Note: Descriptions are shown in the official language in which they were submitted.
1032
This invention relates to crucibles which are
used in melt spinning devices, and more particu-
larly, to an improved crucible formed of boron
nitride and having a wind deflecting skirt which
ellminates air currents from the area where the
molten jet of material meets a moving chill sur-
face.
Devices for forming a continuous filament of
material by cooling a jet of molten material by
contacting it with a moving chill surface are well
known in the art. Tsuya and Arai report on exper-
iments with such apparatus in the Japan Journal of
Applied Physics, Vol. 18, 207-208 (1979~. The
process for forming this ribbon, called the melt
spin process, operates on the principle that a
liquid jet of molten material will solidify on a
moving heat sink to produce a continuous filament
of material. Molten material is placed inside a
reservoir portion o a crucible, where it is main-
tained at a temperature above the melting point ofthe material. By providing a means for effecting
expulsion of the molten material through a nozzle
or orifice in the crucible/ a jet of molten mate-
rial is impinged upon the surface of a moving
chill surface. The jet of molten material then
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wets the surface of the chill surface, causing a
puddle of material to form. The puddle of materi
al can then solidify on the chill surface, thus
producing a continuous ribbon or filament which
can be removed from the chill surface.
The melt spin process and the melt drag pro-
cess are both discussed in the ~ackground Section
of U.S. Patent 4~147,571 issued to Narasimhan on
March 6, 1979. In that patent, it is noted that
"the key to success in the melt spin process is to
stabilize the liquid jet until it solidifies.l'
Stability of the jet is affected by a number of
factors including the viscosity of the molten ma-
terial which in turn is controlled by the tempera-
ture of the melt. ~s noted at Column 5, Lines 43to 47 of the patent, gas currents created by the
moving chill surface can also disturb the molten
jet and/or the puddle which normally forms at the
point where the molten jet meets the moving chill
surfaceO It is of interest that these problems
are recognized in the Narasi n patent even
though it discloses a "planar flow casting" pro-
cess in which there is actually no free flight re-
gion between the nozxle and the moving chill sur-
face.
We have found that the above discussed dis-
advantages may be overcome by employing a crucible
formed from a nonconductive refratory material,
and having an orifice through which material is
expelled toward a moving chill surface and a wind
deflecting skirt formed around the orifice. The
refractory material, which can be quartz, alumina,
or, in a preferred embodiment, boron nitride, is
preferred because it has a higher thermal conduc-
tivity than materials used in prior art cruci-
bles. Therefore, this preférred crucible would
maintain a more uniform temperature within the
molten mass of material and would thereby reduce
possihle defects in the ribbon caused by tempera-
ture gradients.
A melt spinning crucible according to thepresent invention includes a reservoir portion for
holding molten material and having an orifice
through which the material is expelled towards a
moving chill surface and a wind deflecting skirt
formed around said orifice to protect the molten
jet from gas currents as it travels from the cru
cible orifice to the moving chill surface. In a
preferred form, ~et stability is also improved by
close temperature control achieved by forming the
crucible for boron nitride material.
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The preferred embodiment of this invention
will now be described by way of example, with re-
ference to the drawings accompanying this specifi-
cation in which:
Fig. 1 is a simplified view of melt spinning
apparatus including a crucible according to the
present invention as it would appear in operation;
and
Fig. 2 is a cross-sectional view of a portion
of the apparatus of Fig. 1.
With reference now to Fig. 1, melt spinning
apparatus including an improved crucible 10 ac-
cording to the present invention is illustrated.
Crucible 10 may be formed from a nonconductive re-
fractory material such as quartz, alumina, zirco-
nia, or titania. In a preferred embodiment, cru-
cible 10 is formed Erom boron nitride as e~plained
in more detail below. A heating coil 12 is formed
about crucible 10 to maintain a mass of material
20 within the crucible in the molten state. Heating
element 12 may be either an RF induction coil or a
resistive heating element. Either heating tech-
nique is well known. The upper end 14 of crucible
10 is provided with an airtight seal and a gas in-
5 let 16 for providing force to control ~he flow of--4--
~u~3~a3
the molten material within the crucible through an
orifice on its lower end. The orifice (not shown
in Fig. 1) is positioned above a rotating chill
s~rface in the form of a wheel or roll 18. ~r-
rangements for rotating and cooling wheel 18 arewell known. One technique for cooling wheel 18
involves directing a cool gas stream at the sur-
face of wheel 18, for example, by means of a con-
duit 20. For this or other possible reasons, the
melt spin process may be conducted in the presence
of a gas atmosphere. For this reason, a skirt 22
is formed around the lower end of crucible 10 to
protect the jet of molten material 24 from gas
currents. The gas currents are generated primari-
ly by movement of wheel 18 itself and are, thus,present primarily at the surface of the wheel
where stability of the jet 24 is most critical.
