Eleftherios M. Logothetis

CO/HC sensors based on thin films of LaCoO3 and La0.8Sr0.2CoO3−δ metal oxides

Abstract We have demonstrated a new type of mixed potential, zirconia-based sensor that utilizes dense, thin films of either La–Sr–Co–O or La–Co–O perovskite transition metal oxide vs. a Au counter electrode to generate an EMF that is proportional to the oxidizable gas species (carbon monoxide (CO), C3H6, and C3H8) concentration in a gas stream containing oxygen. The devices reported in this work were tested at 600°C and 700°C and in gas mixtures containing 0.1% to 20% O2 concentrations. The metal-oxide-based sensors exhibited an improvement in operating temperature and level stability at elevated temperatures compared to Au–zirconia–Pt mixed potential devices already reported in the literature. However, as with Au–zirconia–Pt devices previously reported, the response behavior reproducibility from device to device was dependent on the Au morphology, which could vary significantly between samples under identical thermal histories. The changing Au morphology on both the Au counter electrode and the Au current collector on the metal oxide electrode were responsible for sensor aging and changes in device response over time. No change in the crystal structure of the perovskite thin film could be seen from XRD. A significant hysteresis in sensor response was found as the background oxygen concentration was cycled through stoichiometry, and this may be attributed to a change in the oxidation state of the cobaltate-based metal oxide electrode. In an effort to mitigate device aging, we replaced the Au counter electrode with a second metal oxide thin film, doped LaMnO3, and demonstrated the operation of a mixed potential sensor based on dual metal oxide electrodes.

Oxygen sensors using CoO ceramics

Ceramic CoO has been found to be a material suitable for oxygen sensing. The CoO sensor has good sensitivity and fast response time and appears particularly suited for applications involving high temperatures and corrosive or contaminating environments, e.g., combustion processes.

Stability of magnets levitated above superconductors

The stability of a permanent magnet levitated above a slab of hard superconductor is considered. The force on a dipole magnet over a perfectly diamagnetic disk is calculated. It is found that the radial component of the force is directed outward and is 10%–20% of the image (vertical) force near the edge. Estimates of the magnetic friction force due to flux motion in a hard superconductor are made using Bean’s model. The magnitude of the magnetic friction is large enough to stabilize the magnet over most of the disk for typical values of the critical current in ceramic superconductors (∼103 A/cm2), but too small for the highest values reported (>106 A/cm2). It is conjectured that flux trapping due to inhomogeneities gives rise to transient restoring forces.

An XPS study of interactions in thin films containing a noble metal with valence-invariant and reducible oxides

X-ray photoelectron spectroscopy was employed to examine the reducibility of oxides possessing multiple oxidation states (titania, niobia, and ceria) in the presence of palladium and a valence-invariant oxide (lanthana). The materials consisted of sputter-deposited ultra-thin films containing palladium with either titania, niobia, or ceria, with and without lanthana, on a sapphire substrate. For lanthana/palladium films, no reduction of the lanthana surface was observed after in situ treatment in hydrogen up to 600°C. After similar treatment, in the absence of lanthana a partial reduction to the lower oxidation states of niobia and titania and a total reduction to the lower oxidation state of ceria was observed. The presence of lanthana in the films inhibits the reduction of the titania completely and that of the niobia and ceria partially. Also, in a complementing set of films containing ceria an overlayer of high-surface-area γ-Al2O3, from the decomposition of an aqueous film of aluminum propoxide, was put over the smooth sapphire surface before the deposition of the other film components. In this case, the inhibiting effect on ceria-reducibility was attenuated. This points to the redistribution of lanthana over the available oxide surfaces. The preponderant portion of the lanthana accommodated on the porous γ-Al2O3 has rendered a significant part of the ceria unprotected and, in the presence of Pd, susceptible to reduction by H2 at 600°C. The results of these studies are a further manifestation of the known surface reactivity of lanthana, which while remaining itself irreducible, may strongly affect the behavior of other irreducible oxides such as alumina, or as presently shown, of reducible oxides such as titania, niobia, and ceria.

Theory of gas sensors

Abstract A first-principles model describing the electrical response of metal oxide gas sensors is presented. The model is applied to the operation of the electrochemical ZrO2 sensor.

A first-principles model of the zirconia oxygen sensor

Zirconia oxygen sensors are used extensively to monitor the air-to-fuel ratio of internal combustion engines. The basic physical and chemical processes involved in the sensing operation have been examined in detail to develop a first-principles model whose results agree well with experiments. Additionally, a wide variety of response is predicted from the model, depending upon the gas mixture, the electrode material, the reactivities of the gases, etc. The analysis is helpful in providing insight and design guidance in optimizing sensor operation for specific applications.

Preparation of oriented silicon carbide films by laser ablation of ceramic silicon carbide targets

Stoichiometric films of SiC, 60–150 nm thick, were deposited on [001] and [111] Si wafers by laser ablation of ceramic stoichiometric SiC targets. Films grown at substrate temperatures above 1050 °C show orientation epitaxial to the Si substrate along the film normal. Depending on the deposition conditions, the oriented crystallite dimension along this direction ranges from 20 nm up to the film thickness. The crystallite dimensions in the film plane range from 20 to 70 nm. Raman spectra indicate that the films often contain material other than crystalline SiC. Some of that is in the form of small (3–5 nm) graphitic inclusions.

Selected aspects of gas sensing

Abstract Based on the insight gained from the analysis of a previously developed model of metal oxide gas sensors and its comparison with results of experimental studies of automotive ZrO 2 oxygen sensors, this paper discusses our present understanding of the physical and chemical processes involved in the operation of these devices. In addition, areas where further research could enhance their usefulness and that of other gas sensors are discussed.

Oscillations in the emf of solid‐state electrochemical oxygen sensors

Oscillations have been found in the emf of ZrO2 electrochemical cells used for high‐temperature oxygen sensing in nonequilibrium CO,O2 environments. This behavior is shown to arise from an oscillation in the platinum‐catalyzed oxidation of CO. The use of a ZrO2 cell provides a new technique for studying oscillating reactions of this type.

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.