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42 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 44, NO. 1, FEBRUARY 1995

A Zirconia-Based Lambda Gas Sensor with Pseudo-Reference

Mohieddine Benammar and William C. Maskell

Abstract- A sensor operating on the potentiometric principle without the need for a stable reference gas is described. This was achieved by using a miniature zirconia pump-gauge device including a diffusion path between its internal volume and exter- nal atmosphere. A pseudo-reference oxygen partial pressure was generated within the internal volume and acted as a reference for potentiometric determination of the external oxygen partial pressure. The sensor was tested in the normalized air-to-fuel range 0.7 to 1.7 and displayed a characteristic similar to that of a conventional potentiometric sensor with a fixed stable reference, e.g. air.

I. INTRODUCTION

XYGEN sensors have a range of applications including 0 the determination of air-to-fuel ratio (A@) in fossil- fuelled combustion systems (automotive, domestic and in- dustrial); appropriate control of the A/F leads to improved fuel economy and reduced noxious emissions. Zirconia-based oxygen sensors are well suited to these elevated temperature applications due to the nonvolatility of the components.

Zirconia is the generic title for solid solutions prepared by mixing and sintering ZrOz with lower valent oxides such as CaO or Y2O3 [l] . In these solutions a fraction of the Zr4f ions is replaced by, for example, Ca2+ or Y3+ ions. For charge balance, oxygen ion vacancies are created in the solid solution which allow oxygen ions to hop from an occupied to a vacant site. This process is analogous to that of electrons in a semiconductor. In a P-type semiconductor, doping results in the generation of positive holes (vacancies) in the electronic energy levels. This allows electrons to move by jumping from an occupied to an unoccupied energy level.

The motion of oxygen ions in zirconia is thermally ac- tivated, becoming significant at temperatures above 300OC. As a result, stabilized zirconia behaves as an electrolyte when operated at elevated temperatures; this property enables zirconia to be used for both oxygen sensing and electro- chemical pumping. A zirconia cell used for either purpose requires the provision of a porous, electronically conducting, electrode to act as an interface between the electrolyte and the external electronic circuitry. The electrochemical reactions involved in the process of sensing and pumping occur at the interface between the electrode, the electrolyte, and the surrounding gas; this interface is referred to as the three-phase boundary.

Manuscript received March 29, 1994. The authors are with the Energy Technology Centre, Middlesex University,

IEEE Log Number 9407773. London N11 2NQ, UK.

diffusion hole \r

\ CURRENT SOURCL\

Fig. 1. unit are not shown.

Schematic diagram of the sensor. The heater and temperature control

In general an electrochemical cell (Fig. 1) containing a solid electrolyte with mobile oxygen ions may be represented as follows [2]:

where MI and M2 are normally porous metal electrodes (e.g., Pt or Ag) in contact with the solid electrolyte, SE, separating two atmospheres with the oxygen partial pressure Po2 and P 0 2 , ~ ~ f . When operated at a sufficiently high temperature (> 500OC) the cell (1) may be used for oxygen pumping and sensing as described below.

Without any external electric source applied to the cell (l), the latter develops an EMF generated by the tendency of oxygen ions within the electrolyte to diffuse from the high to low oxygen partial pressure side. Provided that the partial electronic conductivity of the ceramic is low (0.01), the EMF is given by the Nemst equation

where R,T, and F are respectively the gas constant, the absolute temperature, and the Faraday constant. If P02,ref is known (reference gas), then Po2 may be determined from a measurement of E (at a known operating temperature). Thus the cell may be used as a gauge for the measurement of unknown oxygen partial pressures.

The cell (1) can also act as an electrochemical oxygen pump: by applying a potential difference across the electrodes of cell (1) oxygen may be electrochemically transferred from the cathode to the anode side of the cell. The electrode reaction is

(3) 02 + 4e 2 202-.

0018-9456/95$04.00 0 1995 IEEE

BENAMMAR AND MASKELL: A ZIRCONIA-BASED LAMBDA GAS SENSOR WITH PSEUDO-REFERENCE

Oxygen is reduced on the cathode, and the resulting oxygen ions migrate through the electrolyte to produce free oxygen at the anode. The flux of oxygen electrochemically “transferred” through the electrolyte is related to the applied current, I, by Faraday’s law

J, = d n / d t = - I /4F. (4)

The negative sign in (4) is a consequence of the oxygen flux being in the opposite direction to the pumping current. The factor 4 in (4) is due to the fact that four electrons are involved in the electrochemical pumping of one molecule of oxygen.

