Kumar Rarnohalli: Stephen rod,’ James Colvin, Richard Crockett, and Nat nael Seymour- The University o f Arizona Tucson, A

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T A CE

Kumar Ramohalli,* Stephen Brod: James Colvin: Richard Crockett,% and Nathanael Seymourt

The University of Arizona Tucson, Arizona

Abstract Several ISRU technologies that have the potential

not only for dual use, but also for immediate payo@ through science experiments are presented. Whle the long-term vision for sample return missions, and extraterrestrial bases and colonies, is not lost, the point is made that the in situ resource utilization (ISRU) technologies have the ability to introduce efficiency, economy, and long-term ecology into important missions in the short term, also. Solid state electrolysis cf carbon dioxide to produce oxygen and casbon monoxide is described. The spent stream is mixed with hydrogen and processed into methane and higher hydrocarbons. Hydrogen is produced through solid polymer electrolysis (SPE) of water. Results m quoted from earIier studies of these individuals components. The overall system is integrated and run for extended durations. The separation of useful gases from the spent stream involves a disproportionation system that uses adsorptioddesorption cycles with no moving parts. The same process can separate inert gases on Mars, for example, and provide a stream for gas chromatography for much longer durations than what cdn be done with Earth-transported gases. The other near-term uses include high-power devices that can be operated for short durations after a lightweight ISRU unit produces the necessw chemicals (fuel> over a long duration. A summary cf recent advances in packaging some cf the ISRU plants, along with the necessary controls through modem PC-based software, is also presented.

Introduction Cost-conscious mission planners have a new tech-

nology in their arsenal-ISRU-but, unfortunately, it is still regarded in many circles as a f i r 4 dream. At

'Professor, Department cf Aerospace & Mechanical Engineering; Associate Fellow AIAA 'NASA SERC Graduate Fellow 'Graduate Research Assistant Copyright 0 1995 by K. N. R. Ramohalli. Published by the American Institute d Aeronautics and Astro- nautics, Inc. with permission.

the UA/NASA Space Enpeering Research Center, several key ISRU technologies have been advanced from their pmof-ofancept stage to full system enpeering plants that have operated for thousands af hours. In the process of these experiments, several fundamental issues have been addressed in h g h technologies. The production cf oxygen (and fuel) from carbon &oxide, methanation cf d o n dioxide, and water electrolysis have all involved advancing the state cf the art in (1) high-temperature solid state electrolysis, (2) ceramic- metal seals at high temperatures, (3) application cf thin porous electrodes cf noble metals over ceramic substrates, (4) optimization d reactor operation in methane production, (5 ) component matching and system optimizations, and (6) offdesign performance and transients.

The advantages cf ISRU in reducing the costs cf space missions have been described elsewhere.'-'' The basic concept has been analyzed in the specific context oflunar and Martian missions. A quantitative method- ology for evaluating competing technologies was pioneered at our Center and the fip-of-merit (FoM) has been useful in quick evaluations of various mission architectures. Since mass in LEO represents launch costs, any reduction in this mass should be an indicator of merit. Since most of tlus background information has been reported elsewhere, only the results from a recently completed solid state electrolysis cell m presented here. The fundamental aim is to evolve simple, reliable, economical, and ecologically acceptable production techniques to produce fuels and oxidizers from c h n dioxide.

The Components The single-cell testbed consists cf two sections:

the singlecell zirconia reactor subsystem and the control and data-acquisition subsystem.

The Cell

A single tubular cell was used for the zirconia reactor subsystem. A detailed sthematic af the tubular reactor can be found elsewhere. The cell was purchased from Ceramatec, Inc. (Salt Lake City, Utah). Using energy

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dlspersive X-ray spectroscopy (EDX), the tube was shown to be stabilized zirconia with a mole percent af approximately 4% yttria Or,@) and 4y0 ytterbium ma). These values were determined by obtaining readings for mole percentages at three random locations on the ZrCh and taking the average.

. - I

Electrolytt

Fig. 1. Zirconia solid state electrolyte electrode sandwich.

