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Electrical and Thermal Conductivity of
Carbon Fiber-Polymer Composites Plates
Carbon fiber-polymer composite plates were fabricated using 0°-90°
woven fabrics of a variety of pristine and bromine intercalated carbon
fibers. The fibers had electrical resistivities varying from 50 to 1800
µ ohm-cm, and thermal conductivities varying from 8.5 to 520 W/m-K.
Anisotropic composites were also fabricated from fabrics with low
conductivity fibers in the warp direction and high conductivity in the
weft. Composite electrical resistivity was measured using an eddy
current technique and a four-point technique, and calculated using a
geometry- corrected rule of mixtures. Composite thermal conductivity
was measured using an optical heating technique and infrared scanning
of the surface as well as being calculated from the rule of mixtures.
Woven fabrics were shown to behave like homogeneous, isotropic plates
both electrically and thermally as long as the samples are large with
respect to the weave size of the fabric. The four-point resistivity was
somewhat higher
than that predicted by the rule of mixtures. The resistivity as
measured by the eddy current method was in all cases higher than both
the four-point and rule of mixture resistivities. The thermal
conductivities of the composite were in fairly good agreement with the
rule of mixtures for relatively low conductivity fibers, but much lower
than predicted for high conductivity fibers. Anisotropic composites
could only be made by stacking the anisotropic fabrics in a 0°-0°
geometry. Even under those conditions the anisotropy, especially
of the thermal conductivity, was considerably less than would be
expected
from the rule of mixtures.
![[horizontal rule]](../images/rule.gif)
Thermal Peformance of an Annealed
Pyrolytic Graphite
Solar Collector
A solar collector having the combined properties of high solar
absorptance, low infrared emittance, and high thermal conductivity is
needed for applications where solar energy is to be absorbed and
transported for use in minisatellites. Such a solar collector may be
used with a low temperature differential heat engine to provide power
or with a thermal bus for thermal switching applications. One concept
being considered for the solar collector is an Al-Al2O3 cermet coating
applied to a thermal conductivity enhanced polished aluminum substrate.
The cermet coating provides high solar absorptance, and the polished
aluminum provides low infrared emittance. Annealed pyrolytic graphite
embedded in
the aluminum substrate provides enhanced thermal conductivity. The
as-measured thermal performance of an annealed pyrolytic graphite
thermal conductivity enhanced polished aluminum solar collector, coated
with a cermet coating, will be presented.
Thermal Emittance Measurements on
Candidate Refractory
Materials for Application in Nuclear Space Power Systems
The development of a highly efficient General Purpose Heat Source
(GPHS) space power system requires that all of the available thermal
energy from the GPHS modules be utilized in the most thermally
efficient manner. This includes defining the heat transfer/thermal
gradient profile between the surface of the GPHS’s and the surface of
the energy converter’s hot end whose geometry is dependent on the
converter technology selected. Control of the radiant heat transfer
between these two surfaces is done by regulating how efficiently the
selected converter’s hot end surface can reject heat compared to a
perfect blackbody, i.e. its infrared emittance. Several refractory
materials of interest including niobium-1% zirconium, molybdenum-44.5%
rhenium and L-605
(a cobalt-based alloy) were subjected to various surface treatments
(grit
blasting with either SiC or WC particulates) and heat treatments (up to
1198
K for up to 3000 hours). Room temperature infrared emittance values
were
then obtained using two different infrared reflectometers. Grit
blasting with
either SiC or WC tended to increase the emittance of flat or curved
L-605
coupons by ~0.2-0.3 independent of heat treatment. Heat treating L-605
coupons
under 773 K for up to 2000 hours had only a slight effect on their
emittance,
while heat treating L-605 coupons at 973 K for over 150 hours appeared
to
significantly increase their emittance. For the temperatures and times
studied
here, the emittance values obtained on niobium-1% zirconium and
molybdenum-44.5%
rhenium coupons were independent of heat treat times and temperatures
(except
for the niobium-1% zirconium coupon that was heat treated at 1198 K for
150
hours).
Portable Infrared Reflectometer for
Evaluating Emittance
Optical methods are frequently used to evaluate the emittance of
candidate spacecraft thermal control materials. One new optical method
utilizes a
portable infrared reflectometer capable of obtaining spectral
reflectance
of an opaque surface in the range of 2 to 25 microns using a
Michelson-Type
FTIR interferometer. This miniature interferometer collects many
infrared
spectra over a short period of time. It also allows the size of the
instrument
to be small such that spectra can be collected in the laboratory or in
the
field. Infrared spectra are averaged and integrated with respect to the
room
temperature black body spectrum to yield emittance at 300 K.
