Patent Application
20090159447
The invention relates generally to the area of gas sensing. More specifically, the invention relates to the sensing of NOx gas.
Environmental considerations are the primary motivating factors to develop NOx gas sensors. NOx emissions react with gases such as SOx, CO and moisture (water vapor) in the air to produce smog and acid rain. One of the major sources of NOx emissions is internal combustion engine exhaust.
The European Euro VI emission standards for light commercial vehicles (category N1-I, N1-II and N1-III), to be implemented by September 2015, require NOx emission levels below 0.5 gm/hp-hr. This typically translates to less than 50 ppm of NOx tail pipe emissions. Development of cost-effective gas sensors that can give reliable readout at such low concentration levels of analyte, and which can deliver robust performance even in harsh environments, is one of the major challenges facing present day emissions monitoring technology.
The current paradigm in improving the efficiency of internal combustion engines utilizes the technology of lean burn, whereby very high air:fuel ratios (˜102:1), as compared to conventional stoichiometric ratio (typically 20:1), are used. While the lean burn technology improves the efficiency of the engine, it also results in higher NOx emissions.
Any emissions control scheme that adversely impacts or limits efficiency will not be commercially viable. This necessitates real time monitoring of NOx emission levels and use of this information to dynamically control engine operating parameters (such as compression ratio etc) and exhaust after-treatment systems (such as catalytic filters etc) to achieve enhanced engine efficiency and enhanced emissions control respectively.
One of the current NOx gas sensing technologies in the market employs yttria stabilized zirconia (YSZ) based gas sensors. The gas sensors are essentially a multi-chamber electrochemical cell measuring the oxygen changes as a result of NOx decomposition. Such technology requires catalysts such as Pt. However, the performance of the catalyst degrades upon exposure to SOx and water vapor, as are commonly present in the exhaust from automobiles. This is one of the factors contributing to lowering the working life of such gas sensors. Further, the relatively intricate design of these gas sensors makes them expensive to replace on a regular basis.
Another current gas sensing technology in the market employs semiconductor gas sensors. As with any technology, this technology presents situation specific disadvantages and advantages. For example, gas emissions monitoring applications often require quantitative estimation of a particular or few gas species (e.g., NOx) in a multiple gas species environment. These gas sensors however, are sensitive to a broad range of gases, and therefore are of limited utility in such applications. Furthermore, these gas sensors are prone to long term instability because of their polycrystalline nature. On the other hand, this technology has the advantages of being solid-state, such as rigid construction and compact size. Further, the technology is amenable to readout using simple electronics thereby reducing cost of system manufacture, operation, maintenance and replacement. In addition, semiconductor gas sensors admit wide range of response tunability via introduction of suitable dopants, control of morphology of gas sensing surface, control of gas sensor operating parameters, amongst other factors.
A gas sensor that is semiconductor based, can make quantitative estimation of NOx gas even at low concentration levels, and has a long working life, would therefore, be highly desirable.
Embodiments of the invention are directed towards a gas sensor and a method for making the gas sensor.
In accordance with one exemplary embodiment of the invention, a gas sensor is provided. The gas sensor includes a gas sensing layer including at least one chemical compound with the general chemical formula MαOβNγ, wherein M is at least one chemical element selected from the group consisting of W, Ti, Ta, Sr, Mo, and combinations thereof, and α, β, γ are self-consistent, said gas sensing layer being capable of detecting at least one gas selected from the group consisting of NO, NO2, SO2, O2, H2O, CO, H2, and NH3, at least one electrode positioned within a adhesion layer composed of a material selected from the group consisting of Ti, Cr, and combinations thereof, and a response modification layer composed of a material selected from the group consisting of Mg, Ti, V, Cr, Mn, Co, Ni, Zn, Nb, Ru, Rh, Pd, Ta, W, Re, Pt, and combinations thereof. The at least one electrode is in communication with the sensing layer.
