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Growth and morphology of tin iodide whiskers
Journal of Crystal Growth 51(1981)457—460 © North-Holland Publishing Company
GROWTH AND MORPHOLOGY OF TIN IODIDE WHISKERS C.C. DESAI and J.L. RAI Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India Received 16 August 1980; manuscript received in final form 8 November 1980
An account has been presented of the whisker growth of tin iodide crystals by the gel method. Whisker crystals as long as 10 mm have been grown. The whisker crystals have been identified by X-ray analysis. The whiskers are quite transparent with flat, smooth and well developed {oio} habit faces. The growth mechanism of Sn1 2 whisker crystals has been suggested. The ultimate tensile strength of Sn12 whisker crystals has been determined and the implications are discussed.
1. Introduction Growth of crystal interest for the past
whiskers has been of scientific many years because of their strength approaches the theoretical strength of perfect crystals [1], due to presence of few dislocations in them. In the present paper, the authors describe whiskers of tin iodine, which have been grown by gel technique. Investigations made on the fracture strenth of whiskers are also briefly described. The growth of single crystals and whiskers of Sn!2 are of considerable interest as an additive for mercury vapour arc lamp. In this case tin iodide has several interesting properties, including moderate efficiency and excellent colour rendition, which result largely from the strong continuum radiation [2]. They are also useful in preparing positive electrodes for solid electrolyte cells [3].
2. Experimental and observations: whisker growth Standard test tubes were used in this study. The required quantity of doubly distilled water was added to sodium silicate 3. to The give 1resulting solution of specific .ON stannous chloride solugravity 1.04prepared g cm by dissolving it in appropriate tion was amount of concentrated HCI acid and then diluting to the required strength. Gels were prepared by mixing sodium silicate solution with this 1 .ON stannous
Fig. 1.
Growth of Sn1 2 whiskers inside the silica gel.
457
458
C C Desai, J. L. Rat
/ Growth
and morphology of thin iodine whiskers
10~
/
2V
__
36
72
¶08
1/./.
TIME ( Hours
Fig. 2. Growth rate of Sn1
2 whiskers.
chloride solution. After setting of the gels, the whisker growth experiments were started by adding the feed solution of 2.ON KI solution. The chemicals used for growing the whisker crystals were: (i) BDH Analar HC1 acid (35.4%), (ii) extra pure SnC12 2 H20 (97%) and extra pure KI (99.5%). The production of Sn!2 whiskers is in accordance with the following equation: SnCl2 + 2 1(1
-~
Snl2 + 2 KCI
Within a few hours, a thin red-coloured disc appeared below the gel solution interface. Diffusion took place through this disc and the whiskers of tin iodide exhibited their presence in about 12 Ii. Immediately, the growth spread through out the gel media and the crystallization of whiskers were usually completed in 4 to 6 days. Fig. 1 shows whisker crystals of tin iodide growing inside the tube. The average length of whiskers was found to be about 10 mm and thickness 30 to 250 pm. The growth of these whisker crystals at different time was recorded and is shown in fig. 2. Increasing the concentration of the incorporated solution (SnCl2) to 1 .5N and decreasing the coflcentration of feed solution (KI) to 1 SN, one obtains along with tin iodide whiskers, the nucleation of tin tetraiodide (Sn1 4) single crystals [4]. Orange to reddish octahedral Sn14 crystals up to 3—4 nirn in size have been obtained (fig. 3.).
Fig. 3. Growth of Sn12 whiskers along s~ithSnl4 crystals inside the silica gel.
3. Characterization Chemical analysis of the whisker crystals indicated the presence of tin and iodine ions. X-ray rotation photograph were obtained and lattice parameters were determined which agreed well with the values reported in the literature [5]. for Sn12 crystals: a = 14.17 A, b = 4.679 A, c = 10.87 A, ~ = 92°. A rotation photograph taken along the whisker axis gives a (010) orientation for the whiskers. The average density of whisker crystals measured pykenometrically. 3. was found to be 5.2 10 g cm Tin iodide whisker crystals exhibit differeist forms as a response to the supersaturation in the growth region. They were usually found in the straight form
C. C. Desai, J.L. Rat / Growth and morphology of thin iodine whiskers
459
IT
Fig. 4. Overgrowth of microcry~ta1htesoriented in (010) direction on surfaces of whiskers. Magn. X80.
