Design for ultra sensitive oxide semiconductor gas sensor by controlling meso-pore and high order structure

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Recently, numerous kinds of gases are emitted from various origins into our living space, working space or outdoors. Many of them, like air pollutants and VOCs (volatile organic compounds), are hazardous to human beings and environments. Most of these gases are present at low concentrations so that extremely good sensing characteristics are requested for their monitoring. It seems imperative to establish sensor design principles on the basis of fundamental understandings of semiconductor gas sensors. In this research project, the influences of gas transport on the gas sensing properties of semiconductor gas sensor are investigated theoretically, based on the diffusion equation assuming Knudsen diffusion and first order surface reaction of a target gas. The followings are found to be important factors in order to realize such high sensitive gas sensors by theoretical study(1) Control of mesopores where competition between gas diffusion and surface reaction takes place.(2) Control of size of the secondary particles involving mesopores.(3) Control of macropores through which gas diffusion is rapid.The followings are also found in this research project.(1) Crystallite size of SnO_2 could be enlarged and controlled in the range from 6 nm to 12 nm by controlling preparation procedure of stannic acid gel and hydrothermal treatment conditions.(2) SnO_2 could be coated thinly on SiO_2 spherical particles by adjusting pH.(3) Large size pores could be introduced into thin film of SnO_2 by mixing organic reagent.Besides, aqueous sols of tungsten oxide dehydrate could be prepared by an ion-exchange method. The size of the crystallites could be controlled by washing, centrifugal treatments and/or ultrasonic agitation of the sol, providing a method to control the crystallite size. It was also found that thick film device derived from tungsten oxide dehydrate sol gave an extremely high sensitivity to NO_2.