Embodiments are generally related to gas sensors. Embodiments are also related to NOx gas sensors. Embodiments are also related to techniques for measuring NOx gas content from automotive exhaust in high temperature harsh environments. Embodiments are also related to techniques for measuring NO, NO2 and NOx gas during air quality monitoring.
Environmental pollution, such as air pollution, is a serious problem that is particularly acute in urban areas. Much of this pollution is produced by exhaust emissions from motor vehicles. Governmental standards have been set for regulating the allowable amounts of certain pollutants in automobile exhausts. Additionally, in many geographic areas, periodic inspections are required in order to ensure that vehicles meet these standards. The ability to measure exhaust pollutants during a realistic operating period of a vehicle is a growing need in light of recent efforts to regulate and stem the flow of automotive exhaust pollution.
NOx gases, which are present in automotive exhaust pollution, are known to cause various environmental problems such as smog and acid rain. The term NOx actually refers to several forms of nitrogen oxides such as NO (nitric oxide), NO2 (nitrogen-di-oxide) and/or N2O (nitrous oxide). An NOx sensor is one solution for detecting NOx gases. A NOx sensor is typically implemented as a high temperature device that detects nitrogen oxides in combustion environments, such as automobile or truck tailpipes or in factory smokestacks or air pollution in ambient air or cabin air quality.
The main problems that have limited the development of a successful NOx sensor (which are often composed of many sensors) are: selectivity, sensitivity, stability, reproducibility, response time, along with detection limitations and cost issues. Additionally, due to the harsh environment of combustion, a high gas flow rate can cool the sensor, which alters the signal or de-laminates the electrodes over time. Soot particles can also degrade the sensor materials. A NOx sensor should be stable at a temperature of approximately 900° C. and should constantly withstand harsh environments, particulate matter, unburnt hydrocarbons, carbon monoxide, nitrogen, oxygen and water vapor exposures. The sensitivity to NOx of such a sensor should also be great in comparison to other gases and should ideally demonstrate response and recovery times below one second.
Solid-state metal oxide sensors are widely regarded as a low-cost option for exhaust sensors, but offer questionable performance characteristics. Recent development work has significantly improved the performance of solid-state sensors, without increasing the sensor cost. Most semiconductor metal oxides undergo surface interactions, such as physisorption and chemisorption, with gas molecules at elevated temperatures (e.g., 300° C.-600° C.). Because most semiconductor sensors are polycrystalline-composed of multiple crystallite grains pressed or sintered into a continuous structure incorporating grain boundaries, the adsorbed gases have significant electronic effects on the individual crystalline particles.
These gas-solid interactions result in a change in electron or hole density at the surface, forming a space charge, which in turn results in a change in overall conductivity of the semiconductor oxide. This sensing mechanism, however, also tends to result in poor selectivity and excessive baseline drift. Modification of the sensor materials and processing methods can significantly reduce these problems. The careful selection of sensing materials is critical for improving sensor performance. Recently, substantial performance increases have occurred in semi-conducting metal oxide sensors when grain sizes are reduced to the nanoscale level.
The role of gases and the measurement of the concentration have always received wide spread applications in many fields of science and technology. In nano-sized materials, the surface-to-bulk ratio is much greater than for coarse materials, so that the surface properties become paramount, which makes them particularly appealing in applications where such properties are exploited, as in gas sensors. Grain size reduction is one of the main factors enhancing the gas sensing properties of semi conducting oxides and indeed sharp increases in sensitivity are to be expected when the grain size becomes smaller than the space-charge depth according to currently-accepted mechanisms. Thus, the application of nano-structured materials, both as powders and thin films, in gas sensors is rapidly arousing the scientific community interest.
In an effort to address the foregoing difficulties, it is believed that nanocrystalline yttrium manganese oxide (YMnO3) can be used as a sensing element whose conductivity is very stable in reducing atmospheres for long exposures, while maintaining a melting point is above 1600° C. It is believed that nano-crystalline powders of material such as YMnO3 can be employed for configuring thin films on platinum comb type electrodes preformed on aluminium substrates as described in greater detail herein.
The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide for an improved gas sensor.
It is another aspect of the present invention to provide for an NOx gas sensor configured using nanocrystalline Yttrium Manganese Oxide (YMnO3) and doped Y1-x RxMn1-y TyO3 (where R and T represent rare-earth metals and transition metals respectively and x and y values ranging from 0 to 0.4) a sensing component.
