US2214976A

US2214976A - Apparatus for the manufacture of crystalline bodies

Info

Publication number: US2214976A
Application number: US249505A
Authority: US
Inventors: Donald C Stockbarger
Original assignee: Research Corp
Current assignee: Research Corp
Priority date: 1939-01-05
Filing date: 1939-01-05
Publication date: 1940-09-17
Anticipated expiration: 1957-09-17

Description

Sep 17, 1940- 0. c. STOCKBARGER 2,214,976

APPARATUS FOR THE MANUFACTURE OF CRYSTALLINE BODIES FiJed 5, 19:9 5 Sheets-Sheet 1 Power Lines Sep 17. 1940- D. c. STOCKBARG-ER APPARATUS FOR THE MANUFACTURE OF CRYSTALLINE BODIES Filed Jan. 5, 1939 5 Shggts-Sheei 3 awue/wfml Patented Sept. 17, 1940 UNITED STATES FFiQE Donald C. Stockbarger,

Belmont, Mass, assignor to Research. Corporation, New York, N. Y., a corporation of New York Application January 5, 1939, Serial No. 249,505

7 Claims.

This invention relates to the manufacture of crystalline bodies from fused material and has for its object the provision of apparatus for the production of strong crystalline bodies of great purity and of desired size. The invention makes possible the production of a new article of manufacture heretofore unknown.

In accordance with the invention the molten mass to be crystallized is moved at a controlled 10 rate from a region hotter than the solidification temperature of the mass to a region cooler than the solidification temperature of the mass. At the same time a sharply localized zone of steep temperature gradient is maintained between the relatively hot and cool regions. Consequently the temperature of the edge portion of successive layers of the mass corresponds to the solidification temperature of the mass as these edge portions reach a substantially fixed location in the path of travel of the mass and solidification begins and progresses inwardly. Purer crystals are obtained when this temperature gradient is maintained'as steep as possible, and better crystals are obtained when the zone of solidification approxi- 26 mates a plane. In certain instances, such as when the thermal conductivity of the mass being solidified is small or the size of the crystal being produced is large, it is desirable to control the rate of heat flow through the inner portion of the 30 mass from the hotter to the cooler region. Thus if the rate of heat flow through the inner portion of the mass is too slow the zone of solidification tends to be concave. By properly controlling the rate of heat flow through the inner portion of the mass the zone of solidification may approximate a plane.

Substances which heretofore could not be crystallized to provide bodies of sufiicient strength and purity to make them available for practical scientific and commercial uses may be crystallized by the practice of the invention to provide crystalline bodies of commercial size and of such strength and purity that they may be shaped and used for various purposes. Thus the invention makes possible the production of strong crystalline bodies of substances which have heretofore been available only as natural crystals and of substances which do not occur in the form of natural crystals and which are relatively free' of inclusions. Fused lithium fluoride is an example of a material which may be crystallized in accordance with the invention to provide bodies of desired size and of such quality that they may be V shaped to' render them valuable for practical scientific and commercial uses. Crystals made in accordance with this invention have a high degree of purity which increases their value. Other materials may be crystallized by the practice of the invention such as sodium chloride.

Crystalline bodies of lithium fluoride produced by the practice of my invention are themselves new and are characterized by optical properties not possessed by any other known optically useful solid material obtainable in practical size. Such bodies are characterized by unusually low dispersion in the visible region and by transmission of ultra-violet radiation or ultra-violet light rays having a wave length at least as short as 0.11 micron.v The difference in index of refraction for any two wave lengths, which is a measure of the dispersion for these wave lengths, is small for lithium fluoride. In the trade the dispersive power of any optical substance is expressed as NF=index of refraction for light of wave length 0.4861 micron;

Nn il'ldBX of refraction for light of wave length 0.5893 micronf Nc index of refraction for light of wave length 0.6563 micron;

*Mean Wave length for the doublet.

The quantity NFNC is known as the mean dispersion.

(Ref: Hardy and Perrin, The Principles of Optics, pp. 113, 117, and 118.)

11 for lithium fluoride produced by the practice of my invention is found to lie between 99 and 100.

1 for optical glass may lie between 20 and 17, depending on the composition of the glass.

