Peggy A. Cornell, Frances I. Hurwitz, and David L. Ellis

Glenn Research Center, Cleveland, Ohio

Paul C. Schmitz

Vantage Partners, LLC, Brook Park, Ohio

Investigation of Insulation Materials for Future

Radioisotope Power Systems

NASA/TM22013-216562

December 2013

AIAA 2013 3727
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Peggy A. Cornell, Frances I. Hurwitz, and David L. Ellis

Glenn Research Center, Cleveland, Ohio

Paul C. Schmitz

Vantage Partners, LLC, Brook Park, Ohio

Investigation of Insulation Materials for Future

Radioisotope Power Systems

NASA/TM22013-216562

December 2013

AIAA 2013 3727

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

11th International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

San Jose, California, July 14±17, 2013
Acknowledgments

This work is funded through the NASA Science Mission Directorate. The authors wish to acknowledge the people who made this

effort possible including Katie Shaw and Justin Elchert for providing considerable analytical and modeling guidance; Haiquan

Guo for her materials expertise and composite fabrication; Adrienne Veverka, Jesse Bierer, and Grant Feichter for their technical

dedication to our experimental testing; Don Jaworske for enabling our emissivity testing; and Terry O¶Malley, Lee Mason, and

Duane Beach for supporting this effort under the Radioisotope Power Systems (RPS) program.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076±1320

National Technical Information Service

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identi¿ cation

only. Their usage does not constitute an of¿ cial endorsement,

either expressed or implied, by the National Aeronautics and

Space Administration.

Level of Review: This material has been technically reviewed by technical management.

This report contains preliminary ¿ ndings,

subject to revision as analysis proceeds.
NASA/TM22013-216562 1

Investigation of Insulation Materials for Future

Radioisotope Power Systems

Peggy A. Cornell, Frances I. Hurwitz, and David L. Ellis

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Paul C. Schmitz

Vantage Partners, LLC

Brook Park, Ohio 44142

Abstract

NASA¶s Radioisotope Power Systems (RPS) Technology Advancement Project is developing nextgeneration high-temperature insulation materials that directly benefit thermal management and improve

performance of RPS for future science missions. Preliminary studies on the use of multilayer insulation

(MLI) for Stirling convertors used on the Advanced Stirling Radioisotope Generator (ASRG) have

shown the potential benefits of MLI for space vacuum applications in reducing generator size and

increasing specific power (W/kg) as compared to the baseline Microtherm HT (Microtherm, Inc.)

insulation. Further studies are currently being conducted at NASA Glenn Research Center on candidate

MLI foils and aerogel composite spacers. This paper presents the method of testing of foils and spacers

and experimental results to date.

Nomenclature

Al2O3 aluminum oxide (alumina)

ASC Advanced Stirling Convertor

ASRG Advanced Stirling Radioisotope Generator

atm atmosphere

DAQ data acquisition

Fe2O3 iron oxide

GPHS General Purpose Heat Source

ID inside diameter

MLI multilayer insulation

MPS Moulded Pipe Section

Na2O3 sodium oxide

OD outside diameter

RPS Radioisotope Power Systems

SiO2 silicon dioxide (silica)

TEOS tetraethyl orthosilicate

ZrO2 zirconium dioxide (zirconia)

Introduction

The Advanced Stirling Radioisotope Generator (ASRG) is under development by the Department of

Energy and Lockheed Martin Space Systems Company. It comprises two Advanced Stirling Convertors

(ASCs), which are being developed by an integrated team of Sunpower, Inc., and the NASA Glenn

Research Center. The ASC development, funded by NASA¶s Science Mission Directorate, started as a

technology development effort in 2003 and has since evolved through progressive convertor builds and
NASA/TM22013-216562 2

successful testing to demonstrate high conversion efficiency, low mass, and capability to meet long-life

Radioisotope Power Systems (RPS) requirements. The objective of the ASRG¶s insulation package is

twofold: to minimize the loss of thermal energy from the Plutonium-238 General Purpose Heat Source

(GPHS) to the environment and to act as a passive safety device in the event of single convertor stoppage.

In the latter case, the insulation would permanently shrink to allow a sufficient heat leak, ensuring that the

iridium cladding in the GPHS never exceeds its design temperature.

Preliminary studies have been done on the use of multilayer insulation (MLI) in the ASRG. These

studies show the potential benefits of MLI for space vacuum applications in reducing generator size and

increasing specific power (W/kg) as compared to the baseline Microtherm HT (Microtherm, Inc.)

insulation. Thermal losses from the Microtherm HT insulation currently used in the ASRG are estimated

to exceed 25 percent (62.3 Wth on the ground (1 atm Argon)) and 15 percent (37.8 Wth) in a vacuum of

the total 244.0 Wth generated by the GPHS (Ref. 1). Studies show these losses can be reduced through

the substitution of MLI for the Microtherm HT currently used in the ASRG. A schematic of the ASRG

and its insulation package is shown in Figure 1.