Integrating
with respect to other black body spectra yields emittance values at
other
temperatures. Absorption bands in the spectra may also be used for
chemical
species identification. The emittance of several samples was evaluated
using
this portable infrared reflectometer, an old infrared reflectometer
equipped
with dual rotating black body cavities, and a bench top thermal control
vacuum
chamber. Samples for evaluation were purposely selected such that a
range
of emittance values and thermal control material types would be
represented,
including polished aluminum, Kapton®, silvered Teflon®, and the
inorganic
paint Z-93-P. Results indicate an excellent linear relationship between
the
room temperature emittance calculated from infrared spectral data and
the
emittance obtained from the dual rotating black body cavities and
thermal
vacuum chamber. The prospect of using the infrared spectral date for
chemical
species identification will also be discussed.
Optical and Calorimetric Evaluation of
Z-93-P and Other Thermal Control Coatings
The hemispherical total emissivity of two thermal control coatings,
Z-93-P and black anodized aluminum, was calculated from hemispherical
total reflectivity measured in wavelength range of 2 to 40 m m. These
data were compared to hemispherical total emissivity values obtained on
the same samples measured in a thermal vacuum chamber with a
calorimetric technique. The comparison showed close agreement in the
vicinity of room temperature and above, with differing trends at lower
temperatures.
Emittance of Thermal Control Materials
Between 100K and 400K
The hemispherical total emittances of several common thermal coatings
tapes were evaluated calorimetrically over a wide temperature range.
The calorimetric technique used here to evaluate thermal control
materials allows a thermally isolated sample to cool solely through
radiant heat transfer to a liquid nitrogen cooled cold wall. The
mechanism of cooling is similar to that found in space, providing a
functional evaluation of hemispherical total emittance. The five
samples that were evaluated included several first and second surface
mirrored Kapton tapes, one carbon-filled Kapton tape, and one black
film. These tapes were adhesively bonded to an aluminum substrate,
which provided the sensible heat for the calorimetric calculation.
Temperature-time data were collected as heat flowed from the aluminum,
through the adhesive layer, through the thermal control tape, and
ultimately through the surface to the surroundings. Hemispherical total
emittances were calculated over the temperature range of 100 to 400 K.
Emissivity Characterization of Plastics,
Ceramics, and Coatings Using a Calorimetric Technique
The emissivity of several different materials, including commercially
available plastics, ceramics, and coatings, was evaluated by both
optical and calorimetric means to select appropriate materials of
construction for a microgravity combustion experiment. Samples of grit
blasted black anodized aluminum, grit blasted black oxidized stainless
steel, Bakelite®, Noryl®, Zelux®, Macor®,
and Eccocoat®, were all characterized in the range of 200 to
400 K. Several of these materials exceeded the combustion experiment
emissivity requirement of 0.8 over a wide range of temperatures,
suggesting their promising use as materials of construction. Emissivity
values from all of the materials will be summarized, and those
materials selected for the microgravity combustion experiment will be
identified.
Emittance Characterization of Thermal
Control Paints, Coatings, and Surfaces using a Calorimetric Technique
Thermal control surfaces are used in every spacecraft thermal
management system to dissipate heat through heat transfer. This paper
describes the thermal performance of several thermal control paints,
coatings, and surfaces, as characterized by a calorimetric vacuum
emissometer. The emissometer is designed to measure the functional
emittance of a surface based on heat transfer from an underlying
substrate to the surface and from the surface or near surface
to a surrounding cold wall. Emittance measurements were made between
200
and 350 K. Polished aluminum, used here as a standard, was found to
have a
total hemisperical emittance of 0.06, as expected. A velvet black
paint, also
used as a standard, was found to have an emittance of 0.94 at room
temperature. Other surfaces of interest included a polyurethane-based
black paint designated Z-306, a highly polished 316L stainless steel,
and an atomic beam-textured carbon-carbon composite.
Thermal Modeling of a Calorimetric
Technique for Measuring the Emittance of Surfaces and Coatings
A finite element analysis model of a transient technique used to
measure the emittance of surface and coatings was developed and used to
estimate the uncertainty in emittance. The dimensions used in the model
matched the dimensions used in the design of a low temperature
calorimetric vacuum emissometer being built to characterize the thermal
properties of space power materials in the temperature range 173-673 K.
Radiant energy from a quartz halogen lamp
impinged on an aluminum sample that was coated with a thermal control
coating
and suspended in a liquid-nitrogen-cooled vacuum chamber by narrow
gauge
thermocouple wires. After removing the heat source, the temperature of
the
sample was monitored vs. time and the temperature-time curve was used
to
calculate the emittance. Factors contributing to the uncertainty in the
emittance
included uncertainties in time, temperature, area of the sample, heat
capacity
of the sample, and heat loss from the uncoated back of the sample. Heat
losses
from the thermocouple wires were found to be negligible. The total
probable
error in the emittance obtained from the low temperature calorimetric
vacuum
emissometer design was estimated to be less than 4% for emittance
values
greater than 0.5 at temperatures between 173 and 673 K.
