Note: Descriptions are shown in the official language in which they were submitted.
1 1 7~85 1
This invention relates generally to refrigeration and
more particularly the invention relates to microminiature
refrigerators and methods of making the same.
Certain materials, called superconductors, have the
ability to pass electric current without resistance. Since
superconductivity is observed only at temperatures close to
absolute zero, one of the main obstacles to extensive use of
superconductins devices is the need for reliable, continuous
refrigeration Superconducting devices, such as supersensitive
magnetometers, voltmeters, ammeters, voltage standards, current
comparators, etc., require a cryogenic environment to operate.
Traditionally this has been provided by a bath of liquid helium.
The helium is liquified elsewhere and transported to, and
transferred to the device Dewar. The labor and complexity of
such an operation has severely limited the use of these devices.
Many of the above superconducting devices dissipate only a few
microwatts in operatlon whlle the available cryogenic systems
provide a refrigeration capacity of watts, thus the devices are
poorly matched to the refrigeration.
In addition, ~any devices such as optical microscope
stages, x-ray diffraction sample holders, electron microscope
cold stages, devices for cryosurgery in the brain, for ECG, ~CG
and EKG measurements, and low noise amplifiers require or benefit
from suhambient operating temperatures.
Additionally, there are a number of high speed, high
power devices such as VLSI (very large scale integration) chips
and transmitters that are small, on the order of a centimeter
square, and dissipate large amounts of heat, on the order of
*;
1 1 7085 ~
10 to 50 watts. Traditional cooling devices, such as fans for
convection cooling, are not capable of dissipating this amount
of heat without significant increases in te~perature above
a~ient.
Miniature closed cycle refrigerators such as those
based on the Gifford-Mc~ahon cycle, Vuilleumier, ~tirling, etc.,
have been developed. These refrigerators, with capacities in
the range of 0.5-13 watts, are convenient and compact but, be-
cause of their moving parts, they introduce a large amount of
vihration and magnetic noise which interferes with the operation
of the devices. ~iniature Joule-Thomson refrigeration systems
have been developed which have a cooling capacity typically be-
tween O.S-10 watts. mhe design configurations of these compact
systems are generally helicallY finned tubes coiled around a
mandrel, the high-pressure gas flowing inside the tubes and the
low-pressure gas flowing outside the tubes. Such helically
finned and coiled heat exchangers are fabricated ~y laborious
welding or soldering of the individual components. Because of
the intricacy of the device, microminiature refrigerators with
milliwatt capacities until now have not been made practically
available.
What is needed for many devices is a microminiature
refrigerator of approximately 1/2" to 4" in size with a cooling
capacity in the milliwatt ran~e. Also needed are microminiature
refrigerator fabrication methods which avoid conventional
laborious welding or soldering techniques and allow the forma-
tion of very small gas lines to operate the heat exchangers in
the laminar flow regime and still have an efficient exchange of
heat. The consequent absence of turbulence in the gas stream
eliminates vibration and noise, both important considerations
for superconducting device applications. The miniature size
would allow the incorporation of an entire cryogenic system -
superconducting sensor as a hybrid component in electronic
~ ~ 7~8~ 1
circuitry. me microminiature refrigeration capacity would allow the match-
ing of the refrigeration system to the load. me invention meets these
needs.
Also needed are microminiature refrigerators of the same general
dimensions as discussed above that can dissipate large amounts of heat, 10-
-50 watts, generated by certain small devices while maintaining ambient or
subambient operating temperatures. And such refrigerators should be easy to
manufacture and in configurations that are compatible with standard elec-
tronic packaging.
As explained in greater detall below, the microminiature refriger-
ator of the present invention comprises, in a unique form and scale a plura-
lity of sealed plate-like members which form between them a cooling chamber,
heat exchanger capillary passages and fluid passages for conveying incoming
high pressure gas successively through the heat exchanger the capillary sec-
tion and into the cooling chamber. Return or outflow passages conduct the
fluid from the cooling chamber through the heat exchanger in counterflow
relation with the incoming gas and then to the exterior of the device.
Such a microminiature refrigerator requires scaling down a conven-
tional refrigerator by a factor of about a thousand. The design parameters
for a microminiat~re refrigerator of the same efficiency as a conventional
refrigerator using turbulent flow are described in "Scaling of Miniature
Cryocoolers to Microminiature Size", by W.A. Little, published in NBS
Special Publication in April, 1978.
In summary, the diameter d of the heat exchanger tubing, L the
length of the exchanger and _ the cooldown time are related to the capacity
which is proportional to _ the mass flow, in the following manner:
. 0.5
d ~ (m)
. 0.6
1 ~ (m)
. 0.6
t ~ (m)
7~85 ?.
A microminiature turbulent flow refrigerator with a capacity of a
few milliwatts should have d = 25~ and 1 a few centimeters.
As the device becomes smaller and smaller, eventually the mass
flcw becomes too small to allow turbulent flow of the fluid to occur.
Laminar flow operation then becomes possible without loss of refrigeration
efficiency and gives improved performance.
The theoretical basis for designing microminiature refrigerators
using laminar flow heat exchangers is discussed in "Design Considerations
for Microminiature Refrigerators Using Laminar Flow Heat Exchangers", pre-
sented by W.A. Little at the Conference on Refrigeration for CryogenicSensors and Electronic Systems, Eoulder, Colorado, October 6 and 7, 1980.
For microminiature heat exchangers operating in the laminar flow
region over the same pressure regime and having the same efficiency, the
length of the exchanger (1) should be made proportional to the square of the
diameter (d) of the exchanger tubing. For example, a Joule-Thomson exchanger
operating with N2 at 120 atmospheres, with a capillary channel passage 5 cm
long, 110 microns wide and 6 microns deep should provide approximately 25
milliwatts cooling. Different refrigeration capacities can be obtained by
varying the width of the channel with no change of the efficiency. Gne may
thus operate under streamlined conditions free of vibration and turbulen oe
noise, an advantage, particularly for superconducting d~vices, which require
a very low noise envirolment.
l ~7~851
In a Joule-Thomson refrigerator of this type it is
normally convenient to use a capillary channel to throttle
the compressed gas; however, it is common knowledge that a
porous structure such as porous metal, sintered ceramic, etc.
can be used equally well for throttling the gas.
To increase the efficiency of the refrigerator for
certain applications one form of the invention provides two
capillary sections arranged in series or in parallel and
passages for conduting a substantial portion of the gas directly
to the outflow passage of the heat exchanger after passage
through only one of the capillary sections.
In order to construct microminiature refrigerators,
new fabrication techniques are needed for producing heat
exchangers and expansion nozzles, a factor of 100 to 1,000 times
smaller than those of conventional refrigerators.
Conventional fabrication techniques are ill-suited
for microminiaturization since channels of the order to 5-500
microns must be formed accurately and the device must be sealed
so as to withstand high pressure of the order of 150-3000 psi
for refrigeration efficiency.
Accordingly, the major object of the invention is to
provide a novel microminiature refrigerator particularly for
cryogenic cooling and mode of assembly.
Another object of the i.nvention is to provide a novel
microminiature refrigerator with a cooling capacity ranging
from milliwatts up to 50 watts or more.
Another object of the invention is a novel multilayer
microminiature refrigerator.
