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Advances in Making Perfect Gas Sensors
January 28, 2011 11:49

Tin oxide

Researchers from Russia and China have discovered that adding small amounts of zirconium in tin oxide films changed electrophysical and gas-sensing properties of these films. For instance, tin oxide films become much more sensitive to vapours of ethanol and acetone at temperatures 100 degrees less, than without zirconium.

Sensor elements are commonly used for monitoring toxic and explosive substances in the air. Many sensors are based upon a common principle ability of metal-oxide semiconductors to absorb gas molecules. After gas molecules stick to a film surface, its electric resistance changes, which help calculate gas concentration.

Films, made of different materials, have different gas sensitivities. This parameter can be calculated as a ratio between resistance of a pure film and resistance of this film in a chamber with gas of known concentration. Tin oxide films, both pure and alloyed with various substances, are frequently used as gas sensors. Alien atoms in tin oxide crystal structure affect size of tin grains, which means they modulate electrophysical properties of tin oxides.

Russian researchers have tested zirconium oxide as an additive to tin oxide films. Concentration of zirconium atoms varied between 0 and 4.6%. Films have been made by means of ion sputtering of a mixture of tin and zirconium in an atmosphere of oxygen and argon. Oxygen concentration was adjusted in order to make metals precipitate on glass or silicon cariers in the form of oxides.

Element composition of the films was analyzed by means of x-ray microanalysis, and their thickness by means of atomic force microscopy. Experiments showed that mentioned parameters had a strong correlation the more zirconium a film contained, the thinner it was. An explanation for this effect is that molecules of zirconium oxide do not form any crystal structure and find themselves at intergranular borders of tin oxide molecules, preventing large crystals from being formed. High (4.6%) content of zirconium oxide led to films of 0.5 micron thick, and tin oxide grains of 10 microns. Adding zirconium oxide changes not only size, but also electrophysical properties of a composite material. The smaller oxide granules are, the more atoms are located close to the surface. Unalloyed film of tin oxide has about 7.5% atoms, located close to the surface, and adding zirconium increases this number to 30% due to grain size reduction. Researchers have demonstrated that increase of free surface led to significant improvement of a films gas sensor properties. A film with high zirconium content showed massive reduction in temperature of maximum gas sensitivity 200 degrees Celsius instead of usual 300-400 degrees Celsius.

Temperature of maximum gas sensitivity to vapours of ethanol, acetone and propanol drops with zirconium content being increased. These films are most sensitive to acetone. This research shows that composites of tin and zirconium oxides are promising materials for solid gas sensing systems.

Source: Science & Technologies

Kizilova Anna


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