Zirconia Oxygen Sensor Research Articles

A proportional air-fuel ratio zirconia oxygen sensor

A novel time-sharing control circuit is used to convert a linear lean exhaust gas oxygen sensor into a proportional extended range (lean or rich exhaust) sensor. The time-sharing oscillator allows a single electrochemical cell to act as both the voltage sensor and as an oxygen pump, thus reducing the complexity of sensor geometry and fabrication compared to the universal exhaust gas oxygen (UEGO) sensor design. The control circuit has been demonstrated in the laboratory using two forms of commercially available ZrO 2 cells: (1) a design with a small chamber that opens to the exhaust by a small hole; (2) a design that provides for a limited diffusion region by use of a porous ceramic layer over the outer electrode. The porous-layer design (or a ceramic coating with one or more holes) appears to be a potential candidate for a linear exhaust gas sensor where interest is primarily in a linear response around stoichiometry.

Use of fractals to describe microstructures of porous thick-film platinum electrodes

Zirconia oxygen sensors, in such applications as power plants and automobiles, generally utilize platinum electrodes for the catalytic reaction of dissociating O2 at the surface. The microstructure of the platinum electrode defines the resulting electrical response. The electrode must be porous enough to allow the oxygen to reach the zirconia surface while still remaining electrically continuous. At low sintering temperatures, the platinum is highly porous and fine grained. The platinum particles sinter together as the firing temperatures are increased. As the sintering temperatures are raised even further, the surface of the platinum begins to facet with lower energy surfaces. These microstructural changes can be seen in Figures 1 and 2, but the goal of the work is to characterize the microstructure by its fractal dimension and then relate the fractal dimension to the electrical response. The sensors were fabricated from zirconia powder stabilized in the cubic phase with 8 mol% percent yttria. Each substrate was sintered for 14 hours at 1200°C. The resulting zirconia pellets, 13mm in diameter and 2mm in thickness, were roughly 97 to 98 percent of theoretical density. The Engelhard #6082 platinum paste was applied to the zirconia disks after they were mechanically polished ( diamond). The electrodes were then sintered at temperatures ranging from 600°C to 1000°C. Each sensor was tested to determine the impedance response from 1Hz to 5,000Hz. These frequencies correspond to the electrode at the test temperature of 600°C.

CO/CO 2 electrochemistry on zirconia electrolyte with platinum electrodes in relation to amperometric oxygen sensors

An amperometric zirconia oxygen sensor operated in the flue of a gas-burning combustion system showed a current linearly related to oxygen partial pressure under excess-air conditions (diffusion-controlled) and a current of the same sign but sensitive to sensor operating temperature (kinetically-controlled) for fuel-rich operation. Hence the sensor did not unambiguously distinguish the two sides of stoichiometry. Electrochemical investigations were carried out in CO/CO 2/N 2 and CO 2/O 2/N 2 mixtures to help resolve this problem. Electrode conductance values in CO/CO 2/N 2 mixtures were sometimes higher when approached from the anodic compared with the cathodic direction suggesting electrode blocking by a reduced and non-reversibly adsorbed species (CO or C). An abrupt decrease of more than an order of magnitude in conductance was observed as the temperature was raised through a critical temperature (near 700°C). This was thought to be associated with the change in stability of CO at that temperature via the disproportionation reaction (2CO⇄CO 2+C). Results indicated that the problem of distinguishing the two sides of stoichiometry could be ameliorated by constructing an amperometric sensor having a high ratio of cathode to anode 3-phase boundary lengths and operating at relatively low temperatures.

Characterisation of amperometric zirconia oxygen sensors prepared using planar thick film technology

Thick film planar amperometric zirconia oxygen sensors were prepared and the influence of sintering temperature from 1350–1450°C for one hour investigated. Characteristics of sensors at 1400°C and 1450°C were superior to those of the sensor sintered at 1350°C. Linearity of output with oxygen concentration up to 21% was observed. Temperature coefficients were positive and negative for sensors sintered at 1400 and 1450°C, respectively, offering the prospect of producing devices with a zero temperature coefficient. Encouraging results were also obtained for sensor operation in gases derived from the combustion of natural gas. The low manufacturing cost of these sensors will promote their incorporation into domestic boilers and automobile engines.

