Power degradation in the solar cells was consistent with the exposure environment and appears to be produced principally by the radiation and atomic oxygen environments. Atomic oxygen erosion was generally as expected; atomic oxygen effects dominated for the most part in materials that were both atomic oxygen and ultraviolet vulnerable. Silicone coatings appear to protect Kapton, and adhesive systems contained under photon opaque materials were surprisingly environmentally resistant. A high density of small micro-meteroid/space debris impacts were observed on mirrors, protective coatings, paints, and composites. New synergistic effects of the space environment were noted in the interaction of atomic oxygen and copious amounts of contamination and in the induced luminescence of many materials

Whitaker, Ann F.

Young, Leighton E.

English

NASA Technical Reports Server

N92-27087
AN OVERVIEW OF THE FIRST RESULTS ON THE SOLAR ARRAY
MATERIALS PASSIVE LDEF EXPERIMENT (SAMPLE), AOI71
Ann F. Whitaker and Leighton E. Young
NASA Marshall Space Flight Center
MSFC, AL 35812
Phone: 205/544-2510, Fax: 205/544-0212
SUMMARY
Power degradation in experiment solar cells was consistent with the exposure
environment and appears to be produced principally by the radiation and atomic oxygen
environments. Atomic oxygen erosion was generally as expected; atomic oxygen effects
dominated for the most part in materials that were both atomic oxygen and ultraviolet
vulnerable. Silicone coatings appear to protect Kapton, and adhesive systems contained under
photon opaque materials were surprisingly environmentally resistant. A high density of small
micro-meteoroid/space debris impacts were observed on mirrors, protective coatings, paints and
composites. New synergistic effects of the space environment were noted in the interaction of
atomic oxygen and copious amounts of contamination and in the induced luminescence of many
materials.
INTRODUCTION
This paper describes some of the key results of AO171 materials evaluations performed
to date. This passive experiment contains about 100 materials and material processes which
address primarily solar array materials although other materials of interest to the aerospace
industry were included in the experiment complement. Figure 1 shows the experiment after
space exposure. The experiment objective was to determine the electrical, mechanical and
optical property changes induced by the combined space environments. The space environments
in which this experiment were exposed as a result of its location on LDEF row 8 are described
in Table I.
Although this experiment was configured for 18 months of exposure, the longer than
anticipated exposure of about 70 months generally enhanced the experiment even though most of
the thin film polymeric materials were eroded away as a result of atomic oxygen exposure.
From an analyses standpoint it is important to consider that well heat sunk, thermally conductive
materials on row 8 probably saw no temperature greater than 100°F and that isolated materials of
low thermal conductivity, mass and emittance saw wide temperature cycling. Outgassing of
materials would have occurred early in the mission, and the atomic oxygen environment was
most severe during the last 6 months of the mission.
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https://ntrs.nasa.gov/search.jsp?R=19920017844 2020-03-17T11:02:14+00:00Z
RESULTS
GeneralObservations
From a molecularcontaminationstandpoint,the experimentwasgenerallyconsideredto
bevery cleanwith the exceptionof onelocalizedareawherethe materialsoutgassedasa result
of insufficient thermalvacuumbakeoutprior to flight. Particulatecontaminationwaswide
spreadover theexperimentpresumed,primarily, from disintegratedsiliconecoatingsand
metallizedfilms which were transferredduring thepostretrievalprocess. Dark depositswere
foundon theinterior sidesof theexperimenttray at vents to theexternalenvironmentconsistent
with theangleof atomicoxygenimpingementbut not consistentwith sunangle. This
phenomenasuggestsa new synergisticeffectbetweenatomicoxygenandcopiouscontamination.
The chemistryof thedepositshasnot beenfully identifiedbut theunbakedpolyurethanepaint
andprimer systemon theinterior of theLDEF surfacearesuspectedto be theprincipal
contributorsof materialsexposedto the incomingatomicoxygenat thesevent locations. Many
phenomenawere observedon this experimentthat haveneverbeenreportedon in materials
exposureexperimentseither from flight or in groundbasedsimulations. Someof these
phenomenaare listed asfollows andaredescribedastheyapply to variouscategoriesof
materialsevaluations.
