Note: Descriptions are shown in the official language in which they were submitted.
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TUBULAR CERAMIC ARTICLES, METHODS AND APPARATUS
FOR THEIR MANFACTURE
BACKGROUND O~ THE INVENTION
The ~resent invention relates to tubular ceramic articles
comprised of silicon and silicon carbide, and to processes and
apparatus for the manfacture of such articles.
Silicon carbide, a crystalline compound of silicon and
carbon, has long been known for its hardness, strength, and
excellent resistance to oxidation and corrosion~ Silicon
carbide h~s a low coefficient of expansion, good heat transfer
properties and exhibits high strength and excellent creep
re~istance at elevated temperatures. These desirable
properties may be attributed to a strong covalent chemical
bonding, w~ich also is the cause of an undesirable property of
llicon carbide, that of being difficult to work or fabricate
the mater-ial into useful shapes. For example, because silicon
carbide dissociates at high temperatures, rather than melting,
it is not feasible to fabricate articles by melt processes,
and becau~e silicon carbide has a very slow diffusion rate,
fabric~tion by plastic deformation processes is not viable.
It has been proposed to produce shaped silicon carbide
articles by forming bodies of silicon carbide particles and
e~ther bonding or sintering the particles at high temperature~
to form a consolidated body. If the particulate silicon
carbide starting materia~ is fine enough, and suitable
~intering aid~ are added, the fine, particulate material will
exhibit sufficient self-diffusion at high temperatures that
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the particulate material will sinter and form into
qubstantially dense single phase material. Sintering
processes, in general, require fine powder starting materials
and pre-qsurele~s sintering processes, in particular, require
an even finer starting material. Because of the needed
fineness and high purity of the starting materials, articles
formed by sintering processes are relatively expensive.
Coar~er and less pure silicon carbide powders are known
to bond together at high temperatures. However, the resultant
products have considerable porosity and for that reason are
usually not as strong, or as corrosion resistant, as more
fully densified materials. The properties of such materials
may be substantially improved by infiltrating the pores of
uch materi~ls with silicon, in either vapor or liquid form,
to produce a two phase, silicon-silicon carbide product.
Although such processes utilize relatively inexpensive coarse
powders as starting materials, they require two high
temperature furnacings, one to form thè silicon carbide to
flilicon carbide bond and a second, separate furnacing, to
infiltrate the formed body with silicon.
Mixtures of coarser and less pure silicon carbide powder~
with partlculate carbon or with a carbon source material may
be preformed and subsequently impregnated with silicon at high
te~perature to form ~reaction bonded~ or "reaction sintered"
illcon carbide products. The carbon component may be in the
form of particulate graphite or amorphous carbon, or may be in
he form of a carbon source material, for example a
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carbonizable organic material, such as, pitch, resin or
similar ~aterials, which will decompose during furnacing to
yield carbon. The infiltrating silicon reacts with the carbon
in the preformed body to form additional silicon carbide which
bonds with the orginal ~ilicon carbide particles to produce a
dense ellicon carbide article. Typically reaction bonded
~ilicon carbide materials are characterized by almost zero
porosity and the presence of a second phase, or residual, -of
silicon, usually greater than about 8% by volume.
In typical siliciding or typical reaction bonding
proce~es, the particulate silicon carbide and carbon starting
material 1B initially preformed or preshaped into an article,
commonly referred to as a "green body", which is subsequently
fired. The particulate silicon carbide and carbon starting
mixture is co~monly blended with a binder to aid in shaping.
If the binder is dry, or relatively dry, the powder may be
compacted to the desired shaped green body using a press or
isopress. If the binder i8 liquid, or seml-liquid, and i~
used in sufficient quantity, the mixture may suitably be 81ip
~ast, extruded or injection molded to form a shaped green
body.
