NASA/TM--2001-210245 AIAA-2000-3077

Organic Materials Ionizing Radiation

Susceptibility for the Outer Planet/Solar

Probe Radioisotope Power Source

Eric L. Golliher, Stephen V. Pepper and James K. Sutter

Glenn Research Center, Cleveland, Ohio

Prepared for the

35th Intersociety Energy Conversion Engineering Conference

sponsored by the American Institute of Aeronautics and Astronautics

Las Vegas, Nevada, July 24-28, 2000

National Aeronautics and

Space Administration

Glenn Research Center

January 2001

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ORGANIC MATERIALS IONIZING RADIATION SUSCEPTIBILITY FOR THE OUTER PLANET/SOLAR

PROBE RADIOISOTOPE POWER SOURCE

E.L. Golliher, S.V. Pepper and J.K. Sutter

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

ABSTRACT

The Department of Energy is considering the

current Stirling Technology Corporation's 55 We

Stirling Technology Demonstration Convertor as

a baseline option for an advanced radioisotope

power source for the Outer Planets/Solar Probe

project of Jet Propulsion Laboratory and other

missions. However, since the Technology

Demonstration Convertor contains organic

materials chosen without any special

consideration of flight readiness, and without any

consideration of the extremely high radiation

environment of Europa, a preliminary

investigation was performed to address the

radiation susceptibility of the current organic

materials used in the Technology Demonstration

Convertor.

This report documents the results of the

investigation. The results of the investigation

show that candidate replacement materials have

been identified to be acceptable in the harsh

Europa radiation environment.

INTRODUCTION

The mission to Europa imposes a very severe

ionizing radiation environment to the 55-We

Technology Demonstration Convertor (TDC).

The radiation from the isotope power source is

insignificant when compared to radiation from

the environment for 30 days in the Europa orbit.

The alternator magnets and organic materials

capabilities to meet these requirements were

initially unknown. As it turns out, the magnets

meet the requirements, and data from literature

shows practically no degradation at the expected

dose. However, the 55-We TDC's built to date

contain some organic materials which will not survive

the harsh Europa environment. A detailed

assessment has shown that acceptable substitute

materials exist for the organics. An effort was made

to choose substitute materials close to the same

family and function as the original. These were

evaluated by Stirling Technology Corporation (STC)

and thought to be acceptable for future TDC's.

Should unexpected problems arise with the

substitutes, and a second substitute must be made,

there is a sufficient list of candidate radiation tolerant

organic materials from which to choose.

All of the organic materials are sealed inside the

pressure vessel and cannot escape to contaminate

the rest of the spacecraft. The pressure of the helium

inside the vessel should also act to inhibit outgassing

from the organic materials. Since the coldest part of

the Convertor is the pressure vessel, any outgassed

organic material would deposit on this wall and

should in no way affect the function of the convertor.

NASA Glenn through consultation with members of

the Jet Propulsion Laboratory (JPL) staff, with

Lockheed Martin in Valley Forge, PA, the STC, and

commercial material vendors has carried out this

evaluation. Additional coordination with the JPL

Europa spacecraft materials engineers is expected

as the Europa Orbiter spacecraft design progresses.

Further confirmation of functional suitability of the

new materials will come in the following months as

two 55-We TDC's (S/N 005 and 006) will be built with

these new materials to the maximum extent possible.

These TDC's are scheduled for delivery and testing

at NASA Glenn Research Center (GRC) in mid2000. Revised organic materials and magnets will

survive the Europa radiation environment with

margin.

NASA/TM--2001-210245 1
DESCRIPTION OF ISSUES

The two key issues are the radiation tolerance of

organic materials inside the Stirling convertor

and the radiation tolerance of the magnets. The

approach taken was to 1) collect information

from a variety of sources including a literature

search, vendor inquiries, consultation with

radiation, space materials, and lubrication

experts; 2) estimate expected total ionizing dose

inside the convertor; and 3) choose alternate

materials for the flight article as warranted.

ORGANIC MATERIALS- REQUIREMENTS

The total ionizing dose from the natural space

environment, Europa Orbiter Mission 01, is

estimated to be 10 Mrad (Si) with a 50 - mil

aluminum housing for shielding and 4 Mrad with

a 100 - mil shielding.

