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					                                                                                                  Project Report

                                                                    2011 MVK160 Heat and Mass Transport

                                                                                 May 13, 2011, Lund, Sweden




    Mass Transfer: A look at atomic oxygen erosion of spacecraft material in the
                                  low earth orbit



                                         Patrick Colvin
                        Dept. of Energy Sciences, Faculty of Engineering,
                         Lund University, Box 118, 22100 Lund, Sweden




                                                           Subscripts
ABSTRACT                                                   S              sample material
The presence of atomic oxygen in the low earth orbit
contributes to degradation of spacecraft material. This    K              Kapton H witness sample
material experiences erosion that leads to mass loss,
while the remaining material is mechanically
                                                           INTRODUCTION
compromised. Investigation is ongoing to improve the
                                                           Space is often thought of as a volume containing
resistance of spacecraft material to the harmful
                                                           nothing; an environment that exists in the essence of
effects of atomic oxygen. The ITO-coating and nano-
                                                           nothingness. This, however, is not true. According to
SiO2 methods of material enhancement are discussed
                                                           Tori Woods of NASA’s Glenn Research Center “space
and evaluated. Injection with nano-SiO2 is predicted
                                                           is considered an environment . . . filled with entities
as being superior for it predicted to maintain
                                                           that can be harmful to spacecraft.” Woods lists the
resistance throughout the material. Research should
                                                           various physical and chemical threats: “ultraviolet rays
be conducted to evaluate this prediction over long
                                                           and x-rays from the sun; solar wind particle radiation;
term exposure of atomic oxygen.
                                                           thermal cycling (hot and cold cycles); space particles
                                                           (micrometeoroids and debris); and atomic oxygen.”
NOMENCLATURE                                               (Woods, 2011)
              erosion yield (cm3 atom-1)                   This report will focus on the last environmental
                                                           hazard: atomic oxygen (AO). According to a 2005
              erosion yield of Kapton H witness
                                                           study by the Beijing University of Aeronautics and
              sample (3 x 10-24 cm3 atom-1)
                                                           Astronautics, atomic oxygen is the “predominant
              mass loss (g)                                component and most active species of the LEO
                                                           atmosphere” (Wang, Zhao, Wang, & Shen, 2005).
              surface area exposed to AO (cm2)             Atomic oxygen levels in the low earth orbit (LEO) may
F             fluence of atomic oxygen (atom cm-2)         vary with solar cycle, altitude, season of the year, etc.
                                                           A spacecraft travelling with a velocity of 7 or 8 km s-1
Greek Symbols                                              in the LEO (an altitude between 160 and 2000 km) can
              density of the sample material (g cm-3)      experience an average atomic oxygen flux between
                                                           1012 and 1015 atoms cm-2 s-1 (Wang X. , Zhao, Wang, &
              density of Kapton H witness sample           Shen, 2007). This flux of atomic oxygen poses possible
              (1.4273 g cm-3)
physical threats such as “elastic scattering, scattering
with partial or full thermal accommodation,
recombination, or excitation of ram species” (Groh,
Banks, McCarthy, Rucker, Roberts, & Berger, 2008).
However, studies are most greatly concerned with the
reactive nature of atomic oxygen and the chemical
reactions that occur with the surface material, leading
to material degradation.

