Vector 750 Watt Power Inverter User Manual download pdf (Page 4)

into each other horizontally, they were now pushed

linearly into each other vertically. This allowed much

more graphite rod length to be consumed in a typical run

(1/2 inch total graphite rod consumption for the rotary

approach, versus over 3 inches for the linear approach).

Air leakage during the synthesis was now minimal, as

the starting and final pressures were the same, and the

mass lost by the consumption of the graphite rods

balanced with the total mass of the end products.

However, new problems arose with the longer

duration of a run. Heating and pressure rise required the

process to be broken into short arc synthesis runs

followed by a cool-down of several minutes, after which

the arc synthesis would resume. While the yields rose to

about 3.5 %, the frequent rapid thermal cycling resulted

in the failure of a bell jar, as shown in Figure 1.

Another problem resulted from both graphite rods

being supported by cantilevered copper clamps. The

linear feedthrough also permitted rotary motion, so both

clamps had to be aligned along a common axis before

they could be pushed into each other. This was easy at

the start of a run, when the bell jar was clean and visual

alignment was possible. But as the interior of the bell jar

became coated with soot, is would be easy to misalign

the two rods so that they missed each other and

contacted the other rods’ copper clamp. This resulted in

a lot of melted copper.

The use of a linear motion feedthrough significantly

improved yields. This also generated much more soot so

that the total production mass of fullerene per run

increased dramatically. However, the increased mainte-

nance in frequently making new copper clamps, and loss

of a bell jar, made it a mixed blessing. Reducing the

duration of the arc synthesis segment and allowing more

cooling time between segments together reduced

thermal stress cycles and the magnitude of the pressure

swings. This also improved yield, and started to suggest

an optimum pressure range of around 150 Torr

III. MORE IMPROVEMENTS

Elimination of the cantilevered copper clamp on the

linear feedthrough became an obvious improvement.

This graphite rod was now mounted coaxial with the

linear feedthrough rod, so only linear motion was

needed. Sometimes, if the two graphite rods were

pushed into each other too forcefully, they would spot

weld. A slight rotation of the linear feedthrough would

easily break this weld, and the procedure could

continue. The mounting of the lower graphite rod is

shown in the center of Figure 2. To the left is an electri-

cally insulated copper feedthrough (¼ inch diameter)

used to mount the upper graphite electrode.

A collar is placed on the linear feedthrough shaft

outside the bell jar. It is locked in place to act as a stop

so that when most of the graphite rods are consumed by

the arc, you cannot accidentally go too far and touch the

copper clamps together, again, producing a lot of melted

copper.

A cooling coil was then added to remove excess

heat and stabilize the pressure swings. This consisted of

a section of copper tubing with 3” id and 4” long,

outside of which was wound with four turns of ¼ inch

copper tubing, soldered in place. Swagelock bulkhead

connectors were used to connect the cooling coil to the

baseplate.

Normally, tap water was used as a coolant, flowing

at approximately ½ gallon per minute. Maximum

pressure variations were now of the order of only a few

Torr. In one experiment, liquid nitrogen was used as a

coolant. Curiously, it had no impact on yield. Perhaps

the temperature differential from arc plasma tempera-

tures (a few thousand Kelvins) to a water cooled surface

(at about 285 K) versus to a LN

cooled surface (77 K)

is not too different. Figure 3 shows the upper graphite

electrode and its holder in place, as well as the cooling

coil.

A refrigeration service vacuum pump, with a base

pressure of 25 milliTorr, did not significantly increase

yields when used for the pumpdown-backfill prepara-

tion sequence, but using 5N9 helium (99.999%) did

show a slight improvement in yield. A prepped system

ready to initiate the arc is shown in Figure 4, with the

safety shield removed.

At the end of a run, the system was allowed to cool

to room temperature, and then vented to atmospheric

pressure. When removing the bell jar, a vent hose was

used to collect any soot that may have been dislodged.

Soot can be seen covering just about everything in

Figure 5. The silvery material around the ends of the

rods is sometimes called “popcorn” and is rich in

carbon

the Bell Jar, Vol. 10, No. 3/4, Summer/Fall 2001

Figure 1 - Fractured Bell Jar

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