In accordance with another exemplary embodiment of the invention, an automobile including a system for gas sensing is provided. The automobile includes an exhaust system to transport gases, and a gas sensor. The gas sensor includes a gas sensing layer including at least one chemical compound with the general chemical formula MαOβNγ, wherein M is at least one chemical element selected from the group consisting of W, Ti, Ta, Sr, Mo, and combinations thereof, and α, β, γ are self-consistent, said gas sensing layer being capable of detecting at least one gas selected from the group consisting of NO, NO2, SO2, O2, H2O, CO, H2, and NH3, at least one electrode positioned within a adhesion layer composed of a material selected from the group consisting of Ti, Cr, and combinations thereof, and a response modification layer composed of a material selected from the group consisting of Mg, Ti, V, Cr, Mn, Co, Ni, Zn, Nb, Ru, Rh, Pd, Ta, W, Re, Pt, and combinations thereof. The at least one electrode is in communication with the sensing layer.
In accordance with another exemplary embodiment of the invention, a method for making a gas sensor is provided. The method includes providing a substrate, disposing a heating layer adjacent to the substrate layer, disposing a first glass layer adjacent to the heating layer, disposing a temperature sensing layer adjacent to the first glass layer, disposing a second glass layer adjacent to the temperature sensing layer, disposing at least one electrode adjacent to the second glass layer, disposing a adhesion layer adjacent to the at least one electrode, disposing a response modification layer adjacent to the adhesion layer, and disposing a gas sensing layer including at least one chemical compound with the general chemical formula MαOβNγ, wherein M is at least one chemical element selected from the group consisting of W, Ti, Ta, Sr, Mo, and combinations thereof, and α, β, γ are self-consistent, adjacent to the adhesion layer.
These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.
In the following description, whenever a particular aspect or feature of an embodiment of the invention is said to comprise or consist of at least one element of a group and combinations thereof, it is understood that the aspect or feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
A gas sensor may be used to determine if an “analyte” is present and/or to quantify an amount of the analyte. As used herein, the term “analyte” may refer to any substance to be detected and/or quantified, including but not limited to a gas, a vapor, a bioanalyte, particulate matter, and a combination thereof.
Since the primary constituents of NOx, i.e., NO and NO2 are interconvertible, reliable estimation of total NOx may be achieved if the response of the gas sensor, i.e., the NOx concentration dependent change in resistance of the gas sensor is equal (in terms of magnitude and sign) for both NO and NO2. Thus, if ΔR(NO, c) and ΔR(NO2, c) be the response of the senor to concentration “c” of NO and NO2 respectively, then a response ratio “rr” (defined below) close to unity would be desirable.
rr ≡ΔR(NO2, c)/ΔR(NO, c) (1)
As used herein, the term “equi-sensitive” refers to “rr” defined according to equation (1) when it is in a range from about 0.5 to about 3.
As used herein, the term “adjacent,” when used in context of discussion of different components comprising the gas sensor refers to “immediately next to” or it refers to the situation wherein other components present between the components under discussion.
As used herein, the term “communication,” when used in context of discussion of more than one component comprising the gas sensor may mean that any change in an electrical characteristic of one component is reflected at, and therefore, detectable and measurable via, the other component.
As used herein, the term “self-consistent,” when used in the context of discussion of the chemical formula of a chemical compound composing a host component of a gas sensing layer of a gas sensor may mean that the said chemical formula is consistent with established scientific principles that would be known to one skilled in the art. Necessarily implied in the said definition of the term “self-consistent” is the situation wherein the said chemical formula represents a metal oxynitride inorganic compound. In other words, the said chemical formula must represent an inorganic metal oxynitride chemical composition. A general example of chemical formula of host component of the gas sensor may be WOxN(2-0.67x), wherein 0<x<3.
As used herein, the term “harsh environment” or “harsh environments” refers to an environment within a volume that is in the vicinity of the gas sensing layer, and in which are present the analytes whose detection and/or estimation is being sought. The temperature within this volume may not be uniform, i.e., the temperature at/of different locations within this volume can be different, and can be from about 200° C. to about 800° C. At different locations within this volume can also be present different amounts of corrosive chemical species including but not limited to NOx, SOx, H2O, particulate matter, hydrocarbons, and a combination thereof.