of needles, ribbon or thin platelets, ribbon whiskers grow over the other ribbon whiskers were also observed. In other cases two independent whiskers meeting at a point and still continued their growth, forming an X-shape and a Y-shape. This suggests that even though the tip of a growing whisker accidently met another whisker, the growth continued further. Aggregates of whiskers oriented along one direction forming thin platelets also occur as does the branching of the whiskers. A large number of microcrystallites overgrowing (fig. 4) were also found on some whiskers,
4. Growth mechanism In the present investigations, some interesting growth features have been observed on Sn! 2 whisker crystals. A characteristic growth feature on ribbon whiskers reveals layer growth. These layers are oriented along (010> direction. The different shapes found in Sn12 whiskers lead one to assume a twodimensional nucleation mechanism and pronounced growth in (010> direction. This can be explained by a variation in growth conditions due to the change in supersaturation [6]. The fact that ribbon whiskers have been found, which do not show any sign of screw dislocations even under high magnifications, leads us to believe that the two dimensional nucleation has been the responsible mechanism in the formation of ribbon whiskers, during periods of large supersaturation.
Striations parallel to (010) direction were found on the outer surface of whiskers. In crystal grown from solution, these are plannar regions which are strained either by a departure from stoichiometric composition or by a higher impurity content or both [7].
5. Variation of fracture strength When a whisker having thickness (t) and Young’s modulus (E) is bent into a loop of radius (r) then the strain (e) at the surface is given by [8] e = t/2r, and the stress (cm) at the surface is a = = Et/2r. The radius (r) is reduced until fracture occurs, cm then represents the ultimate tensile strength and e the ultimate extension. The uniform whiskers were fixed firmly to polyethylene sheets with quick fix and then were uniformly bent perpendicular to the whisker axis. The whiskers were kept under observation in an optical microscope and were gradually bent until they fractured. The radius of curvature and the whisker thickness at the point of fracture were measured with the help of the filer-eye piece attached with an optical microscope. The plot of the radius of curvature (R) on fracture against the whisker thickness (t) is shown in fig. 5. The graph is a straight line with uniform slope in the range of 30 to 220 pm thickness of the whiskers. Since the slope is constant, the fracture strength (cm = Et/2r) is a constant for the whiskers and no variation of it with thickness is exhibited,
CC Desai, J.L. Rai / Growth and morphology of thin iodine whiskers
460
kers can be related with the thickening stages of whisker growth.
500
I
~,
Acknowledgements Commission (UGC), New Delhi, for the award of a The authors thank Professor A.R. Patel and Professor M.S. Joshi for their keen interest in the work, One of us (JLR) thanks University Grants
3013
_____________________________________________ 60
180 tIp)
200
220
Fig. 5. Graph of radius of cUrvature (R) on fracture plotted against the whisker thickness (t) for uniform whisker.
teacher fellowship, during the tenure of which the present work was carried out and also to Government of Rajasthan, Jaipur (Rajasthan). for granting study leave. One of us (CCD.) is very much grateful to Mrs. K,C. Desai for the interest in the work.
References unlike the case of silicon carbide and tungsten crystals [9—11], for which the strength rapidly increases with a decrease in thickness for low values of whisker thickness.
Ill C. Herring and J.K. Gait, Phys Rev. 85 (1952) 1060. [2] R.J. Zollweg and L.S. Frost, Bull. Am. Phys. Soc. 11 (1966) ~l45. [3] Ikeda, Hironosuke and Narukawa, (Satoshi, Sanyo Elect. Co. Ltd.) Japan, Kokai, 7602 934 9 (CIHO im) 21 Jan.
6. Conclusions (1) Whisker crystals as long as 10 mm have been grown by the gel technique. (2) Needle shape and ribbon shaped whiskers crystals have been observed throughout the growth. (3) Snl 2 whiskers had (010> orientation and were bounded by {0l0} faces. (4) The ultimate tensile strength of SnI2 whiskers is indepepdent of its thickness. (5) Two dimensional nucleation appears to be the responsible mechanism for the growth of Sn12 whiske rs. (6) The striated structure on the surfaces of whis-
1976.
[41 CC. Desai and J.L. Rai, J. Crystal Growth 50 (1980) 562. [5] R.A. 1-lowie, W. Moser and IC. Trevvena, Acta Cryst. 28 B (1972) 2965. [6] J.C.J.M. Terhell and R.M.A. Lieth, J. Crystal Growth 16 (1972) 54.
[71 L.A. Giess, D.C. Cronemeyer, L.L. Rosier and ID. Kuptsis, Mater. Res. Bull. 5 (1970) 445. 181 i.E. Gordon, Endeavour 23 (1964) 8. [9] AR. Levit, Whisker Technology (Wiley, New York, 1970).
[10] W.W. Webb, lID. Batha and P.T.B. Shaffer, Strengthening Mechanism Metals and Ceramics (Syracuse University Press, Syracuse, NY, 1966). Ill] UN. Alexandrov and A.N. Kogan, Soviet Phys.-Solid State 6 (1964) 246.
