6.8.
CARBON FILMS PREPARED ON PLASTIC SUBSTRATE
207
pure carbon or even pure plastic. For very high resolution work the pure
carbon films described below are preferable.
6.8. Pure Carbon Films Prepared on a Plastic
Substrate
Pure carbon films mounted on grids can be prepared by beginning with
grids having Parlodion or Formvar films upon them. Carbon is then evaporated on top of this substrate, and then finally the substrate is dissolved
away from the regions of the grid holes leaving just the carbon (Watson,
1955). Obviously, however, the substrate must not be removed completely
from between the wire and the carbon film or the latter will float away.
If carbon is to be the sole supporting film, it necessarily must be deposited in a thicker layer than when it is used to reinforce and stabilize
a plastic film. One should evaporate sufficient carbon so that the porcelain indicator is decidedly darkened as suggested by Fig. 23(e). The operator will have to gain experience in judging this by trial and error.
If a Parlodion substrate has been used, reagent grade acetone is a satisfactory solvent. Coated screens are slid under the surface of the acetone
at a steep angle (at least 45°). They are removed in perhaps 2 or 3 minutes, but experience and practice will determine just how long they
should be left submerged to remove all of the substrate in the vicinity
of the grid holes and yet not dissolve the plastic between the wire and
the carbon film. If Formvar is the substrate, chloroform may be used as
a solvent, in which case it will dissolve the Formvar in a matter of minutes, or ethylene dichloride may be used, which will take several hours.
When the screens are submerged, it is convenient to place them on a
piece of wire mesh (window screening). They then can be picked up without fumbling and delay when it is time to remove them from the solvent.
A virtue of this method is that the carbon films remain firmly adherent
to the grid when they are prepared exactly correctly. The disadvantage
of the method, however, is that this timing is difficult to standardize, and
there is, after all, unavoidably rough treatment as the screens go in and
out of the solvent. There are also surface tension forces at work as the
solvent finally evaporates. Thus, the method has a number of hazards.
One might paraphrase the nursery rhyme as follows: "When this method
is good, it is very very good, but when it is bad, it is horrid."
CARBON FILMS PREPARED ON PLASTIC SUBSTRATE
207
pure carbon or even pure plastic. For very high resolution work the pure
carbon films described below are preferable.
6.8. Pure Carbon Films Prepared on a Plastic
Substrate
Pure carbon films mounted on grids can be prepared by beginning with
grids having Parlodion or Formvar films upon them. Carbon is then evaporated on top of this substrate, and then finally the substrate is dissolved
away from the regions of the grid holes leaving just the carbon (Watson,
1955). Obviously, however, the substrate must not be removed completely
from between the wire and the carbon film or the latter will float away.
If carbon is to be the sole supporting film, it necessarily must be deposited in a thicker layer than when it is used to reinforce and stabilize
a plastic film. One should evaporate sufficient carbon so that the porcelain indicator is decidedly darkened as suggested by Fig. 23(e). The operator will have to gain experience in judging this by trial and error.
If a Parlodion substrate has been used, reagent grade acetone is a satisfactory solvent. Coated screens are slid under the surface of the acetone
at a steep angle (at least 45°). They are removed in perhaps 2 or 3 minutes, but experience and practice will determine just how long they
should be left submerged to remove all of the substrate in the vicinity
of the grid holes and yet not dissolve the plastic between the wire and
the carbon film. If Formvar is the substrate, chloroform may be used as
a solvent, in which case it will dissolve the Formvar in a matter of minutes, or ethylene dichloride may be used, which will take several hours.
When the screens are submerged, it is convenient to place them on a
piece of wire mesh (window screening). They then can be picked up without fumbling and delay when it is time to remove them from the solvent.
A virtue of this method is that the carbon films remain firmly adherent
to the grid when they are prepared exactly correctly. The disadvantage
of the method, however, is that this timing is difficult to standardize, and
there is, after all, unavoidably rough treatment as the screens go in and
out of the solvent. There are also surface tension forces at work as the
solvent finally evaporates. Thus, the method has a number of hazards.
One might paraphrase the nursery rhyme as follows: "When this method
is good, it is very very good, but when it is bad, it is horrid."