US3915662A

US3915662A - Method of growing mono crystalline tubular bodies from the melt

Info

Publication number: US3915662A
Application number: US388597A
Authority: US
Inventors: Harold E Labelle; Charles J Cronan
Original assignee: Tyco Laboratories Inc
Current assignee: Saint Gobain Ceramics and Plastics Inc
Priority date: 1971-05-19
Filing date: 1973-08-15
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28

Abstract

The invention is a method of growing a monocrystalline extension such as an end wall or flange onto the end of a monocrystalline tube. Typically the tube is made of alumina.

Description

United States Patent 11 1 1111 3,915,662

Labelle et al. Oct. 28, 1975 METHOD OF GROWING MONO 264/164 CRYSTALLINE TUBULAR BODIES FROM THE MELT [56] References Cited :75] Inventors: Harold E. Labelle, Hanover; Charles UNITED STATES PATENTS J- Cronan, n on, h f Ma 3,765,843 10/1973 Labelle et a1 23/301 SP 3,801,309 4/1974 Mlavsky 1 v 23/30] SP Assgnee' Labmatones 3,826,625 7 1974 Bailey 23/301 SP Mass.

22] Filed: Aug. 15, 1973 Primary Examiner-Norman Yudkoff Assistant Examiner-D. Sanders Appl' 388597 Attorney, Agent, or Firm-Schiller & Pandiscio Related US. Application Data 60] Division of Ser. No. 165,087, July 23, 1971, [57] ABSTRACT abandoned which is a of The invention is a method of growing a monocrystal- 144920 May 1971 abardoned' line extension such as an end wall or flange onto the end of a monocrystalline tube. Typically the tube is 52] US. Cl 23/301 SP; 23/273 SP; 264/164 51 1111. C1. B01J 17/20 made 31mm 58] Field of Search 23/301 SP, 273 SP; 10 Claims, 11 Drawing Figures US. Patent Oct.28, 1975 v. Sheet10f3 3,915,662

PULLING MECHANISM US. Patent Oct. 28, 1975 Sheet20f3 3,915,662

wv mm W y/ e w P LN\M US. Patent Oct.28, 1975 Sheet3of3 3,915,662

mmw

METHOD OF GROWING MONO CRYSTALLINE TUBULAR BODIES FROM TIIE MELT This application is a division of our copending application Ser. No. 165,087 filed July 23, 1971 now abandoned which in turn is a continuation-in-part of a copending application Ser. No. 144,920, filed May 19, 1971 now abandoned.

This invention relates to production of substantially monocrystalline tubular bodies having end walls or flanges.

The present invention pertains to an improvement in growing crystalline bodies from the melt according to what is called the edge-defined, film-fed growth technique (also known as the EFG process). Details of this process are described in the copending U.S. Pat. application of Harold E. LaBelle, Jr., Ser. No. 700,126 filed .Ian. 24, 1968 for Method of Growing Crystalline Materials.

In the EFG process the shape of the crystalline body is determined by the external or edge configuration of the end surface of a forming member which for want of a better name is called a die. An advantage of the process is that bodies of selected shapes such as round tubes or flat ribbons can be produced commencing with the simplest of seed crystal geometries, namely, a round small diameter seed crystal. The process involves growth on a seed from a liquid film of feed material sandwiched between the growing body and the end surface of the die, with the liquid in the film being continuously replenished from a suitable melt reservoir via one or more capillaries in the die member. By appropriately controlling the pulling speed of the growing body and the temperature of the liquid film, the film can be made to spread (under the influence of the surface tension at its periphery) across the full expanse of the end surface of the die until it reaches the perimeter or perimeters thereof formed by intersection of that surface with the side surface or surfaces of the die. The angle of intersection of the aforesaid surfaces of the die is such relative to the contact angle of the liquid film that the liquids surface tension will prevent it from overrunning the edge or edges of the dies end surface. Preferably the angle of intersection is a right angle which is simplest to achieve and thus most practical to have. The growing body grows to the shape of the film which conforms to the edge configuration of the dies end surface. Since the liquid film has no way of discriminating between an outside edge and an inside edge of the dies end surface, a continuous hole may be grown in the crystalline body by providing in that surface a blind hole of the same shape as the hole desired in the growing body, provided, however, that any such hole in the dies end surface is made large enough so that surface tension will not cause the film around the hole to fill in over the hole. From the foregoing brief description it is believed clear that the term edge-defined, film-fed growth denotes the essential feature of the EFG process-the shape of the growing crystalline body is defined by the edge configuration of the die and growth takes place from a film of liquid which is constantaly replenished. v

The primary object of the present invention is to provide a method, using the aforesaid EFG process, of growing substantially monocrystalline extensions of selected shape on substantially monocrystalline tubes.

