10. Cryogenic packaging

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10. ?? ??? ??(Cryogenic packaging)
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Cryogenic packaging issue

• Material selection
• Minimize convection heat transfer
• Minimize radiation heat transfer
• Minimize conduction heat transfer
• Sealing against vacuum leak
• Thermal stress problem from cooling process

He was right, he is right, Murphy will always
be right. - An exasperated cryogenic
engineer
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1. Material selection lt High thermal
conductivity gt (1) Copper 99.999 high purity
(RRR2000) Grade 101 or 102 OFHC (RRR100) ETP
(RRR100) for wire, rods, plates containing about
0.3 oxygen ---gt cannot be used for H2
brazed. Phosphorus deoxidized copper for most
tubes, k is about 1/50 of ETP OFHC The most
important characteristic of Oxygen Free High
Conductivity copper is that it can be heated in
a reducing atmosphere without embrittlement.
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OFHC(CDA 102) minimum purity of 99.95 (silver
counted as copper), Certified OFHC (CDA 101) is
available with a 99.99 copper content. ETP
Copper (CDA 110) Electrolytic Tough Pitch
copper is the most widely used copper for
electronic wire products based on its optimum
combination of workability, performance and
economy. Beryllium Copper (CDA 172) This
alloy is characterized by high strength and good
electrical and thermal conductivity. CDA alloy
172 provides tensile strengths of over 200,000
psi, the highest strength of any copper-base
alloy.
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Cartridge Brass (CDA 260) This is the most
widely used copper alloy for cold headed products
such as machine, wood and cap screws, special
hollow rivets and fasteners with exceptionally
large or intricately formed heads. Phosphor
Bronze (CDA 510) The most widely used
copper-base alloy for almost every type of spring
because of its high strength, resilience,
excellent corrosion and fatigue resistance.
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(2) Aluminum 99.999 high purity
(RRR1000) Grade 1100 (RRR14) Grade 6063 Grade
5052 (3) Sapphire (Al2O3) or quartz (SiO2) at
low temperature
Al2O3
SiO2
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lt Low thermal conductivity gt (1) Metals S.S.
304, 316, 321, Inconel 718, Copper-Nickel 70 -30
, Titanium, Incoloy 903, 905, 908, Monel ? (2)
Non-metals Glass, G-10 fiberglass epoxy,
Phenolics, Nylon, Mylar, Kapton (Ch.1 ??)
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• 2. Minimize heat transfer
• Convection lt 1 mTorr
• Radiation Low emissivity
• Conduction Wiedemann-Franz law k/sT
  constant Platinum-Iridium, Manganin,
  Constantan, Titanium, Inconel alloy

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• 3. Seal materials against vacuum leak (Ch. 10)
• Real leaks Cracks in glass feedthrough, poor
  weld, solder joints. Permeation from
  plastics, rubber o-rings, and epoxies.
  Thermal stress at lower temperature.
• Virtual leaks Gas pockets in screw, epoxies.
  Outgassing from finger prints, skin oil,
  Teflon, organic dielectric materials.
  Unacceptably high outgassing rate at low
  pressure.
• (e.g.) The smaller and thinner a joint, generally
  the better, as a gas pocket of just .020" in
  diameter releases enough gas into a 6 cubic inch
  volume to render it above the critical pressure
  level of around 10-3 Torr.

