US 3591348 A
Description (OCR text may contain errors)
July 6, 1971 H. E. LA BELLE, JR 3,591,343
METHOD OF GROWING CRYSTALLINE MATERIALS Filed Jan. 24, 1968 3 Sheets-Sheet 1 PULLING MECHANISM 4- 1 32 I I L H 5 fi Q: u rfi"-"4 4 25 INVENTOFI.
P16 HAROLD E. LABELLE ,JR.
ATTORNEY July 6, 1971 LA-BELLE, JR 3,591,348
METHOD OF GROWING CRYSTALLINE MATERIALS 5 1 Pk m HAROLD 51455115,. 96 I I W INVENTOR.
. i 4, "77 1-'i"-f ;/7/ BY July 6, 1911 H. E. LA'BELLE, JR 3,591,348
METHOD OF GROWING CRYSTALLINE MATERIALS Filed Jan. 24. 1968 3 Sheets-Sheet 3 HAROZO E MBEZZE, Jr.
United States Patent 3,591,348 METHGD 0F GROWING CRYSTALLINE MATERIALS Harold E. La Belle, Jr., Quincy, Mass, assignor to Tyco Laboratories, Inc., Waltham, Mass. Filed .lan. 24, 1968, Ser. No. 700,126 Int. Cl. K301i 17/18 US. Cl. 23-301 17 Claims ABSTRACT OF THE DISCLOSURE This invention relates to growth of materials from the melt and more particularly to growth of elongate crystalline bodies of predetermined configuration.
It is recognized that many solid high temperature materials produced in the form of elongate crystalline bodies exhibit properties which, depending upon the nature and configuration of such bodies, render them useful for a wide variety of applications. Thus elongate single crystal or essentially monocrystalline bodies of selected materials such as a-alumina have utility as reinforcement elements for metal alloy matrices to provide composite materials useful in fabricating structural parts for jet engines, turbines, etc., as substrates for epitaxially grOWn integrated circuit devices, and as optical components.
Heretofore it has not been possible to control, except within relatively broad limits, the cross-sectional size and configuration of high temperature crystallline materials pulled from the melt as elongate bodies. This lack of control is particularly apparent when attempting to grow elongate essentially monocrystalline bodies. Furthermore prior methods of growing from the melt have not permitted growing to various arbitrary cross-sectional shapes. It also has been extremely difiicult to grow elongate bodies that have relatively smooth surfaces. This latter problem exists in growing in a growth orifice and is particularly prevalent in the case of so-called extrusion processes. In the latter case the smoothness of the surfaces of the product is limited by the texture of the surface of the extrusion orifice. A further difficulty resides in the fact that many congruently melting materials are difiicult to grow from a crucible as elonate single crystals or as extended bodies that are essentially monocrystalline, andthis difiiculty is greatly increased when attempting to grow to predetermined cross-sectional configurations due to excessive sensitivity to growth rate and melt temperature.
Accordingly the primary object of this invention is to provide a new method and means by which solid materials can be pulled from the melt as elongate crystalline bodies of indefinite lengths and predetermined cross-sectional configurations.
A further object is to provide method and apparatus whereby elongate bodies of solid materials may be grown from the melt to various arbitrary shapes and sizes and with smooth surfaces.
Another object of this invention is to provide a method and means by which congruently melting solid materials may be continuously pulled from the melt in the form of crystalline bodies of controlled cross-sectional configurations.
A further object of this invention is to provide a novel method and means for growing from the melt elongate single crystal, essentially monocrystalline, and polycrystalline bodies of indefinite length and predetermined crosssectional configuration.
Still another object is to produce elongate crystalline bodies of the character described by a growth process in which the melt temperature and pulling speeds may be varied over relatively Wide limits without effecting any substantial change in product cross-section.
A specific object of this invention is to grow from melts of selected material extended crystalline bodies characterized by a variety of predetermined cross-sectional configurations, including but not limited to round filaments, flat ribbons, round hollow tubes, etc.
The foregoing and other objects and advantages which are rendered obvious to persons skilled in the art by the following detailed description are achieved by a novel process characterized by what I call edge defined film fed growth. Briefly described the process involves growth of a selected material in the form of an elongate body with an arbitrary, constant cross-section from a shaping member having a substantially horizontal surface whose gross geometry with respect to its bounding edges is the same as that of the elongate body to be produced. The shaping member has one or more orifices through which material in a molten state is fed to form on the aforesaid surface a film having the same geometry as said surface. The elongate body is grown from the film by causing the molten material to solidify on a seed body which is withdrawn in a substantially vertical direction at a speed consistent with the rate at which molten material can be supplied to maintain the film and/ or the rate at which liberated heat of solidification can be rejected. The shaping member is made from a material which is compatible with the liquid and solid phases of the material to be grown and which is wetted by the material. The shaping member is disposed in a reservoir supply of the material in the molten state and each orifice is dimensioned so that the surface tension forces are sufiicient to cause continuous feeding of said material from said reservoir supply by capillarity. By proper adjustment of melt temperature and rate of seed withdrawal, a crystalline body having the same cross-sectional geometry as the aforesaid surface of the shaping member may be produced on a continuous basis so long as the liquid material continues to be supplied at a rate sufficient to maintain the film between the shaping member and the growing body and sustain the desired growth rate.
