Note: Descriptions are shown in the official language in which they were submitted.
,t~3.
PREALLOYED CATALYST FOR GROWING
SILICON CARBIDE WHISKERS
BACKGROUND OF THE INVENTION
The present invention relates generally to a
prealloyed catalyst and the use of such a catalyst in
manufacturing silicon carbide whiskers. More
05 particularly, the present invention relates to a
prealloyed catalyst containing a sufficiently high
percentage of carbon, silicon, or carbon and silicon to
allow growth of silicon carbide whiskers to commence
immediately upon reaching the growth temperature.
Silicon carbide whiskers are valued for their needle-
like single crystal structure which leads to such
excellent properties as high strength, high elastic
modulus, heat resistance, chemical stability, and so on.
Tha whisXers have been used as a composite reinforcing
material for metalsO plastics, and ceramics. The most
desirable whiskers are ~ silicon carbide single crystals
which have a high length to diameter ratio.
Various methods of using a catalyst to promote the
growth of silicon carbide whiskers have been proposed. In
U. S. Patent ~,500,504 issued ~o Yamamoto, a silica gel
wi~h 6 to 25% by weight metal catalyst was mixed with
furnace carbon black. The mixture t~en was placed in a
non-oxidative atmosphere at a temperature of 1300 to
8~3
1700C to produce silicon carbide whiskers. The metal
catalyst was selected from the group of iron, nickel, and
cobalt. However, the metal was used as is, i.e., with no
pretreatment. In U. S. Paten~ 3,622,272 issued to Shyne
05 et ~al., the use of powdered me~als such as iron,
manganese, nickel, aluminum, and stainless ~teel was
disclosed for its role as a surface nucleation site. The
powdered metal coatings were applied to a substrate such
as graphite for the growing of silicon carbide whiskers.
The only pretreatment disclosed for the me~al powders was
to suspend them in a liquid carrier vehicle such that the
metal powders could be applied to the surface of the
grow~h substrate.
In the article "Growth of sic Whiskers in the System
sio2 2 Nucleated by Iron," authored by J. A.
Bootsma et al., which appeared in J. Cryst. Growth 11,
297-309 (1971), the nucleation phenomenon of growing
silicon carbide whiskers using iron particles as
nucleating agents was studied. It was found that the iron
par~icles first took up silicon and carbon from the vapor
when ~he furnace temperature reached 1200C. After
sufficient uptake, an Fe-Si-C alloy droplet was obtained
from which the silicon carbide whisker grew. However,
again there was no suggestion of pretreating the iron
particle beore the furnace was heated up toward
nucleation temperature.
U. S. Patent 3,721,732 issued ~o Knippenberg et al. on
March 20, 1973, contained contradictory advice concerning
the addition of silicon to iron catalyst par~icles. In
Claim 4 as well as while discussing cubic growth, the
patent speculated that admixing silicon to iron may
facilitate cubic crystal growth. However, earlier in the
patent text, it was s~ated that iron may without objection
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contain carbon and silicon or other elements, but
improvement in the grow~h of crys~als, as a resul~, had
no~ been found. At best, thîs patent lePt doubt abou~
wbe~her silicon addi~ion to iron catalys~ particles would
05 help~ in the overall growth of the silicon carbide
whiskers. The role of specially treated catalyst
particles in prompt initiation of nucleation at catalyst
sites was not commented upon in this patent.
Overall, a need still existed for a me~hod to avoid
0 the delay in the growth of silicon carbide whiskers after
reaching the growth temperature while the melted catalyst
takes up from the surrounding furnace atmosphere the
necessary percen~ages of carbon and silicon to ini~iate
growth. When the catalyst particles were placed upon a
carbon subs~rate, the particle interacted with the
substrate in order to absorb carbon, which along with the
accretion of silicon from the vapor results in an
incubation time before growth can be initiated.
SUMMARY OF THE INVENTION
The object of this invention is to provide a catalyst
and a method of using the catalyst to manufacture silicon
carbide whiskers which yield whiskers with a desirable
length to diameter ratio.
A ~ur~her object of the presen~ invention is to
provide a catalyst and a method for using the catalyst to
manufacture silicon carbide whiskers where whisker growth
can immediately commence upon reaching the grow~h
tempera~ure.
