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Cellular pre-freezing and pre-melting phenomena in alkali halides
Journal of Crysta!
Growth 2 (1968) 181-183 0 North-Holland
CELLULAR
PRE-FREEZING
Publishing
Co., Amsterdam
AND PRE-MELTING
PHENOMENA
IN ALKALI
HALIDES
M. HAMALAINEN Department
of Physics,
Turku University,
Turku, Finland
Received 5 February 1968
Melting of very thin crystals of alkali halides with regular dislocation arrays was followed in a hot stage of a light microscope. The material around the thermal etch pits corresponding to vertical dislocations and around the lines joining the etch pits melted preferentially. As a result a pattern with elongated cells was es-
The purpose of the present work is to study premonitory phenomena on either side of the melting point of alkali halides. The specimens contained equal amounts of potassium chloride and sodium bromide and about 0.3 weight % silver chloride as an impurity. A thin mixed crystal was placed in the hot stage “Vacutherm” of a metallurgical microscope “Reichert MeF” as shown in fig. 1. Argon was used as an inert gas in the chamber. The pressure of the gas was 1 atm. The crystal was melted and allowed to solidify. In this way a very thin crystal of uniform thickness was obtained on the quartz plate. The temperature was raised rapidly up to just below the melting point of the crystal and was then allowed
I-1
k-2 07 Fig. 1. Arrangement for hating of the specimens from above. (1) steel cylinder, (2) molybdenum heating strips, (3) thermocouple, (4) quartz ring, (5) specimen, (6) quartz plate, (7) objective.
181
tablished. These elongated cells broke into circular cells in the melt. The circular cells remained for a while and then disappeared. These effects were explained in terms of defect pre-melting. In freezing a reverse process took place.
Fig. 2.
Thermal etch pits revealing vertical dislocation lines in a thin (K, Na) (Br, Cl) mixed crystal. x 170.
to raise slowly. The boundary and the dislocation structure were revealed by thermal etching as shown in fig. 2. The thermal etch pits refer probably to the vertical dislocation lines. The dislocation density calculated from this pattern is about lo6 lines cm-‘. It was found that the grain boundaries melted at a somewhat lower temperature than the grains. In the first stage of melting of the grains the material around the etch pits and around the lines joining the etch pits melted preferentially as shown in fig. 3. As a result the molten lines formed boundaries of elongated cells as shown in fig. 4. During melting these elongated cells broke into circular cells (fig. 5). The circular cells could easily move in the melt. Sometimes they had a tendency to grow and as a result a nearly hexagonal network developed in the melt. Finally the circular cells disappeared. The diameter of these circular cells is about 10
182
M. HXMXLjiINEN a crystal
Fig. 3.
Preferential
melting of the material around the dislocation lines. X 170.
Fig. 4.
Elongated
cells formed in the first stage of melting. x 170.
Fig. 5.
Circular cells in the (K, Na) (Br, Cl) salt melt.
microns tion
which
network
Thereafter
process moved
took
is equal before
170.
size of the disloca-
melting.
the melt
place.
in the melt
to the mesh
x
with a regular pattern of etch pits like that shown in fig. 2 was found again. The “cellular melting” described above can be explained as follows: Impurities have been segregated to dislocations (ref. 1, p. 259). It is well known that grain boundaries begin to melt at a lower temperature than the mass of the material (ref. 1, p. 589). It seems also probable that the material around a dislocation melts at a slightly lower temperature than the rest of the material in the subgrain. In the present case the thickness of the crystal was 10 to 20 microns. The vertical dislocation lines were revealed by thermal etching. It is probable that between vertical lines there are also horizontal dislocations in the crystal. Therefore in the first stage of melting cells of dislocation-free material surrounded by molten boundaries are formed. Impurities have been segregated at the boundaries. In melting of the rest of the material the impurity cells remain for a while and become circular in the melt. This kind of premelting may be termed as “defect pre-melting”2). Silver chloride was used as an impurity because the silver halides melt at a somewhat lower temperature than the rest of the material and in addition various properties of silver halides point to extensive pre-melting disorder2). Also pure (pro analysis) potassium chloride was used and traces of similar phenomena were found. The reversibility of the process is somewhat surprising. As the first step of solidification the circular cells appear in the melt. This means that the equilibrium state of the system at a certain temperature just above the freezing point is characterized by a cellular array of pre-freezed regions. In some cases a dendritic
was slowly
Circular
and formed
cooled and a reverse cells appeared again,
elongated
cells.
Finally
Fig. 6.
A dendrite
array of impurities
in a pre-frozen
state
CELLULAR
PRE-FREEZING
AND PRE-MELTING
array was formed which then broke into circular cells as shown in fig. 6. This seemed to be no real solid-state dendrite but a dendrite in some pre-frozen state. It is natural to think that during freezing the regular array of impurities associated with the cellular pattern in the melt leads to a regular dislocation structure in the solid state. The present writer found convection cells in microscale in thin layers of alkali halides heated from below394). It seems, however, that the present cells in the melt are not thermal convection cells: (1) The present specimens were heated from above. (2) The pre-
PHENOMENA
IN ALKALI
HALIDES
183
sent cells were found only at a temperature just above the melting point. (3) The present cells seem to be separated circles rather than hexagonal convection cells. References 1) .I. W. Christian, The Theory of Transformations in Metals and Alloys (Pergamon Press, Oxford, 1965). 2) A. R. Ubbelohde, Melting and Crystal Structure (Clarendon Press, Oxford, 1965) p. 218. 3) M. HBmiilBinen, J. Crystal Crowth 1 (1967) 125. 4) M. HLmlllinen, J. Crystal Growth 2 (1968) 131.
