2011MVK160 Heat and Mass Transport

Project Report

2011MVK160 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

ABSTRACT

The presence of atomic oxygen in the low earth orbit contributes to degradation of spacecraft material. This material experiences erosion that leads to mass loss, while the remaining material is mechanically compromised. Investigation is ongoing to improve the resistance of spacecraft material to the harmful effects of atomic oxygen. The ITO-coating and nano-SiO2 methods of material enhancement are discussed and evaluated. Injection with nano-SiO2 is predicted as being superior for it predicted to maintain resistance throughout the material. Research should be conducted to evaluate this prediction over long term exposure of atomic oxygen.

NOMENCLATURE

erosion yield (cm3 atom-1)

erosion yield of Kapton H witness sample (3 x 10-24 cm3 atom-1)

mass loss (g)

surface area exposed to AO (cm2)

F fluence of atomic oxygen (atom cm-2)

Greek Symbols

density of the sample material (g cm-3)

density of Kapton H witness sample (1.4273 g cm-3)

Subscripts

Ssample material

KKapton H witness sample

INTRODUCTION

Space is often thought of as a volume containing nothing; an environment that exists in the essence of nothingness. This, however, is not true. According to Tori Woods of NASA’s Glenn Research Center “space is considered an environment . . . filled with entities that can be harmful to spacecraft.” Woods lists the various physical and chemical threats: “ultraviolet rays and x-rays from the sun; solar wind particle radiation; thermal cycling (hot and cold cycles); space particles (micrometeoroids and debris); and atomic oxygen.” (Woods, 2011)

This report will focus on the last environmental hazard: atomic oxygen (AO). According to a 2005 study by the Beijing University of Aeronautics and Astronautics, atomic oxygen is the “predominant component and most active species of the LEO atmosphere” (Wang, Zhao, Wang, & Shen, 2005). Atomic oxygen levels in the low earth orbit (LEO) may vary with solar cycle, altitude, season of the year, etc. A spacecraft travelling with a velocity of 7 or 8 km s-1in the LEO (an altitude between 160 and 2000 km) can experience an average atomic oxygen flux between 1012 and 1015 atoms cm-2 s-1(Wang X. , Zhao, Wang, & Shen, 2007). This flux of atomic oxygen poses possible 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 reduce the harmful effects of this foreign environment?

PROBLEM STATEMENT

Atomic oxygen found in the LEO is reactive with polymers and will cause degradation and mechanical failure from exposure. It has been shown that AO will target material defects as illustrated in Figure 1. As AO molecules strike the surface, a chemical reaction occurs to release gases. When an AO molecule encounters a material defect, there is simply more surface area to interact with, which increases the rate of chemical reaction. Any defect is an open target for AO degradation.

Figure 1: Degradation of surface topography due to AO exposure; a) material defects; b) AO-irradiated sample(Shimamura & Nakamura, 2009)

A material exposed to atomic oxygen will suffer mechanical property degradation. Not only will some of the material react and the mass reduce, but the sample as a whole will decrease in tensile strength and elongation. The material surface becomes rough when previously smooth, and the chance of a discontinuity greatly increases (Shimamura & Nakamura, 2009). This may have detrimental consequences for a spacecraft experiencing continuous AO exposure. As the outer surface remains in exposure to the space environment, it will slowly degrade and eventually discontinuities will appear 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 atomic oxygen (F) may vary significantly from one location to another, and from one time to another. As such, a control material—Kapton H witness sample—is exposed to the same environment as all other samples, allowing for comparative analysis. Kapton H has a known and stable erosion yield in the LEO. Equation 2 uses the measured mass loss of Kapton H witness sample to calculate the fluence of atomic oxygen.

Equation 2: Fluence of Atomic Oxygen according to Kapton H witness sample

(Groh, Banks, McCarthy, Rucker, Roberts, & Berger, 2008)

Equations 1 and 2 may be combined to give an explicit equation (Equation 3) for the erosion yield of any material exposed to the same environment as Kapton H witness sample.

Equation 3: Explicit Erosion Yield based on Kapton H witness sample(Groh, Banks, McCarthy, Rucker, Roberts, & Berger, 2008)

These equations provide the rate at which atomic oxygen is expected to degrade a specific material. The equations follow from logical reason that an increase in material density will lower the erosion yield as atomic oxygen will encounter greater resistance to penetration; an increase in exposed surface area would mathematically decrease the erosion yield, however, due to a greater exposure surface the mass loss would inevitably be increased. As such, a balance can be afforded between the exposed surface area and mass loss due to erosion.

LITERATURE SURVERY

An ongoing struggle exists to evaluate and improve materials used in spacecraft construction. NASA performs practical in-environment experiments to evaluate the erosion yield of various applied and newly invented materials.Other independent organizations perform specificlaboratory testing of methods to improve materials. Each experimental resourcehelps us to optimize the performance of space materials.

NASA’s MISSE

Among the highly practical, NASA performs periodic material tests called Materials International Space Station Experiment (MISSE). Currently, NASA has completed 7 of these tests, with 2 on a return journey and 2 more to have just begun (Woods, 2011). During these tests, a spacecraft orbits at a specified altitude from earth. Attached to the spacecraft are either one or two Passive Experiment Containers (PECs). A 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) Data is collected throughout the experiment and the results provide some basis on how to improve current material, or which characteristics are desirable for future material design. NASA’s experiments provide critical data that encompasses all factors of space environment; even those we may not be aware exist. The erosion yields measured on the second MISSE 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, Roberts, & Berger, 2008).