Other features of the invention are set forth in or rendered obvious by the following detailed description which is to be considered together with the accompanying drawings, wherein:
FIG. 1 is an elevational view, partly in section of one form of furnace that may be employed in the practice of the invention;
FIG. 2 is a sectional view in elevation of a crucibleforming member arrangement for growing round filaments;
FIG. 3 is an enlarged schematic view showing growth of a filament;
FIG. 4 is plan view of another forming member used to grow round filaments;
FIG. 5 is a view similar to FIGS. 2 and 4 but pertaining to an arrangement for growing triangular filaments;
FIG. 5A is a plan view of the forming member shown in FIG. '5;
FIG. 6 is an enlarged perspective view showing growth of a flat ribbon using one form of shaping member;
FIG. 7 is a perspective view of the upper end of another form of shaping member for growing fiat ribbon; and
FIG. 8 and 8A are longitudinal sectional and plan views respectively of a forming member for growing tubular bodies.
While the following detailed description and several examples of the invention are directed to growth of extended crystalline bodies of tat-alumina, barium titanate, lithium niobate and yttrium aluminum garnet, it is to be appreciated that the invention is also applicable to other materials, including but not limtied to materials that melt congruently (i.e., compounds that melt to a liquid of the same composition at an invariant temperature). It is to be noted also that in growing a-alumina and other materials whose crystal structure is characterized by a unique c-axis, it is preferred but not necessary for the seed crystal to be mounted with its c-axis 000l extending normal to the horizontal surface of the shaping member and parallel to the axis of movement of the seed crystal holder, so that the crystal growth occurs along its c-axis. It has been determined that with this preferred seed crystal orientation the products exhibit maximum tensile strength and tend to be essentially monocrystalline. As used herein the term essentially monocrystalline is intended to embrace a filament, tube, ribbon or any other body of indefinite length grown from the melt according to this invention which over any given portion of its length exceeding its maximum cross-sectional dimension is comprised of a single crystal or two or more single crystals growing together longitudinally but separated by a relatively small angle (i.e. less than about 4) grain boundary.
Since the cross-sectional configuration of the product is determined by the plan view geometry (as determined by its bounding edges) of the substantially horizontal surface on which the film of melt is established, the contiguous longitudinally extending surface or surfaces of the shaping member should meet the aforesaid horizontal surface at an angle such that the bounding edges are sharply demarcated, preferably at an angle of about 90. Having sharp bounding edges not only assures that the film will be prevented by surface tension from running off of the horizontal surface down along the sides of the shaping member but also helps control the texture of the products longitudinally extending surface or surfaces. In this connection it is to be appreciated that the product is characen'zed by a surface smoothness which is better than that of the bounding edges of the horizontal surface of the shaping member. This is believed due to surface tension, but it also may be due at least in part to surface diffusion of atoms as the growing crystal mass is pulled clear of the film of melt. It also has been determined thta the surface texture of the product is not affected greatly by the smoothness of the horizontal surface of the shaping member. Although it is preferred that the horizontal surface be relatively smooth, it need not be optically smooth. The horizontal film-supporting surface of the shaping member need not be exactly horizontal or flat (i.e. planar). For example, it may be slightly concave or convex. However, the less horizontal and planar the film-supporting surface of the shaping member, the less easy it is to grow to the desired cross-section and the more the growth process tends to be affected by melt temperature and pulling rates. In this connection it is appreciated that the allowable degree of deviation of the film-supporting surface with respect to being planar and also to being disposed horizontally may depend on the surface tension of the film and thus may vary according to the particular material of which the melt is constituted.
As indicated above, capillary action is employed to continuously feed melt from the reservoir supply to the filmsupporting surface of the shaping member so as to replenish the melt consumed in growing the product from the film. This capillary action is an inverse function of the diameter of the orifice or capillary in the shaping member. The surface energy of the melt material being known, the distance a column of melt can rise by capillary action in a given round capillary above the surface of the reservoir body of melt in the crucible can be approximated by the equation h=2T/drg Where h =the distance in cm. that the column will rise, T=the surface tension in dynes/cm., d=the density of the melt material in gms./cc., r=the internal radius of the tube in cm., and g=the gravitational constant in cm./ sec. With materials of the type that may be employed in the practice of this invention, relatively long columns can be achieved by capillary action. By way of example, in a capilllary of 0.75 mm. diameter, a column of molten alumina may be expected to rise more than 11 cm. Of course, the capillary need not be round and, as disclosed hereinafter, may even comprise an open slot.
