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Annealing of silver chloride crystals containing internal print-out
J. Phys. Chem. Solids
PergamonPress 1959. Vol. 12. pp. 119-121.
ANNEALING
Printed in Great Britain.
OF SILVER CHLORIDE
CONTAINING
INTERNAL
C. CHILDSt University
CRYSTALS
PRINT-OUT*
and L. SLIPKIN
of North Carolina, Chapel Hill, N.C. (Received 23 June 1959)
Abstract-The HAYNES-SHOCKLEY technique was used to obtain internal print-out in single silver chloride crystals. Print-out was produced in all crystals from various boulss after observing that, in some discs from the boules, the surface deformation from manufacturer disc-shaping had a depth of 254 mm. After internal print-out was produced, the crystals were given a short MgO anneal at 425X, which decorated substructure and produced small internal precipitates with definite geometric configurations. Annealing at 425°C for 40-50 br produced uniform cylindrical precipitates with diameters of approximately 2 p and lengths < 0.7 mm. An investigation of these elongated The growth process precipitates showed that they were in the six [llO] crystallographic directions. of these precipitates are shown and discussed.
1. INTRODUC3’ION
HAYNES and SHOCKLJZY(~) utilized synchronous pulses of electric field and ultraviolet light to drive electrons deep into silver chloride crystals, resulting in the formation of specks of metallic silver at trapping sites distributed throughout the crystal. Modifications by WEBB@), HAMILTONet al. (a), and %PTITZ(*) have further extended the potentialities of this technique. The present paper is concerned with the effects of subsequent annealing on the distribution of the colloidal silver. In particular, under appropriate conditions long, perfectly straight whisker-like formations may be produced in the interior. Observations of the distribution of silver in small and large grain boundaries near the surface are also reported. ‘2. EXPERIMENTAL The specimens were in the form of singlecrystal slabs milled from Harshaw material. During preparation they were exposed only to light from a yellow “safe-1ite”. In the subsequent grinding and polishing on metallographic papers, at least l-75 mm were removed from the surfaces * Supported in part by the office of Ordnance and Research, Department of the Army. t Present address: National Aeronautics Administration, Washington, D.C.
and Space
to be exposed to eliminate the material which had been distorted by the machining. Later work showed that some surfaces required 2.54 mm removal before internal print-out was obtained. The crystals were then etched in 3 per cent KCN and washed, resulting in specimens N 1 cm2 in area and 2-5 mm thick. Prior to the ultraviolet exposure, they were annealed for 40-50 hr in air on a powdered MgO surface at ~425°C and cooled at a rate of 20”/hr. Another light etch in 3 per cent KCN served to render the surface clear again. The crystals were exposed to flashes of ultraviolet light provided by a GE BH-6 mercury lamp powered by the repetitive discharges of a Sprague H-850 pulse-forming network. The discharges were initiated by the firing of a 4C35 hydrogen thyratron, which in turn was triggered 1000 times per second. By applying the network potential across the crystal through insulating layers in a manner similar to that employed by HAYNES and SHOCKLEY,voltage pulses of 2 kV were synchronized with the light flashes. These pulses, produced as a result of the short-circuiting of the network, were due to the ionic polarization at the crystal surfaces and served to drive the photoelectrons into and through the crystal. Each flash created approximately 10s photoelectrons/cm2 of crystal
119
120
C.
