Electrochemical oxygen gas sensors
overpotential and a parallel R-C
the current, combination
so that the system and consequently
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
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
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
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