Bulk defect sensors

Ti02 devices could be used as thermodynamically controlled bulk defect sensors to detect oxygen over a large range of partial pressures. At low temperatures, V ITi02 Schottky diodes made extremely sensitive oxygen detection possible [298, 303]. 

SrTi03 may serve as a well-investigated material for such a bulk conductivity sensor. Its defect thermodynamics and also the relevant kinetic parameters have been discussed in detail in Part I.2 In particular at low temperatures and at small sample thicknesses L, the kinetics of oxygen incorporation becomes surface reaction controlled, and ks the decisive kinetic parameter. 

Chemical sensors for gas molecules may, in principle, monitor physisorp-tion, chemisorption, surface defects, grain boundaries or bulk defect reactions [40]. Several chemical sensors are available mass-sensitive sensors, conducting polymers and semiconductors. Mass-sensitive sensors include quartz resonators, piezoelectric sensors or surface acoustic wave sensors [41-43]. The basis is a quartz resonator coated with a sensing membrane which works as a chemical sensor. 

Zakrzewska found that titanium dioxide doped with Nb and Cr should be considered as a bulk sensor. Its performance was governed by the diffusion of point defects, i.e. very slow diffusion of Ti vacancies for Ti02. 9.5 at% of Nb and fast diffusion of oxygen vacancies in the case of HOt. 2.5 at% Cr sensor. The corresponding response times were 55 min for TiOT. 9.5 at% of Nb and 20 s for HOt. 2.5 at% Cr [292]. 

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