Gas carburizing of steel with furnace atmospheres formedin situ from methane and air and from butane and air

Carburizing experiments were conducted at 927 °C (1700 °F) and 843 °C (1550 °F) using furnace atmospheres formed from methane and air and from butane and air introduced directly into the carburizing furnace. Gas flow rates were low to promote equilibration of the reaction products within the furnace. The air flow rate was held constant while the methane or butane flow was automatically regulated to maintain a constant oxygen potential, as measured by a zirconia oxygen sensor, within the furnace. In comparing the results of these experiments with earlier results obtained using propane and air, several differences were noted: (a) The methane content of the furnace atmosphere, measured by infrared analysis, was about twice as great when methane was the feed gas rather than propane or butane. This was true despite the fact that the mean residence time of the gas within the furnace was greater in the methane experiments. Methane appears to be less effective than propane or butane in reducing the CO2 and H2O contents to the levels required for carburizing. (b) There was a greater tendency for the CO content of the furnace atmosphere to decrease at high carbon potentials when methane is used instead of propane or butane. The decrease in CO content is due to hydrogen dilution caused by sooting in the furnace vestibule. These differences in behavior make propane or butane better suited than methane for in situ generation of carburizing atmospheres. However, there is no difference in the amount of carburizing occurring at a specified carbon potential when methane, propane, or butane are used as the feed gas in this process.

Continuous measurement of oxygen concentration in citrus soil by means of a waterproof zirconia oxygen sensor

A limiting-current type of zirconia oxygen sensor has been developed and its efficiency was tested for the determination of oxygen in soil air. This oxygen sensor is waterproof and has a very short response time. By the use of the sensor, changes in soil oxygen concentration, due to undecomposed organic material and waterlogging, were observed. The sensor was found to be highly stable in the soil and suitable for continuous oxygen monitoring in the field.

Recent developments of the sensors for carbon oxides using solid electrolytes

A small tip-type sensor for CO 2 gives an electromotive force (EMF) which depends only on the partial pressure of CO 2. This sensor is composed of Na 2CO 3 and an Na + ion conductor such as NASICON or β-alumina. The simultaneous use of a zirconia oxygen sensor, and the sensor based on the CO-CO 2-O 2 concentration cell provides the feasibility to determine partial pressures of CO and CO 2.

Voltage Step Characteristics of Oxygen Concentration Cell Sensors for Nonequilibrium Gas Mixtures

Concentration cell zirconia oxygen sensors show steady‐state output characteristics having an abrupt voltage step at critical oxygen combustible gas concentration ratio. The voltage step characteristics were investigated experimentally at various temperatures using simple gas mixtures containing oxygen and combustible gas. Experimental results indicated that there are three temperature regions for the voltage step characteristics: (i) a high temperature region (region I) in which the voltage step occurs near the stoichiometric composition. This results from equilibrium reaction occurring in the gas mixtures; (ii) a middle temperature region (region II) in which the voltage step deviates from the stoichiometric composition of the nonequilibrium gas mixtures. The magnitude of the deviation does not depend on temperature; (iii) a low temperature region (region III) in which the voltage characteristics strongly depend on temperature. In the region II , the magnitude of the deviation in the voltage step characteristics is somewhat influenced by the flow rates of the gas mixtures and the thickness of a porous coating on the sensor electrode for protection. This behavior of the sensor was analyzed by a kinetic model in which the sensor electrode is assumed to have a high catalytic activity for the reaction of the combustible gas and oxygen. The model also considers mass transport through the porous coating and gaseous boundary layers. The sensor characteristics in the region II , in which the sensors are mainly used in practice, can be explained by this model.

Multiple Charge‐Transfer Reactions in Zirconia Electrolytic Cells: NO x Reduction on Platinum

This paper investigates the electrochemical activity of nitrogen, , and on platinum paste electrodes in zirconia oxygen sensors. Open‐circuit voltages and steady‐state current‐voltage characteristics were measured at 600°–800°C as a function of the partial pressures of oxygen, nitrogen, and nitrogen oxides. The open‐circuit voltage and the current density both increased monotonically with the and pressures in the sensor feed stream; both were most sensitive to the concentration. These data are well described by a reaction network in which two parallel charge‐transfer reactions occur on platinum: oxygen reduction to oxygen dianions; and reduction to and oxygen dianions. The electrochemical behavior is not consistent with a reaction network in which and are reduced catalytically, nor is it consistent with networks in which the oxides are completely reduced to nitrogen and oxygen dianions. These results indicate that and are electroactive on platinum. They also demonstrate that oxygen exchange cannot always be assumed to be the dominant charge‐transfer reaction in zirconia electrolytic cells.

