J. Gerblinger

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

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.

Auger and SIMS study of segregation and corrosion behaviour of some semiconducting oxide gas-sensor materials

Abstract The influence of corrosive gases (H 2 , CO, NO, Cl 2 , SO 2 ) on the composition of sputtered polycrystalline StTiO 3 , CeO 2 and Ga 2 O 3 thin films was studied. Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) were applied for studying the depth profiles of atomic composition of the thin films, treated for several hundred hours at 800 °C in diluted (20 ppm, 5%) corrosive gases. SrTiO 3 was strongly attacked by Cl 2 and SO 2 . During the Cl 2 treatment the layer thickness was reduced and the structure altered near the grain boundaries. SO 2 treatment caused a new surface phase formation (probably SrO). the Ga 2 O 3 layer had a fast reaction with Cl 2 giving rise to volatile Ga products but the other corrosive gases had no deteriorating effect on the layer. CeO 2 proved to be the most resistant to all the gases mentioned above.

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.

Chemosensors for motor management systems of the future

The growing demands to reduce polluting emissions from motor vehicles can be satisfied only by using new concepts of engine management. These concepts require new chemosensors for their implementation. Their center piece is a new lambda probe for the improved monitoring of idling operation or cylinder fluctuations. Sensors are also required for monitoring catalytic converters and regulating the catalytic reactions of nitrogen oxides in the exhaust from Diesel engines. At Siemens, research is being pursued into new sensors for these applications. The sensors are based on the effect of the gases on the electrical conductivity of semiconducting metal oxides. The latter can be manufactured as a thin film by reactive sputtering, thus allowing their easy integration into microsensor systems. In addition to the fast lambda probe for cylinder-selective measurement, research is being focused on sensors for hydrocarbons, nitrogen oxides and ammonia. The laboratory investigations have led to the discovery of suitable metal oxides for detecting the relevant gases. Prototypes of the fast lambda probe with a response time of less than 10 ms and a constant operating temperature up to 1000°C have been successfully tested on engine test benches.

Segregation driving forces in perovskite titanates

Abstract The surface-segregation properties of perovskite titanate-based solid-state sensor materials are investigated by Auger and secondary ion mass spectroscopy (SIMS) methods. The experimental findings are interpreted in terms of a thermodynamic model, which considers the so-called ‘generalized surface free enthalpies’ and the effect of the ambient gas atmosphere as the major driving forces of segregation. As well as the qualitative comparison of the experimental results and the model predictions, a semi-empirical calculation is also performed to estimate the actual values of these main driving forces. It is found that, although both effects have considerable impact on the surface composition, the influence of the ‘generalized surface free enthalpies’ is predominant.

Comparison of the water effect on the resistance of different semiconducting metal oxides

Abstract In this work the time and temperature dependence of the complex interaction between water and the different oxide semiconductor surface was examined by resistance measurements in inert and in oxygen containing atmospheres. The experimental temperature varied between 823 and 1093 K. The content of the atmospheric pressure nitrogen or 5% oxygen containing nitrogen carrier was 15 or 30 mbar.

Study of the behaviour of CeO2 in SO2-containing environment

The undoped n-type semiconductor CeO 2 is a less studied but in many respects promising potential new sensor material. As in our former experiments (using different reactive gases) CeO 2 proved to be a highly corrosion-resistant material, in this work detailed investigations have been carried out on its electrical and corrosion behaviour in a wide temperature range (300-800 °C) in oxygen- and SO 2 -containing atmospheres. Our most important results can be summarized as : (a) the resistance of CeO 2 is practically uninfluenced either at 350 or 800 °C by SO 2 if oxygen is present ; (b) around these two temperatures no corrosive effect of SO 2 can be observed either ; (c) but at 550 °C in the presence of oxygen, the acceptor effect of SO 2 can be observed ; (d) at 550 °C during long-term heat treatment (100 h) in an SO 2 - and oxygen-containing atmosphere, the CeO 2 in a thin surface layer transforms into CeO and at the same time a reversible incorporation of a few percent of S into this layer can be observed. In the present work the connection between the sensitivity maximum for SO 2 found at 550 °C in the presence of oxygen and a possible surface phase transformation is discussed.

Sensitivity mechanism of metal oxides to oxygen detected by means of kinetic studies at high temperatures

Abstract Investigating the kinetic properties of screen-printed SrTiO 3 layers and sputtered films of SrTiO 3 and CeO 2 due to very fast changes in the oxygen partial pressure of their surroundings, three fundamentally distinct steps dominating the kinetics of such oxygen-sensitive materials can be distinguished. These steps of the reaction of gaseous oxygen and oxygen vacancies of the metal oxides are the diffusion of gaseous molecules from or to the surface, the reaction of oxygen molecules on the surface and the diffusion of oxygen vacancies.