U.S. patent application number 10/782319 was filed with the patent office on 2004-10-21 for method for reducing the partial pressure of undesired gases in a small vacuum vessel.
Invention is credited to McCambridge, James D..
Application Number | 20040208752 10/782319 |
Document ID | / |
Family ID | 32908662 |
Filed Date | 2004-10-21 |
United States Patent
Application |
20040208752 |
Kind Code |
A1 |
McCambridge, James D. |
October 21, 2004 |
Method for reducing the partial pressure of undesired gases in a
small vacuum vessel
Abstract
This invention relates to a method for reducing the partial
pressure of undesired gases in a small vacuum vessel. The method
comprises using a getter pump with a sufficient sorption capacity
to eliminate the need for a bakeout step.
Inventors: |
McCambridge, James D.;
(Swarthmore, PA) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
32908662 |
Appl. No.: |
10/782319 |
Filed: |
February 19, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60448852 |
Feb 20, 2003 |
|
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Current U.S.
Class: |
417/51 ;
417/53 |
Current CPC
Class: |
H01J 9/38 20130101; F04B
37/02 20130101 |
Class at
Publication: |
417/051 ;
417/053 |
International
Class: |
F04B 037/02 |
Claims
What is claimed is:
1. A method for reducing the partial pressure of undesired gases in
a small vacuum vessel with a vacuum volume of about 1 liter or
less, comprising: (a) providing a getter pump as an integral part
of said small vacuum vessel, wherein said getter pump has
sufficient sorption capacity to adsorb undesired gases; (b)
evacuating said small vacuum vessel; (c) activating said getter
pump; and (d) sealing said small vacuum vessel.
2. The method of claim 1, wherein said getter pump is an appendage
getter pump.
3. The method of claims 1 or 2, wherein said getter pump is
activated by heating with an external heater.
4. The method of claim 3, wherein the getter material in said
getter pump is a zirconium alloy.
5. A method for reducing the partial pressure of undesired gases in
a small vacuum vessel with a vacuum volume of about 1 liter or
less, comprising providing a getter pump as an integral part of
said small vacuum vessel, wherein said getter pump has sufficient
sorption capacity to adsorb undesired gases and thereby eliminates
the need for a bakeout cycle.
6. The method of claim 5, wherein said getter pump is an appendage
getter pump.
7. The method of claims 5 or 6, wherein said getter pump is
activated by heating with an external heater.
8. The method of claim 7, wherein the getter material in said
getter pump is a zirconium alloy.
9. A method for reducing the partial pressure of undesired gases in
a small vacuum vessel with a vacuum volume of about 1 liter or
less, said small vacuum vessel having undergone an incomplete
outgassing bakeout step, comprising providing a getter as an
integral part of said small vacuum vessel, wherein said getter has
sufficient sorption capacity to adsorb remaining undesired
gasses.
10. The method of claim 9, wherein said getter pump is an appendage
getter pump.
11. The method of claims 9 or 10, wherein said getter pump is
activated by heating with an external heater.
12. The method of claim 11, wherein the getter material in said
getter pump is a zirconium alloy.
13. A small vacuum vessel with a vacuum volume of about 1 liter or
less, comprising a getter pump as an integral part of said small
vacuum vessel, wherein said getter pump has sufficient sorption
capacity to adsorb undesired gases without the necessity of an
outgassing bakeout step.
14. The small vacuum vessel of claim 13, wherein said getter pump
is an appendage getter pump.
15. The small vacuum vessel of claim 13 or 14, wherein the getter
material in said getter pump is a zirconium alloy.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/448,852, filed on Feb. 20, 2003, which is
incorporated in its entirety as a part hereof for all purposes.
FIELD OF THE INVENTION
[0002] This invention relates to a method for reducing the partial
pressure of undesired gases in a small vacuum vessel that comprises
using a getter pump as an integral part of the small vacuum
vessel.
BACKGROUND OF THE INVENTION
[0003] Vacuum vessels are used in a wide range of applications in
which it is advantageous or necessary to have electronic circuits,
sensors or other devices located in a vacuum either to maintain a
given temperature or to improve reliability, e.g., to provide
corrosion resistance.
[0004] The discovery and use of high temperature superconductor
(HTS) materials that superconduct at temperatures of 77K or higher
have increased the need for small vacuum vessels, which permit
operation at cryogenic temperatures. They have been used in various
industrial, medical, research and military applications. As a
result of the growth in the telecommunications industry, one of the
fastest growing commercial applications has been in the area of
electronics and associated microwave engineering. In this area, an
essential part of many devices is the filter element. HTS filters
have significant advantages in insertion loss and selectivity due
to the extremely low radio frequency (RF) loss in HTS materials.
