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Crystal growth of KCl in a reactive atmosphere II: The crystal growth technique
Mat. Res. Bull., Vol. 19, pp. i195-i199, 1984. Printed in the USA. 0025-5408/84 $3.00 + .00 C o p y r i g h t (e) 1984 Pergamon P r e s s Ltd.
CRYSTAL GROWTH OF KC1 IN A REACTIVE ATMOSPHERE II: THE CRYSTAL GROWTH TECHNIQUE A.C. Pastor and R.C. Pastor Hughes Research Laboratories Malibu, California 90265
(Reeeived June 25, 1984; Refereed)
ABSTRACT The crystal growth technique employed in the growth of large single crystals of KCI while the material is undergoing reactive-atmosphere processing is described in detail.
Introduction Reactlve-atmosphere processing (RAP) has been established as an effective method for the anionic purification of alkali metal halldes. The hydroxyl ion, OH , is the most problematical of the anionic impurities in these compounds because of (a) the ubiquity of its primary source, water, in the air and in both charge and container materials, and (b) its chemical resemblance to the members of the halide ion family. Thus, even the commercially available alkali metal halide powders with the best cationic purity specifications are contaminated with OH if the last processing they underwent prior to packaging was carried out in open air or, worse yet, was an aqueous chemical process. The avoidance of contamination with this impurity is made difficult by (a), and, once contamination has occurred, its removal is made difficult by (b). To reduce the possibility of contamination of the charge material while it is in transit from the purification process to the crystal growth process, it was almost a merely logical conclusion that the two processes should be carried out in the same reactor, i.e., that the charge material be subjected to RAP in the same crucible as that in which the crystal of the purified material was to be grown. We refer to such a combined operation as RAP crystal growth. Specifically, this paper will describe the RAP crystal growth of potassium chloride (KCI), emphasis being placed on the apparatus and the method developed by the authors for that purpose. The chemisty of this process has been elaborated upon in previous publications (I, 2), and only directly pertinent highlights of it will be mentioned here. The method of crystal growth employed was the Bridgman method without the Stockbarger modification but with modifications that to our knowledge are original. 1195
1196
A.C.
PASTOR, et al.
Vol. 19, No. 9
The writing of this paper was prompted by a recent publication (3) which described certain difficulties that its authors presumably encountered in their attempts at RAP crystal growth of KCI and reported as inherent limitations to the process. The purpose of the present paper is to elaborate on the crystal growth technique developed by the originators of the RAP crystal growth of KCI and other alkali metal halides, who indeed encountered some difficulties in the development of the process, but having failed to recognize them as limitations, surmounted them. Backsround The RAP gas mixture used was carbon tetrachloride (CC14) at its saturation pressure at 24°C and carbon dioxide (C09) at a nominal i atm pressure. Even at 500°C this mixture was capable of yielding 0.07 mole per cent CI_, and around sixty times that at 800°C (i). These figures represent around I/~ and 20 percent conversion, respectively, of the chlorine content of the CCI 4 in the gas mixture. RAP was carried out in two steps, the first step being at the lower of these two temperatures and the second step at the higher. In both RAP steps the basis for RAP action is the heterogeneous reaction, Cl2(g) + O H - ( c ) ~ C l - ( c ) which tends to be slow at 500°C because between the condensed (i.e., solid) and KCI charge is in powder form during the to-volume ratio will compensate for the This process was allowed to proceed for
+ HCI(g) + ½02(g),
it is confined mainly to the interface gaseous reactant phases. However, the first RAP step, and its high surfacelow reaction (per unit area) rate. four hours.
