- Email: [email protected]
The sintering of amorphous selenium-arsenic powders
JOURNALOF NON-CRYSTALLINESOLIDS7 (1972) 86--92 © North-Holland Publishing Co.
T H E SINTERING OF A M O R P H O U S S E L E N I U M - A R S E N I C POWDERS R. N. LOTT and Z. A. MUNIR Department of Materials Science, School of Engineering, San Jose State College, San Jose, California 95114, U.S.A.
Received 16 July 1971 The sintering behavior of an 80 at~ Se-20 at HAs glass was studied at 90, 100, 110, and 120°C. Results of shrinkage measurements of powder compacts indicated that viscous flow is the controlling mechanism in the sintering of this material. Furthermore, the activation energy for this process was calculated to be 52 kcal/mole. In agreement with previous observations, no evidence of crystallization in any of the samples during the relatively long sintering times could be detected. 1. Introduction
Largely because of their optical and electrical properties, the chalcogenides of arsenic have received a great deal of attention during the past decade 1-7). In part, interest in these materials originates from their ability to exist in both the crystalline and glassy modifications. Of the four arsenic-chalcogen binaries, that of As-Se has been studied in depth relatively recently by Dembovskii and Luzhnaya s). These investigators found that compositions ranging from 0 to 60 a t ~ As readily formed glasses while only elemental selenium and the compounds As2Se3 and AsSe could be completely crystallized when annealed. Furthermore, the eutectic composition with the highest selenium content, 80 at~o Se, was found to persist in the glassy state even after prolonged annealing. For this reason, and as part of an investigation of some properties of the As-Se binary and the As-Se-I ternary systems, the sintering behavior of an 80 a t ~ Se-20 at~o As glass was studied. This composition offers the opportunity of observing the sintering process without possible complications arising from devitrification. In the sintering of powder compacts, the neck growth between idealized spherical particles is customarily defined by the equation x / a = A t m,
(1)
where x is the radius of the neck between two spheres, a is the radius of the 86
SINTERING OF AMORPHOUS
Se-As POWDERS
87
spherical particle, t is time, and A and m are constants. Values for m depend on the mechanism controlling the sintering process, for example, m = 0 . 5 corresponds to a sintering behavior controlled by viscous flow, as is the case for glassy substances 9,10). The sintering of spheres with various composition in the ternary system As-S-Se has been studied by Carrell and WilderS). Their results indicated, in agreement with previous investigations on more traditional glasses 9,10), that viscous flow is the dominant mechanism in the sintering of these arsenic chalcogenides glasses. However, their investigation, which did not include the composition on the As-Se binary previously shown not to devitrify, did indicate a dependence of the constant m on composition even in the As-Se and As-S binaries. In this paper we report results of sintering experiments on an 80 at% Se-20 at% As glass which were based on shrinkage measurements of powder compacts.
2. Experimental Appropriate amounts of relatively pure (99.999%) selenium and arsenic were weighed into pyrex ampules which were then repeatedly flushed with argon and vacuum sealed. The sealed ampules are attached to the end of a graphite rod and extended into a tube furnace whose temperature was set at 600°C. By means of an electric motor, the graphite rod was rotated to insure complete mixing of the elements in the ampule while being heated for at least three hours. At the end of the homogenizing period, the ampules were allowed to cool, then broken open and their contents removed for analysis. X-ray powder diffraction and Laue back reflection techniques showed that the samples prepared were non-crystalline. The resulting glasses were ground to a powder and then screened to obtain a reasonably well defined particle-size range. The powders used in the sintering experiments constituted the portion that passed through the 150 mesh screen but not the 325 mesh screen. Thus the particles ranged in size from approximately 43 to 103 pm. Approximately 1.5 g of powdered material was compressed under pressures of between 2.3 x 103 and 2.4 x 10 3 arm to form a cylindrical compact about 7.62 mm long with a diameter of equal value. These compacts were then sintered at various temperatures inside a tube furnace and under an atmosphere of argon. Temperatures were measured by means of a calibrated chromel-alumel thermocouple which was positioned next to the powder compacts in the middle region of the tube furnace. Previous determination of the temperature profile of the furnace indicated that the samples were all at the same temperature. Maximum uncertainty in the recorded temperatures was believed not to exceed 2 °C at any of the 90, 100, 110, and 120 °C temperature settings which were utilized in the sintering
88
R.N. LOTT AND Z. A. MUNIR
study. To minimize sintering times at undefinable temperatures, samples were quickly introduced to the furnace which had already equilibriated to the desired temperature. Measurements of shrinkage of the powder compacts were made at chosen time intervals after removing the samples from the furnace and allowing them to cool to room temperature. These measurements were made by means of iO-t IlO*C F l . j ~ ~ = 0 " 4 7 ' "J ~ J
IZO~,.'~M = 0.44 ,b
J?ooc
~.~
°;Y'.°°~2
r~
10-2
i0'
I
[
9o.cJ
A
M=0.74
I I I I III
10
oZ
1
I
i
I I I III
I
I
I
1,000
I00
I I III
I0,000
t (min)
Fig. I. Time-dependence of shrinkage of 80 at % Se-20 at % As powders.
