| Each sample was 0.32 X 0.64 X
0.16 cm (g X'TX I~fin.). After the sample was placed on the plate and
secured with a tungsten ribbon, the chamber was evacuated, and both
ionization gauges were turned on. The radiation from the ionization gauge
within the sample volume was sufficient to quickly bring the temperature
of the sample to 120°C. The samples were baked at this temperature for
four hours. After this bake, the pressure in the sample volume was
typically 9.3-10.7 X 10-5 Pa (7-•8 X 10-7 Torr) while the pressure in
the main chamber was 6.7-8.0 X 10-5 Pa (5-6 X 10-' Torr). The temperature
of the sample plate was then elevated from 120° to 400°C over
approximately a 13-min interval. Though the precision of the plate
temperature measurement was estimated to be t 10°C, the bulk temperature
uncertainties are significantly greater due to poor conductive heat
transfer across surface boundaries, the low thermal conductivities of the
samples, and radiative transfer processes. However, since the materials
have similar surface and heat transfer properties, each sample should
have experienced approximately the same heating rate because of the use of
the programmable supply. Since the rate of temperature rise with time was
slow relative to the flow-related equilibrium times for the essentially
incondensable gases, the outgassing rate qo was computed in terms of
nitrogen equivalents using the relationship
qo=S(P2-Pi)'
where P2 was the pressure in the sample volume, Pl was
the pressure in the main chamber, and S was the speed of the pump out
orifice. A pumping speed of 11.6 1/s cm2 was assumed, which for this
orifice yielded a speed of 33 1/s. In Fig. 2 are shown the outgassing
rates as a function of plate temperature for ECO-006, Viton, and a
stainless-steel blank which was used as a reference.
B. Valve test
In order to check the mechanical properties of ECD-006
at elevated temperatures in a typical high vacuum application, the
following test was performed. A 1.5-in. (3.8-cm) right-angle, high-vacuum
valve (Varian 951-5072) was attached to a 30-1/s VacIon pump. The valve
was initially provided with polyimide gaskets on both the bellows and main
seals so that the valve could be baked to 300°C. Because the VacIon pump
had seen extensive service during which it was |
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100
200 300
400
TEMPERATURE (°C)
FIG. 2.
Outgassing rates as functions of temperature after
4-h bake (120°C).
exposed to large quantities of hydrogen and noble gases,
the terminal pressure after a 48-h bake at 300°C was 5.3-6.7 X 10-6 Pa
(4-5 X 10-8 Torr), even though the pump was covered and the only load was
the polyimide valve. The pressure was determined from the operating
current of the pump. After this base pressure was established, the
polyimide main seal was replaced with an O-ring cut from ECD-006 sheet
stock (Compound 01018). The valve was reassembled and then baked at 300°C
for 48 h. The terminal pressure after this bake was again 5.3-6.7 X 10-8
Pa (4-5 X 10-8 Torr). The valve was then
disassembled, and the O-ring was inspected. The cracking and thermal set
normally encountered with Viton when operated at temperatures in excess of
200°C were not present. The ECD-006 was still pliable. The valve was
opened and closed several times with no apparent leaks across the main
seal.
RESULTS AND DISCUSSION
Despite the uncertainties associated with the outgassing
measurements, e.g., the nitrogen equivalence assumption, the limited
sample preparation, etc., Fig. 2 indicates that ECD-006 has an outgassing
rate at least comparable if not superior to Viton in the temperature |
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