Evaluating Degradation on Thermal Control Materials for ...

Trans. JSASS Aerospace Tech. JapanVol. 8, No. ists27, pp. Pc_11-Pc_16, 2010

Original Paper

Copyright© 2010 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved.

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Evaluating Degradation on Thermal Control Materials for GPM/DPR

By Junichiro ISHIZAWA , Yasutoshi HYAKUSOKU , Hiroyuki SHIMAMURA ,

Yugo KIMOTO and Masahiro KOJIMA

1) Aerospace Research and Development Directorate, Japan Aerospace Exploration Agency, Tsukuba, Japan 2) Space Applications Mission Directorate, Japan Aerospace Exploration Agency, Tsukuba, Japan

(Received July 17th, 2009)

Thermal control materials such as white paints and germanium-coated polyimide film were evaluated with respect to their space environmental tolerance for materials selection of the Dual-frequency Precipitation Radar of the Global Precipitation Measurement satellite (GPM/DPR). Though peeling off and cracking occurred in one paint material during the thermal shock test, other paints showed good tolerance against thermal shock, atomic oxygen, and ultraviolet ray irradiation. Germanium coating on polyimide film was also verified as high atomic oxygen tolerant barrier. Comparing different thickness germanium coatings, it seems that a 1000 angstrom Germanium film has fewer defects and risk of AO undercutting than a 525 angstrom Germanium film.

Key Words: Degradation, GPM/DPR, LEO, Atomic Oxygen, Ultraviolet Ray

1. Introduction

JAXA has been developing the Dual-frequency Precipitation Radar (DPR) of the Global Precipitation Measurement (GPM) satellite (Fig. 1) with the National Institute of Information and Communications Technology (NICT) 1). DPR consists of two radars and has functions for precipitation observation and rainfall parameter estimation 2).

Because GPM’s operation altitude is 407 km, in the low-Earth-orbit (LEO), its thermal control materials are affected by high fluence of atomic oxygen (AO). AO is generated by dissociation of oxygen or ozone molecules by solar ultraviolet rays (UV); AO is very active on materials. It strikes spacecraft surfaces at a relative velocity of 8 km/s and reacts chemically with materials, manifesting itself eventually as erosion or oxidation.

For the purpose of materials selection, we have conducted environmental tests such as AO and ultraviolet ray (UV) and electron beam (EB) irradiation on candidate materials for GPM/DPR. In this paper, we report on the degradation of several white paints and germanium-coated polyimide film.

Fig. 1. Image of the GPM satellite.

2. Experimental Parameters

2.1. Material (a) White paint

Five types of white paints, listed in Table 1, were selected for this evaluation. They are inorganic or silicone based paints which are expected to be AO tolerant.

Table 1. White paint specimen.

White paint specimens were coated onto aluminum alloy plates. Each aluminum plate was weighed to ascertain the paint film’s weight later.

(b) Germanium coated polyimide After a MIDORI-2 (ADEOS-2) malfunction caused by a

discharging event, conductive materials should be used for thermal control of JAXA’s satellites. Germanium (Ge) coated black Kapton XC film (conductive) is selected for substitution of Beta-cloth (nonconductive) for the outermost layer of MLI (multi layer insulation).

Black Kapton films coated with Ge of two different thicknesses ( 525 Å and 1000 Å) were manufactured by

Name Manufacturer Remarks

Z-93 Alion Science & technology Inorganic

AZ-93 AZ technology Inorganic

S13G/L0-1 Alion Science & technology Silicone based

AZ-2000IECW AZ technology Inorganic / Conductive

AZ-2100IECW AZ technology Inorganic / Conductive

DPR

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Sheldhall and used in this study. The manufacturer requires germanium material to be stored in a moisture-free environment for keeping its property. Storage conditions of specimens were shown in table 2.

Table 2. Germanium thickness and storage condition of germanium coated polyimide film.

2.2. Environmental test To evaluate space environmental degradation in specimens,

the following ground irradiation tests were conducted: single AO, UV, and electron beam (EB). Both AO and EB irradiations were performed at the combined space-effects test facility, which is equipped with PSI’s FAST™ AO source 3),EB source, and a deuterium UV source. The AO flux was greater than 1019 atoms·m-2·s. For irradiation of UV in this study, another UV irradiation facility was used, with a conventional xenon UV-ray source that can emit 10 equivalent solar days (ESD) /day at wavelengths between 200 nm and 400 nm for solar UV radiation.

As shown in Table 3, irradiation tests were conducted in two stages, Test 1 was irradiation of EB, AO and UV on white paints and germanium coated black Kapton, test 2 was three different AO fluence irradiation on only germanium coated films.

Table 3. Fluence of irradiation tests.

Before and after the irradiation tests, we measured the thermo-optical properties (e.g., solar absorptance ( S) and normal infrared emittance ( N)) and observed the surfaces by using Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). The mass change of AO-irradiated specimens was measured to evaluate erosion by AO easily.

For paint materials, a thermal-shock test was conducted prior to irradiation tests, in order to evaluate the adhesion of paint on the aluminum plate. The test conditions are shown in Table 4.

