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  Sensing element and method of making the sameUSPTO Application #: 20070108047Title: Sensing element and method of making the same Abstract: Disclosed herein is a sensing element comprising: an electrochemical cell; wherein the sensing element comprises a metal selected from the group consisting of Pd and alloys and combinations comprising at least one of the foregoing; and wherein the electrically conductive element is thermally stable at temperatures of greater than or equal to about 1,200° C. (end of abstract)
Agent: Paul L. Marshall Delphi Technologies, Inc. - Troy, MI, US Inventors: Fenglian Chang, Kerry J. Kruske, Rick D. Kerr USPTO Applicaton #: 20070108047 - Class: 204400000 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Apparatus, Electrolytic, Analysis And Testing The Patent Description & Claims data below is from USPTO Patent Application 20070108047. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present disclosure is related to a sensing element and a method of making and, in particular, to a sensing element containing a palladium and/or palladium alloy electrically conductive element and a method of making the same. BACKGROUND [0002] Sensors, in particular gas sensors, have been utilized for many years in several industries (e.g., flues in factories, in furnaces and in other enclosures; in exhaust streams such as flues, exhaust conduits, and the like; and in other areas). For example, the automotive industry has used exhaust gas sensors in automotive vehicles to sense the composition of exhaust gases, namely, oxygen. A sensor may be used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion. [0003] One type of sensor employs an ionically conductive solid electrolyte between porous electrodes. For oxygen detection, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the application of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, i.e., reference gas, is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine's exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (EMF) is developed between the electrodes according to the Nernst equation. [0004] According to the Nernst principle, chemical energy is converted into electromotive force. Thus, a gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases ("sensing electrode"), and a porous electrode exposed to the partial pressure of a known gas ("reference electrode"). Sensors used for automotive applications typically employ a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine's exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor's ionic conductivity at low exhaust temperatures. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation: E = ( - R .times. .times. T 4 .times. .times. F ) .times. ln .function. ( P O 2 ref P O 2 ) [0005] where: [0006] E=electromotive force [0007] R=universal gas constant [0008] F=Faraday constant [0009] T=absolute temperature of the gas [0010] P.sub.O.sub.2.sup.ref=oxygen partial pressure of the reference gas [0011] P.sub.O.sub.2=oxygen partial pressure of the exhaust gas [0012] Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (EMF) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating in fuel rich or fuel lean conditions, without quantifying the actual air to fuel ratio of the exhaust mixture. [0013] In addition to oxygen, the exhaust gas contains many components including carbon monoxide, carbon dioxide, hydrogen, water, nitrogen oxides, nitrogen, and a variety of hydrocarbons and hydrocarbon derivatives. Because the exhaust gas is a non-equilibrium mixture containing products of incomplete combustion, the oxygen partial pressure is not an equilibrium pressure. Because the oxygen partial pressure is not at equilibrium, sensors do not operate at stoichiometric air to fuel ratios per the Nernst equation. In addition, the use of zirconia-based electrolyte materials contributes to non-ideal sensor behavior. [0014] To provide a means of monitoring the cell potential and to circumvent at least some of the difficulties associated with non-equilibrium conditions, catalytic electrodes are used to both catalyze the oxidation reactions and to equilibrate the local oxygen concentrations. Ideal sensors produce a sharp EMF or voltage step at a stoichiometric air to fuel ratio per the Nernst equation. Manufactured sensors, however, exhibit non-ideal behaviors, for example, a broadened voltage transition that occurs over a range of air to fuel ratios near the stoichiometric ratio. In addition, the sensor EMF may depend upon mass transport processes, adsorption, desorption and chemical reactions that occur at the electrodes. [0015] Platinum (Pt) is widely used as the material for various electrically conductive components of exhaust sensors such as, for example, electrodes, sensing elements, heaters, ground planes, leads, vias, contact pads, and the like. The use of Pt in exhaust sensors such as oxygen, nitrogen oxide and ammonia is desirable because it can withstand many process application temperatures without degradation due to its exceptional physical and chemical properties. [0016] Accordingly, a need exists in the sensor manufacturing art for less expensive methods and/or materials for producing such sensors. SUMMARY [0017] Disclosed herein is a sensing element comprising: an electrochemical cell; wherein the sensing element comprises a metal selected from the group consisting of Pd and alloys and combinations comprising at least one of the foregoing; and wherein the electrically conductive element is thermally stable at temperatures of greater than or equal to about 1,200.degree. C. [0018] Also disclosed herein is a method of making a sensing element, comprising: forming a precursor material comprising a plurality of irregularly shaped Pd particles and an organic vehicle; disposing the precursor material on a supporting surface to define an electrically conductive element; and heating the precursor material to a temperature of greater than or equal to about 1450.degree. C. for a sufficient period of time to sinter the precursor material and form the sensing element; wherein the electrically conductive element is thermally stable at a temperature of about 1,200.degree. C. [0019] The above described and other features are exemplified by the following figures and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS [0020] Refer now to the figures, which are exemplary embodiments, and wherein like elements are numbered alike. [0021] FIG. 1 is an expanded isometric view of an oxygen sensing element. [0022] FIG. 2A shows a scanning electron microscope (SEM) image of an irregularly shaped Pd powder. [0023] FIG. 2B shows a SEM image of a spherically shaped, relatively high surface area Pd powder. [0024] FIG. 3A is a copy of an optical micrograph showing cracks and delamination in a Pd thick film electrically conductive element formed from a relatively high surface area Pd powder, after firing at about 1,530.degree. C. [0025] FIG. 3B is a copy of an optical micrograph showing a Pd thick film electrically conductive element formed from a relatively low surface area Pd powder, after firing at about 1,530.degree. C. [0026] FIG. 4A is a copy of an optical micrograph of a Pd--Rh alloy thick film electrically conductive element formed from a relatively high surface area Pd powder, after firing at about 1,530.degree. C. Continue reading... Full patent description for Sensing element and method of making the same Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Sensing element and method of making the same patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Sensing element and method of making the same or other areas of interest. ### Previous Patent Application: Sputtering target and method for finishing surface of such target Next Patent Application: Sensors Industry Class: Chemistry: electrical and wave energy ### FreshPatents.com Support Thank you for viewing the Sensing element and method of making the same patent info. 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