- Email: [email protected]
Characterization of directionally solidified lead chloride
80
Journal of Crystal Growth 89 (1988) 80—85 North-Holland, Amsterdam
CHARACTERIZATION OF DIRECTIONALLY SOLIDIFIED LEAD CHLORIDE Narsingh Bahadur SINGH Westinghouse R&D Center, 1310 Beulah Road, Pittsburgh, Pennsylvania 15235, USA
and W.M.B. DUVAL and B.N. ROSENTHAL NASA Lewis Research Center, Cleveland, Ohio 44135, USA
A complete analysis has been carried Out Ofl directionally solidified lead(II) chloride material. Purification by directional freezing consistently produced high purity material suitable for subsequent growth of single crystals. It was observed that silicon, magnesium, halogens, sulfur and phosphorous were the hardest impurities to remove from the supplied material. Direct photographic observations of the solid—liquid interface were taken at several values of G/v ratios (denoting the temperature gradient and the translation velocity, respectively) to study the morphology of the interface and optimize the growth conditions. The solid—liquid interface morphology varied from a smooth convex shape to dendritic as the G/v ratio decreased. Single crystals subsequently grown from the material purified by the present method showed no optical distortion, exhibited a transmission range from 0.30 to 20 gm, and displayed extremely low beam scattering.
1. Infroduction Lead molybdate has become an important material for the fabrication of acousto-optic devices since the early seventies [1,2]. Since this material transmits only up to 5.5 ~tm its application is limited to low wavelength regions. Recent research on the lead halides [3—6]has shown that these crystals have relatively high refractive indices and low rates of propagation of acoustic vibrations. The combination of these properties infer a high acousto-optical figure of merit, approximately 10 times higher than PbMoO4. Lead chloride (PbCl2) transmits from 0.30 to 20 ~.tm, lead bromide to 30 ~tm and lead iodide to 40 ~tm. The wide range of transparency, high acousto-opti merit and reasonably good mechanical properties make this class of compounds suitable candidates for device applications. To achieve high performance from the device, high quality single crystals are needed. This can be achieved by optimizing the growth conditions and using extremely pure source material. Such ground-based experimentation is essential in order to decide whether sub-
sequent microgravity experimentation is desirable or required for further improvements of crystal quality. In the present article we discuss the purification and analysis of lead chloride. The results on the optical quality of the crystals grown from the high purity material are also summarized.
2. Experimental technique .
2.1. Material and purification Lead chloride supplied by Aldrich Chemicals with a listed purity of 99.999% was further punfied by directional solidification. A well cleaned quartz tube was baked in a furnace (— 1000 °C) and loaded with supplied material. The material was sealed in high vacuum and passed through a vertical furnace with a high temperature gradient of 40°C/cm at a translation velocity of 2 cm/day. The impurity analysis was carried out by spark source mass spectrometry.
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
N.B. Singh et at
/
Characterization of directionally solidified lead chloride
2.2. Single crystal growth
81 b
a
M
Crystals were grown by Bridgman technique in a transparent furnace which could be operated with a temperature gradient between 2 to 50°C/cm by using two independent temperature controllers. This transparent furnace facilitated the observation of the solid—liquid interface at
_______________________________ ______________
6~
Growth direction
Fig. 1. Sectioning of the directionally solidified PbCI
2.
vanous G/v values. 2.3. Optical transmission and scattering measurements
The scattering of light through a polished single crystal sample was observed by a method described in ref. [4]. This method consists of passing a He—Ne probe beam through a 4 mm thick optically polished crystal slice producing scattered light which was subsequently imaged on a screen to photograph its distribution. A Perkin-Elmer
Model-330 and a Nicolet model 7199 spectrometer were used to determine the optical transmission through a 4 mm thick crystal.