.
.
3.
.
.
Organic materials such as polyimides, silicones, and polyurethanes were found to
luminesce after flight upon exposure to far UV irradiation.
Fiber.us "ash" material was observed on carbon fiber based composite materials.
Silicone adhesives were found to function surprisingly well after 5.8 years in
space.
A high density of small (< lmm) micrometeoroid and space debris impacts was
obvious on all materials.
New synergistic effects were noted where atomic oxygen and copious amount of
contamination were interactive.
1242
Solar Cells
SevenseparateMSFC photovoltaic(solarcell) testarticleswere flown on this
experiment. They were fabricatedfor MSFC by LockheedMissilesand SpaceCompany
(LMSC) who also suppliedthepre-flight electricalperformancetestdata.
As a result of the longer thanplanneddurationof the flight, all testarticlesunderwent
substantialatomicoxygenerosionof their polyimide (Kapton)substratestructureswith the
result that one modulewas lost prior to Orbiter rendezvouswith LDEF, onecamelooseand
drifted awaywhenLDEF wasgrappled,and one(M3) wasattachedat only one comerwhen
LDEF was retrieved. The latter was foundon the floor of thecargobay whenLDEF was
removed. A descriptionof the solarcell testarticleson handpost-flight is found in Table II.
Atomic oxygendegradationwasobviousfrom visual examinationof thepolyimide
substratesof the cells. Solarcell to solarcell interconnectbondswithstoodtheeffectsof
thermalcycling well with no debondingfoundat theparallel-gapweld of thecopper interconnect
to thecell contactpads. Thermalanalyseshavenot as yet beenperformed,but sincethe solar
cell moduleswere thermally isolatedfrom the LDEF structureby polyurethanefoampadding,
temperatureexcursionsareexpectedto be reasonablybroadover the orbit. The modulethat was
retrievedfrom thecargobay had5 (of 12)cells that containedcracksin either the solarcell or
cell cover. Micrometeoroids/spacedebris impactswereevidenton all the testarticles. These
rangedfrom small surfacenicks to solarcell coverslidebreakageandratherdeeppenetrations
within the cells. At this time no correlationbetweencell/moduleelectrical performanceand
impactshasbeenestablished.
During post flight electricalperformancetestingof moduleM3, it wasdiscoveredthat an
apparentvoltagecalibrationerror wasmadeon this moduleasevidencedby post-flight open
circuit voltagehigher thanthat measuredpre-flight. For this reason,pre-flight performancefor
M3 was fabricatedby usingthepre-flight performanceof the singlecell C8 andexpandingit to
theconfigurationof M3 (4 cells in seriesby 3 cells in parallel). Cell C8 performancewas
selectedbecauseits shortcircuit current, whenmultipliedby 3, wasthe closestmatchto the
actualM3 moduleperformance. Additionally, therewere no other indicatorsto suggesta
current calibrationerror of any significance. After theoverall electrical performanceof M3 was
establishedthroughtesting, the modulewascut apartto determineindividual cell performance.
Solarcell and solarcell modulemaximumpowerpoint (Pmp)degradationrangedfrom
4.3% to 80% with 76% of the singlecells testedhaving lessthan 10%degradation,Figure 2.
Therewere4 cells out of 17 that hadPmpdegradationgreaterthan20%. Threeof thesewere
from the modulewhich was retrievedfrom thecargobay. The otherwas thecell (C6) which
was flown without a coverslide. Discountingthesecells, theaveragecell Pmpdegradationwas
6.5% with a standarddeviationof 1.75%. The thin cell module(M4), which was retrieved
intact but with severedamageto its Kapton substratedegraded5.2% in Prop. Four cells had
discernibledegradationin opencircuit voltage(Voc) which is typical of a decreasein cell shunt
1243
resistance.Threeof thesecellswere from the moduleretrievedfrom the cargobaywith the
otherbeing the cell flown without a coverslide. Two of the cells with Voc degradationwere
from thefour which degradedmore than20%. Threecells showedevidenceof severeseries
resistanceincreases. All threeof thesewerefrom thecargobay module. Two are from the
four havingdiscernibledegradationin Voc. Figure 3 illustratesthe differencesin
current/voltage(IV) characteristicsbetweenthreepoor performancecells andcell C1 that
underwentthe lowestdegradation.