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(1) Static seals Buna N (pressures), Viton
(vacuum, low outgassing), soft metal plating,
polymer seals (teflon), gaskets (indium, lead,
silver, gold, copper such as CONFLAT, Cajon VCR,
teflon) (2) Dynamic seals Rubber O-rings
(require lubrication), Glass-filled teflon
(Flourogold, abrasive), Nylon-filled teflon
(Rulon), Graphite-filled teflon (abrasive in dry
He), Coil spring or flat spring to maintain force
(American Variseal, Bal Seal, etc.)
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Baking-out process before pinch-off 100105 oC
for several days (7 to 10 days) Getters (Ch. 10)
Commonly used in cryostat, cryogenic
packaging device Activated charcoal or
zeolite Ta, Nb, Ti, Zr, Th used for getter
pump at high temperature
In cryocooler for IR detector Helium gas
retention capability Leak rate 1 x 10-8 cc/s
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4. Joining techniques (thermal stress
problem) (1) Bolting Clean joints of gold vs.
gold (plated copper), indium gasket, high
pressure, 8 micro-inch finishing. (2) Grease
Easy to disassemble, vacuum grease to give low
outgassing, Apiezon N, Cryocon. Minimize joint
thickness. (3) GE varnish (GE7031) For
semi-permanent bonding.
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(4) Epoxy Surfaces must be degreased and
preferable etched. Tight fits. Stycast 1266 (low
viscosity, unfilled, high thermal
expansion) Stycast 2850 FT or GT (high
viscosity, filled with silica or alumina powder,
low thermal expansion) (5) Soft solder Gap
should be about 0.05 mm 0.2 mm. For high
thermal conductance, pure In _at_ 170 C S.S. must
be tinned with solder using HCl flux. Use ZnCl
fluxes with most metals --gt washed off with warm
water. 98 In - 2 Ag _at_ 150 C 63 Sn - 37
Pb _at_ 183 C 96 Sn - 4 Ag _at_ 221 C Most
metals can be soldered with modifying surface
condition. (Example of GaAs upon Si in
semiconductor)
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Surface structure modification technique
(Si-SiO2-SrTiO2-GaAs)
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(6) Hard (Silver) solder Gap should be about
0.05 mm 0.1 mm. Use boric acit fluxes --gt
washed off with hot water. Melting _at_ 600
C Compositions Cu-Ag-Cd or Cu-Ag-Zn (7)
Vacuum brazing (no flux) H2 atmosphere is
possible if no embrittlement. Gap should be
about 0.025 mm to 0.05 mm. Vacuum should be
better than 5x10-5 Torr. Use braze stop material
to control unwanted flow.
(8) Welding TIG is good for S.S. EB welding for
more precise, reliable, and expensive
stuff. Chiller is necessary for welding thin
tubes. Aluminum welds are not as reliable as S.S.
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Space InfraRed Telescope Facility (SIRTF)

• With its great sensitivity, large-format
  infrared detector arrays, high observing
  efficiency, and long cryogenic life, SIRTF offers
  unprecedented capabilities over previous and
  existing facilities.
• The observatory, when launched in about one
  year, will have a cryogenic lifetime of at least
  2.5 years.

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30 Doradus Across the Spectrum
A composite of three pictures taken in three
different wavelength bands of light. Red
represents X-ray emission created by gas as hot
as 1 million degrees Kelvin. Green represents
emission from ionized hydrogen gas, and blue
represents ultraviolet radiation primarily
emitted by hot stars.
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Innovations Cryogenic Architecture

• Previous infrared telescopes in space have been
  enshrouded within a giant cryostat containing the
  super-fluid liquid helium necessary to
  refrigerate the telescope to operating
  temperatures near absolute zero. This system
  configuration has come to be known as the
  "cold-launch" architecture (below left).
• Unlike IRAS and ISO, however, SIRTF adopts an
  innovative "warm-launch" cryogenic architecture
  (see Figure).
• The Observatory is launched at ambient
  temperature and radiatively (or passively) cooled
  in the deep recesses of space.
• Only the focal-plane instruments and the liquid
  helium cryostat are enclosed in a vacuum shell
  containing the cryostat.

