US3593775A

US3593775A - Heat transfer means in inviscid melt spinning apparatus

Info

Publication number: US3593775A
Application number: US815428A
Authority: US
Inventors: Wilbur J Privott Jr; Wallace C Lawrence
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1969-04-11
Filing date: 1969-04-11
Publication date: 1971-07-20
Anticipated expiration: 1988-07-20
Also published as: FR2043192A5; JPS4910623B1; CA945712A; DE2017260A1; GB1305643A

Abstract

The severe thermal gradients which are ordinarily present in the base of a crucible employed in high-temperature spinning operations are significantly reduced through utilization of a thermally anisotropic shelf. The bottom surface of the crucible base is positioned substantially parallel with the plane of high thermal conductivity of the shelf and in thermal conduction contact therewith. Concurrently, the crucible extrusion opening and an aperture within the shelf are aligned substantially coaxially. A means is employed for heating the outer periphery regions of the shelf, causing a heat flow toward the aperture. The substantially constant temperature along the thermal plane of the shelf and at the region of the shelf adjacent to the aperture therein preclude the formation of significant lateral thermal gradients which are responsible for fracture of the crucible base. Precipitation buildup near or in the extrusion hole is also significantly reduced by keeping the extrusion hole at least as hot as the surrounding melt.

Description

United States Patent [72] Inventors WilburJ. Privott, Jr.

' Raleigh;

Wallace C. Lawrence, Durham, both of, N.C. [2|] Appl. No. 815,428 [22) Filed Apr. 11,1969 [45] Patented July 20,197] [73] Assignee MonsantoCompany St. Louis, Mo.

[54] HEAT TRANSFER MEANS 1N INVISCID MELT [56] References Cited UNITED STATES PATENTS 2,532,389 12/1950 Batcheller 13/27 X 2,826,624 3/1958 Reese et al.. 13/27 2,826,666 3/1958 Cater 2l9/10.49 X

3,100,250 8/1963 Herczogetalr Primary Examiner'R. Spencer Annear I Att0rneysVance A. Smith, Russell E. Weinkauf and John D.

Upham ABSTRACT: The severe thermal gradients which are ordinarily present in the base of a crucible employed in hightemperature spinning operations are significantly reduced through utilization of a thermally anisotropic shelf. The bottom surface of the crucible base is positioned substantially parallel with the plane of high thermal conductivity of the shelf and in thermal conduction contact therewith. Concurrently, the crucible extrusion opening and an aperture within the shelf are aligned substantially coaxially. A means is, employed for heating the outer periphery regions of the shelf, causing a heat flow toward the aperture. The substantially constant temperature along the thermal plane of the shelf and at the region of the shelf adjacent to the aperture therein preclude the formation of significant lateral thermal gradients which are responsible for fracture of the crucible base. Precipitation buildup near or in the extrusion hole is also significantly reduced by keeping the extrusion hole at least as hot as the surrounding melt.

HEAT TRANSFER MEANS IN INVISCID MELT SPINNING APPARATUS FIELD OF .T HE INVENTION This invention relates to a high-temperature spinning operation with a means for preventing crucible fracture and precipitation buildup near or within the extrusion hole. More particularly, this invention relates to a thermally anisotropic shelf utilized in combination with a heating means for precluding formation of significant thermal gradients in a crucible base having an extrusion hole therein.

DESCRIPTION OF PRIOR ART As is well known in melt spinning, particularly in high temperatures involved in some melt-spinning operations, thermal gradients form along the base of the melt-containing crucible due to the heat loss down through the base with the greatest heat loss being near the extrusion opening. .At low temperatures, the small thermal gradients causing the stresses in the crucible base are ordinarily insignificant since the materials employed in crucible construction have physical characteristics which enable the crucible to withstand the stresses.