High temperature heat exchanqe components desirably have
relatively thin wall~ to facilitate high rates of heat
transfer. There have been previou~ attempts to fabricate
tubular article~ of silicon carbide by variou~ methods,
however, none have proven commercially successful. For
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example, US Patent 801,296 discloses a method of producing a
hollow silicon carbide tube by siliciding a solid carbon rod
to form an outer layer of silicon carbide and subsequently
burning out the carbon interior leaving the outer layer of
silicon c~rbide. US Patent 1,266,478 describes a typical
method of preforming a tubular body of silicon carbide and
cnrbon and siliciding to obtain a tubular silicon carbide
article. US Patent 1,756,457 teaches the reaction of silicon
dioxide and carbon in preformed column~ to produce a silicon
carbide tube. US Patent 3,495,939 teaches making tubular
~ilicon carbide by preforming a tube of particulate silicon
carbide and carbon, positioning the tube vertically in a
furnace and siliciding with the bottom of the tube in contact
w~th liquid silicon. US Patent 3,882,210 teaches siliciding a
preformed tube of alpha silicon carbide and graphite to
produce a tube of silicon carbide. US Patent 4,265,843
de~cribes the manfacture of silicon carbide in tubular form by
initially heating at low temperature a rotating preformed
carbon tube in the presence of silicon to impregnate the tube
and subsequently heating at a higher temperature to react the
silicon and Carbon to form a tube of silicon carbide.
It will be appreciated that the fabrication of long,
~e.g., four to eight foot), large diameter, (e.g., four to
eight inch OD), thin-walled, (e.g., 1/8 to 1/4 inch), tubes
presents a difficult problem. The tubular green bodies that
are required to be initially formed by the prior art processes
are inherently structrually weak and easily deformed or broken
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unless h~ndled with utmost care. In subsequent processing
steps, the tubular green bodies must be carefully dried,
and/or baked, and positioned in a furnace for siliciding. The
fragility of the preformed bodies and the required multiple
handling entaillng high labor imput have been major factors in
preventinq the use of tubular silicon carbide in many
applications purely on the basis of cost. ~he term
~reaction sinter~ as used herein means consolidation by
chemical reaction and includes the reaction of silicon with
carbon either alone or in mixture with silicon carbide.
The term "carbon" as used herein means carbon or a carbon
source material that produces carbon upon heating that will
react with the infiltrating silicon to form additional silicon
carbide, in ~itu.
The ter~ ~tubular~ as used herein means that the article
h~s the for~ of a tube, that is, it i8 fistulous. Although
the pre~ent invention will hereinafter be described in terms
of tubes having generally round cross-sections, it will be
understood that the i~nvention is not so limited and that tubes
having eliptical, square or multi-sided cross-section , or
having an external surface of one cross-sectional type and an
internal surface of another, may as easily be produced. It
wlll aloo be understood that the -present invention also
contemplates tubular articles that have internal separa~ions,
or septums, providing multiple passageways within the tube.
GEN~AL DESCRIPTION OF THE INVENTION
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The preient invention relates to tubular articles of
manufactur~ comprised of silicon carbide and silicon and to
methods and apparatus for the production of such articles.
The articles are c~aracterized in that they contain silicon in
metallic and in chemically com~ined form.
The method consists of the steps of concentrically
po~itioning at least one hollow, vertical tubular column of
particulate silicon adjacent to, or contiguous to, at least
one hollow, vertical tubular column of particulate silicon
c~rbide, carbon, or mixtures of silicon and carbon, and
heating the adjacent columns to a siliciding temperature, that
i8, a temperature above the melting point of silicon (about
1410 to 1~20 degrees C.) and less than about 2400 degree~ C.
At such temperatures the particulate silicon component melts
or vaporizes and infiltrates into the pores of the column
containing the particulate silicon carbide, carbon, or
mixture~ thereof, forming a tubular ceramic product.
The apparatus consists of a plurality of supply hoppers
for holding a particulate feed material, a loading means
comprised of at leàst two spaced, concentrically arranged,
dimensionally stable, tubular form members. The loading means
is of a si~e to spacedly fit within the furnace tube of
a vertically positioned electrical induction furnace and i8
~oveable in and out of the furnace tube. Selected particulate
feed materialo ~re dry cast, suitably by flowing, into the
opaceo between and around the form members. For example, the
lo-ding means is initially centrally, or coaxially, positioned
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in the furnace tube, spacedly surrounding the furnace heating
element. After the filling, or dry casting, operation i8
completed, the loading means is removed from the furnace. The
~psce between the outer form member and the inner furnace wall
is Ruitably filled with a particulate heat-insulating
material, the space between the form members is selectively
filled with silicon, carbon, or mixtures thereof and the space
between the inner form and the heating element is selectively
filled with silicon. After the dry casting operation,
eparate vertical, hollow columns of particulate feed
~aterial- remain concentrically arranged within the furnace.