The Lambda sensor is based upon a single electrochemical cell (1) operated as a gauge (2). The reference oxygen partial pressure is generally obtained from uncontaminated air. This sensor enables measurement of oxygen partial pressure in the excess air region, accurate determination of the stoichiometric point, and distinction between the excess air and fuel-rich regions. The disadvantage of this sensor is that the reference gas must be piped to the sensor. It is possible to eliminate this problem using a device with an internal reference generated from a metal-metal oxide redox couple. However, this solution suffers from high temperature sensitivity and is susceptible to leakage effects which can limit sensor life 121.

In the following, a novel Lambda sensor (Fig. 1) not re- quiring a stable reference oxygen partial pressure is described 131. This sensor includes a pump, a gauge, and an intemal volume. The device includes also a pore acting as the diffusion path between the intemal volume and external atmosphere. The principle of operation is based upon maintaining excess air within the internal volume of the sensor by continuous electrochemical pumping. This may be achieved using a constant current applied to the pump, the magnitude of which is chosen to be sufficiently high for all possible compositions of the external gas including fuel-rich conditions. The intemal oxygen concentration is then dependent upon the external gas composition, the amplitude of the current, and the hole dimensions. This internal gas acts as a pseudo-reference for the potentiometric determination of the oxygen concentration in the sample gas. This is achieved by a simple measurement of the Nemst EMF provided by the gauge cell.

11. THEORY OF OPERATION

The theory below is presented for a pump-gauge device operating in oxygen-inert gas mixture, corresponding to the lean region in a combustion system. The device incorporates a diffusion path with a leak conductance, ~ 0 2 . For a diffusion path of length, L, and uniform cross-sectional area, S, the leak conductance is given by

where 0 0 2 is the oxygen diffusion coefficient. When a neg- ative current, I , is applied to the pump, oxygen is electro- chemically pumped into the internal volume; the resulting flux is given by Faraday’s law (4). Simultaneously oxygen leaks out of the internal volume through the diffusion path. The

43

flux of oxygen leaking out is given by Fick’s first law of diffusion,

(6)

where PO^,^ and Po2 are respectively the oxygen partial pressures inside the inner volume and in the surrounding atmosphere. Po2 is the oxygen partial pressure the sensor is used to measure. At steady state the total effective flux into or out of the internal volume is equal to zero, revealing

J d = ~02(POZ,w - PO21

PO^,^ Po2 - IRTL/4Fao2. (7)

E = ( R T / 4 F ) 1n(poz/Pon,t,). (8)

The Nemst EMF provided by the gauge is given by

Combining (7) and (8) leads to

E = - (RT /4F) ln ( l - {I/4Fgo2Po2}). (9)

For pure bulk diffusion the oxygen diffusion coefficient 0 0 2

and go2 as a consequence are inversely proportional to the barometric pressure. In this case, the gauge EMF is depen- dent on the oxygen concentration in the sample gas and is independent of the barometric pressure.

When operated in the excess oxygen region the sensor output, E , is typically below 200 mV at 1000 K. In the fuel- rich region, provided that the pumping current is sufficient to maintain excess oxygen within the intemal volume of the device, the output EMF is typically higher than 600 mV. In the substoichiometric region, oxygen pumping also involves the reaction

(10) CO:! + 2e 2 CO + o’-.

111. EXPERIMENTAL

A device with an internal volume of approximately 1 mm3 was constructed as previously described 141, [5]. The diffusion hole was laser-drilled in the zirconia 161 and was approx- imately 700 pm long. Thick-film platinum heaters [5] , [7] were affixed with a high-temperature cement on either side of the device, enabling operation in the range 500-800°C. The schematic diagram of the electronic circuit used for operating the sensor is shown in Fig. I .

The device was first tested in a mixture of air and nitro- gen, precisely controlled using commercial mass-flow meters (Brooks Instruments). Tests were then conducted in the flue of a premixed gas-bumer where the air-to-fuel ratio was set by adjusting the flows of air and natural gas (92-96% methane, the remaining 4 8 % divided approximately equally between nitrogen and propane) at the inlet to the bumer. All tests were carried out at a sensor temperature of 680°C.

Iv. RESULTS AND DISCUSSION Fig. 2 shows typical results obtained in air-nitrogen gas

mixtures using two different values for the pumping current. Conformance of the gauge EMF characteristics to (9) was investigated as follows. Equation (9) may be rewritten as

I / ( 4 F { 1 - exp{ -4FE/RT}] ) = ao2P02 (9a)

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 44, NO. 1, FEBRUARY 1995

.. II

5 0" c

-45-

-55

' " I I

-

-70 1 2 3 4 5 6 7 8 9 1 0

Oxygen concentration / %

Fig. 2. Characteristics of the sensor when operated in air-nitrogen gas mixtures at 953 K. Values for the pumping current: U. -2mA: H, -4mA. Lines shown are theoretical (9) using the values: hole length, 0.7 mm; hole diameter, 150 pm; diffusion coefficient of oxygen, 160 mm2s-1.

