Richard E. Soltis

Chemical and physical sensors based on oxygen pumping with solid-state electrochemical cells

Abstract Oxygen sensors based on ZrO 2 electrochemical cells have found extensive use in many applications. The most sensitive of these sensors employ the oxygen-pumping principle with ZrO 2 cells. Oxygen pumping, however, can also be used to generate other chemical and also physical sensors. This paper discusses several of these sensors, e.g., sensors for measuring CO, H 2 , H 2 O, hydrocarbons, gas flow, and gas pressure, and presents results from studies of the properties of some of these devices.

Sensors for measuring combustibles in the absence of oxygen

Calorimetric and semiconductor types of sensors for combustibles generally require the presence of oxygen in the measurement gas. This paper describes sensors incorporating at least one ZrO2 electrochemical cell together with a combustibles-sensing element that are capable of measuring combustibles even in the absence of oxygen. The ZrO2 cell is used as an oxygen pump to introduce into the sensor structure the amount of oxygen necessary for the proper operation of the sensing element. Devices of this type can also be used to determine the composition of simple fuel mixtures. Results of experimental investigation of these devices concepts are presented.

A study of factors that influence zirconia/platinum interfacial impedance using equivalent circuit analysis

Abstract An equivalent circuit model based on complex impedance analysis was proposed to characterize the zirconia electrochemical cell, and to correlate the performance of the cell with the material set and processing parameters. The empirical model that emerged is an extension of previous models [1] , [2] and was derived from a series of fabrication experiments. The resulting model can be used to describe the conduction mechanisms of zirconia grain and grain boundaries, along with components of the zirconia/platinum interfacial and electrode conduction, the porosity and roughness of the electrode, and the diffusion characteristics within electrode and at the interface. The model can be utilized to predict the performance of the zirconia cell, and to gain a better understanding of the impact of the materials and processing parameters so as to optimize the fabrication process for planar-type sensors.

Experimental and theoretical study of the response of ZrO2 oxygen sensors to simple one-reducing-gas mixtures

Abstract The emf of commercial automotive zirconia oxygen sensors exposed to O 2 /CO/CO 2 /N 2 or O 2 /H 2 /H 2 O/N 2 gas mixtures was measured as a function of R ′=(2P O 2 /P CO ) or (2P O 2 /P H 2 ) at several temperatures. A step-like transition from high to low emf occurs at the switch-point R s0 ′ that is almost independent of temperature. The high emf (at say R ′=0.5) first increases and then decreases with temperature for O 2 /CO/CO 2 /N 2 gas mixture, whereas it only decreases for O 2 /H 2 /H 2 O/N 2 . These results may be analyzed using the steady state model of gas sensors [A.D. Brailsford, M. Yussouff, E.M. Logothetis, M. Shane, Steady-state model of a zirconia oxygen sensor in a simple gas mixture, Sensors and Actuators, B 24/25 (1995) 362–365]. Two lumped parameters, R s ′ and ξ , are needed in the model to compute the emf. R s ′ represents the theoretical switch-point and ξ determines the theoretical high emf. To obtain good fit with experiment, we choose R s ′ close to the observed R s0 ′ and then vary ξ . The quantity ξ depends upon the fundamental processes taking place on the sensor electrode; the temperature dependence of its fitted values (and hence the high emf) may be understood in terms of rate constants for adsorption, desorption and reaction.

Reactive codeposition of in situ Y‐Ba‐Cu‐O superconducting films using dilute mixtures of ozone in oxygen

This investigation reports the effect of dilute additions of ozone in molecular oxygen on the limiting reactive gas pressures required for in situ deposition of YBa2Cu3O7−x thin films. Thermodynamic analysis indicates that the high oxidizing potential of ozone assures stability at equilibrium under virtually all experimental conditions of ozone pressure and substrate temperature. Thus the generation of a minor fraction of ozone in oxygen enables in situ deposition to be carried out without further enrichment at gas pressures well below those required in pure molecular oxygen. In situ superconductor deposition at various rates using coevaporated metals revealed a lower limit at an ozone‐to‐copper ratio of approximately unity. Superconducting films deposited at O3/Cu flux ratios above this threshold were highly (00l) oriented with critical temperatures of 87–90 K and critical currents of 2×106 A/cm2 that exhibited only small or ambiguous trends with increasing ozone above the threshold. Films deposited at O...