Old

Material

Xylan

1620-

560 Blue

Polyimide

(Kapton)

Dolph

1057

Epoxy

Loctite

420

Scotchweld

3M heat

shrink

tubing

Kapton

Location

Piston OD

Wire

Insulation

Coils

Stator

Lamination

Magnets,

Stator

Wire

connector

Edge of

Purpose

Bearing

Coating

Wire

Insulation

Bonding

Agent

Wicking

Adhesive

Epoxy

Adhesive

Electrical

insulation

Electrical

New

Material

/Mission

Rating

Xylan

1054//

Acceptable as is

/__

EC2216

/_/

Loctite

4014/2

EC2216

//

Raychem

PVDF//

Permacel

tape

(KA00)

Garolite

G-10

TFE

Coating

TFE

Bumper

Stator

Coils,

Laminates

Wires for

Feed

Through

End of

Piston

Wire

Electrical

Insulation

Electrical

Insulation

Power

Piston

Bumper

or 3M

Y966//

Acceptable as is

//

Raychem

PVDF//

PVDF or

EFTE

machined

Mission Rating

1= Current Materials acceptable

2= Acceptable, requires dose calculations

3= Acceptable, with dose calculations and test data

4= Questionable, conclusive proof required

5= Unacceptable

DESCRIPTION OF ORGANICS

Xylan 1620-560 blue is a Teflon-based lubricant

coating. Teflon cannot survive in the Europa

radiation environment. It can be replaced by Xylan

1054, which is a MoS2-based coating. STC has

experience with Xylan coatings, so staying within

same family of lubricants is low-risk. The highpressure dry helium environment will negate any

worries of MoS 2 humidity and water vulnerability.

Xylan is used as a lubricant on the outer diameter of

the piston. Although there is no contact during

nominal operation, the Xylan is needed to facilitate

components/parts installation in the build-up process

and to ensure no metal-to-metal contact during nonoperating and launch vehicle ascent conditions.

There was an extensive investigation of a

replacement for the original Xylan-blue, since the

original contained Teflon, a material which cannot

withstand the 4 Mrad expected dose. Teflon is

notably very poor in even low-level ionizing radiation

environments. A type of Xylan that uses

Molybdenum disulfide as the active lubricant

component was selected as a replacement. Since

NASA/TM--2001-210245 2
the components of this type of Xylan are very

radiation resistant, it follows that the Xylan itself

is rad-hard. No radiation testing data is available

on the chosen Xylan.

The interior of the 55 W Stirling Convertor that is

proposed for the Europa mission will be

subjected to an ionizing radiation dose of about 4

Mrads (Si) during the mission. The lubricant on

the Convertor's piston must survive this dose.

The present lubricant is not a satisfactory choice

for this service condition and a substitute must

be found. A substitute is recommended in this

memo.

The present lubricant is Xylan 1620-560 (blue)

manufactured by Whitford Corporation, West

Chester, PA. The active lubricant in this product

is PTFE (polytetrafluoroethylene, Teflon) and

this PTFE is, according to Whitford held in a

polyamide-imide resin binder. The piston

lubricant is present to protect the piston and

cylinder wall during handling, assembly and

initial operation (run-in). It is also expected to be

protective during the vibration of launch and

other initial vehicle maneuvers. It is not expected

to be operational past this point, since the

Convertor is designed for a piston-cylinder wall

clearance of .001" during normal operation,

during which there is no contact of the piston

with the cylinder wall. Although these service

requirements on the piston lubricant appear to

be very mild, the radiation environment is very

severe on the PTFE and results in undesirable

emission of gaseous fluorocarbon breakdown

products and embrittlement of the PTFE. Such

embrittlement may result in the formation of a

wear particle that could be caught between the

piston and cylinder wall and jam the Convertor, a

catastrophic event. Thus a lubricant such as the

one presently used should be avoided and a

recommendation should be made for a substitute

that is not as vulnerable to ionizing radiation.

This recommendation is motivated by the wish to

retain the positive experience that STC has had

with the Xylan product they have used up to the

present. The 1620-560 is really one of a family of

products, all going by the generic name of Xylan,

but formulated with different active lubricants in

the binder. Xylan 1054 is identified here as a

radiation-resistant substitute for the present

lubricant. This product uses MoS 2 (Molybdenum

Disulfide) powder lubricant formulated in the

polyamide-imide resin binder. MoS 2 is an inorganic

mineral that is invulnerable to ionizing radiation. It is

also noted that the helium environment in the

Convertor is an ideal environment for the tribological

application of MoS 2, which does not perform well in

either moisture or oxygen. The binder, however, is

still an organic material that merits some comment.

