Single crystal fibers are made from miniature extruded ceramic feed rods. A decomposable binder is mixed with powders to inform a slurry which is extruded into a small rod which may be sintered, either in air or in vacuum, or it may be used in the extruded and dried condition. A pair of laser beams focuses onto the tip of the rod to melt it thereby forming a liquid portion. A single crystal seed fiber of the same material as the feed rod contacts this liquid portion to establish a zone of liquid material between the feed rod and the single crystal seed fiber. The feed rod and the single crystal feed fiber are moved at a predetermined speed to solidify the molten zone onto the seed fiber while simultaneously melting additional feed rod. In this manner a single crystal fiber is formed from the liquid portion

Westfall, Leonard J.

English

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AWARDS ABSTRACT
METHOD OF MAKING SINGLE CRYSTAL FIBERS
Single crystal fibers are made from miniature extruded ceramic feed
rods. A decomposable binder is mixed with powders to form a slurry which
is extruded into a small rod which may be sintered, either in air or in
vacuum, or it may be used in the extruded and dried condition. A pair of
laser beams focuses onto the tip of the rod to melt it thereby forming a
liquid portion. A single crystal seed fiber of the same material as the
feed rod contacts this liquid portion to establish a zone of liquid
material between the feed rod and the single crystal seed fiber. The feed
rod and the single crystal feed fiber are moved at a predetermined speed to
solidify the molten zone onto the seed fiber while simultaneously melting
additional feed rod. In this manner a single crystal fiber is formed from
the liquid portion.
INVENTOR:
EMPLOYER:
EVALUATOR:
SERIAL NO.:
FILING DATE:
Leonard J. Westfall
NASA Lewis Research Center
Salvatore J. Grisaffe
571,059
August 23, 1990
NASA Case No. LEW-14,921-1 PATENT
METHOD OF MAKING SINGLE CRYSTAL FIBERS
Origin of the Invention
The invention described herein was made by an employee of the United
States Government and may be manufactured and used by or for the government
for governmental purposes without the payment of any royalties thereon or
5 therefore.
Technical Field
This invention is concerned with making single crystal fibers. The
invention is particularly directed to forming small feed rods for use in
the production of single crystal fibers.
10 It is customary in the art to produce feed rods from a hot pressed disk
of the desired material. The disk is approximately four inches in diame-
ter, and it is fully dense. The disk is then cut into small slices that
are centerless ground to produce a round rod or surface ground into precise
squares.
15 Each rod is about four inches or less in length depending on the size of
the hot pressed disk. The cost of each rod is approximately $200, and the
time required to make a new material into these small rods is about four to
five weeks.
One of the disadvantages of the prior art procedure is the high cost.
20 Another disadvantage is the long lead time required to obtain a new
material. Still another disadvantage is that feed rods produced in accor-
dance with prior art procedures have short lengths.
It is, therefore, an object of the present invention to produce ceramic
or metal feed rods quickly. Another object of the invention is to provide
25 small diameter feed rods economically.
Backqround Art
U.S. patent No. 4,044,082 to Rosette is directed to the extrusion and
re-extrusion of ionic salts which are transparent to various radiations,
including laser beams. The invention deforms the salts by extrusion to
30 fully densify them at a temperature below the melting point. The extruded
billet is then extruded a second time to produce a shape without the loss
of its transmission properties.
U.S. patent No. 4,163,140 to Bardet relates to sintering or melting
ceramic or refractory products in a waveguide or sounding cavity wherein
2the materials are subjected to the action of a microwave field. A close
loop temperature measurementand control is utilized to improve the end
product.
U.S. patent No. 4,725,391 to Bardhan et al teaches extruding ceramic
5 powders, binders, and oxidizers to form ceramic tubes which are fired to
produce coaxial capacitors. The disclosed process is a typical extrusion
and is not concerned with making single crystals.
Disclosure of the Invention
The aforementioned objects are achieved by making small ceramic or metal
10 feed rods having diameters between about O.OlO inch and about 0.050 inch
from powder using a decomposable binder. The extruded rods may be sintered
or used in the extruded and dried condition.
