IL31601A - Large crystalline bodies and method of production thereof - Google Patents

Description

X-AR&E CRYSTALLINE BODIES AHD METHOD OF PRODUCTION THEREOF This Invention relates to the production of large bodies of monocrystalline corundum.

Synthetic crystals of corundum (variously described in the literature as -alumina, corundum and sapphire) have been produced in the past in a number of shapes using various techniques. In the well-known Verneuil or flame fusion growth process, a single crystal corundum rod or boule is produced as follows:^ powdered crystal constituent material is dispensed into an oxygen stream which is then carried to a post-mixed oxy-hydrogen burner; the powder is melted and deposited on the fused cap of a seed crystal; proper growth conditions are established through control of the flame conditions and powder feed rate and the seed is withdrawn progressively from the flame as the boule of deposited crystalline material grows. Cylindrical corundum rods can be produced in this manner with diameters up to an inch or more and with lengths of a foot or more depending on the diameter. n response to a need for other than rod-like shapes of single crystal corundum, modifications have been made to the flame fusion process whereby such shapes as discs and domes, with diameters of up to about five inches, are produced as shown in United States Patents 2,852,890 and 2,962,838. Transparent discs or other desired shapes are cut from the as grown material and polished so as to have parallel faces to provide windows having surface areas of up to about 20 square inches.

These windows are useful in scientific and military applications strength to weight ratio of monocrystalllne corundum compared to other transparent materials. Such windows are considered to be useful as transparent armor for military vehicles and the like.

There Is now a demand for still larger bodies of monocrystalllne corundum, particularly large slab-like bodies which can be fashioned into large area windows, i.e., larger than those which can be produced with the flame fusion disc process. While the Czochralski process, as presently developed, can be used to produce monocrystalllne boules with diameters of up to 2 inches and a length of 12 Inches or more, the shape of the body is not particularly suited for fabrication into large area single piece windows. In the Czochralski process, a molten bath of crystalline material is established and a seed crystal is dipped into the melt and slowly withdrawn to form an elongated crystalline body. The boule is slowly rotated during pulling to equalize the temperature and growth conditions at the solid liquid interface. Windows cut from such bodies can have widths of only about 2 inches or so, which does not fill the present needs .

It is the primary object of this invention therefore to provide monocrystalllne as-grown bodies or slabs of corundum or other suitable materials having large usable surface areas.

It is another object of this invention to provide large polished bodies of monocrystalllne material, for example rectangles, squares, etc., of sizes and shapes not presently attainable with currently practiced processes.

Other aims and advantages of this invention, including the equipment and processes for producing the aforementioned βlabs and shapes, will be apparent from the following description, the drawings and the appended claims.

Accordingly, a process for producing large bodies of monocrystalline material, and in particular monocrystalline corundum is provided comprising establishing a melt of crystal constituent material, establishing a thin elongated region on the surface of said melt at a temperature suitable for crystal growth and at a lower temperature than regions immediately adjacent both sides of the elongated region and at its ends, dipping a seed crystal into the melt along the elongated region and initiating crystal growth on the seed, said seed crystal being oriented relative to the melt and the elongated region therein with the direction of most rapid growth of the crystal extending vertically into the melt and with the planes of least rapid growth parallel to the longitudinal axis of the elongated region, and pulling the seed from the melt as crystal growth occurs principally in the direction of pull and along the axis of the elongated region whereby a crystalline slab of substantially greater length and width than thickness is produced, and growing continuing to pull the^ aaiia&t crystal from the melt until a 8lab of a desired length is produced.

The as-grown corundum bodies produced by this process are massive monocrystalline elongated slabs of substantially uniform thickness, the longitudinal axis of the slab being parallel to the a-axis of the crystal and the lateral faces of the slab lying in substantially flat/ parallel basal planes, with any growth lines occuring in the crystal being substantially flat and perpendicular to the a-axis.