The furnace i3 subsequently progressively, or incrementially,
heated from top to bottom to a siliciding temperature. The
silicon component infiltrates the column containing silicon
carbide, carbon or mixtures thereof. The infiltrated column
i8 subsequently cooled to form a dense, tubular silicon-
sillcon carbide product.
The particulate silicon carbide starting material is of a
sufficiently coarse particle size that the material is easily
flowable through the apparatus without plugging. Suitably the
particles are greater than 50 microns and less than 500
~lcrons in diameter, and the term ~particulate" as used herein
carries this Deaning. The particulate silicon carbide
starting Fateri~l may be of a single particle size or may
consist of a co~bination of separate particle sizes to enable
higher packing efficiencies.
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Particulate carbon may be utilized as a sole feed
material, or may be used in mixture with particulate silicon
carbide. The particulate carbon component reacts with the
infiltrating 9i licon to form silicon carbide, in situ,
reducing t~e amount of free unreacted silicon remaining in the
finished product. The amount of free silicon desired in the
finished product is dependent upon the use of the product.
For exami-ie, it the end product requires abrasion, oxidation,
or corro~ion re~ist~nce, a minimal, or minor, amount of free
~ilicon and free carbon would be desired in the product, as
the hardness and chemical inertness of silicon and carbon i8
les~ than that of silicon carbide.
In a particularly useful alternate embodiment of the
invention, particulate graphite and particulate silicon are
utilized as the starting materials. In such embodiment
~eparate, adiacent columns of particulate graphite and
particulate silicon are heated to a siliciding temperature.
The silicon infiltrates the graphite column reacting with the
graphite to form silicon carbide. If the particulate graphite
is fine, less than about SO microns, the graphite will be
substantially completely converted to silicon carbide. If the
graphite particles are larger than about 50 microns in
~iameter, a thin layer of silicon carbide will form on the
qraphite particles and a three phase material will be
produced. Such material has silicon and graphite as major
pbases with a minor phase of silicon carbide.
The preQence of a large amount of graphite phase affects
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the phy~ical properties of the present products. Graphite, a
crystalline form of carbon, has a low elastic modulus, low
thermal expansion rate, and a high thermal conductivity. When
incorportated in the present products in amount~ over about
ten percent by volume, the products show improved thermal
~ock and thermal stress resistance. Amounts of greater than
about ~ixty percent by volume are difficult to achieve using
~ilicon infiltration processing.
The pre~ent process may be characterized in that no green
body, a prerequsite of the previous tube forming methods, is
required or produced by the present invention. Particulate
material is fed into the furnace, and after firing, a finished
ceramic tube i~ removed rom the furnace.
DETAILED DESCRIPTION OF THE INVENTION
The present tubular articles are composites which contain
silicon in free, unreacted, and in chemically combined form~.
Tbe compo4ites are comprised of free, unreacted, silicon and a
material ~elected from silicon carbide, carbon, or mixtures
thereof. The final product contains from about five to about
sixty percent, and more typically from about ten to 55 percent
by volume ~ree ~ilicon; from about forty to about 95 percent,
~nd more u~ually from about 45 to about ninety percent by
volume silicon carbide, and; from about zero to about forty
percent by volume free carbon.
The composite tubular articles are produced by dry
casting, that i8 by forming, suitably by flowing, adjacent, or
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contiguous hollow, coaxially arranged, vertical columns of
selected particulate starting materials. The hollow
concentric columns of particulate material are subsequently
heated to react the materials in the columns to form a tubular
product. The columns individually consist of particulate
silicon and of particulate silicon carbide, carbon, or
mixtures thereof. The attached drawings, discussed in detail
below, illustrate apparatus particularly suited to carrying
out a dry casting process. The heating step is preferably
carried out by induction heating, under an inert atmosphere,
or in a vAcuum. Suitable siliciding temperatures are above
the melting point of silicon, usually at least about 1500
degrees C., but below about 2400 degrees C.
