THIN-FILM GAS SENSORS BASED ON SEMICONDUCTING METAL-OXIDES

In recent years, there has been a gradual realization that an intact environment and properly functioning ecosystems are essential to the continuance of human life; this has led to the tightening up of environmental legislation. As far as air pollution is concerned, gases like sulfur dioxide, the oxides of nitrogen (NOx), carbon monoxide and carbon dioxide are considered to be the main culprits. Most sulfur dioxide is produced by the combustion of fossil fuels that contain sulfur (coal, oil and natural gas), the sulfur being oxidized to produce sulfur dioxide. This is why SO2 emissions can only be prevented by chemical binding. Oxides of nitrogen are also produced when fossil fuels are burnt. These oxides are largely the result of reactions between oxygen and nitrogen from the air. The main source of NOx emissions is automobiles. In 1986 alone, more than half (approximate to 2 million tonnes in the former FRG) of all NOx emissions could be directly attributed to this source. In the same year, somewhat less NOx (1 million tonnes in the former FRG) was produced by power stations, distant heating plants and other conversion areas. Air pollution by CO emissions from automobiles is an even more clear-cut case. As CO is produced by the incomplete combustion of fossil fuels, it is not surprising that automobiles alone produced 6.5 million tonnes of this gas in 1986 (former FRG), while the three other user groups (industry/households, trade and business/power stations and distance heating plant, other conversion areas) are only responsible for approximately 2.5 million tonnes (former FRG). The presence of hydrocarbons (CHx) in exhaust gases is also due to incomplete combustion. Hydrocarbons along with carbon dioxide and water vapour are also considered to be the main causes of the greenhouse effect. With hydrocarbons too, the main source of emissions is the automobile. The emission of CO and CHx from automobiles is particularly abundant when there is an excess of fuel (rich mixture, air coefficient lambda < 1). The introduction of catalytic converters only provides sufficient recombustion of these components in a very narrow range around lambda = 1. Therefore, to obtain the most efficient combustion and the feast emission of pollutants, it would seem necessary to monitor the combustion process in each cylinder of a vehicle and to control it so that the exhaust gas expelled from each cylinder has a lambda value corresponding to the maximum degree of conversion for the catalytic converter. Fast lambda measurements are, therefore, of crucial importance, as they make regulation possible during non-stationary phases of engine operations (starting, braking, acceleration) that account for 80% of the total operating time. Attempts are also being made to introduce combustion regulation in incinerators by means of lambda coefficients. This type of emission would also require sensors that are capable of continuously monitoring the concentration of pollutant components. Sensor elements based on semiconducting metal oxides seem to be promising both for individual cylinder control in automobiles and for monitoring pollutant components.