In summarythe electricalperformanceof the cells is consistent with the expected
performance for the environment experienced. To date no partition of cell degradation to
specific environmental effect has been made. Loss of Kapton substrate from atomic oxygen
attack was not a surprise since the materials were in space much longer than planned with no
protective coatings. Some portions of the Kapton substrate containing adhesives are still
intact and require more evaluation to determine the reasons for their survival in the atomic
oxygen environment.
Composites
The carbon fiber and glass fiber composites contained in this experiment are described,
along with a summary of their data generated to date, in Table III. The most obvious visible
effect noted on these composites is the increased optical diffuseness of the surfaces which is
commonly observed in materials exposed to orbital atomic oxygen. All of the carbon fiber
composites have a charred appearance and show the fiber direction as a result of preferential
atomic oxygen erosion of the matrix material. A porous ash structure formed on the exposed
carbon fiber composite surfaces with the greatest concentration being on the polysulfone
materials. Micrometeoroid/space debris impacts less than 1 millimeter in diameter were present
on all composites. Concentration of these impacts ranged from 2 to 7 impacts per 25 in 2. Non-
penetrating splattered contaminents, probably from spacecraft fluid dumps, were also noted on
the composites.
Mass loss was consistent with thickness loss with the carbon fiber based composites
losing approximately 1 ply. Atomic oxygen reactivity numbers are not inconsistent with those
generated for carbon composites but no short term data is available currently for comparison of
short and long term reactivity numbers for these specific composites. Another factor to consider
is that short term exposure data probably would not be correlatible to the long term exposure
data since the composite surfaces are matrix material rich. Aluminized tape was effective in
protecting the composites from atomic oxygen erosion although penetrations of the protective
aluminized tape were made by small impacts. The silicone adhesives on these tapes were well
preserved and as effective as the controls. Further evaluations of the tapes are planned.
1244
Thin Films
A cursoryassessmentof metallizedand thin polymeric films is providedin Table IV
which is a summaryof post-flight visual observationsof the films. No analysesother thana
SEM examinationof thewhite Tedlar havebeenmadeto date. Visually, the white Tedlar
appearsintact but the inert particlesin the materialare basicallythe only remainingcomponent
of the material. The intact Kaptonprotectedby thesiliconecoatingsprovide somepromisethat
siliconescan furnish long term protectionfor Kaptonagainstatomicoxygenattack. However,
theextentof the effectivenessof their protectionremainsto bequantified. Thin films of
Kapton,Mylar andTeflon which were aluminizedwith thealuminumsurfaceexposedappearto
havebeenprotectedfrom atomicoxygenerosion.
Paints
During the post-flight examinationof this experiment under black light illumination, it
was discovered that various types of materials were luminescing (fluorescing). Among these
fluorescing materials were the polyurethane and silicone based paints. The intensity and
wavelength dependence of this fluorescence varies with material and are the subject of a separate
report.
Upon exposure, specular paints became diffuse and diffuse paints became more diffuse -
a phenomena which is known from short term exposures and results principally from atomic
oxygen induced surface texturing and also from preferential atomic oxygen attack of diffuse paint
constituents. Inert particles in the paint accumulate on the paint surface as it erodes thereby
providing more shielding of the underlying surface with increased exposure. Atomic oxygen
erosion is, as observed from these results and those from the STS flight, non-linear in time in those
paints subject to this preferential attack so their mass loss cannot be used to predict erosion rates.
Optical properties and mass loss of these paints, Table V, are compared to data from control paints
prepared at about the same time as the flight samples. As noted by the data, exposure of pigments
in white and yellow diffuse paints tend to decrease absorptivity. S13GLO silicone paint which
resists atomic oxygen attack showed darkening, presumably from ultraviolet/particulate irradiation.
One of the more interesting effects noted in the paints is that atomic oxygen effects dominate in
those paints which are both ultraviolet and atomic oxygen vulnerable. Without evaluating this
phenomena in great detail, it would appear that any synergism, if it exists, between atomic oxygen
and irradiation effects under these exposure conditions for these paints is minimal.
