Sensors and Actuators B, 3 (1991) 15-22 IS

Development of a solid electrolyte sensor for oxygen in hot gases

Edward B. Makovos and Cbung-Cbhm Liu* Chemical Engineering Department and Electronics Design Center, Case Western Reserve University, Cleveland, OH 44106 (U.S.A.)

(Received April 20, 1990; in revised fonn August 6, 1990; accepted August 8, 1990)

Abstract

High-temperature oxygen detection is used in the emission control and optimization of combustion and metalhtrgic processes. To overcome the disadvantages of potentiometric sensors, we have investigated an actively operated three-electrode system on yttria-stabilized xirconia solid electrolyte. At various selected potentials, there is a strong and stable relationship between the current and the concentration of gaseous oxygen. The current is not limited by the gas diffusion and is, therefore, unaffected by the gas flow. It is demonstrated that the electrode and electrolyte degradation can be considerably retarded by the proper selection of the electrode voltage protile. A metal&n resistance thermometer deposited directly on the sensor is found to be suitable for the on-site temperature monitoring. This sensor also responds to combustion products present in a hot gas. A possible development of a multicomponent sensor is discussed.

Introduction

Measurements of oxygen at high temperatures, such as in controlling emissions in internal com- bustion engines and industrial burners, in metal- lurgic processes, etc.; are highly desirable. Electrochemical sensors are often used in those applications. The most commonly used technol- ogy employs a solid electrolyte in a concentration cell operating in the potentiometric mode [ 11. The disadvantages of this system are related to the aging of the solid electrolyte at high temperature and to the potentiometric method. The shortened life of solid electrolytes is important because the structural complexity of a concentration cell is often associated with high manufacturing costs. Potentiometric measurements can be complicated by the differences between the local environment of a sensing electrode and the bulk gas [2,3], and are susceptible to chemical interferences.

To overcome the limitations of the concentra- tion cell, efforts have been made to adapt other electrochemical techniques, particularly the limit- ing current method [4], to high-temperature oxy gen sensing. Uncertainty concerning the reaction mechanism has impeded the development of non- equilibrium methods. The overall oxygen reduc- tion reaction, expressed in the Kroeger-Vink notation [5] is

O,(gas) +2Vo+4e’=200X

*Author to whom correspondence should be addressed.

0925-4005/91/33.50

where V, is an oxygen vacancy (hole) in the electrolyte, e is a free electron in the electrode, Oo is an oxygen atom at its site in the electrolyte structure and the superscripts .” are respectively the positive, negative and neutral charges.

Isaacs has demonstrated [6] that a one-step interaction of gaseous oxygen, holes and electrons at the three-phase boundary line (TPBL) between the gas, electrode and electrolyte does not occur. Various rate-controlling steps of a complex reac- tion mechanism have been considered:

(1) gas diffusion control external to the elec- trode surface or within pores;

(2) adsorption and dissociation on the elec- trode/electrolyte interface;

(3) diffusion of the dissolved oxygen in the electrode or electrolyte;

(4) charge transfer; (5) diffusion of the electron holes in the elec-

trolyte; (6) diffusion of adsorbed oxygen on the elec-

trode or electrolyte into the TPBL or the elec- trode/electrolyte interface.

Although step (1) is rate controlling in most aqueous systems, it is not the case here. Therefore, the diffusion-limited current method cannot be directly adapted for the high-temperature sensor. Isaacs showed [6J that the probable slow step is the migration of adsorbed oxygen to the elec- trode/electrolyte interface followed by the fast charge-transfer reaction.

Despite the differences between the high- and low-temperature mechanisms of oxygen reduc-

0 Elsevier Sequoia/Printed in The Netherlands

16

tion, Kondo and colleagues [7] have developed a galvanometric method of high-temperature oxy- gen sensing by artificially limiting the transport of gaseous Oz. This was achieved by covering the cathode with a gas-impermeable ceramic contain- ing a thin hole, and applying a potential difference between the cathode and anode of up to 2.5 V. Unfortunately, the high applied potential is ex- pected to result in a fast electrolyte degradation by reducing the metal oxides of the ceramic to the corresponding metals and encouraging the disso- lution of the cathode in the electrolyte [8]. Both processes will eventually result in a short circuit. However, Kondo et al. reported that cur- rent versus voltage characteristics of their sensor were not seriously affected by the cathode degra- dation while the current was still limited by 0, diffusion.