Nernstian behaviour of zirconia oxygen sensors incorporating composite electrodes

The Nernstian behaviour of zirconia oxygen sensors incorporating composite electrodes has been described in the low temperature (<600°C) range. The composite electrodes consisting of a semiconducting metal oxide and platinum lower the operating temperature of oxygen sensors to about 300–350°C, well below that achievable with conventional metal or metal oxide electrodes. Complex impedance measurements made as a function of Pt/metal oxide ratio show that both the electrode resistance and the time constant go through a minimum when plotted against Pt content in the composite.

Interpretation of the output signal from a zirconia oxygen sensor operating under applied current

Under conditions of applied current flow, oxygen can be transported through a ZrO2 sensor so as to create a localized, relatively oxygen-rich atmosphere at the measuring electrode and thereby provide protection for the sensor in hostile reducing atmospheres. The rate of dissipation of this protective atmosphere can be controlled by the use of a porous ceramic muffle around the sensor's measuring electrode. Analysis of the output signal from the sensor not only allows the oxygen content in the bulk atmosphere to be determined, but can also yield much useful information relevant to the operation of the sensor, such as the onset of electrode limiting and electronic conduction.

SCr420鋼の制御ガス浸炭プロセスの開発

A new gas carburizing process which is capable of easily determining a suitable parameter has been developed using an explicit finite-difference method and a direct-search method of optimization. In this process, a weighting factor was introduced into this process for optimizing the carbon profile and time. Increasing the weighting factor made the difference between the calculated and target carbon profiles smaller but the treating time longer. Calculated carbon profiles of SCr420 steel were compared with experimental results by a atmosphere-control system using a zirconia oxygen-sensor. Agreement between the calculated and measured profiles was very good.The dependence of the diffusion coefficient on carbon concentration in SCr420 steel is expressed experimentally as follows:(This article is not displayable. Please see full text pdf.) \ oindentwhere C was the carbon concentration (mass%).The influence of the carburizing parameter on a variety of the initial conditions in the calculation process was also discussed in relation to the carbon profiles.

Potentiometric gas sensor for carbon dioxide using solid electrolytes

Small insert-type CO 2 sensors are constructed by coating Na 2CO 3 on the one side of a tip-shaped solid electrolyte of NASICON or “beta”-alumina, and the electromotive force (EMF) is examined at temperatures between 600 and 1000 K. The EMF depends only on the partial pressure of CO 2 and is expressed by the Nernst equation because the activity of Na 2O in solid electrolytes remains constant. The high actyivity of Na 2O results in the formulation of Na 2CO 3 on the other side of the solid electrolyte tip, giving the EMF to be zero. The response of the sensor is quite rapid. The hybrid CO 2O 2 sensor is designed by joining the NASICON CO 2 sensor with a stabilized zirconia oxygen sensor. It is revealed by the EMF measurement on the hybrid sensor that the CO 2 sensor functions at oxygen partial pressures above 10 −4Pa.

Low temperature response of the reaction-bonded zirconia oxygen sensor

When operating the reaction-bonded zirconia sensor as an electrolytic cell, in the temperature range 250–500°C, a relationship is shown to exist between oxygen partial pressure at the external platinum electrode and the cell overpotential occurring as a result of an applied cathodic potential of between 0.1 and 4 V. To investigate the nature of this relationship, the variation in sensor resistance polarization Rp and cathodic current flow I with oxygen partial pressure is measured over the range of pO2 of 1 to 10−3 atm. The magnitude of current flowing through the sensor is related to a number of variables - I α pO2, T, (VB-η), 1/Rp, where pO2 is oxygen partial pressure at the cathode, T is temperature, VB is applied cathodic potential, and η is sensor overpotential. All response is recorded as a change from a “reference condition”, applying when anode and cathode are both at air atmosphere; this allows compensation to be made for polarization effects normally occurring when operating the sensor as an electrolytic cell. It is shown that there is a linear relationship between the cell overpotential η and oxygen partial pressure, and that the applied potential VB will determine both the magnitude of η and the electrode reaction rates of the sensor. In this mode of operation, the sensitivity of the electrode reaction to oxygen partial pressure is shown to be increased dramatically, such that usable response can be achieved well below the normal minimum operating temperature for zirconia sensors.

Current mode operation: a technique for improving the reliability of zirconia oxygen probes in reducing atmospheres

A technique is described whereby zirconia oxygen sensors with platinum electrodes can be protected from deterioration in reducing atmospheres, typical of metallurgical heat treatment conditions. By the application of current to the sensor, the technique provides a form of anodic protection, while at the same time the oxygen concentration in an atmosphere varying in temperature and composition can be continuously detected and measured.