The tube is 27.94 cm in length with a nominal outer diameter af 0.95 cm. The wall hckness is approximately 0.12 cm. The ZIOz is sandwiched between two electrodes (Fig. 1). The thickness cf the electrode layer is approximately 50 pm. The electrode materials were silver and LSM, with the Ag at the h stream edge and the LSM between the Ag and the ZrCh. For each tube, the negative electrode (the cathode) and the positive electrode (the anode) are constructed af similar materials. The tubular cell was manufactured by Ceramatec, Inc. The electrode leads for the silver- electroded cell are 0.1 cm dlameter silver wires.

All major active components af the ZIOZ reactor system are described in Fig. 2. Table 1 descriies the major components pictured in the figure. A more detailed reactor drawing and component description stating the materials used are given in Ref. 1 1.

I D I H

Fig. 2. Major components of the tubular zirconia reactor.

Table 1. Description of the major Components of thl tubular reactor.

Description

Martian "air" input Cathode exhaust stream Oxygen production stream Ceramic fiber heater Alumina feed tube nconel600 reactor vessel Zirconia solid electrolyte Cathode lead wire Anode lead wire Heater control thermocouple

"Maman air," or the cathode feed gas, enters the reactor through a 1/8-inch O.D. alumina tube. This tube extends to the innermost end of the ZI02 tube. The ZIQ tube is closed at this end, causing the gas to reverse direction and return toward the h n t cf the reactor. Heating cf the gas begins in the alumina tube and may continue after the gas reverses direction. Clam- shell ceramic-fiber heaters a~ used for the heating and were purchased from Electro Heat Systems (Washington, Pennsylvania). The heaters a~ 17.78 cm in length, with an I.D. cf 2.54 cm. The heating a€ the gas causes thermal dissociation, resulting in 0 2 to be available for the solid electrolyte. The 0 2 or CG, depending of the reaction mechanism, then reacts at the interface between the cathode and the ZIOZ, resulting in oxide ions to be ionically conducted through the electrolyte toward the anode. The oxide ions will combine to form 0 2 . The 0 2 is now outside the ZIOz tube, but inside the outermost tube, which is Inconel 600 Schedule 80 pipe purchased from Wiiliams and Co. (Nashville, Tennessee). The remaining gas, or exhaust, on the inside af the ZIOz will consist af a mixture of mostly unreacted CQ and CO. The front cf the reactor is manifolded so that the exhaust can pass out of the reactor.

The front of the reactor consists af two mating stainless steel flanges. The ZrOz tube seats and is sealed with Teflon ferules against the inside of the front haK cf the flange with a 3/8-inch Swagelock fitting. The m half cf the flange is welded to the front end d the Inconel pipe. The anode lead wire is wound around the Swagelock fitting. The ZrCh tube, now connected to the front half af the flange, is inserted into the m half of the flange and into the Inconel pipe. Finally, the flange bolts are secured, and the unit is ready, except for the cathode lead wire, which will be described next.

The reactor is encased in box made c f 0.64-thick aluminum, 27.94 cm long and 13.53 cm square. At either end of the square faces, a 2.54-cm hole is drilled in the center. These holes support the Inconel reactor vessel tube. The box also encloses the thermal

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insulation. The insulation is Kaowool. The R value af the insulation is 4.15 per inch at 70'F.

The Cathode Gas

The cathode gases used for the characterization d the cells were Q, air, and "Martian air." The "Martian air" was purchased as a specialty gas h m Au Products and Chemicals (Long Beach, California). The aim was to have a gas that would closely model the Martial atmosphere. Tests were run with CQ as the cathode feed gas modeling the Martian atmosphere. This was done because the Martian atmosphere consists af approximately 95% CQ. By using a Martian atmosphere simulant instead, any long-term adverse efFm h m the action cf the additional species would become clear (not including any Martian dust). Table 2 lists the species composition af the specialty gas as given by Au Products. The mole percentages listed agree very closely with the actual mole percentages obtained from Viking data.

Table 2. Reported "Martian air" species composition.

DC and AC Power Systems

The control and data-acquisition subsystem is the permanent section designed to function with various types af ZrQ reactor geometries. The reactor requires two types af power: DC power for the electrochemical conduction af O2 and AC power for heating gas and electrolyte.

