| Thin film mixed potential sensors -> Monitor Keywords |
|
|
  Thin film mixed potential sensorsUSPTO Application #: 20070193883Title: Thin film mixed potential sensors Abstract: A mixed potential sensor for oxidizable or reducible gases and a method of making. A substrate is provided and two electrodes are formed on a first surface of the substrate, each electrode being formed of a different catalytic material selected to produce a differential voltage between the electrodes from electrochemical reactions of the gases catalyzed by the electrode materials. An electrolytic layer of an electrolyte is formed over the electrodes to cover a first portion of the electrodes from direct exposure to the gases with a second portion of the electrodes uncovered for direct exposure to the gases. (end of abstract) Agent: Los Alamos National Security, LLC - Los Alamos, NM, US Inventors: Fernando H. Garzon, Eric L. Brosha, Rangachary Mukundan USPTO Applicaton #: 20070193883 - Class: 204426000 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Apparatus, Electrolytic, Analysis And Testing, Solid Electrolyte, Gas Sample Sensor, Planar Electrode Surface The Patent Description & Claims data below is from USPTO Patent Application 20070193883. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0002] The present invention relates generally to gas sensors, and, more particularly, to mixed-potential sensors for the detection of various oxidizable and reducible gases. BACKGROUND OF THE INVENTION [0003] Mixed potential sensors are used for the detection of various oxidizable and reducible gases. Oxidizable gases include hydrogen, hydrocarbons, carbon monoxide, nitric oxide, ethanol, and the like, while reducible gases include oxygen, nitrogen dioxide, and the like. Typical sensors utilize an ionic conducting electrolyte, such as yttria stabilized-zirconia (YSZ), and thin film metal and/or metal oxide electrodes, such as platinum (Pt) and perovskite-type oxides. Multiple reduction/oxidation reactions occurring between the gas phase and the electrode/electrolyte interface cause mixed potentials of differing magnitude to develop at the dissimilar electrodes. The selectivity of such sensors is achieved by the proper selection of the metal and metal oxide electrodes, while the stability of the sensor is achieved by the precise control of the surface area of the electrode and the 3-phase interface region (gas-electrolyte-electrode) of the sensor. [0004] The mixed-potential that is developed at an electrode-electrolyte interface in the presence of oxidizable gases, such as CO or hydrocarbons, is fixed by the rates of reduction and oxidation of the oxygen and the oxidizable gas, respectively (Reactions 1 and 2). Carbon monoxide is used here as an example but the theory is also applicable to other oxidizable/reducible gases: 1/2O.sub.2+V.sub.o+2e.revreaction.O.sub.0 Reduction (1) CO+O.sub.0CO.sub.2+V.sub.o+2e.sup.- Oxidation (2) [0005] When the kinetic rate for the reduction reaction equals that of the oxidation reaction a stable potential is established. Similar reactions with different kinetics occur on the other electrode triple-phase boundary area. The difference in potential between the electrodes is the device output voltage. The preferred mixed potential CO sensing device consists of an electrode that kinetically inhibits the oxygen reduction reaction yet is fast at CO electro-oxidation and a second electrode that exhibits fast oxygen reduction kinetics yet is poor at CO electrochemical oxidation. [0006] Since the mixed-potential is controlled by the kinetics of various reactions, control of the 3-phase (electrolyte/electrode/gas) area is of importance. Moreover, since the response time of these sensors is fixed by the speed of various reactions, the sensor design is preferably optimized to maximize the rates of reactions 1 and 2. [0007] In the prior art, a dense YSZ electrolyte is used as the substrate and a metal or metal oxide electrode is deposited on top of this electrolyte. The length of the active 3-phase interface is controlled by the morphology of the electrode. A highly porous electrode results in better gas access and greater 3-phase interface, whereas a denser electrode leads to poorer gas access and less 3-phase interface. One drawback of this type of arrangement is that the gas has to meander through the pores of a catalytically active material (the electrode) before reaching the 3-phase interface where the reduction and oxidation reactions occur. Hence, the hydrocarbons (or other reducing gases) are heterogeneously oxidized at the metal (or metal oxide) electrode before they reach the 3-phase interface with the electrolyte, with a concomitant loss in sensor sensitivity and increase in response time. [0008] Mukundan et al. (U.S. Pat. No. 6,656,336, issued Dec. 2, 2003) disclose a non-methane hydrocarbon sensor that measures the amount of hydrocarbons present in an exhaust stream containing oxygen. The selectivity of the device is achieved by the proper selection of the oxide electrode, while the stability of the device is achieved by the precise