For a society that has endeavored to explore space,
we place a great deal of stress on properly
manufacturing that which we send into the unknown.
Among those subjects highlighted is the choice of
spacecraft material. In modern day science, humanity
has reached a point where new materials may be
synthesized or made from a combination of known
materials. The possibilities for materials to own
different characteristics are thereby infinite. The
question remains: how do we select the proper
material with which to outfit a spacecraft in order to      Figure 1: Degradation of surface topography due to AO exposure;
                                                            a) material defects; b) AO-irradiated sample (Shimamura &
reduce the harmful effects of this foreign                  Nakamura, 2009)
environment?
                                                            A material exposed to atomic oxygen will suffer
                                                            mechanical property degradation. Not only will some
PROBLEM STATEMENT                                           of the material react and the mass reduce, but the
Atomic oxygen found in the LEO is reactive with             sample as a whole will decrease in tensile strength
polymers and will cause degradation and mechanical          and elongation. The material surface becomes rough
failure from exposure. It has been shown that AO will       when previously smooth, and the chance of a
target material defects as illustrated in Figure 1. As AO   discontinuity greatly increases (Shimamura &
molecules strike the surface, a chemical reaction           Nakamura, 2009). This may have detrimental
occurs to release gases. When an AO molecule                consequences for a spacecraft experiencing
encounters a material defect, there is simply more          continuous AO exposure. As the outer surface remains
surface area to interact with, which increases the rate     in exposure to the space environment, it will slowly
of chemical reaction. Any defect is an open target for      degrade and eventually discontinuities will appear
AO degradation.                                             that expose sub-surface layers. These sub-surface
                                                            layers will then be exposed unintentionally to space
                                                            environment, and themselves be mechanically
                                                            compromised.

                                                            The erosion yield allows researchers and designers to
                                                            compare the resistance of various materials against
                                                            the impact of atomic oxygen. Equation 1 illustrates
                                                            how the atomic oxygen erosion yield is calculated
                                                            based on several variables.




