Note: Descriptions are shown in the official language in which they were submitted.
1047743
This invention relates to a method for forming refrac-
tory materials into the form of tubing, and more particularly to
forming refractory tubing in amorphous, single crystal or poly-
crystalline forms.
The term "refractory" is used hereinafter to designate
materials which have relatively high melting points and which may
or may not be excessively corrosive. The term is meant to in-
clude amorphous and crystalline materials, including glass, single
crystal and polycrystalline forms; compounds such as alumina, silica
thoria, zirconia, ytteria, etc.; intermetallics such as gallium
arsenide and pseudobinary compounds such as Al~s-GaAs; as well as
elements such as germanium and silicon. Any refractory suitable
for this invention must be capable of existing in a molten state
whether under ambient conditions or controlled environment.
Although a number of refractories have been made into
tube forms by conventional powder processes, at present alumina is
the most important of these refractories where high-temperature
strength, high electrical resistivity and chemical inertness are
required. Therefore, alumina may be taken as exemplary in discuss- -
~
20 ing prior art and utility. -
An important application for alumina tubing is enclo-
sures for high-pressure sodium-halide lights. The emitted light
from a high-pressure sodium-halide (normally sodium iodide) light
is more pleasing than the yellow light emitted by low-pressure
sodium vapor lamps. Moreover, these lamps are smaller and more
efficient than alternative lamps such as mercury vapor lamps or
fluorescent sources. At the increased operating temperatures
characteristic of the high-pressure sodium lamps, the gases be-
come too corrosive to permit the use of previously acceptable
vitreous silica enclosures. These more severe conditions have
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1(~47743
led to the use of alumina enclosures which are formed either by
sintering the alumina in the desired configuration, or by adapta-
tion of the Czochralski crystal pulling technique.
Even with the most sophisticated sintering techniques,
the sintering process rarely produces materials which attain full
or theoretical density. Such failure to attain full density means
that alumina tubes or envelopes formed by sintering alone pro-
bably have residual porosity which provides light scattering
sites, thus detracting from the efficiency of any lamps formed
from the sintered tubing.
To minimize porosity in materials produced by this
prior art sintering techni~ue, it is necessary to fire the pieces
in an atmosphere made up of a gas which has a sufficien.ly high
solubility and mobility for diffusion of the gas entrapped in .
closed-off pores out of the sintered materia~. Such an atmosphere
is, for practical purposes, limited to hydrogen. Alternatively,
the sintering may be done in a vacuum. Thu* in the prior art pro-
cesses, there is yirtually no freedom to select ambient atmos-
pheres to maximize purification of the final tubing material or
to attain other desirable results such as the ability to adjust
the valence state of intentionally added dopants, additives or Of
residual impurities.
It may be desirable to be able to form such tubing of
a single crystal. If the material is not optically isotropic,
for example materials having hexagonal crystal structures, the
presence of a plurality of different grain boundaries in the
optical path ~ill degrade the potential image quality of trans-
mitted light. Such grain boundaries may he effectively eliminated
by forming the tubing as a single crystal. This is not the case
with cubic crystals which are optically isotropic. However, grain
1047743
boundaries in most materials act as concentrators as well as high-
mobility pathsof impurities. Formation of refractory tubings as
single crystals has several additional important advantages. Thus,
for example, it is possible to eliminate, reduce or control the
stresses in ~grown tubing by eliminating stresses due to thermal
expansion anistropy between the grains. Forming single-crystal f
tubing can also provide a crystallographic orientation that is
favorable for relaxing as-grown stresses.
The adaptation of the Czochralski method of growing
crystals to the formation of tubing by pulling the tubing from a
hot melt contained in a hot crucible presents the serious dis- ¦
advantage of introducing contaminants into the tubing from the
crucibles. Such contaminants may interact with the active gases
within the lamp enclosure serving as "getters" for these small
quantities of gases or they may increase the total absorption
across the emitted spectrum of the lamp thus decreasing its
efficiency. Ambient atmospheres must be limited to those which
do not result in degradation of the crucible. Finally, there
are many materials for which no known crucible material exits.
20- The principal disadvantages of t~e prior art methods-- '
failure to attain full density with the resulting undesirable
degree of porosity, introduction of impurities, and restrictions
imposed on processing atmospheres -- are minimized or eliminated
by the method of this invention.
It is therefore a primary object of this invention
to provide improved method and apparatus for forming amorphous
or crystalline refractory tubing. It is another object to pro-
vide method and apparatus of the character described which make
possible the forming of refractory tubings of materials exhibit-
3~ ing full density or near full density and hence of materials in
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1047743
whieh light scattering sites are reduced to a minlmum. Another
objeet is the providing of such method and apparatus which make
it possible to make refractory tubing of extremely high purity.
