Several NASA programs designed to monitor the Earth's atmosphere from space utilize infrared detectors which operate at or below 4.2 K for optimum performance. At present, the detectors are maintained at cryogenic temperatures by a stored volume of liquid helium. These detectors must be electrically linked to amplification electronics data storage instruments maintained at 80 K. The electrical connections over the temperature gradient account for approximately 20% of the total heat load on the Dewar for some systems, accelerating the boil-off of liquid helium cryogen and reducing the operational lifetime of the space-borne instruments. The recent discovery of high temperature superconductors has provided an opportunity to develop electrically conductive, thermally insulating links to bridge this thermal gradient. This paper describes the modelling of the thermal transport properties of thick film, high T(sub c) electrical bridges across a 4.2-80 K temperature gradient and the impact of such devices on a spaceborne remote sensing system

Buoncristiani, A. M.

Caton, R.

Hooker, M. W.

Selim, R.

Wise, S. A.

English

NASA Technical Reports Server

(NASA-CR-199743) HIGH T(SUB C) N96-15745
LEADS FOR REMOTE SENSING
APPLICATIONS (NASA. Langley
Research Center) 4 p Unclas
G3/35 0083571
https://ntrs.nasa.gov/search.jsp?R=19960008579 2020-06-16T06:01:34+00:00Z
NASA-CR-199743
/,.
High 7~c leads for remote sensing c
applications
M.W. Hooker, S.A. Wise*, R. Selim , R. Caton1 and
A.M. Buoncristianif
Science and Technology Corporation, Hampton, VA 23666, USA
*NASA-Langley Research Center, Hampton, VA 23681, USA
Department of Physics and Computer Science, Christopher Newport University,
Newport News, VA 23606, USA
Received 23 February 1993
Several NASA programmes designed to monitor the earth's atmosphere from space
utilize infrared detectors which operate at or below 4.2 K for optimum performance. At
present, the detectors are maintained at cryogenic temperatures by a stored volume of
liquid helium. These detectors must be electrically linked to amplification electronics
and data storage instruments maintained at 80 K. The electrical connections over the
temperature gradient account for =20% of the total heat load on the Dewar for some
systems, accelerating the boil-off of liquid helium cryogen and reducing the operational
lifetime of the space-borne instruments. The recent discovery of high temperature
superconductors has provided an opportunity to develop electrically conductive,
thermally insulating links to bridge this thermal gradient. This paper describes the
modelling of the thermal transport properties of thick film, high 7C electrical bridges
across a 4.2-80 K temperature gradient and the impact of such devices on a space-
borne remote sensing system.
Keywords: high 7"c superconductivity; space cryogenics; remote sensing
Several future NASA programmes designed to monitor
the upper atmosphere utilize infrared (IR) detectors.
These detectors operate at or below 4.2 K for improved
signal-to-noise characteristics and are electrically
linked to data acquisition and storage electronics at
80 K1, as shown in Figure 1. One of these projects
currently under consideration by NASA is the spectros-
copy of the atmosphere using the far infrared emissions
(SAFIRE)2 experiment, designed to detect chemical
radicals in the upper atmosphere. SAFIRE will employ
gallium-doped germanium detectors operating in a
liquid helium environment and the signals from these
sensors must be linked to data acquisition instruments
which are maintained at 80 K by a mechanical cryo-
cooler. The current design utilizes — 150 manganin
(85% Cu, 12% Mn, 3% Ni) wires (40AWG) as the
electrical connections, due to the low thermal conduc-
tivity of manganin at cryogenic temperatures3. The
temperature gradient spans =15 cm in length, and each
lead carries less than 1 p,A of current.
In this design, the heat load due to the electrical
leads constitutes =20% of the total heat load on the
liquid helium Dewar. Furthermore, these connections
comprise the only non-parasitic heat load that can be
modified to reduce the thermal load on the liquid
helium Dewar, and thus increase the operational
lifetime of the space-borne instrument.
The discovery4'5 of high Tc superconductors provides
an alternative to the currently employed manganin
