US2984626A

US2984626A - Production of metal halide ingots

Info

Publication number: US2984626A
Application number: US616170A
Authority: US
Inventors: Robert A Lefever
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1956-10-16
Filing date: 1956-10-16
Publication date: 1961-05-16
Anticipated expiration: 1978-05-16

Description

y 16, 1951 R. A. LEFEVER 2,984,626

PRODUCTION OF METAL HALIDE INGOTS Filed 001;. 16, 1956 INVENTOR ROBERT A. LEFEVER A T TORNE V United States Patent PRODUCTION OF METAL HALIDE INGOTS Robert Lefever, Bon Air, Va., assignor to Union Carbide Corporation, a corporation of New York Filed Oct. 16, 1956, Ser. No. 616,170

8 Claims. (Cl. 252-3014) This invention relates to an improved process for the production of large ingots of fusible materials, particularly of certain metal halides with or without small quantities of activators. Transparent masses are used to construct prisms and windows for use in spectroscopy, for example, in infra-red analyzers. When such ingots contain an activator, they are also useful for detecting and measuring radiation energy.

Ingots of certain metal halides, for example, the alkali metal group, are widely used as prisms for infra-red spectrometers, mainly because glass is only useful for wave lengths up to approximately 2.5 microns and the various alkali halides are suitable for transmission of infra-red wave lengths as high as 30 microns, each metal halide being adaptable for a special range. Furthermore, these materials may be converted into phosphors by the inclusion of small quantities of activators such as thallium which excite the absorbed light to fluorescence and phosphorescence, and form long wave-length absorption peaks. These phosphor ingots may be used in scintillationcounters for the detection of gamma rays.

Large ingots of metal halides have been produced for many years by several processes. For example, one process involves the slow passage of a container with the raw constituents at a controlled rate through a hot zoneto fuse the charge. As the lower portion of the container emerges from the hot zone and cools, solidification initiates and gradually progresses up through the fused melt. As the ingot is grown, the solid is maintained at a temperature only slightly below its melting point to avoid spontaneous nucleation in the melt and the formation of voids in the ingot. This necessitates a slow growth rate to allow sufficient time for dissipation of the heat of solidification. Furthermore, only a limited part of the available container surface is utilized to conduct heat away from the melt since duringmost of the solidification stage, asubstantial part of the container is still in the hot zone.

.In another prior art process, the raw constituents are placed in a hemispherical bowl and heated substantially above the melting point of the charge. The temperature isthen allowed to slowly decrease with maximum heat dissipation at the bottom of the bowl. When the temperature at the bottom reaches the solidification point of the charge, solidification or crystallization occurs and the solid-liquid interface gradually progresses upwardly to the top surface. As in the first mentioned process, the cooling rate must be low to avoid spontaneous nucleation. To increase the solidification rate, it would be necessary to provide a high cooling rate at the bottom of the container and also provide a heat source to prevent s'pontaneousnucleation in the melt. In such an arrangement, there is again the problem of utilizing only a limited portion of the available container surface for conducting heat away from the melt. In principle, this process is similar to. the first mentioned prior art process except that in'the latter the container is held stationary ice crystalline masses as large as 2 inches in diameter and 2 inches long. In addition, the ingot growth rates are low. The first mentioned prior art process, for example, normally employs rates of 1-5 millimeters of linear growth per hour.

A further disadvantage of these processes when used for producing activated metal halide ingots, for example, sodium iodide ingots activated with thallium iodide, is uneven distribution of the activator which in turn reduces the ingots efiiciency as a scintillation counter. This problem may be illustrated by reference to the sodium iodide-thallium iodide system. As the ingot is grown, thallium iodide is ejected from the solid into the melt thus increasing the thallium concentration of the melt. The distribution coeflicient for thallium iodide between the ingot and the melt is about 0.2. This means that the initial thallium iodide concentration in the melt must be at least 0.5 weight percent to provide the minimum amount of thallium iodide (0.1 percent) in the ingot required for maximum pulse amplitude or generated signal in the scintillation counter. As the thallium iodide is ejected into the quiescent melt during the prior art growth processes, it must be given suificient time to diffuse away from the solid-melt interface to prevent engulfment by the growing ingot. This places a limiting condition on the rate of growth and introduces the undesirable possibility of high local and general thallium concentration regions due to growth rate fluctuations.

