Sensor for determining the concentration of oxidizable...

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

1998-09-22

2001-01-02

Tung, T. (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S424000, C204S426000, C204S429000, C205S787000

Utility Patent

active

06168700

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a sensor for determining the concentration of oxidizable components in a gas mixture, in particular for determining saturated and unsaturated hydrocarbons.
BACKGROUND INFORMATION
Formula A
2-x
A′
x
BO
4
are described in German Patent No. 23 34 044. They are used for detecting oxidizable gases. It is also described in German Patent No. 42 44 723 that cuprates of rare earths of the formula A
2-x
L
x
CuO
4
can be used to detect oxygen in gas mixtures, in particular in exhaust gases from internal combustion engines and combustion systems. However, it has proven difficult so far to find suitable materials for determining saturated and unsaturated hydrocarbons. This is due to the low corrosion resistance of the electrode materials used, which have so far had a high tendency to form interfering sulfates on the electrode surface. The only relatively satisfactory option available so far is described in Japanese Patent No. 60 61 654, where hydrocarbons can be determined on metallic measuring electrodes of platinum.
SUMMARY OF THE INVENTION
A sensor according to the present invention is composed of a material of the general formula AB
2
O
6
for the measuring electrode, i.e., materials from the class of eschynites in the narrower sense, which are also known as titanium niobates of the rare earths according to their most common representatives, samarskites, which are also known as yttrocolumbites, and euxenites, have a very high corrosion resistance at high temperatures and a low tendency to form sulfates. Due to the possible structural variations of the class of compounds referred to below in general as eschynites, it is possible to make available appropriate combinations of metals for A and B in the AB
2
O
6
structures for various gases to be determined. In addition, this great structural variety permits the corresponding octahedral positions in the AB
2
O
6
structure to be doped with a plurality of cations of comparable ionic radii if necessary.
A special advantage of the sensor according to the present invention is the use of natural eschynites as measuring electrodes, e.g., samarskite wiikite, yttrotantalite and clopinite which occur frequently in such deposits and are easily extracted. This leads to inexpensive application of eschynites which can be varied as needed.
For a specific application, however, it is also possible to synthesize the corresponding compounds with the desired stoichiometry easily and on a large scale in an especially advantageous manner, e.g., by the generally known method of hydrothermal synthesis at a low temperature and low pressure or by high temperature synthesis in quartz ampules. Hydrothermal synthesis first yields compounds in the eschynite phase, which at higher temperatures are converted monotropically to the high temperature euxenite phase.
In an especially preferred exemplary embodiment according to the present invention, item B of the mixed metal oxides of the eschynite family is at least partially replaced by transition metals. These are selected from the platinum metals, i.e., metals of the group ruthenium, rhodium, palladium, osmium, iridium and platinum, the coinage metals copper, silver, gold, and the iron group metals, i.e., iron, cobalt and nickel, as well as rhenium or transition metals in an oxidation state of +5. Compounds with nickel doping have proven especially advantageous. B of the eschynite is replaced by Ni
2+
cations in an amount of 0.005 to 0.2 mol %. This increases the sensitivity to hydrocarbons, as well as the electric conductivity of eschynites doped in this way.
In another preferred exemplary embodiment, oxygen in the AB
2
O
6
structure may be partially replaced by hydroxide or fluoride anions, which causes a greater thermodynamic and kinetic stability while also yielding an improved electric conductivity of the material.
Another preferred exemplary embodiment uses an eschynite which is also deficient in A (A
1-x
), i.e., has an occupancy defect. This measure also greatly improves the electric conductivity of the eschynite. Therefore, in a preferred general embodiment, the structural formula of eschynite can be given as
A
1-x
B
2-y
B′
y
(O,Oh,F)
6±z
The thickness of the eschynite measuring electrode is preferably 5 to 100 micrometers, but preferably 20 to 30 micrometers, and the eschynite measuring electrode can be applied using a conventional method, preferably by a thick-film method.
In a preferred exemplary embodiment, the sensor is composed of various layers, i.e., a reference electrode made of platinum, for example, is applied to a flat, electrically insulating substrate, e.g., made of aluminum oxide, with a layer of ion-conducting solid electrolytes and the eschynite measuring electrode above that.
In an advantageous exemplary embodiment, the above sensor is used to determine saturated and unsaturated hydrocarbons in combustion exhaust gases. The layered structure of the sensor permits a high degree of miniaturization and therefore simplifies the design and also permits inexpensive manufacture because the solid electrolyte is sintered to become porous. This eliminates the need for a reference gas, which greatly simplifies probe design.


REFERENCES:
patent: 3849282 (1974-11-01), Degueldre et al.
patent: 5393397 (1995-02-01), Fukaya et al.
patent: 5522979 (1996-06-01), Tatumoto et al.
patent: 5643429 (1997-07-01), Wachsman
patent: 23 34 044 (1974-01-01), None
patent: 42 44 723 (1994-03-01), None
patent: 60-61654 (1985-04-01), None

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