3. Results and discussion The results of the spark source mass spectrometry analyses performed on three samples are given
Table 1 Major impurities detected in PbCl2 after directional freezing (listed purity of source material was 99.999%) Impurity
Uranium Thorium Bismuth Thallium Tungsten Hafnium Ytterbium Thulium Erbium Dysprosium Molybdenum Niobium Zirconium Yttrium Rubidium Bromine Selenium Arsenic Germanium Gallium Zinc Copper Nickel Iron Vanadium Titanium
Concentration (ppm wt)
Impurity
Sample A, first section
Sample B, middle section
Sample C, last section
<0.26 <0.35 < 3.9 <1.5 <0.51 <0.58 <0.29 <0.12 <0.37 <0.12 2.7 <0.1 0.38 <0.1 <0.1 2.5 <0.1 <0.32 0.36 <1.7 0.51 <0.31 0.1 0.37 0.16 <0.1
<0.26 <0.35 < 3.9 <1.5 <0.51 <0.58 <0.29 <0.12 <0.37 <0.12 1.3 0.23 0.19 <0.1 <0.1 2.5 <0.1 <0.32 0.18 <0.25 1.0 0.15 0.1 0.25 <0.1 0.15
<0.26 <0.35 < 3.9 <1.5 <0.51 <0.58 <0.29 <0.12 <0.37 <0.12 2.7 0.47 <0.14 0.15 0.10 4.4 0.22 0.89 0.18 <1.7 3.8 0.61 0.20 1.1 0.16 0.15
Calcium Potassium Sulfur Phosphorus Silicon Aluminum Magnesium Sodium Fluorine Boron Beryllium Terbium Gadolinium Europium Samarium Neodymium Praseodymium Cerium Barium Cesium Antimony Cadmium Silver Iodine Tin
Concentration (ppm wt) Sample A, first section
Sample B, middle section
Sample C, last section
2.9 <0.94 1.7 10 20 0.94 <2.0 <7.0 1.6 0.18 <0.14 <0.14 <0.35 <0.16 <0.37 <0.26 <0.14 <0.11 0.16 <0.1 <0.21 <0.22 0.82 <0.17 <0.34
1.2 1.3 1.7 5.8 15 0.33 < 2.0 < 7.0 5.7 0.10 <0.14 <0.14 <0.35 <0.16 <0.37 <0.26 <0.14 <0.11 0.16 <0.1 <0.21 0.35 0.22 <0.17 0.48
4.9 1.3 1.7 12 20 0.94 <2.0 <7.0 1.9 0.36 0.19 <0.14 <0.35 <0.16 <0.37 <0.26 <0.14 <0.11 0.42 0.13 1.3 0.30 0.33 <0.17 <0.34
82
N.B. Singh et a!.
/ Characteri:ation of directionally solidified lead chloride
JO)
it
4
~j8
(~)
_____
(B)
-
(0
Fig.
2. Solid—liquid
interface shapc~ ,iL
itO
the temperature gradient, G = 2°C cm. for the corresponding translation velocities (in cm/day): (A) 0; (B) 5; (C) 10: (Dl 20.
in table 1. These samples were taken from the 11
portion, respectively. Comparison of the data
mm diameter rod which was purified by directional freezing as illustrated in fig. 1. Sections A, B
showed that the best quality material was within the first frozen section of the crystal, whereas the
and C correspond to first, middle, and last frozen
last frozen section produced the poorest quality
NB. Singh et aL
/ Characterization of directionally solidified lead chloride
Table 2 G/v values and observed interface shape
V
S 2 °E/ c m 5 5 ~ / ~ m =
=
cm
~ K
d~j
li 2
•~ =
2
L
Z
S
5
1 ~
25
IZ
5
14
2Z~~ *
H
L-LIOUIO
83
temperature gradient, G = 6°C/cm, illustrated in fig. 3, show similar trends for various translation velocities with slight variations. The base state, v = 0 cm/day, has a skewed convex interface shape. The effect of translation becomes apparent at v 20 cm/day, exhibiting dendritic formation. The optical quality of the purified single crystal was evaluated for a 4 mm thick polished sample sliced from the grown crystal. The crystal was water-white, and free from apparent twins and cracks. Some voids were observed on the crystal surface but there was no significant distortion in the crystal. The crystal transmitted from 0.30 to 20 ~m (fig. 4) without any abso~tionpeak. the =
7~ S-SOLID
Equilibrium base state before start of translation.