Lab-based research

The great advantage of lab-based research is that results can be deduced by way of relating a specific reaction resulting from a known cause. Two primary methods will be discussed for reducing the mass loss of material exposed to atomic oxygen: (1) coated polyimides and (2) filled polyimides.

Coated Polyimides

Hokkaldo University in Japan conducted a study in 2009 to evaluate the protective characteristics of indium tin oxide (ITO) against atomic oxygen degradation. Polyimide films of 125 µm thickness were coated with 25 µm-thick ITO and compared against the pristine samples. Table 1 below illustrates the erosion yields of the ITO coated polyimide and pristine polyimide samples. Erosion yields are significantly reduced for the ITO-coated polyimide, which indicates a high durability for AO erosion (Shimamura & Nakamura, 2009). An increase in the AO fluence caused the erosion yield to increase; this demonstrates the limited resistance of the ITO coating.

Table 1: Atomic oxygen erosion yield of pristine and ITO-coated polyimide(Shimamura & Nakamura, 2009)

Using the indium tin oxide coating method has some promise, as results have shown lesser material 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.

Filled Polyimides

An alternative to applying a layer of coating has been evaluated by the Beijing University of Aeronautics and Astronautics. Unlike the ITO-coating method, which only protects the exposed surface of the material from AO erosion, this alternate method enriches the whole material with AO resistant nano-particles. Through a process of impregnation, clipping, superposing, and cure moldingthat will not be described here, the material is embedded with nano-SiO2 particles. The particles are chosen for their AO resistant characteristics.

The specific experiment conducted by the Beijing University injected 0, 5, 10, and 15 phr nano-SiO2 into a polyimide resin. An AO fluence of 1.036 x 1021 atoms cm-2 was applied and the mass of each sample weighed every 10 hours. After 40 hours of AO exposure the polyimide resins injected with 5, 10, and 15 phr nano-SiO2reduced their erosion yields to 58.2%, 34.3%, and 16.4% of the pristine polyimide resin. As all other variables are maintained constant, these percentages directly relate to the mass loss as per Equation 1. These results verify that injection with AO-resistant nano-particles significantly reduces the degradation of the polyimide resin. Figure 2 compares the polyimide resistance to AO erosion before and after injection with nano-SiO2. Some of the atomic oxygen molecules that come in contact with the polyimide resin undergo a chemical reaction to release gases (CO and RO), while others are cast away. The nano composite material effectively reduces the quantity of AO molecules that undergo these damaging chemical reactions. The AO molecules that do not undergo chemical reaction will not contribute to the release of gases or to the loss of material mass.

Figure 2: Schematic diagram of resistance to AO(Wang X. , Zhao, Wang, & Shen, 2007)

Method Comparison

The ITO-coating method provides AO resistance to the exposed surface of a material while the nano-particle injection method enriches the entire material with AO-resistant characteristics. The main purpose of either method is to reduce the erosion yield caused by atomic oxygen and thereby reduce the material degradation and mass loss. Based on the experimental studies performed by Hokkaldo University and Beijing University, both methods are comparable for reducing the erosion yield. The ITO-coating method reduces the erosion yield of its respective polyimide resin to 26.6% with an AO fluence of 1.30 x 1021 atoms cm-2. The nano-SiO2 injection method reduces the erosion yield to 16.4% with an AO fluence of 1.036 x 1021. Unfortunately, the AO fluence for each experiment is not the same, so direct comparison cannot be made. However, both methods do achieve high reduction.

Research should be conducted to analyze how a material reacts to prolonged AO exposure. Once the exposed layer of the ITO-coating has been compromised, the material may begin to exhibit erosion yields closer to that of the pristine material. The nano-SiO2 injection method may maintain resistance for the lifetime of the material. With further research, it may be proved if the nano-SiO2 injection method is superior for long term application. After applying the ITO-coating, the polyimide resin would be exposed to AO until significant discontinuities appear on the material surface. The erosion yield would then be compared against time to determine the effectiveness of this coating over the lifetime of the spacecraft material.

CONCLUSION

NASA continues to provide valuable data regarding the exposure limits of spacecraft material in space, and particularly in the low earth orbit. Their experiments provide comparative data for material manufacturers and designers as well as engineers. Independent researchers are investigating the prospect of enhancing materials to become more resistant to AO exposure. Current experiments validate the application of AO resistant layers to polyimide resin and injection of AO resistant nano-particles. Both methods show promising reduction of material mass loss. Further research is recommended in order to evaluate material behavior during long term exposure.

REFERENCES

Groh, K. K., Banks, B. A., McCarthy, C. E., Rucker, R. N., Roberts, L. M., & Berger, L. A. (2008). MISSE 2 PEACE Polymers Atomic Oxygen Erosion Experiment on the International Space Station. High Performance Polymers , 388-409.

Shimamura, H., & Nakamura, T. (2009). Mechanical properties degradation of polyimide films irradiated by atomic oxygen. Polymer Degradation and Stability , 1389-1396.

Wang, X., Zhao, X., Wang, M., & Shen, Z. (2007). An Experimental Study on Improving the Atomic Oxygen Resistance of Epoxy Resin/Silica Nanocomposites. WorldWideWeb: Wiley InterScience.

Wang, X., Zhao, X., Wang, M., & Shen, Z. (2005). The effects of atomic oxygen on polyimide resin matrix composite containing nano-silicon dioxide. Beijing: Institute of Fluid Mechanics, Beijing University of Aeronautics and Astronautics.

Woods, T. (den 28 April 2011). Materials: Out of this World.Hämtat från NASA: den 11 May 2011