Gas-sensors based on electronically conducting ceramics

The science of gas-sensors based on changes in electronic conductivity of a semiconducting ceramic is a complex matter depending upon a combination of surface chemistry and electron transport in mainly transition metal oxides. As is usual in exploiting electroceramics, the applications technology runs ahead of 

With the chemisorption of, for example, oxygen a surface density of electron acceptor states leads to the establishment of a Schottky barrier. The process is essentially the same as that which occurs in the case of the PTC thermistor (see Figs 4.21 and 4.10). The electron potential barrier height (p) is given by 

Under the same simplifying assumptions as for the PTC thermistor (where the surface states are shared between two grains) the depleted layer thickness d is given by 

Summarizing the basics of the gas-sensing mechanism, oxygen molecules from the surroundings become chemisorbed (that is chemically bonded) to the surface atoms, the process being described by the reaction  

The above description is on the basis of the semiconductor being n-type. In the case of a p-type semiconductor (e.g. Cr2 xTix03 0.01 x 0.45 [29]) then a decrease in resistance would accompany an increase in the amount of adsorbed oxygen. 

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