Method for Detecting Methane in a Gas Mixture

1. Field of the Invention

This invention relates to gas sensors. More specifically, this invention relates to method and apparatus for detecting methane in a gas mixture also containing known interference gases.

2. Description of the Prior Art

Sensors for the detection of methane in air are known. Currently available methane sensors typically utilize semiconducting metal oxides. Also, reaction heat sensors or heat tone sensors called pellistors are also used to detect methane.

The metal oxide most often used as a sensor material is tin oxide, which is a semiconductor at higher temperatures. A tin oxide sensor that has been doped with platinum will react to the presence of methane and other reducing gases. The operating temperature of a Pt-doped SnO.sub.2 sensor is typically low, around 350.degree. C.

Another methane sensor known in the art has a thin film of Ga.sub.2 O.sub.3 as a gas-sensitive element. At temperatures greater than 700.degree. C., a Ga.sub.2 O.sub.3 sensor will generate a very strong measurement signal even when the methane concentration is well below the lower explosive limit of 5%. The Ga.sub.2 O.sub.3 sensor does not react to the presence of water vapor in air. However, like all known SnO.sub.2 sensors and pellistors, the Ga.sub.2 O.sub.3 sensors are extremely sensitive to the presence of alcohols. The reaction of a Ga.sub.2 O.sub.3 sensor to the presence of alcohol--a commonly used solvent--is called cross-sensitivity, and in some circumstances, will completely mask the reaction of the Ga.sub.2 O.sub.3 sensor to methane.

Therefore, a need exists for an improved methane gas sensor which is useful at high temperatures and in the presence of commonly used solvent vapors.

The method of the present invention enables the construction of gas warning devices by means of which the concentration of methane in air can be continuously monitored. Such devices can be used in areas where known methane sensors often set off false alarms due to alcohol vapors and/or other solvent vapors present in the air.

In one method for detecting methane gas in accordance with the present invention, the gas to be tested is heated to a temperature exceeding 740.degree. C. before it is exposed to a methane-sensitive metal oxide whose temperature exceeds 740.degree. C. The gas is preheated to oxidize any solvents present in the gas. Changes in the resistivity, conductivity or relative permeability of the metal oxide layer are then measured to detect the reaction between the methane and the metal oxide layer.

In an embodiment, a methane sensor is provided which includes two spaced-apart electrodes that are covered by a methane-sensitive semi-conducting metal oxide film. A means for heating the gas to be tested and metal oxide film to a temperature exceeding 740.degree. C. is provided. A means for measuring changes in the resistivity, conductivity or permeability of the metal oxide is used to detect a reaction between methane and the metal oxide film.

An advantage of the invention is the creation of a method and apparatus by which methane can be unambiguously detected in a gas mixture containing solvent vapors.

The invention is illustrated diagrammatically in the accompanying figures wherein:

FIG. 1 is a perspective view of a methane sensor fabricated in accordance with the present invention;

FIG. 2 is a top view of the comb electrodes of a methane sensor fabricated in accordance with the present invention;

FIG. 3 is a bottom view of the sensor shown in FIG. 2, particularly illustrating the resistance heating means;

FIG. 4 is a sectional view taken through the gas-sensitive area and the covering layer of a methane sensor fabricated in accordance with the present invention;

FIG. 5 illustrates, graphically, the alcohol sensitivity at varying temperatures of a Ga.sub.2 O.sub.3 sensor, fabricated in accordance with the present invention:

FIG. 6 illustrates, graphically, the acetone sensitivity at varying temperatures of a Ga.sub.2 O.sub.3 sensor, fabricated in accordance with the present invention;

FIG. 7 illustrates, graphically, the alcohol sensitivity of an untreated or non-Pt-doped Ga.sub.2 O.sub.3 sensor layer fabricated in accordance with the present invention;

FIG. 8, illustrates, graphically, the alcohol sensitivity of a Pt-doped Ga.sub.2 O.sub.3 sensor layer fabricated in accordance with the present invention;

FIG. 9, illustrates, graphically, the methane sensitivity of a Ga.sub.2 O.sub.3 sensor layer fabricated in accordance with the present invention; and

FIG. 10, illustrates, graphically the carbon monoxide cross-sensitivity of the Ga.sub.2 O.sub.3 sensor layer.