The work described in this paper will further investigate the potential of non-equilibrium elec- trochemical oxygen sensing at high temperature. However, no attempt will be made to limit the diffusion in the gas phase. Instead, a three-elec- trode system will be used to find the most suitable parameter related to the oxygen content of a hot gas. Structural simplification of the system will be introduced by using the solid electrolyte to sup- port the electrodes and by investigating the possi- bility of temperature measurements by film resistor thermometry. The problem of long-term instability inherent in all high-temperature appli- cations will be addressed.

Materials

Yttria-stabilized zirconia (YSZ) (composition 1373, Zircoa Inc., Solon, OH) was used as the solid electrolyte and the base to support the elec- trodes. The material contained 8 _+ 1 wt.% Y20, in ZrO,. This ceramic has high thermal and shock resistance, is impermeable to gases and has high ionic conductivity over a wide range of tempera- tures. In fact, the material becomes an essentially ionic conductor above 900 K. The YSZ substrates were used either as prepared or polished by the manufacturer (to an average surface roughness of 32 pm).

The electrodes were prepared from inks for the thick-film printing based on palladium and plat- inum (compositions A4855 and A4731 respec- tively, Engelhard Corporation, East Newark, NJ), and .gold (composition 8835-18, Electra-Science Laboratories, Inc., King of Prussia, PA). The connections between the electrodes and the instru- mentation were made by platinum wires, 0.25 mm in diameter. Parts of the YSZ substrate or the metal surfaces which were to be protected from

the atmosphere were covered by a high-tempera- ture adhesive (composition Ultra-Tempm 516, Aremco Products, Inc., Ossining, NY). Drops of the same adhesive were also used to position the sensor about 5 mm above an alumina support.

The sensor was tested in a capped quartz tube, 46 mm inside diameter, 55 cm long including the cap (manufactured by Quartz Scientific, Inc., Fairport Harbor, OH). Ceramic tube insulators (model Omegatite 450, Omega Engineering, Inc., Stamford, CT) were used to separate and insulate the wires inside the quartz tube. The inlet of the tube was connected to a four-component gas blender (model FM4587, Linde Division, Union Carbide Corp., Somerset, NJ), whereas the outlet was vented at 1 atmosphere. Our experiments employed research grade helium and the primary standard mixtures in helium of 20.00% oxygen, 0.994% hydrogen (all supplied by Air Products and Chemicals, Inc., Tamaqua, PA), 1.21% propane (Linde Specialty Gases, Danbury, CT) and 3.999% hydrogen (Matheson Gas Products, Inc., &caucus, NJ).

Fheedure and equiplnent

Two configurations of YSZ substrates were employed in this study. One was a disk, 24.6 mm in diameter, 3.5 mm in thickness; the other was a 13.8 x 20.4 x 2.1 mm plate. The electrodes were deposited on both sides of the substrates by the thick-film metallization process and fired at 1300 K for 30 min. A detailed description of this process may be found elsewhere [9]. The resulting electrodes were porous, well attached to the sub- strates, and 10 +_ 2 urn thick. Figures 1 and 2 show the patterns used in this study. All patems included a three-electrode system and an ohmic resistor in the form of a narrow metal strip. The resistor was analysed as a potential temperature sensor. Each sensor contained two working elec- trodes of different metals to facilitate the selection of the material for the working electrode. The disk substrates had a 1 mm deep square notch cut along the edge perpendicular to the surface. The patterns on the opposite sides of the disk were aligned by using this notch and the edge of the disk as references. The patterns for the plate substrates were aligned relative to the four edges.