leads. High Tc materials possess adequate curent
transport properties for sensor lead applications, as
well as low thermal conductivities6. However, fine
diameter wires of these materials have not demons-
trated the required mechanical durability to withstand
operational stresses in space-borne systems. As an
alternative, the use of thick film superconductors
screen-printed on to low thermal conductivity sub-
strates is currently under investigation. Thick film
technology provides the ability to deposit long length
superconductive elements with cross-sectional areas
similar to the currently used manganin wires on to rigid
ceramic substrates7.
The preparation of yBa2CujO7_x and Bi2Sr2Ca2-
Cu3O^ thick films on ceramic substrates has been
demonstrated by several researchers8"10. For this parti-
cular application, a current density of 0.065 A cm~2 is
required, assuming a similar cross-sectional area to that
of 40AWG wire. This value is attainable for thick f i lm
superconductors, which exhibit critical current densi-
ties in excess of this value.
High Tc leads for remote sensing applications: M.W. Hooker et al.
Signal Pulses (1
Eleclronics
Focal Rane Array
Heat How 80°K
Figure 1 Schematic representation of remote sensing instru-
ment for space flight
This paper demonstrates the feasibility of using a
thick film superconductive lead assembly as an alter-
native to manganin wires. The thermal characteristics
of various superconductor/substrate combinations are
compared to those of the existing manganin leads and
the potential impact on mission lifetimes is discussed.
Thermal modelling
Heat transfer calculations
To calculate the heat flow through both manganin wires
and superconductive thick film lead assemblies, a
thermal model was developed" using the design
criteria for SAFIRE2. In this design, two isolated
thermal reservoirs are linked by an element with
thermal conductivity k and electrical resistivity pe|ect,
as shown in Figure 1. According to conventional
thermal transport theory, the heat flow will proceed
from the higher temperature reservoir to the lower
temperature reservoir. Additionally, when an electric
current is passed along this link, heat is generated due
to I2R losses. The steady state behaviour for such a
system is governed by the following ordinary differen-
tial equation12
d
—dx J (D
where x is the position along the link. Because the
thermal conductivity and electrical resistance vary with
temperature, numerical methods must be employed to
determine the temperature distribution over the
thermal gradient. The numerical calculations for the
one-dimensional heat transfer across the thermal
gradient were performed using a Runge-Kutta method
with MathCAD™.
Design criteria
The above calculations were performed for both
YEa2Cu3O7^ (7C = 93K) and Bi2Sr9Ca2Cu3O^ (Tc =
110 K) films on A12O3, MgO, 8mol°/o Y2O3-stablized
ZrO2 (cubic YSZ) and fused silica (SiO2) substrates.
Printed superconducting elements with dimensions
0.076mm x 0.020mm, with 0.076mm spacing between
each printed lead were employed. A substrate thick-
ness of 0.152mm was assumed for each case. The
calculations were performed on a per lead basis to
determine the heat load of each superconductive
element and the corresponding substrate.
A similar calculation was also performed assuming
that superconductive elements were printed on both
sides of the ceramic substrate. Such a design should
Superconductive Leads
\ \
Superconductive Leads
\ \
**v-
? '*< N~
I ^«^
Ceramic Substraie
0.076 mm
Ceramic Substrate
a 0.076 mm
Figure 2 Printing configurations for superconductive leads
printed on (a) single side of substrate and (b) both sides of
substrate
reduce the heat loss due to the ceramic substrate by a
factor of two. The high Tc device with superconductive
leads on one side of the substrate is referred to as
model 1, while the case in which superconductors are
printed on both sides of the substrate is referred to as
model 2. Schematic illustrations of the two designs are
shown in Figure 2. In each instance, the calculated
values were compared to those for 40 AWG manganin
wires. The thermal and electrical conductivity values
for the various materials addressed in this work were
obtained from the technical literature3'6'13"17.
Calculation of thermal savings
Once the heat flow through the various candidate lead
assemblies was determined, the expected lifetime of the