The DC power circuit will be considered first. A rough schematic of the circuit is shown in Fig. 3 . The DC power supply is an Acopian model number P06HX1600. This unit can control the DC power either as a constant voltage source cf as a constant current source. American Alliance, Inc., digital panel meters are used to display the system operating voltage and current. The range af the volt meter is 0-20 VDC and the range of the amp meter is 0-20 ADC. The data- acquisition system will measure the voltage at the flange for the cell voltage. This will eliminate any IR voltage drop occurring in the external DC power circuit. Current data acquisition will be obtained by measuring the IR voltage drop across a resistor.

Fig. 3. DC power and control circuit. As it is necessary to have very low contact resistance

connections, the cathode (-) lead from the power supply is soldered to the cathode lead coming out cf the reactor. The anode (+) lead is connected to the &ont flange cf the reactor using a mechanical ring connector secured by one af the flange bolts. As the connections are ''solid," an on/off circuit switch is required to enable the DC power to remain off during the initial heating cf the system. Once the system Teaches operating temperature, the switch can be turned on to begin the experiment. The AC power circuit is needed to supply the ceramic fiber heaters. Gas Flow Systems

The gas flow systems and controls consist cf three regions: cathode feed, cathode exhaust, and anode outflow. All tubing for the testbed is either 1/4-inch stainless steel 304 tubing or 1/4-inch Teflon tubing. The choice of material was based on chemical compatability; both are resistant to O2 and CQ. Stainless steel was used in areas where h g h tempera- tures would be an issue and also where the tubing was used as the means af support, such as supporting pressure transducers or electronic flow meters.

The cathode feed gas line begins with the "Martian air" supply. The regulator is a CGA 350 two-stage regulator with a maximum second-stage pressure cf 60 psig. It is constructed d stainless steel to help prevent any impurities h m being introduced into the system. Figure 4 presents a rough schematic cf the cathode supply portion cf the gas circuit. In addition to the "Martian air" supply, there is also a helium purge gas supply bottle. This is connected to the cathode gas supply line through a three-way valve. T h s allows He to be passed through the system as the system is coming to operating temperature and as the system is coollng after an experiment. The purpose af purging is to prevent carbon deposition by removing carbon- bearing species from the reactor during these phases d the test.

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I

Fig. 4. Cathode feed supply circuit.

Flow rate is controlled with a high-resolution needle valve that is part cf a rotameter purchased from Omega (part number FL-3845C-HRV). The rotameter allows a visual inspection to be made d the cathode feed flow rate. After the rotameter, the gas passes through an Omega electronic flowmeter (model number FMA- 5605). Th~s meter will measure a flow with the range af 0-200 ml/min. The output for data acquisition is a 0-5 VDC signal. After the flowmeter, the flow enters the Teflon tube and enters the ZIQ reactor at the alumina tube. Connection is made using a 1/4-inch to 1W-inch tube union

The exhaust from the cathode exits the reactor from the union cross at the front half af the flange. It first enters a stainless steel tube. Stainless steel is used because of the possible high temperature of the exit gas. It also provides support for the cathode pressure transducer). The pressure transducer measures the pressure cf the cathode compartment. It was purchased from Omega (part number PX302-050AV) and will measure from 0-50 psia. The output for data acquisition is a 0-100 mV signal. After the pressure transducer, the flow enters the Teflon tube and can be either vented to the outside cf the lab or passed on to a gas chromatograph (Varian model 3700) for analysis.

Finally, the O2 line is explained. Figure 5 presents t h~s portion af the flow circuit. The O2 exits the 2 1 0 2 reactor at the rear. It first enters a stainless steel tube (stainless steel is used for the same reasons as above). The pressure transducer is the same model as that for the cathode exhaust line. It will measure the pressure in the anode compartment cf the ZIQ reactor. After the pressure transducer, it flows through a Teflon tube back to the control unit where it enters an Omega electronic flowmeter.

Next, the Q flow will pass through an Omega micro rotameter (part number FL-300). This will measure a flow of the range 0.02-15 ml/min. The purpose cf this rotameter is for visual inspection af the 0 2 flow rate. Finally, the 02 is passed into another Teflon tube, where it can either be delivered to the gas chromatograph for analysis or be vented to the outside.