Turning now to FIG. 1, there is shown one form of furnace that may be used in practicing the present invention. The furnace comprises a vertically moveable horizontal bed 2 on which is supported a furnace enclosure consisting of two concentric-spaced quartz tubes 4 and 6. At its bottom end the inner tube 4 is positioned in an L- 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 12 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 14 and a spacer 15 compressed between a collar 16 that screws onto sleeve 8. Sleeve 8 is provided with an inlet port fitted with a flexi ble 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 12 and 14, and collars 10 and 16 for holding the two tubes in concentric sealed relation. Pipes 20 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 ofa conventional crystal pulling mechanism represented schematically at .34. It is to be noted that the type of crystal-pulling mechanism is not critical to the invention and that the construction thereof may be varied substantially. Preferably, however, I 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. Re gardless 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 a metal rod 36 that is adapted to function as a holder for a seed crystal 3%.
Located within the furnace enclosure is a cyindrical 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 (shown in phantom at 46) adapted to contain a suitable supply of melt material. The crucible is made of a material that will withstand the operating temperatures and will not react with or dissolve in the melt. For example, where the material to be grown is ot-alumina, the crucible is made of molybdenum, but it also may be made of 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 euteciic reaction between carbon and molybdenum at about 2200 C. 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 is wrapped around the carbon susceptor. The carbon cloth does not appear to couple directly to the RF field but greatly reduces the heat loss from the carbon susceptor. At a given RF. power setting the shield 50 increases the susceptor temperature by as much as 500 C.
The furnace described may be employed for growing a variety of materials to which the invention is applicable using any of the crucible-shaping member arrangements shown in FIGS. 2-8.
Referring now to FIG. 2, there is shown an arrangement adapted for growing filaments of substantially circular cross section. This arrangement comprises the crucible 46 which is fitted with a cover 52 that preferably is made of the same material as the crucible. Cover 52 functions as a heat shield for the crucibles contents. Cover 52 has a hole 54 therein. Situated in the crucible is a shaping member 56 comprising a cylindrical rod 58 formed integral with a flat base 60 that rests on the bottom of the crucible. Base 60 is shaped and sized so as to make a snug fit with the interior side surface of the crucible. The outside diameter of rod 58 is smaller than the hole 54 in cover 52 and the rod is long enough so as to project slightly above cover 52. The rod also has an axial bore 62 and one or more radial openings 63 near its bottom end to permit inflow of melt from the crucible. Bore 62 is sized to function as a capillary. The upper end of rod 58 terminates in a flat surface 64 which intersects the rods outer surface at a right angle.
The dimensions of crucible 4'6 and shaping member 56 may be varied over a relatively wide range according to the size and heating capability of the furnace and the size and length of the filament to be produced. Nevertheless it is essential that the axial bore 62 (or equivalent melt delivery passageways in the other embodiments herein described) be sized so that the melt material can rise therein by capillary action up to its top end, so as to permit establishing and maintaining a growth pool of melt on the upper end surface of the rod in the manner herein described.
Operation of the apparatus shown in FIGS. 1 and 2 and a specific example of the method of growing Ot-al'llmina filaments according to my invention will now be described, with further reference to FIG. 3. Positioned in the furnace in the manner described above is a molybdenum crucible having an internal diameter of about inch, a wall thickness of about n inch, and an internal depth of about inch. The crucible is fitted with a molybdenum cover measuring about inch thick and having a center hole of about 0.20 inch diameter. Disposed in the crucible is a shaping member 56 constructed generally as shown in FIG. 2. The dimensions of the shaping member are as follows: a rod diameter of about 0.125 inch, a rod length such that its upper end projects about 1 inch above the cover, and an axial capillary 62 having a diameter of about 0.040 inch. The crucible is filled with substantially pure polycrystalline tat-alumina and an a-alumina seed crystal 38 is mounted in holder 36 so that its c-axis is aligned parallel to the holders path of movement. The lower end of the seed crystal is sufiiciently small in cross section to be inserted into the upper end of capillary 64. Access to the seed holder and the susceptor is achieved by lowering bed 2 away from the furnace enclosure and lowering the seed holder below the bottom end of tube 4. The crucible is disposed so that the shaping member 56 is in line with the seed crystal and also so that its top end surface 64 extends at a right angle to the seed crystals c-axis. With the bed restored to the position of FIG. 1, cooling water is introduced between the two quartz tubes, and the enclosure is evacuated and filled with helium to a pressure of about one atmosphere which is maintained during the growth period. Then the R.F. coil is energized and operated so that the alumina is brought to a molten condition (alumina has a melting point in the vicinity of 2000 C.). The melt is shown at 66. As the alumina is converted to a liquid, a column 68 (see FIG. 3) of molten alumina will rise in and fill capillary 62. The column will rise until its meniscus is substantially fiush with the top of the rod. After afiiording time for temperature equilibrium to be established, the pulling mechanism is actuated and operated so that the seed crystal is moved into contact with the meniscus of the column of alumina in capillary 62, allowed to rest there for about 5 seconds, and then withdrawn slowly, e.g., at a rate of about /2 to one inch per minute. The temperature of the melt in capillary 62 is critical. If it is too cold, the portion in the upper end of capillary 62 will tend to solidify and no growth will occur on the seed crystal. If the melt is too hot, it will cause the seed to melt. Hence the initial withdrawal of the seed may be unaccompanied with any crystal growth. Accordingly the rate of heating the melt is adjusted (increased or decreased depending upon whether the film is too cold or too hot) and then the seed is again brought into contact with the column of melt, held there for about five seconds, and then withdrawn again at the aforementioned rate. Attainment of the proper melt temperature is revealed by commencement of crystal growth on the end of the relatively cooler seed. Normally as the seed continues to be withdrawn slowly, the affinity of the melt with respect to the newly grown material on the seed causes it to spread out from the capillary onto the end surface 64 as a film '70 whose geometry is defined by the outer edge of surface 64.