The invention will be more clearly understood from the following description in. conjunction with the accompanying drawings, in which:

Fig. l is a diagrammatic view of a suitable apparatus for the practice of the invention;

Fig. 2 is an elevation view, partly in section, of a portion of the apparatus;

Fig. 3 is a detail sectional view of one of the parts of the apparatus;

Fig. 4 is a detailperspective View of another portion of the apparatus;

Fig. 5 is a sectional view taken upon. the line 5-5 of Fig. 2;

Fi 6 is a e cvational View, partly section.

of a modified construction of a portion of the apparatus corresponding to the portion illustrated in Fig. 2; and

Fig. '7 is a detailed sectional view of one of the parts of the apparatus shown in Fig. 6,

Before explaining in detail the present invention it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of be ing practiced or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation, and it is not intended to limit the invention claimed herein beyond the requirements of the prior art.

A suitable apparatus for use in practice of the invention is illustrated in the accompanying drawings and comp-rises a support it] carrying a frame H for supporting a furnace H2. The furnace l2 comprises an annular casing having an outer cylindrical wall l5, top wall IE, bottom wall H and an inner cylindrical wall i8 which is supported in position by a fiange I9 extending outwardly from its upper end and resting upon the top wall 3. The inner wall l8 defines the outer limit of the chamber 20 within the furnace which is closed at its upper end by a cover plate 2! which may carry suitable insulation 22.

Heating coils

23 and 24 surround a tubular core 25 and are separated from the outer cylindrical wall l5 by suitable insulation 26. The chamber 29 is divided into an upper chamber 28 and a lower chamber 29 by an annular metallic ring 38 carried by the inner wall l8 and positioned in the plane between the adjacent ends of the

coils

23 and 24. Preferably, the upper surface of the annular ring 30 is covered with a thin polished platinum sheet 3| to deflect heat and maintain a sharp temperature gradient at the plane of di vision between the two chambers of the furnace. The coil 24 is connected through

leads

33 and 34 to asuitable source of electrical energy and the coil 23 is connected by leads 35 and 36 to a suitable source of electrical energy. The furnace wall may be provided with an opening or window (not shown) at the level of the ring 38 through which the interior of the furnace may be observed and through which a rod or feeler may be inserted to contact the wall of the crucible 49 for the purpose of determining the location of the freezing zone. When the crucible is made of, e, g., thin platinum the feel of a rod pressed against it with the fingers is sufficient to enable the operator to determine quite accurately the boundary between the solid and liquid contents of the crucible.

A crucible 40, adapted to contain the liquid to be crystallized, is carried by a support 4| which is fixed upon a rod 42, the lower end of which is suitably secured to a rack bar 43 longitudinally movable in suitable guides M and 45 fixed upon the support 10. The passage in the annular ring 30 is such as to permit the crucible 40 and the support ll to move therethrough without excessive clearance. The rack bar 43 is engaged by a pinion 46 which is actuated by a synchronous clock motor 4'! through speed reduction gearing 48. The crucible 50 should be made of material, e. g., platinum, not attacked by the substance being crystallized and may have any desired shape to correspond to that of the crystalline body to be produced. It should be thin Walled, e. g., 0.003 inch thick, to insure a low rate of heat conduction in the vertical direction. If the crucible wall conducts heat too rapidly the temperature gradient within the freezing layer may be less than desired. The bottom of the crucible should be tapered to a point at which the crystallization begins, and the top surface of the support M is preferably of shape complementary to that of the bottom of the crucible and is coated with a layer 49 of alundum cement, through which the upper end of the rod 42 extends to engage the point of the crucible All.

A sleeve 59 is adjustably mounted upon the rod 32 and is held in desired position thereon by a clamp 51. A disk 52, having a hole through which the rod 42 extends, is fixed upon the upper end of the sleeve 59. The disk 52 may be brass and its upper surface, preferably, is blackened. A pancake cooling coil 53 is soldered to the lower surface of the disk 52 and is provided with a wa: ter inlet 5 and outlet 55. In order to control the rate of flow of heat through the inner portion of the mass in the crucible and to cool the upper portion of the rod 52 and provide a cold spot at the tip of the crucible, a heat conducting split tube 56 having complementary portions 5'! and 5B is held upon the rod 42 by a clamp 59. The

portions

51 and 58 of the split tube 56 are provided each with an outwardly extending flange 60 adapted to rest upon the disk 52.