Another object is to provide essentially monocrystalline tubes having substantially monocrystalline end walls or flanges.

In this connection it is to be noted that the EFG process may be used to grow monocrystalline tubes of selected ceramic materials such as alumina and that tubes made of polycrystalline or substantially monocrystalline alumina have utility as envelopes for high intensity vapor lamps. In the manufacture of such lamps the practice is to mount the electrodes in end caps that are attached to the ends of the envelopes by brazing or other suitable technique. It is recognized that mounting of the electrodes may be facilitated by forming the tubes with end walls each having an opening for direct mounting of an electrode without need for an end cap. Alternatively the tubes may be formed with inner or outer end flanges to facilitate attachment of end caps. It also is dedirable for other applications to form ceramic tubes each having an imperforate end wall at one end. However, heretofore it has not been possible to form monocrystalline tubes having integral end walls or flanges of like material and crystallinity, and particularly end flanges of closely controlled diameter and thickness. Accordingly a more specific object of this invention is to provide a method of producing monocrystalline tubes of ceramic materials such as alphaalumina that terminate in integral end walls or flanges.

Described briefly, the method of this invention comprises taking a previously grown monocrystalline tube of a selected material such as alumina and growing a monocrystalline extension such as an end wall or flange onto the end thereof by the EFG process using a die member having a film supporting end surface that conforms in shape to the desired shape of the extension to be grown.

Other features and many of the attendant advantages of this invention are set forth or rendered obvious in the following detailed description which is to be consid ered together with the accompanying drawings wherein:

FIG. 1 is a fragmentary elevational view, partly in section of apparatus comprising a furnace, crucible and die assembly used in practicing the invention;

FIG. 2 is an enlarged view of a portion of the apparatus of FIG. 1 showing the initial step in growing a closed end wall on a tube;

FIGS. 3-5 are views similar to FIG. 2 illustrating how crystal growth occurs in forming an end wall on a tube;

FIGS. 69 show how an internal flange is grown on the end of a tube; and

FIGS. 10 and 11 illustrate how an end wall may be grown using a modification of the die assembly shown in FIG. 2.

The present invention may be used to produce integral monocrystalline extensions on substantially monocrystalline bodies made of any one of a variety of congruently melting materials that solidify in identifiable crystal lattices. By way of example, the material may be alumina, barium titanate, lithium niobate and yttrium aluminum garnet. The invention is also applicable to other materials that melt congruently (i.e., compounds that melt to a liquid of the same composition at an invariant temperature). The following detailed description of the invention and the apparatus used in practicing the same is directed to growing monocrystalline extensions of selected geometry onto sapphire tubes.

FIG. 1 shows one form of furnace that may be used to practice the invention. The furnace consists of a vertically moveable horizontal bed 2 which engages a stationary furnace enclosure consisting of two

concentricspaced quartz tubes

4 and 6. At its bottom end the inner tube 4 is positioned in a gasket 5 in the bed. Surrounding tube 4 is a sleeve 8 that screws into a collar 10. Between sleeve 8 and collar 10 is an O-ring 12 and a spacer 13. The O-ring is compressed against tube 4 to form a seal. The upper end of sleeve 8 is spaced from tube 4 so as to accommodate the bottom end of tube 6. The bottom end of tube 6 is secured in place by an O-ring l4 and a spacer l5 compressed between a collar 16 that screws onto sleeve 8. Sleeve 8 is provided with an inlet port fitted with a flexible pipe 20. The upper ends of