Another important requirement in efficient production of ingots is adequate heat removal from the ingot during the growing step. If the amount of heat withdrawn from the ingot is small, the heat input to the melt and thus the temperature ofthe melt must be kept relatively low to maintain ingot growth. As the temperature of the melt falls towards the melting point of the charge, the quantity of melt approaching the crystallization temperature at the solid-liquid interface gradually increases. If the overall temperature of the melt is reduced to a level too close to the melting point, spontaneous nucleation occurs near the interface. The resulting crystals attach to the interface, trapping liquid and eventually causing voids during contraction and solidification. In the case of sodium iodide ingots activated with thallium iodide, the voids lead to light scattering and reduced efliciency of the material as a scintillation counter.

A still further disadvantage of the previously described well-known ingot growing processes is the occurrence of high internal strain during the final cooling step. If the ingot is grown in a container, the outer ingot surface is in direct contact with the container. Due to the difference in coefiicients of thermal expansion between the container and the ingot, internal strain occurs and, if sufficiently high, the ingot will crack.

Principal objects of the present invention are: to provide an improved process for the production of large ingots of fusible materials, particularly of metal halides with or without small quantities of activators, process providing substantially higher ingot growth rates than heretofore obtained, and a process for the production of activated metal halide ingots in which the distribution of the activator is substantially uniform throughout the ingot.

In accordance with one aspect of the present invention, a process is provided for producing metal halide ingots in which the raw constituents are first heated in a container to an elevated temperature above the melting 3 points of such raw constituents to form a fused melt. The fused state is then maintained and the melt is agitated. Next, the fused melt is gradually cooled and the ingot is grown up a dl mt b om p ti n toward he p ofsaid 'melt; Ihthe preferred embodiment, heat is intro ced n a nau h z ta ban a u h umference of the raw constituted charge near the top surface of such charge during the crystallization step. Also, the rate'of crystal growth may be controlled by varying the heat input and the agitation of the melt, the former being preferred, Internalingotstrain during the cooling step ma 9? m nimized by heating the ingot sufficiently to melt at large part of the outer surface of the ingot in direct contact with the container wall, and removing this melted part from the container to leave a vacant space between the container'walls and the ingot except for the small remaining unmelted part. The novel apparatus required for the process of this invention is described in the ensuing portion of the specification,

Although the invention will be further described in terms of" growing sodium iodide ingots activated with thallium iodide, it is also applicable to growth of other metal halide ingots such as the alkali metal halides; that is, halides .of lithium, sodium, potassium, rubidium and cesium. The process of the present invention are equally suitable for growth of other metal halides such as thallium bromide-iodide and calcium fluoride. Small quantities of activators may be added to these raw constituent compounds for growth of phosphor ingots.

By agitating the melt, heat already introduced in the system during the fusing stage is uniformly distributed. When cooling is initiated, the agitation continues to uniformly distribute the available heat through the melt so as to prevent the occurrence of cold spots and resultant spontaneous nucleation. In the previously described prior art processes, the melt was not agitated during the fusing or the crystallization stages and the heat was necessarily transferred to and from the inner or central portion of'the melt through the surrounding ring or outer portion of the melt. Only the outer portion of the melt was in direct heat exchange contact with the container surface, and the heat transfer was limited by the thermal conductivity of the charge. In the present invention, agitation exposes the entire body of melt to the container surface thus shortening the heat transfer path and increasing the average temperature difference between the container wall and the melt directly in contact with such wall. In this manner, agitation of the melt permits the attainment of remarkably high ingot growth rates. For example, ingot growth rates of 1-5 millimeters per hour have been considered excellent by the prior art, and commercially attractive ingots of sodium iodide have been grown by the present invention at rates of 25-100 millimeters linear growth per hour.

The agitation technique also facilitates the rapid growth of large metal halide crystalline masses containing sm qua i ies of uniformly distributed activating material. For example, thallium iodide activated sodium iodide ingots have been grown at remarkably high rates of 75 millimeters per hour utilizing the novel process of the present invention. Substantially uniform distribution of thallium iodide is initially obtained during the fusing step and maintained during the initial cooling and crystallizing step by the aforementioned agitation. The prior art has established that the most desirable thallium iodide concentration in sodium iodide phosphor ingots is about 0.1 weight percent. As previously discussed, this requires a concentration of about 0.5 weight percent in the melt. One could employ up to about 2 percent thallium iodide in the melt and obtain a satisfactory phosphor ingot, but still higher concentrations would tend to produce cloudy ingots which are generally unsatisfactory for spectroscopic applications.