The ~ilicon component can be particulate, commercial
grade sillcon, h~ving an average particle size ranging from
about 1500 to le~s than about forty microns. A particularly
useful silicon material ranges from about 100 to about 1000
microns in diameter. The size of the silicon particles is not
cr$tical, except fo~r flow and packing characteristics, as the
silicon component i8 completely melted during the siliciding
process .
The silicon carbide component i8 also particulate, that
1~ tbe particles have an diameter of less than about 500
microns, and more preferably have an average particle~ size
between about 75 and about 300 microns in diameter. The
~ilicon carbide component may suitably be selected from alpha
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or bet~ ph~se s11icon carbide. Mixtures o alpha and beta
phase mater;al may be used. The silicon carbide starting
material does not require seperation or purification, minor
amounts of unreacted carbon, silicon and silicon dioxide, a3
~ell a~ minor amounts of impu~ities isuch as iron, calcium,
magnesium and aluminum, may be present without deleterious
affect.
The carbon component may be either amorphous carbon or
graphite provided that it i~ of a size that it is free~flowing
and is free-flowing when used in mixtures with silicon
carbide. Free-flowing carbon materials having a particle size
ranging between about 0.01 and about fifty microns, and
preferably having an averaqe particle size between about 0.5
~nd about 25 microng are aptly suited to use, if no unreacted
carbon is desired in the final product. If unreacted carbon
i~ desir~d in the final product, a coarser carbon starting
material is utilized, and in such case, carbon materials
having particle sizes ranging from about~50 to about 1500
microns and more preferably between about 100 to about lO00
micron~ are typlcally useful.
In carrying out the present siliciding operation the
hollow column of particulate silicon melts and infiltrate~
into the hollow column containing particulate silicon carbide,
carbon, or mixtures thereof. In such event the wall of the
latter column may be subject to partial collapse because of
, the 1088 . of support of the adjacent wall as the silicon
component is removed by melting. This situation can be greatly
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minimized, or prevented entirely, by blending a small amount
of a dry particulate temporary binder, for example, a
thermosetting resin, suitably a phenolic resin, in the feed
material used for the column.comprised of silicon carbide,
carbon, or mixture~ thereof. Alternatively resin may be added
to the ~eed material by dissolving the resin in a solvent,
such as, acetone, and blending the resin solution into the
feed material. Subsequent drying will deposit the resin in a
coherent, ~ubstantially even manner on the particles of the
feed material. Amounts of resin between about one-half and
about ten percent by weight of the feed material are generally
useful. The binder should be one that will leave a carbon
residue in the column upon heating, in such case the residue
will provide additional carbon for reaction with the silicon
component.
The' siliciding step is carried out at temperatures above
the melting point of silicon, about 1410 to 1420 degrees C.,
and at a temperature less than about 2400 -degrees C. The
siliciding step is carried out in an inert atmosphere, or in a
vacuum, the latter being preferred. Vacuums between about
0.001 Torr and about 2.0 Torr are eminently suited to uQe. If
an inert atmo~phere is utilized slightly higher siliciding
temperatures will usually be required. Suitable inert
atmospheres are for example, nitrogen and nobel gases,~ s~ch
as, argon and helium. An inert atmosphere is one that will
not deleteriously affect the siliciding process. After the
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~llicidin~ process is complete, the tubular product is
preferably allowed to cool in the furnace to a temperature
below about 1200 degrees C. while the inert atmosphere or
vacuum maintained to prevent oxidation of the product.
The weight amount of silicon to completely infiltrate the
hollow column of particulate silicon carbide, carbon, or
mixtures thereof, can be calculated from the packing density
of the silicon carbide or carbon grain, the amount and type of
carbon, the particle size of the components and the de~ired
thickness and composition of the tubular product. The proper
amount may be calculated from such data, or may be determined
emphirically.
The preferred form of heating is by electrical induction
heating and a preferred furnace is a vertical vacuum induction
furnace which mdy be of a core-type or a coreless type.