tubes

4 and 6 are secured in a head 22 so that they remain stationary when the bed is lowered. Head 22 has an outlet port with a flexible pipe 24. Although not shown it is to be understood that head 22 includes means similar to sleeve 8, O-rings l2 and 14, and collars l0 and 16 for holding the two tubes in concentric sealed relation. Pipes and 24 are connected to a pump (not shown) that continuously circulates cooling water through the space between the two quartz tubes. The interior of the furnace enclosure is connected by a pipe 28 to a vacuum pump or to a regulated source of inert gas such as argon or helium. The furnace enclosure also is surrounded by an RF. heating coil 30 that is coupled to a controllable 500 kc. power supply (not shown) of conventional construction. The heating coil may be moved up or down along the length of the furnace enclosure and means (not shown) are provided for supporting the coil in any selected elevation. At this point it is to be noted that the circulating water not only keeps the inner quartz tube at a safe temperature but also absorbs most of the infrared energy and thereby makes visual observation of crystal growth more comfortable to the observer.

The head 22 is adapted to provide entry into the furnace enclosure of an elongate pulling rod 32 that is connected to and forms part of a conventional crystal pulling mechanism represented schematically at 34. It is to be noted that the type of crystalpulling mechanism is not critical to the invention and that the construction thereof may be varied substantially. Preferably, however, we prefer to employ a crystal pulling mechanism that is hydraulically controlled since it offers the advantage of being vibration-free and providing a uniform pulling speed. Regardless of its exact construction which is not required to be described in detail, it is to be understood that the pulling mechanism 34 is adapted to move pulling rod 32 axially at a controlled rate. Pulling rod 32 is disposed coaxially with the

quartz tubes

4 and 6 and its lower end has an extension in the form of metal holder 36 that is adapted to releasably hold a monocrystalline tube 38 on which an integral monocrystalline extension is to be grown as hereafter described.

Located within the furnace enclosure is a cylindrical heat susceptor 40 made of carbon. The top end of susceptor 40 is open but its bottom end is closed off by an end wall. The susceptor is supported on a tungsten rod 42 that is mounted in bed 2. Supported within susceptor 40 on a short tungsten rod 44 is a crucible 46 adapted to contain a melt 48 of the material to be grown onto the tube 38. The crucible is made of a material that will withstand the operating temperatures and will not react with or dissolve in the melt. With an alumina melt, the crucible is made of molybdenum, but it also may be made of tungsten, iridium or some other material with similar properties with respect to molten alumina. Where a molybdenum crucible is used, it must be spaced from the susceptor since there is a eutectic reaction between carbon and molybdenum at about 2,200C. The inside of the crucible is of suitable size and shape, preferably with a constant diameter. To help obtain the high operating temperatures necessary for the process, a cylindrical radiation shield 50 made of carbon cloth may be wrapped around the carbon susceptor. The carbon cloth greatly reduces the heat loss from the carbon susceptor.

Referring now to FIGS. 1 and 2, mounted in crucible 46 is a die assembly 56 comprising a cylindrical rod 58 that is affixed (e.g. by welding or press fit) to a supporting disc 60 that rests on a shoulder 62 formed in the side wall of the crucible at the upper end thereof. The rod has a plurality of axial bores 64 and one or more radial openings 66 near its bottom end to permit inflow of melt to the several bores from the crucible. Bores 64 are sized to function as capillaries for molten alumina. The upper end of rod 58 terminates in a flat horizontal surface 68 which intersects the rods outer surface at a right angle It is to be noted that rod 58 projects above disc 60 so as to be visible to the operator. The length of the rod 58 and diameter of the capillaries 64 are such that molten alumina can rise in and fully fill the capillaries by action of capillary rise so long as the level of the melt in the crucible is high enough to fill the openings 66. The height to which a column of melt can rise is determined by the equation h=2Tcos6/drg, where h is the distance in cm. that the column will rise; Tis the surface tension of the melt in dynes/cm.; 6 is the contact angle, d is the density of the liquid, r is the internal radius of the capillary in cm.; and g is the gravitational constant in cm/sec By way of example in a capillary of 0.75 mm diameter in a molybdenum member, a column of molten alumina may be expected to rise more than 11 cm. by capillary action.

