Patent Description:
The above types of sensors have been used with varying degrees of success in the industrial or laboratory settings where they have been employed. However, many such sensors have limitations that can impact their effectiveness in demanding new and existing applications. For example, pellistor sensors are prone to false alarms due to cross-sensitivity. NDIR sensors have been used in low-volume applications, but can be difficult and expensive to manufacture to commercial tolerances. Electrochemical sensors rely on redox reactions involving tested gas components at electrodes separated by an electrolyte that produce or affect electrical current in a circuit connecting the electrodes. However, solid state electrochemical sensors can be difficult to implement for some materials. For example, solid state electrochemical sensors testing for combustible hydrocarbons may utilize solid electrolytes formed from ceramics such as perovskite, which can require high temperatures (typically in excess of <NUM>) that render them impractical for many applications. Some electrochemical sensors that operate at lower temperatures (e.g., carbon monoxide sensors, hydrogen sulfide sensors) require the presence of water at the electrode/electrolyte interface for the electrochemical redox reactions, which can render them impractical for many applications.

MOS sensors rely on interaction between gas test components such as hydrogen sulfide or hydrocarbons with adsorbed oxygen on the metal oxide semiconductor surface. In the absence of the gas test components, the metal oxide semiconductor adsorbs atmospheric oxygen at the surface, and this adsorbed oxygen captures free electrons from the metal oxide semiconductor material, resulting in a measurable level of base resistance of the semiconductor at a relatively high level. Upon exposure to gas test components such as hydrogen sulfide or hydrocarbon, the gas test component interacts with the adsorbed oxygen, causing it to release free electrons back to the semiconductor material, resulting in a measurable decrease in resistance that can be correlated with a measured level of test gas component.

In view of the demanding requirements for gas sensors, there remains a need for new alternatives for various environments and applications.

A gas-sensing element according to a first aspect of the invention is described in claim <NUM>. This gas-sensing element comprises a body comprising a semiconductor that is a metal oxide of a first metal. This semiconductor is also referred to herein as a "metal oxide semiconductor" or "MOS". The gas-sensing element includes a gas-sensing surface over the body. The gas-sensing surface comprises metal oxide semiconductor of the first metal and a dopant comprising a second metal that is a transition metal and is different than the first metal. The gas-sensing element also includes an auxiliary component comprising: a metal chalcogenide disposed at the gas-sensing surface, wherein the gas-sensing element comprises a plurality of alternating deposits of the metal oxide semiconductor of the first metal and deposits of the second metal, disposed in the gas-sensing element between the body and the gas-sensing surface.

In any one or combination of the foregoing embodiments, the metal chalcogenide may comprise a metal sulfide.

In any one or combination of the foregoing embodiments, the second metal may comprise one or more group <NUM> to group <NUM> transition metals.

In any one or combination of the foregoing embodiments, the first metal may comprise any one of the commonly used metals for metal oxide semiconductors, including aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, iron, gallium, indium, molybdenum, niobium, tantalum, tin, titanium, tungsten, vanadium or zinc.

In any one or combination of the foregoing embodiments, the first metal may comprise tin and the second metal comprises copper.

In some embodiments, a gas sensor comprises the gas-sensing element of any one or combination of the foregoing embodiments disposed between electrodes connected by a voltage-measuring circuit, current-measuring circuit, resistance-measuring circuit, impedance-measuring circuit, or conductance-measuring circuit.

In some embodiments, the resistance-measuring circuit of the gas sensor comprises a signal processor calibrated to determine hydrogen sulfide concentration based on measured resistance at the gas-sensing surface.

In a second aspect according to claim <NUM> a method of using the gas sensor of any one or combination of the foregoing embodiments comprises exposing the gas-sensing surface to a gas to be tested, and measuring resistance of the gas-sensing element between the electrodes to determine a presence or concentration of a gas component.

In any one or combination of the foregoing embodiments, the gas sensor may test for or may be configured to test for hydrogen sulfide.

In a third aspect according to claim <NUM>, a method of making a gas-sensing element comprises disposing a transition metal dopant comprising a second metal at a surface of a semiconductor that is a metal oxide of a first metal, and disposing a metal chalcogenide on top of the doped surface, wherein the method comprises alternately depositing second metal and metal oxide semiconductor of the first metal to form a plurality of alternating deposits of second metal and metal oxide semiconductor of the first metal between the body and the doped surface.