Another object of the invention is a novel single-
stage cryogenic microminiature refrigerator.
1 1 708~ ~
Another object of the invention is a novel multistage
cryogenic microminiature refrigerator.
Another object of the invention is a novel method of
manufactur-ng a microminiature refrigerator.
Yet another object of the invention is a novel method
of making a microminiature refrigerator us~ng photolithographic
and chemical etching techniaues.
Still another object of the invention is a novel
method of making a microminiature refrigerator using a fine-
particle san~blastin~ technique.
A further object of the invention is to provide a
novel refrigerator of small size comprising two or more plates
of a low thermal conductivity material such as glass bonded
pressure-tight and containing at one or more plate interfaces
micron-sized gas supply and return passages to a chamber that
is adapted to continuously cool a superconductor or like device.
Pursuant to rhis object the inlet gas pressures may be in the
order of 150-3Q~n pounds per square inch, and the passages may
be in the range of 5-500 microns wide and 5-6n microns deep.
Pursuant to the fore~oing object of the ~assages and
cooling chamber may be formed by recessing plate surface areas
or forming raised channel walls at the interface or interfaces
of the plates.
And still another object of the invention is a novel
method of making a microminiature refriaerator hy forming
raised channel walls.
Another object of the invention is to provide a novel
refrigerator assembly having two or more multilayer refrigerators
for cascade cooling to reach low cryogenic temperatures.
1 t ~ ~ 8 5 1
A further object of the invention is to provide a
novel microminiature refrigerator composed of three or more
similar plates of glass or equivalent material assembled in a
stack with micron-sized fluid flow high-pressure inlet and low-
pressure outlet passages cor.nected to a cooling chamber and
being arranged in separate layers at interfaces between adjacent
plates.
Further to this object it is contemplated that in a
lamination of three plates hish-pressure inlet, heat exchange
a~d czpillary e~pansion passage sections are formed in series
in one plate surface-bonded to an intermediate plate, one sur-
face of which forms one side of the passages and low-pressure
pressure return recess means is formed in the surface of the
other plate that is bonded to said intermediate plate.
~ nother object of the invention is to provide a novel
refrigeration assembly having a special mounting and gas passage
defining holder at one end, as for mounting the refriserator to
ex.end cantilever fashion within an evacuated enclosure.
Further objects of invention will appear as the
description proceeds in connection with the appended claims and
the accompanying drawings.
In accordance with the invention, a microminiature
refrigerator and method of ma~.ing the same is provided. It
should be noted that the microminiature refrigerator can be
scaled up both in size and capacity for certain applications.
The refrigerator is novel, irrespective of size; however, it is
the miniaturization of the refrigerator that is difficult and
therefore this is 2escribed in 2etail here. The turbulent or
laminar flow microminiature refr:igerator includes one or more
plates on some of which micron-sized gas channels are formed
and completed by one or more bonded plates resistant to high
pressures. The microminiature refrigerator may be used for,
but is not limited to, cryogenic refrigeration.
~-7-
1 1 7~85 1
According to a first broad aspect of the present invention,
there is provided a multilayer microminiature refrigerator comprising
a laminate of three planar-surfaced thin plates bonded pressure-tight
at the interfaces between adjacent plates, means providing in one inter-
face between two adjacent plates a first continuous inflow passage means
extending from an inlet to a cooling chamber and including capillary
passage means, means for connecting said inlet to a source of refrigerant
gas at high pressure, means providing in the other interface between
two adjacent plates a further continuous outflow passage means leading
to an outlet, and a passage interconnecting said chamber and said further
passage means whereby fluid at reduced pressure from said cooling chamber
may pass through said further passage means to said outlet in counter-
flow heat exchange relation with fluid in said first passage means for
regenerative precooling, each of said passage means being of micron size
whereby to promote laminar flow therein.
According to a second broad aspect of the present invention,
there is provided a nnethod of making a refrigerator wherein a thin glass
plate is surface-bonded pressure-tight to another plate of about the
same coefficient of the:rmal expansion, comprising the steps of: a)
forming a layer of fine-particlc sandblast-resistant material on sai.d
gla.ss plate; b) definillg a. flow path pattern in sa:i.d rcsistant material
by lithographic masking and selective etching to thereby expose the
underlying surface of said glass plate in said pattern; c) scanning a
miniature air abrasive device across said glass plate so as to form
recessed channels in said surface by fine-particle blasting to the
required depth; and then d) bonding said other plate upon the recessed
surface of said glass plate.
~ he invention will now be clescribed in greater detail with
reference to the accompanying drawings.
-7a-
1 1 7û8~ ~
BP`IEF DE~CRIPTION OF DRAWINGS
-
Fig. 1 is an exploded view showing a microminiature
refrigerator according to one embodiment of the invention;
Fig. 2 is a generally plan view showing a micromini-
ature refrigerator part having a fluid passage pat'ern accord-
ing to a further em~odiment;
Fig. 3 is a generally perspective view showing a
microminiature refrigerator part e~hi~lting a further
modification;
Fig. 4 is a plan view showing a multiple unit module
embodiment of a microminiature refrigerator;
Fis. 5 is a cross sectional view showing another
embodiment of a microminiature refrigerator;
Fig. 6 is a cross sectional view showing another
embodiment of a multiple unit module microminiature refrigerator;
Figs. 7A-7E are cross section views illustrating ste~s
in a method of fabricating a microminiature refrigerator;
Figs. 8A-8D are cross section views showing steps in
ar.other method of fabricating a microminiature re.frigerator;
Figs. 9A-9C are cross section views sho~!ing steps in
a method of fa~ricating a microminiature refrigerator having
raised channel walls;
Figs. lOA-lOD are cross section views showing steps
in another method of fabricating a microminiature refrigerator;
Fig. 11 is a cross section view showing fabrication
of a multiple unit module embodiment.
~ I 7~5 ~
Fig. 12 is a plan view illustrating a use and environ-
ment of the refrigerator of the invention;
Fig. 13 is a schematic view illustrating the fluid
circuit in the refrigerator of the invention;
Fig. 14 is an illustration section showing the laminated
nature of the refrigerator and the multilayer fluid flow paths,
details of which are shown in later Figures;
Fig. 15 is a plan view partly broken away showing a
refrigerator holder;
Fig. 16 is a section substantially on line 16-16 of Fig. 15;
Figs. 17, 18 and 19 are top plan views of the three (upper,
middle and lower) elements that comprise the laminate or
sandwich construction of the refrigerator of the invention
according to a preferred embodiment particularly illustrating
the various fluid flow passages and chambers;
Figs. 20 and 21 are section-s substantially along lines
20-20 and 21-21 in Fig. 17;
Fig. 22 is a section substantially along lines 22-22
of Fig. 18;
Figs. 23 and 2~ are sections substantially along lines
23-23 and 24-24 in Fig. 19;
Fig. 25 is an exploded view showing an embodiment
wherein the flow passages are differently located;
Fig. 26 is an exploded view showing an embodimer.t
wherein two low-pressure flow paths are provided;
Fig. 27 is a fragmentary view showing a pad of
increased thermal conductivity on the cool end of the refrigerator;
Fig. 2~ is an exploded view showing a multilayer
refrigerator according to a further embodiment;
. .