The platinum lead wires were attached to the bonding pad portion of the electrodes by parallel gap welding. A Type R thermocouple junction made of 0.20 mm diameter wires was placed in the alignment notch of the disk sensor or pressed against the side of the plate sensor. This thermo- couple indicated the sensor temperature. The high-temperature adhesive was used to wver the

17

Exposed surface

Botlom Surface Materials:

Fig. 1 The pattern for the thick-film metal deposition on the surface of a plate sensor (dimensions in mm). The metal surfaces covered with insulation are the bonding pads. The top surface has the reference electrode (middle contact) and two working electrodes (side contacts). The bottom surface has the counter electrode (middle contact) and the film strip resistor.

bonding pad portion of the electrodes, the ther- mocouple and the area around it. The thermocou- ple was connected through a cold junction compensator to a thermocouple thermometer (models Omega-CJ and DT81 respectively, Omega Engineering, Inc., Stamford, CT). The overall uncertainty in temperature measurements reported by the manufacturer was less than 1 K.

The sensor was placed in the middle of the quartz tube in such a way that the electrodes would not be in contact with any solid surface. The lead wires of the electrodes and the thermo- couple were extended through specially cut grooves in the cap of the quartz tube to be connected to the appropriate inputs of a poten- tiostat (model 273, Princeton Applied Research Corporation, Princeton, NJ). One working elec- trode was operated at a time. The lead wires of the film resistor were connected to a multimeter

Top Surface Malerlals

I Platinum

m Allernak metal

&q lnsulatlan

0 Exposed surlace

L c,x,z Of symmetry

Fig. 2 The pattern for the thick-film metal deposition on the surface of a disk sensor (dimensions in mm). The metal surfaces covered with insulation are the bonding pads. The top surface has the reference electrode (middle contact) and two working electrodes (side contacts). The bottom surface has the counter electrode (middle contact) and the film strip resistor.

(model 175, Keithley Instruments, Inc., Cleve- land, OH). The ohmmeter part of the instrument had a 0.01 R resolution.

The quartz tube with the sensor inside was heated by a tube furnace (type 21100, Thermolyne Corporation, Dubuque, IO). The furnace power was regulated manually based on the readings of the thermocouple attached to the sensor. One to four different gases were supplied to the testing tube at the total volumetric flow rate of 200 to 500 cm3 mm’. The ratio of the gases was con- trolled automatically according to the specified individual flow rates or mole fractions. The total flow rate was maintained constant for an experi- ment, except where the influence of the flow rate on the sensor performance was examined. Over the range of the flow rates used, the gas in the tube could be assumed to be at chemical equi- librium at temperatures above 1000 K [lo].

The sensor temperature in all experiments- was maintained at 1250 K or 1150 K to a 0.5 K preci- sion. Experiments were designed so that the sup-

18

plied gas mixture would have the desired compo- sition at equilibrium. The equilibrium composi- tions were calculated by using a computer program (code CET-85, National Aeronautics and Space Administration, Washington, DC) [ 1 l] adapted for use with microcomputers [12]. Be- tween experiments the sensor was kept at the test site inside the quartz tube at 500 to 700 K in an uncontrolled atmosphere. The temperature was raised to the desired value prior to each experi- ment at the rate of x0.1 K s-‘. Resistance mea- surements on the proposed film strip thermometer were performed during the temperature adjust- ments and recorded as a function of temperature measured by the thermocouple.

The three-electrode system was operated in the cyclic voltammetric mode with a potential scan rate of 0.100 V s-‘. The analog output signals from the potentiostat representing the potential and current were converted to digital form by a 16 bit accuracy A/D converter (model DT2801/ 5716A, Data Translation, Inc., Marlboro, MA), sampled at 100 Hz frequency and stored on mag- netic media. Data acquisition, storage and analy- sis were assisted by commercial software ( UnkelScopeR, release 3.01, Unkel Software, Lex- ington, MA) and a microcomputer (PS/2 model 30, IBM Corporation, Armonk, NY).