As the temperatures necessary to form a melt are increased, however, significant stresses may occur which are first ascertainable 'at the juncture between crucible wall and base. Because certain crucible materials are expensive and others are subject to chemical attack by the melt, economics and practicality may limit the materials utilized to those subject to fracture due to thermally caused stress. The problem of fracture has been alleviated partially by the advent of a two-piece crucible which allows unimpeded expansion of the crucible wall and bottom plate. The two-piece crucible has been found to perform satisfactorily with melts such as glass. When, however, the spinning operation utilizes a high-temperature melt (above l,000 C.) such as, for example, steel, fractures readily occur in the bottom plate of even two-piece crucibles due to the severe thermal gradients within the crucible base and/or supporting pedestal. The occurrence of thermally induced fracture is particularly in evidence when refractory materials such as ceramics are employed for crucible construction. As stated before, the thermal gradients which form within crucible base are due to the loss of heat through the base, primarily near the extrusion opening therein. The gradients within the pedestal are similarly caused by the loss of heat downward. Because the extrusion opening readily radiates heat downward and is often cooler than the surrounding melt, precipitation products may formnear or within the extrusion hole causing hole plugging. This latter effect is particularly evident when the equilibrium level of a reaction between the melt and crucible material is dependent upon the temperature of the melt.

It is therefore a primary object of our present invention to provide for a means which precludes formation of significant thermal gradients in the base of a crucible.

Another important object of our present invention is to provide for a means for maintaining the extrusion opening temperature at levels at which precipitation within or near the extrusion hole is kept minimal.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one embodiment of our present invention, we provide for a crucible having the base thereof in physical contact with a thermally anisotropic shelf. The plane of high thermal conductivity of the shelf hereinafter called the a-b" plane, isarranged so as to be substantially parallel to the bottom surface of the base. An aperture in the plane and directional routing of the heat along the base of the crucible base, ensure that the thermal gradients in the bottom plate are minimized. Concomitantly, because the heat is readily conducted to points on the upper surface of the shelf adjacent to the extrusion opening, the extrusion opening may be kept at an optimum temperature level which prevents precipitation buildup therein.

To prevent possible fracture due to thermal expansion of the crucible base and resistance thereto by the crucible wall, a two-piece crucible having a separate base and wall may be employed with the anisotropic shelf. Thus, the baseplate and the crucible wall may expand independently. Leakage therebetween is prevented by lapping the surfaces of the crucible wall and base which are in mutual contact and by using melt which is nonwetting with walls of the crucible.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may be best understood with reference to the following description, taken in connection with the appended drawings in which:

FIG. 1 is a vertical cross-sectional view of an apparatus for spinning a low-viscosity melt in accord with one embodiment of our present invention.

FIG. 2 is a vertical cross-sectional view of an apparatus for spinning a low-viscosity melt in accord with another embodiment of our present invention.

FIG. 3 is a top plan view of the flux concentrator seen in FIG. 2 in accord with our present invention.

FIG. 4 is a vertical cross-sectional view of an apparatus for spinning a low-viscosity melt in accord with still another embodiment of our present invention.

' DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a vertical cross-sectional view of an inviscid melt-spinning apparatus wherein a pedestal 10 supports a thermally anisotropic shelf 11 which in turn supports a susceptor l2 and crucible assembly 13. Crucible assembly 13 comprises baseplate 14, cover plate 15 and wall member 16. For the sake of brevity, shelf 11, susceptor l2 and crucible assembly 13 are discussed as being cylindrically shaped with crucible assembly 13 as being enclosed substantially coaxial within susceptor 12. It should be understood, however, that different configurations may be utilized when desired.

The use of inductive coils, such as coil 17, to heat the crucible and melt are well known. As illustrated, inductive coil 17 which serves to supply the energy necessary to melt the spinning charge in crucible assembly 13 forms a helix about shelf ll and susceptor 12. The source of alternating current utilized to create the appropriate electromagnetic flux is not shown. When activated, coil 17 induces or couples a secondary current into susceptor l2 and the periphery region of shelf 11. The heat generated by the induced alternating current is radiated and/or conducted from susceptor 12 to crucible assembly 13 causing the charge to melt.

As stated before, integral crucible structures are susceptible to thermal fracture, particularly in the case of large crucible structures operating at temperatures needed for inviscid melt spinning. Small crucibles exhibit greater resistance to thermal fracture due to the ease of maintaining uniform temperatures throughout. Small crucible structures however, may not be practical for various reasons. For example, the size unduly restricts the number of extrusion holes that may be employed. Further, small integral crucibles are susceptible to the undesired thermal fracture although to a lesser degree as stated above.

