X hits on this document





12 / 13

Electrochemical oxygen gas sensors

overpotential and a parallel R-C

the current, combination

so that the system and consequently

behaves displays

as an

RC time constant. In (1 - l/e) or 63% of states (see standard Yarwood 1956).

fact RC is the time taken for transport of the charge between the two equilibrium texts on electricity, e.g. Fewkes and

A2. Stoichiometry changes in the double layer The equilibrium between the oxygen-ion conductor and the gas phase can be expressed as

The equilibrium constant, K , for this process is K = [Ob] [h']'/[V&]p#;.

Thus, the stoichiometry of the oxide varies with po2,the extent depending upon the value of K which varies from one oxide to another: with Zr02-based or Biz03-basedmaterials changes in stoichiometry will be small in the first case but substantial in the second.

Clearly, when a change in po2is made then given sufficient time an equilibrium is established throughout the ionic conductor. However, as has been noted, if t= 1 then as far as the EMF is concerned only changes in the double layer region are important; this region must come to chemical equilibrium with the gas phase before a stable EMF can be established and this may effectively introduce a further capacitance with a consequent delay in the response.

AS. Eflect on the gas phase of changes in stoichiometry At short times after a change in po, the [h' ] profile at the inter- face is very steep. These holes are minority carriers and hence move by diffusion (Heyne and Beekmans 1971). There are two consequences of the high diffusion rate of holes at the interface. The first is that a significant electrode overvoltage results due to charge transfer, which perturbs the measured EMF. The second is that a po2gradient is set up in the gas phase adjacent to the electrode-electrolyteinterface, which results in a diffusion overvoltage. Both the above overvoltages effectively slow the electrode response.

A4. Electrode reversibility The electrode reaction involves several consecutive steps (Pizzini 1973, Gur et a1 1980) any of which may, in principle, be rate determining. In practice the particular rate-determining step depends upon the parameters of the system. A low electrode resistance is necessary to reduce the RC time constant and allow

rapid electrode response. In order to achieve this,

sensors care must be taken to ensure boundary (i.e. electrolyte-electrode-gas)

a long and this

in practical three-phase entails the

preparation deteriorates

of a thin porous electrode. Response time if the electrode contains glassy material, sometimes












pretreated at too the electrode (e.g.

high a temperature the Pt) may sinter resulting



in reduced



A5. Hydrodynamics in the gas phase Before the electrode can respond to a po2change, that change must be transmitted to the electrode surface. In many practical situations (e.g. P O , 2 10' Pa, ZrOz-YzO? ceramic, 700 "C) it is the hydrodynamics in the gas phase that limits the sensor response. For example, in a sampling system the gas must pass along a tube before it reaches the sensing electrode: the response is limited by the time taken for the gas to traverse the tube; less obviously the front carrying thep,, change becomes diffuse as a result of viscosity effects in the tube; finally the gas may not impinge directly onto the sensing electrode so as to avoid

cooling the electrode. Consequently there is also a diffusion step which delays the transmission of thepO2change to the electrode. On the other hand, at low po, values (e.g. < 10 Pa in mixture with an inert gas) or at low temperatures ( < 500 "C, ZrOz-based sensor) the electrode response may be limiting.

In practical sensors the electrodes are sometimes coated with a porous layer for protection. This additional coating acts

as a diffusion barrier diffusion coefficient (N 100 mmZs-I) and are usually short.

and also delays of oxygen at response delays

response. However, the

  • -

    700 'C is very high

due to a porous barrier

References Anderson J E and Graves Y B 1981 J. Electrochem. Soc. 128 294

Anderson J E and Graves Y B 1982 J. Appl. Electrochem. 12 335

Badwal S P S, Bannister M J and Garrett W G 1984 Science and Technology of Zirconia II, Advances in Ceramics, vol. 12 ed. N Claussen et a1(Columbus, Ohio: Am. Ceram. Soc.) p 598

Badwal S P S and De Bruin H J 1979 Aust. Chem.Eng. 20 9

Badwal S P S and De Bruin H J 1982 J. Electrochem. Soc. 129 1921

Bauerle J E 1969 J. Phq's. Chem. Solids 30 2657

Crank J 1956 The Mathematics of Diffusion (Oxford: Oxford University Press) p 48

De Bruin H J and Badwal S P S 1980 J. Solid State Chem. 34 133

De Jong H L 1983 US Patent Specification 4 384 935

Dietz H 1982 Solid State Ionics 6 175

Dietz H, Haecker W and Jahnke H 1977Advances in Electrochemistry and Electrochemical Engineering. vol. 10 ed. H Gerischer and C W Tobias (New York: Wiley) pp 1-90

Fewkes J H and Yarwood J 1956Electricity and Magnetism vol. 1(London: University Tutorial) p 123ff

Fleming W J 1977 J. Electrochem. Soc. 124 21

Fouletier J 1982/3 Sensors and Actuators 3 295

Fouletier J, Fabry P and Kleitz M 1976 J. Electrochem. Soc. 123 204

Fouletier J, Seinera H and Kleitz M 1974 J. Appl. Electrochem. 4 305

Goge M, Especel D, Heggestadt K and Gouet M 1985 Transport-Structure Relations in Fast Ion and Mixed Conductors ed. F W Poulsen et a1(6thRisa Int. Symp. On Met. and Mat. Sci.. Risu Nat. Lab.) p 291

Gur T M, Raistrick I D and Huggins R A 1980 J. Electrochem. Soc. 121 2620

Haaland D M 1977 Anal. Chem. 49 1813

Haaland D M 1980 J. Electrochem. Soc. 121 796

Hetrick R E, Fate W A and Vassell W C 1981 Appl. Phys. Lett. 38 390

Hetrick R E, Fate W A and Vassell W C 1982 IEEE Trans. Electron Devices ED-29 129


Document info
Document views20
Page views20
Page last viewedWed Oct 26 21:58:15 UTC 2016