                                                               Equation 1: Erosion Yield (Groh, Banks, McCarthy, Rucker,
                                                                                Roberts, & Berger, 2008)
Because space is an environment, the fluence of                  NASA’s MISSE
atomic oxygen (F) may vary significantly from one                Among the highly practical, NASA performs periodic
location to another, and from one time to another. As            material tests called Materials International Space
such, a control material—Kapton H witness sample—                Station Experiment (MISSE). Currently, NASA has
is exposed to the same environment as all other                  completed 7 of these tests, with 2 on a return journey
samples, allowing for comparative analysis. Kapton H             and 2 more to have just begun (Woods, 2011). During
has a known and stable erosion yield in the LEO.                 these tests, a spacecraft orbits at a specified altitude
Equation 2 uses the measured mass loss of Kapton H               from earth. Attached to the spacecraft are either one
witness sample to calculate the fluence of atomic                or two Passive Experiment Containers (PECs). A
oxygen.                                                          square of proportions approximately 60 cm by 60 cm,
                                                                 the PEC is layered with hundreds of material samples
                                                                 and exposed to the space environment for a
                                                                 predetermined time; sometimes years. (Woods, 2011)
 Equation 2: Fluence of Atomic Oxygen according to Kapton H      Data is collected throughout the experiment and the
                        witness sample
                                                                 results provide some basis on how to improve current
   (Groh, Banks, McCarthy, Rucker, Roberts, & Berger, 2008)      material, or which characteristics are desirable for
                                                                 future material design. NASA’s experiments provide
Equations 1 and 2 may be combined to give an explicit
                                                                 critical data that encompasses all factors of space
equation (Equation 3) for the erosion yield of any
                                                                 environment; even those we may not be aware exist.
material exposed to the same environment as Kapton
                                                                 The erosion yields measured on the second MISSE
H witness sample.
                                                                 mission varied between 1-9 x 10-24 cm3 atom-1 and
                                                                 contributed to estimated mass losses of ~10%,
                                                                 although some mass losses were as low as 1% over a
                                                                 period of 3.95 years (Groh, Banks, McCarthy, Rucker,
  Equation 3: Explicit Erosion Yield based on Kapton H witness
  sample (Groh, Banks, McCarthy, Rucker, Roberts, & Berger,      Roberts, & Berger, 2008).
                              2008)
                                                                 Lab-based research
These equations provide the rate at which atomic
                                                                 The great advantage of lab-based research is that
oxygen is expected to degrade a specific material. The
                                                                 results can be deduced by way of relating a specific
equations follow from logical reason that an increase
                                                                 reaction resulting from a known cause. Two primary
in material density will lower the erosion yield as
                                                                 methods will be discussed for reducing the mass loss
atomic oxygen will encounter greater resistance to
                                                                 of material exposed to atomic oxygen: (1) coated
penetration; an increase in exposed surface area
                                                                 polyimides and (2) filled polyimides.
would mathematically decrease the erosion yield,
however, due to a greater exposure surface the mass              Coated Polyimides
loss would inevitably be increased. As such, a balance           Hokkaldo University in Japan conducted a study in
can be afforded between the exposed surface area                 2009 to evaluate the protective characteristics of
and mass loss due to erosion.                                    indium tin oxide (ITO) against atomic oxygen
                                                                 degradation. Polyimide films of 125 µm thickness
                                                                 were coated with 25 µm-thick ITO and compared
LITERATURE SURVERY                                               against the pristine samples. Table 1 below illustrates
An ongoing struggle exists to evaluate and improve               the erosion yields of the ITO coated polyimide and
materials used in spacecraft construction. NASA                  pristine polyimide samples. Erosion yields are
performs practical in-environment experiments to                 significantly reduced for the ITO-coated polyimide,
evaluate the erosion yield of various applied and                which indicates a high durability for AO erosion
newly invented materials. Other independent                      (Shimamura & Nakamura, 2009). An increase in the
organizations perform specific laboratory testing of             AO fluence caused the erosion yield to increase; this
methods to improve materials. Each experimental                  demonstrates the limited resistance of the ITO
resource helps us to optimize the performance of                 coating.
space materials.
                                                                  cm-2 was applied and the mass of each sample
                                                                  weighed every 10 hours. After 40 hours of AO
Sample           AO              Erosion         Percent          exposure the polyimide resins injected with 5, 10, and
                 fluence         yield           Reduction
                                                                  15 phr nano-SiO2 reduced their erosion yields to
                 (atoms          (cm3 atom-      (%)
                 cm-2)           1
                                  )                               58.2%, 34.3%, and 16.4% of the pristine polyimide
Pristine         0.3 x 1021      1.7 x 10-24     --               resin. As all other variables are maintained constant,
Polyimide        0.85 x          1.7 x 10-24     --               these percentages directly relate to the mass loss as
                 1021            < x < 3.0 x                      per Equation 1. These results verify that injection with
                                 10-24                            AO-resistant nano-particles significantly reduces the
                 1.30 x          3.0 x 10-24     --               degradation of the polyimide resin. Figure 2
                 1021
                                                                  compares the polyimide resistance to AO erosion
ITO-coated       0.3 x 1021      0.1 x 10-24     5.8
                                                                  before and after injection with nano-SiO2. Some of the
Polyimide        0.85 x          0.1 x 10-24     5.8 < x <
                 1021            < x < 0.8 x     26.6             atomic oxygen molecules that come in contact with
                                 10-24                            the polyimide resin undergo a chemical reaction to
                 1.30 x          0.8 x 10-24     26.6             release gases (CO and RO), while others are cast away.
                 1021                                             The nano composite material effectively reduces the
Table 1: Atomic oxygen erosion yield of pristine and ITO-coated
                                                                  quantity of AO molecules that undergo these
polyimide (Shimamura & Nakamura, 2009)
                                                                  damaging chemical reactions. The AO molecules that
Using the indium tin oxide coating method has some                do not undergo chemical reaction will not contribute
promise, as results have shown lesser material                    to the release of gases or to the loss of material mass.
degradation. The method of application is also rather
simple as it involves direct application to the exposed
polyimide surface. One concerning issue with this
method however, is that any exposed weakness in the
ITO coating may eliminate its effectiveness. If for
example, a hole appears in the coating due to AO
erosion, or some form of physical abrasion, the
sample material will be once again directly exposed to
AO in the environment. AO would then degrade the
material from this source location and erosion yield
might approach that of the pristine polyimide. Thus is
the significant weakness of this protective layer.
                                                                  Figure 2: Schematic diagram of resistance to AO (Wang X. , Zhao,
                                                                  Wang, & Shen, 2007)