FIGS. 2-5 illustrate how an end wall of a ceramic material may be grown onto a ceramic tube 38. Initially the tube is inserted vertically and is disposed in axial alignment with the die assembly. Then with the capillaries filled with melt by action of capillary rise and the power input to coil 30 adjusted so that the upper surface of 68 of the die assembly is preferably at least about 1040C higher than the melting point of the tube 38, the tube is lowered into contact with the surface 68 and held there long enough for a portion of the end of the tube to melt and form a liquid film 70 that extends laterally far enough to connect with the melt in the capillaries. It is to be noted that the capillaries are shown empty in FIGS. 2-5 (and also 6-11) in order to render the capillaries more distinct to the reader and that in fact the capillaries are filled with melt. Further it is to be understood with reference to FIG. 2 that be fore the end of tube 38 is melted to form film 70, the melt in each capillary has a concave meniscus with the edge of the meniscus being substantially flush with surface 68. The temperature gradient along the length of the tube and the temperature of surface 68 are factors influencing how much of the tube melts and the thickness of film 70. In this connection it is to be noted that v the tube functions as a heat sink and the temperature of the tube at successively higher points thereon is affected by the height of coil 30 and susceptor 40 and also the power input to the coil. In practice these parameters are adjusted so that the initial film 70 has a thickness in the order of 0.1 mm.

Once the film 70 has connected with melt in the capillaries, the pulling mechanism 34 is actuated to pull tube 38 upwardly away from surface 58. The pulling speed is set so that the film adhering to the tube because of surface tension will crystallize due to a drop in temperature at the solid tube-liquid film interface which occurs as a result of the pulling. The pulling speed also must be such that surface tension will cause the film to spread inwardly toward the center of surface 68 (see FIG. 3). As the film spreads inwardly, crystal growth will occur at all points along the horizontal expanse of the film with the result that a tubular monocrystalline extension is formed on the tube which has a constant outside diameter but a progressively decreasing inside diameter. The film consumed by the crystal growth is replaced by additional melt which is supplied by the capillaries 64. Initially the crystal growth on the tube appears to form a tapered inside flange 72 (FIG. 3). As the growth proceeds, the film continues to spread until it fully covers surface 58. Concurrently the flange 72 continues to grow inwardly until it completely closes off the tube and forms an end wall 72A (FIG. 4). Growth is continued until the end wall 72A has grown to the desired thickness, where upon the pulling speed is quickly increased enough to cause the tube to pull free of the film (FIG. 5). Alternatively, growth may be continued so as to causethe wall 72A to be extended into a solid rod having the same outside diameter as the tube. It is to be noted that the pulling speed and the temperature of the film may be varied during crystal growth. However, the pulling speed should not be so great nor the temperature so high as to cause the tube to pull free of the melt film. In growing alpha-alumina, it is preferred to have an initial pulling speed of about 0.1 in/min and to increase the speed to about 0.2 in/min after the film has expanded enough to fully cover the end surface 68 of the die assembly as above described. The pulling speed of the tube and the temperature of the film control the film thickness which controls the rate of film spreading. Increasing the temperature of surface 68 (and hence the temperature of the film) and increasing the pulling speed each have the effect of increasing the film thickness.

FIGS. 6-9 illustrate growth of an internal flange onto the end of a ceramic tube. In this case the rod 58 is replaced with a round sleeve 58A in which is coaxially disposed a round rod 74. Rod 74 is sized so as to form an annular capillary 64A which functions the same as capillaries 64. Sleeve 58A and rod 74 are secured together by a pin 75 and have flat end surfaces 68A and 688 that together function like surface 68 of rod 58. Surface 688 has a cylindrical coaxial cavity 76 with a diameter corresponding to the desired internal diameter of the flange to be grown and must be large enough in diameter so that surface tension will not cause the film to close over it. Here again the preformed tube 38 has the same outer diameter as surface 68; however, its inside diameter is greater than the diameter of cavity 76. The procedure followed is essentially the same as described above in connection with FIGS. 2-5. Initially the film 78 that is formed by melting the tube has substantially the same inside and outside diameters as the tube. This film gradually expands inwardly but stops when it reaches cavity 76 (see FIGS. 7 and 8). As the crystal growth occurs axially, it also expands inwardly in the same manner as the film so as to form a tapered flange 80 (FIG. 7). However, once the film has stabilized at'the edge of cavity 76, the crystal growth stops expanding inwardly of the tube and continues vertically to the full horizontal expanse of the film, with the result as shown in FIGS. 8 and 9 that the flange 80 acquires a cylindrical inner surface 82. Growth may be discontinued after the flange has developed to a desired thickness as indicated in FIG. 9, or it may be continued so that the grown crystal forms a tube having the same o.d. but a smaller i.d. than the tube 38. The process of FIGS. 6l0 may be carried out with the die assembly of FIG. 2, provided a cavity like cavity 76 is formed in the upper end of rod 58.