Advances in Optical Property Measurements
of Spacecraft Materials
This report describes some of the instruments and experimental
approaches available for measuring optical properties of thermal
control materials. It also describes the instruments' uses in
laboratory studies of the effects of combined contaminants and the
space environment on these materials, and in the qualification of
hardware for spacecraft. In recent years, several instruments for
measurement of solar absorptance (a ) and infrared emittance (e ) have
been introduced. These instruments offer improved speed, accuracy, and
data-handling, all of which substantially improve the study of
contaminated thermal control materials. A transient method for directly
measuring material e is also described, and the results are compared
with other instruments. In addition, our understanding of oxygen
exposure effects on the ( of materials following contamination or
exposure to simulated space conditions shows
that oxygen exposure before measuring of e should be avoided.
Reflectivity of Silver and Silver-coated Substrates from 25°C
to 800°C
A bench top facility was used to evaluate the reflectivity of several
candidate coating-substrate combinations in vacuum at elevated
temperatures. Silver was selected as the reflective coating of choice,
while copper, nickel, electroless nickel on copper, and 304 stainless
steel were selected as substrates. Pure silver, with no coating at all,
was also evaluated. An optically flat silver-coated sapphire substrate
was used as a standard. All metal substrates were either
metallurgically polished or diamond turned to mirror finish
prior to silver deposition. Silicon dioxide was used as a protective
coating
in most cases. Reflectivity measurements were made at room temperature
in
the visible range with a spectrophotometer, and at elevated
temperatures
up to 800°C with a helium-neon laser at 632 nm. Results from the high
temperature reflectivity measurements will be presented.
Correlation of Predicted and Observed
Optical Properties of Multi-Layer Thermal Control Coatings
Thermal control coatings on spacecraft will be increasingly important
as spacecraft grow smaller and more compact. New thermal control
coatings will be needed to meet the demanding requirements of next
generation spacecraft. Computer programs are now available to design
optical coatings, and one such program was used to design several
thermal control coatings consisting of alternating layers of WO3
and SiO2. The coatings were subsequently manufactured with
electron beam evaporation and characterized with both optical and
thermal techniques. Optical data were collected in both the visible
region of the spectrum and the infrared. Solar absorptance values were
predicted in the range of 0.177-0.196 and were observed in the range of
0.155-0.228. Infrared emittance values were predicted in the range of
0.074-0.083 and were observed optically in the range 0.048-0.093 and
calorimetrically in the range of 0.069-0.100.
Performance and Durability of High
Emittance Heat Receiver Surfaces for Solar Dynamic Power Systems
Haynes 188, a cobalt-based super-alloy, will be used to make thermal
energy storage (TES) containment canisters for a 2 kW solar dynamic
ground
test demonstrator (SDGTD). Haynes 188 containment canisters with a high
thermal emittance (e ) are desired for radiating heat away from local
hot spots, improving
the heat distribution, which will in turn improve canister service
life.
In addition to needing a high emittance, the surface needs to be
durable in
an elevated temperature, high vacuum (» 830° C, <10-7 torr)
environment for an extended time period. Thirty-five Haynes 188 samples
were exposed to
14 different types of surface modification techniques for emittance and
vacuum
heat treatment (VHT) durability enhancement. Optical properties were
obtained
for the modified surfaces. Emittance enhanced samples were exposed to
VHT
for up to 2692 hours at 827° C and <10-6 torr with integral thermal
cycling.
Optical properties were taken intermittently during exposure, and after
final
VHT exposure. The various surface modification treatments increased the
emittance
of pristine Haynes 188 from 0.11 to 0.86. Seven different surface
modification
techniques were found to provide surfaces which met the SDGTD receiver
VHT
durability requirement (e ³ 0.70 after 1000 hours). Of the 7 surface
treatments,
2 were found to display excellent VHT durability: alumina-titania
(AlTi)
coatings (e = 0.85 after 2695 VHT hours) and zirconia-titania-yttria
coatings
(e = 0.86 after 2024.3 VHT hours). The AlTi coating was chosen for the
e
enhancement surface modification technique for the SDGTD receiver.
Details
of the alumina-titania coating and other Haynes 188 emittance surface
modification
techniques are discussed. Technology from this program will lead to
successful
demonstration of solar dynamic power for space applications, and has
potential
for applications in other systems requiring high emittance.