In the second RAP step the charge is in the molten state, and RAP action is then aided by dissolution of the nascent chlorine, and convection currents, in the melt. This process was allowed to proceed for another four hours before crystal growth was begun and was sustained until the growth of the crystal had been completed. Desisn of Apparatus (a) Crucible - This is shown in cross section in Fig. I. It is made entirely of vitreous silica. The nucleating tip, a, will either permit spontaneous nucleation or accommodate a seed crystal for oriented seeded growth. The feedthru for the RAP gas inlet tube, e, is rigid and the interior end of this tube is not immersed in the melt, as the 4-hr soaking of the melt in RAP gas renders bubbling of the RAP gas through the melt unnecessary. The liquid trap, b, in the outlet tube, d, will prevent any condensate from the effluent gas, such as unconverted CCI 4, to drip into the crucible. The solid trap, c, is filled with quartz wool and will prevent sublimate from the effluent gas, such as C?C16, from clogging up the plumbing lines. The plumbing lines, not shown in Fig. I, consist of Teflon tubing coupled to the inlet and outlet tubes with Teflon fittings. For the growth interface to be flat the heat flow lines in the grown crystal had to be as purely longitudinal as possible. Therefore, it was of utmost importance to suppress heat loss by radiation from the sides of the crucible and to let heat loss by axial conduction predominate. For this purpose the crucible was wrapped in a sleeve of insulation which traveled with it during growth. The insulating sleeve of Kaowool is outlined by the dashed lines in
Vol. 19, No. 9
KCI CRYSTALS
1197
Fig. i. No provision for cooling the crucible stem was necessary up to crystal diameters of I0 cm, but for the growth of larger crystals it was a relatively minor task to incorporate a water-cooling accessory into the design of the crucible stem. This accessory is shown in Fig. 1 and is labeled f. (b) Furnace - This is shown in cross section in Fig. 2. The heating elements, a, are Lindberg Hevi-Duty semicylindrical Kanthal-wound heaters. The control thermocouple, c, is placed midway between the ends of the heaters, while the recording thermocouple, d, is placed at the growth end. The heaters are enclosed in firebricks, b, and the top opening is insulated with Kaowool, not shown, during operation to minimize heat losses due to air convection. For crystal diameters up to i0 cm the heating elements were powered by a Barber-Colman Model 621A 208V SCR. Temperature control was effected with a nonprogrammable Barber-Colman Model 543C PID controller. e
a
b
b
i .... i
1__
D FIG. i RAP Bridgman crystal growth crucible
FIG. 2 Bridgman crystal growth furnace
(c) RAP Gas Flow Accessories - Fig. 3 is a schematic diagram of the CCI 4 vapor generator and injector. Its main body is made of borosilicate glass. The throttling valves are the greaseless Teflon-in-glass type, of which several designs are commercially available. With the ejector assembly
1198
A.C.
PASTOR, et al.
Vol. 19, No. 9
it is possible to obtain flow mixtures that are undersaturated with CCI.. It is also possible to reduce the CCI, concentration to near zero, and this4 will be necessary when the growth of the4 crystal is completed and the furnace is being cooled down to room temperature so that crystal and crucible may be retrieved. Fig. 4 is a schematic diagram of the effluent gas scrubber. It is also made of borosilicate glass. It was designed to strip the effluent of CI_ and 2 unconverted CCI.. Since the effluent could have only a few mole percent CO, and the efflux ~ate was around 3 liters per hour (I), it was deemed unnecessary to run the effluent through a CO scrubber. The top liquid layer in the scrubber is a NaOH solution. The bottom liquid layer is a hlgh-denslty fluorocarbon liquid manufactured by Commercial Chemicals Division of 3M. Besides its function as an absorbent for unconverted CC14, it serves to block any back diffusion of water vapor from the NaOH solution. RAP E~FLUENT
n
. . . .
l.... H .......iiii ........... ......................
FIG. 3 CCI 4 vapor generator and injector
FIG. 4 RAP effluent gas scrubber
Experimental Procedure The crucible is fabricated in two segments. The division into these segments is indicated by the dotted llne in Pig. I. The lower segment is filled with KCI powder, and the two segments are welded together. The crucible is then wrapped a few times around with Kaowool felt to a minimum thickness of i~ cm, depending on the inside diameter of the furnace. The wrapped crucible should fit snugly into the furnace in order to eliminate convection currents of air which tend to rob the furnace to thermal uniformity and stability. The wrapped crucible is slid into the furnace to such a level that the nucleating tip is Just above the recording thermocouple. The plumbing for the RAP gas flow is connected to the inlet and outlet tubes, and the CO_ flow is started. The furnace is then brought up to 500°C for the first RAPZstep, and CCI 4 is gradually injected into the CO? flowstream, up to full saturation at room temperature. After 4 hrs of soaking at this temperature the furnace
Vol. 19, No. 9
KCI CRYSTALS
1199