If growth occurs on the seed but the melt does not immediately spread to form the film 70, then steps are taken to force the melt to spread as desired. This can be accomplished by increasing the average temperature of the melt or by increasing the pulling speed. Preferably the speed is held constant at /2 to one inch per minute and the temperature is increased slowly until formation of the film is observed. This film functions as a growth pool of melt. Accordingly as the film spreads out to the edge of surface 64, 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 diameter of the growing round crystalling body 72 will be substantially the same as that of surface 64. As growth continues the pulling speed and rate of heating are adjusted to and held at levels consistent with optimum growth. In practice the pulling speed is adjusted to about 2-3 inches/minute. The growing filament has a circular symmetry with an OD. substantially the same as that of the surface 64 of the forming member at the temperature of growth, and its maximum length is limited only by the maximum pulling distance afforded by pulling mechanism 34 or, if a continuous pulling mechanism is used, by the available supply of molten alumina. The filaments surface is very smooth. Inspection of a-alumina filaments grown according to the foregoing example reveals that they usually comprise a single crystal. However, in some cases, they comprise two, three or four crystals growing together longitudinally and separated by a low angle (within 3-4 of the c-direction) grain boundary. The same thing is true of other tit-alumina bodies such as ribbons, tubes, etc. which are grown using a seed crystal oriented so that its c-axis is parallel to the pulling axis.
It has been found that if the operating temperature (as determined by the average temperature of film 70) is held constant close to but slightly above the melting point of the material to be grown, the pulling speed may be varied substantially (e.g. by as much as 50% for a 0.010 inch filament depending upon the operating temperature) without any substantial change in the cross-section of the product. Similarly if the pulling speed is held constant, the operating temperature may be varied substantially (e.g., a change of as much as -30 degrees with respect to the melting point of alumina) without any substantial change in the cross-section of the product. Hence the process affords a great advantage-it will yield a product of constant cross-section despite substantial variations in operating temperatures and pulling speed. If the pulling speed is increased excessively so that the capillary cannot supply melt fast enough to replenish the film which is being consumed in the growth process, the product crosssection will no longer correspond to the geometry of the surface 64 and growth may actually terminate.
The fact that the grown filament has substantially the same shape and size as the periphery of surface 6 4 suggests that the film 70 comprises a growth zone which is substantially isothermal in a direction parallel to surface 64 and has the same configuration and diameter as surface 64. It is to be noted that the film has a depth in the order of about 0.010 inch under usual growth conditions and has a vertical temperature gradient, being hottest at its interface with rod 58. The latter functions substantially as an isothermal heater so that the interface between its top surface 64 and film 70 is substantially isothermal.
Essentially monocrystalline filaments of a-alumina grown according to this invention have been found to exhibit tensile strengths as high as 400,000 p.s.i., with an elastic modulus of 40--70 10 p.s.i. Substantial reductions in tensile strength, elastic modulus and flexure modulus occur if the seed crystal is mounted so that its c-axis is not parallel to the axis of movement of the crystal holder and perpendicular to the melt surface. By way of example but not limitation, filaments measuring 0.005- 0.10 inch in diameter have been grown at pulling rates of 3-4 inches/min. using different size forming members of the type shown in FIG. 2 or of a type hereinafter described.
It is to be appreciated that it is possible to start with a seed crystal that is larger in diameter than the capillary of forming member 56. In such case the seed is brought directly into contact with the end surface 64 and held there for a period of time, e.g., 10 seconds, for its extremity to melt and form a film of melt on the end surface and then withdrawn slowly. Withdrawal of the seed is accompanied by a crystal growth resulting from solidification of a portion of the material of which the film is comprised. As the growth occurs, the aflinity of the liquid for the grown material causes additional melt in the cap illary to spread out onto the end surface of the forming member. The capillary continues to supply melt at a rate consistent with the rate of crystal growth, with the result that the film continues to cover the end surface and the growing body has a diameter conforming to the CD. of the forming member. Where the seed crystal has a diameter larger than the upper end surface of the capillary member, it may be necessary to increase the rate of heating slightly so that the temperature of the upper end surface of the capillary member before it is contacted by the seed is greater than that normally required to be maintained for continuous growth. This higher temperature offsets the heat sink effect of the relatively large seed which causes the growth pool i.e., the film of melt, to momentarily have a lower average temperature than would otherwise be the case when the seed is withdrawn. Unless this heat sink effect is offset by an increase in the rate of heating, the seed crystal may pull clear of the melt without growth occurring thereon. Once growth is visible above the film, the heating rate may be cut back so that the rate at which liberated heat of solidification is rejected by radiant cooling is consistent with the preferred pulling rate for continuous growth along the entire expanse of the film of melt.