Theoretical Anticipants and Tests Both Martian and lunar oxygen processing schemes involve the electrochemical reduction of carbon dioxide to produce 02. The gas used for the experiment will contain CO,, Ar, N2, CO, and O2 in the relative amounts found in the Martian atmosphere. The reason for using a simulated "Martian air" is to investigate the consequences, if any, that other gases besides CQ will have on the long-term operation of the ZIo2 cell. Lunar processing schemes will also have other gases in the cathode stream dependent on the O2 recovery scheme being employed. For example, Erstfeld and Williams" report that the carbochlorination process will have a high percentage cf CO in the CO, in the cathode gas stream. They report that the equilibrium mixture will be 0636 Co and 0.364 CQ, with the actual mixture being somewhere between pure CO and the equilibrium mixture dependent on processing engineering and reaction kinetics.

Thermodynamic Equilibrium Before bcginning the model, i t is informativc to examine thc thcrmodynamic cquilibrium species composition of CO, when it is hcatcd to the anticipated operating ternpcratures of betwecn 800°C and 1000°C. To help solvc the complicatcd mixture equilibrium equations, Gordon and McBridc's computcr p r o g m CET89 was uscd. This prograni calculates thc spccics mole fractions under conditions af thcrmodynamic cquilibrium at a spccilied tempcrature and pressurc. A list d thc mole fractions d all thc spccies down to a molar percentagc d 1.OE-39 was obtaincd. Figure 6 presents thc species composition as a function cf tcrnperature at a prcssure of 0.93 aim.

Pig. 5. Anode circuit.

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Short-Term Tests

The cell voltage was plotted as a function d the current to make it possible to obtain the total cell resistance by takmg the slope af the voltagecurrent line. In Fig. 7, the line drawn through the data in the region d higher current was used for thls purpose. This was done in accordance with the work described by Richter13 and by Etsell and Flengas.14 The slope d this line was obtained by using the h e a r regression option in the software program GRAFTOOL.0 This was determined to be 0.1202 Q. It should be noted that this is the apparent total resistance of the system. T h s is the value that will be used in theory for obtaining the theoretical voltage. An important point should be made here, how- ever-the total resistance should be obtained with O2 at both sides of the electrolyte. As this was only an initial test, this value will be used as an approximation to the actual total call resistance Fig. 6. Thermodynamic equilibrium

dissociation species composition.

Experimental Results Initial test results were obtained using "Martian air" as a feed gas and using the Ag- and LSM-coated cell purchased h m Ceramatec. The reason for using t h ~ s combination first was to get a "feel" for the operational characteristics of the testbed, as well as to establish % accuracy d the O2 production theory developed. Because its intended use was for understanding the new testbed, the test matrix was not entirely completed. Only a partial matrix was developed, which included one attempt at varying the cell potential h m approximately 0.6 VDC to approximately 2.5 VDC while at an operating set-point temperature d 900'C. The "Martian air" flow rate was set at 74 sccm. These values were chosen because they represent average values for thel operational parameters presented in previous work. Also, five extended runs were made to obtain a better understanding d the system when operated for longer runs.

1

2s !

Fig. 7. Initial voltage-current curve with Ag and LSM electrodes.

Fig. 8. Theoretical anticipations versus experimental results.

Using t h s approximate value for the total cell resis- tance, a spreadsheet was used to develop theoretical values for a plot that would allow a comparison between the theoretical anticipations and the actual experimental values (Fig. 8). As can be seen, there is close agreement between the theory and the experi- ments. Possible explanations for the discrepancies between theory and experiment are presented in Ref. 11.

Long-Tern Tests

To evalmte the long-term operational characteristics d the testbed system, five runs weE perforned. The first three runs were conducted only one day at a time, with the system being shut down at the end d the day. The fourth run was performed for several days continuously. The last run lasted for only one day.

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The "Martian air" flow rate was maintained at approxi- mately 74 sccm as before. There was some slight variation, as the controlling needle valve would allow the flow rate to drift slightly with time, about 4 5 sccm. "Ius was corrected manually as needed. The voltage at the DC power source was maintained at 2.0 VDC whle the voltage over the flange varied between approxi- mately 1.7 VDC and 1.8 VDC. The cell set-point temperature was maintained at 9OOOC for the entire set d runs, with no noticeable drifting from this temperature.

