Zirconia Sensor Systems

It is evident that conducting even the simplest tests of the zirconia gas sensors at high temperatures entails certain expenses, and the efQciency of experiments is determined as a ratio of the positive effect obtained during experiments to the test expenses. Therefore, the strategy of planning the sensors tests is usually focused on the development of those algorithms that provide the maximum efQciency of the tests. 

Structure of the test planning and execution of the zirconia gas sensors should cover the following main tasks  

Planning. Planning experiments is one of the main stages of zirconia sensors testing. There are many methods of planning experiments [1-5]. Their complexity usually depends on the diversity of the sensors applications and as far as zirconia sensors testing is concerned, the most commonly used methods can be presented as follows  

Dispersion analysis. The task is formulated like this it is necessary to propose such a scheme of the sensor testing, which would allow factorizing the summary dispersion on the constituents. This method has been widely used in testing the different zirconia gas sensors. 

Factor experiment. The task is formulated like this it is necessary to estimate linear effects and interaction effects at a large number of the independent variables. In this case, the variation of several factors (variables) takes place, which ultimately increases the efficiency of the experiment. The significant effects assessment is usually done afterwards by the dispersion analysis. 

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Such stabilized zirconia sensor devices are widely used for combustion control in automobiles. All new cars sold in the US contain one of these devices in the exhaust system to sense the partial pressure of oxygen. This sensor is a key component... 

Zirconia sensors have been used primarily in the exhaust system of automobiles to control the air-to-fuel ratio for meeting the federal requirements on such noxious gases as carbon monoxide, methane and nitrogen oxides. The applicability of zirconia sensors for this particular application is based on the assumption that, under thermodynamic equilibrium, the partial pressure of oxygen in the exhaust gas depends primarily on the air-to-fuel ratio. To compensate for the fact that in reality equilibrium is not reached, catalytic platinum electrics are incorporated in the zirconia sensor design [Stevens, 1986]. In the zirconia sensor, the outside of the zirconia tube is exposed to the exhaust gas while the inside is exposed to the ambient air as a reference atmosphere. 

The zirconia sensor is installed between the engine and the catalytic converter in the exhaust system of automobiles. Hence, the oxygen sensor is exposed to severe conditions ... 

Application The zirconia oxygen sensor is widely used for combustion control processes and for air/fuel ratio regulation in internal combustion engines. The closed-end portion of the electrode tube is inserted into the exhaust gas stream. In the control of industrial combustion processes, no out stack sampling system is required. 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

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. 

Oxygen sensors, in low volume use as part of a closed loop emission control system for automotive applications since 1977, have seen wide-spread use starting with the 1981 model year. At the present time, a partially stabilized zirconia electrolyte using yttrium oxide as the stabilizer appears to be the most common choice for this application. 

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. 

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]. 

PEVD has been applied to deposit auxiliary phases (Na COj, NaNOj and Na SO ) for solid potenfiometric gaseous oxide (CO, NO, and SO ) sensors, as well as a yttria stabilized zirconia (YSZ) ceramic phase to form composite anodes for solid oxide fuel cells. In both cases, the theoretically ideal interfacial microstructures were realized. The performances of these solid state ionic devices improved significantly. Eurthermore, in order to set the foundation for future PEVD applications, a well-defined PEVD system has been studied both thermodynamically and kinetically, indicating that PEVD shows promise for a wide range of technological applications. 

Titanium and tin are important in terms of photocatalytic and sensor appHcations. There is insufficient space to continue this description here, but the Hterature on these topics can be found elsewhere [222-232]. Tungstated zirconia, for example, is a promising candidate for an ammonia exhaust gas sensor required to control a Urea-SCR (Selective Catalytic Reduction) system in diesel-engined cars [233, 234]. 

Another type of electrical conductivity observed in ceramics is ionic conductivity, which often occurs appreciably at elevated temperature a widely used material exhibiting this behavior is zirconia doped with other oxides such as calcia (CaO) or yttria (Y2O3). For this material, atomic oxygen is the mobile ionic species. Doped zirconia finds widespread use as oxygen sensors, especially as part of automobile emission control systems, where the oxygen content of the exhaust gas is monitored to control the air-to-fuel ratio. Other applications of ionic conducting ceramics are as the electrolyte phases in solid-oxide fuel cells and in sodium-sulfur batteries. 

The zirconia-based pump-sensor device can be used for controlling the oxygen partial pressure in closed systems typical applications include the oxygen permeation flux measurements, oxygen monitoring in molten metals, and coulometric titration. 

FIGURE 3.1 Catalytic converter system equipped with NO sensors for the exhaust gas emitted from a new-type car engine. (Reprinted from Miura, N., Nakatou M., and Zhuiykov, S., Impedancemetric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NO at high temperature. Sens. Actuators B, Chem. 93 (2003) 221-228, with permission from Elsevier Science.)... 

Based on information about various oxides used for the SE in NO sensors given above, the new concept of potentiometric measurement of total NO regardless of the NO2 (NO) ratio in the exhaust gases at high temperatures has been developed recently [24, 34, 35]. In 1999, the ability to measure total NO in exhaust gases by a new zirconia-based laminated-type sensor was reported by Riken Corporation, Japan [34]. Since then, this sensor structure has been modified, and the new total-NOx detection system is shown conceptually in Figure 3.6 [35]. The main functions in this system are as follows ... 

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