The capillary in the forming member need not have a circular cross-section and may even be open to the melt along its entire length. Thus as shown in FIG. 4, for example, it is possible to grow round filaments using a forming member '76 which is similar to forming member 56 in that it consists of a cylindrical rod 78 formed integral with a circular base 80 but differs in that it has an axially disposed slot 82 extending up from the base 80 which is sized to function as a capillary and effectively subdivides the fiat upper end surface into two circular sections 84 and 86. Forming member 76 is disposed in crucible 46 in the same manner as forming member 56 and is employed in the same way to grow filaments. A film of melt may be established on the end surface sections 84 and 86 in the manner described above by using a seed crystal small enough to be inserted in slot 82 or by using a larger seed which is brought into contact with the forming member. As growth proceeds, melt will continuously flow up out of slot 82 onto the circular sections 84 and 86 of the rods end surface, so that the film will support growth to a diameter substantially the same as that of the rod. Of course, the liquid-solid growth interface is not confined to the film supported by the sections 84 and 86 of the upper end surface of the forming member but continues across the slot 82.
Although not shown it is to be appreciated that the forming member may project high enough above the crucible cover 52 to permit use of one or more radiation shields to help control the rate at which liberated heat of solidification is rejected. Preferably but not necessarily the radiation shields are formed as flat plates adapted to be stacked one upon the other. This allows different length capillary members to be used in the same crucible.
FIGS. 5 and 5A illustrate another form of crucibleforming member arrangement adapted for growing filaments with a triangular cross-section. In this embodiment the forming member 92 may consist of a base adapted to lie flat on the bottom of the crucible like the bases of the forming member of FIGS. 2 and 4 or may comprise a fiat plate 94 with a depending skirt 96 that holds it above the bottom of the crucible. Plate 94 has a plurality of holes 98 and supports a rod 100 having a triangular cross-section and an axially extending capillary in the form of a bore 102. The latter may be coaxial with rod 100 or disposed eccentric to its longitudinal axis as shown. The upper end of rod 100 terminates in a fiat end surface 104. The method of growing triangular filaments using the arrangement of FIGS. 5 and 5A is the same as the method for growing round filaments, except that the thin film growth pool of melt that is formed on and covers the end surface 104 has a substantially triangular configuration and has a thermal distribution conducive to crystal growth propagating vertically throughout a horizontal growth zone that is substantially fully coextensive with the entire expanse of surface 104. By way of example, triangular filaments of a-alumina produced with this arrangement have smooth substantially flat surfaces that terminate in rounded corners. Such filaments are essentially monocrystalline. Triangular filaments of a-alumina measuring about inch from each corner to the center of the opposite side surface have been grown at speeds of about one inch/minute.
FIG. 6 is an enlarged perspective view showing growth of an elongate fiat ribbon using one form of shaping member designed for that purpose. The forming member consists of a rod 108 of rectangular cross-section that terminates in a flat upper end surface 110* that extends at substantially a right angle to the rods four side surfaces. The bottom end (not shown) of rod 108 is attached to a suitable supporting base similar to those of forming members 56 and 76. Rod 108 has a plurality of capillaries in the form of round axial bores 112 which extend from end surface 110 down to the bottom end of the rod and communicate with melt in the crucible via suitable ports (not shown) corresponding to the radial openings 63 of forming member 56. The number of capillaries is not critical and may be varied as desired. Also not critical is the shape of the capillary. Thus as shown in FIG. 7, the forming member may comprise a rod 116 corresponding in shape to rod 108 but including instead of axial bores 112 a single capillary in the form of a slot 118 which is open along one side of the rod. Slot 118 extends down from the end surface 112 of rod 116 and, since it is open along one side, ports similar to the radial openings 63 of forming member 56 are not required to provide communication between the slot and the melt in the crucible. Growth of ribbon using the capillary members shown in FIGS. 6 and 7 is achieved by the same method as growing round filaments, except that the thin films of melt that are formed and maintained on the end surfaces 110 and 120 enable growth of ribbons having substantially rectangular cross sections characterized by widths and thicknesses corresponding closely to the overall widths and thicknesses of rods 108 and 116. FIG. 6 illustrates such a ribbon 122 growing from a liquid film 124 which fully covers surface 110 and is constantly fed by capillaries 112. Ribbons grown using capillary members similar to those shown in FIGS. 6 and 7 have fiat surfaces that are smooth to within a maximum deviation of about 1000 angstroms. By way of further example, ribbons of a-alumina measuring /1 inch by .005-010 inch in cross-section have been grown at pulling speeds in excess of one inch/minute. These and other rat-alumina ribbons grown on seeds oriented with their c-axes parallel to the pulling axis have exhibited average tensile strengths on the order of 112000 p.s.i.