In the practice of the invention power is supplied to the

coils

23 and 24 so as to maintain the upper chamber 28 at a temperature above the solidification temperature of the liquid to be crystallized and to maintain the temperature of the chamber 29 below the solidification temperature of the liquid to be crystallized. In order to control and maintain the desired temperature within the chamber 28 a temperature regulator may be provided comprising a thermal couple i2 projecting into the chamber and electrically connected by a lead l3 to one terminal of a galvanometer M and by a lead 15 to the slide wire 15 of a potentiometer Ill. The other terminal of the galvanometer I4 is connected by a lead F8 to the potentiometer H. The galvanometer i4 is provided With a mirror adapted to reflect light rays from a lamp M on or off a photronic cell 82 depending upon the position of the galvanometer mirror. A lens 83 is interposed between the lamp 8i and the mirror 89 so as to concentrate the rays upon the latter. A light chopper 84 in the form of a perforated disk is interposed between the lens 83 and the lamp 9i and is carried upon the armature shaft of a motor 85 which is connected through the leads 86 and 87 to a source of electrical energy. The lamp 8| is connected by leads S8 and 89 to the leads 8'! and 86 respectively.

The photronic cell 82 is connected by leads 90 and 9! to an amplifier 93 which in turn is connected by leads 94 and 95 to a rectifier 96. The rectifier 96 is connected by leads 91 and 98 to a master relay 99 which is connected by leads I99 and ID! to the terminals of a single relay I02. The leads W9 and Illl are connected by'leads I03 and I04 respectively to a double relay I05. The relay I05 is connected by leads I06, I01, and N18 to a demand regulator I NJ. The lead 36 from the coil 23 is connected to the demand regulator which in turn is connected by a lead I l I to a source of electrical energy for heating the coil 23. The relay I92 is connected by leads H2 and M3 to the lead 36 and lead Hi respectively.

In order to permit the temperature within the chamber 28 to be increased or decreased, the slide which is rotated by a synchronous motor Il'l through suitable reduction gearing I 18. The synchronous motor I I1 is connected by leads I I9 and I20 to a suitable source of electrical energy.

The potentiometer 11 is set so that the galvanometer mirror 80 reflects light from the lamp 8| on the light-sensitive cell 82 when the temperature of the couple 12 is adjusted as desired. Because of the intermittent character of the light reaching the cell 82, due to the action of the light chopper 84, the electrical response of the cell 82 contains an alternating current component which is amplified by the amplifier 93 to produce alternating current power. This power is suitable, after rectification by the rectifier 96, for operating the master relay 99. The master relay 99 controls the operation of the relays I02 and I05. The relay I05 controls the direction of motion of a motor-driven rheostat called demand regulator H0 in such a manner that when no chopped light falls on the cell 82, the demand regulator I I!) slowly decreases the amount of current flowing through the heating coil 23; whereas, when a predetermined amount of chopped light falls on the cell 82, the current flowing through the coil 23 is slowly increased. The relay Hi2 acts with the demand regulator I H3 to decrease or increase the current flowing through the coil 23 by disconnecting or connecting, respectively, a suitable resistance R1 across thecombination comprising the demand regulator I it! and the series resistance R2. The action of the relay W2 is nearly instantaneous 'so that practically as soon as the galvanometer mirror 80 has deflected sui ciently to send a small amount of chopped light to the cell 82 the current flowing through the coil 23 is increased thereby raising the temperature of the furnace. The said increase in temperature produces an increase in the electrical response of the couple 12 so that the galvanometer mirror 86 deflects in the opposite direction thereby decreasing the amount of chopped light reaching the cell 82 to the extent that the relay I02 causes less cur rent to flow through coil 23. The consequent decrease in furnace temperature produces a decrease in the electrical response of the couple 12 so that the galvanometer mirror 80 deflects in the direction to increase the amount of chopped light reaching the cell 82, and so on.

It is clear that the hereinbefore described per formance of the temperature regulator is such that the furnace cannot wander greatly as long as conditionsare nearly constant, e. g., when the room temperature'and line voltage do not change much. If, however, conditions are not sufficiently nearly constant, the demand regulator l H! plays an important role. During successive steps of the hereinbefore described performance the reversible motor of the demand regulator Hi1 acts power due to a drop in line voltage, for example,

the on times of the relay Hi2 exceed the off times, and hence the motor of the demand regulator H0 runs on the average more in the direction to reduce the amount of series resistance than it runs in the opposite direction. Similarly,

the series resistance is increased when the line voltage is too high.

It is now clear that the temperature regulator is capable of maintaining practically constant furnace temperature even when the average power required to do so is not constant.