' P Y

16

14

12

10

8 0 25 50 75 100 125 150 175

TIME (mbr)

Figures 9-12 present the results af this runs. For each run, the aL production rate is plotted both as a function d time and as the ionic transfer number. The transference number is important because it represents the percentage of the observed cell current resulting from ionic conduction

Fig. 9. Run 1: production.

f

RUN2 . .t ........ I

i 8 ' A w l ~ l ~ ~ ~ ~ ~ ~ 4 5 0 A I TIME (min)

Fig. 11. Run 2: Ch production.

Fig. 12. Run 2: ionic transference number. The experimental data, combined with a theoretical understanding of the solid state devices, should inspire confidence in mission planners for space, for terrestrial users for fossil fuel exhaust and clean-up and global warming sensors.

In the area d fuel production, as contrasted with oxidizer production, a Sabatier reactor has been built and characterized for methane production At the time d this reporting, tests have been concluded in the catalytic production d methane using c d o n dioxide and hydrogen as the feed gases. Several molar ratios and temperatures were used. Using a thermodynamic equilibrium code (CET89 from the NASA Lewis Research Center), the conversion fraction was calculated and compared with experimental results (Fig. 13)

Fig. 10. Run 1: ionic transference number.

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ZcxuVnwmOFCARBONMOXIDE I

Fig. 13. The Sabatier reactor: results with the CH4/COt versus percentage conversion of COZ plot; theoretical results compared with experimental data at three mole ratios.

The hardware exists in good working condition for further studies and possible integration with hydrogen generators.

Current Work

In an effort to understand the factors effecting cell performance, a disk system was developed. The disk geometty eliminates the thermal gradients and makes the electrolysis process easier to understand. The new system was designed to allow rapid changeout cf disks, reducing the time between data runs. Once again, the electrolyte was made cf 2102, but this time it was stabilized with 9.4% YzQ. The electrodes were hand applied platinum paint. Future tests will use electrodes applied with more precise techniques.

0 0.1 0 1 O J 0.4 0 3 OI 07 M U 1

Gg. 14. Voltage vs. current density data for dis

At this stage, the results are preliminary, but they do show a sigmficant improvement over the older systems. Figure 14 shows the results cf the first data run. This experiment was run in oxygen at 950"C, and shows current densities up to 0.95 Akmz.

geometry.

This paper has shown some of the advantages of using ISRU. Earlier work on using Martian atmosphere as a source cf ISRU, along with more recent work, has shownthat space missions can be more economical by having a lower launch mass, which is directly related to lower launch cost. Much cf th~s has been contingent upon using the Sabatier process as the main system for ISRU propellant production. The Sabatier works in conjunction with a water electrolysis device to produce this propellant. Through a literature review and by experiment, i t has beenshown that a hgh conversion cf CQ to CH4 can be attained (+99%) by using the Sabatier process. The CH, can be used as rocket propellant or as fuel for rovers, and the HzO produced can be converted to HZ and Oz. By experiment, it was also shown at The University cf Arizona SERC that the experimental tabletop Sabatier system could produce CH, when using HZ directly from the water electrolysis subsystem. This experimental research has confirmed the proposed ideas and methods mentioned above. By successll operation cf these experiments, the use of ISRU in space missions is another step closer and will soon be a reality.

An important issue for these oxidizer (fuel) plants has to do with the power needed for their operation. The power is directly related to the current and the voltage used to operate the cell. An extensive literature survey has shown two sets cf the conductivity value range for the zirconia cell. The M e r e m is approximately one order cf magnitude. In order to explain, or to avoid, th~s major inconsistency, a series of carehl experiments is underway. In this connection, a modem PC band data management application (LabVIEW) has been installed on an IBM 486 PC. The system can handle temperatures at eight locations, the flow rates aE COz (input) and oxygen (output) and the spent stream (CO + CG) . The other data include the voltage and current cf the cell and the power needed to maintain the required temperature cf the cell ( b e ) . Full details regarding the acquisition and installation af this data system are available in Ref. 15.

Summary Th~s paper has summarized some recent advances in the technologies cf ISRU. Traditionally speaking, ISRU has been associated with long-tern, "big" missions, such as colonies, bases, and a permanent presence at extraterrestnal sites. At the minimum, a sample return mission has been thought of as an application that could show some of the advantages ISRU offers. While long-term applications are definitely in the ISRU domain, several immehate applications for enhancing, or enabling, science experiments are also possible.

