Solid‐State Gas Sensors: A Review

During the past three decades, gas sensors based either on the surface characteristics or the bulk electrolytic properties of ceramics, have been the subject of extensive research and development. The application of these sensors range from air-to-fuel ratio control in combustion processes such as in automotive engines and industrial furnaces to the detection of leakage of inflammable and toxic gases in domestic and industrial environments. While the solid-state physical sensors, measuring pressure, temperature, and other physical parameters have been commercially successful, less success has been achieved by their chemical analogs, to measure moderate to very low concentrations of gases of importance. These gases include: 02, H2, CO, CO2, NOx, SO=, propane, methane, ethanol, and so on. The semiconductor-based chemical sensors owe their popularity to their small size, simple operation, high sensitivity, and relatively simple associated electronics. However, most of them still suffer from nonselectivity. They also have poor shelf-life and are relatively less stable at higher temperatures. The sensing characteristics and performance of some of the solid-state gas sensors are reviewed in this paper, together with their sensing mechanism, which still is a gray area and has not been fully understood. The recent emergence of concern over environmental pollution and effleieney in a variety of combustion processes and of increased awareness over a need to monitor potentially hazardous gases has stimulated substantial research and development in the field of gas sensors. These gases include, CO, CO2, NOx (x = 0.5, 1, and 2), SOx (x = 2 and 3), a fraction of unburned hydrocarbons [ineluding liquefied petroleum gas (LPG) also known as cooking or town gas] and hydrogen. Requirements to detect and monitor these gases has already led to the development of a wide variety of devices and methods, as evideneed by a large number of instruments now available commercially. The majority of these detection and/or metering devices involve the application of surface or bulk chemical and physical properties of materials and can, therefore, be called solidstate chemical sensors. 1-5 However, before addressing the topic of these gas sensors, it is appropriate to briefly outline some of the environmental concerns. LP gas can form explosive mixture with ambient air and ean lead to personal injury upon continuous exposure, even in small concentrations. A number of industrially important eatalytie processes require either producer gas (CO + H2) in various compositions or H2 and CO separately as feed stocks. The producer gas is synthesized by the steam reforming of natural gases and other hydrocarbons under conditions of high temperatures and pressures. The accidental eseape or leak of either CO or H2 from sueh a high temperature and pressure system can be a source of explosion and health hazard. The combustion of hydrocarbon fuels is a widely used method of obtaining useful energy in various industries. Burning of fossil fuel in the "lean-burn combustion" manner is regarded as the most fuel and energy efficient mode of fuel utilization, since under these eonditions, the hydrocarbons in the fuel are fully eonverted into CO2 and H20. However, the compositional analysis of the flue gases in most of the combustion furnaces, indicates the presence of CHJCO/CO2/H2 in the final reaction product, in accordance with the thermodynamic predictions under the conditions of these processes. This could be attributed to the occurrence of the following reaetions side-by-side