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Czochralski growth of single crystal sodium beta alumina
Journal of Crystal Growth 39 (1977) 180—184 © North-Holland Publishing Company
CZOCHRALSKI GROWTH OF SINGLE CRYSTAL SODIUM BETA ALUMINA L.R ROTHROCK Union Carbide Corporation, Crystal Products Department, 8888 Balboa Avenue, San Diego, California 92123, USA Received 10 December 1976;revised manuscript received 29 December 1976
Single crystal growth of sodium beta alumina has been performed using the Czochralski technique. Loss of Na
20 from the melt is inhibited by the use of 5,000—15,000 ppm oxygen in a flowing nitrogen atmosphere. Crystals produced were 2—3 cm in diameter and up to 15 cm long. Boules were grown along the a-axis.
The recently intensified work in solid ionic conductors, especially as applied to electric battery devices such as the sodium—sulfur secondary battery, has been brought on, in part, by the world-wide energy situation faced today. There are obvious advantages to be achieved by the use of a solid electrolyte, but a major impetus is the high energy density (Elm) that has been achieved in such cells. The theoretical energy density for a sodium—sulfur battery employing sodium beta-alumina is 795 W-hr/kg while advanced lead-acid batteries have a theoretical value of 165 W-hr/kg [1]. While present sodium—sulfur batteries use polycrystalline sodium beta-alumina because of the relative ease of obtaining polycrystalline material, single crystals are obviously simpler components in basic experiments designed to measure conductivity and transport mechanisms as the data can be treated in a much more straightforward manner. While it is unlikely that single crystal material can be produced in the sizes necessary for use in practical high power batteries in the near future, sizes and quality appropriate for laboratory experiments have been achieved. Single crystal sodium beta alumina can now be grown in boules of up to 2—3 cm diameter and 15 cm long (fig. 1). Sodium beta alumina, nominally Na20 1 lA1203, has been reported to exist as a single phase in the range of Na20 5.9Al203 to Na20 9.5A1203 accommodating a high excess3+sodium The in structure and 02 [2,3]. arranged spinelcan beblocks described as A1 type which are supported and separated by
Fig. 1. Boule of sodium beta alumina grown along a-axis.
bridging 02— ions thereby forming relatively open planes wherein also are found Na~ions. The degree of openness of these periododic planes is responsible for relatively high diffusivity of the Na~ions in two dimensions. Thus conduction along this, the c-plane, is possible while conductivity perpendicular to this plane is greatly reduced. It can be seen, therefore, that the diffusion path in randomly oriented grains of polycrystalline material is much greater than that in properly oriented single crystals. This is borne out by measurements indicating an order of magnitude greater conductivity for single crystals. Several phase diagrams for the Na20 A1203 system have been proposed and commented on in the literature, but it is clear that the beta alumina solid solution phase exists quite far into the Na20-rich region [4,5]. The principal difficulty in preparing single crystals by growth from the melt has been the loss of sodium oxide due to the high vapor pressure exhibited above 1400°C.As growth from the melt is performed in the range of 1930°C,a substantial loss of Na20 is encountered in the absence of compensating mechanisms. Two such compensating mechanisms have been
.
180
L.R Rothrock / Czochralski growth of single crystalsodium beta alumina
reported in the area of single crystal growth. In 1974, Cocks and Stormont reported on the use of the EFG method in experiments designed to produce tubes of sodium beta alumina [6]. In these experiments crystals were grown from melt compositions (initially) of Na20 1 1Al203 and Na20 4Al203. Growth was carried out using iridium crucible and EFG dies in a sealed growth chamber. The technique used to inhibit the vaporization of Na20 was to pressurize the chamber with argon. Pressures up to 300 psig were employed. Using Na2O~1 1A1203 as the starting cornposition it was found that the beta alumina content of the tubes increased with increased argon overpressure, but single crystal tubes were not obtained. When growth was carried out with a starting composition containing 20 mole percent excess Na20 and argon overpressure of 200 psig a single crystal tube of Na20 7.lAl203 was achieved. Baughman and Lefever contend that the excess Na20 used in the starting material is the major reason that single crystal material was obtained [5]. Their earlier work suggests that the argon overpressure is likely to lower the vaporization rate by only a factor of two or three [7]. They were able to grow sodium beta alumina using only 50 psig overpressure of argon, if they used excess Na20 in the melt. Interestingly, the crystals so grown were the same composition (Na20 7.lAl2O3) as Cocks and Stormont’s. Single crystals obtained from Monofrax H brick (Carborundum Company) have been reported to be Na20 9.5Al2O3 and Na20 7.8M203 [8,9]. Earlier, in 1972, Yancey found that single crystal sodium beta alumina could be produced at atmospheric pressure [10]. The technique employed was the use of a mixture of flowing gasses of N2 and 02 such that the chemical effect of oxygen suppressed the vaporization of Na20. The method used is as follows. Growth is carried out in a traditional Czochralski growth apparatus contained in a bell jar which is unsealed. The melt is 3 contained in anis iridium of aboutinsulation. 130 cm volume which situatedcrucible inside zirconia Heating is provided by an induction coil placed about the ceramic furnace parts and driven by an rf generator. A flowing mixture of N 2/02 containing about 10,000 ppm oxygen is introduced into the furnace and allowed to escape to the atmosphere. The bell jar and furnace are purged for a time period corre-
181
sponding to many fill times. High purity A1203 powder and Na2CO3 (99.999% purity) are mixed in the ratios desired for the starting composition and are placed in a platinum closed, but not sealed, container. The powder is fired at 1200°C for about 12 h in a muffle furnace. The loss in weight corresponds quite well to the expected loss due to the release of CO2. This indicates that any loss of sodium under these conditions is insignificant. The resulting raw material is loaded into the crucible and the system purged as described. The material is heated over a period of about four hours to melting.