Heating ~8 carried out from the top of the furnace to the
bottom, that is, the siliciding process is progressively
carried out starting from the top~ of particulate,
concentric~lly arranged, hollow columns of the starting
material~ and procèeding to the bottom, or base, of the
columns.
THE DRAWINGS
; The invention will now be further illustrated in greater
detail by reference to the attached drawings which illustrate
apparatu~ particularly suited to carry out the present
invention. Similar components are designated by li~e
reference numbers throughout the several views.
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Figure 1 ~s an front elevational sectional view showing a
preferred furnace loading apparatus.
Figure 2 is a sectional top view of the loading apparatus
taken along lines 1-1' of Figure 1 and also illustrates a
centering, or spacing, means that may be utilized.
Figure 3 is a partial vertical section of the induction
furnace of Figure 1 showing the loading means positioned
within the furnace and the furnace being charged by the
present dry casting method.
Figure 4 is a partial vertical section of the induction
furnace of Figure 1 as the furnace would appear when fully
charged and the loading means has been removed.
Figure 5 is a partial vertical view of the induction
furnace of Figure 1 showing the furnace being heated to
siliciding temperature and the tubular product being produced.
Figure 6 is a partial vertical view of the induction
furnace of Figure 1 showing an alternative arrangement wherein
the furnace is of the coreless type, that is, no internal
heating element is utilized.
Figure 7 is a partial vertical view of the induction
furnace of Figure 1 showing an alternate arrangement in which
the particulate columns have an additional, temporary support
means.
Looking now in detail at Figures 1 tbrough 4,
particulate feed material is supplied through supply hopper
~eans, such a~, 11, 13, 15. Suitably there is one hopper
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provided for each hollow vertical column of particulate
material that is to be formed in the furnace. For example, as
shown, one hopper would supply particulate silicon, one would
supply particulate silicon carbide, carbon, or mixtures
thereor, and one would suppiy particulate heat-insulating
material. Loading means 17 is comprised of a plurality, at
least two, concentrically arranged, dimensionally stable,
hollow, open-ended cylindrical form members, 19 and 21,
~uitably fabricated of thin metal tubes. Loading means 17 is
of a size that will spacedly fit within vertical vacuum
induction furnace 23. Loading means 17 is positioned on
insulating material, such as, 28, which suitably is
particulate fuQed quartz. Induction furnace 23 is suitably
comprised of a furnace tube 25, a vertically moveable
induction coil 27 electrically connected to an electrical
induction power supply 29. As shown, in Figures 1 through 5
and 7, induction furnace 23 also includes a heating core, or
element, for example, 31. Furnace tube 25 is suitably
fabricated of fused quartz, as such material i8 a good
electrical insulator, is substantially impervious, can
withstand reasonably high temperatures, has good thermal shock
resi~t~ce, and i8 commercially available in large tubular
forms. Heating element 31 is suitably fabricated of graphite
and may be i~ the form of a simple hollow tube of graphlte
without spirals or cuts usually required in resi~tive heating
elements. Loading means 17 is moveable in and out of furnace
tube 25, suitably by means of a reversible lift, such as 30,
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of a type well known in the art, for example, an adjustable
sc~ew, rac~ and pinion, or worm gear arrangement.
Figure 2 illustrates a centering, or spacing, means, 39,
that ~ay be positioned on the outside periphery of loading
means 17. Spacing means 39 are eminently useful in enabling
loading means 17 to be centered within furnace tube 25.
Spacing means 39 may be in the form of extentions such a~
feet, or in the form of narrow, preferably intermittent,
strips positioned along the periphery of outer form member 19.
To load furnace 23, loading means 17 is lowered to the
position shown in Figure 3, by a reversible lift mean~, such
as 30, into contact with insulation 28 positioned on the
interior base portion of furnace tube 25. Loading means 17 is
positioned so that it is centered ,or substantially centered,
within furnace tube 25. Hopper means 11, 13 and 15 have a
plurality of feed means, or supply lines, such as, 33, 35 and
37, which may be in the form of hoses or chutes, separately
connectinq the individual hoppers with the the spaces around
and between form members 19 and 21. The feed means may
include valve~, such as 36, to control the flow therethrough.