An apparatus suitable for producing these crystalline bodies comprises a crucible having a rectangular cross section, said crucible surrounded on its sides and base by insulating members, an induction heating coil having a rectangular cross-section symmetrically arranged around the insulating members and crucible for inducing a heating current in the walls of the crucible, a lid over the crucible opening, said lid having a centrally located rectangular opening having its long axis parallel to the long side of the crucible and with a duct for exhausting gases from said crucible interior extending continuously around the edge of said lid opening, the length and width of the opening in the ducted lid being slightly greater than the width and thickness of the slab to be pulled, and means for drawing gases from the crucible through said ducts, whereby when said crucible is heated by said induction coils to melt a charge of crystal constituent material by transfer of heat from the walls of said crucible to the melt, the molten material will flow vertically up the sides of the crucible to the surface and then horizontally in towards the center of the melt surface where heat will be extracted from said flowing material by radiation vertically through the opening in the lid and by a drawn cooling draft of gas -4rem&f\from outside the crucible into the space over the melt surface and out through the ducts thereby forming an elongated region of lower temperature at the center cmicible whereby the mplten material will reenter the melt along ■theis) elongated region and flow downwards . A corundum seed crystal is oriented with its a-axie vertical and its basal planes parallel to the longitudinal axis of the elongated region, which is maintained at a crystal growing temperature whereas the areas of the melt adjacent the elongated region are at a higher temperature not supporting crystal growth . The so oriented seed crystal is dipped into the center of the elongated region and as crystal growth occurs the crystal is pulled from the melt.

In the drawings: FIG. 1 is an illustration of an as-grown monocrystalline corundum body or slab produced by the process of this invnetion, showing the outline of a rectangular window shape which could be cut from the slab; FIG. 2 is a representation of the crystal structure of corundum showing principal planes upon which crystal growth can occur and showing a superimposed monocrystalline window cut from an as-grown slab produced according to the process of this invention; FIG. 3 is a schematic perspective view, partially in section, showing a furnace apparatus and contained crucible for holding the molten crystalline material; FIG. 4 is another perspective view of the crucible with a lid thereover having means for aiding in the control of the temperature of various portions of the melt to produce the desired elongated region of lower temperature; FIG. 5 is a schematic view, in vertical section, of the monocrystalline slab from the melt along the elongated region established In the melt according to the process of this invention; FIG. 6 is a schematic view, in vertical section, of a crystal growing station showing an arrangement of the furnace and crucible therein and the arrangement of the various utility lines.

Referring to FIG. 1, a massive monocrystalline corundum \ shtnm body or slab 10 is ohowiaj in its as-grown form and having the shape of a paddle. This body was grown from a properly oriented seed crystal, represented at 11, by dipping the seed crystal into a molten bath of aluminum oxide having properly maintained temperature conditions therein, and then slowly pulling the seed from the melt as the aluminum oxide crystallizes on the seed.

By reason of the proper orientation of the seed crystal and maintainance of the elongated crystal growing region in the melt, the crystal is induced to grow in length and in width to a much greater extent than its growth in thickness, so that the resulting body has a length L and maximum width W greatly in excess of its thickness or breadth T. The portion of the body designated Li represents an initial period of the growth process wherein the width of the crystal is gradually increased to its maximum width W. The central portion of the body, designated Le, represents the period of the growth process wherein the crystal is pulled with a uniform maximum width. It is this portion of the body, Le, which measures the effective or useful length of the body in terms of the ability to supply a rectangle or other shape of by dotted lines. The lower portion of the body, designated L^, represents the final period of the growth process where the crystal Is rapidly pulled from the then near depleted bath of molten material.

The thickness T of the crystal Is more or less uniform, except at the extreme ends 13 and 14 of the crystal body which taper sharply. The figure 12 outlined by the dotted lines represents only one possible shape, a rectangular or square body having a maximum width, which can be cut from the body 10. It is obvious that a variety of other shapes, of uniform or varying thickness, can be cut from the body with any desired combination of length and width, including the use of almost the entire length L of the crystal and not merely the portion Le of maximum width.

In FIG. 2 there is shown a representation of the hexagonal crystal structure of corundum. Crystal growth occurs by addition of atoms to the lattice in any of the crystal planes. Growth by addition of atoms in an "a plane" is indicated as growth in the "a-direction", which is perpendicular to the a planes. Growth in the "c-direction" represents crystal growth by addition of atoms in the planes which are perpendicular to the c direction, i.e., the basal planes or planes of closest atomic packing for the hexagonal corundum crystal.

Growth in the "m-direc ion" represents growth of the crystal in a direction perpendicular to the a direction. Growth of the crystal can also be in an "r-direction" or perpendicular to the r ^IOIL^ planes (not shown), a direction which is at an angle to the main a- and c-directions .