material. Total impurity content exclusive of 0, N and C was approximately 55 ppm wt in the last frozen section. The metallic impurities in the first frozen section was approximately 15 ppm less than the last section. There are some questions regarding the silicon content of the material which is not well quantified. The concentration of bromine, silicon, sodium, fluorine, molybdenum, calcium, magnesium, sulfur and phosphorus were reasonably high. Although these elements were reduced by purification, their elimination by single directional freezing is not possible. In spite of the fact that the quartz tube was baked at 1000°C,it also may be possible that sodium and silicon are being released from the quartz tube during the crystal growth. Tabulation of G/v values for which the solid—liquid interface shape was recorded is shown in table 2. Fig. 2 illustrates the trends of the interface shape for the case where G = 2°C/cm. For the base state, v = 0 cm/day, the liquid—solid interface has a very smooth convex two-dimensional shape which resembles an elliptic paraboloid in three dimensions, and for a translation velocity of 5 cm/day the interface maintains its convexity. Whereas for V 10 cm/day the interface becomes diffuse and turns dendritic. The effects of higher
N .
~
4
I
4
~. .
. . Fig. 3. Solid—liquid interface shapes at the temperature gradient, G = 60 C/cm, for the corresponding translation velocities (in cm/day): (A) 0; (B) 5; (C) 10; (D) 20.
84
NB. Singh et al.
/
Characterization of directionally solidified leadchloride
0’ MIcron
Fig. 4. Transmittance of 4 mm thick slice of crystal.
•1.
Fig. 5. Light scattering through (a) the reference (without crv~taliand (hI th~cr\ ~tal grou n ironi the purified material.
light scattering through the crystal is shown in fig. 5. The reference case was a He—Ne beam focused on a glass screen. The beam quality scattered through the crystal indicated high optical quality, There is some scattering which can be improved by further purification of the material and optimization of the growth conditions, 4. Summary We have purified PbCl2 by the directional freezing method and analyzed the major impuri-
ties. Silicon, halogens, sulfur, magnesium and phosphorus were the hardest impurities to remove by the single pass directional freezing. Single crystals grown from the purified material showed no absorption peak between 0.30 to 20 ~.cmand good scattering beam quality. Observation of the solid—liquid interface for various G/v values shows that the interface varies from a convex surface to dendritic. The simple and inexpensive procedure for purification and crystal growth suggests that PbCl2 is a promising material for the acoustooptical devices. Further studies are needed to cor-
N.B. Singh eta!.
/
Characterization of directionally solidified lead chloride
85
relate growth conditions with optical quality of the crystal and to determine the necessity and requirements for a microgravity experiment.
References
Acknowledgements
Crystal Growth 21(1974)1. [3] A.V. Zamlcov, IT. Kokov and AT. Anistratov, Soviet Phys.-Cryst. 24 (1979) 355. [4] NB. Singh, RH. Hopkins, R Mazelsky and M. Gottlieb. J. Crystal Growth 85 (1987) 240. [5] Yu A. - Popkov. By. Beznosikov and L.T. Kharchenko. Soviet Phys.-Cryst. 20 (1975) 406. 16] N.B. Singh and ME. Glicksman, Mater. Letters 5 (1987) 453.
The authors are grateful to Drs. H. Gray, M. Gottlieb, R. Mazelsky and R.H. Hopkins for their encouragements. Fruitful discussions with Professor M.E. Glicksman and Dr. S. Coriell and services from E.P. Metz and Ms. D. Todd are gratefully acknowledged.
[1] D.A. Pinnow, L.G. Van Uitert, A W. Warner and W A. Bonner, Appl. Phys. Letters 15 (1969) 83. [2] G.M. Loiacono, J.F. Balascio, R. Bonner and A. Savage, J.
