Chemistry of Materials in Outer Space

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Chemistry of Materials in Outer
Space by Kenneth T. Nicholson October 16,
2007 Northeastern Illinois University
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Outline

• Motivation
• Low Earth Orbit (LEO)--A Dynamic Chemical
  Environment
• Modeling LEO in the Laboratory
• Results
• Future MISSE-6 Experiment
• Summary and Conclusions

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Low Earth Orbit--A Dynamic Chemical Environment

• LEO commonly refers to the small portion of
  outer space
• where satellites and space stations reside,
  300-600 km
• above the Earths surface.

Isnt outer space, just space??

• Space is a vacuummuch less than one
  thousandth of an
• atmosphere (the air we breath) exists at this
  level.
• BUT, there is light (energy) from the sun and a
  few high-
• energy atomic and molecular species

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Low Earth Orbit--A Dynamic Chemical Environment
Radio Microwave Infrared
Visible Ultraviolet X-ray Gamma
Ray
104 -102 1
10-2 10-5 10-6
10-8 10-10-10-12
Wavelength in centimeters
Size Comparison
Buildings/ Insects
Pinhead Single Cell Single
Single Atomic Humans Organisms
Molecules Atoms Nuclei
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Low Earth Orbit--A Dynamic Chemical Environment

• There is no atmosphere to protect space stations
  and satellites from the high energy
• radiation from the sun in low-earth orbit
• Therefore, Vacuum Ultraviolet ( 100-200 nm) and
  X-Ray radiation exist in LEO--photons
• with wavelengths on the order of atoms to
  molecules.

• Energy can be localized to atomic and molecular
  fragments on
• surfaces--play a critical role in chemical
  reactivity

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Low Earth Orbit--A Dynamic Chemical Environment

• Few atomic and molecular species is a misnomer
  when
• considering chemical reactivity.
• There are 1015 atoms/cm2 that compose the
  surface (exposed) layer of most materials.
• The volume of atomic oxygen in LEO alone
  indicates 1015 atoms/cm2 impinge upon a given
  surface every SECOND!!
• If every O atom reacts with the surface, the
  composition of the material changes every
  (!!)second..Even if the chemistry is slow,
  chemical and morphological changes could occur in
  a matter of minutes.

• These chemical species are moving very
  fastgreater than
• 1 km/second!!!

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Experimental Models

• Design experimental systems that are model LEO
  environments.
• High/Ultra-High Vacuum
• Source for Fast Atomic Oxygen and other Chemical
  Reactants
• Ultraviolet and X-ray Light Sources
• Surface Analysis Tools
• Surface Imaging Tools
• Atomic Force Microscopy--Morphology
• Scanning Tunnelling Microscopy---Atomic Structure

• Collaborate with theory groups in order to
  derive a predictive
• scheme for materials chemistry in LEO.

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Experimental Materials--Highly-Ordered Pyrolitic
Graphite

• Graphite is an excellent model system to begin
  LEO modeling.
• Well-defined structure--ease of theoretical
  study
• Unreactive in air
• Study on space shuttle Atlantis for comparison
• Backbone of many polymers and materials used by
  NASA as
• thermal controls

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Erosion of Graphite Profilometry Measurements
Gridded Sample
Reaction Rate 1 C atom removed for every 22
incident O atoms T 373 K 3.0 x 10-25 cm3
per O atom
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Hyperthermal Atomic Oxygen Erosion
of Highly-Ordered Pyrolytic Graphite
PRISTINE HOPG
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Hyperthermal Atomic Oxygen Erosion of HOPG as a
Function of Exposure at 373 K
Clean
C)
B)
A)
3.4 nm
238 nm
238 nm
0 nm
0 nm
0 nm
2 mm
5 nm
2 mm
Pit is gt 200 nm deep!
D)
E)
72 nm
123 nm
0 nm
0 nm
1 mm
1 mm
Pit Growing Within A Larger Pit
Pit Merging Event
Nicholson, K. T., Minton, T. K., Sibener, S. J.,
Proceedings for the 9th International Symposium
for Materials in the Space Environment,
Noordwijk, The Netherlands, 2003.
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Cylinder Diameter as a Function of Atomic Oxygen
Exposure
Cylinder diameter is linear with respect to
exposure time Fluence 3.0 x 1015 O atoms /
second Permits the calculation of of a rate
constant, kprism 2.43 x 10-1 s-1 kprism gtgt
kbasal
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Depth of Circular Pits as a Function of
Hyperthermal Atomic Oxygen Fluence

• Wide distribution of pit depths with
• respect to atomic oxygen fluence
• Pits deepen, on average, as they become
• larger in diametertime.
• The deepening is not linear.
• O atom scattering from the side walls
• causes rate enhancement around the
• periphery.