Additional objects, advantages and novel features of
the invention will be set forth in part in the descrip~ion
which follows, and in part will become apparent to those
skilled in the art upon examination of the following or
may be learned by practice of the invention. The objects
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and advantages of the inventi,on may be realized and attained by
means of the instrumentalities and combinations particularly
pointed ou-t in the appended claims.
To achieve the foregoing and other objects, and in accordance
with the purposes of the present inven-tion, as embodied and
broadly described herein, the method of this invention may
comprise a method for manufacturing silicon carbide whiskers
comprising: a. prea]loying a metallic catalyst with carbon
silicon, or a mixture of carbon and silicon; b. applying the
prealloyed catalyst to a carbon substrate; c. heating the
substrate with applied prealloyed catalyst in a gaseous
environment comprised of a reducing gas; d. introducing silicon
containing gas and carbon containing gas in the gaseous
environment; e~ maintaining the flow of the silicon containing
gas while the temperature of the substrate is maintained above a
minimum temperature above which silicon carbide whiskers will
form; and f. cooling the silicon carbide whiskers after
completion of whisker growth.
The prealloyed catalyst of carbon and silicon will contain 1
to 45 weight percent silicon.
In a further embodiment, the invention contemplates a method
for manufacturing silicon carbide whiskers wherein manufacturing
time is reduced which comprises providing a growth substrate
having a coating comprised of metallic catalyst particles alloyed
with silicon where the particles contain from about 1 to about 45
weight % silicon, heating the coated growth substrate in a gaseous
environment comprised of a reducing gas, adding to the gaseous
environment a gas comprised of silicon and a gas comprised of
carbon, main-taining for a time period the coated growth substrate
and gaseous environment at a temperature above a minimum
tempera-ture at which silicon carbide whiskers will form, and
removing the gas comprised of silicon and the gas comprised of
carbon from the gaseous environment cooling the coated growth
substrate and recovering silicon carbide whiskers from the coated
growth substrate.
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4a
The present invention may also comprise, in accordance with
its objects and purposes, a catalyst for producing silicon carbide
whiskers on a carbon substrate comprising a metal prealloyed with
carbon and silicon before heating to the silicon carbide whisker
'5 growth temperature.
An advantage of the present invention is the manufacturing of
silicon carbide whiskers in the ~ form with adequate diameters to
give desirable properties~
Yet another advantage of the present invention is the rapid
initiation of growth of silicon carbide whiskers when the furnace
atmosphere reaches the growth temperature
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are photomicrographs
`` ~ 2~;~5~
and are incorporated in and form a part of the
speci~ication, illustrate the embodiments of the present
invention and, together with the description, serve to
explain ~he principles o~ the invention. In the drawings:
05 ~FIGURE 1 is a photomicrograph at 80 times
magnification for growth from a ferrosilicon catalyst
growth wherein the ferrosilicon has not ]been carburized.
FIGURE 2 is a photomicrograph a~ 80 times
magnification wherein a ferrosilicon catalyst particle has
been car~urized.
FIGURE 3 is a photomicrograph at 240 times
magnification where Alloy 62 catalyst particles have not
been diluted with silicon.
FIGURE 4 is a photomicrograph at 2~0 times
magnification wherein Alloy 62 catalyst particles have
been diluted with silicon.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Untreated catalyst particles are available from many
sources. For instance, stainless steel flakes are
a~ailable from Novamet in Wykoff, New Jersey. Catalyst
particles composed of a substance designated as Alloy 62
which has the following weight percent composition:
Mn 65, Co 19, Ni 20, Si 0.25, Ee 0.1, Cr 0.2. B 0.25,
Zn 0.25, are available from Metallurgical Technology in
~inslow, New Jersey. Other catalysts such as Perrosilicon
are commonly available from many sources. Before
treatment the catalyst particles are sieved for size such
that, as nearly as possible, no spherical particle is
larger than 15 ~m or smaller 10 ~m or no flake
particle upon melting would yield a sphere larger than
15 ~m or smaller than 10 ~m.
Various methods for pretreating the catalyst particles
to increase the percentage of carbon or silicon or both
` ~75~`38~
are available. Silicon may be added by melting the
catalyst particles and adding solid silicon to the melt.
The prealloyed substance is then pulverized into a desired
size. Carbon may be added by this melting method also.