As used herein, the term “response modification layer” refers to a layer which serves to introduce dopants into a gas sensing layer via surface doping through the mechanism of diffusion. This surface doping may result in a modification of the response of the gas sensing layer for a given set of operating parameters and/or operating environments.
As used herein, the term “adhesion layer”, refers to a layer which serves to provide adhesion between any two or more components of a gas sensor when it is interposed between the said two or more components. It may be composed of at least one chemical element selected from the group consisting of Ti, Cr, and combinations thereof.
As used herein, the term “glass” refers to any suitable material that may be used to form a separating layer, that in a given embodiment of the gas sensor, has sufficient thermal conductivity to provide a sufficiently large heat link between the elements that the separating layer segregates, and which has sufficient electrical resistivity to provide sufficient electrical resistance between the elements that the separating layer segregates.
If the response of a particular embodiment of the gas sensor changes as a result of introduction or withdrawal of the analyte, then as used herein in context of the time of response of the particular embodiment of the gas sensor, the term “fast”, “slow”, and “medium” should be understood as follows: let the gas sensor be exposed to a given analyte for a duration “ton” of time, subsequent to which let the analyte be withdrawn for a duration “toff” of time. Let the response of the particular embodiment of the gas sensor at the end of “ton” and “toff” be “rfon” and “rfoff” respectively. Then the term “fast response time” refers typically to the situation where the particular embodiment of the gas sensor achieves at least 0.9 times rf within the first minute of exposure to the analyte, i.e., the response of the embodiment of the gas sensor tends to “plateau” off after the first minute of exposure to the analyte. Another way of saying this is that an anomalous change in time derivative of the response versus time profile occurs within the first minute or thereabouts of exposure to analyte. In similar vein, if the response of the gas sensor does not exhibit a plateau for the entire duration of exposure to analyte then we the term “slow response time” is used to characterize the time response. The term “medium response time” is used to characterize the time response when the response of the gas sensor can be characterized as neither “fast” nor “slow”, typically, when the gas sensor achieves, within the first minute of exposure to analyte, a response of less than 0.5 times rf. If the response of the particular embodiment of the gas sensor changes as a result of withdrawal of the analyte, then as used herein in context of the time of recovery of the particular embodiment of the gas sensor, the term “fast recovery time” refers typically to the situation where the particular embodiment of the gas sensor withdraws at least 0.9 times rf within about two to three minutes of withdrawal of the analyte. Another way of saying this is that an anomalous change in time derivative of the response versus time profile occurs within the two or three minutes or thereabouts of withdrawal of analyte. In similar vein, if the response of the gas sensor does not exhibit any such anomalous change in the previously mentioned time derivative up to the entire duration of twenty minutes, then the term “slow recovery time” is used to characterize the recovery time. The term “medium recovery time” is used to characterize the recovery times when the recovery of the gas sensor can be characterized as neither “fast” nor “slow”, typically, when it takes the gas sensor response about ten to about fifteen minutes to come to within 0.9 times rf.
A gas sensor can be any device capable of producing an electrical signal proportional to an electrical characteristic that can be modulated upon exposure to gases. Examples of suitable devices include, but are not limited to, a resistor, a field effect transistor, a capacitor, a diode, and a combination thereof.
Examples of suitable gases to be sensed include, but not limited to, NO, NO2, SOx, O2, H2O, CO, H2, NH3, and combinations thereof. In one embodiment, the gas sensor is not susceptible to poisoning by SO2 and CO2 gases.