It is yet another object of this invention to provide method and
apparatus for forming refractory tubing eontaining additives,
the valence state of which may be adjusted. An additional object
is to provide method and apparatus of the character described
whieh make possible, if desired, the formation of refraetory tub-
ings in single-erystal form. It is yet another object of this
invention to provide method and apparatus of forming refractory
tubing, the cross sectional configuration and wall thickness of
whieh may be varied. It is another object of this invention to
provide such method and apparatus which are applicable to a wide
range of high-temperature materials including those normally con-
sidered to be too corrosive to be contained in crueibles. Other
objects of the invention will in part be obvious and will in part --
be apparent hereinafter.
By the method and apparatus of this invention there
is formed a refraetory tubing exhibiting improved physical pro-
perties. According to this invention this method of forming re-
fraetory tubing comprises the steps of providing a preformed
tubing blank of a refractory material as a feed tube; providing
around the tubing blank a concentrated heating zone of laser
energy of substantially uniform intensity thereby to form in the
tubing a molten ring of essentially uniform height completely
through the tubing, the height being such that the forces of sur-
face tension and gravity maintain the molten ring connected to - -
solid sections on either side of the molten ring of said tubing,
the step of providing said heating zone of laser energy compris-
ing forming a pluralit~7 or laser beams of half-annular configura-
, _5_
104774~
tion equally spaced around the tubing, and focusinq each of thelaser beams of half-annular configuration onto the refractory tub-
ing to form a molten ring in the tubing, the focusing being per-
formed in a manner to adjust the energy density distribution
of each of the beams to essentially equalize the absorption of
laser energy around the entire surface of the tubing within said
molten ring; and controllably effecting relative translational
movement between the refractory tubing and the heating zone there-
by to advance the molten ring through the tubing.
The solid sections on either side of the molten ring
may be rotated in the same or opposite directions; and the tubing
may be formed as a single crystal by using appropriately configured
seed crystals. When each end of the tubing is separately held
and when separate moving means are associated with one or both
of the two solid sections (one on each side of the molten ring)
it is possible by adjusting the speed of one of the moving means
relative to the speed of-the other moving means, or the speed --
at which the heat~ng means are moved, to control the thickness -~
of the final tubing wall. Other method modifications are also
possible.
According to this invention, there is provided apparatus
for forming refractory tubing. This apparatus comprises holdina
means adapted to hold separate solid sections of a refractory
tubing; heating means for forming a molten ring in the refractory
tubing, the rlng being of essentially uniform heignt throughout
the tubing, and the height being such that the forces of surface
tension and gravity maintain the molten ring connected to the
solid sections, the heating means comprising (1) laser means pro-
viding at least one beam of radiant energy and (2) optical means
including means for expanding said beam, means for splitting the
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1(~47743
resulting expanded beam into a plurality of beams essentially
equally spaced around the surface of the tubing, energy distri-
bution means to adjust the energy density distribution of each
of the beams making up the plurality of beams to essentially equal-
ize the absorption of the radiant energy around the entire sur-
face of the tubing within the molten ring and means for focusing
each of said plurality of beams onto the surface of the tubing
within a heating zone surrounding the tubing and providing essen-
tially uniform heating of the tubing surface; and means to effect
controlled relative translational movement between the holding
means and the heating means whereby said molten ring advances
through the tubing.
The apparatus of this invention may also optionally
include radiation shielding means surrounding the heating zone,
means to rotate the solid sections of the tubing either in the
same or opposite directions, means to define a controlled atmos-
phere around the tubing; and means to control the movement of
a pick-up member to initiate the movement of the molten ring.