Table 4. Thermal shock test conditions on paint material.

2.3. Resistivity measurement For analysis of the charging level of GPM/DPR, volume

resistivity and surface resistivity were measured. Resistivity measurements were done in the atmosphere and in vacuum. The test in vacuum was conducted to avoid the apparent decrease in resistivity due to moisture adsorption (which is limited in orbit). The measurement meets the JIS C 2141 (Testing methods of ceramic insulators for electrical and electronic applications). Relative humidity was between 52 and 61 % RH in the atmosphere test, and the pressure level was 10-3 Pa in the vacuum test.

3. Results and Discussion

3.1. White paint During the thermal-shock test, peel-off and crack formation

on AZ-2100 paint were observed before 27 thermal cycles (Fig. 2). No defect such as peeling off, cracks, etc. was detected in any other paints after 500 cycles. As a result, irradiation tests were conducted on four types of white paint (without AZ-2100) .

Fig. 2. Crack on AZ-2100 after thermal shock test.

Fig. 3 and Fig. 4 depict changes in solar absorptance and normal infrared emittance of white paints in test 1. A small amount of increasing tendency is shown in several paints’ solar absorption by UV irradiation. Fig. 5 portrays the mass change per AO-irradiated area in irradiated specimens. S-13G, only silicone based paint, was the most stable paint against AO. Silicone is an exceptionally AO-tolerant polymeric material for forming a passivating layer on the surface through oxidization with AO 4).

However, the mass loss caused by AO was sufficiently small in other paints for the GPM/DPR environment.

Ge thickness ( Å )

Storage conditions Experiments

in air EB, AO, UVirradiation

525in nitrogen gas

purged

1000 in vacuum

AO irradiation,XPS, SEM

Environment factor Test 1 Test 2

Electron beam 20 kGy

200 kV,2.0 mA -

Atomic oxygen 3 1020 /cm2 3, 6, 9 1020 /cm2

Ultraviolet rays 134 ESD -

Temperature range ( ) - 100 ~ + 80

Number of cycles 500

Atmosphere In air

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Z 93P

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S 13G

AZ 2000

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Z 93P

AZ 93

S 13G

AZ 2000

Fig. 3. Solar absorptance changes of white paints.

Fig. 4. Normal infrared emittance changes of white paints.

Fig. 5. Mass changes of white paints by AO irradiation.

The measured volume resistivity and surface resistivity of white paints are listed in Table 5.

The resistivity of Z-93 and AZ-93 varied widely. These are inorganic paint and brittle rather than silicone based S-13G. It is thought that small cracks (e.g. Fig. 6) cause variations in resistivity. AZ-2000, which is a conductive paint, shows high conductivity even in vacuum.

Table 5. Measured resistivity of white paints.

Volume resistivity cm Surface resistivity /

Paintin atmosphere in vacuum in atmosphere in vacuum

1.3×108 7.3×109 1.6×1011 1.9×1015

6.2×109 3.6×1013 5.3×1010 7.1×1013Z-93

4.1×107 8.3×109 2.5×109 8.8×1013

1.2×108 1.8×1011 1.4×1011 2.1×1014

8.0×108 8.0×1011 2.7×109 5.3×1013AZ-93

4.7×107 2.3×1011 3.3×109 4.4×1014

6.2×1010 3.8×1011 8.0×1013 1.1×1015

9.9×1011 2.3×1012 5.6×1013 3.0×1014S-13G

2.0×1011 6.8×1011 3.2×1013 1.8×1014

1.3×106 1.1×106 5.2×107 1.3×107

1.3×106 9.4×105 7.8×106 7.9×107AZ-2000

1.7×106 1.0×106 2.2×107 2.3×108

Fig. 6. Small cracks on AZ-93 white paint.

3.2. Germanium coated polyimide As a results of test 1, the solar absorptance (Fig. 7) and

normal infrared emittance (Fig. 8) of germanium-coated black Kapton (525 thickness Ge, stored in air) were measured. 525 -thick Ge specimens were stored in nitrogen gas purged, and 1000 -thick Ge specimens in vacuum before AO irradiation, preventing moisture effect on germanium.

No noticeable change in thermo-optical properties was detected in the irradiated specimens.

Fig. 7. Solar absorptance changes of germanium coated blackKapton (525 Å Ge).

Z-93 AZ-93 S-13G AZ-2000

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-0.15

-0.1

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0

Mas

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AO Flunence (× 1020 atom/cm2 )

Fig. 8. Normal infrared emittance changes of germanium coated black Kapton (525 Å Ge).

For careful evaluation of Ge-coated polyimide, irradiation tests with different AO fluence or Ge thickness were conducted. Solar absorptance (Fig. 9) and mass (Fig. 10) changes by AO irradiation were illustrated. Solar absorptance of the unirradiated 525 -thick Ge specimen was different from the previous measurement (Fig. 7) despite the fact that the same specification film was used. The thermo-optical properties of Ge-coated black Kapton show interlot variation. They are stable during different AO fluence. Black Kapton erodes and loses mass easily, whereas Ge coating has very high tolerance and provides protection against AO. To analyze the AO tolerance mechanism of Ge, XPS depth profiles were obtained with AO-irradiated specimens.