1 17~5 .
Fig. 29 is a plan view showing the plate of Fig. 28
in which the low-pressure gas passage means is formed;
Fig. 30 is a plan view of the plate of Fig. 28 in
which high-pressure passage means is formed;
Fig. 31 is an illustration view showing the laminated
nature of two multilayer refrigerators combined in one unit
to provide cascade cooling;
Fig. 32 is a section substantially along line 32-32
of Fig. 31;
Fig. 33 is an exploded view of a further embodiment of
the invention; and
Fig. 34 is a plan view of one component OL a further
embodiment of the invention.
PREFERRED EMBODIMENTS
Referring now to the drawings, Fig. 1 is a perspective
exploded view of one embodiment of a microminiature re-
refrigerator in accordance with the present invention and includes
a body 12 in sealed surface contact with substrate 10. ~he
body 12 is of crystalline (e.g., such as silicon), an amorphous
(e.g., glass), or metallic (e.g., copper, stainless steel)
material; and substrate 10 is a material such as*Pyrex, soda
gl&ss or~Kovar, having a coefficient of thermal expansion which
is compatible with the coefficient of thermal expansion of
the body 12. Preferably, the body 12 and substrate 10
are coextensive thin glass plates.
J~ k
-10-
~ 17~85~
The body 12 and substrate 10 must be thic~ and/or
strong enough to withstand the pressure of the incoming gas
typically of the order of 150-3000 pounds per s~uare inch
pressure. For example, a silicon body may be approxi~ately
300 microns (.0118") thick, a glass hody may he approximately
0.010" to .020" thic~s, and a glass substrate is about 0.010"
to 0.020" thick. Formed on or etched in the surface of the
body 12 which is mounted on substrate 10 are parallel "serpen-
tine" channels 14 and 16, 5-300 microns wide and separated by
150-S00 microns walls 15. Channels 14 and 16 interconnect an
outlet port 18 (the low-pressure return) and input port 20
(high-pressure inlet) respectively to a reservoir or cooling
chamber 24. The size of reservoir 24 is determined by the de-
sired reserve capacity neeced for fluctuating demandsO The
foregoing channels and reservoir are formed in the bottom sur-
face 13 of body 12 which is bonded flush onto the flat top sur-
face 17 which effectively closes them. The interface ccnsist-
ing of surfaces 13 and 17 is bonded pressure-tight.
The channels 14 and 16 define respective low-pressure
and high-pressure cooling lines which run in juxtaposition for
an initial length and thereby define a heat exchanser section
shown senerally at 22. A fine channel filter section 21 is
provided between the inlet and the heat exchange section.
Beyond the heat exchanger section 22 the input channel 16 be-
comes independently sinuous and narrower at 26 of substrate 10
allowing the fluid to drop in pressure and then expand. As an
example, channel section 22 is about 250 microns wide and 50
microns deep while channel 26 is about 125 microns wide and 10
microns deep. The end of the expansion line 26 is connected
directly into the reservoir 24 and the output channel 14 extends
from tne reservoir 24 back through the heat exchanger to the
i 1 7~85 ~
output port 18. Reservoir 7~ is preferably about 20-50
microns deep.
~ ounted on the other or lower surface of substrate 10
is an interface unit 30 of suitable metal alloy such as~ ovar
which is an alloy of iron, nickel and cobalt with a coefficient
of e~pansion about the same as the material of substrate 10
having holes 32 and 34 e~tending therethrough and com~unicatlng
through aligred bores in substraie 10 with the ports 18 and 20
respectively of the body 12. Bonded to the interface unit 30
are a pair of miniature tubes 36 and 38 which communicate a
fluid to and from the refrigerator. Tubes 36 and 38 may com-
pr se stainless steel material as used in hypodermic needles
or Teflon tubing. The interface unit 30 is attached pressure-
tight to the suhstrate 10 by a suitable sealant such as epoxy.
Suitably mounted on the top surface of body 12 in
direct abutment with the wall of the reservoir 24 is a device
40 to be cooled. The device 40 may be any one of a number of
devices operated 2t a low temperature (e.g., supersensitive
magnetometers, gradiometers, bolometers an~ other like devices
which are based on the Josephson effect or other devices which
are well known in the art) or devices for operating at higher
temperatures (e.g., infra-red detectors, solid state lasers
or samples whose physical properties are to be determined).
The entire assembly may be contained in a Dewar or vacuum
vessel to reduce the heat transfer to the parts
The illustrated microminiature refrigerator is a
Joule-Thomson, open-cycle refrigeration system in which tube 38
is connected through a control valve 39 to a container 37 of
highly compressed refrigerant gas such as nltrogen, hydrogen
or helium.
The highly compressed gas enters at an inlet pressure
of approximately 150-3000 pounds per square inch and a flow
rate of approximately 5-50 milliliters/sec ~STP) through port
20 and passes through the heat exchanger 22 where the gas is
t~ra~ ma~ -12-
8 5 ~
cooled by lower-pressure supercooled gas eYiting the device
through channel 14, port 18 and tube 36~ The high-pressure gas
exits the heat e~changer 22 and passes through the capillary
expander 26 where the drop in pressure reduces the temperature
of the sas which enters the reservoir 2~ as a supercooled or
cryogenic fluid. The low-temperature reservoir 24 in turn
cools the device 40 mounted on reservoir 24 and the absorbed
heat causes the fluid to vaporize and it flows through channel
14 to the e~haust port 18.
An illustrative microminiature refrigerator whose hody
12 is of 0.020" thick glass 1/2 inches wide by 3 inches long
has a refrigeration capacity of 100 milliwatts at 122K;
channel dimensions are of the order o~ 100 microns and the
flow rate about 30 milliliters/sec (STP) of nitroaen at an
inlet pressure of 1600 pounds per square inch. The substrate
10 is a glass plate of the same size
Another illustrative microminiature refrigerator whose
body is silicon measures 75 by 12 by 2 millimeters, has a 30-
centimeter-lona heat exchanger section and operates from room
temperature to 200K using CO2.
A microminiature refrigerator can be appro~.imately
1/2" to 4" in length, 1/2 inches wide, 0.040-0.060 inches thick
with typical channel di~ensions between 5-500 microns with
separating walls 150-500 microns wide, can have a cooling
capacity between 1.0-50,000 milliwatts at temperatures ranging
from 2-3Q0K, and can withstand input pressures from 150-3000
pounds per square inch. However, it should be appreciated that
the microminiature refrigerator as described could be scaled
up or down both in size and capacity for certain applications.
In addition to an open-cycle described above, it
will be appreciated that the refrigerator can be a closed-cycle
system using a compressor to recompress the gas. In addition,
the method of fabrication described here could be used for the
construction of parts of refrigerators using other cycles such as
the Servel, Gifford-McMahon, pulsed tube and Vuille~mier systems.