Unified crucible assemblies also suffer from a further disadvantage in that thermal structural failure of any part results in complete loss of the assembly. The unified assemblies are particularly susceptible in the region of juncture between the crucible base and wall. The base and wall mutually impede We have found it advantageous, therefore, to employ a twoiece meltspinning crucible when spinning nonwetting melts such'as that in FIG. 1. As seen therein, wall member 16 is positioned upon a base which in this instance comprises both baseplate l4 and cover plate 15. Because wall member 16 and the base (referringto both baseplate l4 and cover plate expand or contract freely, the thermal stress noted at the juncture of wall and base in integral crucibles is not present. Inviscid melts (used to describe melts having viscosities g1 poise) have a propensity to seepbetween w'all member 16 and the base. To preclude the necessity of expensive and bulky clamps, we have found it convenient to provide lapped matting surfaces for wall member 16 and the base in the area of mutual contact. Thus, the weight of wall member 16 is sufficient to maintain a tight seal in the contact area. Clamps, however, may be used if desired.

The enclosure defined by crucible assembly 13 encloses inviscid melt 18. A cover plate 15 functions to secure baseplate l4 and an orifice insert member when utilized as discussed below. Cover plate 15 may also function to help prevent leakage between wall member 16 and baseplate 14. The term crucible base" is therefore used to define the entire base of crucible assembly including those instances in which the base is a single member or a plurality of members. Although crucible 13 is illustrated and discussed as being a two-piece" crucible, it should be understood that this is in reference to the wall and base being separate elements. Crucibles may be constructed which have a multiplicity of parts and still be two a piece crucibles in the above definition.

Baseplate 14 has an opening" 19 therein which may be adapted to receive a separate insert member defining a' spinning orifice or, alternatively, maybe a spinning orifice itself. The size of the extrusion orifice is dictated primarily by the diameter of fiber desired. As depicted in FIG. 1, opening 19 is aligned substantially coaxial with conically shaped aperture" 20 in shelf ll which in turn communicates with chamber 21 defined by'pedestal [0. Thus, melt 18 when forced from crucible assembly 13, streams unimpeded through aperture" 20 into chamber 21. To facilitate simplicity, baseplate 14 is illustrated as having a single opening 19, though it is to be understood that a base with a plurality of openings in a chosen pattern may be employed when desired to extrude a plurality of fibers. The word-opening" is used herein and throughout the disclosure to broadly cover either an orifice or an opening adapted to receive an orifice insert. The word aperture is also employed throughout the disclosure in reference to the passage through the anisotropic shelf.

lt is desirable to use ceramic materials in the construction of crucibles when spinning some inviscid materials, for example,

copper andiron. Such ceramic materials may be, for example, alumina, beryllia, and zirconia. These are typical refractory materials having relatively low thermal conductivities, high moduli and thermal expansion coefficients, and low tensile strengths. Consequently, when not heated uniformly, ceramic materials are subject to fracture from thermal stress. in hightemperature inviscid-metal-spinning as, for example, in

copperor steel-spinning operation, it is ordinarily most convenient to employinductive heating as depicted in FIG. 1 in order to reach temperatures sufficiently high enough to obtain a melt. When, for example, the member supporting the cruelble assembly has a significant thermal gradient across it, the uneven. heat distribution along the bottom surface of the crucible base'frequently causes thermal fracture thereof.

We have found it highly advantageous to utilize the physical characteristics of thermally anisotropic materials to solve the problem-as detailed above. Examples of thermally anisotropic materials are pyrolytic boron nitride and pyrolytic graphite. For pyrolytic graphite, the thermal conductivity in one planar direction is several orders of magnitude higher than in a direction normal to that plane. The unique thermal property 'of this material is believed to be caused by unusually high degree of preferred orientation of the crystallites. The crystallites tend to have their basal planes aligned parallel in one direction. The high thermal conductivity is in this direction (a-b plane).

