Conductometric gas sensors

The magnitude of the dissociation constant A plays an important role in the response characteristics of the sensor. For a weakly dissociated gas (e.g., CO2, K = 4.4 x 10-7), the sensor can reach its equilibrium value in less than 100 s and no accumulation of CO2 takes place in the interior layer. On the other hand, SO2, which is a much stronger acid (K = 1.3 x 10-2), accumulates inside the sensor and its rep-sonse time is in minutes. The detection limit and sensitivity of the conductometric gas sensors also depend on the value of the dissociation constant, on the solubility of the gas in the internal filling solution, and, to some extent, on the equivalent ionic conductances of the ions involved. Although an aqueous filling solution has been used in all conductometric gas sensors described to date, it is possible, in principle, to use any liquid for that purpose. The choice of the dielectric constant and solubility would then provide additional experimental parameters that could be optimized in order to obtain higher selectivity and/or a lower detection limit. 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

Gardner derived expressions which dehned the response of a pair of planar conductometric gas sensors according to the electrode thickness and an electrode gap. The steady-state conductance in air, G , of a homogeneous film of conductivity, and thickness, L, lying on semi-infinite electrodes can... 

Schematic of a conductometric gas sensor with electrodes placed high up in the sensor face. 

Then, the oxygen partial pressure of the surrounding gas atmosphere is the driver for a change in the electron concentration.These examples clearly show that DTEGs are based on the same physical principles as conductometric gas sensors, since in both cases the analyte concentration modulates the electron density. However, the measurand is different. In conductometric devices, the material property conductivity changes and, hence, the resistance of a sensor varies with the analyte concentration. In contrast, the determination of thermopower is more complicated, since not only the thermovoltage has to be measured, but also a known temperature difference has to be apphed or, at least, measured. 

Figure 7.13 shows the results. The thermopower of the gas sensitive layer, fisn02> is barely influenced after milling out a portion of the gas sensitive layer (Fig. 7.13(a)), but the resistance i snOj is - as expected from the preceding discussion - significantly increased (Fig. 7.13(b)). If a propane concentration of 100 ppm were present in the ambience of the gas sensor, a milled-out DTEG would measure a concentration of 80 ppm propane. However, a milled-out conductometric gas sensor (Fig. 7.13(b)) would only measure a concentration of 30 ppm propane. 

DTEGs are an alternative to resistive gas sensors. Accurate, rapid and long-term stable gas sensors have been presented in this chapter. The main advantage of DTEGs is the measurand thermopower or Seebeck coefficient . In contrast to conductometric gas sensors, the measurand thermo-power is not influenced by changes in the geometry of the gas sensitive... 

Barillaro, G., Lazzerini, G. M. and Strambini, L. M. A novel power-controlhng approach for integrated, conductometric gas sensors , (2009) Procedia Chemistry 1,188-91. 

G. F. Li, C. Martinez, J. Janata, J. A. Smith, M. Josowicz, and S. Semancik, Effect of morphology on the response of polyaniline-based conductometric gas sensors Nanofibers vs. thin films, Electrochem. Solid-State Lett., 7, H44—H48 (2004). 

Surface Modifiers for Metal Oxides in Conductometric Gas Sensors. 273... 

Table 2.3 Characteristics of several metal oxide nanofiber-based conductometric gas sensors...

Table 3.1 Table of conductometric gas sensors based on ID nanostructiures of metal oxide classified by sensing oxide... 

Regarding NT-based conductometric gas sensors, one can say that chemiresistors based on MNPs were first demonstrated in 1998 by Wohltjen and Snow (1998). They fabricated a chemiresistor by deposition of a thin film of octanethiol-coated AuNPs (tf 2 mn) onto an interdigitated microelectrode. A rapid decrease in the conductance due to film swelling was observed in the presence of toluene, tetrachloroethylene, 1-propanol, and water vapor with a detection limit of -1 ppm. 

At present it is known that various types of polymer-based composites can be applied to design conductometric gas sensors capable of operating at room temperature (Wohltjen et al. 1985 Middlehoek and Audet 1989 Unde et al. 1996 Cespedes et al. 1996 Suri et al. 2002 Densakulprasert et al. 2005 Santhanam et al. 2005 Watcharaphalakom et al. 2005 Wojkiewicz et al. 2011 Potts et al. 2011 Hands et al. 2012). Several examples of such composites are shown in Table 13.1. Several specific combinations of polymers applied for design NH sensors are listed in Table 13.2. 

Metal Oxide-Based Nanocomposites for Conductometric Gas Sensors... 

Metal oxide-metal oxide-based nanocomposites, Me 0-Me 0, are also interesting for gas sensor design (Yamazoe et al. 1983 Yamazoe 1991 Ferroni et al. 1999 Yamaura et al. 2000 Comini et al. 2002 Korotcenkov 2007 Gas kov and Rumyantseva 2009). It was established that one of the ways for improving selectivity and stability of metal oxide conductometric gas sensors is the modification of metal oxide, Me O by the introduction of catalytic or structure modifiers, Me 0, in the nanostruc-tured metal oxide matrix and, thereby, the development of nonhomogeneous complex materials, i.e., nanocomposites Me 0-Me 0. It was also expected that other highly sophisticated surface-related properties important for gas sensor applications such as optical, electronic, catalytic, mechanical, and chemical can also be obtained in complex metal oxides and composites. 

Korotcenkov G,ChoBK(2011) Instability of metal oxide-based conductometric gas sensors and approaches to stability improvement. Sens Actuators B 156 527-538... 

Korotcenkov G, Cho BK (2011) Instability of metal oxide-based conductometric gas sensors euid approaches to stability improvement (short survey). Sens Actuators B Chem 156 527-538 Korotcenkov G, Han SD (2009) (Cu, Fe, Co and Ni)-doped SnO films deposited by spray pyrolysis doping influence on thermal stability of SnO film structure. Mater Chem Phys 113 756-763 Korotcenkov G, Boris I, Brinzari V, Luchkovsky Y, Karkotsky G, Golovanov V, Comet A, Rossinyol E, Rodriguez J, Cirera A (2004) Gas sensing characteristics of one-electrode gas sensors on the base of doped In O ceramics. Sens Actuators B Chem 103 13-22... 

Thus, the above-mentioned discussion testifies that the problems of thermal stability are also peculiar to ID structure. As is well known, the laws of physics are the same for everyone. We can only hope that these problems will not be as strong as for polycrystaUine materials. Basing on the discussions presented, one can conclude that prospects of ID structures used for elaboration of devices intended for work at higher temperature (such as conductometric gas sensors) should be estimated more realistically. Otherwise, in the future one s disappointment could be very intense, as in the case of low-temperature superconductivity. 

Korotcenkov G, Cho BK (2010b) Silicon porosification state of the art. Crit Rev Solid State 35(3) 153-260 Korotcenkov G, Cho BK (2011) Instability of metal oxide-based conductometric gas sensors and approaches to stability improvement. Sens Actuators B 156 527-538... 

---