The feed rod is then utilized in a floating zone laser system which
melts the tip of the feed rod. A single crystal seed rod or fiber of the
15 same material is brought into contact with the molten tip which causes part
of the single crystal to melt and maintain a stable liquid zone of the
material between the feed rod and the single crystal seed rod. The feed
rod and the single crystal seed rod are moved in controlled speeds to
produce a single crystal fiber having the same orientation as the single
20 crystal seed rod.
Brief Description of the Drawinqs
The foregoing, as well as other objects, features and advantages of the
invention will become more apparent from the following detailed description
when taken in conjunction with the appended figures in which
25 FIG. I is an axial cross section of a die used in a ram type extruder
containing a ceramic powder slurry;
FIG. 2 is an axial cross section of a ram type extruder utilizing the
die shown in FIG. I for extruding a ceramic feed rod from the powder
slurry;
30 FIG. 3 is an axial cross section of the extruder shown in FIG. 2 used
in conjunction with an oven illustrating an alternate embodiment of the
invention;
FIGS. 4, 5, and 6 are schematic views illustrating the process of
producing a single crystal fiber from the feed rod produced in a manner
35 shown in FIGS. I-3; and
3FIG. 7 is an axial cross section of an extrusion die used to produce a
continuous feed rod in the manner shown in FIGS. I-3 that is used to pro-
duce a continuous single crystal fiber in the manner shown in FIGS. 4-6.
Best Mode for Carryinq Out the Invention
5 Referring now to the drawings there is shown in FIG. I an extrusion die
10 that is used to produce small diameter feed rods in accordance with the
invention. More particularly, ceramic powders are mixed with a suitable
decomposable binder to form a slurry 12 that is placed in the die 10.
As shown in FIG. 2 an extrusion ram 14 forces the ceramic powder
10 slurry 12 through an opening 16 to form an extruded ceramic rod 18. The
diameter of the opening 16 is quite small, being on the order of 0.050 to
0.010 inch. By proper control of the slurry mixture, temperature and speed
of the ram 14, the resulting feed rod 18 is straight and uniform in the as-
extruded and dried condition. The feed rods 18 may be used in their as-
15 extruded and dried condition.
The process for using the feed rod I8 is illustrated in FIGS. 4, 5 and
6. More particularly, the feed rod 18 extends through a suitable feeding
mechanism in the form of a pull head 20 in a floating zone laser system.
Two laser beams 22 and 24 shown in FIGS. 4-6 from lasers 26 and 28, respec-
20 tively, are focused onto the feed rod tip. This causes a small portion of
the feed rod 18 at the tip to melt, thereby forming a molten portion or
liquid zone 30.
A single crystal 32 is in the form of a seed rod or fiber and supported
by a rod 34 and pull head 36. This single crystal 32 is spaced from the
25 molten tip of the feed rod 18 as shown in FIG.4. This single crystal 32 is
of the same material as the feed rod 18.
The single crystal 32 is brought into contact with the molten tip 30 of
the feed rod 18 as shown in FIG. 5. This causes the end portion of the
single crystal 32 to melt and become a part of a stable liquid zone 30 of
30 the material between the feed rod 18 and the single crystal rod 32.
By moving both the feed rod 18 with the pull head 20 and the single
crystal rod 32 with its pull head 36 at a controlled speed, a single
crystal fiber 38 of the ceramic material is solidified from the liquid 30
and formed with the same orientation as the seed rod 32. At the same time
35 the diameter of the single crystal fiber 38 is controlled by the speed of
4the pull head 36 that is connected to the single crystal rod support 34
relative to the speed of the pull head 20.
The liquid pool formed by the molten portion 30 remains a constant
volume and is fixed in space. The crystal 38 is grown out of the liquid
5 pool, either up or down, while the feed rod 18 enriches the pool 30 causing
its volume to remain constant. The liquid is always at the tip of the feed
rod, and it is fixed in space by the focused laser beams.
