Advances in carbon monoxide sensors

Metal oxide semiconductor gas sensors have been extensively used to measure the concentration of CO in the environment. Approaches have almost exclusively focused on tin dioxide-based materials, with the focus on a variety of different scenarios. 

Wiegleb and Heitbaum (1994) reported the use of metal oxide semiconductor gas sensors for monitoring NO and CO gas concentrations in automobiles. They studied the variation in CO gas concentration over an extensive period of time. Sn02 was used to detect the change in concentration of CO (Fig. 13.6) and In203 to detect the concentration of NO. It was found that the Sn02 sensor gave results comparable to a near infra-red (NIR) optical sensor but with a faster response time. 

6 Comparison of sensor response to varying concentrations of carbon monoxide for both a SnOj sensor (top) and an infrared gas sensor (bottom). Reproduced with permission (Wiegleb and Heitbaum, 1994). 

Various methods have been used to fabricate tin dioxide metal oxide semiconductor gas sensors. Riviere et al (2003) used screen-printing to produce a tin dioxide film. The composition of the printed ink was varied. The materials showed a significant response to CO gas - typically, a resistance change of 1 log (Ohm i) a QQ concentration of 300 ppm, at an operating temperature 

Various other materials have been examined for their gas-sensing properties to CO. Li et al. (2009) compared different phases of titanium oxide (T1O2) films. Anatase Ti02 was found to show an n-type response to the presence of CO gas. Films of the rutile material, however, exhibit n- and p-type responses depending on the operating temperature. The magnitude 

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