Fig. 9. Solar absorptance changes of 525 Å and 1000 Å thickness germanium coated black Kapton by AO irradiation.

To analyze AO tolerance mechanism of Ge, XPS depth profile, which were calibrated by the etched crater depth of the SiO2, were obtained with AO irradiated specimens. Fig. 11 and Fig. 12 are the XPS depth profiles of 525 Å- and 1000 Å- thick Ge coated black Kapton. We observed that the peak shifted to a lower binding energy as the depth increased. Ge3d5/2 signals of GeO2 and Ge are observed at 32.5 eV and 29 eV, respectively 5). Only outermost layer of each unirradiated germanium coating was oxidized. The surfaces of both materials were highly oxidized by AO; however, the oxidized layer was within 6 nm. The deeper layers were not

oxidized. Ge coatings can form good passivating layer (oxidized germanium) against AO reaction.

Fig. 10. Mass changes of 525 and 1000 Å thickness germanium coated black Kapton by AO irradiation.

To select the Ge thickness, we observed 525 and 1000 Å Ge on black Kapton film, after AO irradiation (fluence of 9 E20 atoms/cm2). SEM images of AO-irradiated Ge-coated black Kapton film surfaces are shown in Fig. 13 (525 Ge) and Fig. 14 (1000 Ge). More defects (pinholes or cracks) were observed on the 525 Ge than on the 1000 Ge. Fig. 15 is a magnified image of defects with AO eroded black Kapton coated with 525 Ge. The tolerance of a polymeric material with AO protective coating depends on the coating soundness (defects) 6). It should be considered for the prevention of AO undercutting effect. It seems that the 1000 angstrom Ge film has fewer defects and risk of AO undercutting than the 525 Å Ge film.

However, the comparison of Ge thickness was done with different storage condition. Further studies should be undertaken to examine storage environmental effects on a germanium coating.

Binding Energy (eV) (a) 525 Å thickness Ge (b) 1000Å thickness Ge

Fig. 11. Ge3d5/2 XPS depth profile spectra of germanium coated black Kapton (unirradiated).

0.45

0.5

0.55

0.6

0.65

0 2 4 6 8 10

SolarAbsorptance

AO Flunence (�1020 atom/cm2 )

Arb

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nit

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Binding Energy (eV) (a) 525 Å thickness Ge (b) 1000Å thickness Ge

Fig. 12. Ge3d5/2 XPS depth profile spectra of germanium coated black Kapton (AO irradiated 9 1020 /cm2).

Fig. 13. SEM image of AO irradiated Ge coated black Kapton (525 Å Ge).

Fig. 14. SEM image of AO irradiated Ge coated black Kapton (1000 Å Ge).

Fig. 15. SEM image of defect on Ge coated black Kapton (525 Å Ge).

4. Summary We evaluated degradation of thermal control materials for

GPM/DPR. The following results were obtained from this study.(1) White paints were demonstrated their high tolerance in environmental tests without AZ-2100 (cracked by thermal shock test) . (2) Resistivity property of white paints were obtained. AZ-2100 has high electrical conductivity even in vacuum. (3) Germanium coated black Kapton film is highly tolerant against AO. Germanium can form good passivating layer. (4) Comparing 525 Å- and 1000 Å- thickness germanium coatings, it seems that a 1000 Å Germanium film has fewer defects and risk of AO undercutting than a 525 Å Germanium film.

Acknowledgements

The authors thank Dr. Toshio Iguchi of NICT and the GPM/DPR project team of NEC Toshiba Space Systems for specimen provision; and Mr. Susumu Baba (JAXA) and Mr.Tomoyuki Nakazono (AES) for XPS measurements.

This work was conducted as part of the GPM/DPR project development.

References

1) Smith, E.A, Asrar, G., et al: International Global Precipitation Measurement (GPM) Program and Mission: An Overview, Measuring Precipitation From Space, 2007.

2) Hyakusoku, A., Furukawa, K., Ishikari, T., Kojima, M., Hanado, H., Takahashi, N., Iguchi, T. and Okumura, M.: Development status of the Dual-Frequency Precipitation Radar for the Global Precipitation Measurement, 26th ISTS, 2008.

3) Caledonia, G.E., Krech, R.H. and Greent, B.D.:A high flux source of energetic oxygen atoms for materials degradation studies, AIAA J., 25, (1987), pp.59-63.

4) Ishizawa, J.: Material Aging of Siloxane Coated Polyimide Film and Silicone-Based White Paint on SM/SEED Exposure Experiments, Proceedings of International Symposium on “SM/MPAC&SEED Experiment” JAXA-Sp-08-015E, (2009), pp.139-147.

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5) Briggs, D.: Practical surface analysis, John WILLEY & SONS. Vol. 1, second edition, 1993.

6) Banks, B. A., Snyder, A. and Miller, S.K. : Issues and Consequences of Atomic Oxygen Undercutting of Protected Polymers in Low Earth Orbit , NASA/TM-2002-211577, 2002.