Reference is now had to FIGS. 8 and 8A which illustrate a forming member that has been used for growing tubular bodies of u-alumina. This particular embodiment comprises a round rod 128 having a circular base 130 for supporting it in the crucible. The upper end of rod 128 has an axially dispposed bore 132 and terminates in a fiat annular surface 134. Rod 128 also has four capillaries in the form of small round bores 136 that extend from its bottom end to the annular surface 134, plus four radially extending holes 138 that permit melt to flow into the capillaries from the crucible. Growing tubular bodies using a capillary member as shown in FIGS. 8 and 8A involves essentially the same procedure as described above for growing filaments and ribbon. Hence the seed that is employed may be small enough to be inserted into one of the capillaries or may be large enough to contact all or a portion'of the annular surface 134. The seed may even be a tubular body grown previously with the same capillary member. If the end of the seed is not a previously grown tubular body, the initial pulling speed must be low enough to permit the growth to expand horizontally to the full expanse of surface 134 before any substantial vertical growth has occurred. Even at low pulling speeds the initial growth will be primarily vertical. However, as pulling continues, the crystal growth progresses laterally as well as vertically until a complete tube is formed. Thereafter the pulling rate is increased to the maximum level at which a tubular body continues to be formed with a wall thickness substantially equal to the width of annular surface 134. Essentially monocrystalline tubes of a-alumina have been grown using a capillary member of the type shown in FIGS. 8 and 8A. By way of example, I have grown rat-alumina tubes having an interior diameter of inch and a wall thickness of & inch. At a pulling speed of about 2 inches/minutes, such tubes are characterized by smooth inner and outer surfaces.
While a cover plate for the crucible is not essential, its use does facilitate better control of the average temperature of the melt in the crucible and in the capillary since it limits heat loss and thereby helps establish thermal 10 equilibrium. It also is not essential to carry out the process in an argon or helium atmosphere. Instead the furnace may be evacuated to a suitable level.
It is contemplated that for continuous growth the pulling mechanism may comprise two or more pairs of pulling rolls adapted to grip and pull the product from the melt. Furthermore the crucible should be designed so as to permit periodic replenishment of the melt without having to interrupt the growth process, as taught, for example, by US Pat. 3,265,469 issued Aug. 9, 1966 to R. N. Hall. Depending on their cross-sectional shape, the a-alumina bodies produced as herein described have a certain degree of flexibility which permits them to be bent around wide diameter rolls to effect a change of direction, e.g., when it is desired to move them horizontally in continuous fashion through apparatus designed to perform a selected operation such as severing into pieces of selected lengths.
An important advantage of the invention is that it is applicable to growth of a wide variety of crystalline materials other than alumina. It is not limited to congruently melting materials and encompasses growth of materials that solidify in cubic, rhombohedral, hexagonal and tetragonal crystal structures. By way of example but not limitation, the following additional materials may be grown according to this invention: barium titanate, yttrium aluminum garnet, and lithium niobate. The process also may be used to grow a variety of metals. The process involved in growing these materials to predetermined cross-sectional configurations is essentially the same as the process described above for ot-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. Application of the invention to such other materials is illustrated by several specific examples set forth below. In each example the process involves use of a furnace as shown in FIG. 1 and the crucible-forming member arrangement of FIG. 2 employed in the foregoing example of growing tit-alumina filament. However, it is understood that these materials may be grown to other shapes and that other furnaces may be employed in the process.
The first example involves growth of a round filament of barium titanate. An elongate thin seed crystal of barium titanate is mounted in the seed holder 36 without consideration as to its orientation. At the same time a quantity of barium titanate is placed in a crucible containing the forming member 56. The crucible and forming member are made of iridium. With its cover (also of iridium) set in place, the crucible is mounted on the susceptor 40. With the bed 2 restored to the position shown in FIG. 1, cooling water is introduced between the walls of the two quartz tubes, and the enclosure is evacuated and filled with argon. The pressure in the furnace is adjusted to about one atmosphere. Then the RF coil is energized and operated so that the barium titanate in the crucible is heated to an average temperature slightly above about 1620 C. Once the melt has risen in the capillary to the point Where its meniscus is visible and temperature equilibrium has been established, the pulling mechanism is actuated to bring the seed down into contact with the meniscus of the melt in the capillary. After about 5l0 seconds, the seed is withdrawn at about /2 inch per minute. With the rate of heating adjusted so as to achieve a thermal distribution in the melt conducive to growth (as with alumina this is a trial and error adjustment requiring one or more withdrawals of the seed to see if growth occurs at the given rate of heating), withdrawal of seed is accompanied by crystal growth thereon. The rate of withdrawal of the seed is adjusted until the upper end surface of the forming member is covered by a thin film of melt capable of supporting growth to the outside diameter of the forming member. Thereafter the pulling speed and rate of heating are held constant until the crystal growth has reached the desired length or until the contents of the crucible have been substantially exhausted, whichever occurs sooner. The average pulling speed is about inch per minute. The grown product is characterized by a smooth surface and is essentially monocrystalline.
