Electronic Conductance Sensors

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. ... 

The "man-made" inorganic materials and, in particular, the electron conducting sensor materials will make it possible to develop "bottom-up" concepts in which individual active sites of chemical sensors are addressed and controlled Of particular interest in this context is the scanning tunneling spectroscopy (STS) combined with STM and AFM imaging in different contact modes... 

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.

J. Janata and M. Josowicz, Nature Materials, 2 (1), (2003) 19-24, Conducting polymers in electronic chemical sensors ... 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

When a solid electrolyte component is interfaced with two electronically conducting (e.g. metal) films (electrodes) a solid electrolyte galvanic cell is formed (Fig. 3.3). Cells of this type with YSZ solid electrolyte are used as oxygen sensors.8 The potential difference U R that develops spontaneously between the two electrodes (W and R designate working and reference electrode, respectively) is given by ... 

Electrolytes, oxygen sensors, fuel cells, electronic conduction coatings, and furnace elements. 

Apart from transistor-like devices, single-electron junctions can also be useful for sensor applications. The simplest one might be the monitoring of H2S. Since the formation of CdS nanogranules takes place when an initial cadmium arachidate layer is exposed to this gas, we can expect the appearance of single-electron conductivity only when it is present in the atmosphere. 

Another problem that is common for all membrane-based solid-state sensors is the ill-defined membrane-metal interface. A large exchange current density is required to produce a reversible interface for a stable potentiometric sensor response. One approach to improving this interface is to use conducting polymers. Conducting polymers are electroactive n-conjugated polymers with mixed ionic and electronic conductivity. They... 

Conductivity sensors are most commonly used for safety purposes in household appliances. Presence and absence of washing liquor, detergency, and water softener can be easily measured and proper operation ensured [71]. The various applications mainly differ by their design of electrode geometry and methods for electrical measurement. Due to the close relation between ionic conductivity and water hardness, the automatic water softener in an automatic dishwasher can be controlled by a conductivity sensor [72]. To isolate the transmission of the measured value from the process controller, the conductivity sensor could incorporate an opto-electronical coupling [73]. Thus, protective insulation of the electrodes in a washer-dryer could be ensured. 

In general Zr02 oxygen sensors consist of a tube-like solid-state Zr02 electrolyte where the electronic conductivity is based on oxygen ion charge carrier transport. The inner and outer surface of the yttrium-doped and stabilized zirconia tube is covered by porous platinum electrodes. 

Gopel, W. Schierbaum, K.D. Electronic Conductance and Capacitance Sensors. In Sensors A comprehensive Survey. 

A number of oxides with the fluorite structure are used in solid-state electrochemical systems. They have formulas A02 xCaO or A02 xM203, where A is typically Zr, Hf, and Th, and M is usually La, Sm, Y, Yb, or Sc. Calcia-stabilized zirconia, ZrC)2.xCaO, typifies the group. The technological importance of these materials lies in the fact that they are fast ion conductors for oxygen ions at moderate temperatures and are stable to high temperatures. This property is enhanced by the fact that there is negligible cation diffusion or electronic conductivity in these materials, which makes them ideal for use in a diverse variety of batteries and sensors. 

Boyle A, Genies EM, Lapkowski M. Application of electronic conducting polymers as sensors— polyaniline in the solid-state for detection of solvent vapors and polypyrrole for detection of biological ions in solutions. Synth Met 1989 28 C769-C774. 

The isopiestic and manometric methods (units A2j A2.4) for determination of water activity have the limitation of being dependent on fixed laboratory equipment. The electronic-type sensors have advantages of portability, speed, and simplicity of measurement. The characteristics of a sensor depend upon the manufacturer and each instrument must be calibrated separately. The anodized sensors have advantages of ruggedness, small dimensions, and fast response, as well as freedom from large temperature coefficients and less susceptibility to contamination of the lithium chloride conductivity sensors (Smith, 1971). 

In summary, phthalocyanines modified with crown ethers are interesting synthetic targets as they are prone to form columnar phases. Their electron conductivity and complexation properties make them interesting candidates for the design of sensor materials or supramolecular switches. 

Here, the potentiometric selectivity coefficient is given with respect to the hydroxyl ion. Single-crystal lanthanum fluoride is a wide bandgap semiconductor in which the electrical conductivity is due only to the hopping mobility of fluoride ions through the defects in the crystal. It does not respond to the La3+ ion because of the slow ion exchange of that ion. Hydroxyl ion is the only other ion that has appreciable mobility, and is the only known interference. For this reason, the measurements with a fluoride electrode are always done below pH 7, which circumvents this interference. As shown later, the consideration of ionic and/or electronic conductivity of the membrane plays a critical role also in the design of the internal contact in nonsymmetric potentiometric sensors. 

Owing to a negligible electronic conductivity (amn =0) in the electrolyte, the internal electronic current is zero and since i = / jti + ieon = 0, there is also no ionic current consequently, disappears because of the nonzero value of electrochemical potential of the electrons, i.e., a nonzero cell voltage, can be established over the sample. In the case of the well-known X -sensor,32 which is extensively used in automobiles... 

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