Referring to the drawings in general and to
The illustrated embodiment 100 includes a substrate 102. On this substrate layer is disposed a heater 104. A first glass layer 106 is positioned above the heater. On the first glass layer 106 is disposed a temperature sensing layer 108. A second glass layer 110 is positioned above the temperature sensing layer. On the second glass layer 110 is disposed at least one electrode 112. An adhesion layer 114 that completely covers the at least one electrode, and which is also in contact with the second glass layer 110 upon which is disposed at least one electrode. A response modification layer 116 is disposed on to the adhesion layer. Upon this response modification layer is disposed a gas sensing layer 118 which has a gas sensing surface 120. In some embodiments, the gas sensor may include an element for heating the gas sensor. In one embodiment, an element for heating the gas sensor may be disposed adjacent to the gas sensing layer or, adjacent to the substrate layer, or on the packaging and any combinations thereof, and/or be covered with an electrically insulating and thermally conducting layer. The heating means may be an element that is separate from the main gas sensor, such as a metal (e.g., Pt) layer disposed adjacent to the gas sensing layer. In the embodiment illustrated in
In one embodiment, the substrate 102 shown in
The glass layers 106 and 110 as shown in
In one embodiment, the dopant in the gas sensing layer 118 may be selected from the group consisting of Mg, Ti, V, Cr, Mn, Co, Ni, Zn, Nb, Ru, Rh, Pd, Ta, W, Re, Pt, and combinations thereof. In another embodiment, the dopant in the gas sensing layer 118 can be selected from the group consisting of Re, Mg, Mo, Ta, V, Mn, Ru, Cr, and combinations thereof.
In one embodiment, the oxidation state of the dopant may be such that the dopant is of n type or p type, e.g., when the gas sensing layer is composed of a material with chemical formula MαOβNγ and a dopant, wherein M is a chemical element selected from the group consisting of W, Ti, Ta, Sr, Mo, and combinations thereof, and wherein β and γ are self consistent, and wherein the dopant is a chemical element which is in a suitable oxidation state and is selected from the group consisting of Mg, Ti, V, Cr, Mn, Co, Ni, Zn, Nb, Ru, Rh, Pd, Ta, W, Re, Pt, and combinations thereof. In one example, the response to a given concentration of at least one analyte may be enhanced by varying the thickness of the gas sensing layer. In one embodiment, the gas sensing layer 118 can have a thickness from about 300 Å to about 2000000 Å. In another embodiment, the gas sensing layer 118 can have a thickness from about 500 Å to about 1500 Å. In yet another embodiment, the gas sensing layer 118 can have a thickness from about 300 Å to about 1200 Å.
In one embodiment, the gas sensing layer may be composed of more than one material selected from the group represented by the general chemical formula MαOβNγ, wherein M is a chemical element selected from the group consisting of W, Ti, Ta, Sr, Mo, and combinations thereof, and α, β, γ, are self-consistent. In such a situation the said more than one materials may together be present in forms, including but not limited to, mixture, solid solution, and combinations thereof.
A response modification layer 116 is interposed between the gas sensing layer and the adhesion layer. This response modification layer is composed of a material selected from the group consisting of Mg, Ti, V, Cr, Mn, Co, Ni, Zn, Nb, Ru, Rh, Pd, Ta, W, Re, Pt, and combinations thereof. In one embodiment, the response modification layer 116 can have a thickness from about 10 Å to about 100 Å. In another embodiment, the response modification layer 116 can have a thickness from about 20 Å to about 80 Å. In yet another embodiment, the response modification layer 116 can have a thickness from about 30 Å to about 60 Å. In one embodiment of the gas sensor, the response modification layer may aid the gas sensor in having an equi-sensitive response to any two given gases. In another embodiment of the gas sensor, the response modification layer may aid the gas sensor in having a desired value of baseline resistance. In yet another embodiment of the gas sensor, the response modification layer may aid the gas sensor in having desired levels of response and recovery times upon exposure to and withdrawal of analyte respectively. Not to be limited by any particular theory, it is possible that the response modification layer helps improve the working characteristics of the gas sensor by hindering direct physical contact between the gas sensing layer and the adhesion layer.
In one embodiment, the response modification layer may be composed of a mixture of Ti with at least one chemical element selected from the group consisting of Mg, V, Cr, Mn, Co, Ni, Zn, Nb, Ru, Rh, Pd, Ta, W, Re, Pt, and combinations thereof.