r
- E ) - 90* C
0.06 ~-
|
|
/
/
-z~- ,oo- c
- . - ,,o.~
o.oz
0.01 ~
o
~/~
t
~o
2o
3o
40
50 t~
Fig. 2.
....
J_
6o
J
7o
. _~
8o
(min)~
Kinetics of shrinkage of powder compacts.
9~
ioo
SINTERING
OF AMORPHOUS
Se-As
89
POWDERS
traveling microscope capable of recording changes in length as little as 2.5 x 10 -4 mm. In order to insure consistent orientation of each sample with each measurement, precision-ground gauge blocks were used for alignment. At the end of the sintering experiments, the samples were X-rayed to verify the existence of the glassy phase only. 3. Results and discussion
Results of shrinkage measurments of the powder compacts are given in table 1. Fig. 1 shows a plot of l o g ( A l / l o ) versus logt where AI is the measured shrinkage, lo is the initial length, and t is the sintering time. For the sintering of compacts, eq. (1) can be expressed in the following form 11) (2)
(At~to) = h ' t m ,
and thus the slopes of the lines of fig. 1 represent the values of the exponential, m. With the exception of the 90 °C data, the values of m, shown on fig. 1, are reasonably close to ½, the accepted value for viscous flow-controlled sintering of glasses. Thus a plot of ( A l / l o ) v e r s u s t t/2 gives a straight line whose slope equals to the numerical value of the constant A'. Fig. 2 shows such a plot for each temperature. The rate constant for the sintering process is related 10-4
\ 10-5
tO .¢.03 E: 0
~J ~.)
10-$
rr
10-7
2.5
I 2,6
I 2,7
;).8
103/T (OK)-' Fig. 3.
Temperature-dependence of the rate constant of sintering.
90
R . N . LOTT AND Z . A . MUNIR
TABLE 1 S h r i n k a g e d a t a of A s - S e c o m p a c t s Run No.
t
1
AI
(min)
(mm)
(mm)
Al/ lo
0 1338 5628 8508 0 1338 5628 8508
7.787 7.737 7.620 7.516 7.805 7.747 7.638 7.541
0 30 81 173 293 498 1582 0 30 81 173 293 498 1582
7.772 7.709 7.678 7.645 7.595 7.523 7.409 7.572 7.536 7.503 7.455 7. 394 7.366 7.247
0 33 65 137 279 402 0 33 65 137 179 402
7.419 7.338 7.269 7.231 7.137 7.080 8.3ll 8.186 8.151 8.062 7.963 7.910
0 10 20 30 0 10 20 30 45
7.800 7.686 7.597 7.557 7.543 7.419 7.308 7.252 7.226
po (g/cm a)
pt (g/cm a)
T = 90°C 1
2
0.0050 0.0167 0.0271 0.0058 0.0167 0.0264 T :
3
4
0.0064 0.0214 0.0348 0.0074 0.0214 0.0338
3.57
-
3.57
4.1 -
0.0081 0.0121 0.0163 0.0228 0.0320 0.0467 0.0048 0.0091 0.0155 0.0235
3.51
-
3.53
4.2 -
4.1
100°C
0.0063 0.0094 0.0127 0.0177 0.0249 0.0363 0.0036 0.0069 0.0117 0.0178 0.0206 0.0325
0.0429
4.2
T = 110°C 5
0.0081 0.0150 0.0188 0.0282 0.0337 0.0125 0.0160 0.0249 0.0348 0.0401
0.0110 0.0202 0.0253 0.0380 0.0454 0.0150 0.0193 0.0300 0.0419 0.0482
3.52
-
3.44
4.1 -
4.1
T = 120°C 7
8
0.0114 0.0203 0.0243 0.0115 0.0226 0.0282 0.0308
0.0146 0.0260 0.0312 0.0153 0.0300 0.0374 0.0409
3.50
-
4.2 3.52
4.1
SINTERING OF AMORPHOUS
Se-As POWDERS
91
to A' by K=(A') l/r" or K = ( A ' ) 2 (refs. 12, 13), and thus a plot of log K versus reciprocal of the absolute temperature gives a straight line whose slope represents the activation energy for sintering as described by the following equation K = c e -Q/Rr, (3) where Kis the rate constant, Q is the activation energy, R is the gas constant, T is the absolute temperature, and C is a constant. Such a plot, shown in fig. 3, gave a value for Q of 52 kcal/mole. Table 2 lists values of K and A'. TABLE 2