CHILDS
and
surface. The decay time of the flash was 10 psec, and the subsequent build-up time of the network to one-quarter of its original maximum potential was approximately 200 psec, thus allowing adequate time for the photoelectrons to complete their trajectories before the polarization field was neutralized. After exposures of the order of 12 hr, a column of colloidal silver could be seen extending throughout the thickness of the crystal. It is the redistribution of this silver upon further annealing with which this paper is concerned. 3. SURFACE OBSERVATIONS Immediately after exposure, one observes a surface mosaic pattern of print-out consisting of regions of silver of varying intensity, demarcated by lighter lines which are presumably small-angle boundaries. Heating to 225°C produced no visible change, but a brief anneal to 425°C resulted in a clearing up of the dark regions on the surface. The sub-boundaries had now become delineated by an intense collection of silver. This substructure was visible only within about O-1 or 0.2 mm of the surface. Examples of such decorated boundaries are shown in Figs. 1 and 2. Interestingly, the boundary often appears as a pair of planes instead of a single surface. It has been suggested by ESHELBY et al.(s) that in alkali halides the charges on dislocations would cause a dislocation wall to really consist of two parallel walls. It may well be that the double patterns seen here are due to a similar effect in silver chloride. It was also observed, as already reported by MITCHELL@), that large-angle boundaries were not decorated. Fig. 3 is a photograph of the surface of a specimen consisting of two grains. The thermal etch-pits clearly show the presence of a large-angle boundary, but no silver is in evidence. Prolonged annealing at 425°C results in the complete disappearance of the silver in the surface regions. Presumably, the silver diffuses out of the crystal along the small-angle boundaries at which it had been collected. The fact that no silver is observed on large-angle boundaries, even after a brief anneal at 425”C, may be due to a faster diffusion rate along the large-angle boundaries. 4. INTERNAL OBSERVATIONS After exposure for 12 hr, the crystals had a cloudy appearance due to the colloidal silver
L.
SLIFKIN
dispersed in the interior. No distinct patterns were observable in the interior, perhaps because of the large amount of scattered light. A brief anneal at 225°C imparted a pink coloration which disappeared within 5 days upon standing in the dark at room temperature, but otherwise there were no apparent effects resulting from the anneal. Heating to 425°C and immediately cooling at 20”/hr produced some aggregation of the silver into microscopically visible triangular and cylindrical formations with dimensions of the order of a few microns. Examples of these are shown in Figs. 4 and 5. The axes of the rods were parallel to the sides of the triangles. In cases where these patterns were sharply developed, as in Fig. 5, one observed formations identical to those described by AMELINCKX(‘) in sodium chloride. These formations were never observed within 0.2 mm of the surface of the crystal. Prolonged annealing at 425°C (- 40-50 hr) produced an unusual effect. After etching, the crystals were perfectly clear, indicating complete disappearance or aggregation of the colloidal silver. Closer examination, however, revealed the presence of very long, whisker-like rods in the interior. Fig. 6 is a dark-field photograph showing these rods (but with diameters exaggerated by the scattered light). These rods were perfectly straight, of constant cross-section and, within the resolving power of the microscope, absolutely smooth. Their higher magnification is appearance under illustrated in Figs. 7 and 8. The density and lengths of these rods varied with the extent of exposure and the initial distribution of print-out silver. Typically, the widths were 2 or 3 TVand the lengths of the order of tenths of a millimeter. The longest rod, shown in Fig. 8, had a length > 0.6 mm. A measurement of the direction cosines of 90 rods in one crystal demonstrated that they were oriented in six directions with angles between them characteristic of the crystallographic [l lo] axes. In only one case out of hundreds of observations was there found a rod which was fused with another. No rods of any sort were found within 0.2 mm of the surface after the prolonged hightemperature treatment. One question which arises is whether these whisker-like precipitates have the mechanical strength exhibited by the more conventional whiskers. It was found that, upon etching away the
ANNEALING
OF SILVER
CHLORIDE
CRYSTALS
matrix with hypo, the exposed rods simply crumbled, the only surface change being that the etch-pits at the protruding rods were about three times larger than other etch-pits. It would be surprising if such regular growths were not single crystals, or at least cohesive. Further efforts to extract whole whiskers from the silver chloride matrix are in progress. In some cases, specimens were re-exposed after producing the rods. It was observed that foliagelike growths appeared at one end of each of those rods which were not approximately normal to the applied field (Fig. 9). This “foliage” always formed at the end toward which the field would drive photoelectrons, thus demonstrating that the rods conduct electrons. After short exposures, this foliage could be seen to have a [llO] dendritic structure. It is interesting that whereas the “whiskers” are stable against a 450°C anneal, such an anneal “disolves” the foliage. Elongated precipitates in crystals have been observed before, as in copper-doped aluminum by RUFF and KUSHNER@, but the present phenomenon is unusual in that the precipitates appear to be straight and regular. 5. GROWTH MECHANISM A possible mechanism for the growth of these whiskers is suggested by the observations of prismatic dislocation “smoke rings” around inclusions in AgCl by JONES and MITCHELL@), BARTLETT and MITCHELL(~@ and PARASNIS and MITCHELL. These rings move out along the [llO] slip directions, each leaving behind a vacant disc one atom distance thick. It thus
CONTAINING
INTERNAL
PRINT-OUT
121
appears that, in the early agglomeration of the silver, some particles become large enough to “blow out” such rings. At high temperatures, AgCl is very soft and the stress required to generate and move such rings may well be quite small. One then imagines that the finer particles ionize, evaporate and condense at the ends of the larger rods, generating new dislocation rings as more space is needed. Since no large protrusions are observed at the crystal surfaces, the dislocation rings presumably interact with other dislocations before reaching the surface. REFERENCES 1. HAYNE~J. H. and SHOCKLEY W., Whys.Rev. 82,935 (1951). 2. WEBBJ. H., J. Appl. Phys. 26, 1309 (1955). 3. HAMILTON J. F., HAMMF. A., and BRADYL. E., J. Appl.
