Oxygen, sensors

Oxygen is transferred from the high-pressure side (p q2) to the low-pressure (p q2) side of the cell. The cell voltage is related to the oxygen pressures by the Nemst equation, Eq. (6.4)  

In this case, the number of electrons transferred, n, is 4 and the appropriate reaction quotient is 

The high-pressure p )2 is taken as a reference pressure so that the unknown pressure p 02 is readily determined. These equations are often seen in the form  

Noting that E° for this cell reaction is zero  

Solving this equation for the oxygen partial pressure gives 

The applications based on chemical properties are given in Table 12.22. 

There is an obvious overlap among various applications categories. An example of the overlap is alumina which is both a structural refractory ceramic as well as a catalyst support. The additives modify the interconversion of various AI2O3 phases and the high surface area of y-Al203 is maintained by the added 3 wt% ceria or lanthana. Additives like yttria stabilize zirconia with respect to inertness and mechanical stability. Addition of yttrium or lanthanide to Fe-Cr-Al alloys reduces the spallation of oxide film. 

The vast growth in electronic equipment owes to capacitors which are essential in almost all the devices. Barium titanate forms the heart of the capacitors. The perovskite structure contains a small ion of high charge at the centre of an MC 6 octahedron. The high polarizability is the basis of a high dielectric constant in the capacitor. The addition of Nd to the mixed titanate gives a stable capacitance over a wide temperature range. 

MODERN ASPECTS OF RARE EARTHS AND THEIR COMPLEXES 

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Oxygen sensor Lean mixture sensor Knock sensor... 

Ionic conductivity is used in oxygen sensors and in batteries (qv). Stabilized zirconia, Zr Ca 02 has a very large number of oxygen vacancies and very high conductivity. P-Alurnina/72(9(9j5 -4< -(y, NaAl O y, is an excellent cation conductor because of the high mobiUty of Na" ions. Ceramics of P-alurnina are used as the electrolyte in sodium-sulfur batteries. 

In 1957, Ethyl Corp. announced anew antiknock compound, methylcyclopentadienyknanganese tricarbonyl [12108-13-3] (MMT). MMT is almost as effective as lead on a per gram of metal basis, but because manganese was more expensive than lead, MMT was not widely used until limits were placed on the lead content of gasoline. MMT was used in unleaded fuel between 1975 and 1978. After a large fleet test suggested that MMT could increase exhaust emissions because it interfered with catalysts and oxygen sensors, EPA banned its use in unleaded fuel in 1978. MMT is used in Canada in unleaded fuel. 

Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust control, automotive). It is an electrochemical sensor that uses a soHd electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a process-control situation and has very fast (- 100 ms) response time for feedback control. It is relatively inexpensive because it is designed specifically for one task and is mass-produced. It is relatively immune to other chemical species found in exhaust that could act as interferants. It performs in a very hostile environment and is reHable over a long period of time (36). 

In an actual exhaust system controlled by the signal of the oxygen sensor, stoichiometry is never maintained, rather, it cycles periodically rich and lean one to three times per second, ie, one-half of the time there is too much oxygen and one-half of the time there is too Httle. Incorporation of cerium oxide or other oxygen storage components solves this problem. The ceria adsorbs O2 that would otherwise escape during the lean half cycle, and during the rich half cycle the CO reacts with the adsorbed O2 (32,44,59—63). The TWC catalyst effectiveness is dependent on the use of Rh to reduce NO and... 

Oxygen Sensor and the Closed Loop Fuel Metering System... 

The function of the oxygen sensor and the closed loop fuel metering system is to maintain the air and fuel mixture at the stoichiometric condition as it passes into the engine for combustion ie, there should be no excess air or excess fuel. The main purpose is to permit the TWC catalyst to operate effectively to control HC, CO, and NO emissions. The oxygen sensor is located in the exhaust system ahead of the catalyst so that it is exposed to the exhaust of aU cylinders (see Fig. 4). The sensor analyzes the combustion event after it happens. Therefore, the system is sometimes caUed a closed loop feedback system. There is an inherent time delay in such a system and thus the system is constandy correcting the air/fuel mixture cycles around the stoichiometric control point rather than maintaining a desired air/fuel mixture. 