During the crystallization step, a temperature gradient is established between the bottom and the top of the melt and crystallization is initiated in the cooler bottom portion. The ingot is then grown from the bottom toward the top of the melt, and control of the growth rate is readily achieved by varying the heat input or the agitation rate, or a combination of the two. Control of the heat input is the preferred means of controlling the ingot growth rate as decreased stirring tends to permit cold spots and uneven distribution of the activatorthe problems that agitation is intended to overcome.

A novel feature of the preferred embodiment of the present invention is the introduction of heat in a narrow horizontal band around the container circumference near the top layer of the charge. By agitation, the heat so introduced is uniformly distributed throughout the charge as previously described, and during the crystallization step sufficient heat to avoid spontaneous nucleation is provided by this method. By introducing the heat in a narrow band, the maximum available container surface is utilized for radiation and conduction of heat away from the melt during the crystallization step. This arrangement provides another method of increasing the ingot growth rate.

A further novel feature of the present invention is the avoidance of excessive internal strain during the cooling step following growth of the ingot. This is achieved by first heating the ingot sufliciently to melt a large part of the outer surface of the ingot in direct contact with the container, and then removing this melted outer surface leaving a vacant space between the container walls and the ingot except for the small remaining unmelted part of the outer surface supporting the ingot. The ingot is then reheated sufiiciently to obtain a substantially uni form temperature throughout the mass. When the ingot is subsequently cooled, the vacant space permits thermal contraction with minimum strain because of the small contact area between the ingot and the container. The cooled ingot may easily be removed from the container by severing the small remaining unmelted outer surface of the ingot.

The drawing shows a vertical longitudinal section of one embodiment of the novel apparatus for producing ingots by the process of the present invention. The container 10 holding the charge 11 is enclosed by a cap 12 at its upper or top end. The container 10 and cap 12 are constructed of heat-resistant material which is thermally stable above the fusion temperature of the charge and which does not shed impurities into the charge 11. A concentric ring burner 13 is placed around the container 10 with ports 14 therein for the passage of a flame 15. These holes 14 are preferably uniformly spaced around the circumference of the ring burner 13 to provide uniform distribution of heat around the container circumference in a narrow horizontal band 21. Agitation is facilitated by a disk 16 moving in a longitudinal direction, up and down, on the end of a rod 17 passing through a port 18 in the container cap 12. The rod may be actuated manually, but preferably is oscillated by power transmitted from a small electric motor. The preferred amplitude and rod-disk stroke rate depend on the type of ingot to be grown and the specific apparatus used for such growth. I have successfully grown ingots by the process and apparatus of the present invention using amplitudes of inch to inch with stroke rates of 60 to 200 per minute.

Agitation may be started during the fusing step as soon as the charge 11 has melted sutficiently to permit movement of the disk 16 therein, but agitation is not essential until the entire charge has melted or fused and the crystallizationstep is to be initiated. Either or both the amplitude and the position of the rod and disk assembly are controlled during the crystallization step so that effectiveagitation isobtained without allowing the disk to actually contactthe upwardly growing solid-liquid interface nor so agitating the top surface of .the meltthat gas bubbles are entrained and carried down to the growing ingot. When producing ingots of oxygenesensitive or moisture,-

ifi im l for ex mp S d um d e fluoride, potassium iodide, and calcium fluoride, an inert atmosphere may be maintained in the container to avoid oxidation or hydrolysis and resultant contamination of the charge. An inert gas such as argon is introduced through conduit 19, the latterpassing through a port 20 in the container cap. 12. The inert gas is discharged through the port 18 in the container cap 12 to avoid pressure buildup in the container 10. In this manner, the charge is blanketed fiom the atmosphere by a continuous flow of circulating inert gas.