FIGS. 10 and 11 relate to a modification of the process of FIGS. 2-5. In this case the upper surface 68 of the rod 58 is concave. The concave shape may be confined to a circular area bounded by the capillaries 64 as shown or may extend out to the edge of surface 68. In either event, when a film 84 is formed and caused to extend fully across surface 68 as above described, it will fill the concave depression in that surface but will be relatively flat on top. In other words, the film will tend to be thicker above the low point of the concave depression. Because of the concave nature of surface 68, the film initially formed by melting the tube will quickly flow to the center of the surface, with the result that as the tube is pulled the initial crystal growth will not be limited to the annular region of the film directly in line with the end of the tube but will extend inwardly and will expand quickly to the full expanse of the film. Consequently, the inner surface 86 of the end wall 88 that is grown onto the end of the tube will be shaped less like a cone (see FIG. 4) and more like a shallow dish. When the tube is pulled free of the film, the outer surface of the end wall 88 will tend to have a contour in vertical section that is somewhat similar to that of surface 68. However, the peripheral configuration of the end wall will correspond almost exactly to that of surface 68.

In the foregoing modes of practicing the invention, the upper surface of the die assembly has substantially the same outer diameter as the tube 38, with the result that the monocrystalline flange or end wall and the tube onto which it is grown have substantially the same outside diameter. However, it also is possible to grow a flange or end wall which has a smaller or larger outside diameter. For example, an end wall with a smaller outside diameter may be grown by using a die assembly as hereabove described having an end surface 68 (as shown in FIG. 2) with a smaller o.d. than the tube 38 (but not smaller than the tubes i.d.); correspondingly, a larger diameter may be grown by using a die assembly with an end surface 68 (as shown in FIG. 2) that has a larger o.d. than the tube. Growing an extension having a larger o.d. and a smaller i.d. than the tube, i.e., an extension that forms both an outside and inside flange on the tube, can be achieved witha die assembly like that shown in FIG. 6 in which the o.d. of surface 68A and the diameter of cavity 76 are larger and smaller than the o.d. and i.d. respectively of the tube.

The following example illustrates a preferred mode of practicing the invention. A molybdenum crucible having an internal diameter of about 1%. inch, a wall thickness of about 3/16 inch, and an internal depth of about 9/16 inch is positioned in the furnace in the manner shown in FIG. 1. Disposed in the crucible is a die assembly constructed generally shown in FIG. 2. The rod 58 has four capillaries 64 spaced uniformly about its axis. The dimensions of rod 58 are as follows: a rod diameter of about inch and a rod length such that its upper end projects about l/l6 inch above the crucible. The four capillaries each have a diameter of about 0.03 inch. The crucible is filled with substantially pure polycrystalline alpha-alumina and a monocrystalline alphaalumina tube 38 grown previously by the EFG technique is mounted in holder 38. Tube 38 is cylindrical and was grown so that the c-axis of its crystal lattice extends parallel to its geometric axis. Additionally tube 38 has an outside diameter identical to the diameter of rod 58 and a wall thickness of about 0.03 inch. Tube 38 is mounted in holder 36 so that it is aligned axially with rod 58. Access to seed holder 36 and the susceptor 40 is achieved by lowering bed 2 away from the furnace enclosure and lowering the seed holder below the bottom end of furnace tube 4. With the bed restored to the position of FIG. I, cooling water is introduced between the two quartz tubes. and the enclosure is evacuated and filled with argon to a pressure of about one atomosphere which is maintained during the growth period. Then the RF. coil 30 is energized and operated so that the alumina in the crucible is brought to a molten condition (alumina has a melting point in the vicinity of 2050C) and the surface 68 reaches a temperature of about 2,070C. As the solid alumina is converted to the melt 48, columns of the melt will rise in and fill capillaries 64. Each column of melt will rise until its meniscus is substantially flush with the top of the rod. After affording time for temperature equilibrium to be established, the pulling mechanism is actuated and operated so that the tube 38 is moved into contact with the upper surface 68 of the die assembly and allowed to rest in that position to allow the bottom end of the tube to meet and form film 70. After about 60 seconds, the tube is withdrawn vertically at the rate of about 0.1-0.2 inch per minute. As the tube is withdrawn, crystal growth will occur on the seed and the film of melt will begin to spread over the surface 68 due to its affinity with the newly grown material on the tube and the films surface tension. The latter force also causes additional melt to flow out of the capillaries and add to the total volume of film.