As shown the spaces in and around form member~ 19 and 21 are
annularly defined by the inside of the furnace tube, the form
member~ and the core, or heating element.
As shown in Figure 3 loading means 17 is centered, or
substantially centered, within furnace tube 25. The annular
spaces defined by the interior of furnace tube 25, loading
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means 17 and by heating element 31 are being charged with
particulate material from hoppers 11, 13 and 15. As shown,
the annular space between the interior of furnace tube 25 and
the outside of cylindrical form member 19 of loading device 17
is partially filled with particulate insulating material 41.
Insulating material 4~ functions to provide physical support
for one ~urf~ce of t~e particulate reactant material, insulate
the furnaoe tube from high temperatures and allow the final
product to be easily removed from the furnace tube.
In~ulating material 41 may be of any material which does not
react with silicon, silicon carbide, carbon, or the material
of the furnace tube. The material is one that is not wetted
by molted ~ilicon, that is, it is not silicon infilitrated.
Boron nitride, aluminum nitride, silicon nitride, and oxides
such as aluminum oxide, zirconia oxide and fused guartz are
useful, boron nitride and aluminum nitride and fused quartz
Rhave been found to be eminently useful. The annular space
defined by the outside of cylindrical form member 21 and the
inside of cylinder~ 19 is filled with particulate silicon
carbide, carbon, or mixtures thereof, 43. The annular space
defined by the inside of cylindrical form member 21 and
heating element 31 is filled with particulate silicon, 45.
The out~ide surface of heating element 31 is coated with a
thin layer of boron nitride, aluminum nitride or a~licon
nitride to prevent molten silicon from wetting or reacting
with it. It has been found that heating elements of high
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density, fine grain graphite are more resistant to moltensilicon than low density, coarse grain graphite.
Figure 4 shows the arrangement of Figure 3 after furnace
tube 25 has been charged. Loading means 17 has been removed
from furnace tube 25 leaving hollow columns of reactant
materials and insulatlng material. Cover 32 has been placed
on tube 25. The space between the furnace charge and cover 32
may suitably be filled with an insulation material similar to
that used in the base portion of the furnace.
Moveable induction coil 27 is then positioned at the top
portion of furnace tube 25 and activated causing heatlng
element 31 to increase in temperature. When heating element
reaches a sufficiently high temperature, the hollow column of
particulate silicon is melted and infuqes, or infiltrates,
into an appropriate adjacent column of particulate material.
Induction coil 27 is then progressively moved downward along
furnace tube 25, suitably by reversible lift means, such as
30, thus incrementally carrying out the siliciding process.
Pigure 5 illustrates the arrangement of Figure 4 after
in~tial heating of the furnace has begun. As ~hown the top
portion of the silicon column has partially melted and
infiltrated into the column containing silicon carbide,
carbon, or mixtures thereof forming tube 34. Heating to
silicidinq temperature is progressively carried out from the
top to the bottom of the furnace.
Fiqure 6 illustrates an alternative arrangement whereby a
coreless type furnace is employed, that is no heating element,
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as such, is utilized. In this arrangement a vertical column
of carbon, preferably graphite, 47, is used as both a reactant
and as the heating element. The center portion, or core, of
furnace tube 25 is filled with insulating material 49.
Figure 7 illustrates a further alternative in which
temporary supports 5i and 53 are provided to give additional
stability to the columns of particulate materials and enable
easier removal of loading means 17. Supports 51 and 53 are
suitably fabricated of a combustible material such as paper.
Waxed or coated paper may be used. Materials that are
completely combustible or materials that leave a carbon
residue are equally suitable.
The invention will now be described in greater detail by
reference to the following examples, which are intended to
illustrate, and not limit the scope of the invention. In the
following examples, all parts are parts by weight and all
temperatures are in degrees Centigrade.
EXAMPLE 1
A loading apparatus and vertical vacuum induction furnace
a- lllustrated above was utilized. The loading means had an
outside tube, corresponding to 19 in the drawings, having an
outside diameter of 2.250 inches, and an inside diameter of
2.152 inches. One end of the tube was beveled toward the
inside surface to form knife edge having a diameter of about
2.15 inches. The device had an inner tube, corresponding to
21 in the drawings, having an outside diameter of 2.000 inches
19
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and an in~ide diameter of 1.902 inches. One end of the inner
tube was beveled toward the outside to form a knife edge with
a diameter of 2.000 inches. The inner tube was held in
concentric position within the outer tube by mean~ of set
8C rew~.