-13-
1 17~5~
In sore of the laminar flow ~evices the design of
the channels may be modified by having them straight as illus-
trated in Fig. 2 rather than "serpentine." Fig, 2 shows the
bottom surface of such a body~ In this embodiment, high-
pressure gas enters at port 42, flows through the parallel
heat exchanger channels 43 to the sinuous capillary channel
section 44 thence to the reservoir or cooling chamber 45,
through the lo~-pressure return 46 to the outlet port 47. An
advantage of this design is that the low-pressure return 46 as
shown completely surrounds the high-pressure channel lines so
that any minor aas leak from a high-pressure line is captured
by the low-pressure return and does not escape into the sur-
rounding vacuum, ~7hich insulates the refrigerator from the
environment. A possible disadvanta~e of this design is the
long path for the heat to travel through the alass at the heat
exchange section bet~7een the incoming lines 43 and outgoing
channel 46. This difficulty may ~e avoided by combining the
glass body with a glass substrate 48 shown in Fig. 3 which has
highly conducting transverse metal strips or wires ~9 printed
or bonded respectively. These stripes or strips may be bonded
upon the surface of the glass substrate that form the lnter-
face with body 41. These transverse conducting pieces ~9 pro-
vide a high thermal conductive path laterally across the
refrigerator while maintaining the thermal conductivity length-
~ise along the refrigerator at a low value determined by the
glass.
The microminiature cryogenic device and refrigerator
of this invention also lends itself to multiple u~it config-
urations. For example, a plurality of refrigerators, each
using a different coolant, will provide cascade cooling of one
gas by another and thus produce refrigeration at extremely low
temperatures. Additional poxts can be included in the device
with channels interconnecting the additional ports as described
and additional reservoirs for further cooling of a cryogenic
`~ :
1 ~ 7~85 '
device. Fig. 4 shows in perspective a multiple glass unit body
module 51 of a microminiature refrigerator in accordance with
this phase of the present invention.
There are two reservoirs or coolant chambers 52 and
53. Cha.~er 52 has an input line 54 leading from input port 55
through a sinuous heat~e~change section 56 and a fine capillary
section 57 into the chamber, and an output line 58 that has a
sinuous heat exchan~e section 5q coe~tensive with section 56
leading to output port 60. Chamber 53 has an input line 61
leading from an input port 62 through a sinuous heat e;~change
section 63 and a capillary section 64 into the cham~er, and an
output line 65 that has a sinuo~s heat exchange section 66
coextensive with section 63 le2dino to output port ~7. A
source of high-pressure nitrogen is connected to port 55, and a
source of high-pressure hs~drogen is attached to port 62. The
two circuits thus are partially interactive whereby the fluid
going to chamber 53 is precooled more extensively hefore
expansion. One circuit uses Joule-Thomson expansion of nitrogen
to cool hydrogen to 77K at chamber 52. The other circuit uses
Joule-Thomson expansion of the precooled hvdrogen to reach ~1K
at chamber 53. The entire refrigerator as shown comprises two
heat exchangers, two expansion sections, two cold licuid reser-
voirs, and the input and output ports. The size of the device
is approximately 2" to 5". Similarly, a three-stage system
using nitrogen, hydrogen and helium will produce cooling at 4.5K.
Fig. 5 is a cross sectional view of another embodiment
showing a multilayer microminiature refrigerator where the in-
coming and outgoing channel formations 73 and 74 are formed on
either side or both sides of a glass body 75 and on either side
or both sides of which are bonded two glass substrates or cover
plates 76 and 77. High-pressure fluid flows along the channels
73 and returns through channels 74. The channels 73 and 74 may
be etched or otherwise formed il the surfaces of body 75 as
shown in full lines or etched or other~7ise formed in the
-15-
:
l ~ 7~85 1
surfaces of either or both glass cover plates 76 and 77 t~at
are bonded to the body as shown at 73' and 74' in dotted lines.
The channels 73 and 74 may also be formed with raised channel
walls on the body 75 or on the substrates 76 and 77. The
channel formations 73 and 74 in each instance consist of inlet
sections, heat exchange sections, capillary sections and cool-
ing chambers which may be constructed and related as in ~igs.
1-4. The difference between Fig. 5 and Fig. 1 is that in Pig. 1
the incoming and outgoing channels may be described as formed in
a single layer- that is, they are formed in the same or an
abutting planar surface -whereas in Fig. 5 they are formed in a
nonabutting surface. It is conte~plate~ that in Fig. 1 the in-
coming ~nd out~oing channels may be formed in the op~osed sur-
faces 13 and 17 respectively.
Fig. 6 is a cross sectional view showino another
e~bodiment of a multiple unit module microminiature refrigerator.
Channels 80, 81, 82 and 83 are forme~ in facing surfaces of thin
glass bodies ~, 85, 86, 87 and 88 by etching, particle blastina
or by forming raised channel walls. For example, hiah-pressure
nitrogen enters chanr.el 80 ~nd returns via low-pressure channel
81; an~ high-pressure hydrogen enters via channel 82 and is
precooled by nitrogen in channel 81. Lo~t-pressure hydrogen exits
via channel 83. The inlets, heat exchange sections, expansion
lines and reservoirs are included in the respecti~e channels as
in the earlier embodiments.
FP~P~ICATION ~ETHODS
Fig. 7A-7E illustrate one method of fabricating the
refrigerator by etching channels in glass or silicon ~lates.
The techniquesto be described are to some extent well ~no~n in
the manufacture of semiconductor devices, such as integrated
circuits, and may include conventional photoresist masking and
etching techniques. In one instance, by using a silicon plate
-16-
1 17~8~'
material having a surface crystalline orientation on the (1,0,0)
plane, anisotropic etching can be employed to form V-shaped
grooves in the silicon plate surface. ~lternately, vertical
walls can be made using a silicon plate with a surface o-ientation
of the (l,l,O)plane. In Figure 7A a portion of a silicon plate
9Q is shown in cross section and a silicon oxide layer 91
is provided on one major surface thereof. The plate 90 is on the
order of 300 microns in thickness and o~ide layer 91 is approxi-
mately 9,000 angstroms in thickness. The oxide layer may be
formed by heating the silicon wafer in a wet oxygen atmosphere.
Photoresist 92 is applied to the surface of silicon oxide 91
and is exposed under a photomask having the desired channel
pattern. The photoresist is removed and the e~posed o~ide is
etched leaving it in the pattern 93 as shown in Figure 7s.
The silicon oxide now acts asa mask and the exposed silicon is
etched using an anisotropic etchant, such as ethylene diamine,
resulting in the V-grooves 94 shown in Fig. 7C. Upon completion
of the etching of the V-grooves, the remaining oxide layer 93
is removed from the silicon plate and the plate is cleaned.
An optically flat Pyrex or equivalent glass plate 95 is then
bonded to the etched surface of silicon plate 90, as sho~m in Fig.
7D. The bonding of the glass to the silicon surface is performed
by known field assisted or anodic bonding techniques.
Thereafter, as shown in Fig. 7E, the silicon plate 90 is eLched
or otherwise cut from the backside to reduce the thic}:ness
of the assembly and hence the thermal conductance of the laminated
refrigerator structure.
Input and output lines are then drilled or etched in
the reverse side of the glass substrate 95, and the tubing gas
lines are then bonded to the reverse side of the glass plate
by means of epoxy. ~y using photolithographic definition
-17-
~ 1 7~85 '
and chemical etching, the entire refrigerator including heat
exchanger, expansion line and liquid reservoir channel ormations
can be formed in one step. Electron beam or x-ray lithography
electrolytic and plasma etching can be employed as well as
chemical etching. The foregoing photoresist method may be used
where the body and substrate are both glass plates, using csnven-
tional materials and techni~ues.