                                                                  Method Comparison
 Filled Polyimides
                                                                  The ITO-coating method provides AO resistance to the
An alternative to applying a layer of coating has been
                                                                  exposed surface of a material while the nano-particle
evaluated by the Beijing University of Aeronautics and
                                                                  injection method enriches the entire material with
Astronautics. Unlike the ITO-coating method, which
                                                                  AO-resistant characteristics. The main purpose of
only protects the exposed surface of the material
                                                                  either method is to reduce the erosion yield caused by
from AO erosion, this alternate method enriches the
                                                                  atomic oxygen and thereby reduce the material
whole material with AO resistant nano-particles.
                                                                  degradation and mass loss. Based on the experimental
Through a process of impregnation, clipping,
                                                                  studies performed by Hokkaldo University and Beijing
superposing, and cure molding that will not be
                                                                  University, both methods are comparable for reducing
described here, the material is embedded with nano-
                                                                  the erosion yield. The ITO-coating method reduces the
SiO2 particles. The particles are chosen for their AO
                                                                  erosion yield of its respective polyimide resin to 26.6%
resistant characteristics.
                                                                  with an AO fluence of 1.30 x 1021 atoms cm-2. The
The specific experiment conducted by the Beijing                  nano-SiO2 injection method reduces the erosion yield
University injected 0, 5, 10, and 15 phr nano-SiO2 into           to 16.4% with an AO fluence of 1.036 x 1021.
a polyimide resin. An AO fluence of 1.036 x 1021 atoms            Unfortunately, the AO fluence for each experiment is
not the same, so direct comparison cannot be made.        material mass loss. Further research is recommended
However, both methods do achieve high reduction.          in order to evaluate material behavior during long
                                                          term exposure.
Research should be conducted to analyze how a
material reacts to prolonged AO exposure. Once the
exposed layer of the ITO-coating has been                 REFERENCES
compromised, the material may begin to exhibit            Groh, K. K., Banks, B. A., McCarthy, C. E., Rucker, R. N.,
erosion yields closer to that of the pristine material.   Roberts, L. M., & Berger, L. A. (2008). MISSE 2 PEACE
The nano-SiO2 injection method may maintain               Polymers Atomic Oxygen Erosion Experiment on the
resistance for the lifetime of the material. With         International Space Station. High Performance
further research, it may be proved if the nano-SiO2       Polymers , 388-409.
injection method is superior for long term application.
After applying the ITO-coating, the polyimide resin       Shimamura, H., & Nakamura, T. (2009). Mechanical
would be exposed to AO until significant                  properties degradation of polyimide films irradiated
discontinuities appear on the material surface. The       by atomic oxygen. Polymer Degradation and Stability ,
erosion yield would then be compared against time to      1389-1396.
determine the effectiveness of this coating over the
                                                          Wang, X., Zhao, X., Wang, M., & Shen, Z. (2007). An
lifetime of the spacecraft material.
                                                          Experimental Study on Improving the Atomic Oxygen
                                                          Resistance of Epoxy Resin/Silica Nanocomposites.
CONCLUSION                                                WorldWideWeb: Wiley InterScience.
NASA continues to provide valuable data regarding         Wang, X., Zhao, X., Wang, M., & Shen, Z. (2005). The
the exposure limits of spacecraft material in space,      effects of atomic oxygen on polyimide resin matrix
and particularly in the low earth orbit. Their            composite containing nano-silicon dioxide. Beijing:
experiments provide comparative data for material         Institute of Fluid Mechanics, Beijing University of
manufacturers and designers as well as engineers.         Aeronautics and Astronautics.
Independent researchers are investigating the
prospect of enhancing materials to become more            Woods, T. (den 28 April 2011). Materials: Out of this
resistant to AO exposure. Current experiments             World. Hämtat från NASA:
validate the application of AO resistant layers to        http://www.nasa.gov/centers/glenn/shuttlestation/st
polyimide resin and injection of AO resistant nano-       ation/misse.html den 11 May 2011
particles. Both methods show promising reduction of

				
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