SAFIRE experiment was calculated for each of the
high 7"c sensor lead designs. To translate the heat flow
calculations into lifetime savings, the SAFIRE experi-
ment was again used as a basis. In the current design of
the SAFIRE experiment, 158 leads bridge the thermal
gradient. Thermal analyses of the system design indi-
cate that the manganin connections will comprise 33%
of the instrument heat load on the Dewar, or 17% of
the total-heat load. The proposed mission duration for
the SAFIRE experiment is five to seven years, requir-
ing that a minimum of 125dm3 of liquid helium be
launched into orbit.
The per cent reduction in thermal loss for each of the
superconductive lead assemblies was calculated by
dividing the heat flow through each lead assembly by
the heat flow through the manganin wires and subtract-
ing this value from one. Determination of the thermal
savings for the system was then performed by substitut-
ing the various lead assemblies for the manganin wires.
The heat load due to each lead assembly was calcu-
lated, added to the baseline, and divided by the current
heat load value. Lifetime extensions were then calcu-
lated based on these savings for both five year (60
month) and seven year (84 month) missions.
Results and discussion
Heat loss through high Tc leads
The heat transfer calculations show that the lowest heat
flows are obtained when B\2Sr2Ca2Cu-iOj. elements
High ~TC leads for remote sensing applications: M.W. Hooker et al.
Table 1 Calculated heat flows through candidate high 7"c
superconductive lead assemblies (calculated on a per lead
basis)
Lead assembly
123/fused silica
2223/fused silica
123/cubic YSZ
2223/cubic YSZ
Manganin
123/alumina
2223/alumina
123/magnesia
2223/magnesia
Heat flow (MW):
model 1
4.9
3.7
14
12.8
16.6
884
882
8020
8019
Heat flow (/*W):
model 2
3.2
2.0
7.8
6.6
16.6
443
441
4012
4010
are printed on to both sides of a fused silica sub-
strate. Similar printing configurations using either
YBazCujC^ or Bi2Sr2Ca2Cu3O^ on YSZ substrates
also possess heat flows that compare favourably to
manganin wires. The heat flows through thick film
leads printed on fused silica or YSZ range from 2.0 to
7.8/iW per lead as compared to 16.6/iW per
manganin wire.
Similar lead assemblies printed onto A12O3 and MgO
substrates exhibited significantly higher heat flows (i.e.
>440/iW per lead) than the manganin wires, which
would result in a significant decrease in the operational
lifetime of the experiment in space. In these instances,
the substrate is the major contributor to the thermal
losses over the 4.2-80 K temperature gradient, with no
discernible differences attributable to the supercon-
ductive compound employed. A summary of the heat
flows through each of the candidate lead assemblies is
given in Table I.
These results show that substrate selection is the
most critical parameter in the design of a high Tc
thermal bridge. While superconductor selection is
important (e.g. bismuth cuprate superconductors
possess lower thermal conductivities than YBa2Cu3-
O7_Ar)6-13, the superconductor's contribution to the
heat load is minor due to the small cross-sectional
area of the conductor as compared to the substrate
material.
Thermal savings due to high Tc leads
The overall impact on the mission lifetime was found to
be dependent upon the superconductive compound,
substrate material and printing pattern (i.e. printing
on one or two sides of the substrate) employed in
the production of the device. Replacement of the
manganin wires in the SAFIRE system with printed
superconductive elements on either YSZ or fused silica
substrates would result in mission lifetime extensions
up to almost 10% based on the choice of substrate,
superconductor and printing configuration. This reduc-
tion in thermal losses translates into an additional two
to twelve months of operation for a seven year mission
depending on the lead assembly design used.
The per cent reduction in thermal loss and the
percentage of lifetime increase for the entire system for
each superconductor-substrate printing combination
are shown in Table 2. The calculated lifetime exten-
sions for the SAFIRE instrument (in months) for both
five and seven year missions are also shown in Table 2.
Electrical considerations
In each of the candidate lead assemblies modelled,
including manganin wires, Joule heating was not found