Experimental rear&

Cyclic voltammograms obtained from the disk and plate sensors were similar in shape. Figure 3 shows a typical cyclic voltammetric response of a plate sensor at the indicated conditions. The mag- nitude of the current at various points between the potential scan limits of -0.700 and +0.450 V

-0.70 -0.50 -0.30 -0.10 0.10 0.30 0.50

Potential, V

Fig. 3 A cyclic voltammetric response of a plate sensor at 1250 K in the atmosphere of helium and oxygen at the indi- cated levels. Working electrode area 1.3 x lo-’ m2. Potential scan limits -0.700 and +0.450 V vs. the Pt reference elec- trode. Potential scan rate 0.100 V s- ‘. Volumetric gas flow rate 3.75 x 10e6 m3 s-l (linear gas flow rate 0.25 cm s-9.

was appreciably influenced by the oxygen content of the gas phase, but had a non-zero value even in the absence of oxygen. Stable voltammograms were usually obtained within 5 to 10 min (depend- ing on the gas flow rate) after the change in the gas composition entering the quartz tube. This delay is comparable to the estimated time neces- sary for the gas composition inside the tube to stabilize at a new value.

The values of the current at the two selected potentials are shown in Fig. 4 as a function of the mole fraction of oxygen in the gas. The plotted values were sampled at the decreasing potentials in the cathodic region and at the increasing poten- tials in the anodic region. A highly linear relation- ship between the sampled current and the logarithm of the oxygen mole fraction is demon- strated. The Figure contains data points collected at the linear gas flow rates of 0.25 and 0.56 cm s-i. No influence of the flow rates on the current output of the sensor is observed at the oxygen levels of 0.4% and above, which are reported on Fig. 4. As the oxygen concentration in the gas approached zero, the gas flow rate began to influ-

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-10 0.1 1 10 loo

02 fractm hole %)

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15

14

13

12

11

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6

7 0.1 1 10 100

02 fraction (mole Kf

Fig. 4 The calibration data collected at the indicated poten- tials under the conditions described for Fig. 3, for volumetric gas flow rates of 3.75 and 8.33 x 10m6 m3 s-l (linear gas tlow rates 0.25 and 0.56 cm s-l).

19

-14

-16

-18

-20

-22

-24 0.1 1 10 100

Oxygen Concentration, mole %

Fig. 5 Long-term variability of calibration characteristics of a Pt working electrode on a disk sensor. Temperature 1150 K. Potential scan limits -0.700 and +0.450 V vs. the Pt reference electrode. Potential scan rate 0.100 V s- ‘. Current sampled at -0.650 and +0.400 V. Working electrode area 3.3 x lop5 m*. Volumetric gas flow rate 3.75 x lop6 m3 s-’ (linear gas flow rate 0.25 cm s-l).

ence the current: lower values of the flow rate resulted in a larger current. The voltammogram stability remained as long as the flow rate was constant.

The long-term stability was evaluated on one of the disk sensors. Figure 5 presents the calibration lines obtained with a platinum working electrode at the indicated conditions. The magnitude of the response decreased noticeably during the week of continuous operation. After a month of operation, the platinum electrodes appeared as a dull oxidized foil almost completely separated from the ceramic substrate. Other working electrode materials tested, e.g., palladium, gold, indium and tin oxides on platinum, provided similar or lower current densities in the presence of oxygen, and inferior long-term stability. Therefore, only platinum working electrodes were extensively tested.

An identical disk electrode was tested at the reduced potential scan limits of -0.625 and +0.325 V versus the platinum reference electrode, and lower temperature ( 1150 K). An oxygen sen- sitivity similar to that in the prior experiments was obtained with the current sampled at -0.600 V and +0.300 V. Figure 6 shows the cali- bration lines obtained with the platinum electrode in the first week of operation. A considerable improvement in the long-term stability of the sensor is recognized compared to that obtained with the wider potential window.