Journal of Crystal Growth 89 (1988) 80—85 North-Holland, Amsterdam
CHARACTERIZATION OF DIRECTIONALLY SOLIDIFIED LEAD CHLORIDE Narsingh Bahadur SINGH Westinghouse R&D Center, 1310 Beulah Road, Pittsburgh, Pennsylvania 15235, USA
and W.M.B. DUVAL and B.N. ROSENTHAL NASA Lewis Research Center, Cleveland, Ohio 44135, USA
A complete analysis has been carried Out Ofl directionally solidified lead(II) chloride material. Purification by directional freezing consistently produced high purity material suitable for subsequent growth of single crystals. It was observed that silicon, magnesium, halogens, sulfur and phosphorous were the hardest impurities to remove from the supplied material. Direct photographic observations of the solid—liquid interface were taken at several values of G/v ratios (denoting the temperature gradient and the translation velocity, respectively) to study the morphology of the interface and optimize the growth conditions. The solid—liquid interface morphology varied from a smooth convex shape to dendritic as the G/v ratio decreased. Single crystals subsequently grown from the material purified by the present method showed no optical distortion, exhibited a transmission range from 0.30 to 20 gm, and displayed extremely low beam scattering.
1. Infroduction Lead molybdate has become an important material for the fabrication of acousto-optic devices since the early seventies [1,2]. Since this material transmits only up to 5.5 ~tm its application is limited to low wavelength regions. Recent research on the lead halides [3—6]has shown that these crystals have relatively high refractive indices and low rates of propagation of acoustic vibrations. The combination of these properties infer a high acousto-optical figure of merit, approximately 10 times higher than PbMoO4. Lead chloride (PbCl2) transmits from 0.30 to 20 ~.tm, lead bromide to 30 ~tm and lead iodide to 40 ~tm. The wide range of transparency, high acousto-opti merit and reasonably good mechanical properties make this class of compounds suitable candidates for device applications. To achieve high performance from the device, high quality single crystals are needed. This can be achieved by optimizing the growth conditions and using extremely pure source material. Such ground-based experimentation is essential in order to decide whether sub-
sequent microgravity experimentation is desirable or required for further improvements of crystal quality. In the present article we discuss the purification and analysis of lead chloride. The results on the optical quality of the crystals grown from the high purity material are also summarized.
2. Experimental technique .
2.1. Material and purification Lead chloride supplied by Aldrich Chemicals with a listed purity of 99.999% was further punfied by directional solidification. A well cleaned quartz tube was baked in a furnace (— 1000 °C) and loaded with supplied material. The material was sealed in high vacuum and passed through a vertical furnace with a high temperature gradient of 40°C/cm at a translation velocity of 2 cm/day. The impurity analysis was carried out by spark source mass spectrometry.
0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
N.B. Singh et at
/
Characterization of directionally solidified lead chloride
2.2. Single crystal growth
81 b
a
M
Crystals were grown by Bridgman technique in a transparent furnace which could be operated with a temperature gradient between 2 to 50°C/cm by using two independent temperature controllers. This transparent furnace facilitated the observation of the solid—liquid interface at
_______________________________ ______________
6~
Growth direction
Fig. 1. Sectioning of the directionally solidified PbCI
2.
vanous G/v values. 2.3. Optical transmission and scattering measurements
The scattering of light through a polished single crystal sample was observed by a method described in ref. [4]. This method consists of passing a He—Ne probe beam through a 4 mm thick optically polished crystal slice producing scattered light which was subsequently imaged on a screen to photograph its distribution. A Perkin-Elmer
Model-330 and a Nicolet model 7199 spectrometer were used to determine the optical transmission through a 4 mm thick crystal.