By way of example, at the samespeeds a O.025-inch diameter 50%dense
extruded feed rod 18 produces a 0.017 inch single crystal fiber 36. At
10 twice the speed of the extruded feed rod 18, a single crystal fiber 36
having a diameter of 0.012 inches is formed.
Alternate Embodiment of the Invention
Referring now to FIG. 3 there is shown a ram extruder having the same
construction as that shown in FIG. 2. However, the ceramic feed rod 18 is
15 extruded into a furnace 40 comprising a hollow cylindrical wall 42 that
contains electrical heating elements 44. The feed rod 18 is oven-dried and
sintered at approximately 2600"F or higher in the furnace 40. This sinter-
ing may be done in air or inert gas in the furnace 40 or in a vacuum.
Still another embodiment of the invention is shown in FIG. 7 wherein
20 the ram extruder shown in FIG. 2 extrudes a small diameter feed rod 18 in a
continuous manner to the feeding mechanism 20 where the continuous single
crystal fiber is formed in the same manner as discussed above with regards
to FIGS. 4, 5 and 6.
From the aforementioned description it is apparent a major advantage of
25 this invention is that a large number of small diameter feed rods of almost
any size can be made in a short time. Also, single crystal fibers can be
produced from these feed rods within a short time after the initial feed
rod fabrication.
Another advantage is that the cost of the feed rods is essentially only
30 the cost of the powder material. By way of example, five grams of powder
produces 10 feed rods that are 6 inches long and 0.025 inches in diameter.
It is apparent that the as-extruded and dried feed rods shown in FIG. 2
are easily handled and support themselves so that the feed rod motor drive
20 can move the rod 18 smoothly into the laser beams 22 and 24. The
35 extruded feed rod 18 becomes heated as it approaches laser beams 22 and 24
which cause the volatile binder to evaporate and the powders of the feed
10
15
20
rod 18 to sinter as the temperature increases. The final stage is for the
small section of sintered feed rod 18 to begin to melt and form the molten
zone 30 of the liquid material that is supported only by the sintered
powders beneath the liquid.
Dopedceramics, complex ceramics, or metal alloys may be formed into
feed rods 18 in a short time and into single crystal fibers in an equally
short time by the use of this process. As shown in FIG. 7 the invention
maybe performed in a continuous fashion by extruding the feed rods slowly
and directly into the focused laser beams22 and 24. This makespossible
the production of continuous single crystal fibers from powdered feed rod
at low cost and with no contamination from any container.
The total cost savings utilized in this process is evident. Also, the
possibility of continuous single fiber production cannot be underestimated.
It is further contemplated that the feed rod 18 maybe extruded using a
binder that is flexible which enables the feed rod to be dried and spooled
onto a coil. The coiled feed rod material may be uncoiled and fed into the
focus laser beamsto produce a semicontinuous single crystal fiber that
could be several thousand feet long.
While several embodimentsof the invention have been disclosed and
described, it will be apparent that various structural modifications may be
madeto the invention without departing from the spirit thereof or the
scope of the subjoined claims.
°"v
ABSTRACT OF THE DISCLOSURE
NETHOD OF RAKING SINGLE CRYSTAL FIBERS
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Single crystal fibers are made from miniature extruded ceramic feed
rods. A decomposable binder is mixed with powders to form a slurry which
is extruded into a small rod which may be sintered, either in air or in
vacuum, or it may be used in the extruded and dried condition. A pair of
laser beams focuses onto the tip of the rod to melt it thereby forming a
liquid portion. A single crystal seed fiber of the same material as the
feed rod contacts this liquid portion to establish a zone of liquid1 material between the feed rod and the single crystal seed fiber. The feed
rod and the single crystal feed fiber are moved at a predetermined speed to
solidify the molten zone onto the seed fiber while simultaneously melting
additional feed rod. In this manner a single crystal fiber is formed from
the liquid portion.
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Method of making single crystal fibers

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