Oxygen Sensors
Many processes use zirconia
based oxygen sensors for monitoring and control. Solid-state
sensors have found uses in a wide range of applications,
including, control of atmosphere in materials processing
and control of air-to-fuel ratio in combustion.
Technox® 802 a fully stabilised Yttria zirconia (FSZ)
material is used to produce a wide range of components
for application as oxygen sensors. Typically these include
thin walled, open and closed end tubes, flat plates and
sheets.
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Technox® 802 Zirconia Oxygen Sensors
The characteristics required for an oxygen monitoring device
will vary with its application. Thus a flue gas monitor would
be required to operate between 200°C and 600°C when
interpretation of the EMF generated would be difficult, due
to the temperature dependence of the electrode kinetics and
the variation in EMF due to temperature.
As the boiler output is changed the flue gas temperature will
change similarly so that the EMF output would vary. To obtain
a constant and representative EMF the ZrO2 electrolyte is maintained
at constant high temperature (700°C-800°C) as shown
in figure 1 by incorporating the sensor in an oven.
Atmosphere Control
Atmosphere control using a dedicated monitor requires operation
at low partial pressure of oxygen and temperatures in the range
800°C-1200°C. The gas carburising process used to harden
steel components is a typical application. However at the high
end of this temperature range the electronic conductivity can
become significant.
Care must be taken to avoid impurities such as Fe3+ which
could enhance this reaction. Further problems are encountered
(i) with the removal of grain boundary phases by volatilisation
allowing the electrolyte to become permeable and (ii) with the
high thermal stresses often generated when carbon deposits are
regularly burnt off.
The legal requirements in some countries to control exhaust
gas emission and the rapid increase in fuel prices have led
to the demand for greater control of the internal combustion
engine.
The effectiveness of the equipment added to reduce pollution
depends on accurate control of the air to fuel ratio, which
may be monitored with an oxygen sensor, either before combustion
or more usually from the exhaust gas composition.
The exhaust gas is usually reducing, hence there is only a
small pO2 present. Since the amount of O2 present under thermodynamic
equilibrium depends greatly on the air to fuel ratio, it is
essential for the sensor, particularly the electrode surfaces,
to have catalytic properties in order to equilibrate the pO2
as quickly as possible.
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| Figure 1. Schematic of
an oxygen level sensor |
Oxygen Sensor Design
The device most widely used at present consists of a stabilised
ZrO2 electrolyte tube with platinum electrodes deposited on
the inner and outer surfaces. With different pO2 on inner and
outer surfaces an EMF is generated. If carbon monoxide is present
a further reaction is possible:
CO (gas) + O2 (electrolyte) -> CO2 (gas) + 2e- (electrode)
The catalytic reaction at the platinum electrode
CO (gas) + 1/2 O2 ->CO2
can minimise the above effect. Further reactions can occur
when H2, H2O and NOx are present. The successful application
of an exhaust monitor requires a simple and inexpensive device
which is able to operate in a harsh environment at temperatures
in the region of 900°C in the presence of thermal shock.
A typical sensor is shown in the diagram, figure 2, with the
format being similar to an 18mm diameter sparking plug.
Electrolytes and Electrodes
Yttria stabilised zirconia (YSZ) is used as the electrolyte
with platinum coated electrodes, with the outer layer of Pt
coated with a porous oxide to protect the electrode from erosion.
The microstructure of this layer is of importance since it governs
the oxygen equilibrium conditions and also the response time
of the device.
For control devices, a well made zirconia electrode has a response
time < 200ms above 350°C.
Another factor of importance is the degradation of the sensor
due to ageing of the system, where the main change is an increase
in the response time and a decrease in the EMF output. Poisoning
of the catalytic activity of the Pt electrode can occur by the
deposition of lead oxides or the formation of oil rich deposits
on the sensor.
In spite of these difficulties, ZrO2 exhaust sensors have been
developed successfully particularly for applications with stoichiometric
air to fuel mixtures.
Figure 2. Schematic of a section through a ZrO2 oxygen sensor
for use in an internal combustion engine
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