With reference now to the Figures, <FIG> schematically depict an cross-section view of an example embodiment of a gas-sensing element. As shown in <FIG>, gas-sensing element <NUM>, <NUM>' includes a metal oxide semiconductor body <NUM> disposed on a substrate having a gas-sensing surface <NUM> that comprises the metal oxide semiconductor material and a transition metal dopant. Typically, the gas-sensing elements <NUM>, <NUM>' are disposed on a substrate <NUM> as illustrated in <FIG>. Examples of metal oxide semiconductors include but are not limited to aluminum (III) oxide, bismuth (III) oxide, cadmium oxide, cerium (IV) oxide, chromium (III) oxide, cobalt (III) oxide, copper (II) oxide, iron (III) oxide, gallium (III) oxide, Indium (III) oxide, molybdenum (VI) oxide, niobium (V) oxide, nickel (II) oxide, tantalum (V) oxide, tin (IV) oxide, titanium (IV) oxide, tungsten (VI) oxide, vanadium (<NUM>) oxide, zinc (II) oxide and mixtures of these. Mixed metal oxides (e.g., SnO<NUM>-CuO or other mixed oxides of the above metal oxides) can also be utilized, and the term "first metal" as used herein includes metal mixtures. Transition metal dopants are used to enhance the responsiveness of the metal oxide semiconductor to target gases being sensed for, such as hydrogen sulfide, and to allow for the target gas to be distinguished from other gases that may also produce a change in electrical resistance at the gas-sensing surface <NUM>. In some embodiments, the dopant is a group <NUM> to group <NUM> transition metal. Examples of transition metal dopants include copper, silver, gold, iron, ruthenium, nickel, platinum, palladium, or vanadium. Although any of the above materials can exhibit a change in electrical resistance in response to exposure to various test gas components, the use of some materials for particular applications has been more widespread than other materials. For example, copper-doped tin oxide can be used for hydrogen sulfide sensing elements and platinum and palladium doping is commonly used in sensing for hydrogen or hydrocarbons. Such combinations and others are included within this disclosure. Various other materials can be included in the metal oxide semiconductor at the gas-sensing surface <NUM>, including but not limited to noble metals (e.g., silver, gold). Dopants, metal oxide semiconductors, other materials, and combinations thereof are disclosed in <NPL>. , and in <NPL>.

As mentioned above, the gas-sensing element includes an auxiliary component comprising a metal chalcogenide disposed at the gas-sensing surface that stabilizes the second metal at the gas-sensing surface. An example embodiment of internally-disposed second metal <NUM> between the metal oxide semiconductor body <NUM> and the gas-sensing surface (not claimed) is schematically depicted in <FIG>. The element may also include metal oxide semiconductor <NUM> that is free of second metal (e.g., high purity metal oxide semiconductor) disposed between the internally disposed second metal <NUM> and the second metal-doped gas-sensing surface <NUM>. In the claimed invention, the sensing element includes a plurality of deposits of second metal alternating with deposits of metal oxide semiconductor, as illustrated in <FIG> with additional second metal <NUM> and additional metal oxide semiconductor <NUM>. Four deposits are illustrated in <FIG>, but larger numbers (e.g., more than <NUM>) of such alternating deposits can also be used.

Deposition of second metal or metal oxide semiconductor onto the metal oxide semiconductor body can be performed using thermal deposition techniques such as sputtering, physical vapor deposition, chemical vapor deposition, or thermal spray. Alternatively, any or all of the deposits can be grown layer by layer, for example, using solution-based epitaxy techniques such as sol-gel processing to form the individual layers. The term "layer" as used herein means any deposit of material, including islands and partial layers, as well as contiguous layers of material. Layers of internally-disposed second metal can range in thickness from <NUM> (meaning no contiguous layer such as where areas (e.g., islands) of deposited second metal having thicknesses as low as the mass equivalent of <NUM> Angstroms) to <NUM>. Layers of internally-disposed metal oxide semiconductor, which can be interspersed with deposits of the second metal, can range in thickness from <NUM> to <NUM>.

As mentioned above, the gas-sensing element includes an auxiliary component comprising a metal chalcogenide disposed at the gas-sensing surface that stabilizes the second metal at the gas-sensing surface. An example embodiment of a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface is schematically depicted in <FIG>. As shown in <FIG>, gas-sensing element <NUM>' includes metal oxide semiconductor body <NUM> and doped metal oxide semiconductor gas-sensing surface <NUM>. In some embodiments, the metal chalcogenide can be applied internal to the gas-sensing element adjacent to the gas-sensing surface <NUM>, as depicted by metal chalcogenide <NUM> in <FIG>. In the claimed invention, the metal chalcogenide is applied over the gas-sensing surface, as depicted by metal chalcogenide <NUM> in <FIG>. In some embodiments, the metal chalcogenide can be disposed (not shown) in the gas-sensing surface <NUM>. In some embodiments, the metal chalcogenide can be disposed in a combination of more than one of the specified locations (e.g., both over and under the gas-sensing surface <NUM>, or mixed in with and underneath the gas-sensing surface <NUM>). In some embodiments (not shown), the auxiliary component can include both (<NUM>) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, and (<NUM>) a metal chalcogenide disposed at the gas-sensing surface that stabilizes the second metal at the gas-sensing surface. For example, a gas-sensing element could have a metal chalcogenide over (<NUM>) the gas-sensing surface <NUM> and second metal disposed <NUM>, <NUM> between the gas-sensing surface <NUM> and the metal oxide semiconductor body <NUM>.

