Development of a Solid State Planar Carbon Dioxide Sensor

Aspects relating to and including the development of thick film screen printed CO2 sensors were investigated. Modified sensor structures with potential for prevent- ing sensor degradation due to humidity and solid electrolyte/alumina substrate interaction have been developed and hence tested. Sensor testing characteristics such as the response and recovery times, repeatability and importantly sensor stability and humidity cross-sensitivity were thus determined and investigated.
Firstly, in the quest to lower the cost of sensor production, new and superior synthesis procedures for sensor components (NASICON-solid electrolyte and CuAlO2-oxygen buffer) have been developed which require low thermal input. Bespoke inks from the functional powders were also developed. Sintering schedule optimisation of the NASICON thick films demonstrated that 1100 °C for 60 minutes was the optimal recipe to obtaining a perfectly sintered NASICON film.
To investigate the electrical properties of NASICON having commonly used electrode configurations in sensors, the effect of film thickness and film electrode configurations on electrical properties was explored. No significant dependence on thickness at high temperatures is observed while lower temperature conductivity is dominated by extrinsic behaviour. The film whose electrode configuration was in a basal plane showed superior conductivity properties than when the electrodes were place across the film.
Thermodynamics of reference electrodes using the EMF method using cells (-)Pt, NT6-TiO2( or NT6 - NT3) | NASICON | (α + β) - Al2O3, Pt(+) revealed new information that may have been ignored by other workers; the existence of two temperature regimes and phase transformation of Na2Ti6O13 to higher(lower sodium content) titanates (Na2Ti7O15 and Na2Ti9O19). This data presents the first indications from EMF thermodynamic studies for the possible transformation
as was proposed by other workers.
Three types of sensor structures were developed; (a)the conventional planar sensor architecture(PSA)- fabricated by printing sensing and reference electrodes side by side on the electrolyte (b) the modified planar sensor architecture(M-PSA)- a PSA with an oxygen buffer material deposited on top of the reference electrode and (c) a multi-layered structure referred to as stacked sensor architecture (SSA)- fabricated by first depositing the oxygen buffer on the alumina substrate followed by the reference electrode then NASICON and finally the sensing electrode.
A new sensing material combination (Na2CO3-Li2CO3-BaCO3) used on the PSA improved sensing response and recovery times of the sensors as well as sensor stability. The new sensing electrodes were used in the multi-layered structure (SSA) and performed just as well in terms of response and recovery times as well as stability.
Both the modified sensors(M-PSA and SSA) demonstrated long term signal stability as well as humidity immunity. The sensors also demonstrated that they could operate even at as low a temperature as 225 °C and sensor performance improved as the operating temperature was increased demonstrating how wide a temperature they can operate at. The measurements in this study were done up to 430 °C. Furthermore, since the CO2 concentrations measured were < 500 ppm, it demonstrates how robust the sensors are as they were responsive to such low concentration changes.