Values of A' and K of eqs. (2) and (3) (°K)
T
A' (min)-l/2
K (rain)-1
363 373 383 393
3.15 × 10-4 1.19 × 10-a 2.36 × 10-3 5.82 × 10 z
9.75 × 10-s 1.42 × 10 6 5.58 × 10 6 3 . 3 8 × 10-2
With respect to the value of the exponential m, the results presented here are in good agreement with the expected value of ½ and the experimentally determined values from the sintering of silica-based 11) and As-Se glasses 5). The only line of fig. 1 whose slope is appreciably different from ½ is that generated by the data obtained at the lowest temperature, 90 °C. The activation energy obtained in this study, 52 kcal/mole, compares to the value 37 kcal/mole which is reported to be the activation energy for viscous flow for the compound As2Se314,15), a compound containing twice as much As as the glass investigated in thus study. Carrell and Wilder ~) have shown that the value of m, and thus by implication the activation energy, is composition dependent along the As-Se binary. Since sintering processes have been shown to be influenced by impurities in the materials or the atmosphere under which sintering takes place, all preparatory steps were conducted in such a manner that contamination is minimized. In their work on glasses in the As-S-Se system, Carrell and Wilder 5) could not determine differences in the composition of spheres of glasses formed in nitrogen and others formed in air. X-ray examination of the compacts immediately after sintering showed them to be glassy. However, examinations on arsenic-selenium compositions closer to the boundary between the glassy and crystalline regions in the ternary As-Se-116), showed measurable crystallization after annealing at conditions of temperature and time approximating those described in this work.
92
R . N . L O T r AND Z . A . M U N I R
Acknowledgments The technical assistance of Walter M o u n t s a n d L o u Schallberger is gratefully acknowledged. We are appreciative of Phyllis Ashe's help in the preparation of the manuscript.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)
S. S. Flaschen, A. D. Pearson and W. R. Northover, J. Appl. Phys. 31 (1960) 219 A. A. Vatpolin and E. A. Porai-Koshits, Soviet Phys.-Solid State 2 (1960) 1500. Z. U. Borisova, Izv. Akad. Nauk SSSR, Ser. Fiz. 28 (1964) 1293. R. L. Myuller and Z. U. Borisova, Solid State Chemistry (Consultants Bureau, New York, 1966). M. A. Carrell and D. R. Wilder, J. Am. Ceram. Soc. 50 (1967) 604. G. B. Street and Z. A. Munir, J. lnorg. Nucl. Chem. 32 (1970) 3769. Z. A. Munir, G. B. Street and H. F. Winters, J. Chem. Phys. 55 (1971) 4520. S. A. Dembovskii and N. P. Luzhnaya, Russ. J. Inorg. Chem. 9 (1964) 365. G. C. Kuczynski, J. Appl. Phys. 20 (1949) 1160. W. D. Kingery and M. Berg, J. Appl. Phys. 26 (1955) 1205. D. L. Johnson and I. B. Cutler, J. Am. Ceram. Soc. 46 (1963) 541. G. C. Kuczynski, Trans. AIME 185 (1949) 169. A. G. Elliot and Z. A. Munir, J. Mater. Sci. 3 (1968), 150. S. V. Nemilov and G. T. Petrovskii, Zh. Prikl. Khim. 36 (1963), 1909. R. L. Myuller, Zh. Prikl. Khim. 28 (1955) 363, 1077. L. M. Fuke and Z. A. Munir, to be published.