It is another aspect of the present invention to provide for a method for measuring NOx gas content from automotive exhaust in high temperature harsh environments.
It is another aspect of the present invention to provide for a method for NO, NO2 and NOx gas content measuring for pollution control in ambient as well as cabin air quality environments.
The aforementioned aspects and other objectives and advantages can now be achieved as described herein. An NOx gas sensor for measuring NOx gas content from automotive exhaust is described herein. Such a sensor can be located in the exhaust system of an automotive internal combustion engine. Also disclosed is a method for producing such a gas sensor.
The NOx gas sensor apparatus generally includes a substrate, and a plurality of electrodes preformed and located on one side of the substrate. A platinum heater is generally located the other and opposite side of the substrate. A coating of nano-crystalline powders of a semi-conducting oxide material located and configured on the plurality of electrodes preformed on the substrate, thereby forming a gas sensor for the detection of NOx. The substrate may comprise a ceramic material, glass, alumina and/or another type of high-melting material. The electrodes, along with the heater are preferably composed of platinum. The semi-conducting oxide material preferably YMnO3.
YMnO3 and doped Y1-x RxMn1-y TyO3 compounds can provide a semi conducting oxide material in which conductivity is very stable in reducing atmospheres for long exposures. Additionally, the melting point of YMnO3 is above a temperature of 1600° C. The NOx gas sensor operates based on the electrophillic absorption of NOx gas in which the change in conductivity is measured and the NOx gas sensor calibrated with known concentrations. Harsh gases such as CO and hydrocarbons will burn off very fast on the surface of the NOx gas sensor at and above 800° C. NOx diffuses into the sensor film to provide enhanced sensitivity. A catalytic mesh can be provided to prevent the CO and hydrocarbons from entering into the NOx gas sensor and avoiding cross-sensitivity and interference from other gases.
The NOx gas sensor described herein is very simple to fabricate and possesses a fast response and recovery time for the NOx gas because of the nano-size particles employed for this purpose. YMnO3 can be synthesized with various dopants such as lanthanum, cobalt, chromium, copper and nickel by employing a Sol-Gel process to produce the nano-sized powders along with permitting the fabrication of thin and thick films by electrophoretic deposition, dip coating and also RF magnetron sputtering on preformed platinum electrodes and a platinum heater on the ceramic substrate.
The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein.
The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.
Referring to
The gases, for example, NO, NO2 or NOx gas 104 and/or dry air 105, are delivered by the gas tanks 141 and/or 140, which is then passed through the mass flow controller 110. By adjusting the flow rate of gas using the mass flow controller 110, the concentration of NOx gas 104 and dry air 105 can be varied. Similarly, by adjusting the two way gas valve 115, the NOx gas 104 and/or dry air 105 can flow to the sensor 120, which functions as a NOx sensor, which detects the gas content. Current voltage properties can be measured using the high voltage source or power supply 135 in association with the digital multimeter 130 and the computer 125. The conductance of the NOx sensor 120 can be measured using the digital multimeter 130. The change in resistance and relative work function can be simultaneously monitored by the digital multimeter 130. The control computer 125 is generally operable to control and manage the overall operation of the testing apparatus 100. Note that the sensitivity of the gas sensor 120 can be defined as the ratio of the resistance of a sensor element of gas sensor 120 in air with respect to the resistance of the sensor element in the test gas atmosphere as indicated by the following equation (1):
S=R
air
/R
gas (1)
Referring to
The gas sensor element 220 functions based on the changes of an oxide film resistance resulting from physisorption, chemisorption and catalytic reactions of the gases in the surface of the film. The electrodes 205 are preferably configured as an arrangement of interdigital comb type platinum electrodes 205 formed on one side of the alumina ceramic substrate 215. On the other side of the sensor element 200, the platinum heater 220 is provided to maintain the sensor element 200 at high temperatures. YMnO3 can be synthesized with various dopants like lanthanum, cobalt, chromium, copper and nickel by employing a Sol-Gel process to configure nano-size powders and to fabricate thin and thick films by electrophoretic deposition, dip coating, RF magnetron sputtering on the preformed platinum electrodes 205 and the platinum heater 220 on the alumina ceramic substrate 215. A semi-conducting material 210 can also be configured upon the electrodes 205. Note that material 210 can be, for example, YMnO3.