.
.
b Fig. 2. (a) View of sodium beta-alumina boule grown along the a-axis. Evidence of pronounced faceting is shown. The photograph is taken looking normal to the c-plane. (b) View of can the same boule looking parallel to the c-plane. Surface cracks be seen lying along the c-plane.
182
L.R Rothrock / Czochralski growth of single crystal sodium beta alumina
The melting point measured by optical pyrometer is 1930°C.It should be noted, however, that this measurement is made through the quartz bell jar and, therefore, is likely to be somewhat in error. A seed crystal cut along the a-axis from an earlier boule is then equilibrated with the melt and with appropriate rotation, pull is begun. The typical pull rate for 2—3 cm diameter crystals is 3 mm/h. Quite careful control is required during the process of expanding the diameter of the crystal from that of the seed to the desired boule dimension. There is a strong tendency for basal and (to a lesser degree) a-plane facets to develop which gives a stairstep appearance to the shoulder area (fig. 2). Growth is continued for the desired amount of time (boules up to 15 cm in length have been grown) and the crystal withdrawn a few miimeters from the melt. Cool down is done approximately linearly over a period of about 24 h. Oxygen partial pressures used are in the range of 5,000 to 15,000 ppm. Na20/A12O3 ratios used in the crucible have ranged from 1: 4 to 1: 9. A typical boule produced under a 10,000 ppm atmosphere was grown from astartingmaterial of Na20 : 7Al2O3. This boule was analyzed and found to be of composition Na20 8Al2O3 [81. In general agreement with the work of Baughman and Lefever it is found that single crystal beta alumina can be grown even at low oxygen partial pressures, if a substantial excess of Na20 is used. It should, however, be noted that this work is done at atmospheric pressure relying on the oxygen in the atmosphere to suppress Na20 loss rather than overpressure of inert gas. At higher oxygen partial pressures Na20/A1203 as high as 1 : 9 results in single crystal material. Crystals show a pronounced tendency to grow with the boule axis oriented at 90°to the c-axis. In cases where control has been lost for one reason or another, single crystal growth would often nucleate and proceed to produce a boule perpendicular to the c-axis. This effect has been noted by others, as well [5]. From structural considerations a strong cleavage plane would be expected to coincide with the basal plane and this is indeed observed. Longitudinal cracks in boules are frequently observed. These cracks may be quite shallow in a newly grown boule, but sometimes propagate 1—4 mm into the boule with the passage of time. Although the effects of water on the
Fig. 3. End view of boule sawed and polished on ends. Surface cracks can be seen and one crack has propagated cornpletely across the boule.
sodium beta alumina have been investigated, and it has been shown that water molecules are occluded by cleavage planes as well as hydronium ions diffusing into the crystal, water is not the sole cause for this progressive cleavage [11]. This is shown by the fact that boules have been placed in a dessicator while still relatively hot and the progression of cracks is still observed. Further, once the cracks have attained the 12
10
—
B —
6
-
2
0
0
20
ROTATION
RATE
30
(
40
50
~
Fig. 4. Relationship between rateused. and 2cm diameter boules inrotation the system length of
interface
L.R Rothrock / Czochralski growth of single crystal sodium beta alumina
183
Fig. 5. Typical a-axis grown sodium beta alumina boules.
