NASA CASE NO. LAR 14964-1-CU

PRINT FIG. 1

NOTICE

The invention disclosed in this document resulted from research in aeronautical

and space activities performed under programs of the National Aeronautics and

Space Administration. The invention is owned by NASA and is, therefore,

available for licensing in accordance with the NASA Patent Licensing Regulation

(14 Code of Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy

to grant licenses to commercial concerns. Although NASA encourages

nonexclusive licensing to promote competition and achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve

early practical application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23681-0001.

Serial No.: 08/215,793

03/15/94 LaRC

(NASA-Case-LAR-14964-I-CU)

ELECTRICALLY CONDUCTIVE, THERMALLY

INSULATING CRYOGENIC CURRENT LEADS

Patent Application (NASA. Langley

Research Center) 8 p

N94-36781

Unc! a s

G3/33 0019644
LAR- 1 4964- 1 AWARDS ABSTRACT 

ELECTRICALLY CONDUCTIVE, THERMALLY INSULATING 

CRYOGENIC CURRENT LEADS 

The present invention relates generally to electrically conductive current leads and 

more particularly to thermally insulating electrically conductive current leads. 

According to the present invention, a thermally insulating, electrically conductive 

current lead assembly consisting of a low thermal loss substrate and a series of 

thick film high temperature superconductive elements deposited on the substrate is 

provided. The substrate is a ceramic, preferably yttria-stabilized zirconia or fused 

silica. The high temperature superconductor may be YBa,Cu,O, or a 

superconductive compound in the Bi-Sr-Ca-Cu-0 or TI-Ca-Ba-Cu-0 systems. The 

superconductor is deposited on the substrate using screen-printing techniques and 

fired at elevated temperatures (800-950°C) to densify the superconductive 

elements. 

Novel aspects of the present invention include providing a current lead assembly 

which exhibits low thermal conductivity in the cryogenic region, which can 

withstand the operational stresses imposed on space-borne systems and which 

can be used to electrically connect an infrared detector to data acquisition 

electronics. The lead assembly is generally applicable to any cryogenic system in 

which heat transfer across the lead is to be minimized. 

Inventors: Stephanie A. Wise Matthew W. Hooker 

600 River Bend Court, #204 600 River Bend Court, #304 

Newport News, VA 23602 New~ort News. VA 23602 

Employer: NASA LaRC 

Initial Evaluator: Preston I. Carraway, Ill 

Serial No.: 08/21 5,793 

Filed: 311 5/94 
LAR 14964-1-CU -1- PATENT APPLICATION

ELECTRICALLY CONDUCTIVE, THERMALLY INSULATING

CRYOGENIC CURRENT LEADS

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Origin of the Invenl;ion

The invention described herein was made by an employee of the

United States Government and a contract employee in the performance of

work under NASA Grant Number NGT 50548 and is subject to the provisions

of Public Law 96-517 (35 U.S.C. 202) in which the contractor has elected

not to retain title.

Background of the Invention

15

1.__. Technical Field of the Invention

The present invention relates generally to electrically conductive

current leads and more particularly to thermally insulating electrically

conductive current leads.

20 2.__. Discussion of the Related Art

25

Current leads are an integral part of cryogenically-cooled infrared

detector systems which are used by NASA for atmospheric remote sensing.

In general, these infrared sensors operate more efficiently at cryogenic

temperatures due to improved signal-to-noise characteristics, thus requiring

liquid helium refrigeration systems for cooling. As a result, the duration of a

space mission using these sensors is limited by the evaporation rate of the

liquid cryogen, making all heat loads on the system critical. One
LAR 14964-1-CU -2- PATENT APPLICATION

non-parasitic heat load on the dewar system (i.e., not related to the dewar

design and structural composition) is the electrical leads connecting the

sensors to the data acquisition and storage electronics. These electronic

systems are maintained at higher temperatures, resulting in a thermal

5 gradient from 4 to 80K. The electrical connections over the temperature

gradient produce a significant thermal load, accelerating the boil-off of the

liquid helium cryogen and reducing the mission lifetime. This problem

becomes especially critical for long duration space missions (i.e., > 5 years).

Manganin (84% Cu, 12% Ni, and 4% Mn) wires are currently used for

10 electrical connections in space-borne cryogenic systems due to their low

thermal conductivity as compared to other metals and metallic alloys.

However, the discovery of superconductivity above 80K in ceramic materials

has provided an alternative to manganin connections. High temperature

superconductors possess extremely low thermal conductivities in the

15 cryogenic region, while maintaining sufficient electrical conductivity for

detector applications, where typical currents are on the order of 1 pA.

However, fine diameter wires of the high temperature superconductors have

not demonstrated the required mechanical durability to withstand operational

stresses imposed on space-borne systems.

20 It is accordingly an object of the present invention to provide a current

lead assembly which exhibits low thermal conductivity in the cryogenic

region.

It is a further object of the present invention to provide a current lead

assembly which can withstand the operational stresses imposed on

25 space-borne systems.

It is a further object of the present invention to provide a current lead

assembly which could be used to electrically connect an infrared detector to

data acquisition electronics.
5

LAR 14964-1 -CU -3- PATENT APPLICATION

It is yet another object of the present invention to accomplish the

foregoing objects in a simple manner.