The second example involves growth of a round filament of yttrium aluminum garnet. The crucible, its cover and the forming member are made of iridium. The procedure is the same as that described above in connection with growth of barium titanate filament except that an alumina seed is used and the yttrium aluminum garnet in the crucible is heated to an average temperature slightly above about 2000 C. The average pulling speed is about inch/minute. The grown filament has a diameter approximately the same as the outside diameter of the forming member. Its surface is smoth and it is essentially monocrystalline.
A further example involves growth of a filament of lithium niobate. The procedure is the same as that described above in connection with growth of a-alumina, barium titanate and yttrium aluminum garnet filaments except that the crucible, its cover and the forming member are made of platinum, the seed consists of strontium titanate, and the lithium niobate is melted to an average temperature slightly above 1300 C. An average pulling speed of about /2 inch/minute is employed in order to achieve a filament with a diameter approximately the same as that of the forming member. The grown lithium niobate filament has a smooth surface and is essentially monocrystalline.
By way of further example, it is possible to grow by the same procedure a filament of sodium chloride on a suitable seed, e.g., a seed of tit-alumina, from a melt heated in a molybdenum crucible to a temperature of about 800 C.
It is to be observed that barium titanate crystallizes in a hexagonal or cubic crystal structure, whereas yttrium aluminum garnet crystallizes in a cubic structure and lithium niobate crystallizes in a rhombohedral structure.
The invention is susceptible of a number of other variations and extensions. For example, the forming member need not be a separate element and need not include a base for supporting it in the crucible; instead it may be attached to and form a part of the crucible cover.
Certain other variations and extensions may be made. For example two or more essentially crystalline bodies may be pulled simultaneously using two or more forming members in one crucible. The holders for the several seeds required to grow several bodies at the same time may be mounted on the same or different pulling mechanisms depending upon whether the process is to be carried out on a limited or continuous basis. A further variation involves using a shaping member adapted for growing a crystalline body with a plurality of parallel axial-extending holes. Construction of such a shaping member is easily accomplished in view of the above teachings and may, for example, be done by uniting a plurality of rods such as shown at 128 in FIG. 8. Another modification involves means other than a capillary for delivering melt to the film-supporting surface of the forming member. Thus melt may be delivered to the aforesaid surface under positive pressure or the influence of gravity, in which case the surface may be displaced from and/or located below the level of the crucible.
The invention offers several important advantages. The apparatus required is relatively simple. The process is essentially continuous so that the products may be grown in any suitable length. By using different forming members, it is possible to pull extended monocrystalline bodies of different cross-sectional configurations and sizes. The pull rates are relatively fast. The products have smooth surfaces (notably to within about 1000 angstroms smoothness), and exhibit physical properties that render them useful for a variety of applications.
Particularly important is the fact that the process is not extremely sensitive to pulling speed and temperature, with the result that the products can be grown continuously to a constant size cross-section of predetermined shape despite small changes in pulling speed and temperature. This facilitates growing a number of elongate bodies, e.g., ribbons, simultaneously from a common crucible despite the fact that the products are not pulled at exactly the same speeds or that the films of melt supported by the several forming members are not at the same average temperature.
It is believed to be apparent that with respect to determining the cross section of the body that is grown, the effective shape of the upper film-supporting surface of each forming member is its shape as it would be if undisturbed by any capillary orifice. Accordingly as used herein the terms gross geometry and gross edge configuration denote the peripheral configurations of the film supporting surfaces of the forming members as such surfaces would appear if the capillaries therein were omitted. Thus with respect to FIG. 4 the gross geometry is a full circle, while in the case of FIG. 7 the effective shape and hence the gross geometry of the upper surface is a rectangle. The capillaries are ignored in the gross geometry since they do not determine the cross-sectional shape of the product but function merely as melt-feeding passageways. On the other hand the bore 132 in FIG. 8 is not ignored in the gross geometry since it is not a capillary but functions as an edge-defining boundary. Hence the gross geometry of upper end surface rod 128 is considered to be an annulus rather than a simple circular area.
It is to he understood that the invention is not limited in its application to the details of apparatus and method specifically described or illustrated, and that within the scope of the appended claims, it may be practiced otherwise than as specifically described or illustrated.
1. Method of growing a solid crystalline material in the form of an elongate body having a substantially constant cross section of predetermined shape, comprising providing a liquid reservoir supply of said material in which is disposed a product-shaping member having (1) a substantially horizontal surface that is located above said supply and has a gross edge configuration corresponding to said predetermined shape and (2) a capillary leading from said reservoir supply to said surface, allowing said capillary to fill with liquid material from said reservoir supply, positioning a seed with respect to said productshaping member so as to establish a liquid film of said material on said surface, controlling the temperature of said film and withdrawing said seed from said productshaping member in a vertical direction at a rate such that an elongate crystalline body of said material grows on said seed by solidification of a portion of the material comprising said film, continuously feeding liquid material to said surface via said capillary so as to replace the material of said film consumed in growing said body, adjusting the rate of Withdrawal of said seed so that said film substantially entirely covers said surface, and maintaining said film on said surface during crystal growth so that the film continues to substantially entirely cover said surface without exceeding it and said body grows to said predetermined shape.
