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Nucleation and growth of KClO4 single crystals in silica gels
Journal of Crystal Growth 43(1978) 351—356 © North-Holland Publishing Company
NUCLEATION AND GROWTH OF KCIO4 SINGLE CRYSTALS IN SILICA GELS A.R. PATEL and A. VENKATESWARA RAO Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat State, India Received 10 September 1977; manuscript received in final form 17 November 1977.
As part of a continuing effort on the growth of KC1O4 single crystals in gels, this paper describes the variation of nucleation density and growth of these crystals as a function of concentration of feed solution, gel ageing, gel density, gel pH and intermediate neutral gel column. While high density and high pH gels have been found to produce opaque crystals, good quality transparent single crystals have been obtained at low density and low pH gels. It is observed that the intermediate neutral gel column and gel ageing considerably reduce the number of nucleation sites. A simple procedure to control nucleation sites in gels has been adopted, which has resulted in a dramatic increase in the size of the crystals.
1. Introduction
with the required amount of 1 N perchioric acid. The variation of pH with the amount of 1 N perchloric acid for different specific gravities of sodium silicate solution, is shown in fig. 1. The gel solutions thus prepared were then transferred to 2.5 cm diameter and 30 cm length test tubes and allowed to set at a constant temperature of 22°C. After the gels were set,
It has been reported [l—6] that a variety of crystals suitable for research and technology can be grown in silica gel. The method is specifically useful for substances which, because of their low solubilities or low dissociation temperatures (or both as in this case), can not readily be grown in other ways. The gel medium prevents turbulence and remaining chemi cally inert, provides a three-dimensional structure, which permits the reagents to diffuse at a desirable controlled rate. Further, its softness and the uniform nature of constraining forces that it exerts upon the growing crystals encourages orderly growth. So far nothing has been reported regarding the nucleation studies on KC1O4 single crystals in gels. In the preliminary studies on the growth of KC1O4 single crystals, Patel and Venkateswara Rao [7] have reported the successful growth of these crystals in gels. The present paper reports the results of the detailed studies made on nucleation and growth of these crystals as a function of gel parameters.
-________________________
HCIU4
1.06 70
40
30
1.03
10
2. Experiniental procedure
4
The silica gel (acidified sodium silicate) was used as the growth media throughout this study. The gel was prepared by mixing pure sodium silicate solution
~
g
to
pH
Fig. 1. Variation of gel pH with different amounts of 1 N HCIO4 solution added to 80 ml of aqueous pure sodium .. . . . silicate solutions of specific gravities varying from 1.03 to 1.06. 351
352
A. R. Patel, A. Venkatesst’ara Rao
/ Nucleatioo
the feed solution of KCI was placed above the gels for crystallization. Analytical reagent grade chemicals
and doubly distilled water were used throughout this study. Completion of crystallization (with crystal size
To study the effect of concentration of feed solution, gels of same pH and density were prepared. Feed solutions of different concentrations varying from 0.4 to 1.8 N were added over the set gels. Fig. 2
and growth of Kc10
4 single crystals in silica gels
N
• 52.0
2.
0.4
0.8
Fig. 3. Nucleation density (N) versus normality of KCI solution (n) for two different gel ages.
shows crystals growing at three different concentra-
tions of feed solutions. The variation of nucleation density with concentration of feed solution is shown in fig. 3. This indicates that at higher concentrations of feed solutions, because of the enhanced availability
of potassium ions, the nucleation density increases,
bi
ci
Fig. 2. Crystals growing at three different normalities of KCI solution: (a) 0.8 N KC1, (b) 1.2 N KC1, (c) 1.8 N KCI.
3.2. Effect of ageing of gels
To investigate the effect of gel ageing, gels were allowed to age for various periods before adding the feed solution. Crystals growing in gels allowed to age
11
b~
ci
Jig. 4. Cr~stats growing at three different get ages: (at 5 days, (b) 15 days. (c) 25 dayS.