BdGas carburizing of steel with furnace atmospheres formedin situ from methane and air and from butane and air

Carburizing experiments were conducted at 927°C (1700°F) and 843°C (1550°F) using furnace atmospheres formed from methane and air and from butane and air introduced directly into the carburizing furnace. Gas flow rates were low to promote equilibration of the reaction products within the furnace. The air flow rate was held constant while the methane or butane flow was automatically regulated to maintain a constant oxygen potential, as measured by a zirconia oxygen sensor, within the furnace. In comparing the results of these experiments with earlier results obtained using propane and air, several differences were noted: (a) The methane content of the furnace atmosphere, measured by infrared analysis, was about twice as great when methane was the feed gas rather than propane or butane. This was true despite the fact that the mean residence time of the gas within the furnace was greater in the methane experiments. Methane appears to be less effective than propane or butane in reducing the CO2 and H2O contents to the levels required for carburizing. (b) There was a greater tendency for the CO content of the furnace atmosphere to decrease at high carbon potentials when methane is used instead of propane or butane. The decrease in CO content is due to hydrogen dilution caused by sooting in the furnace vestibule. These differences in behavior make propane or butane better suited than methane forin situ generation of carburizing atmospheres. However, there is no difference in the amount of carburizing occurring at a specified carbon potential when methane, propane, or butane are used as the feed gas in this process.

Efficiency of Noble Metal Electrodes for Zirconia Oxygen Sensors in Detecting Oxygen at Lower Temperatures

Abstract Metal Au, Ag, or Pt was deposited as electrode on the closed end of a stabilized electrolyte tube in the ZrO2 oxygen sensor, and emf of the cell, O2|m|ZrO2–Y2O3|m|O′2; (m=Au, Ag, Pt) with PO2=8.8 and PO′2=159 Torr (1 Torr=133.322 Pa), was measured as a function of temperature between 150 and 950 °C. For cells with Ag electrodes, a good agreement is found between theoretical and measured emf’s at lower temperatures (ca. 340 °C).

Non-ideal emf behavior of zirconia oxygen sensors

The emf of zirconia galvanic cells, O 2( P o 2 ), CO( P co), Pt/stabilized ZrO 2/Pt, O 2( P ó 2 ), which deviates from that calculated from Nernst's equation at temperatures around 350°C has been studied by observing CO oxidation behavior, including the reaction rate and the amount of CO absorbed on Pt (as determined by infrared spectroscopy), as well as by direct measurement. The observed emf is intimately related to CO oxidation on Pt. It is considered to be caused by a mixed electrode potential brought about by electrochemical reactions of O 2− in the solid electrolyte with the CO and O adsorbed on Pt during CO oxidation on Pt.

Complex-impedance analysis for the development of zirconia oxygen sensors

The complex impedance of Pt, Au and Ag electrodes in contact with a solid electrolyte, yttria-stabilized zirconia, was examined in the range 300–1000°C. The real and imaginary components were measured over the frequency range from dc to 100 kHz in three ways. The complex-impedance plot was characterized by two semi-circles. Considering the correlation between different thicknesses of the solid electrolyte, the semicircle in the high-frequency range was found to be due to oxygen ion relaxation due to the impurity phase formed in the electrolyte. On the other hand, the semicircle in the low-frequency range showed a characteristic distortion to an arc depending largely on the process by which the electrode was prepared and also on the electrode material. The impedance-plane plot for different equivalent circuits was calculated with various parameters. A reasonable equivalent circuit for the arc was obtained, and the analysis has been proved to be very useful for the development of oxygen sensors.

The control of very low oxygen partial pressures with stabilized zirconia cell

Control of very low oxygen partial pressures was performed in the range of 10 −21-10 −24 Pa at 750°C by pumping oxygen into the purified hydrogen stream with a stabilized zirconia cell. The oxygen partial pressures were monitored by a stabilized zirconia sensor. The stabilized zirconia oxygen sensor was calibrated by H 2-CO 2 gas buffer mixture in the range of oxygen pressure from 10 −17 to 10 −21 Pa, and oxygen partial pressures below 10 −21 Pa were measured by extrapolating the calibration line to very low oxygen partial pressures. The lowest oxygen partial pressure controlled was 10 −24 Pa at 750°C, which was limited by gas leaks in the system and also by the reduction of the ionic transference number in solid electrolyte used as the oxygen pump.