The sensitivity of the method to combustion

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Oxygen Concentration, mole %

Fig. 6 Long-term variability of calibration characteristics of a Pt working electrode on a disk sensor. Temperature 1150 K. Potential scan limits -0.625 and +0.325 V vs. the Pt reference electrode. Potential scan rate 0.100 V s-l. Cm-rent sampled at -0.600 and +0.300 V. Working electrode area 3.3 x lo-’ m*. Volumetric gas flow rate 3.75 x 10e6 m3 s-’ (linear gas flow rate 0.25 cm s-l).

products was investigated by a series of experi- ments in the presence of various levels of carbon dioxide and water, with oxygen at 0.4, 2.0 and 8.0 mol%. These conditions were achieved at equilibrium by supplying appropriate quantities of oxygen, propane and helium to the furnace. The equilibrium gas compositions were calculated with the aid of the NASA code CET-85. The results of these experiments are summarized by Fig. 7. The infiuence of the combustion products on the sampled current was present in both the anodic and cathodic regions. However, the magni- tude of this influence appears to be inversely related to the concentration of oxygen.

The sensitivity of the proposed detection method to water vapor was examined quantita- tively in the presence of oxygen at 0.4, 2.0 and 8.0 mol%. The desired equilibrium conditions were achieved by supplying appropriate quantities of helium, oxygen and hydrogen to the tube fur- nace at a constant flow rate. Figure 8 presents the current values sampled at the indicated conditions as a function of mole fraction of water in the gas phase. A linear dependence between the sampled current and the Hz0 concentration between 0.0 and 3.2% was observed at the constant levels of oxygen. Again, the sensitivity to water was in- versely related to the concentration of O2 in the analysed range.

The temperature responses of the platinum fihn resistor on the plate sensor and gold film resistor

Component: rUP 1 run II run III run IV --- co2 0.00 % 0.36 % 0.72 % 1.43 % w20 0.00 % 0.48 % 0.96 % 1.91 96

Fig. 7 Dependence of the current values sampled at -0.650 and +0.4OOV vs. the Pt reference electrode on the level of combustion products. Pt working electrode on a disk sensor operated continuously under the conditions described for Fig. 3. Volumetric gas flow rate 6.25 x 10e6 m3 s-’ (linear gas flow rate 0.42 cm SC’).

on the disk sensor are summarized by Figs. 9 and 10 respectively. Since the resistors were printed directly on the YSZ substrates, the conductivity of the ceramic might contribute to the overall con- ductivity, especially at high temperatures. The platinum film offered a stable and relatively high resistance and a highly linear temperature re- sponse below 110 R with a sensitivity of 110 mf2 K- ‘. As the temperatures were raised above 920 K, the conductivity of the tested circuit peaked and began to decrease. The conductivity of the gold resistor circuit was much higher and responsive to a higher temperature. The tempera- ture sensitivity gradually increased from 6 to 8 rnQ K-i between 600 and 1260 K. Neither Pt nor Au resistors produced any detectable change in response or physical appearance within one week of testing.

Discussion

The cyclic voltammetric investigation of the high-temperature oxygen-helium system showed that the current sampled at a chosen value of potential is a reliable indicator of the O2 concen- tration in the gas. It was also important to ob-

0 0.50 1 1.50 2 2.50 3 3.50 4

Water Concentration (mole %)

b.4% - 02 D 2.0% - .8.0% on 02

0 0.50 1 1.50 2 2.5~3 3.50 4

Water Concentration (mole %I

Fig. 8 The moisture sensitivity of a Pt working electrode on a disk sensor. Temperature 1150 K. Potential scan limits -0.635 and +0.325 V vs. the Pt reference electrode. Potential scan rate 0.100 V s-i. Working electrode area 3.3 x 10-r m*. Volu- metric gas flow rate 3.75 x 1O-6 m3 s-l (linear gas flow rate 0.25 cm s- ‘).

serve that the current was not affected by the flow rate of the gas. One realizes that the gas move- ment does affect the thickness of the diffusion boundary layer, or the resistance to the electro- chemical reaction due to the gas diffusion. Our observation, therefore, confirms the known fact that the reaction rate in this case is not limited by the gas diffusion. The absence of this limitation is highly advantageous, as the sensor will require no protection from the effects of gas turbulence.