T H E SINTERING OF A M O R P H O U S S E L E N I U M - A R S E N I C POWDERS R. N. LOTT and Z. A. MUNIR Department of Materials Science, School of Engineering, San Jose State College, San Jose, California 95114, U.S.A.
Received 16 July 1971 The sintering behavior of an 80 at~ Se-20 at HAs glass was studied at 90, 100, 110, and 120°C. Results of shrinkage measurements of powder compacts indicated that viscous flow is the controlling mechanism in the sintering of this material. Furthermore, the activation energy for this process was calculated to be 52 kcal/mole. In agreement with previous observations, no evidence of crystallization in any of the samples during the relatively long sintering times could be detected. 1. Introduction
Largely because of their optical and electrical properties, the chalcogenides of arsenic have received a great deal of attention during the past decade 1-7). In part, interest in these materials originates from their ability to exist in both the crystalline and glassy modifications. Of the four arsenic-chalcogen binaries, that of As-Se has been studied in depth relatively recently by Dembovskii and Luzhnaya s). These investigators found that compositions ranging from 0 to 60 a t ~ As readily formed glasses while only elemental selenium and the compounds As2Se3 and AsSe could be completely crystallized when annealed. Furthermore, the eutectic composition with the highest selenium content, 80 at~o Se, was found to persist in the glassy state even after prolonged annealing. For this reason, and as part of an investigation of some properties of the As-Se binary and the As-Se-I ternary systems, the sintering behavior of an 80 a t ~ Se-20 at~o As glass was studied. This composition offers the opportunity of observing the sintering process without possible complications arising from devitrification. In the sintering of powder compacts, the neck growth between idealized spherical particles is customarily defined by the equation x / a = A t m,
(1)
where x is the radius of the neck between two spheres, a is the radius of the 86
SINTERING OF AMORPHOUS
Se-As POWDERS
87
spherical particle, t is time, and A and m are constants. Values for m depend on the mechanism controlling the sintering process, for example, m = 0 . 5 corresponds to a sintering behavior controlled by viscous flow, as is the case for glassy substances 9,10). The sintering of spheres with various composition in the ternary system As-S-Se has been studied by Carrell and WilderS). Their results indicated, in agreement with previous investigations on more traditional glasses 9,10), that viscous flow is the dominant mechanism in the sintering of these arsenic chalcogenides glasses. However, their investigation, which did not include the composition on the As-Se binary previously shown not to devitrify, did indicate a dependence of the constant m on composition even in the As-Se and As-S binaries. In this paper we report results of sintering experiments on an 80 at% Se-20 at% As glass which were based on shrinkage measurements of powder compacts.
2. Experimental Appropriate amounts of relatively pure (99.999%) selenium and arsenic were weighed into pyrex ampules which were then repeatedly flushed with argon and vacuum sealed. The sealed ampules are attached to the end of a graphite rod and extended into a tube furnace whose temperature was set at 600°C. By means of an electric motor, the graphite rod was rotated to insure complete mixing of the elements in the ampule while being heated for at least three hours. At the end of the homogenizing period, the ampules were allowed to cool, then broken open and their contents removed for analysis. X-ray powder diffraction and Laue back reflection techniques showed that the samples prepared were non-crystalline. The resulting glasses were ground to a powder and then screened to obtain a reasonably well defined particle-size range. The powders used in the sintering experiments constituted the portion that passed through the 150 mesh screen but not the 325 mesh screen. Thus the particles ranged in size from approximately 43 to 103 pm. Approximately 1.5 g of powdered material was compressed under pressures of between 2.3 x 103 and 2.4 x 10 3 arm to form a cylindrical compact about 7.62 mm long with a diameter of equal value. These compacts were then sintered at various temperatures inside a tube furnace and under an atmosphere of argon. Temperatures were measured by means of a calibrated chromel-alumel thermocouple which was positioned next to the powder compacts in the middle region of the tube furnace. Previous determination of the temperature profile of the furnace indicated that the samples were all at the same temperature. Maximum uncertainty in the recorded temperatures was believed not to exceed 2 °C at any of the 90, 100, 110, and 120 °C temperature settings which were utilized in the sintering
88
R.N. LOTT AND Z. A. MUNIR
study. To minimize sintering times at undefinable temperatures, samples were quickly introduced to the furnace which had already equilibriated to the desired temperature. Measurements of shrinkage of the powder compacts were made at chosen time intervals after removing the samples from the furnace and allowing them to cool to room temperature. These measurements were made by means of iO-t IlO*C F l . j ~ ~ = 0 " 4 7 ' "J ~ J
IZO~,.'~M = 0.44 ,b
J?ooc
~.~
°;Y'.°°~2
r~
10-2
i0'
I
[
9o.cJ
A
M=0.74
I I I I III
10
oZ
1
I
i
I I I III
I
I
I
1,000
I00
I I III
I0,000
t (min)
Fig. I. Time-dependence of shrinkage of 80 at % Se-20 at % As powders.