to contribute significantly to the heat load on the liquid
helium Dewar. The negligible heat loads due to I2R
losses may be attributed to the low electric currents
passed through the leads. For applications with high
current transport requirements, the use of manganin
leads may result in higher thermal loads on the liquid
helium Dewar due to resistive heating. However,
resistive heating would not affect the thermal
properties of a superconductive lead assembly, as
electrical resistance would be non-existent in this
* ISinstance .
Manufacture of high Tc lead assemblies
While the use of screen-printing has been successfully
demonstrated for both YBa2Cu3O7_A: and Bi2Sr2Ca2-
Cu3Ox on YSZ substrates8"10, the use of fused silica
substrates has resulted in delamination of the super-
conductive element due to thermal expansion
Table 2 Estimated thermal savings for space-based system, such as SAFIRE, using various superconductive lead assemblies (data
based on 158 leads, with leads comprising 17% of total heat load)
Lead assembly
123/fused silica
Model 1
ModeM
2223/fused silica
Model 1
Model 2
123/cubic YSZ
Model 1
Model 2
2223/cubic YSZ
Model 1
Model 2
% Reduction in
thermal loss of lead
wires
70.5
80.7
77.7
87.9
15.8
53.4
23.0
60.6
%Thermal savings
for system
1 1 .4
13.2
12.7
14.4
2.1
8.5
3.4
9.8
Lifetime extension
for 5 year mission
(months)
6.9 '
7.9
7.6
8.6
1.3
5.1
2.0
5.9
Lifetime extension
for 7 year mission
(months)
9.6
11.1
10.6
12.1
1.8
7.2
2.8
8.2
-
High Tc leads for remote sensing applications: M.W. Hooker et al.
mismatches19. Additionally, a chemical reaction
between YBa2Cu3O7_x and SiO2 has been reported,
resulting in the formation of an intermediate compound
and degradation of the superconductive properties20.
At present, YSZ appears to be the most viable
candidate material for use as the substrate in the
fabrication of the thermal isolator. However, other
alternative printing schemes such as the use of multi-
layer printing of YBa2Cu3O7_^., with the electrically
insulating Y2BaCuO5 compound serving as a dielectric
layer21, and the use of sputter deposited ZrO2 buffer
layers22 on fused silica substrates are currently under
investigation at NASA-Langley Research Center.
Conclusions
Modelling of the heat transfer characteristics of high Tc
lead assemblies over a 4.2-80 K temperature gradient
has shown that the replacement of the manganin leads
with thick films of a ceramic superconductor printed on
a low thermal conductivity substrate can significantly
decrease the thermal loads on liquid helium Dewars.
The critical parameter in designing a thermal isolator
was found to be the selection of a low thermal
conductivity substrate, as the substrate contribution to
the heat load is larger than that of the superconductive
elements.
This study was designed to determine the feasibility
of employing superconductive lead assemblies for
cryogenic detector systems. While cryogenic thermal
conductivity data are limited for ceramic compounds,
other materials may offer further benefits for thermal
savings. In general, materials with random (i.e. glasses)
or complex crystal structures, high atomic weight
constituents and/or defect structures exhibit low
thermal conductivities at cryogenic temperatures23.
However, the primary concern in selecting a substrate
for this application must be chemical compatibility with
YBa2Cu3O7_J or Bi2Sr2Ca2Cu3OAr at high temperatures
in order to successfully co-fire the two materials to
produce a useful device.
Beyond SAFIRE, NASA has several other planned
missions with similar designs and cryogenic
requirements1. Some of these experiments will require
several hundred electrical connections to span a
similar thermal gradient. As the number of electrical
leads increases, the thermal load on the liquid helium
Dewar will also increase, making low thermal loss,
high Tc sensor leads even more beneficial to these
programmes.
Acknowledgements
The authors wish to thank I. Nolt of NASA-Langley
Research Center and J. Lee of Ball Aerospace for
providing system parameters for the SAFIRE experi-
ment. Additionally, R. Selim, R. Caton and A.M.
Buoncristiani wish to acknowledge the support of
NASA grant no. NAG-1-1242.
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High T(sub c) leads for remote sensing applications

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