3. Results and discussion The results of the spark source mass spectrometry analyses performed on three samples are given
Table 1 Major impurities detected in PbCl2 after directional freezing (listed purity of source material was 99.999%) Impurity
Uranium Thorium Bismuth Thallium Tungsten Hafnium Ytterbium Thulium Erbium Dysprosium Molybdenum Niobium Zirconium Yttrium Rubidium Bromine Selenium Arsenic Germanium Gallium Zinc Copper Nickel Iron Vanadium Titanium
Concentration (ppm wt)
Impurity
Sample A, first section
Sample B, middle section
Sample C, last section
<0.26 <0.35 < 3.9 <1.5 <0.51 <0.58 <0.29 <0.12 <0.37 <0.12 2.7 <0.1 0.38 <0.1 <0.1 2.5 <0.1 <0.32 0.36 <1.7 0.51 <0.31 0.1 0.37 0.16 <0.1
<0.26 <0.35 < 3.9 <1.5 <0.51 <0.58 <0.29 <0.12 <0.37 <0.12 1.3 0.23 0.19 <0.1 <0.1 2.5 <0.1 <0.32 0.18 <0.25 1.0 0.15 0.1 0.25 <0.1 0.15
<0.26 <0.35 < 3.9 <1.5 <0.51 <0.58 <0.29 <0.12 <0.37 <0.12 2.7 0.47 <0.14 0.15 0.10 4.4 0.22 0.89 0.18 <1.7 3.8 0.61 0.20 1.1 0.16 0.15
Calcium Potassium Sulfur Phosphorus Silicon Aluminum Magnesium Sodium Fluorine Boron Beryllium Terbium Gadolinium Europium Samarium Neodymium Praseodymium Cerium Barium Cesium Antimony Cadmium Silver Iodine Tin
Concentration (ppm wt) Sample A, first section
Sample B, middle section
Sample C, last section
2.9 <0.94 1.7 10 20 0.94 <2.0 <7.0 1.6 0.18 <0.14 <0.14 <0.35 <0.16 <0.37 <0.26 <0.14 <0.11 0.16 <0.1 <0.21 <0.22 0.82 <0.17 <0.34
1.2 1.3 1.7 5.8 15 0.33 < 2.0 < 7.0 5.7 0.10 <0.14 <0.14 <0.35 <0.16 <0.37 <0.26 <0.14 <0.11 0.16 <0.1 <0.21 0.35 0.22 <0.17 0.48
4.9 1.3 1.7 12 20 0.94 <2.0 <7.0 1.9 0.36 0.19 <0.14 <0.35 <0.16 <0.37 <0.26 <0.14 <0.11 0.42 0.13 1.3 0.30 0.33 <0.17 <0.34
82
N.B. Singh et a!.
/ Characteri:ation of directionally solidified lead chloride
JO)
it
4
~j8
(~)
_____
(B)
-
(0
Fig.
2. Solid—liquid
interface shapc~ ,iL
itO
the temperature gradient, G = 2°C cm. for the corresponding translation velocities (in cm/day): (A) 0; (B) 5; (C) 10: (Dl 20.
in table 1. These samples were taken from the 11
portion, respectively. Comparison of the data
mm diameter rod which was purified by directional freezing as illustrated in fig. 1. Sections A, B
showed that the best quality material was within the first frozen section of the crystal, whereas the
and C correspond to first, middle, and last frozen
last frozen section produced the poorest quality
NB. Singh et aL
/ Characterization of directionally solidified lead chloride
Table 2 G/v values and observed interface shape
V
S 2 °E/ c m 5 5 ~ / ~ m =
=
cm
~ K
d~j
li 2
•~ =
2
L
Z
S
5
1 ~
25
IZ
5
14
2Z~~ *
H
L-LIOUIO
83
temperature gradient, G = 6°C/cm, illustrated in fig. 3, show similar trends for various translation velocities with slight variations. The base state, v = 0 cm/day, has a skewed convex interface shape. The effect of translation becomes apparent at v 20 cm/day, exhibiting dendritic formation. The optical quality of the purified single crystal was evaluated for a 4 mm thick polished sample sliced from the grown crystal. The crystal was water-white, and free from apparent twins and cracks. Some voids were observed on the crystal surface but there was no significant distortion in the crystal. The crystal transmitted from 0.30 to 20 ~m (fig. 4) without any abso~tionpeak. the =
7~ S-SOLID
Equilibrium base state before start of translation.