It has been found that crystal growth In corundum proceeds more rapidly in the a-direction than in the m; r-or c-directions. Crystal growth is least rapid in the c-direction because the basal planes have a lower energy for crystal growth and tend to facet out as flat faces on either side of the crystal. The r-planes tend to facet out as well. Growth in the m-direction is more rapid then these but not as rapid as growth in the a-direction. According to the present invention, a massive mono-crystalline corundum slab is grown by inducing growth of the crystal in a direction of more rapid growth, while at the same time inhibiting growth even in the direction of slower crystal growth so that the crystal has sizable lateral dimensions, the length and width, and a thickness which is considerably less.

As shown in FIG. 2, the slab 15, which represents the slab shown by outline in FIG. 1, has a long side of length 1 which extends in the a-direction since the crystal is pulled from the melt in a direction parallel to the a-axis of the crystal. This orientation is produced by selecting and orienting a seed crystal, upon which growth of the massive crystal occurs, so that an "a plane" of the seed crystal is perpendicular to the direction of pull and additionally with the basal planes of the seed crystal parallel to the elongated region of lower temperature maintained in the melt. The crystal pulled using this method will have its length extending it the a-direction, the direction of most rapid growth; its width extending in the m-direction, a direction of less rapid growth; and its thickness extending in the c-direction, the direction of least rapid growth and a condition in which the broad sides of the crystal tend to facet out as flat faces on either side, giving the fully grown massive crystal slab the flat side faces which form the large faces of windows or other bodies cut from the as-grown slab, as shown in (Fig.2 the drawings(ndmarked "face". The set of arrows above the as-grown crystal in FIG. 1 shows the orientation of the as-grown crystal. The a-direction is the direction of pull. The m-direction is parallel to the width of the body and the c-direction is perpendicular to the broad flat face of the body.

As stated previously, the growth of the crystal in the c-direction, which is intrinsically slow, is further reduced by proper orientation of the seed crystal In a molten bath of crystal constituent material which is maintained in a particular configuration favoring growth in all directions except the thickness direction of the crystal so that long and broad, but relatively thin slabs can be produced. An elongated narrow crystal growing region is created on the surface of the melt by control of the temperature conditions so that a lower temperature at which crystal growth readily occurs exists along the length or longitudinal axis of the region, but a higher temperature at which crystal growth does not readily occur exists to the sides and ends of the region. FIGS. 3, 4 and 5 show the apparatus for holding the melt and maintaining therein these selective crystal growing conditions.

Referring to FIG. 3 there is shown a crucible 16 composed of a high temperature resistant non-contaminating metal, for example iridium. The crucible has a rectangular dimensions of the crystalline slab to be pulled.

The rectangular crucible is fitted inside a box-like structure 17 composed of insulating materials (broken away to show the crucible). An example of a suitable insulation structure comprises an outer enclosure 18 formed of four slabs of a high temperature insulating material, for example fused silica slabs, fitted together as shown. An inner enclosure 19 is formed of four additional slabs of a high temperature insulating material, which are preferably zirconia slabs. Granulated refractory material, such as zirconia grog 20 is packed into the space between the inner and outer enclosures. The structure rests on a base slab of fused silica 21 and has a layer 22 of zirconia grog in the bottom of the thus formed box. The crucible rests on this layer of grog and is surrounded by the insulating walls.

An r-f heating coil 23 is placed around the box-like insulation structure in the form of a helix having a rectangular cross-section (broken away to show the crucible). The flow of electrical current through the coil induces a flow of current in the iridium crucible thereby heating the crucible to a high temperature. Heat is transferred by conduction from the walls of the crucible to the charge of crystal constituent materials in the crucible to melt these materials. By using a rectangular crucible situated within a rectangular coil, a uniform heating of the melt in the crucible is provided. This is necessary so that a uniform crystal growing region can be formed in the melt.

FIG. 4 shows the crucible 16 without its insulating enclosure and without the coil for ease of illustration of a forming the required crystal growing conditions In the melt.

