Sensor for determining the concentration of oxidizable elements

Details Sensor for determining the concentration of oxidizable elements Sensor for determining the concentration of oxidizable elements

1998-02-11

2000-02-08

Warden, Sr., Robert J.

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204291, 204426, 204429, 4292181, G01N 2726

Patent

active

060224649

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention is based on a sensor for determining the concentration of oxidizable constituents in a gas mixture, in particular for determining one or several of the gases NOx, CO, H2 and preferably unsaturated hydrocarbons, of the generic type as in the main claim. The exhaust gases from Otto and diesel motors, internal combustion engines and combustion plants can contain increased concentrations of oxidizable constituents, in particular NOx, CO, H2 and hydrocarbons, e.g., as a result of malfunctions of components such as an injection valve, or a as a result of an incomplete combustion. Thus, it is necessary to know the concentration of these exhaust-gas constituents to optimize the combustion reactions. A method for determining oxidizable gases is described in the JP-0S 60061654, according to which a stoichiometric conversion with oxygen takes place at a first measuring electrode of platinum metals, and according to which quasi equilibrium states are adjusted for the oxygen balance reaction at one or several additional metallic measuring electrodes with reduced catalytic activity. The Nernst voltages E1 and E2 are measured between the measuring electrodes and a reference electrode that is subjected to a reference gas with constant oxygen partial pressure, and their difference based on calibration curves is used to calculate the concentration of the gas constituents.


SUMMARY OF THE INVENTION

The present invention provides a sensor for determining the concentration of oxidizable constituents in a gas mixture, in particular for determining one or several of the gases No.sub.x, Co, H.sub.2, and preferably unsaturated hydrocarbons, for which a reference electrode that catalyzes the equilibrium adjustment of the gas mixture, an ion-conducting solid electrolyte, and at least one measuring electrode that is subjected to the measuring gas and which cannot or only to a small degree catalyze the equilibrium adjustment of the gas mixture are arranged in superimposed layers on one of the large surfaces of a planar, electrically insulating substrate, characterized in that the measuring electrode contains one or several semiconductors as the main component. In contrast, the sensor according to the invention, has improved analytical performance characteristics, in particular an increased sensitivity and selectivity relative to individual measuring gas constituents to be determined.
Advantageous modifications and improvements of the sensor specified in the main claim are possible through the measures listed in the dependent claims. Preferably, the sensor has two measuring electrodes which are arranged side by side and at a distance to each other on the solid electrolyte. Preferably, the semiconductor of the sensor is doped with acceptors and/or donors. Preferably, the semiconductor is an oxide or a single-phase or multi-phase mixed oxide, in particular a rutile or dirutile or a mixture thereof. The semiconductor preferably consists of titanium oxide. Preferably, the donor is present in a higher concentration than the acceptor. Preferably, the donor is an element with higher valence than the metal or metals forming the semiconductor. Preferably, the donor is tantalum and/or niobium. Preferably the semiconductor contains one or several transition elements as acceptor, in particular nickel, copper, cobalt and/or chromium, preferably nickel, copper and/or cobalt and/or rare earths. Preferably, the acceptor is contained in the semiconductor as solid solution or as segregated constituent. Preferably, the semiconductor contains donors and/or acceptors in a concentration of respectively 0.01 to 25%. More preferably, the semiconductor contains 0.5 to 15% niobium and 0.25 to 7% nickel, preferably 7% niobium and 3% nickel.
The sensitivity and selectivity of the measuring electrodes are improved through doping of the in particular oxidic or mixed oxidic semiconductors with acceptors and/or donors.
The conductivity of the measuring electrodes is improved by adding donors, especially at higher concentra

REFERENCES:
patent: 4879016 (1989-11-01), Joshi
patent: 4931213 (1990-06-01), Cass
patent: 5023153 (1991-06-01), Weppner
patent: 5037525 (1991-08-01), Badwal
patent: 5443711 (1995-08-01), Kojima et al.
patent: 5505837 (1996-04-01), Friese et al.
patent: 5630920 (1997-05-01), Friese et al.
patent: 5849165 (1998-12-01), Kojima et al.

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