By providing a shelf ll of thermally anisotropic material such as, for example, pyrolytic graphite commercially available from Pyrogenics Inc., located in Woodside, New York, we are able to significantly reduce the thermal gradients caused by heat loss through the orifice opening in baseplate 14, thereby prevent thermal shock. The bottom surface of the baseplate is arranged substantially parallel to the a-b plane orientation in shelf 11. Because inductive coil 17 is coupled to the periphery regions of shelf 11 adjacent the coils, heat is induced therein which flows along the a-b plane toward aperture 20 as denoted by arrows 22. The temperature is almost constant along the a-b plane, therefore providing equal heat distribution to baseplate 14 along its bottom surface.'Since the top surface of the crucible base is also uniformly heated by melt 17, the fomration of significant temperature gradients is precluded, preventing thermal fracture from occurring in the crucible base.

The bottom surface of baseplate 14 adjacent to opening [9 acts as a radiant body and radiates heat downward through aperture 20, causing considerable heat loss which results in opening 19 and the immediately surrounding melt to have a relatively lower temperature than the remaining melt. Precipitation products often fonn adjacentto or within opening 19 causing plugging. This problem takes on particularly significant proportions when the melt employed and the baseplate are in a chemical equilibrium and the equilibrium is temperature dependent. It is also preferable to have the orifice opening at least as hot as the melt to facilitate streaming (low viscosity)and rapid formation of a stabilizing film. ln inviscidmelt-spinning, it is important that the jet of free-streaming melt react quickly with a stabilizing gas to form a film to prevent surface tension breakup. The rapidity of the reaction ordinarily increases with increased temperature of the molten stream. a

It is desirable, therefore, that the temperature of shelf 11 in the region immediately adjacent to opening 19 be maintained at levels which most effectively prevent precipitation buildup. As may readily be seen from a cursory examination of FIG. 1, the nearest points in shelf 11 to opening 19 are in the upper surface of shelf 11 in the region adjacent to aperture 20. Thus, due to the anisotropic properties of shelf 11, the heat which is readily conducted along the a-b plane direction thereof protive coils to accomplish the above serves only to increase the a temperature of the melt. This may be objectionable in that a particular material employed for a melt may have an increasing propensity for reacting with the crucible with increasing temperatures. Furthermore, because the temperature of the entire spinning apparatus is raised through this procedure when utilizing a single inductive coil for heating both susceptor and shelf, the temperature difference between the orifice opening is not eradicated, thus increasing the possibility that reaction products may precipitate into the opening.

We have found it profitable to space'the turns of the inductive coil closer together i.e., decrease the pitch, in the immediate vicinity of the anisotropic shelf which increases the density of electromagnetic flux lines intersecting the periphery regions of the shelf. This in turn raises the temperature of the region adjacent the shelf aperture which, as stated before, contains the points on the shelf closest to the extrusion opening of the crucible.

By again referring to FIG. 1, the compactness of coils 17 adjacent helices shelf ll may be observed. The interval between turns may be varied for the results desired which in the case of a steel melt may be, for example, l/ 16-inch interval adjacent shelf 11 and l/8-inch interval adjacent to susceptor 12. The frequency utilized to couple coils 17 to shelf ll and susceptor 12 is chosen, among other things, to be commensurate with the dimensions of the objects to be heated, but may be, for example, kHz.

Though it is preferable to have the anisotropic shelf in physical contact with the crucible base, it may be desirable under some circumstances that the anisotropic shelf not be in physical contact with the base of a crucible. For example, it is advantageous to prevent the reactive or stabilizing gas from immediately reacting with the melt as it leaves the extrusion opening. In so doing, it may be necessary to separate the member having the extrusion opening from the anisotropic shelf as illustrated in FIG. 2 and explained below. In the illustrated apparatus, the melt is separated into several compartments or reservoirs for greater temperature control. The lower reservoir holding a smaller amount of melt is easier to maintain at a specific temperature, which temperature is ordinarily higher than the melt temperature of the upper reservoir. Such a temperature difference between reservoirs is desirable to further inhibit precipitation buildup in the extrusion opening. In this arrangement, pedestal 30 supports a pair of thermally

anisotropic shelves

31, 32. Thus, pedestal 30 being thermally insulated by shelves 3], 32 may be comprised of stainless steel. Melt-stabilizing gas separating member 33 is utilized to reduce immediate contact between the stabilizing gas and the nascent melt leaving the lower reservoir 34 is supported by shelf 32 and in turn supports the base 35 of crucible 34 (lower reservoir), thus, separating shelf 32 from base 35.