The lid has a central rectangular opening 25 through which a portion of the surface of the melt 26 Is visible. The rectangular opening In the lid Is longer and wider than the width and thickness of the crystal to be pulled from the melt. This opening Is narrowed and shortened so as to be large enough to allow clearance of the maximum sized crystal to be pulled from the melt by an overhanging duct system 27 extending around the periphery of the lid opening, with an open side of the ducts facing the melt surface. The ducts can be formed of cylinders of iridium split down the center with one edge 28 welded to the top surface of the lid at a point about a half-diameters length back from the edge 25 of the opening and with the other side 29 of the half tube hanging over the edge so as to partially cover and to be open to the space above the melt. The legs of the ducts are closed off at two diagonally opposite points 30 and 31 so that the legs 32 and 33 form one continuous conduit and the legs 34 and 35 form another separate conduit. Tube 36 is connected to the duct 34 so as to communicate with the interior of the ducts 34 and 35. Tube 37 is connected to the duct 32 so as to communicate with the interior of ducts 32 and 33. When a suction is applied to these tubes 36 and 37, gases are drawn from the space above the melt surface underlying the ducts into the ducts and out through the tubes. Additional gas will flow into the crucible through the opening in the lid, pass over the central portion of the surface of the melt 26 and be drawn out through the exhaust tubes. F G. 5 shows more clearly how the ducts extend over the edge of the opening in the lid and form an opening just slightly larger than the width of the crystal 10 being pulled.

In operation, the crucible is placed in the insulating box-like structure and surrounding heating coil. The crucible is charged with solid crystal constituent materials for example cracked Veraeuil grown alumina crystal or spheroidized alumina powder placed over the mouth of the crucible. The power is turned on to energize the heating coils. The charge is melted in the induction heated crucible. Once the melt is established thermal conditions will exist in the system as follows: heat is transferred from the sides of the crucible to the melt; the insulation surrounding the crucible limits the loss of heat from the system except at the surface where radiation of heat from the melt through the opening in the lid occurs, and the molten material according flows vertically up the sides of the crucible to the surface and then horizontally over the surface of the melt. FIG. 4 shows the surface 26 of the melt as seen through the opening in the lid 16. While the horizontal flow of molten material (represented by arrows) is under the overhang of the lid and duct system, it does not lose substantial amounts of heat because thermal energy radiated upwards from the melt surface at this point is reflected back to the surface from the under side of the lid. However, when this horizontal flow of molten material passes the overhang of the lid and duct/-s^cutem, into the central rectangular area exposed under the opening in the lid, it immediately loses heat by radiation and drops in temperature. Because of the symmetrical heating coll around rectangular crucible and because of the shapg^-rectangular (≤Epe of the lid opening, an elongated region or line 38 of minimum temperature conditions occurs at the center of the melt surface, parallel to the long sides of the opening and terminated at either end just inside the rectangular opening because of the reflection of heat from the overhanging lid and ducts 32 and 34 at the short ends of the opening. This elongated and region 38 is a linear heat sinkr¾ue- the molten material which flowed horizontally from the sides of the crucible now reenters the melt along this line and flows toward the bottom of the crucible where it is reheated by the walls of the crucible and starts to flow upward again in the previously described pattern. It is along this line 38 of minimum temperature in the melt that the supercooled conditions required for crystal growth are created by proper control of the heat input to the system. In other words, a temperature at which crystal growth can occur, a supercooled region, exists along the line 38. The portions of the melt to the left or right of this line are at a higher temperature because of reflection of radiated heat back from the overhanging lid and duct, and will not readily support crystal growth. Similarly, the regions of the melt past the ends of the elongated region are also at the higher temperature which does not favor crystal growth.

The shape and linearity of this elongated region is further controlled by the cooling draft of gas produced by the suction in the ducts 32, 33, 34 and 35 and exhaust tubes 36 and 37. This flow of gas removes heat mainly from the central temperature along the elongated region 38. By Increasing the suction, the volume of gases flowing over the melt surface is increased thereby lowering the temperature along the elongated region.

In operation, after the melt is established (at a temperature of about 2050°C), the seed crystal is dipped into the surface of the melt along the line 38 which should be at a supercooled temperature of about 2030°C. the line 38 is visible as a dark line on the surface of the melt. The seed crystal can be an elongated Vemeuil grown single crystal rod about 1/8 inch in diameter with the c-axis of the seed crystal perpendicular to the line 38. The a-direction of the seed crystal will extend down into the melt and the m-direction will extend along the axis of the line 38. Crystal growth will readily occur in the m-direction because the growing crystal will find a supercooled region extending in this direction. Crystal growth will occur less readily in the c-directiori because the growing crystal will soon extend past the supercooled region along the line 38 into a higher temperature portion of the melt where crystal growth will not occur and where crystallized material will remelt. The width of the crystal will increase therefore at a faster rate than its breadth. As growth occurs, the crystal is pulled from the melt in the a-direction until a body of the desired width and length is obtained.