The loading means was centered in a fused quartz furnace
tube having a 2.772 inch inside diameter, a 3.025 inch outside
diameter and a length of 24 inches. The furnace tube was
positioned vertically in a support frame. The bottom end of
the furnace tube was closed with a flat rubber vacuum gasket
held by an aluminum plate to which a vacuum hose and pump were
connected. The bottom three lnches of the quartz tube were
filled with 1/2 inch thick carbon felt discs to thermally
insulate the rubber vacuum gasket.
Three feed hoppers were used. One for boron nitride
grain SHP-40 grade, a product of Sohio Engineered Materials
Company, one for silicon grain, grade Siligrain SGl-20 mesh, a
product of Elkem Metals Company, and one for silicon carbide
grain, a blend of 95% 50/100 mesh size No. 1, a product of
Exolon-ESX Company ànd 5% dry phenolic resin, Dyphene* grade
877P, a product of Sherwin-Williams Company.
The particulate Eeed material was fed through eighteen
plastic 1!4 inch inside diameter feeder tubes, six tubes for
each of the feed materials. The feed tubes were arranged
around the periphery of the top of the loading means in 60
degree increments. The feed tubes were arranged to feed
particulate boron nitride in the space between the outer tube
*Trade Mark
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on the loading device and the inside of the furnace tube,
particulate silicon carbide within the tubes of the loading
means, and particulate silicon in the space between the inside
of the loading means and the outside of furnace heating tube.
After filling the loading mean~ was 810wly raised to
leave seperate concentrically arranged, hollow columns of
particulate boron nitride, silicon carbide-resin, and silicon.
After removal of the loading means, the top space was filled
with carbon felt discs and capped with a rubber gasket and a
metal plate. A vacuum was applied to the lower end of the
furnace tube.
An induction coil having twelve turns of 3/16 inch
outside diameter copper tubing with a coil inside diameter of
3 1/8 inches and a length of three inches, was connected to a
450KHZ, 2 1/2XW Lepel induction power supply and the coil
~tarted at the top of the furnace tube using a 0.8 plate
current power input. The coil was lowered along the furnace
tube at a rate of 0.33 inch per minute. The coil was stopped
and the power turned off when the bottom of the quartz tube
was approached. The furnace tube was then allowed to cool to
room temperature, opened and the tubular product removed. The
siliconized ~ilicon carbide tube was easily removed from the
quartz furnace tube as the boron nitride was still in loose
granular form and unaffected by infiltration process~ The
heating element was easily removed from the siliconized
silicon carblde tube a~ the silicon column had been removed by
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infiltration into the silicon carbide column. The siliconized
silicon carbide tube product was found to be round and staight
with little porosity in the microstructure. The silicon
carbide volume in the mic~ostructure was estimated by visual
inspection to be about 50~. The outside diameter of the
product was about 2.160 inches an~ the inside diameter of the
the product was about 1.970 inch.
EXAMPLE 2
In this Example, graphite powder having an approximate
grain size of minus 150 mesh and a tap density of 0.58 g/cm
was used in place of the silicon carbide-resin component as
was used in Example 1 and in place of the furnace heating
element -a ,core of insulating grain was used. The procedure
u~ed otherwise followed that of example 1. A photomicrograph
of a polished section of the tubular product revealed that thè
graphite particles were not completely converted to silicon
carbide and that only a thin layer of silicon carbide was
present on the surface of the graphite particles which were in
turn surrounded by a matri~ of silicon.
~ EXAMPLE 3
In this Example the furnace tube was charged with
concentric layers of insulation grain, silicon grain and
graphite powder packed around a core of insulation grain.
Otherwise t~e procedure of Example 1 was followed. The
product was similar to that in Example 2.
While the pre~ent invention has been described in detail
in connection with specific embodiments thereof, it will be
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understood that further embodiments and modifications may be
may be made without departing from the spirit and scope of the
appended claims.
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