The adhesion layer 114 serves to anchor the response modification layer upon which is disposed the gas sensing layer. In one embodiment, the adhesion layer 114 can have a thickness from about 5 Å to about 300 Å. In another embodiment, the response modification layer can have a thickness from about 50 Å to about 300 Å. In yet another embodiment, the adhesion layer 114 can have a thickness from about 10 Å to about 50 Å. In yet another embodiment, the adhesion layer 114 can have a thickness from about 15 Å to about 30 Å.
At least one of the electrodes 112 may be made from any material capable of physical adhesion and electrical contact to the adjacent layers. Examples of suitable materials for the at least one electrode include, but are not limited to, Pt, Pd, Au, Ag, Ni, Ti, In, Sn, Cr, nickel nitride, tantalum nitride, titanium nitride, aluminum doped zinc oxide (ZAO), indium tin oxide (ITO), chrome, and any combination thereof.
In one embodiment, the electrodes 112 can have a thickness from about 500 Å to about 10000 Å. In another embodiment, the electrodes 112 can have a thickness from about 800 Å to about 3000 Å.
In one embodiment, at least one of the at least one electrode 112 may be a multiple layer stack of materials. Examples of suitable materials to comprise the different layers of the multilayer stack include, but are not limited to, Pt, Pd, Ti, Al, Au, Ag, Ni, In, Cr, nickel oxide, tantalum nitride, titanium nitride, aluminum doped zinc oxide, indium tin oxide, chrome, and combinations thereof.
In one embodiment in which at least one of the at least one electrode 112 is a multiple layer stack of materials, the thickness of each layer can be from about 100 Å to about 2000 Å. In another embodiment, in which the electrodes are a multiple layer stack of materials, the thickness of each layer can be from about 300 Å to about 1500 Å. In yet another embodiment, in which the electrodes are a multiple layer stack of materials, the thickness of each layer can be from about 500 Å to about 1000 Å.
In one embodiment, the electrodes 112 can be placed in an interdigitated geometry 202 as shown in the embodiment 200 in
In one embodiment, the at least one electrode may be placed adjacent to the gas sensing layer in a “sandwich” geometry, i.e., at least one electrode is disposed on either side of the gas sensing layer along its thickness direction. In another embodiment, the at least one electrode may be placed in a “side-by-side” geometry, i.e., at least two electrodes are disposed adjacent each other on the same side of the gas sensing layer. In another embodiment, an adhesion layer may be placed along those surfaces of the at least one electrode that are closest to adjacent components of the gas sensor.
In one embodiment, the gas sensing layer 118 can have a dopant concentration from about 0.2 mol % to about 5 mol %. In another embodiment, the gas sensing layer 118 can have a dopant concentration from about 0.5 mol % to about 4 mol %. In yet another embodiment, the gas sensing layer 118 can have a dopant concentration from about 2 mol % to about 3 mol %.
In some embodiments, the gas sensor may include a way of measuring the temperature of the device. A means of measuring the temperature may be disposed anywhere within the gas sensor. For example, it may be disposed adjacent to the gas sensing layer or, adjacent to the substrate layer, or on the packaging, or any combination thereof, and/or be covered with an electrically insulating and thermally conducting layer. The temperature sensing means may include but not limited to, a resistive temperature device, a thermocouple, a silicon bandgap temperature sensor and a combination thereof. The temperature sensing means may be a separate element, such as a metal (e.g., Pt) layer disposed adjacent to the gas sensing layer.