Optical Property Enhancement and
Durability Evaluation of Heat and Receiver Aperture Shield Materials
Solar Dynamic (SD) power systems have been investigated by the National
Aeronautics and Space Administration (NASA) for electrical power
generation in space. As part of the International Space Station (ISS)
program, NASA Glenn Research Center (GRC) teamed with the Russian Space
Agency (RSA) to build a SD system to be flown on the Russian Space
Station MIR. Under the US/Russian SD Flight Demonstration (SDFD)
program, GRC worked with AlliedSignal Aerospace, the heat receiver
contractor, on the development, characterization, and durability
testing of materials to obtain appropriate optical and thermal
properties for the SDFD heat receiver aperture shield. The aperture
shield is composed of refractory metal multi-foil insulation (MFI)
attached to an
aperture back plate. Because of anticipated off-pointing periods, the
aperture
shield was designed to withstand the extreme temperatures that 80 W/cm²
would
produce. To minimize the temperature that the aperture shield will
reach
during off-pointing, it was desired for the aperture shield exterior
layer
to have a solar absorptance (a s) to thermal emittance (e )
ratio
as small as possible. In addition, a very low specular reflectance (r s
< 0.1) was also necessary, because reflected concentrated sunlight
could cause overheating of the concentrator which is undesirable.
Testing was
conducted at GRC to evaluate pristine and optical property enhanced
molybdenum
and tungsten foils and screen covered foils. Molybdenum and tungsten
foils
were grit-blasted using silicon carbide or alumina grit under various
grit-blasting conditions for optical property enhancement. Black
rhenium coated tungsten foil was also evaluated. Tungsten, black
rhenium-coated tungsten, and grit-blasted tungsten screens of various
mesh sizes were placed over the pristine and
grit-blasted foils for optical property characterization. Grit-blasting
was
found to be effective in decreasing the specular reflectance and
absorptance/emittance ratio of the refractory foils. The placement of a
screen further enhanced these optical properties, with a grit-blasted
screen over a grit-blasted
foil producing the best results. Based on the optical property
enhancement
results, samples were tested for atomic oxygen (AO) and vacuum heat
treatment
(VHT) durability. Grit-blasted (Al2O3 grit) 2 mil
tungsten foil was chosen for the exterior layer of the SDFD heat
receiver shield.
A 0.007 in. diameter, 20x20 mesh tungsten screen was chosen to cover
the
tungsten foil. Based on these test results, a heat receiver aperture
shield
test unit has been built by Aerospace Design and Development (A.D.D.)
with
the screen covered grit-blast tungsten foil exterior layers. The
aperture shield was tested in GRC's Solar Dynamic Ground Test
Demonstration (SDGTD) system to verify the thermal and structural
durability of the outer foil
layers during an off-pointing period.
Effects of Ambient High Temperature
Exposure on Alumina-Titania High Emittance Surfaces for Solar Dynamic
Systems
Solar dynamic (SD) space power systems require durable, high emittance
surfaces on a number of critical components, such as heat receiver
interior surfaces and parasitic load radiator (PLR) elements. To
enhance surface
characteristics, an alumina-titania coating has been applied to 500
heat
receiver thermal energy containment canisters and the PLR of NASA Glenn
Research
Center's (GRC) 2kW SD ground test demonstrator (GTD). The
alumina-titania
coating was chosen because it had been found to maintain its high
emittance
under vacuum at high temperatures for an extended period. However,
preflight
verification of SD system components, such as the PLR, require
operation
at ambient pressure and high temperatures. Therefore, the purpose of
this
research was to evaluate the durability of he alumina-titania coating
at
high temperature in air. Fifteen of sixteen alumina-titania coated
Incoloy
samples were exposed to high temperatures for various durations (2 to
32
hours). Samples were characterized prior to, and after, heat treatment
for
reflectance, solar absorptance, room temperature emittance, and
emittance
at 1200° F. Samples were also examined to detect physical defects and
to
determine surface chemistry using optical microscopy, scanning electron
microscopy,
operated with an energy dispersive spectroscopy (EDS) system, and x-ray
photoelectron
spectroscopy (XPS). Visual examination of the heat-treated samples
showed
a whitening of samples exposed to temperatures of 1000° F and above.
Correspondingly,
the optical properties of these samples degraded. A sample exposed to
1500°
F for 24 hours had whitened and the thermal emittance at 1200° F had
decreased
from the non-heat treated value of 0.94 to 0.62. The coating on this
sample
had become embrittled, with spalling off the substrate noticeable at
several
locations. Based on this research it is recommended that preflight
testing
of SD components with alumina-titania coatings be restricted to
temperatures
no greater than 600° F in air to avoid optical degradation. Moreover,
components
with the alumina-titania coating are likely to experience optical
property
degradation with direct atomic oxygen exposure in space. |
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