Advances in carbon dioxide sensors

Various materials and fabrication methods have been investigated to produce metal oxide semiconductor CO2 gas sensors. The most widely-used materials based on tin dioxide have largely been found to be insuffident for the task. Typically, poor response behaviour to CO2 and high cross-sensitivity are major problems for tin dioxide-based materials. Examples are work conducted by Patel et al. (1994) and Hoefer et al. (1994). The sensors in the study by Hoefer et al showed a marked response in the concentration range of 1000-10000 ppm CO2 at an operating temperature of 270°C. The most significant responses were above CO2 concentrations of 5000 ppm but. 

The gas response change of the lanthanum based film to CO2 on increasing concentration (temperature unreported). Reproduced with permission (Kim et al., 2000). 

Marsal et al. (2003a) also reported on the fabrication of a CO2 sensor based on LaOCl. The films were made by screen-printing. The resulting sensor showed a gas response of 3.4 to 2 500 ppm CO2 at an operating temperature of 260°C. The authors also tested the response to CO and found that their sensor was more responsive to CO2 than to CO by a factor of 3. 

Lee and Meyer (2000) produced microcrystalline films with thicknesses of 2-5 pm made from LaCls doped BaTiOj that showed a good response to CO2 gas. A paste was screen-printed onto a sensor substrate surface and then dried at 80°C. The substrate was subsequently heated at 750°C for 20 minutes. Materials with a range of compositions were tested in air with 1% CO2 content. The greatest response was obtained with BaTiOs- with 10 weight percent LaCls at an operating temperature of 550°C. Sensor responses were also found to be rapid under these conditions (Fig. 13.9). 

10 Results demonstrating the effect of changing the lanthanum content on the gas response of the SnOj film, also showing poor discrimination between carbon monoxide and carbon dioxide. Reproduced with permission (Marsal etal., 2003b). 

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