The following example describes the subject process in operation: EXAMPLE Preparation of sodium iodide ingots activated by thallium iodide About 150480 grams of sodium iodide and 0.75-0.9 gram of thallium iodide were placed in a 1% inch outside diameter by 1% inch inside diameter fused silica container similar to that illustrated in the drawing. The charge was fused under a continuous flow of argon gas. Rapid vertical agitation, approximately 200 strokes per minute, was maintained by use of a fused silica disk (about /2 inch in diameter) moving up and down on the end of a fused silica rod (Mi-V2 inch'stroke) by power transmitted from a small electrical mctor. Sufficient heat was supplied by means of a ring burner placed concentrically around the container, and the flame was directed in a narrow horizontal band near the top of the charge to maintain such charge in the molten state. The melt temperature during this step was in the range of 690 C.- 750 C., the melting point of sodium-thallium iodide mix being 660 C. Next, the stirring rate was slowly decreased uutil solidification began at the bottom of the container. By gradually decreasing the stirring rate and the heat supplied by the burner, the solid-liquid interface was made to progress toward the burner. At the same time, the liquid level gradually lowered due to contraction of sodium iodide on solidification. Growth proceeded at the rate of about one inch (25.4 millimeters) per hour. After growth of about 1% to 1 /2 inch length of solid ingot, the container was inverted and the remaining molten liquid was poured oflF. The outer surface of the ingot in direct contact with the container was then melted away except for a small connection so that the ingot could not drop out. This greatly reduced strain on the ingot during further cooling, and prevented cracking. The container and the ingot were then placed in a furnace at about 650 C. and slowly cooled to room temperature during a period of about 4-5 hours to minimize internal thermal stress and avoid cleavage.

Although the preferred embodiment of the invention has been described in detail, it is contemplated that modifications of the process maybe made and that some features may be employed without others, all within the spirit of the invention and the scope thereof as set forth in the claims. For example, although a rod and disk stirring arrangement has been described, other forms of agitation might also be employed, such as bubbling an inert gas through the melt, or rotating mixers.

Another possible modification is control of the crystal growth rate by the regulation of the heat loss through the lower part of the container; that is, forcibly cooling the portion of the container containing the growing ingot. However, as previously discussed, the preferred method of controlling the ingot growth rate is by control of the heat input.

A further variation is that of fusing the charge and growing the ingots in dilferent containers. Such an arrangement is adaptable to mass production techniques by, for example, fusing the charge in a large single container and then transferring the molten mass to a battery of smaller containers for ingot growth therein. In the case of growing phosphor ingots, the additive could either be injected in the large single container or in the battery of smaller containers.

. v "6 It was found during the experimental work that agent grade sodium iodide was not uniform from lot to lot and that a considerable amount of suspended impuri-- ties was obtained in some cases. Consequently, to insure uniform quality ingot products, the raw material sodium iodide was recrystallized priorto use in the subject process. This is not a critical feature of the invention how? ever. Reagent grade sodium iodide may be used. for many. applications where optimum quality and light output are not required.

Although theoretical considerations and experimental evidence indicate that transparent polycrystalline sodium iodide-thallium iodide ingots are satisfactory scintillation counters, slight process modifications such as utilization of a container with a conical-shaped bottom may be used to facilitate the growth of large single crystal ingots by the present invention. The use of a conical-shaped bottom enables ingot growth to start at the apex of the cone and then grow upward. .Another method of obtaining large single crystal ingots is the use of a seed crystal in the bas'eof the container. As is known from other ingot growth methods, a seed crystal may be used to predetermine the orientation of the growing ingot.

What is claimed is: I. In a process for growing a metal halideingot, the steps comprising heating raw constituents of such ingot to an elevated temperature above the melting points of said raw constituents to form a fused melt, maintaining such fused state and substantially continuously agitating said fused melt to uniformly distribute the heat through out the melt, gradually cooling said fused melt, initiating crystallization in the bottom portion of the melt and growing the ingot upwardly from said bottom portion toward the top of said fused melt.

2. In a process for growing a metal halide ingot, the steps comprising heating raw constituents of such ingot to an elevated temperature above the melting points of said raw constituents to form a fused melt, maintaining such fused state and substantially continuously agitating said fused melt to uniformly distribute the heat throughout the melt, gradually decreasing the temperature and agitation of said fused melt, initiating-crystallization in the bottom portion of the melt and growing the ingot upwardly from said bottom portion toward the top of said fused melt.

3. In a process for growing a metal halide ingot, the steps comprising heating raw constituents of such ingot to an elevated temperature above the melting points of said raw constituents to form a fused melt, introducing heat to the top portion of such melt to maintain the fused state, substantially continuously agitating the melt to uniformly distribute the heat throughout such melt, gradually cooling said fused melt, initiating crystallization in the bottom portion of the melt and growing the ingot upwardly from said bottom portion toward the top of said fused melt.