An opening 26 is formed in one side of skirt 22 to
allow a filament or ribbon 28 to move along with
2`0 the surfa~e of wheel 18 without contacting skirt
22. In the Figures, the opening 26 is greatly ex-
aggerated or the purposes of illustra~ion. As is
conventional in the melt spin processl the ribbon
material 28 typically separates from wheel 18, for
5 example, at point 30r due to centrifugal forces.
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l~V~
With reference now to Fig. 2, more details of
the crucible 10 are illustrated in a cross-sec-
tional viewO In Fig. 2, it is seen that the bulk
of crucible 10 forms an inner reservoir 32 in the
lower portion of which is positioned a mass of
molten material 34. Material 34 may be, for exam-
ple, molten silicon material intended for use in
manufacturing semiconductor devices. An upper
portion 3S of reservoir 34 is typically filled
t
with an e*e~ gas such as argon. The pressure of
gas in region 36 is controlled through inlet 16 of
Fig. 1 to control the rate of material flow
through the orifice 38 at the bottom of reservoir
32. Gas pressure is typically controlled in the
range of 0.5 to 2.0 pounds per square inch to re-
gulate flow rate through orifice 38. Negative
pressure in the upper portion oE reservoir 32 re-
lative to external pressure may be used to retain
material 34 within reservoir 32 while appropriate
process conditions are established. As illustrat-
ed in Fig. 2, the jet 24 tends to form a puddle of
molten material 40 ~t its point of contact with
moving chill surface 18. Ideally/ the molten ma-
terial solidifies to some extent before the ribbon
5 28 exits through opening 26. The solidified por~
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tion 2a is indicated by the cross hatched section
thereof. It will be appreciated that skirt 22
should essentially contact the chill surface 18 in
order to provide maximum isolation of the zone 42
from moving gas currents~ In practice, some
spacing must be provided between the lower edges
of skirt 22 and the chill surface 18. We believe
that a maximum distance of about 0.40 inches may
be allowed between the skirt 22 and chill surface
18. The frontal opening 26 is sized to allow the
filament 28 to exit from the protected zone 42
with a like amount of clearance. The opening 26
has been exaggerated in the drawings for purposes
of illustration only. ~hus, if filament 28 is
0.20 inches thick, the height of opening 26 would
likewise be 0.20 inches~
The crucible 10 will normally be constructed
from refractory materials such as quartz, alumina,
zirconia, or titania or, in a pre~erred embodi-
ment, boron nitride. The use of b~ron nitride ispreferred because it has a hi~her thermal conduc-
tivity than materials used in prior art cruci-
bles. This preferred crucible would, ~herefore,
have a more uniform temperature within the molten
mass of material and would thereby reduce possible
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8~
defects in the ribbon caused by temperature gradi-
ents. For example, by maintaining a more uniform
temperature within the melt, the viscosity of the
molten mass is more easily controlled and uniform
and as a result, the flow rate through orifice 38
will remain more constant.
The crucible 10 can be constructed by a hot
pressing technique. For some materials sintering
may be required. For fused silica, the crucible
could be formed by glass-blowing techniques. Some
materials which contain binders must be heat
treated at elevated temperatures to remove the
binders prior to use of the crucible in a melt
spinning device. It is preferred that the base of
the crucible be formed in a solid orm which is
then machined to generate the skirt 22 and orifice
38. This machining step is primarily a matter of
boring out the protected zone 42 and orifice 38.
In the initial manufacturing step, it is preferred
2~ that skirt 22 have some excess length. The excess
length may then be machined off as the optimum
distance between orifice 38 and chill surface 18
is determined~ The thickness of skirt 22 is not
believed to be critical so long as there is suffi-
cient thickness for appropriate mechanicalstrength.
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The actual dimensions of a crucible 10 will
vary depending upon the type of crucible material
and the scale of the desired process. An initial
design of a crucible intended for casting a ther
moelectric material may have the following dimen-
sions. The overall length of the crucible from
the lower end of skirt 22 to the upper end 14
would be approximately 100 millimeters. The inner
diameter of reservoir 3~ would be approximately
six millimeters while the outer diameter of the
crucible would be approximately twelve milli-
meters. Orifice 38 would have a length of about
one millimeter and a diameter of about 0.3 milli-
meters. The depth of cavity 42 and, therefore,
the length of skirt 22 would be approximately two
millimeters. The inner diameter of cavity 42
would be about eleven millimeters.
The height of frontal opening 26 could cor-
respond to the thickness of ribbon 28, while the
width of opening 26 would be about one millimeter
wider than ribbon 28.
While the present invention has been illus-
trated and described with respect to specific ap-
paratus and methods of use, various modifications
and changes can be made within the scope of the
appended claims.
_g
