Sensor boundary conductivity

Bulk and boundary conductivity sensors have already been discussed in Chapter 5 in relation to equilibrium defect chemistry, potentiometric sensors in the previous section. Nevertheless, we wish — in view of the importance of this application — to sketch out some of the fundamentals of electrochemical (composition) sensors . The fact that a variation in the chemical composition (ck) ehcits a physical signal is the rule rather than the exception. This is merely a necessary sensor criterion. In addition, it is important that a sensor signal exhibits adequate sensitivity , is sufficiently selective, stable and as free from drift as possible , and displays an ad-... 

The semiconductor boundary sensor (surface conductivity sensor in Table 7.2) avoids the disadvantage of the high temperatures that were important for the bulk sensors... 

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

In a modern dew-point instrument, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared thermometer (Figure A2.2.2). At equilibrium, the relative humidity of the air in the chamber is the same as the water activity of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears a beam of infrared light is directed onto the mirror and reflected back to a photodetector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation to reduce vapor equilibrium time and to control the boundary layer conductance of the mirror surface (Campbell and Lewis, 1998). Additionally, an infrared thermometer measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the water activity. The range of a commercially available dew-point meter is 0.030 to 1.000 aw, with a resolution of 0.001 aw and accuracy of 0.003 aw. Measurement time is typically less than 5 min. The performance of the instrument should be routinely verified as described in the Support Protocol. 

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... 

Thin metal Hlms (Pt, Pd) have been used for the adsorption and detection of gases such as H2 and NH3 [138,139]. While the interaction mechanisms for these sensors were not specified, it is well known that H2 dissolves to a significant extent in Pd, with concomitant changes in the density, electrical conductivity, and mechanical properties of the fllm. The H2/Pt interaction as well as the interaction of NHa with both Pd and Pt undoubtedly involves chemisorption on surface sites. Metal thin films deposited by nearly all techniques are polycrystalline chemisorption along grain boundaries can often lead to a response that is considerably larger than predicted from the properties of metal single crystals. 

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. 

FIGURE 1.13 1 Low boundary of the oxygen-ionic conductivity for the solid (Zr02)o.9(Y203)o 1 electrolyte and 2 and the temperature dependence of logarithm pressure of dissociation for oxide Na20. (From Zhuiykov, S., Electron model of solid oxygen-ionic electrolytes used in gas sensors, Int. J. Applied Ceramic Techn. 3 (2006) 401-411. With permission.)... 

As shown in Fig. 7.26, when the sensor is exposed to vapor, individual molecules can diffuse into the semiconductor thin film and be adsorbed mostly at the grain boundaries [13], If the adsorbed analytes have large dipole moment, such as H2O ( 2 debye) and DMMP ( 3 debye), the adsorption of those analyte molecules at the grain boundaries close to or at the semiconductor-dielectric interface can locally perturb the electrical profile around the conduction channel, and hence change the trap density in the active layer. We can interpret the trapping effects by a simple electrostatic model discussed briefly in Sect. 7.2. The electric field induced by a dipole with dipole moment of p (magnitude qL in Fig. 7.4) is ... 

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