.
ICE z CRYSTALS=
37
.
takes place into a Vacuum during—certain” freeze drying îoperati0ns (gure 20) and ‘also
‘
during evaporation of large ice
in Vacuum. ‘
Ih‘this
appears that the surface
temperature of the specimen falls qmckly during the
stages,
occurs at the lower temperatures to give… the observed hexagonal faces.
__
'
We are now in a position to assess the role of the various processes in controlling
growth or evaporatioh of ice
Inso far as grdwth always g‘iVes rise to dendrites or
—
other crystal forms with well dened
faces, it _follOw5'that‘
faces are always present, although they may not be present at the leading
dendrites
growing in extreme conditions, In the case of crystals growth froni_the
probably also in growth from the liquid, these smooth areas4groWonlÿ by Spreading of
layers which may begin at dislocations', or at Crystal tips where” Surface nüclætion is taking
place. The transition from planar to dendritic growth for crystals growing from the vapeur
will take place as soon as the vapour and diffusion eld begin to favour the corners of a
'
growing crystal, which will depend on the crystal size as well as such factors as ventilation
or stirring. Habit changes will take place depending on the role of surface nucleation on
different crystal faces and the relative rate of propagation of layers on different faces. A
summary of these processes is given in table 2. The role of polar imputies can be seen
as inuencing the propagation of layers in different ways on different crystal faces.
When appreciable areas of the crystal are molecularly smooth with layer separations
signicantly in excess of xs, the collection distance for molecules, the boundary conditions
of equations 2,3, which assume equilibrium everywhere with the crystal surface, will no
longer be valid, and the growth rate will be signicantly less than that predicted.
CONCLUSION
‘
In this review I have given an account of experimental work on ice crystal growth and
.
evaporation, much of which has been carried out against a background of atmospheric
physics. In so far as an understanding of the growth properties of ice are fundamental
to an understanding of the freeze drying process, Ifeel that this approach needs no apology.
—
It is evident that there are still large gaps in our, knowledge of the behaviour of ice crystals,
both on the molecular scale and on the scale of the dendrite morphology which exists
under a variety of growth conditions. We also lack information on the ”growth of ice
…
crystals in contact with membranes; As these gaps are
tQËPPÏŸ
,
this fundamental knowledge more usefully to the practical
Wh10hasell
,
freeze drying process.
»
—}«_
_
'
…
(* References cOtaihextensrveblbhographY)
1. *MASON B. J. (1960), Discussio‘ns.ofrhe
*
j
"
’
‘
-
_
'
2. PRUPPACHER H. -and’NBmURGÉR-M, (1‘963).Îiïaùmalî0fÆMOËÏ£CÏËË
376—
.
3_ Hmm
P. and
and
hkin‘etics.
‘
,
4. ËÏÊ‘ÎÏÊÎËÎ
n"‘\‘Hï‘iÎii
,
_
,
«‘… ';]“ï ,
_
_
w
'
ICE z CRYSTALS=
37
.
takes place into a Vacuum during—certain” freeze drying îoperati0ns (gure 20) and ‘also
‘
during evaporation of large ice
in Vacuum. ‘
Ih‘this
appears that the surface
temperature of the specimen falls qmckly during the
stages,
occurs at the lower temperatures to give… the observed hexagonal faces.
__
'
We are now in a position to assess the role of the various processes in controlling
growth or evaporatioh of ice
Inso far as grdwth always g‘iVes rise to dendrites or
—
other crystal forms with well dened
faces, it _follOw5'that‘
faces are always present, although they may not be present at the leading
dendrites
growing in extreme conditions, In the case of crystals growth froni_the
probably also in growth from the liquid, these smooth areas4groWonlÿ by Spreading of
layers which may begin at dislocations', or at Crystal tips where” Surface nüclætion is taking
place. The transition from planar to dendritic growth for crystals growing from the vapeur
will take place as soon as the vapour and diffusion eld begin to favour the corners of a
'
growing crystal, which will depend on the crystal size as well as such factors as ventilation
or stirring. Habit changes will take place depending on the role of surface nucleation on
different crystal faces and the relative rate of propagation of layers on different faces. A
summary of these processes is given in table 2. The role of polar imputies can be seen
as inuencing the propagation of layers in different ways on different crystal faces.
When appreciable areas of the crystal are molecularly smooth with layer separations
signicantly in excess of xs, the collection distance for molecules, the boundary conditions
of equations 2,3, which assume equilibrium everywhere with the crystal surface, will no
longer be valid, and the growth rate will be signicantly less than that predicted.
CONCLUSION
‘
In this review I have given an account of experimental work on ice crystal growth and
.
evaporation, much of which has been carried out against a background of atmospheric
physics. In so far as an understanding of the growth properties of ice are fundamental
to an understanding of the freeze drying process, Ifeel that this approach needs no apology.
—
It is evident that there are still large gaps in our, knowledge of the behaviour of ice crystals,
both on the molecular scale and on the scale of the dendrite morphology which exists
under a variety of growth conditions. We also lack information on the ”growth of ice
…
crystals in contact with membranes; As these gaps are
tQËPPÏŸ
,
this fundamental knowledge more usefully to the practical
Wh10hasell
,
freeze drying process.
»
—}«_
_
'
…
(* References cOtaihextensrveblbhographY)
1. *MASON B. J. (1960), Discussio‘ns.ofrhe
*
j
"
’
‘
-
_
'
2. PRUPPACHER H. -and’NBmURGÉR-M, (1‘963).Îiïaùmalî0fÆMOËÏ£CÏËË
376—
.
3_ Hmm
P. and
and
hkin‘etics.
‘
,
4. ËÏÊ‘ÎÏÊÎËÎ
n"‘\‘Hï‘iÎii
,
_
,
«‘… ';]“ï ,
_
_
w
'