diameters ranging from 
about 12 microns to about 42 microns. The filament strength appeared to 
depend strongly on diameter for these fibers with strengths decreasing 
from about 687 kpsi for the 11.6 micron filament to about 33.5 kpsi for 
42.5 micron filaments. 
EXAMPLE 2 
Spin Mix Preparation 
33.55 gms of alumina slurry with 54.93% alumina was weighed into 
three-necked glass flask and stirred slowly in water bath at 30.degree. C. 
The pH of the slurry was measured to be 4.234 using a Corning pH/Ion Meter 
135. 14.72 gms of zirconyl acetate solution (TGA residue =28.59%) was 
added and the mixture stirred well for 5 minutes. The pH of the mixture 
was then measured to be 3.256. 35 gms of deionized water was added to the 
mixture and the mixture stirred well for 5 minutes. 31.04 gms of 
Chlorhydrol (Ctrl no. 7536) and 35 gms of deionized water were then added 
and the mixture stirred well for 5 minutes. 2.0 gms of Concentated HCl 
(37.8%) was then added and the mixture stirred well for 5 minutes. 14.34 
gms of Hafnium Chloride oxide (TGA residue =50.17%) and 0.600 gms of 
YCl.sub.3.6H.sub.2 O (Aldrich Chemicals) were added. The pH was measured 
to be 2.350. The mixture was stirred overnight with the bath at 30.degree. 
C. The pH was measured to be 2.651 the next morning. Vacuum was applied to 
the system to remove about 80 ml of water. The residue after heating the 
mix to 600.degree. C. was determined to be 50.1%. The mix was then poured 
into a metal spin cell and spun through a 0.004 inch and 0.003 inch 
diameter spinnerets. The extruded filament was dried in a drying zone at 
95.degree. C. about 8 inches below the spinneret. The fibers were 
low-fired in a staged fashion as in Example 1 and then sintered in a 
Bernzomatic Propane-air flame for 3, 6, 9 and 12 seconds. 
Grain Refinement 
These filaments which were about 30 micron in diameter exhibited a unique 
grain refinement phenomenon, see FIGS. 1 and 2. In polycrystalline 
ceramics, the individual grains grow larger in size with increasing 
sintering times. However, in ceramics of this invention, the grain size 
decreases with increasing sintering times as shown in the attached 
micrographs of the surface and cross-section of the fibers. The average 
quarter inch strength of these approximately 30 micron fibers also 
increases as the grain size decreases from 148 kpsi at 3 seconds to 173 
kpsi at 9 seconds.