If growth occurs on the tube but the film does not immediately begin to spread, steps are taken to force the melt to spread as desired. This can be accomplished by adjusting the temperature of the film or by adjusting the pulling speed. Preferably, the temperature of the surface 68 is held constant and the pulling speed adjusted until spreading of the film is observed. Since the film functions as a growth pool of melt, as the film spreads out over the surface 68, the growth also expands horizontally. At the aforesaid pulling speed growth will propagate vertically throughout the entire horizontal expanse of the film, with the result that the growing crystal will also begin to grow radially inward as shown in FIGS. 3 and 4 until after about 3 minutes it will conform in cross-sectional area and shape to surface 68. As growth continues, the end wall 72A that is formed will be found to have a circular symmetry with an o.d. substantially the same as that of the surface 68 of the die assembly. After about 5 minutes of pulling the tube, the pulling speed is immediately increased to about 1.0 inch/minute, whereby the tube 38 pulls free of film 70. Thereafter the furnace is cooled and tube 38 retrieved from holder 36. The extension grown on the tube is found to have a reasonably flat bottom surface and a conically shaped interior surface as shown in FIG. 5. The thickness of the end wall 72A measured at the center is about 1 cm. The grown crystal is found to be essentially monocrystalline and a crystallographic extension of the crystal lattice of tube 38.

It is to be noted that after the film has fully covered surface 68, if the operating temperature (as determined by the average temperature of film is held constant close to but slightly above the melting point of the material to be grown, the pulling speed may be varied within limits (depending upon the operating temperature) without any substantial change in the crosssection of the grown crystal. Similarly if the pulling speed is held constant, the operating temperature may be varied substantially (e.g., a change of as much as l530 with respect to the melting point of alumina) without any substantial change in the cross-section of the grown crystal.

The fact that the grown crystalline extension has substantially the same shape and size as the surface 68 confirms that the film 70 comprises a growth zone which is substantially isothermal in a direction parallel to surface 68. It is to be noted that the film has a depth in the order of about 0.1 mm. under usual growth conditions and has a vertical temperature gradient. Surface 68 functions substantially as an isothermal heater. Where the tube has a relatively large o.d. and wall thickness, it may be necessary to increase the rate of heating slightly so that the temperature of the upper end surface 68 of the die assembly before it is contacted by the tube is greater than that normally required to be maintained for satisfactory growth. This higher temperature offsets the heat sink effect of the tube which may cause the growth pool, i.e., the film of melt, to have a lower average temperature than expected. Unless this heat sink effect is offset by an increase in the rate of heating, the tube may not melt, or the film may not spread rapidly over the surface 68, unless the pulling speed is adjusted to compensate for the heat sink effect.

It is believed obvious that the process of this invention may be used to grow internal or external end flanges on both ends of a tube. Thus using the apparatus of FIG. 6 an internal flange may be grown on both ends of the tube 38. This is accomplished by (a) growing an internal flange on one end according to the procedure described above and illustrated in FIGS. 6-9, (b) reversing the tube 38 in the holder 36, and (0) growing a like internal flange on the opposite end of the tube according to the same procedure used to grow the first flange. External end flanges or extensions forming both internal and external flanges or end walls may also be grown on both ends of the same tube. Tubes having monocrystalline integral end flanges at both ends have utility as lamp envelopes as above described.