2. Method of claim 1 wherein said surface is annular and said body is tubular.
3. Method of claim 1 wherein said body is essentially monocrystalline.
4. Method of claim 1 wherein said material crystallizes in a rhombohedral, hexagonal, cubic or tetragonal lattice structure.
5. Method of claim 1 wherein said material is alumina.
6. Method of claim 1 wherein said material comprises 13 one of the following: alumina, barium titanate, yttrium aluminum garnet and lithium niobate.
7. Method of claim 1 wherein said surface is substantially planar.
8. Method of producing a solid crystalline material in the form of an elongate tubular body comprising, providing in a crucible a supply of said material and a shaping member having a substantially horizontal surface with an annular gross edge configuration that is disposed above said crucible and a capillary leading from said surface to said supply of material, heating said crucible to render said material liquid and allowing said liquid material to fill said capillary, establishing over substantially the entire area of said surface a liquid film of said material with a seed in contact therewith, withdrawing said seed from said film and controlling the rate of heating and the rate of withdrawal of said seed so that from said liquid film crystalline growth of said material occurs on said seed in the form of an elongate body with an annular cross-sectional configuration, continuously feeding liquid material to said surface from said crucible via said capillary so as to replace the material consumed in growing said body, and maintaining said film during crystalline growth so that said film continues to cover substantially the entire area of said surface Without exceeding said surface.
9. Method of producing a crystal body of a selected material having a predetermined arbitrary cross-sectional geometry comprising disposing in a crucible a member having a substantially horizontal surface with a gross geometry determined by one or more substantially vertical edge surfaces that corresponds to said cross-sectional geometry and at least one capillary having an upper end terminating in an orifice in said surface and a lower end communicating with the interior of said crucible, said member being made of a composition that is wetted by said material in liquid form, providing in said crucible a melt of said material to a level below the level of said surface but above the lower end of said at least one capillary, filling said at least one capillary with melt by action of capillary rise, establishing on said surface a molten film of said material that is continuous with the melt in said at least one capillary and has a thermal distribution conducive to crystal growth of said material to the full horizontal expanse of said film, growing and withdrawing a crystal from said film, continually feeding more melt to said surface by way of said at least one capillary during crystal growth so as to replace the material of said film consumed in growing said crystal, controlling the rate of withdrawal of said crystal and the temperature of said film so that said film substantially entirely covers said surface, and maintaining said film on said surface during crystal growth so that said film continues to substantially entirely cover said surface without exceeding said surface and said crystal grows to the full horizontal expanse of said film.
10. Method of claim 9 wherein said surface is circular.
11. Method of claim 9 wherein said surface is annular and said body is tubular.
12. Method of claim 9 wherein said surface is rectangular.
13. Method of claim 9 wherein said material in a member of the group consisting of alumina, barium titanate, yttrium aluminum garnet and lithium niobate.
14. Method of claim 9 wherein formation of said film is initiated by melting a portion of a seed crystal in contact with said surface.
15. Method of growing an elongate solid body of a crystalline material so that said body has a substantially constant cross-section of predetermined shape comprising, providing in a crucible a liquid supply of said material in which is disposed a product-shaping member hav ing (1) a substantially planar horizontal surface that projects above said supply and has a gross edge configuration corresponding to said predetermined shape and (2) a capillary having an upper end terminating in an orifice in said surface and a lower end communicating with said supply in said crucible, filling said capillary with liquid material from said supply, positioning a seed with respect to said product-shaping member so as to establish a liquid film of said material on said surface, withdrawing said seed from said film in a vertical direction at a rate such that a crystalline body of said material occurs on said seed by solidification of a portion of the material comprising said film, continually feeding liquid material to said surface from said supply via said capillary as said seed is withdrawn so as to replace the material of said film consumed in growing said body, controlling the temperature of said film and the rate of withdrawal of said seed so that said film substantially entirely covers said surface, and maintaining said film on said surface during crystal growth so that the film continues to substantially entirely cover said surface without exceeding said surface and said body grows to the full horizontal expanse of said film.
16. Method of claim 1 wherein said surface is circular.
17. Method of claim 9 wherein formation of said film is initiated by contacting the melt in said capillary with a seed and then withdrawing said seed at a rate such that the aflfinity of the melt for said seed causes said melt to flow out of said capillary onto said surface.
References Cited UNITED STATES PATENTS 2,944,875 7/1960 Leverton 23-273SP 3,033,660 5/1962 Okkerse 23-273 3,078,151 2/ 1963 Kappelmeyer 23-273 3,471,266 10/1969 La Belle 23-301 NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner US. Cl. X.R. 23-273SP Disclaimer 3,591,348.-Har0ld E. La Belle, Jr., Quincy, Mass. METHOD OF GROVING CRYSTALLINE MATERIALS. Patent dated July 6, 1971. Disclaimer filed Mar. 25, 1971, by the assignee, Tyco Laboratories, Inc. Hereby disclaims the portion of the term of the patent subsequent to Oct. 7 1986.
[Official Gazette April 18, 1.972.]