For a fuller understanding of the nature and objects of
the invention, reference should be had to the following detailed
description taken in connection with the accompany drawings in -
which
Fig. 1 is an enlarged longitudinal cross section of the
tubing during formation wherein the finis~ed refractory tubing
has essentially the same wall thickness as the preformed feed
tubing;
Fig. 2 is an enlarged longitudinal cross section of the
tubing during formation wherein the finished refractory tubing has
an attenuated wall thickness;
Figs. 3 and 4 are typical cross sections taken in a
~047743
plane normal to the tubing axis;
Fig. S illustrates in diagrammatic perspective the use
of a ring to start the melting of the tubing at one end
Figs. 6 and 7 illustrate in diagrammatic perspective the
use of a flat plate to start the melting of the tubing at one end;
Fig. 8 illustrates the use of a rod ~o start the melting
of the tubing at one end;
Figs. 9-12 illustrate the steps of initiating the for-
mation of a single crystal refractory tu~ing in accordance with one
embodiment of this invention using one combination of feed tube
and pick-up seed crystal configurations;
Figs. 13-17 illustrate other combinations of feed tube
and pick-up seed crystal configurations;
Fig. 18 is a top plan view of an optical system suit-
able for use with a laser to form the heating means, this optical
system including means to focus the laser beam to form four beams
as lines with controlled energy densities;
Fig. 19 illustrates in diagrammatic fashion the effect ~ -
of the focusing means of Fig. 18;
Fig. 20 is a side elevational view, partly in cross
section, of one embodiment of radiation shielding means surround- -
ina the heating zone; and
Fig. 21 illustrates in somewhat ~iagrammatic cross sec-
tional form the providinq of a controlled fluid surrounding for
the tubing during formation using the laser heating system of
Fig. 18.
There is, of course, a great deal of prior art on float-
zone melting of solid rods of various materials. See for example
"Zone Melting" by ~illiam G. Pfann, John ~iley & Sons, Inc., New
3~ York, New Yor~, 158, and United States Patent 3,1~1,619. However,
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1047743
this techniqu~ has not previously been applied in any workable
manner to the formation of tubings which presents particular pro- -
blems not encountered in the processing of solid rods. British
Patent Specification 1,226,473 mentions in passing that a laser
beam may be focused at a point onto a tubing surface, but no work-
able method is taught which makes it possible to overcome the pro
blems associated with tubing or to form refractory tubings o~ the
character sought and achieved by the method of this invention.
Those particular problems which are associated with tubings in-
clude the maintenance and adjust~ent of wall thicknesses, the
maintaining of a desired cross section of tubing, the homogeniza-
tion of tne refractory materials, the optional formation of a
single crystal,and the like. Tubings have also been pulled from
molten material contained within a crucible as shown in United
States Patent 3,015,592. However, the physics, as well as the
apparatus, of the floating zone process are comple~ely different
from the crucible-contained process.
There is, o~ course, con~iderable art on the drawing
of glass tubings using dies and mandrels (see for example U.S.
Patents 3,620j704 and 3,672,201). However, such processes are ;
not applicable to the formation of refractory tubings and parti-
cularly to forming tubings of materials having very high melting
points or tubings of exceptionally high purity and/or in single
crystal form.
Before describing various embodiments of the method and
apparatus of this invention, it will be helpful to present the
method generally with reference to Fig. 1 which is an enlarged
longitudinal cross section of the molten zone forming section of
the system. As a solid preformed tubing feed blank 20 (generally
formed by pressing the refractory powder and presintering if
1047743
necessary) is moved upwardly through a heating zone 21 created
by directing laser energy in the manner and with apparatus to be
described, a ring 22 of molten refractory is formed and continu-
ously, in effect, advances through the tubing in a downwardly
direction, forming a refractory tubing 23. The dimensions of j~
heating zone 21 are defined by the energy input distribution which
in turn is determined by the optics of the heating means, the
thermal losses into the feed and drawing rods and the thermal
losses to the surrounding environment.
The height h of the heating zone must be so-controlled
through the adjustment of these parameters, and the solid sections
20 and 23 of the tubi~ must be moved a~ such a rate as to always
keep them joined through molten ring 22 which effects such joining
through the forces of surface tension and gravity.
The solid preformed feed tubing 20 has an internal radius
of ra 1' an external radius of ra 2' a wall thickness of ta and a
fractional density of Pa. In like manner, the solid tubing 23
has an internal~radius of rb 1' an external radius of rb 2' a wall
thickness of tb and a fractional density of p~ generally 100%.
In order to establish a stable system permitting the continuous
advancement of the molten ring 22, the mass flow rate crossing the
solid-liquid boundary 24 between tubing feed section 20 and molten ~
ring 22 and the mass flow rate crossing the liquid-solid boundary ~ -
25 between molten ring 22 and tubing section 23 must be equal.
Since the velocities at which the tubing feed section 20 and the
tubing section 23 are moved may be separately controlled, it is
possible to adjust the wall thickness, and to some extent the out-
side diameter of the tubing formed, by moving the tubing section
23 at a greater or lesser velocity. The situation diagra~ed in
Fig. 2 shows how the wall thickness may be attenuated by moving
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~(~47743
section 23 faster than section 20.