It is to be noted also that the invention may be used in growing extensions of other cross-sectional shapes, e.g., rectangular, square, etc., on tubes of the same or different cross-sections. Thus by using a die assembly with a square film-supporting surface. it is possible to grown an extension or termination of square crosssection onto a round or square tube.

An important advantage of the invention is that it is applicable to crystalline materials other than alumina. It is not limited to conqruently melting materials and encompasses growth of materials that solidify in cubic, rhombohedral, hexagonal and tetragonal crystal structures, including barium titanate, yttrium aluminum garnet, and lithium niobate mentioned above. With respect to such other materials, the process is essentially the same as that described above for alpha-alumina, except that it requires different operating temperatures because of different melting points. Additionally, certain minor changes may be required in the apparatus, e.g., different crucible materials in order to avoid reaction between the melt and the crucible.

Laue X-ray back reflection photographs of alphaalumina crystal growth produced according to the foregoing invention reveals that the crystal growth usually comprises one or two, and in some cases three or four crystals, growing together longitudinally separated by a low angle (usually within 4 of the c-direction) grain boundary. Therefore, for convenience and in the interest of avoiding any suggestion that the crystal growth is polycrystalline in character, we prefer to describe it as substantially monocrystalline, it being understood this term is intended to embrace a crystalline body that is comprised of a single crystal or two or more crystals, e.g., a bicrystal or tricrystal, growing together longitudinally but separated by a relatively small angle (i.e., less than about 4) grain boundary. The same term is used to denote the crystalographic nature of the seed tube.

It also has been found that best results are achieved if the c-axis of the crystal lattice of the seed tube extends parallel to the tubes longitudinal axis, so that the extension forming a flange or end wall also grows vertically along the c-axis. Growth in the c-direction is characterized by smooth surfaces and superior strength.

With respect to the die assembly, it is to be understood that in the following claims the term end surface is intended to cover the effective film-supporting surface of the die, whether made as a single piece (see surface 68 of FIGS. 2 and 10) or as two pieces (see

surfaces

68A and 68B of FIG. 6), and the term capillary is intended to denote a passageway that can take a variety of forms, such as the discrete bores 64 or the annular space 64A. The term effective film-supporting surface denotes the end surface of the die, e.g. surface 68 (or surfaces 68A, 688) as it would appear if the capillary opening or openings, e.g. capillary 64 (or 64A) were omitted, since when a film fully covers the end surface it extends over the capillary openings as shown in FIGS. 4 and 8.

What is claimed is:

1. Method of providing an integral laterallyextending monocrystalline extension on a monocrystalline tube where both said extension and tube are formed of a congruently melting material comprising providing a die assembly having an end surface that is larger in at least one direction than a corresponding dimension of said tube and a capillary that extends down from said end surface, filling said capillary with a melt of said material, contacting said end surface with an end of said tube while maintaining said end surface at a temperature at which the end of said tube will melt and form a film on said surface, melting enough of said tube end to form a film on said surface that connects with the melt in said capillary, pulling said tube up away from said surface at a rate at which said film will spread over said entire end surface and controlling the temperature of said film so that crystal growth will occur on said tube at its interface with said film, simultaneously supplying additional melt to said film via said capillary to replenish the melt consumed by said crystal growth, and terminating crystal growth after a desired amount of crystal growth conforming in cross section to substantially the full area of said end surface has occurred on said tube.

2. Method according to claim 1 wherein said end surface is annular and the said one end of said tube has substantially the same internal diameter and a different external diameter than said end surface.

3. Method according to claim 1 wherein said end surface is annular and the same one end of said tube has substantially the same external diameter and a different internal diameter than said end surface.

4. Method according to claim 1 wherein said end surface is annular and the said one end of said tube has a smaller external diameter and a larger internal diameter than said end surface.

5. Method according to claim 1 wherein said end surface is concave.

6. Method according to claim 1 wherein said tube is pulled at a rate in the order of 0.l0.2 inch per minute.

7. Method according to claim 1 wherein said end surface has a single perimeter corresponding in configuration to the outer perimeter of the end of said tube, whereby said crystal growth forms a continuous end wall on said tube.

8. Method according to claim 1 wherein said tube is pulled at one selected rate until said film has spread across the full expanse of said end surface, and thereafter is pulled at a faster rate.