r
- E ) - 90* C
0.06 ~-
|
|
/
/
-z~- ,oo- c
- . - ,,o.~
o.oz
0.01 ~
o
~/~
t
~o
2o
3o
40
50 t~
Fig. 2.
....
J_
6o
J
7o
. _~
8o
(min)~
Kinetics of shrinkage of powder compacts.
9~
ioo
SINTERING
OF AMORPHOUS
Se-As
89
POWDERS
traveling microscope capable of recording changes in length as little as 2.5 x 10 -4 mm. In order to insure consistent orientation of each sample with each measurement, precision-ground gauge blocks were used for alignment. At the end of the sintering experiments, the samples were X-rayed to verify the existence of the glassy phase only. 3. Results and discussion
Results of shrinkage measurments of the powder compacts are given in table 1. Fig. 1 shows a plot of l o g ( A l / l o ) versus logt where AI is the measured shrinkage, lo is the initial length, and t is the sintering time. For the sintering of compacts, eq. (1) can be expressed in the following form 11) (2)
(At~to) = h ' t m ,
and thus the slopes of the lines of fig. 1 represent the values of the exponential, m. With the exception of the 90 °C data, the values of m, shown on fig. 1, are reasonably close to ½, the accepted value for viscous flow-controlled sintering of glasses. Thus a plot of ( A l / l o ) v e r s u s t t/2 gives a straight line whose slope equals to the numerical value of the constant A'. Fig. 2 shows such a plot for each temperature. The rate constant for the sintering process is related 10-4
\ 10-5
tO .¢.03 E: 0
~J ~.)
10-$
rr
10-7
2.5
I 2,6
I 2,7
;).8
103/T (OK)-' Fig. 3.
Temperature-dependence of the rate constant of sintering.
90
R . N . LOTT AND Z . A . MUNIR
TABLE 1 S h r i n k a g e d a t a of A s - S e c o m p a c t s Run No.
t
1
AI
(min)
(mm)
(mm)
Al/ lo
0 1338 5628 8508 0 1338 5628 8508
7.787 7.737 7.620 7.516 7.805 7.747 7.638 7.541
0 30 81 173 293 498 1582 0 30 81 173 293 498 1582
7.772 7.709 7.678 7.645 7.595 7.523 7.409 7.572 7.536 7.503 7.455 7. 394 7.366 7.247
0 33 65 137 279 402 0 33 65 137 179 402
7.419 7.338 7.269 7.231 7.137 7.080 8.3ll 8.186 8.151 8.062 7.963 7.910
0 10 20 30 0 10 20 30 45
7.800 7.686 7.597 7.557 7.543 7.419 7.308 7.252 7.226
po (g/cm a)
pt (g/cm a)
T = 90°C 1
2
0.0050 0.0167 0.0271 0.0058 0.0167 0.0264 T :
3
4
0.0064 0.0214 0.0348 0.0074 0.0214 0.0338
3.57
-
3.57
4.1 -
0.0081 0.0121 0.0163 0.0228 0.0320 0.0467 0.0048 0.0091 0.0155 0.0235
3.51
-
3.53
4.2 -
4.1
100°C
0.0063 0.0094 0.0127 0.0177 0.0249 0.0363 0.0036 0.0069 0.0117 0.0178 0.0206 0.0325
0.0429
4.2
T = 110°C 5
0.0081 0.0150 0.0188 0.0282 0.0337 0.0125 0.0160 0.0249 0.0348 0.0401
0.0110 0.0202 0.0253 0.0380 0.0454 0.0150 0.0193 0.0300 0.0419 0.0482
3.52
-
3.44
4.1 -
4.1
T = 120°C 7
8
0.0114 0.0203 0.0243 0.0115 0.0226 0.0282 0.0308
0.0146 0.0260 0.0312 0.0153 0.0300 0.0374 0.0409
3.50
-
4.2 3.52
4.1
SINTERING OF AMORPHOUS
Se-As POWDERS
91
to A' by K=(A') l/r" or K = ( A ' ) 2 (refs. 12, 13), and thus a plot of log K versus reciprocal of the absolute temperature gives a straight line whose slope represents the activation energy for sintering as described by the following equation K = c e -Q/Rr, (3) where Kis the rate constant, Q is the activation energy, R is the gas constant, T is the absolute temperature, and C is a constant. Such a plot, shown in fig. 3, gave a value for Q of 52 kcal/mole. Table 2 lists values of K and A'. TABLE 2