A Sol-Gel operation or a co-precipitation technique can be utilized to easily control the film structure and introduction of dopants by changing the composition of solution and has a low process cost than other techniques. A sintering operation can be carried out to enhance the adherence of these films to the alumina ceramic substrate 215. Ceramic substrates that can be used may typically select from alumina, zirconia, metal silicates or phosphates or glasses. The gases are absorbed onto the sensor surface 225 and depending on the nature of their interaction electrons, can be trapped or released into the bulk. Changes in the ambient atmosphere are generally reflected in changes in the resistance of the sensor element 200.
Referring to
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The sensor element 200 can be then processed at a specific high temperature, which determines the specific characteristics of the finished sensor element 200 and hence the gas sensor 120 depicted in
Referring to
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Referring to
The sensor described herein is relatively simple to fabricate and possesses a fast response and recovery for the NOx gas because of the nano-sized particles employed for this purpose. Due to a large surface area and the reactive nature of nano-crystalline powders, such benefits can be achieved. The electronics used to measure conductivity change are much simpler in nature and cost less compared to that of electro-chemical and high-conducting materials.
Referring to
Based on the foregoing, it can be appreciated that an NOx gas sensor apparatus can be implemented, which includes a substrate and a plurality of electrodes pre-formed and located on one side of the substrate. A platinum heater can be located on another and opposite side of the substrate. A coating of nano-crystalline powders of a semi-conducting oxide material can then be located and configured on electrodes pre-formed on the substrate, thereby forming a gas sensor for the detection of gases selected from a group comprising NO, NO2 and/or NOx. The coating of nano-crystalline Yttrium Manganese Oxide (YMnO3) can be provided by Y1-x RxMn1-y TyO3, wherein the variables R and T respectively represent rare-earth metals and transition metals and the x and y values range from 0 to 0.4. The substrate may comprise a ceramic material selected from the group comprising of alumina, zirconia, metal silicates, glass and metal phosphates. The ceramic material can comprise a material that has a melting point in a range between about 1000° C. and about 2000° C. The semi-conducting oxide material can comprise nano-crystalline Yttrium Manganese Oxide (YMnO3) and doped Y1-x RxMn1-y TyO3 (where R and T represent rare-earth metals and transition metals respectively and x and y values ranging from 0 to 0.4).
Additionally, coating of nano-crystalline powders of the semi-conducting oxide material can be located and configured on the plurality of electrodes pre-formed on the substrate by: (a) synthesizing the semi-conducting oxide material with a plurality of dopants by employing a sol-gel process in order to provide a plurality of nano-sized powders; (b) fabricating a thick and a thin film by an electrophoretic deposition, dip coating and RF magnetron sputtering on the plurality of electrodes and the platinum heater; and (c) providing a catalytic mesh in order to eliminate a plurality of gases other than NOx from entering into the gas sensor.
A catalyst material can be provided in order to convert NO to NO2 and thereby detect NOx gas for any combination of NO and NO2 by the YMnO3 or doped YMnO3 NOx gas sensor and provide a same output thereof. Additionally, two similar YMnO3 sensor elements can be mounted in the exhaust environmental compatible metal housing and maintained at two different temperatures to measure the NO and NO2 gas concentrations by the use of simple algorithms. The sensitivities and the sensing properties for NO and NO2 are opposite to each other. The sensitivities for NO and NO2 at different temperatures are different for the same sensing element. Thus, by maintaining the two sensor elements at two different temperatures, the signals generated by each sensor are different. A combination of an NO2 sensor, which senses only NO2 and does not sense NO and two YMnO3 or doped YMnO3 sensors, can be used detect NO, NO2 and NOx separately.
Additionally, a catalyst material (e.g., WO3, BaO, Ga2O3, BaWO4, CaWO4, Ba2WO5, and Ca2WO5) can be provided on top of the NOx gas sensor element in order to convert NO to NO2 and thereby detect NOx gas for any combination of NO and NO2 by the YMnO3 NOx gas sensor and provide a same output thereof. The heater described herein can be formed utilizing a screen printing on the substrates following a sintering operation at a temperature of 1200° C.
It will be appreciated that variations of the above disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.