Improvements to Zirconia Thick-Film Oxygen Sensors

Thick-film zirconia gas sensors are normally screen-printed onto a planar substrate. A sandwich of electrode-electrolyte-electrode is fired at a temperature sufficient to instigate sintering of the zirconia electrolyte. The resulting porous zirconia film acts as both the electrolyte and as the diffusion barrier through which oxygen diffuses. The high sintering temperature results in de-activation of the electrodes so that sensors must be operated at around 800 °C for measurements in the percentage range of oxygen concentration. This work shows that the use of cobalt oxide as a sintering aid allows reduction of the sensor operating temperature by 100–200 °C with clear benefits. Furthermore, an interesting and new technique is presented for the investigation of the influence of dopants and of the through-porosity of ionically-conducting materials.


Introduction
Oxygen gas sensors have many applications including, and in particular, the monitoring and control of combustion processes. The incorporation of oxygen sensors in the latter systems can enable the air-to-fuel ratio to be maintained at an optimum value with respect to both efficiency and emissions. Domestic systems are cost-sensitive and require low-cost sensors. Screen-printed thick-film sensors [1] do offer the prospect of low cost as they can be produced in substantial numbers per substrate [2]. The high temperature necessary for sintering the zirconia results in deactivation of the electrodes due to grain growth of the platinum metal particles in the electrodes. Deactivated electrodes, combined with insufficiently low through-porosity, necessitate high sensor operating temperatures in order to support sufficient currents to drive the sensor into the diffusion-limited region. Power input rises with increasing operating temperature but also operating life is expected to be reduced. It is shown that the use of cobalt oxide as a sintering promoter reduces the through-porosity of the zirconia for given firing conditions; this allows substantial decrease of the sensor operating temperature without adversely affecting electrochemical performance.
Various studies have shown the importance of matching of thermal expansion coefficient (CTE) of thick-film and substrate. Alumina substrates, which are widely used in the electronics industry, have a CTE of 7×10 -6 o C -1 while zirconia displays a value of 10-11×10 -6 o C -1 . Hence, upon cooling from the sintering temperature of 1300-1400 o C the zirconia film is prone to crack [1][2][3]. In this work a substrate was used consisting of a mix of magnesia (MgO) and the spinel, MgAl 2 O 4 . By choosing an appropriate mix proportion the CTE can be selected between 9-14×10 -6 o C -1 .

General
All preparative work was done under clean conditions in a Class 100 enclosure. Substrates of MgO-MgAl 2 O 4 with a CTE to match zirconia, manufactured and supplied by Advanced Ceramics of Stafford, UK, were used throughout.

Printing, Doping and Firing of Films
A DEK 1202 screen printer was used for all printing operations. Three types of samples were prepared: 1. A platinum electrode was printed and dried then over-printed with a zirconia electrolyte film and dried. 2. As above then overprinted with a second electrode and dried. 3. A zirconia electrolyte film only was printed and dried with no platinum electrodes. These samples were then either A. fired without doping or B. doped with cobalt (nitrate) and then fired (the cobalt nitrate decomposed to the oxide during the firing process). In order to dope the films a solution of cobalt nitrate hexahydrate in ethanol (50:50 by weight) was prepared (reagents from Sigma-Aldrich, Analytical Reagent grade) which was then applied to the printed layer with a brush and dried prior to firing to provide a level of cobalt doping in the zirconia of approximately 5 cation %. Films were fired in air using a muffle furnace with a maximum operating temperature of 1600 o C.

Characterisation of Films
Electrodes of the Type 2 films were electrically connected to a voltage supply and the current flowing in the circuit measured. The sensor was operated in a furnace in a mixture of air and nitrogen, using the experimental arrangement described previously [4], gas flow rates being controlled by mass-flow valves. Substrates were scribed on the reverse side to the film, snapped and the fracture edges of Types 1 and 3 films examined using a scanning electron microscope (Cambridge Instruments 240 Stereoscan operated at 30kV).

Micrographs
Types 3A, 3B 1A and 1B zirconia films fired at 1000 o C showed no significant differences as this was below the temperature at which significant sintering occurred. The zirconia thick-