Pedestal 30,

shelves

31, 32 and member 33 define a conical chamber 37 which communicates at the upper end thereof with the opening 36 in crucible 34. Shelf 38 which may be comprised of ordinary graphite (to ensure proper thermal expansion snatch) serves to position member 33 and to securely support upright wall member 39 of crucible 34 and susceptor 40. Base 42 and wall member 43 define upper reservoir 41.

As stated above,

shelves

31, 32 serve to insulate pedestal 30 from the high temperatures employed in the spinning aperture. Thus, it should be understood that it is advantageous to restrict heat flow to the upper portion of shelf 32. Ideally, it is desirable to limit the flow only to the upper surface of shelf 32 since any heat flow in regions below the surface serves no useful purpose. Furthermore, it is advantageous to increase the density of flux lines intersecting the periphery regions of shelf 32 in order to promote proper heating of orifice opening 36. Concentration of flux lines in relatively large regions of an object to be heated may be accomplished adequately through a decrease in coil pitch as illustrated in FIG. 1. There are, however, limitations to the minimum pitch which may be employed. That is, the diameter of the inductor coil is ordinarily constant, regardless of the change in pitch, and becomes a factor when a further decrease in pitch is needed in order to induce heat into a relatively narrow region of the shelf. Watercooled coils are particularly limiting since a certain rate of water flow is needed for proper cooling. The rate of water flow largely dictates minimum requirements for internal coil diameters.

Inductive coil 44 which helically encircles susceptor 40 is utilized as an element in a flux concentrator 45 for concentratingflux into the upper periphery region of shelf 32. Flux concentrator 45 comprises a plurality of turns of coil 44 in conjunction with a washerlike conductor member 46, which substantially encircles the upper periphery region of shelf 32.

Inner turn 47 is preferably mounted along its length to and in electrical contact with member 46 which acts as a support and in turn is cooled by the water-cooled inductor coil 44. Thus, it is preferable that member 46 be made of a material, copper, for example, which is not only an electrical conductor but a good thermal conductor also. As can be seen in FIG. 2, turn 47 is fixed to member46 while outer turns 48, 49 are displaced slightly above member 44 to facilitate electrical insulation though other insulating means may be employed when desired. Alternatively, inner turn 47 may be similarly displaced above but in coupling relation with member 46.

FIG. 3 which is a top plan view of flux concentrator 45 without the spinningassembly depicts a gap 50 in conductor member 46. Inner turn 47 extends along the circumference thereof from one side of gap 50 to the other side. When coil 44 is energized, outer turns 48, 49 couple to member 46. The flux around turn 47 is increased due to this coupling effect; consequently, the concentration of flux in the adjacent upper periphery region of shelf 32 is increased. Gap 50 functions to prevent a short circuit of inner turn 47.

The advantages of flux concentrator 45 are twofold. First, as stated before, it is possible to concentrate the heat-inducting flux into small regions. Second, only a nominal increase in longitudinal space requirements is needed. Although FIGS. 2 and 3 depict only three laterally orientated, spiraling turns of coil 44 along conductor member 46, it is understood that the number utilized may be increased or decreased as desired. Similarly, while it is convenient to heat susceptor 40 and the periphery region of shelf 32 with the same coil, it is not essential. Thus, in the illustration of FIG. 2, coil 44 and concentrator 45 may be powered by separate power units.

To further ensure that thermal profile of shelf 32 is continuous only in the upper regions, one or more slots such as annular slot 51 may be employed to interrupt heat transfer. Annular slot 51 which is substantially coaxial to the longitudinal axis of conical chamber 37 as defined by shelf 32 is cut substantially normal to the a-b of shelf 32, thus, interrupting the heat transfer in the lower regions thereof. Arrow 52 denotes the continuous thermal profile above slot 51.