A.R. Patel, A. Venkateswara Rao/Nucleation and growth of KCIO
4 single crystals in silica gels
353
Fig. 5. Nucleation density (N) versus gel ageing (7) in days for two different concentrations of KC1 solution.
for three different periods are shown in fig. 4. Fig. 5 is a plot of age of gels versus the nucleation density. As reported by Henisch [1], gel ageing reduces the cell size and consequently the rate of diffusion of ions into the gel. The gel ageing has no pronounced effect on the size or the quality of the crystals.
-I’~,
Fig. 6. Crystals grown at three different specific gravities of pure sodium silicate (actual size): (a) 1.055, (b) 1.045, (c) 1.035.
354
A.K J-’atel, A. Venkateswara Rao
/ Nucleation and
growth of KC1O
4 single crrstals in silica gels
~
40 2.0
~H OF(N)THE GEL 8 gel solutions io for Fig. 8. Nucleation density versus pH of two different concentrations of KCI solution. .
1.30
i40
140
S
Fig. 7. Nucleation density (N) against specific gravity (S) of sodium silicate solution for two different concentrations of KCI solution,
lacks cross-linkages; the cellular nature becomes less
~:
distinct [1 Fig. 8 shows that the nucleation density decreases with increase in the pH value which may be due to the improper formation of cells at higher pH
values of gels.
3.3. Effect of gel density
3.5. Effect of intermediate neutral gel
The gels of different densities were obtained by mixing sodium silicate of sp. gr. 1.03 to 1.06 to 1 N perchioric acid keeping pH constant at 5 by adding a few drops of CH3COOH. It is observed that the transparency of the gel increases as the gel density decreases. It is seen from figs. 6a—6c that an increase in gel density increased the contamination of the crystals with silica gel and thereby affected their qual ity and shape. It may be noted that well defined and transparent single crystals were obtained with sodium silicate of specific gravities below 1.04. It is clear from fig. 7 that an increase of gel density decreases the nucleation density which is due to the fact that greater gel density results in smaller pore size as demonstrated first by Henisch [1].
To study the effect of the height of the intermediate neutral gel column, different aniounts of acetic
~_,_ -
,
-________ ~.
______
. -
~.
_______ .
-
______
3.4. Effect ofpH of gels The pH values of gels were varied from 2 to 10 by adding few drops of acetic acid before setting of the perchloric acid gel solutions. It is observed that as the pH increases, the transparency of gel decreases. The crystals growing at higher pH values are not transparent and well defined. This is due to the contamination of crystals with silica gel, because as the pH increases the box-like net-work structure of the gel changes to a loosely bound platelet structure which
tie. 9. Crystals growing in tubes with three different heiytits = 0 cm, (b)
(L) of intermediate neutral gel columns: (a) I. L = 4 cm, (c) L 6 cm.
/
A.R. Patel, A. Venkateswara Rao Nucleation and growth of KCIO
2.
4
~
L(cm~)
Fig. 10. Variation of nucleation density (N) against the length (L) of the intermediate gel column for two different concentrations of KC1 solution.
4 single crystals in silica gels
_ ____
355
bi
________
acid set gel solutions were added over the perchloric
acid set gels. The neutral gels were also allowed to set and the feed solution was then added. Fig. 9 shows the crystals growing in tubes with different heights of intermediate neutral gel columns. A graph of nucleation density versus the length of the intermediate neutral gel column is shown in fig. 10. The intermediate neutral gel will slow down the rate of diffusion of KC1 thus slowing down the reaction between the reactants, and reducing the number of nuclei. The intermediate neutral gel does not have remarkable effect on size or quality of the crystals. 3.6. Nucleation control by concentration programming
3 of 0.2 N Over the perchloric acid set gel, 30 cm KC1 solution was placed. The strength of this feed solution was increased at the rate of 0.01 N per day by removing 20 cc of the above feed solution and replacing it by an equal amount of higher concentrated feed solution. The process was continued till the concentration of KC1 reached upto around 2.5 N when nucleation started and resulted in a few nucleation centres. Figs. I la and 1 lb show crystals growing without and with concentration programming, respectively. It is considered that with very dilute reactants, the amount of material diffused through the gel is small and hence the rate at which the supersaturation is attained. Under these circumstances, a few nuclei are formed. On increasing the concentration, further growth of the existing nuclei is preferred to the formation of additional ones. Crystals grown by this method are transparent and larger than those grown without this method.
