Ceramic-based sensor materials

The first term of Equation 8.4 comes from the dipole moment of the nanofiber web at constant thickness, which mainly depends on the number of effective dipoles (N ) present in the nanofiber web. The second term is known as the elastic compressibility constant (cry) directly related to the dimensional changes. One of the vital advantages of the nanofiber web-based pressure sensor or generator is that the expected piezoelectric effect is much higher due to the high compressibihty associated with the large thickness change at the identical pressure compared to its film form and to ceramic-based piezoelectric materials. 

Foulds and Lowe (1986) combined mass production of the base sensor and enzyme immobilization as follows. Using gold or platinum ink, a working and counter electrode were deposited on a ceramic substrate. After thermal treatment of the electrode material a solution containing GOD and a pyrrole derivative of ferrocene was electrochemically polymerized at the electrode. The pyrrole component forms a conducting polymer and the immobilized ferrocene acts as electron acceptor for GOD. The structured immobilization permits this technique to be used for successive enzyme fixation to multiparameter sensors. 

Szabo, N., Lee, C., Trimboh, J., Figueroa, O., Ramamoorthy, R., Midlam-Mohler, S., Soliman, A., Verweij, H., Dutta, P. and Akbar, S. (2003) Ceramic-based chemical sensors, probes and field-tests in automobile engines. Journal of Materials Science 38,4239-45. 

Oxygen Sensor. An application for ceramics whose electrical conductivity is sensitive to changes in the oxygen partial pressure of the surrounding atmosphere. Zirconia is a potentiometric sensor material, whose conductivity is based on a... 

Piezoelectric transducers either convert an electrical signal to physical motion or vice versa. In the past, the sensor material was usually a ceramic based on titanates of barium or lead zirconium. Their rigidity made them especially useful for converting electrical energy to mechanical motion in products like audio speakers and signaling alarm devices. Unfortunately, ceramics are brittle and cannot be made with large surface areas or into complex shapes where plastics are also used. 

The resistance variation is measured by Keithley unit as shown. The chemiresistor-type sensor is mounted on hot plate (filament heater) which is coupled with ceramic base stand. The two leads of thin copper wires are attached to interdigited electrodes by silver paste for electrical connections. The electrical connections for gas sensing measurements, thermocouple and temperature variation are made using instrumentation feed through. The response of sensor is monitored in terms of the normalized resistance calculated by response = Ro/Rg and the sensitivity factor is monitored in terms of % sensitivity = dRJRo. Where dR is the variation in resistance of sensing materials after exposure to gas, Rg is the resistance of the sensor in presence of gas and Rq is the initial resistance with gas. 

A potentiometric sensor was developed by Martinez-Manez and co-workers in thick film technology. Ru02 was used as sensitive material whereas this active electrode surface was covered with a semipermeable polymeric (poly-isophthalamide diphenylsulfone, PIDS) or ceramic-based (Ti02) membrane. The measurement of DO could only be carried out effectively if other redox processes were excluded except the desired reaction which was produced by oxygen. This was achieved by covering the active electrode with a semipermeable polymeric or ceramic membrane which allowed to pass DO and excluded any other redox-active species present in solution. A schematic view of the DO potentiometric sensor is displayed in Fig. 3.2. 

These lead-based materials (PZT, PLZT, PMN) form a class of ceramics with either important dielectric, relaxor, pie2oelectric, or electrooptic properties, and are thus used for appHcations ia actuator and sensor devices. Resistive properties of these materials ia film form mirror the conduction processes ia the bulk material. Common problems associated with their use are low dielectric breakdown, iacreased aging, and electrode iajection, decreasiag the resistivity and degrading the properties. 

The sol-gel process allows the preparation of glass films into which indicator chemistry can be incorporated. The production of ceramic materials and glassy networks is based on the polymerisation of suitable precursors at low temperature. The increasing popularity of sol-gels in sensor applications results from the processing versatility2. 

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