The behavior of the system as the 0, concen- tration was at or approaching zero made an ex- ception. This was the case when our system was supplied with pure helium, but could possibly contain some residual oxygen. Since the current was inversely related to the gas flow rate, the diffusion limitation of the oxygen transport in the

Plate Sensor

Pt Resistance Thermometer

500 600 700 800 900 1000 1100

Temperature (K)

Fig. 9 Calibration characteristics of a platinum tihn resistor printed on a plate sensor.

Dirk Smmr

Au Resistance Thermometer 0 Day 2 A Day 5

Temprrrture. K Fig. 10 Calibration characteristics of a gold film resistor printed on a disk sensor.

gas phase may not explain the observed behavior. This and the fact that considerable current was present in the virtually pure helium point to the importance of the electrochemical side reactions in the oxygen-free environment. Among those, a possible steady-state oxidation reaction is the con- version of the electrode metal to its respective oxide, while the reduction reactions are the re- verse of the oxide formation on the electrode or the formation of the reduced zirconium in the

electrolyte phase. Any reactions involving a net consumption of the electrode or the solid elec- trolyte material lead to the reduced life span of the sensor, and are obviously undesirable.

The long-term performance of the sensor within the -0.700 to +0.450 V potential window and the eventual detachment of the electrodes conf%med that the electrode oxidation was, in- deed, one of the undesirable side reactions. The net electrode oxidation was accompanied by a gradual decrease of the current. This observation is consistent with the suggestion of Isaacs [6] that the overall reaction rate is limited by the migra- tion of adsorbed oxygen at the electrode/elec- trolyte interface.

The magnitude of the current due to the side reactions increased at higher applied potentials, especially in the ,anodic region. We could, there- fore, expect that the reduction of the potential scan limits and the reduction of the net positive charge transfer would improve the long-term stability of the signal. The dramatic stability improvement within the -0.625 to +0.325 V potential window, as illustrated by Fig. 6, justified this approach. The optimal operating regime for a specific application should be selected on the basis of the long-term stability and the magnitude of the sensitivity.

The ability of the sensor to detect moisture at high temperatures was also important, since water is a common combustion product. The mecha- nism for this sensitivity is not known. However, we expect a direct electrochemical reaction of water to be unlikely. A more plausible explana- tion would involve a moisture-induced change in the sensor materials, particularly in the electrode/ electrolyte region, that may accelerate the rate-de- termining step.

Unlike potentiometric sensors, the actively op- erated sensor benefits from multiple sensitivities. This distinction is due to the availability of mul- tiple sensing parameters, e.g., current values sam- pled at different potentials. If the sensitivities of the selected parameters to the various gas compo- nents do not completely overlap, all active com- ponents may be detected [ 131.

Although the performance of our sensor has not been investigated as a function of tempera- ture, this is known to affect the current directly. Therefore, continuous monitoring of the sensor temperature is necessary. The temperature-depen- dent responses of the platinum- and gold-film resistors demonstrated the feasibility of the pro- posed approach to the on-site temperature mea- surements. A sensor incorporating this method is structurally simpler than one with a thermo- couple, and is easier to miniaturize.

The advantage of the platinum film was its relatively high resistance, which is easier to mea-

22

sure accurately than that of gold. However, the high-resistance film is not suitable for use at high temperatures if it is printed directly on the solid electrolyte. The future development of this method should investigate the possibility of de- positing a thin layer of electrically insulating ma- terial between the film resistor and the electrolyte substrate. The maximum operating temperature of the gold resistor is probably limited by the melting point of gold, 1234 K, which is signifi- cantly lower than that of platinum, 2045 K.

coaehr3ioas

The results of our work have demonstrated that a system of three fihn electrodes deposited directly on a solid electrolyte substrate and oper- ated in a cyclic voltammetric regime can act as a high-temperature oxygen sensor. The presence of a diffusion limitation in the gas phase is not necessary or desirable for the sensor operation. On-site temperature monitoring can be performed by a film resistor thermometer. The availability of multiple sensing parameters allows the proposed method to be integrated into a possible multicom- ponent sensor.