Phys. 27, 874 (1956).
4. S~~PTITZ P., 2. Phys. 153, 174 (1958). 5. ESHELBYJ. D., NEWEYC. W. A. PRATTP. L. and LIDIARD A. B., Phil. Mug. 3,75
6. MITCHELL J. W., Properties
of Crystals
(1958). and Mechanical (Ed. FISHERJ. C. et al.), Lake
Dislocations
Placid Conference (1956). 7. AMMELINCKX S.. Dislocations and Mechanical Properties of C&taZs (Ed. FISHERJ. C. et al.), Lake Placid Conference i1956). L. M.. Proceedinas of 8. RIJFFA. W.. Tr. and KUSHNER the 2nd I&rnational Symposium on. X-Ray M&iscopy and X-Ray Microanalysis, Stockholm (1959)
(to be published).
9. JONESD. A. and MITCHELLJ. W., Phil. Mug. 3, 1 (1958).
J. T. and MITCHELLJ. W., Phil. Mug. 3, 10. BARTLETT 334 (1958).
A. S. and MITCHELLJ. W., Phil. Mug. 4, 11. PARASNIS 171 (1959).
FIG. 1. Substructure formed after acquiring print-out and annealing by taking crystal to 425°C and then cooling to room temperature at an initial rate of 20”Cihr.
[See CHILDS and SLIFKIN, pp. 119-121.1
[facirg
p.
118
_
.
_
-.._- .--- “_._
_-
-_-
. -_---.
._.-...-_-_---_-
FIG. 2. Substructure formed after acquiring print-out and annealing by taking crystal to 425°C and then cooling to room temperature at an initial rate of ZO”C/hr. All substructure, like that of Figs. 1 and 2, had a “three-branch network” where it “joined” with other substructure, as similarly reported by MITCHELL(~).
-
_
FIG. 3. Large-angle grain boundary on surface of a crystal. The thermal etch-pits were obtained after the crystal had extensive internal print-out and an anneal at 425°C for 10 hr.
FIG. 4. Internal precipitates after annealing to 425°C and then cooling to room temperature at an initial rate of ZO”C/hr, following the formation of internal print-out.
FIG. 5. Internal precipitates after annealing to 425°C and then cooling to room temperature at an initial rate of ZO”C/hr, following the formation of internal print-out. Similar silver formations have been observed by AMELINCKXt7) in sodium chloride.
FIG. 6. Typical dark-field illumination of elongated precipitates after annealing in MgO for 40 hr at 425°C. The illumination distorts the diameter of several of the line-precipitates. The precipitates formed throughout volume containing print-out. Shown crystal thickness was approximately 3 mm.
;
‘“A$
---QWpF
-:o’--
-.,
./
. I.
-
-... L
‘.
:::
1
,f
I,
’
_
*:
.
FIG. 7. Line precipitates with larger magnification.
f
‘
FIG. 8. One of the longer line precipitates observed.
_‘I ‘
!
a
-O.lmm-
Frc. 9. “Foilage” formed at elongated preeipirares The microscope was focused on the precipitate-end polarization field; consequently,
when crystals containing the lines are rezxposed. to which electrons would be swept by the decaying the rod appears as a “comet”.