FIG. 11 is a perspective view of a methane sensor fabricated in accordance with the present invention, particularly illustrating a thin film heater disposed above the metal oxide layer as well as an insulating housing shown in phantom.

Due to the sp.sup.3 -hybridization of its electron orbits, the methane molecule has a tetrahedral structure. Methane is thus chemically more stable than the molecules of organic solvents especially in regards to a tendency to resist oxidation, even at high temperatures. The invention exploits this characteristic for the detection of the comparatively less reactive methane, without disturbing cross-sensitivities, in a gas mixture containing solvent vapors by means of the sensor schematically represented in FIG. 1.

The main body 1 of the sensor, which is about 0.5 .mu.m thick, is made of an electrically insulating temperature-stable material such as beryllium oxide, aluminum oxide or silicon. Two platinum electrodes 2, 2', a Ga.sub.2 O.sub.3 thin film 3 as a CH.sub.4 -sensitive element that conductively connects the comb electrodes 2, 2', and a temperature probe 4 are arranged on the main body 1. An SiO.sub.2 passivation layer 5 is intended to shield the connection leads 6, 6' which extend from the two comb electrodes 2, 2' and the leads 7, 7' which extend from the temperature probe 4, from the oxygen present in the gas being tested. The dimensions of the comb electrodes 2, 2' depend on the specific resistance of the sensor layer 4 in the desired temperature region. The sensor layer 4 is manufactured by means of a sputtering or vacuum evaporation process and has a thickness from about 1 .mu.m to about 2 .mu.m. The electrodes 2, 2' of the interdigital structure, shown to scale in a top view in FIG. 2, can comprise thicknesses from about 0.1 to about 10 .mu.m, widths from about 1 to about 1000 .mu.m and spacings from about 1 to about 100 .mu.m.

In order to enable the operating temperature, which is preferably between 700.degree. C. and 1000.degree. C., to be set and held constant independent of external influences, the sensor 1 is actively heated by a resistance layer 8, which is arranged on the back side of the main body 1, which is passivated if warranted. This resistance layer 8 is shown to scale in FIG. 3 and is preferably made of platinum, gold or an electrically conductive ceramic.

The porous layer 9, which is shown in FIG. 4, completely covers the CH.sub.4 -sensitive element 3. In porous layer 9, the comparatively reactive organic solvents (e.g., alcohol and acetone) are oxidized to the products CO, CO.sub.2 or H.sub.2 O before reaching the thin film 3. Therefore, the reactive organic solvents do not influence the conductivity of the Ga.sub.2 O.sub.3 because these components are oxidized before they reach the layer 3. The oxidation of the solvents takes place at a temperature predetermined by the heating element 8. One suitable temperature is about 800.degree. C. or higher. As shown in FIG. 8, when a dopant, such at Pt, is utilized, the solvent vapors will be oxidized at a lower temperature of about 740.degree. C. In contrast, the less active methane is not oxidized at these temperatures.

Therefore, the methane reaches the surface of the Ga.sub.2 O.sub.3 thin film 3 and reacts there with the oxygen of the metal oxide. The oxygen vacancies that arise emit electrons to the crystal lattice, so that the conductivity of the Ga.sub.2 O.sub.3 thin film 3 increases according to the concentration of methane in the gas being tested. Due to the high temperature (greater than 800.degree. C.) in the covering layer 9, which is about 1 to 50 .mu.m thick, in particular 10 .mu.m, the methane is partially converted by cracking processes into more reactive constituents which reach the thin film 3, and change its conductivity more than methane itself thereby yielding an enhanced sensor response.