Since the mass flow rate is equal to the product of
factional density, cross sectional area and velocity, the required
stable system is attained when
~r (ra_2 ~ ra-~ ' PaVa = 7r (rb_2 ~ rb_l) PbVb
.
where va and vb are the velocites at which the tubing sections are
moved. Assuming that the fractional densities of both tubes are
substantially 100%, and that rb 2 is essentially.equal to ra Z'
then
2~ra_2taPaVa ~ 2~rb-2tbPb b
By increasing vb it is possible to decrease tb~ or by decreasing
Vb it is possible to increase tb~ so long of course as the basic
requirement is met that the two sections are continuously joined
through the molten ring. Thus there is provided a way of control-
ling the wall thickness of the finished refractory tubing.
The inside and outsidediameters of the final tubing
are functions of zone height and dimensions of the feed tube.
If there is no attenuation during the formation of the final
tubing, the resulting tubing will generally have a somewhat
small diameter and a greater wall thickness. With attenuation,
it is possible to change the relationship between the outside
; diameter and wall thickness; but the outside diameter of the
growing tube will always to equal to or smaller than the outside
diameter of the feed tube.
It is, of course, within the scope of this invention to
form refractory tubings having a range of cross sectional con~ig-
urations in which the wall thickness may be uniform or nonuniform.
Exemplary of a circular cross section of uniform wall thic~ness
1~)47743
is tubing 28 of Fig. 3, and of an eliptical cross sec-
tion with nonuniform wall thickness is tubing 29 of Fig.
4. The ultimate cross sectional configuration may be
controlled in forming the feed tubing blank and to some
extent by the design of the heating zone.
Although it is possible to start the growth of the
final tubing by placing the feed tube in the melting
zone so that melting is begun somewhere between the
ends of the feed tube, it is usually more desirable to
begin the melting at one end of the feed tube. To do
this, it is necessary to bring one end of the feed tube
located in the melt zone into contact with a contacting
solid surface member, hereinafter referred to as a pick-
up, which is affixed to a load-bearing rod such as the -
rod described in connection with Fig. 17. With the
melting of the end of the feed tube and the formation
of a melt in the contacting surface it is possible to
make contact and "weld" the tubing to the pick-up. The
pick-up may take any desirable form and may, in some
instances, be a single crystal used in a seeding func-
tion to start the formation of a single-crystal tubing
Several exemplary forms of pick-ups are shown in Figs.
5-17.
In Fig. 5 the pick-up is in the form of an annular
ring (or other suitable cross section) of any desired -
length which is brought into contact with a molten ring
31 formed on one end of the feed tubing 20 while the end
of the tubing is in the heating zone and it heated by
means diagrammatically represented by arrows 32. Figs.
6 and 7 illustrate the use of a pick-up in the form of
a flat plate 34 which, like the annular ring 30, is con-
~ - 12 -
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10~7743
tacted with molten ring 31 in the heating zone. Sub-
sequent to contacting the tubing molten ring 31 to
the pick-up, the process of tubing formation is contin-
ued as described. When the flat plate 34 is used
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1047743
as a seed crystal, a joint 35 may be formed, as shown in Fig. 7,
and later removed.
In Fig. 8, the pick-up is a rod 36, the end 37 of which
is melted and joined to the molten end of feed tube 20. In some
cases, the rod has been found to be a preferred form of surface
contacting member since it may be used to form a stronger weld with
the feed tube than the thin ring 30 or flat plate 34 of Figs. 5
and 6.
Figs. 9-17 illustrate the use of several embodiments
of a pick-up in tubing form. This type of pick-up has been
found to be the preferred embodiments in forming a single-
crystal tubir.g. Figs. 9-12 illustrate the use of a tubular pick-
up in starting the formation of a tubing by the method of this
invention. A preformed feed tube 38 having its upper edge 39
cut straight across is held in chuck 40; and a pick-up tubing 41 -
having a frustoconically configured contacting end 42 is held in
chuck 43. These chucks ~0 and 43 are affixed to load bearing rods
44 and 45, respectively. Separate means, not shown, are provided
to impart translational and rotational motion to load-bearing rods
44 and 45 and through them to the sections of the tubing. Since
each load-bearing rod has its own individual moving means the trans-
lational and rotational motions of the feed rod and of the pick- - -
up can be separately controlled. Exemplary apparatus for moving
s~ch load-bearing rods at desired axial and angular velocities
are described in U.S. Patent 3,552,931. Since the load-bearing
rods such as in the apparatus of U.S. Patent 3,552,931 are moved
by separate mechanisms, it is possible to achieve adjustments in
wall thickness and tubing diameter through different rates of trans-
lational motion as well as uniformity in tubing size and homogeni-
zation of material through rotational motion.