1245
Metals
Metals flown on this experimentincludealuminum,titanium, silver (disk andcold rolled
ribbon), niobium, magnesium,copper(disk andcold rolled ribbon), molybdenum,tantalum,and
Tophet-30,HOS-875andNi-Cr alloys in the asreceivedconditionandpreoxidized. Several
othercombinationof metalsandcoatingswere alsoflown but are not part of this report. All of
these metals gained weight from interaction with atomic oxygen except the preoxidized alloys
which showed a slight mass loss. Macroscopic atomic oxygen effects were observed on silver
and copper. Of all the metals silver was the most affected, as expected, and tantalum was the
least affected. Accommodation of atomic oxygen in silver ranged from one atom per 1000
incident atoms in disk samples which were well heat sunk to 7 atoms per 1000 incident atoms in
cold rolled stressed ribbon samples which were therm_dly isolated from the base structure. Mass
increase in these materials, toral incident fluence of atomic oxygen zmd the assumption that the
highest oxidation state of the oxide was achieved, were used to generate these accommodation
numbers.
Other Polymers
Materials including RTV-511, Halar and the PEEK resin, configured into tensile, shear
and miscellaneous rectangular samples, and TFE Teflon washers comprised the majority of
structural polymers on the experiment. The RTV-511 materials received an insufficient thermal
vacuum bakeout (for contamination control) prior to flight resulting in outgassing with deposition
on the near adjacent experiment area.
Considerable darkening of all elastomers occurred as a result of exposure and a 10%
increase in Shore A hardness was noted in RTV 511. Halar, which is both ultraviolet and
atomic oxygen susceptible, appears to be dominated by irradiation effects but further evaluations
are required to identify and quantify these effects. Mass loss in these materials ranged up to 7%
of their initial weight. Thirty six TFE washers were examined for atomic oxygen erosion and an
atomic oxygen reactivity number of (1.9 4- .4) x 10 .25 cm3/atom generated for this material.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the contribution of Messrs. Doug Willowby in the
solar cell characterization and Ed White for profllometry measurements, Ms. Whitney Hubbs for
coordination of the major portions of the evaluations and Ms. Freda Summers for typing the
text.
1246
TABLE I. - EXPERIMENT AO171EXPOSURECONDITIONS
• High Vacuum
• UV Radiation
• ProtonFluence
• ElectronFluence
• Atomic Oxygen
• Micrometeoroid/
SpaceDebris
• Thermal Cycles
10.5- 10-7Torr (estimated)
10,041ESH
109p+/cm2(.05- 200 MeV)
(1012- 108)e-/cm2(.05 - 3.0 MeV)
*7.68 x 1021atoms/cm2
2 to 7 impactsper composite,< 1mm
32,000cycles (tempTBD)
*This numberwasderivedfor Row 8 from the JSCprovidednumberof 9.75x 1021atoms/cm2
for total atomicoxygenfluenceon theRAM surface.All reactivityandaccommodation um-
bersprovidedin this paperarebasedon this number.
1247
M3
TABLE II. - SOLAR CELL TEST ARTICLES
ASEC 12 Cell Module
Applied Solar Energy Corp. (ASEC) 200 micron,
2 cm x 4 cm silicon N on P, 2 ohm-cm,
back-surface reflector cells
Chemically Vapor Deposited (CVD) dielectric, wrap-around
contacts
150 micron microsheet coverslides with UV filters and
AR coatings
Rear surface of module facing space
M4 ASEC 6 Cell Module
ASEC 50 micron, 2 cm x 4 cm silicon N on P, 10 ohm-cm
back-surface field cells
Wrap-around contacts with wrap-around junction
2 each 50 micron, 4.7 cm x 6.7 cm coverslides
Cells mounted facing space
Cells C6 through C10 - ASEC cells same as M3 but with coverslides as follows:
C6 - None
C7 - 150 micron microsheet with AR coating
C8 - 150 micron microsheet with UV filter and AR coating
C9 - 150 micron frosted fused silica with UV filter and
AR coating
C10 - 150 micron fused silica with UV filter and AR coating
Note: Each test article has a copper interconnect system laminated between 2 sheets of Kapton
that comprise the structural substrate for the solar cells.