The oxygen sensor closed loop system automatically compensates for changes in fuel content or air density. For instance, the stoichiometric air/fuel mixture is maintained even when the vehicle climbs from sea level to high altitudes where the air density is lower. 

One system for measuring catalyst failure is based on two oxygen sensors, one located in the normal control location, the other downstream of the catalyst (102,103). The second O2 sensor indicates relative catalyst performance by measuring the abiUty to respond to a change in air/fuel mixture. Other techniques using temperatures sensors have also been described (104—107). Whereas the dual O2 sensor method is likely to be used initially, a criticism of the two O2 sensors system has been reported (44) showing that properly functioning catalysts would be detected as a failure by the method. 

EGR can seriously degrade engine performance, especially at idle, under load at low speed, and during cold start. Control of the amount of EGR during these phases can be accompHshed by the same electronic computer controller used in the closed loop oxygen sensor TWC system. Thus the desired NO reduction is achieved while at the same time retaining good driveabiUty. 

Engines are also designed to use either gasoline or methanol and any mixture thereof (132—136). Such a system utilizes the same fuel storage system, and is called a flexible fueled vehicle (EEV). The closed loop oxygen sensor and TWC catalyst system is perfect for the flexible fueled vehicle. Optimal emissions control requires a fuel sensor to detect the ratio of each fuel being metered at any time and to correct total fuel flow. 

Electrochemistry plays an important role in the large domain of. sensors, especially for gas analysis, that turn the chemical concentration of a gas component into an electrical signal. The longest-established sensors of this kind depend on superionic conductors, notably stabilised zirconia. The most important is probably the oxygen sensor used for analysing automobile exhaust gases (Figure 11.10). The space on one side of a solid-oxide electrolyte is filled with the gas to be analysed, the other side... 

Voltage Cell Type Oxygen Sensor The operation of the zirconia oxygen sensor utilizes the conduction of oxygen ions by virtue of anion or oxygen ion vacancies in the crystalline lattice. " The anion vacancies are created when the... 

The basic zirconia oxygen sensor design is illustrated in Fig. 13.52, which shows the principle of the zirconia solid oxygen-ion electrolyte. The sensor consists of a closed-end tube of ceramic zirconia ZrO )). The zirconia ceramic... 

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. 

Tends to burn visually cleaner at the exliaust tailpipe, since it operates in a closed-loop electronic mode (oxygen sensors interacting with the powertrain control module) to maintain an ideal air/fucl ratio of 14.7 1. 

Such tight mixture control is beyond the capability of the traditional carburetor. Consequently, after sorting through a number of alternatives, industry has settled on closed-loop-controlled port-fuel injection. Typically, an electronically controlled fuel injector is mounted in the intake port to each cylinder. A sensor in the air intake system tells an onboard computer what the airdow rate is, and the computer tells the fuel injectors how much fuel to inject for a stoichiometric ratio. An oxygen sensor checks the oxygen content in the exliaust stream and tells the computer to make a correction if the air/fuel ratio has drifted outside the desired range. This closed-loop control avoids unnecessary use ot an inefficient rich mixture during vehicle cruise. 

Fatt, Polarographic Oxygen Sensors, CRC Press, Cleveland, OH, 1976... 

Industrial bioreactors can withstand up to 3 atmospheres positive pressure. Large fermenters are equipped with a lit vertical sight glass for inspecting the contents of the reactor. Side parts for pH, temperature and dissolved oxygen sensors are a minimum requirement. A steam sterilisation sample port is provided. Mechanical agitators are installed on the top or bottom of the tank for adequate mixing. 

Fig. 9. Principle of single catalytic bed for simultaneous reduction and oxidation with oxygen sensor and feedback control on air-to-fuel ratio.

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 ... 

T. Arakawa, A. Saito, and J. Shiokawa, Surface study of a Ag electrode on a solid electrolyte used as oxygen sensor, Applications of Surface Science 16, 365-372 (1983). 

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