Values of A' and K of eqs. (2) and (3) (°K)
T
A' (min)-l/2
K (rain)-1
363 373 383 393
3.15 × 10-4 1.19 × 10-a 2.36 × 10-3 5.82 × 10 z
9.75 × 10-s 1.42 × 10 6 5.58 × 10 6 3 . 3 8 × 10-2
With respect to the value of the exponential m, the results presented here are in good agreement with the expected value of ½ and the experimentally determined values from the sintering of silica-based 11) and As-Se glasses 5). The only line of fig. 1 whose slope is appreciably different from ½ is that generated by the data obtained at the lowest temperature, 90 °C. The activation energy obtained in this study, 52 kcal/mole, compares to the value 37 kcal/mole which is reported to be the activation energy for viscous flow for the compound As2Se314,15), a compound containing twice as much As as the glass investigated in thus study. Carrell and Wilder ~) have shown that the value of m, and thus by implication the activation energy, is composition dependent along the As-Se binary. Since sintering processes have been shown to be influenced by impurities in the materials or the atmosphere under which sintering takes place, all preparatory steps were conducted in such a manner that contamination is minimized. In their work on glasses in the As-S-Se system, Carrell and Wilder 5) could not determine differences in the composition of spheres of glasses formed in nitrogen and others formed in air. X-ray examination of the compacts immediately after sintering showed them to be glassy. However, examinations on arsenic-selenium compositions closer to the boundary between the glassy and crystalline regions in the ternary As-Se-116), showed measurable crystallization after annealing at conditions of temperature and time approximating those described in this work.
92
R . N . L O T r AND Z . A . M U N I R
Acknowledgments The technical assistance of Walter M o u n t s a n d L o u Schallberger is gratefully acknowledged. We are appreciative of Phyllis Ashe's help in the preparation of the manuscript.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)
S. S. Flaschen, A. D. Pearson and W. R. Northover, J. Appl. Phys. 31 (1960) 219 A. A. Vatpolin and E. A. Porai-Koshits, Soviet Phys.-Solid State 2 (1960) 1500. Z. U. Borisova, Izv. Akad. Nauk SSSR, Ser. Fiz. 28 (1964) 1293. R. L. Myuller and Z. U. Borisova, Solid State Chemistry (Consultants Bureau, New York, 1966). M. A. Carrell and D. R. Wilder, J. Am. Ceram. Soc. 50 (1967) 604. G. B. Street and Z. A. Munir, J. lnorg. Nucl. Chem. 32 (1970) 3769. Z. A. Munir, G. B. Street and H. F. Winters, J. Chem. Phys. 55 (1971) 4520. S. A. Dembovskii and N. P. Luzhnaya, Russ. J. Inorg. Chem. 9 (1964) 365. G. C. Kuczynski, J. Appl. Phys. 20 (1949) 1160. W. D. Kingery and M. Berg, J. Appl. Phys. 26 (1955) 1205. D. L. Johnson and I. B. Cutler, J. Am. Ceram. Soc. 46 (1963) 541. G. C. Kuczynski, Trans. AIME 185 (1949) 169. A. G. Elliot and Z. A. Munir, J. Mater. Sci. 3 (1968), 150. S. V. Nemilov and G. T. Petrovskii, Zh. Prikl. Khim. 36 (1963), 1909. R. L. Myuller, Zh. Prikl. Khim. 28 (1955) 363, 1077. L. M. Fuke and Z. A. Munir, to be published.