1047743
As shown in Fig. 10, Feed tube 38 is introduced into
heating zone 21 to form an initial molten ring 46, the cross s~c-
tion of which, through the forces of surface tension, tends to
approach a circle. Once this initial molten ring 46 is formed
around the top of feed tube 38, the pick-up seed tube 41 is lowered
to make physical contact with initial molten ring 46 as shown in
Fig. 11. This leads to the weiding of the pick-up seed ~ube to
the initial molten ring 47 (Fig. 12) which is then advanced along
the length of the feed tube to form the final tubing of this in-
10 vention.
It is also within the scope of this invention to bringthe pick-up tube tor rod) into contact with the feed tube prior to
any heating, i.e., prior to the formation ~f the molten ring.
It is also within the scope of this invention to move either or
both the pick-up and feed tube to make the contact.
The relative sizes and configurations of the feed tube
and pick-up tube must be such that once the initial molten ring
46 is formed on the end of the feed tube, ~he pick-up tube is able
to make physical contact with it so that a molten ring is formed
between the two which extends all the way ~round and all the way
through the tubing.
In one preferred embodiment of t~s method of this inven-
tion, as indicated in Fig. 12, the feed tu~e 38 and pick-up tube,
along with subsequently formed tubing attached thereto, are rota-
ted in opposite directions while the molte~ ring 47 is advanced
through the tubing. This counterrotation ~f the two solid sec-
tions on either side of the molten ring brr~gs about a homogeni-
zation of the material in the tubing forme~ and minimizes any var-
iations in heating intensity within the he~ting zone 21.
~he relative size/configuration (combination of the feed
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1~47743
tube and pick-up tube may be realized in a number of different
èmbodiments, exemplary ones of which are illustrated in fragmen-
tary cross sections in Figs. 9 and 13-17. For example, in Figs
9, 13 and 14, such uniform and complete contacting of the pick-up
tubing with the initial molten ring on the end of the feed tube
is insured by using a pick-up tube having an outside diameter,
ODp, greater than the inside diameter, I~f, of the feed tube, the
wall thickness of the pick-up tubing being essentially the same
or somewhat less than that of the feed tube. In Fig. 13, both
10 the feed tube 38 and the pick-up tube 41 have straight-cut con-
tacting ends 39 and 48, while in Fig. 14, feed tube 38 has a con-
tacting end 49 in which the tube wall is cut to have an inwardly
slanting configuration. These Figures illustrate the contacting
of the pick-up and feed tubes prior to the formation of the molten
ring.
In the embodiment of Fig. 15, the feed and pick-up - ;
tubes have essentially the same cross sectional dimensions and
the contacting ends of the feed tube and pick-up tube are cut
to present complementary frustoconically configured faces 49 and
42, respectively. It is, of course, within the scope of this in-
vention to use the reverse of the tubing cuts shown in Fig. 16.
In the embodiment of Fig. 16 the pick-up tube is sized `
to fit within the feed tube. This is acceptable so long as the
increase in melt volume experienced by the molten ring 46 in its
formation is sufficient to insure the desired contact with the
end of pick-up tube 41. Finally, as shown in Fig. 17, it is also
possible to use a pick-up tube which is larger in cross section
than the feed tube. Fig. 17 also shows the use of a feed tube
with a frustoconically configured contacting end 5~.
Although Figs. 9-17 have illustrated the feed tube as
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1047743
the lower tube and the pick-up tube as the upper tube, the re-
verse arrangement may be used since the forces of surface ten-
sion may be relied upon to hold the initial molten ring 46 onto
the feed tube end. The optimum choice of pic~-up tube configura-
tion and location may be readily determined for any one refractory
material and tubinq size.
The use of a laser has distinct advantages for provid- -
ing the required heating zone of this invention. Where incandes-
cent heating systems emit a large part of their radiation in a
wavelength range to which many of the refractories in their molten
state are transparent, the laser can be chosen to avoid this dif-
ficulty. Incandenscent systems may pose problems of heat transferto the tubing but laser energy can be directed to avOla such pro-
blems. Laser energy has no characteristic temperature of its
own, and thus there are no upper temperature limitations; and
the use of lasers imposes few restrictions on the atmosphere
in which the tubing is formed and provides the opportunity for
using a number of different atmospheres including vacuums, pres-
sures and reducing and oxidizing conditions. The use of a laser
also permits visual observation of the process.
Fig. 18 illustrates a preferred embodiment of the heating
means of the apparatus of this invention. The features of this
apparatus include means to form the laser beam into an annulus,
means to split the beam then into two annuli, means to redivide
both annuli into half annuli and means to focus the annuli to form
beams in line configurations having energy density distributions
particularly suitable for heating a portion of a tubing surface.