Figs. 8A-8D illustrate another method of
fabricating recessed channel formations in a hard, amorphous
isotropic material such as glass or crystalline material such
as silicon. The method allows good si~e control, improved reso-
lution compared to chemical etching, eliminates undercutting
and allows the formation of vertical walls. This method is not
limited to the manufacture of the microminiature refrigera'or.
In Fig. 8A, a portion of a glass plate lO0 approximately
.020" thick is shown in cross section and a resist layer lO1
is provided on one major surface thereof. The purpose of this
resist layer is to protect the underlying surface and to provide
a pattern for channel layout. The resist may be a photo-
sensitive or non-photosensitive resist but must be resilient or
tenacious enough to }~e able to withstand fine-particle "sand-
blasting" as will be described below. The resist may cover the
surface of the entire plate lO0 or be screen-printed on plate
lO0 so as to form a pattern If the resist is a photoresist,
it is exposed through a conventional photo mask to ultraviolet
light in order to define a pattern. A novel photoresist which
meets the requirements of being able to withstand fine-particle
sand-blasting is comprised of: 7 grams gelatin (e.g., Knox)
and l gram ammonia bichromate dissolved in 50 cc hot water.
The resist forms a thick, spongy layer approximately 20-30
times the thickness of conventional resists. The unexposed
-18-
1 1 7~85 1
portions of the resist can be removed by hot water, or by using
the enzyme, protease, to digest the unexposed portion, and result
in the structure ~hown in Fig. 8B. The remaining resist 102 is
tough and resilient, able to withstand the abrasive action of fine-
particle "sand-blasting" while allowing exposed areas of the glass
plate surface be abraded away. A miniature air abrasive device
(e.g., Airbrasive Unit, ~odel K; S.S. White~, which entrains a
stream of fine alumina particles at 30 pounds ~er square inch acts
as a fine particle "sand-blast". The "sand-blast" device is
scanned at a constant rate across the resist carrying plate
surface of Fig. ~B; and a jet of 17 micron particle abrasive
powder can be used to remove approximately 2 microns of
material at each pass. Larger powder ~articles (e.g., 27 and
50 microns) etch more rapidly but may give poorer definition.
They may be usea for fabricating larger devices with adequate
accuracy.
Channels 103 formed by this particle-blast method,
as shown in Fig. 8C, have a precisely controlled depth (2-300
microns), vertical walls and edge definition of approY.imately
5 microns roughness. Vpon completion of the formation of the
recessed channels, the remaining photoresist is removed and
the plate 100 cleaned. The entire refrigerator cooling chamber
and passage system can thus be formed in one step. As shown in
Fig. 8D a substrate 104, such as a soda glass cover slide, is
bonded to the etched surface of glass plate 100 with an adhesive
bond less than 10 microns thic~ but able to withstand 500-3,000 psi.
This is the same mode of bonding used in all embodiments for securing
two or more glass plates together to form a permanent pressure-
tight assembly. Such a bond can be made with epoxy or ultra-
violet curable cement (e.g., Norland's Optical Adhesive). A
micron-thick seal can be made by drawing diluted adhesive into
the space between the plate 100 and substrate 104 by capillary
-lg-
l ~ 7~85 1
action. The re~rigerator is then illuminated with intense
ultraviolet radiation until the adhesive polymerizes and forms
a bond.
Alternately the cover plate-or suhstrate may be fused
to the etched body plate with a thin film of solder glass screen-
printed on either plate or both plates, using conventional
methods for the fabrication of liquid crystal displays. Input
and output lines are drilled or etched in the reverse side of
the glass substrate 104, and stainless steel hypodermic or
Teflon tubing gas lines are then bonded to the reverse side of
the glass plate lO~ as by ~eans of epoxy. By using the fine
particle sandblasting technique, a hard amorphous isotropic
material such as glass can be used for the body plate lO0 of
the refrigerator. The use of such materials avoids the problems
associated with the high thermal conductivity of silicon and
therefore lower te~peratures can be achieved at the co~d chamber
end of the refrigerator.
Figs. 9A-9C illustrate a method of fabricating a
microminiat~re refrigerator by forming raised channel walls on
a surface as opposed to recessed channels in a surface. The
plate 110 may be a crystalline, amorphous or metallic material,
prefexably glass on the plane surface 111 of which channels are
to be formed. ~aterial 112 such as glass frit powder, epoxy,
solder glass, ultraviolet cu^able cement, etc. is screen-printed
as a pattern onto the surface 111 as shown in Fig. 9B. Upon
firing, the glass frit powder melts and defines the channel
w211s. Likewise, as the epoY.y cures, solder glass hardens, or
ultraviolet curable ce~ent is exposed to ultraviolet radiation,
the channel walls are formed. 5-300 micron spacings are accur-
ately made using this technique. Fig. 9C shows a glass sub-
strate plate 113 boncled by any of the previously mentioned
bonding methods to seal the gas eYchanger lines. As before,
input and output lines are then drilled or etched in the
-20-
~ l 7~85 1
reverse side of plate 113, and the gas lines are then bonded
to the reverse side of plate 113.
Figs. lOA-lOD show a methoc1 for the fabrication of
the refrigerator such as that of Fig. 5 where the channels are
etched on either or both planar sides of a glass plate 120 on
which are to be honded the two glass plates 121 and 122
(Fig. lOD). Particularly for the laminar flow design, this
simplifies the design of multistage devices.
To fabricate the device the plate 12~ is screen-
printed with a thin continuous layer of glass frit or covered
with sol~er glass 123 on each side (the same process may be
done on one side only); these layers are then fused, as in the
method step shown in Fig. lOA. Then a resist 124 is either
printed on the top and bottom surfaces; or a photoresist is
used, exposed and developed on each side at 124 (Pig. lOB),
so that each side of the plate 120 bears the desire~ pattern,
using one of the above disclosed methods, Plate 120 now is
abraded and the resist removed, leaving the ~late 120 as sho~n
in Fig. lOC. It will be noted that at this point the channel
formations 125 and 126 respectively appear in separate layers,
and the glass surfaces between them are covered with the
fused-on frit 123.
Cover glass plates 121 and 122 are then bonded upon
the top and bottom surfaces by a program that may include first
heating the entire assembl~ to the softening temperature of the
solder glass or frit. This also seals the inlet port and the
outlet port to complete the refrigerator (Fig. l~D). Holes
through the plate at the cold or cooling chamber end connect
the xeservoir, which is connected via the capillary to the
high-pressure channels 125, to the low-pressure channels 126.
High-pressure gas passes through channels 125 to the cooling
chamber and then back through channels 126. The channel layers
may be formed in either or both suhstrate plates 121 and 122
instead of entirely in body plate 12~.
-21-
1 ~ 7 ~ 8 ~ 1
Fig. 11 illustrates a method of fabricating a stacked
multistage refrigerator 130 such as that of Fig. 6. By this pro-
cedure multistage devices may be conveniently constructed where
the different gases pass in spaced layers through passages in a
bonded stack of plates as illustrated in Fig. 11.
