Multi-electrode gas sensors and methods of making and using them

Details Multi-electrode gas sensors and methods of making and using them Multi-electrode gas sensors and methods of making and using them

1997-08-20

1999-06-29

Brock, Michael

Measuring and testing

Gas analysis

Detector detail

338 34, G01N 2712

Patent

active

059182610

DESCRIPTION:

BRIEF SUMMARY
This invention relates to resistive gas sensors (also referred to as gas-sensitive resistors, or sensing devices), of the multi-electrode kind, i.e. a resistive gas sensor having three or more electrodes for receiving signals from different regions of a gas sensing element of the device. The invention also relates to methods of making such sensors, and to methods of detecting a target gas using a multi-electrode resistive gas sensor.
Such sensors will also be referred to herein as multi-electrode array devices.
Multi-electrode resistive gas sensors are disclosed in, for example, the document W092/21018, which teaches operating principles of multi-electrode gas-sensitive resistors which are self-diagnostic. These principles are developed further in the document W095/04927. Reference is invited to those documents for more information; and to the papers by D. E. Williams and K. F. E. Pratt, in J. Chem. Soc. Faraday Trans., 1995, 91, 1961 (referred to herein, for convenience, as "Williams I"), which presents the theoretical basis for the operation of the sensors, and in J. Chem. Soc. Faraday Trans., 1995, 91, 3307 (referred to herein, for convenience, as "Williams II"), which describes the experimental demonstration of the ability of such devices to detect poisoning of their own surfaces.
The gas sensing element consists generally of a porous body (typically a thin layer) of an oxide, which is to be understood to include combinations (such as a mixture) of more than one oxide, with or without additives for various purposes. Such optional additives may include catalytic material, for example to promote combustion of a particular gas in the mixture to which the sensor is exposed.
One of the objects of this invention is to provide a resistive gas sensor capable of distinguishing between two gases (e.g. a reactive gas and a less reactive gas) in a gaseous mixture. One example of a reactive gas, in this context, is ethanol, and one example of a less reactive gas is carbon monoxide.
A further object of the invention is to provide a sensor which makes an optimal distinction between a real hazard, such as CO, and a false alarm caused by, for example, ethanol.
According to the invention in a first aspect, a resistive gas sensor including: a porous gas sensing element comprising an oxide as active gas-sensitive material, the sensing element having a working surface for contact with an atmosphere; and at least three electrodes in electrical contact with the sensing element, for receiving signals from different regions of the latter, is characterised in that the microstructure of the sensing element is graded as between different regions of the sensing element.
Preferably, the said microstructure is finer in the basal region than in regions of the element closer to its working surface.
The sensing element is typically in the form of a layer, which preferably comprises a plurality of sub-layers, overlaid one on another, with each sub-layer having a different microstructure from the other sub-layer or sub-layers.
In preferred embodiments of the invention the electrodes comprise a first electrode, a common second electrode defining a narrow gap between the first and second electrodes, and a third electrode defining a wide gap between the second and third electrodes, whereby output signals from the first electrode represent electrical resistance in a basal region of the sensing element close to the electrodes, and output signals from the third electrode represent resistance across the whole thickness of the sensing element defined between the electrodes and the working surface.
Preferably, the active sensing material is chromium titanium oxide, with an impurity content comprising Cr.sub.2 O.sub.3 in the inclusive range 0-30 mol % and/or TiO.sub.2.
It will be understood from this that such impurities may be entirely absent, though these and other impurities can be present, as is discussed later herein.
In some embodiments the sensing element includes up to 30% by weight of catalytically active material.
According to the invent

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