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• It is important to recognize that the
  warm-launch architecture is enabled by the choice
  of orbit. With SIRTF inserted into a relatively
  cold space environment (i.e., away from the warm
  Earth), the ambient temperature of deep space
  passively cools the Observatory to less than 40 K
  within a few weeks of launch.
• At that time, the telescope is thermally
  de-coupled from the cryogenic telescope assembly
  (CTA) outer shell. The boil-off of liquid helium
  produces a vapor that cools the CTA to the
  operational temperature of 5.5 K.
• This innovative launch architecture, combined
  with 360 liters of liquid helium, yields an
  estimated mission lifetime of about five years.
  For the sake of comparison, IRAS used 520 liters
  of cryogen during its 10-month mission and ISO
  used 2140 liters to achieve a mission lifetime of
  nearly 2.5 years.
• The most obvious benefit of this architecture is
  the substantial reduction in the size of the
  Observatory, and its resultant launch costs.

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IDA (Integrated Detector Assembly)
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1 - Test data incomplete, cooler malfunction2 -
Bottom out temperature 88 K after 1330 till
beyond 20003 - Bottom out temperature 79K
after 1800
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Multichannel HTS transmitter/receiver
(Conductus) ( final configuration based on
Leybold Stirling cryocooler )
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Cryogenic packaging and integration issues of
cryocooler for IR sensor, cryoelectronics and HTS
General requirements of cryocooler packaging

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Expander or cold-head interface of cryocooler
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Cryocooler selection
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• Some aspects of design
• Target definition
• Reliability
• Cold finger design
• Balancing
• Bearings
• Seals
• Materials
• Cooling
• Electrical and electronic systems