This refrigerator co~prises five bonded glass plates
preferably of the same size in a stack. Here the three inter-
mediate plates 131, 132 and 133 are ~ormed with surface recesses
according to one of the foresoing methods. ~or example, plates
131 and 132 are formed with surface channels 134 and 135
respectively in accord with ~he methods of Figs. 7A-7~, Figs.
8A-8D or Figs. 9A-9C; and plate 133 is formed on opposite
sides with surface channels 136 and 137 respectively as by the
method of Figs. lOA-lOD. In each case the particle blast mode
is preferred whereby the open ends of each channel are spaced by
fused frit layers on the glass surface.
In the two stage device illustrated high pressure
nitrogen may enter via channel 134 and return via 10W pressure
channel 135~ high pressure hydrogen may enter via channel 136
and is precooled by heat exchange with nitrogen in channel 135,
The low pressure hydrogen returns to the outlet port via channels
137. The whole assembly is bonded together with solder glass
as disclosed earlier. Alternately plates having raised channel
walls may be used instead of the recessed channels in this two-
stage refrigeration assembly. The communicating ports between
the various layers are suitably formed.
1 1 7~8S 1
While in the above~described embodiments, the refrigerator
has a crystalline or amorphous body, in some cases the
refrigerator can be photoetched in a copper film on the surface
of a circuit board or a thin sheet of stainless steel. While
the described refrigerator is of the open-cycle type, as
indicated above, closed-cycle refrigerators may be fabricated
using the techniques in accordance with the present invention.
It should be appreciated that the channels can be also formed
as described above on capillary tubing with the tube in
abutment with the confining internal surface of another tube
to form a cylindrical heat exchanger refrigerator. The sealing
of this tube to the inner one may be accomplished by using
a heat shrinkable tubing such as Betalloy (Raychem Corp.) for
the outer tube.
Channel dimensions, surface bonding techniques, channel
forming and other characteristics of the refrigerator may be
as described below in connection with the embodiments of
Figs. 12-32.
Referring to Fig. 12 which illustrates a typical
installation, the refrigerator 211 is mounted at one end in
a holder 212 within which it i5 fixed so that the refrigerator
and holder usually comprise a unitary assembly indicated at 213.
In the illustrated assembly, the refrigerator contains
flow passages that are connected through the holder to fluid
inlet and outlet means.
Fig. 12 shows the assembly mounted in a well 214 of
a typical boxlike enclosure 215 having a suitable airtight
cover indicated at 216. In the enclosure the holder 212 is affixed
suitably at the bottom of the well and the refrigerator extends
cantilever fashion through the well. Preferably the well is
subjected to subatmospheric pressure through the conduit 217
leading to a source of vacuum 218.
1 170851
The device 219 to be continuously cooled which may be a
small superconductor chip or like device is suitably mounted
within the well 214 preferably in contact with the coolest region
of the refrigerator as indicated in dotted lines in the drawings
and any wiring therefrom (not shown) passes through sealed
ports in the enclosure. In the illustrated embodiment this
device is preferably mounted in direct contact with the
refrigerator glass surface that serves as a cover for chamber 224.
Referring to Fig. 13 the fluid path is at least schematically
shown. The refrigerator 211 has an inlet port bore 221 for
admitting highly compressed gas that flows through a passage
having heat exchange section indicated at 222 and a smaller
diameter capillary section 223 into a cooling chamber 224.
The device 219 to be cooled is located as closely as possible
to chamber 224, which is the coolest part of the refrigerator
as will appear, and fluid leaving the chamber 224 returns along
a passage 225 extending adjacent and in heat exchange relation
to the inlet passage section 222 and leading to an outlet port
226. These passages are micron-sized for very low temperature
refrigerations of milliwatt capacity as will appear.
In the invention inlet port 221 and outlet port 226
connect into bores 227 and 228, respectively, in holder 212,
and the holder is so mounted in the enclosure as to connect
bores 227 and 228 with fluid inlet conduit 229 and fluid
outlet conduit 231 projecting from the enclosure. Where the
system is an open cycle refrigerator, conduit 229 is connected
to a source of pressurized refrigerant gas and conduit 231 is
connected to a suitable -exhaust. In a closed cycle system
the conduits 229 and 231 are connected through a loop containing
a condensor and pressurizing assembly.
-24-
~ 37~85~
As shown best in Fig 14 the refrigerator 211 comprises
a three-element sandwich consisting essentially of three bonded
similar accurately flat glass or o~her similarly low thermally
conductive plates 232, 233 and 234 of approximately the same
length and widthJ having thickness preferably in the order of
0.020". The middle plate may be thinner than the others to
enhance the heat exchange between inflow and outflow channels.
The refrigerator 211 may overall be about 1/2" wide and 2 1/4"
long with a total thickness of about 0.060", this representing
a workable embodiment that has been successfully tested. Another
workable embodiment is 0.2" wide, 1.0" long and .060" thick.
The refrigerator is of multigas-layer construction.
That is, the fluid supply passages 222 and 223 and the chamber
224 are formed to provide one fluid flow passage layer within
sandwich substantially in one plane while the return passage
is formed to provide a separate fluid flow passage layer within
the sandwich substantially in a spaced plane As will appear
these layers are in fluid flow connection through an opening
in the chamber wall.
Figs. 17-24 illustrate detail of anotl~er embodiment.
The thin flat glass plates 232, 233 and 234 are of the same size.
The intermediate glass plate 233 is smooth with opposite flat
smooth coplanar surfaces 235 and 236 (Fig. 22). The upper plate
232 as illustrated is transparent. As shown in Fig. 17, plate
232 has recessed regions or channels in its bottom surface 237
defining the inlet port 221, the heat exchange passage section
222, the capillary passage section 223 and the chamber 224
connected in series providing a continuous fluid flow path
from the inlet port 221 to chamber 224. Port 221 is in the nature
of a closed bottom well.
-25-
I ~ 7~85 1
Intermediate plate 233 closes one side of the passages
in plate 232, and at one end it has a through bore 238 aligned
with port 221 of plate 232 in the assembly. Plate 233 at the
other end has a through port 239 aligned with chamber 224
in the assembly.
Bottom plate 234 which is formed on its top surface 240
with the low-pressure fluid return path that comprises a
generally rectangular large area surface recess 241. A series
of spaced ribs 242 and several rows of projections 243 are
provided on the bottom of recess to supportingly contact the
lower surface 236 of the intermediate plate in the assembly
while not interfering appreciably with fluid flow. Plate 234
is formed with a through bore 243 that aligns with bores 238
and 221 in the assembly of Fig. 16. A second bore 244 in plate
234 opens into recess 241. Fluid exhausted from chamber 224
in the assembly passes through port 239 into recess 241 and
exhausts through port 244.
In the assembly glass plates 232 and 233 and 234 are
bonded in the stack pressure tight at their interfaces. This
laminate is placed in holder 212 where one end i6 bonded to the
metal holder by adhesive layers 249 and 250 with ports 243
and 244 in alignment with bores 227 and 228, respectively.
This provides support for the refrigerator and sealed leakproof
inl.et and outlet connections for the refrigerator flow
passages.
The passage sizes are selected for cooling capacities.
