6.5.
MECHANICS OF CARBON
EVAPORATION
203
transformer (1 kva capacity), and the secondary leads, of course, must be
of very heavy wire or cable so that they do not become unduly heated.
The actual design of the apparatus to hold the carbon rods is not critical. A commercially available apparatus is shown in Fig. 23(a). There
also are various ways of preparing the tips of the carbon rods that are
to be vaporized. Most simply, one carbon rod can be sharpened in an
ordinary pencil sharpener, and its sharpened tip butted against a flat end
filed on the other rod. With such a system one must be careful not to
place the specimens to be carbonized in the shadow of the broad-ended
carbon. It must be realized that the carbon atoms will travel in straight
lines from the point of vaporization, and anything intervening between
this spot and the specimen will shield the specimen so that it does not
get properly coated.
A better way to make carbon tips is to turn short, thin shafts of the
type illustrated in Figs. 23(a) and (b). A tool to do this can be purchased
commercially or can be made in a machine shop without much difficulty,
using pieces of tool steel or carbide for the actual cutting edges. A commercially available tool is illustrated in Figs. 23(c) and (d). In some laboratories carbon rods are tipped in this way on a lathe. In this author's laboratory short lengths of carbon rod are mounted in the chuck of an electric
hand drill, and spun in the commercial sharpening tool. Twisting the
rods by hand will suffice, however, with such a tool. A nearly standardized product can be obtained with any of these more refined methods.
FIG. 23. (a) A commercially available holder for the carbon rods used in carbon
evaporation. One §-in. rod is fixed, the other is pushed into it by the spring-driven
sliding carriage, (b) An enlargement of the tips of the carbon rods. These are shaped
by the commercially available sharpener visible to the left in (c) and (d). Note that
one tip is squared and the other pointed. It is important to center these properly in
preparation for use. (c) This includes, besides the carbon rod sharpener, a piece of
broken porcelain with a drop of vacuum-pump oil in its center. This serves as an
indicator of the amount of carbon deposited. The porcelain is placed about the same
distance away from the carbon points as is the specimen, (d) When carbon is to be
evaporated directly upon a naked section overlying a support screen, or when carbon
is to be used to stabilize a plastic supporting film, the carbon layer should be very
thin, almost the least amount detectable on the porcelain indicator. The carbon does
not show in the region of the oil drop on the porcelain, and so one can visually
estimate the thin deposit by the contrast differential as seen in this figure, (e) When
carbon is to be evaporated on a mica surface to be used as a self-sustaining carbon
film, it must have a substantial thickness. Then carbon is deposited until the porcelain
is decidedly darkened as indicated in this
figure.
MECHANICS OF CARBON
EVAPORATION
203
transformer (1 kva capacity), and the secondary leads, of course, must be
of very heavy wire or cable so that they do not become unduly heated.
The actual design of the apparatus to hold the carbon rods is not critical. A commercially available apparatus is shown in Fig. 23(a). There
also are various ways of preparing the tips of the carbon rods that are
to be vaporized. Most simply, one carbon rod can be sharpened in an
ordinary pencil sharpener, and its sharpened tip butted against a flat end
filed on the other rod. With such a system one must be careful not to
place the specimens to be carbonized in the shadow of the broad-ended
carbon. It must be realized that the carbon atoms will travel in straight
lines from the point of vaporization, and anything intervening between
this spot and the specimen will shield the specimen so that it does not
get properly coated.
A better way to make carbon tips is to turn short, thin shafts of the
type illustrated in Figs. 23(a) and (b). A tool to do this can be purchased
commercially or can be made in a machine shop without much difficulty,
using pieces of tool steel or carbide for the actual cutting edges. A commercially available tool is illustrated in Figs. 23(c) and (d). In some laboratories carbon rods are tipped in this way on a lathe. In this author's laboratory short lengths of carbon rod are mounted in the chuck of an electric
hand drill, and spun in the commercial sharpening tool. Twisting the
rods by hand will suffice, however, with such a tool. A nearly standardized product can be obtained with any of these more refined methods.
FIG. 23. (a) A commercially available holder for the carbon rods used in carbon
evaporation. One §-in. rod is fixed, the other is pushed into it by the spring-driven
sliding carriage, (b) An enlargement of the tips of the carbon rods. These are shaped
by the commercially available sharpener visible to the left in (c) and (d). Note that
one tip is squared and the other pointed. It is important to center these properly in
preparation for use. (c) This includes, besides the carbon rod sharpener, a piece of
broken porcelain with a drop of vacuum-pump oil in its center. This serves as an
indicator of the amount of carbon deposited. The porcelain is placed about the same
distance away from the carbon points as is the specimen, (d) When carbon is to be
evaporated directly upon a naked section overlying a support screen, or when carbon
is to be used to stabilize a plastic supporting film, the carbon layer should be very
thin, almost the least amount detectable on the porcelain indicator. The carbon does
not show in the region of the oil drop on the porcelain, and so one can visually
estimate the thin deposit by the contrast differential as seen in this figure, (e) When
carbon is to be evaporated on a mica surface to be used as a self-sustaining carbon
film, it must have a substantial thickness. Then carbon is deposited until the porcelain
is decidedly darkened as indicated in this
figure.