Additional objects and advantages of the present invention are

apparent from the drawing and specification that follow.

Summary of the Invention

A thermally insulating, electrically conductive current lead assembly

consists of a low thermal loss substrate; and a series of thick film high

10 temperature superconductive elements deposited on the substrate. The

substrate is a ceramic, preferably yttria-stabilized zirconia or fused silica.

The high temperature superconductor may be YBa2Cu3OT. x, or a

superconductive compound in the Bi-Sr-Ca-Cu-O or TI-Ca-Ba-Cu-O systems.

The superconductor is deposited on the substrate using screen-printing

15 techniques and fired at elevated temperatures (800-950 °C) to densify the

superconductive elements.

Brief Description of the Drawing

2O Fig. 1 is a schematic showing the position of the lead assembly

related to the infrared detector focal plane array and the data acquisition

electronics.

25

Detailed Description of the Invention

A unique current lead assembly is described in detail below. The lead

assembly is generally applicable to any cryogenic system in which heat

transfer across the lead is to be minimized.
LAR 14964-1-CU -4- PATENT APPLICATION

This invention involves the use of thick film elements of high

temperature superconductors deposited onto low thermal conductivity

substrates as electrically conductive, thermally insulating leads for low

current applications. This lead assembly consists of a series of

5 superconductive elements screen-printed onto ceramic substrates and

electrically contacted at each end. Each high temperature superconductive

thick film element must exhibit low thermal conductivity, be electrically

conductive at cryogenic temperatures, and be able to transport at least 1 pA

of current. Suitable high temperature superconductors include YBa2Cu3OT_"

10 with a superconductive transition temperature of 93K, compounds in the

Bi-Sr-Ca-Cu-O system with transitions from 110K to 80K, and compounds in

the TI-Ca-Ba-Cu-O system with transitions from 125K to 8OK. The

requirements for the substrate include low thermal conductivity at cryogenic

temperatures and chemical compatibility with the superconductive compound

15 during thermal treatment (800-950°C maximum temperature). Suitable

substrate materials include yttria-stabilized zirconia (YSZ) and fused silica.

High temperature superconductive compounds exhibit low thermal

conductivity in the cryogenic region. Although ceramic materials are

typically insulators at high temperatures, some ceramics exhibit high thermal

20 conductivity as the temperature approaches absolute zero (e.g., alumina and

magnesia). The combination of low thermal conductivity and high electrical

conductivity exhibited by the high temperature superconductors make them

ideal candidates for thermally insulating electrical connections designed to

reduce heat loads on cryogenic systems.

25 The thermal conductivity of the substrate material onto which the

superconductive elements are deposited is also critical in determining the

total heat load on the system. Substrate thicknesses on the order of O. 15 to

0.25 mm or greater may be required for mechanical durability, resulting in a
LAR 14964-1-CU -5- PATENT APPLICATION

much greater thermal load contribution for the substrate due to the increased

area as compared to the superconductor. Materials such as YSZ and fused

silica possess extremely low thermal conductivities, making them ideal

candidates for use with thick film superconductors in this application.

5 To illustrate the thermal savings estimated for a cryogenic system

employing high temperature superconductive electrical lead assemblies, a

thermal model was calculated. Manganin (84% Cu, 12% Ni, and 4% Mn)

wires, which are currently employed in several aerospace systems, were

used as a reference. Additionally, preliminary design criteria for SAFIRE

10 (Spectroscopy of the Atmosphere using Far Infrared Emissions) were used to

set the device parameters including the temperature gradient (4-80K), wire

diameter (40AWG), and connection length (15 cm). The heat flow

calculated for a single manganin wire over the temperature gradient was

16.6mW. As an example, a superconductive lead assembly using the

15 Bi2Sr2Ca 2 CuzO x (Tc = 110K) superconductive compound 16 screen-printed

onto a fused silica substrate 14 exhibited a heat flow 20 of only 3.4mW or

approximately 20% of the heat load induced by the manganin connection.

The model showed that the use of superconductive leads over the currently

employed manganin wires in cryogenic detector systems such as SAFIRE

20 would decrease the heat load placed on the dewar system 12 by the

electronic systems 18 by 60-80%, depending on the

superconductor/substrate combination used.

Although our invention has been illustrated and described with

reference to the preferred embodiment thereof, we wish to have it

25 understood that it is in no way limited to the details of such embodiment,

but is capable of numerous modifications for many mechanisms, and is

capable of numerous modifications within the scope of the appended claims.
LAR 14964-1 -CU PATENT APPLICATION

ELECTRICALLY CONDUCTIVE, THERMALLY INSULATING

CURRENT LEADS

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Abstract of the Disclosure

An electrically conductive, thermally insulating current lead assembly

has been developed for cryogenic systems. This lead assembly consists of

thick film elements of high temperature superconductive materials deposited

onto a low thermal conductivity substrate. The superconductor elements

provide current transport but minimize heat transfer. The substrate provides

the mechanical durability necessary for cryogenic and other environments.
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Fig. 1