Nicholson, K. T., Minton, T. K., Sibener, S. J.,
J. Phys. Chem. B, 2005, 109, 8476
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Microscale Cylinders in Eroded HOPG Surfaces

• Line scans of the cylindrical etch pits
• illustrate the deepest point is at the
• peripheral edge.
• This demonstrates these large
• pits grow deeper in a cylindrical
• fashion from the outside.
• From this line scan, it appears that
• only 1 step (z-direction), on average, is
• taken for every 80 carbon atoms from
• the outside edge of the pit. This slope
• is dependent upon the cylinder diameter.
• The cylinders also have non-abrupt
• edges, on the order of 200 nm for this
• specific pit

Z
Side-View-Schematic
Lateral Etching
X
Line Scan
0
75
Depth (nm)
150
1
3
5
7
Distance (mm)
Nicholson, K. T., Minton, T. K., Sibener, S. J.,
J. Phys. Chem. B, 2005, 109, 8476
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Graphite Erosion--Directional Etching--45o
Incident Angle
Short Exposure--8.3 x 1019 atoms
Long Exposure--1.5 x 1020 atoms
2 mm
2 mm
255 nm
100 nm
75
191.2
50
127.5
Z Data
Z Data
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63.8
0
0
1.0
2.0
3.0
4.0
5.0 µm
0.96
1.92
2.88
3.84
4.8 µm
Distance
Distance
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Arrhenius Plot for k prism Calculation of
Activation Energy
Rate constant, kprism, has Arrhenius
like dependence when surface temperature is
considered. Assuming zeroth order kinetics, Ea
10.5 kJ/mol (T 300-423 K)
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Implications for Surface Morphology
Z
Side-View-Schematic
Difference in height between the center of the
pit and edge increases with temperature Conclusio
n rate of downward etching approaches rate of
lateral etching At T 473 K, morphology no
longer involves pits
Lateral Etching
X
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Hyperthermal Atomic Oxygen Reacts with Synthetic
Diamond-- Residual Polishing Artifacts Produce
Additional Reactivity
15 nm
0 nm
5 eV Atomic Oxygen
Tsample 373 K Normal Incidence Fluence
3.3x1020 O Atoms
1 mm
Note grooves are due to polishing

• Pits, similar in shape to the graphite
  experiment, but much smaller in size, are
• found in exposed single-crystal diamond lt100gt.
• The polishing grooves are deeper (3x) after
  exposure which implies the atomic
• oxygen becomes trapped in the grooves before
  reaction
• Experiments are currently underway on samples
  whose peak to valley
• roughness is 1nm.

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Experimental Materials--Kapton
Kapton is commonly used as an atomic oxygen flux
(number of atomic oxygen atoms) meter in LEO.
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Erosion of Kapton by Hyperthermal Atomic
Oxygen-- Temperature Dependence
100 oC
25 oC
5 mm
Etching Depth 4 mm Roughness 31 nm
Etching Depth 7 mm Roughness 69 nm
Exposure 1.5 x 1020 atoms/cm2
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VUV Irradiation of Poly(Methyl Methacrylate)
Decrease in intensity of C-O-C bands
0.10
Decrease in intensity of CO band
Absorbance
0.05
Pristine sample
Irradiated sample
0

• Drastic decrease in intensities of C-O-C
  stretching bands confirming side chain scission
  as main photochemical process
• Reduction in intensity of CO band,
• and slight shift ( 8 cm-1) to lower cm-1 could
  be a sign of conjugation or over-lapping
  absorption from newly created aldehydes or
  ketones

4000
3000
2000
1000
Irradiated Sample (x 10)
0.010
0.0075
Absorbance
0.0050
0.0025
0
Wavenumbers (cm-1)
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Methyl Formate Formation Upon VUV Irradiation of
PMMA
Volatile methyl formate also observed upon
irradiation of PMMA by mass spectrometry (in
situ) This observation is concurrent with the
thin film structure changes. Found the rate of
desorption of HCOOCH3 has a dependence on the
original film thickness. Investigated the
synergistic effects between thermal
atomic oxygen (0.1- 1 eV) and VUV on reaction
rate and surface morphology
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Photographs of Misse-6 SamplesExpected Return
Fall 2008
Graphite--HOPG
Diamond
C-Grade
A-Grade
CVD (small crystallites)
Carbon Nanotubes
Synthetic Diamond Type 1b lt100gt (Colorless)
Natural Diamond Type 2A lt100gt
20 nm Length
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AFM Images of MISSE-6 Samples (Before Trip)
CVD Diamond
HOPG-Grade C
HOPG-Grade A
Carbon Nanotubes 20 nm (z-scale 0-150 nm)
Natural Diamond Type 2A lt100gt
Synthetic Diamond Type 1b lt100gt
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Summary and Conclusions

• Model systems have been developed to begin
  investigating
• the chemical reactivity of materials in LEO.
• Erosion of graphite in LEO appears to be
  dependent on both
• exposure time and direction as well as the sample
  temperature.
• Shapes and sizes of cylinders
• Morphology shift
• Roughness change
• Degradation of kapton has similar
  characteristics, albeit more
• extreme, to graphite.
• Chemistry of PMMA film changes upon the release
  of volatile
• products

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Acknowledgements

• Profs. Steven Sibener, Timothy Minton, John
  Tully,
• George Schatz, and Barbara Garrison
• Sibener Research Group
• Amadou Cisse
• Devon Pennington
• Scott Kelber (Cal-Tech Graduate Student)

• Air Force Office of Scientific Research
• James Franck Institute, The University of
  Chicago
• National Science Foundation--Materials Research
  Science and
• Engineering Center at The University of Chicago