The MLI consists of several layers of closely spaced, highly reflective shields, or foils, which are

placed perpendicular to the heat flow direction. These low emissivity foils provide a thermal high

resistance to radiative heat transfer between layers. To avoid direct contact between the foils, and

therefore heat conduction between the sheets, low-conductivity and nonmetallic spacers are used. A

sketch of a typical MLI is shown in Figure 2.

The theoretical thermal conductivity of MLI (using nickel foils and ZrO2 spacers) has been found to

be approximately two orders of magnitude less than that of fibrous bulk insulations at 1000 °C in a

vacuum environment (Ref. 2). To help better quantify the characteristics and potentially increase the

temperature range of state-of-the-art MLI, a development effort is underway to expand our experimental

test rig capabilities; identify, test, and analyze candidate materials; and optimize data collection and

thermal modeling at Glenn. This effort is key in that insulation materials directly benefit thermal

management and could improve performance of RPS for future science missions.

Figure 1.2Advanced Stirling Radioisotope Generator (ASRG) featuring insulation package.

Figure 2.2Sketch of a typical multilayer insulation (MLI).

Foils are denoted by a solid line and spacers by a

dashed line.
NASA/TM22013-216562 3

Experimental Testing

Insulation Test Rig and Test Methodology

The design goal of the experimental test rig was to establish one-dimensional heat flow through an

insulation sample to compare MLI candidates including foil/aerogel hybrid insulations for future RPS. A

secondary goal was to configure the rig for relatively convenient interchangeability of the test samples

and addition of instrumentation as needed. The MLI rig and test station can be seen in Figure 3.

The test rig consisted of a heat source and a vertical MLI stack surrounded by a Microtherm

Moulded Pipe Section (MPS) holder. The heat source was a HeatWave Laboratory Model #102104

(HeatWave Labs, Inc.) heater enclosed in a custom housing. The inside diameter (ID) of the MPS

holder was made to slip over the outside diameter (OD) of the custom heater housing and enclose the

heater and MLI stack. The MPS holder insulated the MLI stack and the heater, minimizing heat loss in

the radial direction, as well as optically enclosing the test sample. The MPS holder was made of a

microporous silica insulation with a nominally 102 mm (4 in.) ID and 114 mm (4.5 in.) OD that was

wrapped in E-Glass cloth for easy handling. Its rated thermal conductivity in air was 0.034 W/mK at a

mean temperature of 800 °C, the highest temperature tested by the manufacturer (Ref. 3). The heater,

which was made of tungsten, has a three-layer heat shield on the back side to minimize heat loss in the

downward direction. Its heating elements were covered by a molybdenum plate that acted as a heat

spreader. The heater was rated to operate at 1100 °C continuously in vacuum. Installation of the MPS

holder around the tungsten heater is shown in Figure 4. The vertical MLI test sample stacks were

constructed by alternating layers of metal foils and spacers. The spacers were ceramic fabric or paper with

or without aerogel impregnation.

Figure 3.2Multilayer insulation (MLI) rig and test station.
NASA/TM22013-216562 4

Figure 4.2Moulded Pipe Section (MPS) holder installed around tungsten heater in multilayer

insulation (MLI) rig. Image on left shows MPS holder being installed around the heater. Image on

right shows MPS holder in its testing configuration.

Figure 5.2Heat flux sensor installed on side of Microtherm MPS holder.

An RdF Model 27036 3 (RdF Corporation) heat flux sensor with integrated Type T thermocouple

was adhered to the top foil in the MLI stack using OMEGABOND 200 (OMEGA Engineering, Inc.)

high thermal conductivity two-part epoxy. The temperature and heat flux of the outside of the holder and

backside of the heater were measured using RdF Models 27036 1 and 27036 3 heat flux sensors,

respectively. The heat flux sensors were limited to a maximum temperature of 260 °C. The bottom

heat flux sensor was consistently the hottest and therefore limited the maximum reference temperature of

the test rig to approximately 625 °C. Figure 5 shows the heat flux sensor affixed to the outside of the

MPS holder.

The entire assembly was mounted in a stainless steel bell jar. The bell jar was evacuated using a

Leybold Model TMP 1000 C (Oerlikon Leybold Vacuum) turbopump to a pressure less than 8 10±6

torr

to ensure that convection did not play a significant role in the heat transfer.