maximum penetration into the boule surface, no further propagation is observed in some boules stored in the open air for a year or more. This evidence is not conclusive because sawing attempts have not been made on such material and it has been noted that fresh boules can be sawed under the roughest and most harsh conditions without fracture, but the same boule may fracture during the most gentle sawing attempts a few weeks later (fig. 3). Other growth problems encountered were twins and bubbles. The interface of a boule is observed to be highly faceted and it was speculated that interface breakdown occurs and trapping of second phases and bubbles might be the case. It was found by Brandle that the interface was less faceted if lengthened [12]. An investigation of interface length was done and the relationship between boule rotation rate and interface established for the system in use (fig. 4). It was found that growth at 40 rpm produced a highly faceted mterface of about 6.7 mm length while 20 rpm gave a length of 9.9 mm with significantly reduced faceting. Subsequent growth at the lower rotation rate showed
marked improvement in both bubbles and incidence of twinning. Using the described process acceptable boules totaling over 200 cm in length and having diameters of about 2 cm have been produced. While surface cracks (and subsequent propagation) remain a problem, they generally only extend 1—4 mm into the boule and newly.grown boules can be sawed without danger of fracture. These boules are free of second phase inclusions and twins and (on a macroscopic scale, at least) are single crystal sodium beta alumina. Typical boules are shown in fig. 5.
References [1] Chemical Week, 1 Dec 1976, p. 31. [21M. Mirata, Mater. Res. Bull. 6 (1971) 461. [3] Ray and Subbarao, Mater. Res. Bull. 10 (1975) 583. [4] DeVries and Roth, JAm. Ceram. Soc. 52 (1969) 364. [51 RJ. Baughman and R.A. Lefever Mater. Res. Bull. 10 (1975) 607. [61Cocks and Stormont, J. Electrochem. Soc. 121 (1974) 596.
184
L.R Rothrock / Czochralski growth of single crystal sodium beta alumina
[7] R.J. Baughman, R.A, Lefever and W.R. Wilcox, J. Crystal Growth 8 (1971) 317. 18] Fielder Kautz Fordyce and Singer NASA Technical Memorandum NASA TM X 71546 (1974) [9] Peters, Bettman, Moore and Glick, Acta Cryst. B27 (1971) 1826.
[10] Yancey, US Patent No. 3,917,462, Method of Producing Sodium Beta-Alumina Single Crystals. [11) Will 149th Meeting Electrochem Soc 2—7 May 1976 [121 C D Brandle and L R Rothrock AACG II Meeting July 1975.
CZOCHRALSKI GROWTH OF SINGLE CRYSTAL SODIUM BETA ALUMINA L.R ROTHROCK Union Carbide Corporation, Crystal Products Department, 8888 Balboa Avenue, San Diego, California 92123, USA Received 10 December 1976;revised manuscript received 29 December 1976
Single crystal growth of sodium beta alumina has been performed using the Czochralski technique. Loss of Na
20 from the melt is inhibited by the use of 5,000—15,000 ppm oxygen in a flowing nitrogen atmosphere. Crystals produced were 2—3 cm in diameter and up to 15 cm long. Boules were grown along the a-axis.
The recently intensified work in solid ionic conductors, especially as applied to electric battery devices such as the sodium—sulfur secondary battery, has been brought on, in part, by the world-wide energy situation faced today. There are obvious advantages to be achieved by the use of a solid electrolyte, but a major impetus is the high energy density (Elm) that has been achieved in such cells. The theoretical energy density for a sodium—sulfur battery employing sodium beta-alumina is 795 W-hr/kg while advanced lead-acid batteries have a theoretical value of 165 W-hr/kg [1]. While present sodium—sulfur batteries use polycrystalline sodium beta-alumina because of the relative ease of obtaining polycrystalline material, single crystals are obviously simpler components in basic experiments designed to measure conductivity and transport mechanisms as the data can be treated in a much more straightforward manner. While it is unlikely that single crystal material can be produced in the sizes necessary for use in practical high power batteries in the near future, sizes and quality appropriate for laboratory experiments have been achieved. Single crystal sodium beta alumina can now be grown in boules of up to 2—3 cm diameter and 15 cm long (fig. 1). Sodium beta alumina, nominally Na20 1 lA1203, has been reported to exist as a single phase in the range of Na20 5.9Al203 to Na20 9.5A1203 accommodating a high excess3+sodium The in structure and 02 [2,3]. arranged spinelcan beblocks described as A1 type which are supported and separated by
Fig. 1. Boule of sodium beta alumina grown along a-axis.