1248
TABLE II1. - ATOMIC OXYGEN EFFECTS IN AOI71 COMPOSITE SAMPLES
Composite Materials
(No. of Specimens)
Average Thickness
Loss (Mils)
Atomic Oxygen
Reactivity
10.24cm/Atom
ttMF 322/P1700/+45 ° 4.7 to 11.5" (1.6 - 3.8)
(5)
HMS 934/0 ° (5) 2.5** 0.8
HMS 934/90" (6) 2.7 0.9
P75S/934/90 ° (6) 2.7 0.9
P75S/934/0 ° (5) 2.8 0.9
"S" Glass-epoxy (3) 0.36*** 0.11
Thermal Control Indeterminate --
Aluminized Taped
"S" Glass-epoxy (3)
* Matrix erosion much greater than fiber
** Average of rates from 2 ends of sample; contamination likely on forward end
*** Fibers uneroded and become protective after initial matrix mass loss
General Observations
Materials (1-5) "blacker",
i.e., more diffuse. "S"
glass- epoxy much darker
- probably from UV
effects.
Fibers evident in
materials (1-6). Closer
observations show
micrometeoroid/ space
debris hits on all
specimens.
Material (7) shows
unexplained corrugated
features on aluminum
tape.
1249
TABLE IV
POST FLIGHT ASSESSMENT OF
METALLIZED AND POLYMERIC FILMS
Materials/Configuration
(Exposed =, inner material)
Post Flight Condition
1. DC-2577 Coated Kapton
2. RTV-670 Coated Kapton
3. 1.0 mil Kapton H
4. Sandwich configuration
of 1/2 mil Kapton,
1 mil Black Kapton/2 mil Kapton
5. Aluminized Mylar
6. 5 mil Kapton H
7. Aluminized Teflon
8. Kapton F
9. 1/2 mil Teflon
10. 1.0 mil White Tedlar
11. Aluminized Kapton
12. Aluminized tape on composite
Intact
Intact
Eroded away
Eroded away
Intact
Eroded away
Intact
Eroded away
Eroded away
Still white but
matrix eroded away
Intact
Intact, surface fatigue,
adhesive good
1250
TABLE V. - PROPERTY DATA ON PAINTS ON AO171 EXPERIMENT
PAINT MASS LOSS "(MG/cm 2) SOLAR ABSORPTIVITY
Unexposed Ex__x_posed
A-276 (White diffuse)
401-C 10 (Black diffuse)
Z-853 (Yellow diffuse)
Z-306 (Black diffuse)
Z-302 (Black specular)
S 13GLO (White)
!.00
1.13
0.94
1.60
4.06
Negligible
.284 .237
.990 .995
.491 .428
.978 .973
.973 .974
.186 .319
*Inert pigments in diffuse paints tend to retard atomic oxygen produced erosion.
Specular Z-302 was eroded to the metal substrate
1251
\Figure 1. AO171 Experiment After 5.8 Years Exposure in LEO
1252
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPo.i
70-
z
<
rn 80-
<
r¢
I.d
50-
e¢
t,d
o4o-
._30-
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Figure 2.
1L-4R CELLS OF M3 ]
_ ._P__J _
Percent Degradation in Solar Cell Test Article Maximum Power Output
1253
0.50
O3
E
<
v
Z
LU
CC
n-
O
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.00
Figure 3.
-....
3L
2c
0.10 0.20
* 1C - LI)W DEG ::IADATIE_N CELL
* 3C - HIGH DEE RADATI()N CELL
DECRI'ASED _HUNT RI'SISTAk DE
* 3L- H GH DEGRADATI()N CELL
LOWE qED SHLINT & IN_;REASE 3
SERIES RESIS'FANCE
* 2C- HIGH DEE RADATI()N CELL
INCRE&SED SEiRIES RESlSTAN(;E
,,,,. __c
0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
VOLTAGE (Volts)
Variations in Current/Voltage Characteristics of Poor Performance
Cells and Best Performing Cell C 1
1254


An overview of the first results on the solar array materials passive LDEF experiment (sample), A0171

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