The heat generated in shelf 32 by flux concentrator 45 is conducted along the a-b plane in the region near the upper surface of shelf 32. When the heat reaches the interface between shelf 32 and member 33, it is conducted by member 33 up to the crucible base 35 and to the region immediately adjacent opening 36. It should be noted that member 33 being not anisotropic conducts heat equally in all directions. Thus, the temperature along base 35 is maintained substantially constant and the temperature level at opening 36 may be regulated to a level most conductive to efficient spinning operation.

As may be observed, shelf 32 though not in physical contact with crucible base 35 is in thermal conduction contact" therewith. For purposes of description herein, thermal conduction contact is used to define either contact between a pair of objects which are physically touching or mutually touching an object which conducts heat therebetween.

Member 33, therefore, also serves as a thermal conductor between shelf 32 and base 35. It is preferably that member 33 made of material like, for example, graphite which has a good thermal conductivity. When, however, a high-temperature melt is being used such as steel, the close proximity of the freestreaming melt may make it more practical to employ material such as, for example, beryllia for the construction of member 33.

As in the embodiment of FIG. 2, it may be convenient or necessary to reduce the immediate contact between the melt and stabilizing gas when the melt streams into the spinning chamber. Because of the reactive nature of some stabilizing gases employed in the spinning of selected inviscid melts, a troublesome situation arises when the stabilizing gas reacts due to the presence of a heated object such as the melt-stabilizing gas separating member and/or walls of the chamber. Thus, it is important to maintain the temperature of the chamber walls, particularly the region thereof in contact with the melt-stabilizing gas separating member, at a level that precludes significant and undesirable stabilizing gas reactions.

FIG. 4 is an illustration of still another embodiment of our present invention, which readily allows the above condition to be met. As seen therein a pedestal 60 supports two thermally

anisotropic shelves

61, 62. Shelf 62 is cylindrical in shape with the lower portion thereof having a reduced diameter. Said another way, the upper portion or extension 63 of shelf 62 extends laterally beyond the lower portion. Pedestal 60 and shelves 6], 62 define a chamber 64 into which the inviscid melt streams. The a-b planes of

shelves

61, 62 are aligned essentially normal to the longitudinalaxis of the spinning ap-' paratus thereby insulating pedestal 60 from the spinning temperatures. Thus pedestal 60 may be fabricated from a material such as stainless steel, for example.

Shelf 62 has a cylindrical-shaped aperture or bore centered in the upper region thereof which has a depth essentially the same as the thickness of extension 63. The upper end of chamber 64 defined by shelf 62 communicates with the bore. A member 65 with an extrusion opening 66 is centered in the bore of shelf 62 by an anisotropic centering ring 67. The thickness of member 65, ring 67, and the depth of the bore of shelf .62 have been greatly exaggerated to permit ease of description. Ring 67 is essentially circumscribed by shelf 62 and has its a-bplane' parallel to the a-b plane of shelf 62. Thus, ring 67 may be considered to be an extension of shelf 62.

A melt-stabilizing gas separating member 68 is secured to shelf 62 at the upper end of chamber 64. Member 68 functions to maintain'a minimum degree of contact between the stabilizing gas'and the nascent melt as it passes through member 68.

A susceptor 69 is secured to extension 63 and encloses a crucible 70 having a base 71 supported by shelf 72. Crucible wall 72 and base 71 define a container or upper reservoir for melt 73. The center of base 71 and member 68 defines a lower reservoir having a smaller capacity..As stated previously, the temperature of the melt in the lower reservoir is ordinarily kept higher than the melt in upper reservoir.

Susceptor 69 is circled by a helically wound inductor coil 74, the lower turns of which are utilized as part of a flux consource (notfshown). The electromagnetic flux lines couple with the periphery region of extension 63 (and susceptor 69).