bridging 02— ions thereby forming relatively open planes wherein also are found Na~ions. The degree of openness of these periododic planes is responsible for relatively high diffusivity of the Na~ions in two dimensions. Thus conduction along this, the c-plane, is possible while conductivity perpendicular to this plane is greatly reduced. It can be seen, therefore, that the diffusion path in randomly oriented grains of polycrystalline material is much greater than that in properly oriented single crystals. This is borne out by measurements indicating an order of magnitude greater conductivity for single crystals. Several phase diagrams for the Na20 A1203 system have been proposed and commented on in the literature, but it is clear that the beta alumina solid solution phase exists quite far into the Na20-rich region [4,5]. The principal difficulty in preparing single crystals by growth from the melt has been the loss of sodium oxide due to the high vapor pressure exhibited above 1400°C.As growth from the melt is performed in the range of 1930°C,a substantial loss of Na20 is encountered in the absence of compensating mechanisms. Two such compensating mechanisms have been
.
180
L.R Rothrock / Czochralski growth of single crystalsodium beta alumina
reported in the area of single crystal growth. In 1974, Cocks and Stormont reported on the use of the EFG method in experiments designed to produce tubes of sodium beta alumina [6]. In these experiments crystals were grown from melt compositions (initially) of Na20 1 1Al203 and Na20 4Al203. Growth was carried out using iridium crucible and EFG dies in a sealed growth chamber. The technique used to inhibit the vaporization of Na20 was to pressurize the chamber with argon. Pressures up to 300 psig were employed. Using Na2O~1 1A1203 as the starting cornposition it was found that the beta alumina content of the tubes increased with increased argon overpressure, but single crystal tubes were not obtained. When growth was carried out with a starting composition containing 20 mole percent excess Na20 and argon overpressure of 200 psig a single crystal tube of Na20 7.lAl203 was achieved. Baughman and Lefever contend that the excess Na20 used in the starting material is the major reason that single crystal material was obtained [5]. Their earlier work suggests that the argon overpressure is likely to lower the vaporization rate by only a factor of two or three [7]. They were able to grow sodium beta alumina using only 50 psig overpressure of argon, if they used excess Na20 in the melt. Interestingly, the crystals so grown were the same composition (Na20 7.lAl2O3) as Cocks and Stormont’s. Single crystals obtained from Monofrax H brick (Carborundum Company) have been reported to be Na20 9.5Al2O3 and Na20 7.8M203 [8,9]. Earlier, in 1972, Yancey found that single crystal sodium beta alumina could be produced at atmospheric pressure [10]. The technique employed was the use of a mixture of flowing gasses of N2 and 02 such that the chemical effect of oxygen suppressed the vaporization of Na20. The method used is as follows. Growth is carried out in a traditional Czochralski growth apparatus contained in a bell jar which is unsealed. The melt is 3 contained in anis iridium of aboutinsulation. 130 cm volume which situatedcrucible inside zirconia Heating is provided by an induction coil placed about the ceramic furnace parts and driven by an rf generator. A flowing mixture of N 2/02 containing about 10,000 ppm oxygen is introduced into the furnace and allowed to escape to the atmosphere. The bell jar and furnace are purged for a time period corre-
181
sponding to many fill times. High purity A1203 powder and Na2CO3 (99.999% purity) are mixed in the ratios desired for the starting composition and are placed in a platinum closed, but not sealed, container. The powder is fired at 1200°C for about 12 h in a muffle furnace. The loss in weight corresponds quite well to the expected loss due to the release of CO2. This indicates that any loss of sodium under these conditions is insignificant. The resulting raw material is loaded into the crucible and the system purged as described. The material is heated over a period of about four hours to melting.
.
.
b Fig. 2. (a) View of sodium beta-alumina boule grown along the a-axis. Evidence of pronounced faceting is shown. The photograph is taken looking normal to the c-plane. (b) View of can the same boule looking parallel to the c-plane. Surface cracks be seen lying along the c-plane.