PergamonPress 1959. Vol. 12. pp. 119-121.
ANNEALING
Printed in Great Britain.
OF SILVER CHLORIDE
CONTAINING
INTERNAL
C. CHILDSt University
CRYSTALS
PRINT-OUT*
and L. SLIPKIN
of North Carolina, Chapel Hill, N.C. (Received 23 June 1959)
Abstract-The HAYNES-SHOCKLEY technique was used to obtain internal print-out in single silver chloride crystals. Print-out was produced in all crystals from various boulss after observing that, in some discs from the boules, the surface deformation from manufacturer disc-shaping had a depth of 254 mm. After internal print-out was produced, the crystals were given a short MgO anneal at 425X, which decorated substructure and produced small internal precipitates with definite geometric configurations. Annealing at 425°C for 40-50 br produced uniform cylindrical precipitates with diameters of approximately 2 p and lengths < 0.7 mm. An investigation of these elongated The growth process precipitates showed that they were in the six [llO] crystallographic directions. of these precipitates are shown and discussed.
1. INTRODUC3’ION
HAYNES and SHOCKLJZY(~) utilized synchronous pulses of electric field and ultraviolet light to drive electrons deep into silver chloride crystals, resulting in the formation of specks of metallic silver at trapping sites distributed throughout the crystal. Modifications by WEBB@), HAMILTONet al. (a), and %PTITZ(*) have further extended the potentialities of this technique. The present paper is concerned with the effects of subsequent annealing on the distribution of the colloidal silver. In particular, under appropriate conditions long, perfectly straight whisker-like formations may be produced in the interior. Observations of the distribution of silver in small and large grain boundaries near the surface are also reported. ‘2. EXPERIMENTAL The specimens were in the form of singlecrystal slabs milled from Harshaw material. During preparation they were exposed only to light from a yellow “safe-1ite”. In the subsequent grinding and polishing on metallographic papers, at least l-75 mm were removed from the surfaces * Supported in part by the office of Ordnance and Research, Department of the Army. t Present address: National Aeronautics Administration, Washington, D.C.
and Space
to be exposed to eliminate the material which had been distorted by the machining. Later work showed that some surfaces required 2.54 mm removal before internal print-out was obtained. The crystals were then etched in 3 per cent KCN and washed, resulting in specimens N 1 cm2 in area and 2-5 mm thick. Prior to the ultraviolet exposure, they were annealed for 40-50 hr in air on a powdered MgO surface at ~425°C and cooled at a rate of 20”/hr. Another light etch in 3 per cent KCN served to render the surface clear again. The crystals were exposed to flashes of ultraviolet light provided by a GE BH-6 mercury lamp powered by the repetitive discharges of a Sprague H-850 pulse-forming network. The discharges were initiated by the firing of a 4C35 hydrogen thyratron, which in turn was triggered 1000 times per second. By applying the network potential across the crystal through insulating layers in a manner similar to that employed by HAYNES and SHOCKLEY,voltage pulses of 2 kV were synchronized with the light flashes. These pulses, produced as a result of the short-circuiting of the network, were due to the ionic polarization at the crystal surfaces and served to drive the photoelectrons into and through the crystal. Each flash created approximately 10s photoelectrons/cm2 of crystal
119
120
C.