4. In a process for growing a metal halide phosphor ingot containing a small quantity of activator material, the steps comprising heating raw constituents of such ingot to an elevated temperature above the melting points of said raw constituents to form a fused melt, maintaining such fused state and substantially continuously agitating said fused melt thereby substantially uniformly distributing the activator material and the heat throughout the fused melt, gradually cooling said fused melt, initiating crystallization in the bottom portion of the melt and growing the ingot upwardly from said bottom portion toward the top of said fused melt.

5. In a process for growing a metal halide ingot, the steps comprising heating the raw constituents in a container to an elevated temperature above the melting points of said raw constituents to form a fused melt, maintaining such fused state and substantially continuously agitating said fused melt to uniformly distribute the heat 15 throughout the melt, gradually cooling said fused melt,

initiating crystallization in the bottom po tionof the melt, growing gqtuswaraiy from saidbottoni portion tothe top of said fused men, heating'tlie ingot Suffi: cientl'y to melt a large part of the outer surface of'the ingot in direct contact with the container, removing such melted large part from the container leaving a vacant space between the container and the ingot except for a remaining unmelted small part of such outer surface, reheating the ingot to substantially uniform temperature throughout, gradually cooling the ingot'wi'th minimum strain 'due to said vacant space, and removing the cooled ingot from the container by severing the remaining small part of the outer'i'ngot surface in direct contaet with the cont iine 6. In a process for growing a sodium iodide ingot, the steps comprising heating the raw constituent of such ingot to aneleyated temperature above the melting point of said raw constituent to form a fused melt, maintaining such fused state and substantially continuously agitating said fused melt to uniformly distribute the'heat throughout the melt, gradually cooling said fused melt, initiating crystallization in the bottom portion of the melt and growing the ingot upwardly from said bottom portion toward the top of'said fused melt.

7. In a process for growing a sodium iodide ingot containing a small quantity of thallium iodide, the steps comprising heating raw constituents of such ingot to an elevatedtemperature above the melting points of said raw constituents to form a fused melt, maintaining'such fused state and substantially continuously agitating said fused melt thereby substantially uniformly distributing the thalliuin iodide, throughout" the sodium iodide and the heat iiielt, gradually cooling said fusedr'nelt, initiating 'ct star liz'atioli' in thebot'toi'n portion of the melt and growing the ingot upwardly from's'aid bottom portion toward the top of"saidfusedmelt. 8. In a process for growing a sodium iodide ingotcontaininga small quantity of thallium iodide, the steps comprising heating raw constituents of such ingot abovethe melting points of said raw constituentsto"forrn a fused melt, introducing heat to the top portion of such melt to maintain the fused state, substantially continuously agitating the melt to uniformly distribute the thalli'umiodide throughout the sodium iodide and the heat fused melt, gradually cooling said fusedmelt, initiatingcrystallization in the bottom portion of the melt and growing the ingot upwardly from said bot-tom portion toward the top of said fused melt.

References Cited in the file of this patent UNITED STATES PATENTS 1,353,571 Dreibrodt Sept. 2l, 19 20 2,589,310 Tournier Mar. 18, 1952 2,727,863 Fonda Dec. 20, 1955 2,766,105 Washken 'Oct. 9, 1956 2 3 6 0 2 Smith ww OTHER REFERENCES Curran: Luminescense and Scintillation Counter,"

Academic Press Inc., London, pp. 114 to 117 (1953).

Claims

7. IN A PROCESS FOR GROWING A SODIUM IODIDE INGOT CONTAINING A SMALL QUANTITY OF THALLIUM IODIDE, THE STEPS COMPRISING HEATING RAW CONSTITUENTS OF SUCH INGOT TO AN ELEVATED TEMPERATURE ABOVE THE MELTING POINTS OF SAID RAW CONSTITUENTS TO FORM A FUSED MELT, MAINTAINING SUCH FUSED STATE AND SUBSTANTIALLY CONTINUOUSLY AGITATING SAID FUSED MELT THEREBY SUBSTANTIALLY UNIFORMLY DISTRIBUTING THE THALLIUM IODIDE THROUGHOUT THE SODIUM IODIDE AND THE HEAT MELT, GRADUALLY COOLING SAID FUSED MELT, INITIATING CRYSTAL-