The covering layer 9 is made of an electrically non-conductive, temperature-stable material, whereby in particular SiO.sub.2, SiN or SiON (silicon oxinitride) in polycrystalline or amorphous form, as well as non-conductive ceramics (e.g. ceramics based on the metal oxides Al.sub.2 O.sub.3 and BeO) or combinations of the cited materials are suitable. The porous layer 9 serves as a covering layer over the metal oxide sensing layer of FIG. 4. The depositing of the covering layer 9 may for example be achieved by means of silk-screen printing techniques, sedimentation, precipitation or vacuum coating. The effect of the covering layer 9 is improved by providing it with a dispersion of oxidation catalysts such as Pt, Pd or Fe.sub.2 O.sub.3. As an alternative to the porous layer 9, a housing or thin heating layer may be provided for heating of the test gas before it reaches the metal oxide layer 3. FIG. 11 illustrates a main body 1 of a methane sensor disposed beneath a thin heating layer 11 which, in the embodiment illustrated in FIG. 11, is a thin film heating resistance layer. Current flows through the electrode 12 which results in infrared radiation extending downward to the sensor body 1. Alternatively, an insulated housing 13 may be provided as an alternative to or in addition to the heating layer 11. The housing 13 preferably includes a heating element such as an electrode in order to heat the space inside the housing within which the sensor body 1 is located.

In order to determine the extent the preheating of the gas to be detected causes the desired oxidation of the organic solvents (alcohol or acetone), the Ga.sub.2 O.sub.3 sensor represented in FIG. 1 was arranged in a test chamber of a high-temperature oven and exposed to solvent vapors. In a first series of tests, the hot air supplied to the sensor contained 1000 ppm ethanol, and in a second series it contained 1000 ppm acetone. The gas temperatures corresponded to the sensor temperature. The sensitivities are defined as the ratio of the resistance measured for air divided by the resistance measured for the air including the solvent vapors, e.g.

R.sub.air /R.sub.ethanol or, respectively, R.sub.air /R.sub.acetone where

R.sub.air =sensor resistivity to air

R.sub.ethanol =sensor resistivity to air +1000 ppm ethanol

R.sub.acetone =sensor resistivity to air +1000 ppm acetone.

The corresponding measurement curves are illustrated in FIGS. 5 and 6. It is clearly illustrated that the cross-sensitivity of the Ga.sub.2 O.sub.3 thin film to ethanol and acetone approaches zero as the gas temperatures exceed 800.degree. C. A mass spectroscopic analysis of the gas flowing through the test chamber confirmed the suspected conversion of the alcohol into CO.sub.2 and H.sub.2 O.

As shown in FIGS. 7 and 8, the oxidation of the solvent can also be achieved by a catalytic activation of the sensor surface as opposed to oxidation of the solvent in a porous layer. Turning to FIG. 7, the Ga.sub.2 O.sub.3 thin film that is not doped with Pt displays the expected sensitivity to alcohol. In contrast, referring now to FIG. 8, the Ga.sub.2 O.sub.3 thin film treated with a Pt dispersion no longer reacts to ethanol at a temperature approaching 740.degree. C.

As shown in FIG. 9, the Ga.sub.2 O.sub.3 thin film sensor of the present invention is very sensitive to methane. The conductance value thus changes by a factor of three, even if the air contains only 0.4% methane. Further, as shown in FIG. 10, a concentration of 1000 ppm CO, well above the MAC value, influences the measurement signal of the Ga.sub.2 O.sub.3 sensor of the present invention only insignificantly.

The invention is of course not limited to the exemplary embodiments specified above. The covering layer 9 for heating the gas under test can thus be omitted if the sensor is arranged in a housing that permits the exchange of gases, such a housing would need to be heated. It is further possible to arrange a thin layer provided with a heating resistance layer parallel to the sensor surface. The air gap between the thin layer and the sensor surface would ensure the heating of the test gas and the required gas exchange. It can be advantageous to provide the surface on the sensor side of the thin layer with a catalytically active layer or, respectively, dispersion of a noble metal such as Pt or Pd or of a subgroup metal oxide such as Fe.sub.2 O.sub.3.

In place of Ga.sub.2 O.sub.3, the oxygen-sensitive semiconducting metal oxides TiO.sub.2, Fe.sub.2 O.sub.3, CeO.sub.3, SrTiO.sub.3, Nb.sub.2 O.sub.3 or HfO.sub.2 may also in particular be used in order to measure their methane-concentration-dependent resistance, conductance or relative permeability.

Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications that reasonably and properly come within the scope of their contribution to the art.