ICE CRYSTALS
'
35
heat near the step. If we assume that this heat is released at the step itself, the temperature
eld is of the form
'
U
.
7exp.—UB(x+r)
Where Ti is the initial temperature.
r, is the radial distance from the step.
_
-
x, is the distance along the surface from the step.
A and B are constants.
'
‘
The temperature rise is signicant over distances of the order of a few tenths of a micron.
It varies only slowly With temperature and cannot in itself be responsible for thevariation
of xs shown in gure 17.
'
The crystal surface at a distance of several xs
from the growing step must be in equilibrium
With the supersaturated vapeur, and Will be covered With an adsorbed layer
of greater
surface density or thickness than that in the neighbourhood of the step (gure 18). The
'
_
vapeur
diffusion
//
\\
/
\
//
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rowîn
step , velocity.U
‘
\
9
x
/
.‘ê
|
:
|
8
l
‘
—5
|
.
E
:
.
.
.5
|
G)
.
|
—2
|
8
|
-o
i
o
|
_
»
“ô
___—___—
_—_———————————
::
saturation
_
.Q
@
_
,
…
._
,
m
.
Ë
_
5
o
_
.
—
Dîstdnce
‘
.
FIGURE 18. The structure of the adsbrbed layer near a growing step.
‘
equilibrium of the layer near the step is, controlled by the
diffusion of
molecules
Α
into the step and condensation
vapeur which gives an
3
approximate cylindrical
In
theçdusion
vapour
_
Ïf, ‘
in air is inverse}yproportional
should» decrease With decre‘.asmg pressure .
until the heat eld
‘
…
[