. Fig. 11. (a) Crystals growing without concentration programming. (b) Crystals growing with concentration programming.
4. Conclusions Transparent single crystals of KC1O 4 can be obtamed at low pH values (below 6) with low density of gels (below 1.04 sp. gr. of sodium silicate). Gel ageing and intermediate neutral gel column reduce the nucleation centres without affecting the quality of the crystals. The nucleation density increases with the concentration of the feed solution. By concentration programming, the size of the crystals can be increased.
Acknowledgements It is a pleasure to thank Dr. T.C. Pate! and Mr. G.K. Shivakumar for their helpful discussions and suggestions. One of us (A.V.R.) wishes to acknowledge the UGC, New Delhi (India) for the financial assistance.
356
AR. Patel, A. Venkateswara Rao /Nucleation and grosvth of KGb
References
4 single crystals in silica gels
141 K. Sangwal and AR. Patel, J. Crystal GrOwth 23(1974) 282.
[11 H.K. Henisch, Crystal Growth ~ Gels (The Pennsylvania State University Press, 1970). [2] A.R. Patel and H.L. Bhat, J. Crystal Growth 12 (1972) 268. 131 AR. Patel and S.K. Arora, J. Crystal Growth 18 (1973) 199.
151 AR. Patel and S.K. ~ora, J. Mater. Sci. 11(1976)843. [61 AR. Patel and S.K. Arora, J. Crystal Growth 37 (1977) [7] A.R. Patel and A. Venkateswara Rao, J. Crystal Growth 38 (1977) 288.
NUCLEATION AND GROWTH OF KCIO4 SINGLE CRYSTALS IN SILICA GELS A.R. PATEL and A. VENKATESWARA RAO Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat State, India Received 10 September 1977; manuscript received in final form 17 November 1977.
As part of a continuing effort on the growth of KC1O4 single crystals in gels, this paper describes the variation of nucleation density and growth of these crystals as a function of concentration of feed solution, gel ageing, gel density, gel pH and intermediate neutral gel column. While high density and high pH gels have been found to produce opaque crystals, good quality transparent single crystals have been obtained at low density and low pH gels. It is observed that the intermediate neutral gel column and gel ageing considerably reduce the number of nucleation sites. A simple procedure to control nucleation sites in gels has been adopted, which has resulted in a dramatic increase in the size of the crystals.
1. Introduction
with the required amount of 1 N perchioric acid. The variation of pH with the amount of 1 N perchloric acid for different specific gravities of sodium silicate solution, is shown in fig. 1. The gel solutions thus prepared were then transferred to 2.5 cm diameter and 30 cm length test tubes and allowed to set at a constant temperature of 22°C. After the gels were set,
It has been reported [l—6] that a variety of crystals suitable for research and technology can be grown in silica gel. The method is specifically useful for substances which, because of their low solubilities or low dissociation temperatures (or both as in this case), can not readily be grown in other ways. The gel medium prevents turbulence and remaining chemi cally inert, provides a three-dimensional structure, which permits the reagents to diffuse at a desirable controlled rate. Further, its softness and the uniform nature of constraining forces that it exerts upon the growing crystals encourages orderly growth. So far nothing has been reported regarding the nucleation studies on KC1O4 single crystals in gels. In the preliminary studies on the growth of KC1O4 single crystals, Patel and Venkateswara Rao [7] have reported the successful growth of these crystals in gels. The present paper reports the results of the detailed studies made on nucleation and growth of these crystals as a function of gel parameters.