As the crystal grows, the latent heat of crystallization is absorbed by the melt raising the temperature of liquid at the crystal interface and tending to terminate the growth must therefore be reduced. The power input to the coils is gradually reduced throughout the crystal growing process to maintain the proper crystal growing invironment. It has been found convenient to maintain a more or less uniform pulling rate during the entire crystal growing process and to reduce the heat input to the system to control the process. A withdrawal rate of ¼ of one inch per hour has been found suitable.

In the initial phase of the crystal growing process, represented in FIG. 1 by the portion of the crystal body designated Li, the crystal is brought out to its desired width by reducing the heat input in stages, each of which allow large increases in width accompanied by relatively small increases in breadth. When the step-like gradations on the upper portion of the crystal has reached its maximum width W, the thickness should be no more than the maximum thickness T which can pass through the opening in the lid.

Once the crystal has reached its maximum width, the phase of crystal growth represented in FIG. 1 as the effective or usable portion of the crystal, designated Le, is started.

At about this point, there is considerable loss of heat from the melt due to escape of radiant energy up the transparent crystal, but there is a reduced amount of molten material in the melt and increased radiation of heat from the sides of the crucible with the need for still further reduction in heat input to the crucible from the heating coils. The power delivered to the heating coils is therefore reduced still more in gradual steplike reductions so as to favor rapid growth of the crystal in can occur is maintained essen ially the same whereby no further increases in width or thickness occur during this phase of the crystal growing process.

The actual rate of reduction in power supplied to the heating coils depends on the size of the crucible and crystal to be grown and can be determined by trial and error manipulation of the system. Once determined for a particular system, the proper changes can be programed so that the system largely runs itself, or visual observation and control can be performed. n the final phase of the operation, represented in FIG. 1 by the portion of the body designated L^, the amount of melt has been substantially depleted and the power input to the heating coils can be reduced considerably and then turned off. The crystal will grow rapidly In the tapered step wise fashion shown in the drawing until the bottom of the crystal touches the floor of the crucible.

Instead of using a small seed crystal and allowing it to grow selectively to increase its width, it is also feasible to start the process with a long narrow seed crystal, for example a seed crystal having the shape 39 outlined in FIG. 5 and having its a-axis running perpendicular to its long side with the m-axis parallel to the long side. This seed crystal is dipped into the melt along the length of the elongated region and rapid growth in the a-direction is Immediately initiated by proper control of heat input and suction rate. In this way a crystalline slab of maximum width is produced without the truncated top portion 40 shown In the drawings and designated As shown in FIG. 1 the crystalline body has horizontal lines 41 which represent any growth lines which might occur in the crystal. Growth lines as used herein means all optical inhomogenl les which may be produced at the growth interface due to slight changes in growth rate through variations in melt temperature or pull rate due to localized changes in dopant or impurity concentrations occuring during growth because of temperature changes or changes in pull rate, as well as lattice strains or bubbles resulting from such changes in dopant or impurity concentrations, or due to changes in growth conditions. By proper control of the crystal growing process, such growth lines can be kept to a minimum, but whatever growth lines dvte do occur, they will tend to have a substantially horizontal axis as suggested in FIG. 1. Slabs cut from as grown bodies produced by the process will also have any growth lines which are present occuring substantially in planes perpendicular to the a-axis of the body. The as-grown slab 10 may have some small angle mis-orientations 57 showing up on the surface of the slab. These ridge like protuberances can be eliminated by polishing, leaving a face of high optical quality. These misorientations are due to nonuniform thermal gradients and can be limited by careful control of the heating system.

FIG. 6 shows a suitable crystal growing station for performing the process described herein. The station comprises a platform or table 42 having mounted thereon a glass bell jar 43 in which stands a support 44 formed of a cylindrical section having several cut out openings 45 around its sides. The- box lidded crucible Inside the insulation. The heating coll 47 surrounds the Insulation and has Its ends 48-49 passing through Insulated holes in the table 42 to a power supply unit 50. The suction tubes 36 and 37 from the crucible lid are connected to conduits 51 and 52 which run down through openings in the table to suitable gas drawing pumps. Purging gas which may predominantly be nitrogen with a small content of oxygen, is introduced into the bell jar from a conduit 53 which opens under the cylinder 44.