In the apparatus of Fig. 18, the radiation beam 150
from laser 100 is first directed through a beam-expanding means
151 which comprises a spherical mirror 152 and a spherical mirror
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104~743
153. By proper choice of focal length of the two spherical mirrors,
it is possible to expand the laser beam by a factor of two, four or
greater. A rotating Dove prism 154 is used to form the expanded
beam 155 into a beam 156 having an annular configuration. Dove
prisms are known and described in the literature. (See for
example "Modern Optical Engineering" by Warren J. Smith, McGraw-
Hill Book Co., ~ew York, ~ew York, 1966, paye 87.) By focusing
expanded beam 155 through Dove prism 154 above or below the
optical axis of the prism and by rotating the prism about its
axis by suitable means such as motor 157, it is possible to form
the laser beam into an annular configuration.
The annular beam 156 is then passed through a beam
spliter 158 which typicall,v comprises a water-cooled, coated GaAs
window 159 and a front surface mirror 160. The coating thicknesses
on the surfaces of window 159 are designed to reflect 50% of the
beam from the front surface of the window to form annular beam 161 -
and to have no reflection losses from the rear surfaces. ~alf of
the incident beam 156 is transmitted to mirror 160 where it is
reflected as annular beam 162 parallel to annular beam 161. These
two annular beams are each then split to form two half-annular
beams and the resulting four half-annular beams are then focused
as line beams onto that pGrtion of the tubing surface which is
within the heating zone 21.
Beginning first with annular beam 161, it is directed
onto a semicircular mirror 165 which is so positioned as to per-
mit one-half of the annular beam in the form of a half-annulus
beam 166 to strike plane mirror 167 for reflection to a variable
focal length cylindrical mirror 168 which focuses the beam as a
line (line beam 169) onto the tubing to form molten ring 22.
The optical elements of Fig. 18 are mounted on or suspended from
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1~47743
a support base in accordance with well-known optical engineering
techniques, the elements being so spatially positioned as to make
these optical paths possible. In Fig. 18, the line beam 169 is
shown as a dotted line to show that it is distinct from half-
amlulus beam 166. However, it will be appreciated that these
two beams are in two different planes and not side-by-side. In
a similar manner, the half-annulus beam 172 is reflected by cylin-
drical mirror 173 which returns a line beam 174 to be directed
onto the tubing to form molten ring 22. Similarly annular beam
162 is split into two half-annulus beams 175 and 176 by semi-
circular mirror 177; beam 175 is focused by plane mirror 178 onto
cylindrical mirror 179 to be transformed into line beam 180; and
half-annular beam 176 is focused by cylindrical mirror 181 into
line beam 182.
~ toroidal radiation shield 185 having four ports 186 to
permit passage of line beams 169, 174, 180 and 182 is provided to
radiate energy reflected by the molten tubing surface back to the
surface to concentrate and conserve the energy used in melting the
tubing. This radiation shield is described in detai 1 in connection
20 with the discussion of Fig. 2 0.
Fig. 19 is presented to describe the advantages real-
ized by the optical system of Fig. 18. When a half-annulus laser
beam, such as beam 166, is focused into a line, such as line beam
169, it can be shown to have an energy density curve which peaks
at or near each end of the line beam. When a beam of radiation
strikes a circular surface such as that of the tubing, he amount
of energy absorbed by the surface decreases as the angle e, formed
between a line drawn normal to the tubing surface and the line
of the beam, becomes greater than zero. It thus becomes apparent
30 that when a line beam having a constant energy density along its
--18--
.
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1~47743
length strikes a circular surface, the portions of the surface re-
ceiving radiation from the ends of the beam will absorb less of it.
However, by focusing a half-annulus laser beam down to a line beam,
it is possible to form a beam having increased energy densities
towards it ends just where such added energy is required to over-
come the differential in energy absorption as described. This is
shown in Fig. 20. Hence the apparatus of Fig. 18 provides means
to heat the tubing in the ~.elt zone more uniformly around the
entire periphery of the tubing, since each of the four line beams
10 have this unique energy density distribution. It will be appre-
ciated that the term "line beam" is used to designate a beam
which has, in fact, some heignt, the height being controlled
through the focusing of the cylindrical mirrors.