According to Whitford, the 500°F cure that the neat

polyamide-imide resin undergoes during its

application on the workpiece converts it entirely to

polyimide. Polyimide is extremely resistant to

ionizing radiation. Polyimide retains its properties up

to a dose of 100 Mrads'. In addition, the polyamideimide is a good tribological material in its own right

and is marketed for this purpose by BPAmoco under

the name of Torlon. Quoting from the BPAmoco

website:

"Torton_ polyamide-imide provides exceptional

strength at high temperatures, excellent resistance to

creep and wear plus the benefits of injection

mouldability. The material has unmatched frictionwear characteristics in the most severe service

environments. Typical applications for Torlon

polyamide-imide include chemical processing

equipment, bearings and seals, automotive/aviation

components, electrical/electronic devices, rotating

machinery, oil drilling equipment, and industrial

equipment."*

The BPAmoco design handbook also indicates

radiation resistance up to at least 10 Mrads

Polyimide (Kapton) wire insulation does not need to

be replaced. The radiation tolerance of Kapton is

excellent, as it shows no degradation until about 400

Mrads. This will be used on the square wire in the

magnet windings, "Polyimide-ML". Kapton is used as

wire insulation. It shows only incipient to mild

damage below 108 Rads according to page 9 of

reference 1. Our dose is less than 4 X 106 Rads as

listed in on page 152 of Reference 2.

Epoxy Resin Paste (with aromatic-type curing agent)

is acceptable to 1E+09 rads per NASA-CR-1781,

Hanks and Hamman, July 1971, as referenced by

JPL.

*01999 BP Amoco p.l.c, and its affiliates

NASA/TM--2001-210245 3
Loctite 420 can be replaced with Loctite 4014.

This is almost same adhesive, but has test data

for radiation hardness. 4014 retains 83% of

shear strength after exposure to 7 Mrads.

Loctite 420 was used as a wicking adhesive to

bond the stator laminations together. The

laminations are a stack of Hiperco 50 plates. The

plates are put in a moist oven to form an oxide

layer. The oxide layer acts as an electrical

insulator. The mover laminations are bolted to

the moving rod. These laminations interact with

the magnetic field and cause a current to flow in

the stationary coils. The viscosity of the uncured

Loctite 420 is around 1 to 5 centipoise (1 cp is

typical for water). The low viscosity is needed so

that the adhesive will "wick" in between the

sandwich layers, after the sandwich is

assembled. The Loctite technical representative

recommended 4014 as a substitute when asked

about a similar adhesive to 420 suitable for a

radiation environment. The technical

representative knew of Loctite application in the

medical field for use in devices that must

undergo radiation sterilization. Loctite 4014 is

also similar in viscosity to 420 and can handle

high temperature (80 °C). The Loctite 4014 data

sheet appears to imply that Loctite has test data

for 4014 showing only small degradation to 7

Mrads. A careful reading of the datasheet

leaves some doubt that data for 4014 is the

actual material data being reported.

Nevertheless, 4014 is the chosen substitute.

NASA materials engineers identified through

contacts with technical representatives at

Loctite, that the 4014 glass transition

temperature of 120 °C is near that of 420.

Tra-Con Tra-Bond F113 was a material initially

considered as a candidate replacement for

Loctite 420. After GRC obtained a copy of the

technical data sheet from Tra-Con, the glass

transition temperature of 45 °C was found to be

too low for our application.

Scotch-weld DP-460 adhesive is used on the

stator magnets. According to 3M, a good

substitute is 3M-EC2216. This is used in the

terrestrial nuclear power industry. Also, JPL

identifies 3M-EC2216 as a typical spacecraft

adhesive good to 1E+08 RADs or better. STC

has used this recently in assembling some stator

magnets. The materials used at three locations

were replaced with EC2216, a material commonly

used in JPL spacecraft in high radiation

environments and one specifically recommended in

by JPL'. The application is a little higher

temperature than used in JPL spacecraft, so some

investigation as to the adequacy for the Stirling

application on the part of a NASA Glenn materials

expert was undertaken.

EC 2216 will replace Dolph CD-1057 Epoxy and

Permabond 919, 920 on the stator coils.

Dolph CD-1057 Epoxy is a two-part epoxy used on

the stator coils. A layer of Dolph CD-1057 is applied

over the coils, which are square copper wires. It

covers the coils, holds the coils together, has a

glassy, varnish-like appearance, and is brittle.