The purging gas flows into the bell jar through the openings 45 where it can be drawn into the crucible by the suction in the duct system around the Lid so as to cool the central portion of the melt and assist in maintaining the elongated crystal growing region in the melt. The excess purging gas escapes through an opening 54 in the top of the bell jar. The crystal seed rod 55 also extends through this opening to the melt and a partially grown crystalline slab 56 is seen being pulled from the melt.

Using the apparatus illustrated herein and following the process steps described, a large monocrystalline corundum slab was pulled and had the configuration shown in FIG. 1. The slab had a length of about 6 inches, a width of about 3 3/8 inches, and a maximum thickness of about ½ inch. This slab was pulled from a rectangular crucible 3 Inches in depth and having a top opening of 5 by 3 inches. The lid which covered the crucible had a rectangular opening of about 4 by 1 Inches and had a duct system which overhung the lid opening by about 1/8 inch on each side, so the overall opening therefore was about 3 3/4 inches by 3/4 inch, through which the 3 3/8 inch by ½ inch crystal increasing the size of the crucible and the rectangular opening in the lid.

It is to be understood that the new features of process operation and the apparatus disclosed herein may be employed in ways and forms different from those of the preferred embodiment without departing from the spirit and scope of the appended claims Additionally it is to be noted that the method and apparatus of this invention is applicable to the growth of other crystalline materials which have growth rates selectively greater in one crystal direction than in another.

Claims (8)

31601/2 . CLAIMSI

1. An as-grown, produced lay growth on a seed crystal from a*tmelt» massive, monocrystalline, elongated corundum slab of substantially uniform thickness, the longitudinal axis of the slab being parallel to the a-axis of the crystal and the lateral faces of the slab lying in substantially flat, parallel basal planes, with any growth lines occurring in the crystal being substantially flat and perpendicular to the a-axis. o

2. A transparent noncrystalline corundum window Λ cut from the as-grown slab of claim 1, the fa£es of the window lying in parallel basal planes of the crystal.

3. A method or producing large monocrystalline bodies comprising establishing. a melt, of crystal constituents material, establishing a thin elongated region on the surface of said melt at a temperature suitable or crystal growth while maintaining regions immediately adjacent both sides and the ends of said region at a higher temperature not supporting crystal growth, dipping a seed crystal into the melt along the Elongated region and initiating crystal growth on the seed crystal, said seed cryst&l being oriented relative to t he melt and the elongated region therein with the direction of most rapid growth of the crystal extending vertically into the melt and with the planes of least rapid growth parallel to the elongated region, and pulling the crystal vertically from the melt as crystal growth occurs principally in the direction of pull and along the elongated region whereby a crystalline slab of substantially greater length and width than thickness Is produced, and continuing to pull the growing crystal from the melt until a slab of desired dimensions is produced.

4. The method of claim 3 wherein the crystalline body is corundum and wherein the seed crystal is oriented with an a-axis parallel to the direction of pull and its basal planes parallel to the elongated region.

5. The process of claim 3 wherein the melt of crystal constituent material is heated by an inflow of heat principally from its lateral sides and wherein the escape of heat from the melt is substantially limited to a central rectangular portion of the surface of the melt to form the thin elongated region of lower temperature supporting crystal growth.

6. The process of claim 5 wherein the thin elongated region of lower temperature in the central rectangular portion of the melt surface is formed by the escape of heat by radiation mainly from the central portion only of the melt surface and by an induced flow of cooling gas oyer that portion of the melt surface.

7. The method of claim 3 wherein the seed crystal is the end of an elongated monocrystalline corundum rod wherein the a-axis of the crystal is parallel to the longitudinal axis of the rod and the a-axis of the crystalline rod is oriented perpendicular to the longitudinal axis of the elongated region on the melt.

8. The method of claim 3 wherein the seed crystal is 3t601/2 an elongated monoeryatalllne body having a length no greater than the length of the elongated region on the melt surface, said elongated body being supported for dripping into the melt with the a-axls of the body extending into the melt and the basal planes of the body parallel to the longitudinal axia of the elongated region. i f VA.E. Mulford Attorney for Applicants