-26
... .. . . . . . ... . , . . . . ., . ..... , .. , ... . . . , . . . ... . .. .... _
I 1 7~85 s
The passages 222, 223, 225 are micron-sized. The
passages 222 and 223 particularly are micron-sized having
dimensions in the order of 5-500 microns. In a typical
refrigerator these passages may be as shallow as 5-10 microns
and as narrow as 150-200 microns. In passage 222 the gas passes
in laminar flow, thereby reducing vibration, noise and other
problems incident to turbulent flow. Recess 241 is typically
about 20-240 microns deep.
In a modification of the foregoing, the inlet and outlet
passage means may be recessed or otherwise formed in opposite
surfaces of intermediate plate 233, or one of the passage means
formed in plate 233, while the other is formed in one of the
other two plates.
In operation in Figs. 12-24, compressed gas such as
nitrogen or ammonia at ambient temperature (60F - 90F) is
introduced through line 229 and port 221. These gas pressures
are in the region of about 150-3000 pounds per square inch.
The gas flows through the heat e~change section 222 of
the micron-sized passageways and then through the smaller cross
section aapillary section 223 where the gas expands and reduces
in temperature and enters cooling chamber 224. The fluid in
chamber 224 may be supercooled gas, liquid or a mixture, and in
any event this is the coolest part of the refrigerator.
Fluid leaving chamber 224 through port 239 flows at
reduced pressure through the return passage 225 in heat exchange
relation with inlet passage 222 and then to outlet port 226 and
line 231. It will be noted that this heat exchange takes place
substantially directly through the small uniform thickness of
the intermediate glass plate 233 and is efficient and accurate,
the cool low-pressure outgoing gas precooling the incoming
highly compressed incoming gas.
-27-
.. . .. ..
1 17~85 1
In the invention the heat exchange between the heat
exchange sections of the high-pressure gas passages and the
low-pressure return passages takes place through walls having
the dimension of the thickness of respective plates whereby
accurate relative location is possible, the heat exchange being
controlled by the thickness of plate.
One of the major advantages in providing the laminar
flow low-pressure return in a separate layer is that the
refrigerator may operate at a lower temperature than refrigerators
designed for turbulent flow in the return passage. This
results because the laminar flow channels produce lower back
pressures and hence lower operating temperatures.
Experience has shown that mutlilayer refrigerators
of the foregoing type may be fabricated using up to one-third less
material volume than a single-layer refrigerator (single-layer
is where inlet and return passages are formed in one interface)
of the same cooling capacity, and hence operate more efficiently.
Plates 232, 233 and 234 are preferably of soda lime glass,
Pyrex or other similarly low thermally conductive material.
They must be flat, of low thermal conductivity and capable of
being worked to form the surface recess passages and chamber
above described. This is particularly desirable where cooling
down to -50C or below is required, However, when the required
temperature is not so low, it is preferable to provide a top
plate of a very high conductivity material but about the same
coefficient of thermal expansion such as beryllium oxide,
silicon or a crystalline aluminum oxide whereby to effect a
more efficient exchange of heat.
In some embodiments the high-pressure inlet passage
system and the low-pressure passage system may be etched or
otherwise recessed into opposite sides of the intermediate plate,
while the other two plates of the stack are planar-surfaced to
-28-
~ 1 7~85 '
close the recess sides. Also it is within the scope of the
invent.ion to provide one layer (high- or low~pressure passageways)
in the intermediate plate and the other layer in one of the
top or bottom plates.
Fig. 25 discloses an embodiment wherein three similar
plates 251, 252 and 253 about 0.020 inches thick provide the
micron-sized passages. The plates are bonded in a stack in use,
but are here shown in exploded view to better illustrate
detail of the refrigerator components.
The high-pressure gas is introduced at inflow port 254
of plate 251 which is a flat glass plate with planar surfaces,
and continues through bore 255 in plate 252 to a closed bo~tom
well 256 in the upper surface of lower plate 253 which may be
the same as plate 232 of Fig. 12-24. The higher pressure
circuit continues in plate 253 as a heat exchange section 257
and a capillary expansion section 258 opening into the cooling
chamber recess 259.
Intermediate plate 252 which may be the same as plate
234 of Figs. 12-24 is flat and planar on its bottom surface
to complete the passages in plate 253, and its upper surface
is recessed at 261 to provide the low-pressure return. A
through bore 262 in the bottom of recess 251 connects the
low-pressure recess 261 to an external circuit.
Fig. 26 shows an embodiment similar to Fig. 25 but
providing two low-pressure returns. Four similar-sized plates
are here bonded in a stack, shown exploded in Fi.g. 26 for
clarity of disclosure. Plates 251, 252 and 253 are as in
Fig. 25, and a fourth plate 264 is added to provide a second
low-pressure return. Plate 264 may be a duplicate of plate 252
with the low-pressure return recess being formed in its upper
surface. However, a through bore 265 in plate 253 in the bottom
of chamber 259 connects the chamber to low-pressure recesses 266
-29-
~ 1 7~85 ~
in the upper surface of plate 264. Also, there is no bore
corresponding to bore 262 in recess 266, but the low-pressure
gas from recess 266 flows through a through bore 267 in plate
253 and a through bore 268 in plate 252 to join the exhaust
gas from recess 261 in passing to outflow port 263.
Thus in this embodiment there are two heat exchange
paths providing quicker and lower precooling of the incoming
high-pressure gas.
In the embodiment of Fig. 27, the cool end of the
refrigerator 211 is modified to the extent that the cooling
chamber, instead of being a recess as at 224 in Fig. 14 is a
through opening 270 in the cover glass plate 251, and over this
opening is bonded pressure-tight a thin flat pad 271 of a
material that has very high thermal conductivity. The device
219 to be cooled is mounted directly upon the pad 271. Thus
the fluid at its lowest temperature contacts the undersurface of
the pad 217.
Preferred materials for pad 271 are silicon, beryllium
and sapphire. All have high thermal conductivities that
increase dramatically at very low temperatures and can be
matched to an appropriate plate material of about the same
coefficient of thermal expansion. The preferred material is
beryllium oxide. This material combines high hardness with a
coefficient of thermal expansion closer to the preferred glass
plate material.
Figs. 28-30 illustrate another form of multilayer
refrigerator operating on the same principle.
As shown in Fig. 28 the refrigerator laminate comprises
three similar thin planar plates 280, 281 and 282 of a
material that ~ay be etched. Preferably plates 280 and 281 are
glass plates and plate 282 may be of glass for many purposes
-30-
~ 1 7~85 ~
but for other purposes may be of a higher thermal conductivity
glasslike material such as crystalline aluminum oxide ~sapphire),
beryllium or silicon.
Upon the top surface 283 of plate 280 is etched or
equivalently formed the high-pressure gas inlet passage means,
here consisting essentially of a micron-sized capillary dimension
recess 284 that travels from inlet port 285 in a labyrinth
to a central recessed cooling chamber 286.
Plate 281 has a continuous planar bottom surface 287
bonded pressure-tight onto plate 280 to complete the inlet
passage and chamber and in its upper surface 288 there is a
large low-pressure gas return recess 289 connected to chamber 286
by a port 290. A series of raised radial ribs 291 in the recess
289 supportingly engage plate 280 to increase the mechanical
strength of the assembly.