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The simple electrochemical production of oxygen from carbon &oxide, electrochemical pumping (with no moving parts) to high pressures, disproportionation reactions for separating gases from mixtures, fuel production through a Sabatier reactor-all have applications, even in the absence d ernterrestrial colonies or sample returns. A hgh-pressure gas source can keep SUrFaces clean for photovoltaics or radators. Fuel and oxidzer gases can provide a hgh-power burst for subsurface drilling. Isolated inert gases (nitrogen and argon) can extend the life d gas chromatographic measurements. Down here on Earth, the ability to convert carbon dioxide into oxygen and methane (or higher hydrocarbons) can be useful in the next generation d clean cars. The pumping of oxygen to high pressures from low-density air can be very useful in generating on-board oxygen in commercial aircraft where, currently, high-pressure bottled oxygen is carried according to regulations.

The technologes developed at The University of Arizona/NASA Space Enweering Research Center have demonstrated not only applications to a long-term presence on other planets and moons, but also dual use for immediate payoffs here on Earth.

Acknowledgments The work reported here was funded through NASA Grant NAGW 1332. We thank Drs. Robert Hayduk and Murray Hirschbein for their support.

References 'Ash, R., Dowler, W., and Varsi, G., "Feasibility of

Rocket Propellant Production on Mars," Acta Astronaufica, Vol. 5, 1978, pp. 705-724.

Ramohalli, K., Dowler, W., French, J., and Ash, R. , "Some Aspects rf Space Propulsion with Extrater- restrial Resources," Journal of Spacecraft and Rockets,

Ramohalli, K., Lawton, E., and Ash, R. L., "Recent Concepts in Missions to Mars: Extraterrestrial Processes," Journal of Propulsion and Power, Vol. 5 ,

Lewis, J. S., Ramohalli, K., and TnEet, T., "Extraterrestrial Resource Utilization for Economy in Space Missions," Paper IAA-90-604, 41st Congress af the International Astronautical Federation, Dresden, Gy~nany, October 1990.

Ramohalli, K. N. R., "Technologies cf ISRU/ ISMU," Plenary Paper IAA-91-659, 42nd Congress of the International Astronautical Federation, Montreal, C y & , October 1991.

Ramohalli, K., "Overview of In Situ Resource Utilization Activities at the University d Ari- zonalNASA Space Engineering Research Center," Plenary Paper, Space 92, World Space Congress, Washington, D.C., August-September 1992.

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Ramohalli, K. and Sridhar, K. R., "Extraterrestrial Materials Processing and Related Transport Phe- nomena," Plenary Lecture (AIAA-91-0309), 29th Aerospace Sciences Meeting, Reno, Nevada, Janua~y 1991.

'Ramohalli, K., Kirsch, T . , and Preiss, B. , "Figure- of-Merit Approach to ExZraterrestrial Resource Utilization," Journal of Propulsion, Vol. 8, 1992, pp.

Ramohalli, K. N., "Fueling a Revolution in Space Processing," Aerospace America, Vol. 3 1, 1993, pp.

'kewis, J. L. and Lewis, R. S . , Space Resources, Columbia University Press, New York, 1987.

Colvin, J. E., "A Single-cell Testbed for Evalua- tions rf Extraterrestrial Oxygen Production," M. S. Thesis, University of AI-IZOM, Tucson, 1993.

I2Erstfeld, T . and Williams, R., "High Temperature Electrolpc Recovery d Oxygen from CarboChlorina- tion of Lunar Anorhte and the Hydrogenation d Ilmenite: A Theoretical Study," NASA Techcal Memorandum 58214, 1979.

Richter, R., "Basic Investigation Into the Produc- tion d Oxygen in a Solid Electrolyte Process," AIAA Paper 81-1175, AIAA 16th Thermophysics Conference, Palo Alto, Cahfornia, June 1981.

Etsell, T. and Flengas, S., "Overpotential Behavior cf Stabilized Zirconia Solid Electrolyte Fuel Cells," Journal Electrochemical Society: Electrochemical Science and Technology, Vol. 118, 1971, pp. 1890- 1900.

"Wygle, B. S . , "A Digital Control and Data Acquisition System for an Extraterrestrial Oxygen Production Plant," M.S. Thesis, University of Arizona, Tucson, 1994.

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