The temperature sensor can be of various types, including but not limited to, a thermocouple, a resistance temperature detector, a silicon bandgap temperature sensor, or a thermistor. The thermocouple temperature sensor can be of various types, including but not limited to Type K (Chromel®/Alumel®), Type J (Iron/Constantan), Type N (Nicrosil®), Type B, Type R, Type S, Type T (Copper/Constantan), Type C. The resistance temperature detector can be composed of various metals, but are usually made from Pt. The silicon bandgap temperature sensor can be composed of pure silicon or of chemical compounds of silicon including but not limited to silicon carbide. The thermistor temperature sensor can be composed of various materials including but not limited to ceramics and polymers. These materials can have a positive or a negative temperature coefficient of resistance. The temperature sensors may be biased in various ways, including but not limited to, voltage biasing and current biasing. Furthermore, the response of the temperature sensors may be recorded by means including but not limited to resistive measurement, potentiometric measurement and a combination thereof. The temperature sensor layer 108 shown in
Other gas sensor operating and geometry parameters being fixed, the response to a given concentration of any particular analyte may be enhanced when the gas sensing layer is maintained at particular temperatures. In certain embodiments, enhanced sensitivity may be achieved by maintaining the temperature within the range from about 250° C. to about 550° C.
Conceivably, different applied direct current (DC), alternating current (AC), or a combination thereof, of bias levels to the gas sensing layer may enhance the gas sensing characteristics such as selectivity and sensitivity towards one or another analyte. For example, according to some embodiments, varying levels of a DC bias may be used to adjust the sensitivity of the gas sensor 100 to different gas species in an analyte. For example, a gas sensor operating under a given first DC bias level might be preferentially sensitive to a first analyte species, a gas sensor operating under a second DC bias level might be preferentially sensitive to a second analyte species, and so on for different DC bias levels. This property may be used to selectively detect and measure different species of analyte. The AC and/or DC bias used in the operation of the gas sensor may be an electrical current, an electrical voltage or a combination thereof. The AC or DC response of the gas sensor during operation of the gas sensor may be an electrical current, an electrical voltage or a combination thereof.
The gas sensor may also be configured so as to have suitable one or more filters that allow only specific analytes to pass through and impinge onto, i.e., make contact, with the gas sensing layer. Conceivably, such filters may aid selective detection of given one or more analytes. Such filters may also aid in limiting the passage of certain analytes such as, for instance, particulate matter, towards the gas sensing layer. In some embodiments, membranes that serve as filters towards particular chemical or physical species present in the environment of the gas sensor may be disposed adjacent to the gas sensing layer. Such filters would provide a means for limiting or regulating the type and/or the amount of gas or particulate species that contact the gas sensing layer of the gas sensor. Examples of suitable means for limiting or regulating the type and/or amount of gas species include, but are not limited to, a thin film, such as of porous membrane filter medium (e.g., steel wool or quartz wool), an about 10 Å thick film of Pd, porous ceramic materials such as Al2O3, YSZ, SiO2 doped with Pt, Pd or other catalytic metals or alloys, and any combinations thereof. Conceivably, more than one gas sensors may be placed adjacent to each other or at different locations within the environment. Each of these gas sensors may share with each other the same filter, or may individually have one or more, same or different filters. Such an “array” of gas sensors may be used to selectively detect and measure the concentrations of different analyte species at different locations within the environment.
In one embodiment, the response of the gas sensor, or of the material composing a sensing layer of the gas sensor, may be monitored via resistive measurement, potentiometric measurement, or combinations thereof.
In one embodiment, the response of the gas sensor may be tuned to be equi-sensitive to NO and NO2 which are the two primary constituents of NOx emissions. The response ratio of the gas sensor for different concentrations of NOx may depend upon a plurality of system and environment parameters, including but not limited to, the chemical composition of the gas sensing layer, the one or more dopants that are doped into the gas sensing layer, the level of doping of the dopant, the microstructure/morphology of the of the gas sensing layer, the level of crystallinity of the gas sensing layer, the level of strain present in the gas sensing layer, the level of strain present in the adhesion layer, the level of strain present in the response modification layer, the temperature at which the gas sensing layer is maintained while performing the gas sensing, the type and nature of the bias applied across the gas sensing layer, the presence or absence of the response modification layer, the level of adhesion of the gas sensing layer to the electrodes and to the underlying glass, the material, size, design and placement of the electrodes. The microstructure/morphology of the gas sensing layer film may depend on the method used to grow the film. Some of the above mentioned system parameters are likely inter-related. In another embodiment, the response ratio of the gas sensor may depend upon the specific set of gas species that are present in the environment and on the individual concentrations of the different species present. For example, the presence of H2O in the environment being sensed may result in a modification in the response ratio of the gas sensor.