GROWTH AND MORPHOLOGY OF TIN IODIDE WHISKERS C.C. DESAI and J.L. RAI Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India Received 16 August 1980; manuscript received in final form 8 November 1980
An account has been presented of the whisker growth of tin iodide crystals by the gel method. Whisker crystals as long as 10 mm have been grown. The whisker crystals have been identified by X-ray analysis. The whiskers are quite transparent with flat, smooth and well developed {oio} habit faces. The growth mechanism of Sn1 2 whisker crystals has been suggested. The ultimate tensile strength of Sn12 whisker crystals has been determined and the implications are discussed.
1. Introduction Growth of crystal interest for the past
whiskers has been of scientific many years because of their strength approaches the theoretical strength of perfect crystals [1], due to presence of few dislocations in them. In the present paper, the authors describe whiskers of tin iodine, which have been grown by gel technique. Investigations made on the fracture strenth of whiskers are also briefly described. The growth of single crystals and whiskers of Sn!2 are of considerable interest as an additive for mercury vapour arc lamp. In this case tin iodide has several interesting properties, including moderate efficiency and excellent colour rendition, which result largely from the strong continuum radiation [2]. They are also useful in preparing positive electrodes for solid electrolyte cells [3].
2. Experimental and observations: whisker growth Standard test tubes were used in this study. The required quantity of doubly distilled water was added to sodium silicate 3. to The give 1resulting solution of specific .ON stannous chloride solugravity 1.04prepared g cm by dissolving it in appropriate tion was amount of concentrated HCI acid and then diluting to the required strength. Gels were prepared by mixing sodium silicate solution with this 1 .ON stannous
Fig. 1.
Growth of Sn1 2 whiskers inside the silica gel.
457
458
C C Desai, J. L. Rat
/ Growth
and morphology of thin iodine whiskers
10~
/
2V
__
36
72
¶08
1/./.
TIME ( Hours
Fig. 2. Growth rate of Sn1
2 whiskers.
chloride solution. After setting of the gels, the whisker growth experiments were started by adding the feed solution of 2.ON KI solution. The chemicals used for growing the whisker crystals were: (i) BDH Analar HC1 acid (35.4%), (ii) extra pure SnC12 2 H20 (97%) and extra pure KI (99.5%). The production of Sn!2 whiskers is in accordance with the following equation: SnCl2 + 2 1(1
-~
Snl2 + 2 KCI
Within a few hours, a thin red-coloured disc appeared below the gel solution interface. Diffusion took place through this disc and the whiskers of tin iodide exhibited their presence in about 12 Ii. Immediately, the growth spread through out the gel media and the crystallization of whiskers were usually completed in 4 to 6 days. Fig. 1 shows whisker crystals of tin iodide growing inside the tube. The average length of whiskers was found to be about 10 mm and thickness 30 to 250 pm. The growth of these whisker crystals at different time was recorded and is shown in fig. 2. Increasing the concentration of the incorporated solution (SnCl2) to 1 .5N and decreasing the coflcentration of feed solution (KI) to 1 SN, one obtains along with tin iodide whiskers, the nucleation of tin tetraiodide (Sn1 4) single crystals [4]. Orange to reddish octahedral Sn14 crystals up to 3—4 nirn in size have been obtained (fig. 3.).
Fig. 3. Growth of Sn12 whiskers along s~ithSnl4 crystals inside the silica gel.
3. Characterization Chemical analysis of the whisker crystals indicated the presence of tin and iodine ions. X-ray rotation photograph were obtained and lattice parameters were determined which agreed well with the values reported in the literature [5]. for Sn12 crystals: a = 14.17 A, b = 4.679 A, c = 10.87 A, ~ = 92°. A rotation photograph taken along the whisker axis gives a (010) orientation for the whiskers. The average density of whisker crystals measured pykenometrically. 3. was found to be 5.2 10 g cm Tin iodide whisker crystals exhibit differeist forms as a response to the supersaturation in the growth region. They were usually found in the straight form
C. C. Desai, J.L. Rat / Growth and morphology of thin iodine whiskers
459
IT
Fig. 4. Overgrowth of microcry~ta1htesoriented in (010) direction on surfaces of whiskers. Magn. X80.