Amplifiers and other circuit components, as well as the HTS
filters, can be contained within a small vacuum vessel. One such
application is a cryogenic receiver front-end in which
cryoelectronic components such as RF filters and amplifiers are
contained within the small vacuum vessel.
[0005] The small vacuum vessel is evacuated to a high vacuum in
order to more readily maintain the cryoelectronic components at
cryogenic temperatures. Maintaining this high vacuum is critical to
the performance of the cryoelectronic components contained in the
small vacuum vessel.
[0006] Desorption of gases adsorbed on the inner surfaces of the
small vacuum vessel and on the surfaces of its contents, i.e.,
outgassing, is therefore important. Typical outgassing practice is
to subject the small vacuum vessel to a bakeout step, i.e.,
maintaining the small vacuum vessel and its contents at a high
temperature while under a high vacuum, i.e., 10.sup.-5-10.sup.-7
mbar. The materials used in the construction of the small vacuum
vessel and its contents and the temperature to which they can be
subjected must be considered when deciding on the temperature used
for this bakeout. This often dictates the use of lower
temperatures. Longer bakeout times are needed at these lower
temperatures to provide the necessary outgassing. The small vacuum
vessel and its contents are typically heated to a temperature in
excess of 50.degree. C. for a length of time up to a week. Even
with bakeout temperatures on the order of 100.degree. C., bakeout
times can be on the order of 48 hours.
[0007] A getter pump is usually provided in the small vacuum vessel
to adsorb gases that are present as a result of any additional
outgassing that occurs after sealing the vacuum vessel or as a
result of small leaks. The getter pump is activated by heating to
remove any oxide and nitride coatings. Getter pump activation is
carried out before sealing the small vacuum vessel. This process
requires a time on the order of 4 hours for the small vacuum vessel
to cool down after the bakeout, and a time on the order of an hour
for the pulse activation of the internal electrical heaters of the
getter pump to avoid overheating the internal parts of the small
vacuum vessel. The small vacuum vessel can then be sealed.
[0008] The specific times actually required for the steps indicated
above depend on the size and contents of the small vacuum vessel.
However, the typical times given are indicative of the considerable
time and cost to the production of the small vacuum vessel. In
addition, this outgassing process requires the purchase and
maintenance of a high vacuum system dedicated to the bakeout cycle
and equipment needed to bakeout the small vacuum vessel.
[0009] An object of the present invention is to provide an improved
method for more efficiently reducing the partial pressure of
undesired gases in a small vacuum vessel.
SUMMARY OF THE INVENTION
[0010] This invention relates to a method for reducing the partial
pressure of undesired gases in a small vacuum vessel that comprises
the use of a getter pump as an integral part of the small vacuum
vessel. The method comprises (a) providing a getter pump as an
integral part of the small vacuum vessel, wherein the getter pump
has sufficient sorption capacity to adsorb undesired gases; (b)
evacuating the vacuum vessel; (c) activating the getter pump; and
(d) sealing the vacuum vessel.
[0011] Preferably the getter pump is in the form of an appendage
getter pump that is attached to and made a part of the small vacuum
vessel.
[0012] Yet another embodiment of this invention is a method for
reducing the partial pressure of undesired gases in a small vacuum
vessel with a vacuum volume of about 1 liter or less, said small
vacuum vessel having undergone an incomplete outgassing bakeout
step, by providing a getter pump as an integral part of said small
vacuum vessel, wherein said getter pump has sufficient sorption
capacity to adsorb remaining undesired gasses.
[0013] Yet another embodiment of this invention is a small vacuum
vessel with a vacuum volume of about 1 liter or less, that includes
a getter pump as an integral part of said small vacuum vessel,
wherein said getter pump has sufficient sorption capacity to adsorb
undesired gases without the necessity of an outgassing bakeout
step.