Because there was no radiation testing data from the

manufacturer, the decision to substitute this epoxy

with EC2216 was made. EC2216 is on the JPL

recommended list for adhesives for the Outer

Planet/Solar Probe project. However, specific

radiation testing data is not available. STC also

used another adhesive for this same purpose,

Permabond 919 and 920. Both are being replaced

by EC2216. Permabond also has no data on

radiation hardness. The adhesive is actually called

"Powerbond." It requires a secondary cure at a high

temperature to get the cross-linking for high

temperature capability. EC2216 does not require a

secondary cure for its high radiation tolerant

properties. In the initial efforts with the 2216, STC

found it to be more pliable that the Dolph or

Permabond. STC also tested a coil bonded with

2216 in a furnace at 100 °C. The 2216 strength

appeared to be acceptable to STC. This will be

further evaluated during testing of the TDC's 5/6 at

STC and NASA GRC.

EC2216 will replace Scotch-weld DP-460 on the

stator magnets/laminations bond and stator

lamination/bobbin bond.

Scotchweld DP-460 is used to bond the stator

magnet to the laminations, and to bond the bobbin to

the laminations. The bobbin is a center core of

plastic material around which the coils are wound.

No radiation data was available. The 3M technical

representative was contacted to suggest a

substitute, and she suggested 3M-EC2216. The

shear strength for DP-460 is higher than for 2216 at

the operating temperature (700 psi versus 400 psi

lap shear at 82 °C). Both degrade with temperature.

NASA/TM----2001-210245 4
Dexter Hysol was considered as an alternative to

EC 2216. NASA asked Schaeffer Magnetics to

help with a recommendation for a wicking

adhesive typically used for electric motor coils

for space application. Shaeffer Magnetics is a

company known for supplying electric motors

used in satellites for defense, commercial, and

NASA missions. The company revealed they use

Dexter Hysol EA 9396 on some coils in their

space-rated motors. However, the glass

transition temperature was reported to be only

76 °C by the Dexter Hysol tech rep. This may

not be adequate for our slightly higher

temperature application, where the alternator

magnets may operate near 80 °C. Dexter Hysol

was not recommended for this application.

3M heat shrink tubing used around the wire

connectors. Wire insulation, in general, could be

Crosslinked PVDF and ETFE insulation; both

pass 500 Mrads.

Kapton Tape (KA00) Kapton portion is

acceptable. Several sources are available for

flight-qualified Kapton tape.

Heat shrink tubing currently used will be

replaced with tubing made of Kynar, which is

PVDF, polyvinylidene flouride. PVDF is

identified in reference 1 as incipient to mild

damage to almost 107 Rads. This agrees with

JPL information, which reports PVDF hard to

500 Mrads or better.

Garolite G-10. G-10 is not a problem at 4 Mrad.

This is an epoxy-glass laminate. Reference 1,

page 25, reports that epoxy glass laminate is

capable of withstanding 108 Rads.

TFE Wire insulation for general wire insulation is

not acceptable. JPL recommends replacement

with Crosslinked PVDF or EFTE. Teflon cannot

withstand even low levels of ionizing radiation.

The Teflon must be replaced. PVDF is a

common alternative for wire insulation. PVDF,

polyvinylidene flouride, is identified in reference

1, page 9, as incipient to mild damage to almost

107 Rads. This agrees with JPL information,

which reports PVDF hard to 500 Mrads or better.

TFE as the piston bumper material is

unacceptable. STC will replace TFE with PVDF

or EFTE.

SUMMARY

In the Fall of 1999, several critical issues were

identified as possible "show-stoppers" for the

application of a Stirling Convertor to an advanced

radioisotope power source. This investigation has

shown that a list of suitable organic material

replacements has been found, and that there are no

"show-stoppers." Interested parties should be

aware of upcoming functional testing of additional

baseline 55 We Stirling convertor units. These

additional units will be the first time the replacement

organics are used in a Stirling convertor. No issues

are expected. At the time of this report, the testing

will take place in CY 2000 at NASA Glenn Research

Center and at STC. The overall Stirling technology

plan at Glenn is described in Reference 4.

A note about the nuclear fuel radiation is warranted.

The radiation produced by the nuclear fuel was

considered negligible with respect to damaging the

organics inside the convertor, in comparison to the

environmental radiation. This should be re-visited

when the final SRPS design integration concept is

available. The distance of the organics from the

GPHS modules is one of the key parameters. It is

expected that, in any design, the nuclear fuel

radiation will always be negligible when compared to

the Europa environment radiation.