In the practice of the invention, the support 4| is raised into the chamber 28 and the crucible 40 containing the liquid to be crystallized is positioned thereon. When lithium fluoride is being crystallized, a metallic crucible, such as platinum, is used, the outer surface of which may be dark-- eneclv as by copper plating which is subsequently oxidized. If, for example, it is desired to produce a crystalline body of lithium fluoride, the salt in granular or other solid form is placed in the crucible M3 and melted.- When the desired quantity of material to be crystallized, e. g., lithium fluoride, has been melted and brought to the desired temperature which, preferably, should be substantially above the fusion point, the motor 4? is started to move the support 4| downwardly at a slow rate. The motor t! and the reduction gearing 48 may be so arranged so as to impart any desired rate of movement to the support 4i. Good results have been obtained by moving the support M downwardly at the rate of about one inch in twenty hours. the crucible d3 substantially coincides with the plane of division between the

chambers

28 and 29 crystallization of the liquid begins and successive layers of the liquid are progressively crystallized as these successive layers are progressively brought substantially into the plane of division between the

chambers

28 and 29. EX- cellent results have been obtained when maintaining the temperatures of the

chambers

28 and 29 such that the temperature just above the partition 36 is about 930 C. and the temperature just below the partition is about 810 C., the melting point of lithium fluoride being about 850 0. Under such conditions, the thickness of the partition 3i! being about 0.10 mm. a temperature gradient of about 1000 C. per millimeter is obtained. The maintenance of a high temperature gradient at the partition 30 is favored by the use of a temperature in chamber 28 which is substantially, e. g., 100 to 200 C. above the melting point of the mass. Storage of the solidified mass in chamber 29 at a high temperature serves to avoid shattering of the crystal such as would occur if it were rapidly cooled to atmospheric tem-- perature.

There appears to be a relationship between the temperature gradient dividing the hotter and cooler regions and the rate at which the mass is moved from the hotter to the cooler region. Thus, when the temperature gradient is less than that stated, the rate of movement of the mass must be less than that stated in order to produce a crystal approaching the quality desired. While no accurate limits of minimum temperature gradient or maximum and minimum rate of movement of the mass have been established, a temperature gradient as low as 1 0. per millimeter is impractical.

At the beginning of the crystallization process the point of the crucible bottom is chilled by contact with the end of the support rod 42 with the result that a small crystal is formed there. This small crystal acts as a nucleus so that no seed crystal is required and it also insures that no supercooling of the liquid can take place. It is, of course, recognized that during the subsequent solidification of the material in the conical bot- When the lowest point of tom part of the crucible, which occurs as the crucible is lowered, the high temperature gradient is of relatively small effect because here the crucible is enveloped by the support 4! and the alundum cement 49. The steep temperature gradient comes into play as soon as the conical bottom part of the crucible has passed down through the annular ring 30.

It is believed that the improved results obtained by the practice of the invention are due to the sharply localized steep gradient near the surface of the crucible, and an equi-temperature surface which is plane or nearly plane and extends across the crucible and its contents at this level. While the existence and location 31 the steep gradient region are determined by the annular ring 36 and the difference between the temperatures of the chambers 28. and 29, the location and shape of the freezing layer are determined by the actual temperature of the

chambers

28 and 29. If the' temperature of the upper chamber 28 is too high relative to the temperature of the lower chamber 29 the freezing layer surface is concave upwards, the layer is too low and an inferior crystal is obtained. If the temperature of the upper chamber 28 is too low relative to the temperature of the lower chamber 29 the freezing layer is convex upwards, the layer is too high and an inferior crystal is obtained. The temperatures of the two

chambers

28 and 29 are adjusted by trial until the freezing layer surface is nearly plane and practically coincident with the steep gradient region and under these conditions a good crystal is obtained.

In the practice of the invention each layer starts its'growth at the surface of the crucible. Moreover, as the crystal passes slowly downward the timing of new layers is regular. This is because, since the gradient is steep, it remains substantially fixed in space as the crucible moves. Hence the time when a given point in the material passes through the temperature of solidification is determined precisely near the surface of the crucible. If the gradient were not abrupt at this point the casual wanderings of temperature always present in a furnace would cause layers to spring out in an irregular manner, and occasionally in bunches, with inevitable entrapment. The fixed steep gradient with the consequent regular succession of independently growing layers thus prevents entrapment of impurities.