05 Carbon may also be added by carburizing the catalyst
particles before they are placed in the silicon carbide
whisker growth furnace. An addi~ional method for
carburizing the catalyst particles is to allow a carbon
rich ga5 to flow over the untreated catalys~ particles
applied to growth surfaces in the furnace at a temperature
below the growth temperature for silicon carbide
whiskers. A colloidal carbon, such as dgf 123
produced by Miracle Power Products Co. of Cleveland, Ohio,
can be sprayed over the catalyst before or after applying
to the growth substrate which is put in the furnace.
Finally, a catalyst particle may be made from a mixture of
elements chosen to resemble the weight percentage
composition of various elements found in the cat~lyst ball
present at the end of the silicon carbide whisker.
The percen~age of silicon or carbon or both silicon
and carbon to be added to catalyst particles varies over a
wide range. The catalyst can be composed of from 1 to
about 45 ~eight percent silicon. The prealloyed catalyst
can also contain from O.l to 5.0 weight percent carbon.
The metals which may serve as catalyst particles include
one or more of the following metals: manganese, iron,
nickel, cobalt, chromium, and niobium. One typical
composition of a prealloyed catalyst is ~he following:
1.6 weight percent manganese, 23.4 weight percent cobalt,
ZZ.g weight percent nicXel, 40.9 weigh~ percen~ silicon,
10.2 weight percent iron, and 1 weight-percent chromium.
This particular composition represents the element weight
percentages found in a catalyst ball in the end of the
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silicon carbide whisker after growth was initia~ed with an
untreated Alloy 62 ca~alyst particle.
The ca~alyst after the prealloying treatment, is then
deposited upon the growth substrate. Most often this
05 growt~h substrate is graphi~e. The ~wo most common methods
for depositing the catalyst particles upon the substrate
are painting and spraying. For both methods it i~
necessary to suspend the catalyst particles in some type
of solution. For painting, i.e,, applying the catalyst
particle suspension with a brush, a typical suspension
Yehicle is made of a weight of Cabosil , which is a
product o~ the Cabot Corporation, Boston, Massachusetts,
equal to 4.5 parts by weight of catalyst particles ~hich
is then further mixed with 50 parts each of a liquid
acrylic resin and methyl ethyl ketone. Catalyst particles
can also be suspended in a product of Micromeritics, Inc.,
of Atlanta, Georgia called 14A Sedisperse
tapproximate composition: 0.1% phosphatidyl choline, 0.1%
phosphatidyl ethanolamine, 0.1% inositol phosphatides,
1.7g isopropyl myristate blended with alkyl
polyoxyethylene ethanols, and 98~ base liquid.)
Once the growth plates have been coated with catalyst
particles, ~hey are placed in the growth zone of a
furnace. The plates are typically graphite of 6 in. by
13 in. dimensions. The furnace can be a heated by SiC
resistance elements, and use a quartz muffle to contain
the growth plates. Such furnaces allow the silicon
carbide whisker growth to occur under a reducing
at~osphere which is typically a hydrogen atmosphere. The
furnace also allows for a flow of various gases ~hrough
the growth zone. Typically these gases are SiO, SiC19,
SiCH3C13, or silane. The growth ~emperature can be
anywhere from 1200 to 1600 C, but preferably are 1350
~ z7~38
to 1430C. After placing ~he coated growth substrates
into the furnace, it is necessary to heat the furnace up.
Typically the grow~h period lasts for eight hour~. After
reducing the temperature setting to 1000C, the growth
05 plates'are removed and cooled to room temperature. Then
the silicon carbide whiskers are har~vested by careful
scraping.
E~AMPLE 1
Figures 1 and 2 were produced by placing ferrosilicon
catalyst particles on a solid carbon substrate in a
Centorr Model 10-2.5 x 8 furnace. This electrically
heated furnace was supplied with electrical power
controlled by a Helmar Model TA-l power controller and
used a 60 hertz current of 1000 amps at 10 vclts duriny
the heat up time. The growth zone inside ~he furnace was
monitored for temperature by means of thermocouples. The
furnace allowed gases to flow through it during both heat
up and growth periods. The predominan-t gas flowing
through the furnace during both heat up and growth
periods, after an argon purge, was hydrogen, a reducing
gas. Figure 1 shows the result of heating ferrosilicon
catalyst particles on a carbon substrate where the
temperature was promptly lowered after reaching grvwth
temperature. Yigure 1 also shows the result of not
prealloying ~he catalyst particles with carbon. The ~low
gases used during the heat up period did not contain a
methane or other carbon containing component which would
allow ~he catalyst particles during the hea~ up period to
carburize.
