Sensors based on solid electrolytes

Sensing oxygen with cubic stabilized zirconia 

Potentiometric mode There is no essentially different principle involved from that on which the fuel cell is based. The distinction is that in the case of the fuel cell the required output is power whereas with the sensor it is either a small voltage or small current that monitors some chemical characteristic of the ambient. 

A common type of oxygen sensor takes the form of an yttria stabilized zirconia (YSZ, see earlier) tube electroded on the inner and outer surfaces with a porous catalytic platinum electrode. The electrode allows rapid equilibrium to be established between the ambient, the electrode and the tube. Such a system is shown schematically in Fig. 4.36. 

Platinum catalyses the dissociation and recombination of the oxygen molecules so that O2 ions can be formed at one electrode and converted to 02 molecules at the other. The e.m.f. developed by such a cell is given by the Nernst equation (see Eq. (4.33)). In the present case z = 4 because an oxygen molecule consists of two atoms each acquiring two electrons on being ionized. Hence, the equation becomes 

The general form of a lambda sensor is illustrated in Fig. 4.37. The closed end projects into the hot exhaust gas stream heating the sensor to a temperature at which it is sufficiently conductive for the e.m.f. to be measured by a high impedance meter. The input impedance of the meter must be 100 times that of the cell for 1% precision. Corrections for changes in cell temperature (Eq. (4.38)) can be made by monitoring the cell resistance. 

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Kaneyasu, J. Otsuka, K. Setoguchi, Y. Sonoda, S. Nakahara, T. Aso, I. Naka-gaichi, N. A carbon dioxide gas sensor based on solid electrolyte for air quality control. Sensors and Actuators B, 66 (2000) 1-3, p. 56-58. 

Different types of sensor based on solid electrolytes have been developed following a report by Kiukkola and Wagner (1957). These sensors are based on one of two principles (a) the chemical potential difference across the solid electrolyte (potentiometric sensor), or (b) the charge passed through the electrolyte (amperometric sensor). In the following galvanic cell,... 

Many - gas sensors based on - solid electrolytes operate under potentiometric conditions [iii]. The sensors for oxygen use oxide -> conductors, such as ZrC>2 -based ceramic, those for halogens use halide conductors (e.g., KAg s), while -> hydrogen sensors use protonic conductors. There are sensors for C02, N02, NH3, S03) H2S, HCN, HF, etc. (see -> lambda probe). 

Dubbe, A., Wiemhdfer, H.-D., Sadaoka, Y. and Gdpel, W. (1995) Microstructure and response behaviour of electrodes for CO2 gas sensors based on solid electrolytes. Sens. Actuators B, 24-25, 600-2. 

Imanaka, N., Oda, A., Tamura, S. and Adachi, G.-Y. (2004) Total nitrogen oxides gas sensor based on solid electrolytes with refractory oxide-based auxiliary electrode. J. Electrochem. Soc., 151 (5), H113-16. 

Miura, N. and Yamazoe, N. (2001) Approach to high-performance electrochemical NOx sensors based on solid electrolytes. Sensors Update, 6, 191-210. 

These types of sensors are represented by semiconductor devices and devices incorporating solid electrolytes. These have certain advantages over sensors utilising liquid electrolytes. These are the inherent, robust nature of such sensors, the lack of problems due to evaporation of solvent or corrosion due to spillage, the possibilities of miniturisation and the possibility of mass production. Sensors based on solid electrolytes usually operate in the potentiometric or amperometric mode but the semiconductor devices usually operate by measuring changes in conductivity. 

The fundamental principles of potentiometric sensors based on solid electrolytes have been reviewed in several papers. " " Potentiometric sensors are based on the existence of an equilibrium at the interface between a SIC and the analyzed medium, by exchange of electrochemical species. This hypothesis is acceptable (after a transient response) if there is... 

Fig. 19.2 Basic principles of gas sensors based on solid electrolytes ...

Sensors based on solid electrolytes are also known for C02. The general setup of a thick film sensor is given in Fig. 19.4. As a solid electrolyte sodium ion conductors like B-alumina or Nasicon together with sodium carbonate are used. The electrochemical reactions take place at temperatures between 350 and 550 °C. The electrode reactions of the measuring and reference electrode are... 