Growth 2 (1968) 181-183 0 North-Holland
CELLULAR
PRE-FREEZING
Publishing
Co., Amsterdam
AND PRE-MELTING
PHENOMENA
IN ALKALI
HALIDES
M. HAMALAINEN Department
of Physics,
Turku University,
Turku, Finland
Received 5 February 1968
Melting of very thin crystals of alkali halides with regular dislocation arrays was followed in a hot stage of a light microscope. The material around the thermal etch pits corresponding to vertical dislocations and around the lines joining the etch pits melted preferentially. As a result a pattern with elongated cells was es-
The purpose of the present work is to study premonitory phenomena on either side of the melting point of alkali halides. The specimens contained equal amounts of potassium chloride and sodium bromide and about 0.3 weight % silver chloride as an impurity. A thin mixed crystal was placed in the hot stage “Vacutherm” of a metallurgical microscope “Reichert MeF” as shown in fig. 1. Argon was used as an inert gas in the chamber. The pressure of the gas was 1 atm. The crystal was melted and allowed to solidify. In this way a very thin crystal of uniform thickness was obtained on the quartz plate. The temperature was raised rapidly up to just below the melting point of the crystal and was then allowed
I-1
k-2 07 Fig. 1. Arrangement for hating of the specimens from above. (1) steel cylinder, (2) molybdenum heating strips, (3) thermocouple, (4) quartz ring, (5) specimen, (6) quartz plate, (7) objective.
181
tablished. These elongated cells broke into circular cells in the melt. The circular cells remained for a while and then disappeared. These effects were explained in terms of defect pre-melting. In freezing a reverse process took place.
Fig. 2.
Thermal etch pits revealing vertical dislocation lines in a thin (K, Na) (Br, Cl) mixed crystal. x 170.
to raise slowly. The boundary and the dislocation structure were revealed by thermal etching as shown in fig. 2. The thermal etch pits refer probably to the vertical dislocation lines. The dislocation density calculated from this pattern is about lo6 lines cm-‘. It was found that the grain boundaries melted at a somewhat lower temperature than the grains. In the first stage of melting of the grains the material around the etch pits and around the lines joining the etch pits melted preferentially as shown in fig. 3. As a result the molten lines formed boundaries of elongated cells as shown in fig. 4. During melting these elongated cells broke into circular cells (fig. 5). The circular cells could easily move in the melt. Sometimes they had a tendency to grow and as a result a nearly hexagonal network developed in the melt. Finally the circular cells disappeared. The diameter of these circular cells is about 10
182
M. HXMXLjiINEN a crystal
Fig. 3.
Preferential
melting of the material around the dislocation lines. X 170.
Fig. 4.
Elongated
cells formed in the first stage of melting. x 170.
Fig. 5.
Circular cells in the (K, Na) (Br, Cl) salt melt.
microns tion
which
network
Thereafter
process moved
took
is equal before
170.
size of the disloca-
melting.
the melt
place.
in the melt
to the mesh
x
with a regular pattern of etch pits like that shown in fig. 2 was found again. The “cellular melting” described above can be explained as follows: Impurities have been segregated to dislocations (ref. 1, p. 259). It is well known that grain boundaries begin to melt at a lower temperature than the mass of the material (ref. 1, p. 589). It seems also probable that the material around a dislocation melts at a slightly lower temperature than the rest of the material in the subgrain. In the present case the thickness of the crystal was 10 to 20 microns. The vertical dislocation lines were revealed by thermal etching. It is probable that between vertical lines there are also horizontal dislocations in the crystal. Therefore in the first stage of melting cells of dislocation-free material surrounded by molten boundaries are formed. Impurities have been segregated at the boundaries. In melting of the rest of the material the impurity cells remain for a while and become circular in the melt. This kind of premelting may be termed as “defect pre-melting”2). Silver chloride was used as an impurity because the silver halides melt at a somewhat lower temperature than the rest of the material and in addition various properties of silver halides point to extensive pre-melting disorder2). Also pure (pro analysis) potassium chloride was used and traces of similar phenomena were found. The reversibility of the process is somewhat surprising. As the first step of solidification the circular cells appear in the melt. This means that the equilibrium state of the system at a certain temperature just above the freezing point is characterized by a cellular array of pre-freezed regions. In some cases a dendritic
was slowly
Circular
and formed
cooled and a reverse cells appeared again,
elongated
cells.
Finally
Fig. 6.
A dendrite
array of impurities
in a pre-frozen
state
CELLULAR
PRE-FREEZING
AND PRE-MELTING
array was formed which then broke into circular cells as shown in fig. 6. This seemed to be no real solid-state dendrite but a dendrite in some pre-frozen state. It is natural to think that during freezing the regular array of impurities associated with the cellular pattern in the melt leads to a regular dislocation structure in the solid state. The present writer found convection cells in microscale in thin layers of alkali halides heated from below394). It seems, however, that the present cells in the melt are not thermal convection cells: (1) The present specimens were heated from above. (2) The pre-
PHENOMENA
IN ALKALI
HALIDES
183
sent cells were found only at a temperature just above the melting point. (3) The present cells seem to be separated circles rather than hexagonal convection cells. References 1) .I. W. Christian, The Theory of Transformations in Metals and Alloys (Pergamon Press, Oxford, 1965). 2) A. R. Ubbelohde, Melting and Crystal Structure (Clarendon Press, Oxford, 1965) p. 218. 3) M. HBmiilBinen, J. Crystal Crowth 1 (1967) 125. 4) M. HLmlllinen, J. Crystal Growth 2 (1968) 131.