As indicated by arrows 77, heat is transferred along extension 63 to member 65. Because the lower portion of shelf 63 is not coupled to flux concentrator 75, the walls of chamber 64 bordering the lower region and consequently member 68 are easily maintained at temperature levels commensurate with proper operation of the spinning apparatus. As before, the temperature gradients along the base of crucible 70 are substantially eliminated. The extrusion opening 66 is kept at least as hot as the adjacent melt due to the thermal conduction characteristics of shelf 62.

Thespinning apparatus of FIGS. 2 and 4 are similar to that of FIG. 1 so that in the following operative summation, the apparat'us of FIG. '1 is used to facilitate brevity. Initially to melt the spinning charge, the heat is induced into susceptor l2 and the periphery region of thennallyanisotropic shelf 11. The plane of high thennal conductivity of shelf I1 is oriented substantially parallel to the bottom surface of baseplate l4 and of the anisotropic properties of shelf ll. The constant temperature precludes significant temperature gradients from developing the baseplate 14 thus preventing possible thermal fracture. Concurrently, heat is transferred to those points on the surface of shelf I 1 closest to the opening 19 which tends to be the coolest spot in crucible 13. The proximity of the hot surface of shelf 11 to opening 19 is utilized to maintain temperature of opening 19 at an optimum level. Such a level may be at or above the temperature of the surrounding melt.

The configuration of the anisotropic shelf may be utilized to maintain heat transfer only in those regions closest to the upper surface of the shelf. In one configuration slots may be employed extending normally up through the 04; plane of the shelf to the desired region of heat transfer. Another configuration is in the form of a lateral extension of the upper shelf region so as to allow only the extension to intersect the magnetic flux lines of the inductor coils. Alternatively, a flux concentrator may be employed to concentrate the flux lines in the upper periphery region of the anisotropic shelf. In any of the above mentioned arrangements the result is the same-heat transfer only in the upper region of the shelf.

From the foregoing discussion, it is apparent that objects set forth have been obtained. Therefore, the invention having been set forth with respect to certain examples, it is believed thatmany modifications and changes thereof will be within the purview of those skilled in the art. Accordingly, by the appended claims, we intend to cover all such modifications and changes as full within the true spirit and scope of the present invention.

We claim: I I v I. In an apparatus including a crucible assembly for spinning an inviscid melt as a free stream, the combination of:

a member having an extrusion opening therein; an anisotropic shelf having an aperture which is positioned substantially. coaxial with said extrusion opening, said shelf having the plane of high thermal conductivity extending from the periphery region of said shelf to said aperture;

heating means for heating the periphery region of said shelf; and wherein the region of said shelf adjacent the aperture is in thermal conduction contact with the region of said member adjacent said extrusion opening.

2. In the apparatus of claim 1, said shelf comprising pyrolytic graphite.

3. In the apparatus of claim 1, said member being the base of a crucible for containing the inviscid melt, said thermally anisotropic shelf physically contacting essentially the entire bottom surface of said crucible base and having the plane of high thermal conductivity substantially parallel to said bottom surface.

4. In the apparatus of claim 3, said crucible further comprisceramic material at least in the area of contact with the melt.

6. In the apparatus of claim 5, said crucible being made from a material selected from the group consisting of alumina, beryllia, and zirconia. I

7. In the apparatus of claim I, the region of said member adjacent said extrusion opening and the region of said shelf adjacent said aperture mutually contacting a thermal-conducting second member. i r

I 8. In the apparatus of claim 1, said shelf and said member being substantially cylindrical in shape with said shelf circumscribing said member.

9. In the apparatus of claim 1,

a crucible,

a susceptor encircling said crucible, said heating means being an inductive coil in a helical configuration about said susceptor and the outer periphery region of said shelf, said inductive coil further having turns spaced closer together adjacent said shelf than adjacent said susceptor.

10. In the apparatus of claim I, said shelf being provided with an essentially annular slot in the lower surface thereof ing an inductive coil and an electromagnetic flux concentrator positioned adjacent the upper periphery region of said shelf so as to increase the density of flux lines intersecting said upper periphery region.