Examples of chalcogens for the metal chalcogenide include sulfur, selenium and tellurium. In some embodiments, the chalcogen is a chalcogen having a higher number on the periodic table than oxygen. Metals for the metal chalcogenide include but are not limited to silver, lead, zinc, iron, cadmium or other metals that provide a stable chalcogenide at the operating temperature of the sensing element. In some embodiments, the metal chalcogenide comprises a metal sulfide. Examples of metal sulfides include but are not limited to silver sulfide, lead sulfide, zinc sulfide, iron sulfide or cadmium sulfide. In some embodiments, the metal chalcogenide comprises silver sulfide. The metal chalcogenide can be introduced by applying the metal (e.g., silver, lead, zinc, iron) below or above the gas-sensing surface <NUM> using sputtering or any of the techniques referenced above for application of second metal <NUM> or metal oxide semiconductor <NUM>, reacting with a reactive chalcogenide such as hydrogen sulfide, and sintering. Sintering may promote spreading of the metal chalcogenide through the gas-sensing surface <NUM>.

The above-described sensing element can be incorporated into a sensor <NUM> as schematically depicted in <FIG>. As shown in <FIG>, gas sensor <NUM> comprises the gas-sensing element <NUM> with metal oxide semiconductor body <NUM> and gas-sensing surface <NUM>, integrated with either parallel or interdigitated (as shown, for higher gain) electrodes <NUM> and <NUM> configured to have doped metal oxide semiconductor at the gas-sensing surface <NUM> disposed between the interdigitated electrodes <NUM> and <NUM>. The electrodes <NUM>, <NUM> are depicted on top of the sensing element <NUM>, but can also be disposed at the bottom. The electrodes are connected externally to the gas-sensing element <NUM> by an electrical circuit <NUM> that includes a signal processor <NUM>. Signal processor <NUM> can be a voltmeter or ampere meter, but in many cases comprises a potentiostatic circuit, voltage divider circuit, bridge circuit, microprocessor, electronic control unit (ECU), or similar electronic device with integrated voltage and or amperage measurement functions and also can apply a voltage bias between the electrodes <NUM> and <NUM>. Other sensor components including but not limited housings, mounting hardware, gas flow conduits, fluid chambers are not shown in <FIG>, but can be incorporated into the sensor by the skilled person.

Additional disclosure is provided in the following Examples:.

As demonstrated by the following non-limiting example embodiments, some embodiments can provide a technical effect that promotes gas sensor stability and can mitigate gas sensor drift.

This Example is directed to disposing second metal between the gas-sensing surface of a sensing element and its metal oxide semiconductor body. Sensing elements were prepared by doping a tin oxide semiconductor surface with copper deposited by physical deposition means. Sensing element A was prepared as a control with the copper and gold dopants deposited onto the surface of a tin oxide body. Sensing element <NUM> was prepared by depositing copper and tin oxide in alternating layers to a tin oxide body, finishing with copper. Sensing element <NUM> was prepared similar to sensing element <NUM>, except that silver was deposited and sulfided after the top copper dopant application. All three sensors were sintered. The sensing elements were exposed to varying concentrations of hydrogen sulfide over time, and the sensor output was recorded in measured hydrogen sulfide content. The results are shown in <FIG> (sensing element A), 4B (sensing element <NUM>), and 4C (sensing element <NUM>). <FIG> depict overlaying plot of delivered concentration of hydrogen sulfide and the gas sensor output result in measured hydrogen sulfide content. The plotted sensor output in <FIG> is typical of hydrogen sulfide sensor experiencing a phenomenon known as "sleeping", where the sensor response to hydrogen sulfide is initially reduced, but grows stronger with each exposure. In contrast, <FIG> shows a longer term stabilizing effect of the layered sensor with improved stability and reduced sleep effect. In <FIG>, it is seen that with the silver co-catalyst at the surface produces a refined response, also with improved stability and reduced sleep effect.

Claim 1:
A gas-sensing element (<NUM>), comprising:
a body (<NUM>) comprising a semiconductor that is a metal oxide of a first metal;
a gas-sensing surface (<NUM>) over the body, comprising metal oxide semiconductor of the first metal and a dopant comprising a second metal that is a transition metal and is different than the first metal; and
an auxiliary component comprising a metal chalcogenide disposed at the gas-sensing surface;
wherein the sensing element comprises a plurality of alternating deposits of the metal oxide semiconductor of the first metal and deposits of the second metal, disposed in the gas-sensing element between the body and the gas-sensing surface.