Molecular sieve abstract
A non-evacuated dewar 10 advantageously employs a molecular sieve
30 that serves to adsorb gasses in the dewar when cooled during
operation of the detector 24 thereby preventing liquid formation
onto the detector. The effects of outgassing and permeation during
storage are substantially eliminated because the dewar package is
in partial pressure equilibrium with its environment since the interior
of the dewar is backfilled with the same inert gas as is in the
surrounding outside environment. A second molecular sieve 40 may
be used to adsorb moisture which may permeate into the housing.
Molecular sieve claims
What is claimed is:
1. In a dewar having a non-evacuated housing at substantially ambient
temperature and a detector mounted to the tip of a coldfinger for
cooling the detector during operation thereof, the improvement comprising:
means including a molecular sieve mounted to the coldfinger adjacent
the detector for adsorbing gas in the housing adjacent the detector
when the coldfinger is cooled during operation of the detector;
wherein the dewar is backfilled with an inert gas at substantially
one atmosphere.
2. The improvement of claim 1 which further comprises:
a second molecular sieve for adsorbing moisture in the housing.
3. The improvement of claim 2 which further comprises insulation
in the housing between the coldfinger and interior walls of the
housing.
4. A non-evacuated dewar comprising:
a housing having a mounting flange at one end and a lens cap assembly
at an opposite end;
a coldfinger substantially concentrically mounted in the housing
and extending from the mounting flange end and having a tip terminating
adjacent to the lens cap assembly;
an infrared detector mounted to the tip of the coldfinger;
a molecular sieve mounted to the coldfinger adjacent to the tip
thereof;
said dewar being backfilled with an inert gas at substantially
one atmosphere pressure;
cryoengine means coupled to the coldfinger for cooling the coldfinger
when it is desired to operate the infrared detector; and
said molecular sieve comprising means to adsorb said inert gas
to reduce the pressure in the housing adjacent the sensor below
the gas triple point of the gas to prevent liquid forming on the
detector in response to operation of the cryoengine.
5. The dewar of claim 4 which further comprises:
second molecular sieve comprising means for adsorbing moisture
that may permeate the interior of the housing.
6. The dewar of claim 5 wherein said second molecular sieve has
a larger surface area than presented by the surface area of the
molecular sieve adjacent to the tip of the coldfinger.
7. The dewar of claim 6 wherein said second molecular sieve is
positioned in the housing adjacent the flange.
8. The dewar of claim 7 wherein said second molecular sieve surrounds
the coldfinger and is spaced from the other molecular sieve by insulation.
9. The dewar of claim 8 which further comprises:
foam insulation between the coldfinger and interior walls of the
housing.
10. The dewar of claim 4 wherein said inert gas is nitrogen.
11. A method of detecting infrared radiation by a detector in a
dewar mounted on a tip of a coldfinger that is selectively cooled
by a cryoengine when it is desired to operate the detector, said
method comprising:
mounting a molecular sieve to the coldfinger adjacent the detector;
backfilling the dewar with an inert gas at substantially one atmosphere
pressure;
using the cryoengine to substantially simultaneously cool the detector
and molecular sieve to below about 80.degree. K.;
said molecular sieve adsorbing said inert gas, when cooled, sufficiently
to reduce the partial pressure within the dewar to below the gas
triple point of the inert gas to prevent liquid from forming on
the detector; and
using the detector array, when cooled, to sense infrared radiation
incident thereon.
12. The method of claim 11 which further comprises:
mounting a second molecular sieve within the dewar; and
using said second molecular sieve to adsorb moisture that may permeate
the dewar.
13. The method of claim 11 wherein insulation is provided between
the coldfinger and interior walls of the dewar.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to hermetically sealed packages and, more
particularly, to dewars containing infrared detectors.
2. Discussion
Some sensors, particularly mercury-cadmiumtelluride infrared detectors,
are most sensitive when operating at approximately 77.degree. K.
These detectors are typically used in conjunction with an evacuated
dewar in which the detector is placed. The evacuation of the dewar
is used to remove gasses which would otherwise occupy the region
surrounding the detector so that the potential heat loss through
convection and conduction during operation is minimized, as well
as to eliminate the formation of liquid onto the detector. The detector
is generally mounted onto the tip of a coldfinger which is in communication
with a cryoengine assembly. During operation the cryoengine serves
to expand a fluid such as helium in the coldfinger which, in turn,
adsorbs thermal energy causing the detector to be cooled.
While the traditional evacuated dewar has generally operated satisfactorily,
it does have some drawbacks. For example, the choice of materials
that are used to fabricate the dewar is somewhat limited and expensive
because it is necessary to choose materials having special characteristics
such as low diffusivity, low outgassing and other properties. Furthermore,
implementing the necessary closure techniques required to create
the vacuum inside the dewar is often costly and it is sometimes
difficult to ensure that the vacuum is maintained over a long period
of time.
U.S. Pat. No. 4719353 discloses a non-evacuated dewar in which
polymeric foam is disposed between the expander or coldfinger and
the housing. While the above document discloses a dewar which has
its advantages, it also has its own set of shortcomings and can
be further improved.
SUMMARY OF THE INVENTION
In accordance with the teachings of the preferred embodiment of
this invention, cryopumping means include a molecular sieve which
is mounted to the dewar coldfinger adjacent the detector. When the
coldfinger is cooled by the cryoengine, it also cools the molecular
sieve causing it to adsorb gas in the dewar housing next to the
detector. As a result, the pressure in the dewar is reduced to prevent
liquid formation on the detector as well as minimizing convection
and conduction losses. These advantages are economically obtained
while avoiding the problems of the traditional evacuated dewar construction.