182
L.R Rothrock / Czochralski growth of single crystal sodium beta alumina
The melting point measured by optical pyrometer is 1930°C.It should be noted, however, that this measurement is made through the quartz bell jar and, therefore, is likely to be somewhat in error. A seed crystal cut along the a-axis from an earlier boule is then equilibrated with the melt and with appropriate rotation, pull is begun. The typical pull rate for 2—3 cm diameter crystals is 3 mm/h. Quite careful control is required during the process of expanding the diameter of the crystal from that of the seed to the desired boule dimension. There is a strong tendency for basal and (to a lesser degree) a-plane facets to develop which gives a stairstep appearance to the shoulder area (fig. 2). Growth is continued for the desired amount of time (boules up to 15 cm in length have been grown) and the crystal withdrawn a few miimeters from the melt. Cool down is done approximately linearly over a period of about 24 h. Oxygen partial pressures used are in the range of 5,000 to 15,000 ppm. Na20/A12O3 ratios used in the crucible have ranged from 1: 4 to 1: 9. A typical boule produced under a 10,000 ppm atmosphere was grown from astartingmaterial of Na20 : 7Al2O3. This boule was analyzed and found to be of composition Na20 8Al2O3 [81. In general agreement with the work of Baughman and Lefever it is found that single crystal beta alumina can be grown even at low oxygen partial pressures, if a substantial excess of Na20 is used. It should, however, be noted that this work is done at atmospheric pressure relying on the oxygen in the atmosphere to suppress Na20 loss rather than overpressure of inert gas. At higher oxygen partial pressures Na20/A1203 as high as 1 : 9 results in single crystal material. Crystals show a pronounced tendency to grow with the boule axis oriented at 90°to the c-axis. In cases where control has been lost for one reason or another, single crystal growth would often nucleate and proceed to produce a boule perpendicular to the c-axis. This effect has been noted by others, as well [5]. From structural considerations a strong cleavage plane would be expected to coincide with the basal plane and this is indeed observed. Longitudinal cracks in boules are frequently observed. These cracks may be quite shallow in a newly grown boule, but sometimes propagate 1—4 mm into the boule with the passage of time. Although the effects of water on the
Fig. 3. End view of boule sawed and polished on ends. Surface cracks can be seen and one crack has propagated cornpletely across the boule.
sodium beta alumina have been investigated, and it has been shown that water molecules are occluded by cleavage planes as well as hydronium ions diffusing into the crystal, water is not the sole cause for this progressive cleavage [11]. This is shown by the fact that boules have been placed in a dessicator while still relatively hot and the progression of cracks is still observed. Further, once the cracks have attained the 12
10
—
B —
6
-
2
0
0
20
ROTATION
RATE
30
(
40
50
~
Fig. 4. Relationship between rateused. and 2cm diameter boules inrotation the system length of
interface
L.R Rothrock / Czochralski growth of single crystal sodium beta alumina
183
Fig. 5. Typical a-axis grown sodium beta alumina boules.
maximum penetration into the boule surface, no further propagation is observed in some boules stored in the open air for a year or more. This evidence is not conclusive because sawing attempts have not been made on such material and it has been noted that fresh boules can be sawed under the roughest and most harsh conditions without fracture, but the same boule may fracture during the most gentle sawing attempts a few weeks later (fig. 3). Other growth problems encountered were twins and bubbles. The interface of a boule is observed to be highly faceted and it was speculated that interface breakdown occurs and trapping of second phases and bubbles might be the case. It was found by Brandle that the interface was less faceted if lengthened [12]. An investigation of interface length was done and the relationship between boule rotation rate and interface established for the system in use (fig. 4). It was found that growth at 40 rpm produced a highly faceted mterface of about 6.7 mm length while 20 rpm gave a length of 9.9 mm with significantly reduced faceting. Subsequent growth at the lower rotation rate showed
marked improvement in both bubbles and incidence of twinning. Using the described process acceptable boules totaling over 200 cm in length and having diameters of about 2 cm have been produced. While surface cracks (and subsequent propagation) remain a problem, they generally only extend 1—4 mm into the boule and newly.grown boules can be sawed without danger of fracture. These boules are free of second phase inclusions and twins and (on a macroscopic scale, at least) are single crystal sodium beta alumina. Typical boules are shown in fig. 5.
References [1] Chemical Week, 1 Dec 1976, p. 31. [21M. Mirata, Mater. Res. Bull. 6 (1971) 461. [3] Ray and Subbarao, Mater. Res. Bull. 10 (1975) 583. [4] DeVries and Roth, JAm. Ceram. Soc. 52 (1969) 364. [51 RJ. Baughman and R.A. Lefever Mater. Res. Bull. 10 (1975) 607. [61Cocks and Stormont, J. Electrochem. Soc. 121 (1974) 596.
184
L.R Rothrock / Czochralski growth of single crystal sodium beta alumina
[7] R.J. Baughman, R.A, Lefever and W.R. Wilcox, J. Crystal Growth 8 (1971) 317. 18] Fielder Kautz Fordyce and Singer NASA Technical Memorandum NASA TM X 71546 (1974) [9] Peters, Bettman, Moore and Glick, Acta Cryst. B27 (1971) 1826.
[10] Yancey, US Patent No. 3,917,462, Method of Producing Sodium Beta-Alumina Single Crystals. [11) Will 149th Meeting Electrochem Soc 2—7 May 1976 [121 C D Brandle and L R Rothrock AACG II Meeting July 1975.