Zirconia Oxygen Sensors - Principles of Operation and Design

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 Zirconia Oxygen Sensors
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.

Figure 1. Technox® 802 zirconia oxygen sensors.
Operating Temperatures for 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 zirconia electrolyte is maintained at constant high temperature (700°C-800°C) as shown in figure 2 by incorporating the sensor in an oven.

Figure 2. Schematic of an oxygen level sensor.
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:
• With the removal of grain boundary phases by volatilisation allowing the electrolyte to become permeable and
• With the high thermal stresses often generated when carbon deposits are regularly burnt off.
Reducing Emissions
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.
Control of Air/Fuel Ratios
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.
Oxygen Sensor Design
The device most widely used at present consists of a stabilised zirconia 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) + ½ 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.

Figure 2. Schematic of a section through a ZrO2 oxygen sensor for use in an internal combustion engine.
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.
Degradation of the Oxygen Sensor
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.
Summary
In spite of these difficulties, zirconia exhaust sensors have been developed successfully particularly for applications with stoichiometric air to fuel mixtures.

 iSweek（http://www.isweek.com/）- Industry sourcing & Wholesale industrial products