| INTRODUCTION
The elastomeric O-rings used in high vacuum
valves and flanges generally limit system bake temperatures to a
maximum of 200°C and, typically, to below 150°C. Above these
temperatures most elastomers have poor mechanical properties and outgas
excessively. In order to bake a system at 300°C, gold, oxygen-free high
conductivity (OFHC) copper, or polyimide seals are required. The
advantages of metal seals are well documented.' Polyimide has excellent
outgassing properties at elevated temperatures if properly cured2;
however, it does not have the desirable mechanical properties of an
elastomer like Viton. In fact, polyimide is relatively brittle, subject to
chipping, and typically requires considerable stress to effect a seal.
High temperature, atmospheric pressure experiments
conducted at this laboratory on a new fluorocarbon
Fm. 1. Outgassing
measurement apparatus. |
elastomer indicated that this
elastomer might be a suitable replacement for metal and polyimide seals in
systems bakable to 300°C. This elastomer, developed by E. I. duPont de
Nemours & Co., is sold under the name ECD-006 perfluoroelastomer.s It
possesses many of the mechanical properties of Viton but with a
significantly superior high temperature limit. Most of sit mechanical and
chemical properties at atmospheric pressure and high temperatures have
been documented.'' However, there has been no effort, to date, to
investigate its high vacuum properties. This brief communication presents
some data on these properties.
EXPERIMENTAL PROCEDURES AND RESULTS
Two types of measurements were made. The first was a simple
comparison of the outgassing rates of Viton and ECD-006. The second was an
evaluation of ECD-006 as an O-ring in a bakable valve.
A. Outgassing
Figure 1 shows the apparatus used to compare the
outgassing rates of Viton and ECD-006. The sample was placed in an
enclosed volume on a square-shaped stainless-steel (Type 304) plate which
was 1.27 cm (0.5 in.) on a side and 0.38 mm (0.015 in.) thick. The
temperature of the plate was monitored with a chromelalumel thermocouple.
The heater was constructed to minimize temperature gradients in the center
of the plate. Heating of the plate was accomplished by radiation from
tungsten filaments. Pressure within the enclosed volume was measured by a
nude ionization gauge. A shield was interposed between this gauge and the
sample plate to reduce charged particle bombardment of the sample. The
only orifice connecting the sample volume with the main chamber was a
1.91-cm (0.75-in.) orifice through a 0.38-mm (0.015-in.) stainless-steel
removable cover plate. The volume of the sample enclosure was 2.5 1. The
pressure in the main chamber was also determined by a nude ionization
gauge. Thoriated iridium filaments were used on both gauges. Two separate
ionization gauge control units |
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