-________________________
HCIU4
1.06 70
40
30
1.03
10
2. Experiniental procedure
4
The silica gel (acidified sodium silicate) was used as the growth media throughout this study. The gel was prepared by mixing pure sodium silicate solution
~
g
to
pH
Fig. 1. Variation of gel pH with different amounts of 1 N HCIO4 solution added to 80 ml of aqueous pure sodium .. . . . silicate solutions of specific gravities varying from 1.03 to 1.06. 351
352
A. R. Patel, A. Venkatesst’ara Rao
/ Nucleatioo
the feed solution of KCI was placed above the gels for crystallization. Analytical reagent grade chemicals
and doubly distilled water were used throughout this study. Completion of crystallization (with crystal size
To study the effect of concentration of feed solution, gels of same pH and density were prepared. Feed solutions of different concentrations varying from 0.4 to 1.8 N were added over the set gels. Fig. 2
and growth of Kc10
4 single crystals in silica gels
N
• 52.0
2.
0.4
0.8
Fig. 3. Nucleation density (N) versus normality of KCI solution (n) for two different gel ages.
shows crystals growing at three different concentra-
tions of feed solutions. The variation of nucleation density with concentration of feed solution is shown in fig. 3. This indicates that at higher concentrations of feed solutions, because of the enhanced availability
of potassium ions, the nucleation density increases,
bi
ci
Fig. 2. Crystals growing at three different normalities of KCI solution: (a) 0.8 N KC1, (b) 1.2 N KC1, (c) 1.8 N KCI.
3.2. Effect of ageing of gels
To investigate the effect of gel ageing, gels were allowed to age for various periods before adding the feed solution. Crystals growing in gels allowed to age
11
b~
ci
Jig. 4. Cr~stats growing at three different get ages: (at 5 days, (b) 15 days. (c) 25 dayS.
A.R. Patel, A. Venkateswara Rao/Nucleation and growth of KCIO
4 single crystals in silica gels
353
Fig. 5. Nucleation density (N) versus gel ageing (7) in days for two different concentrations of KC1 solution.
for three different periods are shown in fig. 4. Fig. 5 is a plot of age of gels versus the nucleation density. As reported by Henisch [1], gel ageing reduces the cell size and consequently the rate of diffusion of ions into the gel. The gel ageing has no pronounced effect on the size or the quality of the crystals.
-I’~,
Fig. 6. Crystals grown at three different specific gravities of pure sodium silicate (actual size): (a) 1.055, (b) 1.045, (c) 1.035.
354
A.K J-’atel, A. Venkateswara Rao
/ Nucleation and
growth of KC1O
4 single crrstals in silica gels
~
40 2.0
~H OF(N)THE GEL 8 gel solutions io for Fig. 8. Nucleation density versus pH of two different concentrations of KCI solution. .
1.30
i40
140
S
Fig. 7. Nucleation density (N) against specific gravity (S) of sodium silicate solution for two different concentrations of KCI solution,
lacks cross-linkages; the cellular nature becomes less
~:
distinct [1 Fig. 8 shows that the nucleation density decreases with increase in the pH value which may be due to the improper formation of cells at higher pH
values of gels.
3.3. Effect of gel density
3.5. Effect of intermediate neutral gel
The gels of different densities were obtained by mixing sodium silicate of sp. gr. 1.03 to 1.06 to 1 N perchioric acid keeping pH constant at 5 by adding a few drops of CH3COOH. It is observed that the transparency of the gel increases as the gel density decreases. It is seen from figs. 6a—6c that an increase in gel density increased the contamination of the crystals with silica gel and thereby affected their qual ity and shape. It may be noted that well defined and transparent single crystals were obtained with sodium silicate of specific gravities below 1.04. It is clear from fig. 7 that an increase of gel density decreases the nucleation density which is due to the fact that greater gel density results in smaller pore size as demonstrated first by Henisch [1].