BRIEF DESCRIPTION OF THE DRAWING
The various advantages of the present invention will become apparent
to one skilled in the art after reading the following specification
and by reference to the drawing in which:
FIG. 1 is a cross-sectional view of a dewar made in accordance
with the teachings of the preferred embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The aforementioned U.S. Pat. No. 4719353 discloses many of the
details of a dewar of the general type to which the present invention
pertains. The '353 patent is hereby incorporated by reference and
the reader's attention is drawn to that patent for background information.
The following specification accordingly focuses on a concise description
of the contribution to the art made by this invention.
Briefly, the dewar 10 includes a housing 12 with a lens cap assembly
14 at one end thereof and a mounting flange 16 at an opposite lower
end thereof. Flange 16 is suitably connected to a mounting plate
18 which, in turn, carries a suitable cryoengine 20. Cryoengine
20 is coupled to a coldfinger 22 which projects upwardly through
the major extent of housing 12. An infrared detector 24 is mounted
to the tip 26 of coldfinger 22. A cold shield 28 surrounds detector
24 and includes an aperture in an upper portion thereof acting as
a field stop to restrict the field of view of detector 24 in a known
manner.
A molecular sieve 30 is in thermal contact with the tip 26 of coldfinger
22. As will appear, the purpose of molecular sieve 30 is to remove
gasses from the area adjacent detector 24 when it is operating.
When detector 24 is operating, the cryoengine 20 is energized to
cause fluid contained within coldfinger 22 to expand thereby absorbing
thermal energy to cool detector 24 to the preferred 77.degree. K.
Molecular sieve 30 has a particular affinity for the type of gasses
in the area adjacent detector 24. Preferably, the dewar housing
12 is backfilled with an inert gas such as nitrogen at one atmosphere
or atmospheric pressure. Thus, the gas adjacent .detector 24 is
predominately nitrogen in the preferred embodiment. However, other
gasses such as argon and xenon can be alternatively used to backfill
the package.
Molecular sieve 30 can be made of a variety of zeolite materials
such as activated crystalline silicoaluminate with organic binders.
This preferred material is commercially available from Multiform
Desiccants under the trade designation NATRASORB 900. It is approximately
0.100 inch thick and about 0.400 inch in diameter. Sieve 30 is attached
to the outer walls of coldfinger 22 adjacent tip 26 by way of adhesive.
In the drawing, sieve 30 is shown with a plurality of annular grooves
32 which are for the purpose of increasing surface area to enhance
gas adsorption.
Provision is made for reducing the amount of gas adjacent detector
24 that needs to be adsorbed by the sieve 30 during operation. To
this end, much of the interior space within housing 12 is filled
with insulation 34. Preferably, the insulation is made of a polymeric
foam such as a polystyrene composite material. As can be seen in
the drawing, there is no insulation in the space above detector
24 which could otherwise block thermal radiation to be sensed by
the detector. The insulation serves a variety of functions such
as reducing heat loss due to gas conduction and convection until
the gas is adsorbed by the sieve 30 acting as a stiffener for the
coldfinger 22 and it can also aid in positioning of control cables.
The dewar 10 is typically located in an outside environment containing
nitrogen gas at one atmosphere pressure. Although the dewar 10 is
also backfilled with nitrogen at atmospheric pressure and is therefore
at equilibrium with the outside environment when in a non-operational
state, there still exists a possibility of moisture permeating the
interior of package, for example, through seals 38 between the mounting
flange 16 and mounting plate 18. Moisture in an appreciable amount
can degrade the cryopumping operation of molecular sieve 30. To
remove moisture within the housing 12 a second molecular sieve 40
with a larger surface area is contained within the dewar 10. It
is preferably located at the lower end of the housing 12 adjacent
the seals 38 which represent the most likely point of entry of moisture.
Molecular sieve 40 likewise can be made of the same material as
sieve 30.
When the detector 24 is not operating, the dewar components are
substantially at room or ambient temperature, i.e., the cryoengine
20 is not functioning to cool the detector to its operating point
which is below 80.degree. K. and preferably about 77.degree. K.
In this non-operating condition, there exists nitrogen gas in the
area above the detector 24 since the molecular sieve 30 is at equilibrium.
Any moisture that permeates the seals and enters the interior of
the dewar 12 is adsorbed primarily by the molecular sieve 40. Thus,
the dewar 10 can exhibit extended shelf life. This is important
since infrared detectors of this type may remain in their non-operating
state for some period of time.
When it is desired to utilize the detector 24 the cryoengine 20
is operated to cool the coldfinger 22. As is known in the art, the
tip 26 of coldfinger 22 is cooled more quickly than the lower portions
thereof. The cooling of coldfinger tip 26 simultaneously cools the
detector 24 and molecular sieve 30. The cooling of sieve 30 causes
it to change from its equilibrium condition to a condition at which
it adsorbs or getters gasses surrounding the detector 24. This adsorbing
of gasses creates a "cryopumping" action in which the
pressure in the dewar housing 12 is kept below the gas triple point,
e.g., 94 torr for nitrogen, during operation of the detector 24.
Thus, the possibility of a liquid forming onto the detector 24 is
substantially eliminated. In addition, heat losses through convection
and conduction are also substantially reduced.
When the dewar returns to its non-operational mode, the molecular
sieve 30 desorbs the adsorbed gasses and the package returns to
its equilibrium condition. Permeation during storage is kept to
a minimum because the package is in partial pressure equilibrium
with its outside environment. As noted above, moisture which may
enter the package is removed by the molecular sieve 40.
Those skilled in the art can now appreciate that the present invention
provides an economical, yet reliable dewar package construction
that eliminates many of the problems associated with traditional
evacuated dewars. It should be understood that while this invention
was described in connection with one particular example, many modifications
can be made thereto without departing from the spirit of this invention
after having the benefit of studying the specification, drawing
and following claims.
|