As discussed in the former paragraphs there are high- and low-temperature gas sensors (Table 19.2). Which sensor for which application can be recommended is a question of expected information, response time and lifetime. This depends on the matrix to be analysed and on the temperature. For long-term applications at high as well as low temperatures with a short response time high-temperature sensors based on solid electrolytes are preferred. They can be used for the most part maintenance free. On the other hand, the price of those sensors is mostly much higher than that of normal-temperature sensors. Normal-temperature sensors need calibrations from time to time. 

Lu X, Wu S, Wang L, Su Z (2005) Solid-state amperometric hydrogen sensor based on polymer electrolyte membrane fuel ceU. Sens Actuators B 107 812-817... 

Ruangchuay L, Sirivat A, Schwank J (2004) Electrical conductivity response of polypyrrole to acetone vapor effect of dopant anions and interaction mechanisms. Synth Met 140 15-21 Sakthivel M, Weppner W (2006a) Response behaviour of a hydrogen sensor based on ionic conducting polymer-metal interfaces prepared by the chemical reduction method. Sensors 6 284-297 Sakthivel M, Weppner W (2006b) Development of a hydrogen sensor based on solid polymer electrolyte membranes. Sens Actuators B 113 998-1004... 

As shown in Table 6.1, at present three types of potentiometric gas sensors can be designed based on solid electrolytes (Yamazoe and Miura 1998). The well-known oxygen YSZ-based probe with an oxygen ion conductor is a sensor of Type I. This means that a gas sensor of Type I is constructed from a solid electrolyte for which the mobile ion is the same as that electrochemically derived from the gas phase. In this case, the interface potential can be obtained from the local equilibrium reaction, occurring at the electrode/electrolyte/gas three-phase boundary involving the mobile 0 ions in the electrolyte, electrons... 

Miura N, Yan Y, Sato M, Yao S, Nonaka S, Shimizu Y, Yamazoe N (1995) Solid state potentiometric CO sensors using anion conductor and metal carbonate. Sens Actuators B 24-25 260-265 Miura N, Lu G, Yamazoe N (2000) Progress in mixed-potential type devices based on solid electrolyte for sensing redox gases. Solid State Ionics 136-137 533-542... 

For example, based on solid electrolytes, electronically conducting ceramic films, or humidity sensors... 

To allow commercial applications of sensors at various potential emission sites, the construction of these sensors from solid materials is desirable, so as to minimize the size of the sensors and simplify the manufacturing process. To date, a number of small CO2 gas sensors have been developed, and these may be categorized by their sensing mechanism, whether based on optical cells, resistance/capacitance of semiconductors, or electromotive force (EMF)/current measurements based on solid electrolytes. However, such sensors continue to exhibit deficits, including low selectivity, poor chemical and physical stability, or high cost, and these problems must be mitigated to improve their usefulness. 

Electrochemical cells where ions dissipate through a solid electrolyte are mainly used for measuring oxygen concentrations. The most common sensors based on this principle are the A-probe for monitoring combustion engines in motor vehi-... 

The most thoroughly developed sensor based on a solid electrolyte is the oxygen sensor using a stabilised zirconia electrolyte. This type of sensor is one of the most successful commercial sensors to date. They are widely used in industry, especially in the analysis of exhaust gases from combustion engines. The following configuration is used in the Oj sensors ... 

Instead of the system silica/silicate also other systems such as titania/titanate, zirconia/zirconate can be used as a reference system [xiv]. The response time of freshly fabricated thick-film sensors based on thin-film /3-alumina is very short (about 15 ms at 650 °C). After several weeks of operating this time increases 10 times (150 ms) [xv]. Solid electrolyte C02 sensors using Ni/carbonate composite as measuring electrode are suited for measuring of C02 in equilibrated water gases [xiv]. Using semiconducting oxides and carbonates like ITO (indium tin oxide) Nasicon-based C02 sensors are able to measure at room temperature [xvi]. 

Jasinski, G., Jasinski, P.. Nowakowski. A. and Chachulski, B. (2006) Properties of a lithium solid electrolyte gas sensor based on reaction kinetics. Meas. Sci. Technoi.. 17, 17-21. 

Maffei, N. and Kuriakose, A.K. (1999) A hydrogen sensor based on a hydrogen ion conducting solid electrolyte. Sens. Actuators B, 56, 243-6. 

Inaba. Y, Tamura, S. and Imanaka, N. (2007) New type of sulfur dioxide gas sensor based on trivalent Al ion-conducting solid electrolyte. Solid State Ionics, 179, 1625-7. 

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