Oxygen sensors made of alkaline-earth-doped lanthanum ferrites

Details Oxygen sensors made of alkaline-earth-doped lanthanum ferrites Oxygen sensors made of alkaline-earth-doped lanthanum ferrites

1997-03-03

1998-12-01

Bonner, Melissa

Compositions: ceramic

Ceramic compositions

Refractory

204421, 204425, 204427, 204428, 338 34, C04B 3540

Patent

active

058438580

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to oxygen sensors according to the preamble of the main claim.
Oxygen sensors that contain strontium ferrate, barium ferrate, or strontium-barium ferrate are known (U.S. Pat. No. 4,454,494). In these materials, the central iron atoms of the ferrate lattice are replaced by one to seventy atom-percent of an element from the group composed of titanium, cerium, tantalum, or niobium. The chemical stability limit of these materials is at a temperature of 850.degree. C. with an O.sub.2 partial pressure of pO.sub.2 =10.sup.-18 bar, i.e. these materials decompose during prolonged operation in a reducing atmosphere, in a rich exhaust mixture for example.
The transition from p- to n- semiconducting sensor material in the materials of U.S. Pat. No. 4, 454, 494 takes place at a relatively high O.sub.2 partial pressure (>10.sup.-10 bar). This results in ambiguity of the sensor signal or insufficient signal at a gas change between rich and lean exhaust. In addition, the materials exhibit a clearly different temperature dependence for different O.sub.2 partial pressure ranges.
An oxygen sensor is known (Chem. Abstr. 112 (1990), reference number 126, 210t) whose sensor material consists of an alkaline-earth-doped lanthanum ferrite. However, this sensor material is not used to measure the change in resistance based on the recorded oxygen partial pressure, but the thermo electric force. In these sensor materials, a significant temperature dependence prevails. To compensate for this temperature dependence, additional measures must be adopted, temperature-regulating measures or precise settings of currents and/or voltages for example. This document therefore contains technical prejudice against oxygen sensors that use the resistive properties of the sensor material. The thermo electromotive force is measured instead.
The goal of the present invention is to provide improved sensor materials for oxygen sensors that are suitable for applications in lambda sensors for exhaust from combustion processes. In particular, these materials should exhibit a temperature-independent sensor resistance in the range of lean exhaust mixtures (.lambda.>1), while in the rich area they are temperature-activated in such fashion that the influence of the temperature on the sensor resistance can be compensated to the greatest degree possible by the temperature dependence of the thermodynamic equilibrium reactions of the gas components typically contained in the exhaust. As a result, the influence of temperature on the fluctuation and resistance between rich and lean ranges can be largely eliminated.
This goal is achieved by oxygen sensors made of alkaline-earth-doped perovskitic lanthanum ferrites according to the characterizing clause of the main claim.
The alkaline-earth-doped lanthanum ferrites used in the oxygen sensors according to the invention have the general formula Ca, Sr, and Ba, and the degree of doping x=0.1 to 0.3. The oxygen deficit of the anion is .delta.=0 to 0.25.
It has been found that the material properties in such alkaline-earth-doped lanthanum ferrites can be significantly improved by a careful choice of the mixed-ceramic composition, i.e. by varying the ratio of lanthanum to alkaline earth With a degree of doping in the range from 0.1 to 0.3, the ferrites exhibit a sensor characteristic that is independent of temperature and pO.sub.2, provided the oxygen partial pressure remains in the range from 10.sup.-3 to 10.sup.-2 bar (lambda greater than 1). In the rich range (lambda less than 1) these materials exhibit an increased dependence on oxygen partial pressure and temperature. This influence of temperature on resistance however is largely compensated by the temperature dependence of the oxygen partial pressure that is produced by the thermodynamic equilibrium reactions of the exhaust components. This results in an approximately constant temperature-dependent sensor resistance in the range of rich exhaust mixtures.
This result is surprising, since U.S. Pat. No. 4,454,494 states that increased

REFERENCES:
patent: 3951603 (1976-04-01), Obayashi et al.
patent: 4322968 (1982-04-01), Takami et al.

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