CHILDS
and
surface. The decay time of the flash was 10 psec, and the subsequent build-up time of the network to one-quarter of its original maximum potential was approximately 200 psec, thus allowing adequate time for the photoelectrons to complete their trajectories before the polarization field was neutralized. After exposures of the order of 12 hr, a column of colloidal silver could be seen extending throughout the thickness of the crystal. It is the redistribution of this silver upon further annealing with which this paper is concerned. 3. SURFACE OBSERVATIONS Immediately after exposure, one observes a surface mosaic pattern of print-out consisting of regions of silver of varying intensity, demarcated by lighter lines which are presumably small-angle boundaries. Heating to 225°C produced no visible change, but a brief anneal to 425°C resulted in a clearing up of the dark regions on the surface. The sub-boundaries had now become delineated by an intense collection of silver. This substructure was visible only within about O-1 or 0.2 mm of the surface. Examples of such decorated boundaries are shown in Figs. 1 and 2. Interestingly, the boundary often appears as a pair of planes instead of a single surface. It has been suggested by ESHELBY et al.(s) that in alkali halides the charges on dislocations would cause a dislocation wall to really consist of two parallel walls. It may well be that the double patterns seen here are due to a similar effect in silver chloride. It was also observed, as already reported by MITCHELL@), that large-angle boundaries were not decorated. Fig. 3 is a photograph of the surface of a specimen consisting of two grains. The thermal etch-pits clearly show the presence of a large-angle boundary, but no silver is in evidence. Prolonged annealing at 425°C results in the complete disappearance of the silver in the surface regions. Presumably, the silver diffuses out of the crystal along the small-angle boundaries at which it had been collected. The fact that no silver is observed on large-angle boundaries, even after a brief anneal at 425”C, may be due to a faster diffusion rate along the large-angle boundaries. 4. INTERNAL OBSERVATIONS After exposure for 12 hr, the crystals had a cloudy appearance due to the colloidal silver
L.
SLIFKIN
dispersed in the interior. No distinct patterns were observable in the interior, perhaps because of the large amount of scattered light. A brief anneal at 225°C imparted a pink coloration which disappeared within 5 days upon standing in the dark at room temperature, but otherwise there were no apparent effects resulting from the anneal. Heating to 425°C and immediately cooling at 20”/hr produced some aggregation of the silver into microscopically visible triangular and cylindrical formations with dimensions of the order of a few microns. Examples of these are shown in Figs. 4 and 5. The axes of the rods were parallel to the sides of the triangles. In cases where these patterns were sharply developed, as in Fig. 5, one observed formations identical to those described by AMELINCKX(‘) in sodium chloride. These formations were never observed within 0.2 mm of the surface of the crystal. Prolonged annealing at 425°C (- 40-50 hr) produced an unusual effect. After etching, the crystals were perfectly clear, indicating complete disappearance or aggregation of the colloidal silver. Closer examination, however, revealed the presence of very long, whisker-like rods in the interior. Fig. 6 is a dark-field photograph showing these rods (but with diameters exaggerated by the scattered light). These rods were perfectly straight, of constant cross-section and, within the resolving power of the microscope, absolutely smooth. Their higher magnification is appearance under illustrated in Figs. 7 and 8. The density and lengths of these rods varied with the extent of exposure and the initial distribution of print-out silver. Typically, the widths were 2 or 3 TVand the lengths of the order of tenths of a millimeter. The longest rod, shown in Fig. 8, had a length > 0.6 mm. A measurement of the direction cosines of 90 rods in one crystal demonstrated that they were oriented in six directions with angles between them characteristic of the crystallographic [l lo] axes. In only one case out of hundreds of observations was there found a rod which was fused with another. No rods of any sort were found within 0.2 mm of the surface after the prolonged hightemperature treatment. One question which arises is whether these whisker-like precipitates have the mechanical strength exhibited by the more conventional whiskers. It was found that, upon etching away the
ANNEALING
OF SILVER
CHLORIDE
CRYSTALS
matrix with hypo, the exposed rods simply crumbled, the only surface change being that the etch-pits at the protruding rods were about three times larger than other etch-pits. It would be surprising if such regular growths were not single crystals, or at least cohesive. Further efforts to extract whole whiskers from the silver chloride matrix are in progress. In some cases, specimens were re-exposed after producing the rods. It was observed that foliagelike growths appeared at one end of each of those rods which were not approximately normal to the applied field (Fig. 9). This “foliage” always formed at the end toward which the field would drive photoelectrons, thus demonstrating that the rods conduct electrons. After short exposures, this foliage could be seen to have a [llO] dendritic structure. It is interesting that whereas the “whiskers” are stable against a 450°C anneal, such an anneal “disolves” the foliage. Elongated precipitates in crystals have been observed before, as in copper-doped aluminum by RUFF and KUSHNER@, but the present phenomenon is unusual in that the precipitates appear to be straight and regular. 5. GROWTH MECHANISM A possible mechanism for the growth of these whiskers is suggested by the observations of prismatic dislocation “smoke rings” around inclusions in AgCl by JONES and MITCHELL@), BARTLETT and MITCHELL(~@ and PARASNIS and MITCHELL. These rings move out along the [llO] slip directions, each leaving behind a vacant disc one atom distance thick. It thus
CONTAINING
INTERNAL
PRINT-OUT
121
appears that, in the early agglomeration of the silver, some particles become large enough to “blow out” such rings. At high temperatures, AgCl is very soft and the stress required to generate and move such rings may well be quite small. One then imagines that the finer particles ionize, evaporate and condense at the ends of the larger rods, generating new dislocation rings as more space is needed. Since no large protrusions are observed at the crystal surfaces, the dislocation rings presumably interact with other dislocations before reaching the surface. REFERENCES 1. HAYNE~J. H. and SHOCKLEY W., Whys.Rev. 82,935 (1951). 2. WEBBJ. H., J. Appl. Phys. 26, 1309 (1955). 3. HAMILTON J. F., HAMMF. A., and BRADYL. E., J. Appl.