To study the effect of the height of the intermediate neutral gel column, different aniounts of acetic
~_,_ -
,
-________ ~.
______
. -
~.
_______ .
-
______
3.4. Effect ofpH of gels The pH values of gels were varied from 2 to 10 by adding few drops of acetic acid before setting of the perchloric acid gel solutions. It is observed that as the pH increases, the transparency of gel decreases. The crystals growing at higher pH values are not transparent and well defined. This is due to the contamination of crystals with silica gel, because as the pH increases the box-like net-work structure of the gel changes to a loosely bound platelet structure which
tie. 9. Crystals growing in tubes with three different heiytits = 0 cm, (b)
(L) of intermediate neutral gel columns: (a) I. L = 4 cm, (c) L 6 cm.
/
A.R. Patel, A. Venkateswara Rao Nucleation and growth of KCIO
2.
4
~
L(cm~)
Fig. 10. Variation of nucleation density (N) against the length (L) of the intermediate gel column for two different concentrations of KC1 solution.
4 single crystals in silica gels
_ ____
355
bi
________
acid set gel solutions were added over the perchloric
acid set gels. The neutral gels were also allowed to set and the feed solution was then added. Fig. 9 shows the crystals growing in tubes with different heights of intermediate neutral gel columns. A graph of nucleation density versus the length of the intermediate neutral gel column is shown in fig. 10. The intermediate neutral gel will slow down the rate of diffusion of KC1 thus slowing down the reaction between the reactants, and reducing the number of nuclei. The intermediate neutral gel does not have remarkable effect on size or quality of the crystals. 3.6. Nucleation control by concentration programming
3 of 0.2 N Over the perchloric acid set gel, 30 cm KC1 solution was placed. The strength of this feed solution was increased at the rate of 0.01 N per day by removing 20 cc of the above feed solution and replacing it by an equal amount of higher concentrated feed solution. The process was continued till the concentration of KC1 reached upto around 2.5 N when nucleation started and resulted in a few nucleation centres. Figs. I la and 1 lb show crystals growing without and with concentration programming, respectively. It is considered that with very dilute reactants, the amount of material diffused through the gel is small and hence the rate at which the supersaturation is attained. Under these circumstances, a few nuclei are formed. On increasing the concentration, further growth of the existing nuclei is preferred to the formation of additional ones. Crystals grown by this method are transparent and larger than those grown without this method.
. Fig. 11. (a) Crystals growing without concentration programming. (b) Crystals growing with concentration programming.
4. Conclusions Transparent single crystals of KC1O 4 can be obtamed at low pH values (below 6) with low density of gels (below 1.04 sp. gr. of sodium silicate). Gel ageing and intermediate neutral gel column reduce the nucleation centres without affecting the quality of the crystals. The nucleation density increases with the concentration of the feed solution. By concentration programming, the size of the crystals can be increased.
Acknowledgements It is a pleasure to thank Dr. T.C. Pate! and Mr. G.K. Shivakumar for their helpful discussions and suggestions. One of us (A.V.R.) wishes to acknowledge the UGC, New Delhi (India) for the financial assistance.
356
AR. Patel, A. Venkateswara Rao /Nucleation and grosvth of KGb
References
4 single crystals in silica gels
141 K. Sangwal and AR. Patel, J. Crystal GrOwth 23(1974) 282.
[11 H.K. Henisch, Crystal Growth ~ Gels (The Pennsylvania State University Press, 1970). [2] A.R. Patel and H.L. Bhat, J. Crystal Growth 12 (1972) 268. 131 AR. Patel and S.K. Arora, J. Crystal Growth 18 (1973) 199.
151 AR. Patel and S.K. ~ora, J. Mater. Sci. 11(1976)843. [61 AR. Patel and S.K. Arora, J. Crystal Growth 37 (1977) [7] A.R. Patel and A. Venkateswara Rao, J. Crystal Growth 38 (1977) 288.