of needles, ribbon or thin platelets, ribbon whiskers grow over the other ribbon whiskers were also observed. In other cases two independent whiskers meeting at a point and still continued their growth, forming an X-shape and a Y-shape. This suggests that even though the tip of a growing whisker accidently met another whisker, the growth continued further. Aggregates of whiskers oriented along one direction forming thin platelets also occur as does the branching of the whiskers. A large number of microcrystallites overgrowing (fig. 4) were also found on some whiskers,
4. Growth mechanism In the present investigations, some interesting growth features have been observed on Sn! 2 whisker crystals. A characteristic growth feature on ribbon whiskers reveals layer growth. These layers are oriented along (010> direction. The different shapes found in Sn12 whiskers lead one to assume a twodimensional nucleation mechanism and pronounced growth in (010> direction. This can be explained by a variation in growth conditions due to the change in supersaturation [6]. The fact that ribbon whiskers have been found, which do not show any sign of screw dislocations even under high magnifications, leads us to believe that the two dimensional nucleation has been the responsible mechanism in the formation of ribbon whiskers, during periods of large supersaturation.
Striations parallel to (010) direction were found on the outer surface of whiskers. In crystal grown from solution, these are plannar regions which are strained either by a departure from stoichiometric composition or by a higher impurity content or both [7].
5. Variation of fracture strength When a whisker having thickness (t) and Young’s modulus (E) is bent into a loop of radius (r) then the strain (e) at the surface is given by [8] e = t/2r, and the stress (cm) at the surface is a = = Et/2r. The radius (r) is reduced until fracture occurs, cm then represents the ultimate tensile strength and e the ultimate extension. The uniform whiskers were fixed firmly to polyethylene sheets with quick fix and then were uniformly bent perpendicular to the whisker axis. The whiskers were kept under observation in an optical microscope and were gradually bent until they fractured. The radius of curvature and the whisker thickness at the point of fracture were measured with the help of the filer-eye piece attached with an optical microscope. The plot of the radius of curvature (R) on fracture against the whisker thickness (t) is shown in fig. 5. The graph is a straight line with uniform slope in the range of 30 to 220 pm thickness of the whiskers. Since the slope is constant, the fracture strength (cm = Et/2r) is a constant for the whiskers and no variation of it with thickness is exhibited,
CC Desai, J.L. Rai / Growth and morphology of thin iodine whiskers
460
kers can be related with the thickening stages of whisker growth.
500
I
~,
Acknowledgements Commission (UGC), New Delhi, for the award of a The authors thank Professor A.R. Patel and Professor M.S. Joshi for their keen interest in the work, One of us (JLR) thanks University Grants
3013
_____________________________________________ 60
180 tIp)
200
220
Fig. 5. Graph of radius of cUrvature (R) on fracture plotted against the whisker thickness (t) for uniform whisker.
teacher fellowship, during the tenure of which the present work was carried out and also to Government of Rajasthan, Jaipur (Rajasthan). for granting study leave. One of us (CCD.) is very much grateful to Mrs. K,C. Desai for the interest in the work.
References unlike the case of silicon carbide and tungsten crystals [9—11], for which the strength rapidly increases with a decrease in thickness for low values of whisker thickness.
Ill C. Herring and J.K. Gait, Phys Rev. 85 (1952) 1060. [2] R.J. Zollweg and L.S. Frost, Bull. Am. Phys. Soc. 11 (1966) ~l45. [3] Ikeda, Hironosuke and Narukawa, (Satoshi, Sanyo Elect. Co. Ltd.) Japan, Kokai, 7602 934 9 (CIHO im) 21 Jan.
6. Conclusions (1) Whisker crystals as long as 10 mm have been grown by the gel technique. (2) Needle shape and ribbon shaped whiskers crystals have been observed throughout the growth. (3) Snl 2 whiskers had (010> orientation and were bounded by {0l0} faces. (4) The ultimate tensile strength of SnI2 whiskers is indepepdent of its thickness. (5) Two dimensional nucleation appears to be the responsible mechanism for the growth of Sn12 whiske rs. (6) The striated structure on the surfaces of whis-
1976.
[41 CC. Desai and J.L. Rai, J. Crystal Growth 50 (1980) 562. [5] R.A. 1-lowie, W. Moser and IC. Trevvena, Acta Cryst. 28 B (1972) 2965. [6] J.C.J.M. Terhell and R.M.A. Lieth, J. Crystal Growth 16 (1972) 54.
[71 L.A. Giess, D.C. Cronemeyer, L.L. Rosier and ID. Kuptsis, Mater. Res. Bull. 5 (1970) 445. 181 i.E. Gordon, Endeavour 23 (1964) 8. [9] AR. Levit, Whisker Technology (Wiley, New York, 1970).
[10] W.W. Webb, lID. Batha and P.T.B. Shaffer, Strengthening Mechanism Metals and Ceramics (Syracuse University Press, Syracuse, NY, 1966). Ill] UN. Alexandrov and A.N. Kogan, Soviet Phys.-Solid State 6 (1964) 246.