Aging Studies of Sr-Doped LaCrO3 ∕ YSZ ∕ Pt Cells for an Electrochemical NO x Sensor

The stability and NO x sensing performance of electrochemical cells of the structure Sr-doped LaCrO 3-δ (LSC)/yttria-stabilized zirconia (YSZ)/Pt are being investigated for use in NO x aftertreatment systems in diesel vehicles. Among the requirements for NO x sensor materials in these systems are stability and long lifetime (up to 10 years) in the exhaust environment. In this study, cell aging effects were explored following extended exposure to a test environment of 10% O 2 at operating temperatures of 600-700°C. The data show that aging results in changes in particle morphology, chemical composition, and interfacial structure. Impedance spectroscopy indicates an initial increase in the cell resistance during the early stages of aging, which is correlated principally to densification of the Pt electrode. Also, X-ray photoelectron spectroscopy indicates formation of SrZrO 2 solid-state reaction product in the LSC, a process which is of finite duration. Subsequently, the overall cell resistance decreases with aging time, due in part to roughening of YSZ-LSC interface, which improves interface adherence and enhances charge transfer kinetics at the gas phase/YSZ/LSC triple-phase boundary. This study constitutes a first step in the development of a basic understanding of aging phenomena in solid-state electrochemical systems with applications not only to sensors, but also to fuel cells, membranes, and electrolyzers.

Investigating the Stability and Accuracy of the Phase Response for NOx Sensing 5% Mg-modified LaCrO3

Impedance spectroscopy measurements were carried out on LaCr{sub 0.95}Mg{sub 0.05}O{sub 3} (LCM) asymmetric interdigitated electrodes supported on fully stabilized 8-mol% Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} (YSZ) electrolytes. Experiments were carried out using 0-50 ppm NO{sub x}, 5-15% O{sub 2} with N{sub 2} as the balance, over temperatures ranging from 600-700 C. AC measurements taken at a constant frequency between 1-100 Hz indicated the phase response of the sensor was less sensitive to fluctuations in the O{sub 2} concentration and the baseline drift was limited. Specific frequencies were observed where the sensor response was essentially temperature independent.

Development of Highly Active Catalyst for Si-Microcalorimetric Gas Sensor

We are interested in the development of catalytic materials that are suitable for incorporation onto micromachined silicon gas sensors. The goal of the work is to explore methods to increase the activity and surface area of Pd and Pt catalysts. We employ sputtering techniques to fabricate catalytic layers with small grain size. In this paper, we study the catalytic properties of Pt/SiO2, Pd/SiO2, Pd/Al2O3, Pt/Al2O3, Pd/Au and Pt/Cr multilayer stacks formed by successive step-by-step deposition of ultra-thin films of the corresponding materials.

AC Impedance Studies of AgI / Al2 O 3 Composites

The objective of the present study was, through ac and dc electrical measurements, to clearly separate bulk properties from electrode effects, search for the possible presence of a second conducting phase, and determine the frequency dependence of the conductivity of the AgI/Al 2 O 3 composites

Chemical and physical sensors based on oxygen pumping with solid state electrochemical cells

The authors discuss the use of the oxygen pumping principle with ZrO/sub 2/ solid-state electrochemical cells to generate a variety of sensors for measuring chemical species such as oxygen, CO, hydrocarbons, and H/sub 2/O, as well as physical parameters such as gas pressure and flow. Laboratory results on the behavior of some of these sensors are also presented. The variety of devices discussed shows the versatility of the oxygen pumping principle in realizing unique sensors. Although most of these are basically chemical sensors, it is possible in some cases to also generate sensors for physical variables such as gas pressure and flow.<<ETX>>