CA1047743A

CA1047743A - Method and apparatus for forming refractory tubing

Info

Publication number: CA1047743A
Application number: CA240,349A
Authority: CA
Inventors: John S. Haggerty
Original assignee: Arthur D Little Inc
Current assignee: Arthur D Little Inc
Priority date: 1975-08-14
Filing date: 1975-11-21
Publication date: 1979-02-06
Also published as: DE2555551A1; JPS5223112A

Abstract

ABSTRACT OF THE DISCLOSURE
Refractory tubings, either in amorphous, polycrystal-line or single crystal form, are made by moving a preformed tub-ing of a refractory material and heating zone relative to each other, the heating zone providing sufficient heat to melt through the tubing and form a molten ring which is continuously advanced through the tubing. The heating zone is provided by focusing a plurality of laser beams in a manner to adjust the energy distri-bution of each beam to essentially equalize the absorption of the laser energy around the entire surface of the tubing. The tubings may be formed as single crystals by using appropriate seeds; and by controlling the rate of movement of the tubing sections on either side of the molten ring the wall thickness and diameter of the final tubing may be adjusted.

Description

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 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 _a_ 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 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, .

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 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 - .

~(~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 -. ~ -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 - 12a -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.

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 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 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 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 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 .

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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-1~47743 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 1~47743 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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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 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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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.

Claims

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of forming refractory tubing, comprising the steps of (a) providing a preformed tubing blank of a refractory material as a feed tube;
(b) providing around said tubing blank a concentrated heating zone of laser energy of substantially uniform intensity thereby to form in said tubing a molten ring of essentially uni-form height completely through said tubing, said height being such that the forces of surface tension and gravity maintain said molten ring connected to solid sections on either side of said molten ring of said tubing, said step of providing said heating zone of laser energy comprising (1) forming a plurality of laser beams of half-annular configuration equally spaced around said tubing, and (2) focusing each of said laser beams of half-annular configuration onto said refractory tubing to form said molten ring in said tubing, said focusing being performed in a manner to ad-just the energy density distribution of each of said beams to es-sentially equalize the absorption of laser energy around the en-tire surface of said tubing within said molten ring; and (c) controllably effecting relative translational move-ment between said refractory tubing and said heating zone thereby to advance said molten ring through said tubing.

2. A method in accordance with claim 1 wherein said step of forming a plurality of laser beams of half-annular config-uration comprises expanding a laser beam, imparting an annular con-figuration to the resulting expanded beam, splitting the annular beam into at least two annular beams and then dividing said annular beams to form said half-annular beams.

3. A method in accordance with claim 1 wherein said focusing each of said laser beams of half-annular configuration comprises focusing said beams into line beams.

4. A method in accordance with claim 1 further includ-ing the step of reflecting radiation reflected and emitted by said molten ring back to said molten ring.

5. A method in accordance with claim 1 including the step of imparting rotational motion to at least one of said solid sections.

6. A method in accordance with claim 1 including the step of imparting rotational motion to both of said solid sections.

7. A method in accordance with claim 6 wherein said ro-tational motion of one of said solid sections is opposite in direc-tion to that of the other of said solid sections.

8. A method in accordance with claim 1 wherein said molten ring is initially formed at one end of said preformed tub-ing, and including the step of bringing into contact with said molten ring within said heating zone a pick-up member which is a rod or tubing of said refractory material of a size and configura-tion such that it makes physical contact with said initially formed molten ring around its entire surface thereby to join said pick-up member and said tubing blank through said molten ring.

9. A method in accordance with claim 8 wherein said pick-up member is a single crystal used in a seeding function.

10. A method in accordance with claim 1 wherein said step of effecting relative motion between said refractory tubing and said heating zone comprises moving said solid sections on either side of said molten ring at predetermined rates of travel along the axis of said tubing.

11. A method in accordance with claim 10 wherein said moving of said solid sections is effected so that the section being moved in the direction opposite to the direction of travel of said molten ring is moved at a different rate than the other section.

12. A method in accordance with claim 1 including the step of maintaining said tubing within controlled fluid surround-ings.

13. A method in accordance with claim 12 wherein said controlled fluid surroundings comprises a gaseous atmosphere.

14. An apparatus for forming refractory tubing compris-ing in combination (a) holding means adapted to hold separate solid sec-tions of a refractory tubing;
(b) heating means for forming a molten ring in said refractory tubing, said ring being of essentially uniform height throughout said tubing, said height being such that the forces of surface tension and gravity maintain said molten ring connected to said solid sections, said heating means comprising (1) laser means providing at least one beam of radiant energy;
(2) optical means including means for expanding said beam, means for splitting the resulting expanded beam into a plural-ity of beams essentially equally spaced around the surface of said tubing, energy distribution means to adjust the energy density dis-tribution of each of the beams making up said plurality of beams to essentially equalize the absorption of said radiant energy around the entire surface of said tubing within said molten ring and means for focusing each of said plurality of beams onto said surface of said tubing within a heating zone surrounding said tubing and providing essentially uniform heating of said tubing surface; and (c) means to effect controlled relative translational movement between said holding means and said heating means whereby said molten ring advances through said tubing.

15. An apparatus in accordance with claim 14 wherein said energy distribution means comprises means to convert said expanded beam into a beam of annular configuration whereby said beams subsequent to splitting are also of annular configuration, and means to form said beams of annular configuration into twice as many beams of half-annular configuration thereby to provide said plurality of beams.

16. An apparatus in accordance with claim 14 wherein said means for focusing each of said plurality of beams onto said surface comprises cylindrical mirror means for focusing said beams of half-annular configuration as line beams onto said surface.

17. An apparatus in accordance with claim 15 wherein said means to convert said expanded beam into a beam of annular configuration comprises a rotatable Dove prism and means to rotate said prism.

18. An apparatus in accordance with claim 15 wherein said means to form beams of annular configuration into beams of half-annular configuration comprises spatially arranged mirrors.

19. An apparatus in accordance with claim 14 including radiation shielding means surrounding said heating zone and being adapted to reflect radiant energy reflected and emitted from said molten ring back to said molten ring.

20. An apparatus in accordance with claim 14 including means to define a controlled atmosphere around at least said molten ring.

21. An apparatus in accordance with claim 14 including means to impart rotational motion to at least one of said solid sections.

22. An apparatus in accordance with claim 14 including means to impart rotational motion to both of said solid sections.

23. An apparatus in accordance with claim 22 wherein said means to impart rotational motion to both of said sections comprises means to rotate said sections in opposite directions.