9. Method according to claim 1 wherein said end surface has a single perimeter and is concave and said film spreads out to all points on said end surface within said perimeter as said tube is being pulled and crystal growth occurs on said tube.

10. Method of providing an integral lateral extension on a monocrystalline tube where both said tube and extension are formed of a congruently melting material comprising:

providing a die having a generally horizontal end surface and at least one capillary that extends down from said end surface; filling said capillary with a melt of said material and maintaining said end surface at a temperature above the melting point of said material;

providing a monocrystalline tube of said material;

positioning said tube above said die and bringing one end of said tube into contact with said end surface long enough for a portion of said tube to melt and form a film on said end surface that connects with the melt in said capillary, thereafter pulling said tube away from said end surface and simultaneously spreading said film over the full expanse of said end surface while controlling the temperature at the interface of said tube and film so that crystal growth will occur on said tube to the full area of said surface; and

feeding additional melt to said capillary and via said capillary to said film as said tube is being pulled so to make-up for the material consumed by said crystal growth.

Claims

1. METHOD OF PROVIDING AN INTEGRAL LATERALLY-EXTENDING MONOCRYSTALLINE EXTENSION ON A MONOCRYSTALLINE TUBE WHERE BOTH SAID EXTENSION AND TUBE ARE FORMED OF A CONGRUENTLY MELTING MATERIAL COMPRISING PROVIDING A DIE ASSEMBLY HAVING AN END SURFACE THAT IS LARGER IN AT LEAST ONE DIRECTION THAN A CORRESPONDING DIMENSION OF SAID TUBE AND A CAPILLARY THAT EXTENDS DOWN FROM SAID END SURFACE, FILLING SAID CAPILLARY WITH A MELT OF SAID MATERIAL CONTACTING SAID END SURFACE WITH AN END OF SAID TUBE WHILE MAINTAINING SAID END SURFACE AT A TEMPERATURE AT WHICH THE END OF SAID TUBE WILL MELT AND FORM A FILM ON SAID SURFACE, MELTING ENOUGH OF SAID TUBE END TO FORM A FILM ON SAID SURFACE THAT CONNECTS WITH THE MELT IN SAID CAPILLARY, PULLING SAID TUBE UP AWAY FROM SAID SURFACE AT A RATE AT WHICH SAID FILM WILL SPREAD OVER SAID ENTIRE END SURFACE AND CONTROLLING THE TEMPERATURE OF SAID FILM SO THAT CRYSTAL GROWTH WILL OCCUR ON SAID TUBE AT ITS INTERFACE WITH SAID FILM SIMULTANEOUSLY SUPPLYING ADDITIONAL MELT TO SAID FILM VIA SAID CAPILLARY TO REPLENISH THE MELT CONSUMED BY SAID CRYSTAL GROWTH AND TERMINATING CRYSTAL GROWTH AFTER A DESIRED AMOUNT OF CRYSTAL GROWTH CONFORMING IN CROSS SECTION TO SUBSTANTIALLY THE FULL AREA OF SAID END SURFACE HAS OCCURED ON SAID TUBE.

3. Method according to claim 1 wherein said end surface is annular and the said one end of said tube has substantially the same external diameter and a different internal diameter than said end surface.

5. Method according to claim 1 wherein said end surface is concave.

6. Method according to claim 1 wherein said tube is pulled at a rate in the order of 0.1-0.2 inch per minute.

10. Method of providing an integral lateral extension on a monocrystalline tube where both said tube and extension are formed of a congruently melting material comprising: providing a die having a generally horizontal end surface and at least one capillary that extends down from said end surface; filling said capillary with a melt of said material and maintaining said end surface at a temperature above the melting point of said material; providing a monocrystalline tube of said material; positioning said tube above said die and bringing one end of said tube into contact with said end surface long enough for a portion of said tube to melt and form a film on said end surface that connects with the melt in said capillary, thereafter pulling said tube away from said end surface and simultaneously spreading said film over the full expanse of said end surface while controlling the temperature at the interface of said tube and film so that crystal growth will occur on said tube to the full area of said surface; and feeding additional melt to said capillary and via said capillary to said film as said tube is being pulled so as to make-up for the material consumed by said crystal growth.