material. Total impurity content exclusive of 0, N and C was approximately 55 ppm wt in the last frozen section. The metallic impurities in the first frozen section was approximately 15 ppm less than the last section. There are some questions regarding the silicon content of the material which is not well quantified. The concentration of bromine, silicon, sodium, fluorine, molybdenum, calcium, magnesium, sulfur and phosphorus were reasonably high. Although these elements were reduced by purification, their elimination by single directional freezing is not possible. In spite of the fact that the quartz tube was baked at 1000°C,it also may be possible that sodium and silicon are being released from the quartz tube during the crystal growth. Tabulation of G/v values for which the solid—liquid interface shape was recorded is shown in table 2. Fig. 2 illustrates the trends of the interface shape for the case where G = 2°C/cm. For the base state, v = 0 cm/day, the liquid—solid interface has a very smooth convex two-dimensional shape which resembles an elliptic paraboloid in three dimensions, and for a translation velocity of 5 cm/day the interface maintains its convexity. Whereas for V 10 cm/day the interface becomes diffuse and turns dendritic. The effects of higher
N .
~
4
I
4
~. .
. . Fig. 3. Solid—liquid interface shapes at the temperature gradient, G = 60 C/cm, for the corresponding translation velocities (in cm/day): (A) 0; (B) 5; (C) 10; (D) 20.
84
NB. Singh et al.
/
Characterization of directionally solidified leadchloride
0’ MIcron
Fig. 4. Transmittance of 4 mm thick slice of crystal.
•1.
Fig. 5. Light scattering through (a) the reference (without crv~taliand (hI th~cr\ ~tal grou n ironi the purified material.
light scattering through the crystal is shown in fig. 5. The reference case was a He—Ne beam focused on a glass screen. The beam quality scattered through the crystal indicated high optical quality, There is some scattering which can be improved by further purification of the material and optimization of the growth conditions, 4. Summary We have purified PbCl2 by the directional freezing method and analyzed the major impuri-
ties. Silicon, halogens, sulfur, magnesium and phosphorus were the hardest impurities to remove by the single pass directional freezing. Single crystals grown from the purified material showed no absorption peak between 0.30 to 20 ~.cmand good scattering beam quality. Observation of the solid—liquid interface for various G/v values shows that the interface varies from a convex surface to dendritic. The simple and inexpensive procedure for purification and crystal growth suggests that PbCl2 is a promising material for the acoustooptical devices. Further studies are needed to cor-
N.B. Singh eta!.
/
Characterization of directionally solidified lead chloride
85
relate growth conditions with optical quality of the crystal and to determine the necessity and requirements for a microgravity experiment.
References
Acknowledgements
Crystal Growth 21(1974)1. [3] A.V. Zamlcov, IT. Kokov and AT. Anistratov, Soviet Phys.-Cryst. 24 (1979) 355. [4] NB. Singh, RH. Hopkins, R Mazelsky and M. Gottlieb. J. Crystal Growth 85 (1987) 240. [5] Yu A. - Popkov. By. Beznosikov and L.T. Kharchenko. Soviet Phys.-Cryst. 20 (1975) 406. 16] N.B. Singh and ME. Glicksman, Mater. Letters 5 (1987) 453.
The authors are grateful to Drs. H. Gray, M. Gottlieb, R. Mazelsky and R.H. Hopkins for their encouragements. Fruitful discussions with Professor M.E. Glicksman and Dr. S. Coriell and services from E.P. Metz and Ms. D. Todd are gratefully acknowledged.
[1] D.A. Pinnow, L.G. Van Uitert, A W. Warner and W A. Bonner, Appl. Phys. Letters 15 (1969) 83. [2] G.M. Loiacono, J.F. Balascio, R. Bonner and A. Savage, J.