The top of the bell jar was water cooled to maintain a constant sink temperature of 20 °C. The

uncooled sides of the bell jar that were outside of the view factor of the top foil heat up minimally

during testing. The maximum temperature of these sidewalls was measured by a combination of

reference thermocouples and infrared imaging of the exterior and found to be less than 35 °C at the

maximum reference temperature.
NASA/TM22013-216562 5

An Ohio Semitronics PC8 002 01D (Ohio Semitronics, Inc.) direct current watt transducer and a

variable frequency alternating current watt transducer measured the electrical power supplied to the

heater and permitted a power balance calculation and determination of the potential heat losses

through thermal shorts and other means not accounted for by the three heat flux sensors, for example,

loss through the copper leads for the heater power and loss through the thermocouples.

The signals from the sensors were collected using up to four ICP DAS Model I 7011 (ICP DAS

USA) data acquisition (DAQ) modules. The modules converted the analog signals to digital signals and

sent the data over a local area network to the DAQ computer. A custom-written DAQ program was

utilized to record data every 60 sec. This interval was sufficient to give adequate statistical data on the

measurements.

During testing, the system was considered to be at steady state and test conditions at thermal

equilibrium when the test environment pressure was less than 8 10±6

torr and the temperature of the top

foil remained constant within 3 °C for 1 hr with no definite trend. The top foil thermocouple was used to

determine thermal equilibrium since it was observed to be the last of the test environment thermocouples

to reach thermal equilibrium. The thermocouple that defined reference temperature, or test rig

temperature, during testing was located on the hottest (bottom) foil.

Candidate Materials

The ASC has an upper temperature limit of approximately 850 °C based on material life

requirements. The ASCs in the ASRG currently operate below this maximum temperature due to

improved overall power output (reduced heat losses through the insulation) at 760 °C. Using the ASC

material limits of a nickel heat acceptor at 850 °C, the temperature of the inner surface of the

insulation is typically 100 °C greater, or 950 °C. The candidate MLI foils and spacers must perform

effectively within this temperature range. The foils and their respective material properties are listed in

Table I.

TABLE I.2CANDIDATE FOILS FOR MLI TESTING

Metal Melting point Emissivity/temperature,a

°C

Note Measured emissivity/temperature,

°C

Tantalum 3000 °C 0.14 to 0.30/727 to 2930 Unoxidized 0.0922/25

Nickel-niobium 1170 °C

(eutectic)2

Ni = 0.05 to 0.19/38 to 1000

Nb = 0.12/1000

Unoxidized2only

first layer Nb

Nickel = 0.0258/25

Niobium = 0.052/25

Molybdenum 2617 °C 0.06 to 0.18/38 to 1093 unoxidized TBD

Gold 1063 °C 0.02 to 0.03/38 to 1093 Polished TBD

a

Per Table of Total Emissivity, www.omega.com .

Emissivity testing was performed at room temperature (25 °C) on the tantalum and nickel and

niobium samples to verify their emissivity values, which can vary due to surface finish and oxidation.

The measurements were obtained using an Infrared Reflectometer Model DB100 from Gier Dunkle

Instruments, Inc. Reflectivity was measured and recorded at various locations on each sample, allowing

to average the emissivity for each foil. The reflectometer was calibrated using a gold and black standard

with reflectively values of 0.976 and 0.089, respectively. These measured emissivity values are also

shown in Table I.

The candidate spacer materials under consideration, which have advanced insulating properties

compared to zirconia spacers that have been used in the past, include APA 2 alumina paper (Zircar

Ceramics, Inc.), Astroquartz silica fiber fabric (JPS Composite Materials), Fiberfrax 972AH alumina

paper (Unifrax, LLC) and Saffil paper (Saffil, Ltd.). As shown in Figure 6, spectral properties of the

candidate spacer materials are a primary factor in material selection for testing. Further consideration

includes commercial availability and physical characteristics such as thickness and density. The physical

and thermal characteristics, as well as composition, of the current candidate spacers are shown in Table II

and their respective microstructures are shown in Figures 7 to 10.
NASA/TM22013-216562 6

Figure 6.2Spectral properties of APA 2 paper with aerogel.