Fig. 20 illustrates an exemplary radiation shield formed
to define a toroidal surface 190 surrounding the heating zone 21
and curved to return radiation reflected by or emitted from the -
molten ring back to the molten ring. Normally the radius of curva-
ture R will be equivalent to twice the focal length of
-the mirror defining surface 190. This mirror is coated to produce
a surface which is highly reflective to infrared radiation, such as
bright gold. The toroid 191, the inner surface of which serves
as mirror 190, is machined as an upper half 192 and lower mating
half 193. Each of these toroid halves is also machined to have an
integral joining and support means which takes the form of a shor-
ter clamping section 194, formed of upper half 195 and lower half
196, and a longer section 197 formed of upper half 198 and lower
half 199, providing means to clamp, cool and align the toroid
halves. The support means must be of such dimensions that it
clears ports 186 in the toroid. Clamping is accomplished with a
series of bolts 200 and nuts 201, and alignment of the toroid
1~)4~743
halves and support means halves is achieved and maintained through
a series of dowel pins and pin holes (not shown) in the longer
sections 198 and 199 of the support means. The entire radiation
shielding means 185 is preferably machined from copper and is
cooled by circulation of a cooling liquid, e.g., water, through
the longer supporting section halves 198 and 199. The cooling
liquid is introduced through an inlet conduit 202 into one or
more passages 203 and withdrawn through passage 204 which -termi-
nates outside supporting section half 195 as an annular outlet
conduit 205 surrounding and concentric with inlet conduit 202.
Suitable sealing rings such as O-rings 206 and 207 provide adequate -
sealing between the mating passages in section halves 198 and 199.
The radiation shielding means is supported by a base support
through the cooling liquid conduit 205 and hence the upper sec-
tion of the shielding remains fixed with respect to the other
optical components. It is therefore a simple matter to remove
and replace the lower section while maintaining the desired align-
ment.
It will, of course, be appreciated that the construction
illustrated in Fig. 20 and described above is only exemplary of
one possible configuration and assembly of the radiation shield-
ing means used. Therefore, it is within the scope of this inven-
tion to use any suitably-configured radiation shield (e.g., a
spherically-shaped one) assembled in any appropriate manner.
Since it will normally be advantageous to use a single
type of laser for forming tubings of different material, the use
of the radiation shielding is particularly valuable when working
with materials which reflect an appreciable amount of the laser's
radiation.
Fig. 21 illustrates the providing of a controlled at-
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mosphere around the tubing during formation. The housing 63 may
be evacuated or charged with a gas of the desired character, e.g.,
reducing, oxidizing or inert. The laser optics are those shown
in Fig. 18, the same reference numerals being used to identify
the optical elements shown for purposes of illustration. Housing
63 has a plurality of windows, such as 210 and 211, a window being
provided for each beam of laser energy striking the tubing surface.
Exemplary of apparatus which may be used to provide a controlled
atmosphere is the pressure-and temperature-controlled furnace
described in United States Patent 3,639~718.
Fig. 21 illustrates means for imparting translational
and, if desired, rotational motion to the solid sections 20 and
23 of the tubing. Solid section 20 is held in chuck 40 which
in turn is supported by or affixed to a load-bearing rod 44. In
similar manner, solid section 23 is held in chuck 43 which is
supported by load-bearing rod 45. In those apparatus embodiments
which include means to define a controlled atmosphere around the
tubing, seals 68 and 69 are provided for load-bearing rods 45
and 44. Any suitable apparatus may be used to impart transla-
tional and rotational motion to load-bearing rods 44 and 45, the
apparatus described in United States Patent No. 3,552,931 being
exemplary. It is also within the scope of this invention to move
the heating means relative to the tubing if this is desired.
A number of different arrangements for effecting the
relative motion of such heating means and the solid sections of
the tubing are of course possible. Moreover, any of the apparatus
embodiments illustrated or discussed may be located within a cham-
ber in which the fluid surroundings may be controlled during the
formation of the tubing.
The method and apparatus of this invention may be used
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to form refractory tubing from any refractory material which is
capable of existing in the liquid state and which can be formed
i~to a tubing blank. In addition to alumina, such refractories
include, but are not limited to, oxides such as zirconia, titania,
thoria, ytteria and the like, carbides such as titanium carbide,
borides such as titanium boride, intermetallics such as gallium
arsenide, ternary compounds such as HgCdTe, pseudobinary compounds
such as AlAs-GaAs, and elements such as boron and silicon. Both
crystalline and amorphous materials may be used. For optical ap-
plications, it is generally necessary to use materials of extremelyhigh purityO For other applications it may be desirable to have
dopants, such as titanium or other additives present in minor
amounts.
