Joachim Frank

Selective gas detection with high-temperature operated metal oxides using catalytic filters ☆

Metal oxide gas sensors in general, show high sensitivity but poor selectivity if pure sensor materials are used. The performance in terms of reproducibility may be enhanced by using very stable materials, which may be operated at quite high temperature but this does not help in the problem of selectivity. This paper discusses the use of gas pretreatment using filters operated at elevated temperatures to overcome this problem for special applications. Three different types of gas filters are discussed: a physical filter which is permeated by hydrogen only, a catalyst filter which removes disturbing solvent vapours by oxidation, and a gas conversion filter which ensures a defined NO/NO2 equilibrium. These filters relate to the applications of selective indoor CH4, H2, and automotive exhaust gas nitrogen oxide detection.

High temperature Ga2O3-gas sensors and SnO2-gas sensors: a comparison

Abstract High temperature Ga 2 O 3 -gas sensors show some differences to other sensors based on other metal oxides like SnO 2 . Among the advantages of Ga 2 O 3 -based sensors good long-term stability, fast response and recovery times, good reproducibility, low cross sensitivity to humidity and short pre-ageing times have to be mentioned. The good stability in sulphur-containing atmospheres makes them suitable for use in domestic burner controls. On the other hand, is the high operation temperature of about 600–800°C which means that power consumption is comparably high ( 2 O 3 -sensors to CO and NO 2 is lower compared to other metal oxide-based sensors. In this article measurements are presented in order to compare the performances of high temperature Ga 2 O 3 -gas sensors and SnO 2 -sensors. Electronic conductivity models are proposed explaining the above mentioned differences and recommendations for the different application areas are given.

Gas-sensitive electrical properties of pure and doped semiconducting Ga2O3 thick films

Abstract The gas-sensitive electrical properties of pure Ga 2 O 3 thick films are investigated. The influence of doping on these properties has also been studied. SnO 2 was used as donator type dopand. It was found that there is an increase in the overall conductivity due to the doping up to two orders of magnitude. Despite this high conductivity, the gas sensitivity remains almost unchanged in all cases. There is no effect of the dopands on the bulk controlled oxygen sensitivity. Pure and doped Ga 2 O 3 thick films have been proved to be reproducible and stable sensor base materials. The result forms the base for the use of screen-printed electrodes or a further reduction of the chip size for a decreased heating power consumption.

Enhancement of sensitivity and conductivity of semiconducting Ga2O3 gas sensors by doping with SnO2

Abstract The use of high temperature operated metal oxides, e.g. Ga 2 O 3 thin films, for gas sensors shows promising properties in terms of reproducibility, long term stability against interfering gases and low cross sensitivity to humidity. It is shown that by employing SnO 2 (0.1–3% At ) as doping material, a very effective donor for sputter deposited polycrystalline Ga 2 O 3 thin films has been found which allows an increase in conductivity of up to two orders of magnitude, as well as an enhancement of the gas-sensitivity. This is the basis for a significant reduction of the sensor chip size to obtain a reduction in the heating power.

Electrical doping of gas-sensitive, semiconducting Ga2O3 thin films

Abstract Recently polycrystalline n-type semiconducting Ga2O3 thin films have been characterized as a new sensor base material for high-temperature gas sensors. The influences of donator type and acceptor type dopants on the overall conductivity and its gas-sensitivity of Ga2O3 thin films are investigated. It was found that the change in conductivity of the polycrystalline thin films is significantly smaller than in the case of single crystals reported in the literature. However, a difference in overall conductivity of two orders of magnitude have been found between acceptor and donor doped specimens. There is no effect of the dopants on the bulk controlled oxygen sensitivity. In contrast, a strong influence of the dopants on the surface controlled sensitivity to reducing gases was found. The results are explained by a model.

On the mechanism of hydrogen sensing with SiO2 modificated high temperature Ga2O3 sensors

Abstract On the way to understand the detection mechanisms inside a high temperature metal oxide sensor with Ga 2 O 3 as its sensitive material and a silicon dioxide top layer for increasing the sensitivity to hydrogen the sensor is exposed to dry and oxygen free gas mixtures. While the absence of oxygen decreases the selectivity of the sensor dramatically, dry air is without a major influence to the selectivity. The detection of hydrogen is possible in all cases. For additional investigations an electrode was mounted directly on top of the sensor influencing the space charges inside the sensor, which seems to be the major detection mechanism. This should lead to a further optimization of the hydrogen sensor, especially increasing the selectivity, and characterization of the environments the sensor can be utilized in.

A new type of gas sensor based on thermionic charge carrier emission

Abstract This paper presents investigations on DC emission currents from heated surfaces at ambient pressure for the purpose of gas sensing. Since applied gases alter the work function (WF) of a material, the emitted electron current changes and becomes gas sensitive. Gas sensitive effects were identified as being dependent upon the emission material used. In contrast to vacuum conditions, additional effects arise from interactions among electrons, ions and atoms. Consequently, the electrical behaviour at atmospheric pressure is completely different. This paper presents preliminary results on this new gas sensing principle with materials such as barium nitrate and scandium oxide.

Enhancement of sensitivity and conductivity of semiconducting Ga/sub 2/O/sub 3/ gas sensors by doping with SnO/sub 2/

The use of high temperature operated metal oxides, like Ga/sub 2/O/sub 3/ thin films, for gas sensors shows promising properties in terms of reproducibility, long-term stability against interfering gases and low cross sensitivity to humidity. It is shown that by employing SnO/sub 2/ (0.1-3%/sub at/) as the doping material a very effective donor for Ga/sub 2/O/sub 3/ has been found which allows an increase in conductivity up to two orders of magnitude as well as an enhancement of the gas-sensitivity. This is the basis for a significant reduction of the sensor chip size to obtain a reduction in the heating power.