temperature is raised to 800°C, and soaking is continued at this temperature for another 4 hrs. Crystal growth may then be started by lowering the wrapped crucible at the desired linear crystal growth rate. For a 15-cm diameter crystal a typical growth rate is 2 mm per hr. When the reckoned length of the crystal has been withdrawn from the hot zone of the furnace, the lowering mechanism and the furnace power are turned off. The CCI. injection is shut off, but the CO 2 is allowed to continue to flow as the c~ystal cools down to room temperature. Cooling down takes several hours because of the insulating wrapping. After the wrapping is removed the crucible may be cut open around the welded join with a dry cutting wheel. The crystal ingot will then slip out of the crucible easily. The two segments of the crucible assembly may be reused after cleaning. The top segment may be reused many times but the bottom segment is progressively etched in the RAP processing, and its wall will become too thin after five cycles or so. Results and Conclusion With the method Just described we were able to grow single crystal ingots of KC1 with diameters ranging from 2 to 15 cm and lengths ranging from 15 to 25 cm. The upper limit on the diameter was set by the structural soundness of the crucible, particularly at its conical bottom. After the conical end of the crystal has cooled and contracted, its taper will not anymore match that of the crucible. The tremendous weight of the ingot will then bear down on a much reduced area of the crucible bottom, at times to the extent of rupturing it. The incidence of crucible breakage at such diameters was somewhat high. There was the option to redesign the crucible and its support for the growth of the larger crystals, but there was not a demand for them great enough to Justify such an undertaking. The quality of the crystals grown by the RAP crystal growth method has been described in the publications already cited. The authors feel that their work has served to demonstrate the RAP crystal growth of KCI is a reliable and economical operation and can serve as the basis for the production of these crystals for infrared window application. Acknowledgments The authors are grateful to C. Fetters and L. Veit of Hughes Research Laboratories for having provided the vast amount of high-quality glassblowlng work that this project required. As was already stated in the publications cited (I), this project was funded in part by ARPA Order Nos. 1256, Contract F2906-71-C-0101, and 2612, Contract F33615-74-C-5115. References I.
R.C. Pastor and A.C. Pastor, Mat. Res. Bull. 10, 117 (1975); R.C. Pastor and A.C. Pastor, Mat. Res. Bull. IO, 251 (1975).
2.
R.C. R.C. R.C. R.C.
3.
I. Ursu, S.V. Nistor, M.M. Voba, L.C. Nistor, and V. Teodorescu, Mat. Res. Bull. 18, 1275 (1983).
Pastor and A.C. Pastor, Pastor, U.S. Patent No. Pastor and A.J. Timper, Pastor and A.C. Pastor,
U.S. Patent No. 3,826,817 (July 1974); 3,932,597 (January 1976); U.S. Patent No. 3,969,491 (July 1976); U.S. Patent No. 4,076,574 (February 1978).
CRYSTAL GROWTH OF KC1 IN A REACTIVE ATMOSPHERE II: THE CRYSTAL GROWTH TECHNIQUE A.C. Pastor and R.C. Pastor Hughes Research Laboratories Malibu, California 90265
(Reeeived June 25, 1984; Refereed)
ABSTRACT The crystal growth technique employed in the growth of large single crystals of KCI while the material is undergoing reactive-atmosphere processing is described in detail.
Introduction Reactlve-atmosphere processing (RAP) has been established as an effective method for the anionic purification of alkali metal halldes. The hydroxyl ion, OH , is the most problematical of the anionic impurities in these compounds because of (a) the ubiquity of its primary source, water, in the air and in both charge and container materials, and (b) its chemical resemblance to the members of the halide ion family. Thus, even the commercially available alkali metal halide powders with the best cationic purity specifications are contaminated with OH if the last processing they underwent prior to packaging was carried out in open air or, worse yet, was an aqueous chemical process. The avoidance of contamination with this impurity is made difficult by (a), and, once contamination has occurred, its removal is made difficult by (b). To reduce the possibility of contamination of the charge material while it is in transit from the purification process to the crystal growth process, it was almost a merely logical conclusion that the two processes should be carried out in the same reactor, i.e., that the charge material be subjected to RAP in the same crucible as that in which the crystal of the purified material was to be grown. We refer to such a combined operation as RAP crystal growth. Specifically, this paper will describe the RAP crystal growth of potassium chloride (KCI), emphasis being placed on the apparatus and the method developed by the authors for that purpose. The chemisty of this process has been elaborated upon in previous publications (I, 2), and only directly pertinent highlights of it will be mentioned here. The method of crystal growth employed was the Bridgman method without the Stockbarger modification but with modifications that to our knowledge are original. 1195
1196
A.C.
PASTOR, et al.