Acknowledgements

Support of this work by Edison Sensor Tech- nology Center is greatfully appreciated. Professor E. A. Fletcher and his research group at the Department of Mechanical Engineering, Univer- sity of Minnesota, provided the microcomputer version of the NASA code CET-85.

References

1 H. Die&, W. Haecker and H. Jahnke, Electrochemical sensors for the analysis of gases; in H. Gerischer and C. W. Tobias (eds.), Advances in Electrochemistry and Electro- chemical Engineering, Vol. 10, Wiley, New York, 1971, p. 1.

2 M. Kleitx, E. Siebert and J. Fouletier, Recent develop ments in oxygen sensing with a solid electrolyte cell, in T. Seiyama, K. Fueki, J. Shiokawa and S. Suzuki (eds.), Proc. Int. Meet. Ckemical Sensors, Fukuoka, Japan, Elsevier, New York, 1983, p. 262.

3 P. Fabry, M. Kleitx and C. Deportes, Sur l’utilisation dune electrode ponctuelle dam3 les celhtles a oxyde elec- trolyte solide, J. Solid State Ckem., 5( 1) (1972) 1.

4 E. M. Logothetis and R. E. Hetrick, in D. Schuetxle and R. Hammerle (eds.), Funabmentals and Applications of Chemical Sensors, American Chemical Society, Washing- ton, DC, 1986, p. 136.

5 F. A. Kroeger, The Chemistry of Imperfect Crystals, North- Holland, Amserdam, 1964, p. 178.

6 H. S. Isaacs, Zircoma fuel cells and electrolyxers, in N. Claussen, M. Ruehle and A. H. Heuer (eds.), Aabances in Ceramics, Vol. 12, The American Ceramic Society, Inc., Columbus, OH, 1984, p. 406.

7 H. Kondo, H. Takahashi, K. Saji, T. Takeichi and I. Igarashi, Thin lilm limiting current-type oxygen sensor, Proc. 6th Sensor Symp., Tokyo, Japan, 1986, pp. 251-256.

8 A. H. Heuer, Case Western Reserve University, Cleveland, OH, personal communication, 1988.

9 V. Karagounis, L. Lun and C. C. Liu, A thick-film compo- nent cathode three-electrode oxygen sensor, IEEE Trans. Biomed. Eng., BME-33(2) (1986) 108.

10 E. B. Makovos, Case Wcsem Reserve University, Cleve- land, OH, unpublished work, 1989.

11 S. Gordon and B. J. McBride, Computer program for calculation of compkx chemical equilibrium compositions, rocket performance, incident and re&cted shocks, and Chapman-Jouguet detonations, NASA Lewis Research Center. Cleveland. OH. NASA SP-273. Interim Revision. 1976.

12 M. Pipho, T. Kappauf, E. Whitby and E. A. Fletcher, AT and PSI2 version of NASA wowam CET-85. Dmartment of Mechanical E&meting, -University of Minnesota, Min- neapolis, MN, 1987.

13 S. Zaromb and J. R. Stetter, Theoretical basis for identifi- cation and measurement of air contaminants using an array of sensors having partly overlapping sekctivities. Sensors and Actuators, 6 (1984) 225.

Biographies

Edward B. Makovos is currently a doctoral student in the Chemical Engineering Department of Case Western Reserve University. He received his B.S. and M.S. degrees, both in chemical engi- neering, from Case Western Reserve University. His research interests are in the areas of electro- chemical sensors, enzyme electrodes and biomedi- cal sensors.

Chung-Chiun Liu is the Wallace R. Persons professor of sensor technology and control as well as the director of the Electronics Design Center at Case Western Reserve University. His research interests include electrochemical sensors, mi- croelectronic fabrication processes and electro- chemical sciences.