13, In the apparatus of claim 12, said flux concentrator further comprising a washerlike conductor member substantially encircling the upper periphery region of said shelf, said conductor member having a gap extending therethrough, and

a plurality of turns of said coil positioned along one surface of said conductor member in a substantially coplanar arrangement, the inner turn of said coil being mounted on and in electrical contact with 'said conductor member from one side of said gap to the other side, the outer turns of said coil being electrically insulated from said conductor member and being coupled to said conductor member when said coil is electrically energized. 14. In a spinning apparatus for spinning an inviscid melt through an extrusion opening in a member wherein the portion of said member containing said extrusion opening is in thermal-conducting contact with {thermally anisotropic shelf having an aperture substantially coaxial with said extrusion opening.

Claims

1. In an apparatus including a crucible assembly for spinning an inviscid melt as a free stream, the combination of: a member having an extrusion opening therein; an anisotropic shelf having an aperture which is positioned substantially coaxial with said extrusion opening, said shelf having the plane of high thermal conductivity extending from the periphery region of said shelf to said aperture; heating means for heating the periphery region of said shelf; and wherein the region of said shelf adjacent the aperture is in thermal conduction contact with the region of said member adjacent said extrusion opening.

2. In the apparatus of claim 1, said shelf comprising pyrolytic graphite.

4. In the apparatus of claim 3, said crucible further comprising an upright wall member separate from and supported by said base, said upright wall member and said base defining a container for the low viscosity melt and having lapped surfaces in the area of mutual contact to prevent leakage of the inviscid melt therebetween.

5. In the apparatus of claim 4, said crucible being made of ceramic material at least in the area of contact with the melt.

6. In the apparatus of claim 5, said crucible being made from a material selected from the group consisting of alumina, beryllia, and zirconia.

7. In the apparatus of claim 1, the region of said member adjacent said extrusion opening and the region of said shelf adjacent said aperture mutually contacting a thermal-conducting second member.

8. In the apparatus of claim 1, said shelf and said member being substantially cylindrical in shape with said shelf circumscribing said member.

9. In the apparatus of claim 1, a crucible, a susceptor encircling said crucible, said heating means being an inductive coil in a helical configuration about said susceptor and the outer periphery region of said shelf, said inductive coil further having turns spaced closer together adjacent said shelf than adjacent said susceptor.

10. In the apparatus of claim 1, said shelf being provided with an essentially annular slot in the lower surface thereof which is substantially coaxial with said aperture and normal to said a-b plane thereby preventing the transfer of heat across said slot.

11. In the apparatus of claim 1, the upper periphery region of said shelf extending laterally beyond the lower periphery region thereof and said heating means comprising an inductor coil positioned so as to have the electromagnetic flux lines intersect the upper periphery region of said shelf but not the lower periphery region of said shelf.

12. In the apparatus of claim 1, said heating means comprising an inductive coil and an electromagnetic flux concentrator positioned adjacent the upper periphery region of said shelf so as to increase the density of flux lines intersecting said upper periphery region. 13, In the apparatus of claim 12, said flux concentrator further comprising a washerlike conductor member substantially encircling the upper periphery region of said shelf, said conductor member having a gap extending therethrough, and a plurality of turns of said coil positioned along one surface of said conductor member in a substantially coplanar arrangement, the inner turn of said coil being mounted on and in electrical contact with said conductor member from one side of said gap to the other side, the outer turns of said coil being electrically insulated from said conductor member and being coupled to said conductor member when said coil is electrically energized.

14. In a spinning apparatus for spinning an inviscid melt through an extrusion opening in a member wherein the portion of said member containing said extrusion opening is in thermal-conducting contact with a thermally anisotropic shelf having an aperture substantially coaxial with said extrusion opening. heating means for supplying heat to said extrusion opening including an electromagnetic flux concentrator, said flux concentrator comprising a washerlike conductor member substantially encircling the upper periphery region of said shelf and having a gap therethrough and a plurality of turns of an inductor coil positioned along one surface of said conductor member in a substantially coplanar arrangement, said turns being coupled to said conductor member when said coil is electrically energized wherein said thermally anisotropic shelf has its plane of high thermal conductivity extending in a direction from said flux concentrator to said extrusion opening.