Uwe Lampe

Metal oxide sensors

New gas sensors based on novel semiconducting metal oxides are developed. In comparison to the sensors based on SnO2 their properties are improved in several items. The main improvements are the lower influence of humidity, the long-term stability, the ability to start measurement immediately after switch on and reduced unit spread. The sensors will be marketed within this year.

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 (NO x ), 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 SO 2 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 NO 2 emissions is automobiles. In 1986 alone, more than half (2 million tonnes in the former FRG) of all NO x emissions could be directly attributed to this source. In the same year, somewhat less NO x (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 (CH x ) 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 CH x from automobiles is particularly abundant when there is an excess of fuel (rich mixture, air coefficient λ<1). The introduction of catalytic converters only provides sufficient recombustion of these components in a very narrow range around λ = 1. Therefore, to obtain the most efficient combustion and the least 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 λ value corresponding to the maximum degree of conversion for the catalytic converter. Fast λ 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 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

Carbon-monoxide sensors based on thin films of BaSnO3

Abstract BaSnO 3 is a very promising sensor material for detecting carbon monoxide. The mechanism of CO sensitivity of this metal oxide is supposed to be a surface reaction process. Thus to get a high gas sensitivity, the CO elementary sensor is realized as a thin film. The sensitivities of the BaSnO 3 thin films to CO are measured as a function of the temperature and the oxygen concentration. As the most important cross-sensitivities, the influence of humidity, methane and hydrogen on the sensor signal has been examined. The CO sensitivity shows a maximum in the range 600–700 °C under dry conditions. Humidity has a different influence on the CO sensitivity, which depends on the oxygen concentration. The cross-sensitivity to methane can be described by the sum of the sensitivities to CO and methane.

Nitrogen oxide sensors based on thin films of BaSnO3

Abstract A very promising sensor material to detect nitrogen oxide is BaSnO3. The mechanism of NO sensitivity of this metal oxide is supposed to be a surface reaction process. Thus to get a high gas sensitivity the NO elementary sensor is realized as a thin film. The sensitivity of the BaSnO3 thin films to NO is measured as a function of the temperature and the oxygen concentration. As the most important cross-sensitivities, the influence of humidty, methane, ammonia and CO on the sensor signal was examined. The NO sensitivity shows under dry conditions a maximum in the range 450–550 °C.

Comparison of transient response of exhaust-gas sensors based on thin films of selected metal oxides

New concepts of engine control are required to meet the increasingly stringent standards for the reduction of pollutants in the exhaust gas. These new concepts can be realized by new fast sensors for lambda measurement by means of which the transient operating condition of a combustion engine and the fluctuation between the individual cylinders of the engine can be recorded. The most suitable materials for a fast exhaust-gas sensor are thin films made from different metal oxides (e.g., SrTiO3, BaTiO3 or CeO2). As bench tests show, use of these materials allows a lambda measurement to be made in a wide range (0.9 < λ < 1.1). In order to permit a cylinder-selective and stroke-selective measurement, the sensors are placed directly behind the outlet valve of the engine (position I), at the point of junction of the exhaust manifold (position II) and several centimetres towards the exhaust cycles of the engine. Nevertheless, the signals indicate that apart from position I directly behind the outlet valve, the exhaust gases of the different cylinders are mixed. This is caused by the valve operation times and the different running times of the gas from the outlet to the sensor.

Preparation of stoichiometric barium stannate thin films: Hall measurements and gas sensitivities

Barium stannate (BaSnO 3 ) is a typical compound with cubic perovskite lattice. For the first time thin films with a Ba/Sn ratio of 1 were prepared by radio frequency (r.f.) sputtering. Hall measurements were performed on the thin films in a temperature range between 600°C and 1050°C. The variation of Hall mobility and charge-carrier density was investigated in the oxygen partial pressure range between 10 2 Pa and 10 5 Pa. The gas sensitivities of the material in wet air were investigated by resistance measurements of sensor chips with sputtered BaSnO 3 thin films. The material showed interesting responses to the reducing gases isobutene, H 2 , NO, CH 4 and CO in application-relevant concentrations. The best sensitivities were obtained at temperatures below 700°C. At temperatures of 600°C the thin films showed response times of typically 3 min. The material was found to be insensitive to CO 2 and NH 3 .