In Fig. 6 another arrangement for controlling the rate of heat flow through the inner portion of the mass being crystallized from the hotter to the cooler region is illustrated. This arrangement includes a cooling device comprising a cylindrical casing 50' having acentral guide passage adapted to receive the rod 42 and upon which the casing is slidably mounted. The casing 59 of the cooling device is provided with an annular passage 52' in which a cooling fluid, such as water, may be circulated. A' pipe 54 is mounted in the bottom of the casing 50 and is connected with a suitable fluid inlet conduit 55. The upper end of the pipe 56 is provided with an outlet nozzle 56' having a depending skirt or sleeve 5'5 and extending downwardly below outlet openings 58 near the top of the pipe whereby fluid supplied through the pipe 54 is directed downwardly along the outer surface of the pipe. A conduit iii? communicates with the top of the annular passage 52 to permit the escape therefrom of air or vapor. A fluid outlet conduit 6! communicates with the annular chamber 52 and.

extendsupwardly therefrom to a fixed point, as indicated at 62', so as to control the level of'fiuid within the annular chamber 52' The casing 50 is supported upon a ring 65' which is slidably mounted upon the rod '42. The ring 65is provided with a pair of arms 66' to each of which one end of a cable 61 is secured. Each of the cables 6'! extends upwardly from the arms 66 over pulleys 68' rotatably carried. by a bracket 59 fixed to the frame H and is secured at its other end to a counterweight 70'.

This application is a continuation-in-part of my Patent No. 2,149,076 patented February 28, 1939.

I claim:

1. In an apparatus for making a crystal from a molten mass, a hot chamber, and a cool chamber closely adjacent each other, means for maintaining the hot chamber at a temperature above the solidification temperature of the said mass, means for maintaining the cool chamber at a temperature below said solidification temperature, a container for said mass, means for moving the container from the hot to the cool chamber, and means for maintaining a sharply 1ocalized zone of steep temperature gradient between said chambers surrounding the path of travel of said container including a thin partition having a heat reflecting surface exposed to said hot chamber.

2. Apparatus for making crystals from a molten mass comprising a furnace having a longitudinally ex ending chamber divided into two portions, means for independently heating said two portions, an annular disc dividing saidtwo portions, a container for the material to be crystallized, a heat conductive support for said container adapted to move the same from one of said portionsof the furnace chamber through said annular disc into the other portion of the furnace chamber, means for moving said support, means for abstracting heat from said support, and means for determining the location of the freezing zone of the material in said container in the vicinity of said disc.

3. Apparatus for making crystals from a molten mass comprising a furnace having a lon- 'gitudinally extending chamber divided into two portions, means for independently heating said two portions, an annular disc dividing said two portions, a container for the material to be crystallized, a heat conductive support for said container adapted to move the same from one of said portions of the furnace chamber through said annular disc into the other portion of the furnace chamber, means for moving said support and means for abstracting heat from said support.

4. Apparatus for making crystals from a molten mass comprising a furnace having a longitudinally extending chamber, an annular disc dividing said chamber into two portions, means 'for independently heating said two portions, a container for the material to be crystallized, a heat conducting support for said container in heat conducting contact with the bottom thereof adapted to move said container from one portion of the chamber to the other through said annular disc, means for moving said support and means for withdrawing heat from the support.

5. Apparatus for making crystals from amolten mass comprising a furnace having a longitudinally extending chamber, an annular disc dividing said chamber into two portions, means for independently heating said portions, a container having a conical bottom for the material to be crystallized, a support for said container adapted to move said container from one portion of the chamber to the other through said annular disc, the shape of the top surface of the support being complementary to that of the bottom of the container and the central portion of said top surface of the support adjacent the vertex of the cone being heat conducting and the surrounding portions of said top surface being relatively heat insulating, and means for movingsaid support.

6. Apparatus for making crystals from a molten mass comprising a furnace having a longitudinally extending chamber divided into two portions, means for independently heating said two portions, a container for the material to be crystallized having a tapered bottom, means for moving said container completely from one of said portions of the chamber to the other portion comprising a support for the container having a supporting surface conforming to the bottom of the container, said chamber being materially larger in cross section than said container, and an annular disc separating said two portions and having an opening only slightly larger than said container.

7. Apparatus for making crystals from a molten mass comprising a furnace having a longitudinally extending chamber divided into twoportions, means for independently heating said two portions, a container for the material to be crystallized, a heat conductive support for said container in heat conductive contact therewith adapted to move the same completely from one portion of said chamber to the other, means for moving said support and means for abstracting heat from said support.

DONALD C. STOCKBARGER.