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TIMETEMPERATUR~
IN C
1:35 power on
1:~5 200
, 1~58 1100
05 2~04 1340
2:07 l9oo
2:08:30 1403 temperature stabilized
2:09 gas on
~:10 1405
2~11:00 1404
2:12:00 1~02
b 2:13:30 1900
2:15:0~ 1396
2:18:00 1390
2.19:00 1389 gas off
2:19:10 power decreased to 1.5
2:38:00 1129
Figures 1 and 2 are photomicrographs taken at 80 ~imes
magnification. Figure 2 is the result of using
ferrosilicon catalyst particles on a solid carbon
substrate, only this time the gas flow was initiated at a
lower temperature and included a methane component. This
methane component carburized the catalyst particles before
they reached the growth temperature. The following table
shows the heating schedule:
TIMETEMPERATURE
IN C
2:40 power on - setting 2.0
argon purge
2:99 150 power increase to ~.0
2~5~ 450
2:56 600 gas on
3:09:351340 power decreased to 2.18
3:111349
3:121393
3:1Z:451400 power decreased to 1.5
3:181338
3:271187 gas off argon on
``` ~.. ;~7~
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As can be seen from ~he figures, when the processed
gas was introduced at a lower temperatur~ and included a
carbon component, the growth of silicon carbide whiskers
was greater than when there was no carburization of the
05 ferrosilicon ca~alyst particles. Figure 2 shows evidence
of the growth of whiskers with the ca~alyst balls intact
at the end of the whiskers when the growth period was
terminated.
EXAMPLE 2
10The same furnace and procedures as used in Example 1
were used in Example 2. Figure 3 shows a photomicrograph
at 240 times magnification of Alloy 62 catalyst particles
after ten minutes a~ growth temperature, 1413 C. The
catalyst was co~posed of the following weight percents:
566% Mn, 16~ Ni, 16% Co, 0.8% B, and 1.2% trace elements.
The heating schedule was as follows:
TIME TEMPERATURE
IN C
2:30 power on
2:50 1095
argon purge
3:01 1402 power backed off to 2.2
3:03 1409 gas on
3:05 1413 power backed off to ~.18
3:08 1414
3:10 1413
3:13 1412 gas off - power backed off
to 1.5
As can be seen from Figure 3, the pho~omicrograph.
there was little nucleation at the catalyst par~icle sites
and there was little growth of the silicon carbide
whiskers where there had been any nucleation.
Figure 4 shows a photomicrograph at 2~0 times
magnifica~ion for Alloy ~2 particles ~o which a
substantial percentage of silicon has ~een added. The
ll
composition of the catalyst particles was as follows in
weight percent: 25% Si, 50.0% Mn, 12.2% Ni, 12.2% Co, and
0.6% B. Again carbon substrats plates coated with
catalyst particles were placed in the same furnace as in
05 Example 1. The heating schedule was as followso
TXM~TEMPERATURE
IN C
9:56power on - setting 3
argon purye
10:04 700
10:08 1000
10:17 13~0 power backed off to 2.18
lOo1~ 1400
lO:l9:q5 1408
10:20:3~ 1~14
10:21 1~16
10:22 1420
10:23 gas on
10:24 1~26
10:25 1428
10:27 1430
10:33 1430
10:34 gas off
flow off
10:34:30 power decreased to 1.5
10:45:30 1245
The difference between ~igure 3 and Figure 4 is striking.
~igure 3, which represents Alloy 62 without the addition
of silicon after ten minutes at growth temperature, shows
littls whisker growth with just a few whiskers and
catalyst balls at their ends present. In contrast, Figure
4, which represents Alloy 62 with silicon added, after ten
minutes a~ growth temperature shows significant whisker
development with ~he associated catalyst balls at the end
30 of the whiskers tha~ had nucleated at catalyst particle
sites.
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The foregoing description oP the preferred embodiments
of the invention have been presented for purposes of
illustration and description. It is not intended to be
exhaustive or ~o limit the invention to the precise form
05 disclosed~,and obviously many modifications and Yariations
are possible in light of the above teaching. The
embodiments were chosen and described in order to best
explain the principles of the invention and its practical
application to thereby enable others s~illed in the art to
best utilize the invention in various embodiments and with
~arious modifications as are suited to the particular use
contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto.