The outgoing gas at reduced-pressure flows in recess
289 to an outlet port 292 that continues through one of the
plates 280 or 282 to an external device as in the earlier
embodiments.
As indicated in Fig. 28 the plate 282 is bonded
pressure-tight onto plate 281 to complete the micron-sized deep
return passage 289.
On the top surface 293 of plate 282 is directly secured
the ckip 294 to be cooled, and a printed or like circuit 295 for
the chip extends over plate 282 to suitable external electrical
connections. The chip is thus exposed to the cooled fluid at
substantially the coldest region of the refrigerator. This
mode of attaching the device to be cooled may be employed in
all of the embodiments herein.
In the foregoing, the capillary passage exhibits heat
exchange relation with the fluid in exhaust recess 289. This
arrangement may be preferable for higher temperature refrigeration
. , .. . .. , ~ _ . .. . . . .
~ ~ 7~8S ~
where cooling to cryogenic temperatures is not required.
For example, ammonia may be used as the refrigerant to achieve
temperatures of -30~C and refrigeration capacities up to 50
watts. As the requirements approach cryogenic cooling a longer
heat exchange region for precooling prior to the capillary
section is provided. For example, the refrigerant may be Freon
introduced at high pressure and which will drop in pressure,
expand and cool in the capillary section 284 while in heat
exchange relation with the return gas in recess 289 for further
cooling. Where the gas is nitrogen for cryogenic cooling as
in the embodiment of Fig.s. 12-24, the longer precooling heat
exchange region is provided.
The foregoing refrigerator may be of particular value
in cooling larger computer chips such as those known as VLSI
(very large scale integration) chips which today are being
designed with greater circuit density and increased power
capacity, thereby dissipating large amounts of heat, i.e.
10-50 watts. Refrigeration enables such chips to operate at
lower temperatures, improves their operating efficiency, speed
and reliability, and increases their useful life.
Figs. 31 and 32 illustrate a refrigerator unit consisting
of two multilayer refrigerators constructed between five
laminated plates. This configuration allows for cascade cooling
the precooling of one fluid by another to enable either
faster cooldown or lower temperatures. For example, ammonia
could be used in the first stage of the cascade, layers 294, to
precool nitrogen in the second stage, layers 295. This will
decrease the cooldown time for nitrogen by a factor of three
or more. As another example, nitrogen could be used in the
shorter stage, layers 294, to precool hydrogen which will then
cool to 20K. Low boiling point fluids such as hydrogen and
helium will not cool in the Joule-Thomson cycle unless they are
precooled to the proper temperature in this manner. Refrigerators
-32-
_~ . .. ... . . .... .. ... . . . . . .
1 17~85 1having three or more stages can be constructed in a similar
manner.
Other channel dimensions and characteristics of the
refrigerator may be as described in connection with the
embodiments of Figs. 1-11. The formation of the channels
and the bonding of the plates herein is accomplished by the
channel-forming techniques and the bonding material described
in connection with said embodiments.
In accordance with another aspect of the invention it
has been discovered that there is a trade off between
efficiency of the heat exchanger and the minimum temperature
which can be reached in the cooling section of the refrigerator.
The minimum temperature is determined by the gas pressure
at the point at which the gas exits from the cooling chamber.
The lower the pressure at this point, the lower the temperature.
On the other hand, the heat exchanger efficiency is a function
of the pressure drop along the outflow channel, an increase
in the pressure drop producing more efficient heat exchange.
Thus to obtain an efficient heat exchange which leads to
faster cool down time and/or lower gas consumption rates the pressure
of the gas as it leaves the cooling chamber must be relatively
high. Conversely, to effect maximum temperature drop in the
cooling chamber the gas pressure at the same point should be
relatively low.
The embodiments of Figs. 33 and 34 represent an
effective solution to this problem. In each of these embodiments
two capillary sections are provided and a portion of the
incoming gas is by-passed directly to the outflow passage of the
heat exchanger after passage through one of the capillary section
thus by-passing the cooling chamber.
The embodiment of Fig. 33 comprises a stack of four
21ateS, 300J 302, 304 and 306 which may be of the same materials
~ 17~851
and bonded together in the same manner as in the previously
described embodiments. In this embodiment two capillary sections
308 and 310 are provided in series relationship. Between the
two capillary sections a small port 312 is provided leading
to the upstream end of the heat exchanger outflow passage 314
formed in plate 300. The downstream end of the second capillary
section 310 is connected to the cooling chamber 316 which in
turn is connected by a port 318 to a second outflow passage
320 formed by the recessed face of plate 304. Ribs 322 are
provided in the plate 304 to impart stiffness to the unit and
assure uniform spacing between the plates 304 and 306.
In operation, high pressure gas flows through the inflow
section of the heat exchanger 324 and expands and drops in
pressure as it flows thereafter through the first capillary
section 308 to the port 312. At this point the gas flow is
divided so that a substantial portion of the gas proceeds
through the port 312 directly to the outflow heat exchanger
passage 314.
The port 312 and the outflow passage 314 are so dimensioned
that a relatively high pressure is maintained at the port 312,
typically 10 to 30 atmospheres. Accordingly, a large pressure
drop will occur in the outflow section 314 of the heat
exchanger producing high efficiency of the heat exchange
function.
The remainder of the gas flows through the second
capillary section 310 to the cooling chamber 316. Here the
gas absorbs heat from the device being cooled and then flows
out through the port 318 and to the exterior of the device
through the second outflow passage 320, at a relatively low
pressure, typically 2 to 3 atmospheres. This low pressure
assures achievement of the desired low pressure in the
cooling chamber.
-34-
~ 1 7~8S 1
It has been found that both heat exchanger efficiency
and the desired cooling can be achieved by permitting from 50%
to as much as ~5% of the gas to flow through the port 112 and
the outflow heat exchanger passage 314.
Fig. 34 illustrates a plate 324 which may be substituted
for the plate 302 in the embodiment of Fig. 33 to achieve
similar results. In this form of the invention the two
capillary sections, indicated at 326 and 328, are arranged in
parallel rather than in series as in the embodiment of Fig. 33.
The incoming gas passes through the inflow section of the heat
exchanger 324, through the first capillary section 326 and
through the port 312 for passage to the first outflow heat
exchanger passage 314. The remainder of the gas passes through
the second capillary section 328 into the cooling section 316
and thence into the alternate outflow passage 320. ~s in the
previously described embodiment good results may be obtained
by by-passing from 50 to 95% of the incoming gas through
the port 312.
Other channel dimensions or characteristics of the
refrigerator may be as disclosed above. The formation of
the channels and the bonding of the plates herein also may be
accomplished by the channel-forming techniques and the
bonding material described above.
Refrigerators as above described are ideal for a
wide range of laboratory and like applications. They provide
convenient very low temperature economic operation as an alternative
to volatile liquid cryogens. They are of small size and low
weight enabling them to be used directly on instruments
such as microscope stages. The small size enables them to be
used to cool very small devices enabling tools or optical
instruments to observe or work directly on the device without
interference. Small gas consumption enables days of continuous
1 1 7~85 '
use from a standard pressurized cylinder of gas. Temperature
control is simple. The refrigerators are simple in structure
and may be constructed and operated relatively simply and
safely.
-36-