High temperature oxygen sensor based on sputtered cerium oxide

Abstract The chemical stability at high temperature and the high diffusion coefficient of oxygen vacancies are reasons for selecting CeO 2 as a promising material for fast oxygen sensors at high temperatures. To reach short response times of oxygen sensors based on semiconducting metal oxides it is necessary to realize the sensitive material in the form of a thin film. This can be done using the sputter process. Sputtered thin films of CeO 2 like polycrystalline bulk materials present pure n-conducting behaviour in the temperature range 800–1100 °C. Above temperature of 900 °C response times of the sensitive material shorter than 50 ms due to a change in the oxygen partial pressure can be reached. The cross-sensitivities of CeO 2 thin films on reactive gases like H 2 and CH 4 have been investigated.

Lambda measurement with Ga2O3

Abstract Sensors based on high-temperature-stable semiconducting gallium oxide thin films may be used to monitor the composition of ot exhaust gases from internal combustion engines or furnace installations. The electrical d.c. conductance of these low-cost devices, operable in the temperature range 700–1000 °C, represents the sensor signal. Investigations have been performed in the laboratory, using a special gas-mixing system that supplies a mixture of the main components of real exhaust gas (N 2 , O 2 , CH 4 , CO, NO, water vapour) to produce a synthetic exhaust gas with very precisely defined composition. Additional investigations have been performed in real exhaust gas on an engine bench test-bed. In the temperature range 1000–900 °C the gallium oxide sensors respond to the oxygen partial pressure of the mixtures thermodynamic equilibirum. With knowledge of the fuel composition (carbon-hydrogen ratio), this yields a simple λ measurement for λ=1.2–0.85 with a resistance jump of about three decades at the stoichiometric point. The main mechanism in this temperature regime is a setting of the bulk defect equilibirum. At lower temperatures, the influence of surface-located mechanisms becomes more important: between 800 and 700 °C the jump at the stoichiometric points becomes smaller and broader, but there is still a monotonic λ dependency. At 600 °C or below, the monotonic λ dependency is lost.

Cross-sensitivity of various doped strontium titanate films to CO, CO2, H2, H2O and CH4

Abstract Doped and undoped films made from SrTiO 3 have proved to be promising for use in high-temperature oxygen sensors (600 2 have only a reducing effect when present in concentration of 1% in oxygen-containing gas mixtures (1% O 2 , the remainder being N 2 ).The strongest effect is seen at about 575 °C, H 2 , H 2 O and Ch 4 also have a reducing effect in oxygen-containing atmospheres below a temperature that is dependent on the reactive gas in question. However, above these temperatures such gases have an ‘oxidizing’ effect. These effects can be explained on the basis of a surface model for SrTiO 3 that assumes that the positive surface charge at the grain surface is at least partially counterbalanced by OH groups.

Coadsorption and cross sensitivity on high temperature semiconducting metal oxides: water effect on the coadsorption process☆

Abstract The operation of high temperature oxide semiconductor gas sensors is strongly influenced by their surface properties and the processes taking place in the topmost atomic layers such as adsorption, coadsorption, gas—gas interactions and the catalytic processes on them. From our experimental and theoretical results a resistance limit was calculated; owing to the measured resistance it is possible to estimate when the surface phenomena dominate. According to our results the effect of water on the resistance is the result of at least two processes. The adsorption of molecular water is fast and results in a donor effect while the parallel formation of surface OH groups is relatively slow and results in an acceptor effect. When the temperature is increased the OH formation becomes the dominant process and at a definite temperature its influence passes a maximum. Our results prove that on β-Ga2O3 and n-SrTiO3 the adsorption and coadsorption processes are strongly influenced by the presence of water. For both substances the effects are very similar. We suggest that similar mechanisms take place on every oxide semiconductor.