='I ,: '" I

0 2 4 6 8 10

Oxygen concentration / %

Fig. 3. tion (9). Data and symbols are as in Fig. 2.

Plot of I / ( 4 F { 1 - exp{ - 4 F E / R T } ] ) versus oxygen concentra-

and was tested as shown in Fig. 3 using the data shown in Fig. 2. The straight line obtained indicates excellent agreement with theory. The slope of the best line was determined using linear regression analysis. Using the approximate values for the length, L = 700pm, of the diffusion path and the value for 0 0 2 of 160 mm2s-l [SI, the effective diameter of the diffusion hole was calculated to be 150 pm which was similar to that estimated using an optical microscope.

Figs. 4 and 5 show the results obtained by operating the sensor in the gas-buming flue. The sensor behaved like a classic potentiometric sensor with a stable reference gas; the stoichiometric point (A = 1) was accurately identified and,

-1000' ' I ' I ' I ' I ' 0.7 0.9 1.1 1.3 1.5 1.7

x Fig. 4. Characteristics of the sensor when operated in the gas-burning flue at 953 K. Symbols are as in Fig. 2. X is the normalized air-to-fuel ratio ( X = {actual air-to-fuel ratio} / { stoichiometric air-to-fuel ratio}).

-25 .-I t

$ -35 c

-65' I I I I I I 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

x Fig. 5. as in Fig. 4.

The results of Fig. 4 in the excess oxygen region only. Symbols are

when using a current of -4 mA, the excess oxygen and fuel- rich regions were clearly distinguished. For low values of the air-to-fuel ratio (A < 0.85), excess oxygen was not maintained within the intemal volume when using a pumping current of -2 mA: the oxygen pumped into the intemal volume was then insufficient to oxidize all CO (and H2) diffusing into the intemal volume of the device.

V. CONCLUSION

A Lambda zirconia oxygen sensor requiring neither a sup- plied reference gas nor a metal-metal oxide redox reference has been described. A pseudo-reference was generated by electro-

BEN4hlMAK AND MASKELL. A ZIRCONIA-BASED LAMBDA GAS SENSOR WITH PSEUDO-REFERENCE 4s

chemical oxygen pumping. The sensor showed conformance with theory in air-nitrogen mixtures. When operated in the flue of a gas-burning system the two sides of stoichiome- try were distinguished and the substoichiometric point clearly iden ti fi ed.

REFERENCES

W, C. Maskell, Techniques und Mechanisms in Gas Sensing. Bristol, CT: Adam Hilger, 1991, pp. 1 3 5 . -, “Inorganic solid state chemically sensitive devices: electro- chemical oxygen gas sensors,” J . Phys. E; Sci. Insrrum., vol. 20, pp. 1156-1 168, 1987. M. Benammar and W. C. Maskell, “Gas analysis apparatus and method,” UK Patent Application No 9401 158.2, 1994. H. Kaneko, W. C. Maskell, and B. C. H. Steele, “Miniature oxygen punip-gauge. 1. Leakage considerations,” Solid State Ionics, vol. 22, pp. 161-172, 1987. hl. Benammar and W. C. Maskell, “Miniaturised solid-state pump- gauge oxygen sensors: Practical aspects,” Proc. Inr. School Materials Sri. Trchnol., July 1-12, 1992, Erice-Sicily, Italy, in press. W. C. Maskell and B. C. H. Steele, “Miniature amperometric oxygen pump-gauge,” Solid State Ionics, vol. 28-30, pp. 1677-1681, 1988. M. Benammar and W. C. Maskell, “Temperature control of thick-film printed heaters,” J. Phys. E: Sci. Instrum., vol. 22, pp. 933-936, 1989. R. H. Perry and D. W. Green, Eds.,Perry’s Chemical Engineer’s H m t f h o o k , 6th ed. New York: McGraw-Hill, 1984, p. 3.285.

Mohieddine Benammar took a “maitrise” (equiva- lent to B.Sc.) in electrical engineering and a “DEA’ (equivalent to M.Sc.) in automation at ENSET, University of Tunis. He received the Ph.D. degree in instrumentation from Middlesex University, Lon- don, UK.

He continued at Middlesex University as a Re- search Fellow. His fields of interest include solid- state gas sensors and general electronic instrumen- tation. He recently took up an academic post at the Institut Preparatoire aux Etudes d’IngCnieur de Nabeul in Tunisia.

William Maskell received the BSc. degree in physics at King’s College, London and the Ph.D. degree in chemistry from London University, London.

He joined the Ever Ready Company in 1969 working on research and development projects relating to batteries. In 1983, he developed solid state electrochemical oxygen sensors at Imperial College, London and is now Reader in Energy Technology at Middlesex University. His research interests include zirconia oxygen sensors and

electrochemical reactions involving gases on solid electrolytes.