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ltTarget Definitiongt (1) ??? ??? ??? ? ? ????
???? ??? ??? ??. (Quick Cooldown
? ???? ??, ?? ??. , ????) ? ??? ??, ?? ??,
??? ?? ??. (2) ???? maintenance free ? ? ???
magnetocardiogram ??.
? Superconducting magnet (MHD or
MRI).
? ??, ??? ???? ??.
? Cooldown ?? ???? ??.
(3) Ground base or airborne base
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ltReliabilitygt ? ???? ???? Cryocooler? ?? ?? ?
?? ???? ?? ?? 4?? ??(NASA) ? Simplicity,
Redundancy, Light loading, ??? ??.
Simplicity ? ??? ????? ??? ? ??? ??. Design
Redundancy ? ???? ??? ???, original unit? ???
???? ??? ???? back-up system? ?? Light loading ?
??? ???? ??? ??? ??? ? ?. ??? ??? ?? ? ??
engineering ???? ? ???.
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ltCold finger Designgt ? Coldfinger design? ???
????? ????? ?? ??? ??? maximize ?? ? ? ??????
???? ?? ?? ??? ???? thermal isolation ?
Thermal isolation? ?? ??? ???? ?? heat leakage?
??? ??.
??? ?? ?? ?? ????? ?? ?? cylinder ! S.S. or
Ti, glass dewar. ? ???,
? ??? Cylinder? ?? ??? ???
?? Cylinder ??,
) ? Stainless Steel? cold finger Cylinder? ???
??.
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? Cooldown
R ? cycle ? ???? ???
P Cooldown
time
N Number of cycles per minute
C ??? ???
???
?? ??? ?? ??? ??
? Cooldown time ? ???? cylinder cap, ????? ???
?? ?? ???? ???? ?? ?? ? ??? ??. ??, ???? ?????
?? ??? ??? ????? ?? ??? detector?? sensitive ?
??, ? ??? ?? perturbation? ??? ???. ??? ??? ????
???? ???? thermal inertia? ?? ??? ?? ??????
cooldown time? ??? ???? ???? ?? ??.
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lt Balancing gt ? ??? ??? ??? ??, bearing ?? ?
stress??. (1) Case 1 Single revolving mass
M? ?? ? ? ??? ?? ???? balance ??? ??? ?? B?
?????? b ?? ??? ?? ?? ?? ?? ?? ??. ?? B? ?? ??
??? b? ???
Fig. 9.3. Balance of single revolving mass
???? ???? ??? ?? ???? ?? Fig.9.4 ?? ???? ??? ???
??? ??.
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Fig. 9.5 ?? balance weight B? ? ??? ??? ????
complete dynamic balance of forces and moments
Fig. 9.4. Balance mass not in same plane as
unbalanced mass
Fig. 9.5. Twin balance masses symmetrically
located about the unbalanced masses
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lt Close tolerance sealsgt Gas lubricated
bearing ?? fine tolerance? clearance? ???? ???
close tolerance seal ??. ? Leakage flow ?
( Clearance )3Leakage flow? ??? ??? groove? ??
? ?? ?? (Fig.9.25).
Fig. 9.25. Close tolerance seal elements
Fig. 9.24. Close tolerance seals
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ltMaterials gt (1) Mechanical Properties. ?
strength? ?? ? cylinder wall, regenerator casing,
other structural applications. ??, ????, ???? ?
??, ??????, austenitic stainless steel ? ??? ???
structural element? ?? ???? ???, fiber ????? ??
???? ?? ??. (2) Physical Properties
specific feat, thermal conductivity, thermal
expansion coefficient ?
? ??? ??? ???? stainless
steel ? ???. ?? glass fiber reinforced epoxy
composite? ?? stainless steel? 50?.
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(3) Fluorocarbons. (PTFE, Kel-F) ? good
electric thermal insulator. ? inert to
most chemicals solvents. ? low friction
coefficients. ? not brittle at low
temperature Cryocooler ? design ??? ??? ???
???? ?? ?? ???.
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lt Cooling gt
TH cooling
temperature, ??
TC
refrigeration temperature, ?? . ?
???? cryoccooler ? 1 of COP(Carnot) ?
Cooler ?? ??? ???? ?? ??? ??? ???. ?T TC -
Tamb ? Q hA?T ?? h, A ? ???
?T? ??? ( ?? cryocooler ) ? Compressor work fn
(TC - Tamb) ??. ? system ? ??? ??? fin?
???? ?? ??? ?? ??? ??? ??? power input? ??? ??
??? ???. ? Cooling?? ???? ???? ??.
? Spacecraft Radiative Cooling ??? Qrad ?A?T4
? ??? small compact radiator ? ???? ???
cooling??? ??? ???, ??? ?? power input? ??? ??. ?
?? ???? ??? cryocooler? ??? ??? cooling ?
radiator ??? ???? ?????, heat pipe (Capillary
Pumped Loop CPL in NICMOS)? ??? ??.
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lt Electrical Electronic System gt (1) Drive
Motors External motor mechanical coupling,
dynamic seal Partially enclosed
motor Completely enclosed motor
(2) Brushless dc Motor (3) Electronic Controls
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Possible problems in cryocooler packaging
1. EMI (Electro Magnetic Interference)
rejection - 60 Hz noise - DC/AC conversion
switching noise
2. Microphonics
Unbalanced displacer causes vibration on the
sensor, causing false signal
3. Thermophonics
Fluctuation of the cold tip T.
Thermal mass between the cold tip and the load
(cold device) may alleviate problem, but
increases the cool-down time.
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4. Gas contamination
Small outgassing is always present Teflon,
Nomex, Polyimide,
low-vapor-pressure epoxy adhesive
Careful for using organic material with outgassing
Vacuum bake all components at 60 C before final
assembly
Use high purity helium for system-charging
Wear debris
5. Cold leak due to thermal contraction mismatch
problem ? Test in LN2 before cooling to Lhe is
desirable.
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6. Helium gas retention
Leak rate 1 x 10-8 cc/s
Rubber O-ring seals, soft copper, indium, gold
Casting leaks - cryocooler pressure vessel
component (Al)
Metal porosity
Weld joints
7. Seal problems
Piston seal, drive seal, displacer seal
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To become a cryogenic expert

• Understand material characteristics at low
  temperature
• Be careful for material selection
• Minimize all heat transfer
• Utilize good vacuum technology
• Be prepared for the worst scenario
• Avoid any unnecessary trial error

Hands-on experience is the most important thing
in cryogenics !