control of the surface area (SA) of the electrode and the 3-phase interface region (3PA) (gas-electrolyte-electrode) of the sensor. By controlling the ratio of the SA to the 3PA, the rates of the heterogeneous catalysis and electrochemical catalysis are controlled for any particular electrode used. Thus, by proper selection of the electrode material and electrode dimensions, the magnitude of sensor response to any particular gas species can be amplified (selectivity). [0009] Mukundan et al. (U.S. Pat. No. 6,605,202, issued Aug. 12, 2003) disclose a mixed-potential electrochemical sensor for the detection of gases, such as CO, NO, and non-methane hydrocarbons, in room air. The sensor utilizes a ceria-based electrolyte, and metal wire electrodes. The stability and reproducibility of the sensor is achieved by using wire electrodes instead of the usual thin or thick film electrodes that are currently employed. The metal wire-electrodes are directly embedded into the electrolyte and co-sintered with the electrolyte in order to produce a stable metal/electrolyte interface. [0010] The present invention uses thin film technology to produce a mixed-potential electrochemical sensor for the detection of gases, such as CO, NO, and non-methane hydrocarbons, where the sensor exhibits fast response time, good reproducibility, and can be produced using inexpensive thin film technology. [0011] Bloemer et al. (U.S. Pat. No. 6,352,631, issued Mar. 5, 2002) teach a mixed-potential sensor formed by depositing electrodes on a dense electrolyte, where the electrodes are exposed directly to the gas stream to be sampled. Muller et al. (U.S. Pat. No. 4,277,323) teach an oxygen sensor where two electrodes are put on a porous substrate and completely covered with a thin YSZ film. While this works well for an O.sub.2 sensor and improves performance, this will not provide a mixed potential sensor. Further, Mueller et al. provide gas access only through a porous substrate onto an electrode embedded in an electrolyte where the 3-phase interface region is not well defined. [0012] Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. SUMMARY OF THE INVENTION [0013] In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a mixed potential sensor for oxidizable or reducible gases and a method of making the mixed potential sensor. A substrate is provided and two electrodes are formed on a first surface of the substrate, each electrode being formed of a different catalytic material selected to produce a differential voltage between the electrodes from electrochemical reactions of the gases catalyzed by the electrode materials. An electrolytic layer of an electrolyte is formed over the electrodes to cover a first portion of the electrodes from direct exposure to the gases with a second portion of the electrodes uncovered for direct exposure to the gases. BRIEF DESCRIPTION OF THE DRAWINGS [0014] The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: [0015] FIGS. 1A and 1B are a partial top view and a side view of one embodiment of a sensor according to the present invention. [0016] FIG. 2 is a pictorial illustration of a sensor shown in FIGS. 1A and 1B. [0017] FIG. 3 graphically compares sensor performance without and with a thin electrolyte cover for the electrodes. [0018] FIG. 4 graphically depicts sensor performance with electrodes on various substrates. [0019] FIG. 5 graphically depicts sensor performance in various oxidizable and reducible gases. DETAILED DESCRIPTION [0020] The sensor design of the present invention provides dense thin film electrodes deposited on an inert substrate in a non-intersecting, preferably parallel, geometry with a second layer of thin film electrolyte that partially covers the two electrode strips. The fabrication of sensors in this manner produces a device with a reproducible triple phase boundary. The triple phase boundary is the region where the gases come in contact with the electrode/electrolyte interface. If the YSZ electrolyte film were 100% dense, then the region is defined by the line at which the YSZ ends and the gas/electrode interface begins. If the YSZ is not completely dense, there is then gas diffusion through the electrolyte to the surface contact between the electrodes and the overlying electrolyte, where the electrode/electrolyte interface forms a triple phase boundary. Continue reading... Full patent description for Thin film mixed potential sensors Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Thin film mixed potential sensors 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 Thin film mixed potential sensors or other areas of interest. ### Previous Patent Application: Electrochemical test strip for multi-functional biosensor Next Patent Application: Method for producing a composite copper wire Industry Class: Chemistry: electrical and wave energy ### FreshPatents.com Support Thank you for viewing the Thin film mixed potential sensors patent info. IP-related news and info Results in 1.38366 seconds Other interesting Feshpatents.com categories: Novartis , Pfizer , Philips , Polaroid , Procter & Gamble , |
||