Vol. 19, No. 9
The writing of this paper was prompted by a recent publication (3) which described certain difficulties that its authors presumably encountered in their attempts at RAP crystal growth of KCI and reported as inherent limitations to the process. The purpose of the present paper is to elaborate on the crystal growth technique developed by the originators of the RAP crystal growth of KCI and other alkali metal halides, who indeed encountered some difficulties in the development of the process, but having failed to recognize them as limitations, surmounted them. Backsround The RAP gas mixture used was carbon tetrachloride (CC14) at its saturation pressure at 24°C and carbon dioxide (C09) at a nominal i atm pressure. Even at 500°C this mixture was capable of yielding 0.07 mole per cent CI_, and around sixty times that at 800°C (i). These figures represent around I/~ and 20 percent conversion, respectively, of the chlorine content of the CCI 4 in the gas mixture. RAP was carried out in two steps, the first step being at the lower of these two temperatures and the second step at the higher. In both RAP steps the basis for RAP action is the heterogeneous reaction, Cl2(g) + O H - ( c ) ~ C l - ( c ) which tends to be slow at 500°C because between the condensed (i.e., solid) and KCI charge is in powder form during the to-volume ratio will compensate for the This process was allowed to proceed for
+ HCI(g) + ½02(g),
it is confined mainly to the interface gaseous reactant phases. However, the first RAP step, and its high surfacelow reaction (per unit area) rate. four hours.
In the second RAP step the charge is in the molten state, and RAP action is then aided by dissolution of the nascent chlorine, and convection currents, in the melt. This process was allowed to proceed for another four hours before crystal growth was begun and was sustained until the growth of the crystal had been completed. Desisn of Apparatus (a) Crucible - This is shown in cross section in Fig. I. It is made entirely of vitreous silica. The nucleating tip, a, will either permit spontaneous nucleation or accommodate a seed crystal for oriented seeded growth. The feedthru for the RAP gas inlet tube, e, is rigid and the interior end of this tube is not immersed in the melt, as the 4-hr soaking of the melt in RAP gas renders bubbling of the RAP gas through the melt unnecessary. The liquid trap, b, in the outlet tube, d, will prevent any condensate from the effluent gas, such as unconverted CCI 4, to drip into the crucible. The solid trap, c, is filled with quartz wool and will prevent sublimate from the effluent gas, such as C?C16, from clogging up the plumbing lines. The plumbing lines, not shown in Fig. I, consist of Teflon tubing coupled to the inlet and outlet tubes with Teflon fittings. For the growth interface to be flat the heat flow lines in the grown crystal had to be as purely longitudinal as possible. Therefore, it was of utmost importance to suppress heat loss by radiation from the sides of the crucible and to let heat loss by axial conduction predominate. For this purpose the crucible was wrapped in a sleeve of insulation which traveled with it during growth. The insulating sleeve of Kaowool is outlined by the dashed lines in
Vol. 19, No. 9
KCI CRYSTALS
1197
Fig. i. No provision for cooling the crucible stem was necessary up to crystal diameters of I0 cm, but for the growth of larger crystals it was a relatively minor task to incorporate a water-cooling accessory into the design of the crucible stem. This accessory is shown in Fig. 1 and is labeled f. (b) Furnace - This is shown in cross section in Fig. 2. The heating elements, a, are Lindberg Hevi-Duty semicylindrical Kanthal-wound heaters. The control thermocouple, c, is placed midway between the ends of the heaters, while the recording thermocouple, d, is placed at the growth end. The heaters are enclosed in firebricks, b, and the top opening is insulated with Kaowool, not shown, during operation to minimize heat losses due to air convection. For crystal diameters up to i0 cm the heating elements were powered by a Barber-Colman Model 621A 208V SCR. Temperature control was effected with a nonprogrammable Barber-Colman Model 543C PID controller. e
a
b
b
i .... i
1__
D FIG. i RAP Bridgman crystal growth crucible
FIG. 2 Bridgman crystal growth furnace
(c) RAP Gas Flow Accessories - Fig. 3 is a schematic diagram of the CCI 4 vapor generator and injector. Its main body is made of borosilicate glass. The throttling valves are the greaseless Teflon-in-glass type, of which several designs are commercially available. With the ejector assembly
1198
A.C.
PASTOR, et al.
