Electrical-Based Gas Sensing

Abstract

Metal oxides represent an assorted and appealing class of materials whose properties cover the entire range from metals to semiconductors and insulators and almost all aspects of material science and physics including superconduc-tivity and magnetism. In the field of chemical sensing, for more than five decades it has been known that the electrical conductivity of semiconductor varies with the composition of the gas atmosphere surrounding them [17]. The first generation of commercial devices was prepared in the 1960s by Taguchi in Japan. They were made of SnO 2 prepared by thick-film technology and employed in the warning of possible leaks of explosive gases. This was and is of major importance in densely built-up Japanese cities characterized by wooden houses and widespread gas furnaces. Taguchi established Figaro Engineering Inc., which remains today the major manufacturer of gas sensors. Since then, the need of cheap, small, low-power-consuming and reliable solid-state gas sensors has grown over the years and, recently, the development of information technology have triggered a large amount of research worldwide to overcome metal oxide sensor drawbacks, summed up in improving the well-known ''3S'': sensitivity, selectivity, and stability. A variety of devices have been developed mainly by an empirical approach, and a lot of basic theoretical research and spectroscopy studies have been carried out. The sensing properties of semiconductor metal oxide other than SnO 2 , like TiO 2 , WO 3 , ZnO, Fe 2 O 3 , and In 2 O 3 , have been studied as well as the benefits from the addition of noble metals-Pd, Pt, Au, Ag-in improving selectivity and stability. In 1991 Yamazoe [171] showed that reduction of crystallite size resulted in a huge improvement in sensor performance. The challenge became to prepare materials with small crystallize size which were stable when operated at high temperature for long periods. Since the incorporation of a second oxide