In one embodiment, the one or more dopants that are incorporated into the gas sensing layer may aid in modifying one or more response characteristics, including but not limited to, baseline resistance, response time, recovery time, of the gas sensor.
In one embodiment, it may be possible to tune the response of the gas sensor to at least one analyte by controlling the level and distribution of oxygen and nitrogen present in the gas sensing layer.
In the following measurements presented, the components of the gas sensor used may be grouped into two distinct sets depending on their source of origin and/or procurement. The five layers, i.e., the substrate 102, the heater 104 the first glass layer 106, the temperature sensing layer 108, and the second glass layer 110 were sourced from a commercial vendor. All other elements, i.e., the electrodes 112, the adhesion layer 114, the response modification layer 116, and the gas sensing layer 118, were designed and implemented by the inventors.
The following results of measurements of the response of certain embodiments of the gas sensor were performed according to the following protocol: A mixture comprised of gases N2, O2, NO, NO2, SO2, CO, was first prepared by mixing the previously mentioned gases at 300° C. This mixture gas is then introduced into the chamber where the gas sensor is mounted. The response of the gas sensor, which is maintained at a temperature of about 400° C., is continuously monitored. The exact composition of the mixture gas is dependent upon the experiment being performed. For example, if the response time of the gas sensor to, say, 50 ppm (parts per million) of NO gas has to be ascertained, the mixture gas composition is 1000 sccm of N2, 100 sccm of O2, and 50 sccm of 1% NO balanced with N2. This NO flow is maintained typically, for duration 8 or 10 minutes. The response time is obtained from the time evolution of the response upon the introduction of the NO gas in to the sample chamber. The recovery time is determined in similar vein by switching off of the flow of the NO gas, all other conditions remaining identical. The flow of NO is withdrawn, typically for duration 20 or 30 minutes. The recovery time is obtained from the time evolution of the response upon the withdrawal of the NO gas in to the sample chamber. This sequence of steps may be repeated to determine the reproducibility of the response.
In one embodiment, the gas sensing layer of the gas sensor may need to be conditioned before it displays desired and/or adequate response to any given one (or more) analyte(s). For example, when the method of deposition of the gas sensing layer is Sputtering, then the as-deposited gas sensing layer is likely amorphous. This as-deposited gas sensing layer may not display desired or adequate response characteristics to, say, NOx. Not to be limited by any particular theory, it is believed that changing the morphology of the gas sensing layer so as to tune its level of crystallinity, and/or grain size, and/or grain boundary interconnectivity, amongst other factors, will result in improved response characteristics of the gas sensor. It was determined that annealing the gas sensing layer at high temperatures in the presence of gases which contain nitrogen and oxygen (e.g., NOx) resulted in the development, in the gas sensing layer, of desired response characteristics towards NOx.
It has been estimated that the delay time associated with the response of the gas sensor as the flow of analyte is introduced/withdrawn is expected to be of the order of few 101 s. For example, referring to
In one embodiment, the gas sensor may be used to monitor and/or measure the concentration of at least one analyte in the exhausts of an automobile. For instance, the gas sensor may be positioned for enhanced monitoring and/or measurement of analytes within the exhaust system of an automobile. In another embodiment, a plurality of gas sensors may be positioned at different locations within the exhaust system of the automobile to monitor and measure the concentration of analytes in the exhausts. In another embodiment, a plurality of gas sensors may be positioned at different locations within the exhaust stream of the automobile. In another embodiment, the gas sensor may be used to monitor and/or measure the concentration of at least one analyte at other locations within the automobile. For instance, the gas sensor may be positioned for enhanced monitoring and/or measurement of analytes within the cabin of an automobile. In another embodiment, a plurality of gas sensors may be positioned at different locations within the automobile to monitor and measure the concentration of analytes within the cabin of the automobile.