TABLE II. CANDIDATE SPACERS FOR MULTILAYER INSULATION (MLI) TESTINGa

Ceramic reinforcement Thickness,

mm

Density,

g/cc

Upper use

temperature

Composition,

percent

APA 2 paper 1.25 0.11 1650 °C 86 Al2O3, 10 SiO2, and four other oxides

Fiberfrax 972AH 0.8 0.192 1176 °C 47-52 Al2O3, 48 to 53 SiO2, <0.5 Na2O3, and <0.5 Fe2O3

Astroquartz (503 plain weave

without binder) 0.11 2.2 1070 °C 99.99 SiO2

Saffil paper 0.5 and 1.0 0.5 to 0.7 1600 °C 95 to 97 Al2O3, 3.0 to 5.0 SiO2, and <0.5 trace elements

aAs per manufacturer data.

Figure 7.2APA 2 microstructure. Figure 8.2Fiberfrax 972AH microstructure.

Figure 9.2Astroquartz fabric microstructure. Figure 10.2Saffil paper microstructure.
NASA/TM22013-216562 7

Figure 11.2Microstructure of the as-supercritically dried aluminosilicate aerogel before and after

20 min of exposure at 1100 °C. Image on left shows before drying (surface area is 415 m2/g).

Image on right shows after drying (surface area is 266 m2/g).

Characterization of insulation materials has substantially progressed and material scientists at Glenn

Research Center are presently studying aerogel materials with increased temperature capability and lower

densities compared with state-of-the- art polymer-based insulating materials. Composites such as those

currently under consideration impregnated with aerogel offer potential as spacers between MLI foils.

Aerogels are mesoporous structures with interconnecting pores and high surface areas; the pore

walls comprise thin struts that provide a lengthy and tortuous path limiting solid conduction from foil to

foil. In an application with any gaseous environment, gas convection is limited by the small-scale pore

structure; in both atmosphere and vacuum, extinction of radiation heat transfer takes place by

absorption (Ref. 4) and is wavelength and temperature dependent. The effective thermal conductivity

also is expected to be influenced by the emissivity of the adjacent foils (Ref. 5).

Although numerous studies have been conducted on silica aerogels, aerogels can be synthesized

using a wide variety of elements, and their upper use temperature is limited by their chemical

composition. Hence, polymeric aerogels are limited by the decomposition temperature of the particular

polymer (> 400 °C for some polyimides). Silica aerogels sinter above 700 °C, losing their high surface

area and becoming less insulating. NASA Glenn Research Center has developed aluminosilicate

aerogels that maintain high surface areas and mesoporous structures to temperatures of 1100 to 1200 °C.

They are produced through the reaction of boehmite and tetraethyl orthosilicate (TEOS), in which the

surface area and pore structure can be controlled by the choice of boehmite precursor and the

synthesis parameters (Refs. 6 and 7). The aluminosilicate aerogels in the present study utilize an

aluminum to silicon ratio of 3 to 1, the ratio found in mullite.

Microstructures of the as-supercritically dried aerogel, and the same aerogel after 20 min of exposure

at 1100 °C, are shown in Figure 11. The surface area decreases from 415 to 266 m2/g on exposure, and

the pore volume decreases from 0.67 to 0.44 cm3/g. A mesoporous structure is maintained, with the

average pore diameter changing from 7.44 to 8.15 nm.

Because of the fragile nature of the aluminosilicate aerogel monoliths, Glenn has developed

techniques for incorporating them into composites, which can be reinforced using ceramic fabrics,

papers, or felts. The work presented here focuses on the use of APA 2 alumina paper. Composites also

have been fabricated using Astroquartz silica fiber fabric and Fiberfrax 972AH alumina paper. The

composites remain flexible (Fig. 12), which would be advantageous in forming an MLI around the

GPHS. The APA 2 composite has a density of 0.15 g/cm3, as compared with 0.3428 g/cm3for

Microtherm HT. The aluminosilicate aerogel bonds well to the alumina fibers (Fig. 13), averting the

spalling of aerogel particles seen in some commercial materials.
NASA/TM22013-216562 8

Figure 12.2Aerogel composite reinforced with

APA 2 alumina paper.

Figure 13.2Microstructure of APA 2 with aerogel

composite showing good wetting of aerogel to

fibers.

Modeling

A Thermal Desktop (Cullimore & Ring Technologies, Inc.) model of the insulation sample and

surrounding environment was created to estimate the thermal performance of the test foils (Fig. 14). The

model included the following test rig components: foils with temperature dependent properties, spacers,

heat source, and test chamber (approximated as a single isothermal boundary).

Currently, poor correlation exists between the test results and the model, therefore, modification of

both the test rig and the model are underway. Heat losses, such as edge effects (radiation from the edges

of the MLI stack), were present primarily due to the small test sample size of 102 mm (4 in.) in

diameter.