[0014] Yet another embodiment of this invention is a method for
reducing an increase in temperature experienced by a vacuum vessel,
having a vacuum volume of about 1 liter or less, from the reduction
of the partial pressure of undesired gases in the vacuum vessel by
(a) providing as an integral part of the vacuum vessel a getter
pump that has sufficient sorption capacity to remove undesired
gases from the vacuum vessel; and (b) before sealing the vacuum
vessel, heating the vacuum vessel only to the extent required to
activate the getter pump.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] The present invention provides a method for reducing the
partial pressure of undesired gases in a small vacuum vessel
containing a getter pump as an integral part of the small vacuum
vessel. No bakeout is necessary with the instant method and the
time interval between the beginning of evacuation of the small
vacuum vessel to sealing of the small vacuum vessel is reduced to
the order of an hour instead of the typical 4-5 days of the current
practice. The work of the getter pump in removing contaminants is
thus performed at the temperature and pressure at which the
pressure vessel is operated (i.e. the temperature at which it is
placed and kept in service), and decontamination is thus performed
in this invention in the absence of the elevated temperatures
typical of previous bakeout methods.
[0016] A small vacuum vessel is a vacuum vessel with a vacuum
volume of about 1 liter or less. Undesired gases are gases adsorbed
on the inner surface area of the small vacuum vessel and on the
surfaces of its contents and subject to outgassing, as well as
gases present as a result of small leaks.
[0017] The getter pump is an integral part of the small vacuum
vessel and must be chosen appropriately to have sufficient sorption
capacity to adsorb the undesired gases.
[0018] The quantity of undesired gases as a result of outgassing
can be estimated based on the inner surface area of the small
vacuum vessel and the surface area of its contents and the
particular materials used in the construction of the small vacuum
vessel and its contents. Typically the volume of adsorbed gases is
equivalent to a few monolayers coverage of the internal surface.
Allowance must be made for the varying degree to which the
different components or materials used tend to outgas. The quantity
of undesired gases as a result of small leaks can be estimated on
the basis of degree of leakage that is to be provided for. A getter
pump is then chosen with a sorption capacity of several times, for
example at least 3-5 times, that estimated to be needed in order to
provide a safe margin of design. This getter pump can then be
tested in a small vacuum vessel to determine if the estimate was
correct and performance is as expected. If not, a getter pump with
even greater sorption capacity, for example at least an additional
2-3 times more sorption capacity, can be used and tested. When the
expected performance has been obtained, such a getter pump can be
used in all similar small vacuum vessels produced. The use of a
getter pump with the greater sorption capacity as described above
will enable relaxation of restrictions with regard to outgassing
placed on materials chosen for the construction of the small vacuum
vessel and its contents.
[0019] The getter pump can be in the form of pellets of
non-evaporable getter material contained in an appendage that is an
integral part of the small vacuum vessel or in the form of an
appendage getter pump available commercially. The getter pump is
activated by heating to remove any oxide or nitride coatings. This
activation is carried out with the small vacuum vessel evacuated
but before the sealing of the small vacuum vessel. This heating can
be accomplished by passing an electrical current through internal
heaters provided with the getter pump, which are located inside the
pressure vessel, but preferably is done by heating the appendage to
a temperature sufficient to activate the getter material contained
therein by means of an external heater outside the pressure vessel
designed to fit around or encompass the appendage. Care must be
taken to avoid overheating the sensitive components of the small
vacuum vessel and its contents during activation. The getter pump
can be placed sufficiently far from or shielded from the small
vacuum vessel contents to protect the contents from a harmfully
high temperature when the getter is activated. Careful design and
the use of an appendage getter pump can therefore result in a
significant reduction in the maximum temperature experienced by the
contents compared to that occurring during a typical outgassing
bakeout step.
[0020] If a shortened incomplete outgassing bakeout step is used
and a significant amount of gas is still adsorbed on the inner
surface of the small vacuum vessel and the surfaces of its
contents, the instant invention provides a method for reducing the
partial pressure of remaining undesired gases, i.e., the remaining
undesired gases adsorbed on the inner surface of the small vacuum
vessel and the surfaces of its contents. In this instance, the
getter pump must be chosen appropriately to have sufficient
sorption capacity to adsorb these remaining undesired gases.
[0021] Typical getter material that can be placed within an
appendage are Zr--V--Fe alloys such as the getter pump St 707.TM.
available from SAES Getters, S.p.A., Milan, Italy. Preferably, the
getter pump is in the form of an appendage getter pump available
from SAES Getters USA Inc., Colorado Springs, Colo. 80906. The
appendage getter pump can be bolted to the small vacuum vessel
using holes in the flange provided by the vendor or it can be
welded to the small vacuum vessel.
[0022] The method of the invention for reducing the partial
pressure of undesired gases in a small vacuum vessel requires
considerably less time than the method currently used and
eliminates the need for a bakeout cycle and the time it consumes as
well as the need for an expensive high vacuum system dedicated to
the bakeout cycle and the need for equipment to bakeout the small
vacuum vessel.
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