Sensor bulk conductivity

The emphasis on the thickness of the selective layer is linked to the mode of interaction between the analyte and the selective layer. If this interaction takes place exclusively at the surface of the selective layer, then the bulk conductivity does not contribute and represents only a shunt which decreases the signal-to-noise ratio. This is a typical case of chemiresistors based on inorganic materials (Fig. 8.6). On the other hand in chemiresistors based on organic semiconductors, the signal usually originates in the bulk of the selective layer. In that case, the response time of the sensor is affected by its thickness. 

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. 

Important advantages of the bulk conductivity sensor are, besides selectivity, its simplicity (no reference needed) and its selectivity an important drawback is the T-dependence which can be quite significant (see above). Improvement via doping or by using a T-reference is straightforward, but partly at the cost of sensitivity, simplicity, or range of application. 

Chapter 1 by Joachim Maier continues the solid state electrochemistry discussion that he began in Volume 39 of the Modem Aspects of Electrochemistry. He begins by introducing the reader to the major electrochemical parameters needed for the treatment of electrochemical cells. In section 2 he discusses various sensors electrochemical (composition), bulk conductivity, surface conductivity, galvanic. He also discusses electrochemical energy storage and conversion devices such as fuel cells. 

The sensitivity of a polycrystalline gas-sensing ceramic depends upon sensor composition, ceramic particle size (since this determines reactive surface area, and particularly its relationship to LD), sensor geometry, temperature (which strongly affects adsorption processes and oxide conductivity) and accessibility of the sensor bulk material (i.e. permeability) to the gas or gases. 

Contrary to the SnC>2-based sensors the conductivity of the equilibrated bulk is measured. Due to the establishing of the equilibrium the operating temperature (>500 °C) is much higher than those of Sn02-sensors. The temperature dependence can be minimized by selection of oxides and by doping [xxv]. It is also possible to measure hydrocarbons by means of such sensors. 

Obviously, high D5 values, e.g., as required for bulk conductivity sensors, demand materials that are free from redox centers, while the minimization of drift phenomena in boundary layer sensors demands just the opposite.239... 

The best known ceramic is Ti02 which is used in oxygen sensors of this conductor type for various vehicle applications [8], The operating principle is based on changes in the bulk conductivity due to the concentration of oxygen vacancies, indicated by the formulation Ti02 x. 

Functional polymers may be used for dihierent types of chemical sensors, including acoustic wave sensors (bulk acoustic wave, surface acoustic wave, and flexural plate wave sensors), electronic conductance sensors (semiconducting and capacitance sensors), and calorimetric sensors. ... 

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