Several heat transfer paths and mechanisms can strongly influence the model predictions and

associated uncertainty and error propagation. One testing attribute being investigated is to experimentally

determine the effect of spacer thickness on MLI performance. Thick or multiple spacer layers allow for

the placement of thermocouples between foil layers with minimum physical perturbation of the foils.

Test runs are also underway with thermocouples at only the hot and cold sides of the stack as well as

each layer of the stack to evaluate the influence of radial heat transfer along the thermocouple leads.

Once the heat transfer paths are identified, the MLI test rig will be redesigned in an attempt to mitigate

heat leaks, improve data fidelity, and reduce associated error. An exploded view of the current test

setup, generated through modeling tools, is shown in Figure 15 with respective callouts in Table III.
NASA/TM22013-216562 9

Figure 14.2Multilayer insulation (MLI) model results using Thermal Desktop.

Figure 15.2Exploded view of the primary test section components and multilayer insulation (MLI) test sample.

TABLE III.2LOCATIONS OF THE PRIMARY

TEST SECTION COMPONENTS AND MULTILAYER

INSULATION (MLI) TEST SAMPLE AS SHOWN IN FIGURE 15

Callout Description

1 Heater assembly

2 Moulded Pipe Section (MPS) holder

3 Foil

4 Spacer
NASA/TM22013-216562 10

Experimental Results

Tantalum Foils With APA 2 Alumina Paper

The first MLI run consisted of tantalum foils and APA 2 alumina paper spacers. The thickness of

the MLI stack was approximately 13 mm (0.5 in.). The temperature within the MLI test rig was increased

as shown in Figure 16 and held until the system was considered to be at steady state and test conditions

at thermal equilibrium (pressure was less than 8 10 6

torr and temperature of the top foil remained

constant within 3 °C for 1 hr with no definite trend).

As seen in Figure 16, the maximum operating temperature that was reached with this foil and spacer

combination was approximately 600 °C. This was because the heat flux sensors that were utilized were

limited to a maximum operating temperature of 260 °C. The bottom heat flux sensor, located on the heat

shield of the heater, was consistently the hottest and therefore limited the maximum reference

temperature of the rig. In this test, a maximum of 600 °C was reached, as shown in Figure 16.

Figure 16.2A typical time versus temperature plot for the tantalum foils with APA 2 alumina paper. The data at

maximum test rig temperature are circled. Loss of signal occurred between approximately 273 and 325 hr.
NASA/TM22013-216562 11

Figure 17.2Comparison of tantalum foils with APA 2 paper and

tantalum foils with APA 2 paper with aerogel. The blue plot

represents the tantalum/APA 2 temperatures and the red plot

represents the tantalum/APA 2 with aerogel temperatures. The

x-axis displays reference temperatures and the y-axis displays

the coldest foil temperature (top of multilayer insulation (MLI)

stack).

Tantalum Foils With APA 2 Alumina Paper Impregnated With Aluminosilicate Aerogel

The subsequent test run consisted of tantalum foils and APA 2 alumina paper spacers impregnated

with aluminosilicate aerogel. The test rig conditions remained the same as the previous test, and the

maximum temperature reached was approximately 625 °C. Test results of the tantalum foils with and

without aerogel-impregnated spacers were similar. The exception was that the temperature of the top

(coldest) tantalum foil within the stack was approximately 40 °C less at a test rig temperature of 600 °C

for the aerogel-impregnated spacers (Fig. 17). The data captured in Glenn¶s MLI test rig indicated that

the composite spacer with aluminosilicate aerogel reduced heat transfer compared to the composite

spacer without aerogel. Further testing and analysis is required to confirm repeatability of these

preliminary results and to determine the basis for the temperature difference.

Conclusion

Insulation materials have a significant effect on decreasing the thermal losses and increasing

specific power (W/kg) of Radioisotope Power Systems (RPS). To help better quantify the characteristics

and potentially increase the temperature range of state-of-the-art multilayer insulation (MLI), a

development effort is underway to expand our experimental test rig capabilities; identify, test and

analyze candidate materials; and optimize data collection and thermal modeling at NASA Glenn

Research Center. Heat flow through insulation samples was achieved in the newly developed MLI test

rig and thermal measurement methods and predictions are progressing. Characterization of insulation

materials has substantially advanced and aerogel materials with increased operating temperature and

decreased density are being studied for use as composite spacers in MLI. Future testing will consider

foil/spacer combinations that are suitable in high temperatures, can remain flexible to form an effective

MLI around the General Purpose Heat Source (GPHS) of the Advanced Stirling Radioisotope Generator

(ASRG), and are thin and low in density to potentially reduce generator size and increase specific power

as compared to the baseline Microtherm HT insulation.
NASA/TM22013-216562 12

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