Some mention of environmental radiation dose

calculations is needed. Interested parties should

probably take a fresh-look at the necessity of dose

calculations when a final SRPS concept becomes

available. There were no computer calculations

performed for dose estimates in the organics for this

investigation. As a guideline, the estimated ionizing

dose in Silicon of 4 Mrads behind 100 mils aluminum

shielding was used. Radiation deposited in the

material Silicon is a typical ionizing radiation

calculation, because most ionizing radiation damage

assessments are interested in survivability of

electronics. It is possible that the dose in the various

organics may be different for the same ionizing

radiation source. The difference from Silicon is

thought to be negligible for the scope of this report.

The flight unit Stirling Convertor pressure vessel is

anticipated to be stainless steel or titanium with wall

thickness near 60 mils. However, there is very high

confidence that the intrinsic radiation shielding

provided by the final design of the pressure vessel,

as is the case with the current design, will exceed

that of an equivalent design provided by 100 mils of

NASA/TM--2001-210245 5
aluminum. Therefore, the total dose of 4 Mrads

is reasonably conservative for this level of

investigation. Two further points to mention

which may decrease conservatism. First,

computer ray-tracing modeling considering the

real geometry and the additional shielding of

various structural metallic and non-organic parts

inside the convertor will probably show a lower

dose than 4 Mrads. Second, consideration of

the radiation spectra near Europa may show less

damaging effects than a Total Ionizing Dose

(TID) of 4 Mrads, although total dose methods

were considered adequate for organics at this

level of investigation.

fresh-look study on this matter is warranted in future

months. JPL, as the overall spacecraft integrator, is

expected to further refine requirements for

spacecraft level and unit level radiation hardness for

all materials and electronics aboard the spacecraft.

.

REFERENCES

Radiation testing was not mentioned in this

report, but is always a consideration for radiation 2.

hardness verification. The decision to perform

radiation testing on individual materials or to

perform radiation testing on the convertor at the 3.

unit level should be made later, in concert with

the spacecraft-level radiation control plan. There

are considerations of radiation design margin

(RDM), which may be different for organics than 4.

for electronics. Although it appears no radiation

testing is needed on the Stirling convertor

organics nor on the convertor at the unit level, a

Hanks, C.L., and Hamman, D.J., Radiation

Effects Desiqn Handbook, Section 3. Electrical

Insulating Materials and Capacitors, NASA CR1787, Prepared for the Radiation Effects

Information Center, Battelle Memorial Institute,

Columbus, Ohio, July 1971.

JPL, Outer Planet/Solar Probe Review, Arcadia,

CA. Veteran's Day, November 12, 1999.

Willis, P.B., JPL, "X-2000 Mission Materials

Radiation Effects," presentation to X-2000

Project, May 26, 1998.

Thieme, L.G., "NASA GRC Technology

Development Project for a Stirling Radioisotope

Power System," 35'" IECEC, Las Vegas, NV,

July 24-28, 2000.

NASA/TM----2001-210245 6

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January 2001 Technical Memorandum

4. TITLE AND SUBTITLE

Organic Materials Ionizing Radiation Susceptibility for the Outer

Planet/Solar Probe Radioisotope Power Source

6. AUTHOR(S)

Eric L. Golliher and Stephen V. Pepper

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135- 3191

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National Aeronautics and Space Administration

Washington, DC 20546-0001

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REPORT NUMBER

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AGENCY REPORT NUMBER

NASA TM--2001-210245

AIAA-2000-3077

11. SUPPLEMENTARY NOTES

Prepared for the 35th Intersociety Energy Conversion Engineering Conference sponsored by the American Institute of

Aeronautics and Astronautics, Las Vegas, Nevada, July 24-28, 2000. Responsible person, Eric L. Golliher, organization

code 5490, 216---433-6575.

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12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The Department of Energy is considering the current Stirling Technology Corporation 55 We Stirling Technology

Demonstration Convertor as a baseline option for an advanced radioisotope power source for the Outer Planets/Solar

Probe project of Jet Propulsion Laboratory and other missions. However, since the Technology Demonstration

Convertor contains organic materials chosen without any special consideration of flight readiness, and without any

consideration of the extremely high radiation environment of Europa, a preliminary investigation was performed to

address the radiation susceptibility of the current organic materials used in the Technology Demonstration Convertor.

This report documents the results of the investigation. The results of the investigation show that candidate replacement

materials have been identified to be acceptable in the harsh Europa radiation environment.

14. SUBJECT TERMS

Stirling; Radiation; Organics; Europa; Survivability; Spacecraft

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