Phys. 27, 874 (1956).
4. S~~PTITZ P., 2. Phys. 153, 174 (1958). 5. ESHELBYJ. D., NEWEYC. W. A. PRATTP. L. and LIDIARD A. B., Phil. Mug. 3,75
6. MITCHELL J. W., Properties
of Crystals
(1958). and Mechanical (Ed. FISHERJ. C. et al.), Lake
Dislocations
Placid Conference (1956). 7. AMMELINCKX S.. Dislocations and Mechanical Properties of C&taZs (Ed. FISHERJ. C. et al.), Lake Placid Conference i1956). L. M.. Proceedinas of 8. RIJFFA. W.. Tr. and KUSHNER the 2nd I&rnational Symposium on. X-Ray M&iscopy and X-Ray Microanalysis, Stockholm (1959)
(to be published).
9. JONESD. A. and MITCHELLJ. W., Phil. Mug. 3, 1 (1958).
J. T. and MITCHELLJ. W., Phil. Mug. 3, 10. BARTLETT 334 (1958).
A. S. and MITCHELLJ. W., Phil. Mug. 4, 11. PARASNIS 171 (1959).
FIG. 1. Substructure formed after acquiring print-out and annealing by taking crystal to 425°C and then cooling to room temperature at an initial rate of 20”Cihr.
[See CHILDS and SLIFKIN, pp. 119-121.1
[facirg
p.
118
_
.
_
-.._- .--- “_._
_-
-_-
. -_---.
._.-...-_-_---_-
FIG. 2. Substructure formed after acquiring print-out and annealing by taking crystal to 425°C and then cooling to room temperature at an initial rate of ZO”C/hr. All substructure, like that of Figs. 1 and 2, had a “three-branch network” where it “joined” with other substructure, as similarly reported by MITCHELL(~).
-
_
FIG. 3. Large-angle grain boundary on surface of a crystal. The thermal etch-pits were obtained after the crystal had extensive internal print-out and an anneal at 425°C for 10 hr.
FIG. 4. Internal precipitates after annealing to 425°C and then cooling to room temperature at an initial rate of ZO”C/hr, following the formation of internal print-out.
FIG. 5. Internal precipitates after annealing to 425°C and then cooling to room temperature at an initial rate of ZO”C/hr, following the formation of internal print-out. Similar silver formations have been observed by AMELINCKXt7) in sodium chloride.
FIG. 6. Typical dark-field illumination of elongated precipitates after annealing in MgO for 40 hr at 425°C. The illumination distorts the diameter of several of the line-precipitates. The precipitates formed throughout volume containing print-out. Shown crystal thickness was approximately 3 mm.
;
‘“A$
---QWpF
-:o’--
-.,
./
. I.
-
-... L
‘.
:::
1
,f
I,
’
_
*:
.
FIG. 7. Line precipitates with larger magnification.
f
‘
FIG. 8. One of the longer line precipitates observed.
_‘I ‘
!
a
-O.lmm-
Frc. 9. “Foilage” formed at elongated preeipirares The microscope was focused on the precipitate-end polarization field; consequently,
when crystals containing the lines are rezxposed. to which electrons would be swept by the decaying the rod appears as a “comet”.