Vol. 19, No. 9
it is possible to obtain flow mixtures that are undersaturated with CCI.. It is also possible to reduce the CCI, concentration to near zero, and this4 will be necessary when the growth of the4 crystal is completed and the furnace is being cooled down to room temperature so that crystal and crucible may be retrieved. Fig. 4 is a schematic diagram of the effluent gas scrubber. It is also made of borosilicate glass. It was designed to strip the effluent of CI_ and 2 unconverted CCI.. Since the effluent could have only a few mole percent CO, and the efflux ~ate was around 3 liters per hour (I), it was deemed unnecessary to run the effluent through a CO scrubber. The top liquid layer in the scrubber is a NaOH solution. The bottom liquid layer is a hlgh-denslty fluorocarbon liquid manufactured by Commercial Chemicals Division of 3M. Besides its function as an absorbent for unconverted CC14, it serves to block any back diffusion of water vapor from the NaOH solution. RAP E~FLUENT
n
. . . .
l.... H .......iiii ........... ......................
FIG. 3 CCI 4 vapor generator and injector
FIG. 4 RAP effluent gas scrubber
Experimental Procedure The crucible is fabricated in two segments. The division into these segments is indicated by the dotted llne in Pig. I. The lower segment is filled with KCI powder, and the two segments are welded together. The crucible is then wrapped a few times around with Kaowool felt to a minimum thickness of i~ cm, depending on the inside diameter of the furnace. The wrapped crucible should fit snugly into the furnace in order to eliminate convection currents of air which tend to rob the furnace to thermal uniformity and stability. The wrapped crucible is slid into the furnace to such a level that the nucleating tip is Just above the recording thermocouple. The plumbing for the RAP gas flow is connected to the inlet and outlet tubes, and the CO_ flow is started. The furnace is then brought up to 500°C for the first RAPZstep, and CCI 4 is gradually injected into the CO? flowstream, up to full saturation at room temperature. After 4 hrs of soaking at this temperature the furnace
Vol. 19, No. 9
KCI CRYSTALS
1199
temperature is raised to 800°C, and soaking is continued at this temperature for another 4 hrs. Crystal growth may then be started by lowering the wrapped crucible at the desired linear crystal growth rate. For a 15-cm diameter crystal a typical growth rate is 2 mm per hr. When the reckoned length of the crystal has been withdrawn from the hot zone of the furnace, the lowering mechanism and the furnace power are turned off. The CCI. injection is shut off, but the CO 2 is allowed to continue to flow as the c~ystal cools down to room temperature. Cooling down takes several hours because of the insulating wrapping. After the wrapping is removed the crucible may be cut open around the welded join with a dry cutting wheel. The crystal ingot will then slip out of the crucible easily. The two segments of the crucible assembly may be reused after cleaning. The top segment may be reused many times but the bottom segment is progressively etched in the RAP processing, and its wall will become too thin after five cycles or so. Results and Conclusion With the method Just described we were able to grow single crystal ingots of KC1 with diameters ranging from 2 to 15 cm and lengths ranging from 15 to 25 cm. The upper limit on the diameter was set by the structural soundness of the crucible, particularly at its conical bottom. After the conical end of the crystal has cooled and contracted, its taper will not anymore match that of the crucible. The tremendous weight of the ingot will then bear down on a much reduced area of the crucible bottom, at times to the extent of rupturing it. The incidence of crucible breakage at such diameters was somewhat high. There was the option to redesign the crucible and its support for the growth of the larger crystals, but there was not a demand for them great enough to Justify such an undertaking. The quality of the crystals grown by the RAP crystal growth method has been described in the publications already cited. The authors feel that their work has served to demonstrate the RAP crystal growth of KCI is a reliable and economical operation and can serve as the basis for the production of these crystals for infrared window application. Acknowledgments The authors are grateful to C. Fetters and L. Veit of Hughes Research Laboratories for having provided the vast amount of high-quality glassblowlng work that this project required. As was already stated in the publications cited (I), this project was funded in part by ARPA Order Nos. 1256, Contract F2906-71-C-0101, and 2612, Contract F33615-74-C-5115. References I.
R.C. Pastor and A.C. Pastor, Mat. Res. Bull. 10, 117 (1975); R.C. Pastor and A.C. Pastor, Mat. Res. Bull. IO, 251 (1975).
2.
R.C. R.C. R.C. R.C.
3.
I. Ursu, S.V. Nistor, M.M. Voba, L.C. Nistor, and V. Teodorescu, Mat. Res. Bull. 18, 1275 (1983).
Pastor and A.C. Pastor, Pastor, U.S. Patent No. Pastor and A.J. Timper, Pastor and A.C. Pastor,
U.S. Patent No. 3,826,817 (July 1974); 3,932,597 (January 1976); U.S. Patent No. 3,969,491 (July 1976); U.S. Patent No. 4,076,574 (February 1978).