Measuring and testing – Gas analysis – Detector detail
Reexamination Certificate
1998-08-11
2001-01-16
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas analysis
Detector detail
C422S088000
Reexamination Certificate
active
06173602
ABSTRACT:
BACKGROUND OF THE INVENTION
A large number of semiconductor gas sensors are presently in use in many parts of the world largely to provide early warning of the development of an explosion hazard (e.g. escaping flammable gas) or the presence of toxic gases or vapors in ambient air.
A sensing element normally comprising a semiconducting material and presenting a high surface-to-bulk ratio is deployed on a heated substrate between two metallic electrodes. The presence of gas posing a hazard is detected by a sensible change in the resistance of the semiconducting element by means of the electrodes that are incorporated in a suitable electric circuit. The device is thus a gas-sensitive resistor.
The most commonly used material in gas sensitive resistors used to measure impure gases in air is tin dioxide. Tin dioxide sensors, though often useful in particular alarm functions, have generally been found to suffer from a lack of selectivity.
The reactions that allow the detection of target gases normally involve the oxidation of the target gas at the semiconductor (oxide) surface and a concomitant change in the charge carrier density of the material. Unfortunately, changes in relative humidity also give rise to a sensible change in the conductivity of tin dioxide even though, in this case, no oxidation process is possible. In other words, changes in relative humidity amount to an interference with the detection of gases by tin dioxide even though the mechanisms involved in the two responses are different.
Since the reactions that generate the resistance response take place at the oxide surface, a very small amount of second phase additive may modify the behavior substantially.
SUMMARY OF THE INVENTION
The present invention relates to sensors and more particularly to sensors suitable for use in gases and gaseous mixtures.
In a preferred embodiment, a sensor is provided that is suitable for use in a gas or gaseous mixture. The sensor includes a gas sensitive material (as hereinafter defined) that is capable of exhibiting a response in the form of an increase or a decrease in an electrical property of the material in the presence of a gas and that exhibits a small response to changes in the moisture content of the atmosphere.
In another preferred embodiment, the gas sensitive material is provided with two or more electrodes in communication with the gas sensitive material and the gas sensitive material is arranged so as to be capable of being contacted with a gas or gaseous mixture.
A sensor in accordance with the present invention may be used as a gas sensor in quantitative and/or qualitative determinations with gases or gaseous mixtures. The electrodes may be in direct communication with the gas sensitive material by being in contact therewith.
In this specification, the term “gas” preferably embraces a gas as such and any material that may be present in a gaseous phase, one example of which is a vapor.
The gas sensitive material is a material which responds to a target gas rather than to changes in relative humidity. Also, it will be appreciated that in this specification the term “gas sensitive material” means a preferred material which is gas (including vapor) sensitive in respect of an electrical property of the material.
It will be appreciated that the resistance and/or capacitance, and/or impedance of the gas sensitive material depends upon the gas or gaseous mixture contacting the gas sensitive material. Thus, by measuring the resistance and/or capacitance, and/or impedance of the gas sensitive material, the composition of a gas or gaseous mixture can be sensed.
Since the resistance and/or capacitance, and/or impedance of the gas sensitive material tends also to be temperature dependent, the sensor also preferably includes a temperature sensing means. The sensor may also include a heating means to enable operating temperature to be adjusted and/or contaminants to be burnt off if required.
It is to be understood that the sensitivity of a gas sensitive material may depend upon the composition of the gas sensitive material. Thus, by selection of the composition of the gas sensitive material its response to a particular gas may be optimized and its response to interferents, such as changes in relative humidity may be minimized.
The resistance and/or conductance, and/or impedance may be measured directly. Alternatively, the measurement may be carried out indirectly by incorporating the sensor in a feedback circuit of an oscillator such that the oscillator frequency varies with composition of the gas or gaseous mixture. Gas composition may then be determined using an electronic counter. The signal thus produced may be used to modulate a radio signal and thereby be transmitted over a distance (e.g. by telemetry or as a pulse train along an optical fibre).
Examples of gases which have shown responses using a sensor in accordance with the present invention are H
2
, C
2
H
4
, NH
3
, C
3
H
8
, H
2
S, CH
4
, and CO.
In one preferred embodiment of the present invention, the gas sensitive material (as herein defined), has two or more electrodes in communication with said gas sensitive material, and the gas sensitive material and the electrodes are in contact with the same gas.
Preferably, the gas sensitive material has porosity to give a satisfactory surface area for contact with the gas or gaseous mixture when in use.
The gas sensitive material, for example, may be prepared from an oxide or from an appropriate precursor. The oxide or precursor may optionally be prepared by a gel process, such as a sol-gel process or a gel precipitation process.
The powder may be dried and calcined (e.g. for approximately sixteen hours) at a temperature in the range of about 700-1000° C. depending upon the particular composition of gas sensitive material being prepared. The product resulting from calcination, which may be in the form of a cake, may be ground as required to give a fine powder. If required, grinding and calcination may be repeated several times in order to obtain a more suitable powder.
Subsequently, the fine powder may be pressed (e. g. with the optional addition of a binder, such as a solution of starch or polyvinyl alcohol) into any suitable shape (e. g. a pellet).
The pressing may be followed by a firing (e. g. at the same temperature as the calcination step(s) described above, or at a somewhat higher temperature, for approximately sixteen hours).
In addition to assisting in the binding of the powder into desired shapes, the binder also burns out during the firing stage giving rise to porosity.
As an alternative, a powder for subsequent calcination may be prepared, for example, by spray drying a solution (e.g. an aqueous solution) of appropriate starting material (e.g. a metal oxalate, metal acetate, or metal nitrate).
Electrodes may be applied to the prepared gas sensitive material in any suitable manner. For example, electrodes (e.g. gold electrodes) may be applied by means of screen printing or sputtering.
Alternatively to preparing a sensor by forming a pellet and applying electrodes as disclosed above, a sensor in accordance with the present invention may be formed in any suitable manner. Thus, for example, a parallel plate configuration may be fabricated by applying a first electrode (e.g. of gold) to an insulating substrate (e.g. by screen printing or sputtering), forming a gas sensitive material layer covering at least a portion of the first electrode (e.g. by deposition, for example by screen printing or doctor blading from a suspension or a colloidal dispersion and firing at a temperature in range of about 450-950° C. to promote adhesion and mechanical integrity) and forming a second electrode (e.g. of gold) on the gas sensitive material layer (e.g. by screen printing or sputtering).
The second electrode is preferably permeable to facilitate access of gas or gaseous mixture in which the sensor is to be used to the gas sensitive material layer.
By way of further example, a coplanar configuration may be used in the preparation of a sensor in accordance with the present invention.
I
Creighton Wray James
Larkin Daniel S.
Narasimhan Meera P.
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