The. feed tube blank used to produce the polycrystalline
or single crystal tubing may be prepared by an~ suitable technique
- such as for example by slip casting, or by pressing the refractory
in powder form (with or without a heat-removable binder) under
sufficient pressure to form a self-supporting structure. This
pressed structure may, if desired, be partially sintered to
enhance its structural strength. Tubing blanks of presintered
materials, e.g., A12O3, are available commercially. (See also,
for example, "Introduction to Ceramics" by W. D. Kingery, John
Wiley ~ Sons, Inc., New York, 1960, particularly Chapter 3 on
"Forming Processes" which describes in detail such forming pro- -~
cesses as powder pressing, extrusion, slip casting and sintering.)
In view of the well developed art in the formation of ceramic or
refractory bodies, the choice of such parameters as particle size,
binders, and density of the material making up the feed tubing is
within the skill of the artisan in this field; and the choice of ~ -
the method by which the feed tubing is formed is also within his
skill.
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. ~ :
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1~)47743
If a single crystal tubing is to be formed using a seed
crystal, the quality of the seed crystal must be consistent with
~he quality desired of the finished tubing. It is also necessary
that suitable pretreatment of the seed crystal is effected to eli-
minate any work damage sustained by the seed crystal in shaping
and/or cutting_ Techinques for providing suitable seed crystals
in any desired form are well known.
The temperature attained within the heating zone must,
of course, be that which is sufficient to melt the tubing. Al-
though somewhat higher temperatures can be tolerated, the temper-
ature should be maintained somewhat below that level which would
cause vaporization or boiling off of the molten material under the
environmental conditions being used. Thus temperature range,
which is readily determinable from existing physical data, will
depend upon the refractory material from which the tubing is form-
ed.
The height of the travelling molten ring is controlled
by well-known physical factors, i.e., the existing temperature gr2-
dient which is a function of the thermal conductivity of the tubing
material and the environment surrounding the tu~ing. As noted
a~ove this environment may include the ambient atmosphere, an
inert pressurized gas with or without a liquid encapsulant, or a
gas providing a special type of atmosphere, e.g., reducing or oxi-
dizing. In addition, the use of radiation shielding to return re-
flected radiation to the molten zone represents another factor in
the environment. The growth rate of the finished tubing is,of
course, equal to the withdrawal rate, Vb.
Because the method of this invention involves the melt-
ing of the tubing, it permits the selection of any desirable
; 3G atmosphere to achieve such results as purification of the
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1~)47743
refractory material and adjusting the valence state of any addi-
tives. For example, if the process is carried out in an oxidizing
atmosphere it is possible to convert to or maintain a titanium
dopant in its higher valence state, i.e., Ti+4. Other types of
environments may, of course, be used to attain other desired
results.
The method of this invention may be further illustrated
by the following example which is meant to be illustrative and
not limiting.
A single crystal alumina tubing was formed by the pro-
cess illustrated in Figs. 9-12. A tubing of 99~ pure alumina, - -
formed by cold pressing to have a fractional density of 97%, and
having an outside diameter of 9.15 mm with a wall thickness of
1.01 mm was used as the feed tube. A single crystal tubing of
alumina having an outside diameter of 7.1 mm, a wall thickness
of 0.5 mm and a frustoconical contacting end served in the dual
-role of pick-up~tube and seed crystal. A CO2 laser using 415-430
watts power was used to form the heating zone. The optics were
essentially those illustrated in Figs. 25 and 27, with the excep-
tion that the radiation shielding was spherically shaped.
Once the molten ring was formed in the contacting feed
and pick-up tubes,the feed tube was moved upwardly at 3.43 inches
; per hour and the solid tube section attached to the pick-up tube
was pulled at a rate of 6.06 inches per hour. Simultaneously with
these translational motions, the feed tube was rotated in one di-
rection at 490 rpm and the solid section of the tube attached to
the pick-up tube was rotated in the opposite direction at 385 rpm.
The finished single crystal tubing had an outside dia-
meter of 7 mm and a wall thickness of 0.7 mm. It was transparent
30 and free from any cracking.
:
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161 47743
The method and apparatus of this invention thereby pro-
vide for upgrading of a refractory tubing structure and, if de-
sired, for the forming of such a structure in the form of a single
crystal, a form which possesses operational advantages when the
resulting crystalline tubing is used as an enclosure for high-pres-
sure lights.
It will thus be seen that the objects set forth above,
among those made apparent from the preceding description, are effi-
'ciently attained and, since certain changes may be made in carrying
out the above method without departing from the scope of the inven-
tion, it is intended that all matter contained in the above des-
cription or shown in the accompanying drawings shall be interpre-
ted as illustrative and not in a limiting sense.
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