Embodiments of the gas sensor of the present invention may also be used to monitor emission of NOx in applications including, but not limited to, aluminum, cement, fertilizer, glass, mineral wool, power, steel, sulphuric acid, and waste incineration industries. In the automobile sector, the gas sensor of the present invention may be used to monitor emissions in a variety of applications including, but not limited to, the emission of NOx from petrol, gasoline and diesel engine automobiles including, but not limited to, passenger cars, light commercial vehicles, lorries, trucks, and buses.
The gas sensor may also be used to meet the U.S. Environmental Protection Agency continuous emissions monitoring standards (CEMS) outlined in 40 C.F.R. §60 and 40 C.F.R. §75. The gas sensor may further be used to meet the European Union CEN emissions limit values. Still further, the gas sensors may be used in a continuous emissions monitoring system to determine “cap and trade” allowances as described by local and federal regulating authorities.
In another aspect, a gas sensor is arranged within an encapsulation in a flip-chip arrangement. In a flip-chip arrangement, the gas sensor is flipped upside down, such that all of the top sensitive surface area of the device including the area surrounding the sensitive areas of the device, are protected from gases to be monitored. An additional protective board protects the back surface of the chip. Directly over the sensitive area of the device, a slit, or opening in the ceramic board to which the chip's top surface is mounted, is created to allow the gases to flow to the gas sensing layer. A layer of high temperature stable conductive material, such as Pt or Au, may be used to interconnect the components of the gas sensor to leads in the encapsulation layer. This flip chip arrangement enables interconnect in a higher vibration and higher temperature, for example greater than 500° C., environments than conventional wire bonds, which are susceptible to fatigue failure. The interconnection using platinum and/or gold “bumps” to connect the components, such as the at least one the electrodes to the leads helps to enable the use of the gas sensor in harsh environments.
In one embodiment the gas sensor may be configured to be operable in harsh environments in which are present locations where the temperature is between about 200° C. and about 800° C. In another embodiment, the gas sensor may be configured to be operable in harsh environments in which are present locations with temperature is between about 200° C. and about 600° C. In yet another embodiment, the gas sensor may be configured to be operable in harsh environments in which are present locations with temperature is between about 300° C. and about 500° C.
The gas sensor is cost effective in that it has a long working life (˜103 hours) and provides highly repeatable readout. The cost effectiveness is further enhanced because of the simple modular design of these gas sensors allowing ready scaling of the manufacturing process to large volumes.
The gas sensor may be encapsulated in a packaging. The encapsulation further protects the gas sensor from the high temperature and corrosive atmosphere in the harsh environments where these gas sensors are likely to be used. The encapsulation acts to cover exposed surfaces of such elements of the device as the adhesion layer, the electrodes, the first glass layer, the thermometer, the second glass layer, the heater, and the substrate, which do not by themselves, sense the gases. This encapsulation may also involve forming a bond with the underlying layer (substrate), so as to not permit flow of gases and corrosive materials (e.g., particulate matter, hydrocarbons) that would be detriment to the device over time. Examples of such suitable materials for encapsulation include, but are not limited to, silicon carbide, ceramic based epoxies such as those containing alumina, glass, quartz, silicon nitride, silicon dioxide and a combination thereof.
The encapsulation layer can be deposited by any known method, such as plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD), and a combination thereof. The encapsulation is such that at least a portion of the gas sensing layer remains exposed to ambient gases. With the application of an encapsulation the gas sensors may be protected in harsh environments and have a longer working life. Such protection against harsh environments would allow for the use of these gas sensors in a wide variety on settings, including but not limited to, boiling water reactor, automotive and locomotive petrol or diesel engine exhaust, industrial process (glass, aluminum, steel, and petroleum) plant exhaust. It would further protect the gas sensor from the particulate matter that may be present in the exhaust streams of the previously mentioned environments. Such particulate matter may potentially be detriment to the gas sensor as they may adhere to and/or corrode the sensor thereby hindering the detection of exhaust gases by hindering contact between the exhaust gases and the gas sensor.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
